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Pt/Ce0.68Zr0.32O2 Washcoated Monoliths for Automotive Emission Control J. R. Gonza´ lez-Velasco,*,† M. A. Gutie´ rrez-Ortiz,† J. L. Marc,† J. A. Botas,† M. P. Gonza´ lez-Marcos,† and G. Blanchard‡ Chemical Engineering Department, Faculty of Sciences, Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea, P.O. Box 644, E-48080 Bilbao, Spain, and Rhodia Recherches, 52 rue de la Haie Coq, F-93308 Aubervilliers Cedex, France
A three-way model catalyst has been prepared with 0.58 wt % Pt on Ce0.68Zr0.32O2 mixed oxide washcoated on a ceramic monolith. Conditions of the washcoating procedure, i.e., powder size distribution and acidic nature of the slurry, have been optimized. The activity on simultaneous elimination of NO, C3H6, and CO of the prepared monolithic catalyst has been determined, fresh and after submitted to mild and severe reducing-oxidizing thermal treatments. Changes in activity have been related with the role of platinum and ceria-zirconia mixed oxide in the reaction environment. The effect of aging under severe oxidation at higher temperature has also been related to the reduction of textural properties of the catalyst. The efficiency of the prepared catalyst was compared with three-way catalyst behavior of two commercial ceria-based monolithic catalysts with similar platinum contents and washcoat percentages, describing the advantages of using ceria-zirconia mixed oxides instead of CeO2/Al2O3 to improve stability and oxygen storage capacity. Introduction Metal-loaded CeO2-ZrO2 mixed oxides are known to be very efficient three-way catalysts (TWCs) for the simultaneous elimination of CO, HC, and NO pollutants present in the automotive exhaust gases.1 The role of such mixed oxides as TWC promoters as well as oxygen storage capacity (OSC) enhancement components has been reviewed by Kasˇpar et al.2 and is currently the subject of different studies.3-7 Although monoliths are the real support for catalysts in the automotive converters,8-10 preliminary research on new formulations is commonly made by testing the catalysts not yet deposited over the monolith but as pellet samples. In the monolithic catalytic converter, the catalyst (active component) is coated on the walls of a monolith substrate by the washcoat procedure.11 Once the good behavior of the catalyst has been checked, the washcoating procedure is conveniently adapted by the automobile converter makers, considering the physicochemical characteristics of support and active phases to be incorporated.12 To achieve the more stringent regulations needed in the future, the thermal stability and resistance to deactivation phenomena of TWCs need to be improved. It has been reported that metal-loaded ceria-zirconia catalysts maintain suitable OSC performance after aging treatment under high temperature and an aggressive atmosphere even with a significant decrease of some textural properties (specific surface area and total pore volume).13,14 The behavior of these kinds of catalysts in the presence of SO2 has also been studied.15,16 * To whom correspondence should be addressed. Tel.: +34 94 601 2681. Fax: +34 94 464 8500. E-mail: iqpgovej@ lg.ehu.es. † Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea. ‡ Rhodia Recherches.
Previous activity tests carried out in our laboratory with fresh and aged metal-free17,18 and metal-loaded19,20 Ce0.68Zr0.32O2 catalyst pellets showed that some specific aging treatments could enhance low-temperature activity. At this time, we decided to check this behavior with the catalyst washcoated on a cordierite monolith after it was submitted to those and other more severe aging treatments. The model catalyst selected has been a low Pt loading, high surface area ceria-rich (Ce0.68Zr0.32O2) mixed oxide. Pt loading has been selected not far from that of the actual TWCs, which is usually in the range of 0.1-0.15 wt % referred to as the total catalyst weight. The composition of the mixed oxide was selected as the best compromise between resistance to thermal treatments and activity as TWC.17,18 Thus, the aim of this work is to optimize washcoating of monoliths to become active TWCs as well as to check the effect of severe aging conditions on their activity. Also, the results of this work shall be connected with those previously reported for pellets,20 and the TWC activity compared with that of two commercial ceria-based catalysts with similar composition but only CeO2 instead of Ce0.68Zr0.32O2. Experimental Section Catalyst Preparation. A Ce/Zr mixed oxide with high surface area (HS; 101 m2/g) and a molar Ce/Zr ratio of 68/32 (HS-Ce0.68Zr0.32O2) was synthesized by Rhodia as a single phase by precipitation. Pt was incorporated by incipient-wetness impregnation from an aqueous solution of a chlorine-free platinum precursor (nitrate). Then the catalyst was dried in air at 383 K for 2 h and subsequently calcined in dry air at 723 K for 5 h. The composition of the catalyst, supplied by Rhodia as a powder (9-30 µm; Figure 1, dash-dotted line, determined in a Malvern Mastersizer X apparatus), was 0.58 wt % Pt/HS-Ce0.68Zr0.32O2.
