Catalysis Under Transient Conditions - ACS Publications - American

3 The Dynamic Behavior of Three-Way Automotive Catalysts

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R I C H A R D K. H E R Z General Motors Research Laboratories, Physical Chemistry Department, Warren, MI 48090

The composition of engine exhaust entering a three-way catalytic converter oscillates about the stoichiometric point at frequencies of 0.5 to 2 Hz when the engine is operating under feedback control. Time-averaged conversion measurements have shown that three-way catalysts do not respond instantaneously under these rapidly varying conditions. Recently, time-resolved conversion measurements have permitted direct observation of the dynamic responses of three-way catalysts. After a step change from lean to rich c o n d i t i o n s , for example, CO conversions over a catalyst may take several seconds to decay to a new, lower steady-state level. One possible explanation of this response is reaction of the CO in rich exhaust with oxygen that was stored on the catalyst during the preceding exposure to lean exhaust. Other measurements have demonstrated that the oxygen content of a base metal-containing three-way catalyst can vary with exhaust composition at rates which are sufficient to explain the observed dynamic responses of CO conversion.

Three-way c a t a l y s t s are used i n most 1981 g a s o l i n e - f u e l e d automobiles to lower the l e v e l s of NO, CO, and hydrocarbons i n engine exhaust. These c a t a l y s t s normally operate under dynamic c o n d i t i o n s : c a t a l y s t temperature i n c r e a s e s r a p i d l y a f t e r the engine s t a r t s (during c a t a l y s t "warmup"), and exhaust f l o w r a t e and composition f l u c t u a t e under most modes of o p e r a t i o n . The warmup of automotive c a t a l y s t s i s reasonably w e l l understood (1). The o p e r a t i o n of three-way c a t a l y s t s i n the dynamic exhaust environment a f t e r warmup i s complex and l e s s w e l l under-stood. In t h i s paper, we summarize the progress t h a t has been made toward understanding the behavior of three-way c a t a l y s t s i n exhaust of r a p i d l y v a r y i n g composition. A f t e r a b r i e f d e s c r i p t i o n of the c o n d i t i o n s under which three-way c a t a l y s t s operate

0097-6156/82/0178-0059$05.00/0 © 1982 A m e r i c a n Chemical Society

Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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i n an automobile, we d i s c u s s time-averaged NO and CO conversions measured under l a b o r a t o r y c o n d i t i o n s which s i m u l a t e a c t u a l o p e r a t i o n . Next, we d i s c u s s t i m e - r e s o l v e d CO conversions measured during step-response experiments. Some of the step-response r e s u l t s may be explained by changes i n the amount of oxygen bound to the c a t a l y s t , and measurements of these changes are reviewed. We conclude by suggesting approaches to be taken i n future studies. C o n d i t i o n s During Automobile Operation Three-way c a t a l y s t s are a b l e to reduce NO as w e l l as o x i d i z e CO and hydrocarbons when the exhaust composition i s h e l d near the s t o i c h i o m e t r i c a l l y balanced composition, or " s t o i c h i o m e t r i c p o i n t . " T h i s c o n t r o l of exhaust composition i s accomp l i s h e d , a f t e r the i n i t i a l warmup p e r i o d , through the use of the feedback c o n t r o l system i l l u s t r a t e d i n F i g u r e 1 (2, 3^, . An oxygen sensor s i g n a l s whether the exhaust e n t e r i n g the converter i s " r i c h " (net reducing) or " l e a n " (net o x i d i z i n g ) . The v o l t a g e response of an oxygen sensor i s shown i n F i g u r e 2. A microcomputer p e r i o d i c a l l y reads the sensor s i g n a l and a d j u s t s the f u e l c o n t r o l s i g n a l to b r i n g the a i r - f u e l r a t i o , and thus the exhaust composition, c l o s e r to the s t o i c h i o m e t r i c p o i n t . The exhaust composition cannot be h e l d e x a c t l y a t the s t o i c h i o m e t r i c p o i n t , however. As a r e s u l t of the s w i t c h - l i k e response of the oxygen sensor and the time r e q u i r e d f o r f l o w from the f u e l metering p o i n t through the engine to the sensor, the cont r o l system e x h i b i t s l i m i t - c y c l e behavior. The exhaust composit i o n o s c i l l a t e s about the s t o i c h i o m e t r i c p o i n t a t a frequency which i n c r e a s e s w i t h exhaust f l o w r a t e over the range 0.5 to 2 Hz. This o s c i l l a t i o n , or c y c l i n g , i s i l l u s t r a t e d by the oxygen sensor s i g n a l shown i n F i g u r e 3. I n a d d i t i o n to these f a i r l y r e g u l a r o s c i l l a t i o n s i n exhaust composition, t r a n s i e n t s i n composition occur d u r i n g r a p i d v e h i c l e a c c e l e r a t i o n and dec e l e r a t i o n . Depending on the response of the c a t a l y s t , l e a n excursions from the s t o i c h i o m e t r i c p o i n t may r e s u l t i n i n c r e a s e d NO emissions and r i c h excursions may r e s u l t i n i n c r e a s e d CO and hydrocarbon emissions. Time-Averaged Conversion Measurements The performance of a three-way c a t a l y s t under dynamic c o n d i t i o n s i s o f t e n s t u d i e d by measuring the time-averaged r e a c t a n t conversions t h a t are obtained as the feedstream comp o s i t i o n i s d e l i b e r a t e l y c y c l e d . The v a r i a b l e s i n these c y c l e d s t u d i e s i n c l u d e time-averaged feedstream composition, c y c l i n g frequency, c y c l i n g amplitude, and c a t a l y s t composition. Cycled s t u d i e s are performed i n engine-dynamometer l a b o r a t o r i e s by s w i t c h i n g the a i r - f u e l r a t i o between two s e t t i n g s a t s e l e c t e d frequencies (5, j6, 7) . Cycled s t u d i e s a l s o have been performed


