J . Phys. Chem. 1990, 94, 2471-2475 over the indicated temperature ranges. The theoretical expressions, eqs 15a and 15b, give slightly higher values than experiment, eq 14, over most of the temperature range; however, the differences in the two expressions are not serious when the accuracy of the data is considered. The spread in experimental values, particularly in the higher temperature region, is typically f 15-25%, and this is about the same as the differences between eq 14, 15a, and 15b. It should be noted that the present TST/W calculation gives a much better representation of the data than the variational transition-state calculation (CVT/SCSAG) of Isaacson and T r ~ h l a r .These ~ workers used the earlier less accurate potential energy s ~ r f a c e ,and ~ ? ~this may account for the difference. A similar comparison for the deuterated reaction -1 is shown in Figure 8. The present data points (Table I) were transformed to k-, through the J A N A F equilibrium constants3 (Le., k-' = k l / K Iwhere K , is given by eq 12), yielding the linear least-squares Arrhenius expression
*
k-, = (6.6 1.7) X lo-', exp(-3320 f 356 K / T ) cm-, molecule-' s-I (16) for 1285 I T I 2261 K. The line given in the figure is the theoretical prediction. In this case there is less curvature, and theory can be represented to within *lo% by the three-parameter expression
k-lthwry= 7.94
X
10-'8T1.98exp(-1820/T) cm3 molecule-' s-I (17)
over the temperature range 200-2500 K. Since the theoretical value for K l , eq 10, is almost identical with eq 12, the resulting theoretical prediction for k-, is in the same relationship to the transformed data as the original data are to the theoretical calculation for k , . This is easily seen in comparisons of Figures 5 and 8, and thus, the theoretical prediction only overlaps the spread of the data over some of the temperature range. At 1200 K, eq 17 predicts a value that is -50% of that from eq 16 whereas at
2471
2200 K, the values from both equations are essentially equal. Lastly, there is only one direct experimental value for O D Dz that is available for comparison to eq 17. Vaghjiani et al. report k-, = (1.88 f 0.30) X cm3 molecule-' s-' at 298 K.33 Equation 17 gives a value that is only -%25% lower (i.e., 1.40 X cm3 molecule-' s-l), in good agreement with experiment. Therefore, eq 17 is probably accurate to within a factor of 2 over the temperature range 300-2200 K. In conclusion, the present work represents the first study on the rate behavior of reaction 1, D DzO. From the experimental evidence obtained for both D D 2 0 and H HzO, it must be concluded that no isotope effect is present within experimental error. Although a simple TST/W calculation approximates the data reasonably well, it fails to accurately predict the experimental isotope effect of unity. A further refinement to the theoretical calculations is probably necessary in order to better describe the data on this reactive system. In particular, the discrepancy may be due to reactive bottlenecks that can only be described, within the framework of transition-state theory, by variation transition-state theory. Hence, the effect may be an interesting example of a dynamical effect rather than an effect that is based solely on the potential energy surface.
+
+
+
+
Acknowledgment. The authors are indebted to Drs. E. Kraka and T. H. Dunning, Jr., for supplying the results of their calculations prior to publication. We thank Drs. Vaghjiani, Ravishankara, and Cohen for communicating the results of their experiments before publication. We also thank colleagues Drs. A. F. Wagner and J. P. Hessler for a thorough reading of the manuscript and useful suggestions. The work was supported by the US. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-3 I - 109Eng-38. (33) Vaghjiani, G.L.; Ravishankara, A. R.; Cohen, N. J . Phys. Chem., in press.
Flash Photolysis-Shock Tube Kinetic Investigation of the Reaction of O(3P) Atoms with Ammonia J. W. Sutherland,* P. M. Patterson, and R. B. Klemm Department of Applied Science, Brookhaven National Laboratory, Upton, New York I 1 973 (Received: July 17, 1989; In Final Form: October 6,1989)
The rate constant for the reaction of O(,P) with NH, was measured over the temperature range of 765-1790 K by the flash exp(-l1155 photolysis-shock tube technique. The results, expressed in Arrhenius form, are k l ( T ) = (1.07 f 0.10) X 217/R7') (cm' molecule-' s-'). When these results were combined with the directly determined, low-temperature data of Perry (448-841 K), k l ( 7') displayed distinct non-Arrhenius behavior over the extended temperature range of 448-1790 K. The three-parameter fit to the combined data set is kl(7') = (1.56 X 10-17)T1.94 exp(-6461/R7') (cm3 molecule-' 8). The overall uncertainty in values of k , ( 7') derived from this expression is estimated to be *25%. The present results are compared with previous high-temperature determinations of k l ( 7').
