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Synthesis and rheological properties of mesoporous nanocrystalline CeO2 via sol–gel process
Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 61–68
Synthesis and rheological properties of mesoporous nanocrystalline CeO2 via sol–gel process N. Phonthammachaia , M. Rumruangwonga , E. Gularib , A.M. Jamiesonc , S. Jitkarnkaa,∗∗ , S. Wongkasemjita,∗ a
c
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, USA Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH, USA Received 5 January 2004; accepted 26 August 2004 Available online 27 September 2004
Abstract Cubic-phase CeO2 with high surface area was prepared using sol–gel process. The effect of sol–gel parameters, viz. HCl:alkoxide and water:alkoxide molar ratios, indicated that the gelation time increased with acid concentration and the lowest gelation time was found at 55 water:alkoxide molar ratio. The XRD patterns of cerium dioxide at different calcination temperature (300–800 ◦ C) and times (1–7 h) showed increasing crystallinity as a function of temperature and time, while the scanning electron microscope (SEM) micrographs showed the agglomeration of crystallites to form large particle cerium dioxide at higher calcination temperatures. The Brunauer–Emmett–Teller (BET) specific surface area increased with decreasing the HCl:alkoxide and water:alkoxide molar ratios, calcination temperature and time. The highest surface area was obtained at 0.8:1:55 HCl:alkoxide:water molar ratio and 400 ◦ C calcination for 1 h (180 m2 /g). The viscoelastic properties of different HCl:alkoxide ratio solutions were studied using oscillatory shear. The fractal dimension determined from the frequency-scaling exponent of the modulus at the gel point indicated a dense critical gel structure at high HCl ratio and tight structure at low amount of acid. The gel strength increased as a function of acid molar ratio and the 0.8 molar ratio gave the highest gel strength. © 2004 Elsevier B.V. All rights reserved. Keywords: Cerium glycolate; Cerium dioxide; Complex viscosity; Rheology and sol–gel process
1. Introduction CeO2 , the most significant oxide of rare earth elements in industrial catalysis, has been widely used and investigated as a structural and electronic promoter to improve the activity, selectivity and thermal stability of catalysts. Specifically, CeO2 has potential uses for removal of soot from diesel engine exhaust, removal of organics from wastewaters (catalytic wet oxidation) and as a promoter of catalysts in environmental clean-up and fuel cell technologies. In addition to these applications, recently much effort has been dedicated to study the role of ceria in well-established industrial processes, such ∗
Corresponding author. Tel.: +66 2 218 4133; fax: +66 2 215 4459. Co-corresponding author. Tel.: +66 2 218 4148; fax: +66 2 215 4459. E-mail addresses: [email protected] (S. Jitkarnka), [email protected] (S. Wongkasemjit). ∗∗
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.030
as FCC, TWCs and ethylbenzene dehydrogenation, where CeO2 is a key component in catalyst formulation [1–5]. It is very well known that the main role of ceria in this complex mixture is to provide oxygen buffering capacity during the rich/lean oscillation of exhaust gases and affect the conversion of the three major pollutants (CO, HCs and NO) under conditions typically encountered in the normal operation of a three-way catalyst. The main problem of ceria is its surface area and stability of catalyst. Generally, the higher surface area leads to the better activity of catalyst and it should be stable at high-temperature operation. In all commercial applications, stability of textural properties and resistance to sintering after aging play an important role. Several methods have recently been described for the preparation of CeO2 to be used as catalyst. These range from the high-temperature firing or high-energy milling and conventional co-precipitation to sol–gel techniques. Some of the
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approaches that have been recently applied to prepare powdered cerium oxide are homogeneous precipitation technique with different precipitating agents and additives, hydrothermal synthesis, spray pyrolysis methods, inert gas condensation of cerium followed by oxidation, thermal decomposition of its carbonate, microemulsion and electrochemical methods [6–11]. In 1988, Hsu et al. [12] prepared colloidal dispersions containing spherical cerium(IV) oxide using ceric sulfate and H2 SO4 . The highest specific surface area with an average particle diameter of 0.10 m was 19.3 m2 /g. Terribile et al. [13] synthesized the mesoporous ceria using a hybrid organic/inorganic route of CeCl3 ·7H2 O containing cetyltrimethyl ammonium bromide as surfactant. They found that it is possible to obtain Brunauer–Emmett–Teller (BET) surface area of 200 m2 /g after calcination at 430 ◦ C. However, the surface area dropped to 40 m2 /g at T > 800 ◦ C. Ding and Liu [14] prepared the ultra-fine cerium(IV) oxide using reversed micelles of cerium nitrate solution and most of the particles were distributed between 2 and 6 nm. The goal of the past studies was to improve the special features of ceria, such as its redox/oxidation properties and its high oxygen mobility. The key challenge is how to increase specific surface area with homogeneous distribution of pore size. For the last two decades, the sol–gel process has become one of the successful techniques for preparing metallic oxide materials with high-specific surface area and homogeneous distribution [15]. Furthermore, the product after sol–gel processing and sintering can be easily prepared in different forms, such as powder, monoliths, thin film and membranes. Previous investigations carried out to characterize the solto-gel transition have used spectroscopic techniques, and a little was done with rheological measurements, which are sensitive to the structural and textural evolution of gels and are complementary to spectroscopic experiments. Knowledge of the evolution in rheological properties during sol–gel processing is a useful guide to the manufacturer when formulating dispersion to optimize the physical properties required in the final product [16]. Thus, in this work, the objectives are not only to synthesize high surface area cubic form of cerium dioxide, but also to study the rheological properties of cerium glycolate complex, synthesized directly from inexpensive and widely available cerium hydroxide and ethylene glycol via the oxide one pot synthesis (OOPS) method [17,18]. The influence of the acid concentration used in acidcatalyzed hydrolysis and the effect of calcination temperature on morphology, surface area and viscoelastic properties are also investigated.
