Ind. Eng. Chem. Prod. Res. Dev. 1085, 24, 43-49
43
Behavior of Automotive Noble Metal Catalysts in Cycled Feedstreams Hldeakl Murakl, Hlrofuml Shlnjoh, Hldeo Sobukawa, Kohjl Yokota, and Yoshlyasu FuJltanl Toyofa Central Research and Development Laboratories, Inc., Nagakute-cho, Aichi-gun, Aichi-ken, 480- 7 7, Japan
The response of Pt, W,and Rh catalysts to perturbations in the feedstream stoichiometry was examined in laboratory experiments. The conversions of NO, CO, and HC were measured by use of the cycled feeds and steady feeds of equivalent net stoichiometry. The activities of Pt and Pd catalysts were improved, particularly for NO,by the cycled feed. Another beneftt of cycling feed was the improvement of the light-off performance due to the periodic operation effect. The order of periodic operation effect was Pt > Pd > Rh. The optimum frequencies for the maximum conversion increased with increasing amplitude and temperature.
Introduction
Automotive catalysts presently in use contain noble metals such as platinum (Pt),palladium (Pd), and rhodium (Rh) as active components. Pt oxidizes saturated and unsaturated hydrocarbons (HC) and carbon monoxide (CO). In particular, Pt is currently the only practical catalyst that possesses a high oxidation activity for saturated hydrocarbons (Kummer, 1980; Yao, 1980) required for automotive catalysts. Pd is more active than Pt for oxidation of CO and unsaturated hydrocarbons and is frequently used to together with Pt as oxidation catalysts. However, it has a poor catalytic activity for oxidation of saturated hydrocarbons and selective reduction of nitric oxide (NO) and is poorly resistant to poisoning. Rh is used in three-way catalysts (TWC) because of its excellent activity for selective reduction of NO to nitrogen with low ammonia,formation (Kummer, 1980; Shelef and Gandhi, 1972; Schlatter and Taylor, 1977). On the other hand, the amount of Rh used in the TWC with sufficient durability is considerablyhigher than that correspondingto the mine ratio. Therefore, for large-scale vehicle application it is necessary to find ways to minimize the use of this scarce material or to find a mode of operation to control the air-fuel ratio which can provide a suitable environment for Rh-free catalysts. In order that the TWC may operate effectively, the air-fuel ratio of the exhaust is controlled close to the stoichiometrically balanced ratio (Grimm et al., 1980; Canale et al., 1978; Engh and Wallman, 1977). The feedback system which controls the exhaust air-fuel ratio using a signal from an oxygen sensor produces small oscillations in the air-fuel ratio. These oscillations occur with a frequency in the order of 1Hz (Canale et al., 1978; Seiter and Clark, 1978; Engh and Wallman, 1977). Gandhi et al. (1976) pointed out that inclusion of an “oxygen storage” component in the catalyst moderates the effects of rapid changes between rich and lean exhaust stoichiometries. Recently, Taylor and Sinkevitch (1983) used both symmetric and asymmetric air-fuel ratio cycles in their laboratory experiments of cyclic operation of TWCs. However, the Rh-containing TWCs require a minimum period and amplitude oscillation in the air-fuel ratio for their effective performance (Schlatter et al., 1983; Herz, 1982; Hegedus et al., 1979; Adavi et al., 1977). Our objectives are to determine how Pt, Pd, and Rh catalysts perform during the oscillations and to identify how the performance of Pt and Pd catalysts is improved by controlling the cycling characteristics. In this paper we report the performance during warm-up and at 400 O C 0196-432 1185/1224-0043$0 1SO10
of fresh catalysts (Pt, Pd, Rh) in the condition in which the air-fuel ratio of a feedstream to the catalyst was oscillated over a wide range (0.2-10.0 Hz) by the use of a simulated engine exhaust of the stoichiometric air-fuel ratio. Experimental Section Laboratory Reactor. The laboratory integral reactor
