Enhanced hydrothermal stability and oxygen storage capacity of La3+ doped CeO2-γ-Al2O3 intergrowth mixed oxides

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Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides Liang Wanga, Meina Huanga, Bin Lia,n, Lihui Donga, Guangzhou Jinb,n, Junbin Gaob, Jiahui Mab, Tangkang Liua a

Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China b School of Chemistry and Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China Received 6 February 2015; received in revised form 9 June 2015; accepted 30 June 2015

Abstract La3 þ -doped CeO2-γ-Al2O3 intergrowth mixed oxide powders were synthesized using an improved amorphous citric precursor (IACP) method and then steam-aged with 90% steam and 10% air for 12 h at 788 1C. The synthesized materials were characterized through X-ray diffraction, energy-dispersive X-ray spectrometry, H2 temperature programmed reduction, Brunauer–Emmett–Teller, and oxygen storage/release capacity (OSC) measurements. La3 þ was successfully doped into the CeO2-γ-Al2O3 lattice to form an intergrowth system. Strong intergrowth interactions were observed between CeO2, La2O3, and γ-Al2O3 crystallites. The (Ce0.92La0.08)0.4Al0.6O2  z(CLA) sample showed a maximum OSC of 979 μmol g  1; however, the pure ceria sample showed a low OSC of 315 μmol g  1. The CLA samples maintained a relatively high OSC of 880 μmol g  1 even after hydrothermal treatment. These results may be attributed to the incorporation of La3 þ ions into the ceria lattice to form a CeO2–La2O3 solid solution as well as to the intergrowth interactions between the CeO2–La2O3 solid solution and γ-Al2O3. Such interactions affected structural homogeneity and oxygen vacancy formation and inhibit the CLA particles from sintering during steam-aging. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: CeO2-γ-Al2O3; La3 þ doped; Intergrowth and coexistence effects; Hydrothermal stability

1. Introduction Ceria-based oxides are widely applied in many catalytic processes, including the water–gas-shift reaction [1,2], carbon monoxide oxidation [3,4], and exhaust gas treatment [5], because of their capacity to cycle easily between reduced and oxidized states (i.e. Ce4 þ /Ce3 þ ) in combination with numerous oxygen vacancies [6,7]. However, pure CeO2 is significantly influenced by thermal aging. In particular, the oxygen storage/release capacity (OSC) of pure CeO2 sharply decreases at high temperatures, such as during thermal

n

Corresponding author. Tel.: þ86 7713233718. Corresponding author. Tel.: þ86 1081292059. E-mail addresses: [email protected] (B. Li), [email protected] (G. Jin). n

sintering. Thus, the OSC and thermal stability of ceria-based oxides can be improved by modifying CeO2. Ceria-based oxides are usually supported on transition aluminas to improve the dispersion of the active phase and oxygen exchange rates [8,9]. However, the intimate contact between ceria and alumina after dispersion facilitates the formation of CeAlO3, which can significantly decrease the OSC by fixing a single oxidation state [10]. Kanazawa et al. [11] inhibited CeZrO2 particle coagulation by placing diffusion barrier layers composed of alumina among primary CeZrO2 particles. We previously found strong intergrowth interactions between CeO2 and γ-Al2O3 oxide crystallites during synthesis. These interactions considerably influence the microstructure and crystal growth of coexisting oxides [12], consequently increasing the hydrothermal stability of γ-Al2O3 [13]. Despite the availability of the literature in this field, intergrowth oxides

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Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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based on biphasic intergrowth CeO2-γ-Al2O3 mixed oxides and hetero-valent cations are rarely reported. Recent studies have examined several mixed oxides to improve their properties. When doping ceria with other oxides, the concentration and size/valence of the dopant ion must be carefully tailored to achieve optimum reducibility [14,15]. Lanthana as a dopant has attracted considerable attention because lanthanum and cerium are homologous elements with great similarities, that is, both of them are lanthanide-series elements that feature fine chemical tuning properties [16]. Meanwhile, the difference between La3 þ and Ce4 þ ionic radii may induce significant lattice deformation and promote oxygen vacancy generation via the charge compensation mechanism in the ceria lattice. This mechanism increases the number of channels for oxygen flow, and the above-mentioned phenomena are the key factors that influence oxidation reactions. We have recently optimized the synthesis of ceria-based nanomaterials via an improved amorphous citric precursor (IACP) method [17,18]. We have also synthesized a series of samples with Ce/(Ce þ Al) molar ratios varying from 0.1 to 0.9 and observed biphasic intergrowth effects in nano-CeO2-γAl2O3 mixed oxides. The OSCs of the samples at 700 1C showed a plateau in the Ce/(Ce þ Al) molar ratio range of 0.3– 0.7, and the Ce0.4Al0.6O2  z sample demonstrated maximum OSCs before and after steam-aging [13]. In the present work, (Ce1  xLax)1  yAlyO2  z (x ¼ 0–0.16, y ¼ 0, 0.6) intergrowth mixed oxides (CLA) were prepared via the same IACP method described in our previous work. This study aims to achieve an improved CLA oxygen storage material with high OSC value and thermal stability.

