J. Phys. Chem. 1991,95, 3224-3229
3224
"matrix" that includes relative concentrations of [CHJO and [C2H310and kl, kllc, and klllc covering a reasonable (though not extensive) range of values normally encountered in such experiments. The quantity R = [C3H6]m2/( [C2H6],[C,H6],) is usually taken to be a "system constant". In Table VI this quantity is calculated by using the ACUCHEM program and it is compared with the approximations, eq 1 and 7 in the text. It can be seen that, if k l / k l l c = 1, the pulsed system behaves as though it were in a quasi-steady state. For other ratios the above product ratios vary depending upon the relative rate constants and upon the initial concentration ratios. For the present experiments where kl:kll&lc = 1:2:2 the ratio R is approximately constant (within
experimental error) and either eq 1 or 7 represents a reasonable approximation. The matrix shown in Table VI shows the limitations of taking R as a constant and the accuracy of using either eq 1 or 7. In the final analysis, numerical integrations should be used to establish best fits to the real time data and the product ratios. Taking R to be constant and employing either eq 1 or 7 as an approximation simply serves to minimize the number of iterations when fitting many runs under different initial conditions. Similar conclusions were drawn p r e v i o ~ s l y . ~ ~ Registry No. CH3, 2229-07-4; C2H3. 2669-89-8; H, 12385-13-6.
Spatlotemporal Temperature Patterns on an Electrlcally Heated Catalytic Ribbon Ceorgios Philippou, Fred Schultz, and Dan Luss* Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792 (Received: September 21, 1990)
Oscillatory and chaotic variations in the overall reaction rate were observed during the oxidation of propylene in air on a thin platinum ribbon, the average temperature (resistance) of which was kept at a preset value via electrical heating. A thermal image showed that back and forth movement of a high-temperature wave on the ribbon and the dynamic change in the length and temperature of the ignited zone were the cause of the variation in the overall reaction rate. The overall reaction rate in the chaotic region oscillates at a much higher frequency than the local temperature. The corresponding power spectrum of the reaction rate decays exponentially, while that of the local temperature decays as a power law.
Introduction Many chemical and electrochemical reacting systems exhibit periodic, quasiperiodic, and chaotic behavior.'" The discovery of the Belousov-Zhabotinski reaction6generated significant interest in spatial and spatiotemporal patterns, and waves in chemically reacting homogeneous systems. Theoretical studies7-I0 predict that propagating waves and dissipative structures exist also on heterogeneous catalytic surfaces. Experimental studies' revealed spatiotemporal temperature patterns on Pt and Ni surfaces on which catalytic oxidation reactions were carried out. As yet there exist no understanding of the cause of these patterns, the conditions and reactions for which they occur, and their impact. This work is an experimental study of the oscillatory and chaotic behavior of an electrically heated catalytic Pt ribbon on which the oxidation of propylene in air was carried out in order to characterize the local temperature oscillations and the overall reaction rate, and the relation between them.
Kodak IRTRAN 2 infrared transparent window in the reactor wall. The infrared image was magnified by a telescopic lens which allowed viewing of 1 cm segment of the ribbon. The imager measures 25 times per second the infrared radiation emitted from a two-dimensional grid of 64 X 128 points. The ribbon images were stored on a computer at the rate of 25 images/s when only 8 center lines of each image of the ribbon were recorded and up to 12.5 images/s when all grid points were recorded. The spatial resolution of the thermal imager was 0.1mm and the temperature resolution was between 1 and 2 OC depending on the temperature and thermal range setting. The infrared camera was mounted on a motorized table and driven parallel to the ribbon. The thermal imager was used to carry out two types of measurements: (a) Temperature profiles of the ribbon were obtained by juxtaposing a series of consecutive infrared images, recorded as the camera was driven parallel to the platinum ribbon. (b) Local temperature images of 1 cm segment of the ribbon were recorded
Experimentnl System
The reaction was carried out on a 14.7 cm long, 0.05 cm wide by 0.0025 cm thick pure platinum ribbon (Johnson Mathey Inc.) placed in a rectangular duct with a cross section of 0.93 cm X 22.9 cm and a height of 27 cm. The platinum ribbon was maintained at a constant total resistance (and therefore constant average temperature) by a constant temperature anemometer (TSI IFA-100). The ribbon was suspended in the center of the reactor channel with the gaseous reacting mixture (linear velocity of 2.9 cm/s) flowing perpendicular to its length. The gases, extra dry grade oxygen, prepurified nitrogen, and propylene (99.0% minimum purity), were controlled by a mass flow controller, purified, dried, and mixed before they entered the reactor at room temperature (22 "C). The catalyst was activated by heating to lo00 OC in air for 1 h before 1% propylene was introduced to the air stream for about 14 h. The infrared radiation from the ribbon was measured by a thermal imager (AGEMA Thermovision 780) placed next to a To whom correspondence should be addressed.
