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Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 288-293
SYMPOSIA SECTION
I.
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
Three-way Catalyst Response to Transients James C. Schlatter‘ and Patricia J. Mitchell Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
Because the exhaust composition in closed-loop emission control systems fluctuates about the stoichiometric set point, there is considerable interest in understanding and improving catalyst behavior in this oscillating environment. We have measured adsorption capacities, responses to step changes in feedstream composition, and performance in cycled laboratory feedstreams in order to clarify features of the transient response of three-way catalysts. Our laboratory results are best explained on the basis of a temporary increase in water-gas shift activity rather than the more common oxygen storage explanation. The phenomenon is associated with the presence of rhodium in the catalyst and is enhanced when cerium is also present. Sulfur dioxide in the feedstream was highly detrimental to the water-gas shift reaction; so the applicability of these results to the exhaust environment remains an open question.
Background
The operating environment of three-way catalysts is characterized by oscillations in the feedstream composition which occur with a frequency on the order of 1Hz (Canale et al., 1978; Seiter and Clark, 1978; Wallman and Engh, 1977). Gandhi et al. (1976) pointed out the potential benefits of including an “oxygen storage” component in the catalyst to moderate the effects of the rapid changes between rich and lean exhaust stoichiometries. Such a storage component would adsorb or react with the excess oxygen present during excursions to the lean side of the stoichiometric set point. During subsequent rich excursions, the stored oxygen would then be available for removing the carbon monoxide and hydrocarbons present in the otherwise oxygen-deficient stream. Cerium has perhaps received the most attention as a base metal additive for three-way catalyst formulations. It was shown in both laboratory and exhaust environments to enhance the performance of a fresh platinum-rhodium catalyst under oscillating conditions (Hegedus et al., 1979). Recently, attention has been focused on the interaction of cerium with supported noble metals (Summers and Ausen, 1979) and on the storage capacity of such systems (Yao, 1979). In our laboratory reactor we have examined the means by which cerium is able to enhance the performance of three-way catalysts under transient feedstream conditions. By simplifying both the feedstream and the transients, we have been able to provide new insight into a t least one aspect of cerium’s contribution to three-way catalyst performance. E x p e r i m e n t a l Section Catalysts. All samples were prepared by impregnation
of Al2O9spheres (3 mm diameter, 112 m2/g) to incipient 0196-4321/80/1219-0288$01.00/0
wetness. For samples containing Ce, the Ce was deposited on the support and calcined in air at 500 “C for 4 h prior to impregnation with the noble metals. The same calcination step (4 h at 500 O C in air) followed the latter impregnations. Pt and Rh, if both present, were deposited simultaneously. Metal loadings will be given when the results are presented. Adsorption. Catalyst capacities for adsorbing oxygen and carbon monoxide were measured in a constant-volume system equipped with a quartz spiral manometer to monitor the pressure. The standard pretreatment given all samples included flowing O2 while heating (- 10 “C/ min) to 500 “C, continuing the O2 flow for 1h, evacuating for 1 h, and cooling to room temperature under vacuum. The same thermal steps were then repeated using flowing H2, after which the adsorption isotherm was measured. For adsorptions at 500 “C, only the last cooling step was omitted. The extent to which adsorbed O2 could be removed by CO or H2was estimated by measuring the O2 uptake at 500 “ C in the manner just described, evacuating the sample for 45 min, exposing it to CO or H2at -20 kPa for -45 min, evacuating for 45 min, and then remeasuring the O2 uptake. This second measurement reflected the amount of O2 adsorption capacity regenerated by a short exposure to a reducing gas. Step Changes. In order to isolate features of catalyst response to cycling conditions, it is instructive to introduce the simplest of transient inputs-a step change in composition. Our laboratory reactor for three-way catalyst studies (Schlatter et al., 1979a; Schlatter, 1978) is ideally suited to such experiments. It has the facilities for independently blending t,wo simulated exhaust streams and then alternating them across the catalyst in any desired manner. The “full feed” experiments used the rich and (C 1980 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Table 111. Effect of Ce Loading on 0, Uptake at 500 C Ce loading 0, uptake wt % pmol/g pmol/g 0.5 36 8 1.0 71 17 5.0 360 46
Table I. Composition of Cycled Feedstreams concn, vol % component
rich
lean
co
1.00 0.33 0.054 0.185 0.045
0.20 0.07 0.040 0.985 0.055 10.0 10.0 balance
H* C3H6 0 2
NO COZ
10.0 10.0
HZO NZ
balance
Table IV. Removal of Adsorbed Oxygen at 500 C amount, pmolig, initial removed by 0 2
Table 11. Cerium Effect on Oxygen Adsorption metal content (son A1,0,)
0, adsorption, pmol/g _ .
