Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 387-396
387
CATALYST SECTION
Dynamic Behavior of Automotive Catalysts. 2. Carbon Monoxide Conversion under Transient Air/Fuel Ratio Conditions Rlchard K. Herz’ and Joanna B. Klela Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
Jeffrey A. Sell phvsics Department, General Motors Research Laboratories, Warren, Michigan 48090
This report presents the first analysis of the CO conversion response of three-way automotive catalysts following step-changes in engine airlfuel ratio (A/F) and during individual A/F cycles. Infrared diode laser spectroscopy was used to measure CO concentrations in exhaust at the inlet and outlet of a catalytic converter as the engine A/F setting (1) was changed in steps or (2) was cycled in square waves. The concentration measurements at the converter inlet and outlet were made simultaneously and with hgh time resolution. Two fresh three-way catalysts were compared: (1) cerium-containing Pt/Pd/Rh/Ce/Al,O, beads and (2) similar but cerium-free Pt/Pd/Rh/Al,O, beads. The results presented contribute to the understanding of the performance of catalysts under the rapidly fluctuating A/F conditions characteristic of automobile operation.
Introduction Catalysts are used in the exhaust systems of most new gasoline-fueled automobiles to control the emission of NO, CO, and hydrocarbons. The term “three-way catalyst” is applied to a catalyst designed to convert the three controlled exhaust components simultaneously. Three-way catalysts typically contain Pt and Pd, which are added to catalyze the oxidation of CO and hydrocarbons, and Rh, which is added to catalyze the reduction of NO to N2. Various base metals may be added to stabilize the A1203 support, promote the water-gas shift reaction, and enhance the performance of the catalyst under dynamic conditions. The precious metals (Pt,Pd, Rh) and the base metals are dispersed over the internal surface of porous Al,O,. High conversion of NO, CO, and hydrocarbons can be achieved simultaneously over a three-way catalyst when an engine is operated at the stoichiometrically balanced air/fuel ratio (“stoichiometric point”). During operation of a warmed-up three-way catalyst in an automobile, however, the mass air/fuel ratio (A/F) cycles about the stoichiometric point at frequencies of 0.5 to 2.0 Hz as a result of the characteristics of the A/F control system (Grimm et al., 1980; Herz, 1982). Under these cycled A/F conditions the time-averaged emissions of the three controlled exhaust components are often greater than would be obtained during steady-state operation at the stoichiometric point: CO and hydrocarbon emissions can be greater during the ”rich” (excess fuel) periods of A/F cycles, and NO emissions can be greater during the “lean” (excess air) periods. In some emission control systems an oxidation catalyst and supplementary air injection follow 0196-4321 I831 1222-0387$01.50/0
the three-way catalyst to provide additional control of CO and hydrocarbons (“dual-bed system”). There are several indications that the behavior of a three-way catalyst during A/F cycling is complex (Herz, 1982). One is that time-averaged conversions obtained under cycled conditions are greater than would be expected if the catalyst were to respond instantaneously to A/F changes (Schlatter et al., 1983). Also, catalysts with different compositions may perform similarly under steadystate conditions but perform differently under cycled conditions (Hegedus et al., 1979). We wish to identify and understand the processes that determine the performance of three-way catalysts under dynamic conditions so that we can design improved catalysts and A/F control strategies and thereby reduce the cost and complexity of emission control systems. For example, the formulation of a three-way catalyst which maintains high conversions of NO, CO, and hydrocarbons over 80 000 km in an automobile that currently requires a dual-bed system would allow the elimination of the supplementary air supply and the oxidation catalyst. In order to analyze the dynamic behavior of a three-way catalyst, one would, ideally, like to measure the concentrations of all reactants and products in the exhaust at the inlet and outlet of the converter simultaneously and with high time resolution. This desire has not yet been fully realized. Conventional exhaust analyzers respond too slowly to follow rapid concentration changes. Because of this limitation, most studies of the dynamic performance of three-way catalysts have involved the use of conventional automotive exhaust analyzers to measure time-av0 1983 American Chemical Society
388
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 Table I. Engine-Out Exhaust Compositions’J Screen
Dvnamometer
i l
airlfuel ratio C0,vol % HC,b vol % as C J , NO,vol % 0,, vol %
14.1
14.6
15.1
15.5
1.3 0.062
0.6 0.052
0.28 0.040
0.2 0.036
0.099 0.28
0.10 0.58
0.10 1.1
0.11 1.3
a All values are averages and are reported o n a dry basis. Both H,O and CO, are present in exhaust a t levels of about 10%. H, is present in exhaust a t about one-third Hydrocarbons. the level of CO.
Figure 1. Block diagram of the apparatus. Infrared diode laser spectroscopy waa used to measure CO concentrations at the inlet and outlet of the catalytic converter simultaneously and with high time resolution.
