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Ind. Eng. Chem. Prod. Res. Dev. 1984, 2 3 , 531-536
suggest. Nevertheless, the results of this study have significant implications on the design of the pelleted TWCs as well as the impregnation strategy for the catalytic . metals. Acknowledgment The authors wish to thank AC Spark Plug Division of General Motors Corporation for providing samples of dynamometer-aged catalysts for this study. The authors would also like to acknowledge the contribution of R. L. for scanning Jordan in X-ray Work, A. microscopy, K. I. Halle for catalyst aging and evaluation, and N, H. Hummer and D. L. Smith for catalyst prepsration and attrition, all of W. R. Grace & Co. Registry No. Pb, 7439-92-1; Zn, 7440-66-6; P , 7723-14-0;Pt,
Conner, W. C.; Lane, A. M.; Ng, K. M.; Goldblatt, M. J. Catal. 1983.83, 336.
g:;r:i
Received for review April 6, 1984 Accepted August 3, 1984
7440-06-4; Pd, 7440-05-3; Rh, 7440-16-6; Ce02, 1306-38-3.
Literature Cited Caracciob. F.; Spearot, J. A. 1979, Paper No. 790941.
SOC.
~ u t o m t Eng. . 1976, Paper NO. 780562;
kEtkr
B & ~ ~ ~ ! ~ Pub,ishing co., Amsterdam, 1958; p 73. Hegedus, L. L. CHmrEcH 1980, IO, 630. Hegedus, L. L.; Summers, J. C. J. Catal. 1977, 48, 345. Hegedus, L. L.;Baron, K. J. Catal. 1978, 5 4 , 115. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J. Cafal. 1979, 56, 321. Kim, G. Ind. Eng. Chem. Prod. Res. ~ e v 1982, . 27, 267. Schiatter, J. C.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 288. Shelef, M.; Otto, K.; Ono, N. C. A&. Cats/. 1978, 27, 311. Summers, J. C.; Baron, K. J. Catal. 1979, 57, 380. Summers, J. C.; Hegedus, L. L. Ind. Eng. Chem. Prod, Res. D e v . 1979, 18, 318. SU"erS, J. c.; Hegedus, L. L., U S . Patent 4 152301, 1979. Yao, H. C.; Yao, Y. F. Y. J. Catal. 1984, 86, 254.
Paper presented at the 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983, INDE 44.
Durability of Palladium Automotive Catalysts: Effects of Trace Lead Levels, Exhaust Composition, and Misfueling W. Burton Wllllamson,' Dezmonla Lewls, James Perry, and Haren S. Gandhl Ford Motor Company, Research Staff, Dearborn, Michigan 48 12 1
The activity and durability of palladium (Pd) catalysts were investigated as a function of trace lead (Pb) levels in fuel, exhaust composition, and misfueling with leaded fuels to determine their potential as automotive emission three-way catalysts. I n laboratory durability studies, Pd catalysts showed significant improvement for nitric oxide (NO) and hydrocarbon (HC) conversions as trace Pb levels decreased in the range of 12 mg of Pb/gal to Pb-sterile fuel. Lean exhaust gas compositlons during catalyst aging significantly improved the performance of the Pd catalyst for NO conversions near stoichiometry, improved HC conversions during rich excursions, and substantially lowered HC l i g h t 4 temperatures. Resutts on Pd catalysts of experiments designed to mimic misfueling with Pb levels typical of commercial leaded fuels are discussed. The practical implications of these resuits toward the design of future automotive emission systems are considered.
