versão On-line ISSN 0104-6632
Braz. J. Chem. Eng. vol.29 no.1 São Paulo jan./mar. 2012
KINETICS AND CATALYSIS; REACTION ENGINEERING; AND MATERIALS SCIENCE
Jiuying Tian*; Jusheng Lu
School of Chemistry & Chemical Engineering, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Phone: +86 516 83403165, Fax: +86 516 83536977, Xuzhou Normal University, Xuzhou 221116, P.R. China. E-mail: email@example.com
Nanophase (Ce, Zr, Pr)O2-doped alumina coatings were prepared by impregnating the cordierite ceramic honeycomb in the sol or in the slurry of already calcined powder, respectively. The effects of preparation methods on the crystal phase, texture, oxygen storage capacity (OSC), reducibility, surface morphology and thermal stability of coatings were investigated by X-ray diffraction (XRD), the Brunauer Emmet Teller (BET) method, the oxygen pulsing technique, H2-temperature-programmed reduction (H2-TPR) and scanning electron microscopy (SEM). These nanophase (Ce, Zr, Pr)O2-doped alumina coatings were used as supports to prepare Pd-only three-way catalysts, and evaluated with respect to catalytic activities. The results indicate that the nanophase (Ce, Zr, Pr)O2-doped alumina coatings prepared by the two methods have high thermal stability. However, the coating derived from the sol shows better crystalline structure, texture, reducibility and oxygen storage capacity than the coating derived from the slurry. SEM observation shows that the morphology of the coating derived from the sol is uniform and smooth. The Pd-only catalyst derived from the sol exhibits high three-way catalytic activity at low temperature and thermal stability, suggesting a great potential for applications.
Keywords: Nanophase (Ce, Zr, Pr)O2; Coating; Preparation; Properties; Three-way catalyst.
Three-way catalysts (TWC) for automobile exhaust purification are comprised mainly of three parts: a cordierite honeycomb substrate, an activity coating and a noble metal (Pt, Rh, Pd) activity element (Muraki and Geng, 2000; Gonzalez-Velasco et al., 2001). The cordierite ceramic substrate (2MgO 2Al2O3 5SiO2) has high mechanical strength, a small pressure drop and a low thermal expansion coefficient (Zotin et al., 2005), but the specific surface area of the cordierite ceramic honeycomb is small (<1m2.g-1). In order to improve the catalytic activity, a coating material of high specific surface area should be deposited on the surface layer of the cordierite ceramic honeycomb (Hirohumi et al., 2001; Stary et al., 2006). γ-Alumina is a coating material that is currently being used to expand the specific surface area (Yue et al., 2005; Aguila et al., 2008). However, the surface area loss becomes pronounced above 1000ºC as the γ-alumina phase is converted to the θ-phase and finally to the α-form (Kucharczyk et al., 2004). Studies have shown that a stabilizer, such as CeO2, La2O3 or Y2O3, can delay the phase transition temperature of γ-alumina and reduce the loss of specific surface area (Agrafiotis et al., 2000; Mokhnachuk et al., 2007; Yassir and VanMao, 2007).
CeO2 has been widely studied in recent years because of its potential usefulness for catalytic applications (Kaspar and Fornasiero, 2003; Kakuta et al., 2006). It is well known that CeO2 can affect: (i) thermal and structure stability of the catalyst supports (Piras et al., 2000); (ii) dispersion of supported metal (Montoya et al., 2000); (iii) oxidation and reduction of noble metals (Bozo et al., 2001); (iv) storage and release of oxygen (Hadi and Yaacob, 2004), etc. Especially when the grain size of CeO2 is nanometer, a large number of lattice defects such as oxygen vacancies are present, which provide the active sites of gas-solid catalytic reaction. The number of defects can be increased by partial substitution of the cerium atoms in the cerium oxide lattice with other metal atoms such as Zr, La, Nd or Pr. Because of these properties, CeO2-based mixed oxides such as (Ce, Zr)O2 and (Ce, Zr, M)O2 (M= La, Y, Pr, Nd, etc.) are efficient oxygen storage materials, which can act as catalysts themselves and significantly reduce the need for noble metals (He et al., 2003; Fagg et al., 2006; Wu et al., 2007). Researchers have focused their attention on (Ce, Zr)O2 or (Ce, Zr, M)O2 doped alumina (Fernandez-Garcia et al., 2000; Damyanova et al., 2002; Wei et al., 2008; Morikawa et al., 2008; Morikawa et al., 2009; Zhu et al., 2010), which is a valid combination for environmental application catalysts, such as catalysts for removal of pollutants from auto-exhaust streams. When nanophase (Ce, Zr, M)O2-doped alumina is deposited on cordierite ceramic honeycomb, the whole coating can be used for high-temperature catalytic applications.
