Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 414-419
414 I
I
Registry No. H20, 7732-18-5; Fe, 7439-89-6.
I
Literature Cited .3
Amelse, J. A.; Butt, J. B.; Schwartz, L. H. J . Phys. Chem. 1978,82, 558. Anderson, R. B. I n Catalysis;Emmett, P. H., Ed.; Reinhold: New York, 1956: Vol. 4. Anderson, R. B.; Hofer, L. J. E.; Cahn, E. M.: Seligman, B. J . Am. Chem. SOC. 1951, 73, 944. Bauminger, R.; Cohan, S. G.; Marinov, A.; Ofer, S.;Segal, E. Phys. Rev. 1961, 122, 1447. Brotz, W.; Rottig, W. 2.Elektrochem 1952, 56, 896. Dry, M. E. In Catalysis, Science and Technology: Anderson, J. R.. Boudart, M.. Eds.; Springer: Berlin, 1981; Vol. 1, p 159. Dwyer, D. J.; Somorjai, G. A. J . Cafal. 1978,52, 291. Frye, C. G.; Pickering, H. L.; Eckstrom, H. C. J , Phys. Chem. 1958, 62, 1508. Greenwood, N. N.; Gibb. T. C. i n Mossbauer Spectroscopy; Chapman and Hall: London, 1971; p 305. Huff, G. A., Jr.; Satterfieid, C. N. Ind. Eng. Chem. Fundam. 1982,21, 479. Huff, G. A., Jr.; Satterfield, C. N. J . Catal. 1984% 8 5 , 370. Huff, G. A.. Jr.; Satterfield, C. N. Ind. Eng. Chem. Process Des. Dev. I984b,23, 696. Huff, G. A.. Jr.; Satterfield, C. N.; Wolf, M. H. Ind. Eng. Chem. Fundam. 1983,22,258. Karn, F. S.; Shultz, J. F.; Anderson, R. B. Actes Congr. Int. Catal., Znd, 1960 1961,2,2439. Kolbel. H.; Engelhardt, P. Erdol Kohle 1949, 2,52. Loktev, S.M.; Bashkirov, A. N.; Sllvinskii, E. V.; Zvezdkina, L. I.; Kagan, Yu. 6.Kinef. Katal. 1973, 1 4 , 217. Madon, R. J.; Taylor, W. F. J . Catal. 1981, 6 9 , 32. McCartney, J. T.; Hofer, L. J. E.; Seligman, 8.; Lecky, J. A,; Peebles, W. C.; Anderson, R. B. J . Phys. Chem. 1953, 5 7 , 730. Niemantsverdriet, J. W.; van der Kraan, A. M.; van Dijk, W. L.; van der Baan, H. S.J . Phys. Chem. 1980, 8 4 , 3363. Rethwisch. D. G.; Phillips, J.; Chen, Y.; Hayden, T. F.; Dumesic, J. A. J . Cafal. 1985,9 7 , 167. Reymond, J. P.; Meriaudeau. P.;Pommier, B.; Bennett, C. 0. J . Cafal. 1980, 6 4 , 163. Satterfield, C. N.; Huff, G. A., Jr. J . Catal. 1982, 73, 187. Satterfield, C. N. et al. Ind. Eng. Chem. Prod. Res. Dev., preceding paper in this issue. . .___ Shultz, J. F.; Hall, W. K.; Seligman, B.; Anderson, R. B. J . Am. Chem. SOC. 1955. 77. 213. S~ndergaard:K. I n Addendum to Bohlbro, H. An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapour over Iron Oxide Based Catalysts; Gjellerup: Copenhagen, 1969; p 159. Tramm, H. Chem.-Ing.-Tech. 1952,2 4 , 237. Vogler, G. L.; Jiang, X.-2.; Dumesic, J. A,; Madon, R. J. J . Cafal. 1984,89, 116.
i
.2
P-OLEF I N - Pd > Ir > Ru > Rh. Also, the results obtained in this study suggest that the mechanism of periodic operation effect is interpreted in terms of the strong adsorbed CO on the catalyst. This conclusion coincides with the case of a CO-O2 system recently reported (Muraki et al., 1985b).
