Reaction between NOp and O3 in Solid N, (32) J. Weaver, J. Meagher, and J. Heicklen, J . Photochem., 60, 111 (1976/77). (33) A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., Chem. fhys. Lett., 42, 205 (1976). (34) L. Batt and R. D. McCulloch, Int. J . Chem. Kinet., 8, 911 (1976).
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979 2311 (35) C. Anastasi and I. M. W. Smith, J . Chem. SOC.,Faraday Trans 2, 72, 1459 (1976). (36) A. M. Winer, A. C. Lloyd, K. R. Darnall, and J. N. Pitts, Jr., J . fhys. Chem., 80, 1635 (1976). (37) E. D. Morris, Jr., and H. Niki, J . fhys. Chem., 75, 3640 (1971).
Reaction between Nitric Oxide and Ozone in Solid Nitrogen
Donald Lucast and George C. Pimentel" Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 (Received April 2, 1979) Publication costs assisted by the National Aeronautics and Space Administration
Nitrogen dioxide, NOz, is produced when nitric oxide, NO, and ozone, 03,are suspended in a nitrogen matrix at 11-20 K. The NOz is formed with first-order kinetics, a 12 K rate constant of (1.4 f 0.2) X s-l, and an apparent activation energy of 106 i~ 10 cal/mol. Isotopic labeling, variation of concentrations, and cold shield experiments show that the growth of NO2is due to reaction between ozone molecules and NO monomers, and that the reaction is neither infrared induced nor does it seem to be a heavy atom tunneling process. Reaction is attributed to nearest-neighbor N0.03 pairs probably held in a specific orientational relationship that affects the kinetic behavior. When the temperature is raised, more such reactive pairs are generated, presumably by local diffusion. Possible mechanisms are discussed.
Introduction The matrix isolation technique was developed to prevent chemical reactions and permit leisurely spectroscopic study of normally transient species.l Because of the unusual environmental conditions, interesting things can be learned when a reaction does take place in the matrix. In 1958, Pimentel discussed the potentials for studying matrix reactions where a low activation energy barrier existed,z but still today, only a little work has been done on the kinetics of diffusion-inhibited reactions in cryogenic solidsa3 Recently, Guillory and Smith investigated the formation of Nz04when NO was suspended in solid ~ x y g e n .The ~ reaction, which involved an NO dimer and an O2 molecule from the surrounding cage, was found to have an activation energy of 103 cal/mol. In the gas phase, the oxidation of NO follows a termolecular rate law, but the presently accepted mechanism involves NO3 rather than (NO), as an intermediate. The new matrix results may be important relative to the gas phase reaction, though in a yet unclear manner. Other kinetic systems studied in matrices include i~omerization,~ H atom t ~ n n e l i n g ?radical ~ recombination: and polymeri~ation.~ In this paper we present the study of reaction between two stable molecules, NO and O,, isolated in a nitrogen matrix. The gas phase reaction between NO and O3 has been extensively studied1@13with recent work centering on the role of vibrational excitation in promoting the reaction.12 Of particular interest is the recent work of Redpath et al.13 These authors add important data about the kinetic energy dependence of the chemiluminescent processes and they Department of Chemistry, Indiana University, Bloomington, Ind. 47401. 0022-3654/79/2083-2311$01 .OO/O
summarize the interpretational questions posed by the observed facts. Most relevant to the present work is the evidence that the reaction is characterized by two activation energies, 2.3 or 4.2 kcal/mol, depending on whether ground state or electronically excited NO2is formed.ll On this basis, it was our first intent to extend these studies to a cryogenic matrix environment. At 10-20 K, no thermal reaction would be expected, so the effect of vibrational excitation on reaction rate could be investigated by illuminating the sample with a blackbody glower source. Surprisingly, however, thermal reaction does take place, even a t these low temperatures, as will be described. Experimental Section Matrices were prepared by the simultaneous deposition of NO/N2 and 03/N2gas mixtures. The two gas mixtures met just in front of a CsI window held a t 10-12 K by an Air Products CSW-202 refrigerator. The temperature was controlled by a resistance heater mounted on the cold tip, and measured with a hydrogen vapor pressure thermometer in the 10-25 K range, and with a 0.6% Co/Au vs. Cu thermocouple at higher temperatures. Temperature stability of h0.5 K could be maintained over the period of an experiment. In addition to the standard radiation shield provided with the refrigerator, an extra cold shield was constructed which could be moved without breaking Dewar vacuum.l* During periods of spectroscopic study, the shield was placed out of the optical path. Between such periods, the shield could be positioned to completely cover the matrix and prevent room temperature background radiation from striking the sample. A separate resistance heater was mounted on the shield, along with a Co/Au vs. Cu thermocouple. Shield temperatures from 50 to >200 K could 0 1979 American Chemical Society
2312
The Journal of Physical Chemistry, Vol. 83,
No. 18, 1979
D. Lucas and G. C. Pimentel
TABLE I: N,O, Absorptions in Cryogenic Matrices (1590-1900 crn-l) sDeciesa
v. cm-'
