J. Phys. Chem. 1991, 95,8771-8775
Direct Rate Constant Measurements for the Reaction 0 300-1341 K
+- NO -tAt
-
8771
NOp
+- Ar at
G.Yamood: J. W. Sutherland, M. A. Wickramaaratchi, and R. B. Klemm* Brookhaven National Laboratory, Building 815, Upton, New York I1973 (Received: April 18. 1991: In Final Form: June 11, 1991) Rate constants for the title reaction were measured by two independent techniques: flash photolysis-resonance fluorescence 300-1001 K, and flash photolysisshock tube, 915-1341 K. In all cases, low-pressure-limit conditions prevailed. The data are well fitted, over the entire experimental temperature span (300-1341 K), by the expression klo(M = Ar) = (6.7 f 0.1) X 10-32(T/300)-1"1M,02 cm6 molecule-2 s-l. The present values, which fill in a gap in experimental measurements from 500 to 1450 K, are compared to those of previous studies and evaluations. Also, the strong collision, low-pressure rate constant (klosc) and the collisional efficiency factor (6,)are calculated over the temperature range 300-2000 K,and oc is compared with previously reported results. light from a N, flash lamp.I*12 The N atoms (generated in the flash) reacted rapidly ( 1200 K) because we were operating at high initial pressures, P I . Since [NO] and [Ar] were always maintained in large excess, the decay of [O], always followed pseuddintsrder kinetics. In these experiments, diffusion of 0 atoms out of the reaction zone was negligible,I7 and thus,
Yarwood et al. 2000,
-
In [0], = -Kobt
+ In [O],
(111)
*.
..
4-
M/ms
..
..
ot
I
I
0
2
4
6
TIME/ms Figure 1. First-order decay of OCP) resonance-fluorescence intensity and (inset) a least-squares fit to a semilog plot of the same experimental data. Conditions: T = 549 K, [NO] = 2.81 X 10” mdmlcs c d , [Ar] = 1.76 X lo1*molecules flash energy 95 J, number of flashes = 365.
where KOb = kl[NO] [Ar]. For 0 atom absorbance values less than about 0.50, [O] is directly proportional to the ab~orbance~~J* (ABS), and thus experimental first-order rate constants, Kh, were TABLE I: Rate Ihta for FP-RF Experiments at 549 K‘ obtained from linear least-squares fits of the experimental absorbance measurements total density, “I, 10” flash energy, J T,K loi8cm-’ In (ABS), = -Kobrt + In (ABS), (IV) 550 1.756 1.054 57 Termolecular rate constants, k l , were derived by dividing Kobsby [NO] and [Ar]. All the gases, except for NO, were of the highest purity obtainable and were used directly from cylinders. Helium and argon were scientific grade (M.G. Industries, 99.9999% purity), and in the 0 2 / H e mixtures the O2 purity was 99.99%. The N O (99% purity, M.G. Industries) was carefully purified by distillation from 90 to 77 K. Mass spectrometric analysis of purified NO samples showed N 2 0 impurity to be typically less than 0.05% and NO2 to be below the detection limit that was estimated to be 10.01%.
Results and Discussion The resonance fluorescence signal observed in a typical FP-RF experiment at 549 K is shown in Figure 1. Also shown in Figure I , inset, is a linear least-squares fit to a semilog plot of the same data which displays good linearity for more than two decay lifetimes (tl& Normally, experiments were repeated 3-4 times under the same conditions, and the decay constants were averaged to obtain a single value for K&. Similar experiments were then performed at constant total pressure for different [NO], and the Kobsvalues were plotted against [NO] as shown in Figure 2. The intercept of each plot corresponded to diffusional loss of 0 atoms while the slope yielded a value for k2”*. At 550 K, experiments were performed at total pressures of both 100 and 200 Torr of Ar and at different flash energies and flow rates. The decay rates obtained were independent of these conditions, as shown in ~
(21) (a) Myerson, A. L.; Thompson, H. M.; Joseph, P. J. Cornell Aeronautical Laboratory Report No. AD-1689-A-3, May 1964. (b) Myerson, A. L.; Thompson, J. M.; Joseph, P. J. J . Chem. Phys. 1965, 42, 3331. (c) Myerson, A. L.: Watt, M. S.J . Chem. Phys. 1968, 49.425. (22) Bradley, J. N. Shock W u m in Chemistry and Physics; Wiley: New York. 1962. (23) Greene, E. F.; Toennies, J. P. Chemical Reactions in Shock Waues; Academic Press: New York, 1964. (24) (a) Skinner, G. B. J . Chem. Phys. 1959, 31, 268. (b) Bernfeld, D.: Skinner, G. 8. J . Phys. Chem. 1983, 87, 3732.