system used in this study is a conventional flow system with a tubular fmed-bed reactor as shown in Figure 1. The characteristic feature of this reactor system is its ability to change the feedstream to the catalyst quickly so that the feedstream can be rapidly cycled between two different gas mixtures. Four fast-acting solenoid values inject O2 and CO-H2 alternately and periodically to simulate an engine exhaust gas (A/F = 14.6) so that an oscillating feedstream can be delivered to the catalyst. The cycling frequency was varied from 0 to 10.0 Hz. The concentration of CO, H2, and 02,selected as representative of the main constituents in engine exhaust, were changed simultaneously, but the stoichiometry was kept to simulate the amplitude (&A/F) of exhaust gas. Figure 2 shows time-average concentration changes of CO, H2,and O2for various &A/F. Table I lists the composition of the gas blends used in this study. The time-averagegas composition (Figure 2) which determined the amplitude of the cycles was adjusted by two mass-flow control valves. The gas leaving the reactor entered a condenser so that the water was trapped and then was analyzed by gas analyzers for time-average concentrations of NO, CO, etc. The feed gas used in these experiments contained only one hydrocarbon species, propylene (0.05 vol % 1. Although propylene is an easy-to-oxidize hydrocarbon, the catalytic activity using the feed gas has been found to agree well with that for engine exhaust. There was a small amount of H 2 0 (about 3 vol % ) in the feed gas used. We had already found that 3 vol % HzO was good enough to investigate the TWC’s activity. Cycled Feedstreams. In this study, catalyst response to feedstream oscillations was evaluated by using a frequency generator which imposes perturbations to rectangular waves on solenoid valves for gas injection. In order that the concentration change of rich and lean feedstreams might take a square-waveshape, the pressure between the Nz line and simulated exhaust gas line was carefully equalized by the back-pressure valve (Figure 1). The instantaneous gas composition in the cycled streams delivered to the catalysts was promptly analyzed by a quadrupole mass analyzer. However, as cycling frequencies 0 1985 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table I. Comoosition of Gas Blends Used to Simulate Exhaust at Different Engine Air-Fuel Ratios in Cycling ExDeriments fAIF -1.2 -0.8 -0.5 -0.3 0" +0.3 t0.5 t0.8 +1.2 0.68 0.44 0.30 0.19 0.15 0.19 0.15 0.19 Hz, v01 % 0.96 1000 1000 1000 1000 1000 1000 1000 1000 1000 NO, ppm 2.01 1.35 0.60 0.60 0.60 0.91 0.60 0.60 2.85 co, vol % 500 500 500 500 500 500 500 500 500 C3H61 ppm 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 co*,vol % bal bal bal bal bal bal bal bal Nz,balance bal 0.57 0.78 0.57 1.07 2.10 0.57 0.57 1.52 0.57 oz,vol % 3 3 3 3 3 3 3 3 3 H20,v01 % ~~
I
-
"A/F = 14.6. BACK PRESSURE VALVE
CONTROLLER
Exhamt Gas(a,
CO
H2
Or
1401
Figure 1. Schematic diagram of system for simulating catalyst to oscillating feedstream compositions.
/I
S +----
5 sec q-
=r
Figure 3. Waveforms a t 0.2, 0.5, and 1.0 Hz.
- 10 0
1 .o
0.5 Simulated
+AIF
Figure 2. Component concentrations used to simulate exhaust a t various hA/F.
were higher than several hertz, the reactor received a feedstream of practically constant composition. Figure 3 shows the waveforms for 0.2-, 0.5, and 1.0-Hz cycling frequencies and Figure 4 shows the amplitudes at corresponding frequencies. The catalyst reached the intended amplitude (&A/F) only when the cycling frequency was considerably slow (e.g., 0.2 Hz). At 0.5 Hz, the amplitude w&s slightly reduced but the square-wave-shapesof H2 and O2 concentrations were kept. However, for faster cycling rates (e.g., 1.0 Hz), the amplitude showed a sinusoidal shape and the amplitude was continuously diminished, as shown in Figure 4. Experimental Procedures. The reactor was a quartz tube (20 mm diameter) heated by an electric furnace. The catalyst charge was 7 cm3. In order to preheat the feedstream, the upper side of the catalyst bed was packed with 7 cm3 of 3-mm-diameter silicon carbide spheres. In all experiments, the gas hourly space velocity was kept at 30000 h-' (STP). The activity of the catalyst for converting NO, propylene, and CO was measured at 200 to 500 O C for stoichiometric steady and cycling feedstreams.
FrequencyIHz)
Figure 4. Amplitude a t various frequencies.
