272
Ind. Eng. Chem. Res. 2000, 39, 272-276
Catalytic Activity Study of Ceria-Zirconia Mixed Oxides Submitted to Different Aging Treatments under Simulated Exhaust Gases Juan R. Gonza´ lez-Velasco,*,† Miguel A. Gutie´ rrez-Ortiz,† Jean-Louis Marc,† Juan A. Botas,† M. Pilar Gonza´ lez-Marcos,† and Gilbert Blanchard‡ Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias, Universidad del Paı´s Vasco/E.H.U., P.O. Box 644, E-48080 Bilbao, Spain, and Rhodia Recherches, 52 rue de la Haie Coq, F-93308 Aubervilliers Cedex, France
A Ce/Zr mixed oxide with a molar ratio of 68/32 was submitted to aging treatments in different environments: one oxidizing, one reducing, and one in which the feedstream was continuously changing between oxidizing and reducing through the stoichiometric composition. The textural evolutions of the differently aged samples have been compared to each other and also to the fresh sample and have been related to the samples’ activities as three-way catalysts. The activities of the samples have been found to depend not only on the samples’ final specific surface areas but also on the reducing or oxidizing nature of the aging treatment. Thus, it has been pointed out that an aging feedstream as similar as possible to the actual one in real operation should be used when performing durability studies to arrive at reliable conclusions. Introduction In the past decade, investigation on three-way catalysts (TWCs) has focused on ceria-zirconia metalsupport oxide applications. Many authors have shown and argued about the advantages of ceria and zirconia together compared to those of ceria alone, one of the most important advantages being their capacity to present a higher oxygen storage capacity.1-3 Also, the presence of zirconia stabilizes ceria crystallites, preventing their sintering, especially under severe aging conditions at high temperature.3,4 The redox properties of CeO2-ZrO2 mixed oxides have been investigated and related to the composition and/ or the structural, chemical, and textural properties, to better understand and to improve the catalytic properties, the goal being to determine which factor influences the catalytic activity. Surface oxygen and oxygen vacancies seem to be involved in the catalytic improvement: an enhancement of the oxygen ion mobility can promote the redox properties and oxygen diffusion through the catalyst at lower temperature, thus improving the activity. Some studies have shown that the properties of these mixed oxides are optimum for a composition range in which the zirconia content does not exceed 50%.5,6 Most of the catalytic reactions investigated on these supports concern CO7 and hydrocarbon8 oxidation, or NO reduction by CO.9,10 The presence of highly reactive O2 radical anions on metal-loaded reduced ceria11 has been suggested as being very effective in CO oxidation. Also, oxygen vacancies associated with reduced ceria in the proximity of noble-metal particles have been suggested as promoting NO and CO conversion.9 From these studies, it seems that the mixed oxide support plays an important role in the formation of active sites.10 * 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. ‡ Rhodia Recherches.