At present, nanophase (Ce, Zr, M)O2-doped alumina material is deposited on cordierite ceramic honeycomb by the dip coating method, which is an economical and simple method used in practice (Agrafiotis et al., 2002; Jiang et al., 2005; Chen et al., 2006; Guo et al., 2007; Tian et al., 2010). Active nanophase (Ce, Zr, M)O2 powder can be effectively synthesized through liquid precursor solutions via wet chemistry methods. This nanophase (Ce, Zr, M)O2 powder needs to be processed as slurries for impregnating the cordierite ceramic honeycomb. After subsequent calcination, this method leads to a coating layer deposited on the honeycomb walls. However, there are few reports of impregnating the honeycomb directly in the sol phase. In addition, (Ce, Zr)O2 and (Ce, Zr, La)O2 have been widely applied in TWC (Wu et al., 2004; Jiang et al., 2005; Guo et al., 2007; Chen et al., 2006), but relatively little attention has been paid to the nanophase (Ce, Zr, Pr)O2 solid solution. Here we report a novel sol-gel method for preparing a nanophase (Ce, Zr, Pr)O2-doped alumina coating on cordierite ceramic honeycomb.
In the previous works, the authors prepared the nanophase (Ce, Zr, Pr)O2 solid solution by coprecipitation (Tian et al., 2006) and studied the preparation conditions of a pure alumina coating on cordierite ceramic honeycomb, using boehmite as the raw material (Tian et al., 2010). In the present work, we prepared (Ce, Zr, Pr)O2-doped alumina coatings by impregnating the cordierite ceramic honeycomb in the sol and in the slurry of already calcined powder, respectively. The effects of preparation methods on the crystalline structure, texture, reducibility, oxygen storage capacity and thermal stability of coatings were studied. These (Ce, Zr, Pr)O2-doped alumina coatings were used as supports to prepare Pd-only catalysts so that the whole support/catalyst system can be evaluated under operating conditions simulating automotive exhaust gases. The aim of this work was to obtain an uniform nanophase (Ce, Zr, Pr)O2-doped alumina coating support with high catalytic activity and thermal stability.
Preparation of Supports
The slurry was prepared as follows: the nanophase (Ce, Zr, Pr)O2 powder was prepared by the co-precipitation method described in the previous work (Tian et al., 2006). Then this (Ce, Zr, Pr)O2 nano-oxide powder was mixed with a boehmite sol, which was prepared by a technique similar to the literature (Tian et al., 2010). After stirring, the slurry was obtained for impregnating honeycombs.
Another method for the coating of cordierite ceramic honeycomb is the sol-gel method. First, the (Ce, Zr, Pr)O2 precursor sol was prepared as follows: Ce(NO3)3·6H2O, Zr(NO3)4·5H2O, Pr(NO3)3·6H2O and citric acid were dissolved in de-ionized water. Its pH value was adjusted with aqueous NH3·H2O solution. This mixture solution was heated at 70ºC in a water bath and constantly stirred until a stable sol was obtained. Then this (Ce, Zr, Pr)O2 precursor sol was mixed with the above-mentioned boehmite sol to form a final sol, which was used to impregnate the cordierite ceramic honeycomb.
