Ind. Eng. Chem. Prod. Res. Dev. 1083, 22,45-51
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Behavior of Automobile Exhaust Catalysts with Cycled Feedstreams Kathleen C. Taylor' and Robert M. Slnkevltch Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
The response of an exhaust-aged three-way catalyst to perturbations in the feedstream stoichiometry was examined in laboratory experiments. The air-fuel ratio was perturbed using both symmetric and asymmetric cycling and a range of net stoichiometries. The conversions of NO and CO were determined using the cycled feeds and steady feeds of equivalent net stoichiometry. At 550 O C cycling the feed benefited conversion at average air-fuel ratios away from the stoichiometric composition. This effect was more pronounced with asymmetric than symmetric cycles. Cycling the air-fuel ratio of the simulated exhaust gas during warm-up of the catalyst did not significantly benefit either NO or CO conversion.
Introduction The air-fuel ratio of the exhaust of most 1981 model year vehicles is controlled close to the stoichiometrically balanced ratio so that the rhodium containing three-way catalysts can optimally promote the reduction of nitric oxide (NO) to nitrogen as well as oxidize carbon monoxide (CO) and unburned hydrocarbons (HC) (Canale et al., 1978; Grimm et al., 1980). The feedback system which controls the exhaust air-fuel ratio produces small oscillations in the air-fuel ratio. While these oscillations are an inherent feature of the feedback system, the form of the oscillations is manipulated in order to minimize the period and amplitude of the oscillations. That is, the electronic control unit is programmed to establish both an integral and proportional change in the air-fuel ratio upon receiving a signal from the oxygen sensor. Grimm et al. (1980) have described how the input signals to the control unit could be expanded. Recent studies of three-way catalysts have shown that the rate at which a catalyst responds to an air-fuel ratio change as monitored by the conversion of CO depends upon the direction of the change (e.g., rich to lean vs. lean to rich) (Sell et al., 1980). Schlatter and Mitchell (1980) observed that the conversion of CO immediately following a lean to rich switch exceeded the level that would be predicted by the instantaneously available feedstream oxygen concentration. Both the reaction of CO with oxygen stored on the catalyst during the lean part of the cycle and the conversion of CO via the water-gas shift reaction have been proposed to explain this activity (Schlatter and Mitchell, 1980; Herz, 1981). As a result of these observations we propose that symmetrically cycling between two fixed feedstream compositions may not always give the best catalyst performance. The overall conversions of NO, CO, and HC might be improved by manipulating the duration of exposure to rich or lean exhaust during a full cycle. In this paper we report results of laboratory experiments in which the air-fuel ratio of the feedstream to the catalyst was oscillated over a wide range, using both symmetric and asymmetric air-fuel ratio cycles. Our objective was to determine how a typical exhaust-aged catalyst performs during these oscillations and to identify how catalyst performance might be improved by manipulation of the cycling characteristics. Both performance during warm-up of the catalyst and performance at 550 OC were monitored. A detailed computational study of the effect of oscillations on conversion will be reported by Cho (1982). Experimental Section Laboratory Reactor. The integral laboratory reactor system used in this study has been described by Schlatter 0196-4321/83/1222-0045$01.50/0
