SIGMUND JAFFEAND HADLEY W. FORD
1832
The Photolysis of Nitrogen Dioxide in the Presence of Nitric Acid at 3660 A and 25"'
by Sigmund Jaffe and Hadley W. Ford Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Califurnia (Received December 14, 1966)
Nitrogen dioxide was irradiated a t 3660 A in the presence of "03. Both NO2 and HNOI decomposition is 0 "0, --+ were decomposed. The proposed mechanism for "03 OH NO3 followed by OH "01 + H2O Nos. The specific rate constant for the 0 HN03 reaction is estimated to be -1O'O 1. mole-' sec-'. Quantum yields are reported as a function of NO2, "08, and NO pressures.
+
+ +
Introduction This work was performed as part of a continuing study of the photochemistry of NO2. Its specific purpose was to determine the mechanism of nitric acid decomposition which results from irradiation in the presence of X02. Nitric acid shows continuous absorption below 3300 A indicating dissociation with a primary yield of unity. The process that appears to be energetically possible in solar radiation2 is
HN03
+ hv -+ OH + NOz (-
53 kcal)
However, in the presence of NOz, decomposition takes place above 3300 A. Dissociation must, therefore, result from secondary reactions with the products of KO, photolysis. It is well established3-5 that the primary process in the photolysis of NO2 at 3660 A is
+ hv
n'02
--f
Thus, decomposition of "03 action with oxygen atoms
0
+
NO(Z~)
o(3~)
could result from re-
+ H N 0 3 + O H + NO,
Such reactions of 0 atoms and OH radicals are of special interest in the photochemistry of air pollution.
Experimental Section The apparatus and procedures used in this work were the same as those reported earlier.6a A 10-cm long quartz cell containing the gas samples was irradiated The Journal of Physical Chemistry
+
+
with monochromatic light derived from a high-pressure mercury arc. The initial and final NO2 pressures were determined at 4350 A with a Cary Model 11 spectrophotometer. Spectra of pure "Os, pure NO2, and the products of photolysis showed that NO2 was the only species absorbing appreciably at 4350 A. The initial pressure of "03 was measured with a stainless steel Wallace and Tiernan Bourdon gauge. The use of the stainless steel gauge and Kel-F stopcock grease was necessary because H N 0 3 reacts with most stopcock greases and manometer oils. The NO2 pressure was followed as a function of time by a continuous record of the ouput of a solid-state photocell. The photocell had a window measuring 1.5 in. square and, when placed at the end of the quartz cell, absorbed the transmitted light which was focused on it. The intensity of the incident and transmitted light was determined by the photolysis of pure NOz for which the quantum yields have been established. The NO2 pressure was calculated from the measurement of the light absorbed using the extinction coef~~
~
~~
(1) This paper presents results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aero-
nautics and Space Administration. ( 2 ) P. A. Leighton, "Photochemistry of Air Pollution," Academic Press Inc., New York, N. Y . , 1961, pp 62-64. (3) H. W. Ford, Can. J . Chem., 38, 1780 (1960). (4) F. E. Blacet, T. C. Hall, and P. A. Leighton, J . A m . Chem. SOC., 84, 4011 (1962).
(5) (a) H. W. Ford and 5. Jaffe, J . Chem. Phys., 38, 2935 (1963); (b) J. N. Pitts, Jr., J. H. Sharp, and 9. I. Chan, ibid., 40, 3655 (1964).
PHOTOLYSIS OF NITROGEN DIOXIDE IN
THE
PRESENCE OF NITRICACID
1833
ficient 8.15 X mm-' cm-' at 3660 A, reported by Holmes and Daniels.6 Corrections were made for the absorption of light by N204 using 3.21 X lo-* mm-I cm-' for the extinction coefficient.'j The final pressure of NO was determined by the addition of excess 0 2 to the photolysis cell a t the end of a run. N O was converted t o NO2by the reaction 2N0
+
0 2 ---t
2N02
0
The "03 used in this work was prepared by the reaction of Baker's Analyzed anhydrous K N 0 3 with pure HzS04. The acid, distilled under vacuum a t room temperature, was 99.8% pure and was colorless. When stored in the dark a t -lo", the " 0 3 did not decompose. Small samples were stored a t -196' for use in the present work.
50
100 150 TME IN MINUTES
200
Figure 1. NOz pressure as a function of time.