Experimental Section Catalyst. In order to lower the inevitable support effects which affect the intrinsic character of noble-metal catalysts, chemically inactive a-A1203(2-3-mm-diameter spheres, BET surface area = 10 m2/g, bulk density = 0.79 g/cm3) was selected as the support. The a-A1203was prepared by calcination of 6-Alz03 (Rh6ne-Poulence
SCS-79) at 1200 "C for 3 h in air. The catalysts were all prepared by impregnation of the a-A1203supports with the aqueous solutions of ruthenium chloride, iridium chloride, rhodium nitrate, platinum nitrate, and palladium nitrate. The concentration of respective solutions were adjusted to give 0.006 and 0.24 wt % metal on the finished catalyst. The impregnation spheres were dried overnight at 110 "C and calcined at 600 "C for 3 h in air for Pt, Pd, and Rh and in hydrogen for Ru and Ir. Activity Measurements. The laboratory reactor system used in the present experiment is similar to the previously used system (Muraki et al., 1985b). In the light-off experiments, the catalyst temperature was gradually raised at a rate of 2 "C/min from 200 to 500 "C in the steady and cycled feedstreams of NO/CO in N1. The CO and NO were alternately and periodically injected into the cycled feedstreams. The period of the cycled feed composition fluctuation was varied from 0.2 to 2.0 s, the injection period of NO was fixed a t 0.1 s, and the CO injection period was varied from 0.1 to 0.4 s at constant partial pressure. The same procedure was repeated for fixed CO and varied NO. The stoichiometry number, S, which identifies the redox characteristic of the gas mixture, is defined as S = [NO]/[CO] The nature of the feedstream is reducing, stoichiometric, and oxidizing for S < 1.0, S = 1.0, and S > 1.0, respectively. TPD Measurements. The reactor consisted of a 6mm-i.d. quartz tube with a 12-mm-i.d. bulb-shaped midsection. A quartz fritted disk was fused to the inside of the reactor. Helium (44 mL/min), as a carrier, was passed through a Deoxo purifier and a molecular sieve liquid nitrogen trap. Hydrogen was purified by passing through a Pt catalyst at 200 "C and a molecular sieve liquid nitrogen trap. Nitric oxide (99.9%) and carbon monoxide (99.98%) were used without further purification. One milliliter of 0.24 wt % catalyst set in the reactor was pretreated by (1)increasing the catalyst temperature up to 500 "C at the heating rate of 500 "C/h in flowing H2(50 mL/min), (2) reducing the catalyst with H2 (50 mL/min) at 500 "C for 1 h, (3) flowing to the He stream (44 mL/min) for 1 h at 500 "C, and (4) cooling down to room temperature. A gas injection valve was connected in series with the line to the reactor for permitting pulsative injection of the adsorbate. Ten NO or CO pulses were fed to the catalyst every 3 min during 0.5 h. The reactor was heated to 500 "C, and a programmable power supply was used to control catalyst heating at a linear rate of 40 "C/min. The carrier gas, helium, at atmospheric pressure was flowed to the catalyst at 50 mL/min. The effluent from the reactor was analyzed by a quadrupole mass spectrometer.
Results and Discussion Comparison of Noble Metals under Noncycled Feedstream. The conversions of NO and CO under the noncycled feedstream were determined as a function of the catalyst temperature. In order to simulate the feedstreams from lean (S = 1.5) to rich ( S = 0.25) conditions, the gas compositions were chosen to contain 0.2, 0.3, and 1.2 vol % of CO and a constant NO concentration (0.3 vol '70). The gas space velocity was kept at 30000/h. The catalyst activity for the NO reduction by CO was expressed in terms of percentage of NO conversion against the catalyst temperature. Figures 1-3 show activity data of 0.006 wt % catalysts a t S = 1.5, 1.0, and 0.25, respec-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986 100
.-6
7 %
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f
$ 40 0
0
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20 200
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200
300
400
300
400
Temperature Temperature
500
500 ("C)
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Figure 1. Relative activity curves for reduction of NO with CO over supported noble-metal catalysts a t S = 1.5.
