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Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 17–23

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

atalytic performance of CexZr1−xO2 solid solutions promoted viaodification on synthesis methods

ei Lia,b,∗, Zhaogang Liua, Yanhong Hua, Mitang Wanga

Material and Metallurgy School, Inner Mongolia University of Science and Technology, Baotou 014010, ChinaInner Mongolia Autonomous Universities’ Key Laboratory of New Technologies of Modern Metallurgy and Application of Rare Earth Materials,

nner Mongolia University of Science and Technology, Baotou 014010, China

r t i c l e i n f o

rticle history:eceived 2 January 2010eceived in revised form 27 May 2010ccepted 2 June 2010vailable online 11 June 2010

a b s t r a c t

CexZr1−xO2 solid solutions were synthesized via various methods which were modified based on con-ventional precipitation and sol–gel methods. X-ray diffraction (XRD) analysis revealed that CexZr1−xO2

solid solutions obtained via these methods exhibited cubic structure. Specific surface area and oxygenstorage capacity (OSC) characterizations displayed that CexZr1−xO2 solid solutions prepared via these

eywords:exZr1−xO2 solid solutionsxygen storage capacitypecific surface areaatalytic activity

modified methods exhibited higher specific surface area, thermal stability, and OSC than synthesized viathose common approaches. Encapsulation with Al2O3 and SiO2 via a dipping method also increases thespecific surface area of CexZr1−xO2 solid solutions. Catalytic activity of these solid solutions was assessedusing a 4-channel catalysis device. The results revealed that CexZr1−xO2 solid solutions synthesized viathose modified methods exhibited higher catalytic activity than the common method. The solid solutionprepared via NH3·H2O precipitation with the addition of dodecanoic acid method exhibited the lowest

high

light-off temperature and

. Introduction

Cerium-base oxides are broadly used as oxygen storage mate-ials of three-way catalysts (TWC) for the automobile emissionontrol systems owing to its high oxygen storage capacity (OSC)1–3]. Normally, the working temperature of TWC catalysts reachesver 900 ◦C [4], so high thermal stability of catalysts is required.gglomeration often occurs when pure CeO2 powder is sintered at

emperatures over 800 ◦C, resulting in decreases in specific surfacerea as well as in OSC [5]. This severely restricts the application ofeO2 in TWC area. Thus, it is critical important to generate novelaterials for TWC catalysts with high oxygen storage capacity and

hermal resistance.Recently, CexZr1−xO2 solid solutions have gradually replaced

eO2 powder, to be used as TWC catalysts due to the fact that their

hermal stability is higher than CeO2 powder [6–10]. Comparedo CeO2 powder, the sintering resistance of the CexZr1−xO2 solidolutions also increased significantly in redox reactions [9]. Thetructural distortion in the crystal lattice of CexZr1−xO2 solid solu-ions caused by the introduction of zirconium is a great benefit for

∗ Corresponding author at: Material and Metallurgy School, Inner Mongolia Uni-ersity of Science and Technology, Baotou 014010, China. Tel.: +86 472 5954390;ax: +86 472 5951505.

E-mail addresses: limei@imust.cn, btlm65@163.com (M. Li).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.06.003

est catalytic activity.© 2010 Elsevier B.V. All rights reserved.

oxygen migration and diffusion [7]. As a consequence, the kineticof redox reaction is improved due to the decreases of the reduc-tion temperature at the surface and inside of the catalysts [11].The OSC, sintering resistance and redox property of CexZr1−xO2solid solutions is prominently enhanced. Furthermore, the for-mation of CexZr1−xO2 solid solutions inhibits the formation ofCeAlO3 which is generally believed to paralyze the catalytic activity[12–14].