549 549 551 549 549 550 5 50 549 549 548 548 549 549 549 549 549 549 549 549 549 549 549 549 549 549 549 548 548 548 548
1.759 1.759 1.753 1.759 1.759 1.756 1.756 1.759 1.759 1.762 1.762 1.759 1.759 1.759 1.759 3.518 3.518 3.518 3.518 3.518 3.518 3.518 3.518 3.518 3.518 3.518 3.524 3.524 3.524 3.524
1.056 1.056 1.052 1.759 1.759 1.756 1.756 2.459 2.459 2.464 2.464 2.8 13 2.8 13 2.813 2.813 0.704 0.704 0.104 0.704 1.407 1.407 1.407 1.407 2.107 2.107 2.107 2.814 2.814 2.814 2.814
36 83 90 90 90 90 90 90 90 90 90 90 86 90 90 101 101 101 101 94 98 98 98 90 98 98 94 94 60 126
K*,b s-’ 71 f 1 70 f 0 69 f 1 68 f 1‘ 103 f 2 103 f 1 103 f 2 103 f 3 137 f 2 140 f 4 138 f 1 136 f 1 I74 f 1 I74 f 1 175 f 1 177 5 78 f 2 78 f 1 82 f 2 77 f 2 143 f 3 148 f 2 143 f 2 146 f 2 218 f 4 217 2 217 f 4 292 f 6 292 f 2 283 f 3 285 f 3
*
*
Flow velocities of the gas reaction mixtures were about 2.0 cm s-I for 100-Torr runs and about 1.5 cm s-I for 200-Torrruns, unless otherwise specified. buncertainties are given at the 1u level. ‘Flow velocity was about 4.0 cm s-l.
Table 1. Values for k l , obtained between 300 and 1001 K, are summarized in Table 11. In initial experiments performed at 300 K and SO-Torr total pressure, significant emission was observed during the first 3-5
Rate Constants for 0
+ NO + Ar
-
+
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8113
NO2 Ar
TABLE II: Summary of Rate Data For the FP-RF Experiments"
T. K
total press., Torr
total density, ioi8 6m-I
300 f 1
50
1.61
k l o = (6.67 f 0.09) 1.76
IO0
549 f I
kiO= (2.84 f 0.03) 200
549 f 0
200
X
250
2.84
1001 f 1
2.89
300
kI0 = (1.12 f 0.02)
0.70 78.8 f 2.2 1.41 145.0 f 2.4 2.11 217.3 f 0.6 2.81 288.0 f 4.7 cm6 molecule-2 s-I, KD = (7.3 f 1.9) s-'
4 3 4 4
1.10 2.20 3.30 4.40 cm6 molecule-2 s-I, KD = (19.1 f 2.4)
73.2 f 1.0 127.5 f 2.4 187.8 f 3.7 235.8 f 4.3 s-'
4 4 3 3
1.15 60.5 f 1.0 2.31 102.0 f 0.8 3.47 137.0 f 3.4 4.62 172.0 f 2.2 5.78 213.8 f 2.1 cm6 molecule-2 s-l, KD = (24.0 f 1.4) s-I
4 4 4 4 4
cm6 molecule-l
s-I,
76-90
X
4 4 4 4
2.27 112.0 f 0.8 3.41 159.2 f 2.2 4.50 202.0 f 3.5 5.67 251.3 f 2.3 KD = (19.8 f 1.9) s-'
76-90
k I o = (1.44 f 0.2) X
4 2 4 3 4 4 4 4
69.5 f 1.3 103.0 f 0 2.46 137.8 f 1.7 2.8 1 175.3 f 1.9 cm6 molecule-2s-l, KD = (15.8 f 1.2) s-'
49-94
X
no. of runs
s-I
60-126
2.75
klo = (1.81 f 0.03) 851 f I
X
&, s-' 118.2 f 1.7 169.0 f 1.4 224.2 f 3.6 272.3 f 3.1
I .05 I .76
36-90
3.52
k l o = (2.83 f 0.3) 703 f 0
X
[NO19 10l5 cm-3 23-43 0.97 1.44 1.93 2.41 cm6 molecule-2 s-I, KD = (14.9 f 2.6)
flash energy, J
"Uncertainties are given at the 1u level.