In the catalyst light-off experiments, the reactivities of CO, NO, and propylene were monitored by warm-up of the catalyst at a heating rate of 2 OC/min from 200 to 500 " C for steady and cycled feedstreams. The catalyst bed temperature represented that of the bed center. In other experiments, the catalyst was kept at a reaction temperature and the feedstream composition was cycled at 5.0 Hz. After the catalyst bed temperature and the outlet concentrations were stabilized, the oscillation frequency was changed in steps from 5.0 to 0.4 Hz. Catalysts. In order to lower the inevitable carrier effects which affect the intrinsic character of noble metals on catalysts, chemically inactive a-A1203pellet (2-3 mm diameter; BET surface area: 10 m2/g; bulk density: 0.79 g/cm2) was selected as the carrier. a-A1203was obtained by calcination of 6-A1203(RhGne-Poulenc: SCS-79) at 1200 "C for 3 h.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24,
-z
100
No. 1, 1985 45
a
v
C 0 y1
?c
50-
0
0
8 C a t a l y s t B e d Temperature
('c)
C a t a l y s t B e d T e m p e r a t u r e ("2)
J 300 C a t a l y s t B e d Temperature
('c)
C a t a l y s t B e d Temperature
('C)
Figure 5. NO, CO, and propylene conversion data of Pt catalyst for steady and cycled stoichiometric feeds plotted against the catalyst bed temperature: (-) steady; (- - -) 2 Hz;(- - -) 1 Hz;(--) 0.5 Hz.
Practical fresh catalysts provide such fast reaction rates that the catalytic process cannot be followed precisely. There are two methods to examine the process. That is, one is use of low loading catalysts and the other is use of aged Catalysts. Since deactivation of automobile exhaust catalysts is a rather complex event involving various chemical and physical processes, the aged catalysts have different surface structures that cannot be well-defined in comparison with fresh catalysts and are not suitable to show the native character of noble metals. Thus, we used low loading fresh catalysts and the reason why we used the noble metal of the 0.05 g/L will be described subsequently. The amount of CO adsorption on a 0.05 g/L catalyst was almost equal to that of exhaust-aged catalyst (100 h at 750 "C in Toyota 4M-E engine exhaust gas). Pt, Pd, and Rh catalysts were prepared by impregnating a-A120,with aqueous solutions of platinum, palladium, and rhodium nitrate, respectively. The concentrations of the nitrate solutions were adjusted to give 0.05 g/L metal on the finished catalysts. The impregnated pellets were dried overnight at 110 "C, then heated slowly in flowing air up to 600 "C, and calcined at 600 "C for 3 h in air. Results and Discussion Light-off Behaviors of Catalyst under Cycled and Steady Feedstreams. This section will describe light-off performances of Pt, Pd, and Rh catalysts in steady and cycled feeds. Our experiment was undertaken to determine whether cycling the feedstream composition during
400
500
C a t a l y s t B e d Temperature ('C)
C a t a l y s t B e d Temperature
('C)
Figure 6. NO, CO, and propylene conversion data of P d for steady and cycled stoichiometric feeds plotted against the catalyst bed temperature: (-1 steady; (- --) 2 Hz;(---) 1 Hz;(--) 0.5 Hz.