2. Experimental section 2.1. Sample preparation (Ce1  xLax)1  yAlyO2  z (x ¼ 0–0.16, y ¼ 0, 0.6, z¼ number of oxygen vacancies) mixed oxides were prepared by the IACP method. An aqueous salt solution containing 3 M Ce (NO3)3  6H2O (AR, Beijing Chemical Reagent Plant, China), 2 M Al(NO3)3  9H2O (AR, Beijing Chemical Reagent Plant), and 0.2 M La(NO3)3  6H2O (AR, Beijing Chemical Reagent Plant) was prepared. Citric acid (AR) was then added to the mixed-solution until the molar ratio of citric acid to the total amount of metal ions was 0.6:1. The solution and citric acid were mixed through agitation. After 12 h, the solution was added dropwise onto carbon black (Cabot VXC-72; specific surface area, 254 m2 g  1). The wet carbon black was dried at 110 1C, ground into fine powders, decomposed at 230 1C, and then calcined at 650 1C under air flow for 3 h. The products were referred to as fresh CL (x ¼ 0.04–0.16, y¼ 0) and CLA (x¼ 0.04–0.16, y¼ 0.6). For comparison purposes, a mechanical mixture sample with the same molar ratio of (Ce0.92La0.08)0.4Al0.6O2  z was prepared by mixing CL (x¼ 0.08) and γ-Al2O3. Steam-aged samples were prepared by treating the oxides with 90% steam and 10% air for 12 h at 788 1C.

2.2. Characterization methods Powder X-ray diffraction (XRD) measurements were conducted at room temperature (RT) by using a Rigaku D/ max2000 diffractometer with Cu Kα radiation (λ¼ 1.5406 Å). The instrument was operated at 40 kV and 100 mA. Step scans were obtained over the 2θ range of 25–751 with a step size of 0.021. The intensities of each point were determined for 2 s for phase identification and lattice parameter calculation. The data used to determine the average particle sizes and strain of CeO2 samples were obtained over the 2θ range of 44–521 in steps of 0.011 at 10 s/step. The lattice parameters were determined using Fullproful 2000 [19]. The peak at 2θ ¼ 47.41, which corresponds to the (220) reflection of CeO2, was nearly symmetrical and fitted a Voigt line shape well [20]. This diffraction peak was selected to determine the average particle sizes (D220). A wellcrystallized CeO2 sample was used as a standard to calibrate instrumental broadening contributions. The specific surface areas of the samples were measured through single-point Brunauer–Emmett–Teller (BET) analysis of the nitrogen adsorption isotherms obtained at liquid N2 temperature (  196 1C) and recorded using a SMART SORB92/93 instrument with a thermal conductivity detector. All samples were pretreated in vacuum at 220 1C for 3 h and 20 min before measurement. The reducibility of the samples was examined through H2 temperature programmed reduction (H2-TPR) with a FINESORB-3010 Chemisorption Analyzer. Approximately 200 mg of the samples (40–60 mesh) was employed for each test. The reducing gas was 15% H2/Ar flowing at a rate of 30 sccm from RT to 900 1C (10 1C min  1); the carrier gas was Ar (30 sccm). The elemental distribution of the samples was determined by an FEI Navo NanoSEM 430 scanning electron microscope equipped with an X-ray energy dispersive spectrometer. CLA powders were dispersed in ethanol and deposited onto a Si substrate. The entire substrate was attached to a sample holder with a carbon conductive belt prior to scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectrometry. 2.3. OSC determination OSC measurements were performed using a quartz fixedbed micro-reactor, and the effluents were analyzed with a GC7890T gas chromatograph (Tianjin Xianquan Instrument Co., Ltd., China). The powder samples were pressed into tablets at 10 MPa, crushed into small pieces, and then sieved. Each sample (20 mg, 40–60 mesh) was homogeneously mixed with the same amount of quartz (40–60 mesh). The mixture was charged into a quartz tube reactor and fixed by two pieces of silica wool. The bed temperature was maintained within 7007 0.1 1C. An appropriate amount of the sample was chosen to ensure that the amount of remaining CO ranges from of 40% to 60% in the blank experiment. Approximately 12 mL of the mixture gas (5.12% O2 þ 94.88% He) was

Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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injected into the reactor five times at 2 min intervals to completely oxidize the sample. Approximately 12 mL of the reducing gas (4.93% CO þ 95.07% He) was then passed through the reactor, and the integrated area of the remaining CO was recorded. The oxidation and reduction cycle described above was repeated 10 times and the results were averaged. The OSC (μmol g  1) of each sample was calculated as: OSC ¼ ð1 SR =SB Þ  M B =W

ð1Þ

where SR is the integrated area of the remaining CO (μV s), SB is the integrated area of CO in the blank analysis (μV s), MB is the amount of CO in the blank analysis (μmol), and W is the sample mass (g). 3. Results and discussion 3.1. Crystal structure 3.1.1. XRD patterns of the (Ce1  xLax)1  yAly O2  z samples Fig. 1(a) demonstrates that the XRD diffraction peaks can be easily indexed. All of the samples show well-defined characteristic diffraction peaks corresponding to the (111), (200), (220), (311), (222), and (400) crystal faces; these peaks are characteristic of a cubic structure belonging to the Fm3m space group (JCPDF 34-0394). Diffraction peaks related to La2O3 are not observed in the pattern, which confirms the high purity of the samples. When the value of x is increased, all of the peaks of the CL sample shift toward smaller diffraction angles compared with those of pure CeO2. This shift can be attributed to the substitution of large La3 þ (0.130 nm) for Ce4 þ (0.111 nm) [21] in the sample. A large x indicates a high degree of angular deflection. Thus, La3 þ ions may be inferred to have been successfully incorporated into the ceria lattice to form a uniform CeO2–La2O3 solid solution. The diffraction patterns of CLA with different amounts of La3 þ are presented in Fig. 1(b). Similar to Fig. 1(a), no impurity peaks are observed in any of the patterns. The diffraction peaks of the CLA samples are weaker and wider than those of the CL samples. This result indicates that the CLA samples have lower crystallinity and form smaller crystallites than the CL samples. To further demonstrate the effect of γ-Al2O3 addition the XRD patterns of CeO2, CL (x¼ 0.08), and CLA (x ¼ 0.08, y¼ 0.6) are shown in Fig. 1(c). The diffraction peaks of CLA are much wider and weaker and their degree of angular shift is smaller, than those of CL. These variations result from the intrinsic irregularities of γ-Al2O3 and the strong interaction between CeO2–La2O3 and γ-Al2O3. 3.1.2. Textural properties The crystal structure of the samples is further investigated in terms of their textural properties. Lattice distortion is often observed in nanocrystalline materials with unique lattice relaxation energies [22]. On the basis of the least-squares refinement of the d-spacings measured from the XRD traces, the lattice parameters of the samples are determined and summarized in Table 1. The lattice parameters of both CL and CLA increase with increasing La3 þ content because of the