eraged concentrations during A/F cycling. Recently, Shulman et al. (1982) have used a timed-sampling technique with conventional exhaust analyzers to determine the concentration profiles of CO, NO, O,, and hydrocarbons during an average A/F cycle at the inlet and outlet of a catalytic converter. The technique is limited to the analysis of low frequency cycles (0.5 Hz or slower) and the profiles are obtained by averaging over many cycles. We have developed an unconventional exhaust analysis technique for CO, infrared diode laser spectroscopy, and have used it to measure CO concentrations in exhaust with high time resoltuion (Sell et al., 1980,1981). The rapid response of our laser system has allowed us to measure CO concentrations at the inlet and outlet of a converter during individual A/F cycles at the cycling frequencies characteristic of three-way systems. In part 1of this work (Herz, 1981) we reviewed transient chemical processes which can affect the dynamic behavior of three-way catalysts and determined that the oxygen content of a cerium-containing catalyst changed with A/F at rates which were sufficiently great to affect the CO conversion performance of the catalyst under cycled A/F conditions. In this report we present the first analysis of changes in CO conversion which occur over three-way catalysts following A/F step-changes and during individual A/F cycles. The responses of a Pt/Pd/Rh/Ce/A1203 catalyst and a similar but cerium-free Pt/Pd/Rh/A1203 catalyst are compared with each other. Cerium has been found to enhance the CO conversion performance of three-way catalysts during A/F cycling (Hegedus et al., 1979; Schlatter and Mitchell, 1980; Kim, 1982). We also introduce a new procedure for evaluating the dynamic response of a catalyst: the measured responses of each catalyst are compared with the responses that would have been measured if the catalyst had responded instantaneously to the A / F changes. Experimental Methods Apparatus and Procedure. A block diagram of the experimental system is shown in Figure 1. The 5.7-L V8 gasoline engine was operated at 1700 rpm with an intake manifold pressure of 54 kPa (abs) on Indolene Clear fuel. Exhaust from both manifolds joined at a tee and flowed through 50 mm i.d. tubing to the inlet of a standard GM catalytic converter. The converter was filled with 2620 cm3 (1370 g) of catalyst beads. The flow rate of exhaust
through the converter was 3.6 X lo4cm3s-l (including HzO vapor and corrected to STP) which resulted in a space velocity of 50 O00 h-’ (STP). Exhaust temperatures at the converter inlet ranged between 740 and 750 K, depending upon the engine operating conditions. Taps before and after the converter (not shown) carried small streams of exhaust through 5 mm i.d. tubing to a set of standard exhaust analyzers. These analyzers were used to determine the concentrations of CO, NO, 02,CO,, and hydrocarbons at the inlet and outlet of the converter. Because of their slow characteristic response times (ca. 1-10 s), the analyzers were used only to determine the time-averaged values of the concentrations under constant engine operating conditions. The average A/F obtained under constant operating conditions was computed using the average concentrations of CO, NO, O,, and hydrocarbons, the hydrogen/carbon atom ratio of the fuel (1.91), and the following assumptions: (1)H, is present in exhaust at one-third the level of CO, and (2) hydrocarbons can be expressed as propylene equivalents. The stoichiometrically balanced mass A/F, the “stoichiometric point”, with this fuel is 14.6. Table I lists the engine-out exhaust compositions obtained at the stoichiometric point and at the rich and lean A/F settings used in this work. All concentrations listed in this report are given in units of volume % or ppm of the component in the exhaust after removal of H20 from the exhaust. H20 is present in exhaust at a level of about 10%. The “lean” (excess air) A/F setting in our experiments was obtained by adjusting the carburetor. The “rich” (excess fuel) A/F setting was obtained by adding additional fuel through a standard fuel injector which was fitted into a spacer mounted between the carburetor and the intake manifold. The A/F setting was changed in steps or was cycled in square waves by turning the fuel injector on and off with the use of an electronic timing circuit (A/F controller). CO concentrations in the exhaust flowing into and out of the converter were determined by measuring the absorption of infrared radiation from a semiconductor diode laser tuned to the center of the CO P7 absorption line at 2127.685 cm-’. The beam of the infrared diode laser was chopped at 3.5 kHz, passed through a monochromator for selection of the desired laser mode, and split into two infrared beams. Sapphire windows mounted on the exhaust pipe allowed one infrared beam to pass through the exhaust flow at the inlet of the converter and the other beam to pass through the exhaust flow at the outlet of the converter. The components of signals from the HgCdTe infrared detectors that were in phase with the chopper’s reference signal were separated by two lock-in amplifiers and were recorded by a minicomputer. The relatively high chopping frequency was needed to enable the lock-in amplifiers to distinguish between the signal from the laser and
Ind. Eng. Chem. Prod. Res. Dev., VOl. 22, NO. 3, 1983 389
Table 11. Catalyst Properties Support: r-Al,O, pellet diameter, cm pellet density, g cm-3 surface area, m z g-l pore volumes, cm3 g-’ macro micro pore radii, nm, volume averaged macro micro Metals Cat. P loadings, w t % Pt Pd
Rh Ce impregnation depths, pm Pt Pd
Rh Ce
0.32 0.76 105
0.41 0.57 430 10 Cat. P/Ce
0.084 0.024 0.004 0
0.087 0.032 0.006 2.6
40-290 30-330 0-450
0-60 60-190 0-55 uniform
the strong infrared background from the hot engine and exhaust piping. The 1040% response time of the laser system to a step change is 0.025 s at the output of the lock-in amplifiers. This response time, which is limited by the filtering (RC) time constant used with the amplifiers, is sufficiently fast to follow accurately all concentration changes which occur in exhaust (Sell et al., 1980). The CO concentration at a given time t was calculated using the equation a t ) = (1/LuL) x In V o / I ( t ) l (1) where C ( t ) is the CO concentration in percent on a dry basis, a is the absorption coefficient, L is the cell pathlength, Io is the signal which would be obtained in the absence of CO, and I ( t ) is the signal recorded by the computer. A longer pathlength was used at the converter outlet (14.8 cm) than at the inlet (8.0 cm) because the conversion of CO in the catalyst bed resulted in lower CO concentrations at the converter outlet. In each experiment the laser response was calibrated using the standard exhaust analyzer for CO (a nondispersive infrared analyzer calibrated using known mixtures of CO in N2). To do this, we determined the average laser signals and the average CO concentrations measured by the analyzer at both the steady-state rich and lean A/F settings used in the experiment. Using these readings, we computed the a’s and Io’s for the inlet and outlet measurement points. Because the laser measures only a narrow wavenumber range (ca. lo4 cm-’) a t the maximum of the CO absorption line, the detector signal remains constant for a constant percentage of CO when the exhaust pressure fluctuates: an increase in the line area due to an increase in total pressure is counterbalanced at the line maximum by a broadening of the line. Further details of the experimental system are given elsewhere (Sell et al., 1980, 1981). Catalysts. The characteristics of the two catalysts studied in this work are listed in Table 11. One catalyst contained only Pt, Pd, and Rh and is labeled “Catalyst P”, where P stands for “precious metal”. The other catalyst contained the base metal Ce in addition to the precious metals and is labeled ”Catalyst P/Ce”. This is the same catalyst that was studied in part 1. Note in Table I1 that the precious mettal loadings are somewhat higher in Catalyst P/Ce than in Catalyst P. Prior to the measurements reported here, Catalyst P/Ce had been “aged” for 100 h in an engine-dynamometer test