0022-3654/91/2095-3224$02.50/0
( 1 ) Field, R. J.; Burger, M. Oscillatiotu and Trawling Waves in Chemical Systems; Wiley: New York, 1985. (2) Razbn, L. F.; Chang, S.M.; Schmitz, R. A. Chem. Eng. Sci. 1986,41, 1561. (3) Lev, 0.; Wolffberg, A.; Pismen, L. M.; Sheintuch, M. J. Phys. Chem. 1989, 93, 1663. (4) Lev, 0.;Sheintuch, M.; Yarnitzky, C.; Pismen, L. M. Chem. Eng. Sci. 1990.45, 839. ( 5 ) Bassett, M. R.; Hudson, J. L. J. Phys. Chem. 1989, 93, 2731. (6) Zaikin, A. N.; Zhabotinski, A. M. Nature 1970, 255, 535. (7) Pismen, L. Chem. Eng. Sci. 1980, 35, 1950. (8) Sheintuch, M.; Pismen, L. Chem. Eng. Sci. 1981, 36, 893. (9) Schmitz, R.; Tsotsis, T. Chem. Eng. Sci. 1983, 38, 1421. (10) Bykov, V.; Gorban, A.; Kamenshehikov, L.; Yabloskii, G. Kinef. Catal. 1983, 24, 520. (11) Imbihl, R.; Cox, M. P.; Ertl. G. J . Chem. Phys. 1986. 84, 3518. (12) Imbihl, R.; Ladas, S.; Ertl, G. Surf. Sci. 1989. 215, L307. (13) Pawlicki, P. C.; Schmitz. R. A. Chem. Eng. frog. 1987, 83, 40. (14) Lobban, L.; Philippou, G.; Luss, D. J . Phys. Chem. 1989, 93, 733. (15) Lobban, L.; Luss, D. J . Phys. Chem. 1989, 93, 6530. (16) Kellow, J. C.; Wolf, E. Chem. Eng. Sci. 1990, 45, 2597. (17) Cordonier, G. A.; Schmidt, L. D. Chem. Eng. Sci. 1989, 44, 1983.
Q 1991 American Chemical Society
Temperature Patterns on a Heated Catalytic Ribbon
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3225
400r
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Transition Chaos
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0.2 Oh Propylene
0. 200
250
300
350
400
1
450
Average Temperature ("C) Figure 1. The overall heat generated by the reaction versus average ribbon temperature for a mixture containing 0.2% propylene. Bars de-
note amplitude of oscillations.
100 -8
(18) Packard, N. H.; Crutchfield, J. P.;Farmer, J. D.; Shaw, R. S. Phys. Reu. Len. 1980, IS, 712. (19) Fraser, A. M.;Swinney, H.L. Phys. Rev. A 1987, 33, 1134. (20) Eckmann, J. P.; Ruelle, D. Reu. Mod. Phys. 1985. 57, 617. (21) Bendat, J. S.; Piersol, A. G. Random Data: Analysis and Measurement Procedures; Wiley: New York, 1985. (22) Gransberger, P.;Procaccia, 1. Physica D 1983, 9, 189. (23) Brandstater, A.; Swinney, H. L.Phys. Rev. A 1987, 35, 2207. (24) Grebogi, C.; Ott, E.;Yorke, J. A. Physica D, 1983, 7 , 181.
