Rh exhaust gas catalysts studied by XRD

Applied Catalysis A: General 277 (2004) 107–117 www.elsevier.com/locate/apcata

Effect of the ageing atmosphere on catalytic activity and textural properties of Pd/Rh exhaust gas catalysts studied by XRD M. Hietikkoa, U. Lassib,c, K. Kallinend, A. Savima¨kid, M. Ha¨rko¨nend, J. Pursiainena, R.S. Laitinena,*, R.L. Keiskic a

Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 University of Oulu, Finland Central Ostrobothnia Polytechnic, Department of Technology, Talonpojankatu 2, FIN-67100 Kokkola, Finland c Department of Process and Environmental Engineering, University of Oulu, P.O. Box 4300, FIN-90014 University of Oulu, Finland d Ecocat Oy, Catalyst Research, P.O. Box 171, FIN-90101 Oulu, Finland b

Received 27 April 2004; received in revised form 1 September 2004; accepted 2 September 2004

Abstract Effect of thermal, engine bench, and vehicle ageings on catalytic activity and support properties was evaluated mainly by XRD on a real catalytic system. The solid–solid phase transitions in the bulk material are of particular importance to catalyst behaviour after ageings. It was observed that the ageing atmosphere either accelerated or inhibited the phase transitions. The formation of aluminates was observed after ageings in inert and reducing atmospheres as well as after engine and vehicle ageings. The formation of aluminates is associated with the loss of specific surface areas that remained higher after reducing and inert ageings. The formation of cerium and lanthanum aluminates prevents the formation of low surface area a-Al2O3 that is responsible for the decrease in the total surface area. Catalytic activities also remained higher after reducing and inert ageings than after oxidizing ageing. Furthermore, laboratory ageing in reducing atmosphere seems to correlate best with the real vehicle ageing from the textural point of view. # 2004 Elsevier B.V. All rights reserved. Keywords: Activity; Deactivation; Ageing; X-ray diffraction; Catalyst

1. Introduction The requirement of high thermal stability and activity, which the current three-way catalysts have to fulfil, is one of the most crucial demands for a successful commercial application. Therefore, the understanding of deactivation phenomena and deactivation correlations is an important issue in the design and preparation of a catalytic system. Temperature has become an increasingly important factor for the deactivation of a three-way catalyst due to the fact that pre-converter is installed near to the engine to confirm the efficient purification of hydrocarbons, and therefore, catalytic materials are exposed to high operation tempera* Corresponding author. Tel.: +358 855 316 11; fax: +358 855 316 08. E-mail address: [email protected] (R.S. Laitinen). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.09.029

tures. Catalyst testing and ageing in an engine bench or onroad is slow and expensive, and thus, there is a need for a rapid laboratory scale ageing procedures [1,2]. Several additives promote and stabilize commercial catalysts and facilitate the manufacturing of the catalyst. Catalysts are not in a stable form during their use and can undergo several undesired structural and chemical transformations. These changes mainly comprise the crystallization and solid–solid phase transitions of non-amorphous phases and are normally detectable by XRD diffraction. The phase transitions in the bulk material that is used in exhaust gas catalysts have increasingly attracted attention during recent years [3–5]. In particular, the chemistry of aluminium oxide and its many phase transformations have extensively been studied, since Al2O3 provides a high surface area carrier for the supported precious metal catalysts.

