Application of Rare Earth Modified Zr-based Ceria-Zirconia Solid

Environ. Sci. Technol. 2010, 44, 3870–3875

Application of Rare Earth Modified Zr-based Ceria-Zirconia Solid Solution in Three-Way Catalyst for Automotive Emission Control Q I U Y A N W A N G , † B O Z H A O , †,‡ G U A N G F E N G L I , † A N D R E N X I A N Z H O U * ,† Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China, and School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou, 317000, PR China

Received September 23, 2009. Revised manuscript received February 26, 2010. Accepted April 13, 2010.

Automotive exhaust emission is a major cause of air pollution. Three-way catalyst (TWC) which can eliminate CO, HC (hydrocarbons), and NOx simultaneously has been used to control exhaust emissions. Ceria-zirconia is a key component in TWC and most researchers pay attention to Ceria-Zirconia (Cerich) solid solution. The research presented in this paper is focused on the intrinsic structure of Ceria-Zirconia (Zr-based) solid solution and its application in TWC. A series of Ce0.2Zr0.8O2 modified with rare earths (La, Nd, Pr, Sm, and Y) have been prepared by coprecipitation method combined with supercritical drying technique. All samples showed single tetragonal solid solution, indicating that the rare earth ion inserted into the lattice structure completely, and an approximately linearly relationship between lattice parameter a and the ionic radius of doped rare earth was observed. The catalytic performances of corresponding Pdonly catalysts were investigated in simulated exhaust gas. The presence of La, Nd, and Pr was favorable to the catalytic activity and wide air/fuel operation window. The relationship between the intrinsic structure of the Zr-based ceria-zirconia solid solution and catalytic activity was discussed in detail, which has some reference value for catalyst design and application.

1. Introduction At this time, the catalytic converter is the most popular and only efficient solution for automotive pollution control (1). Three-way catalyst (TWC) has proven to be a suitable alternative to simultaneously reduce emissions of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) from gasoline engine powered vehicles (2, 3). As it is well-known, TWC works under oscillating feed gas composition around the stoichiometric value of air/fuel (A/ F) ratio at which the catalytic activity is the most efficiency (4, 5). In practice, the exposure of catalysts to high temperature will induce agglomeration and sintering of noble metal, eventually leading to the loss of catalytic activity and selectivity. So it is highly desirable to solve the two problems mentioned below in the development of TWC: (i) how to improve the oxygen storage capacity (OSC) since it enables * Corresponding author phone: 86-571-8827-3290; fax: 86-5718827-3283; e-mail: [email protected]. † Zhejiang University. ‡ Taizhou University. 3870

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TWC to enlarge the operating A/F window lowering the sensitivity to small temporary variations; (ii) how to enhance the thermal stability against high-temperature sintering. Classical components of TWC usually include Rh, Pt, and/ or Pd as the active metals, ceria-containing oxide as the promoter, and high surface alumina as the support. The use of Pd as the single active metal component in TWC has recently received considerable attention as a result of the high cost and scarcity of Rh, the availability of cleaner fuels, and its remarkable activity for oxidation reactions (6). The specific promotion by ceria has been extended to include other oxide components in order to increase or maintain the durability of the TWC. Among the latter, CeO2-ZrO2 solid solution, showing good thermal stability and high OSC, has achieved widespread utilization in TWC (7). Several research groups have investigated the properties of Ceria-Zirconia, which are related to their OSC (8-14). It has been found that the composition (Ce/Zr ratio) can significantly change the reduction behavior, which is a consequence of the change in the concentration of active redox element Ce4+ and of different structural features in the solid solution (15). The results seem to indicate that cubic CexZr1-xO2 (0.5 < × < 0.8) has superior performance, and a lot of attention has been focused on the Ce-based CeriaZirconia solid solution. To our knowledge, only a few researchers paid attention to investigations of properties and catalytic performance of Zr-based Ceria-Zirconia solid solution with a noble metal loaded on it. According to our previous study, a series of CexZr1-xO2 (0 e × e 0.5) were prepared, and the samples exhibited obviously higher surface area compared with Cebased samples after calcination at 1100 °C for 4 h. In fact, the rate of oxygen release and storage depends on the surface area of the oxygen storage material, suggesting the practicality of Zr-based solid solution in TWC. A final Ce/Zr ratio was attained considering the optimum catalytic activity over Pd/ Ce0.2Zr0.8O2. However, the low content of ceria results in unsatisfied OSC, which restricts the development of Zr-based solid solution. When Zr4+ is inserted into the cubic CeO2, distortion of the O2- sublattice in the mixed oxide permits a higher mobility of the lattice oxygen and reduction is no longer confined to the surface but extends deep into the bulk, resulting in the advanced OSC. The composition of the binary oxide impacts on the OSC (16). Therefore, the addition of other metal cations such as rare earths to CeO2-ZrO2 lattice structure may improve the OSC in a similar way, and one of the most important reasons for the wide use of rare earths in catalytic field is its significant thermal stability which is also important for CeO2-ZrO2 solid solution. In this study, a series of Ce0.2Zr0.8O2 (CZ) modified with rare earths (La, Nd, Pr, Sm, and Y) were prepared and characterized from a structural and textural point of view. The effects of doping on OSC and thermal stability of the solid solution and on supported Pd-only catalysts were studied. The catalytic performance over theoretical and a wide A/F operation window were also investigated.

