Journal of Colloid and Interface Science 450 (2015) 404–416

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Preparation of ceria–zirconia by modified coprecipitation method and its supported Pd-only three-way catalyst Li Lan a, Shanhu Chen b, Yi Cao a, Ming Zhao a, Maochu Gong a, Yaoqiang Chen a,c,d,⇑ a

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Sichuan, Chengdu 610064, China Sichuan Zhongzi Exhaust Gas Cleaning Co., Ltd., Sichuan, Chengdu 611731, China c Center of Engineering of Vehicular Exhaust Gases Abatement, Sichuan Province, Sichuan, Chengdu 610064, China d Center of Engineering of Environmental Catalytic Material, Sichuan Province, Sichuan, Chengdu 610064, China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 2 November 2014 Accepted 23 March 2015 Available online 30 March 2015 Keywords: Modified coprecipitation method Mixed phase composition Interface sites Pd-only three-way catalysts

a b s t r a c t A CeO2–ZrO2 compound with mixed phase composition (CZ4) was prepared by modified co-precipitation method, and for comparison, single-phase Ce0.2Zr0.8O2, Ce0.5Zr0.5O2 and Ce0.8Zr0.2O2 were synthesized via simultaneous co-precipitation method. The textural, structural and redox properties, together with the catalytic performance of the supported Pd-only three-way catalysts were investigated systematically. The results revealed that the generation of numerous interface sites in Pd/CZ4 due to its mixed phase composition (as confirmed by TEM observation) had a positive influence on modifying its structural, redox properties and thermal stability. The XRD and Raman results revealed that the highest structural stability was obtained by Pd/CZ4 with negligible lattice variation and slightest grain growth after aging treatment. The XPS analysis demonstrated that the compositional heterogeneity of Pd/CZ4 could facilitate the formation of Ce3+, and was beneficial to preserve high dispersion of Pd as well as maintain Pd at a more oxidized state. The H2-TPR and oxygen storage capacity measurements indicated that Pd/CZ4 possessed highest reduction ability as well as largest oxygen storage capacity regardless of thermal aging treatment. And consequently Pd/CZ4 exhibited improved three-way catalytic activity compared with the catalysts supported on single-phase CexZr1xO2 both before and after thermal aging treatment. Ó 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: College of Chemistry, Sichuan University, Wangjiang Road 29, Sichuan, Chengdu 610064, China. Fax: +86 28 85418451. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.jcis.2015.03.042 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

L. Lan et al. / Journal of Colloid and Interface Science 450 (2015) 404–416

1. Introduction The regulations concerning automotive emissions are becoming more and more stringent due to the serious air pollution caused by the emission from gasoline engine powered vehicles [1], thus the development of automotive emission control technology is still of great environmental and practical importance [2]. And the socalled three-way catalyst (TWC) is considered to be highly efficient for its ability to simultaneously convert hydrocarbons (HC), CO and NOx in the automotive exhaust to environmentally friendly CO2, H2O, and N2 [3,4]. The conventional TWCs employed in the early stage were mostly composed of platinum group metals (Pt, Pd, Rh) as active components and CeO2 as an oxygen storage material [5]. Generally speaking, the highest conversions of the contaminations are obtained when the air-to-fuel ratio (A/F) is close to the stoichiometric value, that is, A/F = 14.6, while fluctuations to rich or lean conditions would result in severe deactivation of the catalysts, and consequently the usage of the TWC is limited [5]. CeO2 is declared to play a key role in compensating the fluctuations in the exhaust stream on account of its ability to store/release oxygen under the lean/rich conditions and consequently the degradation of the conversion efficiency can be suppressed [5–7]. In addition, CeO2 is also a good stabilizer for noble metals against sintering, and it could promote the CO oxidation process through the water–gas shift reaction [8–10]. The catalytic performance is more desirable when Zr is introduced into the CeO2-based system, because the formation of CeO2–ZrO2 solid solutions could facilitate the generation of oxygen vacancies and thus significantly improve the reducibility of Ce4+ [10,11], in the meantime increased resistance to sintering under high temperature treatment is achieved [9,12,13]. Previous researchers have demonstrated that the phase compositions of CeO2–ZrO2 are strongly dependent on the content of cerium [8,12,14], the crystalline structure of CeO2–ZrO2 may undergo a change from the monoclinic form of ZrO2 to the metastable tetragonal phase and then to the fluorite cubic form of CeO2 with increasing content of CeO2 [14]. And on the other hand, the treating conditions, e.g., temperatures, atmospheres, etc., could lead to various phase transformations [8,12,15]. As reported by Colón etc. [15], CeO2–ZrO2 mixed oxide with intermediate Ce content exhibits higher thermal stability from the surface area and crystallite size points of view, but at the same time, phase segregation is most notable for CexZr1xO2 when x approaches 0.5, fortunately, the phase separation process could in turn modify the redox property of the mixed oxide. Previous studies [8,12] have predicted that phase separation process takes place for CexZr1xO2 with intermediate compositions when submitted to high-temperature treatment, thus a mixture of cubic and tetragonal phases is obtained. It has been also proposed that the phase segregation process may be considered as surface energy driven, during the thermal aging process, aggregation of the primary particles takes place, leading to increasing crystallite size and when the crystallite size exceeds a critical value, the contribution of the surface energy is not large enough to stabilize the existed phase, and consequently phase segregation occurs to form Cericher and Zr-richer phases with higher stability. As a result, new interfacial boundaries between the crystallites with different phase compositions are formed, therefore, crystallites with the same composition can hardly contact with each other. Generally, it is considered that the growth of CexZr1xO2 nano-particle is induced by contact of the primary particles [16]. Therefore, the presence of abundant interface can effectively inhibit the grain growth process. In addition, it is generally accepted that the redox property of CexZr1xO2 is strongly relied on the structural variation [17], and the high efficiency of oxygen mobility near the interfacial regions