et al. (1981). The characteristic feature of this reactor system is the ability to change the feedstream to the catalyst quickly so that the feedstream can be rapidly cycled between two differently blended gas mixtures. Two fast-acting solenoid selector valves are operated simultaneously to deliver an oscillating feedstream composition to the catalyst. The cycling frequency can be varied from 0.0 to 10 Hz. The two gas blends which determine the amplitude of the cycles are made up in the blending section of the apparatus. Each gas blend is described by its S number, defined by Schlatter (1978) as the ratio of the concentrations (in volume percent) of the oxidizing species to the reducing species as 2 ( 0 2 ) + (NO) S= (1) (CO) + (H2) + 3n(CnHd + (3, + l)(CnH2n+Z) Figure 1 shows the component concentrations used to simulate exhaust at various S numbers and corresponding air-fuel ratio values. The 02, CO, Hz, propylene, and NO concentrations were changed simultaneously because such a total composition change typifies a change in exhaust air-fuel ratio. The gases leaving the reactor enter a mixing tank so that the gases are blended, and the gas analyzers measure average concentrations of NO, CO, etc., during a period of steady-state cycling. The feed gas contains only one hydrocarbon, propylene. Propylene is an easy-to-oxidize hydrocarbon. Exhaust contains a mixture of hydrocarbons. Representative hard-to-oxidize saturated hydrocarbons should be included in a comprehensive study of hydrocarbon conversion. Cycled Feedstreams. Catalyst response to feedstream oscillations was evaluated in this study by cycling the feedstreams in two different ways: symmetrically and asymmetrically. For the symmetric cycling the flow time for each half of the two half-cycles was equal. For the asymmetric cycling the flow time for one of the feedstreams was longer than for the other feedstream during a complete cycle. For a symmetric cycle the S number of the combined feedstreams is determined by the average composition of each component and is not the average of the two S numbers. For an asymmetric cycle, the S number of the combined feedstream is determined by allowing for the flow time of each of the two feedstreams. For example, during a 10-s cycle (0.1 Hz cycle frequency) instead of using 5-s intervals for each of the two feedstreams (symmetriccycling),we may use an 8-s interval for the net reducing feedstream followed by a 2-s interval for the net oxidizing feedstream. We designate such an asymmetric cycle as 80120 because for 80% of the cycling time the net reducing feed is utilized and for 20% of the cycling time the net oxidizing feed is utilized. The S number of the 0 1983 American Chemical Society
46
Ind. Eng. Chem. Prod. Res.
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Table I. Composition of Gas Blends Used to Simulate Exhaust at Different Engine Air-Fuel Ratios in Cycling Experiments feed
H, , vol % NO, P P ~ c o , vol % C & , , ppm co, , vol 9% N, , balance 0, , vol % H,O, vol % AIF AA/F S
R3
R2
Rl
S
L1
L2
L3
0.285 464 0.885 517 10 bal 0.305 10 14.27 -0.36 0.40
0.265 475 0.8 502 10 bal 0.385 10 14.37 -0.26 0.54
0.230 488 0.69 384 10 bal 0.493 10 14.51 -0.12 0.76
0.2 500 0.6 468 10 bal 0.585 10 14.63 0 1.00
0.170 512 0.505 452 10 bal 0.680 10 14.75 +0.12 1.31
0.135 525 0.4 434 10 bal 0.785 10 14.89 +0.26 1.76
0.110 535 0.320 420 10 bal 0.865 10 14.99 +0.36 2.21
Table 11. Composition of Cycled Feedstreams (Averaged over Cycle Duration) calcd blends
calcd blends
calcd blends
' L3
' L1
feeds blended cyclesymmetry
R2 80120
50/50
L2 20180
R1 80120
50/50
20/80
R3 + 80120
50150
20180
H, , vol % NO, ppm co, vol % C,H,, ppm co, , vol % N, , balance 0, , vol % H,O, vol % AI F AAIF S
0.239 485 0.71 488 10 bal 0.465 10 14.48 -0.1 5 0.71
0.2 500 0.6 468 10 bal 0.585 10 14.63 0 1.00
0.161 51 5 0.48 446 10 bal 0.705 10 14.78 +0.15 1.40
0.206 450 0.616 471 10 bal 0.567 10 14.61 -0.02 0.95
0.17 512 0.505 452 10 bal 0.679 10 14.75 +0.12 1.31
0.134 526 0.394 433 10 bal 0.791 10 14.90 + 0.27 1.78
0.262 474 0.809 504 10 bal 0.380 10 14.36 -0.27 0.53
0.228 488 0.695 48 5 10 bal 0.493 10 14.51 -0.12 0.76
0.193 502 0.581 465
-------------f
.8
10 bal 0.605 10 14.68 +0.05 1.06
- \ co
.6 Component Concentration (VOl%)
50/50
i-10
set+
.4
i
2 C3H6 142
',
NO
. I
144 146 148 Simulated A / F Ratio I
40 54
l
l
150
80/20 I
76 1 0 0 1 31 1 76
S
+lo
s e c 4
Figure 1. Component concentrations used to simulate exhaust at various air-fuel ratios.
