A

smaim©

K

1

mpeeVcasi

iimfitts[

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 153–157

Effects of the synthesis methods on the physicochemicalproperties of cerium dioxide powder

Mei Li a,b, Zhaogang Liu b, Yanhong Hu b, Zhenxue Shi b, Hangquan Li a,∗a College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

b Material and Metallurgy School, Inner Mongolia University of Science and Technology, Baotou 014010, China

Received 1 October 2006; received in revised form 3 December 2006; accepted 12 December 2006Available online 16 December 2006

bstract

The CeO2 powders were synthesized using several different methods and characterized by X-ray diffraction (XRD), laser diffraction particleize analyzers, BET surface area analysis and transmission electron microscopy (TEM). The results show that CeO2 crystals prepared by different

ethods possess all the same fluorite structure. On the other hand, the physicochemical properties including particle size, cubic specific surface

rea, fluidity and density of cerium dioxide powder depend on the different synthesis procedures. This means that the properties requested forndustrial applications can be tuned by generative methods. The calcined temperature is another very important parameter for the preparation of

onodisperse superfine powder. 2007 Elsevier B.V. All rights reserved.

e; Flu

f[TipTi[[

mismtur

eywords: Cerium dioxide; Specific surface area; Particle size; Crystal structur

. Introduction

The rare earth composites were praised as the treasury ofaterials due to their special optical, electric and magnetic

roperties that were given by the specific ‘f’ electron of rarearth elements [1]. Studies on rare earth materials attracted anxtensive interest with the development of new technologies.arious rare earth materials possessing different physicochemi-al properties have been produced due to the demand of differentpplications. For example, a powder having different particleize, density, specific surface area and morphology can be usedn different industry areas.

CeO2 powder is widely used in ceramic, catalyst, polish-ng, luminescence and solid electrolyte materials [2]. However,ts performance in different application areas is directly deter-ined by its physicochemical properties. For example, superne CeO2 can decrease the calcination temperature and increase

he density of ceramic; various particle size of CeO2 can meet

he different demands of polishing, in general, the polishing ofemiconductor will need CeO2 powder with a fine particle size3]; in catalytic area, using CeO2 with a higher specific sur-

∗ Corresponding author. Tel.: +86 10 64445339; fax: +86 10 64445339.E-mail address: lihq@mail.buct.edu.cn (H. Li).

2

2

M

927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.12.042

idity

ace area as carriers can enhance catalytic activity of catalyst4]; the glass additive need a higher loose density of CeO2 [5].he physicochemical properties of CeO2 are so important for

ts application that the synthesis of CeO2 powder with differenthysicochemical properties attracts widely investigate interests.he preparation methods of CeO2 powder are mainly precip-

tation [6], hydrothermal [7,8], sol–gel [9], alkoxide method10], microemulsion [11], template synthesize [12], gas reaction13,14], etc.

In this study, several improved precipitation and sol–gelethods were used to prepare CeO2 powders. The physicochem-

cal properties of CeO2 powders were characterized. The crystaltructure of cerium dioxide was determined by XRD and TEMeasurements. The effect of calcination temperature on crys-

al properties was also investigated. This work provides muchseful information for the industry and fundamental research ofare earth materials.

. Materials and methods

.1. Sample preparation

Cerium carbonate [Ce2(CO3)3] was provided by Innerongolia Baotou Steel Rare-Earth Hi-Tech Co., Ltd.

1 ysicoc

(wCCm

m

•

•

a

2

tsict(B

3

3

uf

t

TeIp

paiaabioOp

nhabcbc

iccmil

ictbvvaCd

TT

M

OEANCCFAS

54 M. Li et al. / Colloids and Surfaces A: Ph

CeO2/∑

REO ≥ 99.9%,∑

REO ≥ 45%). Cerium carbonateas purified and dissolved by nitric acid in order to get 1 Me(NO3)3 solution. A 2% dispersant was then added to thee(NO3)3 solution and were stored for the following experi-ents. All other reagents were of analytical grade.Cerium dioxide powders were synthesized by the following

ethods:

Precipitation method: A given concentration (0.5–1.2 M,depending on the different precipitation methods) precipitatorwas added to the Ce(NO3)3 solution at a given tempera-ture varying between 30 and 80 ◦C and stirring intensity(500 r/min). Keep stirring for 10 min after deposition isentirely finished. The deposition was dried at 100 ◦C afterfiltrating and washing with water for several times.Sol–gel method: Citric acid was added to Ce(NO3)3 solutions,the final ratio of Ce3+ and citric acid is 1:1.2, followed byslowly evaporating to a transparent sol status under stirring at95–100 ◦C, it will become a gel by the slowly and continuousevaporating, finally, the gel was dried at 100 ◦C.