Introduction Automotive exhaust catalysts use platinum (Pt),palladium (Pd), and rhodium (Rh) extensively because of their high intrinsic activity and durability in automotive exhaust conditions. Of these precious metals, palladium is relatively abundant, domestically available, and significantly less expensive than Pt or Rh. Since the automotive catalyst industry is the largest consumer of noble metals (Burke, 1979),the use of Pd-based catalysts is continually explored as a potential substitute for the more expensive metals. Because of the better thermal stability of Pd compared to Pt (Klimisch et al., 1975),Pd-based catalysts could be mounted, in the absence of P b poisoning, closer to the engine's exhaust manifold where exposure to higher temperatures occurs. The usage and poisoning of noble metals in automotive catalysts has been covered extensively in recent reviews by Kummer (1980),Taylor (1984), Shelef and co-workers (1978), and Hegedus and McCabe (1981). Conventional oxidation catalysts (COCs) contain Pt and Pd for the oxidation of hydrocarbons (HC) and carbon monoxide (CO). Pd has a higher specific activity than Pt for the oxidation of CO and olefinic hydrocarbons (Yao, 1975). However, Pt has a higher specific activity for the difficult-to-oxidize, 0 198-432 118411223-053 1$0 1.5010
-
paraffinic hydrocarbons which comprise 20-3070 of the total HC (Kummer, 1980). Three-way catalysts (TWCs) use Rh for the removal of nitric oxide (NO) due to its good activity and selectivity for the NO to N2 reaction (Kobylinski and Taylor, 1974), as well as good activity for CO and the easy-to-oxidize unsaturated HC. The deactivation of Pd catalysts by lead (Pb) is more severe than for Pt (Klimisch et al., 1975). Also, while poisoning by the ethylene dibromide (EDB) scavenger in tetraethyllead (TEL) was found reversible when the halide was completely removed, Pd-containing catalysts were more severely deactivated by EDB than Pt-only catalysts (Barnes et al., 1974; Otto and Montreuil, 1976). Under high temperature (900 "C) oxidizing conditions, Pd disperses as an oxide with considerable thermal stability on alumina catalysb (Klimisch et al., 1975),whereas Pt sinters substantially under similar conditions (Yao et al., 1979). The effect of lead on Pd differed from that on Pt in Auger studies on foils (Williams and Baron, 1975) where Pb was present on the surface of Pt, but absent on the Pd surface. Under oxidizing conditions these results were explained by the diffusion of P b ions into the oxidized Pd surface layer with the formation of a solid solution of Pb and Pd oxides. In overall lean exhaust gas compositions with 0 1984 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 4, 1984
slightly rich modulations, the formation of PbPds has been detected (Gandhi et al., 1984). Thus, Pd is susceptible to poisoning by P b due to interactions between surface Pd species and lead resulting in an inactive state. The deactivation of Pt-Pd oxidation catalysts decreases as the fuel P b levels decrease (Shelef et al., 1973, 1978). Current field audit data indicate average contaminant levels of 3 mg of Pb/gal in “unleaded” gasoline marketed across the nation with a range of 1 represents an overall reducing gas mixture.
