559
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 559-565
are due to Mr. D. Kowalczyk for providing cracking activity data of commercial catalysts. Literature Cited Barrett, G. P.; Joyner, L. G.; Halenda, P. H. J . Am. Chem. SOC. 1950, 73, 373. Boiton, A. P.; Lanewala, M. A. J. Catal. 1970, 18, 1. Brueckmann, F. G. U.S. Patent 2 303 680, 1942. Brindley, G. W.; Sempels, R . E. Clay Miner. 1977, 12, 229. Ciapetta, F. G.; Anderson, D. Oil Gas J. 1967, 65, 88. Dorogochinskii. A. S. Chem. Techno/. 1974, 26, 8. Dessau, R. M. "Shape-Selective Acid Catalyzed Reactions of Olefins Over Crystalline Zeolites", E.P. No. 037671-A1 (1981). Garwood, W. E.; Lee, W. U S . Patent 4227 992, 1980. Gohr, E. T.; Thompson, W. I.; Martin, H. 2. U.S. Patent 2320273, 1943. Grim, R. E. "Clay Mineralogy"; McGraw-Hill: New York, 1968. Hall, W. K.; F. E. Lutinski, F.E.; Gerberick, H. R. J. Cafal. 1964, 3, 512. Hemminger, C. E. U.S. Patent 2303047, 1942. Hightower, J. W.; Emmett, P. H. J . Am. Chem. SOC. 1965, 87, 939. Johansson, G. Acta Chem. Scand. 1960, 14, 769. Lahav, N.; U. Shani, U.; Shabtai. J. Clays and Clay Mlner. 1978, 26, 107. Lussier, R. J.; Magee, J. S.; Voughan, D. E. W. Preprints, 7th Canadian Symposium on Catalysis, 1980; p 88. Magee, J. S.;Blazek, J. J. ACS Monogr. 1976, 171, 615. Marshall, S. Pet. Refiner 1952, 31(9), 263. Occelli, M. L.; Tindwa, R. M. Clays Clay Miner. 1963, 31, 22. Occelli, M. L.; Hwu, F.S.S.: Hightower, J. W. Preprints, 182nd National Meeting of the American Chemical Society, New York, 1981. Occelli, M. L.; Lester, J. E. to be submitted to Ind. Eng. Chem. Prod. Res. D e v . , 1983. O'Deii, W. W. U.S. Patent 1984380, 1934. Parry, E. P. J. Catai. 1963, 2 , 371. Poutsma, M. L.; Schaffer, S. R. J . Phys. Chem. 1983, 77, 158. Shabtai, J.: Lazar, R.; Oblad, A. G. "Acidic Forms of Crosslinked Smectites. A Novel Type of Cracking Catalysts"; Proceedings, 7th International Con-
gress of Catalysis"; Seiyama, T.; Tanabe, K., Ed.; Kodansha-Elsevier: Tokyo, 1980; p 828. Shepard, J. W.; Peters, E. P.; Juveland, D.O. Belg. Patent 633,473, 1963. Svoboda, A. R.; Kunze, G. W. "Infrared Study of Pyridine Adsorbed on Montmorillonite Surfaces", Clays and Clay Minerals, Proceedings, 5th National Conference, Pittsburgh, Baliey, S. W., Ed.; Pergamon Press: New York, 1966, p 277. Thomas, C. L. Ind. Eng. Chem. 1949?41, 2564. Thomas, C. L.; Hickey, J.; Strecker, G. Ind. Eng. Chem. 1950, 4 2 , 866. Tyson, C. W. US. Patent 2322075, 1943. Vaughan, D. E. W.; Lussier, R. J.; Magee. J. S. US. Patent 4 176090, 1979. Vaughan, D. E. W.; Lussier, R. J. "Preparation of Molecular Sieves Based on Pillared Interbyered Clays (PILC)"; Proceedings, 5th International Conference Zeolites, Rees, L. V., Ed.; Heyden: London, 1980, p 94. Vaughan, D. E. W. "Industrial Use of Zeolite Catalysts": "Properties and Applications of Zeolites"; Townsend, R. P., Ed.; British Chem. Soc.; London, 1979; p 294. Van den Berg, J. P., et al. "The Conversion of Dimethylether to Hydrocarbons on Zeolite HZSM-5. The reaction Mechanism for Formation of Primary Olefins"; "Proceedings, 5th International Conference on Zeolites"; Rees, L. V., Ed., Hyden: London, 1980; p 649. Van Hook, W. A,; Emmett, P. H. J . Am. Chem. SOC. 1963, 84, 4421. Venuto, P. B.; Habib, E. T., Jr. "Fluid Catalytic Cracking with Zeolites"; Marcel Dekker; New York, 1979. Venuto, P. B. Chem. Tech. 1971, 1215. Vedrine, J. C.; Dejaifve, P.; Garbowski, E. D. "Aromatics Formation from Methanol and Light Oleflns Conversion on HZSM-5 Zeolite: Mechanism and Intermediate Species"; "Catalysis by Zeolites"; Vol. V., Imeiik 8.. et al., Ed.; Eisevier Publ. Co.: Amsterdam, 1980. Waish, D. E.; Roilmann, L. D. J. Cafal. 1977, 4 9 , 369. Ward, J. W. ACS Monogr. 1976, No. 171. Yamanaka, S.;Brindley, G. W. Clays and Clay Miner. 1979, 27, 119.