10.1021/ie020157v CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002
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Figure 1. Particle size distribution of the catalyst powders for washcoating: as supplied (dash-dotted line); after 3 h of milling in nitric acid (pH ) 5; dashed line); after 3 h of milling in citric acid (pH ) 4; dotted line); and after 3 h of milling in citric acid (pH ) 6; continuous line).
Figure 2. SEM image of the cleaned cordierite monolith before washcoating.
The monoliths were substrates of cordierite supplied by Corning with the following characteristics: 62 cells cm-2, wall thickness of 0.15 mm, and both a diameter and length of 21 mm after cutting. Before any manipulation, small monoliths were cleaned by thermal treatment at 1173 K in air for 1 h. Figure 2 shows a scanning electron microscopy (SEM) image of the cleaned monolith (JEOL 6400 microscope, at Departamento de Fı´sica de la Materia Condensada, UPV/EHU), where the cavities in the walls of the monolith, necessary for a good sticking of the washcoat, can be seen. The slurry solution for washcoating consisted of 30 g of the previous catalyst powder in 150 mL of water, resulting in a pH of 6.6. The particle size of this slurry was too large to stick correctly to the monolith walls. Thus, wet crushing of the original powder was needed for a better adhesion of particles to the ceramic walls. On the other hand, a better suspension of particles in the slurry was achieved at lower pH, provided by the addition of a more acidic aqueous solution, which affected the crushing procedure. Typically, 60 mL of water was added to 30 g of catalyst powder to be crushed in a bowls grinder for 3 h, because it was checked that a longer time did not produce any additional decrease in the particle size, and the pH adjusted. After crushing,
the slurry solution was recovered and stirred, its volume was increased to 150 mL with water, and the pH was again adjusted. Three slurries were prepared: one at pH ) 5 by the addition of a nitric acid aqueous solution and two others at pH ) 4 and 6 by the addition of a citric acid aqueous solution, resulting in the granulometric distributions, after stirring for 12 h, shown in Figure 1: dashed, dotted, and continuous lines, respectively. A shift of the average particle size to smaller values can be seen in the range of 0.5-10 µm. The particle size seems to decrease at lower pH, and a nitric acid solution provides smaller particles than a citric acid solution. The washcoating was carried out by immersion of the cordierite monolith in the slurry for 10 s, increasing the stirring to be sure the solution went into the channels of the monolith. Afterward, the washcoated monolith was blown with air to remove the excess of slurry and finally dried at 393 K for 1 h. This procedure was repeated in order to increase the amount of catalyst washcoated on the surface of cordierite until the desired percentage of washcoat weight was obtained. The pH of the solution was maintained with the selected acid during the complete procedure. To homogenize as much as possible the thickness of washcoating in the channel walls, the relative position of the immersed monolith with the slurry flow was continuously changed. The reproducibility of the preparation was checked by simultaneous immersion of three monoliths in the slurry, which finally resulted in very similar catalyst deposition (less than 1% difference) in all three samples. Thermal Treatments and Aging Procedure. The so-called “fresh” monolithic catalyst (FM) was obtained after submitting the washcoated sample to a cleaning treatment consisting of oxidation in a flow of 5% O2/N2 at 823 K for 1 h to eliminate possible nitrates and/or carbonates left behind after preparation. Then, the sample was slowly cooled to 423 K and, after switching the flow to pure N2, cooled to room temperature. Reduction of the catalyst (FM-R) was carried out under 5% H2/N2 at 823 K for 1 h. The catalyst was also submitted to mild reoxidation (FM-R-O) under 5% O2/ N2 at 823 K for 1 h. Finally, the aged sample (AM) was obtained by submitting the catalyst to calcination (severe oxidation) at 1273 K for 11 h, followed by