Figure 4. Effect of temperature on N20 selectivity over noble-metal catalysts a t S = 1.0. 100
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20 0
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Figure 2. Relative activity curves for reduction of NO with CO over supported noble-metal catalysts a t S = 1.0.
500 ("C)
Figure 5. Effect of temperature on NO conversion over Pt catalyst under cycling conditions a t S = 1.0.
and N2 is mainly formed. On the contrary, if the ratio is nearly two, the main product is N20 via reaction 2. The catalyst selectivity for N20 formation is given by consumed NO - consumed CO (3) yN20 = consumed NO 0
p
20 0
200
300 Temperature
400
500
('c)
Figure 3. Relative activity curves for reduction of NO with CO over supported noble-metal catalysts a t S = 0.25.
tively. Arbitrarily choosing the temperature of 50% NO conversion as an activity indicator gave the relative activity sequence of catalyst as Rh > Ru > Ir > P d > Pt for an S value. This result is very similar to that reported by Ohara et al. (1978) using the engine exhaust. It was noted, however, that this result is different from that of Kobylinski and Taylor (1974). The difference between these may be due to the difference of supports; i.e., the catalyst support of this study is inactive a-A1203but that of their study is active alumina. In the reduction of NO by CO, the following set of reactions may occur simultaneously: 2 N 0 + 2CO N2 + 2C02 (1) 2 N 0 + CO N,O + COz (2) Reaction 1 represents the reaction between 1 mol of NO and 1 mol of CO, and Nz is formed. The formation of NzO is the reaction between 2 mol of NO and 1 mol of CO, as illustrated by reaction 2. If the ratio between consumed NO and CO is nearly one, reaction 1 is the main reaction
--
and is illustrated as a function of catalyst temperature in Figure 4. Figure 4 illustrates the effects of temperature on the selectivity of N 2 0 as observed at S = 1.0. As the temperature increased, the N20 selectivity was decreased except for the Rh catalyst. For Rh catalyst, YN20increased until 275 "C and then decreased with increasing temperature. Similar phenomena has been found by Hecker and Bell (1983). From the results of our experiments, Yh20 decreased in the order of Pt > Pd > Ir = Ru > Rh. Comparison of Noble Metals under Cycled Feedstream. The conversions of NO and CO as a function of the catalyst temperature were measured under the cycled feedstream of NO/CO in N2. The time-average gas concentration of the feedstream was chosen to be 0.3 vol % NO and 0.3 vol '70CO ( S = 1.0). The period of the cycled feedstream was varied from 0.2 to 2.0 s. The conversion of NO for the Pt catalyst as a function of the catalyst temperature is shown in Figure 5 under the various periods. This figure shows that as the period becomes longer, the light-off temperature gets lower. That is, the cycling feed improved the light-off performance due t o the periodic operation effect. The conversions of NO for the five catalysts of 0.006 wt % loading as a function of the period a t 400 "C are shown in Figure 6. Data a t 0 s represent the results under noncycled feestream. In the appearances of periodic operation effect, the active catalysts, Rh, Ru, and Ir, were weak, Pt was remarkable, and P d was medium. The activities of Pt and Pd catalysts are improved by the cycled feedstream of NO/CO, although these activities
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
0
0
1 .o
2.0
Figure 9. Effect of period on NO conversion over Pt catalyst at S = 0.25.
8
>
1.o
2.0
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Figure 6. Effect of period on NO conversion over noble-metal catalysts under stoichiometric conditions at 400 O C .
0
1.o
0
Period ( s e d
417
2.0
0.8
1
0
1
Period (sec)
Figure 7. Effect of period on NO conversion over Pt catalyst at S = 1.5.