The presence of oxygen vacancies in the bulk and at the surface of the mixed oxides induced by the introduction of zirconia in the ceria lattice and enhanced by some aging treatment under reducing or cycled redox conditions seems to be one of the most interesting aspects. Concerning the catalytic reactions involved in TWCs, many investigations have already been published about the promoting effect of ceria on the reactivity of noble metals12-14 or about the activity of ceria alone. With ceria/zirconia mixed oxides, as mentioned above, mostly reactions with simple gas mixtures were tested.5,7-10 Simple reactions are usually used as tests to characterize the catalysts, while the situation in which a complex gas mixture is used is much more difficult to explain and there is no longer any agreement about the effects. However, we have to keep in mind that the catalytic properties of the final catalyst are dependent on the support properties. Because of that, it is interesting to measure the activity of the support alone under a complex gas mixture in which CO2, CO, a hydrocarbon, NO, water, and N2 are present, to have an idea about the gas mixture’s contribution to the activity of the catalyst.15,16 Another very important problem in TWCs is the deactivation phenomena induced by aging treatments, as a high durability of the catalyst is required for practical applications. Generally, the approaches to determining the catalytic resistance to aging consist of studying the durability of the catalyst with extreme aging conditions. In that way, the stability of ceria/ zirconia mixed oxides has been studied by analyzing the chemical, structural, and textural modifications suffered by the samples after aging treatments at high temperature using H2, O2, and H2O as reacting gases.2,17 We have studied in this work the durability of ceria/ zirconia mixed oxides using during the aging treatments gas compositions close to the real applications. Thus, a Ce/Zr 68/32 mixed oxide has been submitted to different aging treatments, in oxidizing, reducing, and redox cycled conditions with a complex gas mixture. After the aging treatments, the catalytic activity of the mixed oxide as a TWC was determined under simulated
10.1021/ie990433e CCC: $19.00 © 2000 American Chemical Society Published on Web 01/15/2000
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 273
exhaust gases, and the textural properties were measured and compared to those of the fresh material. The objective was to determine how the redox nature of the treatment influences the catalytic properties, especially when the aging treatment is performed in the presence of reducing species such as NO or CO and oxidizing species such as O2, CO2, or H2O in the reacting gas stream. The study was carried out using a cerium-zirconium mixed oxide with a molar Ce/Zr ratio of 68/32. The reasons to choose this composition have been put forward in a previous paper:15 being a combination of good intrinsic activity and resistance to sintering under moderate temperature redox treatments as well as being the only composition to present NO conversion after a moderate temperature redox cycling treatment in a stoichiometric feedstream. Experimental Section Sample Preparation. Rhodia supplied the Ce0.68Zr0.32O2 mixed oxide used in this study. The samples, constituted of pellets with a size range between 0.5 and 1 mm, were previously cleaned by a mild oxidizing pretreatment (cleaning pretreatment) and afterward submitted to either a reducing, an oxidizing, or a redox cycling aging treatment. The sample after cleaning constitutes for us the “fresh” sample. The cleaning pretreatment consisted of heating the sample at 10 K/min up to 823 K in a flow of O2/N2 (5: 95). Then, after 1 h, the sample was slowly cooled down to 423 K under the same flow, and after the flow was switched to N2 flow, the sample was slowly cooled to room temperature. The objective of this treatment was to clean the sample of nitrates and/or carbonates remaining from preparation. Before aging, the samples were degassed for 12 h in N2 flow at 373 K. Aging treatments consisted of heating the sample at 8 K/min from 373 up to 1173 K in a gas flow with a space velocity of 125 000 h-1. The temperature was maintained at 1173 K for 5 h and then slowly cooled to 423 K under the same flow; after the flow was switched to N2, the sample was slowly cooled to room temperature. The gas composition during the thermal treatment determined the oxidizing, reducing, or redox cycling character of the aging applied to the sample: A. Oxidizing Aging (SO, Severe Oxidation). The gases flowing through the sample were oxidizing (airto-fuel ratio of 15.13; stoichiometric number of 2.16), with the following composition, in volume: CO2, 10%; CO, 0.4%; C3H6, 900 ppm; NO, 900 ppm; O2, 1.26%; H2O, 10%; nitrogen, the balance. B. Reducing Aging (SR, Severe Reduction). The gases flowing through the sample during aging had a reducing character (air-to-fuel ratio of 14.13; stoichiometric number of 0.42), with the following composition, in volume: CO2, 10%; CO, 1.6%; C3H6, 900 ppm; NO, 900 ppm; O2, 0.46%; H2O, 10%; nitrogen, the balance. C. Redox Cycling Aging (SRC, Severe Redox Cycling). During redox cycling aging, the gases corresponding to oxidizing and reducing aging were alternatively fed with a frequency of 0.017 Hz. Considering the air-to-fuel ratio of both streams, their mixture produced the stoichiometric composition (A/F ) 14.63). Thus, a gas composition oscillating around the stoichiometric value, with an amplitude of (0.5 units, was continuously flowing through the sample. This aging