cell to stimulate 6400 km of exposure to automobile exhaust. Catalyst P was not aged prior to use but was exposed to engine exhaust in the experimental apparatus for several hours prior to the measurements reported here. The different pretreatment conditions probably did not have a significant effect upon the results presented here. Several experiments performed with a fresh Ce-containing catalyst showed this catalyst to behave similarly to the 100 h-aged catalyst P/Ce. Estimation of Instantaneous Response. We define the terms “measured response curve” and “measured outlet curve’’ to be equivalent and to refer to the measured outlet CO concentration plotted vs. time after a step-change in A/F setting or during A/F cycling. For each of the experiments discussed below, the measured response curve is compared with the “estimated instantaneous response curve” or equivalently, the “estimated outlet curve”. In the following paragraphs we define our terminology further and describe how we estimate instantaneous response curves. We define the term “catalyst response” to refer to the physical and chemical changes occuring within catalyst beads that affect the rate at which reactant and product concentrations change at the converter outlet following a change in inlet exhaust composition. The rate at which a change is observed at the converter outlet is also affected by the flow of exhaust through the converter over the catalyst beads. However, the flow of exhaust through the converter closely approximates that through a plug-flow reactor (demonstrated by the first experiment in the Results and Discussion section) and merely introduces a delay between changes at the converter inlet and outlet. The plug-flow of exhaust allows us to consider the flow of a train of volume-elements of gas through the converter in the following development. We define instantaneous catalyst response by specifying the following. When a new volume-element of gas flows over a volume-element of catalyst at any point in the converter (1) the intrinsic activity of the catalyst either remains constant or changes instantaneously to reach a new level as a result of a difference in composition between the new and the preceding volume-elements of gas, and (2) there are no changes in the amounts of reactive species held by the catalyst that affect the composition of the new volume-element of gas, where the reactive species may be held by adsorption and by reaction with the catalyst. Under these specifications, the conversion of CO in a volume-element of gas is a function of the composition of that volume-element but is independent of the composition of all preceding volume-elements. In other words, the catalyst is specified to have no “memory” of its previous exposure to exhaust. This means that the conversion of CO in a volume-element of gas with a given composition will be the same when preceding volume-elements have differing compositions (i.e., under dynamic conditions) as when preceding volume-elements have the same composition (i.e., under steady-state conditions). Stated simply, when catalyst response is instantaneous under dynamic conditions, steady-state conversions are obtained instantaneously. This behavior allows us to use CO concentrations measured at the converter inlet and outlet under steady-state conditions to estimate instantaneous response curves. The first step in the estimation of instantaneous response curves was the measurement of inlet and outlet GO concentrations over a range of constant A/F settings. Figure 2 is a plot of the ratio of outlet to inlet CO concentrations at “steady-state”, (Gout/ Ci,Js-, vs. inlet GO
390
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 i. I
15515’ I
1
hI
1425
I
I
c 0
02
04
Pcrllu
1465
I OR
’4‘
I I Pa
I 10
I
I
12
14
I
Figure 2. The ratio of the converter outlet CO concentration to the converter inlet CO concentration plotted vs. the inlet CO concentration. Each point plotted was determined from average concentration measurements obtained as the engine was operated with a constant air/fuel ratio setting.
concentration, Ch The CO concentrations measured under these conditions are averages since the exhaust composition fluctuates when the engine is run at a constant A/F setting. The CO conversion performances of the two catalysts are similar under these relatively constant conditions. Under lean conditions CO conversion approaches 100% over both catalysts, and under rich conditions CO conversion is limited over both catalysts by the essentially complete conversion of 02. (Data obtained for catalyst P/Ce for Ci, between 0.6 and 0.8% were unreliable and are not shown. In previous work we have found that Ce addition produces no significant differences in steady-state CO conversion in this operating range. Also, the inlet composition crosses this operating range only briefly during the transient experiments.) Before we can use these measurements to estimate instantaneous response curves, we must assume that volume-elements of exhaust at the converter inlet that have the same CO concentration also have the same overall composition, whether the CO concentrations are measured under steady-state or dynamic conditions. This is a reasonable assumption because the composition of a volume-element of exhaust will be determined solely by the intake and firing events of individual cylinders in the absence of significant mixing of exhaust from individual cylinders in the exhaust manifolds and piping. Although mixing must occur to some extent in the exhaust manifolds, we demonstrated in earlier work (Sell et al., 1981) that cylinder-to-cylinder variations in CO levels can be distinguished at the converter inlet. CO concentrations measured at the converter inlet under dynamic conditions and the steady-state data of Figure 2 were used to estimate the CO concentrations that would have been observed at the converter outlet if the catalyst’s response were instantaneous. In the following equation C*out(t + 7 ) is the estimated ,outlet CO concentration at time t + 7,where 7 is the time required for a volume-element of exhaust to flow from the inlet to the outlet measurement point, Ci,(t) is the measured inlet CO concentration at time t , and the ratio (Cou,/C,)sa is obtained at each time t from Figure 2 by using the value of Cin(t) on the Cin axis. The value of 7 was determined to be 0.06 s from measurements made with blank M 2 0 3beads in the converter. This equation was used to calculate the estimated outlet curves shown below.
,
I
0;
5
I 1 15
10
20
T ~ m eIs1
Figure 3. Step-responses measured over blank A1203beads. The mass air/fuel ratio (A/F) setting was stepped from 14.0 (“rich”)to 16.9 (“lean”)and then back to 14.0. The inlet curve is displaced up by 0.5% to separate it from the nearly identical outlet curve.
-Measured
Outlet
Time I s /
Figure 4. Rich(l4.1)-to-lean(15.5)step-response measured over Catalyst P/Ce. Solid curves: CO concentrations measured at the converter inlet (A) and at the converter outlet (B).Dashed curves in A and B: “estimated outlet” concentration curve that would have been obtained if the catalyst had responded instantaneously.
Results and Discussion The responses of blank AlZO3beads, Catalyst P, and Catalyst P/Ce to A/F step-changes are discussed first. Next, the responses of the two catalysts to A/F cycling are presented. This section is concluded by a comparison of step-change and cycled experiments. Blank A1203Beads. Figure 3 shows CO concentrations measured at the inlet and outlet of the converter which was loaded with blank M 2 0 3beads. The A/F setting was stepped from 14.0 (rich) to 16.9 (lean) and then back to 14.0. We have displaced the inlet curve upwards by 0.5% to separate it from the outlet curve. The 0.06 s delay ( 7 ) between the changes in the curves is too small to be noticeable on this plot. The result that the inlet and outlet curves are nearly identical demonstrates clearly (1)that the flow through the converter closely approximates that through a plug-flow reactor and (2) that the stainless-steel converter body and the A1203 catalyst support did not contribute to conversion of reactants or the dynamic behavior we discuss below.
Ind. Eng. Chem. Prod. Res.
2or
ESI8maled
-Measured
Figure 5. Lean(l5.5)-to-rich(l4.1)step-responses measured over Catalyst P (left panels) and Catalyst P/Ce (right panels).