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is declared to make an enormous contribution to the low-temperature redox property [17,18]. Accordingly, it is rational to assume that if an initial mixture of cubic and tetragonal phases exists in an as-prepared CexZr1xO2 composite, the small crystallites with large interfacial areas may present fascinating thermal stability and preserve superior redox property under thermal aging treatment compared with the single-phase CexZr1xO2 mixed oxides. As per literatures reported, for Ce0.5Zr0.5O2, phase separation occurs upon heating treatment and two new phases with compositions approximately as Ce0.2Zr0.8O2 and Ce0.8Zr0.2O2 are typically obtained [12,15,16]. In this work, a CeO2–ZrO2 compound with initially mixed phase composition and three single-phase CexZr1xO2 mixed oxides (Ce0.2Zr0.8O2, Ce0.5Zr0.5O2 and Ce0.8Zr0.2O2) were prepared and used as the supports for Pd-only three-way catalysts. The influence of the phase composition on the textural, structural and redox properties, along with the catalytic performance of the supported catalysts were investigated systematically. And consequently a superior Pd/CexZr1xO2 catalyst was developed. 2. Experimental method 2.1. Synthesis of CeO2–ZrO2 composites Three CexZr1xO2 mixed oxides (x = 0.2, 0.5, 0.8) were prepared by simultaneous co-precipitation method. And a simplified schematic showing the precipitation process of our samples could be found in Fig. S1. First, a mixed salt solution was obtained by quantitatively dissolving the corresponding nitrates Ce(NO3)36H2O and ZrO(NO3)2 in water. Then, the aqueous solution was co-precipitated with an ammonia solution (25 wt%) under vigorous stirring and during the process the pH was controlled at around 10. The resulting precipitate was subsequently filtered and washed with distilled water, followed by drying at 120 °C overnight and calcining at 600 °C in air for 3 h to finally get the CexZr1xO2 materials, which was hereafter referred to as CZ1, CZ2 and CZ3, respectively. A CeO2–ZrO2 compound with mixed phase composition was prepared by the modified co-precipitation method, which was firstly used in our previous work [19]. Two salt solutions with different molar ratios, i.e., Ce/Zr = 4:1 and Ce/Zr = 1:4, were obtained by quantitatively dissolving the corresponding nitrates Ce(NO3)36H2O and ZrO(NO3)2 in water, then these two solutions with the appropriate proportion to ensure the final Ce/Zr = 1:1 were simultaneously precipitated with an ammonia solution (25 wt%) under intense agitation and during the process the pH was kept at around 10. The resulting precipitate was treated in the same way as above-mentioned, and was designated as CZ4. The aged materials assigned as CZxa (x = 1, 2, 3, 4) were obtained by thermal treating the corresponding fresh samples in air at 1000 °C for 5 h. 2.2. Catalyst preparation The corresponding Pd/CZx (x = 1, 2, 3 and 4) catalysts were prepared by conventional impregnation method using aqueous solutions of Pd(NO3)2 as metal precursors, and the theoretical loading contents of Pd were 0.5 wt%. Firstly the four support materials CZx were wet-impregnated with Pd(NO3)2 aqueous solutions, respectively. Then the impregnated samples were dried at 120 °C for 2 h and subsequently calcined at 550 °C for 3 h. After that the powders were mixed with a certain amount of water to get slurries with the solid content of 50 wt%, which were then coated onto cylindrical cordierite monoliths (Corning, USA, 400 cells/in.2, 2.5 cm3). After drying at 120 °C for 2 h, the samples were then calcined at 550 °C for 3 h and eventually the fresh catalysts were obtained. In order to estimate the thermal stability of the catalysts,

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aging treatment for the fresh catalysts was performed in static air at 1000 °C for 5 h, and the corresponding aged catalysts were henceforward labeled as Pd/CZxa. 2.3. Catalytic activity tests The three-way catalytic performance was evaluated in a fixedbed continuous flow reactor. The simulated reaction mixture were composed of CO (0.6%), C3H8 (0.03%), NO (0.06%), CO2 (12%), H2O (10%), O2 (adjustable) and N2 was used as the balance gas, the feed stream was regulated using special mass flow controllers before entering the reactor and the GHSV (gas hourly space velocity) was controlled at 38,000 h1 during the testing process. The effluent CO, C3H8, NOx (NO, NO2) and N2O were analyzed by an FT-IR (Antaris IGS-6700, Nicolet, Thermo Fisher Scientific). The catalyst was pretreated under the reaction condition at 550 °C for 1 h prior to the measurement. The k value of the simulated gas mixture, which represents the ratio between available oxygen and oxygen needed for full conversion to CO2, H2O and N2, is defined as k = {2[O2] + [NO]}/{10[C3H8] + [CO]}, and k = 1 which was at stoichiometry was utilized during the catalytic activity measurements. 2.4. Characterization techniques The Powder X-ray diffraction (XRD) experiment was performed on a Rigaku DX-2500 diffractometer employing Cu Ka (k = 0.15406 nm) as radiation. The X-ray tube was operated at 40 kV and 100 mA. The spectra with the range of 10° 6 2h 6 80° were acquired at an interval of 0.03°. The Raman spectra were obtained by a LabRAM HR Laser Raman spectrometer (HORIBA JOBIN YVON, France) with a diode pumped YAG laser of 532 nm excitation wavelength, and the laser power was 73.5 mW. The spectra acquisition consisted of 3 accumulations of 30 s for each sample and the resolution was 0.65 cm1. A frequency range of 100–1000 cm1 was recorded. The textural characteristics were estimated using a Quantachrome automated surface area & pore size analyzer (Autosorb SI) and N2 adsorption/desorption at 77 K was used for the measurement, the specific surface areas of the samples were calculated using the Brunauer–Emmett–Teller (BET) method. The samples were degassed at 300 °C for 3 h under vacuum prior to the measurement.

Transmission electron microscopy (TEM) characterization of the samples was performed using a Tecnai G2 F20 S-TWIN TEM (FEI Company, USA, 200 kV accelerating voltage) equipped with STEM, EDX and HAADF. Prior to the experiment, the powder specimens were suspended in ethanol and dispersed by ultrasonic, after that a droplet was deposited on a perforated carbon film supported by a copper grid. To obtain the composition profiles, the measurement was operated in scanning mode (STEM), and EDX line scan analysis was conducted. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a British Kratos XSAM-800 electron spectrometer using Mg Ka (1253.6 eV) as the radiation, the experiment was operated at 13 kV and 20 mA. The XPS data associated with Ce 3d, Zr 3d, O 1s and Pd 3d core levels were collected for all samples. The binding energies of the elements were calibrated using the C 1s peak at 284.6 eV as the internal standard reference. The reduction features of the catalysts were investigated by hydrogen-temperature programmed reduction (H2-TPR), and the measurement was carried out on a self-assembled system equipped with a gas chromatograph. The pretreatment process consisted of heating the sample (100 mg) with a flow of N2 (25 mL min1) from room temperature to 450 °C, holding for 1 h, and cooling down to room temperature. Afterwards the sample was heated under flowing H2 (5 vol%) in N2 (20 mL min1) at a heating rate of 5 °C min1. The obtained TCD signal was used to estimate the consumption of H2, and CuO was used as a standard to quantify the consumption of H2. OSCC (Oxygen storage capacity complete) of the catalysts was measured by pulse injection technique. The sample was pretreated in H2 (30 mL min1) at 550 °C for 1 h and then cooled to the measurement temperature under N2 flow, after that an oxygen pulse was injected every 5 min to obtain a breakthrough curve, from which the OSCC was calculated. 3. Results and discussion 3.1. XRD and BET results The XRD spectra of the fresh and aged catalysts are shown in Fig. 1(a) and (b), respectively. It can be seen from Fig. 1(a) that the fresh samples crystallize as cubic fluorite-like CeO2 phase, and no peak splitting associated with a second ZrO2 phase is

Fig. 1. X-ray diffraction patterns for (a) fresh and (b) aged catalysts. For comparison’s sake, legends represent the relative intensities of Y-axes are plotted.