The preparation method, however, plays an important role topromote the catalytic activity. In the last several years, manykinds of synthesis methods have been studied in preparation ofCexZr1−xO2 solid solutions, including solid reactions [9,10], chem-istry precipitation [15–17], sol–gel [18–23], and hydrothermalmethods [24–26]. The specific surface area, OSC and thermal resis-tance of CexZr1−xO2 solid solutions synthesized via these methodsvaried significantly. Therefore, investigating the effects of prepa-ration methods on the properties of CexZr1−xO2 solid solutionsis critical for developing rare earth oxygen storage and cata-lyst materials. In this work, several soft chemistry approacheswere applied to synthesize CexZr1−xO2 solid solutions, such asmodified NH3·H2O precipitation, modified NH4HCO3–NH3·H2Oprecipitation, precipitation–hydrothermal, sol–gel with citric acid

and sol–gel with stearic acid. The structure, specific surface area,OSC and thermal stability of CexZr1−xO2 solid solutions synthesizedvia these various methods were characterized. The catalytic perfor-mance of as-synthesized and aged CexZr1−xO2 solid solutions wasassessed using real engine exhaust gas.

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8 M. Li et al. / Colloids and Surfaces A: P

. Materials and methods

.1. Materials

Ce2(CO3)3 was provided by Inner Mongolia Baotou Steelare-Earth Hi-Tech Co., Ltd of Baotou Steel (CeO2/�REO ≥ 99.9%,REO ≥ 45%). Zr(CO3)2 (ZrO2 ≥ 40%), NH3

.H2O, NH4HCO3, ethanol,itric acid and stearic acid were purchased from Sigma. All reagentsre used without further treatment.

.2. Sample preparation

Ce(NO3)3 and Zr(NO3)4 solutions (0.5 M) were prepared byissolving Ce2(CO3)3 and Zr(CO3)2 in nitric acid. According to stoi-hiometric ratio of Zr to Ce in the final molecular formula, a certainmount of Ce(NO3)3 and Zr(NO3)4 was mixed for the following dif-erent preparation methods. The as-synthesized CexZr1−xO2 solidolutions prepared via different methods in this work are listed inable 1. The chemical composition of the CexZr1−xO2 solid solutionsas determined by inductively coupled plasma atomic emission

pectroscopy (ICP/AES) (VARIAN 725-ES, Varian, Inc.). The actualomposition of the CexZr1−xO2 solid solutions is in good agreementith the nominal composition: CeO2:ZrO2 ≈ 7:3.

For precipitation method, NH3·H2O (1.0 M) andH4HCO3–NH3·H2O (1.0 M, ratio of NH4HCO3 to NH3·H2O is:1) was each added into the mixture of Ce(NO3)3 and Zr(NO3)4nder stirring, the mixture was stirred for 10 min after the concen-ration of metal ions reached 0.5 M. The depositions were washednd then dried at 100 ◦C (CZ1 and CZ7). The modified NH3·H2O pre-ipitation methods included five different modifications (Table 1).1) The precipitation was performed in ethanol–water (volumeatio, 1:1) solution (CZ2). (2) The precipitation was performed inthanol solution (CZ3). (3) The suspensions of depositions wereged for 8 h at 90 ◦C (CZ4). (4) Dodecanoic acid (molar ratio ofcid to metal ions is 1:5) was added into the mixture duringhe precipitation process (CZ5). (5) Precipitation–hydrothermal

ethod was performed by putting suspension obtained viaH3·H2O precipitation method into a reaction autoclave under

he pressure of 2.4 MPa at 150 ◦C for 8 h without filtration. Next,he deposits were filtrated, washed and then dried at 100 ◦CCZ6). The modified NH4HCO3–NH3·H2O precipitation methodsre: (1) the suspensions obtained by the NH4HCO3–NH3·H2O

recipitation method were aged for 8 h at 90 ◦C (CZ8); (2)recipitation–hydrothermal method as described above in the fifthodification method of the NH3·H2O precipitation methods, except

he suspension was obtained via NH4HCO3–NH3·H2O precipitationethod (CZ9).

able 1hemical composition of CexZr1−xO2 solid solutions.

