Effects of Hydrocarbons, Carbon Monoxide, and Oxygen on Sulfuric Acid Emission from an Automotive Catalyst Mati Mikkor, Robert H. Hammerle, and Timothy J. Truex' Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 I
The results of a flow reactor study of the sulfur dioxide oxidation over a 1975 production Pt/Pd monolithic catalyst are described. The effects of catalyst temperature, space velocity, oxygen concentration, sulfur dioxide concentration, and feedgas composition have been studied. The sulfur dioxide oxidation is kinetically limited below approximately 475 OC and approaches thermodynamic equilibrium at higher temperatures. In the kinetically limited region and in the absence of reducing gases, sulfur dioxide conversion is relatively independent of oxygen concentration (-0.01-5 % 02). When the reducing gases, carbon monoxide, propylene, or hydrogen, are present, they are oxidized more readily than sulfur dioxide and hence serve to inhibit its oxidation at oxygen concentrations of less than -1.0%. There is a range of oxygen concentrations which gives high conversion of carbon monoxide and propylene simultaneously with low conversion of sulfur dioxide. Catalyst aging causes significant reductions in sulfur dioxide oxidation activity while still maintaining moderate to high carbon monoxide and propylene oxidation activities.
Introduction Gasolines contain a small amount of sulfur, on the average approximately 0.03 wt %, which is converted to sulfur dioxide (S02) during combustion. On passing through a noble metal oxidation catalyst, this SO2 is partially converted to sulfur trioxide (SO3) which rapidly reacts with water vapor in the exhaust to form sulfuric acid (H2SO.J. A recent review by Pierson (1976) describes the chemical and physical processes which underlie the emission of sulfuric acid from catalystequipped vehicles and also covers the current literature in this field. Of particular interest are the effects of operating temperature, space velocity, oxygen ( 0 2 ) concentration, SO2 concentration, and feedgas composition on the SO2 oxidation reaction.
The values of the equilibrium constant for reaction 1 over the range of temperature experienced in automotive emission control applications (400-800 "C) are such that equilibrium conversion to SO3 with 5% 0 2 concentration approaches 100% in the bottom part of the temperature range (Pierson et al., 1974). Toward higher temperatures, the equilibrium conversion drops such that it is approximately 15% a t 800 "C. The equilibrium S03/S02 ratio is proportional to the square root of 0 2 concentration (see eq I), independent of the SO2 level, and independent of transit time through the catalyst. There are a number of reports of catalysts which operate in or near the equilibrium regime under automotive operating conditions (see references in Pierson, 1976). Outside the equilibrium regime, the SO2 oxidation may be kinetically limited or diffusion limited. The extent of SO2 oxidation will be lower and will increase with increasing temperature. The percent oxidation will no longer be independent of transit time through the catalyst, nor will it necessarily be independent of SO2 concentration. The dependence upon 0 2 concentration and feedgas composition may also be more complex. The determination and understanding of sulfate emission levels will be dependent upon careful
studies of catalyst activities for reaction 1. In particular, determination of the operating regime of the catalyst (thermodynamic equilibrium or kinetic limitation) and elucidation of the effects of operating parameters are of importance. In this paper the results of a laboratory flow reactor study of the catalytic conversion of SO2 over a 1975 production Pt/Pd monolithic catalyst are presented. The effects of catalyst temperature, gas flow rate (space velocity), 0 2 concentration, SO2 concentration, and feedgas composition on SO2 conversion have been determined. In addition, the SO2 conversion by catalysts aged for 50 OOO miles on a vehicle has been studied. Experimental Method The flow reactor used in these experiments has been described by Hammerle and Mikkor (1975). Briefly, a number of gases from cylinders are mixed to make a simulated exhaust gas. The mixture is humidified, heated, and then passed over a catalyst. The sulfuric acid formed by the catalyst and the unreacted SO2 are collected in a Goksdyr-Ross coil and a hydrogen peroxide impinger, respectively. The data described here are taken at steady-state operation of the catalyst; that is, the catalyst temperature and the gas flow rate and composition are constant during measurements. Whenever