Figure 2. Waveforms for the asymmetric 80/20 cycle and 50/50 cycle at 0.1 Hz cycle frequency. These waveforms are the hot wire anemometer output.
combined feedstream is calculated from the total amounts of CO, NO, Hz, Oz, and HC used in each full cycle. The instantaneous gas composition delivered to the catalyst as a result of cycling the feedstream has been carefully analyzed. In general, when the cycling frequency is very fast (e.g., 10 Hz), the cycling between two feedstreams delivers to the reactor a feedstream of constant composition. The catalyst only sees the two starting S numbers when the cycling frequency is much slower (e.g., 0.5 Hz). Even at 0.5 Hz cycling frequency the amplitude is attenuated with respect to the square-wave shape of the S number a t the valve outlets. For this reason very slow 0.1 Hz cycles were chosen for some experiments. Figure 2 shows the waveform for the 80120 cycle and 50/50 cycle for a 0.1 Hz cycling frequency. The slow cycling rate (0.1 Hz) ensured that the catalyst saw the full S-number amplitude of the feedstream used for the shorter 2-s interval during part of the interval (approximately 1 s). The time-average S-number or air-fuel ratio calculated with asymmetric cycling is of course different than the
time-average S-number calculated for symmetric cycling of the same two feedstreams. Figure 3 shows the waveform for the 80/20 cycle and 50/50 cycle for 1.0 Hz cycling frequency. In order to compare the catalyst response upon cycling the feed with the response without cycling, feedstreams were blended to have the time-average composition characteristic of the cycled feed following complete mixing. Table I lists the composition of the gas blends used in this study. Table 11 lists the composition of the cycled feedstreams. Experimental Procedure. The catalyst charge was 15 cm3. The gas hourly space velocity was 20000 h-I (STP). The activity of the catalyst for converting NO, propylene, and CO was monitored both during warm-up and at a catalyst temperature of 550 "C. For the measurements at 550 "C the catalyst was always warmed up to 550 "C with the more reducing of the two feedstreams flowing over the catalyst. The feedstream cycling was done in the order 80f 20, 50f 50, and 20180 last. The steady feedstream ex-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 47 sr0.64
I ________________
I
-' - -I
a
1
I
250 300 350 Catalyst Bed Temperature ("C)
s = 0.54
1
40(
________________
~ ~ 1 . 7 6
+5
80/20 s e c 4
Figure 3. Waveforms for the symmetric 50/50 cycle and asymmetric 80/20 cycle at 1.0 Hz cycling frequency. Table 111. Catalyst Characteristics wt%
noble metals (fresh catalyst) plat in um palladium rhodium base metal (fresh catalyst) cerium oxide (-0,) impurities in aged catalyst phosphorous sulfur lead
0.0682 0.0313
0.007
toz/260 in.3 0.046 0.021 0.0048
I I I 300 350 250 Catalyst Bed Temperature ("C)
s1001
I
/
400
I 0
I
2.68
0.04 0.12 0.1
periments were done in the order most reducing to most oxidizing. These procedures were adopted because the activity of the catalyst for converting CO under net reducing conditions was sensitive to the sample history (i.e., the sequence of performing the experiments). For example, following exposure of the catalyst to feed L2 (net oxidizing) at 550 "C, the activity of the aged catalyst for converting CO in a slightly reducing feedstream was lower than following warm-up to 550 "C with feed R2. Schlatter et al. (1979) have reported on the sensitivity of the activity of rhodium containing three-way catalysts to preconditioning. In several experiments the reactions of CO, NO, and propylene were monitored during warm-up of the catalyst from room temperature to 400 "C.These so-calledlight-off curves were generated using both steady (constant stoichiometry) and cycled feedstreams. The heating rate of the reactor was monitored by a thermocouple at the inlet to the reactor. The heating rate was 1OC/min. This slow heating rate was chosen in order to avoid large temperature gradients in the catalyst bed. Additional thermocouples in the catalyst bed monitored the temperature along the reactor. The cycling frequency in these experiments was 1 Hz. Recall that a fast cycling frequency produces a mixing of the feeds which results in a decrease in the apparent amplitude of the feedstream entering the reactor as shown in Figure 3. Catalyst. The noble metal and base metal content of the catalyst used in this study is given in Table 111. The metals were supported on low-density commercial alumina pellets (0.497 g/cm3). The surface area of the fresh catalyst was 135 m2/g (BET). The catalyst was aged on an AMA driving schedule for 16000 km on a vehicle. The surface area of the aged catalyst was 1 2 1 m2/g (BET). The major impurities present following the aging were sulfur, lead, and phosphorus. The analysis results for the poisons are given in Table 111. An aged catalyst was chosen for this study
Catalyst Bed Temperature ("C)
Figure 4. CO, NO, and propylene conversion data for steady stoichiometric (-) and cycled (- - -) feeds from Figure 10 plotted vs. the catalyst bed temperature. Dotted line is determined from the amount of CO (NO and propylene) reacted using feeds L2 and R2 such that 100 X (total amount reacted with L2 and R2/sum of L2 and R2 inlet).