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repulsive forces between extra-interstitial anions accompanying the substitution of La3 þ for Ce4 þ . This finding further proves that La3 þ ions have been successfully incorporated into the ceria lattice. Our previous work demonstrated that CeO 2 and γAl 2 O 3 oxide crystallites show intergrowth properties and coexist in the Ce–Al–O samples synthesized via the IACP method [13]. In the present study, La 3 þ ions are successfully incorporated into the ceria lattice to form a solid solution, and the CeO 2 –La 2 O 3 solid solution and γ-Al 2 O 3 form an intergrowth system (i.e., CLA intergrowth mixed oxides). As further evidenced by the average particle size of these samples determined from the diffraction peak of the (220) plane, the average particle size of the CL solid solution (16.512–11.319 nm) significantly differs with increasing La 3 þ content. By contrast, the average particle sizes of all CLA (6.183–5.593 nm) are similar in the range of La 3 þ -doped content. This result may be attributed to the strong interaction between CeO 2 –La 2 O 3 and γ-Al 2 O 3 . Moreover the average particle sizes of all CLA samples are much smaller than those of the CL samples. This result agrees with the specific surface area results. The above results clearly reveal that La3 þ ions can be easily incorporated into the ceria lattice to form a CeO2–La2O3 solid solution and that the amount of doped La3 þ ions exerts significant effects on the resulting average particle size. Furthermore, the γ-Al2O3 and CeO2–La2O3 solid solution form a uniform intergrowth system, in which strong interactions between CeO2–La2O3 and γ-Al2O3 are observed; γ-Al2O3 induces CeO2 lattice distortions so that more lattice defects and oxygen vacancies [23] are produced and the average particle size of CeO2–La2O3 is inhibited from increasing. 3.2. EDX results The uniformity of the intergrowth mixed oxides can be further confirmed through EDX characterization of CLA (x¼ 0.08, y¼ 0.6) and corresponding mechanically mixed CL þ A samples (Fig. 2). The SEM results of CLA [Fig. 2 (a1)] and CL þ A [Fig. 2(b1)] clearly reveal that CLA is composed of uniform, well-distributed particles; by contrast, CL þ A is composed of a large number of nonuniform particles. Fig. 2(a2)–(a4) shows that the Ce, La, and Al elements of the CLA samples are well distributed. Further EDX characterization in point-scan mode indicates that the molar ratio of the three elements [Fig. 2(a1), point 1] is consistent with their theoretical values (Table 2). This finding further confirms that the Ce, La, and Al in the CLA samples are well distributed. To confirm the intergrowth effect, EDX mapping images of the CL þ A samples are shown in Fig. 2(b2)–(b4). These images reveal that the elemental composition of the mechanically mixed samples is not well-distributed; in addition, EDX in point-scan mode shows that the actual molar ratio of different positions [Fig. 2(b1), points 2, 3] has significant discrepancies (Table 2). These results suggest that CLA is an

Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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Fig. 1. XRD patterns of (Ce1  xLax)1  yAlyO2  z samples: (a) Ce1  xLaxO2  z (x¼0–0.16, y ¼0), (b) (Ce1  x Lax)0.4Al0.6 O2  z (x ¼0–0.16, y¼ 0.6), (c) CeO2, CL (x ¼0.08, y¼ 0), CLA (x¼ 0.08, y ¼0.6).

Table 1 Lattice parameter (a0), average particle size (D), and specific surface area (S) of (Ce1  xLax)1  yAlyO2  z samples. Sample

CL

CLA

y

x

0 0 0 0 0 0.6 0.6 0.6 0.6 0.6

0.00 0.04 0.08 0.12 0.16 0.00 0.04 0.08 0.12 0.16

a0 (nm)

D(220) (nm)

S (m2 g  1)

0.5415 0.5427 0.5440 0.5452 0.5464 0.5411 0.5416 0.5422 0.5430 0.5435

16.512 15.454 12.791 14.626 11.319 6.183 6.105 5.688 5.421 5.593

49 47 56 56 58 61 59 65 61 64

intergrowth mixed oxide that has a strong interaction between CL solid solution and alumina. Thus, the elements are welldistributed, which agrees with the XRD results.

3.3. Hydrothermal stability 3.3.1. XRD patterns of aged (Ce1  xLax)1  yAly O2  z Fig. 3 shows the XRD patterns of the steam-aged (Ce1  xLax)1  yAlyO2  z samples. These samples display no new characteristic peaks compared with the corresponding samples in Fig. 1(a) and (b). The diffraction peaks of aged samples are sharper than those of their counterparts. This result can be attributed to the increase in grain size during hydrothermal treatment because of high-temperature sintering. These results demonstrate that the structural features of all of the samples are not destroyed during heat treatment.

3.3.2. Textural properties of aged (Ce1  xLax)1  yAlyO2  z The hydrothermal stability of (Ce1  xLax)1  yAlyO2  z can be evaluated using the textural properties of the aged samples (Table 3). After steam-aging, the particle size of pure CeO2 is much larger than that of fresh CeO2. The increase rate (R) of

Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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Fig. 2. EDX mapping result of (Ce0.92La0.08)0.4Al0.6O2  z (CLA) intergrowth mixed oxides: (a1) SEM image; (a2) Al3 þ signal; (a3) La3 þ signal; (a4) Ce4 þ signal and the mechanical mixture of CeO2, La2O3 and γ-Al2O3 (CL þA): (b1) SEM image; (b2) Al3 þ signal; (b3) La3 þ signal; (b4) Ce4 þ signal.