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three-way catalytic converter Figure 1. Schematic diagram of an engine and emission control system. The microcomputer also reads signals from sensors measuring other engine operating parameters. In some emission control systems, the three-way catalyst is followed by supplementary air injection and an oxidizing catalyst to provide additional control of CO and hydrocarbon emissions.

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Exhaust oxygen sensor characteristics (4).

RICH SENSOR SIGNAL (Volts) LEAN TIME (s) Figure 3. Oxygen sensor signal showing oscillation of the exhaust composition around the stoichiometric point during feedback control of the engine.


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(6) u s i n g the l a b o r a t o r y r e a c t o r system developed by S c h l a t t e r , S i n k e v i t c h , and M i t c h e l l ( 8 ) , shown i n F i g u r e 4. Two simulated exhaust streams of d i f f e r e n t compositions are a l t e r n a t e l y fed to the r e a c t o r i n t h i s system. Time-averaged i n l e t and o u t l e t compositions are measured i n these c y c l e d s t u d i e s because of the l i m i t e d response r a t e s of c o n v e n t i o n a l exhaust a n a l y z e r s . F i g u r e 5 presents CO and NO conversion data which are charact e r i s t i c of the c y c l e d performance of three-way c a t a l y s t s . Note that c y c l i n g r e s u l t s i n lower average conversions f o r mean a i r f u e l r a t i o s near the s t o i c h i o m e t r i c p o i n t . The extent to which c y c l i n g a f f e c t s average conversions over a c a t a l y s t has been found to be s e n s i t i v e to changes i n the composition of the c a t a l y s t and to aging of the c a t a l y s t i n engine exhaust ( 9 ) . Since c y c l i n g o f t e n r e s u l t s i n the lowering of conversions, there i s a s u b s t a n t i a l i n c e n t i v e to study the dynamic behavior of three-way c a t a l y s t s . One i n c e n t i v e i s that an understanding of the e f f e c t s of c a t a l y s t composition on dynamic behavior may l e a d to the f o r m u l a t i o n of c a t a l y s t s which g i v e r e q u i r e d conv e r s i o n s under c y c l e d a i r - f u e l r a t i o c o n d i t i o n s w i t h the use of minimum amounts of noble metals. Note i n F i g u r e 5 that average conversions are dependent on c y c l i n g frequency. Some of t h i s e f f e c t i s r e l a t e d to mixing i n the feedstrearn which decreased the amplitude of the i n l e t comp o s i t i o n o s c i l l a t i o n s as the frequency was i n c r e a s e d . However, t h i s frequency e f f e c t does i n d i c a t e that the dynamic response of the c a t a l y s t was complex such that conversions over the c a t a l y s t d i d not change i n s t a n t a n e o u s l y as the feedstream composition changed. This complexity i s shown more s p e c i f i c a l l y f o r CO by F i g u r e 6. The s o l i d l i n e g i v e s conversions measured under s t e a d y - s t a t e c o n d i t i o n s . The dotted l i n e represents the average conversions that would be expected i f the o u t l e t CO concentrat i o n i n s t a n t a n e o u s l y reached a new s t e a d y - s t a t e v a l u e as the i n l e t composition i s c y c l e d as a square wave about the mean a i r f u e l r a t i o . A c t u a l conversions obtained when c y c l i n g a t a frequency of 0.25 Hz are given by the dashed l i n e . The r e s u l t that the measured c y c l e d conversions are higher than the p r e d i c t e d conversions i m p l i e s t h a t conversions d u r i n g the r i c h h a l f of a c y c l e are higher than would be obtained under these r i c h c o n d i t i o n s at s t e a d y - s t a t e . Dynamic Gas-Phase Measurements Although time-averaged conversion measurements provide v a l u a b l e i n f o r m a t i o n , a d e t a i l e d a n a l y s i s of the dynamic beh a v i o r of three-way c a t a l y s t s r e q u i r e s d i r e c t o b s e r v a t i o n of c a t a l y s t responses to r a p i d changes i n exhaust composition. Some t r a n s i e n t s over three-way c a t a l y s t s i n simulated exhaust occur s l o w l y enough that they can be observed w i t h c o n v e n t i o n a l exhaust a n a l y z e r s . S c h l a t t e r and M i t c h e l l (9) have s t u d i e d the step responses of Pt/Rh/Ce/Al 0^ c a t a l y s t s i n simulated exhaust. ?