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Recently, the behaviour of CeXZr1 XO2 mixed oxides at high temperatures has thoroughly been studied by Bozo et al. [6], Colo´ n et al. [7], and Fornasiero et al. [8]. The addition of zirconium to the catalyst has been found to prevent the grain growth of CeO2 crystallites at high temperatures and to improve activity and thermal stability of g-alumina support and supported precious metal phases. CeXZr1 XO2 mixed oxides are also effective oxygen storage components in three-way catalysts. They are known to inhibit the formation of the low surface area a-Al2O3 [8– 10]. The loss in surface area of catalysts is associated with the changes in the pore structure and with the crystallite growth that can be considered as a pre-sintering phenomenon, followed by sintering between the crystallites. Some earlier observations by Vidal et al. [4] have shown that high-temperature reducing and oxidizing ageings strongly favour sintering of this kind of oxide materials with high surface areas. Pore sizes were clearly larger in the oxidizing than in the reducing atmosphere with the same ageing temperature and time. Thus, the sintering of the support material was most extensive in air-aged catalysts, which was also observed by Perrichon et al. [11] and Gonza´ lezVelasco et al. [12]. During the sintering process, the particle growth can occur by the migration of single atoms or small particles across the surface of an oxide [11,13]. It has also been reported that pure CeO2 easily sinters under the reducing conditions [4,11], but the stabilizing of ceria on Al2O3 or La2O3 is known to prevent sintering under these conditions [14]. A number of studies have indicated a growth in the crystallite size of CeO2 upon high-temperature calcinations [15], which has lead to a conclusion that deactivation phenomena on these catalysts probably depend on the surface area loss in CeO2 at elevated temperatures [11,16,17], on poor contact between the precious metals and ceria in aged catalysts, or on the decreased reducibility of large ceria crystallites [15]. However, the increased loading of ceria does not solve the problem and other factors must be relevant. Recently, Jen et al. [18] and Ferna´ ndez-Garcı´a et al. [19] have reported that pure or nearly pure CeO2 supports showed poor surface area stability compared to CeXZr1 XO2 mixed oxides, especially under reducing conditions, and the increased Zr content of the mixed oxide leads to higher surface areas [20]. Further, the activity of CeXZr1 XO2 mixed oxides as TWCs in a given Ce/Zr molar ratio is associated with the specific surface areas of the catalysts, as shown by Gonza´ lezVelasco et al. [21–23]. The aim of this study is to explore the solid–solid phase transitions on real catalysts after thermal, engine bench, and vehicle ageings and to evaluate their effect on the catalytic activity and support properties. Especially, the effects of ageing atmosphere (reducing, inert, and oxidizing), temperature, and time are considered. Furthermore, laboratory ageing-induced changes in the catalysts are compared with

the changes in the catalysts after the engine bench and vehicle ageings.

2. Experimental The evaluation of the catalyst performance, ageinginduced solid–solid phase transitions in the bulk washcoat, and the loss of specific surface area are based on activity, XRD, and BET measurements, respectively. They were carried out to study catalyst deactivation and thermal ageing phenomena in reducing, inert, and oxidizing gas atmospheres. 2.1. Catalysts The catalysts used in this study were metallic monoliths coated with a layered support that were impregnated with chlorine-free solutions containing precious metals (Pd/Rh). The support material content of the catalyst was 25 wt.% (62.8 g/m2), and the amount of the active metal in the washcoat was 1.25 wt.%. The catalysts consisted of a thin Fe–Cr–Al foil coated with g-alumina containing washcoat including also CeXZr1 XO2 mixed oxides (both Ce- and Zrrich), as well as pure Ce- and La-oxides as stabilizers and additives. The catalysts were prepared by mixing the washcoat materials with the precursor salts as the water slurry and by coating to a layered structure. Finally, the catalysts were dried at 100–150 8C and then calcined in air at 300–550 8C. The dried and calcined catalysts are referred to as ‘fresh’ in the following text.1 2.2. Ageing procedures Catalysts were aged to simulate the high-temperature conditions in the exhaust gas streams. The aged catalysts were prepared from the fresh catalyst by using different thermal ageing procedures, as described in Table 1. Thermal ageings were accomplished in a tubular furnace and a thermocouple was used to measure the temperature of the furnace. The samples included fresh catalysts and catalysts thermally aged in a static air atmosphere (oxidizing), in a flowing nitrogen atmosphere (inert), and in a 5% H2/N2 flow (reducing) in the temperature range of 800–1200 8C, an engine-aged catalyst, and a vehicleaged catalyst. The ageing time (3, 24, or 42 h) was also one variable. Engine ageing was carried out in the exhaust gas stream of a 5.0 l V8 engine during 40 h of operation. The ageing procedure was composed of both rich (50 min/ 1030 8C, l = 0.98–0.99) and stoichiometric (10 min/1050– 1060 8C, l = 1.00) air-to-fuel ratios carried out consecutively to the engine. The vehicle ageing was accomplished 1

For more information about the composition and preparation of the catalysts, contact Ecocat Oy, Catalyst Research, P.O. Box 171, FIN-90101 Oulu, Finland.

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Table 1 Ageing procedures Gas feed

Temperature (8C)

Duration (h)

Oxidizing (air) Reducing (5% H2/N2) Inert (nitrogen) Exhaust gas (engine ageing) Exhaust gas (vehicle ageing)