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observed, indicating the formation of CexZr1xO2 solid solutions. However, what should be pointed out is that the reflection of Pd/CZ4 is quite broad and somewhat asymmetric, thus it is highly reasonable to speculate that there may exist a mixture of phases in the crystallite [8]. It is also noticed that no extra peaks attributed to Pd or PdO are detected, which is coincided with the small Pd loading. After thermal aging treatment, all diffraction patterns experience a significant peak sharpening owing to the grain growth during the aging process, meanwhile, a visible Zr-rich tetragonal phase can be identified for Pd/CZ2a and Pd/CZ4a, indicating the phase segregation caused by severe sintering. In order to achieve some insight into the structural differences, the amplifying diffraction patterns with a narrower range of 2h (27–31°) are presented in Fig. 2, and Table 1 summarizes the crystalline phases, lattice parameters, and crystallite sizes of the fresh and aged catalysts. It can be seen from Fig. 2(a) that for catalysts supported on single-phase CexZr1xO2, the peak position corresponding to the (1 1 1) crystallographic plane of the cubic phase shifts to lower value with increasing content of Ce, which is in good agreement with the increasing lattice parameters. When it comes to Pd/CZ4, the principal diffraction peak occurs at 2h = 28.88°, which is in good correspondence with the diffraction peak of Ce0.75Zr0.25O2 (PDF28-0271), and the peak is obviously asymmetric on the high-angle side, implying that a Zr-rich phase may coexist in this sample. After thermal aging treatment at 1000 °C, noticeable differences among the four samples can be observed. Notice that the diffraction peak of Pd/CZ1a shifts to higher position and the pattern resembles that of tetragonal ZrO2, which hints us that surface segregation of Ce happens upon aging, taking into account the comparatively high Zr content in this sample, it is not surprising [20]. And the crystalline phase of Pd/CZ3a remains almost unchanged compared with that of fresh Pd/CZ3. In addition, a mixture of Ce-rich cubic phase and Zr-rich tetragonal phase can be identified for Pd/CZ2a and Pd/CZ4a, indicating that phase segregation has taken place during the aging process. It is worthwhile to point out that the peak position corresponding to the (1 1 1) crystallographic plane of the cubic phase for Pd/CZ2a shifts to lower value (Fig. 2(b)), coinciding with the larger cell parameter compared with that of fresh Pd/CZ2 owing to segregation of Zr4+ ions during the aging process, which is confirmed by the existence of Zr-rich tetragonal phase in Pd/

Table 1 The crystalline phases, lattice parameters, crystallite sizes and BET surface areas of the fresh and aged catalysts.

a

Samples

Phase composition

Lattice parameter (nm)

Crystallite size (nm)

Surface area (m2 g1)

Pd/CZ1 Pd/CZ2 Pd/CZ3 Pd/CZ4 Pd/CZ1a Pd/CZ2a

Cubic Cubic Cubic Cubic Tetragonal Cubic tetragonal

6.8 6.1 6.5 5.2 – 17.0a

60 90 107 96 7 12

Pd/CZ3a Pd/CZ4a

Cubic Cubic tetragonal

0.5241 0.5266 0.5369 0.5351 – 0.5364 a = 0.3648; c = 0.5245 0.5367 0.5353 a = 0.3667; c = 0.5271

28.9 10.8a

4 16

Crystallite size based on the cubic phase.

CZ2a. By comparison of the peak positions to the standard curves originated from the diffraction data of the known phases, the two new phases of Pd/CZ2a can be approximately assigned as Ce0.8Zr0.2O2 and Ce0.2Zr0.8O2, respectively, which is in good agreement with the results in the literatures [12,15,16]. However, it is also noticed that the peak position corresponding to the (1 1 1) crystallographic plane for Pd/CZ4a remains almost unchanged after aging treatment, implying that the phase composition of this sample is not sensitive to the thermal aging treatment, and the crystalline phase of Pd/CZ4a appears to be a mixture of Ce0.75Zr0.25O2 and Ce0.25Zr0.75O2. As is well known, Vegard’s rule has been extensively applied to assess the presence of solid solutions [21]. To further clarify the phase compositions of the fresh samples, the corresponding theoretical lattice parameters were calculated based on Vegard’s rule. According to the literature [22], for CeO2-based solid solution, Vegard’s rule can be expressed as follows:

a ¼ 5:411 þ

X ð0:0220Dr k þ 0:0015Dzk Þmk

where 4rk represents the difference in ionic radius, 4rk = rk  rCe, 4zk represents the difference in valence state, 4zk = zk  zCe, and mk stands for the molar percentage of the dopant k, while 5.411 is the standard lattice parameter of pure CeO2. For comparison, the

Fig. 2. The amplifying diffraction patterns for (a) fresh and (b) aged catalysts over narrow range of 2h. The dotted lines correspond to diffraction positions of (1) CeO2; (2) Ce0.75Zr0.25O2; (3) Ce0.6Zr0.4O2; (4) Ce0.5Zr0.5O2 and (5) Ce0.18Zr0.82O2. For comparison’s sake, legends represent the relative intensities of Y-axes are plotted.

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calculated theoretical lattice parameters as well as the experimental values are plotted in Fig. 3. As shown in Fig. 3, the experimental lattice parameter of Pd/CZ1 is obviously larger than the theoretical value, which coincides with its cubic structure that is Ce-richer than expected. Note that the experimental lattice parameters of Pd/CZ2 and Pd/CZ3 are consistent with and approximate to the theoretical values, respectively, demonstrating the formation of homogeneous CZ solid solutions for these two samples. However, particular attention should be paid to Pd/CZ4, the experimental lattice parameter of which shows conspicuous deviation from the theoretical value, which may be attributed to the coexistence of different phases in this sample as discussed above, coinciding with the declaration in the literature that the presence of nanoscale compositional heterogeneity would lead to deviations from Vegard’s rule [21]. It has been proposed in previous studies [8,12] that the phase separation process may be regarded as surface energy driven, the solid solution is stable before the crystallite size exceeds a critical value, above which the surface energy is too small compared with the total energy of the system, thus the existed phase becomes unstable, and phase segregation occurs to form more stable Ce-richer and Zr-richer phases. As a result, new interfacial boundaries between the crystallites with different phase compositions are formed, therefore, crystallites with the same composition seem difficult to contact with each other and consequently the aggregation process is effectively suppressed. The crystallite sizes shown in Table 1 reveal that owing to the phase separation process, Pd/CZ2a maintains relatively smaller crystallite size, whereas the grain growth of Pd/CZ1a and Pd/CZ3a is outstanding. For Pd/CZ4, the initial compositional heterogeneity has a pronounced effect on retarding the grain growth upon aging treatment, and thus Pd/CZ4a displays the smallest crystallite size among the four aged samples. The specific surface areas of the catalysts before and after aging are also listed in Table 1. Notice that the surface areas of the fresh samples decrease in the order: Pd/CZ3 > Pd/CZ4  Pd/CZ2 > Pd/CZ1, indicating that the surface area is predominated by the Ce/Zr ratio of the samples and higher Ce/Zr ratio gives rise to larger specific surface area. In general, higher BET surface area of the support is advantageous to the dispersion of the loaded Pd species, which can in result facilitate the catalytic reaction process. During thermal aging treatment, all samples undergo a severe sintering process and a remarkable loss of specific surface area for the aged samples is observed. As claimed in the literature [23], aggregation of the support often leads to the encapsulation of PdOx species, and the extent of encapsulation of PdOx is approximately