2. Experimental Section All the rare earth doped supports and the parallel CZ were prepared by coprecipitation method combined with supercritical drying technology. The rare earth doped supports calcined at 500 °C are expressed as CZR collectively, and R is substituted by the first letter of La, Nd, Pr, Sm, and Y to denote the given rare earth modified support, respectively, 10.1021/es903957e

 2010 American Chemical Society

Published on Web 04/21/2010

TABLE 1. Surface Area, Pore Volume and Mean Pore Diameter of the Catalysts Calcined at 500 and 1100 °C sample

SBET (m2/g)

cumulative pore volume (cm3/g)

Vmic (cm3/g)

Dmic (nm)

Dmes (nm)

Pd/CZ Pd/CZL Pd/CZN Pd/CZP Pd/CZS Pd/CZY Pd/CZa Pd/CZLa Pd/CZNa Pd/CZPa Pd/CZSa Pd/CZYa

97.1 155.3 139.1 165.0 137.8 132.1 28.1 40.2 31.8 32.9 26.6 25.0

0.285 0.416 0.363 0.348 0.323 0.327 0.037 0.119 0.065 0.051 0.036 0.034

0.029 0.047 0.038 0.046 0.039 0.036 0.003 0.005 0.004 0.003 0.002 0.002

1.2 1.2 1.3 1.3 1.3 1.3 1.7 1.6 1.7 1.7 1.7 1.7

11.2 10.2 9.7 8.6 9.3 9.5 18.7 18.1 16.7 17.8 18.7 18.2

TABLE 2. Lattice Constants and Crystallite Size of Fresh and Aged Supports sample CZ CZL CZN CZP CZS CZY

lattice parameters (Å) a a a a a a

) ) ) ) ) )

3.625/3.632a, 3.661/3.644a, 3.655/3.648a, 3.658/3.644a, 3.651/3.647a, 3.648/3.654a,

c c c c c c

) ) ) ) ) )

crystallite size (nm)

5.218/5.227a 5.228/5.232a 5.217/5.224a 5.224/5.225a 5.215/5.220a 5.215/5.200a

a Lattice parameters and crystallite size responding sample calcined at 1100 °C for 4 h.