wt %
-
0.045 Pt 5.0 Ce 0.038 Pti5.O Ce
pmol/g
25°C
2.3 360 362
1.0 26.0 20.0
289
500°C 4.3 46.0 50.0
lean mixtures given in Table I. The rich and lean feeds simulate engine operation at air/fuel ratios of 14.2 and 15.2, respectively. Catalytic activity was always determined at 550 "C and 104000 h-' space velocity (13 L/min (STP) through 7.5 cm3 of catalyst). The procedure for measuring step responses began with the catalyst stabilized in the rich stream. Then the lean feed was directed through the reactor for 6 min, after which the feed was changed back to the rich composition until the outlet concentrations had restabilized. Thirty minutes or so was usually required to reach stable conversion levels; then the sequence (and results) could be repeated quite reproducibly. We showed in our earlier paper (Schlatter et al., 1979a) that the time scale for the step change in composition is about 1s at the front of the catalyst bed. Inlet and outlet concentrations were monitored continuously with nondispersive infrared (for CO and C02) andl polarographic (for 0,) analyzers. A gas chromatograph with a Molecular Sieve 5A column and N2carrier was used far H2 analyses; sample injections had to be spaced a minimum of 90 s apart. Cycled Feedstreams. Catalyst response to oscillations a t frequencies characteristic of closed-loop automotive systems (0.25 to 2 Hz) was also evaluated in this study by switching between rich and lean feeds a t the appropriate frequencies. The feeds were the same as those used in the step response experiments (Table I). In fact, generating a comparison between the step response and the cycled behavior of a catalyst was simply a matter of changing the timing of the feedstream switching valves. The steady, time-averaged conversions observed a t any particular oscillating frequency were independent of the sample history (i.e., the sequence of performing the experiment was immaterial). Note that the average of the two feedstreams is a stoichiometric mixture.
Results Adsorption Capacities. The extent to which Ce addition affects the catalysts' capacity for oxygen adsorption was determined a t room temperature and at 500 "C. The samples characterized were 0.045 wt % Pt/Al,O,, 5 wt % Ce/A120,, and 0.038 wt % Pt on the 5 wt % Ce/Al,O, support. The O2 uptakes, obtained by extrapolating to zero pressure the (linear) isotherms measured over the range from 10 to 40 kPa, are recorded in Table 11. Certain features of the data in Table I1 are very clear. First, Ce did indeed enlarge the catalyst's capacity for retaining OD Second, this capacity was significantly larger (approximately double) a t 500 "C than at 25 "C. Third,
catalyst Ce Pt/Ce
adsorbed, evacupmol/g ation 44 2 45 2
CO 24 20
M Z
36 33
although Pt has been reported to enhance the rate of 0, uptake by Ce (Yao, 1979), the ultimate capacities which we measured (and also those measured by Yao) were not affected appreciably by the presence of Pt. Fourth, the O2 uptakes were reasonable with respect to the amount of Ce present, representing less than 15% of the amount required to convert the Ce totally to CeO,. Finally, it should be noted that all three samples adsorbed only small amounts of CO relative to O2 at 500 "C after the standard pretreatment; the largest value was 7 pmol/g on the Pt/Ce catalyst. Table I11 shows how the 0, uptake varied with Ce loading; the uptake changed proportionately with loading at low Ce levels but was less than proportional a t the highest Ce level. This latter observation has also been made using Ce loadings ranging from 7 to 22 wt % on A1203 (Yao, 1979). The useful O2storage capacities of three-way catalysts are actually not represented by these total O2uptakes, but by the amount of 0, that can be stored and released during the rapid lean-rich transitions in the exhaust. To estimate the amount of this available storage, we exposed an 0,covered sample at 500 "C to a reducing gas (CO or H,) for 45 min, evacuated the sample, and then remeasured the O2uptake. This second measurement provides an estimate of the amount of O2 adsorption capacity liberated by exposure to rich conditions. It is actually a more important value than the total uptake, since a large uptake is of no consequence unless it is reversible on the time scale of exhaust fluctuations. Because our adsorption experiments were on a time scale three orders of magnitude longer than exhaust fluctuations, the results can only be viewed as upper bounds on the actual dynamic capacities of 0, storage and release. Moreover, since engine exhaust is a complex mixture of both oxidizing and reducing components, laboratory measurements with pure gases can only be approximations for storage behavior in exhaust. Table IV shows the extent of removal of adsorbed O2 a t 500 O C by either evacuation or exposure to a reducing gas. The catalysts were the same as those from Table 11; the differences in the initial 0, adsorption between Table IV and Table I1 show the uncertainty in the measurements. Table IV shows that less than 5% of the initial 0, uptake was reversible by evacuation, implying that the O2 had indeed bonded strongly to the Ce. About half of the O2 incorporated with the Ce could be removed by CO and about three-fourths by H2. Again, since we measured capacities and not rates, we found no influence of the Pt on the sample characteristics. Response to Step Changes. Further information about the dynamics of O2 and CO interactions with Ce-containing catalysts was obtained by observing the response to step changes in feedstream composition. Figure l a depicts the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
5
8
'
R
q
I
Table V. Feedstreams to Study Water-Gas Shift Rich
concn, vol %
c
component
rich
lean
c
co
1.0
0
0 2
0.2 1.0 10.0 balance
e C
8
0 5 .
10.0
KO N* 0
10
20
30
5
Time mtn
r'p
balance
1 .o 'Rich
I
Lean Rich
c o '
c
e
I6
0.5
.
0
0 0
10
20
30
Time. rnin
Figure 1. Inlet (a) and outlet (b) CO concentrations during a step change experiment with a Pt-Rh//Ce/A1203 catalyst.
inlet CO concentration as a function of time during the procedure described in the Experimental Section. With a catalyst containing 0.05 wt 70Pt and 0.005 wt % Rh on 0.7 wt % Ce/A1203,the outlet CO concentration is shown in Figure lb. The total feedstreams were those listed in Table I, but CO was the component most dramatically affected by shifts in the stoichiometry; so we restrict our attention to CO. A comparison of Figure l a and l b shows the catalyst efficiency during the experiment. CO conversion was minimal (-9%) under the stable rich conditions (time