the conditions are changed, no data are taken for 1 h and then two successive samples are taken. These precautions ordinarily ensure that the sulfur storage on the catalyst is a t equilibrium, but if continuing storage or release is detected, the data are not used in such cases. The catalysts used in this study were 1975 Englehard IIB Pt/Pd monoliths. Nominal compositions of the catalyst samples are 0.2% Pt; 0.1% Pd, expressed as weight percent of bulk catalyst. Noble metal surface areas are approximately 0.04 m2/g of bulk catalyst as determined by CO adsorption. The fresh catalysts were pretreated for 3 to 4 h a t 800 "C in air. This lowered the very high activity of a new catalyst and was necessary in order that the activity of the catalyst remain constant throughout the experiments. The aged catalysts used Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
217
Table I. Simulated Exhaust Gas Composition-without Reducing Gases Gas
Partial pressure
Nitrogen Water Carbon dioxide Oxygen Sulfur dioxide
Without C0,C3He,H2 375'C; 102,300hF' With CO,C$l6,H2 0376'C, 74,600hi' $ 4 6 7 O C , 74,600h i '
0
71% 13% 11% 5% 20 PPm
Table 11. Simulated Exhaust Gas Composition-with Reducing Gases Gas
Partial Dressure
Nitrogen Water Carbon dioxide Oxygen Carbon monoxide Hydrogen Nitric oxide Propylene Sulfur dioxide
1 IO
70.5% 13% 11% 5%
3600 ppm 1200 ppm 500 ppm 160 ppm 20 PPm
I
I
,
20
40
60
I
00 100
iNLET SO, (pprn)
Figure 1. The percent conversion of SO2 as a function of inlet [SO21 in the region where the SO2 oxidation is kinetically limited. I
I
I
200
300
400
I
I
I
I
I
500
600
700
800
900
I
in these experiments had been subjected to the EPA durability procedure on a vehicle for 50 000 miles, and extensive testing showed that they are typical of catalysts aged for that period. No preconditioning was given to the aged catalysts. The simulated exhaust gas composition varied somewhat depending on the purpose of the experiment. In general, two main compositions were used; these are shown in Tables I and 11. One was a mixture of N:!, 0 2 , COZ,H2O vapor, and S02, and the other was a mixture of these gases plus CO, C3H6, H2, and NO. Both compositions were used for most of the different types of experiments so that the effect of the reducing gases, CO, C3H6, and H2, could be determined. The effect of nitric oxide on the conversion of sulfur dioxide was found to be negligible.
Figure 2. The conversion of SO2 as a function of temperature and space velocity with gas composition shown in Table I.
Results and Discussion Sulfur Dioxide Concentration. The conversion of SO:!, i.e., ([SO~]ouJISO~]out [S0310ut), was measured in the ratelimited region as a function of the SO2 concentration using simulated exhaust gas with and without the reducing gases. The gas composition was the same as that shown in Tables I and I1 except that the SO2 concentration varied from 10 to 60 ppm. The results are shown in Figure 1.The percent conversion decreases with increasing SO:! concentration; however, the effect is rather small. For example, with reducing gases present a t a catalyst temperature of 376 OC and a space velocity of 74 600 h-l, the conversion decreases from 28% at 10 ppm SO2 to 22% at 60 ppm. Increasing the temperature to 467 "C has little consequence; the conversion still drops very little (from 48% to 42%) between 10 and 60 ppm SO:!. At the catalyst temperatures and space velocities used in these experiments, the SO2 conversion is limited by the rate of oxidation (vide infra). Under conditions where thermodynamic equilibrium is achieved, the percent conversion is a function of only the temperature and O2 concentration and is independent of the SOz concentration. Temperature and Space Velocity. The conversion of SO:! was measured as a function of catalyst temperature and space velocity using a gas stream without reducing gases added (see Table I). The results of these experiments are shown in Figure 2. Both catalyst temperature and space velocity (defined as the volume flow rate of the gas a t S T P divided by the gross volume of the catalyst) have a substantial effect on the conversion of sulfur dioxide. As expected at low temperatures, the