2. Experimental 2.1. Materials Cerium(IV) hydroxide (Ce(OH)4 ) containing 87.4% CeO2 was purchased from Aldrich Chemical Co. Inc. (USA)
and used as received. Ethylene glycol (EG) was purchased from Farmitalia Carlo Erba (Barcelona) and purified by fractional distillation under nitrogen at atmospheric pressure, at 200 ◦ C before use. Sodium hydroxide was purchased from Merck Company Co., Ltd. (Germany) and used as received. Triethylenetetramine (TETA) was purchased from Facai Polytech. Co., Ltd. (Bangkok, Thailand) and distilled under vacuum (0.1 mmHg) at 130 ◦ C prior to use. Methanol and acetonitrile were purchased from Lab-Scan Company Co. Ltd. and purified by standard techniques. 2.2. Instrumental Fourier transform infrared spectra (FT-IR) were recorded on a VECOR3.0 BRUKER spectrometer with a spectral resolution of 4 cm−1 using transparent KBr pellets containing 0.001 g of sample mixed with 0.06 g of KBr. Thermo gravimetric analysis (TGA) was carried out using a Perkin-Elmer thermal analysis system with a heating rate of 10 ◦ C/min over the 30–800 ◦ C temperature range. 2.3. Preparation of cerium glycolate Preparation of cerium glycolate complex was duplicated from previous work [19]. A mixture of 5 mmol (1.04 g) cerium hydroxide, 18 mL of ethylene glycol and 5 mmol (0.73 g) triethylenetetramine with sodium hydroxide (NaOH) at about 10 mol% equivalent to cerium hydroxide were mixed, magnetically stirred and heated to the boiling point of ethylene glycol for 18 h under nitrogen to distill off ethylene glycol along with water liberated from the reaction. The reaction mixture was cooled overnight under nitrogen. The product was filtered and washed with acetonitrile (3 × 15 mL), followed by drying under vacuum (10−2 torr). The precursor obtained was identified using TGA, 1 H NMR and FT-IR. The same characterization results as previous work [19] were obtained. FT-IR shows: 2939 and 2873 cm−1 (υ C H), 1080 cm−1 (υ Ce O C) and 550 cm−1 (υ Ce O). 1 H NMR spectra of product recorded in deuterated DMSO: 3.4 ppm was assigned to chelated glycolate ligands. TGA showed one transition at 400 ◦ C and the percentage ceramic yield was 65.9%. 2.4. Sol–gel processing of cerium glycolate Cerium glycolate complex was mixed with the hydrochloric acid solution. The acid:alkoxide molar ratio was varied from 0 to 2.0 and water:alkoxide molar ratio was varied from 50 to 65 at room temperature. Cerium dioxide was formed after calcining the ceria gel at temperature varying from 300 to 800 ◦ C and calcination time from 1 to 7 h. The calcined products were characterized using FT-IR, XRD, scanning electron microscope (SEM) and BET.
