Ind. Eng. Chem. Prod.
Res. Dev. 1981, 20,
451-457
45 1
CATALYST SECTION
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Dynamic Behavior of Automotive Catalysts. 1 Catalyst Oxidation and Reduction Richard K. Herr Physical Chemistty Department, General Motors Research Laboratories, Warren, Michigan 48090
The oxidation and reduction of base metals in three-way automotive catalysts is a process which has been proposed to explain the dynamic behavior of these catalysts. This study provides direct evidence that the oxygen content of a base metal-containing three-way catalyst varies with engine air-fuel ratio. The oxygen content of the Pt/Pd/Rh/Ce/A1203catalyst was significantly higher after a 30-min exposure to lean exhaust than after a 30-min exposure to rich exhaust. The difference in oxygen content could correspond to complete oxidation of CO and H2 for 3 s after a change in air-fuel ratio from 15.1 to 14.1 at typical converter space velocities. The rates of change of oxygen content measured after abrupt changes in air-fuel ratio were sufficiently large to affect the performance of the catalyst under conditions of rapidly varying air-fuel ratio.
Introduction Background. Most industrial catalysts operate under steady-state or quasi-steady-state conditions. An automotive catalyst, however, is exposed to constantly varying conditions: catalyst temperature increases rapidly after the engine starts, and exhaust flowrate and composition fluctuate under most modes of operation. The dynamic behavior of these catalysts under oxidizing air-fuel ratio Conditions has been analyzed and modeled (Oh et al., 1980; Kuo et al., 1971). Three-way automotive catalysts (which control NO, CO, and hydrocarbon emissions) operate under oscillating air-fuel ratio conditions (Canale et al., 1979), however, and their dynamic behavior has not been fully analyzed. The following transient chemical processes can affect the dynamic behavior of three-way catalysts: (1)Changes in the ACTIVITY of a catalyst (a) through changes in POISONING, and (b) through changes in the OXIDATION STATES of the active metals. (2) Changes in the ACCUMULATION of reactive species in a catalyst, which can affect dynamic behavior (a) through REACTION of the accumulated species, and (b) through INHIBITION of catalytic reactions by reactive species adsorbed on the active metals. Mass transport processes will influence the rate at which these changes occur and thus affect their importance under dynamic conditions. Each of these transient chemical processes will be discussed briefly in the following paragraphs. The major poisons of automotive catalysts are phosphorus, lead, and sulfur compounds. Poisoning of the precious metals in the catalyst by sulfur is reversible. Lean air-fuel ratio excursions may remove sulfur adsorbed under rich conditions (Summers and Baron, 1979), thus temporarily enhancing NO conversion during rich portions of 0 196-4321/81/ 1220-045 1$01.25/0
cycles. Phosphorus and lead poisoning, on the other hand, are relatively slow processes and are not expected to have a direct effect on the dynamic response of a catalyst. The activities of precious metals depend upon their oxidation states. The precious metals in typical three-way catalysts (Pt, Pd, Rh) are dispersed as individual atoms or small clusters of atoms over an oxide support and are subject to oxidation (Lam and Boudart, 1977; Chen and Schmidt, 1979; Yao et al., 1979; Yao and Shelef, 1980). The oxidation and reduction of Rh will affect dynamic behavior because oxidation of Rh severely decreases its NO reduction activity (Schlatter et al., 1979). Lean excursions, when cycling about a rich midpoint, may partially oxidize Rh and lower average NO conversions from what they would be without Rh oxidation. Rich excursions, when cycling about a lean midpoint, may partially reduce oxidized Rh. Wang (1980) had included this process as a correction factor in a procedure designed to calculate emissions during the cycled operation of a three-way converter. In contrast, Schlatter and Mitchell (1980) have shown that Rh oxidized during a lean excursion led to a temporary increase in CO conversion through the water-gas shift reaction under rich conditions. The presence of Ce tended to stabilize the activated state of Rh. The addition of SOz, which is present in exhaust, lessened this effect by poisoning the water-gas shift reaction. Changes in the accumulation of reactive species in automotive catalysts will have significant effects on the dynamic performance of these catalysts. Kaneko et al. (1978) have demonstrated that a pulse of propylene will be subtracted from gas flowing over a Pt/Rh/Al,O, catalyst at 820 K and that a subsequent pulse of NO or O2will react with the carbonaceous species that are retained to form COz. The reversible adsorption of CO on precious metals competes with and inhibits the adsorption of 0 2 , thus inhibiting CO oxidation. Oh, Hegedus, and co-workers (Oh et al., 1978; Hegedus et al., 1980; Oh and Hegedus, 1981) 0 1981 American Chemical Society
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have demonstrated that the reversible adsorption of CO can affect the impulse, step, and frequency responses of Pt/Al2O3 catalysts at temperatures as high as 670 K. Cutlip (1979) has shown how the cycling of two feedstreams, one containing CO and one containing Oz, over a Pt/A1203catalyst can result in CO emissions which are lower than would be obtained if the composition of the feedstream were held steady at the time-averaged value. Oxygen atoms can accumulate as a result of the dissociative adsorption of NO and O2 on clusters of precious metal atoms. Accumulation of oxygen inhibits further dissociative adsorption of NO and O2 (Amirnazi and Boudart, 1975). The rate of desorption of oxygen from the metal is slow at exhaust temperatures (Oh and Hegedus, 1981; Gland and Korchak, 1978). The oxygen atoms will be removed by reaction with CO, H2,and