2
Period ( s e d
Figure 10. Effect of period on NzO selectivity over P t catalyst under cycling conditions at S = 1.0.
rL NO injection
Period ( s e d
CO injection
Figure 8. Effect of period on NO conversion over P t catalyst at S = 1.0.
are the lowest in noble metals in noncycled feedstream. Periodic Operation Effect for Pt. This section will deal with the effect of periodic operation for Pt catalyst, which shows striking effect and is suitable to investigate the mechanism. Effect of S. Figures 7-9 illustrate the effects of the period on the conversion of NO for Pt catalyst at S = 1.5, 1.0, and 0.25, respectively. NO conversions at constant temperature increased with the increasing period until the optimum period (T") for the maximum conversion and then decreased. T,,, shortened with increasing temperature and The product of T,, by S was almost constant in each experimental run, for example, 1.2 a t 350 "C,0.3 a t 400 "C, and 0.2 a t 450 "C, where T,,S is the amount of the injected NO. YN20for Pt catalyst under the cycled feedstream (S = 1.0) is shown in Figure 10. The minimum selectivities were observed a t rather short periods that were equal to those a t maximum conversion in Figure 8. Effect of Injection Period. An experiment was undertaken to determine which injection process of NO or CO may lead to more COPformation. The COz formation was measured when the injection period for NO or CO was longer than 0.1 s. In the former section, the injection
s.
0
0.2
0.4
Injection period (sec)
Figure 11. Effect of injection period of NO or CO on COz formation over Pt catalyst at 400 OC.
periods of CO and NO were the same. However, in this section, the injection period of CO was fixed at 0.1 s, and the NO injection period was changed in the range from 0.1 to 0.4 s. The same procedure was repeated for a fixed NO period and varied CO periods. The partial pressures of NO and CO were kept a t the same value in Figure 8. Under these conditions, the amount of formed COzfor one period was measured for the 0.006 w t % Pt catalyst. The results at 400 "C are presented in Figure 11. When the NO injection period was increased, the amount of COP formation was increased about 4 times. On the contrary, when the CO injection period was increased, the C 0 2 formation was not increased. These results indicate that for oxidation of almost all CO chemisorbed on the catalyst surface, about 0.2 s (residence time) was required. This injection period (0.2 s) was almost half the time for the maximum conversion in Figure 8,0.4 s (the sum of the NO injection period, 0.2 s, and the CO injection period, 0.2 s).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
418
t
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a pt
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1
'
300
400 500-
500
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200
300
400
500
Temperatlire ("C)
-
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Figure 12. 'I'PD spectra of NO, N2, and N 2 0 following NO adsorption on 2.4 wt % (left) Pt, (middle) Pd, and (right) Rh catalysts.
31
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0
100 200 300 400 500 - 5 0 0 keep Temper a t ure
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o
ion
200 300 400 5 0 0 - - - - * 500
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Temperature
( "C)
("C)
Figure 13. TPD spectra of CO and C 0 2 following CO adsorption on 2.4 wt % (left) Pt, (middle) Pd, and (right) Rh catalysts
These results indicate that the optimum period for the maximum conversion was due to scavenging of adsorbed CO with NO. Comparison of the Periodic Operation Effect on Pt, Pd, and Rh Catalysts. This section will show the comparison with the Pt, Pd, and Rh catalysts, which are used in the typical three-way catalyst because of their high durabilities. TPD of NO. The TPD spectra of adsorbed NO were measured to determine the difference of the ability for NO dissociation among the Pt, Pd, and Rh catalysts. Figure 12 shows the spectra of TPD of NO which adsorbed the saturation at room temperature. Three species were detected during the thermal desorption of adsorbed NO: NO, N2,and N20. For three catalysts the desorption of both N2 and N 2 0 was observed from about 150 "C; that is, the adsorbed NO was dissociated at a very low temperature. It is expected that the adsorbed NO for the three catalysts was almost dissociated at the high temperature, 400 O C , in these reaction studies. TPD of CO. The TPD spectra of CO are shown in Figure 13. For three catalysts two distinct CO peaks and one C02peak are observed. Comparison of the three noble metals shows that the spectra have the same pattern but that the desorbed temperatures were different. That is, the desorption temperature of Rh is the lowest and that of Pt is the highest in the three noble metals. These results indicate that CO is strongly chemisorbed on Pt, and the order of strength for CO adsorption is Pt > P d > Rh. Comparing the three noble metals shows that the ionization potential decreases in the order Pt > P d > Rh. Therefore, Pt would show a higher tendency to be in the metallic or zero-valent state, and the surface coverage of CO on P t is expected to be more than that of P d and Rh.