Sample number Preparation method

CZ1 Common NH3·H2O precipitationCZ2 NH3·H2O precipitation (in ethanol–water solution)CZ3 NH3·H2O precipitation (in ethanol solution)CZ4 NH3·H2O precipitation (suspension was aged at 90 ◦C for 8 hCZ5 NH3·H2O precipitation (dodecanoic acid)CZ6 NH3·H2O precipitation–hydrothermalCZ7 NH4HCO3–NH3·H2O precipitationCZ8 NH4HCO3–NH3·H2O precipitation (suspension was aged atCZ9 NH4HCO3–NH3·H2O precipitation–hydrothermalCZ10 Citric acid sol–gelCZ11 Stearic acid sol–gelCZ–Si NH4HCO3–NH3·H2O precipitation (coated with SiO2)CZ–Al NH4HCO3–NH3·H2O precipitation (coated with Al2O3)

chem. Eng. Aspects 367 (2010) 17–23

Sol–gel method was applied to prepare CexZr1−xO2 solid solu-tions, in brief, citric acid (CZ10) or stearic acid (CZ11) was addedinto the mixture of Ce(NO3)3 and Zr(NO3)4 in a molar ratio of acid tometal ions which is equal to 4. The mixture was slowly evaporatedunder stirring at 95–100 ◦C until it transformed into a transparentgel, and then the gel was dried at 100 ◦C.

The as-synthesized samples were obtained by further sinteringthe precursors prepared by the above methods at 500 ◦C for 2 h. Theaged samples were prepared by sintering the precursors at 900 ◦Cfor 4 h.

The Al2O3 and SiO2 coated samples were obtained by dip-ping the precursors, which were prepared by NH4HCO3–NH3·H2Oprecipitation method (CZ7), into 1 M Na4SiO4 (CZ–Si) and 0.4 MAl(NO3)3 (CZ–Al) solutions for 4 h, then was filtered, dried at 100 ◦C,and sintered at 500 ◦C for 2 h. The aged samples with the coating ofSiO2 or Al2O3 were obtained after sintering the precursors at 900 ◦Cfor 4 h.

2.3. Characterizations

Specific surface area was measured using a Coulter SA3100 sur-face area analyzer (Beckman Coulter). X-ray powder diffraction(XRD) patterns were recorded on Philips-PW1700 diffractometerwith a radiation of Cu K� at a working voltage of 40 kV and a work-ing current of 100 mA. Scanning electron microscopy (SEM) in theback-scattered electron imaging mode with X-ray energy disper-sive spectroscopy (EDS) were recorded on EDAX system (EDAXInc.) at voltage of 15 kV. OSC were measured using pulse chro-matographic technique described elsewhere [27], briefly, sampleswere reduced in H2 at 550 ◦C for 40 min and then flowed pureN2 at a flow rate of 40 mL/min until the temperature was cooleddown to 200 ◦C. The samples were oxidized by injecting periodically2 mL gas (O2, 5.29%, He 94.71%) five times at an interval of 2 min,and then reduced with 5 mL of gas (CO, 5.0% and He 95%) pulseinjected at the same periodical interval. OSC was calculated fromthe amount of CO2 formed after the first CO pulse and expressed inmicromoles of CO per gram from the CO consumption. Each mea-surement was repeated five times to get the average value. Thecatalytic activity of the as-synthesized and aged CexZr1−xO2 solidsolutions were assessed in a 4-channel catalysis evaluation appara-tus (Guilin Reecat Environment Industry Co., Ltd.) using real engine

shaped ceramic as carrier. The light-off temperature was measuredat the air–fuel ratio (�) of 1. The catalytic activity was assessed at� = 0.97, 1 and 1.03, which can give us information about the cat-alytic activity at the certain window of the air–fuel ratio, in thiscase, it is ±0.03.