6.7/32.2a 5.8/20.4a 6.5/19.5a 5.1/17.2a 6.0/29.1a 6.5/16.9a for

cor-

whereas the ones calcined at 1100 °C are referred to as CZRa. The corresponding catalysts are labeled as Pd/CZ (Pd/CZa) and Pd/CZR (Pd/CZRa). Catalytic Activity Test. The evaluation of three-way catalytic activity was performed in a fixed-bed quartz reactor. The feed stream was regulated using special mass flow controllers and contained NO (0.1%)-NO2 (0.03%)-C3H6 (0.067%)-C3H8 (0.033%)-CO (0.75%)-O2 (0.745%) with balance Ar and the space velocity was 43 000 h-1 referred to the catalyst volume and to a gas flow rate at room temperature (25 °C). The contents of CO, NO, NO2, and total HC (C3H6 and C3H8) were recorded by a Bruker EQUINOX 55 FTIR spectrometer coupled with a multiple reflection transmission cell (Infrared Analysis Inc.) before and after the simulated gas passed in the reactor. The air/fuel ratio (λ) is defined as λ ) (2VO2 + VNO + 2VNO2)/(VCO + 9VHC) (V means concentration in volume percent unit), λ ) 1 was utilized in all the activity measurements and the air/fuel ratio experiment was carried out at 400 °C with adjusting the concentration of O2.

3. Results and Discussion Textural Study. A detailed investigation of the textual changes for Pd/CZ and Pd/CZR before/after calcination at 1100 °C was conducted. The results are presented in Table 1. In the view of literature reports, the composition of CeO2-ZrO2 solid solution can influence the surface area of sample significantly (17). Compared to the reference Pd/CZ, it is clear that the BET surface area increased for Pd/CZR. From the results of XRD (Table 2), it can be seen that the lattice parameter a for CZR is larger than that of CZ due to the introduction of bigger rare earth cation which may substitute part of Zr4+ cations. The rare earth exhibits significant thermal stability, therefore the thermal stability for CZR could be improved by the presence of rare earth resulting in the thermostable Pd/CZR which possesses higher surface area than Pd/CZ.

The decrease in the surface area and total pore volume as well as the increased average pore diameter with increasing of calcination temperature is a common phenomenon due to sintering of the samples at high temperature (18). These changes before and after calcination can be a measure of thermal stability. It can be seen by the data reported in Table 1, the surface area and pore volume of all the fresh samples undergo a sharp decrease after calcination at 1100 °C for 4 h due to the sintering via pore closure. However, the addition of La, Nd, and Pr results in an enhanced heat resistance of the sample, which maintains higher surface area after aging, while the samples containing Sm and Y exhibit lower surface area than Pd/CZa after equivalent thermal treatment. Analysis of the t-plots reveals that the catalyst has a higher incidence of mesopore, as evidenced by the lower micropore volume, especially in the case of aged samples. Structural Investigation. From the X-ray patterns (Supporting Information (SI) Figure S1), it can be seen that the fresh samples show rather broad peaks attributed to the presence of small crystallites which are formed after calcination at 500 °C. After thermal treatment at 1100 °C for 4 h, the width of the XRD peaks strongly decrease, indicating extensive sintering. The average particle diameter of the aged sample increased obviously. In the case of CZ, the increase in particle diameter is much stronger. The lattice parameters and crystallite size of all samples are listed in Table 2. Researchers have revealed that the composition of the binary CeO2-ZrO2 oxide impacts on the phase structure evidently. At low Zr content, the binary oxide is of cubic, phase segregation occurs in the intermediate composition range, and at even higher Zr content the stable phases are tetragonal and monoclinic (8, 19-21). The phase for all the samples is single tetragonal solid solution (space group P42/ nmcs, Z ) 2, ICSD No. 68590) attributing to Zr-based in the component, irrespective of the treatment temperature applied. At the same time no diffraction peaks of rare earth oxide were detected. There may be two possible reasons: (i) no phase formation for rare earth oxide occurs in the samples, indicating the completely formation of a homogeneous tetragonal structure; (ii) some rare earth oxide crystallites have been formed but are too small ( Pd/CZNa > Pd/CZPa > Pd/CZa > Pd/CZSa > Pd/ CZYa for HC, CO, NO, and NO2. From Table 1, it can be seen that the Pd/CZLa, Pd/CZNa, and Pd/CZPa exhibit high pore volume and surface area among all the aged catalysts. For all the aged catalysts, there is a higher incidence of mesopore, as evidenced by the extremely low micropore volume and t-plot analysis due to the sintering of the micropore at high temperature, that is to say the high pore volume and surface area for the aged catalyst is based on the mespore. The porosity (high pore volume) for Pd/CZL, Pd/CZN, and Pd/ CZP is beneficial to adsorption and desorption, and the high surface area means more active sites, resulting in an enhanced catalytic activity.