The CeO2 powders were gained by calcinated the precursorst 300–1000 ◦C.

.2. Characterizations

The particle size distribution of sample was done with Coul-er LS230 laser diffraction particle size analyzer. The specificurface area was tested with Coulter SA3100 specific area test-ng equipment. The morphology, dispersing character and phaseonformation were analyzed by TEM (JEM-200CX). The crys-al structure and phase conformation were analyzed with XRDPhilips PW170). The loose density and fluidity was tested withT-1000 synthesis performance testing instrument.

. Results and discussion

.1. Preparation methods

Several improved precipitation and sol–gel methods were

sed to prepare CeO2 powder in order to fulfill different demandsor industry materials.

Using the oxalic acid as precipitator to deposit Ce3+ is araditional industry preparation method for CeO2 powder. [15]

pAd

able 1he physical properties of CeO2 powder prepared by different method

ethods Particle size, D50 (�m) Specific s

xalic acid precipitation 0.896 27.21DTA–oxalic acid precipitation 0.543 36.25mmonia–oxalic acid precipitation 0.173 44.04H4HCO3 precipitation 0.397 59.96arbamide–NH4HCO3 precipitation 0.277 55.32arbamide–ammonia precipitation 0.345 55.26ormic acid–hydrazine precipitation 3.01 105.1cetic acid–hydrazine precipitation 2.53 103.3ol–gel method 0.301 99.18

hem. Eng. Aspects 301 (2007) 153–157

he particle size of powder obtained is about 3–6 �m. How-ver, the superfine powder is difficult to obtain by this method.mproving precipitation methods or controlling the condition ofrecipitation can generate the powder with nano size particles.

EDTA–oxalic acid complex and ammonia–oxalic acid com-lex were used as precipitators to improve the traditional oxaliccid method and carried out as the following: first, adding EDTAnto the solution of Ce3+, EDTA combines with Ce3+ to form

very stable complex, then a certain mount of oxalic acid isdded. At this point, no precipitation formed. The depositionegins to form when HNO3 is dropped into this solution becauset destroys the complex of EDTA and Ce3+. In this way, the speedf precipitation is well controlled to avoid the non-uniformity.n the other hand, the super saturation of system could keeproper range that makes the particle uniform.

Ammonia–oxalic acid deposition method is adding ammo-ia into the solution to form a hydroxide deposition. Then theydroxide deposition is transformed into oxalate deposition bydding oxalic acid. This method improved the problem causedy the fine particle size of hydroxide deposition that is very diffi-ult for afterward disposal. However, the fine CeO2 powder cane generated by this method because the oxalate deposition oferium was translated from very fine hydroxide deposition.

Using NH4HCO3 as precipitator has several advantagesncluding the low cost, having less pollution and the easy andontrollable technique. However, the phenomena of local pre-ipitation have been observed by this method. We improved thisethod by gradually releasing the precipitator into the solution

nstead of adding the precipitator at once. The result shows noocal precipitation formed.

Hydrolyzing carbamide to produce NH4OH as precipitators a traditional homogenous precipitation method. This methodan decrease the precipitation speed of metal ions to obtainhe homogenous deposition. However, quite an amount of car-amide is required to produce NH4OH in order to adjust the pHalue since Ce3+ only can completely precipitate at the high pHalue. We used carbamide–ammonia and carbamide–NH4HCO3s co-precipitator and were added into Ce3+ solution to geteO2 powder. It is obvious that the dosage of carbamide canramatically decrease by this method.

Using formic acid and acetic acid as precipitator is anotherrecipitation method to synthesize cerium dioxide powder [16].dding hydrazine in this process of deposition yields the mixtureeposition of formic cerium, acetic cerium, formic hydrazine

urface area, S (m2 g−1) Loose density, ρ (g cm−3) Fluidic index

1.14 621.11 650.98 640.82 560.86 600.89 650.86 510.87 650.84 65

M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 153–157 155

Fig. 1. The particle size distribution of CeO2 prepared by carbamide–NH4HCO3

method.

Fs

agfe

ntpssg

3

piTma(ispm

cttITbh

ig. 2. The XRD patterns of CeO2 powder prepared by (a) common oxalate, (b)ol–gel, and (c) ammonia-oxalic acid method.

nd acetic hydrazine. The cerium dioxide particles do not aggre-ate and have amount of pores after calcinations because theormic hydrazine and acetic hydrazine released a lot of heatnergy and gas during the calcination process.