Receiued for review April 19, 1983 Accepted July 29, 1983
Development of a Low Cost, Thermally Stable, Monolithic Three-way Catalyst System Chlng-Hsong Wu" and Robert H. Hammerle Research Staff, Ford Motor Company, Dearborn, Michigan 48121
A new three-way catalyst (TWC) system consisting of a palladium (Pd) catalyst as the inlet half and a standard platinum (Pt) and rhodium (Rh) TWC as the outlet half has been investigated. This Pd/TWC system offers improved thermal resistance, light-off performance, and reduction in precious metal cost compared to an equal volume standard TWC. However, several open issues, such as the cost of manufacturing a two-part catalyst system, the poison resistance, and vehicle durability, remain to be resolved. The study of the Pd/TWC system was based on the concept that the activity of the outlet portion of a catalyst can be protected from thermal damage by the inlet portion as long as the inlet remains active. To prove this, a series of catalyst systems with different formulations for the inlet and outlet halves was studied using engine dynamometers. The performances of these catalyst systems were evaluated and compared during and after an emission durability cycle, with and without the imposition of 60 1-min long, 2000 O F episodes with excess air to simulate severe thermal environments experienced in some vehicles. Detailed results will be discussed.
Introduction As a part of the effort to reduce automotive emissions, three-way catalysts (TWCs) were developed to simultaneously convert three major pollutants, hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO,) (Jones et al., 1971; Gandhi et al., 1976; Mooney et al., 1977). Vehicles equipped with TWCs have been shown to meet emission standards with minimal loss of fuel economy and driveability (Seiter and Clark, 1978; Engh and Wallman, 1977; Oser, 1979). Today, TWCs followed by conventional oxidation catalysts (COCs) are used in most U S . light-duty passenger cars, and their use is expected to continue for some time.
Because of their wide-spread application, improvements in TWCs would be desirable. Most TWCs contain platinum (Pt) and rhodium (Rh) as active metal components. These precious metals (PMs) are imported and expensive. In addition, Pt is susceptible to sintering at high temperature (Yao et al., 19801, and the minute amount of Rh in TWCs is known to interact with the alumina (A120J washcoat at temperatures above 1650 OF under lean airfuel ratio (A/F) conditions to form a catalytically inactive spinel (Yao et al., 1977). Therefore, in order to maintain good TWC activity after high temperature exposure, the conventional method is to increase the PM loading on TWCs. As a result, the cost of TWCs can be high.
0196-4321/83/ 1222-0559$01.50/0 0 1983 American Chemical Society
560
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
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,
,
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Figure 1. Washcoat BET surface area as a function of distance from the inlet face of a 6 in. long, 40g PM/ft3, 11 Pt/Rh TWC aged under 1 2 000 mile AMA cycle plus 60, 2000 OF-lean spikes.