-
h
I
v)
Y
v)
n0 Y
0
40
80
120
160
PNO/mTorr
Figure 2. Variation of the observed first-order decay constant with PNo at total pressures of 100 (0)and 200 Torr ( 0 )([Ar] = 1.76 X 10l8and 3.52 X 10" molecules cm-], respectively). The intercepts at PNo = 0 correspond to the rate of diffusional loss of 0 atoms. The rate constants calculated from the gradients were k, = (2.84 f 0.03) X and (2.83 f 0.3) X cm6 molecule-2s-I, respectively. T = 549 f 2 K.
ms after the flash. Similar emission, attributed to N O fluorescence, was reported by Slanger and Black25in their flash photolysis-resonance fluorescence kinetic study of reaction 1. The emission observed in the present study decayed rapidly (kl" =I500 s-I) and was superimposed upon the much slower decay of the 0 atom fluorescence signal. Experiments performed with the 0 atom resonance lamp off confirmed the presence of the emission signal, and a careful check of the purity of N O and Ar suggested (25) Slanger, T. G. Black, G. J . Chcm. Phys. 1970, 53, 3717.
that it was not due to an impurity. Subsequent experiments showed that the emission intensity was inversely proportional to the pressure and that the emission could be quenched by raising the argon pressure to 200 Torr. Alternatively, the emission could be removed by reducing the flash energy from about 90 to 30 J. It was noted that a 3-fold reduction in flash energy produced a disproportionally large decrease in the strength of the emission, suggesting that it was not simply due to NO fluorescence. The source of the emission could not be identified because facilities for dispersing the emission were unavailable at the time. However, the emission must have been limited to the range 1350 A < A < 2000 A by the combination of a CaF2 window and solar blind photomultiplier tube. This emission was not observed under any conditions in experiments performed above 300 K, and at 300 K unperturbed 0 atoms decays were obtained by performing experiments at low flash energies (23-43 J), as shown in Table 11. Data obtained in a typical FP-ST experiment are shown in Figure 3. The main part of the figure shows the variation in transmitted resonance lamp intensity through, in order, the arrival of the incident and reflected shock waves, the firing of the flash lamp, and the subsequent decay of [O].The zero of time is set to the point at which the flash lamp was triggered. The inset shows a least-squares fit to the absorbances calculated for the same data which yielded Kabs = 1452 s-I. Values for k, were obtained between 915 and 1341 K and are summarized in Table 111. The possible effect of the reaction of 0 atoms with NOz (produced in reaction 1) 0 NO2 ' 0 2 NO (2)
+
+
was investigated by numerical modeling. Any effect would be larger for the FP-ST experiments than for the FP-RF experiments because the initial [O] was larger by a factor of 10-20. For a representative T = 950 K,[Ar] = 4.2 X IOk8 molecules ~ m - and ~, [NO] = 1.7 X I O t 6 molecules cm-3 and taking k l = 1.3 X cm6 molecule-2 s-I from eq V (below) and k2 = 7 X cm3
Yarwood et al.