The sol–gel method is adding citric acid into the ceriumitrate solution and concentrate it to form gel, then the heatingreatment is performed to generate the powder material. The

rocess includes the following phases: molecule → polymer →ol → gel → crystal (or non-crystal). This method is used toynthesize the powder material with special structure and aggre-ation states [9].

sNrc

Fig. 3. (a) The electron diffraction pattern of CeO2 obtained

Fig. 4. The XRD patterns of CeO2 calcined at different temperature.

.2. The physicochemical properties of CeO2 powder

The physicochemical properties of CeO2 powders includingarticle size, specific surface area, loose density and the fluid-ty index are characterized. The results are listed in Table 1.he particle size of CeO2 decreased with both improvedethods (EDTA–oxalic acid, 0.543 �m and ammonia–oxalic

cid, 0.173 �m) compared to the oxalate precipitation method0.896 �m). At the same time, the specific surface areancreased slightly with both improved method. The loose den-ity and the fluidity index are quite similar. This kind of CeO2owder is good to be used as polishing agent and potteryaterials.Fig. 1 shows the particle size distribution of CeO2 prepared by

arbamide–NH4HCO3 method. The particle size in the middle ofhe cumulative volume (D50) is 0.160 �m, and the width of par-icle size distribution is 0.8 calculated by R = (D90 − D10)/D50.t is obvious that the particle size distribution is very narrow.he volume-particle size distribution shows a normal distri-ution that means CeO2 particle obtained by this method areomogenous.

Comparison with the other methods, the particle size and

pecific area of CeO2 prepared by NH4HCO3, carbamide–H4HCO3, carbamide–ammonia methods is in the medium

ange. This kind of CeO2 powder can be used as pottery andatalyst materials. NH4HCO3 precipitation method is a better

by NH4HCO3 method. (b) The analysis result of (a).

156 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 153–157

rent te

ii

caTm

acbc

ootacp

3

pOcopCrmvws

wpcpo

3

NcstbIpooNsposottt

btaacIp

4

ctc

Fig. 5. TEM of CeO2 calcined at diffe

ndustrial method to prepared CeO2 catalyst material consider-ng the cost.

Using hydrazine improved formic acid and acetic acid pre-ipitation method, the super fine cerium dioxide powders withlarger specific surface are obtained (103.3 m2/g, Table 1).

his kind of cerium dioxide powder is an ideal catalystaterial.The size of the particles prepared by sol–gel method is small

nd its distribution is uniform. The specific surface area is biggeromparing with other methods (Table 1). So the powder preparedy this method is also an ideal catalyst and pottery materialsandidate.

The loose density of powders prepared by different meth-ds are all relatively small except those prepared by threexalate methods. The fluidity index of CeO2 shows a lit-le difference between all different methods (Table 1). Abovell, we found that using different improved methods canontrol the physicochemical properties of cerium dioxideowder.

.3. The crystal structure of CeO2 powder

Fig. 2 shows X-ray diffraction patterns of CeO2 powders pre-ared by common oxalate, sol–gel, and ammonia-acid methods.ne notes that specific diffraction peaks of three samples are

onsistent with the peaks of the cubic structure of CeO2. Itbviously demonstrates that the crystal structures of these sam-les are cubic. On the other hand, comparing with the typicaleO2, The specific peak of CeO2 with a size of nanometer is

elatively lower and broader which is typical character of nano-aterials [7]. This means the particle size of CeO2 crystal is

ery small and the crystallization is not integrity. On the otherords, the crystal structure of nano CeO2 particles possesses

ome defects.Fig. 3a shows the electron diffraction pattern of CeO2 that

as prepared with NH HCO method. The analysis of this

4 3attern (Fig. 3b) shows that the CeO2 has a face-centeredubic (FCC) structure, which is the same structure as the CeO2repared by common oxalate, sol–gel, and ammonia–acid meth-ds.

cai

mperature. (a) 500 ◦C and (b) 900 ◦C.

