Influence of Tetraethyllead and Lead Scavengers on Oxidation of Carbon Monoxide and Hydrocarbons Over Pt and Pd K. Otto* and C. N. Montreuil Research Staff, Ford Motor'Co., Dearborn, Mich. 48121
w The separate and combined effects of T E L and the halide compounds, ethylene dibromide and ethylene dichloride, on Pt and P d oxidation catalysts were investigated in the laboratory. Automobile exhaust was simulated by controlled combustion of isooctane. Of the two scavengers, only ethylene dibromide decreases catalyst activity. Deactivation is especially severe for catalysts containing Pd. Pure isooctane restores activity completely. Scavengers alone do not enhance shrinkage of the noble metal surface. T E L causes permanent deactivation and irreversible loss of the Pt-Pd surface area. Total lead deposit, residual noble metal surface, and catalyst activity are not a simple function of the lead deposit, but depend on the combination of the additives.
Catalysts for the removal of air pollutants have become a reality on most of the 1975 model cars sold in the USA. There are two main causes that shorten the useful life span of catalytic converters by decreasing their exposed noble metal surface. Such a decrease can be caused by excessive temperatures or by ubiquitous contaminants in the exhaust-e.g., Pb, P, S, Zn, and Fe. It has been well established that a fully leaded gasoline will deactivate catalysts based on Pt or P d very quickly. However, recently, questions have been raised ( 1 ) as to what extent catalyst deterioration is caused by lead, and to what extent it is due to lead scavengers, such as ethylene dibromide and ethylene dichloride, added to gasoline along with the lead compound-usually tetraethyllead. This controversy prompted the present investigation concerning the effects of TEL, EDB, and EDC, separately and combined, on the activity of monolithic noble metal catalysts. A laboratory method was chosen to guarantee a satisfactory control of experimental conditions and the absence of extraneous poisons.
the virgin catalyst. The additive was then introduced and the change in the activity followed for about 10 000 simulated miles. Afterward an attempt was made to reactivate the catalyst by switching back to pure isooctane. The experiment was completed by reexposing the catalyst to the investigated additive. Occasional measurement of the noble metal surface area provided additional information of the catalyst condition. The Pt-Pd area was determined by the adsorption of CO a t room temperature, after cleaning of the surface in air a t 660 O F (350 OC). Carbon monoxide was flowed over the sample in a carrier gas and the adsorbed CO evaluated by a gas chromatograph (3). The unit used here, 1 pmol of CO adsorbed per gram of catalyst, is equivalent to a combined surface area of Pt plus P d of 0.05 m2/g. After completion of the poisoning experiments, the lead' deposit was analyzed by x-ray fluorescence. Catalyst specimens of the type PTX-IIB, containing a mixture of Pt and P d were employed in most of the experiments, as the same catalyst formulation was presumably used in the investigation by Teague ( I ) .
Experimental Results Evaluation of PTX-IIB Catalyst. With pure isooctane there exists practically no deterioration of both the HC and the CO activity over a t least 12 000 simulated miles. Figure 1 shows that conversion a t 930 O F (500 "C), taken as an activity index, decreased by less than 3% in the case of HC and remained constant in the case of CO. Conversion of HC and CO as a function of temperature did not show any significant change with increasing mileage either, as shown by the examples a t 0, 1410, and 11 500 simulated miles, which are also plotted in Figure 1. The HC activity increased almost linearly after light-off, up to approximately 1100 O F (600 "C), while CO conversion reached a maximum of 97% a t a relatively low temperature of 750 O F (400 "C). In spite
Experimental Procedure Automotive exhaust was simulated by the combustion of isooctane, free of S and P, in an adiabatic flame pulse. The flame pulse has been described previously ( 2 ) .The exhaust contained 1.1% CO, 650 ppm hydrocarbon (measured as hexane), 150 ppm NO, and 3.5% 0 2 with corresponding amounts of COn and HzO. The gas mixture was passed over a small button of the catalyst monolith (diameter 1.9 cm, length 1.2 cm) a t an average space velocity of 40,000 h-l. The catalyst was subjected to temperature cycling between 700° and 1400 OF (370O and 760 "C). The poisons to be studied, tetraethyllead (TEL, neat lead), ethylene dibromide (EDB, 1,2-dibromoethane), and ethylene dichloride (EDC, 1,2-dichloroethane) were introduced into pure isooctane (2,2,4-trimethylpentane). The concentrations were selected to represent a "TEL Motor Mix" level of 0.5 g Pb/gal, with an atom ratio of Pb:Cl:Br = 1:2:1. These scavenger concentrations relative to the lead level are sometimes referred to as 1.0 and 0.5 theories of EDC and EDB, respectively. The experiments started with the exposure of each catalyst sample t o the combustion products of pure isooctane for about 1500 simulated miles to establish the activity of 154
Environmental Science & Technology
200 400
200 400
6C4
0 mi
100
*
A
200
600
400
1410mi -
y
n
.