inversely proportional to the surface area value. Thus, the extremely low surface areas of Pd/CZ1a (7 m2 g1) and Pd/CZ3a (4 m2 g1) tend to result in more severe encapsulation of PdOx species and consequently only very low dispersion of PdOx can be retained, which coincides with their inferior catalytic performance. And the relatively higher surface areas of Pd/CZ2a (12 m2 g1) and Pd/CZ4a (16 m2 g1) can retard the encapsulation process to some extent, which is beneficial to maintain relatively higher dispersion of PdOx and thus enhanced catalytic performance can be preserved. 3.2. Raman characterization To achieve some deep insight into the fine structural details, the Raman spectra of the fresh and aged catalysts are presented in Fig. 4. Generally, according to the literatures, the band located at around 465 cm1 is associated with the F2g mode of cubic fluorite CeO2, which can be viewed as the symmetric breathing mode of oxygen ions surrounded by cations, and the Raman spectrum of tetragonal ZrO2 phase mainly features six bands centered at 147, 268, 313, 460, 600 and 645 cm1 [24,25]. Accordingly, for the fresh samples, the prominent peak at around 465 cm1 can be attributed to the F2g vibration mode of cubic CeO2, and the shift of Raman frequency to higher wave numbers compared with that of pure CeO2 (465 cm1) is caused by the introduction of Zr into the lattice of CeO2. As can be seen in Fig. 4(a), for Pd/CZ1–Pd/CZ3, with decreasing Zr content in the system, the Raman frequency gradually shifts to lower wave numbers, which agrees well with the increasing lattice parameters as displayed in the above Table 1. With respect to Pd/CZ4, the overall Ce/Zr ratio of which is the same as that of Pd/CZ2, the lower wave number compared with that of Pd/CZ2 indicates that the cubic phase of Pd/CZ4 is Ce-richer, as also evidenced by the XRD results. In addition, for Pd/CZ1–Pd/CZ3, besides the main peak centered at around 465 cm1, another two peaks centered at around 304 and 612 cm1 are observed which can be assigned to the existence of defective structure [25]. Whereas in the spectrum of Pd/CZ4, the additional band appeared at 243 cm1 is indicative of the presence of tetragonal phase. Therefore, it confirms the speculation that the phase composition of Pd/CZ4 is a mixture of cubic and tetragonal phases. After aging treatment, only tetragonal features can be observed in Raman spectrum of Pd/CZ1a, and the Raman spectrum of Pd/CZ3a resembles that of fresh Pd/CZ3. Whereas for Pd/CZ2a and Pd/CZ4a, besides the main peak corresponding to cubic CeO2, four bands centered at around 130, 243, 300 and 611 cm1 can be observed for both samples, which further confirms the coexistence of cubic and tetragonal phases in Pd/CZ2a and Pd/CZ4a, and agrees well with the XRD results. Furthermore, what should be pointed out is that the position of the main peak of Pd/CZ4a (468 cm1) remains almost unaltered, indicating that the phase composition of this sample is insusceptible to the thermal aging process, which coincides with the XRD observations, while the peak position of Pd/CZ2a shifts from 472 cm1 to 466 cm1, which is caused by the segregation of Zr ions out from the cubic phase as mentioned above. It is also noticed that the intensities of the peaks associated with the cubic fluorite phase become much stronger than those of the fresh samples, indicating the significant grain growth caused by severe sintering during the aging process. And the weakest intensity of Pd/CZ4a is in good consistency with its smallest crystallite size as listed in Table 1. 3.3. TEM observations

Fig. 3. Comparison of the experimental (s) and theoretical (j) lattice parameters (based on Vegard’s rule) of Pd/CZx catalysts.

The HAADF-STEM images and the corresponding line scan analysis results of Pd/CZ2 and Pd/CZ4 are displayed in Figs. 5 and 6, respectively. From the line scan profile in Fig. 5(b), it can be found that the Zr/Ce ratios along the scanning line show relatively

L. Lan et al. / Journal of Colloid and Interface Science 450 (2015) 404–416

409

Fig. 4. Raman spectra of (a) fresh and (b) aged Pd/CZx catalysts. For comparison’s sake, legends represent the relative intensities of Y-axes are plotted.

constant values with only slight variations, and the values approach the theoretical Zr/Ce ratio 1, which further confirms the formation of CZ solid solution for Pd/CZ2. However, in the case of Pd/CZ4, the EDX result manifests that the phase composition is significantly different from that of Pd/CZ2. As shown in Fig. 6(a) and (b), compositional line scan across three adjacent individual nanoparticles is obtained, and the result shows that the first grain has an average Zr/Ce ratio of around 0.36, the composition of which is almost the same with that of the third grain (the average Zr/Ce ratio is around 0.35), whereas the average Zr/Ce ratio of the middle grain turns out to be around 3.13, indicating that the middle grain is obviously richer in Zr and the other two grains are apparently Ce-richer. Considering that the average composition of the sample is supposed to be Zr/Ce = 1/1, the EDX line scan analysis clearly demonstrates that substantial variation in the composition exists among different nanocrystals in Pd/CZ4, and the composition of this sample approaches to be a mixture of Ce0.75Zr0.25O2 and Ce0.25Zr0.75O2, which is in good correspondence with the XRD observations. As a result, the compositional heterogeneity of Pd/CZ4 would consequently create large amount of interface sites between crystallites with different compositions [17] which is beneficial to improve its redox property as well as the thermal stability. 3.4. XPS studies XPS investigation is performed to get information of the surface elemental distribution and states of the elements. As presented in Fig. 7, the spectra of Ce 3d are composed of eight peaks, among which those marked as ‘‘u’’ arise from 3d3/2, and the others assigned as ‘‘v’’ are attributed to 3d5/2. The couples labeled as u’ and v’ are corresponded to Ce3+, while the others are all attributed to the diversified states of Ce4+ [26]. The proportion of Ce3+ with regard to the total cerium species is calculated based on the ratio of the sum of peak areas of Ce3+ to the sum of peak areas of the total cerium species, and the results, together with the surface elemental concentrations of Ce, Zr, Pd, O and the corresponding Ce/Zr ratios, are compiled in Table 2. As can be seen in Table 2, the Ce/Zr ratio of Pd/CZ1 is higher than the theoretical value, coinciding with the above XRD and

