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Characterization of Pd-based automotive catalysts
J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary
Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.
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Characterization of Pd-based Automotive Catalysts R. W. McCabe and R. K. Usmen Ford Research Laboratory, MD 3179, SRL, Dearborn, MI 48121-2053 1. ABSTRACT Characterization studies were undertaken to determine the cause of large differences in activity between various commercial automotive catalysts after aging for 75 or 120 h on an accelerated engine-dynamometer cycle. In all, a set of nine catalysts was examined, comprised of both fresh and aged Pal-only, Pd/Rh, and Pt/Rh catalysts. Catalyst activity, as measured by CO/NOx crossover efficiencies in dynamometer airfuel sweep tests, showed no correlation with either noble metal dispersion or noble metal surface area. The amount of stored oxygen required to obtain 100% CO conversion in the A/F-modulated dynamometer sweep experiments was estimated at 15 p-mol O/g-cat.. Bench reactor experiments involving both titration of pre-oxidized catalysts with CO and cyclic CO oxidation confirmed that the 15 p-mol O/g-cat. storage requirement represents a threshold level separating high- and low-activity catalysts. Formation of bulk PdO is the main oxygen storage mode in the 120 h dynamometeraged Pd-based catalysts. Pd loading is important: the higher the Pd loading, the greater the capacity for oxygen storage via PdO. Dispersion of Pd in the dynamometer-aged catalysts (2-6%) was too low to account for significant oxygen storage via chemisorbed oxygen. Furthermore, temperature-programmed reduction experiments and comparisons of oxygen uptakes on catalysts with and without rare earth oxides both indicated that the rare earth oxides play little role in oxygen storage after 120 h dynamometer aging. Not only does the formation of bulk PdO account for the quantities of oxygen stored in the aged catalysts, but the observed rates of oxygen uptake are consistent with bulk Pd oxidation kinetics reported by Remillard et al [J. Appl. Phys. 71 (9), 1992, pp. 4515-4522].
2. INTRODUCTION Three-way automotive catalysts based on palladium, rather than the more expensive metals platinum and rhodium, have long been desired. However, Pd is more sensitive than Pt to poisoning by lead (Pb) compounds [1-4]. Consequently, widespread commercial use of Pd-based automotive three-way catalysts (TWC) was delayed in the U.S. until the early 1990s, by which time residual Pb concentrations in unleaded gasoline had decreased to negligible levels. The past five years have witnessed
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considerable research and development of various types of Pd-based TWCs including Pd-only [5-11 ], Pd/Rh [12-15], and Pt/Pd/Rh '~rimetal" catalysts [16,17]. Aside from its historically low price compared to Pt and Rh, Pd has distinct catalytic properties which make it a desirable component of today's three-way catalysts. Chief among these is thermal durability, particularly Pd's ability to maintain activity under high-temperature lean (i.e. excess O2) conditions. Pd also has excellent light-off characteristics, especially when deployed at higher concentrations than traditional Pt/Rh catalysts. Highly loaded Pd-based catalysts are thus an obvious choice for socalled close-coupled or starter catalysts mounted close to the exhaust manifold [5,18,19]. Such catalysts reach operating temperatures much faster than underbody catalysts but also experience higher warmed-up operating temperatures and greater risk of thermal deactivation. The present study was initiated to understand the causes of large differences in performance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance characteristics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-
25]. Catalyst performance (as evaluated in dynamometer sweep evaluations) did not correlate with noble metal particle dispersion. Instead, bench reactor experiments involving titration of preadsorbed oxygen with CO showed that total Pd load in@ rather than Pd surface area is the key factor affecting performance. Pd serves as its own oxygen storage agent through formation of bulk PdO, and the amount of PdO formed depends primarily on the amount of Pd available, not the surface area of the Pd. 3. EXPERIMENTAL 3.1. Catalysts Table 1 lists characteristics of the catalysts. Those labeled "TWC" are commercial formulations from Ford's catalyst suppliers, each letter designating a different formulation. The commercial catalysts all contained various rare earth and alkaline earth oxide promoters and stabilizers in addition to the noble metals. Two simple Pd-on-alumina reference catalysts (A and B) were prepared in our laboratory by impregnating alumina-coated ceramic monoliths with aqueous solutions of Pd nitrate. The laboratory-prepared catalysts were dried and calcined in air at 550 ~ for 5 h prior to evaluation. Noble metal concentrations were determined by x-ray fluorescence. Some of the commercial formulations were aged on an engine dynamometer for 75 or 120 h according to a standard 4-mode aging procedure with an inlet exhaust gas temperature of 760 ~ (peak catalyst temperature ca. 900 ~ [6]. The dynamometer aging cycle simulates vehicle aging of catalysts - 75 h for 50,000 miles and 120 h for 100,000 miles.
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Table 1 Catalyst Description Catalyst
Source
Aging
A) Pd/AI203 B) Pd/AI203 C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh TWC
Lab Lab Supplier Supplier Supplier Supplier Supplier
Fresh Fresh Fresh 75 h 120 h Fresh 120h
F) Pd/Rh TWC
Supplier
120h
G) Pt/Rh TWC
Supplier
120h
Pd concentration (%) 0.30 0.79 0.66 0.66 0.66 0.60 0.30 (0.026 Rh) 0.33 (0.036 Rh) (0.21 Pt) (0.042 Rh)
Oxygen storage component (~) No No Yes Yes Yes No Yes Yes Yes
(1) rare earth oxide component
32. ~ meUxx~ Conversion efficiencies of the dynamometer-aged catalysts were measured in a standard A/F sweep test on an engine dynamometer [6]. The sweep experiments were carried out at 450 ~ and 85,000 h"1 space velocity (volumetric basis; standard conditions). The sweep ranged from 0.5 A/F lean of stoichiometry to 0.5 A/F rich of stoichiometry with imposed A/F perturbations of +_0.5A/F at 1 Hz. After sweep evaluation, small samples of catalyst were removed from the front region of the brick for chemisorption and flow reactor experiments. Chemisorption measurements employed the CO-methanation technique of Komai et al [26] as modified in our laboratory [27] for application to automotive catalysts. In particular, modifications were made to the pretreatment to avoid the formation of Pd hydrides [27]. We have found the method well-suited to automotive catalysts, both because of its high sensitivity and apparent freedom from complications due to adsorption of CO on sites other than noble metal sites. In previous studies involving a series of Pd/Rh and Pt/Rh TWCs aged on vehicles, we obtained good correlation between apparent dispersions (and noble metal surface areas) measured by the CO methanation technique and those determined from both x-ray diffraction and transmission electron microscopy [28,29]. As with other chemisorption methods, the CO-methanation technique does not distinguish between different noble metals. Thus, Rh was treated equivalently to Pt or Pd, and a standard adsorption stoichiometry of 1 CO molecule per surface metal atom was assumed for all three noble metals. In general, contributions from Rh are expected to be small due to the low concentrations employed (one-tenth the Pd loading in the case of the Pd/Rh catalysts). Thus, the Rh concentration was simply added to the Pd or Pt loading and treated as Pd or Pt.