For the final sol and the slurry, the relative molar ratios of each constitute were (Ce, Zr, Pr)O2/Al2O3 = 1:3, Ce/Zr/Pr = 12:7:1.
A commercial honeycomb cordierite cast (Corning Corporation, 400 cells per square inch) was cut into cylinder samples (10 mm diameter × 25 mm length) and employed for the impregnation experiment. The cordierite specimens were immersed in the sol or slurry prepared by the abovementioned methods. The loaded specimens were withdrawn and the excess slurry was removed by blowing air through the honeycomb channels. Subsequently, the coated honeycombs were dried at 120ºC for 2 h in air and calcined at 550ºC for 5 h in air (heating rate 10ºC min-1), so that the fresh (Ce, Zr, Pr)O2-doped alumina coatings on cordierite ceramic honeycombs were formed. The aged samples were obtained by aging the fresh samples at 1000ºC for 3 h in air (heating rate 10ºC min-1). The (Ce, Zr, Pr)O2-doped alumina coatings prepared by impregnating the honeycomb in the sol and in the slurry were abbreviated as CZPA-S and CZPA-P, respectively.
Preparation of Catalysts
Catalysts were prepared by the classical postimpregnation method. The cordierite ceramic honeycombs loaded with the (Ce, Zr, Pr)O2-doped alumina coatings were subsequently postimpregnated with H2PdCl4 aqueous solutions, whose volumes were adjusted for complete absorption by the coating. The Pd loadings were 0.8 g L-1. After the post-impregnation, the cordierite catalysts were dried at 120ºC for 3 h in air and calcined at 550ºC for 5 h in air (heating rate 10ºC min-1) to obtain the fresh catalysts. The aged catalysts were obtained by thermal treatment of fresh catalysts at 1000ºC for 3 h in air (heating rate 10ºC min-1). The catalyst prepared from CZPA-S was labeled Pd/CZPA-S, while that prepared from CZPA-P was labeled Pd/CZPA-P.
Characterization of Supports and Catalysts
Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-III B powder diffraction instrument, using CuKα radiation, operated at 35 kV and 30 mA. The intensity data were collected in a 2θ range from 10º to 70º.
Surface area analysis was carried out at liquid N2 temperature with the Brunauer-Emmet-Teller (BET) method using ASAP2010 micromeritics.
The surface morphology was examined with a Hitachi S-3400N scanning electron microscope, operated at an accelerated voltage of 20KV. Prior to SEM analysis, the cordierite honeycomb loaded with (Ce, Zr, Pr)O2-doped alumina coating was cut along the channel, so that the coating inside the channel was exposed. Before examination, this coating was covered with a thin layer of gold by sputtering, which was used as an electrically conductive film to prevent charging.
Total oxygen storage capacity (OSC) was measured by gas chromatography in flowing N2 (30 ml min-1) at 200ºC. Prior to the experiment, the sample (200 mg) was reduced at the appropriate temperature (500ºC) for 30 min under a H2 flow (40 ml min-1). After reduction, the sample was cooled to 200ºC under N2 flow (30 mL min-1). Then a given amount of oxygen (0.15 ml) was pulsed every 5 min until the intensity of the peak reached a constant value, which means that no additional oxygen consumption could be detected by the thermal conductivity detector (TCD).
H2-temperature-programmed reduction (H2-TPR) experiments were performed in a quartz tubular microreactor. The sample (200 mg) was placed in a quartz reactor. A thermocouple was located in the middle of the reactor, the reactor was placed in a tubular furnace, and a temperature-programmed controller was used to adjust the reactor temperature. In order to remove the moisture produced during H2-TPR, the outlet of the quartz tubular reactor was connected to a glass column packed with 5A molecular sieve. The sample was first pretreated at 400ºC for 1 h under N2 flow (30 ml min-1), cooled to room temperature, and swept with H2-N2 (H2:N2 5:95) at a rate of 30 ml min-1 until the base line on the recorder remained unchanged. Finally, the sample was heated at the rate of 10ºC min-1 under flowing H2-N2 (H2:N2 5:95, flow rate 30 ml min-1) from room temperature to 900ºC. H2 consumption during reduction was detected by a thermal conductivity detector (TCD).