Propylene
0
-4
4
8
Position (cm) Figure 2. Transient ribbon temperature profiles at different increasing average temperatures and 0.2% propylene.
as a function of time with the infrared camera being stationary. The heat generated by the exothermic propylene oxidation on the ribbon was calculated from the difference in the electric power required to maintain the ribbon at the operating resistance (average temperature) with and without reaction. The time-dependent electric voltage across the wire was recorded simultaneously with the thermal images. The techniques used to characterize the dynamic behavior of the overall heat generated by the reaction and the local temperature included attractor construction by the time delay Poincard sections,20power spectra,20*21and correlation d i m e n s i ~ n . ~ ~ . ~ ~
Results Experiments, in which the average ribbon temperature was kept constant, were conducted using mixtures of air and 0.2% and 0.3% propylene. The bifurcation diagram of the overall heat generated by the reaction per unit surface area, Q4,versus the average ribbon temperature, Taw,for a mixture containing 0.2% propylene (Figure 1) shows that a uniform extinguished state existed for all average temperatures below 228 OC. Two different states, either an extinguished or a chaotic state, existed for average temperatures between 228 and 266 OC. A periodic state existed for average temperatures exceeding 290 "C. The amplitude of these oscillations decreased with increasing average temperatures. Very small oscillations still existed at Taw= 400 OC. A transition region from oscillatory to chaotic states existed between Tavgof 260 and 290 OC. A chaotic attractor existed for Tsvgbelow 260 OC until a boundary crisis (a collision of a chaotic attractor with an unstable fixed point or periodic orbit24)destroyed the attractor at Tavgof 228 OC. The amplitude of the chaotic oscillations decreases with decreasing average temperature. Figure 2 describes several transient temperature profiles measured as the average temperature was increased. For average temperatures between 229 and 347 OC a fraction of the ribbon is at a high temperature while the rest is at a low temperature. The shape of the moving temperature profiles is similar to that of the stationary temperature fronts observed during the oxidation of ammonia on Pt ribbon by Lobban et aI.l4 When a periodic or chaotic state existed, the ignited section moved along the catalyst surface with a change in its length and a slight change in temperature. Only one transient profile is shown in Figure 2 for each average ribbon temperature. Above 347 "C the ribbon is fully ignited and only small temperature changes occur next to the two
0.2 %
Time (sec) 700
900
1100
1300
02
101 L
fs
n
o
Frequency (Hz) Figure 3. Overall heat generated by the reaction as a function of time and the corresponding power spectrum for a mixture of 0.2% propylene and an average temperature of 294 "C (quasiperiodic behavior). ends. The local temperature at either the ignited or extinguished section resembles that of a bistable system. Figure 3 shows the time dependence of the overall heat generated by the reaction at TavBof 294 OC (top) and the corresponding power spectrum (bottom). These oscillations are caused by changes in the length of the ignited section of the ribbon. The heat generation rate of each periodic cycle consists of three peaks of different amplitudes. The power spectrum reveals two fundamental frequenciesf, = 0.0117 Hz (period of 85.5 s) and f2 = 0.0195 Hz (period of 51.3 s) and their harmonics. The overall reaction rate in the transition region ( TavB between 260 and 290 "C)alternates randomly between an almost periodic state and chaotic state. A typical case is shown in Figure 4, top. A series of temperature profiles were measured to gain an understanding of the dynamics of the heat generation. Four such profiles are shown in Figure 4. The times at which they were recorded are marked by arrows in Figure 4,top. The measurements showed that each almost periodic cycle consisted essentially of five stages. In the initial one, the reaction rate was at a maximum and an almost stationary ignited section exists on the left side of the ribbon (profile a in Figure 4). Both the front and tail temperature fronts were essentially stationary during this period. In the second stage, the ignited zone moved to the right with the tail of the wave moving somewhat faster than its front. This decreased the length of the ignited zone and the
3226 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991
N~
Philippou et al.
w;o
Time (sec) .53
0
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1000
Time (sec)
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2000
-
1000
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300
-
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-
- 001 4
0.2 % Propylene Tavg=229"C
-a
-4
a
Figure 4. Overall heat generated by the reaction as a function of time and related temperature profiles for a mixture of 0.2% propylene and an average temperature of 269 OC (transition to chaos).