SO2 oxidation is kinetically limited so the conversion increases with temperature, and at high temperatures the oxidation reaches thermodynamic equilibrium so the conversion decreases with increasing temperature. Thermodynamic equilibrium conversion is reached at -475 "C at 33 700 h-' S.V. and -550 OC for 82 000 h-l S.V. (as shown in Figure 2). In the kinetically limited region, the data show that the conversion decreases with increasing space velocity as expected. The low-temperature conversion for one set of experiments at 82 000 h-1 space velocity is about the same as that for 33 700 h-l, even though the residence time over the catalysts is different by a factor of 2.5. Since two different catalyst plugs were used for these experiments, these lowtemperature results may be a consequence of slight differences in the catalyst samples or may be a result of the relatively low sensitivity of the experiments at low conversions. The latter explanation would appear to be the most reasonable since a repeat of the 82 000 h-1 space velocity experiment in the low-temperature region (solid squares-Figure 2) resulted in slightly lower measured conversions. Since the percent conversion of SO2 in the kinetically limited region is nearly independent of the inlet SO:! concentration (Figure l), it is reasonable to interpret the temperature and space velocity data on the assumption that the oxidation rate is first order in SOz. Calculated first-order rate constants are presented in Figure 3 as a plot of In (12) vs. 1/T for the three different space velocities. The curves are linear from low temperatures to about 400-450 OC indicating that the reaction
+
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Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3,1977
TEMPERATURE "C
IO0
I
1
I
I
O
I
I
I
8
S P I C E VELOCITY 77,000 h i ' O i i d o l i o n Efliciency
a c,u6
- >
438.c
CONVERSION OF SO2 OWilhoul CO, C x H e , H p 438.C
.Q
c
= I -
i
\\\\
02 CONCENTRATION (%I
p\
]
14 15 16 17 18 19 2 0 21 IOOO/T ( O k ? Figure 3. The rate constant for SO2 oxidation as a function of 1/T. The rate constant is calculated assuming that the oxidation is first order in SOz.
9
IO
- G
\ L L
- w
II
12 13
Figure 5. The conversion of SO2 as a function of 0 2 concentration a t about 438 "C and 77 OOO h-l space velocity. The oxidation efficiencies for CO and C3H6 are also shown. 8
h
55
I
I
I I I SPACE VELOCITY 231,000 h? Oxidolion Etficmncy
IO0
0 " z 80
D O u Iz a
SPACE V E L O C I T Y 33.700 h;' O x i d o l i o n Efficirncy co 435.C 435.C A C,He
o e D X
a 0 40 W
> z
0 0
I
I
0 02 CONCENTRATION ( X )
C O N V E R S I O N OF SO2 OWithOul CO. Mz, C s M b 4 3 8 . C 0 With CO,Mz, C s M 6 438.C I I
1.0 2.0 3.0 OzCONCENTRATION
4.0
5.0
Figure 6. The conversion of SO2 as a function of 02 concentration at about 440 "C and 231 000 h-l space velocity. The oxidation efficiencies for CO and C3H6 are also shown.
(%I
Figure 4. The conversion of SO2 as a function of 0 2 concentration at 438 OC and 33 700 h-l space velocity. The oxidation efficiencies for CO and C3He are also shown. is kinetically limited in this region. The slopes of the lines give activation energies of 16.4 kcal/g-mol for the 246 000 h-l and the 82 000 h-l space velocities and 18.6 kcal/g-mol for the 33 700 h-l space velocity. The differences in activation energy appear to be within the experimental error of the results and any errors associated with the assumption that first-order kinetics are strictly obeyed. The effects of temperature and space velocity on the SO2 conversion were not studied with the reducing gases added to the gas stream because their presence makes the experiment difficult to interpret. Depending on the temperature a variable fraction of the reducing gases are oxidized in the catalyst; this will cause a varying interference with the SO2 oxidation. Where vehicle data on the conversions of SO2 to H2S04 for the appropriate type of catalyst are available, the agreement between flow reactor data and the vehicle data is generally good. Careful vehicle dynamometer studies a t steady-state speeds with measurement of exhaust or catalyst temperatures and taking care to equilibrate sulfur storage and release have been reported by Griffing et al. (1975) and Trayser et al. (1976). Griffing and co-workers report that thermodynamic equilibrium conversions of SO2 are closely approached a t catalyst exhaust temperatures of 2440 OC-actual catalyst bed temperatures would be slightly higher. This temperature corresponds to different speeds in different automobiles, but