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2.5. Rheological study of cerium glycolate Gelation occurs when aggregation of particles or molecules takes place in a liquid, under the action of van der Waals forces or via the formation of covalent or noncovalent bonds. The process can be conveniently monitored using rheological measurement techniques [16]. The rheometric measurements were conducted using an ARES rheometer with parallel plate geometry, 25 mm in diameter. The storage (G ) and loss (G ) moduli were determined using oscillatory shear at frequencies in the range 0.2–6.4 rad/s. The strain amplitude was 10%, which is enough to ensure that all experiments were conducted within the linear viscoelastic region, where G and G are independent of the strain amplitude. Cerium glycolate (0.026 g) was hydrolyzed in different HCl:alkoxide molar ratios of 0.8, 0.9, 1.0 or 1.1. The hydrolysis temperature was selected to be 30 ◦ C. The mixtures were stirred until homogeneous before transferring to the rheometer. 2.6. Characterization of calcined material Crystallinity was characterized using a D/MAX-2200H Rigaku diffractometer with CuK␣ radiation on specimens prepared by packing the sample powder into a glass holder. The diffracted intensity was measured by step scanning in the 2θ range from 5◦ to 90◦ . Specific surface areas, nitrogen adsorption–desorption and pore size distributions were determined using an Autosorp-1 gas sorption system (Quantachrome Corporation) via the BET method. A gaseous mixture of nitrogen and helium was allowed to flow through the analyzer at a constant rate of 30 cm3 /min. Nitrogen was used to calibrate the analyzer and also as the adsorbate at liquid nitrogen temperature. The samples were thoroughly outgassed for 2 h at 150 ◦ C, prior to exposure to the adsorbent gas. Material morphology was observed using a JEOL 5200-2AE (MP15152001) scanning electron microscope. Samples were prepared for SEM analysis by attachment to aluminum stubs, after pyrolyzing at 800 ◦ C. Prior to analysis, the specimens were dried in a vacuum oven at 70 ◦ C for 5 h followed by coating with gold via vapor deposition. Micrographs of the pyrolyzed sample surfaces were obtained at ×7500 magnification.
3. Results and discussion To determine the effect of sol–gel parameters on the formation and properties of ceria gel, a series of gels were formed with different acid and water contents at room temperature. First parameter examined was the effect of acid molar ratio, at constant water to cerium glycolate complex with the molar ratio of 50, the gels were formed at different amounts of hydrochloric acid, as shown in Table 1. At acid:alkoxide molar ratios from 0 to 0.6, the alkoxides are insoluble at 5–7 pH range. When the amount of acid is increased from 0.8 to 1.1 molar ratios, a cloudy yellow solution first occurs fol-
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Table 1 The summary of the gelation time and pH at different acid:alkoxide molar ratio and water:alkoxide molar ratio Acid:alkoxide (mol:mol)
Water:alkoxide (mol:mol)
Gelation time (s)
pH
0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50
0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 868 931 1183 5206 Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated
7 6 6 5 5 5 5 3 3 2 2 2 2 2 2 2 1 1 1 1
lowed by the formation of solid gel and the samples have pH between 2 and 3. A larger acid amount beyond this point drastically increases the gel time. When using HCl:alkoxide molar ratios greater than 1.2, clear yellow solutions followed by precipitation are obtained. The pH of these ratios is in the range of 1–2, resulting in fast hydrolysis and condensation, causing immediate precipitation of ceria. From these results, we conclude that at low acid molar ratios giving higher pH value the hydrolysis rate is slow and becomes faster with increasing amount of acid to a molar ratio of 0.8 to form a polymeric gel. Further increasing the amount of acid, leads to increase gelation time. The addition of more hydrochloric acid did not only speed up the hydrolysis rate by protonating OR ligand, providing a better leaving group, but also increased the gel time by decreasing the rate of condensation. The hydroxy ligands of the metal atom are protonated to form M OH2 + , making the oxygen atom no longer nucleophilic, resulting in a decrease in the driving force for substitution. As more acid is added to the system, more hydroxo ligands are protonated and the condensation reaction becomes slow, increasing the gelation time [20]. For the effect of water alkoxide ratio studied at the ratio between 20 and 40, at the pH value of 2, precipitate was formed (Table 2). The results show the same trend as those using high acid ratios, forming precipitate at pH around 1–2. When the water:alkoxide molar ratio increases from 50 to 55, the gelation time decreases dramatically from 868 to 208 s. The decrease in the amount of water reduces the number of hydroxyl groups for hydrolysis; therefore the condensation is slowed down, resulting in an increase of the gelation time. When increasing the water:alkoxide ratio to 60 and 65, the gelation time is also increased. At larger water:alkoxide molar ratios, 100 and 120, the samples turn to a sol without forming
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Table 2 The summary of the gelation time and pH at different acid:alkoxide and water:alkoxide molar ratio Acid:alkoxide (mol:mol) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
Water:alkoxide (mol:mol) 20 40 50 55 60 65 100 120
Gelation time (s)
pH
Precipitated Precipitated 868 208 774 1494 Sol Sol
2 2 3 3 3 3 4 4
a gel. This is an expecting result since, the higher the water content, the slower the condensation rate, due to the reversed condensation reaction. After the ceria gels were calcined at 500 ◦ C for 1 h, the specific surface area (Fig. 1a) decreased with increasing acid:alkoxide molar ratio from 0.8 to 1.1. This is in agreement with the explanation given above that at higher acid amounts the rate of condensation is greatly slowed down and the particles with little or no bridging are formed. The highest specific surface area at 0.8 molar ratio is 148 m2 /g. The specific surface area of the calcined gel at various water:alkoxide ratios, see Fig. 1b, is almost the same (148–152 m2 /g), and
Fig. 1. The specific surface area (m2 /g) after calcining at 500 ◦ C for 1 h using (a) different HCl:alkoxide molar ratio and (b) different water:alkoxide molar ratio.