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T~ration of pre-dosed oxygen by CO was carded out at 500 ~ in a 1" o.d. quartz reactor tube housing a 3/4" diameter by 1/2" long catalyst button. The reactor contained two solenoid-controlled three-way valves th= were used to inject alternating pulses of secondary feed streams (each 0.1 I.Jmin) into a main feed of N2 carder gas (2.9 L/min). Two types of experiments were conducted. One involved pre-dosing the catalyst for 120 s with 0.87% 02 in the main feed. The O2 flow was then stopped, the system was purged for 120 s, and the adsorbed oxygen was titrated from the catalyst by injecting alternating 15 s pulses of CO and N2 into the N2 carder stream (the CO concentration was 0.3% after dilution with the carder N2 stream). The second type of experiment exposed the catalyst to alternating pulses of CO (0.3%) and 02 (0.185%) for durations of 15 s for 02 and variable times between 1 and 7 s for CO. Conversion of CO and formation of CO2 were monitored by non-dispersive infrared analyzers. 4. RESULTS
DyrBmonteter ev uaSons Figure 1 shows sweep data for two of the dynamometer aged catalysts: the 120 h aged Pd-only ((33) catalyst (Fig. 1A), and the 120 h aged Pd/Rh (E) catalyst (Fig. 1B). Despite equivalent aging, the Pd-only catalyst gave much higher conversions, especially around the stoichiometric point. CO/NOx cross-over efficiencies of the other dynamometer-aged catalysts are reported in Table 2. The Pd-only catalyst stands out, showing crossover efficiencies in excess of 95% after both 75 and 120 h aging. In contrast, the formulations containing Pd/Rh or PURh have crossover efficiencies between 50 and 54%.
Figure 1. Engine dynamometer sweep plots of 120-hr aged (A) Pd-only (C3) and (B) Pd/Rh (E) catalysts at 450~ and modulations of +0.5 A/F at 1Hz.
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4.2. Noble metal dispersions and surface areas Table 2 lists the apparent dispersions obtained from the CO methanation technique. No correlation is observed between dispersion and catalyst performance as measured by the CO/NOx crossover efficiencies. The C2 and C3 Pd-only TWCs, despite their extremely high CO/NOx crossover efficiencies, gave apparent dispersions of 3.5 and 3.0% after 75 and 120 h aging versus higher values of 5.9% for the Pd/Rh catalyst (E) and 4.3% for the Pt/Rh catalyst (G), both of which displayed low CO/NOx crossover efficiencies. Even between the two Pd/Rh catalysts, catalyst E has an apparent dispersion more than four times that of catalyst F, yet the two are nearly identical in their CO/NOx crossover efficiencies. Table 2 Catalyst Properties Catalyst
CO/NOx crossover efficiency
(%)
A) Pd/AI203 B) Pd/AI203 C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh TWC F) Pd/Rh TWC G) Pt/Rh TWC
NA NA NA 96 98 NA 50 52 54
Dispersion (%)
10.1 9.5 10.8 3.5 3.0 6.6 5.9 1.6 4.3
NM Surface Area (m2/g-cat.)
0.17 0.42 0.40 0.13 0.11 0.22 0.11 0.03 0.04
Table 2 also lists the noble metal surface areas normalized to the total mass of the catalyst. The surface areas were calculated directly from the dispersion data taking into account the different mass of noble metal in each catalyst and assuming a constant site density of 1x1019/m2. As with dispersion, no clear correlation exists between mass-specific noble metal surface areas and CO/NOx cross-over efficiencies. 4.3. Oxygen tJtr"~n expedrnents The engine dynamometer sweep evaluations were carried out under modulated air-fuel conditions of + 0.5 A/F ratio at 1 Hz. To achieve high conversions under these conditions, the catalyst must store oxygen during lean excursions in order to convert CO and HC under rich excursions. Likewise, some of the oxygen related inhibition of NOx reduction on the lean side is mitigated by replenishment of oxygen to the storage agent. The lack of a correlation between the CO/NOx crossover frequencies of these catalysts and either noble metal dispersion or mass-specific surface area suggests, in turn, that oxygen storage in aged catalysts is not strongly dependent on either noble metal dispersion or noble metal surface area.
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120 h aged Pt/Rh (G) Feed CO A M U
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Figure 2. Trtration of pre-adsorbed oxygen by pulses of CO (0.3% CO in N2) at 500 ~
Experiments involving titration of preadsorbed oxygen with pulses of CO in nitrogen were carded out to assess the oxygen storage capacity of both the fresh and dynamometer aged catalysts. Figure 2 shows the pulse profiles for the two catalysts with the highest (CI) and lowest (G) oxygen uptakes. The areas under the CO pulses were integrated to determine the amount of oxygen pre-adsorbed during the 120 s O2 pretreatment at 500 ~ W'rth the exception of the fresh CI Pd-only catalyst, all of the other catalysts produced CO breakthrough in excess of 90% after the third pulse. The cumulative O-atom uptakes, expressed as /~-mol O/g-cat., are summarized in Table 3. Both the first-pulse uptakes and the cumula'dve uptakes corresponding to the first three pulses are listed. Table 3 also contains the theoretical oxygen uptakes associated with both chemisorbed oxygen (i.e. assuming one chemisorbed O-atom per surface noble metal atom) and formation of bulk noble metal oxides (i.e. PdO or PtO).
Table 3 Oxy.qe.n........capac...ities(#-mol O/Q-~.)... Catalyst
- Theoretical O uptakesSurface O ~ Bulk O 2
- O titrated by CO 1st pulse 1st 3 pulses
A) Pd/AI20 3 B) Pd/AI20 s C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh "IWC F) Pd/Rh TWC G) PURh TWC
1.0 1.7 6.8 2.7 1.8 3.4 1.9 0.4 0.6
11.5 24.8 35.5 27.2 21.6 20.9 9.2 12.7 5.2
28.2 74.2 62.0 62.0 62.0 56.4 30.6 34.4 14.0
1~ ...assumes i'" O-atom per surface""or""buJk noble metal atom. '..........
15.6 32.0 66.8 39.0 32.0 31.0 14.3 23.0 7.2
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Two key observations can be drawn from the data in Table 3: 1) the oxygen consumed in either the first pulse or the first three pulses is much greater than that which can be attributed to oxygen adsorbed on the surface of the noble metal particles, and 2) with the exception of ~ fresh (CI) Pd-only IWC, none of the other catalysts have either first-pulse or cumulat~e CO uptakes that exceed the theoretical oxygen uptake associated with formation of bulk noble me'~ oxide. Even though all of the aged commercial TWCs contain oxygen storage agents, the quantities of oxygen taken up after 120 h dynamometer aging are not sufficient to require storage via those agents. 40
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4.4. Role of bulk PdO The importance of bulk PdO as an oxygen storage component is illustrated in Figure 3 which plots the amount of oxygen consumed in the first CO pulse versus the theoretical bulk oxygen capacity of each Pd-containing catalyst (expressed as/~-mol PdO/g-cat.). The solid curve is fit to the data for the three catalysts which do not contain rare earth oxygen storage agents plus an additional point at the origin reflecting the experimental observation that negligible oxygen is consumed on a blank alumina catalyst. Given the absence of rare earth oxides, and recognizing that the amount of stored oxygen is far greater than that available from chemisorption (as shown by the data points denoted by diamonds in Fig. 3), the only other source of oxygen is from reduction of bulk PdOo Thus the solid curve can be taken as representative of bulk oxidation of Pd during the 120 s exposure to O2 at 500 ~ Note that all of the 12=3h dynamometer aged catalysts have oxygen uptakes on or below the curve defined by the catalysts which do not contain rare earth oxygen storage agents. This suggests that rare earth oxides do not contribute significantly to oxygen uptakes after 120 h dynamometer aging. The only catalysts showing greater oxygen
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storage are the fresh and 75 h aged Pd-only TWCs (C1 and C2), and it is likely that rare earth oxides do contribute to oxygen uptakes in those catalysts. Interestingly, the C1 and C2 catalysts are the only pair which show a correlation between oxygen uptake and noble metal dispersion (i.e. the oxygen titrated by the first CO pulse drops from 35.5 to 27.2/~-mol O/g-cat. as the dispersion drops from 10.8% (C1) to 3.5%