The difference in temperature of T50 (the temperature required to attain 50% conversion) and T90 (the temperature required to attain 90% conversion) for catalysts before and after aging is shown in SI Table S3. The ∆T50 and ∆T90 for La, Nd and Pr doped catalyst is obviously lower than Pd/CZ, indicating the increased thermal stability. On the contrary, the presence of Sm and Y result in higher ∆T50 and ∆T90 than Pd/CZ. The thermal stability increases in the order Pd/CZY < Pd/CZS < Pd/CZ < Pd/CZP < Pd/CZN < Pd/CZL, which is consistent with the activity order for the aged catalysts. In addition, the temperature of PdO reduction peak could be correlated with the catalytic activity. From the results of Figure 1(b), it can be seen that the trend of peak a temperature decreasing is similar to that of their activities increasing, which suggests that the catalytic activity may be related to the reducibility of PdO species finely dispersed on support, and the higher the reducibility of the PdO species, the higher the catalytic activities. The above observation has been also documented in literature. Kasˇpar et al. (35) reported that the active phase of Pd TWC is PdO. The transformation from PdO to Pd determines the catalytic performance of the catalysts. Lower reduction temperature of PdO can give higher catalyst activity. Figure 3 gives the results of Air/Fuel (λ) test for the aged catalysts. After being aged at 1100 °C for 4 h, the operation windows of all the catalysts were destroyed significantly. For example, the conversion of HC approaches 100% at λ > 0.8 over Pd/CZLa and Pd/CZNa, and at λ ) 1.0 over Pd/CZPa, while HC can not be completely converted even at λ > 1.0 VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available The preparation and characterization of supports and/or catalysts, the H2 consumption for fresh and aged catalysts, the analysis of the structure, and the evaluation results for fresh catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 3. Conversion curves of HC, CO, and NOx as a function of air/fuel ratio (λ) over Pd/CZa and Pd/CZRa catalysts. over Pd/CZa, Pd/CZSa, and Pd/CZYa. Fortunately, the presence of La, Nd, and Pr shows wider operation window than undoped catalyst to the utmost extent. From the results of Table 3, it can be seen that the doped supports show higher OSCC than CZa, especially for CZLa, CZNa, and CZPa, which is also one of the reasons for the enhanced tree-way catalytic activity and wider operation window.

Acknowledgments We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (No.2006AA060306, 2009AA064840). 3874