warm-up of the catalyst might lead to higher conversions of NO, CO, and propylene than are obtained by using a steady feed which is blended to the time-average composition of the cycled feed. The periodic operation effect was expected to improve the overall conversion by cycling around stoichiometry (Jain et al., 1982; Cutlip, 1979; AlTaie and Kershenbaum, 1978). Feedstream to the reactor was cycled with 0.5, 1.0-, and 2.0-Hz frequencies and was oscillated with h0.5 A/F amplitude. In these experiments, the temperature of the reactor was increased at 2 "C/min. The conversions of NO, CO, and propylene are shown in Figures 5 through 7. Also, the conversions obtained with the steady stoichiometric feed (solid lines) are shown for comparison. For the Pt catalyst (Figure 5), the conversion of NO between 310 and 460 "C was found to be improved by cycling in comparison with the steady feed. However, the conversion of CO was found to be decreased by cycling in comparison. The conversion of propylene below 430 "C was found to be improved by cycling in comparison with the steady-state result and further below 360 "C the lowfrequency cycling result was more excellent than the high-frequency (2.0-Hz) result. The performance of the Pd catalyst (Figure 6) shows a similar periodic operation effect to that of the Pt catalyst, although the conversions of NO, CO, and propylene under steady feed were higher than Pt catalyst. The conversion
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
/j
I
4 1
/
300
400
500
C a t a l y s t B e d T e m p e r a t u r e ('C)
100
b
300
400
500
C a t a l y s t Bed Temperature ('C)
-
: ic io0
I
C a t a l y s t B e d T e m p e r a t u r e ('C)
Figure 7. NO, CO, and propylene conversion data of Rh catalyst for steady and cycled stoichiometric feeds plotted against the catalyst bed temperature: (-1 steady; ( - - -) 2 Hz;(---) 1 Hz;(--) 0.5
Hz. of NO in 1.0-Hz cycled feed between 270 and 340 "C was excellent and above 340 "C the 1-Hz cycled feed result fell below the 2.0-Hz result. These data suggest that the conversion of NO has a maximum value for particular frequency a t every catalyst bed temperature. For the Rh catalyst (Figure 7), the conversions of NO and CO were found to be decreased by cycling in comparison with the steady feed result a t all catalyst bed temperatures. The performance of NO conversion of the Rh catalyst was clearly different from the Pt and Pd catalysts. On the other hand, the conversion of propylene above 320 "C was found to be improved by 2.0-Hz cycling in comparison with the steady feed result. In the cycling experiments described thus far the light-off performance is different for the three noble metal catalysts. Pt and Pd catalysts around stoichiometry had different characteristics in comparison with the Rh catalyst. In particular, when the Pt catalyst is compared with the Pd catalyst, the NO and propylene conversions of the steady feed for the Pt catalyst (Figure 5 ) are lower than that of the Pd catalyst (Figure 6). Therefore, in the periodic operation effect the Pt catalyst is predominant over the Pd catalyst; that is, the conversions of NO and propylene at low temperature under low-frequency feedstreams are higher than that of high-frequency feeds, and,
as the temperature is increased, the results are opposite. The discrepancy of catalyst performance between the steady feeds and the cycled feeds must be due to the surface state of the catalyst; that is, the catalyst surface under the steady feeds at relatively low temperatures is almost completely covered by the strong admolecules, such as CO, propylene (in these circumstances, now, we presume that propylene is more contributive than CO), and then, the expected reaction rates are remarkably suppressed. On the other hand, under the optimum cycled feeds, these admolecules are suitably eliminated and the surface compositions of reactants are kept suitable to react with each other. Then, the reaction rates attain to the maximum value. Comparing the three noble metals shows that the ionization potential decreases in the order of Pt > Pd > Rh. Therefore, the surface coverage of CO and propylene on Pt is expected to be more than that of Pd and Rh. Similar findings in periodic operation effects are obtained for the more simple reaction systems, such as CO02,NO-CO, NO-CO-propylene, etc., on the same catalysts at relatively low temperatures. These results will be reported in the following paper. From the above-mentioned results, we are able to propose how to operate the automobile catalyst system during warm-up; that is, when the catalyst temperature is relatively low, the perturbations of the engine exhaust gas should be enlarged, and after the catalyst temperature is increased, the cycling of the engine exhaust gas should have small perturbations. The next section of this paper will include cycling data obtained under a wide range of feed frequencies and amplitudes so that the cycling condition for the optimum conversion can be compared. Although Cutlip et al. (1979) reported the periodic operation effect of oxidation of CO and HC, it has not been reported that the periodic operation effect on NO conversion is applied to the TWCs.
Catalyst Performance with Cycled Feed Frequency Effects. The effect of the frequency was examined at 400 "C with cycled feedstreams. The catalyst performance of Pt, Pd, and Rh in the stream with the frequency ranging from 0.4 to 5.0 Hz a t f0.5 AfF amplitude is shown in Figure 8. For Rh catalyst, the NO conversion attained 100% above 2.2 Hz. On the other hand, for Pt and Pd catalysts, the NO conversions similarly increased with the frequency until the optimum frequency (about 1.8 Hz) and then decreased with further increase. The rate of the decrease of the Pt catalyst was larger than that of the Pd catalyst. The Pt catalyst showed a slight decrease in CO conversion, compared with the remarkable decrease in NO conversion. For the Pd and Rh catalysts, the CO conversions increased to a constant value above a particular frequency (about 1.8 Hz). The conversion of propylene showed a similar pattern as to NO conversion for the three catalysts. In other words, the Pt and Pd catalysts have an optimum frequency (about 1.8 Hz), but the Rh catalyst does not. These data suggest that the Pt and Pd catalysts, different from the Rh catalyst, have the particular frequency for maximum conversion. Amplitude Effects. The catalyst performance is affected by the amplitude of air/fuel ratio oscillations as well as frequency. The conversions of NO, CO, and propylene at 400 "C for the amplitude from f0.3 to f1.2 A/F are shown in Figures 9 through 11. For the Pt and Pd catalysts, the optimum frequency for the maximum conversion shifted to high frequency when the amplitude increased. The shift of the Pd catalyst is not so remarkable as that of the Pt catalyst. On the other hand, for the Rh catalyst,
l
o
o
.
