Synthesis and Characterization of Nanosized Ceria Powders and High

Feb 28, 2004 - Nanoscale, high surface area cerium oxide (ceria) powders and stable, ... environment using cerium(IV) nitrate as the precursor. The re...
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Ind. Eng. Chem. Res. 2004, 43, 3019-3025

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Synthesis and Characterization of Nanosized Ceria Powders and High-Concentration Ceria Sols H. Zou,† Y. S. Lin,*,† N. Rane,† and T. He‡ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171, and Honda Research Institute USA, Inc., 1381 Kinnear Road, Columbus, Ohio 43212

Nanoscale, high surface area cerium oxide (ceria) powders and stable, high-concentration (>1 M) ceria sols were prepared by a new method based on homogeneous precipitation in an acidic environment using cerium(IV) nitrate as the precursor. The results are compared with ceria powders and sols prepared by a hydrothermal method in a basic environment with cerium(III) nitrate as the precursor. Hydrolysis and condensation of the cerium(IV) and cerium(III) nitrates yield the ceria precursor precipitates with molecular formula of CeO2‚2H2O [or Ce(OH)4] and CeO2‚H2O [or Ce(OH)2O], respectively. The dried ceria precursor powders from Ce(IV) and Ce(III) are well dispersed in the form of primary particles of about 4 nm in size. Calcination at 450 °C causes phase transformation of the amorphous portion accompanying growth of the Ce(IV)derived ceria crystallite or aggregation of the Ce(III)-derived ceria particles. The Ce(III)-derived ceria crystallites have a smaller lattice parameter than the Ce(IV)-derived sample whose lattice parameter decreases with increasing calcination temperature. Stable ceria sols at a solid concentration up to 300 g of CeO2/L were obtained from a Ce(IV)-derived ceria precursor powder, as compared to a maximum stable solid concentration of 20 g of CeO2/L for the Ce(III)-derived ceria sols. These results are discussed in terms of the shape and aggregation tendency of the primary ceria particles prepared by the two different methods. Introduction Cerium oxide (ceria) has the fluorite structure that is stable from room temperature to its melting point as compared with other pure oxides such as zirconia and bismuth oxide1 having oxygen ion conductivity. It has been considered a useful material in applications as additives for glass, glass polishers, ceramics, phosphors, oxygen gas sensors,2 catalytic supports for automotive exhaust systems3,4 and solid electrolytes for solid oxide fuel cell.5,6 Ceria thick films can be used as advanced low-temperature water gas shift catalysts for the production of carbon monoxide free hydrogen to feed the PEM fuel cells.7 Ceria films are prepared by a large variety of techniques such as sputtering,8,9 chemical vapor deposition,10,11 pulsed-laser deposition,12 electron beam evaporation,13 and sol-gel deposition.14-16 The preparation of oxide films from aqueous solutions has several advantages as compared to the other techniques mentioned. The sol-gel process allows for facile fabrication of large-area coatings at a low cost and offers advantages of controlling the composition and microstructure of the film, an asset for eventual technological applications especially to obtain films of desired thickness. A very important factor in obtaining thick films by the sol-gel dip-coating method is to get stable, highconcentration sols. A few research groups prepared ceria sols by using the commercial 20 wt % CeO2 aqueous colloidal dispersion17 (Johnson Matthey) or peptizing CeO2 precipitation by the addition of an equimolar quantity of HNO3.14,18 However, methods for the preparation of high-concentration CeO2 sols are not well established. The major problem to obtain high-concen* To whom correspondence should be addressed. E-mail: [email protected]. † University of Cincinnati. ‡ Honda Research Institute USA, Inc.