600
IlSOO mi n
.
,
,
co
-
20ISOOCTANE
*
loo;92
752
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o l o 392 m 752 1112 l 392: 752 1112 : 392 r 752 1112 ~ 1~~~~392 752 1112O F
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20
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-0
200 400 600
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I
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"C 200 400 600
60
.60
60-
4 0 1 4 401JI
200 400 600
20
200 400 600
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20
200 400 600
13560 m i
16470 mi
OC
200 400 600 23070 mi
17940 mi
I
I
/I
1
1
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10.000 15.000 SIMULATED MILES '
, ,
,
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,
1
,
, , , ,
I
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25,000
SIMULATED MILES
25.000
Figure 3. Effect of TEL on PTX-IIB activity
Figure 2. Effect of EDB on PTX-IIB activity
of the unchanging conversions with mileage, a drastic decrease of the noble metal surface was observed at the same time. The metal surface of a virgin catalyst adsorbs approximately 2.6 pmol of CO per gram of catalyst, but a fast decrease to about 1.8 pmol/g took place, once the catalyst had been heated to 1400 OF (760 "C) in the combustion products of isooctane. When the catalyst was aged further, the metal surface decreased to about 1 pmollg after 1500 simulated miles, and to 0.3 pmol/g after 12 000 simulated miles. The adsorption values, given in parentheses at the appropriate mileage points in Figure 1;are typical for thermal sintering of this catalyst in the absence of lead. The presence of EDC in isooctane (1.5 theories relative to 0.5 g Pb/gal) does not cause any changes. Again the conversions of CO and HC a t 500 O C were practically constant over the observed 13000 simulated miles. Likewise, the conversion-temperature curves kept the same shape. As before, the CO adsorption decreased from 0.80 in the beginning to 0.32 pmol/g at the end of the run. The combustion products of isooctane containing 1.5 theories EDB result in a drastic deactivation, as shown in Figure 2. HC conversion dropped from 75-52%; CO conversion from 95-70%. Change to pure isooctane a t 11 000 simulated miles results in a complete recovery to the values of the virgin catalyst within 3000 simulated miles. Repeated exposure to EDB again caused loss in activity. The pronounced catalyst deactivation was, however, not accompanied by a corresponding decrease of the noble metal surface. After 10 000 simulated miles, CO adsorption remained a t a relatively large value of 0.53 pmollg. Catalyst poisoning by EDB is reflected by characteristic conversion-temperature curves, as shown in Figure 2. The normal curve shape was observed with pure isooctane (at 1320 and 14 250 simulated miles), while in the presence of EDB, conversions of CO and HC showed a much steeper ascent (examples given a t 2820 and 17 940 simulated miles). Qualitatively the same poisoning characteristics can be noted if a part of EDB is replaced by EDC. This fuel (EDB:O.5 plus EDC:1.0 theories) caused a drop in CO conversion from 96-81%, in HC conversion from 76-55%. The poisoning was completely reversed with pure isooctane. As before, the addition of EDB plus EDC did not enhance the normal shrinkage of the Pt-Pd area. The conversion-temperature curves display the steepness characteristic of the presence of EDB.
