Ind. Eng. Chem. F+rcd.Res.
effectively to expand their thermal comfort zone. Literature Cited American Society of Heating, Refrigerating, and AlrCondkionlng Engineers, Inc., “ASHRAE Standard 55-74R-Thermai Environmental Condltlons for Human Occupancy”, ASHRAE: New York, Apr 25, 1980. Azer, N. Z. ASHRAE Trans. 1978, 82(I), 87-106. Bergland, L. G.; Oonzalez, R. R. ASHRAE Trans. 1978, 84(II), 110-114. Fanger, P. 0. Ann. &cup. Hyg. 1977, 20, 285-291. Fanger, P. 0. “Thermal Comfort-Analysis and Application in Environmental Engineering”, McQraw-Hill: New York, 1973. Gagge, A. P.; Burton, A. C.; Bazett, H. C. Science 1941, 94, 428-430. Gagge, A. P.. Nevins, R. G. “Effect of Energy Conservation GuMellnes on Comfort, Acceptability, and Heath”, Report prepared for the Federal Energy Office, Washington, D.C., by the John B. Pierce Foundation Laboratory, New Haven, CN, Mar 1976. Gagge, A. P.; Stoiwijk, J. A. J.; Nlshl, Y. ASHRAE Trans. 1971, 77(I), 247-262. Nishl, Y.; Gonzalez, R.; Nevins, R. G.; Gagge, A. P. ASHRAE Trans. 1978, 82(II), 248-259. Goidman, R. F. “Clothing Design for Comfort and Work Performance in Extreme Thermal Environments”, Third Shirley International Seminar on Textiles for Comfort, Manchester, England, June 15-17, 1973. Goldman, R. F.; Bergland, L.; Rohles, F. H. ”Human Factors in Dynamic Control of Environmental Condition”. Workshop on Dynamic Response of Environmental Control Process in Buildings, Purdue University, Lafayette, IN, Mar 13-15, 1979, pp 32-48. Gonzalez, R. R.; Nlshl, Y. ASHRAE Trans. 1978, 82(I), 76-86. Hollies, N. R. S.;Goldman, R. F., Ed.; “Clothing Comfort”, Ann Arbor Science Publishers, Inc.: Ann Arbor, MI, 1977.
Dev. 1981, 20, 23-31
23
McIntyre, D. A., Grlffiths, I. D. Ergonomics 1975, 78(II), 205-211. Nevins, R. G. 6uM. Res. Jul-Aq 1988, 27-30. Nlshi, Y.; Oonzalez, R. R.; Gagge, A. P. ASI-IRAE Trans. 1975, 87(II), 183-1 99. Nishi, Y.; Oonzalez, R. R.; Nevins, R. 0.;Gagge, A. P. ASHRAE Trans. 1978, 82(II), 248-259. Rohles, F. H.; Hayter, R. B.; Miliken, G. A. ASHRAE Trans. 1975, 87(II), 148- 156. Rohles, F. H.; Skipton, D. E.; Milliken, G. A.; Krstlc, I. ASHRAE Trans. 1980, 86(II). Rohles, F. H.; Wells, W. V. ASHRAE Trans. 1977, 83(II),21-29. Rohles, F. H.; Woods, J. E.; Nevins, R. G. ASHRAE Trans. 1973, 7S(II), 71-80. McNall, P. E.; Munson, D. M.; Sprague, C. H. ASHRAE Trans. Seppanen, 0.; 1972, 78(I), 120-130. Sprague, C. H.; Munson, D. M. ASHRAE Trans. 1974, 80(I). 120-129. US. Department of Energy, “How to Comply with Emergency Building Temperature Restrictions”, U.S. Department of Energy, Washington, D.C., July, 1979. Winslow, C . I . A.; Gagge, A. P.; Herrington, L. P. Am. J. physbl. 1940, 737, 79. Woodcock, A. H. Text. Res. J . 1982, 32, 628-723.
Received for reuiew April 16, 1980 Accepted August 22,1980
Paper presented at the 179th National Meeting of the American Chemical Society,Cellulose,Paper, & Textile Division Symposium on Textile Comfort, Houston, TX, Mar 26, 1980.