tration ceria sols is that the primary particles used are easy to aggregate. Nonagglomerated or weakly agglomerated ceria primary particles should be spherical with a narrow size distribution. The strength of agglomeration depends on the surface properties of the nanocrystalline particles, and these properties are strongly dependent on the synthesis procedures.19 Numerous “soft chemistry” synthesis methods have extensively been investigated for the preparation of suitable ceria powders.20 Terribile et al. 21,22 used a hybrid organic/inorganic route to successfully prepare high surface area mesoporous cerium oxide. The hydrous cerium(IV) oxide was formed through a progressive precipitation/reaction process from Ce(III). The size of crystallites detected by transmission electron microscopy (TEM) is 2-5 nm after calcination at 450 °C. Djuricic et al.19 reported weakly agglomerated ceria powders obtained by two steps. The first step was to add hydrogen peroxide to cerium(III) nitrate at 5 °C and then obtain precipitate by the addition of ammonium hydroxide. The second step consisted of treating the precipitation hydrothermally at 180 °C. They believed that the function of hydrogen peroxide was to slowly oxidize the cerium cation to a higher valence state and thereby initiate homogeneous precipitation with the formation of dense spherical agglomerates. The precipitation process was then completed by the addition of ammonium hydroxide, which also disrupts the spherical agglomerations leaving behind a weakly agglomerated powder. The process was completed by hydrothermal treatment without an increase in the crystallite size. The weak agglomeration property of the ceria particles in their work was proved by TEM measurement. However, these researchers did not use their weakly agglomerated ceria particles to prepare ceria sols. The objective of the work reported in this paper is to synthesize ultrafine weakly agglomerated ceria precursor powders from cerium(IV) nitrate and to obtain

10.1021/ie030676d CCC: $27.50 © 2004 American Chemical Society Published on Web 02/28/2004

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stable, high-concentration ceria sols from the ceria precursor powders prepared by this method. The solid products and ceria powders were characterized using nitrogen adsorption porosimetry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and TEM. The results are compared with those of the ceria powder and sol prepared by the established approach reported by Diuricic et al.19 using cerium(III) nitrate as the precursor. Experimental Section Synthesis. Two approaches were used to prepare ceria powders. In the first approach [referred to as the Ce(IV) method], ceric nitrate (Johnson Matthey) and H2O were introduced into a three-necked flask equipped with a thermometer and a reflux condenser at ambient temperature. The pH of the solution was 0.34. In the hydrolysis medium, the molar ratio of [H+]/[CeIV] was about 2.14. The reaction medium was maintained under agitation and refluxed for 4 h at 100 °C, and yellow precipitates were formed. Ceria precursor powders were obtained by filtering the precipitates, followed by drying in air at 40 °C for 2 days. For comparison, ceria powders were prepared by the second approach [referred to as the Ce(III) method] from oxidization of the cerium(III) precursor following the procedure published in the literature.19 In this method, a cerium nitrate hexahydrated solution (0.1 M), Ce(NO3)3‚6H2O (Johnson Matthey), was mixed with hydrogen peroxide, H2O2 (29.5 wt %), in a volume ratio of 3:1, at 5 °C with stirring. Ten minutes later, an ammonium hydroxide solution was added to the metal nitrate/hydrogen peroxide solution with stirring to increase the pH value to 10. Yellowish precipitates were formed. After that, the solution with precipitates was hydrothermally treated at 180 °C for 4 h under autogenous pressure without stirring. Upon cooling, the clear supernatant was decanted and the yellowish precipitate was collected and dried in air at 40 °C for 2 days to obtain a ceria precursor powder. The dried ceria precursor powders obtained by the above methods were calcined in air at 450, 600, and 800 °C, respectively, for 3 h in order to examine the structural change of the ceria-based powders at different temperatures. Ceria sols of various solid concentrations were prepared from the ceria precursor powders obtained by drying the filtered ceria precursor precipitates described above (without calcination at 450 °C). The first group of stable ceria sols were prepared by simply redispersing the dried ceria precursor powder from the Ce(IV) method in distilled water without the addition of any other chemical. The second group of ceria sols was prepared by dispersing the dried ceria precursor powder from the Ce(III) method in distilled water followed by careful peptization with the addition of HNO3 with stirring and heating at 70 °C. A clear milky sol was obtained. A sol is considered stable in this work if no precipitates were observed for the sol after setting still for at least 1 month. Characterization. The dried and calcined ceria precursors or ceria powders were characterized using several techniques to determine the particle size, composition, and morphology. XRD analysis (Siemens D-500) was used to determine the phase composition and to estimate the crystallite size of the powders. The average crystallite size was estimated from peak broadening

Figure 1. XRD patterns of CeO2 powders obtained by the Ce(IV) method from Ce(NO3)4: (A) dried at 40 °C; (B) calcined at 450 °C; (C) calcined at 800 °C.