loo392 752
1112 100,392 752
1112 100392 752
m m
200 400 600 4350 mi
0
9
= P 60
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F
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T
200 400 600
200 400 600
9180 mi
12600 mi
14760 mi
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200 400 600
1
I
l)12 100:92 752
I
I
0
,
I0,WO
1 -
15,000
20,000
SIMULATED MILES
Figure 4. Combined effect of TEL and EDC on PTX-IIB activity
The activity change caused by TEL alone is shown in Figure 3. In contrast to the previous cases, a continuous decrease of HC and CO activities is observed. Conversion of CO dropped from 94-7596, while a much more pronounced drop from 77-35% took place for HC conversion. Moreover, most of the deactivation was permanent, although a recovery of more than 30% of the experienced activity loss can be achieved with pure isooctane, as shown. Further poisoning by TEL, after the recovery interval, was quite slow. The continuing decrease in catalyst activity was paralleled by a decrease in metal surface. The numerical values of CO adsorption in Figure 3 illustrate this point. Quite clearly the surface area falls below the value of 0.3 pmol CO/g observed as a minimum for lead free fuels. The difference between poisoning by P b and Br is also reflected by the conversion curves taken as a function of temperature at 13 560 and 16 470 simulated miles. The shape is similar to that observed with pure isooctane, but shifted toward lower conversions. The curves lack the steep slope characteristic of the presence of EDB (Figure 2). Qualitatively and quantitatively deactivation by isooctane containing TEL plus EDC resembles closely that by Volume 10, Number 2, February 1976
155
100-
392 752
1112
100-/
392 752
1112
392 752
1112
0
0
l
1001-
392 752
1112 O F
~
~
Table I . Characterization of the Investigated Catalysts Catalyst
Noble metal surface area @mol CO/g) O/O Conversion at 500 "C
co HC
-e 0
0
1
i
o
.
b
-
,
I
-
'
o
q
o
1
?
;
I
=0-
$;CO ' 0 1 0 I
Environmental Science & Technology
0.98
i
94.6 74.7
i
*
Pt
Pd
0.20
0.39 t 0.10
0 . 7 1 I 0.28
2.1 4.1
93.0 i 0.8 76.2 r 2.5
90.4 71.4
t i
2.6 5.9
-
V
T E L alone. The conversions, shown in Figure 4, exhibit considerable scatter that can be traced to the conversiontemperature curves, exemplified a t three mileage points. The lack of a smooth and monotonic increase of the conversions with temperature indicates an interaction of T E L and EDC. While deactivation is not much different from that caused by T E L alone, recovery of HC activity by pure isooctane, if any, was small. The presence of EDC caused an increase in the permanent lead deposit on the catalyst with possible enhancement of the permanent deactivation. The noble metal surface was not affected by the isooctane treatment (Figure 4). In contrast, the recovery of HC activity from poisoning by T E L alone showed a significant recovery of the metal area from 0.05 to 0.15 wmol/g, as shown in Figure 3. The poisoning effect of isooctane containing T E L plus EDB is what one would expect from the previously discussed cases. Catalyst deactivation is severe and becomes increasingly worse, as shown in Figure 5. The catastrophic decrease in CO activity can be explained by the action of EDB, as found before (Figure 2). Almost complete recovery of CO activity was achieved with pure isooctane. In contrast, HC deactivation, caused predominantly by TEL, was much less reversible. The temperature dependence of the conversions, shown a t 3570 and 17 900 simulated miles, again exhibits the irregular shape observed when lead and scavenger are combined, while in the absence of the poisons the normal curve shape is resumed as shown. Isooctane containing T E L Motor Mix with 0.5 g Pb/gal showed qualitatively the same poisoning characteristics as the preceding mixture. All three combinations of T E L and the two scavengers, tested here, caused approximately the same permanent damage after a poisoning period of 10 000 simulated miles. Permanent deactivation of CO oxidation was fairly small, conversion remained a t 90 f 5%, while HC conversion dropped permanently from 75 to 34 & 4%. There exists a significant difference in the total permanent lead deposit with and without a scavenger. After 10000 simulated miles, lead deposit in the absence of a scavenger was 1.80 wt %, while the presence of the scavengers, separately and combined, resulted in a lead deposit of 2.65 f 0.10 wt %. The much higher lead deposition with scavengers is not reflected by a much higher permanent catalyst deactivation or loss in noble metal area. 156