*
-
agreement among the low-temperature values for kl ( T I 400 K) and the uncertainty as to whether the ONH3* complex (postulated to explain some of the data) exists. In the hightemperature studies ( T I1600 K), O(3P) atoms were produced by the thermal dissociation of N,O in shock tubes and changes in the concentrations of NH,, OH, and N 2 0 were monitored by absorption and emission These investigations
( I ) Salimian, S.; Hanson, R. K. Combust. Sci. Technol. 1980, 23, 225. (2) Lyon, R. K. Int. J . Chem. Kine!. 1976, 8, 315.
(3) Cohen, N. I n t . J . Chem. Kinet. 1987, 19, 319. (4) Dove, J. E.; Nip, W. S. Can. J . Chem. 1974, 52, 1171. ( 5 ) Salimian, S.; Hanson, R.; Kruger, C. H. Int. J . Chem. Kine!.1984, 16, 125.
Introduction The reaction of O(,P) with NH,, reaction 1, is important in the combustion of NH3I and in the Exxon "Thermal De-NO," process2 advanced for NO, abatement in large combustion processes. Several studies have reported experimental measurements O(,P) + NH, OH + N H 2 (1) of the rate constant, and this information has been critically reviewed by C ~ h e n . In ~ his review, Cohen discussed the lack of
0022-3654/90/2094-247 1$02.50/0
0 1990 American Chemical Society
2412
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
Sutherland et al. ”-
,
J3 n
I
i
I
r
\\
-13;
I
-2.5
I -3.5
\ I
c
-4.5
-15 4.0
8.0
12.0
16.0 IO‘IT
20.0
24.0
(K-’)
Figure 1. Summary of previous experimental rate constant data for k l ( T ) : (A) Salimian et al.,5 k , ( T ) = 3.63 X lo-” exp(-8882/RT), 1750-2060 K; (B)Dove and Nip: k,(T) = 1.66 X IO-” exp(-6600/RT), 1620-1920 K; (C) Fujii et al.,’ k l ( T ) = 4.98 X exp(-6080/RT), 1600-2000 K; (D) Perry,8 k l ( T ) = 3.42 X IO‘” exp(-9000/RT), 448-841 K.
required relatively high concentrations of the reagents, and values of the rate constant for reaction 1 were extracted from analysis of a complex kinetic system. Such an indirect procedure can sometimes lead to large errors as has been pointed out by Cohen3 in the case of the data of Fujii et aL6s7 Only one reported study employed a direct experimental technique.8 Perry8 measured k , ( n over the temperature range 448-841 K, using laser flash photolysis of NO2 to produce 0 atoms. NO2 chemiluminescence was used to follow subsequent changes in concentration of 0 atoms with time, The experimental data presently available for reaction 1 are summarized in Figure 1. It is evident that (i) there is marked disagreement among the high-temperature shock tube studies; (ii) there are no experimental measurements available between 850 and 1600 K; and (iii) there are insufficient measurements to determine precisely the degree of non-Arrhenius behavior of kl( r). This paper reports experimental measurements of kl( r ) with use of the flash photolysis-shock tube (FP-ST) technique9J0over the temperature range 76.5-1790 K.
Experimental Section The present FP-ST technique uses atomic resonance absorption to follow the changes in atom concentrations with time.lOJ1 The high sensitivity of this method usually allows the rate measurements to be made under experimental conditions in which the kinetics are pseudo-first-order and in which there are no complications from secondary reactions. Details of the technique can be found in the published studies on the rate constant measurements of O(3P) with CH4I2and H213and of H atoms with NH3,I4 H2O,I5and O2.I6 Ongoing improvements to the present apparatus (6) Fujii, N.;Sato, H.; Fujimoto, S.; Miyama, H. Bull. Chem. SOC.Jpn. 1984, 57, 271.
