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
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983
52
Vent
2 tiz
m
Valve to Equalize Back Pressure
C
O
-
Valve Actuator
C
co
H2 -c
02
co2 h?
H2
-
C3H6-c NO
-C3H6
=
-coz
NO 02 N2
Water Vapor
-i
*-
'i-
Figure 2. Hot-wire anemometer output when valves are actuated at various frequencies. Horizontal lines show the output for the separate streams.
Cycled Blend
h
1
Catalyst
U
Figure 1. Schematic diagram of system for exposing catalyst to an oscillating feedstream composition.
into two feedstreams. The streams can be as simple as binary mixtures or as complex as fully simulated exhaust gases. In either case the blending system gives two stable, repeatable, and precisely controlled gas blends. Cycling between the streams is handled by two fastacting solenoid selector valves (ASCO, 25 ms travel time) operating simultaneously. A schematic diagram of the blending and cycling equipment is shown in Figure 1. The switching frequency can be varied from 0.03 to 10 Hz on the valve actuator. The amplitude and average composition of the oscillations are changed by changing the concentrations in the two gas blends. Although the response time of the switching valves is fast relative to the cycling frequencies in automotive systems (0.5-3 Hz), mixing of the two streams is expected in the volume between valve outlets and the catalyst bed. Thus, the amplitude and shape of the fluctuations encountered at the front of the catalyst bed are attenuated with respect to the approximately square-wave shape existing at the valve outlets. The extent of this attenuation has been investigated experimentally with a hot-wire anemometer. The anemometer probe, sensitive to changes in the gas thermal conductivity, was placed at the inlet of the catalyst bed (at room temperature) while the feedstream composition was cycled between pure nitrogen and nitrogen containing 0.25 vol % hydrogen. (Note: Axial mixing is not expected to be a strong function of temperature; the increased effective dispersion coefficient at typical reaction temperatures is counterbalanced by the decreased residence time for mixing.) The total flow rate was fixed at 10 L/min (STP). The anemometer output was recorded on an oscilloscope and is shown as a function of the cycling frequency in Figure 2. The horizontal lines in Figure 2 show the anemometer output when the individual gases were passed through the system without cycling. When the valves were switched back and forth a t 0.5 Hz or slower, the anemometer indicated that the catalyst was indeed exposed to the in-
tended amplitude of concentration difference,although the "corners" of the square-wave input were rounded by mixing. A t 1 Hz the reactor inlet concentration did not quite reach either end point; the amplitude was attenuated to about 86% of the maximum value. Figure 2 shows that the amplitude continued to decline as frequency increased, but, at least up to a frequency of 6 Hz, the oscillations were still evident. When the cycling was at 8 Hz or faster, the catalyst was simply exposed to a steady stream whose composition was the average of the two blends being cycled. This is a useful circumstance in the experimental catalyst evaluations because it allows a comparison between behavior under cycled and steady conditions with the same time-averaged throughput of reactants. In this report we shall consider catalyst performance at three frequencies - 0.25, 1, and 8 Hz. The first two provide an indication of frequency effects a t nearly constant amplitude, while the last shows catalyst efficiency under steady inlet conditions. In all cases the reactor effluent was passed through a mixing volume (-2 L) so that the analyzers reflected the average conversion regardless of the cycling conditions. Test Conditions Conversion efficiency under a series of near-stoichiometric feedstream conditions is one measure of three-way catalyst performance. Response to oscillating reactant concentrations is also an important aspect of the three-way application, so our laboratory evaluations incorporate both of these features. A typical experiment includes variations in the composition of the two blends being cycled and in the cycling frequency. In this way catalysts can be compared under a variety of conditions of potential practical importance. Specifically, the feedstream compositions can be found in Figure 3; they are plotted as functions of the simulated air/fuel ratio. A stoichiometric blend of reactants occurs, in our case, at an A/F ratio of 14.68. An advantage of varying each component concentration approximately linearly with air/fuel ratio is that, when two feedstreams are cycled, the overall time-averaged composition is the same as the composition represented by the average air/fuel ratio of the two streams. The catalyst used in these studies contained platinum and rhodium supported on 3-mm diameter alumina spheres. The total flow through the 15 cm3 catalyst bed was 13.0 L/min (STP), including (in a nitrogen background) 10 vol % C 0 2 and 10 vol % H 2 0 in addition to the components shown in Figure 3. The resulting space velocity was 52000 h-I (STP). The feedstream did not contain SOz, so extrapolations to the actual exhaust environment must be done with caution (Summers and Baron, 1979). While the catalyst was being heated to the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 53
C
140
150
14 5
15 5
M e a n A / F Ratio
Figure 5. CO and NO conversions measured at various cycling frequencies. Cycling was 10.25 A/F ratio around the mean value. 1M I
14 0
15 2
14 S
144
i
156
Simulated A / F Ratio
Figure 3. Component concentrations used to simulate exhaust at different engine air/fuel ratios. 140
10
15 0
14 5
155
M e a n A / F Ratio
Figure 6. CO and NO conversions measured at various cycling amplitudes. Frequency = 1 Hz.