hydrocarbons, especially during rich air-fuel ratio excursions when there is insufficient NO and O2 in exhaust to completely oxidize CO and hydrocarbons. This process of cyclic oxygen atom accumulation and reaction on the precious metals in automotive catalysts will tend to lower the average emissions of NO, CO, and hydrocarbons during cycled operation. The importance of this process was demonstrated by Kaneko et al. (1978), who measured the rate of increase of oxygen content of a Pt/Rh/A1203catalyst during exposure to lean exhaust. Using these data, Kaneko and co-workers developed a procedure for calculating emissions under cycled air-fuel ratio conditions. These calculations could explain average conversion measurements that could not be explained by assuming the catalyst responded instantaneously to air-fuel ratio changes. Most three-way catalysts also contain base metals in larger amounts than the precious metals (Hegedus et al., 1979; Summers and Ausen, 1979; Falk and Mooney, 1980; Gandhi et al., 1976; Cooper and Keck, 1980). The cyclic oxidation and reduction of base metals may lower emissions in a manner similar to that discussed above for the precious metals. The stoichiometric oxidation and reduction reactions of base metal-containing automotive catalysts have been studied in laboratory tests using the single-component gases 02,CO, and H2 (Schlatter and Mitchell, 1980; Gandhi et al., 1976; Yao, 1979); there has been no demonstration that these reactions contribute to the conversion of CO and hydrocarbons in exhaust. Scope of This Research. The purpose of this research is to analyze the dynamic behavior of three-way automotive catalysts. Our first objective is to determine how Ce affects the dynamic behavior of these catalysts. The addition of Ce to a Pt/Rh/A1203 catalyst has been observed to increase average NO, CO, and hydrocarbon conversions during cycled air-fuel ratio tests using engine exhaust, and average NO and CO conversions during cycled tests using simulated exhaust (Hegedus et ai,, 1979). The addition of Ce also has been shown to greatly increase the amount of oxygen than can react with a catalyst (Schlatter and Mitchell, 1980; Yao, 1979). However, there has been no demonstration that the oxygen content of a Ce-containing catalyst changes significantly with air-fuel ratio in exhaust, or that the effects of Ce addition on cycled performance are due to the oxidation and reduction of this metal. In this report, we present measurements of the oxygen content of a representative Ce-containing three-way catalyst after exposure to exhaust. Experiments were performed to determine the magnitudes and rates of change of oxygen content with changes in air-fuel ratio. Also, preliminary experiments using single reactant gases were performed to determine which species in exhaust might participate as catalyst oxidants or reductants. In a future report, we
Table I. Catalyst Properties Support: r-Al,O, pellet diameter, cm pellet density, g/cm3 surface area, mz/g pore volumes, cm3/g macro micro pore radii, A , volume-averaged macro micro Metals loadings, wt % Pt Pd Rh Ce impregnation depths, fim Pt Pd Rh Ce
0.32 0.76 105
0.41 0.57
4300 100
0.087 0.032 0.006 2.6 0-60 60-190 0-55 uniform
will present the results of experiments in which we have used infrared diode laser spectroscopy (Sell et al., 1980) to study the effect of catalyst composition on the step and frequency responses of three-way catalysts. Experimental Methods Catalyst. The characteristics of the three-way catalyst are listed in Table I. Before use in our experiments, fresh catalyst was aged for 100 h in an engine-dynamometer test cell to simulate 6400 km of exposure to automobile exhaust. Blank A1203pellets, of the same type as the catalyst, and 4 w t % Ce on A1203pellets were used in some tests. The Ce-containing pellets were prepared by impregnating blank pellets to incipient wetness with an aqueous Ce(N03)3solution, and then calcining the dried pellets in air at 770 K for 4 h. Exposure to Single Reactants. A sample of the catalyst, Ce/A1203,or blank A1203was placed in a tubular reactor and heated to 770 K. Either pure He or a mixture of the reactant gas and 0.1% Ar in He could be passed over the sample. The reactants studied were H2, CO, C3Hs (propylene), 02,NO, HzO,and C02. H20 was introduced by flowing 0.1% Ar/He through a bubbler at room temperature. Between successive exposures to reactants, the sample was flushed with He. The gas flowing from the reactor was analyzed continuously using a quadrupole mass spectrometer. After pretreatment and flushing of the sample, the feedstream was switched from He to a reactant/Ar/He mixture. The amount of reactant taken up by the sample was determined by comparing its elution curve to that of Ar. The quantities of reaction products eluted from the reactor were also measured using Ar as an internal concentration standard. Reversible retention of a reactant on a sample was measured by switching from the reactant to He after steady state had been obtained during the exposure. Exposure to Exhaust. A schematic diagram of the apparatus is shown in Figure 1. The exhaust was supplied by a 5.7-L, V8 gasoline engine operating at 1800 rev/min and 53 kPa manifold absolute pressure on Indolene clear fuel. A 2.54-cm i.d. tubular reactor, whose details are shown in Figure 2, contained 5 g of catalyst pellets. The volume of the catalyst bed was 9.6 cm3. An arrangement of four high-temperature solenoid valves (0.95-cm orifice) allowed either exhaust or dry Nzto be directed over the catalyst. A tubular furnace was used to maintain the temperature at the frontal plane of the catalyst bed at 680
Ind. Eng. Chem. Prod. Res. Dav., Vol. 20, NO. 3. 1981 453
Figure 1. Schematic diagram of the experimental apparatus. INJECTION PORT
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VACUUM FLANGE WlTH COPPER OAISIET
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Figure 2. Details of the reactor.