is in the range of 30-40 mph. At temperatures less than 440 OC the SO2 conversion is considerably less than thermodynamic equilibrium and apparently is kinetically limited. Trayser and co-workers report conversions at or near thermodynamic equilibrium for catalyst temperatures 3500 "C. For the vehicle used in their study, these catalyst temperatures were achieved at 320 mph cruise conditions. Considering that the space velocities in the vehicle studies range from -30 000 h-l at 20 mph to -80 000 h-l at 60 mph, these vehicle results are in good agreement with the flow reactor data presented in Figure 2 . Oxygen Concentration. The conversion of SO2 was measured as a function of inlet 0 2 concentration with and without the reducing gases listed in Tables I and 11. In one series of experiments the catalyst temperature was constant at 440 "C and the 0 2 was varied a t each of three space velocities; the data are shown in Figures 4, 5, and 6. Without reducing gases the conversion of SO2 is relatively independent of the 0 2 concentration. With reducing gases present the O2 reacts preferentially with the CO, Hz, and C3H6. Thus, until the C3H6, CO, and H2 are oxidized, the SO2 conversion is pracitcally zero (see Figures 4 5 , and 6). With excess O2 above that required to oxidize the CO, C&, and H2 (the stoichiometric 0 2 concentration is 0.29%) there is still an inhibition of the SO2 conversion which becomes more pronounced as the space velocity increases. At higher 0 2 concentrations, the SO2 conversion again becomes relatively independent of the O2 concentration and is the same as that without reducing gases. Figures 4-6 very definitively show that there is a range of O2 Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
219
SPACE VELOCITi 74,600 kr-l OXIDATION EFFICIENCY CO 526T AC3H6 526OC .~ CONVERSION OF SO& 0 Without CO,C3H6 526.C With CO, C3H6,Hp 526.C
ON
g
L
0
1.0
2.0
3.0
40
02 CONCENTRATION ( X I
5.0
0 2 CONCENTRATION ( X )
Figure 7. The conversion of SO2 as a function of 02 concentration at 526 O C and 74 600 h-l space velocity. The oxidation efficiencies for CO and C ~ H are G also shown. concentrations which allows simultaneous high conversion of co and C3H6 without extensive so2 conversion as long as a sufficient amount of the reducing gases is present in the feed stream. As will be discussed later, the actual boundaries of this range are dependent upon the extent (or concentrations) of the reducing gases over the catalyst surface. At higher temperature (526 "C) the dependence of the SO2 conversion on 0 2 concentration is very similar to that described above (see Figure 7). The reducing gases are preferentially oxidized resulting in a range of 0 2 concentrations which allows simultaneously high conversions of CO and C3H6 without extensive SO2 conversion. The 526 "C results without reducing gases do show a slight reduction in SO2 conversion a t O2 concentrations less than -1% which is not observed at 440 O C . This is a result of the SO2 conversions approaching thermodynamic equilibrium a t 526 OC and without reducing gases present. Thermodynamic equilibrium conversions as a function of O2 concentration for different temperatures are shown in Figure 8 where the decrease in SO2 conversion with decreasing 0 2 concentration at 526 "C is clearly seen. Figure 9 shows the SO2 conversion with each of the reducing gases added separately to the inlet gas stream in the usual concentrations (co 3600 ppm; C3H6 160 ppm, and H2 1200 ppm). With these concentrations, which are typical of those encountered in actual vehicle operation, it is seen that CO has the largest effect on SO2 conversion followed by C3H6 and H2. One series of experiments was performed at 440 "C and 5000 ppm of 0 2 with the individual reducing gases added at concentrations such that they had equal reducing power (1800 ppm of O2 required for complete oxidation). The SO2 conversions under these conditions were: 71.7%without reducing gases; 53.8% with CO; 42.6% with C3H6; and 71.5% with H2. Thus, under conditions of equal reducing power, the order of effectiveness in suppressing the so2 oxidation is C3H6 > co
>> Hz.
The oxidation of the reducing gases is exothermic and results in a 20 to 30 "C temperature rise across the catalyst. In order to keep the catalyst temperature as close as possible to that which would prevail in the absence of reducing gases, the catalyst oven temperature is lowered the corresponding amount. Thus, although the outlet face of the catalyst is the same temperature during each experiment, the inlet face is some 30 "C cooler when the reducing gases are present. At 440 "C and 77 000 h-l space velocity, the 30 "C temperature change can result in only about a 10%change in SO2 conversion and, hence, cannot explain the 0 2 effect observed. This is further demonstrated by an experiment in which 300 ppm of CO was added to a gas stream without the other reducing 220
427T 527'C
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Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3,1977
Figure 8. The conversion of SO:! as a function of [ 0 2 ] . The conversion is calculated assuming that thermodynamic equilibrium is reached.