800–1200 800–1200 800–1200 1030–1060

3, 24, and 42 3, 24, and 42 3 and 24 40 100000 km

under the European driving conditions (100,000 km). Engine-aged and vehicle-aged catalysts were used as reference catalysts. All the catalysts were identical in chemical composition, and thus only the changes induced by ageings were considered. 2.3. Catalyst characterization techniques Several characterization techniques were utilized in this study. The main method was X-ray powder diffraction (XRD). Specific surface area (BET), pore volume, pore size distribution, and catalytic activity measurements were also carried out. XRD was used to determine the ageing-induced phase transitions on real catalysts and to evaluate their effect on catalytic activity and surface areas after ageings. The XRD diffractograms were recorded directly from the catalyst foils on a Siemens D5000 diffractometer employing ˚ , 40 kV, nickel-filtered Cu Ka radiation (l = 1.5406 A 30 mA) at 0.0208 intervals in the range 208  2u  758 with 1 s count accumulation per step. The assignment of the diffraction diagrams was carried out using the PDF2— Diffraction Database File compiled by the International Centre for Diffraction Data [24]. Total surface areas (m2/g) were measured according to the standard BET (Brunauer–Emmett–Teller) analysis by using a Coulter Omnisorp 360CX for the fresh, engine, vehicle, and thermally aged catalysts. The specific surface areas, pore volumes, and pore size distributions were obtained from the nitrogen adsorption–desorption isotherms at 196 8C by assuming the cylindrical shape of pores. Catalysts were outgassed in a vacuum at 140 8C overnight before the measurements. All BET values were measured within a precision of 5%. Catalytic activities were determined by use of laboratory scale light-off measurements. Catalyst light-off is defined as the temperature of 50% conversion, which is used to indicate the efficiency of an automotive exhaust gas catalyst. Activity measurements were carried out at the atmospheric pressure by using a cylindrical catalyst with a volume of 1.4 cm3 (length 28 mm and diameter 8 mm). Catalytic activities were determined by using a simple model reaction: the reduction of NO by CO in lean (1000 ppm NO, 800 ppm CO, and N2 balance) and rich (1000 ppm NO, 1200 ppm CO, and N2 balance) conditions. Before the measurements, the catalysts were reduced in a hydrogen flow at 500 8C for 10 min, followed by 15 min at 550 8C. The total gas flow during the experiments was 1 dm3/min, gas hourly space

Fig. 1. Effect of ageing temperature on BET surface areas of catalysts. Comparison between reducing (3 h (*) or 24 h (*)), oxidizing (3 h (&) or 24 h (&)), and inert (3 h (~)) ageing atmospheres.

velocity (GHSV) was 43,000 h 1, and the heating rate of the furnace was 20 8C/min. The concentrations of CO, NO, CO2, N2O, and NO2 as a function of temperature were measured by a FTIR gas analyser (GasmetTM CR2000) and the gas flow was controlled by mass flow controllers (Brooks 5850TR). All light-off temperatures were measured within the precision of 5 8C. Blank tests were carried out with the uncoated metal foil to ensure the inactivity of metal foil during the thermal treatments.

3. Results and discussion 3.1. Catalyst characterization The results of BET measurements are presented in Figs. 1–3 and in Tables 2 and 3. The results indicate that structural changes take place upon thermal, engine, and vehicle ageings of the catalyst. The catalyst’s surface areas and pore volumes decreased strongly as a function of ageing temperature in all ageing atmospheres, as can be seen in Fig. 1. Thermally aged catalysts retained high specific

Fig. 2. Effect of ageing time on the BET surface areas of the catalysts at the ageing temperatures of 900, 1000, and 1100 8C. The comparison after 3, 24, and 42 h of ageing in the reducing ageing atmosphere (5% H2/N2 balance).

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Fig. 3. Effect of ageing temperature on the sintering rate at 900, 1000, 1100, and 1200 8C after reducing (—) and oxidizing (- - -) ageings.

surface areas up to the ageing temperature of 800 8C. A critical temperature region for ageing in the oxidizing atmosphere was around 850–900 8C, where the significant loss in surface area was observed indicating a close correlation with the catalyst’s deactivation. As reported by Angove et al. [25] and Gonza´ lez-Velasco et al. [21,22], the surface area loss in air becomes pronounced above 1000 8C with the growth of the oxide particles and the formation of low surface area a-Al2O3. As can be seen in Fig. 1, the surface areas of the air-aged catalysts decreased to half of their original value by the ageing temperature of 950 8C. This collapse in the surface area was associated with a significant decrease in the pore volume and an increase in the pore size. The surface area of a fresh catalyst (69 m2/g) was found to be about three times larger than that of the engine-aged (22 m2/g) or vehicle-aged (16 m2/g) catalysts. The effect of ageing on the BET surface areas in reducing and inert atmospheres is rather different. After the reducing (5% H2/N2) and inert (N2) ageings, the surface areas started to decrease at ageing temperatures around 800 8C, but this decrease in surface area was not as extensive as in the oxidizing atmosphere. This is consistent with the results of Piras et al. [3] who found that the surface areas remained higher in reducing conditions over Al2O3 and ceriasupported Al2O3 catalysts. They also found that under oxidizing conditions at 1200 8C, cerium oxide was almost totally ineffective as a stabilizing agent for aluminium oxide,