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Figure 4. 1-12TPR traces of fresh (CI) and b) 120 h aged (C3) Pd-only TWCs (30 ~ heating rate in 9% i"La/Ar). Inset: TPR traces of a 0.32% Pd/15% CeO2/AI=O3 catalyst fresh and after aging for 1 h at 900 ~ in a laboratory flow reactor. 4.5. TPR The absence of a strong contribution from rare earl~ oxygen storage agents is confirmed by hydrogen temperature-programmed reduction (TPR) experiments. Figure 4 shows hydrogen TPR traces for the fresh and 120 h aged Pd-only catalysts (C1 and C3). The fresh catalyst contains a broad peak centered around 100 ~ which has an area of 127/~-mol 1-12/g-cat. (corresponding to twice the theoretical I-Iz uptake of 62 /~-mol I-Iz/g-cat. required to reduce PdO to Pd metal). The "extra" area is attributed to the reduction of surface oxygen associated with ceria in intimate contact with Pd as shown by the close similarity to the TPR trace of a fresh 0.32=,(, Pd/15% CeO2/AI203
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catalyst prepared in our laboratory (Fig. 4 inset). After 120 h dynamometer aging, the low-temperature feature of the C3 catalyst peaks near 40 ~ and decreases in area to 23/~-mol H2/g-cat. (Fig. 4b). Again, the changes upon aging are similar to those of the reference 0.32% Pd/15% CeO2/AI20~ catalyst after aging for 1 h at 900 ~ in a laboratory reactor (Fig. 4 inset). Changes in TPR spectra of the type shown by both the Pal-only catalyst and the model Pd/CeO2/AI20 s catalyst are characteristic of large reductions in contact area between Pd and rare earth oxides effected by thermal aging [30]. Pd in intimate contact with ceria [30,31] or lanthana [32] gives a TPR feature between 100 and 200 ~ whereas Pd on alumina has its chief TPR feature at temperatures between 0 and 60 ~ [23,32,33]. In fact, partial reduction of PdO may be occurring in our experiments prior to the start of the temperature ramp from 0 ~ The TPR data thus support the oxygen uptake experiments in suggesting that rare earth oxides contribute little to the uptakes observed for the 120 h dynamometer-aged catalysts. 5. DISCUSSION Automotive catalysts have traditionally been designed with the objective of maximizing dispersion of the noble metals. Some exceptions exist, such as the well-documented observation that oxidation rates of saturated hydrocarbons increase with increasing noble metal particle size [34,35]. However, one of the main objectives in designing automotive catalysts is to stabilize the dispersion of the noble metals against thermal sintering. Pd undergoes complex changes in oxidation state and structure in response to variations in temperature and gas environment [20-25,33,36-40]. Spreading and/or redispersion of Pd (as a cationic form of Pd) has been reported in oxidizing environments at temperatures between 700 and 900 oC [20-25]. At first glance, such a mechanism would appear to offer a plausible explanation for the thermal durability of Pd-basecl catalysts, particularly in aging cycles of the type reported here involving lean (i.e., oxidizing) modes. The data of this study, however, indicate that dispersions of Pd-based catalysts after thermal aging are quite low, and no greater than comparably aged PURh catalysts. Catalytic activ'rty, as measured in modulated NF sweep experiments, neither correlates with noble metal dispersion nor with noble metal surface area. As shown in Fig. 3, oxygen uptakes (as reflected in amounts of CO oxidized) show an increasing trend with the bulk oxygen capacities of the catalysts (expressed as the theoretical amount of bulk PdO which can be formed). Comparing the oxygen uptake data of Fig. 3 to the CO/NOx crossover efficiencies of Table 2, a threshold level of oxygen uptake is suggested, between 13 and 20/~-mol O/g-cat., above which the catalyst can store enough oxygen to ensure high efficiency during the A/F perturbations encountered in the dynamometer sweep test. This range is consistent with a stored oxygen demand of 15.3/~-mol O/g-cat. which we have estimated as required to ensure stoichiometric oxidation of all reducing species during the 0.5 s, -0.5 delta NF half cycle of the dynamometer sweep (centered at the stoichiometric A/F ratio) [41]. To a first approximation, the high activity of catalyst C3 results simply from its having an oxygen uptake capacity above the threshold requirement, whereas the other
364 dynamometer-aged catalysts have oxygen uptakes below the threshold requirement. The oxygen uptakes shown in Fig. 3 reflect much longer oxygen exposures (120 s) and larger CO titers (48 #-mol CO/g-cat.) than charactedstic of the dynamometer sweep cycle. Therefore, additional bench reactor CO oxidation experiments were carried out at shorter time intervals cycling between 15 s pulses of O2 (0.185 mol%) and 2 to 7 s pulses of CO (0.3 mol%). Results are summarized in Table 4, with the catalysts listed in order of highest to lowest CO conversions. The most active catalyst (C1 Pd-only TWC) stored sufficient oxygen to convert a 7 s CO pulse with 98% efficiency. At the other extreme, the PURh catalyst (G) gave only 48% conversion of a 7 s CO pulse and reached only 9'2% conversion with a 2 s pulse. The most pertinent condition for comparing the bench reactor and dynamometer results is at 5 s, since the bench reactor CO dose at 5 s (16/~-mol CO/g-cat.) is close to the dynamometer stored oxygen demand (15.3/~-mol O/g-cat.) Significantly, we find close quantitative agreement between the CO conversions at 5 s CO pulse length and the CO/NOx crossover efficiencies reported in Table 2. In both cases, catalysts E, F, and G give conversions in the 50-55% range whereas the C2 and C3 catalysts give conversions in excess of 85%. The good quantitative agreement between the engine dynamometer data and the laboratory pulsed CO oxidation experiments supports our interpretation that high CO/NOx crossover efficiencies in dynamometer sweep evaluations reflect oxygen storage above a threshold level. Table 4 Per cent conversion of CO pulses of various len.qths Catalyst 7 C1) Pd-only TWC C2) Pd-only TWC D) Pd-only TWC C3) Pd-only TWC B) Pd-AI20 3 A) Pd-AI20 s E) Pd/Rh TWC F) Pd/Rh TWC G) Pt/Rh TWC
98 NM NM NM 62 NM 42 NM 48
length of CO pulse (s) ............. 5 4 3 >99 95 89 86 78 67 51 51 NM
>99 >99 95 96 89 NM 67 NM 59
>99 >99 >99 > 99 >99 92 82 NM 71
>99 >99 >99 >99 >99 >99 >99 93 92
Given the connection between oxygen uptakes and catalyst sweep performance, the lack of a correlation between noble metal dispersion (and surface area) and catalyst performance implies that oxygen uptake does not depend on noble metal dispersion or surface area. The lack of a correlation is not surprising given that, 1) the quantities of oxygen involved, 2) the similarities in oxygen uptakes between catalysts with and without oxygen storage agents, and 3) the absence of TPR features characteristic of noble metal-rare earth oxide interactions all point to bulk oxidation of PdO as the