Figure 2. XRD patterns of CeO2 powders obtained by the Ce(III) method from Ce(NO3)3: (A) dried at 40 °C; (B) calcined at 450 °C; (C) calcined at 800 °C.

using the Scherrer equation: D ) 0.9λ/(β cos θ),23 where D is the crystallite size, λ is the wavelength of the XRD (Cu KR, 1.5406 Å), θ is the diffraction angle, and β is the corrected half-width of the diffraction peak for the (111) plane. Cerium(IV) oxide powder (Aldrich) having a particle size of about 0.5 µm was used as the reference material in XRD analysis. The Brunauer-EmmettTeller (BET) surface area, pore volume, pore size, and average pore-size distribution of the calcined powders were obtained from N2 sorption isotherms using an adsorption porosimeter (Micromeritics ASAP 2000). The dried precipitates were analyzed by DSC/TGA (SDT 2960 Simultaneous DSC/TGA) in dry air with a heating rate of 5 °C/min. The morphology of the samples was observed by TEM (Philips CM20). For TEM analysis, samples were prepared in absolute ethanol and evaporated on the grid at room temperature. Results Figure 1 shows the XRD patterns of ceria precursor or ceria powder samples prepared by the Ce(IV) method after drying and calcination at 450 and 800 °C. XRD data for the three ceria precursors or ceria powders prepared by the Ce(III) method after drying and calcination under the same conditions are shown in Figure 2 for comparison. All samples exhibit XRD peaks that are attributed to the typical face-centered-cubic fluorite structure. However, the XRD peaks for both of the dried

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3021 Table 1. Summary of Ceria Crystallite and Particle Sizes Measured by Different Methods

sample Ce(IV) method Ce(III) method

heat treatment temp, °C

XRD crystallite size, nm

40 450 800 40 450 800

3.1 6.9 32.9 3.9 6.3 40.0

TEM particle size, nm ∼4 ∼7 ∼4 10-20

Table 2. Values of d Spacing and Lattice Parameter for Ceria Powders Prepared by the Ce(IV) Method after Drying and Calcination at Different Temperatures temp, °C

111

40 450 800

3.217 3.120 3.115

d value 200 220 2.752 2.704 2.698

1.931 1.908 1.907

311

lattice parameter, Å

1.652 1.633 1.628

5.514 5.404 5.395

samples before calcination are very broad, indicating that these samples have low crystallinity or contain a large portion of the amorphous phase (see DSC data next). The XRD peaks become sharper with increasing calcination temperature, indicating an increase in the crystallite size and crystallization of the amorphous phase with calcination temperature. The crystallite sizes calculated by the Scherrer equation are given in Table 1. The crystallite size increases by 3-5-fold after calcination at the higher temperatures. The ceria samples prepared by the Ce(IV) method exhibit an increase in the XRD peak 2θ values with increasing calcination temperature, as shown in Figure 1. The corresponding d-spacing values and the lattice parameters are given in Table 2. As shown, the lattice parameters decrease with increasing calcination temperature. Such a change in the lattice parameters is, however, not observed for the ceria samples prepared by the Ce(III) method, and the lattice parameter for Ce(III) derived samples at different heat treatments is about 5.400 Å. Figures 3 and 4 show the TGA/DSC curves for the dried ceria precursor prepared by the Ce(IV) and Ce(III) methods. The dried samples may consist of three portions of materials: (A) crystalline ceria, CeO2, (B) crystalline ceria with structural water, CeO2‚2H2O, and (C) amorphous cerium hydroxide, Ce(OH)4. For both samples, the endothermic peaks in 50-120 °C could be due to crystallization of the amorphous portion in the sample, Ce(OH)4, to crystalline CeO2. The larger endothermic peak for the Ce(IV)-derived sample as compared to that for the Ce(III)-derived sample indicates that the former contains more amorphous phase than the latter. The total weight loss up to 800 °C for the Ce(IV) sample is about 16.85% (or 1.94 for n, which is the number of water molecules per mole of cerium in the dried sample). This is consistent with the change of [Ce(OH)4 + CeO2‚2H2O] to [CeO2]. The two endothermic peaks at about 313 and 436 °C in Figure 3 can be attributed to the loss of the structural water molecules. In contrast, the total weight loss for the Ce(III)-derived sample is only about 9.58%, indicating less structure water associated with ceria in the dried sample. The less obvious endothermic peaks for the structural water loss for this sample (Figure 4) also agree with the weight loss data. These results suggest that the Ce(IV)-derived sample contains primarily Ce(OH)4 and CeO2‚2H2O, while the Ce(III)-derived sample is a mixture of three materials: Ce(OH)2O, CeO2, and CeO2‚H2O.