cooling to room temperature in air. Reduction of the aged sample (AM-R) was identical with that of the fresh sample. Textural Characteristics. The specific surface area, pore volume, and average pore radius of the pellets and monoliths have been measured by the BrunauerEmmett-Teller method using a Micromeritics ASAP2010 apparatus. Before measurements, samples were outgassed under 100 mPa vacuum at 573 K for 12 h. Activity Measurements. The activity tests were carried out in a stainless steel tubular reactor with an inner diameter of 21 mm, where the reactants circulated downward, at the lower part of which the monolith was allocated for reaction. The reactant temperature was measured at both the inlet and the outlet of the monolith, and an automatic temperature controller connected to an electric furnace was used to get the desired temperature. Before a run, the catalyst was kept in N2 at 373 K for 4 h. During activity tests the temperature was increased from 373 to 873 K at 3 K/min, and conversion data were continuously recorded: by chemiluminiscence for NO, nondispersive infrared for
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Figure 4. SEM image of the fresh monolith, FM (washcoating in nitric acid, pH ) 5).
Figure 3. Weight uptake during washcoating versus number of immersions of the monolith in the slurry: 2, nitric acid (pH ) 5); b, citric acid (pH ) 4); 9, citric acid (pH ) 6).
CO and CO2, flame ionization for C3H6, and magnetic susceptibility for O2. The monolithic catalyst was tested under stationary stoichiometric conditions (A/F ) 14.63) with the following feed-stream composition in volume: CO, 1%; C3H6, 900 ppm; NO, 900 ppm; O2, 0.86%; CO2, 10%; H2O, 10%; N2, balance. The space velocity was 120 600 h-1. Demineralized water (ultrapure) was dosed with a high-precision liquid pump and then vaporized before mixing with the rest of the feed stream. On the other hand, to check the behavior close to real conditions of the FM-R sample in comparison with two commercial ceria-based samples available on the market, the samples were tested by cycling the following two reducing and oxidizing feed streams with a frequency of 1 Hz and an amplitude of (0.5 A/F, the socalled “cyclic stoichiometric feed stream” in our previous work.19-23 Reducing: 10.0% CO2, 1.6% CO, 900 ppm C3H6, 900 ppm NO, 0.46% O2, 10.0% H2O, and N2 to balance (A/F ) 14.13). Oxidizing: 10.0% CO2, 0.4% CO, 900 ppm C3H6, 900 ppm NO, 1.26% O2, 10.0% H2O, and N2 to balance (A/F ) 15.17). The two feed streams were prepared in two independent gas-blending systems. They could be quickly alternated through the reactor by means of two fastacting solenoid valves, in an apparatus similar to that developed previously by Schlatter et al.24 Results and Discussion Washcoating of Monoliths. The weight uptake percentage of the monolith versus the number of immersions is plotted in Figure 3 for the three studied slurries. The ceria-zirconia firmly adhered to cordierite once the average particle size was reduced to around 1 µm. Figure 3 shows that the faster washcoat uptake was obtained with the nitric acid aqueous solution at pH ) 5; after 16 immersions, 30% of washcoat uptake was obtained. When the citric acid aqueous solution was used, the evolution of weight uptake with a number of immersions resulted in no practical dependence of the pH of the solution. Figure 4 shows, as an example, a SEM image of the FM sample washcoated with the slurry acidified at
pH ) 5 with nitric acid. We can see that the washcoat is uniform across the monolith walls, with a thickness in the range of 5-10 µm. At the corners, as expected, thicker deposits of washcoat are encountered. The SEM images of the monoliths prepared with citric acid were very similar, although the washcoat layer is slightly thinner because of the smaller amount of material washcoated (20% vs 30%). Thus, adequate washcoating of these kinds of materials on cordierite monoliths with the structure of cavities necessary for a good sticking does not seem too difficult. Upon analysis of the results, it was found that a good washcoating of these materials is favored by particle size distributions preferably below 10 µm. Although the pH during