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dominant oxygen storage mechanism in the 120 h dynamometer-aged Pd-based catalysts. For bulk oxidation of Pd to be the dominant oxygen storage mechanism, Pd oxidation kinetics must be rapid enough to account for the quantities of oxygen stored. Data reported by Remillard et al [38] on the air oxidation of thin Pd films indicate that the rates are indeed fast enough to account for the oxygen uptakes observed for the aged catalysts in these experiments. The 500 ~ 120 s pre-oxidation employed in the CO titration experiments would produce an oxide film thickness of 89 Angstroms using the growth expression reported in Ref. 38 (assuming equivalent kinetics in air and in the 0.87% O2/N2 mixture of our experiments). Taking the C3 catalyst, for example, with a dispersion of 3% (corresponding to a mean particle diameter of 373 Angstroms [42]), the formation of an outer PdO band of 89 Angstrom thickness would result in shrinkage of the metallic core to 274 Angstroms and growth of the overall diameter to 452 Angstroms. The increase in particle diameter owes to both the lower density of PdO compared to Pd and the presence of about 21% void volume in the PdO band [38]. Approximately 61% of the Pd in the particle is oxidized. Experimentally, 120 s oxidation of the C3 catalyst yielded a 3-pulse CO consumption corresponding to oxidation of about 52% of the Pd in the catalyst. Thus, the bulk oxidation kinetics are fast enough to account for the experimental observations. Even at the shorter 15 s oxygen exposure of the cyclic CO oxidation experiments, the kinetics of Remillard et al predict a 35 Angstrom thick oxide film. This corresponds to 25% oxidation of the Pd (or 15.2/~-mol O/g-cat.), just slightly less than the uptake of 16/~-mol O/g-cat. required for complete conversion of the 5 s CO pulse in the cyclic CO oxidation experiments (Table 4). Note that the oxidation kinetics of Remillard et al were obtained on 1-/~m-thick sputtered films of Pd on quartz, indicating that Pd oxidation kinetics are rapid enough, even for large-grain Pd particles, to account for oxygen uptakes required in both the dynamometer and bench reactor activity evaluations. The conclusions reached in the present study, namely that the catalytic activity of aged Pd-based catalysts depends primarily on Pd loading, begs the question '~hat is the function of the various additives such as ceria?". One obvious answer is that much effort has gone into stabilizing support components against loss of surface area and associated occlusion of noble metal particles. Both rare earth and alkaline earth oxides are important in this regard. In addition, we have addressed catalytic activity under very limited conditions -- modulated A/F sweep experiments carried out with lowsulfur fuel on an engine dynamometer. Factors such as noble metal dispersion and effects of promoters/stabilizers may be more important under other conditions, e.g. during light-off, at different temperatures and space velocities, with high-sulfur fuel, or in different A/F regimes during aging and evaluation. Also, we have focussed exclusively on CO oxidation, whereas different effects of dispersion and promoter/stabilizers may obtain for HC oxidation and for NOx reduction. Our study does suggest, however, that at least part of the Pd should be deployed in the form of large particles to obtain good CO and NOx conversions under modulated A/F conditions. The par~cles will consist of a metallic core with a roughened outer band capable of facile interconversion between Pd oxide and Pd metal.
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6. SUMMARY Chemisorption measurements, combined with oxygen uptake, TPR, and pulsed CO02 experiments were employed to determine the source of large differences in dynamometer sweep performance of a series of Pt/Rh, Pd/Rh and Pd-only TWCs after dynamometer aging. The following observations have been made: 1) Apparent noble metal dispersions of 75 and 120 h dynamometer-aged TWCs range from about 2 to 6%. 2) CO/NOx cross-over efficiencies of aged catalysts in dynamometer sweep experiments do not correlate with either noble metal dispersion or noble metal surface area. 3) Oxygen uptakes in both dynamometer and bench reactor experiments at 500 ~ are much too great to attribute to oxygen chemisorption. 4) After 120 s oxygen exposure (500 *C), all of the dynamometer-aged Palbased catalysts gave oxygen uptakes that could be accounted for by the formation of bulk PdO. 5) Dynamometer-aged (120 h) catalysts showed no evidence for oxygen storage via rare earth oxides. 6) Formation of bulk PdO is the primary oxygen storage mechanism in the dynamometer-aged Pd-based catalysts. 7) A threshold level of oxygen storage (via bulk PdO) is required to reach high CO/NOx conversion levels in dynamometer sweep tests; Pd loading, rather than dispersion or surface area, is the most important factor affecting oxygen uptakes. 8) Rates of oxygen uptake in the dynamometer-aged catalysts are consistent with published oxidation kinetics of 1-/~m-thick Pd films. 7. ACKNOWI.EDGMENTS We thank K. S. Patel and D. M. DiCicco for providing the dynamometer-aged catalysts and sweep evaluation data. E. Gulari and C. Sze (U. of Michigan) assisted with the design of the pulsed reactor system. REFERENCES 1. M. Shelef, K. Otto, and N.C. Otto, Adv. in. Catal. 27 (1978) 311-365. 2. H.S. Gandhi, W.B. Williamson, E.M. Logothetis, J. Tabcock, C. Peters, M.D. Hurley, and M. Shelef, Surf. & Interface Anal. 6(4) (1984) 149. 3. W.B. Williamson, D. Lewis, J. Perry, and H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 531. 4. R.L Klimisch, J.C. Summers, and J.C. Schlatter, Amer. Chem. Soc. Adv. Chem. Ser. 143 (1975) 103. 5. J.C. Summers, J.F. Skowron, and M.J. Miller, Soc. of Automotive Eng., Paper
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930386 (1993). 6. J.S. Hepburn, K.S. Patel, M.G. Meneghel, H.S. Gandhi, and Engelhard and Johnson Matthey Three Way Catalyst Development Teams, Soc. of Automotive Eng., Paper 941058 (1994). 7. M. Harkonen, M. Kivioja, P. Lappi, P. Mannila, T. Maunula, and T. Slotte, Soc. of Automotive Eng., Paper 940935 (1994). 8. J.C. Summers, J.J. White, and W.B. Williamson, Soc. of Automotive Eng., Paper 890794 (1989). 9. H. Muraki, Soc. of Automotive Eng., Paper 910842 (1991). 10. H. Tanaka, H. Fujikawa, and I. Takahashi, Soc. of Automotive Eng., Paper 930251 (1993). 11. T. Yamada, K. Kayano, and M. Funabiki, Soc. of Automotive Eng., Paper 930253 (1993). 12. Y.-K. Lui and J.C. Dettling, Soc. of Automotive Eng., Paper 930249 (1993). 13. J.K. Hochmuth and J.J. Mooney, Soc. of Automotive Eng., Paper 930219 (1993). 14. J.C. Summers, W.B. Williamson, and J.A. Scaparo, Soc. of Automotive Eng., Paper 900495 (1990). 15. H. Muraki, H. Sobukawa, M. Kimura, and A. Isogai, Soc. of Automotive Eng., Paper 900610 (1990). 16. B.H. Engler, E.S. Lox, I~ Ostgathe, T. Ohata, I~ Tsuchitani, S. Ichihara, H. Onoda, G.T. Garr, and D. Psaras, Soc. of Automotive Eng., Paper 940928 (1994). 17. A. Punke, U. Dahle, S.J. Tauster, and H.N. Rabinowitz, Soc. of Automotive Eng., Paper 950255 (1995). 18. D. Ball, Soc. of Automotive Eng., Paper 922338 (1992). 19. Z. Hu and R.M. Heck, Soc. of Automotive Eng., Paper 950254 (1995). 20. E. Ruckenstein and J.J. Chen, J. Catal. 70 (1981) 233. 21. J.J. Chen and E. Ruckenstein, J. Phys. Chem. 85 (1981) 1606. 22. E. Ruckenstein and J.J. Chen, J. Colloid Interface Sci. 86 (1982) 1. 23. H. Lieske and J. Volter, J. Phys. Chem. 89 (1985) 1841. 24. J.W.M. Jacobs and D. Schryvers, J. Catal. 103 (1987) 436. 25. J.G. McCarty and Y-F. Chang, Scripta Metal. et Mater. 31 (1994) 1115. 26. S. Komai, T. Hattori, and Y. Murakami, J. Catal. 120 (1989) 370. 27. R.K. Usmen, R.W. McCabe, and M. Shelef, "Proceedings of the Third Congress on Automotive Pollution Control," Elsevier, Brussels, Belgium, in press. 28. M.H. Yao, D.R. Liu, R.J. Baird, R.K. Usmen, and R.W. McCabe, ext. abstr., "Proc. 52rid Ann. Mtg. Microscopy Soc. of Amer.," C.W. Bailey and A~J. Garratt-Reed, eds., San Francisco Press, Inc., p.776, 1994. 29. M.H. Yao, D.R. Liu, R.J. Baird, R.K. Usmen, and R.W. McCabe, submitted to J. Catal., Nov., 1995. 30. R.K. Usmen, R.W. McCabe, G.W. Graham, W.H. Weber, C.R. Peters, and H.S. Gandhi, Soc. of Automotive Eng., Paper 922336 (1992). 31. B. Harrison, A.F. Diwell, and C. Hallett, Plat. Met. Rev. 32 (1988) 73. 32. S. Subramanian, R.J. Kudla, C.R. Peters, and M.S. Chattha, Catal. Letts. 16 (1992) 323. 33. T.R. Baldwin and R. Burch, Appl. Catal. 66 (1990) 359. 34. T. Tokoro, I~ Hori, T. Nagira, T. Uchyima, and Y. Yoneda, Nippon Kagaku Kaishi
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12 (1979) 1646. H. Yao, Y. Yao, and K. Otto, J. Catal. 56 (1979) 21. R.F. Hicks, Q. Haihua, M.L. Young, and R.G. Lee, J. Catal. 122 (1990) 295. P. Briot and M. Primet, Appl. Catal. 68 (1991) 301. J.T. RemiUard, W.H. Weber, J.R. McBride, and R.E. Soltis, J. Appl. Phys. 71 (1992) 4515. 39. D. Konig, W.H. Weber, B.D. Poindexter, J.R. McBride, G.W. Graham, and I~ Otto, Catal. Letts. 29 (1994) 329. 40. R. Burch and F.J. Urbano, Appl. Catal. A: Gen. 124 (1995) 121. 41. The stored oxygen demand was estimated from the difference between the actual conversions measured at the stoichiometric point with +0.5 A/F perturbations at 1 Hz and steady state conversions measured at the +0.5 and -0.5 A/F extremes. 42. F.H. Ribeiro, M. Chow, and R.A. Dalla Betta, J. Catal. 146 (1994) 537. 35. 36. 37. 38.
Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.
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Characterization of Pd-based Automotive Catalysts R. W. McCabe and R. K. Usmen Ford Research Laboratory, MD 3179, SRL, Dearborn, MI 48121-2053 1. ABSTRACT Characterization studies were undertaken to determine the cause of large differences in activity between various commercial automotive catalysts after aging for 75 or 120 h on an accelerated engine-dynamometer cycle. In all, a set of nine catalysts was examined, comprised of both fresh and aged Pal-only, Pd/Rh, and Pt/Rh catalysts. Catalyst activity, as measured by CO/NOx crossover efficiencies in dynamometer airfuel sweep tests, showed no correlation with either noble metal dispersion or noble metal surface area. The amount of stored oxygen required to obtain 100% CO conversion in the A/F-modulated dynamometer sweep experiments was estimated at 15 p-mol O/g-cat.. Bench reactor experiments involving both titration of pre-oxidized catalysts with CO and cyclic CO oxidation confirmed that the 15 p-mol O/g-cat. storage requirement represents a threshold level separating high- and low-activity catalysts. Formation of bulk PdO is the main oxygen storage mode in the 120 h dynamometeraged Pd-based catalysts. Pd loading is important: the higher the Pd loading, the greater the capacity for oxygen storage via PdO. Dispersion of Pd in the dynamometer-aged catalysts (2-6%) was too low to account for significant oxygen storage via chemisorbed oxygen. Furthermore, temperature-programmed reduction experiments and comparisons of oxygen uptakes on catalysts with and without rare earth oxides both indicated that the rare earth oxides play little role in oxygen storage after 120 h dynamometer aging. Not only does the formation of bulk PdO account for the quantities of oxygen stored in the aged catalysts, but the observed rates of oxygen uptake are consistent with bulk Pd oxidation kinetics reported by Remillard et al [J. Appl. Phys. 71 (9), 1992, pp. 4515-4522].
2. INTRODUCTION Three-way automotive catalysts based on palladium, rather than the more expensive metals platinum and rhodium, have long been desired. However, Pd is more sensitive than Pt to poisoning by lead (Pb) compounds [1-4]. Consequently, widespread commercial use of Pd-based automotive three-way catalysts (TWC) was delayed in the U.S. until the early 1990s, by which time residual Pb concentrations in unleaded gasoline had decreased to negligible levels. The past five years have witnessed
356
considerable research and development of various types of Pd-based TWCs including Pd-only [5-11 ], Pd/Rh [12-15], and Pt/Pd/Rh '~rimetal" catalysts [16,17]. Aside from its historically low price compared to Pt and Rh, Pd has distinct catalytic properties which make it a desirable component of today's three-way catalysts. Chief among these is thermal durability, particularly Pd's ability to maintain activity under high-temperature lean (i.e. excess O2) conditions. Pd also has excellent light-off characteristics, especially when deployed at higher concentrations than traditional Pt/Rh catalysts. Highly loaded Pd-based catalysts are thus an obvious choice for socalled close-coupled or starter catalysts mounted close to the exhaust manifold [5,18,19]. Such catalysts reach operating temperatures much faster than underbody catalysts but also experience higher warmed-up operating temperatures and greater risk of thermal deactivation. The present study was initiated to understand the causes of large differences in performance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance characteristics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-
25]. Catalyst performance (as evaluated in dynamometer sweep evaluations) did not correlate with noble metal particle dispersion. Instead, bench reactor experiments involving titration of preadsorbed oxygen with CO showed that total Pd load in@ rather than Pd surface area is the key factor affecting performance. Pd serves as its own oxygen storage agent through formation of bulk PdO, and the amount of PdO formed depends primarily on the amount of Pd available, not the surface area of the Pd. 3. EXPERIMENTAL 3.1. Catalysts Table 1 lists characteristics of the catalysts. Those labeled "TWC" are commercial formulations from Ford's catalyst suppliers, each letter designating a different formulation. The commercial catalysts all contained various rare earth and alkaline earth oxide promoters and stabilizers in addition to the noble metals. Two simple Pd-on-alumina reference catalysts (A and B) were prepared in our laboratory by impregnating alumina-coated ceramic monoliths with aqueous solutions of Pd nitrate. The laboratory-prepared catalysts were dried and calcined in air at 550 ~ for 5 h prior to evaluation. Noble metal concentrations were determined by x-ray fluorescence. Some of the commercial formulations were aged on an engine dynamometer for 75 or 120 h according to a standard 4-mode aging procedure with an inlet exhaust gas temperature of 760 ~ (peak catalyst temperature ca. 900 ~ [6]. The dynamometer aging cycle simulates vehicle aging of catalysts - 75 h for 50,000 miles and 120 h for 100,000 miles.