Fig. 2. XRD patterns of ceria oxides prepared using the HCl:alkoxide:water ratio of 0.8:1:55, and calcined for 1 h at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C and (f) 800 ◦ C.
starts to decrease as the ratio increases to 65 (118 m2 /g) and 70 (65 m2 /g), due to higher water amount driving the condensation reaction to go backward, resulting in a longer gelation time. The short chain network was thus formed, giving lower surface area after the gels were calcined. The effect of calcination temperature and time of ceria gels at various temperatures (400–800 ◦ C) and times (1, 3, 5 and 7 h) were studied on the samples prepared with the HCl:alkoxide:water molar ratio of 0.8:1:55. The crystallinity of CeO2 was changed with increasing calcination temperature and time. The XRD patterns in Fig. 2 show an amorphous solid at 300 ◦ C whereas at 400 ◦ C the cubic pattern of CeO2 [21] is identifiable, but it exhibits broad and low intensity peaks. The intensity and the sharpness of the XRD peaks increases dramatically for the samples calcined at the temperature of 700 and 800 ◦ C, representing the higher crystallinity of CeO2 . Similarly, the XRD patterns in Fig. 3 show the presence of cubic phase of CeO2 for all calcination times,
Fig. 3. XRD patterns of ceria oxides prepared using the HCl:alkoxide:water ratio of 0.8:1:55, and calcined at 500 ◦ C for (a) 1 h, (b) 3 h, (c) 5 h and (d) 7 h.
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Fig. 4. The specific surface area of the product formed with 0.8:1:55 acid:alkoxide:water molar ratio as a function of calcination temperature (400–800 ◦ C) and time (1, 3, 5, 7 h).
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Fig. 6. The nitrogen adsorption–desorption isoterm of mesoporous ceria obtained from 0.8 HCl:alkoxide molar ratio and calcined at 400 ◦ C for 3 h.
and indicate the same trend as the study of calcinations temperature in which the longer the calcination time, the sharper the peaks with higher intensity. Fig. 4 shows the decrease of specific surface area as a function of calcinations, temperature and time. The highest specific surface area (180 m2 /g) is obtained at the lowest calcinations time and temperature (400 ◦ C, 1 h). This temperature is confirmed by the presence of organic residue using TGA (Fig. 5), and it shows 100% ceramic yield, implying that pure CeO2 is produced at 400 ◦ C calcinations temperature. The surface area decreases to 71 m2 /g when the calcinations temperature reaches 800 ◦ C. The nitrogen adsorption–desorption isotherm of the material obtained at 0.8 HCl:alkoxide molar ratio and 400 ◦ C calcinations temperature for 3 h, shown in Fig. 6, is of type V isoterm with H3-type hysteresis loop. Its morphology is observed as aggregates of plate-like particles [22]. Examples of the SEM micrographs of ceria taken after calcining at 500 ◦ C (Fig. 7a) and 800 ◦ C (Fig. 7b) support the specific surface area results. At higher calcination tempera-
Fig. 5. The TGA thermogram of cerium gel calcined at 400 ◦ C for 1 h.
Fig. 7. Effect of the heat treatment on ceria prepared using the acid:alkoxide:water ratio of 0.8:1:55, and calcined for 1 h: (a) 500 ◦ C and (b) 800 ◦ C.
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Fig. 9. The plots of tan δ with time (s) at HCl:alkoxide molar ratio of 0.8.
nents n and n , G (ω) ∝ G (ω) ∝ ωn
Fig. 8. Effect of the heat treatment on ceria prepared using the acid:alkoxide:water ratio of 0.8:1:55, and calcined at 500 ◦ C for (a) 1 h and (b) 7 h.