Extensive research has been done in recent years to improve thermal resistance or activity, or to lower the cost of TWCs. Various washcoat formulations have been developed for better thermal resistance (Stepien et al., 1980; Gandhi et al., 1979). Different base metals have been proposed for P M substitution to reduce cost (Gandhi et al., 1975; 1982; Yao and Shelef, 1976). Other aspects, such as the variation in geometry of monolithic substrate channels (Hegedus, 1973; Lester and Marinangeli, 1980), melt resistant substrates (Loehman and McNelly, 1981; Oda and Matsushisa, 1981), etc., have been investigated. However, none of these studies has demonstrated the development of a TWC with all three properties: improved performance, better thermal resistance, and reduced cost. This paper describes the development of a new TWC system which has these properties. It was developed using the concept that the inlet portion of a catalyst could protect the outlet portion from thermal damage. Tests verifying this concept using engine dynamometers are described. The results indicate that a TWC system with a 3-in. Pd catalyst (20 g/ft3) as the inlet half and a 3-in. TWC (20 g/ft3, 11 Pt/Rh by weight) as the outlet half performs equivalently to, or better than, a 6411. TWC (20 g/ft3, 11 Pt/Rh) after normal aging and after high-temperature, lean-A/F spike aging. In addition, a substantial saving in PM cost is realized. Background on Catalyst Deactivation. It is wellknown that some catalysts removed from development vehicles exhibit some degree of thermal damage, and the damage tends to be more severe in the inlet portion than in the outlet (Weaver et al., 1976). For example, washcoat cracking or even substrate melting usually begins at about in. from the inlet face of the catalysts. The damage may be limited to a small volume or extended to the entire portion of the catalyst depending on the severity of the catalyst environment. Similar damage has been observed on catalysts aged on engine dynamometers. A post-mortem analysis of a TWC (40 g/ft3 PM, 11 Pt/Rh) aged under high-temperature, lean spike conditions (Hammerle and Wu, 1984) with minimal contaminant accumulation on the catalyst shows more loss of washcoat BET surface area in the inlet portion than in the outlet as shown in Figure 1. The activity of the more damaged portion of the catalyst is worse than that of the less damaged one. Figure 2 shows a comparison of light-off performance of the inlet and outlet halves of 6 in. long TWCs with various P M loadings after aging for 12OOO miles on the U.S. emission durability cycle (Federal Register, 1977), which will be referred to as the AMA durability aging cycle. The temperatures for 50% conversion are higher for all inlet halves than for the outlet
k
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20
40
60
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100
PM LOADING ( g / f t 3 )
Figure 2. Light-off performance comparison of inlet and outlet halves of 6 in. long, 11 Pt/Rh TWCs as a function of precious metal loading aged under 12 000 mile AMA cycle.
ones regardless of PM loading. From these observations, it appears that the catalyst deactivation begins from the inlet portion of the catalyst and progresses to the outlet. A major cause of the deactivation, among others, is believed to be the thermal-induced sintering of washcoat and PM near the reaction zone where the temperature is the highest. Presumably, on a fresh catalyst, the HC and CO oxidation releases heat on the active metal sites near the inlet of the catalyst developing high local temperature before the heat can be dissipated by conduction, radiation, and convection. This high local temperature enhances the sintering of PM and washcoat, or causes the substrate to melt in severe engine malfunction situations where high concentrations of CO and HC are present in the exhaust gas. As the inlet of the catalyst becomes inactive, the reaction zone for subsequent malfunction moves toward the exit. Eventually, the entire catalyst may become deactivated. However, it is postulated that, if the inlet portion of the catalyst has sufficient thermal resistance to maintain some activity after repeated high-temperature exposure, the outlet portion will suffer less damage because it does not convert much reactants and experineces less local heat than that of the inlet portion. Thus, more PM will remain in active forms and the whole catalyst will have better performance. This concept will be called “catalyst protection”. Using this concept, a study was undertaken to identify a good formulation for the inlet portion of the catalyst in hope that the overall TWC system may offer better durability and a possibility of cost reduction. A series of catalyst systems consisting of various formulations in the inlet and outlet halves was aged on an engine dynamometer under two different thermal environments, with or without periodic, high-temperature, lean spikes, in an AMA durability aging cycle. The performances of these catalysts systems under a wide range of conditions were evaluated on an engine dynamometer and compared. Experimental Section A detailed description of the engine dynamometer facilities has been given previously (Hammerle and Wu, 1981). However, additional features important to this study will be described. For catalyst aging, a 1980 production 5.0-L, central-fuel-injection engine was used. The engine operated automatically under control of a computer programmed for the AMA durability aging cycle. In order to periodically expose the catalysts to high temperatures,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 581 INLET W P L E PORT ,
Table I. Description of Catalysts Shown in Figure 3
+3fa+3ntJ
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/
-
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THERMoCoUPLES HALVES
OU~LET HALVES
Figure 3. Configuration of the catalyst systems in an eight-chamber reactor. The notations of each catalyst are listed in Table I.