Oxide content (wt.%)

CeO2 ZrO2 SiO2 Al2O3

70.36 29.84 0 069.68 30.42 0 069.89 30.22 0 0

) 70.52 29.33 0 070.68 29.75 0 069.86 29.96 0 070.24 30.03 0 0

90 ◦C for 8 h) 70.56 29.45 0 070.67 29.66 0 076.87 24.20 0 070.26 29.76 0 068.01 29.08 3.25 069.23 29.56 0 1.58

hysicochem. Eng. Aspects 367 (2010) 17–23 19

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M. Li et al. / Colloids and Surfaces A: P

. Result and discussion

.1. XRD analysis

The CexZr1−xO2 solid solutions synthesized via various methodsn this study were characterized by XRD for both as-synthesizednd aged samples (Fig. 1). The four main peaks with miler indices,1 1 0), (2 0 0), (2 2 0) and (3 1 1) observed from XRD patterns aredentical to the cubic fluorite type CexZr1−xO2 solid solutionsJCPDS-28-271) indicates that Zr4+ enters into CeO2 crystal latticeuring the CexZr1−xO2 solid solutions formation. The aging pro-edure did not change the structure, no phase segregation was

bserved.

The XRDs of CexZr1−xO2 solid solutions coated with Al2O3 andiO2 also exhibit the same cubic structure. No diffraction peaks ofl2O3 or SiO2 are observed, suggesting that dispersive Al2O3 or SiO2articles exist at the surface of CexZr1−xO2 without forming individ-

ig. 1. XRD profiles of CexZr1−xO2 solid solutions synthesized via different methods.A) As-synthesized samples; (B) aged samples.

Fig. 2. Specific surface area of as-synthesized and aged CexZr1−xO2 solid solutions.

ual second phase. The aged samples, however, appeared a slightlyphase segregation (Fig. 1B).

3.2. Specific surface area analysis

Specific surface areas of as-synthesized and aged CexZr1−xO2solid solutions are measured in this work (Fig. 2). The specific sur-face area of the aged samples is lower than the as-synthesizedsamples due to the agglomeration caused by high aging tem-perature (900 ◦C) which is independent from the preparationmethods. The CexZr1−xO2 solid solutions prepared via modi-fied NH3·H2O precipitation and NH4HCO3–NH3·H2O precipitationmethods, however, exhibited higher specific surface area than syn-thesized via the common method in the each case of as-synthesisand aged samples.

For the modified NH3·H2O precipitation methods (Fig. 2, CZ2 toCZ6), the NH3·H2O precipitation–hydrothermal method (CZ6) pro-vided the highest specific surface area of the as-synthesized sample,however, the aged sample synthesized via the NH3·H2O coprecipi-tation with the addition of dodecanoic acid method (CZ5) presentedthe highest specific surface area and thermal stability than others.One notes that the specific surface area of the CexZr1−xO2 solid solu-tions increased with the increased amount of ethanol added intothe mixture of Ce(NO3)3 and Zr(NO3)4 (CZ2 to CZ3). This suggestedthat the decreases in the polarity of the mixture solution result ina better dispersity and higher specific surface area. The NH3·H2Ocoprecipitation with the addition of dodecanoic acid method is sug-gested to be a suitable method for CexZr1−xO2 solid solutions whichhas high specific surface area and best thermal stability.

For the samples synthesized via NH4HCO3–NH3·H2Oprecipitation methods (CZ7 to CZ9), NH4HCO3–NH3·H2Oprecipitation–hydrothermal (CZ9) and the suspension wasaged at 90 ◦C (CZ8) both produced higher specific surface areain the as-synthesized and aged samples than those prepared bythe common method (CZ7), however, the former (CZ9) has thehighest specific surface area among all as-synthesized CexZr1−xO2solid solutions studied in this work. Thus, the NH4HCO3–NH3·H2Oprecipitation–hydrothermal method is suggested to be the mostsuitable method for synthesizing CexZr1−xO2 solid solutions.

The CexZr1−xO2 solid solutions prepared with sol–gel methodshave a lower specific surface area in the as-synthesized samples,

however, the specific surface area decreased less significantly afteraged at 900 ◦C compared to other methods. It suggested the samplessynthesized via modified sol–gel methods have a higher thermalstability than prepared by other methods.

20 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 17–23

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ig. 3. Specific surface area of CexZr1−xO2 solid solutions coated with Al2O3 andiO2.