111. Symposium on Automobile Exhaust Catalysis L. L. Hegedus and W. K. Hall, Chairmen 178th National Meeting of the American Chemical Society Washington, D.C., September 1979 (Continued from September 1980 issue)
Design Factors of Dual Bed Catalysts Jack C. Summers and Davld R. Monroe’ Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
Dual bed catalysts offer one means of meeting increasingly stringent automobile emission standards. The work presented herein is an investigation into the effect of various design parameters on the performance of dual bed systems. For front bed catalysts in which Rh is very active, neither Pt nor Pd adds to the warmed up CO or HC conversions. Pt and W do affect the NO conversion activii of Rh, deteriorating rich and enhancing lean conversions. Pt and Pd assist Rh for lightoff. The three-way durability performance of Pt and Pd Is poorer than that of Rh. The main function of the rear bed of a dual bed converter is to oxidize the CO and HC emitted from the front bed under warmed up operation. Increasingthe noble metal loading of the rear bed increases the warmed up durability of the dual bed catalyst. Poisons are unequally distributed with P and Pb collecting heaviest on the front bed and S heaviest on the rear.
Introduction The simultaneous control of nitrogen oxides, carbon monoxide, and hydrocarbon emissions from automobile exhaust can be achieved by the use of complex three-way catalysts, in connection with an O2sensor and closed loop electronic circuitry (Canale et al., 1978;Engh and Wallman, 1978). This &fuel ratio (A/F) control system is required to keep the exhaust composition near the stoichiometric point where simultaneous conversions of these emissions can be attained. Two alternate approaches have been developed around the closed-loop A/F control system; both represent a compromise between NO and CO control. The single bed (or three-way) approach uses one catalyst bed filled with a three-way catalyst (Ghandi et al., 1976;He-
gedus et al., 1979),operating in a near stoichiometric exhaust. The dual bed approach, which is the subject of this paper, employs a smaller volume of three-way catalyst in the front bed followed by an oxidation catalyst. Air is injected between the two catalyst beds. For the same total volume, the single bed approach favors NO conversion while the dual bed approach favors CO conversion. Dual bed catalysis, like three-way catalysis, requires the use of an O2sensor and closed loop electronic circuitry. During warmed up operation, the exhaust entering the first bed is maintained near the stoichiometricpoint, while air is pumped into the second or oxidizing bed in order to oxidize the CO and HC that were not converted in the front bed. During the warm-up portion of the operation, the system
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
Table I. Dual Bed Test Cell Poisoning Experiments front bed (3-way bed)a
rear bed (oxidizing bed)
Pt
Rh
Pd
Pt
Pd
location loading
location loading
location loading
location loading
location loading
0-78 um 0.006 toz 0-78 bm 0.006 toz 0-78 pm 0.006 toz 0-78 pm 0.006 toz 0-78 pm 0.006 toz 0-78 pm 0.006 toz 0-78 pm 0.006 toz 0-78 pm 0.006 toz 0-124 pm 0.006 toz
0-78 um 0.002 toz 0-78 h m 0.002 toz 0-78 pm 0.002 toz 0-78 pm 0.002 toz 0-78 pm 0.002 toz 0-78 pm 0.002 toz 0-78 pm 0.002 toz 0-78 pm 0.002 toz 0-124 um 0.014 toz
0-100 pm 0.005 toz 0-100 pm 0.030 toz
0-100 pm 0.030 toz 0-100 pm 0.030 toz 0-100 pm 0.030 toz
0-75 um 0.002 toz 0-75 pm 0.005 toz 24-233 pm 0.002 toz 0-75 pm 0.002 toz 0-75 pm 0.002 toz 0-75 pm 0.002 toz 24-233 pm 0.002 toz 0-75 pm 0.002 toz 0-75 pm 0.002toz
Front bed volume: 1450 cm3.
0-84 pm 0.011 toz 0-84 pm 0.011 toz
Rear bed volume: 750 cm3.