Catalytic Tests of Catalysts
The three-way catalytic activity was evaluated in a conventional fixed-bed flow quartz micro-reactor by passing a gas mixture similar to exhaust from a gasoline engine. The gases were regulated using mass-flow controllers. The catalyst was held in a quartz tube by packing quartz wool at both ends of the catalyst bed. The quartz tube was placed in a tubular furnace and the temperature of the catalyst was controlled by a temperature-programmed controller. The simulated exhaust containing a mixture of CO (0.7%), C3H8 (0.05%), NO (0.05%), CO2 (10%), O2 (0.6%) and N2 (balance) was fed to the reactor at a gas space velocity of 30,000h-1. The concentrations of CO, NO and C3H8 were analyzed online by a FGA-4015 analyzer before and after simulated gas passing through the micro-reactor. Conversion data were measured from 50 to 400ºC with a rate of 5ºC min-1. The curves of the relationship between conversion and temperature were obtained. The catalytic activity of catalyst was expressed as the light-off temperature (T50%), at which the target reactant (CO, NO or C3H8) had been converted by 50%.
RESULTS AND DISCUSSION
X-ray diffraction patterns of the supports and the Pd-supported samples are shown in Fig. 1. For CZPA-S and CZPA-P (Fig. 1a), a single fluorite-type phase of (Ce, Zr, Pr)O2 solid solution and the γ-alumina phase are observed in all fresh samples. The diffraction peaks at 2θ = 28.8º, 33.3º, 47.5º, 56.6º, which correspond to the (111), (200), (220), (311) crystal planes for the cubic fluorite structure, are ascribed to a single phase of uniform (Ce, Zr, Pr)O2 solid solution (Tian et al., 2006). The characteristic diffraction peaks of γ-alumina are of low intensity and the most intense peak is around 2θ = 67.3º. Samples did not show any clear diffraction peaks of cerium aluminate (CeAlO3) (2θ = 23.5, 33.5, 41.4 and 60º) (Damyanova et al., 2002) and praseodymium aluminate (PrAlO3) (2θ = 23.3, 33.9, 42.1 and 48.5º) (Pawlak et al., 2005). This indicates that the (Ce, Zr, Pr)O2 solid solution does not combine with alumina to form the new species. After thermal treatment at 1000ºC, the intensities of the diffraction peaks of the (Ce, Zr, Pr)O2 solid solution increase and the line-broadenings decrease. No phase other than alumina and (Ce, Zr, Pr)O2 was observed in the XRD profiles. The supports CZPA-S and CZPA-P show γ- and δ-alumina, respectively. We did not detect any α-alumina, although pure γ-alumina should be converted to αalumina at 1000ºC (Kucharczyk et al., 2004), indicating that the presence of (Ce, Zr, Pr)O2 obviously improved the thermal stability of alumina, as already reported in the literature (Monte et al, 2000; Yue et al, 2005; Wang et al., 2010). It also indicates that the (Ce, Zr, Pr)O2-doped alumina coatings prepared by the two methods have high thermal stability. After thermal treatment, the CZPA-S still shows γ-alumina; however, CZPA-P converted to δ-alumina, indicating that the thermal stability of CZPA-S is higher than that of CZPA-P.
For fresh and aged Pd/CZPA-S and Pd/CZPA-P (Fig. 1b), the characteristic peaks of metallic Pd (2θ = 40.4 and 41.9º) cannot be distinguished from the main peaks of the support. This suggests that Pd particles are well dispersed on the supports, which makes them hard to be detected by XRD. The structures of the supports remain essentially unchanged and alumina and (Ce, Zr, Pr)O2 solid solution phase are detected, which is consistent with results reported in the literature (Zhao et al., 2009; Yue et al., 2005; Cai et al, 2008).