Table 1. Specific Surface Area, Pore Volume, and Average Pore Radius of Fresh and Aged Ce/Zr 68/32 Mixed Oxide Samples sample
SBET, m2/g
VP, cm3/g
rP (average), nm
fresh SO-aged SR-aged SRC-aged
110 41 39 27
0.256 0.200 0.218 0.175
4.7 9.7 11.3 12.9
treatment simulates the actual aging that takes place in the catalytic converter during real operation of a TWC. Textural Characterization. Specific surface area, pore volume, average pore radius, and pore size distribution of fresh and aged mixed oxides were determined from the N2 isotherms at 77 K, using a Micromeritics AccuSorb 2100E apparatus. The sample, weighing about 500 mg, was degassed at 573 K under a vacuum of 100 mPa overnight before the measurements were taken. SBET was determined by linear regression of the adsorption data obtained at relative pressures up to P/P0 ) 0.25 according to the BET equation. The total pore volume was calculated at P/P0 ) 0.98 according to the Gurvitsch rule. The average pore radius was determined by assuming cylindrical pores. The mesopore size distribution was deduced from the desorption branch using the Kelvin equation and the procedure given by Roberts.18 Activity Measurements. The catalytic activity was measured using a conventional continuous-flow reactor.19 The activity runs were carried out with 1.8 g of pellets diluted with quartz, to a total fixed-bed volume of 3.5 cm3. The temperature was programmed from 373 to 873 K at 3 K/min. Before the runs, samples were degassed in a flow of N2 at 373 K for 4 h. The volume composition of the stoichiometric reacting gas was as follows: CO2, 10%; CO, 1.0%; C3H6, 900 ppm; NO, 900 ppm; O2, 0.86%; H2O, 10%; N2, the balance. The gas leaving the reactor was led to a condenser to remove water vapor. The remaining components were continuously analyzed by nondispersive infrared (CO and CO2), flame ionization (C3H6), magnetic susceptibility (O2), and chemiluminiscence (NOx). Results and Discussion Textural Characterization. Table 1 and Figure 1 summarize the textural properties of fresh and aged samples. We can see that the fresh sample presents, as expected, the highest surface area, 110 m2/g, and pore volume, 0.256 cm3/g, with the lowest average pore radius, 4.7 nm. Its pore radii range from about 1.5 to 9 nm, with the distribution centered at 3.5 nm. Figure 1 shows that aging treatments induced a similar evolution in all the samples: the important surface area decrease is associated with a significant pore volume decrease and pore radii increase. We can see that the pore structure of the fresh sample is completely destroyed by sintering and a new, more open mesopore structure with bigger pores is formed in the aged samples. The pore size distribution is similar in all the aged samples, the three of them presenting pores in the range from 3 to 25 nm. The stability of Ce/Zr mixed oxides to aging depends on many factors, such as the preparation method, the zirconia content, and the aging conditionsschemical composition of the reacting gas,20 time, and temperature.6 The preparation method, zirconia content, and
274
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
Figure 1. Pore size distribution of fresh and aged Ce/Zr 68/32 mixed oxide samples.
aging time and temperature were constant in the differently aged samples; thus, the textural differences among the samples are necessarily due to the different gas environments in which aging was performed. As we have commented above, the aging treatment that better simulates the actual aging in the catalytic converter is the SRC. The two other aging treatments were carried out to see what happens in both the oxidizing and reducing ends limiting the redox cycles. A comparison among the three aged samples, to determine the effect of the gas composition during aging in their textural properties, makes it clear that SO aging is milder than SR, and SR milder than SRC. Thus, textural evolution of Ce/Zr mixed oxides during aging is favored by a reducing environment, compared to an oxidizing one, and even more favored when the redox nature of the gaseous environment is alternated. It becomes clear from the former results that SRCaged samples do not present intermediate textural properties between SO- and SR-aged samples, but on the contrary, much more important a surface area decrease that can be related to structural stress in the solid after several steps of oxidation and reduction.4 Aging in reducing environments has usually been found in the literature to favor sintering.17,20 However, differences compared to oxidizing environments are usually much higher than those found in this work. The reason is that reducing conditions are usually much stronger in the literature, where they usually prepare the reducing stream with different concentrations of H2. It should be emphasized that significant differences are observed among the three aged samples, even though neither the oxidizing environment was strongly oxidizing (oxidation in 5% O2/N2, for example, would correspond to A/F ) 19.62) nor the reducing environment was strongly reducing (reduction in 2.5% H2/N2, for example, would correspond to A/F ) 13.75), and thus there was not a dramatic change in the redox nature of the feedstream during redox cycling aging either. We have shown in previous papers that the activity of the Ce/Zr mixed oxides as TWCs, for a given Ce/Zr ratio, is related to the specific surface area of the samples.15,16 Thus, we will try to relate in the following section the textural evolution observed in the aged samples with their performance as TWC.