Table I. Power Law Parameters for NO Reduction by CO
Pt (300 " C ) Pd (350 "C) Rh (220 " C )
--2 -1 0.09
1 1 -0.2
Lorimer and Bell, 1979 Muraki et al., 1986 Hecker and Bell, 1983
CO inhibits NO reduction over most noble metals but not over Ru (Shelef, 1975; Taylor and Klimisch, 1973). The reason why Rh is a good catalyst is perhaps due to weak inhibition by CO. Pt has the lowest activity among noble metals because of its strong CO self-poisoning. The results are also consistent with the chemisorption studies (McKee, 1967) and the infrared studies (Lorimer and Bell, 1979). Kinetic Parameter. Above-mentioned results indicate that, in this reaction system, the CO inhibition may be strong. Table I illustrates the reaction rate equation for the NO reduction by CO. The reaction rate of Pt catalyst exhibited negative second-order dependence on CO partial pressure (Lorimer and Bell, 1979), that of Pd catalyst exhibited a negative first-order dependence (Muraki et al., 1986), and that of Rh catalyst exhibited a weak positive order dependence (Hecker and Bell, 1983). Therefore, the order of the periodic operation effect corresponded to the order of their susceptibilities to CO self-poisoning. These results are consistent with TPD studies. Perhaps these periodic operation effects on the NO conversion must be due to the surface state of catalyst: that is, the catalyst surface under the noncycled feed at relatively low temperature is almost completely covered by the strong admolecules, CO, and the expected reaction rates are remarkably suppressed. On the other hand, under the optimum cycled feeds these admolecules are
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 419-424
419
we have made an attempt to explain the mechanism of the periodic operation effect on the automotive three-way catalyst. We have begun work aimed a t this goal.
suitably eliminated, and the surface compositions of reactants are kept suitable to react each other. Then the reaction rate attains the maximum value. The periodic operation effect can be interpreted in terms of the strong adsorption of CO, and this concept has been confirmed in the CO-02 system (Muraki et al., 198513). N 2 0 selectivity is minimum at the period that the NO conversion is maximum. A t the optimum periods, no oxygen atoms are present on the catalyst surface before NO is injected; 0 atoms are produced when NO is injected and are almost scavenged with CO injection. The result is that the N2 formation on the reducing surface is much better than that on the slightly oxidized surface (Muraki et al., 1986). In the periodic operation effect for NO-CO reaction, Pt catalyst, which is the least active, is remarkable, and Rh catalyst, which is the excellent catalyst, is not influenced. In this reaction system, the CO inhibition is strong, and the activities order corresponded to the order of their susceptibilities to CO self-poisonings. Automotive exhaust catalysts in more complex feedstreams show the marked periodic operation effect on the Pt and Pd catalysts (Muraki et al., 1985a; Yokota et al., 1985). This approximately corresponds to the periodic operation effect of our NO/CO cycling reaction (Figure 6), implying that the effect of the injection period of NO gas on the catalytic activities (Figure 11)is indeed governed by the adsorbed CO scavenging process, as previously suspected (Muraki et al., 198513). In addition to HC, we could observe the striking periodic operation effect on the Pd catalyst (Muraki et al., 1985c), since the HC could be chemisorbed strongly onto the noble metals with increasing temperature. The mechanism of periodic operation effect of HC is also interpreted in terms of the strong adsorption of HC. In order to develop a Rh-free three-way catalyst,
Acknowledgment