Rich-to-Lean Steps. Figure 4 presents CO concentrations measured before and after a step-change in A/F setting from 14.1 (rich) to 15.5 (lean) over Catalyst P/Ce. The rapid decrease in CO concentration occurred over roughly 0.2 s, or the period of three firing cycles of the engine. During rich operation before the A/F transition, the fluctuations in the estimated outlet curve in Figure 4A mimic those in the measured inlet curve except that a conversion of 10-20% is shown and changes in the outlet curve occur 0.06 s (7)later than the corresponding changes in the inlet curve. During lean operation after the step, the estimated outlet concentration is close to zero as a result of the relatively low CO level in the inlet and the expected 95% conversion. Figure 4B presents a comparison of the estimated instantaneous response and measured response curves. The only significant differences between the curves occur during rich operation where the measured outlet CO concentration fluctuates less than it would if the catalyst response were instantaneous. This smoothing behavior was also observed during rich operation over Catalyst P (noted below). The result that no smoothing of CO concentration fluctuations occurred over blank Alz03beads allows us to conclude that this noninstantaneous response of the catalysts during rich operation is related to processes involving only the precious metals and the Ce in the catalysts. Note that the measured and estimated outlet curves coincide in Figure 4B during the rapid decrease in CO concentration. Now, using the estimated outlet curves for comparison and remembering the delay 7,note in Figures 4A and 4B that the measured outlet CO concentration decreased at a faster rate (steeper slope) than the measured inlet CO concentration. This is because the inlet O2 concentration (not measured during the transition) increased as the inlet CO concentration dropped so that the conversion of CO increased at the same time that CO eluted. The results obtained for 14.1-to-15.5steps over Catalyst P and for 14.1-to-15.1 steps over both catalysts are similar to those shown in Figure 4. The outlet responses of other exhaust components, especially those of NO and 02,may not have been as rapid as that of CO and the responses of these components over Catalyst P may have differed from their responses over Catalyst P/Ce. Lean-to-RichSteps. The solid curves in the upper and lower panels of Figure 5 are the inlet and outlet CO concentrations, respectively, measured as the A/F setting was stepped from 15.5to 14.1. The dashed curves in the lower
Dev., Vol. 22, No. 3, 1983 391
panels are the outlet CO concentrations estimated for instantaneous catalyst response. The panels on the left are for Catalyst P, and the panels on the right are for Catalyst P/Ce. There is an obvious qualitative difference between the step responses of the two catalysts: the measured outlet CO concentration increased more slowly over Catalyst P/Ce than over Catalyst P. Such a slow response is desirable since we wish to minimize the emission of CO from the converter. For Catalyst P the measured outlet curve approaches the estimated outlet curve about one second after the estimated outlet curve starts up from the baseline. At longer times the average levels of the two curves match; however, the amplitudes of the fluctuations in the measured outlet curve are less than in the estimated outlet curve. The same behavior was noted above in the left side of Figure 4B. With Catalyst P/Ce the measured outlet curve takes much longer to approach the estimated outlet curve than with Catalyst P; the average levels of the estimated and measured curve for Catalyst P/Ce do not match until about 30 s after a lean-to-rich A/F change. Only 8 s of the response are shown here to allow more detail to be observed. In each experiment the area under the estimated outlet curve multiplied by the flow rate of exhaust equals the amount of CO that we estimate would have come out of the converter if the catalyst had responded instantaneously. The area under the corresponding measured outlet curve multiplied by the flow rate is equal to the amount of CO that actually came out of the converter. The difference between the two areas represents CO that was not emitted from the converter during this time period as a result of the noninstantaneous response of the catalyst. The difference in area, or the amount of CO “missing” following the lean-to-rich step, is 8 mmol over Catalyst P. Over Catalyst P/Ce the total amount of CO missing is 100 mmol (70 mmol for the period shown in the figure). The same results were obtained with initial lean A/F settings of 15.1 and 15.5. This is not surprising since much of the reduced emission of CO over Catalyst P/Ce may be related to reaction of CO with oxygen held by the catalyst, and we found in part 1 that the reactive oxygen content of this catalyst when exposed to 15.1-A/F exhaust was close to the maximum amount of reactive oxygen that the catalyst could hold. The noninstantaneous responses we have observed for both catalysts means that one or both of the specifications of instantaneous response which were listed in the Experimental Methods section do not hold. That is, (1)the intrinsic activities of the catalysts changed at observable rates, and/or (2) the amounts of reactive species held by the catalysts changed significantly. Transient chemical processes that may have contributed to these changes are discussed next. Processes Affecting Catalyst Response. Below we list the most likely transient processes which could have occurred over the precious metals and the Ce in the catalysts and which may have contributed to the noninstantaneous step-responses of the catalysts: (1)reaction of CO with oxygen atoms bonded to the precious metals and with hydroxyl groups or oxygen atoms bonded to Ce; (2) adsorption and accumulation of CO over the precious metals; (3) reaction of CO and HzO to form COP and H z (the wate-as shift reaction) over surface sites activated during lean operation. These processes also may have contributed to the smoothing of CO concentration fluctuations that was observed during operation at constant rich A/F settings.
392
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983
Table 111. Comparison between Metal Loadings and CO ‘‘Missing’’Due to Noninstantaneous Catalyst Response Cat. Cat. P
PICe
8 6
100 70
7
8
0
190
CO missing following
lean-to-rich step
in units (mmol CO) in units (pmol CO g-’ of cat.)a sum of F’t, Pd, and Rh
in catalyst (pmol g-’ ) Ce in catalyst (pmol g-’ )
a
1370 g of cat.