Figure 3. DSC/TGA analysis of powder prepared from the Ce(IV) precursor.

Figure 4. DSC/TGA analysis of powder prepared from the Ce(III) precursor.

Figure 5 shows TEM images of ceria-based powders prepared by both methods after drying at 40 °C and calcination at 450 °C. As shown in Figure 5A, the sample prepared by the Ce(III) method after drying contains crystallites of nonuniform shape and approximately 4 nm in size. After calcination at 450 °C, the sample contains elliptical-shaped particles of about 10-20 nm in size, as shown in Figure 5B. In contrast, the sample prepared by the Ce(IV) method after drying includes crystallites of about 4 nm in size, as shown in Figure 5C. After calcination at 450 °C, the particles grow to only about 7-8 nm and remain in almost spherical shape. Table 3 summarizes the pore structure of the samples prepared by the two methods. The surface area and pore volume of both samples decrease with increasing calcination temperature. Both samples calcined at 450 °C have similar specific surface areas, total pore volumes, and average pore diameters. The average pore diameter increases with increasing calcination temperature. These are consistent with the pore structure change of the solgel-derived alumina, zirconia, and titania.24,25 These changes in the pore structure are due to coarsening of the compacted particles as a result of relocation of materials from the convex (or flat) grain surface to the more concave necks or phase transformation. The particle (or aggregate) size of the ceria-based powders can be estimated from the TEM micrographs. The TEM results are compared with the crystallite size determined by XRD in Table 1 for these two ceria-based samples. At 40 °C, the crystallite size is the same as the particle size for both ceria precursor samples. This indicates that the ceria crystallites of both samples in the dried ceria precursor precipitates are not aggregated. However, after calcination at 450 °C, the particle size for the ceria sample prepared by the Ce-

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Figure 5. TEM photographs of CeO2 particles prepared from Ce(NO3)3 material dried at 40 °C (A) and calcined at 450 °C (B) and from Ce(NO3)4 material dried at 40 °C (C) and calcined at 450 °C (D). Table 3. Pore Structure of Ceria Powders

sample Ce(IV) method Ce(III) method

pore average pore calcination BET surface volume, cm3/g diameter, Å temp, °C area, m2/g 450 600 800 450 600 800

101 92 12 109 70 23

0.162 0.144 0.0269 0.169 0.142 0.066

64 54 90 69 80 113

(III) method is much larger than the crystallite size, while the ceria sample prepared by the Ce(IV) method has the same crystallite and particle size. These results show that the ceria crystallites prepared by the Ce(III) method can easily aggregate upon calcination but the ceria crystallites prepared by the Ce(IV) method are better dispersed. Ceria sols were obtained from the dried ceria precursor precipitates prepared by the Ce(IV) and Ce(III) methods following the procedures described in the Experimental Section. Major characteristics of the two sols are summarized in Table 4. The ceria sol prepared by the Ce(IV) method is stable in a wider pH range and at high concentration up to 300 g of CeO2/L. In contrast, the ceria sol prepared by the Ce(III) method is only stable in a narrow pH range and at a low ceria

Table 4. Comparison of the Major Properties of Ceria Sols Prepared from Ceria Powders of Ce(IV) and Ce(III) Methods Ce(IV)-derived sol pH range highest concn (g of CeO2/L) color transparency viscosity (cP) stability

Ce(III)-derived sol

1.0-4.0 300

1.5-2.5 20

yellow no 1.18 stable

light yellow yes ∼1 stable

concentration. Because of the higher solid concentration, the ceria sol prepared by the Ce(IV) method is more viscous and less transparent compared to the Ce(III) ceria sol. The difference in the properties of the ceria sols prepared by the two methods is related to the properties of the ceria particles formed in the step of hydrolysis and condensation during the material synthesis process, as will be discussed in detail in the next section. Discussion Table 5 compares the material properties in various stages during the synthesis by the Ce(IV) and Ce(III)