the engine was momentarily operated rich of stoichiometry, and appropriate amounts of air were injected into the exhaust system until the desired temperature and O2levels prior to the catalyst were achieved (Hammerle and Wu, 1984). For catalyst performance evaluation, a 6.6-L electronic-fuel-injection engine was used. The A/F could be precisely controlled at one steady-state value or with periodic modulations of various amplitude and frequency. The steady exhaust gas temperature (7') and catalyst space velocity (SV) were controlled independently for TWC performance evaluation. For light-off performance evaluation, the rate of catalyst temperature rise of 25 OF/min was controlled by passing the exhaust gas through a heat absorber which consisted of a 5.0-L stainless steel can filled with blank ceramic pellets. In order to ensure the accuracy of data, the catalysts were aged simultaneously and evaluated sequentially in eight-chamber reactors (Falk and Mooney, 1980). In this manner, the experimental uncertainties due to the variation in engine dynamometer facilities could be minimized. Figure 3 shows a schematic diagram of an eight-chamber reactor and the configuration of the eight catalyst systems used in this study. The chambers are cylindrical in shape and equally spaced around the circumference of the reactor. At the exit end of each chamber an orifice was installed to ensure an equal flow through each chamber. A common sample port was used for monitoring the inlet exhaust gas composition, and a separate sample port at the end of each chamber was used for measuring the activity of each catalyst system. During aging, the temperature of the catalysts was measured with a thermocouple located in the catalyst 1 in. downstream from the inlet of catalyst system 1. During performance evaluation, the gas temperature was monitored with 4 thermocouples located at in. upstream of the inlet of catalyst systems 1,3,5, and 7 to check the uniformity of the temperature. The typical temperature spread is less than 9 O F (5 "C) with good gas mixing. In this study, all catalyst systems were composed of two pieces in tandem. Each was made of a 300 cell/in.2 monolithic cordierite substrate 1.5 in. diameter and 3.0 in. long. For systems 1 and 3 through 6, a 20 g PM/ft3, 11 Pt/Rh TWC was preceded by catalysts of various formulations to identify which formulation offered the best protection. The formulation of inlet halves of these systems were, respectively, 20 g PM/ft3, 11 Pt/RH TWC; blank; 10 g PM/ft3, 11Pt/Rh TWC; 20 g Pd/ft3 COC; and 20 g Pt/ft3 COC. System 1 was considered equivalent to one piece of 20 g/ft3 TWC, 6 in. long, and was used as a reference. The PM loadings in the inlet halves of systems 3 through 6 are lower than the reference to explore the effect of noble metal loading. The system 2 was a 4 0 g PM/ft3, 11Pt/Rh TWC followed by a blank substrate. I t was regarded as
cat.