Each of these processes will now be examined as a possible contributor to the temporary reductions in CO emissions that followed the lean-to-rich A/F step-changes. Previous work has demonstrated that the first two processes can occur in exhaust and affect the step-responses of three-way catalysts. Kaneko et al. (1978) measured the changes in reactive oxygen content of a Pt/Rh/A1203 catalyst during step-changes in engine A/F and found that the conversion of CO during A/F cycling could only be explained when these oxygen content changes were taken into account. In part 1 we measured significant changes in the reactive oxygen content of Catalyst P/Ce after abrupt changes in engine A/F. In laboratory experiments using single reactant gases, Oh and Hegedus (1982) determined that both (1) reaction of CO with adsorbed oxygen and (2) adsorption and accumulation of CO affect step-responses over supported Pt at 723 K. We have shown in a modeling study of CO oxidation (Hen and Marin, 1980) that the surface of supported Pt is covered by oxygen atoms under lean conditions and that, when the gas phase CO concentration is increased, the surface concentration of oxygen decreases by reaction (process 1) and the surface concentration of CO increases (process 2). The extent to which the first two processes could have contributed to the observed step-responses is limited by the amounts of precious metal and Ce in the catalysts. Table I11 lists the total amounts of CO missing in the step-change experiments and the average amounts of CO missing per gram of catalyst in the converter. Table I11 also lists the amounts of precious metal and Ce in the catalysts. Over Catalyst P the difference between the estimated instantaneous and measured CO responses for the leanto-rich steps was 6 pmol 8’.The maximum contribution of transient process (1) to this difference is 7 pmol g-’ under the assumption that one oxygen atom was bonded to every precious metal atom before the stepchange. The maximum contribution of process (2) is also 7 pmol 8-l under assumption that one CO molecule was bonded to every precious metal atom after the step. The total possible contribution of transient processes (1) and (2) under these assumptions is 14 pmol 8’.The result that a smaller amount of CO was actually subtracted indicates that processes (1) and (2) may have been sufficient to explain the observed step-response of Catalyst P. Over Catalyst P/Ce the difference between the estimated instantaneous and measured CO responses for the lean-to-rich steps was 70 pmol gl. The maximum amount of CO that could have been oxidized in transient process (1) following the step is equal to the change in the catalyst’s content of reactive oxygen atoms. In part 1 we found that the oxygen atom content of Catalyst P/Ce decreased by 40 pmol g-l as the A/F was changed from 15.1 to 14.1 (since H2 is present in exhaust at one-third the level of CO, up
to 10 pmol g-’ of this change may have been due to H2). Considering transient process (2) alone, the maximum amount of CO that could have been subtracted from the instantaneous step-response curve over Catalyst P/Ce is 8 pmol g-’ under the assumption that every precious metal atom in the catalyst adsorbed one CO molecule. The sum of the estimated maximum contributions of processes (1) and (2), 48 pmol g-’, is less than the 70 pmol g-’ of enhanced CO removal that was observed. The discrepancy between these figures indicates that transient process (31, the water-gas shift reaction, may have contributed to the enhanced conversion of CO that followed the lean-to-rich steps. Process (3) could have contributed up to about one-half of the enhanced CO removal if Hz competed with CO in process (1) and if the contribution of process (2) was lower due to incomplete dispersion of the precious metal atoms. The extent of the contribution of process (1) and, thus, the contribution of process (3) is not certain since the space velocity used here (50000 h-’) was lower than that used with the small catalyst bed in part 1 for the oxygen content measurements (110 000 h-l). Increased conversion of reductants under lean conditions and oxidants under rich conditions could have caused the oxygen content change to be greater at the lower space velocity used here, thus decreasing the discrepancy between the estimated contributions of processes (1) and (2) and the 70 pmol g-l figure. Transient process (3) was found to occur over supported Rh by Schlatter and Mitchell (1980). Their data are consistent with their proposals that Rh is oxidized under lean conditions, that oxidized Rh is active for the water-gas shift reaction under rich conditions, and that reduction of oxidized Rh occurs under rich conditions following a lean-to-rich transition. Our data are not sufficient to allow us to state whether or not this process contributed to the dynamic behavior of the two catalysts studied here. However, the oxygen content measurements in part 1 and the data presented here do indicate that process (3), the transient enhancement of the water-gas shift reaction, was responsible for one-half or less of the extra CO conversion obtained over Catalyst P/Ce during lean-to-rich steps. This result, obtained in SOz-containing exhaust (ca. 20 ppm SOz), contrasts with the much greater participation of process (3), relative to processes (1) and (2), in SO2-free feedstreams (Schlatter and Mitchell, 1981)). SO2is known to poison the water-gas shift reaction over group 8 transition metals (Joy et al., 1979), and we caution that one should not evaluate the dynamic behavior of three-way catalysts exclusively in S02-freefeedstreams. In a future report we will present data which provide positive evidence for the participation of the water-gas shift reaction in the transient responses of Rh-containing catalysts in exhaust. A/F Cycling at 0.5 Hz. All of the data shown below were taken after the establishment of steady cycling patterns as the A/F setting was switched in a square wave between a rich setting of 14.1 and a lean setting of either 15.1 or 15.5. E k p d periods of time were spent at each A/F setting. In Figure 6 we show representative inlet CO curves measured during cycling between A/F settings of 14.1 and 15.1 at frequencies of 0.5,1.0, and 1.67 Hz. During cycling at 0.5 Hz, the inlet CO concentration roughly reaches its steady-state level during the one second rich and one second lean half-cycles. As the cycling frequency increases, the CO oscillation amplitude decreases. This attenuation is related to the fuel evaporation and air-fuel mixing processes which occur in the intake manifold of the engine. For example, when the fuel injector is turned on, some of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 393 20
1 OH2
05Hz
1 6 7 Hz
,n,
Catalyst P / C e
Catalyst P
E-08
1
-
1 k-4s-4
L-4s-4
I-,,-+
Figure 6. CO concentrations measured at the inlet of the converter aa the A/F setting was switched in a square-wave between a rich setting of 14.1 and a lean setting of 15.1. Equal periods of time were spent at each A/F setting.
the fuel spray collects on the relatively dry manifold wall and thus is removed from the intake air flow, making the A/F leaner than expected during this initial period. After the injector is turned off, a film of fuel on the manifold wall will continue to evaporate until the film is depleted, making the A/F richer than expected. This process tends to decrease the amplitude of A/F cycles. Since CO concentration various nonlinearly with A/F (see Figure 2), the maxima of the CO cycles in Figure 6 decrease with increasing frequency more than the minima increase. As the frequency of symmetric cycling about the stoichiometric point increases, the CO concentration will tend to approach a constant level (0.6%) corresponding to that at the stoichiometric point (14.6). Although the carburetorplus-injector combination used in these experiments is unique, the air-fuel mixing processes which occur in the intake manifolds of engines equipped with standard carburetors and throttle-body injectors are similar and will also attenuate the amplitude of A/F cycles (Sell and Chang, 1982). The noninstantaneous air-fuel mixing processes described above attenuate the amplitude of A/F oscillations at the inlet to a converter and thus tend to reduce CO emission as the cycling frequency increases. This reduction in CO emission will occur no matter what the response characteristics of the catalyst are. The response characteristics of the catalyst may result in a further reduction in CO emission. Below we concentrate on the dynamic behavior of the two catalysts and emphasize how the noninstananeous responses of the catalysts reduce the emission of CO during A/F cycling. In Figures 7 and 8 we show outlet CO concentrations obtained during A/F cycling. Only the estimated and measured outlet response curves are shown for brevity. Remember that an estimated instantaneous response curve has the same shape and is only 10-20% lower than the inlet curve during the rich half of a cycle. We have also scaled the data in the two figures so that all of the estimated instantaneous response cycles have about the same height and width. This scaling procedure deemphasizes the changes in the data that result from air-fuel mixing in the intake manifold and emphasizes changes in the relationships between the estimated and measured response curves. Consider first the data in the top two panels in Figure 7 which were obtained during cycling at 0.5 Hz between A/F settings of 14.1 and 15.1. The amount of CO that was emitted over each catalyst was less than the amount that would have been emitted if the catalyst had responded instantaneously. Since equal periods of time were spent at each setting, the time-averaged A/F was equal to the
T".