whereas under the reducing conditions its stabilizing effects were remarkably enhanced. The stabilizing effects of CeO2 are also associated with its reactions with g-Al2O3, as will be discussed below upon consideration of the ageing-induced solid–solid phase transitions in the bulk material. It was also observed that the reducing and inert ageing atmospheres behaved quite similarly (see Fig. 1). Thus, the formation of aluminates may occur during both ageing procedures. This is discussed in more detail in Section 3.3. The comparison between different ageing atmospheres also showed that in the oxidizing atmosphere the pore sizes were clearly larger than in the reducing or inert atmospheres with the same ageing temperature and time. The effect of ageing time in reducing and oxidizing atmospheres can be evaluated on the basis of results presented in Figs. 1 and 2. Ageing time was a particularly critical variable in the oxidizing ageing atmosphere, although it was not as significant a factor in the deactivation process as ageing temperature and atmosphere. The rate of sintering and surface area loss increased as the ageing temperature increased and led to an increase in the average pore size, which has also been observed by Teixeira and Giudici [26] and Johnson [13]. The rate of sintering was also strongly dependent on the level of catalyst’s surface area. The sintering rates (S/S0 versus time) in reducing and oxidizing ageing atmospheres are presented in Fig. 3. It was observed that the rate of sintering was higher in the oxidizing atmosphere than in the reducing atmosphere, which is consistent with the observations of Gonza´ lez-Velasco et al. [12]. The ageings also induced structural transformations in the catalyst. Both the fresh and aged catalysts gave similar types of adsorption isotherms corresponding to the type IV according to the IUPAC classification. A comparison of adsorption–desorption isotherms showed that in all cases the characteristics hysteresis loops in the range of p/p0 from 0.80 to 0.95 were observed, but the porosity of catalysts decreased during the ageing treatment. Thus, the pore size distributions for the thermally aged, engine-aged, and vehicle-aged catalysts differed from that of the fresh catalyst. The change in the adsorption isotherm was associated with the shift in the pore size distribution

Table 2 Catalyst characterization results for thermally aged catalysts Ageing temperature (8C)

Oxidizing ageing for 3 h 2

Fresh Engine-aged Vehicle-aged 800 900 1000 1100 1200

Oxidizing ageing for 24 h

BET surface area (m /g)

Volume adsorbed (cm3/g support)

69 22 16 70 39 25 9.1 1.5

15.8 5.2 3.7 17.0 8.9 5.7 2.4 0.4

Thermal ageing was carried out in the oxidizing atmosphere (air) for 3 and 24 h.

BET surface area (m2/g)

Volume adsorbed (cm3/g support)

65 31 21 6.6 0.1

15.0 7.1 5.0 1.5 <0.1

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Table 3 Catalyst characterization results for thermally aged catalysts Ageing temperature (8C)

Reducing ageing for 3 h

Reducing ageing for 24 h

2

800 900 1000 1100 1200

BET surface area (m /g)

Volume adsorbed (cm3/g support)

BET surface area (m2/g)

Volume adsorbed (cm3/g support)

65 60 49 30 18

15.0 13.7 11.0 7.0 4.0

67 59 42 27 7.0

15.5 13.5 9.6 6.3 1.7

Thermal ageing was carried out in the reducing atmosphere (5% H2 and N2 balance) for 3 and 24 h.

towards larger pore diameters, when the surface area of the catalyst decreased [12]. The fresh catalyst had small pores under 20 nm, whereas in the case of aged catalysts, the pore sizes had grown and mesopores (40–50 nm) were observed. Small pores disappeared during the thermal ageing procedures in oxidizing, inert, and reducing atmospheres. The corresponding mean pore diameters for the fresh, engine-aged, and vehicle-aged catalysts were 13.7, 39.9, and 45.5 nm, respectively. Graham et al. [27] and Kakuta et al. [28] reported BET results that differ from those presented in this work. They concluded that the ageing has only little effect on the surface areas. The difference may be explained by the small temperature range used in Graham’s and Kakuta’s studies since the trends are small compared to experimental errors. Their results do not facilitate any conclusions of the effect of ageing temperature on surface areas. 3.2. Catalytic activities Catalytic activities were determined after thermal, engine bench, and vehicle ageings in lean and rich conditions by using a model reaction: the reduction of NO by CO. The detailed light-off temperatures of CO and NO are presented in Tables 4 and 5. Light-off temperatures of CO and NO increased systematically as a function of ageing temperature in oxidizing, inert, and reducing ageing atmospheres, as can