K. Conversions were found to be insensitive to temperature variations over a range of 50 K. The reactor outlet line and the vent line (0.85-em i.d. tubing) were each connected to a flow regulating valve, a 2.7-L pulsation dampener, and a diaphragm pump (35 L/min capacity, 1725 rev/min). Flowrates were measured with a positive displacement wet-test meter. Care was taken in the design of the solenoid valve system and the reactor to provide for rapid, controlled switches in the gas flowing over the catalyst. The flowrates of the two streams were balanced, and the N2was preheated so that both streams entered the valve arrangement at about 475 K. A pipe tee connected the two inlet solenoid valves directly to the reactor, and void and dead volumes in the reactor were minimized using aluminum inserts and solid aluminum spheres of the same diameter as the catalyst pellets. All four solenoid valves could be switched si-
multaneously be an electronic controller. The opening and closing times of the valves were specified by their manufacturer to be 4-8 ms. Conventional exhaust analysis instruments were used to measure the compositions of the inlet and outlet gas streams. In addition, the gas stream leaving the reactor could be analyzed using a quadrupole mass spectrometer. Gas was leaked from a flow-through tee valve through a differential pumping stage and an orifice into the spectrometer's vacuum chamber. The exponential response time of the spectrometer system to a step change in concentration in the main gas flow was 0.15 s. After the desired exposure to exhaust, the reactor was flushed with N2 for 10 min, and then the oxygen and carbon contenta of the catalyst bed were determined in the following procedure. First, both solenoid valve at the inlet of the reactor were closed and a small flow of N2injected into the reactor through a four-port switching valve. The switching valve was then turned to inject a stream of 2.3% 02/0.1% Ar/He into the reactor as the flow from the reactor was analyzed continuously with the mass spectrometer. Ar was monitored to determine the elution curve of an inert species, and it also served as an internal concentration standard in the C 0 2 measurement. The elution curve of O2 relative to that of Ar was used to determine the amount of O2that reacted with the catalyst. Part of the O2reacted with carbon on the catalyst to form C02, which was also measured (H20formation was negligible). The oxygen content of the catalyst after exposure to exhaust was calculated by subtracting the net oxygen uptake (measured uptake less the amount reacted to form COJ from the maximum oxygen content, or oxygen capacity of the catalyst bed. The oxygen capacity of the catalyst bed is defined as the maximum amount of oxygen that is retained by the catalyst after treatment with O2and that can be removed by reaction with CO or H2. The oxygen content was also determined by measuring the amount of CO that r e a d with the catalyst to form COPduring injection of a 3.3% CO/O.l% Ar/He stream, after a separate exposure to exhaust. The results of the two procedures agreed to within 15%. Results and Discussion Exposure to Single Reactants. These experiments were performed to investigate the interaction of single components of exhaust with the Pt/Pd/Rh/Ce/Al,O, catalyst. The quantity of a component that reacted and the quantities of products formed could be measured, but the experiments were insensitive to the kinetics of these reactions. Thus, we can identify and determine the extent of reactions, but we cannot estimate their relative importance. In addition, reactions may occur in exhaust which were not identified here. After treatment of the catalyst with Hz, an uptake of 37 rmol of 02/g of catalyst was measured. The oxygen was bound strongly to the catalyst since no significant amount of reversibly adsorbed O2 eluted when the flow over the catalyst was switched from 02/Ar/He to He. Within the accuracy of our experiments,all of the oxygen that r e a d with the catalyst could be removed by CO. When CO was passed over the OTtreated catalyst, 72 pmol of CO/g reacted. A total of 69 rmol of C02/g had eluted by the time the CO uptake was complete, but low levels of C02 contiiued to elute for some time. C02may have been retained by reaction with Ce oxide in the catalyst to form carbonates (Rosynek, 1977; Artamonov and Sazonov, 1971; Breyesse et al., 1973). When the Hz-treated catalyst was expoeed to pure C02for 30 min, the subsequent O2uptake was decreased by 45%. COPappeared to only block the
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Table 11. Oxygen Uptake Measurements oxygen uptakea
Pt/Pd/Rhl CeIALO, WAW, Al,o,.'
Pt+ pmol Pd + Rh, Ce, O/g rrmollg rmollg 74 8 190
pmol OJg 37 55