(2)
the gel point is observed at a crossover point where n = n = n [16], as shown in Fig. 10 (acid:alkoxide molar ratio 0.8). The crossover of n and n is at 868 s, which is equal to the gelation time from the plot of tan δ. The different stages of the critical moments near the gelation are observed in a plot of G , G (Pa) with ω (rad/s) at pregel stage, gel point and postgel stage as seen in Fig. 11. For all investigations, the G (ω) is greater than G (ω) because of the colloidal nature of the system in which solid-like behavior becomes predominant at an initial stage. Both G and G could exhibit a power-law dependence on applied frequency, at the gel point (tg ) n = n = ngel , and the power-law plots for G and G were parallel [24]. Table 3 shows the viscoelastic exponent n at different acid:alkoxide molar ratios, the n values increase from ∼0.14 to 0.24 with increasing acid ratio. The results suggest that the viscoelastic exponent depends
tures the structures are agglomerated and the pores collapse, causing the decrease of the specific surface area. The same trend (Fig. 8) is also seen with increasing calcination times from 1 h (Fig. 8a) to 7 h (Fig. 8b). From the rheological point of view, for the case of different acid:alkoxide molar ratio 0.8, 0.9, 1.0 and 1.1, the gelation time (tg ) is determined by employing the well-established criterion of gelation, proposed by Winter and Chambon [23] tan δ =
nπ G = tan G 2
or δ =
nπ 2
(1)
at a frequency-independent value of tan δ obtained from a multi-frequency plot of tan δ versus gelation time (Fig. 9) in which G and G are storage and loss modulus, respectively, and n is viscoelastic exponent. The result indicates that tg values increase as a function of acid amount which is corresponding to the result in Table 1. An alternative method to determine the gel point is by plotting the viscoelastic expo-
Fig. 10. The plots of the apparent exponents, the storage moduli (n ) and the loss moduli (n ) during the course of gelation for the 0.8 HCl:alkoxide molar ratio.
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Fig. 11. The frequency dependence curves of G (ω) and G (ω) at pregel stage (B = −3), gel point (B = 0) and postgel stage (B = 3) of (a) 0.8, (b) 0.9, (c) 1.0 and (d) 1.1 HCl:alkoxide ratio.
on the acid concentrations and higher crosslink density will probably reduce the value of n. The different viscoelastic exponents reflect different structures at the sol–gel transition point. The experimentally viscoelastic exponent, n, can be further related to the mass fractal dimension, df , and the structure may be described by a fractal dimension as Eq. (3) M = Rdf
(3)
where M is the mass of molecular cluster and R is the radius of gyration [16]. The connection between the fractal dimensions and the viscoelastic exponents is given in Eq. (4) by
Table 3 The viscoelastic exponent, fractal dimension and gelation time at different HCl:alkoxide molar ratios HCl:alkoxide:water (molar ratio)
n
df
Gelation time (s)
0.8:1:50 0.9:1:50 1.0:1:50 1.1:1:50
0.1389 0.1411 0.1560 0.2431
2.3786 2.3766 2.3628 2.2795
868 931 1182 5206
Muthukumar [24] n=
d (d + 2 − 2df ) 2 (d + 2 − df )
(4)
where, n = viscoelastic exponent, d = space dimension and df = fractal dimension, shows that the value of n also depends on the fractal dimension, i.e. also on cluster structure. The estimation of fractal dimensions is calculated because the scaling exponents do not have a direct physical meaning. The connection between the viscoelastic exponents and fractal dimensions is justified because the value for viscoelastic exponent comes from the spanning cluster under stress that is dependent on the polymer structure, meaning that for an aggregate structure having a lower fractal dimension, the molecular weight grows slower with radius than the one having a higher fractal dimension. The aggregated structure having lower fractal dimension can thus be described as a more open structure [25]. The values of fractal dimensions in Table 3 decrease with increasing n value and increasing acid ratio, indicating that the incipient gels with high values of the relaxation exponent have an open structure and gels with lower n values have a tight structure [15,16]. These results are corresponding to the explanation of Tables 1 and 2 such that the
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perature and time affect the specific surface area and the crystallinity. The highest specific surface area was obtained at 0.8:1:55 HCl:alkoxide:water molar ratio gel calcined at 400 ◦ C for 1 h (180 m2 /g). The viscoelastic properties of the incipient ceria gels at different HCl:alkoxide molar ratios indicate that the gelation time increased as a function of the amount of HCl. The fractal dimension determined from the frequency-scaling exponent of the modulus at the gel point indicated a dense critical gel structure at high HCl ratio and tight structure at low amount of acid. The gel strength increased as a function of acid:alkoxide molar ratio, 0.8 molar ratio gave the highest gel strength. Acknowledgements Fig. 12. The plots of gel strength parameter S at the gel point as a function of HCl:alkoxide molar ratio 0.8, 0.9, 1.0 and 1.1.