We are grateful to Professor Y. Murakami and Associate Professor T. Hattori, of Nagoya University, for their helpful suggestions. Registry No. NO, 10102-43-9; CO, 630-08-0; Rh, 7440-16-6; Ru, 7440-18-8; Ir, 7439-88-5; Pd, 7440-05-3; Pt, 7440-06-4. Literature Cited Ashmead, D. R.; Campbell, J. S.; Davies, P.; Farmery, K. SA€ Tech. Pap. Ser. 1974, No. 740249. Bauerle, G. L.; Service, G.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1972, 1 1 , 54. Canab, R. P.; Winegarden, S. R.; Carison, C. R.; Miles, D. L. SA€ Tech. Pap. Ser. 1978, No. 780205. Hecker, W. C.; Bell, A. T. J . Catal. 1983, 8 4 , 200. John, H.; Volker, R.; Ismail, M. I. Pletlnum Met. Rev. 1978, 2 2 , 92. Kobylinski, T. P.; Taylor, B. W. J . Catal. 1974, 33, 376. Kummer, J. T. f r o g . Energy Combust. Sci. 1980, 6 , 177. Lester, G. R.; Joy, 0.C.; Brennan, J. F. SA€ Tech. Pap. Ser. 1978, No. 780 202. Lorimer, D.; Bell, A. T. J . Catal. 1979, 5 9 , 223. McKee, D. W. J . Catal. 1967, 8 , 240. Muraki, H.; Shinjoh, H.; Sobukawa, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. h v . , 1985a, 2 4 , 43. Muraki, H.; Sobukawa. H.; Fujitani, Y. Nippon Kagaku Kaishi 1985b, 2 , 176. Muraki, H.; Sobukawa, H.; Fujitanl, Y. Nippon Kagaku Kaishi 1985c, 4 , 532. Muraki, H.; Shinjoh, H.; Fujiani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1988, following paper in this issue. Ohara, H.; Kondoh, S.; Fujiini, Y., J . SOC.Automot. Eng. Jpn. 1978, 16, 9. Schlatter, J. C.; Taylor, K. C. J . Catal. 1977, 4 9 , 42. Shelef, M. Catal. Rev. 1975, 1 1 , 1. Shelef, M.; Gandhi, H. Ind. Eng. Chem. Prod. Res. Dev. 1972, I f , 393. Tauster, S. J.; Murrell, L. L. J . Catal. 1976, 4 1 , 192. Taylor, K. C.; Klimlsch, R. L. J . Catal. 1973, 3 0 , 476. Yokota, K.; Muraki, H.; Fujitani, Y. SA€ Tech. P a p , Ser. 1985, No. 850 129.
Received for review September 4, 1985 Revised manuscript received January 24, 1986 Accepted February I, 1986
Reduction of NO by CO over Alumina-Supported Palladium Catalyst Hldeakl Muraki,' Hirofumi Shlnjoh, and Yoshlyasu Fujltanl Toyota Central Research and Development Laboratories, Inc., Aichi-gun, Aichi-ken 480- 1 1, Japan
The kinetics of NO reduction by CO have been investigated over a Pd/AI,O, catalyst. These studies have been complemented by pulse reaction, transient response method, and TPD. The performance of the Pd catalyst depended strongly on the ratio of NO/CO and the reaction temperature. I t was shown that the N20 selectivity was minimum at the stoichiometric ratio of NO/CO. For NO conversion below 20% at 350 O C , the kinetics for NO reduction and N, and NO , formation over Pd catalysts were first order in NO and inverse first order in CO. The kinetics and product selectivity were found to be in good agreement with a mechanism based on the chemisorption of NO as the rate-limiting step.
Therefore, for its extensive use, it is necessary to find ways to minimize the use of the scarce material or provide a Rh-free catalyst. From the comparison of catalytic behavior between Pt and Pd, TWC performance of Pd is very similar to that of Rh catalyst (Yokota et al., 1985; Muraki et al., 1985). Also, Pd is relatively abundant, domestically available, and significantly less expensive than Pt and Rh. Therefore, we selected Pd as the main component of the Rh-free TWC. The reduction of NO by CO over noble-metal catalysts has been investigated (Lorimer and Bell, 1979; Hecker and Bell, 1983; Dubois et al., 1980; Taylor and Schlatter, 1980;
Introduction
Automotive exhaust catalysts mainly consist of platinum (Pt), palladium (Pd), and rhodium (Rh) because of their high intrinsic activity and durability in automotive exhaust conditions. Of these precious metals, rhodium is used in three-way catalyst (TWC) for its activity to selectively reduce nitric oxide (NO) to nitrogen (N2)with low ammonia (NH,)formation (Shelef and Gandhi, 1972; Kummer, 1980; Schlatter and Taylor, 1977). The standard TWC formulations are those containing both Pt and Rh. The Rh/Pt ratio in TWC with sufficient durability is considerably higher than mine ratio. 0196-432118611225-0419$01.50/0
0
1986 American Chemical Soclety