Raman results that Pd/CZ1 is Ce-richer than expected. And the Ce/Zr ratios of Pd/CZ2 and Pd/CZ3 approach the theoretical values of 1.0 and 4.0, suggesting the formation of homogeneous CZ solid solutions for these two samples. However, for Pd/CZ4, the Ce/Zr ratio is much smaller though its academic value is the same with that of Pd/CZ2, which means that the outer part of this sample is richer in Zr, considering the fact that the crystalline phase of Pd/CZ4 is a mixture of Ce-rich cubic phase and Zr-rich tetragonal phase as proved by the above TEM analysis result, it is understandable. For CZ system, after aging treatment, the migration of Zr from the bulk to the surface to form a so-called ‘‘core–shell’’ structure with a Zr-richer phase concentrated in the outer sphere of the particles has been observed by many researchers [22,27]. In our work, Pd/CZ2a shows a consistent result with the literatures as certified by the decrease of the Ce/Zr ratio. Whereas Pd/CZ1a exhibits an opposite variation, confirming the surface segregation of Ce as proposed in the XRD section. In addition, it is also noticed that the Ce/Zr ratios of Pd/CZ3a and Pd/CZ4a display only slight variations compared with those of fresh Pd/CZ3 and Pd/CZ4, demonstrating their excellent phase stability. It has been reported in the literatures [28,29] that the presence of Ce3+ can strongly affect the formation of oxygen vacancies. As listed in Table 2, the relative concentration of Ce3+ follows a decreasing tendency of Pd/CZ4 > Pd/CZ2 > Pd/CZ3 > Pd/CZ1, therefore, it seems that the phase heterogeneity of Pd/CZ4 is beneficial to the reduction of Ce4+ to Ce3+ and consequently more oxygen vacancies are formed. After thermal aging treatment, an obvious decrease of the concentration of Ce3+ can be observed for all samples, which is probably caused by the oxidation of Ce3+ to Ce4+ by the PdOx species or the aging treatment in air [27]. It is found that the relative content of Ce3+ for the four aged samples shows the same sequence with that of the fresh samples, declaring that largest amount of oxygen vacancies can be preserved for Pd/CZ4a, which can effectively promote the mobility of oxygen ions and consequently modified redox property is obtained, and the oxygen vacancies are seriously destroyed for Pd/CZ1a and Pd/CZ3a. Furthermore, as demonstrated in the literatures [30,31], for the supported CeO2-based catalysts, the oxygen vacancies associated with the Ce3+ species adjacent to the noble metal particles are considered to be active sites for NO activation, thereby the highest

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Fig. 5. STEM and EDX line scan profile of Pd/CZ2, (a) HAADF-STEM image, (b) Ce/Zr ratio.

Fig. 6. STEM and EDX line scan profile of Pd/CZ4, (a) HAADF-STEM image, (b) Ce/Zr ratio.

concentration of Ce3+ for Pd/CZ4 could contribute to the enhanced NO catalytic activity. Particular attention should be paid to the surface concentration of Pd since the dispersion of Pd species is closely related to the catalytic performance of the catalysts. As revealed in Table 2, the surface concentration of Pd shows a decreasing trend of Pd/CZ3 > Pd/ CZ2 P Pd/CZ4 > Pd/CZ1 , which coincides with the sequence of the BET surface areas of the fresh samples, demonstrating the strong dependence of the dispersion of noble metal species on the surface area. The surface concentration of Pd shows a significant decrease after aging for Pd/CZ1a and Pd/CZ3a, taking into account the extremely low surface areas of these two samples, it is rational to infer that more severe sintering of PdOx on the surface and serious encapsulation of PdOx species have taken place. For Pd/CZ4a, owing to its highest surface area (16 m2 g1) and structural stability among the four aged samples, the encapsulation process can be inhibited to some extent, and the relative increase of the surface concentration of Pd may be interpreted as the inevitable increase of the areal density of Pd on the surface of the support due to the decline of the BET surface area during the aging process [27,32]. In addition, with the surface area in-between, the surface concentration of Pd remains almost unchanged for Pd/CZ2a. To approach some useful insights into the chemical state of surface Pd, the corresponding Pd 3d spectra are depicted in Fig. 8.

According to the literatures, the binding energies within the range of 340.8–342.2 eV and 335.8–336.8 eV are ascribed to the peaks of Pd 3d3/2 and Pd 3d5/2, respectively [33,34]. For fresh catalysts, the Pd 3d spectra exhibit two similar peaks centered at 342.2 and 336.8 eV, implying that Pd species in the fresh catalysts are mainly present in the oxidized Pd2+ states. After aging treatment, two additional peaks appear at ca. 340.4 and 335.2 eV, which are associated with Pd0 3d3/2 and Pd0 3d5/2, respectively, indicating the partial reduction of Pd2+ during the aging process. The ratio of oxidized Pd2+ (Pd2+/(Pd2+ + Pd0)) can be estimated from the peak areas of Pd2+ and Pd0 based on the Pd 3d5/2 core level, and the results which are also listed in Table 2 demonstrate that for Pd/CZ1a, most of the Pd2+ species are reduced during the aging process, and the ratio of Pd2+ for Pd/CZ4a (0.53) is the largest, implying that more Pd species are maintained at oxidized state for Pd/CZ4a, which can be probably owing to the stronger interaction between Pd and CZ in this sample, since the Pd–CZ interaction could lead to the electron transfer from Pd to CZ to maintain the Pd species at a more oxidized state [33,34]. 3.5. H2-TPR studies To investigate the reduction features of the CZx supports and the corresponding Pd/CZx catalysts, H2-TPR experiment is

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Fig. 7. Ce 3d XPS spectra for (a) fresh and (b) aged Pd/CZx catalysts.

Table 2 Surface elemental composition derived from XPS analyses. Samples

Pd/CZ1 Pd/CZ2 Pd/CZ3 Pd/CZ4 Pd/CZ1a Pd/CZ2a Pd/CZ3a Pd/CZ4a

Surface atomic concentration (%) Ce 3d

Zr 3d

Pd 3d

O 1s

5.91 13.81 15.09 6.38 7.05 8.66 13.76 8.52

13.76 14.12 3.89 12.47 13.06 13.24 3.39 14.31

0.37 0.58 0.76 0.53 0.19 0.56 0.46 0.79

79.96 71.49 80.26 80.62 79.70 77.53 82.39 76.38

Ce/Zr ratio

Ce3+ in Ce (%)

Pd2+/ (Pd2+ + Pd0)

0.4295 0.9780 3.8792 0.5116 0.5398 0.6541 4.0590 0.5954

14.79 16.02 15.21 18.18 10.54 13.21 11.77 14.82

1.00 1.00 1.00 1.00 0.13 0.41 0.43 0.53

performed and the results are reported in Figs. 9 and 10, respectively. Furthermore, the values of H2 consumption are calculated using CuO as a standard and the results are summarized in Tables 3 and 4, respectively. From Fig. 9(a), we can see that for single-phase CZ1–CZ3, the profiles feature only one reduction peak with an asymmetric shape, which hints us that the peak may be an overlap of the reduction characteristics of surface and bulk Ce4+ species. And among the three samples, CZ2 with the medium Ce/Zr ratio displays the best reduction ability on account of its lowest reduction temperature as well as the largest H2 consumption (1163 lmol/g). However, the reduction feature of CZ4 is fundamentally different from those of the other three samples, which shows two obvious reduction peaks, considering the mixed phase composition of CZ4 (as certified by XRD, Raman, TEM and XPS measurements), it is understandable due to the coexistence of various oxygen species with different reducibility in this sample, and the H2 consumption of CZ4 is even larger than that of CZ2. In addition, as it can be found in Table 3, the theoretical H2 consumption of the four samples which is obtained assuming that all Ce species are in oxidized state (Ce4+) before H2-TPR test and can be totally reduced to Ce3+ is also listed in Table 3, and the results show that CZ4 owns the largest reduction ratio among the four samples.