Results and Discussion Susceptibility of Pd TWCs to Trace Pb Levels. The effects of trace P b fuel levels (0-12 mg of Pb/gal) on the performance of a Pd TWC formulation are reflected by catalyst activities following -15000 simulated miles (500 h) of pulsator aging at slightly rich conditions (R = 1.3) representative of TWC operation. Decreasing P b levels from 12 to 0 mg of Pb/gal improved NO conversions substantially as shown by steady-state conversions at 550 “C (Figure 1)as a function of the exhaust redox ratio. Peak NO conversions near R = 1.0 increased from 32% with 12 mg of Pb/gal to 68% NO conversion with Pb-sterile isooctane fuel. The 4 mg of Pb/gal fuel, which most closely resembles P b levels in unleaded gasolines currently marketed across the nation, had 51% maximum NO conversion. The values in Figure 1 represent gross NO conversions. Ammonia formation increased to -30% of the NO converted at R = 1.9, but was not significantly different in all cases, resulting in slightly lower net NO conversions during rich operation. Decreasing trace P b levels also improved the HC oxidation activity of the Pd TWC as shown by the steadystate conversions in Figure 2. Hydrocarbon conversions rich of stoichiometry at R = 1.8 improved from 66% with 12 mg of Pb/gal to 84% conversion with the Pb-sterile fuel. With 4 mg of Pb/gal of fuel the Pd TWC retained 75% HC conversion at R = 1.8. Near stoichiometry ( R = 1.0) HC conversions converged over the catalysts aged with the various Pb levels resulting in only slight poisoning differences for operation near stoichiometry and lean of stoichiometry. Carbon monoxide over the Pd TWC was least affected at these trace P b levels but improved slightly with increased Pb levels, as shown in Figure 2. CO conversions rich of stoichiometry improved in small increments as Pb levels increased from 0 to 12 mg of Pb/gal and fell within the shaded area of Figure 2. As noted previously for Pt-Pd COCs (Otto and Montreuil, 1976; Shelef et al., 1973) and Pt-Rh TWCs (Williamson, Stepien et al., 1979), the oxidation of hydrocarbons is a more demanding reaction and poisoning by P b affects the HC oxidation activity more
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 533 Table I. Effect of Pb on t h e Activity of Pulsator-Aged 0.22% Pd TWC % conversion
fuel Pbpulsator (500 "C) level,(' simulated = 1.15 mg of miles Pb/gal (X1000) NO CO HC
cat. (aging)
0 1 4 6 12
(1)Pd (rich)
( 2 ) Pd (lean)c ( 3 ) P t - R h d (rich)b
6 6
16.3 15.3 15.8 15.6 4.6 16.4 15.6 15.6
70 68
49 52
72 75
50 49 39 66 60
52 42 40 54 59
71 74 58 75 65
-
-
-
steadv-state f550"C)
= 1.00
R
R = 1.80
net
grosslne t
NO
CO
HC
NO
CO
HC
68 59 51 42 58 32 61 94
92 92 95 97 91 96 98 98
94 91 90 90 91 92 96 94
55/40 48/35 39/25 32/24 52/43 24/17 35/22 97/80
28 35 27 31 27 31 26 40
84 80 75 69 66 66 77 54
T(80% C), "C 365 4 20
-
490
-
>550 385 280
a Isooctane fuel containing 0.8 mg of P t 0.03 wt % S/gal and the indicated levels of Pb from "TEL Motor Mix" containing tetraethyllead, ethylene dichloride, and ethylene dibromide in an atomic ratio of Pb:Cl:Br of 1:2:1. Rich aging at R = 1 . 3 . Lean aging at R = 0.7. Pt-Rh TWC: 0.21% Pt t 0.02% Rh. 0.22 % Pd CATALYST
0.22 % Pd CATALYST R : 1.0 S.V=60,000 h-l 100
-1
1 S.V.60.000 T~ 5 5 0 *C
100
BO
BO
E 5 60
3 z 2 60
D
w L
>
8
"
B)
8 40
40
? 20
20
0
0 200
100
300
400
500
600
100
200
300
400
500
600
TEMPERATURE ('C)
2o
t-
1
0
0.3
1.0
1.2
1.4 1.6 REDOX RATIO, R
1.8
2.0
Figure 2. Effect of trace Pb levels on the steady-state HC and CO activity of 0.22% Pd after -15000 simulated miles of pulsator aging at R = 1.3.