Results A series of preliminary experiments were carried out with combinations of "03, "03 NO2, HNOa NO2 02,H N 0 3 NOz NO, and "01 NO. These samples were allowed to stand in the dark (up to 15 hr) except for the periodic determination of the NO2 concentration by means of the Cary spectrophotometer. No reactions were observed except in the case of mixtures containing NO, where a relatively slow conversion to NO2was detected. Assuming the simple mechanism
+
+
+
"03
"02
+
+ NO "02
+
+
+ + +
+
NO2 HNO, +H2O NO NO2
+ NO3 +2N02
predominates and the NOz pressure increases. The N O z pressure continues to increase until it reaches a maximum. It is assumed that the "03 is almost completely dissociated at this point and that further photolysis results in the dissociation of NOz in the presence of NO, 02,N2, HzO, and small quantities of "03 and perhaps HXO2. The ratio of (NO2) to ("03)
TIME 100IN MINUTES 150
200
Figure 2. NO2, NO, and HNOa pressures as a function of time (NO calculated from 02 titration).
was about 20 a t the maxima in curves such as that in Figure 1. The dissociation approaches a steady state when the recombination reactions such as
+ 2N02 0 + NO + M +NO2 + M
+
a second-order rate constant for the "03 NO reaction equal to 1 X lo2 1. mole-' sec-' was obtained. This value is much too small to influence the results of the photochemically induced reactions. Mixtures of NO2, HN03, and N2were photolyzed a t 3660 A and 25" to determine the dependence of the quantum yield on the pressures of NO2 and " 0 3 . The results of a typical experiment are shown in Figure 1. It can be seen that there is an initial period during which the rate of dissociation of NO2is greater than the rate of formation of NO2. However, when the NO pressure builds up sufficiently, the formation of NO2, presumably by the reaction
KO
50
2NO
0 2 --j-
are as fast as the dissociation reactions
+ hv--tNO + 0 0 + NO2 +NO + Separate samples of NO2 + were titrated with NO2
0 2
" 0 3
after a s,hort period of photolysis, a t the maximum, and after the system approached the final steady state. Typical results are shown in Figures 2 and 3. The quantity of NO produced at these points suggests that NO builds up slowly a t first, reaches a steady state when the NO2 production becomes linear, and then builds up to the final steady state after the maximum in NO2 is passed. When NO was added to the system before photolysis, no initial decrease in NOz pressure was observed. The NOz pressure built up steadily as shown in Figure 4. Lt appears that the NO NO3 reaction is rapid under these conditions. 0 2
+
(6) H. H. Holmes and F. Daniels, J. Am. Chem. Soc., 56, 630 (1934).
Volume 71, .%-umber 6 M a y 1967
SIGMUND JAFFE
1834
AND
HADLEY W. FORD
as described by Ung and Back' was assumed to have taken place to some extent as evidenced by the appearance of a mass 44 peak in the mass spectrogram of the products of the photolysis. CO does not react readily with 0 atoms nor does CO affect the rate of normal NOz photolysis any more than NZ or COZ does.* The OH radical is assumed to result from the reaction
5 E
E4
I: I
0 M
200
I50
TIME IN MWTES
Figure 3. NO*, NO, and HNOa pressures as a function of of time (NO calculated from 0%titration).
+ HNOa +OH + NOS
Correlation of the quantum yields with the concentrations of NOZ, HNOa, and NO revealed that the data could be presented as a function of the product, (NO)(HNOs). Table I summarizes these data and they are shown in Figure 5. ~~
3
Table I: Quantum Yields as a Function
I
i2
(NO)0
ff'
0
IO
20 30 40 50 TME NMINLTTES
60
Figure 4. NO2 pressure as a function of time with added NO (0.55 mm). 3
-2.2 -1.5 -1.5 -1.4
0 0 0.13 0.46 0.79 1.21 1.76
1.88 2.03 2.30 2.56
2
I
NO, mm
NOz, mm
"Ox,
mm
mms
0.02 0.04
1.66 6.58 4.32 4.38 4.98 4.01 6.28 4.32 1.70 1.03 4.26 3.84 3.42 2.86 2.48
4.93 0.86 1.77 1.40 0.24 1.90 0.73 1.04 1.01 2.02 1.66 2.16 2.66 3.49 3.75
0.10 0.03 0.07 0.07 0.21 0.32 0.33 0.49 0.86 1.03 1.16 1.34 1.44
0.04 0.47 0.87 0.17 0.48 0.47 0.85 0.51 0.70 0.62 0.54 0.47 0.39
("Or),
1.64 1.46
I.* einsteine 1. -1 sec -1
x
108
1.09 3.07 2.53 2.70 2.70 2.42 2.94 2.68 1.91 1.24 2.27 2.12 1.97 1.73 1.56
1
0
I
'I
I
I
1
-2
2
(NOMN4 )mm2
3
Figure 5.