overall reaction rate. The front of the wave stopped its movement before reaching the right support. The wave then reversed its direction of movement and its front moved faster than its tail, increasing the length of the ignited section and the overall rate. Profile b corresponds to a state in this third stage. Eventually the tail of the wave started moving faster than its front, decreasing the length of the ignited section and the overall rate. Eventually the front of the wave came close to the left support and stopped moving. The tail of the wave then reversed its direction and moved to the right, expanding the ignited region and increasing the observed reaction rate. Profile c in Figure 4 is a state in stage 5. Eventually profile c turned into profile a. The average duration of each of the five stages shown in Figure 4 was -46, 19, 18, 16 and 24 s, respectively. The velocity of the moving temperature fronts was of the order of 0.1 cm/s. During the oscillatory behavior, a reduction in the heat generation by the reaction increased the electrical heating needed to keep the ribbon at the present average temperature. This increased slightly the temperatures of both the ignited and extinguished sections as noted by comparing profiles a and b. The inverse occurred when the ignited zone expanded. The experiments revealed that the left section of the ribbon was slightly more active than the right one. Thus, the ignited wave moved closer to the left support than to the right one before reversing its direction. This difference increased as the average temperature was reduced. In the case shown in Figure 4, the wave reversed its rightward movement when it was about 3 cm of the right support, but it got very close to the left support. When the ribbon was turned around the right section was slightly more active, indicating that the observed activity is an intrinsic property of the catalytic ribbon and not caused by flow and transport phenomena nonuniformities. Measurements showed that the chaotic motion in the transition region was caused by the splitting of the ignited section (profile a) into two waves (profile d, Figure 4) which moved along the ribbon a t the same direction but with different velocities. Eventually the split waves collided and formed a single wave. When this happened the ribbon returned to the almost periodic motion. In the chaotic region ( Taq between 228 and 260 "C)the ignited section moved back and forth on the ribbon changing its position and width. The ignited section never penetrated the region close to the right support. The length of this extinguished region
0
a
4
Position ( cm )
Figure 5. Overall heat generated by the reaction as a function of time and related temperature profiles for a mixture of 0.2% propylene and an average temperature of 229 O C (chaotic state).
.-E
0.2 % Propylene TaVg= 229 "C
tom3
IU
0
.03
.06
.09
.12
Frequency ( Hz ) Figure 6. Power spectrum of the overall heat generated by the reaction at 0.2% propylene and 229 OC (chaotic region). increased as the average ribbon temperature was decreased. Figure 5 (bottom) describes three profiles recorded a t 0.2% propylene and Tavgof 229 OC. The corresponding overall heat generation is shown a t the top of that figure. In this chaotic state, which is very close to extinction, the ignited section reversed its direction at about 5 cm from the right-hand support. The overall reaction rate corresponding to the three profiles in Figure 5 is about equal, as is the length of the ignited section. Yet, their position on the ribbon and speed of movement on the ribbon are different. The power spectrum of the overall rate of heat generation (Figure 6) has no significant peaks and decays exponentially with a slope of -51.02, suggesting it has the features of a deterministic system.u The dynamic features of reaction mixtures containing 0.3% propylene were similar to those containing 0.2% propylene. However, the chaotic behavior of the reaction mixture containing 0.3% propylene was less complex (lower correlation dimension) than that containing 0.2% propylene. Figure 7 is a typical trace of the dynamic overall rate of heat generation in the chaotic regime ( 2 5 ) Sigeti, D,;Horthemke, W.Phys. Reu. A 1986, 35, 2276. (26) Kurtanjek, Z.; Sheintuch, M.;Luss, D. J . Coral. 1980, 66, 11. (27) Lindstrom, T.H.; Tsotsis, T. T.Sur$ Sci. 1985, 150, 487.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3227
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Time (sec) Figure 7. Overall rate of heat generated by the reaction as a function of time for a mixture of 0.3% propylene and an average temperature of 225 O C (chaotic state).
0.3 % Propylene, Ta,,=225
"C
Figure 10. The temperature of a 1-cm segment of the ribbon as a function of time and distance from the support, for a mixture of 0.3% propylene and an average temperature of 243 "C.
Time (sec) 4000
4250
4500
4750
5000
.J
Figure 8. A three-dimensional reconstruction of the attractor shown in Figure 7 using a time delay of 5 s. 0.
0.3 % Propylene,TaVg=225 O
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n
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Figure 9. A Poincari section of the attractor shown in Figure 8.