active metals, wt 5%
PM loading, g/ft3
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0 P t + Rh(0.066), Ce(1.2), Ni(3) P t + Rh(0.131), Ce(l.2), Ni(3) P t + Rh(0.186), Ce(1.2), Ni(3) P t t Rh(0.262), Ce(1.2), Ni(3) Pt(0.14), Ce(0.9) Pt(0.14), Ce(0.6)
0 10 20 30 40 20 20
30 40 Pt Pd
Pt/Rh
11 11 11 11
a catalyst with twice the PM loading and half the length, thus, the same PM mass as the reference. The blank substrate was used to hold the inlet piece in place and to further ensure the same flow restriction as the other systems. The systems 7 and 8 consisted of a 30 g PM/ft3, 11 Pt/Rh TWC preceded by a blank substrate and a 10 g PM/ft3, 11 Pt/Rh TWC, respectively. The blank substrates were used as inlet halves in some systems (e.g., 3 + 7) to determine if they offered catalyst protection. A simplified notation consisting of two catalyst designations separated by a slash is used to refer to these catalyst systems. For example, system 1 is described as 20/20, while system 5 is described as Pd/20. The key to the notation is listed in Table I. Two eight-chamberreactors of identical catalyst systems were prepared and aged on an engine dynamometer. One reactor was mounted on the left side of the engine exhaust system, and the other on the right side. Both were aged under the AMA durability cycle for 1 2 000 miles, which had been shown to be equivalent to a catalyst aged for 50 000 miles on vehicles without severe malfunction (Hammerle and Wu, 1981). In addition, the reactor on the right side of the engine exhaust was subjected to sixty evenly spaced high-temperature spikes. In each spike, the peak temperature was 2000 O F (1095 "C) with 1.5% excess of O2 and a duration of 1min. This condition was applied to simulate the severe catalyst environment for some vehicles with engine malfunction or under the conditions at the end of a cold start period. During aging, the performance of the catalyst systems was measured after every 4000 miles (128 h) to monitor the deterioration rates. After the completion of aging, the performance of these systems was evaluated in more detail under constant temperature and light-off conditions. The TWC performance was measured at two constant inlet temperatures: 650 and 800 O F (360 and 430 "C); two A/F controls, steady-state and modulation with an amplitude of 1.0 A/F unit peak-to-peak and frequency of 0.5 Hz; and a space velocity of 4 0 000/ h. Conversions of HC, CO, gross NO,, and net NO, (the conversion of NO, to N2) were measured over an A/F range from 14.2 to 15.0. For light-off performance, the conversions of HC and CO were measured as functions of inlet gas temperature from 200 to 850 OF at a constant space velocity of 40000/h and A/F of 16.5. The lead (Pb), sulfur (S),and phosphorus (P) contents in the engine fuels used for aging and performance evaluation were 5 mg and 10/20 > Pt/20 > B/20. Figure 10 depicts the light-off temperatures for 50% conversions for the same systems. The 40/B system was also included for comparison. The Pd/20 system shows equal light-off temperature to the 40/B systems and lower than the 20/20 system aged under the AMA cycle. In addition, this system exhibits much lower light-off tem-
2 1
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,
.1..20,10.
'8..
2 1
,0130
Figure 12. Light-off performance comparison for the three catalyst systems with constant total PM mass but different loading on inlet and outlet halves after two different aging conditions. Performance measured at A/F = 16.5, SV = 40000/h.
perature compared with other systems when exposed to high-temperature spike indicating its excellent thermal resistance. The ranking for the light-off performance is Pd/20 > 40/B > ZO/ZO > 10/20 = Pt/20 > B/20. These data indicate that for TWC/TWC systems, the light-off temperature decreases with the increased of PM loading in the inlet halves. For COC/TWC systems, Pd shows better early CO and HC light-off than Pt even though they have the same metal loading (20 gift3). It further confirms the excellent light-off Drouertv . . . of the Pd catalvst . .(Kummer et al., 1976). According to the concept of catalyst protection, the P M distribution along the catalyst length may also affect the performance andthermal resistance. Figure 11 shows a comparison of TWC performance of three catalyst systems with constant total PM mass hut different PM loading on inlet and outlet halves after two different aging conditions, The performance was evaluated a t near stoichiometry (A/F = 14.6),800 "V, 40000/h space velocity, and A/F modulation amplitude of 1.0 peak-to-peak and frequency of 0.5 Hz. The 10/30 system is the best performer and is followed by the ZO/ZO system. The 40/B system has the poorest TWC activity and thermal resistance. Figure 12 shows the light-off performance comparison for the same catalyst systems. The 40/B system has lower light-off temperature than the ZO/ZO standard system under both aging conditions indicating again, the depen-
564
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
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HALVES O F C A T A L Y S T S Y S T E M S
Figure 13. CO conversion comparison for seven outlet halves and one inlet half of the catalyst systems after aged under 12000 mile AMA cycle plus 60,2Mw) "F, lean spikes. Performance measured at A/F = 14.6, T = 800 OF, SV = SOOOO/h, modulation amplitude = 1.0 peak-&peak, and frequency = 0.5 Hz. Catalyst notations 0 1 and 03-08 are outlet halves of the catalyst systems 1 and 3-8, respectively, and I1 is the inlet half of the catalyst system 1, where the catalyst systems 1 and 3-8 are: 1 = 20/20; 3 = B/20; 4 = 10/10: 5 = Pd/20; 6 = Pt/20; 7 = B/30; and 8 = 10130.