because its behavior is more representative of the performance to be expected on a vehicle than is a fresh catalyst. Aging suppressed activity for the water-gas shift reaction. Results and Discussion Catalyst Light-Off-Cycled and Steady Feedstreams Compared. In this first section, light-off performance of the aged three-way catalyst will be examined using steady and cycled feeds. We wish to determine whether cycling the feedstream composition during warm-up of the catalyst might lead to greater conversion of CO, NO, or propylene than is obtained using a steady feed which is blended to the time-average composition of the cycled feed. One might expect the overall conversion to be improved with cycling around stoichiometry compared with steady stoichiometric feed whenever the average of the amount of reactant converted at steady state using each of the two feeds which are used for the cycling exceeds the amount of reactant converted using the steady stoichiometric feed. The feedstream to the reactor was cycled symmetrically between two feedstreams, one net reducing (R2) and one net oxidizing (L2), with a 1-Hz frequency. In these experiments the temperature at the inlet to the catalytic reactor was programmed at 1 "C/min. The conversions of CO, NO, and propylene are shown by the dashed lines in Figure 4 with the temperature measured at the hottest point in the catalytic reactor as the parameter. The con-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983
versions obtained with the steady stoichiometric feed (solid lines) are shown for comparison. The dotted lines in Figure 4 are a calculation of the conversion expected upon cycling if the conversion changes instantaneously with a change in feedstream and with no mixing of the two feedstreams. Of course considerable mixing of the two feeds occurs with the fast 1-Hz cycling frequency as was demonstrated by the data shown in Figure 3. Complete mixing of the two feeds yields the time-average feed composition so the effects of cycling will disappear a t very high cycling frequencies. The conversion data obtained for feeds R2 and L2 were used for this calculation. The conversion of CO over the aged catalyst was about the same with and without cycling. We see no benefit of cycling at low temperature and the 3% change above 325 "C may not be significant. The calculated value for the CO conversion upon cycling (dotted line) is lower than the measured value above 275 "C. Schlatter et al. have similarly reported the calculated conversion of CO with cycling to be lower than the experimentally measured conversion at time-average air-fuel ratios close to the stoichiometric composition. The conversion of NO over the aged catalyst below 400 "C was found to be impaired by cycling in comparison with the steady feed result. The conversion of NO with cycled feeds is well approximated by the calculation below 315 "C, but above 315 "C the calculated conversion falls below the experimental data. The conversion of propylene using the symmetrically cycled feedstreams clearly overtook the conversion with the noncycled feedstream once the catalyst bed temperature reached 315 "C. We have no explanation for this result. Perhaps the activity of this aged catalyst for converting propylene a t the higher temperatures depends upon the catalyst's immediate past history, that is exposure to a feedstream richer or leaner than the stoichiometric composition. The calculated propylene conversion (dotted line in Figure 4) does not explain the enhanced conversion with cycling. The calculated result predicts enhancement between 250 and 350 "C. We do not know whether other hydrocarbons characteristic of exhaust gas show similar behavior or how SOz in exhaust modifies the effect of cycling with this catalyst. Schlatter and Mitchell (1980) have reported that with SOz in the feed the characteristics of feedstream cycling are diminished for Rh/Al20,. In another set of experiments we cycled the same two feedstreams (R2 and L2) asymmetrically using a 20/80 cycling scheme. Here the net reducing feedstream is used for 0.2 s, and the net oxidizing feedstream is used for 0.8 s during each 1-s cycle. The conversions of CO, NO, and propylene are shown in Figure 5. For comparison we also determined the conversions without cycling using a feedstream blended to have the same time-average stoichiometry as the cycled feed. In order for the overall conversions of CO and propylene to be improved with such asymmetric cycling, the conversions during the net oxidizing portion of a cycle must compensate for the lower conversions during the net rich portion of a cycle. We observe, however, that only the propylene conversion was improved, and the effect was small. The reader should recall that with the fast 1-Hz cycling frequency the net reducing feedstream flows for only 0.2 s, and its full amplitude is not seen by the catalyst (Figure 3). The mixing of the two feedstreams which are being cycled leads to a smaller final amplitude than with the symmetric cycling. Asymmetric cycling of feeds R2 and L2 using an 80120 cycling scheme and 1-Hz frequency (0.8 s net reducing and 0.2 s net oxidizing) penalized NO conversion compared