(7) Fujii: N.; Chiba, K.; Uchida, S . ; Miyama, H. Chem. Phys. Leu. 1986, 127, 141. (8) Perry, R. A. Chem. Phys. Letr. 1984, 106, 223. (9) (a) Ernst, J.; Wagner, H. Gg.; Zellner, R. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 409 and references therein. (b) Niemitz, K . J.; Wagner, H. Gg.; Zellner, R. Z . Phys. Chem. 1981, NF124, 155. (10) (a) Michael, J. V.; Sutherland, J. W.; Klemm, R. B. Inr. J . Chem. Kiner. 1985, 17, 315. (b) Maki, R. G.; Michael, J. V.; Sutherland, J. W. J . Phys. Chem. 1985, 89, 4815. (c) Michael, J. V.; Sutherland, J. W. Int. J . Chem. Kiner. 1986, 18, 409. ( I I ) Sutherland, J. W.; Klemm, R. B. Sixteenth International Symposium on Shock Tubes and Wows; VCH: Weinheim, FRG, 1988; p 395. (12) Sutherland. J. W.; Michael, J. V.; Klemm, R. B. J . Phys. Chem. 1986, 90. 5941. (13) Sutherland, J. W.; Michael, J. V.; Pirraglia, A. N.; Nesbitt, F. L.; Klemm, R. B. Twenty-Jrsr Symposium (Inrernarional) on Combustion; The Combustion Institute: Pittsburgh, 1987; p 929. (14) Michael, J. V.; Sutherland, J. W.; Klemm, R. B. J. Phys. Chem. 1986, 90, 497. ( 1 5 ) Michael, J. V.; Sutherland, J. W. J . Phys. Chem. 1988, 92, 3853. (16) Pirraglia, A. N.; Michael, J . V.; Sutherland, J. W.; Klemm, R. B. J . Phjs. Chem. 1989. 93, 282.
1 I
I 2.4
2.6
2.8
3.0
3.2
TIME (msec)
Figure 2. (Inset) Typical transmission signal: (A) reflected shock, (signal change is due to compression of the gas mixture); (B)flash; (C) recovery from flash (electronic noise). The base line, zero of 0 atom concentration following decay, lines up quite well with the transmitted resonance light intensity observed in the reflected regime prior to the flash. Conditions P I = 15.03 Torr, XNO = 2.03 X IO-’, A’,,, = 1.015 X p s = 2.55 X 10l8molecules T, = 1279 f 10 K. The graph shows the first-order plot, In (ABS), vs t , of the data given in the inset: kobs= 3751 f 29 s-l; k ( 0 + N O M) = 114 s-I; and kl = 1.40 X cm3 molecule-I SKI.
+
have included replacement of an oil diffusion pump on the shock tube with a turbomolecular pump and the use of high-vacuum epoxy cement to seal the vacuum-UV windows to the shock tube. The turbopump enhances the pumping speed and ultimate vacuum, reduces start-up time, and provides an overall improvement in shock tube cleanliness. The improved vacuum seals on the vacuum-UV windows have significantly reduced the apparent leak rate in the shock tube. In this study, oxygen atoms were produced by the flash photolysis of N O through a sapphire window (A L 145 nm). The concentration of N O was sufficiently large that the N atom, formed along with the O(’P) in the photodecomposition, reacted with N O in less than about 20 M S to give a second O(3P) atom. The 0 atom concentration, at an initial level of -4 X 10l2atoms cm-), was then monitored in real time by the sensitive technique of atomic resonance absorption spectrophotometry (ARAS).I7 The transient signals from the transducers and the photomultiplier were digitized with a dual-channel storage oscilloscope. The common time scale was 4 ms, and the time resolution was 0.5 ws/point. The digitized photomultiplier (voltage) signals were converted to absorbance values ((ABS), = In (Zo/Z,)) and analyzed with a microprocessor. A typical signal is shown in the inset in Figure 2. Since Beer’s law is known to hold for the small absorbances used here, (ABS), is proportional to [O],. The decrease in [ O ] ,depends predominantly on reaction 1. At the higher initial pressures ( P I = 30 Torr) and the lowest temperatures, there is a significant contribution from the third-order reaction of 0 atoms with NO: 0 + N O + Ar NO2 + Ar (2) +
The decrease in 0 atom concentration follows first-order kinetics, and hence, the decrease in the absorbance, (ABS),, is given by eq I. Since the decay in [O] depends entirely on reactions 1 and In (ABS), = -kobst + C (1) 2, kobsis given by eq 11, where k l and k2 are the bimolecular rate kobs
= kl [ N H J +
k2[N01
[Ar]
(11)
constants for reactions 1 and 2. The k2[NO][M] term was evaluated with values for k2 measured in this laboratory.I8 (17) (a) Myerson, A. L.; Thompson, H. M.; Joseph, P. J. Cornel1 Aeronautical Laboratory Report No. AD-1689-A-3, May 1964. (b) Myerson, A. L.; Thompson, H. M.; Joseph, P. J. J . Chem. Phys. 1965, 42, 1331. (c) Myerson, A. L.; Watt, W. S . J . Chem. Phys. 1968, 49, 425. (18) Yarwood, G.; Sutherland, J. W.; Wickramaaratchi, M. A,; Klemm, R. B. To be published.