The presence of OH radicals was qualitatively established by the addition of 1 atm of CO to the system. The reaction OH
+ CO +COz + H
The Journal of Phyeical Chemistry
Experiments were conducted with varying pressures of nitrogen from about 100 to 700 mm. The variation in total pressure did not seem to affect the correlation of quantum yields with the product, (NO)("Os). The quantum yields approached - 2 at the beginning of the process when (NO) was small and they never exceeded + 3 in this study. The quantum yields could again approach -2 at the end of the photolysis when ("03) is approaching zero. The limits of - 2 at the beginning and end of the process are consistent with the quantum yields for the photolysis of pure NOz. (7) A. M. U n g and R. A. Back, Can. J . Chem., 42,753 (1964). (8) H.W. Ford and 5. Jaffe, unpublished work.
PH~TOLYSIS OF NITROGEN DIOXIDEIN
PRESENCE OF NITRICACID
THE
Discussion A mechanism that is consistent with the data during the early stages of photolysis is NO2
Constant
Value
Ref
a
ks
3 . 3 X 109 1. mole-' sec-1 5 . 6 X lo@1. mole-" sec-1 2 . 5 X 106 1. mole-' sec-l 1.48 X lo4 mole-2 sec-1 1.8 X lo@1. mole-' sec-1
k9
0.24 sec-I
(1)
0 2
(2)
k5
(3)
k7
+ NO2 +NO + 0 + H N 0 3 +OH + NO3 OH + +H20 + NO3 NO + NOS +2N02 NO2 + NO3 +NO + NO2 + 2N0 + +2N02 "03
0 2
0 2
(4) (5)
(6)
At the beginning of the photolysis, when the NO pressure is very low, the third and fourth terms in eq I will vanish and the slope of (NO,) vs. time will be negative. As the photolysis proceeds, the NO pressure will increase and the third term will become most important. It will eventually be numerically larger than the first two terms and the slope of the curve will become positive. After some time, the ("Os) will decrease to the point where the curve will reach a maximum after which the slope will be negative once again. The quantum yield could approach -2 when (HN03) is very small, since the second term will approach - 1. However, NO2 regenerating reactions, such as eq 7, will prevent the quantum yield from actually reaching the limit of - 2. By using the data in Table I, the rate constants in Table 11, and eq I, one obtains an average value of ka equal to -10'0 1. mole-' sec-l. A rate constant of a t least this value would be necessary so that "03 molecules could compete with NO2 molecules for reaction with 0 atoms. One could conclude that reaction 3 is at least as fast as
+ NO2
----f
kz
ka
b b 1, p 185 1, p 188 1, p 188
' F. S. Klein and J. T. Herron, J . Chem. Phys., 41, 1285 C . Schott and N. Davidson, J . A m . Chem. Soc., 80, (1964). 1841 (1948).