The crystallite sizes of the (Ce, Zr, Pr)O2 solid solution were calculated by Scherrer's equation from the full-width at half maximum of the XRD profiles. The crystallite sizes of (Ce, Zr, Pr)O2 are 2.3 nm and 4.3 nm for fresh CZPA-S and CZPA-P, respectively. After thermal treatment at high temperature, the crystallite sizes of (Ce, Zr, Pr)O2 are increased to 13.1 nm for CZPA-P, whereas it only increased to 6.8 nm for CZPA-S. The results also suggest that the thermal stability of CZPA-S is higher than that of CZPA-P. The crystallite size is still nanometer scale after thermal treatment. Furthermore, there is no solid phase reaction between alumina and the (Ce, Zr, Pr)O2 solid solution at high temperature, i.e., they still exist as single phases. It is plausible that alumina and (Ce, Zr, Pr)O2 are well dispersed at the nanometer level, which could serve as a diffusion barrier to each other (Morikawa et al., 2008) and inhibit mutual particle growth, consequently resulting in samples with good stability of crystal particles at high temperature. The results indicate that the nanophase (Ce, Zr, Pr)O2 doped into alumina can stabilize and improve the thermal stability compared with pure alumina, which is consistent with the results of the XRD patterns.
For Pd-supported samples, the crystallite sizes are 3.6 nm and 5.1 nm for fresh Pd/CZPA-S and Pd/CZPA-P, respectively. The crystallite sizes are 8.0 nm and 12.6 nm for aged Pd/CZPA-S and Pd/CZPA-P, respectively. The crystallite sizes of Pd/CZPA-S are smaller than those of Pd/CZPA-P before and after aging. Moreover, the crystallite size increase of Pd/CZPA-P is greater than that of Pd/CZPA-S, indicating that Pd/CZPA-S has higher thermal stability than Pd/CZPA-P. This result shows that the thermal stability of the support is relevant for the properties of the catalyst. The catalyst usually works at temperature higher than 600ºC. Strong thermal sintering of the support or the catalyst may cause a serious decrease of catalytic activity. The supports with high thermal stability could resist the sintering of catalyst after thermal treatment. The stability of CZPA-S is more suitable for the support of TWC.
Texture and Oxygen Storage Capacity
The specific surface area, pore volume, pore diameter and oxygen storage capacity of two supports before and after aging are shown in Table 1. The specific surface areas are 152.6 m2/g for fresh CZPA-S and 130.2 m2/g for fresh CZPA-P. After thermal treatment, the specific surface areas of CZPA-S and CZPA-P are still 123.4 m2/g and 96.3 m2/g, respectively. The specific surface area of CZPA-S is larger than that of CZPA-P before and after aging. Moreover, the decrease of the specific surface area of CZPA-S is smaller than that of CZPA-P, indicating that CZPA-S has higher thermal stability than CZPA-P. The pore volume of fresh CZPA-S is 0.47 ml/g, which is slightly larger than that of fresh CZPA-P (0.42 ml/g). The average pore sizes of fresh CZPA-S and CZPA-P supports are 4.02 and 4.81 nm, respectively. After thermal treatment, the average pore size increases to 4.38 nm for CZPA-S and 5.52 nm for CZPA-P. The increase of the average pore size of CZPA-S is 9.0%, while the average pore size increases by 14.8%, indicating that CZPA-S has higher thermal stability than CZPA-P.
The oxygen storage capacity of fresh CZPA-S is 628.2 µmol/g and decreases to 563.7 µmol/g (10.3%) after aging, while that of CZPA-P decreases from 516.3 µmol/g to 443.1 µmol/g (14.2%). The oxygen storage capacity and thermal stability of CZPA-S are higher than those of CZPA-P. It has been reported that the deactivation of TWC results mainly from the decrease of the oxygen storage amount (Kaspar and Fornasiero, 2003). Moreover, in heterogeneous catalysis, the surface of the support could provide the necessary active sites for catalytic reactions. The smaller the support particles are, the more surface sites of the support there are, and thus the higher the catalytic activity of the catalyst. Therefore, the good oxygen storage capacity and texture of the supports play an important role in the catalytic performance of the catalyst. Since CZPA-S exhibits better texture and oxygen storage capacity, it should be more suitable for the support of TWC, which is consistent with the XRD results.