Reaction of O(3P) Atoms with Ammonia TABLE I: Rate Constant Data for the Reaction O(3P) + NH3 P,/Torr M,” kobb/s-I ko+NOC/s-Lklst/s-l pse T5 XNH,P = 1.55 X XNo = 3.074 X 10-3 10.09
2.548
816
73
743
-
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2413 OH kl(
+ NH2
1.947 1617 2.46
1448 5799 2086 3481
74 584 642 608
= 3.0 x 10-3 1374 1.900 1514 5215 6.002 1790 1444 5.112 1332 2873 5.598 1574
XNH, = 4.035 X IO4 30.45 2.332 3515 30.49 2.488 5631 30.48 2.563 8019 30.45 2.483 4925 30.05 2.384 3947 29.76 2.241 2328 30.64 2.147 2147 30.32 2.168 2516 30.24 2.102 2118
675 655 635 655 643 645 695 686 680
XNo = 3.023 X IO-’ 2840 5.233 1336 4976 5.606 1504 7384 5.750 1594 4270 5.588 1498 3304 5.270 1396 1683 4.878 1248 1452 4.815 1162 1830 4.828 1177 1438 4.648 1122
1.35 2.20 3.18 1.89 1.55 0.855 0.747 0.939 0.767
XNH, = 4.331 X IO4 14.86 2.514 4405 15.04 2.677 4564 14.99 2.569 4273 30.06 2.294 4082
52 50 51 215
XNo = 9.986 X IO4 4353 2.829 1559 4514 2.980 1748 4222 2.873 1635 3867 5.051 1302
3.55 3.50 3.39 1.77
XNH, = 2.204 10.05 2.462 30.38 2.718 30.47 2.31 1 30.19 2.536
X
XNH, 10.09 10.10 10.10 10.11 10.13 10.10 10.10 10.15 15.04 15.14 30.22
= 8.98 x
XNH, 15.67 15.00 14.90 15.28 15.31 15.06 15.05 15.00 15.22 15.04 15.04 30.21 29.97 30.10 30.09
= 1.01 x
2.639 2.374 2.639 2.613 2.551 2.638 2.341 2.658 2.451 2.432 2.313 1.888 2.198 2.251 2.151 2.134 2.010 2.065 1.944 2.083 2.426 2.372 2.180 2.128 2.102 2.033
IO4
xNO
xNO
10055 5098 9697 7870 8574 8393 4556 8212 6266 6230 8364
70 76 70 70 72 68 75 68 159 162 638
930 3791 3872 2683 2741 1505 1827 992 1798 6444 5805 4669 4276 3593 2705
134 120 1 I9 129 129 124 124 123 127 116 I22 482 478 483 480
3.28 3.94 1.28 2.33
= 2.99 x 10-3
9985 5022 9627 7800 8502 8325 4481 8144 6107 6068 7726
2.016 1.847 2.011 1.992 1.960 1.997 1.812 2.018 2.781 2.782 5.099
1703 1426 1710 1686 1618 1720 1402 1743 1506 1487 1329
5.52 3.03 5.33 4.36 4.83 4.64 2.75 4.49 2.45 2.43 1.69
XNO= 2.163 X 796 2.205 3671 2.502 3753 2.553 2554 2.511 2612 2.494 1381 2.287 1703 2.358 869 2.188 1671 2.407 6328 2.758 5683 2.732 4187 4.824 3798 4.671 3110 4.631 2225 4.449
974 1258 1305 1205 1189 1080 1130 1022 1146 1481 1410 1191 1145 1122 1065
0.357 1.45 1.46 1.01 1.04 0.598 0.715 0.393 0.687 2.27 2.06 0.859 0.805 0.665 0.495
PIlTorr
M,O
k,bb/s-‘
Ts
k,‘
1301 1093 1297 1279 1232 1191 1130 1071 939 1005 1061
1.36 0.715 1.25 1.40 1.26 1.14 0.700 0.643 0.281 0.488 0.592
174 166 166 171 174 179 172 172 683 674 66 1 649
XNO = 3.009 X lo-’ 6946 2.726 1380 9600 2.886 1549 9121 2.863 1529 6254 2.752 1416 3354 2.416 1164 1096 2.153 966 1356 2.249 1060 3286 2.422 1175 1480 4.245 980 2563 4.476 1067 4278 4.699 1160 5002 4.663 1162
1.93 2.53 2.42 1.73 1.05 0.387 0.458 1.03 0.265 0.435 0.691 0.814 1.69 0.667 0.388 0.149 0.235 0.169 0.163 0.146 0.255 0.116 0.122
ko+NOC/S-I kl,,d/s-‘
pse
XNH, = 1.015 X lo-’ 15.04 2.262 3727 15.03 2.033 1808 15.01 2.252 3414 15.03 2.230 3751 15.03 2.180 3312 15.02 2.132 2930 30.13 2.107 3739 30.12 2.039 3368 30.14 1.893 1647 30.14 1.970 2604 30.16 2.036 3006