+
severely. For rich operation (R > 1)where CO HC I 02,oxygen is partitioned between HC and CO. Any decrease in HC conversion may be compensated by an equivalent enhancement in CO oxidation. Lighboff temperatures for the Pd catalysts pulsator-aged at different P b levels are shown in Figure 3a for HC and in Figure 3b for NO conversion a t stoichiometry. Since the HC feedgas for these steady-state conversions contained 65% propylene and 35% propane, HC conversions up to 65% represent conversion of the easy-to-oxidize unsaturated HC. For the range of 0 to 1 2 mg of Pb/gal the temperature required for this oxidation reaction increases only -30 "C. However, for oxidation of saturated HC the P b levels become increasingly important as higher temperatures are required to achieve similar conversions. For example, temperatures required for 80% HC conversion were lowered from >550 OC with 12 mg of Pb/gal to 420 "C with 1 mg of Pb/gal to 365 "C with Pb-free fuel. (Light-off temperature data for the catalyst aged with 4 mg of Pb/gal was taken in a slightly leaner exhaust mixture and are not included here.) Above 400 OC NO conversions were significantly improved at the lower P b levels (Figure 3b). During steady-state measurements the 0.22% Pd catalyst appeared to deactivate rapidly as far as poisoning of the saturated HC was concerned. The poisoning effect of
Figure 3. Effect of trace Pb levels on the stoichiometric light-off temperature of 0.22% Pd after pulsator aging for -15000 simulated miles for (a) HC and (b) NO conversions. 0 22 % Pd CATALYST 100
80
-zz
I
P v)
60
W K
5 40 8 20 0 0 REDOX RATIO, R
Figure 4. Poisoning effect of 12 mg/gal Pb levels on the steadystate conversions of NO, CO, and HC over 0.22% Pd after pulsator aging for 4600 simulated miles.
12 mg of Pb/gal on the steady-state 3-way conversions of the Pd TWC after 4600 simulated miles (153 h) is shown in Figure 4. The HC activity shown in Figure 4 is identical with that shown in Figure 2 for 12 mg of Pb/gal after a 3-fold longer aging period. At R = 1.8 the HC conversion is 66% in both cases, indicating that all of the unsaturated HC feedgas (65% of the total) is being oxidized, whereas the catalyst is poisoned for saturated HC oxidation after only 4.6K miles with 12 mg of Pb/gal. However, during pulsator modulation measurements,the Pd catalyst appeared to deactivate continuously for HC oxidation with increasing mileage. Three-way conversions of the pulsator-aged Pd catalysts after rich aging with the
534
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 4, 1984 Pd TWC PULSATOR-AGED LEAN (R=0.7)
Pd TWC AFTER LEAN AGING f R a 0 . 7 )
iG - Lo2
u
40 20 0
5
IO
15
20
kJ1 Ad%h'I NH3(aa X NO)
00 3
IO
Figure 5. Effect of lean exhaust gas compositions (R = 0.7) during pulsator durability aging of 0.22% Pd with 6 mg of Pb/gal fuel.
various P b levels are summarized in Table I for pulsator modulation conditions a t 500 "C and steady-state conditions at 550 "C. During pulsator modulation conditions at a slightly rich set point of R = 1.15, the Pd catalyst would be exposed to an alternating rich and lean environment. Measurements under these conditions indicated 74% HC conversion at 4.6K with 12 mg of Pb/gal fuel with further deactivation to 58% HC conversion after 16.4K simulated miles. The NO activity over the Pd TWC showed a continuous decrease with increasing accumulated mileage during steady-state and modulation measurements (Figure 4 and Table I). The pulsator measurements (Table I) show progressive deactivation for NO as P b levels are increased. The NO activity of the Pd TWC is severely depressed with additional aging (from 4.6K to 16K at 12 mg of P b level/gal), as shown by the dashed line in Figure 4 (from Figure 1 for comparison) and by the pulsator conversions of Table I. The CO activity was again least affected by increased exposure of the Pd TWC to P b and the steady-state activity falls within the shaded area of Figure 2. Beneficial Effects of Aging Pd TWCs under Slightly Lean Conditions. The effects of lean exhaust gas compositions