dence of the light-off activity on the PM loading in the inlet half of the catalyst. However, the 10/30 system lights-off at lower temperature than the 20/20 standard catalysts in all cases indicating a significant contribution to the light-off activity by the outlet portion when it is very active. To further study the effect of catalyst protection, the activities of the outlet halves of the systems aged under high-temperature, lean spikes, were measured and compared. Figure 13 shows such a comparison of CO conversions for seven outlet halves and one inlet half of TWCs separated according to PM loading. Among the 20 g/ft3 TWC, the outlet half behind the Pd (05) catalyst remains most active and is followed by the one behind the Pt (06) catalyst. The activities retained in the outlet halves decrease with the decrease of PM loadings in the inlet halves as is shown in Figure 13 by the catalysts 01, 0 4 , and 0 3 (behind catalysts 20,10, and 0 g PM/ft3, respectively). The blank substrate provided no protection for the outlet halves in that the outlet half of the B/20 system (03) had the same activity as the inlet half of the 20/20 system (11). A 10 g/ft3 TWC provided substantial protection as can be seen by the comparison of outlet halves of the 10/20 (04) with B/20 (03) systems and the B/30 (07) with 10/30 (08) systems, respectively. With a 10 g/ft3 TWC as protection during high-temperature aging, a 20 g/ft3 TWC outlet half (04) performed better than an unprotected 30 g/ft3 TWC (07). Thus, the catalyst protection is significant. Conclusions a n d Open Issues This study showed that the performance of the outlet halves of these catalyst systems depends on the formulation of the inlet half, especially after high-temperature, lean aging. This supports the catalyst protection concept described earlier. The protection capability of the TWCs used as inlet catalysts increases with the increase of PM loading. The blank substrate offers no protection for the catalyst behind it. For COC formulations as inlet halves, Pd offers better protection for the outlet catalyst than the Pt with the same active metal loading. The overall assessment of the applicability for these eight catalyst systems depends on their cost and performance. The B/20, B/30,10/20, Pt/20, and 40/Bsystems,
although they offer some cost reduction because of the less PM usage or smaller catalyst volume, are not good alternatives to the ZO/ZO system because they have lower performance. In view of the excellent performance of the 10/30 system, some cost reduction can be achieved by lowering PM loading using this ascending PM distribution manner if equivalent performance to the reference system is required. The Pdj20 systems offers the greatest advantages. It exhibits an equivalent TWC performance and better light-off activity compared to the reference ZO/ZO catalyst system. In addition, the Pd/20 system saves 50% of the Pt and Rh usage by replacing them with cheaper and possibly domestically available Pd. Assuming the cost of Pd is one third of that of Pt, a cost reduction by as much as 33.3% can be realized due to the PM substitution. The system appears to fulfill the above-mentioned three research objectives: cost reduction, improvement in performance, and thermal resistance. Therefore, the potential for automotive application needs to be explored. Some of the possible open issues related to the applicability of the Pd/TWC system are the following. (1) Complexity and cost: there is a concern over the increase of the complexity and cost for catalyst packaging, inventory, and processing of two catalyst pieces. These problems may be reduced by the development of a low cost process to apply two formulations on the same substrate. These problems can also be minimized in vehicles equipped with a TWC light-off catalyst (mounted close to the engine) followed by another TWC or by TWC/COC system further downstream. Here, a Pd catalyst can be used to replace the TWC light-off catalyst without causing the packaging problem. In addition, it could be advantageous if field