The specific surface area of the CexZr1−xO2 solid solutions coatedith Al2O3 or SiO2 have a higher specific surface area and ther-al stability than the sample without coating (Fig. 3), same for

s-synthesized and aged samples. The CexZr1−xO2 solid solutionsoated with SiO2 has higher specific surface area and thermal sta-ility than those coated with Al2O3. Coating SiO2 and Al2O3 onhe surface of CexZr1−xO2 solid solutions is a good method, whichormally results in high specific surface area. Accordingly, highhermal stability of CexZr1−xO2 solid solutions coated with SiO2nd Al2O3 result from the protection of SiO2 and Al2O3 existed onexZr1−xO2 solid solutions surfaces [28].

.3. OSC analysis

The OSC analysis is described in Section 2. The results indi-ated that the OSC of the sample prepared with modified NH3·H2Orecipitation and NH4HCO3–NH3·H2O precipitation methods areigher than synthesized via the common method (Fig. 4). The sam-le (CZ5) prepared by NH3·H2O precipitation with the addition ofodecanoic acid has the best OSC among all samples investigated

n this work, its OSC is two times higher than the sample (CZ1)repared with the common NH3·H2O precipitation method. Theddition of dodecanoic acid during the precipitation improved theSC while increased the specific surface area and thermal stability.

Fig. 4. OSC of CexZr1−xO2 solid solutions prepared via different methods.

Fig. 5. SEM micrograph in the BSE mode and EDS patterns of CexZr1−xO2 solid solu-tions coated with (A) Al2O3 and (B) SiO2.

It is further suggested that the modification of NH3·H2O precip-itation method with the addition of dodecanoic acid is feasibleapproach.

The samples prepared with modified NH4HCO3–NH3·H2O pre-cipitation methods (CZ8 and CZ9) also exhibit higher OSC thanthe sample prepared via the common method (CZ7), althoughthe OSC of the sample (CZ9) prepared with NH4HCO3–NH3·H2Oprecipitation–hydrothermal method is lower than the CexZr1−xO2solid solutions (CZ5) synthesized via NH3·H2O precipitation withthe addition of dodecanoic acid method, it still is the second high-est OSC among other samples, it also suggested that both modifiedmethods are practical methods for synthesizing CexZr1−xO2 solidsolutions.

The sample prepared with sol–gel method with citric acidexhibit a medium OSC value among all samples. These results areconsistent with the values in specific surface area. Although theCexZr1−xO2 solid solutions coated with Al2O3 and SiO2 have higherspecific surface area and thermal stability compared to the samplewithout coating (Fig. 3), the OSC of coated sample decreased. Thisindicates that the surface of coated sample exhibit similar prop-erties of Al2O3 and SiO2, which have higher specific surface areabut lower OSC [28]. SEM and EDS results clearly confirmed theformation of Al2O3 and SiO2 on coated CexZr1−xO2 solid solutions

(Fig. 5).

OSC of aged samples at 900 ◦C (Fig. 4) exhibit lower value thanthe as-synthesized samples as expected, however, the sample (CZ5)prepared by NH3·H2O precipitation with the addition of dodecanoic

M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 17–23 21

Fig. 6. Three-way catalytic activity of as-prepared CexZr1−xO2 solid solutions catalysts at (A) � = 0.97, (B) � = 1.0 and (C) � = 1.03.

Fig. 7. Three-way catalytic activity of aged CexZr1−xO2 solid solutions catalysts at (A) � = 0.97, (B) � = 1.0 and (C) � = 1.03.

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cid still has the best OSC among all samples. The trend of changesf OSC between modified methods and the common method onach preparation approach is same as described above.