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Figure 4. Laboratory reactor system used to measure the performance of catalysts under cycled conditions. Two simulated exhaust streams of differing compositions are alternately fed to the reactor (8).


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Mean A/F Ratio Figure 5. Time-averaged CO and NO conversions measured using the laboratory reactor system shown in Figure 4. A fresh, pelleted Pt/Rh/Al 0 catalyst was operated at a middle-bed temperature of 820 Κ and a space velocity of 52,000 h' (STP). The feedstreams simulated exhaust that would be obtained with various engine air-fuel ratios (A/F) but did not contain S0 . The feedstream compositions were cycled at 0.25 and 1 Hz at an amplitude of ±0.25 A/F about the mean A/F. For the curves labeled Steady-State, conversions were measured with feedstreams at the mean A/F values ($). 2

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Figure 6. CO conversions measured under steady and cycled (±0.25 A/F) conditions. Expected conversions were calculated by assuming that the catalyst would respond instantaneously to changes in feedstream composition (&).


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They found that CO conversions can take up to 15 minutes to decrease to a new s t e a d y - s t a t e l e v e l a f t e r a change from l e a n to r i c h c o n d i t i o n s a t a space v e l o c i t y of 100,000 h " (STP) and a middle-bed temperature of 823 K. The t r a n s i e n t enhancement i n CO c o n v e r s i o n was found to be due to the water-gas s h i f t r e a c t i o n c a t a l y z e d by Rh. An o x i d i z e d form of Rh was proposed to be a c t i v e f o r the water gas s h i f t r e a c t i o n , w i t h r e d u c t i o n of the o x i d i z e d Rh o c c u r r i n g s l o w l y i n the r i c h feedstream. Ce prolonged the t r a n s i e n t enhancement i n CO c o n v e r s i o n , a p p a r e n t l y by r e t a r d i n g the r a t e of Rh r e d u c t i o n . When S 0 was added to the feedstream, CO conversions reached a new s t e a d y - s t a t e l e v e l i n l e s s than 10 s, or f a s t e r than the response r a t e of the a n a l y s i s system. S 0 i s present i n exhaust a t a l e v e l of about 20 ppm and poisons the water-gas s h i f t r e a c t i o n on the Group V I I I t r a n s i t i o n metals (10). Cooper and Keck (11) have s t u d i e d the step responses of N i - c o n t a i n i n g three-way c a t a l y s t s i n S0«-free simul a t e d exhaust. At a space v e l o c i t y of 50,000 h " (STP) and a c a t a l y s t i n l e t temperature of 813 K, they observed t r a n s i e n t s i n conversion over p e r i o d s up to 10 minutes long. The water-gas s h i f t a c t i v i t y of the N i and changes i n the amount of oxygen bound to the N i were proposed to a f f e c t the observed step responses of the c a t a l y s t s . Conversions over three-way c a t a l y s t s change more r a p i d l y a f t e r a change i n i n l e t composition when the feedstream i s a c t u a l engine exhaust r a t h e r than S02~free simulated exhaust. The d i f f e r e n c e between the c a t a l y s t responses i n the two f e e d streams may be r e l a t e d to f a c t o r s which are yet unknown i n a d d i t i o n to S0~ p o i s o n i n g of the water-gas s h i f t r e a c t i o n . Instruments which have 10%-to-90% response times of l e s s than 0.1 s are r e q u i r e d to measure the dynamic response of a c a t a l y s t i n engine exhaust. As we have seen i n F i g u r e 3, the oxygen sensor used i n the engine c o n t r o l system i s f a s t enough to respond to most of the important changes i n exhaust composition. However, the oxygen sensor cannot be used to measure c o n c e n t r a t i o n s of r e a c t a n t s i n exhaust s i n c e the sensor's output i s a n o n l i n e a r f u n c t i o n of the r a t i o of o x i d i z i n g s p e c i e s to reducing s p e c i e s . M e i t z l e r (12) has used oxygen sensors placed b e f o r e and a f t e r a c a t a l y t i c converter to study the dynamic response of a three-way c a t a l y s t . The c a t a l y s t was found to r e t a i n o x i d i z i n g s p e c i e s i n t o the i n i t i a l p e r i o d s of r i c h p o r t i o n s of a i r - f u e l r a t i o c y c l e s . I n f r a r e d diode l a s e r spectroscopy has been used f o r the measurement of hydrocarbon and CO c o n c e n t r a t i o n s i n exhaust (13, 14, 15). The a d s o r p t i o n path l e n g t h , and thus the a b s o r p t i o n c e l l volume, r e q u i r e d f o r hydrocarbon measurement i s r a t h e r l a r g e , l i m i t i n g the t i m e - r e s o l u t i o n of the measurement. The a b s o r p t i o n path l e n g t h r e q u i r e d f o r CO measurement, however, i s r e l a t i v e l y s h o r t and approximately equal to the diameter of a standard exhaust p i p e . T h i s a l l o w s CO to be measured w i t h h i g h t i m e - r e s o l u t i o n by an i n f r a r e d l a s e r beam passed through an 1