8774 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 TABLE 111: R8te Dab for F h b Photolysis-Shock Tube ExDerilnents T5,K P I , Torr XN0 X IO3 hi," p~/10'* Kh, s-'
915 935 940 949 959 968 981 1006 1011
1021 1029 1045 1065 1070 1077 1081 1085 1090 1091 1091 1094 1095 1100 1101
1102 1104 1108 1108 1114 1132 1140 1160 1163 1169 1175 1175 1179 1205 1209 1211 1216 1241 1241 1244 1246 1251 1272 1273 1277 1297 1300 1305 1312
1336 1341
30.05 30.02 30.02 30.05 30.16 29.97 29.98 30.09 30.05 30.04 30.14 30.01 30.00 30.06 30.11 30.04 30.02 30.01 30.02 30.05 30.01 29.99 30.05 30.01 30.05 30.02 30.01 30.05 30.10 30.08 30.00 29.96 30.01 30.00 29.91 29.98 30.16 30.01 30.05 30.09 30.10 30.06 30.07 30.05 30.04 29.97 30.07 30.08 29.92 30.19 29.84 30.09 29.87 30.02 30.07
4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 5.99 4.00 4.00 4.00 4.00 5.99 5.99 4.00 6.00 7.87 4.00 7.87 5.99 7.87 7.87 7.87 5.99 4.00 5.99 6.00 4.00 6.00 8.00 6.00 8.oo 6.00 8.00 6.00 8.00 8.00 8.00 6.00 8.00 8.00 8.00 8.00 8.00 7.87 8.00 8.00 8.00 7.87 8.00
1.856 1.883 1.894 1.905 1.909 1.924 1.929 1.958 1.977 1.988 1.997 2.017 2.051 2.046 2.029 2.033 2.042 2.080 2.071 2.054 2.070 2.063 2.060 2.079 2.080 2.074 2.087 2.083 2.098 2.1 12 2.122 2.140 2.143 2.147 2.162 2.157 2.166 2.186 2.199 2.202 2.203 2.235 2.234 2.232 2.235 2.248 2.263 2.260 2.272 2.316 2.290 2.291 2.305 2.349 2.334
4.037 4.077 4.119 4.153 4.152 4.179 4.151 4.242 4.328 4.354 4.393 4.424 4.485 4.503 4.380 4.379 4.412 4.556 4.560 4.459 4.540 4.493 4.459 4.565 4.569 4.524 4.583 4.563 4.634 4.647 4.610 4.679 4.691 4.634 4.738 4.720 4.787 4.772 4.842 4.850 4.841 4.919 4.919 4.892 4.899 4.931 4.962 4.943 4.979 5.101 4.995 5.021 5.054 5.193 5.157
1043 1125
940 898 1089 1019 864 970 1033 1295 1333 1209 1478 1041 812 940 950 1395 1452 959 1497 1985 973 1953 1437 1919 2067 2016 1353 1046 1451 1548 994 1695 2239 1562 2060 1801 1638 1564 2192 2073 1813 1588 2056 2061 1927 1673 1766 2063 2170 1746 1757 1723 1645
Y
&lob
1.60 1.69 1.39 1.30 1.58 1.61 1.25 1.35 1.38 1.71 1.68 1.54 1.23 1.28 1.06 1.23 1.22 1.12 1.17 1.21 1.21 1.25 1.22 1.19 1.15 1.19 1.25 1.23 1.05 1.21 1.14 1.18 1.13 1.29 1.25 1.17 1.09 1.32 0.87 1.11 1.17 1.07 0.94 1.11 1.07 0.85 0.98 0.86 0.89 1.01 1.09 0.87 0.86 0.81 0.77
OThe error in measuring the Mach number is typically h0.756 corresponding to an uncertainty in TSof 152.0%. *Termolecular rate cm6 moleconstant derived from k l o = Kob/[NO][Ar]; units are cule-2 s-!. molecule-' s - ! , ~ it was calculated that reaction 2 would contribute