Pt-Pd
Comparison of Pt and Pd Poisoning by EDB. The poisoning studies described so far refer to the same catalyst type that has been investigated by Teague ( I ) and other researchers ( 4 ) in engine tests. The catalyst was selected to permit a direct comparison of our laboratory data with the published results. As the particular formulation consists of two noble metals, Pt and Pd, in an atomic ratio of about 1:1, it was of interest to assess the poisoning characteristics of the two components separately. For this purpose, a monolith was prepared that contained the same concentration of Pd present in the mixed Pt-Pd catalyst. Furthermore, another commercial catalyst (Matthey-Bishop) that contained only Pt as noble metal was selected for comparison. The surface areas of the three catalysts, expressed by the amount of CO chemisorbed per gram of catalyst after an initial heat treatment by the exhaust, are listed in Table I. In addition, this table shows the activities a t the same state. There is very little difference in the CO and HC activities; the 500 "C conversions are 92.7 f 2.1 and 74.1 f 2.596, respectively. The most obvious difference is a substantially lower surface area of the Pt catalyst. Significant activity differences are caused by EDB illustrated in Table 11. The data show that on Pt the ratio of unreacted HC with and without EDB is 36/27 = 1.33, while this deterioration factor is much larger for the P d catalyst 51/33 = 1.55, and the mixed Pt-Pd catalyst yields 42/27 = 1.56. The corresponding deterioration factors for CO oxidation are 1.67, 3.00, and 4.17 for Pt, Pt-Pd, and Pd, respectively. The last column in Table I1 shows that complete recovery takes place for all three catalysts when EDB is removed from the fuel. A combination of TEL and EDB again decreases the activity of the two Pd-containing catalysts, more than that of Pt (Table 11). The last column shows that much of the poisoning of the P d catalysts is reversible, which is not the case for Pt. The lower residual activity of Pt is probably a result of the initially lower noble metal area (Table I). In contrast, EDC had no influence on either activity of the three catalysts: CO conversion remained constant and HC conversion decreased by less than 3%, as was the case with pure isooctane. Discussion
Although the individual influence of TEL, EDB, and EDC is straightforward, the poisoning effect of the combined additives is complex and not easily quantified. This point is illustrated by Figure 6. Here the noble metal areas measured during the poisoning experiments on the mixed Pt-Pd catalyst have been plotted as a fmction of mileage. Surprisingly, the most severe loss in surface area is caused by TEL without a scavenger, indicated by the filled circles without a flag and by the lower line in Figure 6. The smallest decrease in surface area appears when lead is absent, as shown by the open symbols and the upper line in Figure 6.
0
Table I I . Comparison o f E D 6 Effect o n Pt and Pd
IS0
d lSOtl5EDC
p fl
Conversion percentaqes
ISOti5EDB l S O t IOLDC+OSEDB
10 000 Recovery Baseline miles -~ CO
HC
CO
HC
CO
HC
Pt Pt-Pd Pd
94 94 94
73 73 67
90 82 75
64
58 49
94 95 96
71 74
68
Pt Pt-Pd Pd
93
79 75 65
55
10 7 8
45 54 81
14 25 33
1.5 E D 6
1.5 EDB + 0.5 g Pb/gal
88 88
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Fuels containing TEL plus scavengers result in surface areas that fall between the two lines. A simple correlation is not apparent between the noble metal area and catalyst activity, as the largest decrease in surface area is found for T E L alone, while in general, TEL does not cause more permanent deactivation than does a combination of TEL and scavenger. Similarly there is no simple relation between total lead deposit and residual P t - P d area. Analyses by x-ray fluorescence show that the lead deposit on the catalyst is about 50% higher when T E L is used with a scavenger than