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3023 Table 5. Comparison of the Material Properties in Various Stages during the Synthesis by the Ce(IV) and Ce(III) Methodsa

stage

method

chemical formula (TGA/DSC)

precipitates

IV III IV

Ce(OH)4 Ce(OH)2O Ce(OH)4 + CeO2‚2H2O

III IV

Ce(OH)2O + CeO2 + CeO2‚H2O CeO2

III IV

CeO2 Ce(OH)4 and CeO2‚2H2O

III

Ce(OH)2O and CeO2‚H2O

dried

calcined (450 °C) sols

a

crystallite structure (XRD) amorphous amorphous amorphous and fluorite amorphous and fluorite fluorite fluorite amorphous and fluorite amorphous and fluorite

crystallite size (XRD)

particle size (TEM)

aggregate

lattice size (XRD)

5.514

3.1

∼4

yes yes no

3.9

∼4

no

5.400

6.9

∼7

no

5.404

6.3

10-20

yes no

5.400

yes

Starting materials: for the Ce(IV) method, Ce(NO3)4; for the Ce(III) method, Ce(NO3)3, H2O2, and NH3‚H2O.

methods. During the hydrolysis step with Ce(NO3)4 as the precursor in the Ce(IV) method, Ce(NO3)4 reacts homogeneously with water molecules surrounding the precursor molecules. Hydrolysis converts Ce(NO3)4 to the fully hydrolyzed Ce(OH)4, which subsequently condensed to form more dense, spherical primary particles. These particles aggregate at low pH (∼0.34) and precipitate. The hydrolysis and condensation reactions under the acidic environment can be represented by

Ce4+ + nH2O S [Ce(H2O)n]4+ [Ce(H2O)n]4+ + mH2O S [Ce(OH)m(H2O)n-m](4-m)+ + mH3O+ [Ce(OH)m(H2O)n-m](4-m)+ + H2O S CeO2‚nH2O + H3O+ The CeO2‚nH2O precipitates continuously separate from solution, driving the reaction toward the right-hand side. In contrast, during the hydrolysis step with Ce(NO3)3 as the precursor in the Ce(III) method, Ce(NO3)3 reacts with H2O2 (oxidation) and H2O (hydrolysis) at the same time, following the addition of a precipitating agent of ammonium hydroxide, resulting in partially hydrolyzed cerium [such as Ce(OH)2O]. Nonspherical, more faceted primary particles are formed through the condensation of the partially hydrolyzed cerium. The addition of ammonium hydroxide promotes aggregation of the primary particles. The aggregates precipitate quickly. The reactions under the basic environment can be represented by

Ce3+ + H2O2 + 2H+(aq) S Ce4+ + 2H2O Ce4+ + nH2O S [Ce(H2O)n]4+ [Ce(H2O)n]4+ + mOH- S [Ce(OH)m (H2O)n-m](4-m)+ + mH2O [Ce(OH)m(H2O)n-m](4-m)+ + OH- S CeO2‚nH2O + H2O The large aggregates from the methods were dried at 40 °C to remove the free water. The drying process creates a large capillary pressure, which redisperses the loosely bound aggregates to the primary particles, as confirmed by the TEM data for the dried samples. The