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(1) Dong, F.; Tanabe, T.; Suda, A.; Takahashi, N.; Sovukawa, H.; Shinjoh, H. Investigation of the OSC performance of Pt/CeO2ZrO2-Y2O3 catalysts by CO oxidation and 18O/16O isotopic exchange reaction. Chem. Eng. Sci. 2008, 63, 5020–5027. (2) Martı´nez-Arı´as, A.; Ferna´ndez-Garcı´a, M.; Hungrı´a, A. B.; IglesiasJuez, A.; Duncan, K.; Smith, R.; Anderson, J. A.; Conesa, J. C.; Soria, J. Effect of thermal sintering on light-off performance of Pd/(Ce, Zr)Ox/Al2O3 three-way catalysts: model gas and engine tests. J. Catal. 2001, 204, 238–248. (3) Morikawa, A.; Suzuki, T.; Kanazawa, T.; Kikuta, K.; Suda, A.; Shinjo, H. A new concept in high performance ceria-zirconia oxygen storage capacity material with Al2O3 as diffusion barrier. Appl. Catal., B 2008, 78, 210–221. (4) Matsumoto, S. Recent advances in automobile exhaust catalysts. Catal. Today. 2004, 90, 183–190. (5) Lambrou, P. S.; Efstathiou, A. M. The effects of Fe on the oxygen storage and release properties of model Pd-Rh/CeO2-Al2O3 threeway catalyst. J. Catal. 2006, 240, 182–193. (6) Ferna´ndez-Garcı´a, M.; Iglesias-Juez, A.; Martı´nez-Arias, A.; Hungrı´a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Role of the state of the metal component on the light-off performance of Pd-based three-way catalysts. J. Catal. 2004, 221, 594–600. (7) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Iglesias-Juez, A.; Hungrı´a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Behaviro of bimetallic Pd-Cr/Al2O3 and Pd-Cr/(Ce, Zr)Ox/Al2O3 catalysts for CO and NO elimination. J. Catal. 2003, 214, 220–233. (8) Fornasiero, P.; Di Monte, R.; Ranga Rao, G.; Kasˇpar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. Rh-loaded CeO2-ZrO2 solidsolution as highly efficicent oxygen exchanger: dependence of the reduction behavior and the oxygen storage capacity on the structural-properties. J. Catal. 1995, 151, 168–177. (9) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kasˇpar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. Modification of the redox behavior of CeO2 induced by structural doping with ZrO2. J. Catal. 1996, 164, 173–183. (10) Hori, C. E.; Permana, H.; Simon Ng, K. Y.; Brenner, A. B.; More, K.; Rahmoeller, K. M.; Belton, D. Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system. Appl. Catal., B 1998, 16, 105–117. (11) Jen, H. W.; Graham, G. W.; Chun, W.; McCabe, R. W.; Cuif, J. P.; Deutsch, S. E.; Touret, O. Characterization of model automotive exhaust catalysts: Pd on ceria and ceria-zirconia supports. Catal. Today. 1999, 50, 309–328. (12) Zhao, M.; Shen, M.; Wang, J. Effect of surface area and bulk structure on oxygen storage capacity of Ce0.67Zr0.33O2. J. Catal. 2007, 248, 258–267. (13) He, H.; Dai, H.; Ng, L. H.; Wong, K. W.; Au, C. T. Pd-, Pt-, and Rh-loaded Ce0.65Zr0.35O2 three-way catalysts: An investigation on performance and redox properties. J. Catal. 2002, 206, 1–13. (14) Christou, S. Y.; Bradshaw, H.; Butler, C.; Darab, J.; Efstathiou, A. M. Effect of thermal aging on the transient kinetics of oxygen storage and release of commercial of CexZr1-xO2-based solids. Top. Catal. 2009, 52, 2013–2018. (15) Boaro, M.; Leitenburg, C.; Dolcetti, G.; Trovarelli, A. The dynamic of oxygen stroage in ceria-zirconia model catalysts measured by CO oxidation under stationary and cycling feedstream composition. J. Catal. 2000, 193, 338–347. (16) Kozlov, A. I.; Kim, D. H.; Yezerets, A.; Andersen, P.; Kung, H. H.; Kung, M. C. Effect of preparation method and redox treatment on the reducibility and structure of supported ceria-zirconia mixed oxide. J. Catal. 2002, 209, 417–426. (17) Guo, Y.; Lu, G.; Zhang, Z.; Zhang, S.; Qi, Y.; Liu, Y. Preparation of CexZr1-xO2 (x ) 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts. Catal. Today 2007, 126, 296– 302. (18) Guo, J.; Wu, D.; Zhang, L.; Gong, M.; Zhao, M.; Chen, Y. Preparation of nanometric CeO2-ZrO2-Nd2O3 solid solution and its catalytic performances. J. Alloy. Compd. 2008, 460, 485–490. (19) Fornasiero, P.; Kasˇpar, J.; Sergo, V.; Graziani, M. Redox behavior of high-surface-area Rh-, Pt-, and Pd-loaded Ce0.5Zr0.5O2 mixed oxide. J. Catal. 1999, 182, 56–69.