replacement of defective catalyst systems would only involve the first piece of a two-catalyst system. (2) Chemical poisoning: Pd is known to be very susceptible to lead (Ph) poisoning (Shelef et al., 1975). Misfueling with high Ph fuel may severely deactivate the Pd catalyst. Hence its ability to protect the catalyst in the outlet half may he lost. In this study the P b level in the aging fuel was 5 mg/gal, which is three times higher than that of the current certification fuel (National Fuel Survey, 1982). Thus, Ph level at 5 mg/gal or lower in fuel should not be a great concern. Other contaminants, such as zinc, phosphorus, and sulfur, which affect catalyst activity independently of PM used should not be a particular problem to the Pd/TWC system. In general, by placing a Pd catalyst in the inlet portion, the outlet catalyst with Pt and Rh can also be better protected from contaminant accumulation since contamination usually starts at the inlet portion. (3) Aging process: In this study, the catalyst systems were aged in eight-chamber reactors (each with 1.5 in. diameter and 6 in. total length) under 12 000 miles AMA cycle with and without sixty 2000 OF, 1 min-duration, lean spikes. Whether this aging method is comparable to vehicle aging for the full-size catalysts has not been verified. However, the present aging method is believed to be a reasonable way to discriminate between the thermal resistance of different catalyst systems. If the vehicle engine malfunction occurs more frequently, lasts longer than l min each time, or the temperature rises more than ZOO0 "F, the extent of catalyst damage would be more severe, and most catalysts would be deactivated regardless of its formulation. If no malfunction occurs, the results of this study aged under 12 000 mile AMA cycle would still be valid. Acknowledgment The authors wish to thank M. R. Vaughen for helping with the experimental work and G. Meguerian of Amoco
Ind. Eng. Chem. Prod.
565
Res. Dev. 1983, 22, 565-570
Oil Company for aging the catalyst on an engine dynamometer.
Registry NO. Pd, 7440-05-3;Pt,7440-06-4;Rh, 7440-16-6; CO, 630-08-0; NO,, 11104-93-1. Literature Cited Engh, 0.T.; Wallman, S. SA€ (Society of Automotive Engineers) 1977, 770295. Falk, C. D.; Mooney. J. J. SA€ 1980, 800462. Fed. Regist. 1977, 42(124), 32906. Gandhl, H. S.; Steplen, H. K.; Shelef, M. Meter. Res. Bull. 1975, IO, 837. Gandhl, H. S.; Plken, A. G.; Shelef, M.; Delosh, R. G. SA€ Trans. 1978, 85, 901; SA€ 1978, 760201. Gandhi, H. S.; Plken, A. G.; Shelef, M.; Delosh, R. G.; Steplen, H. K.; Heyde, M. E. SA€ 1977, 770196. Gandhi, H. S.; Shelef, M.; Yao, H. C.; Kummer, J. T.; Steplen, H. K. U S . Patent 4 172047, 1979. Gandhl, H. S.; Yao, H. C.; Steplen, H. K. ACS Symp. Ser. 1982, 178. 143. Hammerle, R. H.; Wu, C. H. Submitted to SAE International Congress, 1984. Hammerle, R. H.; Wu, C. H. SA€ 1981, 810275. Hegedus, L. L. Am. Chem. SOC. Div. f a t . Chem. frepr. 1973, 18, 487.
Jones, J. H.;Kummer, J. T.; Otto, K.; Shelef, M.; Weaver, E. E. Envfron. Sci. Technol. 1971, 5 , 790. Kummer, J. T.; Yao, Y.. McKee, D. SA€ 1978, 760143. Lachman, I. M.; McNelly. R. N. Preprints of Papers, 83rd Annual Meeting of American Ceramic Society, Washington, DC, 1981. Lester, G. R.; Marlnangell, R. E. SA€ 1980, 800844. Mooney. J. J.; Thompson, C. E.; Dettllng, J. C. SA€ 1977, 770365. National Fuel Survey, Motor Vehlcle Manufacturers Assoc., Detroit, 1982. Oda, I.; Matsuhlsa. T. Preprints of Papers, 83rd Annual Meeting of American Ceramic Society, Washington, DC, 1981. Oser, P. SA€ 1979, 790306. Seler, R. E.; Clark. R. J. SA€ 1978, 780203. Shelef, M.; Koester, D. W.; Mlchel, M. M., Jr., Adv. Chem. Ser. 1975, No. 143, 133. Steplen, H. K.; Wllllamson, W. B.; Gandhi. H. S. SA€ 1980, 800843. Weaver, E. E.; Shlller, J.; Plken. A. 0.AIChE Symp. Ser. 1976, 72(156), 369. Yao, H. C.; Shelef, M. J . Catal. 1978, 43, 393. Yao, H. C.; Japar, S.; Shelef, M. J . M a l . 1977, 5 0 , 407. Yao, H. C.; Wynblatt, P.; Sleg, M.; Plummer, B. K., Jr. In "Sintering Processes"; Kuczynskl, G. C., Ed.; Plenum: New York, 1980; p 561.