Figure 2. Conversion vs temperature for (a) CO and (b) C3H6 on Ce/Zr 68/32 mixed oxides: b, fresh sample; [, SO-aged sample; 2, SR-aged sample; 1, SRC-aged sample.
Activity Measurements. The CO and C3H6 conversion light-off curves for the four samples are reported in Figure 2. Neither the fresh nor the aged samples presented appreciable NO conversion, and so the curves for NO have not been included in the figure. Figure 2 shows that there is a significant loss of activity for both CO and C3H6 from fresh to aged samples, as the light-off curves for the aged samples are shifted to higher temperatures. This significant activity decrease will probably be related to the important surface area loss observed previously. When comparing the light-off curves in Figure 2 for the three aged samples, however, we can realize that the position of the curves cannot be a function exclusively of the specific surface area. This result suggests that the gaseous environment in which aging was carried outsoxidizing, reducing, or redox cyclingsalso has some influence on the activity performance of the mixed oxides. To isolate the effects due to the specific surface area from those corresponding to the redox conditions of the aging procedure, the reaction rate per square meter of surface area was calculated at low conversions, assuming differential reactor operation, from the light-off curves in Figure 2. The reaction rate versus temperature data for each component and each mixed oxide sample submitted to different treatments allow the determination of pre-exponential factors and apparent activation energies. Because of the apparent character of both the pre-exponential factors and activation energies, compensation phenomena occur (linear trend of the points for each pollutant in the graphic ln A0 vs Ea). An arbitrary line in the compensation graphic for each pollutant was obtained by linear regression of the points obtained from experiments carried out with Ce/Zr mixed oxides with the stoichiometric feedstream (reported elsewhere15,16). The deviation of each individual point from this line has been found to be related to the
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 275 Table 2. Values of di for the Reaction of CO and C3H6 on Fresh and Aged Ce/Zr 68/32 Mixed Oxide Samples, with the Stoichiometric Feedstream Ce/Zr 68/32 sample
dCO
dC3H6
fresh SO-aged SR-aged SRC-aged
1.03 1.07 1.03 1.08
1.03 0.99 1.00 1.00
intrinsic activity of the solid and quantified by a parameter, di, calculated as
di )
(ordinate of the point - intercept of the line) (abscissa of the point)(slope of the line)
(1)
that is, a ratio of slopes. The reliability of di has been estimated to be within (0.02 for 95% confidence, calculated as the maximum variation of slope for the point. The higher the value of di, the higher the intrinsic activity of the mixed oxide for that component. Also, di is related to the distribution of reactions undergone by reactant i during an experiment with the rest of the components in the feedstream. Previous studies15 with Ce/Zr mixed oxides have shown that CO and C3H6 oxidation takes place following two reaction paths: either by reduction of the support, where the reduction rate is the limiting step, or by reaction with adsorbed oxygen species (superoxides, and also peroxides on a reduced mixed oxide surface13). Reduction of the support during reaction is compensated by its much faster simultaneous oxidation by the oxidizing components in the feedstream (O2, NO, and also water). A reduced Ce/Zr mixed oxide surface presents higher intrinsic activity for CO in the stoichiometric