network formation of ceria gel prepared at higher acid ratio is shorter than that prepared at lower acid ratio due to the faster hydrolysis and condensation rates, as discussed previously. Moreover, the calcined product shows the lower specific surface area at higher acid ratio meaning lower porosity. The gel-strength parameter (S), which is depended on the cross-linking density and the molecular chain flexibility, is shown in Eq. (5). S=
G ωn Γ (1 − n) cos
δ
(5)
where G is the storage modulus, ω is the frequency (rad/s), Γ (1−n) is the gamma function, n is the viscoelastic exponent and δ is the strain being independent of frequency but proportional to the relaxation exponent (Eq. (6)). δ=
nπ 2
(6)
The effect of viscoelastic exponent on the strength of gel in Fig. 12 shows that the strength slightly increases from 0.8 to 1.0 molar ratio and then dramatically decreases at 1.1 molar ratio. From the relationship between the strength value and the cross-linking density and the molecular chain flexibility, it is indicated that 1.1 molar ratio has the lowest cross-linking density of the cerium dioxide network which is again corresponding to the explanation of Table 1. Thus, at low acid ratio, the critical gel cluster is relatively stronger than the gel formed, at high acid ratio.
4. Conclusions High surface area cerium dioxide was successfully synthesized via the sol–gel process using inexpensive and moisturestable cerium glycolate precursor. The gel can be formed at 0.8–1.1 HCl:alkoxide and 50–65 water:alkoxide molar ratios. Gelation time increases and the specific surface area decreases with increasing amount of HCl. Calcination tem-
This research work is supported by the Postgraduate Education and Research Program in Petroleum and Petrochemical Technology (ADB) Fund, Ratchadapisakesompoch Fund, Chulalongkorn University and the Thailand Research Fund (TRF). References [1] R. Craciun, W. Daniell, H. Knozinger, Appl. Catal. A-Gen. 230 (2002) 153. [2] D. Terribile, A. Trovarelli, C.D. Leitenburg, A. Primavera, G. Dolcetti, Catal. Today 47 (1999) 133. [3] E.S. Putna, X.L. Bunluesin, X.L. Fan, R.J. Gorte, J.M. Vohs, R.E. Lakis, T. Egami, Catal. Today 50 (1999) 343. [4] A. Trovarelli, C.D. Leitenburg, M. Boaro, G. Dolcetti, Catal. Today 50 (1999) 353. [5] Y. Konishi, T. Murai, S. Asai, Ind. Eng. Chem. Res. 36 (1997) 2641. [6] Y. Izumi, Y. Iwata, K.I. Aika, J. Phys. Chem. 100 (1996) 9421. [7] W. Liu, M.F. Stephanopoulos, J. Catal. 153 (1995) 317. [8] H. Cordatos, T. Bunluesin, J. Stubenrauch, J.M. Vohs, R.J. Gorte, J. Phys. Chem. 100 (1996) 785. [9] A. Turkovic, Z.C. Orel, Solid State Ionics 89 (1996) 255. [10] Z.C. Orel, B. Orel, Sol. Energy Mat. Sol. C. 40 (1996) 205. [11] A. Turkovic, Z.C. Orel, Sol. Energy Mat. Sol. C. 45 (1997) 275. [12] W.P. Hsu, L. Ronnquist, E. Matijevic, Langmuir 4 (1988) 31. [13] D. Terribile, A. Trovarelli, J. Llorca, C.D. Leitenburg, G. Dolcetti, J. Catal. 178 (1998) 299. [14] X.Z. Ding, X.H. Liu, Mater. Sci. Eng. A-Struct. 224 (1997) 210. [15] N. Kao, S.N. Bhattacharya, J. Rheol. 42 (1998) 493. [16] A.L. Kjoniksen, B. Nystrom, Macromolecules 29 (1996) 5215. [17] N. Phonthammachai, T. Chairassameewong, E. Gulari, A.M. Jamieson, S. Wongkasemjit, J. Met. Mat. Min. 12 (2002) 23. [18] P. Piboonchaisit, S. Wongkasemjit, R. Laine, Science-Asia, J. Sci. Soc. Thailand 25 (1999) 113–119. [19] B. Ksapabutr, E. Gulari, S. Wongkasemjit, Mater. Chem. Phys. 1 (2004) 34. [20] D.A. Ward, E.I. Ko, Chem. Mater. 5 (1993) 956. [21] A.A. Antonova, O.V. Zhilina, G.G. Kagramanov, K.I. Kienskaya, V.V. Nazarov, I.A. Petropavlovskii, I.E. Fanasyutkina, Colloid J. 63 (2001) 662. [22] K.S.W. Singh, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, I. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [23] H.H. Winter, F. Chambon, J. Rheol. 30 (1986) 367. [24] M. Muthukumar, Macromocules 22 (1989) 4656. [25] M. Jokinen, E. Gyorvary, J.B. Rosenholm, Colloid Surf. A 141 (1998) 205.