357
Table 1 Catalyst Description Catalyst
Source
Aging
A) Pd/AI203 B) Pd/AI203 C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh TWC
Lab Lab Supplier Supplier Supplier Supplier Supplier
Fresh Fresh Fresh 75 h 120 h Fresh 120h
F) Pd/Rh TWC
Supplier
120h
G) Pt/Rh TWC
Supplier
120h
Pd concentration (%) 0.30 0.79 0.66 0.66 0.66 0.60 0.30 (0.026 Rh) 0.33 (0.036 Rh) (0.21 Pt) (0.042 Rh)
Oxygen storage component (~) No No Yes Yes Yes No Yes Yes Yes
(1) rare earth oxide component
32. ~ meUxx~ Conversion efficiencies of the dynamometer-aged catalysts were measured in a standard A/F sweep test on an engine dynamometer [6]. The sweep experiments were carried out at 450 ~ and 85,000 h"1 space velocity (volumetric basis; standard conditions). The sweep ranged from 0.5 A/F lean of stoichiometry to 0.5 A/F rich of stoichiometry with imposed A/F perturbations of +_0.5A/F at 1 Hz. After sweep evaluation, small samples of catalyst were removed from the front region of the brick for chemisorption and flow reactor experiments. Chemisorption measurements employed the CO-methanation technique of Komai et al [26] as modified in our laboratory [27] for application to automotive catalysts. In particular, modifications were made to the pretreatment to avoid the formation of Pd hydrides [27]. We have found the method well-suited to automotive catalysts, both because of its high sensitivity and apparent freedom from complications due to adsorption of CO on sites other than noble metal sites. In previous studies involving a series of Pd/Rh and Pt/Rh TWCs aged on vehicles, we obtained good correlation between apparent dispersions (and noble metal surface areas) measured by the CO methanation technique and those determined from both x-ray diffraction and transmission electron microscopy [28,29]. As with other chemisorption methods, the CO-methanation technique does not distinguish between different noble metals. Thus, Rh was treated equivalently to Pt or Pd, and a standard adsorption stoichiometry of 1 CO molecule per surface metal atom was assumed for all three noble metals. In general, contributions from Rh are expected to be small due to the low concentrations employed (one-tenth the Pd loading in the case of the Pd/Rh catalysts). Thus, the Rh concentration was simply added to the Pd or Pt loading and treated as Pd or Pt.
358
T~ration of pre-dosed oxygen by CO was carded out at 500 ~ in a 1" o.d. quartz reactor tube housing a 3/4" diameter by 1/2" long catalyst button. The reactor contained two solenoid-controlled three-way valves th= were used to inject alternating pulses of secondary feed streams (each 0.1 I.Jmin) into a main feed of N2 carder gas (2.9 L/min). Two types of experiments were conducted. One involved pre-dosing the catalyst for 120 s with 0.87% 02 in the main feed. The O2 flow was then stopped, the system was purged for 120 s, and the adsorbed oxygen was titrated from the catalyst by injecting alternating 15 s pulses of CO and N2 into the N2 carder stream (the CO concentration was 0.3% after dilution with the carder N2 stream). The second type of experiment exposed the catalyst to alternating pulses of CO (0.3%) and 02 (0.185%) for durations of 15 s for 02 and variable times between 1 and 7 s for CO. Conversion of CO and formation of CO2 were monitored by non-dispersive infrared analyzers. 4. RESULTS
DyrBmonteter ev uaSons Figure 1 shows sweep data for two of the dynamometer aged catalysts: the 120 h aged Pd-only ((33) catalyst (Fig. 1A), and the 120 h aged Pd/Rh (E) catalyst (Fig. 1B). Despite equivalent aging, the Pd-only catalyst gave much higher conversions, especially around the stoichiometric point. CO/NOx cross-over efficiencies of the other dynamometer-aged catalysts are reported in Table 2. The Pd-only catalyst stands out, showing crossover efficiencies in excess of 95% after both 75 and 120 h aging. In contrast, the formulations containing Pd/Rh or PURh have crossover efficiencies between 50 and 54%.
Figure 1. Engine dynamometer sweep plots of 120-hr aged (A) Pd-only (C3) and (B) Pd/Rh (E) catalysts at 450~ and modulations of +0.5 A/F at 1Hz.
359
4.2. Noble metal dispersions and surface areas Table 2 lists the apparent dispersions obtained from the CO methanation technique. No correlation is observed between dispersion and catalyst performance as measured by the CO/NOx crossover efficiencies. The C2 and C3 Pd-only TWCs, despite their extremely high CO/NOx crossover efficiencies, gave apparent dispersions of 3.5 and 3.0% after 75 and 120 h aging versus higher values of 5.9% for the Pd/Rh catalyst (E) and 4.3% for the Pt/Rh catalyst (G), both of which displayed low CO/NOx crossover efficiencies. Even between the two Pd/Rh catalysts, catalyst E has an apparent dispersion more than four times that of catalyst F, yet the two are nearly identical in their CO/NOx crossover efficiencies. Table 2 Catalyst Properties Catalyst
CO/NOx crossover efficiency
(%)
A) Pd/AI203 B) Pd/AI203 C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh TWC F) Pd/Rh TWC G) Pt/Rh TWC
NA NA NA 96 98 NA 50 52 54
Dispersion (%)
10.1 9.5 10.8 3.5 3.0 6.6 5.9 1.6 4.3
NM Surface Area (m2/g-cat.)
0.17 0.42 0.40 0.13 0.11 0.22 0.11 0.03 0.04
Table 2 also lists the noble metal surface areas normalized to the total mass of the catalyst. The surface areas were calculated directly from the dispersion data taking into account the different mass of noble metal in each catalyst and assuming a constant site density of 1x1019/m2. As with dispersion, no clear correlation exists between mass-specific noble metal surface areas and CO/NOx cross-over efficiencies. 4.3. Oxygen tJtr"~n expedrnents The engine dynamometer sweep evaluations were carried out under modulated air-fuel conditions of + 0.5 A/F ratio at 1 Hz. To achieve high conversions under these conditions, the catalyst must store oxygen during lean excursions in order to convert CO and HC under rich excursions. Likewise, some of the oxygen related inhibition of NOx reduction on the lean side is mitigated by replenishment of oxygen to the storage agent. The lack of a correlation between the CO/NOx crossover frequencies of these catalysts and either noble metal dispersion or mass-specific surface area suggests, in turn, that oxygen storage in aged catalysts is not strongly dependent on either noble metal dispersion or noble metal surface area.
360
120 h aged Pt/Rh (G) Feed CO A M U
~2 tn q0 c o r m 4r b., t) ,,4
-
Fresh (Cl) Pd-Only
-
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.
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CO Pulse Number
Figure 2. Trtration of pre-adsorbed oxygen by pulses of CO (0.3% CO in N2) at 500 ~
Experiments involving titration of preadsorbed oxygen with pulses of CO in nitrogen were carded out to assess the oxygen storage capacity of both the fresh and dynamometer aged catalysts. Figure 2 shows the pulse profiles for the two catalysts with the highest (CI) and lowest (G) oxygen uptakes. The areas under the CO pulses were integrated to determine the amount of oxygen pre-adsorbed during the 120 s O2 pretreatment at 500 ~ W'rth the exception of the fresh CI Pd-only catalyst, all of the other catalysts produced CO breakthrough in excess of 90% after the third pulse. The cumulative O-atom uptakes, expressed as /~-mol O/g-cat., are summarized in Table 3. Both the first-pulse uptakes and the cumula'dve uptakes corresponding to the first three pulses are listed. Table 3 also contains the theoretical oxygen uptakes associated with both chemisorbed oxygen (i.e. assuming one chemisorbed O-atom per surface noble metal atom) and formation of bulk noble metal oxides (i.e. PdO or PtO).
Table 3 Oxy.qe.n........capac...ities(#-mol O/Q-~.)... Catalyst
- Theoretical O uptakesSurface O ~ Bulk O 2
- O titrated by CO 1st pulse 1st 3 pulses
A) Pd/AI20 3 B) Pd/AI20 s C1) Pd-only TWC C2) Pd-only TWC C3) Pd-only TWC D) Pd-only TWC E) Pd/Rh "IWC F) Pd/Rh TWC G) PURh TWC
1.0 1.7 6.8 2.7 1.8 3.4 1.9 0.4 0.6
11.5 24.8 35.5 27.2 21.6 20.9 9.2 12.7 5.2
28.2 74.2 62.0 62.0 62.0 56.4 30.6 34.4 14.0
1~ ...assumes i'" O-atom per surface""or""buJk noble metal atom. '..........