feedstream, attributed to the second reaction path. Table 2 lists the values of di obtained for the fresh (cleaned) sample as well as the three aged samples for the experiments reported in Figure 2, in the stoichiometric feedstream. If we have a look at the activity of the aged samples in Table 2, we can see that the intrinsic activity for C3H6 is significantly decreased in all of them, compared to the fresh sample. CO, however, is significantly improved in SO- and SRC-aged samples, while it remains nearly the same as that of the fresh sample in SR-aged sample. In principle, one might expect the SO sample to be oxidized, the SR sample reduced, and the SRC sample in an intermediate reduction state. However, we have shown15 that net oxidation of reduced Ce/Zr mixed oxides takes place during reaction with simulated exhaust gases, even if the feedstream had a reducing character, because of the higher rate of oxidation compared to that of reduction of the mixed oxide. During SO, SR, and SRC aging treatments, simulated exhaust gases go through the mixed oxide at 1173 K. Thus, reaction among the different components in the feedstream will take place, and even the sample submitted to SR aging will be, probably, not reduced much. This is confirmed by the values of dCO in Table 2, where the SR-aged sample presents the same CO intrinsic activity as the fresh sample. The intrinsic activity of Ce/Zr mixed oxides for C3H6 has been reported to be a function of the Ce/Zr ratio16 and independent of the oxidation state of the solid.15 Calcination in an oxidizing atmosphere in the presence of water vapor at 1173 K of a Ce/Zr 68/32 mixed oxide
Figure 3. Activity of the Ce/Zr 68/32 mixed oxides expressed as (a) T50 and (b) X773 for black bars, CO, and white bars, C3H6.
does not seem to induce phase segregation, although phase segregation takes place at higher temperatures (1473 K).21 Thus, the decrease in dC3H6 from fresh to aged samples will not be related to a change in the Ce/ Zr ratio in the solid surface compared to the bulk, but rather to a change in the redox properties of the solid induced by temperature. It has recently been reported22 that oxidation at high temperature (1223 K) of a Ce/Zr 68/32 mixed oxide hinders its reducibility, compared to oxidation at lower temperature (823 K). Less reducibility of the solid implies less intrinsic activity for C3H6 (via the first reaction path). Comparison between SO- and SRC-aged samples shows that their activity per unit surface area is nearly identical (Table 2). The improved CO intrinsic activity compared to the fresh sample, not observed in the SRaged sample, must be derived from the thermal treatment in the oxidizing feedstream, which will probably modify the adsorption capacity of the solids for the different species. The previous results refer to the initial points of the light-off curve, where differential reactor operation could be considered. For higher conversions, Figure 3 shows the values of T50, temperature at which 50% conversion is attained, and X773, conversion at 773 K, normal operation in the automotive converter, for CO and C3H6 on fresh and aged 68/32 mixed oxide samples. Although these results cannot be expressed per unit surface area, we can see that they mostly agree with the previous discussion. However, if we have a look at the results of the SRaged sample, we can see that it presents better activity than the SO- and SRC-aged samples at high conversions. Thus, it presents the typical behavior of reduced samples, although at higher temperatures and to a small extent. Conclusions The objective of this study was to determine how aging procedures carried out under different environ-