* E
0
r
s
5
U 0
14 0
14 5
150
15 5
Stmulared A/F Ratio
Figure 4. Catalyst efficiencies measured with a steady feedstream composition at various simulated A/F ratios.
standard reaction temperature of 550 "C the feedstream composition was cycled at 1Hz. Once the temperature and outlet concentrations had stabilized, the frequency of the oscillations was varied in steps from 0.25 to 8 Hz. The conversions measured at any particular steady-state condition were independent of the prior course of the experiment, so the frequency and composition could be changed in any convenient sequence. Frequency Effects The catalyst efficiency as a function of the simulated air/fuel ratio is shown in Figure 4 for the case of steady, uncycled inlet compositions. The curves are characteristic of the situation in automotive exhaust catalysis, and they depict clearly the need to operate very near the stoichiometric &/fuel ratio (denoted by "S" in the figure) in order to convert all three major pollutants simultaneously. As shown in Figure 4 and in work by others (Kaneko et al.,
1978; Gandhi et al., 1977), CO conversion (rather than HC conversion) is typically the limiting factor on the rich side of the stoichiometric point. For clarity, then, we consider only CO and NO conversions in subsequent figures. Two key features of the conversion curves change when oscillations are introduced (Figure 5), and the changes are magnified as the frequency is decreased. On one hand, the limiting conversions (CO on the rich side, NO on the lean side) improve with cycling. That is, the range of feedstream stoichiometry over which a given conversion (say, 50%) is achieved is broader when the feedstream composition oscillates. This has been reported for actual exhaust streams as well (Adawi et al., 1977; Kaneko et al., 1978). On the other hand, cycling impairs the catalyst's efficiency at the stoichiometric air/fuel ratio. As shown in Figure 5, the maximum simultaneous conversion of NO and CO without cycling was 90%, but the value dropped to 77 % with 4~0.25A/F ratio oscillations at 1 Hz and 65% at 0.25 Hz. This effect may temper the benefits of using cycling to widen the catalyst's operating window. Amplitude Effects The amplitude of air/fuel ratio oscillations similarly affects catalyst performance. As the cycle amplitude is increased, as depicted in Figure 6 at a frequency of 1 Hz, a tradeoff again exists between widening the window and lowering the peak efficiency. These effects have also been reported for tests in engine exhaust (Kaneko et al., 1978). The qualitative effects of cycling the feed composition are not unexpected, judging from the shape of the conversion curves for a steady inlet flow (Figure 4). For example, CO conversion is nearly complete under stoichiometric or leaner conditions. With oscillations around the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 100
‘
. !Expecred Conversion
..’-
ap
W h e n Cycling
Measured Converafion
0 c
50 c U
14
e
I
0 14 5
150
/
I
15 5
M e a n A / F Ratio
Figure 7. CO conversions measured under steady and cycled (*0.25 A / F ratio) conditions. Expected conversions were calculated from steady inlet data as described in text.