be seen in Fig. 4. The activity of a catalyst remained clearly higher if ageing was carried out in reducing or inert atmospheres. Furthermore, differences in the light-off temperatures were not found between these two ageing atmospheres. Instead, after the oxidizing ageing, higher light-off temperatures of CO and NO and lower catalytic activities were observed. This is consistent with the BET measurements (see Section 3.1) where the surface areas prevailed higher after the reducing and inert ageings. On the basis of our measurements, the loss of catalytic activity can be associated with the loss of specific surface area. However, the evolution of catalytic activity cannot be explained exclusively in the terms of specific surface areas and textural properties of the catalyst. It is known that not only support properties but also the changes in the active surface phases after the thermal, engine bench, and vehicle ageings affect the catalytic activity [12]. Ageing-induced changes in the oxidation states of the active metals have been studied by XPS and reported in Ref. [29]. Catalytic activity depends on the nature of ageing atmosphere and temperature, as substantiated above. The ageing time is also an essential factor for deactivation although, on the basis of this study, it is of secondary importance to ageing atmosphere and high ageing temperature. It was mostly observed only at lower ageing temperatures, below 950 8C. Thermal laboratory ageings in the temperature range of 800–1200 8C do not cause a

Table 4 Light-off temperatures of CO and NO after 3 and 24 h of ageing in the oxidizing ageing atmosphere Sample

Fresh 800 8C 850 8C 900 8C 950 8C 1000 8C 1050 8C 1200 8C Engine-aged Vehicle-aged

Ageing time, 3 h

Ageing time, 24 h

CO light-off temperature, T50 (8C)

NO light-off temperature, T50 (8C)

CO light-off temperature, T50 (8C)

NO light-off temperature, T50 (8C)

Lean

Rich

Lean

Rich

Lean

Rich

Lean

Rich

<140 153 190 204 226 237 256 291 265 305

<150 187 201 234 238 252 279 322 260 327

<130 135 179 204 230 236 259 298 270 309

<140 158 166 215 222 230 266 312 246 310

215 204 242 255 256 254 311

207 212 248 254 263 249 313

212 207 251 262 260 261 320

176 199 238 241 246 237 308

T50 indicates temperature at which 50% conversion was achieved for CO and NO.

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Table 5 Light-off temperatures of CO and NO after 3 and 24 h of ageing in the reducing ageing atmosphere Sample

Fresh 800 8C 850 8C 900 8C 950 8C 1000 8C 1050 8C 1100 8C 1150 8C 1200 8C Engine-aged Vehicle-aged

Ageing time, 3 h

Ageing time, 24 h

CO light-off temperature, T50 (8C)

NO light-off temperature, T50 (8C)

CO light-off temperature, T50 (8C)

NO light-off temperature, T50 (8C)

Lean

Rich

Lean

Rich

Lean

Rich

Lean

Rich

<140 141 152 167 222 234 256 253 259 272 265 305

<150 171 188 190 210 244 251 262 280 261 260 327

<130 136 154 170 228 237 263 253 262 275 270 309

<140 153 166 172 204 234 241 252 267 255 246 310

157 204 192 210 223 246 261 264 265

189 212 211 207 239 256 250 271 274

160 208 195 216 226 255 255 270 271

174 199 196 219 226 240 250 261 261

T50 indicates temperature at which 50% conversion was achieved for CO and NO.

significant performance decrease for the catalysts. All aged catalysts maintained a high catalytic activity up to the ageing temperature of 800 8C, and were active in decreasing the emissions of CO and NO. Even after the ageings at 1200 8C, high catalytic activities still remained if compared with the vehicle-aged reference catalyst. 3.3. XRD X-ray powder diffraction data showed that the support material of the catalysts is mainly composed of three oxides, i.e. Al2O3 (alumina), CeO2 (ceria), and La2O3 (lanthana). In addition to these pure oxides, the presence of Ce- and Zr-rich CeXZr1 XO2 mixed oxides was observed. The support of the fresh catalyst contained amorphous material and crystal sizes were small. Therefore, the crystalline phases were not detected by XRD in case of a fresh catalyst. The XRD diffractograms due to pure cerium oxides and CeXZr1 XO2

Fig. 4. Effect of ageing atmosphere on catalytic activities (CO light-off temperatures) in lean reaction conditions. Comparison between oxidizing (&), reducing (*), and inert (~) ageing atmospheres. Ageing time, 3 h, was kept constant.