Table 2 Percentage distribution of surface elements in CLA and CLþ A. Sample

Ideal value of CLA Intergrowth (Fig. 2(a) point 1) Mechanical mixing (Fig.2(e) point 2) Mechanical mixing (Fig. 2(e) point 3)

Atom (%) Ce

La

Al

36.80 35.40 3.78 55.26

3.20 3.44 0.41 6.36

60.00 61.16 95.81 38.38

the particle size after steam-aging may be as high as 393.76%. However, R significantly decreases with La3 þ doping. The R of the CL samples ranges from 77.82% to 45.14%, whereas that of the CLA samples ranges from 66.86% to 37.80%. These results reveal that the addition of appropriate amounts of La3 þ can promotes the thermal stability of the CL samples and that thermal stability can be further improved in the CLA samples. The variations in specific surface area before and after aging agree with the particle size results. The results described above clearly reveal that La3 þ doping can significantly inhibit the CLA particles from being sintered during steam-aging. Thus, the strong interaction between CeO2–La2O3 and γ-Al2O3 particles can apparently inhibit crystal growth and enhance hydrothermal stability. 3.4. H2-TPR results The reduction properties of the samples are characterized via H2-TPR, and the results are shown in Fig. 4. Detailed information about the obtained reduction peaks is also summarized in Table 4. Two peaks are observed during H2TPR: (i) the adsorption of H2 generates surface hydroxide groups on the oxide surface; (ii) oxygen vacancies are formed after the recombination of these hydroxide groups.[15] To obtain insights about the surface oxygen species, Gaussian fitting of the curves was performed for all of the profiles. All of the profiles exhibit two reduction peaks (labeled α and β). The

difference in these two reduction peaks is due to the difference in their respective oxygen binding energies.[24] The peak at the low-temperature region (α) is mainly attributed to the reduction of surface oxygen species, whereas that at the hightemperature region (β) is mainly attributed to the reduction of bulk oxygen species.[25] La3 þ -doped CeO2 shows a shift in reduction peaks toward low temperatures, especially for bulk reductions. As shown in Table 4, the β peak temperature of CL is remarkably reduced to 592 1C, whereas that of CLA is much lower at 543 1C. Thus, the reduction properties of the CLA samples are greatly improved. The increase in the peak areas of α þ β is in the order of CLA 4 CL 4 CeO2, which suggests that more oxygen species are reduced in the CLA samples than in the other samples. This result further illustrates that CLA presents more oxygen vacancies, and more easily reduced bulk oxygen compared with the other samples. These results agree with our XRD findings. 3.5. Oxygen storage capacity Fig. 5 shows the dynamic changes in the OSCs of the fresh [Fig. 5(a)] and aged [Fig. 5(b)] samples at 700 1C as a function of La3 þ doping amount. Fig. 5(a) reveals that the CL solid solution has a higher OSC than pure CeO2 and that OSC increases with increasing x. Furthermore, the OSCs of the CLA samples are enhanced by up to several times compared with those of the CL samples. The OSCs of the CLA samples increases with increasing La3 þ dopant concentration at xr 0.08 but decrease at x 4 0.08. The increase in OSC with increasing dopant concentration depends on the number of oxygen vacancies in the sample, which, consequently affects oxygen mobility within the ceria network. Beyond the limit of about x ¼ 0.08, the decreases in OSC may be attributed to excess vacancy density, which leads to stable vacancy–dopant cation association [26] and limits the number of mobile vacancies. The (Ce0.92La0.08)0.4Al0.6O2  z sample shows a maximum OSC of 979 μmol g  1; however, the OSCs of CL þ A and CeO2 are only 267 and 315 μmol g  1,

Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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Fig. 3. XRD patterns of the aged (Ce1  x Lax)1  yAlyO2  z samples: (a) aged Ce1  xLaxO2  z (x¼ 0–0.16, y¼ 0), (b) aged (Ce1  xLax)0.4Al0.6O2  z (x ¼0–0.16, y¼ 0.6).