during catalyst aging were studied by using fuel containing 6 mg of Pb/gal, the same cycling temperatures (730 "C maximum for 6% t), but using a lean aging set point (R = 0.7) with -0.8-1% excess oxygen. The pulsator durability of Pd aged at R = 0.7 for 15600 simulated miles is shown in Figure 5. The catalyst was evaluated at 500 "C and at slightly rich conditions ( R = 1.15) for comparison with the Pd TWC aged at R = 1.3 (shown by the dashed line for NO conversion). A significant difference from rich aging was that only a slight deterioration from initial NO conversions was observed for lean aging with the catalyst retaining 66% NO conversion compared to 50% conversion when aged rich. HC conversion during modulation conditions was -4% higher and CO conversions remained unchanged when Pd was aged under lean conditions (Table I). Three-way steady-state conversions following pulsator aging are shown in Figure 6 and compared to Pd aged at R = 1.3 (shown by the dashed lines for NO and HC). The peak NO conversion near R = 1.0 is 19% higher and the HC conversions during rich operation are 8-10% higher when the Pd TWC was aged at R = 0.7. CO conversions were only slightly affected. Light-off conversions near stoichiometry are shown in Figure 7 after lean aging of the Pd TWC and again compared to a Pd TWC when aged rich (shown by the dashed
12
14
16
18
20
REDOX RATIO, R
M I L E S X10-3 (K1nX16~/1.6)
Figure 6. Effect of lean exhaust gas compositions (R = 0.7) during catalyst aging on the steady-state conversions of 0.22% Pd after 15 600 simulated miles. Pd TWC AFTER LEAN AGING (13.07)
IO0
-1
III I
lGED R'Cn
I
20
0
I00
20%
I
1
300 400 500 TEMPERATURE ('C)
Li
600
Figure 7. Effect of lean exhaust gas compositions during catalyst aging on the light-off temperature of 0.22% Pd after 15600 simulated miles.
lines). The HC light-off curves from 0 to -70% conversion are similar and correspond to the oxidation of propylene which is 65% of the total HC. However, 80% HC conversion, which includes oxidation of some saturated HC, occurs at 490 "C over Pd aged at R = 1.3, but only requires 385 "C over the Pd aged at R = 0.7. The pulsator aging and steady-state measurements are summarized in Table I for rich aging at R = 1.3 and lean aging at R = 0.7, and are compared to a standard production Pt-Rh TWC containing 0.21% P t 0.02% Rh. Pulsator conversions of the lean-aged Pd exceed those of the Pt-Rh TWC (aged rich) for NO and HC, but not for NO conversion during steady-state conditions. Also, the Pt-containing TWC achieves 80% HC conversion light-off at lower temperatures due to its inherently good saturated HC activity. Lean aging at R = 0.7 and rich aging at R = 1.3 resulted in similar amounts of chemical poisoning of the Pd TWCs after 15K miles. Both Pd TWCs retained 0.06 wt % Pb and 0.04-0.05% P. Rich aging resulted in 0.03 wt 7'0 S retention while no S was detected on the lean-aged Pd TWC. Since Pb retention was the same in both cases, the improved catalyst performance could be attributed to higher Pd dispersion and metal area retained by the catalyst during the leaner aging conditions at peak temperatures of 730 "C. Chemisorption experiments to substantiate this possibility were not performed, however. Investigations in this laboratory into the affinity of Pb for noble metals on different supports also indicated the formation of a PbPda alloy which was detected during reducing conditions (Gandhi et al., 1984). Aging of the Pd
+
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 535 PULSATOR AGING
SOL' I
I
I
I
I 1 I I
,
OF 0 22 % Pd CATALYST I , , I I I I I I , 1 ' 3
MlSFUELlNG OF 0.22 '10 Pd CATALYST
I
1
co
I
I
1
I
S.Vm60.000 h'
z 80 0
8
8
60
40
$
20
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0v * ' 0.8 1.0
40 20
0
5 MIL E S
IO X
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20
25
NO
. ~ -; ; ~-p
1.2 1.4 1.6 1.8 2.0 REDOX RATIO, R Figure 9. Steady-state activity of pulsator-aged 0.22% Pd after misfueling with leaded fuel followed by attempted recovery with 6 mg of Pb/gal fuel (24.6K total miles).