Received for review February 22, 1983 Accepted August 1, 1983
Oxidation of I-Butene and Butadiene to Maleic Anhydride. 1. Role of Oxygen Partial Pressure Fabrlzlo Cavanl, Gabrlele Centl, Halo Manentl, Alfred0 Rlva, and Ferrucclo Trlflrb' Instituto di Tecnologle Chlmiche Speciail, Viale Rlsorglmento, 4, 40 136 Bologna, Italy
The oxidation of 1-butene and butadiene to maleic anhydride was tested with an integral reactor with vanadiumphosphorus mixed oxides prepared by different methods and presenting different activities and selectivities. The data were analyzed by utilizing a Langmuir-Hinshelwood type rate expression derived under differential conditions. At high partial pressure, competition between 1-butene and butadiene for the same active centers leads to a decrease in the rate of formatlon of maleic anhydride. This competitlon does not affect the rates of formation of methyl vinyl ketone, acetaldehyde, and carbon oxides. Emphasis is made on the necessity of operating with oxygen-nriched air in order to avoid the negative effect of the reactant on the rate of maleic anhydride formation.
Introduction The selective oxidation of C4 fractions to maleic anhydride (MA) has been of considerable interest in recent years as an alternative process to benzene oxidation. Specific catalysts for this reaction are vanadium and phosphorus mixed oxides, as suggested in the patent literature (Hucknall, 1974; Varma and Saraf, 1979). Three types of kinetic mechanisms have been suggested in V-P mixed oxides for the selective oxidation of butenes to maleic anhydride: (i) a Rideal mechanism, where an adsorbed form of oxygen reacts with gaseous butene, proposed by Sunderland (1976); (ii) a Mars-van Krevelen mechanism, proposed by Varma and Saraf (1978) and Miiller and Baern (1980); (iii) a more complex mechanism in which, in a first stage, butadiene (BTD) is formed via a redox mechanism with structural oxygen and then, in a second stage, BTD is oxidized to MA via an adsorbed form of oxygen (Brkic and Trifirb, 1979; Morselli et al., 1982). A similar mechanism has been proposed by Trifirb et al. (1973) for the oxidation of butenes to MA on MnMoo4. The study of V-P mixed oxides prepared by different methods has revealed a common feature despite the different catalytic behavior of the oxides. This common feature is that the yield of MA decreases with increased concentration of 1-butene (1-BT). The occurrence of this phenomenon was pointed out but not explained by Sun0196-432118311222-0565$01.50/0
derland (1976) and by Varma and Saraf (1978). The objectives of the study presented in this paper were: (i) to identify this effect on three types of catalysts presenting different activities and selectivities; (ii) to explain the nature of this effect; (iii) to find the conditions under which the drop in the yield of MA can be eliminated.
Experimental Section Preparation of Catalysts. The two catalysts, hereinafter called CT A and CT B, were prepared in an aqueous medium as follows: 40.0 g of Vz05was dissolved in 0.500 L of 37% hydrochloric acid. The solution was heated for 2 h with a head condenser, then 65.4 g of 85% orthophosphoric acid for CT A and 51.3 g for CT B was added. The resulting mixtures containing 100% vanadium(1V) were evaporated and dried in air at 125 "C for 16 h. After drying, CT A was introduced directly into a furnace at 500 "C and calcined in air for 16 h. CT B was heated slowly to 370 "C in 2 h and calcined at 370 "C for 3 h in air. The CT B has no vanadium(V). In the case of CT A, the amount of vanadium(V) after calcination was 60%. Following the procedure reported by Poli et al. (1981), the catalyst was washed with water at 25 "C to reduce the amount of vanadium(V). After several washings, the resulting catalyst had 25% vanadium(V) and a P / V ratio of 1.29 (the initial P/V ratio was 1.19). The surface areas, 0 1983 American Chemical Society