Synthesis and rheological properties of mesoporous nanocrystalline CeO2 via sol–gel process N. Phonthammachaia , M. Rumruangwonga , E. Gularib , A.M. Jamiesonc , S. Jitkarnkaa,∗∗ , S. Wongkasemjita,∗ a
c
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, USA Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH, USA Received 5 January 2004; accepted 26 August 2004 Available online 27 September 2004
Abstract Cubic-phase CeO2 with high surface area was prepared using sol–gel process. The effect of sol–gel parameters, viz. HCl:alkoxide and water:alkoxide molar ratios, indicated that the gelation time increased with acid concentration and the lowest gelation time was found at 55 water:alkoxide molar ratio. The XRD patterns of cerium dioxide at different calcination temperature (300–800 ◦ C) and times (1–7 h) showed increasing crystallinity as a function of temperature and time, while the scanning electron microscope (SEM) micrographs showed the agglomeration of crystallites to form large particle cerium dioxide at higher calcination temperatures. The Brunauer–Emmett–Teller (BET) specific surface area increased with decreasing the HCl:alkoxide and water:alkoxide molar ratios, calcination temperature and time. The highest surface area was obtained at 0.8:1:55 HCl:alkoxide:water molar ratio and 400 ◦ C calcination for 1 h (180 m2 /g). The viscoelastic properties of different HCl:alkoxide ratio solutions were studied using oscillatory shear. The fractal dimension determined from the frequency-scaling exponent of the modulus at the gel point indicated a dense critical gel structure at high HCl ratio and tight structure at low amount of acid. The gel strength increased as a function of acid molar ratio and the 0.8 molar ratio gave the highest gel strength. © 2004 Elsevier B.V. All rights reserved. Keywords: Cerium glycolate; Cerium dioxide; Complex viscosity; Rheology and sol–gel process
1. Introduction CeO2 , the most significant oxide of rare earth elements in industrial catalysis, has been widely used and investigated as a structural and electronic promoter to improve the activity, selectivity and thermal stability of catalysts. Specifically, CeO2 has potential uses for removal of soot from diesel engine exhaust, removal of organics from wastewaters (catalytic wet oxidation) and as a promoter of catalysts in environmental clean-up and fuel cell technologies. In addition to these applications, recently much effort has been dedicated to study the role of ceria in well-established industrial processes, such ∗
Corresponding author. Tel.: +66 2 218 4133; fax: +66 2 215 4459. Co-corresponding author. Tel.: +66 2 218 4148; fax: +66 2 215 4459. E-mail addresses: [email protected] (S. Jitkarnka), [email protected] (S. Wongkasemjit). ∗∗
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.030
as FCC, TWCs and ethylbenzene dehydrogenation, where CeO2 is a key component in catalyst formulation [1–5]. It is very well known that the main role of ceria in this complex mixture is to provide oxygen buffering capacity during the rich/lean oscillation of exhaust gases and affect the conversion of the three major pollutants (CO, HCs and NO) under conditions typically encountered in the normal operation of a three-way catalyst. The main problem of ceria is its surface area and stability of catalyst. Generally, the higher surface area leads to the better activity of catalyst and it should be stable at high-temperature operation. In all commercial applications, stability of textural properties and resistance to sintering after aging play an important role. Several methods have recently been described for the preparation of CeO2 to be used as catalyst. These range from the high-temperature firing or high-energy milling and conventional co-precipitation to sol–gel techniques. Some of the
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approaches that have been recently applied to prepare powdered cerium oxide are homogeneous precipitation technique with different precipitating agents and additives, hydrothermal synthesis, spray pyrolysis methods, inert gas condensation of cerium followed by oxidation, thermal decomposition of its carbonate, microemulsion and electrochemical methods [6–11]. In 1988, Hsu et al. [12] prepared colloidal dispersions containing spherical cerium(IV) oxide using ceric sulfate and H2 SO4 . The highest specific surface area with an average particle diameter of 0.10 m was 19.3 m2 /g. Terribile et al. [13] synthesized the mesoporous ceria using a hybrid organic/inorganic route of CeCl3 ·7H2 O containing cetyltrimethyl ammonium bromide as surfactant. They found that it is possible to obtain Brunauer–Emmett–Teller (BET) surface area of 200 m2 /g after calcination at 430 ◦ C. However, the surface area dropped to 40 m2 /g at T > 800 ◦ C. Ding and Liu [14] prepared the ultra-fine cerium(IV) oxide using reversed micelles of cerium nitrate solution and most of the particles were distributed between 2 and 6 nm. The goal of the past studies was to improve the special features of ceria, such as its redox/oxidation properties and its high oxygen mobility. The key challenge is how to increase specific surface area with homogeneous distribution of pore size. For the last two decades, the sol–gel process has become one of the successful techniques for preparing metallic oxide materials with high-specific surface area and homogeneous distribution [15]. Furthermore, the product after sol–gel processing and sintering can be easily prepared in different forms, such as powder, monoliths, thin film and membranes. Previous investigations carried out to characterize the solto-gel transition have used spectroscopic techniques, and a little was done with rheological measurements, which are sensitive to the structural and textural evolution of gels and are complementary to spectroscopic experiments. Knowledge of the evolution in rheological properties during sol–gel processing is a useful guide to the manufacturer when formulating dispersion to optimize the physical properties required in the final product [16]. Thus, in this work, the objectives are not only to synthesize high surface area cubic form of cerium dioxide, but also to study the rheological properties of cerium glycolate complex, synthesized directly from inexpensive and widely available cerium hydroxide and ethylene glycol via the oxide one pot synthesis (OOPS) method [17,18]. The influence of the acid concentration used in acidcatalyzed hydrolysis and the effect of calcination temperature on morphology, surface area and viscoelastic properties are also investigated.