/.I
Figure 7. A/F cycling at 0.5 Hz. Dashed curves: estimated instantaneous response. Solid curves: measured response. The A/F setting was switched between 14.1and 15.1 in the top panels and 14.1 and 15.5 in the bottom panels. The parameter E, the 'conversion enhancement", is a measure of the extent to which the noninstantaneous dynamic response of a catalyst results in increased CO conversion during cycling and is defined in the text. ~
Catalyst P
Catalyst P / C e
E-07
I
r
E=09
Figure 8. A/F cycling at 1.0 and 1.67 Hz between the A/F settings of 14.1 and 15.5. The corresponding curves for cycling at 0.5 Hz are shown in the bottom panels of Figure 7. Dashed curves: estimated instantaneous response. Solid curves: measured response.
stoichiometrically balanced ratio of 14.6. At a constant A/F setting of 14.6 and with either of the two catalysts, the average outlet CO concentration obtained is about 0.03%, a level which is close to the baseline in Figure 7. Thus, the measured outlet CO concentrations for both catalysts are higher than would be obtained if the A/F setting were held constant at the timeaveraged A/F of 14.6 and are lower than the concentrations expected for instantaneous response. The same observation was made by Schlatter et al. (1982),who measured time-averaged CO concentrations during cycling of feedstreams to a laboratory reactor. The differences in area under the estimated and measured curves in the top panels of Figure 7 correspond to 4 mmol (3 pmol g-') CO per cycle over Catalyst P and 6 mmol(5 pmol g-') CO per cycle over Catalyst P/Ce. Since the average A/F during cycling was stoichiometrically balanced, there was sufficient excess oxygen in the lean periods of the 0.5 Hz A/F cycles to partipicate in transient process (1)listed above and react with the "extra" CO that
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was converted during the rich periods of the cycles. Under steady-state 15.1-A/F conditions, the amount of oxygen atoms in the NO and O2 that flows from the converter during a period of one second is 20 mmol, where one second is the duration of a lean half-cycle at 0.5 Hz. The fact that the excess oxygen was sufficient to account for the extra CO conversion, however, does not rule out the participation of transient process (3), the water-gas shift reaction, in the enhanced conversion of CO during cycling. Shulman et al. (1982) found that the amount of excess O2 that was retained during lean-half cycles (over a catalyst of unspecified composition) was only sufficient to account for about one-half of the extra CO that was converted during rich half-cycles. This result is consistent with our conclusion that transient process (3) may have contributed up to one-half of the extra CO converted during lean-torich steps over Catalyst P/Ce. Conversion Enhancement. We now introduce a quantitative measure of the extent to which the noninstantaneous response of a catalyst results in a decrease in CO emission. We define “E”, the average “conversion enhancement” over the period t, to t z ,where tl to tz includes an integral number of cycles.
~ l t * [ ~ * o , -t (Cout(t)l t) dt
E=
(4)
j-,tzC*,ut(t) dt Here, C*outis the estimated, instantaneous response CO concentration (mol ~ m - ~C,), , is the measured CO concentration, and Q is the volumetric flow rate (em3&). For each experiment shown in Figures 7 and 8, E is the ratio of the area that lies between the estimated (dashed) and measured (solid) outlet curves to the total area under the estimated outlet curve. In words, E is the fraction not emitted of the CO that would have been omitted if the catalyst’s response were instantaneous. The second transient process listed in the preceding section, the adsorption and accumulation of CO on the catalyst surface, does not, by itself, involve the conversion of CO to COP. However, much of the CO that is accumulated during a rich half-cycle will be oxidized to C02 during the subsequent lean half-cycle. For this reason we only determine E over an integral number of cycles. The range of E is 0 IE I1. When E equals 0, the dynamic response of the catalyst with respect to CO conversion can be considered “instantaneous” because C, = C*,,,. The average conversion enhancement should not be confused with the average CO conversion: when E equals 0, the average CO conversion may range from 0% to less than 100%. When E equals 1,the dynamic response of the catalyst results in zero emission of CO (the CO conversion is 100%). This is very desirable behavior, of course. Average conversion enhancements are noted in the upper right corner of each panel in Figure 7. Each value was determined by integfating over the three cycles shown in the panel. To examine the cycle-to-cycle variation in E , we have determined the conversion enhancement for the individual cycles shown in the upper left panel: 0.35 (0-2 s), 0.41 (2-4 s), and 0.38 (4-6 s). The result that these values are similar demonstrates that E is a measure of the response of a catalyst which is relatively insensitive to
random fluctuations in inlet exhaust composition during cycling. This insensitivity of E allows us to compare the dynamic responses of the two catalysts to each other, even though the shapes of the inlet composition changes differ in detail from cycle to cycle and from experiment to experiment. Catalyst P ( E = 0.4) performed almost all as well as Catalyst P/Ce ( E = 0.5) during cycling between the 14.1 and 15.1 A/F settings, even though the step-responses of the two catalysts differ greatly. This behavior can be partially explained by noting (1) that the catalysts only see rich exhaust for one second before seeing lean exhaust again during cycling at 0.5 Hz, and (2) that the outlet level of CO in Figure 5 remains low over both catalysts during much of the first one second of exposure to rich exhaust. Although the maximum potential of Catalyst P/Ce for maintaining high conversion of CO after a lean-to-rich transition is greater than that of Catalyst P, only a fraction of this potential is utilized during cycling. The difference between the estimated and measured CO emission over Catalyst P during cycling between A/F settings of 15.1 and 14.1 at 0.5 Hz was 4 mmol per cycle, or one-half of the total difference over Catalyst P following a step-change in A/F setting from 15.1 to 14.1 (Table 111). The difference was 6 mmol per cycle over Catalyst P/Ce, or only one-seventeenth of the difference following a 15.1-to-14.1 step. In the two lower panels of Figure 7, the A / F settings were switched between the same rich setting of 14.1 and the leaner setting of 15.5. The time-averaged A/F, 14.8, was lean during these experiments. Note that the CO concentration scale differs between the top and bottom panels in Figure 7. The difference between the estimated and measured curves in the lower panels is 3 mmol per cycle over Catalyst P, or one-third of the lean-to-rich step-response value, and is 5 mmol per cycle over Catalyst P/Ce, or one-twentieth of the step-response value. These absolute differences are lower than the ones obtained during cycling between the 14.1 and 15.1 A/F settings because the leaner A/F limit of 15.5 and the air-fuel mixing in the intake manifold decreased the extent of the rich excursion during the rich half of the 14.1-15.5 cycles. When the lean A/F setting was increased from 15.1 to 15.5, the performance of Catalyst P did not change: E remained at 0.4. This result indicates that Catalyst P was in the same state, with respect to CO conversion, at the end of each lean half-cycle when the lean A/F settings were 15.1 and 15.5. The step-response results for this catalyst was also similar at the two lean A/F settings. Taken together, the cycled and step-response results suggest that Catalyst P closely approached a steady-state condition within one second under lean exhaust during cycling. However, other results, which are discussed below, indicate that this catalyst may not have attained such a steadystate condition. The behavior of Catalyst P/Ce in these experiments differed from that of Catalyst P. When the lean A/F setting during cycling was increased from 15.1 to 15.5, the performance of Catalyst P/Ce improved E increased from 0.5 to 0.8. However, the steady-state condition of Catalyst P/Ce, with respect to the catalyst’s step-response, was the same under both 15.1 and 15.5-A/F exhaust. Taken together, these results indicate that, unlike Catalyst P, Catalyst P/Ce did not closely approach a steady-state condition within one second under lean exhaust during cycling. The greater ratio of oxidizing to reducing components in 15.5-A/F exhaust apparently left Catalyst P/Ce with a greater reactive oxygen content at the end of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 395 Catalyst P --Sleo
Rich
T me 111
Catalyst P/Ce --Step
Rich
T me I S 1
Figure 9. Comparison of step and cycled responses. The A/F settings were 15.5 (lean) and 14.1 (rich).