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exhaust p i p e . The path l e n g t h r e q u i r e d f o r NO measurement may be short enough to a l l o w dynamic measurement o f t h i s s p e c i e s . 0^ cannot be measured by c o n v e n t i o n a l i n f r a r e d spectroscopy. F i g u r e 7 i s a schematic diagram o f the apparatus used f o r dynamic CO measurements. An i n f r a r e d diode l a s e r beam i s s p l i t and components pass through the exhaust a t the i n l e t and o u t l e t of a standard c a t a l y t i c converter. The l a s e r beam i s chopped a t a h i g h frequency and l o c k - i n a m p l i f i e r s a r e used to separate the d e t e c t o r s i g n a l due to the l a s e r from that due to the high i n f r a r e d background near the engine. A minicomputer i s used to r e c o r d the c a r b u r e t o r o r f u e l - i n j e c t o r c o n t r o l s i g n a l , the two l a s e r s i g n a l s , and s i g n a l s from oxygen sensors placed a t v a r i o u s p o i n t s i n the exhaust f l o w system. The engine can be operated under normal feedback c o n t r o l , or the minicomputer can be used to c o n t r o l the a i r - f u e l r a t i o . The 10%-to-90% response-time of t h i s system i s 0.025 s, which i s s u f f i c i e n t l y f a s t to f o l l o w a c c u r a t e l y any CO c o n c e n t r a t i o n change which occurs i n automot i v e exhaust. F i g u r e 8 shows the CO c o n c e n t r a t i o n s measured a t the i n l e t and o u t l e t of the c a t a l y t i c converter during feedback c o n t r o l of the exhaust composition. The f l o w time between the i n l e t and o u t l e t measurement p o i n t s was 0.06 s a t the space v e l o c i t y of 50,000 h (STP) and converter temperature of 773-823 K. The converter contained a p e l l e t e d Pt/Pd/Rh/Ce/Al 0 c a t a l y s t which had been aged f o r 30,000 km on an automobile. The r e l a t i v e l y l a r g e , low frequency (ca. 1 Hz) o s c i l l a t i o n i n the i n l e t CO l e v e l was due to l i m i t - c y c l i n g of the c o n t r o l system. The lower amplitude, higher frequency (ca. 10 Hz) o s c i l l a t i o n s r e s u l t e d from two superimposed components: (1) a component due t o modul a t i o n of the f u e l - m e t e r i n g rod i n the c a r b u r e t o r , and (2) a component due to m a l d i s t r i b u t i o n of f u e l to the e n g i n e s c y l i n ders (15). The exhaust composition f l u c t u a t i o n s shown by the CO measurements a r e a l s o present i n the oxygen sensor s i g n a l i n F i g u r e 3. The high frequency CO o s c i l l a t i o n s were present a t the converter o u t l e t d u r i n g the r i c h periods o f the l i m i t c y c l e s ; l i t t l e or no CO broke through the converter when the exhaust was l e a n . These r e s u l t s a r e d i r e c t measurements of the CO concentrat i o n s which a r e present a t the i n l e t to a three-way converter under r e a l i s t i c o p e r a t i n g c o n d i t i o n s . The c o n c e n t r a t i o n s of other exhaust components a t the converter i n l e t can be estimated using c o n c e n t r a t i o n c r o s s p l o t s , f o r each component versus CO, obtained from time-averaged measurements a t constant a i r - f u e l ratio settings. One of the goals of an a n a l y s i s of three-way dynamic behavi o r i s to p r e d i c t o u t l e t c o n c e n t r a t i o n s given r a p i d l y v a r y i n g i n l e t c o n c e n t r a t i o n s , such as those shown i n F i g u r e 8. The a n a l y s i s i s made e a s i e r , however, when the dynamic c o n d i t i o n s are s i m p l i f i e d to step changes and s i n g l e frequency, constant amplitude o s c i l l a t i o n s i n a i r - f u e l r a t i o . F i g u r e 9A shows CO 2