Table 3 Lattice parameter (a0), average particle size (D), specific surface area (S), and increasing rate (R) of particle sizes before/after steam-aging of (Ce1  xLax)1  yAlyO2  z samples. Sample

CL

CLA

y

x

0 0 0 0 0 0.6 0.6 0.6 0.6 0.6

0.00 0.04 0.08 0.12 0.16 0.00 0.04 0.08 0.12 0.16

a0 (nm)

D(220) (nm)

S (m2 g  1)

R (%)

0.5414 0.5426 0.5438 0.5448 0.5461 0.5415 0.5421 0.5425 0.5432 0.5439

81.529 25.329 22.745 21.228 18.327 11.957 10.187 8.522 7.696 7.707

15 30 27 28 32 37 38 36 41 38

393.76 63.90 77.82 45.14 61.91 93.39 66.86 49.82 41.97 37.80

: R(%)¼ (D(220)A D(220)F)/D(220)F  100%; D(220)F: average particle sizes of fresh samples; D(220)A: average particle sizes of aged samples.

respectively. The high OSC of (Ce0.92La0.08)0.4Al0.6O2  z may be attributed to La3 þ ion doping, which forms a uniform CLA product. At 700 1C, the OSC of CeO2 is mainly decided by the migration and transfer of bulk oxygen. La3 þ ions are incorporated into the ceria lattice to form a CeO2–La2O3 solid solution, which constrains the forces between metal ions and oxygen atoms because Ce3 þ ions are partly replaced by La3 þ ions. Thus, bulk oxygen is rapidly diffused to the material surface and leads to OSC improvement. In addition, the CeO2– La2O3 solid solution and γ-Al2O3 form a uniform intergrowth system in which strong interactions are produced between the components. These interactions induce CeO2 lattice distortions so that amounts of lattice defects and oxygen vacancies produced are increased and the increase in particle size is inhibited. The increase in specific surface area can also increase the amount of oxygen vacancies produced, thereby improving the mobility of oxygen in the cerium oxide lattice.

Fig. 4. H2-TPR profiles of the CeO2, CL (x ¼0.08, y ¼0), CLA (x¼ 0.08, y¼ 0.6) samples.

Table 4 Peak areas of CeO2, CL (x¼ 0.08, y¼0), CLA (x ¼0.08, y¼ 0.6) samples. Sample

CLA CL CeO2

Peak area

x

y

α

β

αþ β

0.08 0.08 0

0.6 0 0

505 740 886

3830 3011 2058

4335 3756 2944

Consequently, the CLA samples with a known amount of La3 þ dopant show significantly enhanced OSCs. Fig. 5(b) shows the OSCs of the aged samples. Similar to Fig. 5(a), Fig. 5(b) illustrates that the OSCs of the aged CLA samples are enhanced by up to several times compared with those of the aged CL or CeO2 samples. The OSCs of all aged

Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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Fig. 5. OSCs of (a) fresh and (b) aged CL (x¼ 0–0.16, y ¼0) and CLA (x¼ 0–0.16, y¼ 0.6) samples.

samples decrease after hydrothermal treatment, which may be attributed to particle aggregation. As a result, the particle size is increased. Nevertheless, the CLA samples still obtain a relatively high OSC of 880 μmol g  1.

Guangxi Undergraduate Innovation Program (Grant number 201410593116).

4. Conclusions

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CLA samples are prepared using the IACP method and aged by treatment with 90% steam and 10% air for 12 h at 788 1C. Strong intergrowth interactions between CeO2–La2O3 and γAl2O3 are observed in the samples. La3 þ ions in the binary CeO2-γ-Al2O3 system are investigated using multiple analytical techniques, and the following effects are observed: (a) marked decrease in average particle size, (b) effective stabilization of the defective structure, and (c) considerable enhancement of the OSC. The excellent OSC and hydrothermal stabilization of CLA depended on the concentration of the La3 þ dopant. In particular, La3 þ ions increase the OSC and cause stabilization at low-doping concentrations (xr 0.08). However, the OSC decrease at x 40.08. The (Ce0.92La0.08)0.4Al0.6O2  z sample shows a maximum OSC of 979 μmol g  1, and the CLA samples maintain a relatively high OSC of 880 μmol g  1 even after hydrothermal treatment. These results may be attributed to the incorporation of La3 þ ions into the ceria lattice to form a CeO2–La2O3 solid solution as well as to the intergrowth effects between the CeO2–La2O3 solid solution and γ-Al2O3. These intergrowth effects enhance structural homogeneity and oxygen vacancy formation and inhibit the CLA particles from being sintered during steamaging. Thus, both OSC and hydrothermal stability are enhanced. Acknowledgments This work was supported by the Key Project of Chinese National Programs for Fundamental Research and Development (973 program) (Grant number 2012CB21500203).

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Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142

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Please cite this article as: L. Wang, et al., Enhanced hydrothermal stability and oxygen storage capacity of La3 þ doped CeO2-γ-Al2O3 intergrowth mixed oxides, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.142