/ 1.6) 0.22 X Pd CATALYST
Figure 8. Effect of misfueling with 1.58 g/gal Pb levels on the pulsator durability of 0.22% Pd compared to a 0.21% Pt + 0.02% Rh TWC.
TWC under slightly lean conditions would presumably maintain a higher metal dispersion and prevent Pb-Pd alloy formation. Detrimental Effects of Misfueling. The severe effects resulting from misfueling a Pd-based catalyst with a high Pb-containing fuel are shown on the right side of Figure 8. After -15K pulsator miles using simulated certification fuel containing 6 mg of Pb/gal, stabilization of the Pd TWC occurred with the indicated conversions (conversionsvary slightly compared to those in Table I due to different samples). A simulated leaded fuel containing 1.58 g of Pb/gal was then introduced to mimic emergency or deliberate misfueling for -4 consecutive tankfuls (1000 miles) of misfueling, which is a severe case of misfueling (shown by the open symbols in Figure 8). A simulated single tank of misfueling shows correspondingly less deactivation. The results after 1000 miles indicate a severe reduction of activity. For the Pd catalyst the stabilized NO activity of 52% conversion decreased to 6%, the CO activity decreased from 45 to 0% , and the HC activity decreased from 68 to 24% conversion. Upon reverting to 6 mg of Pb/gal levels, only the CO activity at 500 "C was fully restored after -3000 additional miles. The HC activity was partially recovered, whereas the recovery of NO activity was minimal. Superimposed on the 3-way conversions of the Pd TWC in Figure 8 are results reported previously (Williamson, Gandhi et al., 1979) showing the detrimental misfueling effects using fuel with the same P b content with a Pt-Rh TWC. After 29000 simulated miles the Pt-Rh TWC still maintained slightly higher NO conversion due to inherent NO activity of Rh, but had slightly lower HC activity. After -1000 simulated miles with the leaded fuel, reverting to 6 mg of Pb/gal fuel completely restored CO activity and partially restored NO and HC activities. The results of Figure 8 illustrate that while Pb poisoning of Pd and Pt-Rh TWCs for CO oxidation is reversible, significant irreversible poisoning for HC and NO activities occurs. Over the Pt-Rh TWC the ratio of unreacted NO after misfueling and reverting to 6 mg of Pb/gal levels for -6000 miles compared to that before misfueling is 60/40 = 1.5, while the deterioration factor for the Pd catalyst is 90148 = 1.9. Corresponding deterioration factors for HC oxidation are 1.4 (62/44) for the Pt-Rh TWC and 1.7 (54/32) for the Pd TWC.
-
7 - - - - - - -(BEFORE)
I
005
002
1
1 1 1 1 1 1 (
01 ut % Pb ON CATALYST
IO
20
Figure 10. Effect of Pb retention on the steady-state HC and NO activity of pulsator-aged Pd catalysts.
Steady-state conversions of the Pd catalyst following exposure to the leaded fuel for -1000 miles and then reverting to simulated certification fuel containing 6 mg of Pb/gal are shown in Figure 9 after 24.6K of total simulated miles. For comparison the dashed lines (from Figures 1 and 2) indicate typical HC and NO activity of the Pd catalyst at 15K miles before misfueling occurred. These data also reflect the substantial decrease in HC activity (accompanied by a slight increase in CO oxidation activity) and drastic decrease and irreversibility of the NO activity with P b poisoning. These results for HC and CO activities parallel the effects observed over Pt-Pd oxidation catalysts (Otto and Montreuil, 1976). The greater deterioration of NO activity might also have been expected due to the higher susceptibility of Pd-based catalysts to P b poisoning when compared to Pt-based catalysts (Klimisch et al., 1975). The deterioration of Pd activity for NO and HC in the present work indicates that Pd-based catalysts are also more sensitive to P b misfueling than Pt-based TWCs. Vehicle misfueling should become less frequent due to the decreasing usage of leaded fuels (predicted to be 20% in 1985 and