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CO concentrations measured at the inlet and outlet of a three-way catalytic converter during feedback control of the exhaust composition (15).

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concentrations measured as the a i r - f u e l r a t i o c o n t r o l s i g n a l was stepped from r i c h to l e a n over a p e l l e t e d Pt/Pd/Rh/Ce/AÔ^ c a t a l y s t at a space v e l o c i t y of 50,000 h " (STP). Before use, the c a t a l y s t was aged f o r 100 h on an engine-dynamometer to simulate 6400 km of exposure to automobile exhaust. Although d i f f i c u l t to see i n t h i s f i g u r e , the CO at the converter o u t l e t a c t u a l l y dropped f a s t e r than at the i n l e t ( a f t e r c o n s i d e r i n g the f l o w time between the measurement p o i n t s ) . This i s because the inlet c o n c e n t r a t i o n (not measured) increased as the i n l e t CO c o n c e n t r a t i o n dropped so that the conversion of CO increased a t the same time t h a t CO e l u t e d . This r i c h - t o - l e a n CO response was found to be i n s e n s i t i v e to c a t a l y s t composition. A much more i n t e r e s t i n g response i s shown i n Figure 9B. Here, the a i r - f u e l r a t i o s e t t i n g was stepped from l e a n to r i c h . The i n l e t CO t r a c e i s roughly a m i r r o r image of that i n F i g u r e 9A. The o u t l e t CO l e v e l , however, took a much longer time to reach a new s t e a d y - s t a t e l e v e l than f o r the r i c h - t o - l e a n step. At the maximum time shown i n Figure 9B, the o u t l e t CO l e v e l had o n l y r i s e n to about 60% of the r i c h s t e a d y - s t a t e o u t l e t l e v e l , which can be seen on the l e f t s i d e of F i g u r e 9A. Approximately 25 s were r e q u i r e d f o r the o u t l e t CO to reach the new steadys t a t e l e v e l a f t e r the l e a n - t o - r i c h step. This time i s much s h o r t e r than that mentioned above f o r c a t a l y s t s i n S02~free simulated exhaust, but i s s t i l l long w i t h respect to the p e r i o d s of the exhaust composition o s c i l l a t i o n s observed during a c t u a l automotive o p e r a t i o n . The dashed curve i n F i g u r e 9B gives the approximate, smoothed r e s u l t that would have been obtained i f the c a t a l y s t response were instantaneous. The area between the dashed curve and the a c t u a l response represents a d d i t i o n a l CO conversion due to the noninstantaneous dynamic response of the c a t a l y s t . This type of response i s d e s i r a b l e because i t w i l l l e a d to low CO emissions when the a i r - f u e l r a t i o c y c l e s about the s t o i c h i o m e t r i c p o i n t . The a d d i t i o n a l CO conversion that i s observed a f t e r a change from l e a n to r i c h exhaust may be due to a t e m p o r a r i l y enhanced water-gas s h i f t a c t i v i t y as proposed by S c h l a t t e r and M i t c h e l l ( 9 ) , and may a l s o be due to r e a c t i o n of CO w i t h oxygen bound to Ce i n the c a t a l y s t . On the b a s i s of time-averaged conversion measurements and l a b o r a t o r y measurements using s i n g l e component gases, Shelef and coworkers Ç5, 16) proposed that the "storage" of oxygen by base metals i s an important f a c t o r i n the dynamic performance of three-way c a t a l y s t s . I f t h i s mechanism i s important f o r a c a t a l y s t , then the oxygen content of the c a t a l y s t should change by measurable amounts a f t e r sudden changes i n exhaust composition. 1