1001
200
100
401
b
g l
t
250 300 350 Catalyst Bed Temperature (OC)
i
200
250 300 350 Catalyst Bed Temperature ("C)
200
250 300 350 Catalyst Bed Temperature ("C)
o
400
i
Figure 5. CO, NO, and propylene conversion with asymmetrically cycled (20/80 cycling of R2/L2,1.0 Hz cycle frequency) and blended feedstreams with the same time-average stoichiometry (S = 1.4). 100 a
200
1
260 300 350 Catalyst Bed Temperature ("C)
100,
1
Catalyst Bed Temperature (OC)
Figure 6. CO and NO conversion with asymmetricallycycled (80/20 cycling of R2/L2,1.0 Hz cycle frequency) and blended feedstreams with the same time-average stoichiometry (5' = 0.71).
with the results obtained with a steady feed blended to the same time-average reducing stoichiometry (Figure 6). The reader will recall that cycling around the stoichiometric point also gave lower NO conversion compared with a steady stoichiometric feed (Figure 4). CO conversion, on the other hand, was the same with and without cycling up
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 49
I co
Conversion
I%)
10
- I
to 300 OC and improved with this asymmetric cycling scheme above 300 "C. In the cycling experiments described thus far the time-average stoichiometries of the feedstreams are different for each of the three cycling schemes. Using the 20180 cycling scheme for which the time-average stoichiometry is net oxidizing, the percentage conversions of CO and propylene are greater than with the other cycling schemes. The time-average stoichiometry of the feedstream during the warm-up of the catalyst apparently determines the level of conversion, not feedstream perturbations. In practice three-way catalysts are warmed up without closed-loop air-fuel ratio control. In fact, air is added to the exhaust before the three-way catalyst in order to enhance CO and HC oxidation. For this reason we do not consider the cycling experiments done during warm-up of the catalyst to have a current application. In these warm-up experiments only cycled vs. steady feed conversions were determined at the same time-average stoichiometry. The next section of this paper will include cycling data obtained using a wide range of feed compositions so that cycling experiments for overlapping timeaverage stoichiometries can be compared. Catalyst Performance at 550 "C with Cycled Feeds. The performance of the exhaust aged catalyst was examined at 550 "C with cycled and steady feedstreams. This inlet temperature to the reactor is characteristic of the operation of a warmed-up three-way catalytic converter. The feedstreams were cycled asymmetrically as well as symmetrically at a slow (0.1 Hz) cycling frequency. As explained earlier, the asymmetric cycling results in a time-average stoichiometry that is more oxidizing or reducing than symmetric cycling of the same two feeds because of the longer flow time of one of the feeds than the other during each cycle. The conversion of CO with a steady net reducing feed (Rl, R2, and R3) was lower at a catalyst inlet temperature of 550 "C than at 350 O C . The decrease in CO conversion with increasing temperature above 350 "C was greatest with the most reducing feed (R3). The conversion of CO at 550 "C obtained using three cycling schemes is shown in Figure 7. The composition of the feeds used for the cycling was shown in Table I. The composition of the cycled feed averaged over the duration of the cycle was shown in Table 11. The data presented in Figure 7 are explained with the following example. The data point for CO conversion at the far right in Figure 7 at S = 1.78 was obtained by cycling feeds R1 and L3 such
10
that feed R1 flows for 2 s and feed L3 flows for 8 s during each 10-s cycle. The feed composition averaged over the 10-s cycle is considerably more oxidizing than the stoichiometric compositions and is characterized by an S value of 1.78. The data designated by curve C were all obtained using the 20180 cycling scheme. Curve C points at lower S values than 1.78 were obtained by cycling feeds other than R1 and L3. In figure 7 curve A is the CO conversion using an asymmetric80/20 cycling scheme. CO conversion during symmetric cycling is shown by curve B. The conversion of CO in steady feedstreams blended to the time-average composition of the corresponding cycled feedstreams is also shown (solid line). The data presented in Figure 7 show that CO conversion at time-average feed compositionsmore reducing than the stoichiometric (S = 1) composition was greater with the 80120 asymmetric cycling scheme (curve A) than with the symmetric 50150 cycling scheme. The asymmetric 20/80 cycling scheme, on the other hand, was less effective than the