117 119 1 I6 115 1I 6 115 449 45 1 465 462 422
XNn = 2.03 X 3610 2.614 1689 2.328 3298 2.590 3636 2.560 3196 2.502 2815 2.436 3290 4.630 2917 4.467 1182 4.137 2142 4.326 2584 4.297
XNH,= 1.317 X 15.24 2.338 7120 15.08 2.512 9766 15.13 2.488 9287 15.10 2.379 6425 15.02 2.108 3528 15.39 1.879 1275 15.01 1.986 1528 15.05 2.115 3458 30.10 1.940 2163 30.04 2.040 3237 29.87 2.146 4939 29.62 2.148 5651 XNH,= 1.996 X 10.04 2.216 15.02 2.062 15.01 1.926 30.13 1.768 30.11 1.918 30.10 1.847 30.08 1.873 30.05 1.820 30.04 1.865 30.03 1.729 30.26 1.729
5828 3268 1798 1549 2384 1772 1747 1574 2484 1264 1314
51 115 113 429 436 432 433 438 436 42 1 427
XNO = 1.962 X IO-’ 5777 1.717 1272 3153 2.368 1118 1685 2.175 1002 1120 3.757 846 1948 4.161 967 1340 3.966 910 1314 4.036 931 1136 3.911 883 2048 4.021 921 843 3.628 816 887 3.654 816
XNH, = 2.29 x 30.65 1.725 30.49 1.658 30.39 1.907
lo-’ 1688 1077 4321
437 42 1 459
XNO = 1.979 X IO-’ 1251 3.679 816 0.148 656 3.450 765 0.083 3862 4.201 952 0.401
2308 1629
209 208
XNO = 3.627 X lo-’ 2099 2.039 916 0.247 1421 1.945 859 0.176
XNH, = 4.17 X 15.06 1.925 3346
125
XNo = 2.084 X 3221 2.202
x,,, 15.12 15.25
IO-’
= 4.16 x 10-3
1.823 1.752
992 0.351
‘The error in measuring the Mach number is typically about f0.7% at the 1 standard deviation level. bThe error from the linear least-squares fit is * l ~ 3 %at the I standard deviation level. ‘ko+NO is calculated from measured values of k(O+NO+M); see text. dObserved first-order rate constant corrected for contribution of the third-order O+NO+M reaction. ‘Units of density are IO’* molecules ~ m - /Units ~. of k , are cm3 molecule-l s-l. EXi denotes the mole fraction of species i.
The temperature, density, and pressure in the reflected shock regime were calculated from the measured values of the incident shock velocity, the test gas composition, and the initial temperature and pressure with use of ideal shock theory. Corrections for nonideal shock behavior, due to boundary-layer effects in the present shock tube, were made by a procedure based on experimentally measured pressures and the adiabatic equation of state. 10.1 1%I9 The helium and argon were of research grade (M. G. Industries; stated purity 99.9999%) and were used without further purification. The nitric oxide (Matheson; 97%) was purified by outgassing at 77 K and vacuum-distilling at 90 K. Mass spectrometric analysis gave the level of N,O impurity as less than about 0.1%. (19) (a) Skinner, G. B. J . Cfiem. Phys. 1959, 3 1 , 268. (b) Bernfeld, D.; Skinner, G. B. J . Phys. Cfiem. 1983, 87, 3732.
Ammonia was electronic grade (M. G. Industries; 99.999%) and was further purified by trap-to-trap distillation. Results and Discussion Over the temperature range 765-1 790 K, good pseudo-firstorder decays of the 0 atom absorbance were observed in accordance with eq I. This is demonstrated in Figure 2, which shows a typical 0 atom decay for an experiment performed at 1279 K. The plot of In (ABS), vs time is linear over about three half-lives, and this fit is remarkably tight (