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5000 -

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RICH

6 8 Time (s)

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Society of Automotive Engineers

LEAN

B: CO concentrations measured as the A/F control setting was stepped from lean (A/F = 15.5) to rich (A/F = 14.1) (14). The dashed curve gives the approximate, smoothed result that would have been obtained if the catalyst response were instantaneous.

Figure 9. A: CO concentrations measured over a pelleted Pt/Pd/Rh/Ce/Al 0 catalyst at a space velocity of 50,000 h' (STP) as the engine A/F control setting was stepped from rich (A/F — 14.1) to lean(A/F = 15.5).

m

6

CO Outlet (PP ) 4000

CO Inlet (ppm)

15000

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Measurement of C a t a l y s t Oxygen Content Kaneko, e t a l , (17) have made measurements o f the oxygen content o f a Pt/Rh/AlÔ^ c a t a l y s t a f t e r v a r i o u s exposures to exhaust. A f t e r r e d u c t i o n o f the c a t a l y s t w i t h CO, the oxygen content o f the c a t a l y s t was found to i n c r e a s e w i t h exposure to l e a n exhaust. The data obtained were used to develop a procedure f o r c a l c u l a t i n g emissions under c y c l e d a i r - f u e l r a t i o c o n d i t i o n s . The c a l c u l a t i o n s could e x p l a i n average conversion measurements that could not be explained by assuming the c a t a l y s t responded i n s t a n t a n e o u s l y to a i r - f u e l r a t i o changes. The c a t a l y s t s t u d i e d by Kaneko and coworkers contained o n l y Pt and Rh. Most three-way c a t a l y s t s , however, a l s o c o n t a i n base metals i n l a r g e r amounts than the precious metals (5_, 6^, 7_, 9_> 11). We have measured changes i n the oxygen content of a Cec o n t a i n i n g three-way c a t a l y s t a f t e r v a r i o u s exposures to exhaust (18). The apparatus that was used i n our study i s shown i n F i g u r e 10. A f t e r the d e s i r e d exposure of the 5 g c a t a l y s t bed to exhaust, the r e a c t o r was f l u s h e d w i t h N f o r 10 min, and then the oxygen content of the c a t a l y s t bed was determined i n the f o l l o w i n g procedure. F i r s t , both s o l e n o i d v a l v e s a t the i n l e t of the r e a c t o r were closed and a s m a l l flow of N was i n j e c t e d i n t o the r e a c t o r through a 4-port s w i t c h i n g v a l v e . The s w i t c h i n g v a l v e was then turned to i n j e c t a stream of 2.3% 0 /0.1% Ar/He i n t o the r e a c t o r as the f l o w from the r e a c t o r was analyzed c o n t i n u o u s l y w i t h the mass spectrometer. A r was monitored to determine the e l u t i o n curve o f an i n e r t s p e c i e s , and i t a l s o served as an i n t e r n a l c o n c e n t r a t i o n standard i n the C 0 measurement. The e l u t i o n curve of 0 r e l a t i v e t o t h a t of Ar was used to determine the amount o f 0 that reacted w i t h the c a t a l y s t . A s m a l l amount of 0 reacted w i t h carbon on the c a t a l y s t to form C 0 , which was a l s o measured (H«0 formation was n e g l i g i b l e ) . The oxygen content of the c a t a l y s t a f t e r exposure t o exhaust was c a l c u l a t e d by s u b t r a c t i n g the net oxygen uptake (measured uptake l e s s the amount reacted t o form C0 ) from the maximum oxygen content, o r oxygen c a p a c i t y of the c a t a l y s t bed. The oxygen c a p a c i t y of the c a t a l y s t bed i s d e f i n e d as the maximum amount of oxygen that was r e t a i n e d by the c a t a l y s t a f t e r treatment w i t h 0 and that could be removed by r e a c t i o n w i t h CO. The r a t e o f i n c r e a s e o f oxygen content a f t e r a r i c h - t o - l e a n step change i n a i r - f u e l r a t i o was s t u d i e d using the f o l l o w i n g procedure. The r e a c t o r flow was switched to N a f t e r s t a b i l i z a t i o n of the c a t a l y s t i n exhaust a t an a i r - f u e l r a t i o of 14.1, a c a t a l y s t i n l e t temperature of 680 K, and a space v e l o c i t y of 110,000 h (STP). Next, the a i r - f u e l r a t i o was changed t o a s e t t i n g o f 15.1 w h i l e the exhaust bypassed the r e a c t o r . Then the r e a c t o r was given a p u l s e of l e a n exhaust by s w i t c h i n g from N« t o exhaust f o r a s p e c i f i e d p e r i o d , and then back to N«. 2