15.6 32.0 66.8 39.0 32.0 31.0 14.3 23.0 7.2
361
Two key observations can be drawn from the data in Table 3: 1) the oxygen consumed in either the first pulse or the first three pulses is much greater than that which can be attributed to oxygen adsorbed on the surface of the noble metal particles, and 2) with the exception of ~ fresh (CI) Pd-only IWC, none of the other catalysts have either first-pulse or cumulat~e CO uptakes that exceed the theoretical oxygen uptake associated with formation of bulk noble me'~ oxide. Even though all of the aged commercial TWCs contain oxygen storage agents, the quantities of oxygen taken up after 120 h dynamometer aging are not sufficient to require storage via those agents. 40
40
z~Pd w OS A "" Pd w/o OS w 30 • Pt/Rh a. o * Surface O ~ 20
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7O 80 20 30 40 50 60 Bulk oxygen capacity (as PdO) Figure 3. Plot of O-atoms consumed in the first CO pulse (/~-mol O-atoms/g-cat.) vs. the bulk oxygen capacity of the catalyst (PdO basis). 0
10
4.4. Role of bulk PdO The importance of bulk PdO as an oxygen storage component is illustrated in Figure 3 which plots the amount of oxygen consumed in the first CO pulse versus the theoretical bulk oxygen capacity of each Pd-containing catalyst (expressed as/~-mol PdO/g-cat.). The solid curve is fit to the data for the three catalysts which do not contain rare earth oxygen storage agents plus an additional point at the origin reflecting the experimental observation that negligible oxygen is consumed on a blank alumina catalyst. Given the absence of rare earth oxides, and recognizing that the amount of stored oxygen is far greater than that available from chemisorption (as shown by the data points denoted by diamonds in Fig. 3), the only other source of oxygen is from reduction of bulk PdOo Thus the solid curve can be taken as representative of bulk oxidation of Pd during the 120 s exposure to O2 at 500 ~ Note that all of the 12=3h dynamometer aged catalysts have oxygen uptakes on or below the curve defined by the catalysts which do not contain rare earth oxygen storage agents. This suggests that rare earth oxides do not contribute significantly to oxygen uptakes after 120 h dynamometer aging. The only catalysts showing greater oxygen
362
storage are the fresh and 75 h aged Pd-only TWCs (C1 and C2), and it is likely that rare earth oxides do contribute to oxygen uptakes in those catalysts. Interestingly, the C1 and C2 catalysts are the only pair which show a correlation between oxygen uptake and noble metal dispersion (i.e. the oxygen titrated by the first CO pulse drops from 35.5 to 27.2/~-mol O/g-cat. as the dispersion drops from 10.8% (C1) to 3.5%
(C2)). |
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Temperature (OC)
Figure 4. 1-12TPR traces of fresh (CI) and b) 120 h aged (C3) Pd-only TWCs (30 ~ heating rate in 9% i"La/Ar). Inset: TPR traces of a 0.32% Pd/15% CeO2/AI=O3 catalyst fresh and after aging for 1 h at 900 ~ in a laboratory flow reactor. 4.5. TPR The absence of a strong contribution from rare earl~ oxygen storage agents is confirmed by hydrogen temperature-programmed reduction (TPR) experiments. Figure 4 shows hydrogen TPR traces for the fresh and 120 h aged Pd-only catalysts (C1 and C3). The fresh catalyst contains a broad peak centered around 100 ~ which has an area of 127/~-mol 1-12/g-cat. (corresponding to twice the theoretical I-Iz uptake of 62 /~-mol I-Iz/g-cat. required to reduce PdO to Pd metal). The "extra" area is attributed to the reduction of surface oxygen associated with ceria in intimate contact with Pd as shown by the close similarity to the TPR trace of a fresh 0.32=,(, Pd/15% CeO2/AI203
363
catalyst prepared in our laboratory (Fig. 4 inset). After 120 h dynamometer aging, the low-temperature feature of the C3 catalyst peaks near 40 ~ and decreases in area to 23/~-mol H2/g-cat. (Fig. 4b). Again, the changes upon aging are similar to those of the reference 0.32% Pd/15% CeO2/AI20~ catalyst after aging for 1 h at 900 ~ in a laboratory reactor (Fig. 4 inset). Changes in TPR spectra of the type shown by both the Pal-only catalyst and the model Pd/CeO2/AI20 s catalyst are characteristic of large reductions in contact area between Pd and rare earth oxides effected by thermal aging [30]. Pd in intimate contact with ceria [30,31] or lanthana [32] gives a TPR feature between 100 and 200 ~ whereas Pd on alumina has its chief TPR feature at temperatures between 0 and 60 ~ [23,32,33]. In fact, partial reduction of PdO may be occurring in our experiments prior to the start of the temperature ramp from 0 ~ The TPR data thus support the oxygen uptake experiments in suggesting that rare earth oxides contribute little to the uptakes observed for the 120 h dynamometer-aged catalysts. 5. DISCUSSION Automotive catalysts have traditionally been designed with the objective of maximizing dispersion of the noble metals. Some exceptions exist, such as the well-documented observation that oxidation rates of saturated hydrocarbons increase with increasing noble metal particle size [34,35]. However, one of the main objectives in designing automotive catalysts is to stabilize the dispersion of the noble metals against thermal sintering. Pd undergoes complex changes in oxidation state and structure in response to variations in temperature and gas environment [20-25,33,36-40]. Spreading and/or redispersion of Pd (as a cationic form of Pd) has been reported in oxidizing environments at temperatures between 700 and 900 oC [20-25]. At first glance, such a mechanism would appear to offer a plausible explanation for the thermal durability of Pd-basecl catalysts, particularly in aging cycles of the type reported here involving lean (i.e., oxidizing) modes. The data of this study, however, indicate that dispersions of Pd-based catalysts after thermal aging are quite low, and no greater than comparably aged PURh catalysts. Catalytic activ'rty, as measured in modulated NF sweep experiments, neither correlates with noble metal dispersion nor with noble metal surface area. As shown in Fig. 3, oxygen uptakes (as reflected in amounts of CO oxidized) show an increasing trend with the bulk oxygen capacities of the catalysts (expressed as the theoretical amount of bulk PdO which can be formed). Comparing the oxygen uptake data of Fig. 3 to the CO/NOx crossover efficiencies of Table 2, a threshold level of oxygen uptake is suggested, between 13 and 20/~-mol O/g-cat., above which the catalyst can store enough oxygen to ensure high efficiency during the A/F perturbations encountered in the dynamometer sweep test. This range is consistent with a stored oxygen demand of 15.3/~-mol O/g-cat. which we have estimated as required to ensure stoichiometric oxidation of all reducing species during the 0.5 s, -0.5 delta NF half cycle of the dynamometer sweep (centered at the stoichiometric A/F ratio) [41]. To a first approximation, the high activity of catalyst C3 results simply from its having an oxygen uptake capacity above the threshold requirement, whereas the other