SPACE VELOCITY 74,600 h i 1 OXIDATION EFFICIENCY A C ~ H4 3~ 8 T CONVERSION OF SO1 Without CO, C3H6, H2 a With CO* 3600ppm o WithC3H6=IBOppm 4 With Hp 1200ppm
438OC 438OC 438T 438'C
::F EO
20
w
0
I
IO
I
20
I
I
30 4 0 02 CONCENTRATION (w
I
5.0
I
Figure 9. The conversion of SO2 as a function of [O:!]and type of reducing gas. The oxidation efficiencies for CO and C3H6 are also shown. gases. The 0 2 concentration was about 100 ppm, so that not all of the CO was oxidized. The SO2 conversion was about 3% and the CO conversion was 45%. By contrast, in the absence of CO, the SO2 conversion was 35%. These results, which were obtained under conditions that gave a very small temperature rise, confirm the effect of CO on the SO2 conversion reported above under conditions of a larger temperature rise. The most logical explanation of the effect of 0 2 concentration on the SO2 conversion is as follows. The reducing gases evidently compete with the SOz, either for adsorption sites on the catalyst surface or for O2 once they are adsorbed. The oxidation rates of the CO, C3H6,and H2 are first order in 0 2 ; hence, as the O2 concentration increases, the amount of CO, C3H6, and H2 adsorbed on the surface decreases, leaving it free for the 0 2 to react with SO2. In order for there to be a large reduction in SO2 conversion, the reducing gases must be present for a large distance along the catalyst bed. The data presented in Figures 4-7 are consistent with this explanation in that, a t low space velocities where it is expected that the reducing gases are oxidized very early in the catalyst, the range of 0 2 concentrations which gives high CO and C3H6 conversion and simultaneously low SO2 conversion is very narrow. As the space velocity increases and the reducing gases are present for a larger distance along the catalyst bed, the range of 0 2 concentrations which gives high CO and C3H6 conversion and low SO2 conversion becomes much wider. Under conditions where the SO2 conversion reaches thermodynamic equilibrium and all of the reducing gases are oxidized, the H2S04/S02emission ratio should vary as o ~in accordance with the form of eq 1.Figure 8 shows the equilib-
~ / ~
Table 111. Activities of 50 K Aged Catalysts SO2 Conversion @525"C
Conversion Pb, wt%
Catalyst Inlet A Outlet A Inlet B
Space velocity
7.8 2.0 6.5
co
35 000 h-1 97% 28 200 h-1 99% 27 850 h-1 96% 79% 55 700 h-l 98% Outlet B 1.5 33 340 h-' 66 680 h-l 91% 100% Fresh 33 700 h-l 99% 82 000 h-l Gas composition shown in Table I. b Gas composition shown in Table 11. 100
-
-8 I
Therrnodynornic Equilibriun 0 Fresh Cotolyst Wlthout COIC,H~,H2 Aged Catalyst Without
CO. C ~ H ,SH E
h
700
0 Aped Cataly6t With
0
-+ v)
Y
ON
-
60
7 n
0
v)
Y
z 40 0 v) a W
> z
0
2c
0
0
2 1
I
I
I
600 TEMPERATURE ('C)
400
I
I
800
Figure 10. The conversion of SO2 as a function of temperature for a fresh catalyst and for a plug taken from near the inlet of 50 000 mile aged catalyst B. The space velocities are 33 700 h-' for the fresh catalyst and 33 340 h-I for the aged catalyst. rium SOz conversions as a function of 0 2 concentration for a number of temperatures. These calculations show that, under equilibrium conditions, the 0 2 concentration has a modest effect on SO2 conversion, the effect being larger with higher catalyst temperature. Published vehicle data indicate that the SO2 conversion can be greatly reduced by lowering the exhaust 0 2 concentration (Holt et al., 1975). At 60 mph cruise the vehicle data show a greater than 10-fold suppression in the SO2 conversion when the air injection is stopped. Unfortunately, the 0 2 concentrations and catalyst temperatures were not reported in the vehicle data and thus a direct comparision with the results presented here is not possible. The vehicle results do indicate a larger reduction in SO2 conversion than that predicted by the steady-state flow reactor data. The flow reactor data indicate that only a fourfold reduction in SO2 conversion should be possible while still converting most of the co and C3H6 (see Figure 7). Aged Catalyst Studies. The SO2 conversion was measured for two catalysts which had been aged on vehicles for 50 000 miles. Two sample plugs were cut from each catal st, one from near the inlet and the other from near the o u t z t . SO2 conversion data, lead concentrations, and CO and C3H6 conversions for each catalyst sample are presented in Table 111. Summary data for a fresh catalyst are also presented in Table