without it. The increased deposit can be due to the higher volatility of lead halides resulting in a larger portion of lead reaching the catalyst instead of being deposited in the conduits ahead of it. However, the much higher lead deposition is not reflected by a much higher catalyst deactivation, or, as shown in Figure 6, by an increased loss in noble metal surface. I t is also interesting to note that the highest Pt-Pd area is retained when isooctane with EDB is used, suggesting that EDB may have a retarding effect on the sintering of the noble metal crystallites. It is known that many different lead compounds can be formed by the interaction of TEL, EDB, and EDC ( 5 ) , of which lead bromochloride is the most abundant (6). The reaction products are strongly dependent on temperature and oxygen/fuel ratio. Several lead species-Le., lead, lead oxide, lead halides, and lead oxyhalides, have been identified by x-ray analysis from deposits scraped off reactor and pulsator tubes. Some of the lead compounds are probably more selectively deposited onto the noble metal surface than others, and some lead species may be able to diffuse into the inert support with relative ease. In the case of actual automobile exhaust, matters are even more complicated as additional lead compounds, e.g., phosphates and sulfates, are known to be formed (5, 7-9) in appreciable amounts. The experimental results presented here indicate that the combustion of a fuel containing EDB, but not Pb, leads to a gaseous Br species which occupies adsorption sites required for the oxidation of HC and CO. I t is thermodynamically probable that HBr appears in the gas phase when gasoline containing Motor Mix is combusted ( 5 ) .The poisoning effects of EDB and HBr were compared in an attempt to identify and characterize the detrimental adsorbate. In the first experiment, activity of the mixed Pt-Pd catalyst was measured as a function of the EDB concentration in isooctane. The results (Figure 7, square symbols) show that catalyst activity at 500 "C deteriorates fast as the EDB concentration is increased to 0.2 theories of EDB
-
I''
I 0.5
I
I
I
HBr -~ 0 1
1 : t
I.o THEORIES B r
1
0
I
I.5
Figure 7. Conversions of HC and CO at 500 OC as a function of EDB and HBr concentrations
( I theory = 0.5 X 79.91/207.19 = 0.19 g Br/gal). Further increase to 1.5 theories does not result in additional deactivation. In the second experiment, pure isooctane was used as a fuel, and an aqueous solution of HBr was injected into the combustion products before they contacted the catalyst. Conversions of HC and CO as functions of HBr concentration are given in Figure 7 by the round symbols. These curves show reasonable agreement with the EDB results. It appears thus that reversible poisoning is caused by EDB combustion products, either HBr, or bromine that is, in part, produced at the given concentrations of HBr, H20, and 0 2 at the poisoning temperatures, according to thermodynamics ( I O ) . The described results may be summarized as follows: (1) Oxidation of HC and CO over Pt or Pd is not impaired by EDC alone. (2) The oxidation of HC and CO is poisoned by EDB. This poisoning is completely reversible when Br is eliminated. Catalysts containing Pd are deactivated by EDB to a higher degree than catalysts containing Pt, but no Pd. CO oxidation is affected by EDB more strongly than HC oxidation over catalysts containing Pd. (3) TEL with EDC, with EDB, and without a scavenger Volume 10, Number 2, February 1976
157
Table I l l . Comparison of Summarized Results with Literature Data 11,4
Reference Catalyst Type of testa Resistance to deactiv. by EDC on HC oxid. on CO oxid. Poisoning by E D B of HC oxid. of CO oxid. Reversible E D B effect Pd deactiv. > Pt deactiv. CO deactiv. > H C deactiv. (on Pd) Poisoning by TEL with EDC with E D B No scavenger HC deactiv. >CO deactiv. Reversible lead effect i n part Scavengers enhance lead deposit a L,
laboratory data: E, engine data.
12a Pt-Pd
Pt-Pd,Pt
bX
E
E
L
-
(-1
-
+
+
+
+
+ + + (+
E
+
+ +
(+I +
+
(-1
+ (-1
+
indicates only
+ (+I
+
HC
conversion
Acknowledgment
We thank M. Shelef and J. T. Kummer f o r helpful discussions during this work. E. c. Su is credited for evaluation of the surface data in Figure 6.