mixed oxides (Zr-rich) were first observed after ageings at around 800–900 8C, as can be seen in Fig. 5. These oxides have characteristic main peaks at 2u = 28.78 and 29.88, respectively [24]. However, the peaks for the CeO2 are broad and the presence of a Ce-rich mixed oxide is also possible. After 24 h of ageing in the reducing atmosphere at 1000 8C a very broad peak was observed at ca. 298. It could correspond to a Ce-rich mixed oxide (see Fig. 6, trace A). Depending on the cerium content and on the temperature, CeXZr1 XO2 mixed oxides exist in three different crystalline phases, namely monoclinic, tetragonal, and cubic phases [30], in the composition range of 0 mol% < Ce < 100 mol%. Below 1000–1050 8C (i.e. at temperatures where solids are used as catalysts), the cubic (Ce-rich composition) and monoclinic (Zr-rich composition) phases appear to be thermodynamically stable [31]. After 24 h of ageing in the oxidizing atmosphere at 1200 8C (see Fig. 6, trace C) the presence of Ce0.5Zr0.5O2 mixed oxide is evident. Due to the different ionic radii of Ce4+ and Zr4+ (0.97 and 0.84 nm, respectively), a linear relationship between the Zr content and the lattice parameters is expected for the cubic range of compositions [30]. It has been reported that cubic CeXZr1 XO2 mixed oxides obey Vegard’s law [32]. For instance, d(1 1 1) decreases linearly with decreasing CeO2 content in the solid solution [9,31]. Using this relationship it is possible to determine semi-quantitatively the compositions of the CeXZr1 XO2 solid solutions observed after 24 h of ageing in the inert atmosphere at 1050 8C (see Fig. 6, trace B) and in the reducing atmosphere at 1200 8C (see Fig. 6, trace D). The composition appears to be approximately Ce0.9–0.8Zr0.1–0.2O2. Furthermore, the Ce-rich mixed oxide seems to be more resistant against thermal treatments than the Zr-rich mixed oxide, which has decomposed in air after 24 h of ageing at 1200 8C (Fig. 6, trace C). As indicated above, a high ageing temperature induced phase transformations in the bulk of the catalyst. Furthermore, significant differences were observed between

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Fig. 5. The crystallization of the catalysts after reducing and oxidizing ageings: (A) oxidizing ageing at 800 8C/24 h, (B) oxidizing ageing at 950 8C/3 h, (C) reducing ageing at 800 8C/3 h, (D) reducing ageing at 900 8C/3 h. a = CeO2, b = Zr-rich mixed oxide.

reducing, inert, and oxidizing ageing atmospheres indicating that the crystal structure of the catalyst was strongly dependent on the surrounding environment. As can be seen in Fig. 6, the main difference between reducing and inert atmospheres compared to the oxidizing atmosphere comprises the temperature in which the formation of CeAlO3

starts. In reducing and inert atmospheres after 24 h of ageing the formation of CeAlO3 was observed at around 1000 and 1050 8C, respectively. At higher ageing temperatures, CeAlO3 was observed already after shorter ageing times. For the oxidizing atmosphere, CeAlO3 was observed only at higher temperatures, after 24 h of ageing at 1200 8C. The

Fig. 6. The formation of CeAlO3 in different ageing atmospheres: (A) reducing ageing at 1000 8C/24 h, (B) inert ageing at 1050 8C/24 h, (C) oxidizing ageing at 1200 8C/24 h, (D) reducing ageing at 1200 8C/24 h. a = CeO2, a* = Ce-rich mixed oxide, b* = ZrO2, c = CeAlO3, d = Ce0.5Zr0.5O2, e = a-Al2O3, f = CeAl11O18.