HC
Withouta red. gases
Withb red. gases
91% 89% 90% 63% 95% 83% 98% 97%
76% 7 1% 69% 55% 78% 57% 86% 77%
33% 37% 27% 13% 44% 12%
79% '75%
I11 for comparison. Comparison of the fresh and aged catalyst data shows that while the CO and C3H6 activities have remained reasonably high even on those aged catalysts which contain an appreciable amount of lead, there are significant reductions in SO2 conversion depending upon the operating conditions. The most striking feature of the SO2 conversion data for the aged catalysts is the large reduction in conversion in the presence of reducing gases. While the SO2 conversion over fresh catalysts has been shown to be relatively independent of the presence of reducing gases except at low 0 2 levels or high space velocities, the results presented in Table I11 show that addition of reducing gases to the gas stream of the aged catalysts lowers the SO2 conversion by -60%.This is also observed in data showing the temperature dependence of the SO2 conversion for the inlet plug of aged catalyst B (Figure 10). The data in Figure 10 show that without reducing gases present, the SO2 conversions for the aged and fresh catalysts are about the same except in the 450-500 "C temperature range where the SO2 conversion over the aged catalyst appears to be diffusion limited. With reducing gases, the aged catalyst has much lower SO2 conversion at all temperatures until thermodynamic equilibrium is approached at approximately 650 "C. The results presented in Table I11 also indicate that the SO2 conversion is more sensitive to changes in space velocity for the aged catalysts than for fresh catalysts. Thus, while a greater than twofold increase in space velocity for the fresh catalyst results in only a small change in SO2 conversion (note that this should be the case since at 525 "C the fresh catalyst is operating at or near thermodynamic equilibrium),a twofold increase in space velocity for aged catalyst B results in large reductions in SOP conversion. The effect is again most pronounced with the reducing gases present and is accompanied by a reduction in CO and C3H6 conversions. These results indicate that the SO2 oxidation reaction is a more demanding reaction than the oxidation of CO and C3H6. Therefore, deactivation of the catalyst by aging is reflected first in a decrease in the SO2 conversion. The largest reductions in SO2 conversion for the aged catalysts are observed with reducing gases present and are consistent with the preferential oxidation of CO, C3H6, and Hz which is found in the 0 2 concentration studies. The general reduction in catalyst activity which occurs during aging results in the reducing gases being present for a larger distance along the catalyst bed and therefore being more effective in their suppression of the SO2 oxidation reaction. An increase in space velocity enhances this phenomenon. The reduced SO2 conversions which result from the preferential oxidation of the reducing gases occur even at high 0 2 concentrations (5%)for the aged catalysts and effectively result in the catalysts becoming more selective far CO, C3H6, and H2 oxidation. An alternative explanation of the Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
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aged catalysts results may be that selective poisoning of the catalysts for SO2 oxidation has taken place. This does not appear to be consistent with the relatively high SO2 conversions that are observed in the absence of reducing gases although further studies will be necessary to completely rule out this possibility. These results on aged catalysts indicate that under operating conditions comparable to vehicle operation (reducing gases present), a two- to sixfold decrease in sulfate emissions may be expected with aged catalysts in comparison to fresh catalyst systems. Krause and co-workers (1976), using a representative fleet of 20 catalyst-equipped vehicles, indicate a 3.7-fold drop in sulfate emissions on the average (from 22 to 6 mg/mile) from stabilized fresh conditions to 24 000 miles. With further mileage accumulation, the average sulfate emissions remained practically constant. Irish and Stefan (1976) tested one well-characterized vehicle and found a 2.5to 3-fold decrease in sulfate emissions in going from 4000 miles to 50 000 miles of catalyst aging. Both of these results are in general agreement with those observed on the flow reactor. Conclusions Flow reactor studies of the SO2 oxidation reaction over a 1975 Engelhard IIB P t P d monolithic catalyst indicate the following conclusions. 