1875 NO (NO), 1866 1861 i-N,O, 1840 N,O,(A) 1828 i-N,04 1794 1781 (NO),,, (NO),,, 1769 Reference 1 6 .
1762 1761 1737 1690 1661 1630 1628 1617
v, cm-'
70t I
1 I900
I
I
I
I
I
il
i
,
I
i l
a
sDeciesa (NO),,, N2°4
N2°4
N20,(B)
N,OAB) N,O,(A) i-N,O, NO2
I600
FREQUENCY (cm") A,----------
Figure 1. Growth of absorption at 1617 cm-' in an N, matrix at 12 K: NOIOBIN, = 1130l250.
be maintained with a stability of k2 K over a period of several hours. Ozone was prepared by a method suggested by Andrews.lj A separate vacuum line was constructed of stainless steel, teflon, and glass. Pressures were measured with a 4-in. Ashcraft gage. Oxygen at pressures 99%), growth at 1583 cm-l was observed and no growth was seen a t 1617 cm-l. When mixed l60/l80isotopically substituted ozone was used in mixtures with 14N160,growth was observed a t both 1617 and 1604 cm-l. The latter frequency is the same, within experimental error, of that previously reported" for isolated NO2 with the appropriate isotopic substitution,
160N180. The other infrared active bands of NOz, at 2919 and 750 cm-l, were accountably not observed because of their relatively weak absorption strength,l6>l8 at the low NO and NOz concentrations required to avoid complications with dimers and aggregates. The evidence does indicate, however, that the observed growth must be due to monomeric NOz neither complexed nor significantly perturbed. Furthermore, the nitrogen atom and one of the oxygen atoms comes from NO while the other oxygen atom comes from ozone. Growth of NOz. A typical curve of growth for NOz is shown in Figure 2. Each point is the average of several determinations of the optical density measured in a 515-min period, with the error bars representing one standard deviation. No systematic growth was discerned during the relatively brief spectroscopy period. Photometry reproducibility was checked by monitoring the constancy of the water monomer absorption which appeared in several experiments at 1599 cm-l. The width of the NOz band remained constant to within 10% over a
Reaction between NO, and
O3 in Solid N2
period of 24 h and over a temperature range of 11-20 K. The growth rate decreases with time as though reactants are being consumed. Yet, no bulk depletion of the reactants could be measured (as long as the matrix temperature was held constant) and, in view of the small amounts of NO, produced, no measureable change was expected. Consequently, we attribute reaction to N0.03 pairs in close proximity. A t the concentrations used, the number of such nearest neighbor reactants would be small, so their depletion could account for the decreasing growth of product. With this interpretation, the asymptotic limit for NO, growth, A,, provides a measure of the available reactant pair concentration. Infrared Effects. Since the rate of the gas phase reaction between NO and O3 has been shown to be enhanced by vibrational excitation (via infrared light absorption),Iz we made many attempts to alter the reaction rate in the matrix by changing the intensity of the infrared light striking the matrix. An external glower, identical with the spectrometer Nernst glower light source, was mounted -5 cm from the matrix window and provided with a 3-in. diameter concave collecting mirror. The light from this system was filtered through a germanium flat, transmitting only h > 1.8 pm, and used to illuminate the sample for several hours. This glower delivered to the sample about lo1*photons/s in the ozone v3 bandwidth, an order of magnitude more infrared photons per second than the spectrometer glower. In addition, several different filters, both broad and narrow band pass, were used to filter the spectrometer source. I n none of the experiments did a change in the infrared intensity alter noticeably the reaction rate. Even though additional infrared photons do not increase the reaction rate, it is still possible that infrared photolysis plays a role in the reaction mechanism in which the rate-limiting step does not involve absorption of a photon. One possible mechanism of this type is