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CeAlO3 phase is formed in the reaction of g-alumina with the surface CeO2 [9]. Furthermore, the formation of CeAlO3 is thermodynamically favoured in the reducing atmosphere [25]. CeAlO3 was identified from the characteristic diffraction peaks at 2u of 23.68, 33.58, and 41.48 [24]. The formation of CeAlO3 phase prevents the sintering and hinders the formation of low surface area a-alumina at high temperatures, explaining why the surface areas remained higher after the reducing and inert ageings compared to the oxidizing ageing. In an oxidizing atmosphere, a-alumina was detected after 3 h of ageing at 1200 8C, but CeAlO3 was detected only after 24 h of ageing. This explains the collapse of the surface area during the oxidizing ageing at high temperatures: a-alumina was formed before CeAlO3 and, therefore, the formation of CeAlO3 could not prevent the formation of a-alumina. It has been reported [9] that a large amount of the ceria on g-alumina has been reduced to the oxidation state 3+ and that these Ce3+ ions can be converted to CeAlO3 upon reduction. Our measurements confirmed this observation. Tas and Akinc [33] have reported the formation of CeAlO3 in a Ce2O3–Al2O3 system in both inert (Ar) and reducing atmospheres (10% H2/Ar) at elevated temperatures. In a flowing hydrogen atmosphere, CeAlO3 was formed at 1000 8C. Tas and Akinc [33] also reported that CeAlO3 completely decomposed to CeO2 and Al2O3 in air at 800 8C in 1 h. The first phase transition, observed in our measurements, in the reducing atmosphere was the formation of CeAlO3, as mentioned earlier. However, a second phase transition took place at higher ageing temperatures. The formation of the second compound of Ce and Al, CeAl11O18, started at temperatures 1150– 1200 8C in the reducing atmosphere. At these temperatures, the formation of small amounts of low surface area aalumina was also observed. This new phase formed upon the consumption of a-alumina and CeAlO3, and it has also been detected in the three-way catalysts after the operation in rich conditions [25]. The CeAl11O18 phase was identified from its characteristic peaks at 2u of 34.08 and 36.28 [24] after 24 h of reducing ageing at a temperature of 1200 8C, as can be seen in Fig. 6, trace D. It can also be seen that, under these conditions, CeAlO3 has disappeared almost completely. The formation of CeAlO3 was also observed after the engine bench and vehicle ageings as shown in Fig. 8. The formation of CeAlO3 is inversely proportional to the loss of surface area, which remained higher in the reducing and inert ageing atmospheres than in the oxidizing atmosphere. According to Angove et al. [25], the surface area loss becomes pronounced at around 1000 8C, as the starting g-alumina form is converted to the u-form and finally to the a-form. This is consistent with our measurements. The cerium oxide phases, CeO2 for the oxidized and CeAlO3 for the reduced catalyst, were also observed by Haneda et al. [34], who concluded that, on this basis, reduced catalysts had higher oxygen storage capacities compared to the oxidized ones. They suggested two possibilities for the structure around Ce ions in the reduced

catalyst; Ce ions either remained dispersed in the bulk alumina, or they coagulated to form several non-stoichiometric cerium oxides, CeO2 X. An interesting observation of these types of La2O3containing Al2O3-based catalysts is the formation of LaAlO3, which has been observed several times on model catalysts [3,5,35,36]. The formation of LaAlO3 was observed from lanthana-rich washcoat as a result of ageing (see Fig. 7). LaAlO3 has been observed to have a main characteristic peaks at 2u of 23.58, 33.48, and 41.28 [24]. These peaks were typical for samples containing g-alumina. The presence of the LaAlO3 phase increases the stability of g-alumina phase at high temperatures and prevents deactivation processes between alumina and the catalytically active precious metal components. According to Bogdanchikova et al. [5], the formation of the LaAlO3 phase was observed in air ageing at around 1000 8C and it is stable in hydrogen, nitrogen, and air at relatively high temperatures. In this study, the formation of LaAlO3 was detected from the lanthana-rich washcoat after 24 h of reducing and oxidizing ageings at 900 and 1050 8C, respectively. The LaAlO3 phase was formed as a result of ageing mainly in the reactions of pure La-oxides with g-alumina. This is consistent with the results of Piras et al. [3]. They showed that Ce-oxide confers remarkable stability to the alumina surface under reducing conditions with a hydrogen-containing atmosphere. This stability is connected to the formation of CeAlO3, which inhibits the sintering in a similar manner to that observed for La2O3 on Al2O3 under oxidizing conditions. It has also been reported that CeO2 forms a mixed oxide phase with La2O3, which enhanced oxygen storage capacity [37]. The formation of mixed (Ce–La)AlO3 phase is reasonable due to the similarity of the Ce3+ and La3+ ionic radii. However, in this case the likely formation of (Ce–La)AlO3 as a result of ageing was not observed in the lanthana-rich washcoat, because this washcoat does not contain pure Ce-oxides. Similarly to the case of Ce and Al, an increase in the ageing temperature results in the formation of a second compound of La and Al. When the ageing temperature is increased to 1200 8C (24 h) in reducing atmosphere, LaAl11O18, is detected in trace amounts, as can be seen in Fig. 7, trace C. It is identified by its characteristic peaks at 2u of 32.18, 34.08, 36.28, and 42.78 [24]. Be´ guin et al. [38] have reported that on pure La-stabilized alumina LaAl11O18, is formed, probably from the reaction of LaAlO3 and Al2O3, when the sample is aged in air at 1220 8C for 24 h in the presence of 20 vol.% of water. Our results seem to indicate that its formation is possible also on real catalysts. 3.4. Correlations between laboratory, engine bench, and vehicle ageings From the point of view of phase changes observed by XRD measurements, the reducing laboratory ageing for 24 h at 1000 8C correlates best with the phase changes observed after the engine bench and vehicle ageing (see Fig. 8). Also, the BET