276
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
ments affected the intrinsic activity of a Ce/Zr 68/32 mixed oxide, and particularly aging under conditions close to the actual exhaust gases. We have shown that the mixed oxide is not texturally stable and presents an important specific surface area loss during aging. Aging carried out with a feedstream in which the composition of oxidizing and reducing components continuously oscillates around the stoichiometric (SRC aging), that is, simulates the conditions in the actual catalytic converter, has shown to produce the highest loss of specific surface area. SR and, particularly, SO aging proved to be milder. Aging treatments produced an important loss of activity compared to that of the fresh mixed oxide: a shift of 50-80 K in the light-off temperature for CO and a shift of about 100 K for C3H6. This loss of activity is related to the loss of specific surface area, and also to the oxidizing or reducing nature of the aging treatments. Generalizing, aging treatments increase the intrinsic activity for CO, while the intrinsic activity for C3H6 is reduced. From the practical point of view, this work shows the importance of studying the evolution of the texture and activity of supports, and catalysts, with aging treatments in which a feedstream is as similar as possible to the actual one used, to predict their durability in actual operation in the catalytic converter, as the results are strongly dependent on that variable. Acknowledgment The authors wish to thank the European UnionsTMR program of the EU (CEZIRENCAT Project, Contract FMRX-CT-96-0060)sas well as the Universidad del Paı´s Vasco/E.H.U.sUPV 069.310-G40/98sfor their financial support. Nomenclature A/F ) air-to-fuel ratio, parameter used to define the oxidizing (above 14.63) or reducing (below 14.63) character of a gaseous feedstream di ) deviation, parameter used as a measurement of the intrinsic activity for the component i, dimensionless S ) stoichiometric number, parameter used to define the oxidizing (above 1) or reducing (below 1) character of a gaseous feedstream T50 ) light-off temperature, temperature at which 50% conversion is attained, K X773 ) conversion at 773 K, % i ) CO, C3H6, or NO
Literature Cited (1) Putna, E. S.; Bunluesin, T.; Fan, X. L.; Gorte, R. J.; Vohs, J. M.; Lakis, R. E.; Egami, T. Ceria Films on Zirconia Substrates Models for Understanding Oxygen-Storage Properties. Catal. Today 1999, 50, 343. (2) Yao, M. H.; Baird, R. J.; Kunz, F. W.; Hoost, T. E. An XRD and TEM Investigation of the Structure of Alumina-Supported Ceria-Zirconia. J. Catal. 1997, 166, 67. (3) Cuif, J. P.; Blanchard, G.; Touret, O.; Marczi, M.; Que´me´re´, E. New Generation of Rare Earth Compounds for Automotive Catalysis. In Emissions and Emissions Control (SP-1207); Society of Automotive Engineers, Inc.: Warrendale, PA, 1996; p 73. (4) Balducci, G.; Fornasiero, P.; Di Monte, R.; Kasˇpar, J.; Meriani, S.; Graziani, M. An Unusual Promotion of the Redox Behaviour of CeO2-ZrO2 Solid Solutions upon Sintering at High Temperatures. Catal. Lett. 1995, 33, 193. (5) Trovarelli, A.; Zamar, F.; Llorca, J.; De Leitenburg, C.; Dolcetti, G.; Kiss, J. T. Nanophase Fluorite-Structured CeO2-ZrO2