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H E R Z

Automotive

Catalysts

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l&EC Product Research and Development Figure 10. Schematic diagram of the apparatus used to measure changes in the oxygen content of a catalyst with changes in exhaust air-fuel ratio. The 2.54-cm i.d. tubular reactor contained 5 g of catalyst and was heated by an electric furnace (18j.


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A f t e r t h i s exposure, the oxygen content of the c a t a l y s t was measured. F i g u r e 11 shows the r e s u l t s of these experiments f o r the same p e l l e t e d Pt/Pd/Rh/Ce/AlÔ^ c a t a l y s t used f o r the data i n F i g u r e 9. The oxygen content of the c a t a l y s t bed increased l i n e a r l y w i t h pulse d u r a t i o n and reached the s t e a d y - s t a t e l e a n l e v e l w i t h i n 0.5 s. The oxygen c a p a c i t y of the c a t a l y s t was a s s o c i a t e d p r i m a r i l y w i t h o x i d a t i o n of the 190 ymol of Ce cont a i n e d i n each gram of c a t a l y s t ; the c a t a l y s t contained only 8 pmol of precious metal per gram. A comparison of the oxygen c a p a c i t y to the amount of Ce i n the c a t a l y s t suggests that about 76% of the Ce could change between the +3 and +4 o x i d a t i o n s t a t e s (the most common o x i d a t i o n s t a t e s of Ce). The i d e n t i t i e s of the Ce compounds which undergo o x i d a t i o n and r e d u c t i o n i n exhaust are not known, however, dispersed hydroxides and oxyhydroxides are l i k e l y candidates (18). The r a t e of decrease of oxygen content of the c a t a l y s t a f t e r a l e a n - t o - r i c h step change was s t u d i e d using the procedure described above w i t h the l e a n and r i c h exposures reversed. F i g u r e 12 presents the r e s u l t s of these experiments. After exposure to r i c h exhaust f o r 1.0 s, the oxygen content of the c a t a l y s t bed decreased 36% of the way from i t s l e a n s t e a d y - s t a t e l e v e l to i t s r i c h s t e a d y - s t a t e l e v e l . This change i n oxygen content could correspond to a conversion of 58% of the CO and H i n the 1.0 s pulse of r i c h exhaust (hydrocarbons are l e s s r e a c t i v e w i t h the oxygen held by the c a t a l y s t than CO and H ) . The d i f f e r e n c e i n c a t a l y s t oxygen content between the r i c h and l e a n s t e a d y - s t a t e s i s s u f f i c i e n t l y l a r g e to account f o r the e x t r a conversion obtained i n the CO step response experiment shown i n F i g u r e 9B. In a d d i t i o n , the r a t e s of change of c a t a l y s t oxygen content shown i n Figures 11 and 12 are s u f f i c i e n t l y f a s t to a f f e c t the performance of the c a t a l y s t when the a i r - f u e l r a t i o i s c y c l e d about the s t o i c h i o m e t r i c p o i n t at frequencies on the order of 1 Hz. 2

2

Future

Studies

The s t u d i e s described i n the preceding two s e c t i o n s have i d e n t i f i e d s e v e r a l processes that a f f e c t the dynamic behavior of three-way c a t a l y s t s . Further s t u d i e s are r e q u i r e d to i d e n t i f y a l l of the chemical and p h y s i c a l processes that i n f l u e n c e the behavior of these c a t a l y s t s under c y c l e d a i r - f u e l r a t i o c o n d i t i o n s . The approaches used i n f u t u r e s t u d i e s should i n c l u d e (1) d i r e c t measurement of dynamic responses, (2) mathematical a n a l y s i s of experimental data, and (3) f o r m u l a t i o n and v a l i d a t i o n of mathematical models of dynamic converter o p e r a t i o n . Such models must be used i n c o n j u n c t i o n w i t h a model of convert e r warmup (1) and a model of c a t a l y s t d e a c t i v a t i o n (6) when d e s i g n i n g the d i s t r i b u t i o n of a c t i v e components i n a c a t a l y s t and converter and the engine c o n t r o l s t r a t e g y .


Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 1

I&EC Product Research and Development 2

s

Figure 11. Rate of increase in the oxygen content of a pelleted Pt/Pd/Rh/Ce/Al O catalyst in lean exhaust at a space velocity of 110,000 h' (STP) and a catalyst inlet temperature of 680 Κ as;.

3

5

Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 1

2

3

Figure 12. Rate of decrease in the oxygen content of a pelleted Pt/Pd/Rh/Ce/Al 0 catalyst in rich exhaust at a space velocity of 110,000 h' (STP) and a catalyst inlet temperature of 680 Κ

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Three-Way

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77

Literature Cited 1.

Oh, S. H . ; Cavendish, J. C.; Hegedus, L . L . "Mathematical Modeling of Catalytic Converter Lightoff: Single P e l l e t Studies"; AIChE J., 1980, 26, 935.

2.

Canale, R. P . ; Winegarden, S. R.; Carlson, C. R.; Miles, D. L., SAE Paper No. 780205, SAE Trans. 1978, 87, 843.

3.

Seiter, R. E.; Clark, R. J., SAE Paper No. 780203, SAE Trans. 1978, 87, 828.

4.

Hegedus, L . L.; Gumbleton, J. J. CHEMTECH 1980, 10, 630.

5.

Gandhi, H. S.; Piken, A. G.; Shelef, M . ; Delosh, R. G . , SAE Paper No. 760201, SAE Trans. 1976, 85, 901.

6.

Hegedus, L . L.; Summers, J. C.; Schlatter, J. C . ; Baron, K. J. Catal. 1979, 56, 321.

7.

Falk, C. B . ; Mooney, J. J. "Three-Way Conversion Catalysts: Effect of Closed-Loop Feed-Back Control and Other Parameters on Catalyst Efficiency"; SAE Paper No. 800462, 1980.

8.

Schlatter, J. C.; Sinkevitch, R. M . ; M i t c h e l l , P. J. "A Laboratory Reactor System for Three-Way Catalyst Evaluation"; GM Research Publication GMR-2911; presented at 6th N. Amer. Mtg. Catal. Soc., Chicago, IL, March 1979.

9.

Schlatter, J . C . ; M i t c h e l l , P. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 288.

10.

Joy, G. C . ; Molinaro, F. S.; Lester, G. R. "Water-Gas Shift and Steam-Reforming Ability of Group VIII Metals i n Simulated Automotive Exhaust"; UOP Inc. report presented at 6th N. Am. Mtg. Catal. Soc., Chicago, I L , March 1979.

11.

Cooper, B. J.; Keck, L . "NiO Incorporation i n Three Way Catalyst Systems"; SAE Paper No. 800461, 1980.

12.

Meitzler, A. H. "Application of Exhaust-Gas-Oxygen Sensors to the Study of Storage Effects i n Automotive Three-Way Catalysts"; SAE Paper No. 800019, 1980.

13.

Hill, J. C . ; Majkowski, R. F. "Time-Resolved Measurement of Vehicle Sulfate and Methane Emissions with Tunable Diode Lasers"; SAE Paper No. 800510, 1980.


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14.

S e l l , J. Α.; Herz, R. K . ; Monroe, D. R. "Dynamic Measure ment of Carbon Monoxide Concentrations i n Automotive Exhaust Using Infrared Diode Laser Spectroscopy"; SAE Paper No. 800463, 1980.

15.

S e l l , J. Α.; Herz, R. K . ; Perry, E. C. "Time-Resolved Measurement of Carbon Monoxide i n the Exhaust of a Computer Command Controlled Engine"; SAE Paper No. 810276, 1981.

16.

Yao, H. C . ; Shelef, M. J. Catal. 1976, 44, 392.

17.

Kaneko, Y.; Kobayashi, H . ; Komagome, R.; Hirako, O.; Nakayama, O., SAE Paper No. 780607, SAE Trans. 1978, 87, 2225. Herz, R. K. "The Dynamic Behavior of Automotive Catalysts. I. Catalyst Oxidation and Reduction"; Ind. Eng. Chem. Prod. Res. Dev. 1981, i n press.

18.

RECEIVED

July 28, 1981.


Catalysis Under Transient Conditions - ACS Publications - American

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