symmetric cycling scheme. While each cycling scheme was examined over only a limited range of feed stoichiometries, comparison of all three curves close to S = 1suggests that the asymmetric 80120 cycling scheme is the preferred one for improving CO conversion. These data suggest that close to the stoichiometric feed composition (S = 0.6 to S = 1) CO conversion is greatest with a long exposure to the slightly reducing feed (R1 for 0.8 s) coupled with a short exposure to the most oxidizing feed (L3 for 0.2 s) during each cycle. An important question to explore is whether the greater conversion of CO exhibited with the 80/20 asymmetric cyling scheme can be predicted from steady feed data. The conversions of CO expected upon cycling the feed were calculated assuming no mixing of the feeds (square-wave cycling function) and using the conversions obtained at steady state with the feeds which were cycled. The wave forms obtained for the 0.1 Hz cycle frequency (Figure 2) suggest that the square-wave assumption is a reasonable approximation. The results of these calculated conversions of CO are shown in Figure 8. (The steady-state data line has been repeated from Figure 7 so that these calculated conversions may be easily compared with the observed conversions.) Comparison of these calculated conversions with the observed conversions from Figure 7 shows that the calculated conversions did not predict the data well, especially close to the stoichiometric (S = 1)composition. We attribute this discrepancy to changes in the reactivity of the catalyst under the changing feed compositions during cycling and to the rate at which the reactivity of the catalyst responds to a feed composition change. A slow
50
.-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 rnn
NO
Conversion
6o 50
(%) 40
30 2o 10
0
100,
I
’6 O 0 r
0.
i -
c
C
-t
I~
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response of rhodium-containing catalysts to a change in feed composition has been reported earlier for experiments using exhaust gas (Sell et al., 1980; Herz, 1981) and also for carefully monitored laboratory experiments (Schlatter and Mitchell, 1980; Schlatter et al., 1979). Also the fact that the measured conversions were greater than the calculated conversions suggests that the slow 0.1-Hz cycling frequency was still fast enough to generate “cycled behavior. , The conversion of NO at 550 “C obtained using the symmetric and asymmetric cycling schemes is shown in Figure 9. These data were obtained during the same experiments which produced data for CO oxidation displayed in Figure 7. The particular cycling scheme used had a smaller effect on NO conversion than on CO conversion. The conversion of NO at net oxidizing time-average stoichiometries was greater with cycling than with the steady feed. These results suggest that symmetric or asymmetric cycling of the exhaust air-fuel ratio may be a way to improve NO conversion at exhaust compositions considerably more oxidizing than the stoichiometric composition. The NO conversions calculated using steady feed data and simulating the cycling experiments are shown in Figure 10. The calculations show lower NO conversion at time-average reducing conditions than do the experiments. We do not believe that mixing of the feedstreams upon cycling can fully account for this difference. A decision to deliberately manipulate the cycling characteristics of the feedback control system is necessarily tied to the operational constraints of the entire system as well as the amounts of CO, NO, and HC which must be removed from the exhaust in order to meet exhaust emission regulations. Our laboratory experiments have simulated a narrow compositional range of engine-out emissions and probably simulate poorly the warm-up portion of catalytic converter operation. The NO data are believed to be of greater practical significance than the CO data because lean operation is generally preferred over rich operation. [Though not shown here, net reducing conditions increase the selectivity of NO reduction to ammonia which is not the preferred reduction product (Taylor, 197511. Greater NO removal under net oxidizing conditions with cycled than with steady feed composition has been described in the literature as a “widening of the window” of operation (Schlatter et al., 1981; Gandhi et al., 1976; Kaneko et al., 1978; Falk and Mooney, 1980) and has been examined as a function of cycling frequency and amplitude. The influence on catalyst performance of asymmetric vs. sym-
l
8
l
9
l
l
l
l
l
l
l
l
l
7 . 0 1 . 1 1 . 2 7.3 1 . 4 7.5 1 . 8 1 7 1 8
s
S
Figure 9. Effect of cycling scheme on NO conversion. Feeds: (0) R3/L1; (A) R2/L2; (0)Rl/L3. Cycling schemes: (A) 80/20 asymmetric cycling; (B)50/50 symmetric cycling; (C) 20/80 asymmetric cycling. Solid line is NO conversion with steady feedstreams blended to indicate stoichiometry. See Tables I and I1 for explanation of notation.