(20) Kozlov, A. I.; Kim, D. H.; Yezerets, A.; Andersen, P.; Kung, H. H.; Kung, M. C. Effect of preparation method and redox treatment on the reducibility and structure of supported ceria-zirconia mixed oxide. J. Catal. 2002, 209, 417–426. (21) Larese, C.; Lo´pez Grandos, M.; Mariscal, R.; Fierro, J. L. G.; Lambrou, P. S.; Efstathiou, A. M. The effect of calcination temperature on the oxygen storage and release properties of CeO2 and Ce-Zr-O metal oxides modified by phosphorus incorporation. Appl. Catal., B 2005, 59, 13–25. (22) Hickey, N.; Fornasiero, P.; Kasˇpar, J.; Gatica, J. M.; Bernal, S. Effects of the nature of the reducing agent on the transient redox behavior of NM/Ce0.68Zr0.32O2 (NM ) Pt, Pd, and Rh). J. Catal. 2001, 200, 181–193. (23) Daley, R. A.; Christou, S. Y.; Efstathiou, A. M.; Anderson, J. A. Infulence of oxychlorination treatments on the redox and oxygen storage and release properties of thermally aged Pd-Rh/CexZr1-xO2/Al2O3 model three-way catalysts. Appl. Catal., B 2005, 60, 117–127. (24) Yao, H. C.; Yu Yao, Y. F. Ceria in automotive exhuast: I. Oxygen storage. J. Catal. 1984, 86, 254–265. (25) Fornasiero, P.; Di Monte, R.; Ranga Rao, G.; Kasˇpar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. Rh-loaded CeO2-ZrO2 solid solutions as highly efficicent oxygen exchangers: dependence of the reduction behavior and the oxygen storage capacity on the sturctural properties. J. Catal. 1995, 151, 168–177. (26) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kasˇpar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. Modification of the redox behavior of CeO2 induced by structural doping with ZrO2. J. Catal. 1996, 164, 173–183. (27) Zotin, F. M. Z.; Tournayan, L.; Varloud, J.; Perrichon, V.; Frety, R. Temperature-programmed reduction-limitation of the technique for determining the extent of reduction of either pure

(28)

(29)

(30)

(31) (32)

(33) (34)

(35)

ceria or ceria modified by additives. Appl. Catal., A 1993, 98, 99–114. Christou, S. Y.; Costa, C. N.; Efstathiou, A. M. A two-step reaction of mechanism of oxygen release from Pd/CeO2: mathematical modelling based on step gas concentration experiments. Top. Catal. 2004, 30/31, 325–331. Costa, C. N.; Christou, S. Y.; Georgiou, G.; Efstathiou, A. M. Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/ CeO2 catalyst. J. Catal. 2003, 219, 259–272. Fan, J.; Wu, X.; Wu, X.; Liang, Q.; Ran, R.; Weng, D. Thermal ageing of Pt on low-surface-area CeO2-ZrO2-La2O3 mixed oxides: effect on the OSC performance. Appl. Catal., B 2008, 81, 38–48. Chen, G.; Chou, W. T.; Yeh, C. T. The sorption of hydrogen on palladium in a folw system. Appl. Catal. 1983, 8, 389–397. Lin, W.; Zhu, Y.; Wu, N.; Xie, Y.; Murwani, I.; Kemnitz, E. Total oxidatin of methane at low temperature over Pd/TiO2/Al2O3: effects of the support and residual chlorine inos. Appl. Catal., B 2004, 50, 59–66. Su, S. C.; Carstens, J. N.; Bell, A. T. A study of the dynamics of Pd oxidation and PdO reduction by H2 and CH4. J. Catal. 1998, 176, 125–135. Lambrou, P. S.; Costa, C. N.; Christou, S. Y.; Efstathiou, A. M. Dynamics of oxygen storage and release on commercial aged Pd-Rh three-way catalysts and their characterization by transient experiments. Appl. Catal., B 2004, 54, 237–250. Kasˇpar, J.; Fornasiero, P.; Hickey, N. Automotive catalytic coverters: current status and some perspectives. Catal. Today 2003, 77, 419–449.

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