Catalysts Prepared by High-Energy Mechanical MillingsAnalysis of Low-Temperature Redox Activity and Oxygen Storage Capacity. J. Catal. 1997, 169, 490. (6) Hori, C. E.; Permana, H.; Simon Ng, K. Y.; Brenner, A.; More, K.; Rahmoeller, K. M.; Belton, D. Thermal Stability of Oxygen Storage Properties in a Mixed CeO2-ZrO2 System. Appl. Catal. B 1998, 16, 105. (7) Bunluesin, T.; Gorte, R. J.; Graham, G. W. CO Oxidation for the Characterization of Reducibility in Oxygen Storage Components of Three-Way Automotive Catalysts. Appl. Catal. B 1997, 14, 105. (8) De Leitenburg, C.; Trovarelli, A.; Llorca, J.; Cavani, F.; Bini, G. The Effect of Doping CeO2 with Zirconium in the Oxidation of Isobutane. Appl. Catal. A 1996, 139, 161. (9) Ranga Rao, G.; Fornasiero, P.; Di Monte, R.; Kasˇpar, J.; Vlaic, G.; Balducci, G.; Meriani, S.; Gubitosa, G.; Cremona, A.; Graziani, M. Reduction of NO over Partially Reduced MetalLoaded CeO2-ZrO2 Solid Solutions. J. Catal. 1996, 162, 1. (10) Fornasiero, P.; Ranga Rao, G.; Kasˇpar, J.; L’Erario, F.; Graziani, M. Reduction of NO by CO over Rh/CeO2-ZrO2 CatalystssEvidence for a Support-Promoted Catalytic Activity. J. Catal. 1998, 175, 269. (11) Sass, A. S.; Shvets, V. A.; Savel’eva, G. A.; Popova, N. M.; Kazanskii, V. B. Mechanism of Low-Temperature Oxidation of Carbon Monoxide on Supported Mixed Catalysts Containing Noble Metals and Cerium Oxide. Kinet. Katal. 1986, 27, 894. (12) Nunan, J. G.; Robota, H. J.; Cohn, M. J.; Bradley, S. A. Physicochemical Properties of Ce-Containing Three-Way Catalysts and the Effect of Ce on Catalyst Activity. J. Catal. 1992, 133, 309. (13) Trovarelli, A. Catalytic Properties of Ceria and CeO2Containing Materials. Catal. Rev.-Sci. Eng. 1996, 38, 439. (14) Diwell, A. F.; Rajaram, R. R.; Shaw, H. A.; Truex, T. J. The Role of Ceria in Three-Way Catalysts. Catal. Automotive Pollution Control II 1991, 71, 139. (15) Gonza´lez-Velasco, J. R.; Gutie´rrez-Ortiz, M. A.; Marc, J.L.; Botas, J. A.; Gonza´lez-Marcos, M. P.; Blanchard, G. Effects of Redox Thermal Treatments and Feedstream Composition on the Activity of Ce/Zr Mixed Oxides for TWC Applications. Appl. Catal. B, in press. (16) Gonza´lez-Velasco, J. R.; Gutie´rrez-Ortiz, M. A.; Marc, J.L.; Botas, J. A.; Gonza´lez-Marcos, M. P.; Blanchard, G. Contribution of Cerium/Zirconium Mixed Oxides to the Activity of a New Generation of TWC. Appl. Catal. B 1999, 22, 167. (17) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kasˇpar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. Modification of the Redox Behaviour of CeO2 Induced by Structural Doping with ZrO2. J. Catal. 1996, 164, 173. (18) Roberts, B. F. A Procedure for Estimating Pore Volume and Area Distributions from Sorption Isotherms. J. Colloid Interface Sci. 1967, 23, 266. (19) Gonza´lez-Velasco, J. R.; Gutie´rrez-Ortiz, M. A.; Botas, J. A. Competitive Reactions for the Selective Reduction of Nitrogen Monoxide on Platinum Catalysts. In Proceedings of the International Forum on Environmental Catalysis, IFEC’95; Segawa, K., Ed.; Tokyo, Japan, 1995; p 47. (20) Perrichon, V.; Laachir, A.; Abouarnadasse, S.; Touret, O.; Blanchard, G. Thermal Stability of a High Surface Area Ceria under Reducing Atmosphere. Appl. Catal. A 1995, 129, 69. (21) Colo´n, G.; Pijolat, M.; Valdivieso, F.; Vidal, H.; Kasˇpar, J.; Finocchio, E.; Daturi, M.; Binet, C.; Lavalley, J. C.; Baker, R. T.; Bernal, S. Surface and Structural Characterization of CexZr1-xO2 CEZIRENCAT Mixed Oxides as Potential Three-Way Catalyst Promoters. J. Chem. Soc., Faraday Trans. 1998, 94, 3717. (22) Baker, R. T.; Bernal, S.; Blanco, G.; Cordo´n, A. M.; Pintado, J. M.; Rodrı´guez-Izquierdo, J. M.; Fally, F.; Perrichon, V. Reversible Changes in the Redox Behaviour of a Ce0.68Zr0.32O2 Mixed Oxide: Effect of Alternating the Re-oxidation Temperature after Reduction at 1223 K. Chem. Commun. 1999, 149.
Received for review June 16, 1999 Revised manuscript received November 1, 1999 Accepted November 18, 1999 IE990433E