The reducibility of the support and supported catalyst is an important factor influencing its catalytic performance. H2-TPR is a common technique to investigate the reducibility of samples (Salasc et al., 2002; Lin et al., 2004; Yeste et al. 2009). The H2-TPR profiles of supports and Pd-supported catalysts are shown in Fig. 2. From Fig. 2a, the reduction profiles of CZPA-S are obviously different from those of CZPA-P, indicating that the preparation methods significantly affect the reduction behavior of the samples. The fresh CZPA-S shows a single main peak at 570ºC, with broad shoulders at 440 and 533ºC. However, for fresh CZPA-P, two reduction peaks at 619 and 782ºC are observed, respectively. According to the literature (Silva et al., 2008), pure alumina commonly has no hydrogen consumption in the TPR profile. The pure ceria exhibits two reduction peaks at approximately 500 and 830ºC, which are associated with the reduction of surface and bulk, respectively (Yao et al., 1984). The low-temperature reducibility (below 600ºC) of the fresh CZPA-S sample is typically characterized by the presence of more than one overlapping peak. Moreover, no high temperature reduction peak is found. We suggest that there are two possible reasons: (i) according to the XRD and BET results, fresh CZPA-S presents a smaller crystal size and a higher surface area than fresh CZPA-P, which makes the surface Ce4+ easier to reduced at a lower temperature. (ii) the mobility of bulk oxygen in fresh CZPA-S is greatly enhanced due to the doping with ZrO2, Pr6O11 and Al2O3 (Wu et al., 2007; Morikawa et al., 2009). The bulk oxygen can move to the surface or subsurface more easily during the reduction process. Therefore, the multimodal shape of fresh CZAP-S can be attributed to the reduction of surface and subsurface Ce4+ of the mixed oxide crystallites and a successive reduction in the bulk (Wu et al., 2005; Vidal et al., 2000; Fally et al., 2000; Zhang et al., 2009). For fresh CZPA-P, the presence of two reduction peaks at 619 and 782ºC indicates that there are at least two types of Ce4+ located in different chemical environments. The reduction peak at 619ºC could be assigned to the reduction of surface Ce4+, whereas the peak at 782ºC could be assigned to the reduction of bulk Ce4+ (He et al., 2003; Kim et al., 2007; Kim et al., 2009; Thammachart et al., 2001). The temperature of bulk reduction of fresh CZPA-P is lower than that of pure CeO2 (Boaro et al., 2003) due to the increased mobility of bulk oxygen in fresh CZPA-P. After thermal aging, the reduction peaks in the TPR experiment with CZPA-S sample shifts slightly towards lower temperature. A main peak at 562ºC with a shoulder at 409ºC is observed. Furthermore, the low temperature shoulder at 440ºC disappears. This may be related to the loss of surface area and increase of crystal size, which decrease the surface activity of the sample. For the aged CZPA-P, a decrease of the maximum reduction temperature took place, which leads to a merging of TPR peaks. A single peak at 618ºC is observed. It indicates that the promotion of the reducibility at low temperature could be induced by solid solution sintering at high temperature (Balducci et al., 1995; Otsuka-Yao-Matsuo et al., 1998; Fornasiero et al., 1999; Kaspar et al., 2003a; Wei et al., 2008). The results also indicate that CZPA-S and CZPA-P supports have excellent resistance to aging.