l
7
Figure 10. Predicted NO conversion with cycling (- - -) for the cycling schemes and feeds given in Figure 9. Solid line is the observed conversion with no cycling.
metric cycling of the feed has not been demonstrated previously. We have seen that under certain conditions asymmetrically cycling the feed gives higher conversion than does symmetric cycling. Asymmetric cycling may be viewed as a way to further “widen the window”. Of course, asymmetricand symmetric cycling of the feed are probably not the schemes of choice if operation at the stoichiometric air-fuel ratio is both desired and possible. Asymmetric cycling of the feed might be considered whenever operation about the stoichiometric composition does not best meet the exhaust emission regulations. For example, a situation might arise where cycling the exhaust air-fuel ratio with an overall lean air-fuel ratio rather than stoichiometric air-fuel ratio may lead to lower CO emissions, together with lower NO output when the cycling result is compared with steady lean operation. Conclusion In conclusion, these cycling experiments have not revealed a single cycling scheme which gives better conversion of NO and CO under all conditions of temperature and stoichiometry than a steady noncycled feed. We have identified conditions where either the oxidation of CO or reduction of NO is improved. Benefits of cycling the feed are found, in general, under net oxidizing conditions for NO and under net reducing conditions for CO, where the conversions of these gases are lowest. Deliberate perturbation of the exhaust air-fuel ratio would only be recommended for operation away from the stoichiometric composition. The effect of catalyst pretreatment on CO conversion activity needs further clarification. Acknowledgment The authors thank Dr. J. C. Schlatter, who contributed to the initiation of this project. We also thank members of the Analytical Chemistry Department for catalyst analyses. Registry No. Pt, 7440-06-4; Pd, 7440-05-3; Rh, 7440-16-6; CeO,, 1306-38-3; CO, 630-08-0; C3Hs, 115-07-1;NO, 10102-43-9; NO,, 11104-93-1.
Literature Cited Canaie, R. P.; Winegarden, S. R.; Carison, R.; Miles, D. C. Paper No. 780 205 presented to Society of Automotive Engineers, Detroit, MI, 1978. Cho, 6.K., private communication, General Motors Research Laboratories, Warren, MI, 1982. Falk, C. D.; Mooney, J. J. Paper No. 800 462 presented to Society of Automotive Engineers, Detroit, MI, 1980. Gandhi. H. S.; Plken. A. 0.; Shelef, M. Paper No. 760 201 presented to Society of Automotive Engineers, Detroit, MI, 1976. Grimm, R. A,; Bremer, R. J.; Stonestreet, S. P. Paper No. 600 053 presented to Society of Automotive Engineers, Detroit, MI, 1980. Herz, R. K. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 451. Kaneko, Y.; Kobayashi, H.; Komahome, R.; Hirako, H.; Nakayama, 0. Paper No. 780 607 presented to Society of Automotive Engineers, Troy, MI, 1978.
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 51-56 Schlatter, J. C.; Mitchell, P. M. Ind. Eng. Chem. Pmd. Res. D e v . 1980, 19, 288.
Schlatter, J. C.; Sinkevitch, R. M.; Mitchell, P. J. Ind. Eng. Chem. prod. Res. Dev.. submitted 1981. Schl&&, J. C. Paper No. 780 199 presented to Society of Automotive Engineers, Detroit. MI, 1978. Schlatter, J. C., Taylor, K. C., Sinkevitch, R. M., presented to Advances in CataMic Chemistrv SvmDoslum I. Snowbird.. UT.. Oct 1979: to be Dubiished in Symposiim Proceedings.’ Sell, J. A.; Herz, R. K.; Monroe, D. R. Paper No. 800 463 presented to Society of Automothre Engineers, Detroit, MI, 1980.