of a mixture containing 0.3% propylene and Tavg= 225 OC. The behavior is more regular than that shown in Figure 5 for a mixture with 0.2% propylene. The time delay was used to reconstruct the attractor of the dynamic overall reaction rate (Figure 8). Both Figures 7 and 8 show that the chaotic behavior is due to motion around two unstable states, one having a low reaction rate and one with a high reaction rate. The two states correspond to different length of the ignited reaction zone. Figure 8 shows that the motion is either araund one of these states or around both. A Poincare
section of the attractor shown in Figure 8 with a 45' plane perpendicular to the Qg(t)-Qg(t + T ) plane (Figure 9) shows three bands. The two bands along the diagonal are of trajectories close to the two unstable states. The third is of trajectories moving around both unstable states. The corresponding correlation dimension is 3.2. The nature of the temperature wave was studied by recording temperature profiles of a 1-cm segment of the wire, keeping the camera stationary. Figure 10 shows a typical behavior for a mixture containing 0.3% propylene and a ribbon at an average temperature of 243 OC. The temperature profiles are of a section located between 3.3 and 4.3 cm from the right-hand support, close to the position at which one end of the temperature wave reverses its direction. During the 400 s shown in Figure 10 five waves crossed the left end of the section (4.3 cm from the support). Of these four crossed also the right end of the segment (position 3.3 cm) before reversing their direction of movement. One wave (at t = 4350 s) reversed its direction of movement at about the center of the section. Figure 11 describes the overall rate of heat generation along the whole ribbon, a segment of which is shown in Figure 10. The corresponding power spectrum (Figure 11, bottom) shows an exponential decay with a slope of -33.2. The time dependence of the local temperature at two points on the ribbon is shown in Figure 12 (top). The corresponding power spectra (Figure 12, bottom) decays as a power law for
3228 The Journal of Physical Chemistry, Vol. 95, No. 8,1991 Time (sec)
B -
4250
4000
4500
4750
5000
..-- 3.3 cm from end
- 4.3 cm from end
350
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Frequency ( Hz) Figure 12. Local temperature as a function of time located 3.3 cm (dashed line) and 4.3 cm (solid line) from the support and corresponding power spectrums, same conditions as in Figure 10.
frequencies higher than about Hz, with slopes of -2.8 and -2.9. Comparison of Figures 1 1 and 12 shows that the overall reaction rate oscillates a t a much higher frequency than the local temperature. The chaotic attractor of the overall rate of the heat generation at 0.3% propylene and an average temperature of 243 OC had a correlation dimension of 2.8. A small increase in the correlation dimension was observed as the average temperature was decreased at constant propylene concentration. For example, at 0.3% propylene and for average temperatures of 230 and 225 OC, the estimated correlation dimension was 3.1-3.2. A larger increase in the correlation dimension was observed as the reactant concentration was decreased from 0.3% to 0.2% propylene. The chaotic attractor at 0.2%propylene and an average ribbon temperature of 259 OC had a correlation dimension slightly larger than five. The correlation dimension of the local temperature was different and in general smaller than that of the overall heat generated by the reaction. For example, at 0.3%propylene and an average temperature of 243 OC, the correlation dimension of the local temperature was 2.3 at 4.3 cm from the end of ribbon, while that of the overall heat generated by the reaction was 2.8. Clearly, measurements of the overall rate are not sufficient to characterize this spatiotemporal behavior.
Discussion The experiments reveal that periodic and chaotic oscillations in the overall reaction rate (heat generation) during the oxidation of propylene are due to the back and forth movement of temperature waves on the electrically heated Pt ribbon. Theoretical and experimental s t u d i e ~ ’have ~ * ~established ~ that a controller which keeps a constant average temperature may stabilize stationary nonuniform temperature states when a chemical reaction on an electrically heated catalytic wire may have two different uniform temperature steady states. These stationary nonuniform states consist of a high- and low-temperature sections separated by a narrow front and are similar in shape to the transient nonuniform states shown in Figure 2. The oscillatory states are of cases in which the electrical current cannot stabilize nonuniform states. The moving front of the high-temperature section stops its movement as it approaches either support, next to which the (28) Sheintuch, M.; Schmidt, J. J . Phys. Chem. 1986, 92,3404.