+ -
slow, reactive pair formation
NO
+ O3
rate depends on intensity (N0.03) hv
(N0.03) NO2 + O2
If the intensity of the light is sufficiently high to cause the rate of NO2 production to be much faster than the rate of formation of reactive pairs, (N0.03),then additional radiation cannot increase the rate of production of NOz. The plausibility of this model depends upon the amount of NOz produced. This can be estimated from the observed absorbance of a known amount of NO, the ratio of the gas phase absorption coefficients of NO2 and NO, (€(NO2)/ €(NO) 30), and the measured growth of the NOz band. Thus, we calculate that the average rate of NO2 molecule production in our experiments is -1 X 1013s-l, From the geometry of the experimental apparatus it can be calculated that a 300 K background would result in about 1 x 10l6photons/s being absorbed by the matrix in a 2.5-cm-’ band a t 1000 cm-l where the ozone us absorption occurs. This is the most intense band in the O3 spectrum, and an absorption found in the gas phase laser studies to enhance the reaction rate. Thus, the background provides about lo3photons absorbed by the matrix for each NOz molecule produced. T o determine if the background radiation was saturating the matrix, we installed the additional radiation shield described previously, which could completely surround the sample with a thermal environment adjustable down to 50 K. At 50 K the background radiation results in only lo6 photons/s available for absorption in the 2.5-cm-’ band
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
2313
xp 48 O:03:N2= 1:5:600 = 15.9 K
0 300K Bkgd
0
m 5 0 K Bkgd
TIME(HR9 +
5
30
Figure 3. Optical density at 1617 cm-‘ vs. time in a cold (50 K) environment.
TABLE 11: NO, Growth Rates in the Cold Shield Experiments duration of shield period period A t , h temp, K A(OD)/At 0.030 la 2 R T ~ 0.012 150 2a 3 0.0069 3a 3 R T ~ 0.0045 4’” 8 50 R T ~ 0.0034 5’” 5
R T ~
3.5 3 10 5.5
lC 2c
3c 4c
50 50
RT
0.024 0.010 0.0039 0.0038
Experiment 47, NO/O,/N, = 1/4/600, T = 15.9 K. Shield open to expose sample to room temperature radiation. Experiment 48, NO/O,/N, = 1/5/600, T = 15.9 K.
width a t 1000 cm-’. Results of one experiment with the extra shield are shown in Figure 3. Similar results were obtained in another experiment. Table I1 lists the measured growth rates seen in the “cold shield” experiments. We conclude that the rate of the reaction observed is independent of the thermal environment, hence that it does not involve the absorption of an infrared photon. Rate Equation. Attempts to interpret the dependence of the rate on the concentrations of NO and O3 as a second-order process, as observed in the gas phase, were unsuccessful. This led to consideration of a mechanism in which an initial concentration of (NO-OJ complexes slowly react in a first-order process that does not involve radiation: (N0.03) NO2 + O3
-
The term “complex” and symbol (N0.03) are used here to designate an NO and an O3 molecule sufficiently close together in the matrix to react by a first-order process. (At our spectral resolution, no new absorption features were seen that could be assigned to such a complex.) Integrating the first-order rate equation from time zero to t, we obtain [C], = [CIoe+, in which [C] = [(NO-O,)] and t = 0 is taken as the time a t which half the matrix sample has been deposited. Since one NO2 is produced from one complex [NO,], = [C]& - e-kt) Expressed in terms of NOz absorbance, ANOZ,this equation becomes AtN@ = %Oz[C01(1- e-kt) and, a t t =
00
AmNoz
= €NOz[CO]
The Journal of Physical Chemistry, Vol. 83, No. 18, 1979
2314
D. Lucas and G. C. Pimentel
Hence
0.0A ~ N O Z= A , N O Z ( ~ -
-0.5
or
--
7
8
"
O b
+ toI
4
A:
E'
. -