Compared with CZPA-P, the reduction of CZPA-S begins at a lower temperature and its reduction peak area is larger, indicating that the CZPA-S support possesses the better reduction capacity. This result is mainly related to the smaller crystallite particle size and higher specific surface area of CZPA-S. It has been found (Perrichon et al., 1994) that the reduction of ceria occurs first on the surface and then progressively affects the bulk. It is said that the initial progress of reduction is highly sensitive to the surface area of the sample (Pan et al., 2009). Therefore, the specific surface area of CZPA-S is higher, which is more beneficial to the reduction of CZPA-S.
The reducibility of Pd-supported catalysts is an important factor influencing their catalytic activity. H2-TPR profiles of fresh and aged Pd-supported catalysts are shown in Fig. 2b. From Fig. 2b, it can be seen that the H2-TPR profiles of all catalysts exhibit one new reduction peak at low temperature compared to those of the pure supports. According to the literature (Luo et al., 1999; Barrera et al., 2001; Sun et al., 2004; Yue et al., 2005; Barrera et al., 2005; Yue et al., 2006), the H2-TPR profiles of Pd/Ce-Zr-O catalyst show that the PdO species are reduced at 60-90ºC, Thus, for the fresh catalyst, the reduction peak below 100ºC may reflect the consumption of hydrogen by the reduction of PdO. Kaspar et al. (Kaspar et al., 2003b) reported that the active phase of Pd three-way catalyst is PdO. The transformation from PdO to Pd determines the catalytic performance of the catalysts. The lower reduction temperature of PdO can give the higher catalyst activity. The reduction peak temperatures are 73 and 88ºC for fresh Pd/CZPA-S and Pd/CZPA-P, respectively. Therefore, Pd/CZPA-S exhibits the better reduction capacity and catalytic activity. In addition, the reduction peaks above 400ºC, ascribed to the reduction of surface and bulk Ce4+, are still evident, but shift to lower temperature compared with those of pure supports, which can be attributed to Pd-promoted reduction of CeO2 via spillover of hydrogen (Kaspar et al., 2003b; Hickey et al., 2001). After aging, the reduction peaks of PdO shift to higher temperature, indicating that the transformation from PdO to Pd became more difficult, which possibly resulted from sintering of the active components. Moreover, the intensity of the reduction peak in the low-temperature region is weakened for aged catalysts, implying that Pd was partly sintered after aging (Wang et al, 2008). The aged Pd/CZPA-S was reduced at a lower temperature than aged Pd/CZPA-P, and the change in reduction temperature of Pd/CZPA-S is smaller before and after aging. The results indicate that Pd/CZPA-S has a stronger reduction capacity and higher thermal stability than Pd/CZPA-P.
The top views of (Ce, Zr, Pr)O2-doped alumina coatings on monolith channels prepared by the two methods are presented in Fig. 3, where the details of the coating microstructure can be distinguished. First of all, the coating morphology was observed at low magnification (×50). The surface of CZPA-S is smooth and evenly distributed. However, a large number of tiny particles were obviously distributed on the surface of CZPA-P. This indicates that the preparation methods directly affect the surface morphology of the coatings. In order to observe better the microstructure of the coating, the photograph was magnified (×2000). The surface morphology of CZPA-P shows that the particles are aggregated in different sizes and geometries, less uniformly distributed on the substrate. These particles are mainly formed by combination between (Ce, Zr, Pr)O2 powder particles and boehmite sol. However, compared with CZPA-P, CZPA-S shows a smooth layer. The reason is probably that CZPA-S is prepared by impregnating the cordierite ceramic honeycomb in the homogeneous sol phase.