XRD and TGA/DSC presented previously suggest that the particles in the samples dried at 40 °C consist of both crystalline and amorphous phases. During the subsequent heat treatment, the amorphous part was transformed to the crystalline phase. For the Ce(IV)-derived ceria, the amorphous phase is converted to the crystalline phase by crystal growth on the existing crystallites; i.e., the existing crystallites grow by taking the ceria from the amorphous phase in the surrounding area. As a result, the calcined ceria particles are larger in size but not aggregated. The more faceted ceria particles prepared by the Ce(III) method turn to aggregate during crystal growth and phase transformation. The crystalline phase of the dried ceria precursor samples prepared by both methods exhibits a fluorite structure with slightly larger 2θ values (smaller lattice parameter) for the Ce(III)-derived sample, which contains larger Ce3+ ions in the crystal lattice. The 2θ values for the Ce(III)-derived sample do not change with the calcination temperature. In contrast, the lattice parameter for the Ce(IV)-derived sample decreases with increasing calcination temperature or increasing crystallite size. A similar relationship between the lattice parameter and crystallite size for ceria was also noticed by other researchers,26,27 but the reason for the change in the lattice constant was not fully understood. Zhang et al.27 believed that the reason for lattice expansion in small particles (d < 20 nm) is the substitution of Ce4+ sites by larger Ce3+ ions rather than the defects such as inclinations in multiple-twinned particles or volume expansion in high-angle boundaries. However, this is unlikely the reason in the present case for the Ce(IV)derived ceria sample because cerium(IV) nitrate was used as the precursor in preparing the sample. It has been suggested that other factors, such as surface stress and other atomic forces, may affect the lattice expansion.28 The surface stress affecting the lattice expansion derives from the hydrogen atoms. When the crystallites are not aggregated, the existence of the hydrogen atoms, which are bound to the lattice oxygen through hydrogen bonds, could change the surface stress, leading to the lattice expansion. Therefore, the difference in the lattice expansion between ceria samples prepared by the Ce(IV) and Ce(III) methods could be a result of the different degrees in the agglomeration for the ceria crystallites in these two samples. Stable ceria sols could be obtained from the dried, well-dispersed ceria precursor powders prepared by both

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methods. However, the results are different for the different methods. Stable ceria sol at very high ceria concentration could be obtained from the Ce(IV)-derived dried ceria precursor powder at a larger pH range (above pH 1, higher than the pH for the solution in the hydrolysis and condensation step, ∼0.3). Stable, but lowsolid-concentration ceria sol could be obtained from the Ce(III)-derived dried ceria precursor powder only with the addition of acid for peptization. In both cases, the sol formation process consists of dispersion of primary particles into a solution. In liquid, these primary particles turn to aggregate to larger particles under the influence of intramolecular forces. As discussed before, the Ce(IV)-derived ceria consists of dense, more spherical primary particles. In the solution, the particle surface is more uniformly charged because of its spherical geometry. As a result, the particles are less likely to aggregate even at high solid concentration because of the electric repulse force. In contrast, the Ce(III)derived ceria precursor powder contains more faceted particles. The surface charge distribution for these nonspherical particles is less uniform on the particle surface. These primary particles are more likely to aggregate and precipitate, especially at high solid concentration. Conclusions Ultrafine ceria-based powders were prepared by two methods with cerium(IV) and cerium(III) nitrates as the precursors. Hydrolysis and condensation of the cerium(IV) and cerium(III) nitrates yield respectively the precipitates with molecular formulas of CeO2‚2H2O [or Ce(OH)4] and CeO2‚H2O [or Ce(OH)2O]. The Ce(IV)derived ceria precursor primary particles are of spherical shape versus the more faceted primary particles formed from the Ce(III) precursor. The dried ceria precursor powders from Ce(IV) and Ce(III) are well dispersed as primary particles. Redispersing the Ce(IV)derived ceria precursor powder in water results in stable, high-concentration (300 g of CeO2/L) ceria sol because of the low tendency for aggregation of the spherical ceria primary particles prepared by the Ce(IV) method. In contrast, the stable ceria sol can be obtained from the Ce(III)-derived ceria precursor powder at low solid concentration (20 g of CeO2/L) with acid for peptization because of the more faceted shape of the ceria precursor primary particles. The ceria precursor precipitates from Ce(IV) and Ce(III) precursors also exhibit different physical characteristics upon calcination at high temperatures. After calcination at 450 °C, the Ce(IV)-derived ceria particles grow in size when the amorphous part of ceria particles is transformed to the crystalline phase. For the Ce(III)-derived ceria, the phase transformation at 450 °C causes aggregation of the grown crystallite particles. Acknowledgment The authors are grateful to Honda R&D Americas, Inc., for financial support of this work. Note Added after ASAP Posting. This article was released on 2/28/04 with an incorrect author designation. The correct version was posted on 3/3/04. Literature Cited (1) Mochizuki, S. Infrared Optical Properties of Cerium Dioxide. Phys. Status Solidi B 1982, 114, 189.

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Received for review August 18, 2003 Revised manuscript received December 15, 2003 Accepted December 17, 2003 IE030676D