will run open-loop until the O2sensor reaches its operating temperature and can begin controlling the exhaust composition. In open loop operation, the air pump will supply air to the exhaust ahead of the converter in order to provide a highly oxidizing exhaust to facilitate lightoff performance. When the sensor warms up, the air will be diverted through a plenum to the rear bed only. The first questions to be answered when considering the design and operation of such a complex catalytic system are what noble metals should be used, and how should they be distributed between the two catalyst beds? In light of current information concerning the roles of various metals for both three-way and oxidation catalysis,Pt, Pd, and Rh appear to be the most attractive candidates for dual bed catalytic use (Meguerian et al., 1972). Since nitrogen oxide emissions are to be controlled in the front bed, Rh must be placed there (Cooper et al., 1977): any Rh placed in the rear bed would soon be oxidized and rendered catalytically inactive (Yao et al., 1977). What distribution of Pt and Pd should be made between the two beds is a more difficult question to answer. With a fixed quantity of these metals available per vehicle, prior experience with three-way and oxidation catalysis is not directly applicable to answer this question. Consequently, experimentation is necessary. This study, therefore, was undertaken in an attempt to develop guidelines for determining the optimum distribution of noble metals between the front and rear beds of a dual bed catalyst system. Our work was directed toward clarifying the roles of the noble metals for warmed-up performance and for lightoff, and determining the relative importance of the front and rear beds for lightoff. To accomplish these objectives, poisoning experiments were conducted with single noble metal component catalysts and on a well-controlled series of nine dual bed catalysts which was designed, prepared, and poisoned in accelerated engine-dynamometer poisoning tests. The CO conversion-temperature characteristics were obtained for fresh, thermally aged, and poisoned Pt/A1203and Pd/A1203catalysts at variable CO and O2 levels in a laboratory reactor system. These results are applicable to vehicle systems; however, caution must be exercised, since in vehicles the catalysts are subjected to simultaneous poisoning and thermal damage. Experimental Section Catalyst Preparation. Three sets of catalysts were prepared. One set was used to study dual bed designs, the second was employed in preliminary poisoning experiments to determine the roles and durability of the individual metals, and the third was used to study lightoff. All the catalysts were supported on nominal 0.32 cm diameter spherical A1203pellets. The support was a highly porous
(1.03 cm3/g), high surface area (110 m2/g) &alumina with a diffusivity of 0.0227 cm2/s (transient pulse technique, 20 "C, 1 atm) (Chou and Hegedus, 1978). The catalysts studied in the dual bed poisoning experiments are characterized in Table I. With the exception of the last catalyst in this series, the composition and metal profiles of the rear bed were kept constant. For those front bed catalysts in which Rh was located at the surface of the pellet, the Rh was placed on the support by impregnation with an aqueous solution of (NH&RhCl, (pH 1.7-1.8). The catalysts were air dried at ambient conditions and then calcined in air at 500 "C for 4 h. The remaining noble metals were then placed on the support as a chloride (H2PtC16or PdC12) from aqueous solutions. They were then dried in air at ambient temperatures and calcined at 500 "C for 4 h. The subsurface impregnation of Rh was accomplished by the use of HF as a site blocking agent (Hegedus et al., 1978, 1979). The rear bed catalysts were made by coimpregnating aqueous solutions of HC1, H2PtC1,, and PdClz onto the support, air drying at ambient temperatures, and then calcining at 500 "C for 4 h. The three-way and lightoff Catalysts were prepared by similar techniques. The Pt and Pd impregnation profiles were determined by the SnClz staining method (Michalko, 1966), and the Rh impregnation profiles were determined by ion microprobe mass analysis (Hegedus et al., 1979). The noble metal contents and impregnation profiles of the catalysts used in the three-way poisoning and lightoff studies are listed in Tables I1 and 111, respectively. The noble metal loadings of the three-way and dual bed catalysts roughly represent the loadings at current Pt usage levels. The dual bed catalysts are designated by the following terminology: [Rh-Pt + Pt-Pd]. The Rh-Pt term refers to the front bed catalyst while the Pt-Pd term refers to the rear bed catalyst. A subscripted lo refers to low relative metal loading; a subscripted sub indicates the metal was placed subsurface. Accelerated Poisoning Experiments Three-way Poisoning Experiments. A 5.7-L V-8 engine (without EGR), operated at 1800 rpm and 47 kPa manifold vacuum, was used for these experiments. During the aging test, the A/F was continuously changed from 14.0 to 15.0 (in 20 s) and back (in 20 s), by manipulating the choke plate with an electric motor. The activity scans were made by setting the A/F to selected values and measuring the CO, NO, and HC conversions at these A/F values. The catalysts were placed in a four-tray lW-cm3 (Hegedus and Baron, 1978) converter which operated at a space velocity of 130OOO h-' (STP). The converter inlet temperature was maintained between 560 and 575 "C. The fuel was doped with P (0.014 g of P/L added as tricreysol
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Ind. Eng. Chem. pmd. Res. ~ev..vol. 20, No. 1, 1981 25
Table 11. Characterization ofPt/Al,O,, Pd/AI,O, and Rh/AI,O, Catalysts Employed in the Three-way Poisoning Study Rh Pd Pt metal loading, wt % metal penetration, r m poison pickup" P, wt % Pb, wt %
fresh
poisoned
fresh
poisoned
fresh
poisoned
0.028 39 f I
0.028 39 f I
0.016 46f 8
0.016 46 i 8
0.0028 81
0.0028 87
0.21 0.06 0.07
s, wt 7%
poison penetration' P, um Ph, r m 8,r m a
14