.4. The effect of calcinated temperature

Fig. 4 shows X-ray diffraction patterns of CeO2 prepared byH4HCO3 method and calcined at different temperatures. We

an observe that the powder calcined at 100 ◦C has an amorphoustructure. The specific diffraction peak of CeO2 appeared whenhe powder was calcined at 300 ◦C. It means that the powderasically decomposed into CeO2 after it was calcined at 300 ◦C.n the calcinations reaction of all rare earth salts, the decom-osed temperature of the Ce3+ salt was usually lower than forther rare earth salts. For example, the decomposed temperaturef Ce2(C2O4)3 is 500 ◦C, but the decomposed temperature ofd2(C2O4)3 is 800 ◦C [17]. This is because that cerium pos-

esses a different isotope that Ce4+ is more stable than Ce3+ inresence of O2, so normally cerium is present as CeO2 insteadf Ce2O3 in nature. From Fig. 4, one also notes that the producthows an obvious cubic crystal structure. The diffraction peakf CeO2 becomes sharp with increase of the calcined tempera-ure. It means that the degree of crystallization is enhanced andhe particle size increases when powder are calcined at higheremperature.

Fig. 5 shows the TEM images of CeO2 that were obtainedy calcined at 500 and 900 ◦C, respectively. The result showshat CeO2 powder consisted of fine particles and aggregatedfter calcined at 500 ◦C. However, the particle size increaseds a result of aggregation of small particle when the powder isalcined at 900 ◦C. This is consistent with the result of XRD.t means that the calcination is also very important condition torepare monodisperse superfine powder.

. Conclusion

CeO2 powder having different physicochemical propertiesan be obtained with different methods and precipitators. Allhe CeO2 obtained by different methods possess a same cubicrystal structure. Their fluidities are all similar.

The fine CeO2 powder with medium specific surface areaan be obtained by EDTA–oxalic acid and ammonia–oxaliccid precipitation methods. This kind of CeO2 powder is andeal candidate for polishing and pottery material. The particle

Physi

sctfaihg

oiC

A

N2oh

R

[[[[

[14] R.X. Valenzuela, G. Bueno, A. Solbes, et al., Topics Catal. 15 (2001)

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

ize and specific surface area of CeO2 obtained by NH4HCO3,arbamide–NH4HCO3 and carbamide–ammonia methods is inhe medium range. However, the particle size and specific sur-ace area of CeO2 obtained by formic acid–hydrazine and aceticcid–hydrazine methods are big which has advantages in apply-ng as catalyst materials. The CeO2 obtained by sol–gel methodas small particle size and big specific surface area, which isood for catalyst and pottery application.

The calcined temperature also can control the particle sizef CeO2 powder. Indeed, the calcined temperature is also verymportant for the preparation of the monodisperse superfineeO2 powder.

cknowledgements

We greatly acknowledge the financial support by the

ational Basic Research Foundation of China (Grant No.004CCA03900) and the National Natural Science Foundationf China (Grant No. 20566001). We also thank Dr. Andersen forelping us in revising the English.

[[[

cochem. Eng. Aspects 301 (2007) 153–157 157

eferences

[1] Q. Su, Rare Earth Chemistry, Henan Sci. & Technol. Publishing Com.,Zhengzhou, 1993, p. 202.

[2] X.T. Dong, X.G. Qu, G.Y. Hong, et al., Chin. Sci. Bull. 41 (1996) 1396.[3] Y.X. Li, X.Z. Zhou, Y. Wang, et al., Mater. Lett. 58 (2003) 245.[4] B. Jacques, L. Oloviero, B. Renard, et al., Catal. Today 75 (2002) 29.[5] Z.Z. Xu, Y.X. Yang, S.L. Yuan, et al., J. Rare Earth 20 (2002) 119.[6] B. Djuriclic, S. Pickering, J. Eur. Ceram. Soc. 19 (1999) 1925.[7] T. Masui, H. Hirai, N. Imanaka, et al., J. Mater. Sci. Lett. 21 (2002)

489.[8] X.T. Dong, J.H. Yan, Y. Wei, Rare Metal Mater. Eng. 1 (2002) 312.[9] W.H. Hou, L. Xu, J.H. Qiu, et al., J. Nanjing Univ. 7 (1997) 487.10] X.T. Dong, G.X. Liu, J. Sun, et al., Rare Metal Mater. Eng. 31 (2002) 229.11] S. Shi, Y.H. Lu, H.Q. Wang, Chemistry 12 (1998) 51.12] L.D. Zhang, W.P. Cai, C.M. Mo, Prog. Natural Sci. 9 (1999) 103.13] N. Guillou, L.C. Nistor, H. Fuess, et al., Nanostructured Mater. 8 (1997)

545.

181.15] G.X. Xu, Rare Earth, Metallurgical Industry Press, Beijing, 1995, p. 68.16] H.G. Brittain, P.S. Grade, J. Less Common Met. 94 (1983) 277.17] Y.W. Zhang, Z.G. Yan, C.S. Liao, et al., J. Rare Earth 19 (2001) 378.