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Fig. 7. The formation of LaAlO3 in different ageing atmospheres: (A) reducing ageing at 900 8C/24 h, (B) reducing ageing at 1050 8C/24 h, (C) reducing ageing at 1200 8C/24 h, (D) oxidizing ageing at 1050 8C/3 h. a = LaAlO3, b = a-Al2O3, c = LaAl11O18. The peak at approx. 29.88 could not be identified.

results support the correlation between reducing laboratory ageing and vehicle ageing although higher ageing temperature of 1200 8C is required. However, after the engine bench ageing, this correlation in BET results is not as clear. In contrast, when comparing the activity of laboratory aged catalysts to the activity of the vehicle-aged catalyst, 3 h oxidizing ageing at 1200 8C seems to have the best correlation to vehicle ageing, although this correlation is not perfect. The

reason for this is probably due to the presence of fuel and lubricant derived poisons as well as the water vapour in the exhaust gas during the vehicle ageing. As can be seen from our results, no single characterization method can give a reliable correlation between laboratory and engine bench and vehicle ageings. Therefore, a combination of characterization methods is required to understand these deactivation correlations. Furthermore, the laboratory scale ageing should be

Fig. 8. The XRD diffractograms for: (A) engine, (B) vehicle, (C) reducing ageings at 1000 8C/24 h. a = CeO2, a* = Ce-rich mixed oxide, b* = ZrO2, c = CeAlO3, d = Ce0.5Zr0.5O2, e = aAl2O3.

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improved to take into account the role of water vapour and the presence of catalyst poisons (e.g. P, Ca and S).

4. Conclusions Based on the BET results, it can be concluded that gas phase composition (reducing, inert, and oxidizing) and ageing temperature influence the catalytic activity and structural properties of automotive exhaust gas catalysts. Surface areas decreased during the ageing in the oxidizing atmosphere at the temperature range of 850–1000 8C. This temperature range was also critical after the reducing and inert ageings, although the loss in the surface area was not so extensive. The loss in surface area during the thermal ageing (1050 8C, 3 h) was 82% after oxidizing and only 43% after inert and 38% after reducing ageings. Thus, reducing and inert ageings did not decrease the surface area as rapidly as the oxidizing ageing, which was more demanding for the catalyst. The activity of the catalysts remained higher after reducing ageing compared to the oxidizing ageing, which is consistent with the behaviour of the surface area in different ageing atmospheres. The formation of aluminates after ageings in reducing and inert atmospheres at temperatures around 1000 8C as well as after engine and vehicle ageing was observed. This inhibits the crystal growth and prevents the formation of low surface area a-alumina phase responsible for the rapid decrease in surface area. The formation of CeAlO3 in the oxidizing atmosphere could not prevent the collapse of the surface area, because a-alumina was formed before CeAlO3 formation started (1200 8C/3 h versus 1200 8C/24 h). Thus, surface areas remained higher after reducing and inert ageings compared to oxidizing ageing, where the loss of surface area was significant even after short ageing times. Our XRD results on real catalytic systems correspond well with the results observed on the model catalysts. Finally, it seems that 24 h laboratory ageing in reducing atmosphere at 1000 8C seems best to correlate with the real vehicle ageing and engine ageing in terms of phase changes. Surface areas and pore volumes seem to correlate best after 3 h reducing ageing at 1200 8C when compared to the vehicle-aged catalyst. However, best correlation in terms of activity is achieved after 3 h oxidizing ageing at 1200 8C. Clearly, more work needs to be done to find a single ageing method in terms of atmosphere and temperature to achieve good correlation in all measured characteristics. One possibility is to introduce water vapour into the model exhaust gas, as water vapour is present under real driving conditions. Furthermore, the role of fuel and lubricant derived poisons in catalyst ageing needs to be addressed. Acknowledgements Mr. Jorma Penttinen and Ms. Hannele Nurminen are gratefully acknowledged for the help in making a part of the

experiments. The authors also acknowledge the Technology Development Center of Finland (U.L. and M.H.), the Ministry of Education and the Graduate School in Chemical Engineering (U.L.), and the Academy of Finland (M.H and R.S.L.) for financial support.

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