After aging, all samples show declined reduction abilities owing to the aggregation of the particles and the decreased amount of labile oxygen species. The shapes of the reduction features of CZ1a and CZ3a display little changes compared with those of fresh CZ1 and CZ3, only higher reduction temperatures accompanied by lower H2 consumption can be observed. However, the reduction profile of CZ2a changes to a bi-modal shape which is related to the variation of phase composition in CZ2a, as proved by the XRD and Raman results. It must be noted that CZ4a exhibits the lowest reduction temperature as well as the highest H2 consumption (553 lmol/g) among the four aged samples, that is, the reduction ability of CZ4a is preserved to the largest extent, demonstrating that the initial phase heterogeneity presented in CZ4 can suppress the aggregation process and largest amount of labile oxygen ions are preserved. When Pd is loaded on the CZ supports, the reduction characteristics become significantly different. In contrast to blank supports, the reduction of Pd-loaded catalysts occurs at relatively lower temperatures, i.e., below 200 °C [35,36]. As displayed in Fig. 10(a), Pd/ CZ1, Pd/CZ2 and Pd/CZ3 exhibit unimodal reduction features in the low temperature region, while the low-temperature reduction peak of Pd/CZ4 is obviously asymmetric and a distinct shoulder can be observed, which is in accordance with its phase heterogeneity. And the low temperature reduction feature (peak a) can be rationally attributed to the reduction of PdO accompanied by the reduction of a fraction of CZ [11,32], as verified by the observation that the H2 consumption below 200 °C is significantly larger than the theoretical H2 consumption of single PdO species (47 lmol/ gcat), which suggests that the reduction of CZ is apparently promoted by PdO species. And the H2 consumption of peak a shows a decreasing tendency of Pd/CZ4 > Pd/CZ2 > Pd/CZ3 > Pd/CZ1, which is consistent with the difference in Ce3+ concentration (as discussed in the XPS section). In addition, a comparatively weak reduction peak (peak b) is found between 200 °C and 300 °C, which is probably induced by the reduction of bulk ceria, but processes at much lower temperature than that for blank supports. Thermal aging at 1000 °C provides noteworthy changes in the reduction characteristics of the Pd/CZx catalysts. It can be found

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Fig. 8. Pd 3d XPS spectra for (a) fresh and (b) aged Pd/CZx catalysts.

Fig. 9. H2-TPR profiles of the (a) fresh and (b) aged CZx supports. For comparison’s sake, legends represent the relative intensities of Y-axes are plotted.

that the reduction of Pd/CZ4a occurs at lowest temperature and the H2 consumption of peak a is the largest, while Pd/CZ2a ranks secondary, which well matches with the Pd dispersion results of the aged catalysts. For all the aged catalysts, a negative peak assigned as peak c is seen below 100 °C, which can be accordingly ascribed to the decomposition of Pd hydride [32,34], due to the fact that metallic Pd would adsorb hydrogen to form Pd hydride even at low hydrogen pressure and room temperature [34]. As mentioned above (in the XPS analysis section), in Pd/CZ4a largest amount of Pd species stay at oxidized state (Pd2+/(Pd2+ + Pd0) = 0.53) due to the stronger interaction between Pd and CZ, which can effectively suppress the formation of Pd hydride at low temperatures, thus the

lowest intensity of the negative peak is obtained. Note also that a new reduction peak (peak b) appears in the temperature region between 100 °C and 200 °C, which is due to the reduction of Ce4+ to Ce3+ also assisted by Pd [32]. Compared with the fresh catalysts, the H2 consumptions are much smaller, and the associated Pd-assisted reduction of CZ occurs at higher temperatures, moreover, the reduction of bulk cerium species is not found in the low temperature region (below 400 °C), which confirms the severe sintering process during the aging treatment. Among the four aged samples, Pd/CZ4a shows the best reducibility with the lowest reduction temperature and highest H2 consumption (782 lmol/gcat), which is in good correspondence with the TPR results of the blank

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Fig. 10. H2-TPR profiles of the (a) fresh and (b) aged Pd/CZx catalysts. For comparison’s sake, legends represent the relative intensities of Y-axes are plotted.

Table 3 H2 consumption of fresh and aged CZx materials obtained from the H2-TPR profiles. Samples

H2 consumption (lmol/g)

Theoretical H2 consumption (lmol/g)

Reduction ratio (%)

CZ1 CZ2 CZ3 CZ4 CZ1a CZ2a CZ3a CZ4a

472 1163 862 1411 184 458 402 553

753 1695 2466 1695 753 1695 2466 1695

62.68 68.61 34.96 83.24 24.44 27.02 16.30 32.63

supports, suggesting that the initial phase heterogeneity presents a positive influence on preserving the reduction ability during the severe aging process. 3.6. OSCC measurements The oxygen storage capacity of the fresh and aged Pd/CZ2 and Pd/CZ4 is measured by pulse injection experiment, and the results expressed as cumulative O2 uptake versus the number of pulses are displayed in Fig. S2. It can be found therein that the OSCC of Pd/CZ2 (821 lmol O2/gcat) is slightly lower than that of Pd/CZ4

(832 lmol O2/gcat). Mamontov et al. have disclosed that Ce0.5Zr0.5O2 material consisting of different Ce-rich and Zr-rich domains with large interfacial area possesses drastically improved oxygen storage capacity [18]. Thus it appears reasonable to deduce that the creation of plenty of interfacial boundaries owing to the compositional heterogeneity in Pd/CZ4 results in its superior oxygen storage capability. And the intriguingly large OSCC of the catalysts (close to the theoretical OSCC value (867 lmol O2/gcat)) may be attributed to the existence of Pd noble metal which can effectively facilitate the oxygen mobility and consequently enhances the oxygen storage capacity [37–39]. After aging treatment, the OSCC is deteriorated for both samples due to the sintering of the supports and the cohesion of PdOx species [40]. The total oxygen storage capacity of Pd/CZ4a (748 lmol O2/gcat) is significantly larger than that of Pd/CZ2a (501 lmol O2/gcat), which agrees well with the H2 consumption results, indicating the excellent thermal stability of Pd/CZ4a which preserves more labile oxygen atoms. Early studies [41,42] have pointed out that the oxygen transfer is restricted by surface area when the value is below 50 m2 g, so the maintenance of high OSCC for these aged samples may be a result of the participation of ‘‘less reducible’’ oxygen species [43,44]. In addition, other researchers [8,11] have demonstrated that the oxygen storage capacity of ceria–zirconia mixed compound is not directly