51
Taylor, K. C. I n “The Catalytic Chemistry of Nitrogen Oxides”; Klimisch, R. L., Larson, J. G., Ed.; Plenum Press: New York, 1975; p 173.
Received f o r review May 19, 1982 Accepted October 7 , 1982 Presented at the 183rd National Meeting of the American Chemical Society, Las Vegas, NV, March 1982.
Laboratory Reactor System for Three-way Automotive Catalyst Evaluation James C. Schlatter,’ Robert M. Slnkevitch,” and Patrlcla J. Mitchell Physical Chemistry Department, General Motors Research Laboratories, Warren, Mlchigan 48090
The exhaust composition in closed-loop emission control systems typically oscillates about the stoichiometric set point. We describe herein the laboratory system we developed to investigate catalyst behavior in such an environment. The design is such that the frequency, amplitude, and average composition of the oscillations can be varied independently of one another. In laboratory testing of a platinum-rhodium catalyst it was demonstrated that the “window” for three-way conversion could be improved via oscillations of low frequency or large amplitude, but peak conversion efficiencies were sacrificed in the process. On samples aged In engine exhaust or at high temperature, cycling effects were most evident under near-stoichiometric conditions.
Background Since their introduction on 1975 model year automobiles, catalytic converters have been effective for oxidizing both carbon monoxide (CO)and hydrocarbons (HC) in the exhaust (Taylor, 1982). Of course, these oxidation reactions are catalyzed most efficiently when there is a stoichiometric excess of oxygen. This is accomplished either by operating the engine on a lean mixture of air and fuel or by pumping air directly into the exhaust upstream from the converter. The third major automotive pollutant, nitric oxide (NO), requires catalytic conversion as well. Because NO is removed via a reduction reaction, excess oxygen tends to be detrimental to NO conversion. To satisfy the need for catalyzing both oxidation (of CO and HC) and reduction (of NO) reactions simultaneously, automotive systems have been designed to maintain the engine air/fuel ratio (and, hence, the exhaust) at or very near a stoichiometric mixture through all modes of engine operation (Wallman and Engh, 1977; Canale et al., 1978; Seiter and Clark, 1978). Such systems usually require an exhaust sensor to detect any oxygen imbalance and to initiate appropriate corrections via complex hardware. A characteristic of these feedback control systems is that the instantaneous value of the air/fuel ratio fluctuates about the set point (Wallman and Engh, 1977; Canale et al., 1978; Seiter and Clark, 1978), with the frequency and amplitude of the oscillations depending upon a variety of parameters in system design and operation. A consequence of using a “three-way” catalyst for simultaneous control of the three major pollutants, then, is that the catalyst must operate in an environment in which the reactant stream is shifting between fuel-rich and fuel-lean approximately once a second. This cycling enCatalytica Associates, Inc., 3255 Scott Blvd., Suite 7-E, Santa Clara, CA 95051.
vironment has been shown to widen the range of engine &/fuel ratios (that is, the operating “window”)over which a catalyst can yield a specified degree of conversion (Adawi et al., 1977). On the other hand, the peak catalyst efficiency under cycling conditions is lower than under steady-state conditions (Kaneko et al., 1978). Clearly there are tradeoffs made in formulating three-way catalysts and in establishing operating conditions in order to optimize the overall control system performance. To design and evaluate catalysts for these emission control systems, a laboratory-scale reactor system can be quite useful. An obvious application is in providing an initial screening of small samples of various formulations to select promising candidates for actual converter testing. In addition, evaluations under well-controlled and characterized laboratory conditions afford an opportunity for a more detailed understanding of catalyst response than is possible under exhaust conditions. Such understanding can be valuable input for optimizing the overall emission control system. For these reasons we undertook the task of designing a laboratory system for three-way catalyst evaluation. System Design Our basic system for laboratory studies of catalyst behavior has been described previously (Schlatter, 1978); it includes a feedstream blending section, a reactor section, and an analytical train. Major revisions in the blending section were required in order to incorporate the oscillating feedstream composition characteristic of three-way operation. These features (Hegedus et al., 19791, are specified below in somewhat more detail. The objective of the revisions was to provide a fluctuating reactor inlet composition of independently variable frequency and amplitude. Duplicate blending systems, each incorporating eight flow controllers and associated linear mass flowmeters with digital displays, are used to mix the individual components 0 1983 American Chemical Society