grinding affects the particle size distribution of the slurry (compare continuous and dotted lines in Figure 1, corresponding to pH ) 6 and 4, respectively, both in citric acid), it seems not to affect the rate of washcoating uptake (see Figure 3) or the washcoating quality in the studied range. For similar distributions (compare dotted and discontinuous lines in Figure 1, corresponding to citric and nitric acid, respectively), however, the nature of the acid in the slurry seems to play the decisive role, with nitric acid being better than citric acid (see Figure 3). Considering the above results, we have chosen the monolith prepared with nitric acid at pH ) 5 for subsequent activity studies. Also, nitric acid presented the advantage that it prevented the addition of a new ion during the preparation process, because both ceriumzirconium mixed oxide and platinum were derived from nitrates. Light-Off Tests. Figure 5 shows the CO, C3H6, and NO light-off curves obtained with fresh and aged monolithic catalysts (FM and AM) as well as after submitted to the mentioned thermal treatments (FM-R, FM-R-O, and AM-R). The light-off temperature (temperature at which 50% conversion is reached), T50, and conversion percentage measured at 773 K, X773, is reported in Table 1. Differences can be noted between reduced and unreduced catalysts, with an improvement of the activity for the former group, i.e., FM-R > FM and AM-R > AM. The two reduced samples, and only them, present a twostep behavior in the light-off curves for CO. This behavior has already been observed in catalysts of the same composition as the washcoat of this work but in the form of pellets.19,20 The low-temperature activity step was attributed to the existence of reduced Pt19 and associated with the
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Figure 5. CO, C3H6, O2, and NO light-off curves obtained in stationary stoichiometric conditions with the prepared monoliths: 9, FM; b, FM-R; 2, FM-R-O; 1, AM; [, AM-R. Table 1. Values of T50 and X773 Obtained with the Prepared Washcoated Monoliths (Stationary Stoichiometric Conditions) T50, K (X773, %)
Table 2. Textural Properties of the Cordierite Monolith and the Prepared Washcoated Monoliths SBET, m2/g
Vp, cm3/g
cordierite M 0.5 monolith FM ≈ FM-R ≈ FM-R-O 22a 93b AM ≈ AM-R 1.3a 5.5b
0.0005 0.045a 0.192b 0.002a 0.009b
type
catalyst
CO
C3H6
NO
FM FM-R FM-R-O AM AM-R
715 (74.7) 390 (98.2) 560 (98.2) (2.7) 675 (73.9)
803 (49.8) 508 (96.8) 600 (96.7) (4.8) 740 (63.8)
(2.0) 500 (64.5) 648 (53.5) (0.0) (41.0)
reaction of CO and O2 (compare the curves for CO and O2 conversion in Figure 5), both adsorbed on the metal surface,20 following Langmuir-Hinshelwood reaction kinetics. A maximum in the reaction rate between CO and O2 at around 500 K, depending on the CO partial pressure, has been found on platinum surfaces25 and is associated with changes in the surface coverage of both components. At higher temperatures, a change in the reaction path was observed, which produced a high-temperature activity step. This second step is common to both reduced and unreduced samples, and not only CO and O2 but also C3H6 and NO underwent conversion. With the unreduced fresh catalyst, only moderate conversions were obtained at 773 K: 75% for CO, 50% for C3H6, and only 2% for NO. Reduction of this sample produced the most active of the studied samples, enhancing very significantly the activity to 98% for CO, 97% for C3H6, and 64.5% for NO. This NO conversion could be even more improved when running under the “cyclic stoichiometric feed stream”, as will be reported below. To explain the differences in the activity of FM and FM-R samples, we have to consider the final treatment to which each sample has been submitted. The FM sample suffered a mild oxidation treatment (cleaning at 823 K). Thus, platinum will probably be oxidized, mainly as PtOx,26 which is less reactive than reduced Pt.27 The absence of a low-temperature activity step for sample FM is due to the absence of reduced platinum, and the reacting feed stream, being stoichiometric, is
sample
rp (average), nm 8.3
a Relative to the total weight (monolith + washcoat). b Relative to the weight of the washcoat.