Philippou et al. ribbon is much less active than a t its center. This causes a temporary reduction in the size of the ignited region. Consequently, the electrical heating increases the temperature along both sections of the wire, which eventually leads to a reversal in the direction of movement of the ignited section. The reaction at the hot section of the ribbon is the main contributor to the dynamic nature of the overall chemical reaction rate. The observed rate is due mainly to changes in the size and shape of the ignited section. Thus the distinction between a periodic and chaotic behavior is mainly related to the nature of the movement of the temperature wave. The data reveal a large difference in the dynamic features of the overall rate and that of the local temperature oscillations. For example, the overall reaction rate oscillates at a higher frequency and is in general much less regular than the local temperature. The power spectrum of the overall rate decays exponentially (Figures 6 and 1 I ) , which according to Sigeti and H ~ r t h e m k esuggests ~~ features of a deterministic system. On the other hand, the power spectrum of the local temperature decays as a power law (Figure 12), suggesting stochastic behavior. We do not believe that this is the case. The local temperature oscillations are caused by the movement of the temperature wave through the observed point, and its dynamic features depend on the speed, shape, and length of the temperature waves. In the chaotic regime the variations of the local speed and wavelength create power spectrum features resembling that a stochastic process, even though the chaotic wave movement is a deterministic process. This point is emphasized by the fact that the correlation dimensions of the local temperature oscillations are smaller than those of the overall reaction rate. Further theoretical analysis of these and other chaotic waves and the relation between overall and local features would be very beneficial. The motion of the high-temperature waves has many similarities to the local antiphase electrical current waves observed by Lev et al.4 during the electrochemical dissolution of Ni in the galvanostatic mode. Electrically heated Pt wires have been used in many studies of the oscillatory features of exothermic catalytic reactions. Clearly, any analysis or model of this behavior, which ignores the spatiotemporal features of the wire, will lead to pitfalls. A model of the system should account not only for the dynamic features of the observed reaction rate, but also for the observed local features. Many mechanisms leading to periodic and chaotic reaction rates in catalytic systems have been described in the literature.29 It is recognized that a specific mechanism cannot be determined just from the analysis of the dynamic rate. It would, however, be most useful to develop some criteria or diagnostics for predicting when the observed rate is due to temporal behavior of the catalyst or due to a spatiotemporal process. Sheintuch30 has used a simple model to illustrate that a slow modification of the surface properties in a bistable system by some activation4eactivation mechanism may lead to periodic movement of temperature waves on the surface of an electrically heated wire or ribbon maintained at a constant average temperature. H e simplified significantly the analysis by assuming a uniform activity in each section. He considered a special case in which the adsorption and desorption of a poisonous species is the slow activation4eactivation process. Other feasible mechanisms are oxidation-reduction, as observed during the oxidation of hydrogen on a nickel s u r f a ~ e ~or~ ,a~change ’ in the surface structure, as observed by Ertl’s”-12 group during the oxidation of carbon monoxide. The wave movement is related to the deactivation of the ignited region and the activation of extinguished region. Sheintuch3’ showed that very complex dynamic features including chaos may be observed by assuming that the wire consists of many sections. It is of intrinsic academic interest to be able to predict a priori the type of reactions and operation conditions which cause the (29) Ranzon, L. F.; Schmitz, R. A. Carol. Reu. Sci. Eng. 1986, 28, 89. (30)Sheintuch, M.Chem. Eng. Sci. 1989,44, 1081. (31)Sheintuch, M.J . Phys. Chem. 1990, 94, 5889.
3229
J. Phys. Chem. 1991,95, 3229-3237 formation of the spatiotemporal patterns, the size of the ignited region, and rate of movement of the reaction fronts. It is of practical importance to know the impact of the temperature patterns on the overall rate and how it can be affected by changes in the omrating conditions such as reactant concentration, Dreset temperature, Gc. Moreover, it is essential to study how.;obust or sensitive these patterns are and the time-averaged rate and/or
selectivity to slight variations in the activity of the catalyst or to nonuniformities in the activity profile.