364 dynamometer-aged catalysts have oxygen uptakes below the threshold requirement. The oxygen uptakes shown in Fig. 3 reflect much longer oxygen exposures (120 s) and larger CO titers (48 #-mol CO/g-cat.) than charactedstic of the dynamometer sweep cycle. Therefore, additional bench reactor CO oxidation experiments were carried out at shorter time intervals cycling between 15 s pulses of O2 (0.185 mol%) and 2 to 7 s pulses of CO (0.3 mol%). Results are summarized in Table 4, with the catalysts listed in order of highest to lowest CO conversions. The most active catalyst (C1 Pd-only TWC) stored sufficient oxygen to convert a 7 s CO pulse with 98% efficiency. At the other extreme, the PURh catalyst (G) gave only 48% conversion of a 7 s CO pulse and reached only 9'2% conversion with a 2 s pulse. The most pertinent condition for comparing the bench reactor and dynamometer results is at 5 s, since the bench reactor CO dose at 5 s (16/~-mol CO/g-cat.) is close to the dynamometer stored oxygen demand (15.3/~-mol O/g-cat.) Significantly, we find close quantitative agreement between the CO conversions at 5 s CO pulse length and the CO/NOx crossover efficiencies reported in Table 2. In both cases, catalysts E, F, and G give conversions in the 50-55% range whereas the C2 and C3 catalysts give conversions in excess of 85%. The good quantitative agreement between the engine dynamometer data and the laboratory pulsed CO oxidation experiments supports our interpretation that high CO/NOx crossover efficiencies in dynamometer sweep evaluations reflect oxygen storage above a threshold level. Table 4 Per cent conversion of CO pulses of various len.qths Catalyst 7 C1) Pd-only TWC C2) Pd-only TWC D) Pd-only TWC C3) Pd-only TWC B) Pd-AI20 3 A) Pd-AI20 s E) Pd/Rh TWC F) Pd/Rh TWC G) Pt/Rh TWC
98 NM NM NM 62 NM 42 NM 48
length of CO pulse (s) ............. 5 4 3 >99 95 89 86 78 67 51 51 NM
>99 >99 95 96 89 NM 67 NM 59
>99 >99 >99 > 99 >99 92 82 NM 71
>99 >99 >99 >99 >99 >99 >99 93 92
Given the connection between oxygen uptakes and catalyst sweep performance, the lack of a correlation between noble metal dispersion (and surface area) and catalyst performance implies that oxygen uptake does not depend on noble metal dispersion or surface area. The lack of a correlation is not surprising given that, 1) the quantities of oxygen involved, 2) the similarities in oxygen uptakes between catalysts with and without oxygen storage agents, and 3) the absence of TPR features characteristic of noble metal-rare earth oxide interactions all point to bulk oxidation of PdO as the
365
dominant oxygen storage mechanism in the 120 h dynamometer-aged Pd-based catalysts. For bulk oxidation of Pd to be the dominant oxygen storage mechanism, Pd oxidation kinetics must be rapid enough to account for the quantities of oxygen stored. Data reported by Remillard et al [38] on the air oxidation of thin Pd films indicate that the rates are indeed fast enough to account for the oxygen uptakes observed for the aged catalysts in these experiments. The 500 ~ 120 s pre-oxidation employed in the CO titration experiments would produce an oxide film thickness of 89 Angstroms using the growth expression reported in Ref. 38 (assuming equivalent kinetics in air and in the 0.87% O2/N2 mixture of our experiments). Taking the C3 catalyst, for example, with a dispersion of 3% (corresponding to a mean particle diameter of 373 Angstroms [42]), the formation of an outer PdO band of 89 Angstrom thickness would result in shrinkage of the metallic core to 274 Angstroms and growth of the overall diameter to 452 Angstroms. The increase in particle diameter owes to both the lower density of PdO compared to Pd and the presence of about 21% void volume in the PdO band [38]. Approximately 61% of the Pd in the particle is oxidized. Experimentally, 120 s oxidation of the C3 catalyst yielded a 3-pulse CO consumption corresponding to oxidation of about 52% of the Pd in the catalyst. Thus, the bulk oxidation kinetics are fast enough to account for the experimental observations. Even at the shorter 15 s oxygen exposure of the cyclic CO oxidation experiments, the kinetics of Remillard et al predict a 35 Angstrom thick oxide film. This corresponds to 25% oxidation of the Pd (or 15.2/~-mol O/g-cat.), just slightly less than the uptake of 16/~-mol O/g-cat. required for complete conversion of the 5 s CO pulse in the cyclic CO oxidation experiments (Table 4). Note that the oxidation kinetics of Remillard et al were obtained on 1-/~m-thick sputtered films of Pd on quartz, indicating that Pd oxidation kinetics are rapid enough, even for large-grain Pd particles, to account for oxygen uptakes required in both the dynamometer and bench reactor activity evaluations. The conclusions reached in the present study, namely that the catalytic activity of aged Pd-based catalysts depends primarily on Pd loading, begs the question '~hat is the function of the various additives such as ceria?". One obvious answer is that much effort has gone into stabilizing support components against loss of surface area and associated occlusion of noble metal particles. Both rare earth and alkaline earth oxides are important in this regard. In addition, we have addressed catalytic activity under very limited conditions -- modulated A/F sweep experiments carried out with lowsulfur fuel on an engine dynamometer. Factors such as noble metal dispersion and effects of promoters/stabilizers may be more important under other conditions, e.g. during light-off, at different temperatures and space velocities, with high-sulfur fuel, or in different A/F regimes during aging and evaluation. Also, we have focussed exclusively on CO oxidation, whereas different effects of dispersion and promoter/stabilizers may obtain for HC oxidation and for NOx reduction. Our study does suggest, however, that at least part of the Pd should be deployed in the form of large particles to obtain good CO and NOx conversions under modulated A/F conditions. The par~cles will consist of a metallic core with a roughened outer band capable of facile interconversion between Pd oxide and Pd metal.
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6. SUMMARY Chemisorption measurements, combined with oxygen uptake, TPR, and pulsed CO02 experiments were employed to determine the source of large differences in dynamometer sweep performance of a series of Pt/Rh, Pd/Rh and Pd-only TWCs after dynamometer aging. The following observations have been made: 1) Apparent noble metal dispersions of 75 and 120 h dynamometer-aged TWCs range from about 2 to 6%. 2) CO/NOx cross-over efficiencies of aged catalysts in dynamometer sweep experiments do not correlate with either noble metal dispersion or noble metal surface area. 3) Oxygen uptakes in both dynamometer and bench reactor experiments at 500 ~ are much too great to attribute to oxygen chemisorption. 4) After 120 s oxygen exposure (500 *C), all of the dynamometer-aged Palbased catalysts gave oxygen uptakes that could be accounted for by the formation of bulk PdO. 5) Dynamometer-aged (120 h) catalysts showed no evidence for oxygen storage via rare earth oxides. 6) Formation of bulk PdO is the primary oxygen storage mechanism in the dynamometer-aged Pd-based catalysts. 7) A threshold level of oxygen storage (via bulk PdO) is required to reach high CO/NOx conversion levels in dynamometer sweep tests; Pd loading, rather than dispersion or surface area, is the most important factor affecting oxygen uptakes. 8) Rates of oxygen uptake in the dynamometer-aged catalysts are consistent with published oxidation kinetics of 1-/~m-thick Pd films. 7. ACKNOWI.EDGMENTS We thank K. S. Patel and D. M. DiCicco for providing the dynamometer-aged catalysts and sweep evaluation data. E. Gulari and C. Sze (U. of Michigan) assisted with the design of the pulsed reactor system. REFERENCES 1. M. Shelef, K. Otto, and N.C. Otto, Adv. in. Catal. 27 (1978) 311-365. 2. H.S. Gandhi, W.B. Williamson, E.M. Logothetis, J. Tabcock, C. Peters, M.D. Hurley, and M. Shelef, Surf. & Interface Anal. 6(4) (1984) 149. 3. W.B. Williamson, D. Lewis, J. Perry, and H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 531. 4. R.L Klimisch, J.C. Summers, and J.C. Schlatter, Amer. Chem. Soc. Adv. Chem. Ser. 143 (1975) 103. 5. J.C. Summers, J.F. Skowron, and M.J. Miller, Soc. of Automotive Eng., Paper
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