(7)
The quantum yield expression for NO2 production that results from this mechanism, assuming steady state for 0, OH, and NOa, is
0
Table 11: Rate Constants Used in the Calculation of ka
+ h~ +NO + 0
0
1835
n'o3*
for which Klein and Herrong have reported a rate constant equal to 9.6 X lo91. mole-' sec-'. In order for eq I to represent the observed behavior
more closely, it would require that ks(NOz) be of the same magnitude as ks(N0). This is true in the early stages of photolysis when (NO) is low. However, if (NO) builds up as shown in Figure 2, k6 would have to be a few orders of magnitude larger than the value given in Table 11. If reaction 6 involved an excited or metastable Nos, such as OONO, the rate constant, k6, could be as large as lo8 1. mole-' sec-'. (The preexponential factor3 for the equivalent thermal reaction is 4 X 109 1. mole-' sec-l.) There are several reactions that may become significant in the later stages of photolysis. After the maximum in (NO2) is passed, the system approaches a steady state in all components. Nitric acid may be formed by the process
+ NO3 +N2Os No2 + Nos N205 + HzO +2HN03 NO2
N205
--f
(8)
(9) (10)
A calculation was made under steady-state conditions, assuming various values for klo. Interpolation of these results where the quantum efficiency for dissociation of NO2 is equal to that for reformation of NO2 yielded klo approximately equal to lo3 1. mole-' sec-l. Another possible reaction for the formation of " 0 3 is NO2
+ OH + M +
"03
+M
(11)
This reaction is similar to that proposed by Johnston, et u ~ . , ~ O , ~ ' for the thermal dissociation of HNOa. However, no estimate of the value of lcll has been made. It appears a t this time that reactions 1-7 describe the system early in the process and up t o the maximum (9) See footnote a of Table 11. (10) H. 5. Johnston, L. Toering, and R. J. Thompson, J. Phys. Chem., 57, 390 (1953). (11) H.S.Johnston, L. Toering, Yu-Sheng Tao, and G. H. Messerley, J . A m . Chem Soc., 7 3 , 2319 (1951).
Volume 71, Number 6 May 1067
LESLIEBATTAND FRANK R. CRUICKSHANK
1836
in NOz pressure. However, it is necessary to consider reactions 8-10 to explain the behavior at the end of the process when (NO2) is approaching a final steady state.
Further investigation of this system a t low partial pressures of NOz and HN03 should prove of interest in atmospheric photochemistry.
Complex Formation in the Gas-Phase Reaction of Hydrogen Bromide with Di-t-butyl Peroxide
by Leslie Batt and Frank R. Cruickshank Department of Chemistry, Uniuersity of Aberdeen, Old Aberdeen, Scotland Accepted and T~ansmittedby the Faraday Society
(August 10, 1966)
A mechanism for the decomposition of di-t-butyl peroxide (dtBP) in the presence of HBr, HBr + t-BuOH Br (2), t-BuO + 140" and 100 mm, is dtBP + 2t-BuO ( l ) , 1-BuO Me&O Me (3), Me HBr CH, Br (4), Me Br2+ MeBr Br (5), Me Qe --t C2Hs (6), Br Br M --t Br2 M (7), Me Br (M?) --t MeBr (M?) (8). Here reaction 7 is the main termination process with reactions 6 and 8 playing minor roles, in contrast to the normal decomposition of dtBP where reaction 6 is the main termination process. Formation of isobutene oxide (IBO) indicates a catalyzed decomposition of dtBP according to Br dtBP + dtBP-H HBr (9), dtBP-H + IBO t-BuO (10). Alternatively, complex formation occurs between dtBP and Brz or HBr, IBO being subsequently produced as a result of this. The variation of pressure with time and the very low experimental value of Jcg provide some evidence for complex formation. The following Arrhenius parameters have been estimated. EZ = 15.5-17.5 kcal/mole (assuming A2 = 1013.5sec-l, AB = lo9 l./mole sec, E2 = 0-2 kcal/mole); A4 = 108.g5l./mole sec, E4 = 2.9 kcal/mole, E5 = 0.9 kcal/mole; E , = 17 kcal/mole, E-, = 5 kcal/mole.
+
+ +
+
+
Introduction Di-t-butyl peroxide is a very convenient thermal source of t-butoxy radicals (t-BuO) in the gas phase over the temperature range 120-180O (for a static system). This allows a study to be made of the pressure-dependent decomposition of t-BuO,' provided that an efficient radical trap is used to measure its concentration. Raley, Rust, and Vaughan have shown that HBr does not catalyze the decomposition of dtBPz and therefore seems to be a suitable radical trap for studying the decomposition of t-BuO. The Journal of Physical Chemistry
+
+
+ +
+ +
+
+
+
+
+
+
Experimental Section The dtBP was purified as before.' The HBr (Matheson) was dried by passing it through a Dry Ice-acetone trap several times, which also removed traces of bromide, bulb-to-bulb distilled, and stored in a 3-1. bulb at -80' in the dark. The apparatus and essential experimental technique have been described in detail el~ewhere.~Auramine (1) L. Batt and 9. W. Benson, J . Chem. Phgs., 3 6 , 895 (1962). (2) J. H . Raley, F. F. Rust, and W. E. Vaughan, J . A m . Chem. Soc., 7 0 , 2767 (1948).