In order to investigate the effect of coating supports on three-way catalysts, the catalytic activities of fresh and aged Pd-supported catalysts were evaluated. Fig. 4 presents the conversion of CO, C3H8 and NO over Pd/CZPA-S and Pd/CZPA-P catalysts. The light-off temperatures of the two catalysts are listed in Table 2. The light-off temperature of fresh Pd/CZPA-S catalyst is 180ºC for CO, 195ºC for C3H8, and 210ºC for NO. The light-off temperature of fresh Pd/CZPA-P catalyst is 205ºC for CO, 225ºC for C3H8, and 234ºC for NO. As reported by Cai et al. (2009), fresh Pd-only catalysts series supported on CeO2-ZrO2-BaO exhibit light-off temperatures in the range of 200-228ºC for CO, 265-286ºC for C3H8, 218-233ºC for NO, which are higher than those of Pd/CZPA-S. However, they used a Pd-loading of 2 g/L, i.e. two times higher than that used in this work. The results show that Pd-only catalysts supported on a nanophase (Ce, Zr, Pr)O2-doped alumina coating exhibit higher catalytic activity for CO and C3H8 oxidation and NO reduction. After thermal treatment, the light-off temperatures of Pd/CZPA-S and Pd/CZPA-P are 188ºC and 220ºC for CO, 217ºC and 253ºC for C3H8, 239ºC and 278ºC for NO, respectively. According to the report of Zhu et al. (1996), the conventional Pd/Al2O3 three-way catalyst contains a high quantity of precious metal (1.75 g/L), and the light-off temperature of aged catalyst is 287ºC for the three components CO, C3H8 and NO, which is higher than those of the catalysts used in this work. It indicates that Pd/CZPA-S and Pd/CZPA-P have high thermal stability, which is consistent with the results of XRD and H2-TPR. The conversion of NO is very important for TWC. Rh shows excellent activity to remove NO, but its high price and scarcity inhibit the development of TWC. In this work, the Pd-only catalysts supported on nanophase (Ce, Zr, Pr)O2doped alumina coatings show high activity of NO conversion. The light-off temperatures of fresh Pd/CZPA-S and Pd/CZPA-P are as low as 210 and 234ºC, respectively, which are similar to the Pt/Rh TWC (Suopankia et al., 2005).
Based on the above analysis, CZPA-S and CZPA-P are suitable for use as supports of TWC owing to their high thermal stability and catalytic activity. However, compared with Pd/CZPA-P catalyst, the fresh and aged Pd/CZPA-S have lower light-off temperatures for CO, C3H8 and NO, which indicates that the Pd/CZPA-S catalyst exhibits high thermal stability and three-way catalytic activity. The catalytic activities are linked to the performance of the coating supports. Considering the results of support characterizations, we suggest that the main possible reasons are: (i) the CZPA-S support exhibits smaller crystallite size and larger specific surface area than CZPA-P, which could increase the dispersion of Pd species and provide more of the necessary activity sites for catalytic reactions; the catalyst would then have higher catalytic activity (Kim et al., 2000; Kolli et al., 2005). (ii) The CZPA-S support shows higher oxygen storage capacity and reduction capacity than CZPA-P, which can supply enough oxygen transport in the course of the reaction. (iii) The CZPA-S support has higher thermal stability than CZPA-P, which could resist the sintering of the catalyst. Therefore, the CZPA-S coating support derived from sol is more suitable as a support for TWC.
The present investigations have confirmed that the preparation methods have important effects on the crystal phase, texture, reducibility, oxygen storage capacity, surface morphology and thermal stability of the nanophase (Ce, Zr, Pr)O2-doped alumina coating supports. In this work, the nanophase (Ce, Zr, Pr)O2doped alumina coatings prepared by the two methods have high thermal stability. The CZPA-S support demonstrates better texture, reducibility, oxygen storage capacity and thermal stability than the CZPA-P support. This is likely due to the greater interaction between the colloids at the sol stage. The Pd-only three-way catalyst derived from the sol shows low light-off temperature and high three-way catalytic activity, especially the excellent NO conversion and aging resistance.
This study was supported financially by the Natural Science Foundation of Jiangsu Education Committee (07KJB150112), the Preliminary Research of the National Natural Science Foundation of China (08XLY05), and the Natural Science Foundation of Xuzhou Normal University (07XLA06).
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(Submitted: January 4, 2011 ; Revised: April 16, 2011 ; Accepted: September 7, 2011)
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