.4. Catalytic activity

The three-way catalytic activities of as-synthesized CexZr1−xO2olid solution catalysts assessed at 450 ◦C and � = 0.97, 1 and 1.03re shown in Fig. 6. All as-synthesized CexZr1−xO2 solid solutionatalysts exhibited higher activity for NOx reduction than for HCnd CO oxidations in the rich and stoichiometric mixture (� = 0.97nd 1.0, Fig. 6A and B). The activity for NOx reduction decreasedramatically in the lean mixture (� = 1.03, Fig. 6C) while the con-ersion of HC and CO reached to 100%. The CexZr1−xO2 solid solutionrepared by those modified methods exhibited similar activity thanhe sample prepared by the common methods at � = 1.0. The samplerepared by NH3·H2O precipitation with the addition of dodecanoiccid (CZ5) and NH4HCO3–NH3·H2O precipitation–hydrothermalethod (CZ9) did exhibit slightly higher catalytic activity in allixtures than other samples investigated in this work, which is

onsistent with the results of specific surface area and OSC. Oneotes that the conversion of HC and CO remain a similar value inhe � window of ±0.03, however, the conversion of NOx exhibit aramatic decrease at � = 1.03. This observation indicates that theatalytic activity of all samples on the conversion of HC and CO haswider effective air–fuel ratio window than NOx.

The catalytic activities of aged CexZr1−xO2 solid solution (at00 ◦C for 4 h) assessed at 450 ◦C and � = 0.97, 1 and 1.03 are shown

n Fig. 7. All aged CexZr1−xO2 solid solution catalysts exhibitedigher activity for HC and CO oxidations than for NOx reduc-ion in all mixtures. The conversion of HC and CO in the lean

ixture is higher than the rich and stoichiometric mixture. TheexZr1−xO2 solid solution prepared by those modified methodsxhibited higher activity than the sample prepared by the com-on methods. The sample prepared by NH3·H2O precipitation with

he addition of dodecanoic acid (CZ5) and NH4HCO3–NH3·H2Orecipitation–hydrothermal method (CZ9) did exhibit slightlyigher catalytic activity in all mixtures than other samples investi-ated in this work, which is consistent with the results of specificurface area and OSC. The similar trend of the air–fuel ratio win-

ow on HC, CO and NOx with the as-synthesized samples has beenbserved on the aged samples too. The catalytic activity of all agedamples on the conversion of HC and CO has a wider effectiveir–fuel ratio window than NOx.

ig. 8. Light-off temperature of as-synthesized and aged CexZr1−xO2 solid solutions.

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chem. Eng. Aspects 367 (2010) 17–23

The light-off temperature of as-synthesized and agedCexZr1−xO2 solid solution catalysts are shown in Fig. 8. Theas-synthesized samples exhibited lower light-off temperaturethan the aged samples. The sample (CZ5) prepared by NH3·H2Oprecipitation with the addition of dodecanoic acid exhibited thelowest light-off temperature among all samples. The light-offtemperatures of the CexZr1−xO2 solid solution prepared by thosemodified methods (CZ5, CZ6 and CZ9) are lower than the sampleprepared by the common method (CZ1 and CZ7).

4. Conclusion

All as-synthesized and aged CexZr1−xO2 solid solutions preparedwith different methods including those coated with Al2O3 andSiO2 are cubic structure. The specific surface area, thermal stabilityand OSC, however, varied among the CexZr1−xO2 solid solutionsprepared via different methods. In general, the specific surfacearea, OSC and thermal stability are improved by those modifiedmethods, especially the samples prepared via NH3·H2O precipita-tion with the addition of dodecanoic acid and NH4HCO3–NH3·H2Oprecipitation–hydrothermal methods, which have a higher specificsurface area, OSC, and thermal stability. The catalytic activities ofsamples synthesized via those modified methods are higher thanthose prepared via the common methods. The NH3·H2O precipita-tion with the addition of dodecanoic acid and NH4HCO3–NH3·H2Oprecipitation–hydrothermal methods provided CexZr1−xO2 solidsolutions with higher conversion of HC, CO oxidation and NOx

reduction and lower light-off temperature, which indicated thesemethods are the feasible methods for synthesis CexZr1−xO2 solidsolutions for the application in TWC catalyst. This study indicatedthat, even though the performance of the automotive catalystinvolved complex factors, the performance can be improved viamodifications of synthesis methods.

Acknowledgements

We greatly acknowledge the financial support by the Programfor New Century Excellent Talents in University (NCET-07-0407)and the National Natural Science Foundation of China (Grant No.20966007). We also thank Megan Choy for helping us in revisingthe English.

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