stoichiometric point, however, the catalyst spends approximately half of the time under rich conditions, where inlet CO concentrations are higher and conversions are lower. Thus, although the time-averaged feedstream is a stoichiometric composition, the average conversion is lowered because of poor efficiency for CO conversion during rich excursions. Figure 7 illustrates this effect of moving back and forth on the conversion vs. A / F ratio curve. To determine, for example, a crude estimate for the expected conversion when cycling *0.25 A/F ratio about the stoichiometric point (A/F = 14.68), we made the approximation that the catalyst would spend one-half the time at A/F = 14.43 and one-half of the time at A/F = 14.93. (The anemometer work demonstrated that the cycled inlet is not strictly a square wave, but the approximation is convenient in showing qualitative effects of an oscillating feedstream composition.) According to Figures 3 and 4, then, the inlet CO level would switch from 0.8 to 0.4 vol %, and the conversion woil‘d be 20% or 95%, respectively. The measured (average) inlet CO level would therefore be 0.6 vol %, and the outlet level would be 0.33 vol %, yielding an expected CO conversion of 45% on Figure 7. Each point on the curve labeled “expected conversion when cycling” was determined by a similar calculation. An interesting feature of Figure 7 is that, after accounting for the averaging process just discussed, the measured conversions can exceed the predicted values, often by a wide margin. Such observations have given support to the concept of “oxygen storage” in catalysts (Gandhi et al., 1976), whereby oxidizing species are stored on the catalyst during lean excursions and released during the subsequent rich excursions. Such storage would dampen the effects of air/fuel ratio fluctuations. Although the details of possible storage mechanisms remain under study, the idea has been used to explain catalyst performance in exhaust streams of oscillating composition [both with (Hegedus et al., 1979) and without (Kaneko et al., 1978) base metal additives in the catalyst]. Performance of Aged Catalysts The versatility of the laboratory system makes it useful for characterizing the changes in catalyst behavior which result from exposure to exhaust contaminants or extreme temperatures. To provide an example of these changes, we characterized samples which had been exposed to high temperatures in air (2 h at 900 “C) or to exhaust from an engine operated on fuel containing 20 mg/L lead and 20 mg/L phosphorus. A detailed investigation of various catalyst deactivation mechanisms has been reported
8 C
E
p
50
C
V
0
f 01
0
I 50
1
CO Conversion, %
Figure 9. Cross-plot of CO and NO conversions over fresh and thermally aged samples showing effect of varying frequency.
elsewhere (Summers and Hegedus, 1979);our purpose here is simply to show how the laboratory reactor can be used to elucidate changes in catalyst characteristics under cycled conditions. The performance of the sintered sample under cycled (1Hz,f0.25 A/F ratio) conditions is compared to the fresh sample in Figure 8. The conversion efficiencies for CO, HC, and NO all decreased as a result of the 900 “C exposure. It is interesting to note that the thermal aging was relatively more detrimental to CO conversion than NO conversion. We and others have reported such observations on rhodium-containing catalysts before (Schlatter and Taylor, 1977; Lester et al., 1978). The effect is to improve the selectivity for NO removal or, viewed another way, to shift the optimal operating region to leaner stoichiometries. HC efficiency was decreased by the hightemperature treatment, but the three-way window was still determined by the CO and NO conversions. The relationship between CO and NO conversions at near-stoichiometric compositions is a key factor in threeway catalysis, and, as we have noted before (Hegedus et al., 1979), it is informative to plot one against the other (see Figure 9). The curves denoted “1 Hz” are derived from the data in Figure 7. Efficiencies under stoichiometric conditions, particularly important in three-way systems, are shown by the “S”data points in the figure. Points above and to the left of each S represent rich conditions; points below and to the right are for lean conditions. (The goal, of course, is to develop a catalyst which will operate as near as possible to the upper right
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 55 lo(
Table I. Contaminant Accumulation during Exhaust Exposure ~~
exposure time, h
6 14 20
contaminant level, wt %
Pb