T
v
J -0 0
2
1
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State
frequency ( H z ) 1oc
i
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01
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(
)
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P t 400°C
>
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)
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-
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C
0
0
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';j 50
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(
4 H~
)
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a,
>
oo'ooi
t0.5 A / F
C
0
-.
400%
u u I 0
1
2
3
frequency
Pt 400"c
C
4
(
HZ
)
Figure 8. Effect of frequency on the cycled performance of Pt, Pd, and Rh catalyst; amplitude = f0.5 A/F.
when the amplitude was increased at each frequency, the NO, CO, and propylene conversions were decreased. These data suggest that optimum frequencies for the Pt and Pd catalysts increase with increased amplitude. Perhaps this effect is caused by the increase of the reaction rate owing to the increased partial pressure of the reactants when the amplitude increases. Temperature Effects. The catalyst temperature similarly affects catalyst performance. The NO conversions of Pt, Pd, and Rh catalysts under cycled conditions (h0.5 A/F) are shown in Figure 12. For the Pt and Pd catalysts, the optimum frequencies for the maximum conversion increase with increasing temperature. On the other hand, for the Rh catalyst, when the frequency was increased at each temperature, the NO conversion was increased. Similar effects for optimum frequencies of other conversions except NO and CO conversions of the Rh catalysts are recognized, which are also inferred from Figures 5-7. PerhaDs this effect is caused bv the increase of the reaction
Figure 9. Effect of amplitude and frequency on cycled performance of Pt catalyst.
rate owing to the decrease of surface coverage of the strong admolecules, such as CO and propylene, when the temperature increases. For comparison between Pt and Pd, an important question to develop the Rh-free TWC is which performance is more similar to that of Rh catalvst. We should select Pd as a main component of the'lRh-free TWC.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 loo[
Pd
400'C
loor
R h 400°C
a
-0
frequency (
HZ
)
'"[
0
1
2
3
frequency (
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4 HI
)
'"[
Rh
frequency
( HI
frequency (
HI
400'C
\ amplitude
0
1
2
3
\
4
frequency ( H I ) Figure 10. Effect of amplitude and frequency on cycled performance of Pd catalyst.
Figure 11. Effect of amplitude and frequency on cycled performance of Rh catalyst.
Although Pd is less resistant to poisoning than Pt (Hegedus et al., 1979), it is more resistant to sintering than Pt (Yao, 1980). Pd catalyst forms more NH3 than Rh catalyst under the stoichiometric steady feeds, but less NH3 than Pt catalyst (Schlatter and Taylor, 1977; Taylor, 1975). We are going to investigate the NH3 formation on Pd and Pt catalysts under the stoichiometric cycled feeds.