1. With excess oxygen present, the SO2 oxidation is kinetically limited below approximately 475 "C a t 33 700 h-l S.V. and below 550 "C a t 82 000 h-l S.V. Above these temperatures, the SO2 oxidation reaches thermodynamic equilibrium. 2. The extent of SO2 oxidation is only slightly dependent upon the SO2 concentration in the kinetically limited region. The activation energy for SO2 oxidation is 16.4 to 18.6 kcal/ g-mol. 3. The reducing gases CO, C3H6, and H2 effectively compete with SO2 for oxidation over the catalyst such that SO2 oxidation is inhibited a t low 0 2 concentrations. There is a range of 0 2 concentrations which allows simultaneous high conversion of CO and C3H6 while severely limiting the SO2 conversion. 4. Catalyst aging results in significant reductions in SO2 oxidation activity. Under conditions comparable to actual vehicle operation, decreases in SO2 conversion of two- to sixfold are observed with 50 000 mile aged catalysts relative to fresh ones. The results presented in this paper indicate that catalyst aging and 0 2 concentration in the exhaust gas stream are two
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very significant factors in determining vehicle sulfate emissions. The 02 concentration studies indicate that a vehicle emission control strategy based on operation with limited 0 2 concentrations may be feasible. Such a strategy would require very careful 0 2 control to revent CO and h drocarbon breakthrough and still limit E 0 2 conversion. Caralyst aging studies indicate that significant reductions in SO2 conversion result from the aging process. However, it should be pointed out that CO and hydrocarbon activity also decrease with aging and will become a serious problem if the catalyst is severely deactivated. In comparing the flow reactor data presented in this paper with vehicle data it is important to remember that the flow reactor results are for steady-state operation of the catalyst. In cyclic vehicle operation, a number of other factors come into play. Among them are the important effects of sulfur storage and release which have been discussed in another publication (Hammerle and Truex, 1976). In addition, variations in exhaust gas compositions and concentrations-particularly 0 2 concentrations-which occur during cyclic operation will have an effect on sulfate emissions. The flow reactor results should, however, provide a sound basis for understanding of these vehicle results. Acknowledgments The authors thank Ms. Beth Graves for experimental assistance, the analytical staff of the Ford Research Laboratories for performing the sulfate analyses, and W. R. Pierson and J. T. Kummer for many helpful discussions. Literature Cited Griffing. M. E., Gilbert, L. F., Ter Haar, G. L., Immethun, P. A., Zutaut, D. W., SAE Paper 750 697 (1975). Hammerle, R . H., Mikkor, M., SAE Paper 750 097 (1975). Hammerle, R. H. and Truex, T. J., Prep. Pap. Nat. Meet. Div. Petrol. Chem., Am. Chem. SOC.,21, (4),769 (1976). Holt. E. L., Bachman, K. C., Leppard, W. R., Wigg, E. E., Somers, J. H., SAE Paper
750 683 (1975). Irish, D. C., Stefan, R. J., SAE Paper 760 037 (1976). Kraus, B. J., Karmilovich, T. J.. Bouffard, R. A,, SAE Paper 760 091 (1976). Pierson, W. R.. Chemtech, 332 (1976). Pierson, W. R., bmmerle, R. H., and Kummer, J. T., SAE Paper 740 287 (1974); SA€ Trans., 83, 1233 (1975). Trayser, D. A., Creswick, F. A,, Blosser, E. R., Pierson, W. R., Bauer, R. F., SAE Paper 760 036 (1976).
Receiued for review November 1,1976 Accepted April 2 5 , 1977 Presented a t t h e Symposium o n A u t o Emission Catalysts, D i v i s i o n o f Colloid a n d Surface Chemistry, 170th N a t i o n a l M e e t i n g o f t h e American Chemical Society, Chicago, Ill., 1975.