15.5-A/F half-cycles than at the end of 15.1-A/F half-cycles. We conclude that the large reactive oxygen capacity of the Ce in Catalyst P/Ce is responsible for the differing behavior of the two catalysts in these experiments. A/F Cycling at 1.0 and 1.67 Hz. The bottom panels of Figure 7 and the top and bottom panels of Figure 8 show results obtained during cycling between A/F settings of 14.1 and 15.5 at frequencies of 0.5, 1.0, and 1.67, Hz, respectively. Note that the time scales have been expanded at each higher frequency so that three cycles of similar size can be shown in each panel and changes in catalyst response can be seen more directly. Over both catalysts CO emission decreases with increasing frequency. This result agrees with time-averaged measurements made by others using engine-dynamometer test cells and laboratory reactors (Hegedus et al., 1979; Schlatter et al., 1982). Part of the decrease in CO emission with increasing frequency is due to mixing in the engine's intake manifold which reduced the amplitudes of the inlet concentration oscillations (note the change in CO concentration scale from the bottom panels of Figure 7 to the bottom panels of Figure 8). Most of the decrease in CO emission, however, is due to the noninstantaneous dynamic responses of the catalysts. This is shown clearly by the increase in conversion enhancement over Catalyst P: from 0.4 at 0.5 Hz, to 0.7 at 1.0 Hz, to 0.9 at 1.67 Hz. At 1.67 Hz the CO conversion performance of Catalyst P equaled that of Catalyst P/Ce. Comparison of Step and Cycled Responses. In Figure 9 we compare the measured outlet curves obtained in step-change and cycled experiments. The lean A/F setting was 15.5 and the rich A/F setting was 14.1 in the experiments. Each set of curves is aligned so that the lean-to-rich transition occurs at time zero. Before the transition each catalyst was exposed to lean exhaust until steady state was reached in the step experiment, for one second when cycling at 0.5 Hz, and for one-half second when cycling at 1.0 Hz. After the transition the exhaust stayed rich until steady state was reached in the step, for one second when cycling at 0.5 Hz, and for one-half second when cycling at 1.0 Hz. The behaviors of both catalysts are similar except that the outlet levels are lower over Catalyst P/Ce. Note that CO concentrations increased more rapidly following leanto-rich transitions during cycling than during the stepchange experiments. This response indicates that neither of the two catalysts reached a steady-state condition in the lean periods of the A/F cycles. We caution that detailed analyses of the curves in Figure 9 should not be made since the inlet exhaust composition changes were different for each of the curves as a result of (1)random A/F fluctuations and (2) attenuation of the amplitude of the A/F excursions with increasing cycling frequency. However, the general features of these curves allow us to see clearly what happens when the cycling frequency is increased from 0.5 to 1.0 Hz.
First, note in Figures 5 and 7 that the instantaneous conversion enhancement, the difference between the estimated and measured curves at a given time, is large immediately following each lean-to-rich transition and then decreases with time. This behavior indicates that the rates of the processes that contribute to the enhanced conversion of CO decrease with time following the lean-to-rich transition. For example, the rate of CO adsorption and reaction with oxygen bound to the catalyst can be expected to decrease as the concentration of reactive oxygen in the catalyst decreases. Similarly, the rate of CO adsorption and accumulation on the catalyst surface will decrease as the surface concentration of CO increases. Second, refer again to Figure 9. At a cycling frequency of 0.5 Hz, the exhaust stays rich for one second following a lean-to-rich transition, during which time the instantaneous CO conversion enhancement decreases. Since the exhaust stays rich for only one-half second following a lean-to-rich transition at 1.0 Hz,we expect that a greater average conversion enhancement will be obtained and, thus, that less CO will be emitted. In Figure 9 one can see that the amount of CO emitted in each of the 1.0-Hzcycles is less than 50% of the amount emitted in the corresponding 0.5-Hz cycle. Thus, after integration over two or more cycles, one obtains the result that less CO was emitted during operation at the higher cycling frequency. Conclusions Time-resolved measurements of CO concentrations at the inlet and outlet of a catalytic converter enabled us to observe the CO conversion responses of two three-way catalysts to rapid changes in exhaust composition. The results show that the responses of both catalysts are noninstantaneous. CO emissions following lean-to-rich transitions during A/F step-changes and A/F cycling are lower than the CO emissions that would be obtained for instantaneous catalyst response. The CO conversion performance of the Ce-containing catalyst is better than that of the Ce-free catalyst at 0.5 Hz,the lowest cycling frequency encountered during automobile operation. The performances of both catalysts improve with increasing cycling frequency and are equal at 1.67 Hz. The total potential of the Ce-containing catalyst for reducing CO emission following a lean-to-rich step-change is much greater than that of the Ce-free catalyst. In contrast, the differences between the catalysts are much smaller during A/F cycling. This is because cycled performance depends mainly on the rate of enhanced CO conversion immediately following a lean-to-rich transition and this rate does not differ dramatically between the two catalysts. The greater total potential of the Ce-containing catalyst for reducing CO emission following a lean-to-rich step-change, however, should enable the catalyst to perform better during the rich A/F excursions associated with rapid vehicle acceleration. Acknowledgment We thank John C. Price for developing the computer programs used for data analysis and Dennis J. Upton, David F. McCready, Robert W. Richmond, Charles B. Murphy, and Daniel B. Hayden for technical assistance with the experiments. Registry No. CO, 630-08-0; Ce, 7440-45-1; Pt, 7440-06-4; Pd, 7440-05-3; Rh,7440-16-6. Literature Cited Grlmm, R. A.; Bremer, R. J.; Stonestreet. S. P. SAE Paper No. 800053, SA€ Trans. 1980. 89. 357. Hegedus, L. L.;'Summers, J. C.; Schlatter, J. C.; Baron, K. J. Cafal. 1079. 54, 321. Herz, R. K.; Marin, S. P. J. Catal. 1080, 65, 281. Herz, R. K. Ind. Eng. Chem. Prod. Res. D e v . 1981, 20, 451.