In order to overcome some of the disadvantages of Pd catalysts, modified Pd catalysts containing base metals were reported recently (Gandhi et al., 1982; Adams and Gandhi, 1983). The effect of SO, on the performance of the TWC has been investigated by Summers and Baron (1979) and Gandhi et al. (1978). Schlatter and Mitchell (1980) have
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
100 n
ap
w
Y
C
450°C \
.-
z
;
5c
0
0 Z
C
1
2
3
frequency
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-
0
4
49
timum cycling condition in which the Rh-free TWC shows the maximum conversion. We have a good idea that by using a signal of the catalyst temperature, the cycling condition of the A/F feedback system should be decided to meet the optimum performance for respective temperature. Further studies are required to develop the entire Rh-free TWC system. Conclusion Pt and Pd catalysts have different characteristics in comparison with the Rh catalyst. The notable findings for Pt and Pd catalysts are their activities, which give higher conversion, particularly for NO, under cycled feed than steady noncycled feed. Another benefit of cycling feed is improvement of the light-off performance due to the periodic operation effect. The effect of the periodic operation is more significant in the Pt catalyst than in the Pd catalyst. The optimum frequencies for the maximum conversion increase with increased amplitude and temperature. Acknowledgment The authors wish to express their sincere thanks to Professor Y. Murakami of Nagoya University for his helpful discussion. Registry No. NO,10102-43-9;CO,630-08-0; C3H6, 115-07-1; Pt, 7440-06-4;Pd, 7440-05-3;Rh, 7440-16-6;nitrogen oxide,
0
1
2
3
frequency
4 (Hd
100 h
ZR c
W
-m z> 5 0 . 0
e O
L 0 1 2 3 4 frequency
(Hz)
Figure 12. Effect of temperature and frequency on cycled performance of Pt, Pd, and Rh catalysts; amplitude = *0.5 A/F.
reported that, with SOz in the feed, the characteristics of feed stream cycling are diminished for the Rh catalyst. On the other hand, we had found out that the performance of the same catalyst under the cycling of engine exhaust had the same results as the present experiment. These results will be reported in a separate paper. The effect of the cycled feed on Pt and Pd catalyst performances has not been demonstrated previously; we have shown that under certain conditions the cycled feed gives higher conversions than the steady feed. In order that the Rh-free TWC system may meet the exhaust emission regulations, the cycling characteristics of the A/F feedback system must be changed to the op-
11104-93-1.
Literature Cited Adams, K. M.; Gandhi, H. S. Ind. Eng. Chem. Rod. Res. Dev. 1983, 22, 207. Adawi, M. K.; Briggs, A. D.; Delosh, R. G.; Smith, C. S. US. Patent 4 024 706, 1977. Ai-Taie, A. S.;Kershenbaum, L. S. ACS Symp. Ser. 1978, 65, 512. Canale, R. P.; Winegarden, S. R.; Carlson, C. R.; Miles, D. L. Society of Automotive Engineers, 1978, Paper No. 780205. Cutlip, M. B. AIChEJ. 1979, 25, 502. Engh, 0. T.; Wallman, S. Society of Automotive Engineers, 1977, Paper No. 770295. Gandhi, H. S.; Piken, A. G.; Shelef, M.; Delosh, R. G. Society of Automotive Engineers, 1976, Paper No. 760201. Gandhi, H. S.; Yao, H. C.; Stepien, H. K.; Shelef, M. Society of Automotive Engineers, 1978, Paper No. 780606. Gandhi, H. S.; Yao, H. C.; Stepien, H. K. ACS Symp. Ser. 1982, 178, 143. Grimm, R. A.; Bremer, R. J.; Stonestreet, S. P. Society of Automotive Engineers, 1980, Paper No. 800053. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J. Cafal. 1879, 56, 321. Herz, R. K. ACS Symp. Ser. 1882, 178, 59. Jain, A. K.; Silverston, P. L.; Hudgins, R. R. ACS Symp. Ser. 1982, 178, 267. Kummer, J. T. f r o g . Energy Combust. Sci. 1980, 6 , 177. Schlatter, J. C.; Taylor, K. C. J . Cafal. 1977, 4 9 , 42. Schlatter, J. C.; Mitchell, P.J. Ind. Eng. Chem. Rod. Res. Dev. 1980, 19, 288. Schlatter, J. C.; Sinkevitch, R. M.; Mitchell, P. J. Ind. Eng. Chem. Rod. Res. Dev. 1983, 22, 51. Seiter, R. E.; Clark, R. J. Society of Automotive Engineers, 1978, Paper NO. 780203. Shelef, M.; Gandhi, H. S . Ind. End. Chem. R o d . Res. Dev. 1972, 1 1 , 393. Summer, J. C.; Baron, K. J . &tal. 1879, 5 7 , 380. Taylor, K. C. "The Catalytlc Chemistry of Nitrogen Oxides"; Plenum: New York, 1975; p 173. Taylor, K. C.; Sinkevltch, R. M. Ind. Eng. Chem. Prod. Res. D e v . 1883, 22, 45. Yao, Y. F. Y. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 293.
Received for review December 23, 1983 Revised manuscript received June 27, 1984 Accepted September 19,1984