not able to reduce PtOx to Pt. During the reduction treatment suffered by sample FM-R, platinum oxide will be reduced to Pt. Improvement of the low-temperature conversion induced by some specific treatment has also been reported in the literature.28 Mild oxidation treatments (FM-R-O), on the other hand, produced a decrease in the activity. Figure 5 shows that oxidation of the fresh reduced monolith produced a shift in the light-off curves to higher temperatures, although not so high as those obtained with the unreduced fresh catalyst, and conversions at 773 K were maintained for CO and C3H6 and nearly for NO (Table 1). The different activity between FM and FMR-O cannot be justified in this case by differences in the oxidation state of platinum. Temperature-programmed reduction experiments carried out with the catalysts in pellet form have shown that the reduction peak of the catalyst is shifted to higher temperature as it is submitted to oxidation treatments at increasing temperatures.20 The fresh sample is always the one to present the reduction peak at the highest temperature, probably because of the increased reducibility of the mixed oxide after steps of reduction and oxidation.21,29 This can justify the different activity of FM and FM-R-O; although they have similar specific surface areas (see Table 2), both are oxidized at the same temperature and have the same amount of platinum. Better activity of FM-R-O compared to FM could be expected even though some decrease in the specific surface area would have taken
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Figure 6. SEM image of the aged monolith, AM. Table 3. Composition of the FM-R Sample and the Commercial Ceria-Based Catalysts % Pt FM-R C1 C2
0.58 0.50 0.48
% Rh
0.08
% CeO2
support
% washcoat
25 25
Ce0.68Zr0.32O2 Al2O3 Al2O3
30 20 20
place because of extension of the reducibility of the mixed oxide to the bulk.20,21 Aging in oxidizing conditions produced a catalyst (AM) with nearly no activity, which can only be justified in relation to the textural properties shown in Table 2. We can see that aging produced a complete destruction of the porous texture of the washcoat, with probable growth of the support particles and ceria-zirconia phase segregation phenomena.30 The decrease in the surface area was so high that it could not be compensated for with the bulk contribution. Figure 6 shows a SEM image of the aged monolith, in which we can see how aging has induced an evolution of the washcoated material, with the formation of cracks that can be appreciated mainly at the corners of the monolith channels. Also, the aging treatment can induce sintering of the platinum,31 which will be mainly as platinum oxide aging in oxidizing conditions. Upon reduction (AM-R), some of the activity is recovered, particularly because of the reduction of platinum, but the effect of the low surface area can still be observed. This effect not only reduces the surface area and pore volume but can also occlude some platinum, leaving it inaccessible to the reactants.32 Comparison with Commercial Ceria-Based Catalysts. To test the three-way behavior of the prepared catalyst, we have compared it with two commercial ceria-based monolithic catalysts under cyclic stoichiometric conditions, which resembled the exhaust A/F fluctuations in a closed-loop emission control system in automobiles. The commercial catalysts for comparison were chosen with the compositions shown in Table 3, where the composition of the FM-R sample is also given. Figure 7 shows the light-off curves of CO, C3H6, and NO for our reduced fresh catalyst (FM-R; Figure 7a) and the two commercial catalysts after submitted to the same primary reduction treatment (C1 and C2; Figure 7b,c). We can see in Figure 7 that the light-off curve corresponding to CO is practically identical for the three catalysts. Because the lower-temperature conversion is associated with the presence of a reduced platinum surface in the catalysts, this is not surprising considering that the amount of platinum in the three catalysts
Figure 7. CO, C3H6, and NO light-off curves obtained in cycled stoichiometric conditions with monoliths: (a) FM-R; (b) C1; (c) C2. Symbols: 9, CO; b, C3H6; 2, NO.