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Herz, R. K. I n “Catalysis Under Transient Conditions”; Bell, A. T.; Hegedus, L. L., Ed.; ACS Symposium Series No. 178; America1 Chemlcal Society: Washlngton, DC, 1982; Chapter 3, p 59. Joy, G. C.; Lester, 0. R.; Molarlno, F. S.“The Influence of Sulfur Specles on the Laboratory Performance of Automotive Three Component Catalysts”. SAE Paper No. 790943, 1979. Kanedo, Y.; Kobayashl, H.; Komagome, R.; Hirako, 0.;Nakayama, 0. SAE Paper No. 780607, SAE Trans. 1978, 87, 225. Kim, G. Ind. Eng. Chem. Prod. Res. D e v . 1082, 27. 267. Oh, S. H.; Hegedus, L. L. I n “Catalysis Under Transient Conditions”; Bell, A. T.; Hegedus, L. L., Ed.; ACS Symposium Series No. 1 7 8 American Chemical Society: Washington, DC, 1982; Chapter 4, p 79. Schlatter, J. C.; Mitchell, P. J. Ind. €ng. Chem. Prod. Res. D e v . 1080, 79, 288. Schlatter, J. C.; Slnkevitch, R. M.;Mitchell, P. J. Ind. eng. Chem. Prod. Res. Dev. 1083, 22, 51.
Dev. 1983,22,396-401 Sell, J. A.; Herz, R. K.; Monroe, D. R. SAE Paper No. 800463, SA€ Trans. 1080, 8 9 , 1833. Sell, J. A.; Herz, R. K.; Perry, E. C. “Time Resolved Measurements of Carbon Monoxide in the Exhaust of a Computer Command Controlled Engine”, SA€ Paper No. 810276, 1981. Sell, J. A.; Chang, M.-F. “Closed-Loop Control of an Engine’s Carbon Monoxide Emissions Using an Infrared Diode Laser”, SAE Paper No. 820388, 1982. Shuiman, M. A,; Hamburg, D. R.; Throop, M. J. “Comparlson of Measured and Predicted Three-way Cata\yst Conversion Efficiencies under Dynamic Air-Fuel Ratio Condltions”, SAE Paper No. 820276, 1982.
Receiued for review November 15, 1982 Accepted April 25, 1983
Effect of Composition and Pretreatments on the Activity of Copper-Chromium-Based Catalysts for the Oxidation of Carbon Monoxide Francisco Severlno and Jorge Lalne’ Laboratorio de CatSrlisis HeterogGnea, Centro de Odmica. Instituto Venezolano de Investigaciones Cientkcas, Apartado 1827, Caracas 1010-A, Venezuela
The activity behavior for carbon monoxkie oxidation of copper-chromium catalysts was studied by varying both the catalyst composition and pretreatments. Optimum pretreatment conditions were found by empioylng a commercial copper chromite catalyst. Supported and unsupportedcatalysts were prepared from metal nitrate solutions. An optimum metal concentration corresponding to CuCr,O, was found for the unsupported catalysts. The presence of chromium was found to inhibit the reduction of the catalyst and to eliminate an activity induction period observed in copper. These resub led to the postulation of a mechanism of electron transfer between copper and chromium, copper being the main active species. Copper-supported catalysts were more active than copper chromitesupported catalysts when metal concentrations were smaller than approximately 12%. At larger metal concentrations the supported copper chromite catalysts were more active. Chromium was also found to diminish an inhibition effect observed when water was introduced to the reaction mixture.
Introduction The oxidation of carbon monoxide with the object of reducing air pollution is actually an important consideration when one thinks in terms of automobile emission control (Acres, 1974; Dwyer, 1972; Hightower, 1975;Shelef et al., 1978). Among the most efficient and most commercially employed catalysts for the oxidation of CO and hydrocarbons are the precious metals Pt, Pd, and Rh. However, attention has also been given to base metals due to the limited availability of precious metals. Thus, for example, monoliths and pellets impregnated with copper chromite have been reported to have activities near those of precious metal-based auto-emission control catalysts (Barnes, 1975; Kummer, 1975). This comparison has also been extended to the oxidation of CO with NO (Hierl et al., 1981; Ohara, 1975), another important auto-emission pollutant. Previous works on oxidation of CO with various base metal oxides (Elovich et al., 1946; Pedersen and Libby, 1972; Shelef et al., 1968) have reported that copper chromite is the most active catalyst among the series: CuCr20, > Co304> Fe203> MnO > NiO > Cr203> V205. First studies on CO oxidation over copper chromite (Frazer and Albert, 1936; Lory, 1933) have attributed the catalytic activity to surface chromium species, postulating
a mechanism of catalytic action similar to that proven for the oxidation of CO over copper oxide (Dwyer, 1972; Shelef et al., 1968). This consists of a reduction-oxidation cycle in which CO takes structural oxygen from the oxide, occurring a subsequent reoxidation of the catalyst by gaseous oxygen. However, reaction of CO with adsorbed oxygen may also be possible (Stone, 1962), and formation of carbonate species (Winter, 1955), either from structural or adsorbed oxygen, has been supposed to precede the formation of C02 (Hierl et al., 1981; Stone, 1962). The oxidation mechanism of CO over chromia, however, seems to be more complicated than over copper oxide (Kobayashi, 1979; Kobayashi and Kobayashi, 1976; Rienacker, 1949; Bridges et al., 1960). In the former, the rupture of the oxygen-catalyst bond has been suggested to be the limiting step in the mechanism of the catalytic action (Kobayashi, 1979; Shelef et al., 1968). Studies on copper chromite (Farrauto and Hertl, 1973; Kingsbury and Hertl, 1974; Morgan and Farrauto, 1973) employing infrared spectroscopy, gravimetric, and kinetics techniques have shown that surface species involved in reaction are either a copper carbonyl, at temperatures lower than 180 OC, or a carbonate associated with chromium, at higher temperatures. In contrast, by use of an SO2 poisoning technique (Farrauto and Wedding, 1973),
0196-4321/83/1222-0396$01.50/00 1983 American Chemical Society