Table 4 H2 consumption of fresh and aged Pd/CZx catalysts obtained from the H2-TPR profiles. Samples

Pd/CZ1 Pd/CZ2 Pd/CZ3 Pd/CZ4 Pd/CZ1a Pd/CZ2a Pd/CZ3a Pd/CZ4a

H2 consumption (lmol/gcat)

a

b

658 902 693 1125 135 241 168 397

70 45 46 85 249 398 248 385

c

50 52 56 38

Total H2 consumption (lmol/gcat) 728 947 739 1210 384 639 416 782

Theoretical H2 consumption (lmol/gcat)

Reduction ratio (%)

Theoretical H2 consumption of single PdO (lmol/gcat)

796 1734 2501 1734 796 1734 2501 1734

91.46 54.61 29.55 69.78 48.24 36.85 16.63 45.10

47 47 47 47 47 47 47 47

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dependent on the surface area, and a high oxygen storage capacity can be maintained for the CZ solid solution even after severe aging treatment. As declared in the literature [18], the reduction enthalpy of ceria at the surface/grain boundary is much lower than that in the bulk. Thus, the comparatively large OSCC of Pd/CZ4a after aging treatment is mostly related to the larger interfacial area of this sample because of the much higher oxygen mobility at the interfacial region. Moreover, it is proposed that the lattice defects associated with oxygen vacancies make an enormous contribution to the oxygen storage capacity when noble metal is supported on CexZr1xO2 [45], which is well in line with the present work when we notice that the Ce3+ concentration correlated with the oxygen vacancies shows an analogical tendency with the OSCC variation, as testified by XPS investigations. In summary, a conclusion can be drawn from the O2 uptake results of the fresh and aged catalysts that the initial mixed phase composition of the support can accelerate the O2 uptake process to a large extent, and the large interfacial area is beneficial to preserve the high oxygen mobility during severe aging process, thus the excellent OSCC can be well maintained. 3.7. Three-way catalytic performance The results of the catalytic activity evaluation performed over all the fresh and aged catalysts are summarized in Fig. 11. For NOx species, no NO2 can be detected for all catalysts, which can exclude the possibility of NO oxidation. And various amounts of N2O emission are observed, accordingly N2 selectively is calculated and the results are plotted in Fig. 12. With respect to the fresh

catalysts, the catalytic performance shows a decreasing tendency of Pd/CZ4 > Pd/CZ2 > Pd/CZ3 > Pd/CZ1, indicating that for catalysts loaded on single-phase supports, Pd/CZ2 with the medium Ce/Zr ratio owns the best catalytic activity as well as relatively higher N2 selectively (82–97%). And it is interesting to see that in the case of Pd/CZ4, although the theoretical Ce/Zr ratio is the same as that of Pd/CZ2, the conversions of C3H8, CO and NO are dramatically higher than those of Pd/CZ2, and the N2 selectively is slightly higher (84–99%), which should be mainly ascribed to the phase heterogeneity of this sample. As mentioned above, Pd/CZ4 with mixed phase composition exhibits comparatively small crystallite size, high concentration of surface Ce3+ species, excellent reduction ability as well as large oxygen storage capacity, which contributes well to its superior catalytic performance. From the conversion curves of the aged catalysts, it can be observed that the catalytic performance shows a same tendency with that of the fresh catalysts, that is, Pd/CZ4a > Pd/CZ2a > Pd/ CZ3a > Pd/CZ1a, which is in good correspondence with the difference of the Pd dispersion among the four aged samples (as mentioned in the XPS section), indicating that the maintenance of higher Pd dispersion is crucial to the catalytic activity. And the N2 selectively in Fig. 12(b) appears slightly decreased compared with that of the fresh catalysts, whereas the variation tendency is similar with that of the fresh samples. As discussed in XRD and Raman sections, phase segregation happens for Pd/CZ2a and the grain growth process can be effectively suppressed, which consequently modifies the catalytic activity. And with regard to Pd/CZ4 which consists of an initial mixture of different phases, the influence of high temperature treatment on its structural property

Fig. 11. Conversion curves of C3H8, CO and NO as a function of reaction temperature at stoichiometry (k = 1) over the supported fresh (a, b, c) and aged (d, e, f) catalysts.

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415

Fig. 12. N2 selectivity of the (a) fresh and (b) aged catalysts.

seems to be the slightest, leading to its almost fixed phase composition and smallest crystallite size, which coincides with its superior catalytic performance. To the best of our knowledge, the chemical state of Pd can greatly affect the oxidation activities of C3H8 and CO, and the reduction of PdO by cerium species for the aged catalysts is responsible for the decrease of catalytic activity for C3H8 and CO. Therefore, as evidenced by XPS analysis, the strong interaction between the noble metal and the support in Pd/CZ4a plays an important role in stabilizing the oxidized Pd2+, leading to the highest ratio of Pd2+/(Pd2+ + Pd0) in Pd/CZ4a, which further sustains its excellent oxidation activity of C3H8 and CO. In addition, the relative Ce3+ concentration derived by XPS analysis is in good correspondence with the NO reduction activity. Furthermore, a combination of the H2-TPR profiles with the OSCC results reveals that the redox properties of the aged catalysts are well matched with the catalytic activity tendency.

4. Conclusions According to the results and discussion presented above, the following conclusions can be drawn: the initial phase composition of CZ compounds can strongly affect the structural, redox properties and the thermal stability of the supported catalysts. Compared with the conventional single-phase CZ solid solutions, a CZ compound (CZ4) consists of an initial mixture of Ce-rich cubic phase and Zr-rich tetragonal phase possesses large amount of interfacial boundaries between crystallites with different phase compositions, which can effectively retard the aggregation of the primary particles upon thermal aging treatment, and consequently improved structural stability is obtained. In addition, the presence of abundant interface is beneficial to the generation of more lattice defects and can facilitate the migration of oxygen atoms, which gives rise to modified reduction property as well as desirable OSCC even after severe aging treatment. And the stronger interaction between Pd and CZ in Pd/CZ4 helps maintain higher dispersion and more oxidized state of Pd after aging. Therefore, the corresponding catalyst Pd/CZ4 exhibits improved catalytic performance regardless of the thermal aging treatment, showing potential application in TWC.

Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (Grant: 21173153), the National Hi-tech Research and Development Program of China (863 Program, 2013AA065304) and the Sichuan Science and Technology Agency

Supported Project (2012FZ0008) for their generous financial supports to our research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.03.042. References [1] A. Iglesias-Juez, A. Martínez-Arias, M. Fernández-García, J. Catal. 221 (2004) 148–161. [2] A. Papavasiliou, A. Tsetsekou, V. Matsouka, M. Konsolakis, I.V. Yentekakis, N. Boukos, Appl. Catal. B: Environ. 106 (2011) 228–241. [3] Zh.Q. Han, J.Q. Wang, H.J. Yan, M.Q. Shen, J. Wang, W.L. Wang, M. Yang, Catal. Today 158 (2010) 481–489. [4] S.Y. Christou, S. García-Rodríguez, J.L.G. Fierro, A.M. Efstathiou, Appl. Catal. B: Environ. 111–112 (2012) 233–245. [5] J. Kašpar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285–298. [6] Q. Yu, X.X. Wu, Ch.J. Tang, L. Qi, B. Liu, F. Gao, K.Q. Sun, L. Dong, Y. Chen, J. Colloid Interface Sci. 354 (2011) 341–352. [7] I. Atribak, A. Bueno-López, A. García-García, J. Catal. 259 (2008) 123–132. [8] C. Bozo, F. Gaillard, N. Guilhaume, Appl. Catal. A: Gen. 220 (2001) 69–77. [9] M. Alifanti, B. Baps, N. Blangenois, J. Naud, P. Grange, B. Delmon, Chem. Mater. 15 (2003) 395–403. [10] J.C. Yu, L.Zh. Zhang, J. Lin, J. Colloid Interface Sci. 260 (2003) 240–243. [11] P. Fornasiero, R.D. Monte, G.R. Rao, J. Kašpar, S. Meriani, A. Trovarelli, M. Graziani, J. Catal. 151 (1995) 168–177. [12] K. Kenevey, F. Valdivieso, M. Soustelle, M. Pijolat, Appl. Catal. B: Environ. 29 (2001) 93–101. [13] A. Martínez-Arias, M. Fernández-García, A.B. Hungría, J.C. Conesa, J. Soria, J. Alloys Compd. 323–324 (2001) 605–609. [14] T. Nakatani, H. Okamoto, J. Sol-Gel Sci. Technol. 26 (2003) 859–863. [15] G. Colón, F. Valdivieso, M. Pijolat, R.T. Baker, J.J. Calvino, S. Bernal, Catal. Today 50 (1999) 271–284. [16] J. Kašpar, P. Fornasiero, N. Hickey, Catal. Today 77 (2003) 419–449. [17] R.G. Wang, P.A. Crozier, R. Sharma, J.B. Adams, J. Phys. Chem. B 110 (2006) 18278–18285. [18] E. Mamontov, R. Brezny, M. Koranne, T. Egami, J. Phys. Chem. B 107 (2003) 13007–13014. [19] L. Lan, Sh.H. Chen, M. Zhao, M.Ch. Gong, Y.Q. Chen, J. Mol. Catal. A: Chem. 394 (2014) 10–21. [20] G.W. Graham, H.-W. Jen, R.W. McCabe, A.M. Straccia, L.P. Haack, Catal. Lett. 67 (2000) 99–105. [21] R.D. Monte, J. Kašpar, J. Mater. Chem. 15 (2005) 633–648. [22] J. Fan, D. Weng, X.D. Wu, X.D. Wu, R. Ran, J. Catal. 258 (2008) 177–186. [23] G.W. Graham, H.-W. Jen, W. Chun, R.W. McCabe, J. Catal. 182 (1999) 228–233. [24] W.Zh. Huang, J.L. Yang, Ch.J. Wang, B.L. Zou, X.Sh. Meng, Y. Wang, X.Q. Cao, Zh. Wang, Mater. Res. Bull. 47 (2012) 2349–2356. [25] R. Si, Y.W. Zhang, Sh.J. Li, B.X. Lin, Ch.H. Yan, J. Phys. Chem. B 108 (2004) 12481–12488. [26] G.J. Zhang, Zh.R. Shen, M. Liu, Ch.H. Guo, P.Ch. Sun, Zh.Y. Yuan, B.H. Li, D.T. Ding, T.H. Chen, J. Phys. Chem. B 110 (2006) 25782–25790. [27] B. Zhao, G.F. Li, Ch.H. Ge, Q.Y. Wang, R.X. Zhou, Appl. Catal. B: Environ. 96 (2010) 338–349. [28] C. Bozo, N. Guilhaume, J.M. Herrmann, J. Catal. 203 (2001) 393–406. [29] J. Fan, X.D. Wu, X.D. Wu, Q. Liang, R. Ran, D. Weng, Appl. Catal. B: Environ. 81 (2008) 38–48. [30] Q.Y. Wang, G.F. Li, B. Zhao, R.X. Zhou, J. Hazard. Mater. 189 (2011) 150–157.

416

L. Lan et al. / Journal of Colloid and Interface Science 450 (2015) 404–416

[31] Q.Y. Wang, G.F. Li, B. Zhao, R.X. Zhou, Fuel 90 (2011) 3047–3055. [32] H.-W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J.-P. Cuif, S.E. Deutsch, O. Touret, Catal. Today 50 (1999) 309–328. [33] X.L. Weng, J.Y. Zhang, Zh.B. Wu, Y. Liu, H.Q. Wang, J.A. Darr, Appl. Catal. B: Environ. 103 (2011) 453–461. [34] M.Q. Shen, M. Yang, J. Wang, J. Wen, M.W. Zhao, W.L. Wang, J. Phys. Chem. C 113 (2009) 3212–3221. [35] Y. Guo, G.Zh. Lu, Zh.G. Zhang, Sh.H. Zhang, Y. Qi, Y. Liu, Catal. Today 126 (2007) 296–302. [36] G.F. Li, B. Zhao, Q.Y. Wang, R.X. Zhou, Appl. Catal. B: Environ. 97 (2010) 41–48. [37] G.F. Li, Q.Y. Wang, B. Zhao, R.X. Zhou, Catal. Today 158 (2010) 385–392. [38] G.F. Li, Q.Y. Wang, B. Zhao, R.X. Zhou, Appl. Catal. B: Environ. 105 (2011) 151– 162.

[39] C. Descorme, R. Taha, N. Mouaddib-Moral, D. Duprez, Appl. Catal. A: Gen. 223 (2002) 287–299. [40] A. Papavasiliou, A. Tsetsekou, V. Matsouka, M. Konsolakis, I.V. Yentekakis, N. Boukos, Appl. Catal. B: Environ. 90 (2009) 162–174. [41] F. Giordano, A. Trovarelli, C. de Leitenburg, M. Giona, J. Catal. 193 (2000) 273– 282. [42] V. Raju, S. Jaenicke, G.K. Chuah, Appl. Catal. B: Environ. 91 (2009) 92–100. [43] Q.Y. Wang, G.F. Li, B. Zhao, M.Q. Shen, R.X. Zhou, Appl. Catal. B: Environ. 101 (2010) 150–159. [44] C. Larese, M. López Granados, R. Mariscal, J.L.G. Fierro, P.S. Lambrou, A.M. Efstathiouc, Appl. Catal. B: Environ. 59 (2005) 13–25. [45] M.W. Zhao, M.Q. Shen, J. Wang, J. Catal. 248 (2007) 258–267.