Environmental Science 8, Technology
13 Pt-Pd ,Pd ,Pt E L
+
+
+ +
+
14 Pt,Pt-Pd L L
(+ 1
loss
1 Pt-Pd E
+ +
+ +
.xb
(-1
+
+
X X
causes permanent deactivation of catalysts containing Pt or Pd. Poisoning by lead is more pronounced for HC than for CO oxidation. The poisoning attributable to lead is, in part, reversible, especially with respect to CO oxidation. (4) The amount of P b deposited on the catalyst is enhanced by EDB and EDC. It is not known how much of this effect is due to an increased lead transport, on the one hand, and to an increased interaction of lead halides with the catalyst, on the other. By now several industrial laboratories have made public their findings and conclusions about catalyst poisoning by TEL, EDB, and EDC. These studies were executed under widely differing conditions, mostly on engine systems. Experimental differences existed in exhaust temperature, definition of catalyst activity, concentration of additives, and catalyst formulation, which included monolith and pellet substrates, and so forth. A comparison of the available data is useful to identify common facts. Based on the results summarized above, pertinent data were examined and a condensed comparison listed in Table 111. Agreement with our conclusions 1-4 is indicated by a “+”, disagreement with a “-” mark. Parentheses indicate a tentative conclusion. The most obvious disagreement is found for the deactivation by EDC. Under certain conditions significant temporary catalyst deactivation is found (4, 11, 12a, 13), while under other circumstances it is not (1, 14). Note that if deactivation by EDC was observed, this effect was always reversible. Closer examination of the data reveals that loss of activity by EDC is strongly temperature dependent. While deactivation at an inlet temperature of 565 “C (1049 O F ) was negligible (3% loss in HC conversion), a t 455 “C (851 OF), it was more substantial (10%loss) (13).There exists overwhelming evidence that TEL itself must be considered as a permanent poison, especially for HC oxidation.
158
12b Pt-Pd
X X
+ (+I (-1 +
+
+ X
(-1
+
is,consistent.
Literature Cited (1) Teague, D. M., (a) “SO4 Emissions from Oxidation and NonOxidation Catalyst-Equipped Vehicles”, April 18, 1974, Durham, N.C., Symposium, “Health Consequences of Environmental Controls: ImDact of Mobile Emissions Controls“. (h) Chem. Eng. N e w s , 52 (is),5 (1974). (.2 .) Otto. K.. Dalla Betta. R. A.. Yao. H. C.. APCA J.., 24.. 596 (1974): ’ (3) Gruber, H. L., Anal. Chem., 34,1828 (1962). (4) Holt, E. L., Wigg, E. E., Neal, A. H., “Fuel Effects on Oxidation Catalysts and Oxidation Catalyst Systems 11”, Paper No. 740248. Societv of Automotive Eneineers. Dresented a t meeting of February 2C-March 1,1974. (5) Newbv. W. E.. Dumont. L. F., I n d . Ene. Chem.. 45. 1336 (1953). (6) Boyer, K. W., Laitinen, H. A,, Enuiron. Scc. Technol., 8, 1093 (1974). (7) Bomback, J . L., Wheeler, M. A., Tabock, J., Janowski, J. D., ibid , 9,139 (1975). (8) McArthur, D. P., (a) “The Deposition and Distribution of Lead, Phosphorus, Calcium, Zinc, and Sulfur Poisons on Automobile Exhaust NO, Catalysts”, presented a t meeting of American Chemical Society, Division Symposium on Catalysts for Removal of Automobile Pollutants, Los Angeles, April 1-5, 1974. (b) “The Catalytic Chemistry of Nitrogen Oxides”, R. L. Klimisch and J. G. Larson, Eds., pp 263-79, Plenum Press, New York-London, 1975. (9) Shelef, M., Dalla Betta, R. A,, Larson, J . A,, Otto, K., Yao, H. C., “Poisoning of Monolithic Noble Metal Oxidation Catalysts in Automobile Exhaust Environment”, presented at Symposium on Catalyst Poisoning, 74th National Meeting of the AIChE, New Orleans, La., March 11-15, 1973. (10) Stull, D. R., Westrum, E. F., Jr., Sinke, G. C., “The Chemical Thermodynamics of Organic Compounds”, pp 215, 229, John Wiley & Sons, New York, N.Y., 1969. (11) Cecil, R. R., Exxon Research and Engineering Co., letter report to J. DeKany of EPA, June 7,1974. (12) Bowdich, F. W., General Motors, letter report to E. 0. Stork of EPA (a) June 4,1974, (h) June 17 1974. (13) Barnes, G. J., Baron, K., Summers, J. C., “Scavenger and Lead Poisoning of Automotive Oxidation Catalysts”, presented a t Meeting of Society of Automotive Engineers, Toronto, Ont., October 21-25, 1974. (14) Cooper, B. J., Renny, L., Johnson Matthey Group Research, unpublished data, July 15, 1974.
-
Received for review May 21, 1975. Accepted October 8, 1975
0