Acknowledgment. We are thankful to the NSF, the Welch Foundation, and the Texas Advance Research R W a m for SUPPOfi Of this research. Registry No. Pt, 7440-06-4; propylene, 115-07-1
Kinetics of Reactions of CH902and HOCH2CH202Radicals Produced by the Photolysis of Iodomethane and 2-Iodoethanoi Michael E. Jenkin* and Richard A. Cox B551,Hanvell Laboratory, Didcot, Oxfordshire, OX1 1 ORA, UK (Received: October 17, 1989; In Final Form: November 14, 1990)
The molecular modulation technique coupled with UV absorption spectroscopy has been used to investigate the UV spectra and kinetics of reactions of the methylperoxy radical (CH3O2) and the 2-hydroxyethylperoxy radical (HOCH2CH202),generated by the 254-nm photolysis of the organic iodides CH31 and HOCH2CH21: RI hu(X=254 nm) R + I (7) and R + O2 + M R 0 2 + M (8). Measurements of the UV spectra of both R02 radicals were complicated by the production of additional transient species absorbing strongly at wavelengths above 240 nm. These are believed to be CH3001and HOCH2CH2001 formed as intermediates in the R02-chaperoned recombination of iodine atoms. Both CH3O2 and HOCH2CH202were found to obey second-order kinetic behavior owing to removal by a series of reactions initiated by the self-reactions: CH3O2 CH3O2 products (9) and HOCH2CH202+ HOCH2CH202 products (IO). The parameter kkb/u (where kg, is the observed second-order rate coefficient) had a value of (1.01 f 0.09) X los cm s-' at 230 nm, independent of pressure in the range 10.8-760 Torr, at 298 K. Additional measurements made over the temperature range 268-350 K indicated that this parameter displays a weak negative temperature dependence. E / R was found to have a value of -220 72 K at 760 Torr, and a value of -92 f 53 K at 10.8 Torr. The parameter klhbs/u had a value of (6.8 f 0.4) X IOs cm s-I at 230 nm (p = 760 Torr, T = 298 K). Assuming the photolysis of HOCH2CH21leads exclusively to the production of HOCH2CH202, the following values of a(230 nm) and klhb were concluded: u(230nm) = (2.35 0.25) X IO-'* cm2 molecule-' and klOqb = (1.60 0.17) X cm3 molecule-' s-l. At lower pressures, H 0 2 was also generated in the HOCH2CH21system in significant quantities, enabling investigation of reaction 11 at 10 Torr and 298 K: HOCH2CH202+ H 0 2 products (1 1). A value of k l l = (4.8 f 1.5) X cm3 molecule-' s-I was concluded from these measurements, based on the above value of u(230 nm).
-
+
-
-
-
+
*
*
*
1 . Introduction It is well established that the oxidation of organic compounds in the atmosphere occurs by free-radical mechanisms in which organic peroxy radicals (RO,) participate.'V2 In the presence of NO,, O3is formed as a byproduct of these oxidation schemes, since the R 0 2 radicals react with NO to produce NO,, which is photolyzed rapidly to yield O3 in the following manner: RO2 + N O R O + NO2 (1)
- + - +
+ hv(X1420 nm) N O + 0 0 + O2 M O3 M Generally, concentrations of NO, (= N O + NO,) NO2
(2) (3)
in the continental boundary layer are sufficiently high that R 0 2 radicals react exclusively with N O leading to O3 formation. Since the oxidation of the parent organic compound is usually initiated by attack of the O H radical, the potential of a particular organic compound for O3production is governed by its rate of reaction with OH (Le., the concentration of the organic compound X the rate coefficient for its reaction with OH). In the remote continental boundary layer, the marine boundary layer, and the background troposphere, the situation is slightly different, since concentrations of NO, are significantly lower and the reactions
0022-365419112095-3229$02.50/0
-
of R 0 2 radicals with HOz can compete with reaction 1: R02
+ HOz
products
(4)
This type of reaction is terminating since it is believed to produce mainly the relatively stable hydroperoxide species ROOH. Consequently, reaction 4 inhibits the production of O3 by the reaction scheme (1)-(3). In these circumstances, the potential of a given organic compound for the production of O3is not only governed by its rate of reaction with OH but also by the relative rate of the reaction of the derived peroxy radical with NO (reaction 1) and HO, (reaction 4). The oxidation of CH4 is known to make a major contribution to photochemical production of O3in the background troposphere? Non-methane hydrocarbons, although present in much lower concentrations, may also be significant since the rate coefficients for their reaction with O H are much larger.3 For instance, if we compare CH4 at a concentration of 1.7 ppm with CzH4at a concentration of 0.5 ppb? then the rates of reaction with OH a t and 5.4 X s-I, re270 K and 5 km altitude are 9 X and kb = 9 X cm3 molecule-' spectively ( k , = 4.4 X s-I ,), i.e., within a factor of 2.
OH + CH4 OH
( I ) Atkinson. R:; Lloyd, A. C.J. Phys. Chem. Ref. Dura 1984, 13, 315. (2) 'Atmospheric OzontAssessment of our understanding of the processes controlling its distribution and change"; W M O Report No.16, Vol. 1, Chapter 4, 1985.
-
+ C2H4 + M
-
CH3
+ H2O
HOCHzCH,
(5)
+M
(3) Atkinson, R. Chem. Reu. 1986, 86, 69. (4) Rudolph, J., KFA Julich, private communication.
0 1991 American Chemical Society
(6)