is not very different and all of the samples have been reduced in identical conditions. Also, neither of the samples was submitted to high-temperature treatments, and thus no decoration effects by ceria on the platinum are expected.20 Differences in the behavior of the three samples are found, however, when the light-off curves for C3H6 and NO are compared in Figure 7. In particular, our catalyst produces an important increase in the activity for C3H6, with a shift of more than 150 K to lower temperatures in the light-off curve, even though the C2 catalyst contains a small amount of rhodium in its composition. This increase in activity is associated with the presence of Ce0.68Zr0.32O2 instead of CeO2/Al2O3 in the washcoat and the increased OSC of the catalyst that this change produces.19-21 Concerning NO, we can see that its conversion starts at slightly higher temperature (about 20-30 K) with our catalyst than with C1 and C2, but the light-off curve rises quickly above 90% NO conversion, which is not observed with the commercial ceria-based catalysts. The quick rise up to the final conversion in the NO light-off curve is again due to the increased OSC associated with the presence of Ce0.68Zr0.32O2 in the catalyst, while the small delay is probably associated with different selectivities. Conclusions The washcoating procedure of a ceramic monolith with a 0.58 wt % Pt/Ce0.68Zr0.32O2 powdered catalyst has
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been studied. To achieve an adequate washcoating of the material, the slurry should be composed of particles below 10 µm in diameter. Besides the particle size distribution, it is also important to stabilize the slurry in an acidic medium. Although the pH, in the studied range, affected the particle size distribution obtained during wet grinding of the slurry, no effect on washcoating was observed provided that the particles in the slurry were small enough. More significant was the effect of the nature of the acid used to reduce the pH in the slurry solution: nitric acid, compared to citric acid, produced a quicker washcoating uptake, even for almost identical particle size distributions in the slurry. Fine particles of around 1 µm and a pH of 5 obtained by the addition of nitric acid to the slurry solution were chosen to prepare the optimum washcoated monolithic catalyst. In this way, a good adherence of the catalyst to the cordierite substrate up to 30 wt % after 16 subsequent immersions was achieved. The prepared monolithic catalyst (FM) has been submitted to reducing and mild oxidizing treatments and subsequently tested under a stationary feed stream with an A/F ratio of 14.63. The reduced sample (FM-R) showed higher activity than the unreduced sample, and the highest activity of all of the studied samples, as was already observed with pellets prepared with the same composition as the washcoat. This behavior seems to be related to the state of platinum as Pt0 in the reduced sample instead of PtOx in the unreduced sample. For the same reason, mild oxidation (FM-R-O) produced a decrease in the activity, shifting the light-off curves to higher temperatures. However, the sample submitted to mild oxidation (FM-R-O) remained more active than the fresh one (FM), because of extension of the reducibility of the ceria-zirconia mixed oxide to the bulk. Aging in severe conditions produced a catalyst (AM) with nearly no activity mainly because of complete destruction of the porosity of the ceria-zirconia mixed oxide. Also, sintering and probable occlusion of platinum reduces the activity of the catalyst. Reduction of the aged sample (AM-R) allowed the recovery of some activity because of an increase in the Pt0/PtOx ratio. A comparison of the prepared FM-R catalyst with two commercial ceria-based catalysts of similar metallic composition but CeO2/Al2O3 instead of Ce0.68Zr0.32O2 under stoichiometric cyclic feed stream showed notable enhancement of OSC and, subsequently, the model catalyst prepared in this work presented some increase in the activity. Acknowledgment The authors thank the European Union (TMR program, CEZIRENCAT Project, Contract FMRX-CT-960060), the Universidad del Paı´s Vasco/EHU (Project 9/UPV 13517/2001), and the Basque Government (PI1999-109) for their financial support. Literature Cited (1) Muraki, H.; Zhang, G. Design of Advanced Automotive Exhaust Catalysts. Catal. Today 2000, 63, 337. (2) Kasˇpar, J.; Fornasiero, P.; Graziani, P. Use of CeO2-Based Oxides in the Three-Way Catalysis. Catal. Today 1999, 50, 285. (3) Martı´nez-Arias, A.; Ferna´ndez-Garcı´a, M.; Iglesias-Juez, A.; Hungrı´a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. New Pd/CeXZr1-XO2/Al2O3 Three-Way Catalysts Prepared by MicroemulsionsPart 1. Characterization and Catalytic Behaviour for CO Oxidation. Appl. Catal. B: Environ. 2001, 31, 39.
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Resubmitted for review August 4, 2002 Revised manuscript received November 4, 2002 Accepted November 18, 2002 IE020157V