2. Experimental 2.1. Materials Cerium(IV) hydroxide (Ce(OH)4 ) containing 87.4% CeO2 was purchased from Aldrich Chemical Co. Inc. (USA)
and used as received. Ethylene glycol (EG) was purchased from Farmitalia Carlo Erba (Barcelona) and purified by fractional distillation under nitrogen at atmospheric pressure, at 200 ◦ C before use. Sodium hydroxide was purchased from Merck Company Co., Ltd. (Germany) and used as received. Triethylenetetramine (TETA) was purchased from Facai Polytech. Co., Ltd. (Bangkok, Thailand) and distilled under vacuum (0.1 mmHg) at 130 ◦ C prior to use. Methanol and acetonitrile were purchased from Lab-Scan Company Co. Ltd. and purified by standard techniques. 2.2. Instrumental Fourier transform infrared spectra (FT-IR) were recorded on a VECOR3.0 BRUKER spectrometer with a spectral resolution of 4 cm−1 using transparent KBr pellets containing 0.001 g of sample mixed with 0.06 g of KBr. Thermo gravimetric analysis (TGA) was carried out using a Perkin-Elmer thermal analysis system with a heating rate of 10 ◦ C/min over the 30–800 ◦ C temperature range. 2.3. Preparation of cerium glycolate Preparation of cerium glycolate complex was duplicated from previous work [19]. A mixture of 5 mmol (1.04 g) cerium hydroxide, 18 mL of ethylene glycol and 5 mmol (0.73 g) triethylenetetramine with sodium hydroxide (NaOH) at about 10 mol% equivalent to cerium hydroxide were mixed, magnetically stirred and heated to the boiling point of ethylene glycol for 18 h under nitrogen to distill off ethylene glycol along with water liberated from the reaction. The reaction mixture was cooled overnight under nitrogen. The product was filtered and washed with acetonitrile (3 × 15 mL), followed by drying under vacuum (10−2 torr). The precursor obtained was identified using TGA, 1 H NMR and FT-IR. The same characterization results as previous work [19] were obtained. FT-IR shows: 2939 and 2873 cm−1 (υ C H), 1080 cm−1 (υ Ce O C) and 550 cm−1 (υ Ce O). 1 H NMR spectra of product recorded in deuterated DMSO: 3.4 ppm was assigned to chelated glycolate ligands. TGA showed one transition at 400 ◦ C and the percentage ceramic yield was 65.9%. 2.4. Sol–gel processing of cerium glycolate Cerium glycolate complex was mixed with the hydrochloric acid solution. The acid:alkoxide molar ratio was varied from 0 to 2.0 and water:alkoxide molar ratio was varied from 50 to 65 at room temperature. Cerium dioxide was formed after calcining the ceria gel at temperature varying from 300 to 800 ◦ C and calcination time from 1 to 7 h. The calcined products were characterized using FT-IR, XRD, scanning electron microscope (SEM) and BET.
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2.5. Rheological study of cerium glycolate Gelation occurs when aggregation of particles or molecules takes place in a liquid, under the action of van der Waals forces or via the formation of covalent or noncovalent bonds. The process can be conveniently monitored using rheological measurement techniques [16]. The rheometric measurements were conducted using an ARES rheometer with parallel plate geometry, 25 mm in diameter. The storage (G ) and loss (G ) moduli were determined using oscillatory shear at frequencies in the range 0.2–6.4 rad/s. The strain amplitude was 10%, which is enough to ensure that all experiments were conducted within the linear viscoelastic region, where G and G are independent of the strain amplitude. Cerium glycolate (0.026 g) was hydrolyzed in different HCl:alkoxide molar ratios of 0.8, 0.9, 1.0 or 1.1. The hydrolysis temperature was selected to be 30 ◦ C. The mixtures were stirred until homogeneous before transferring to the rheometer. 2.6. Characterization of calcined material Crystallinity was characterized using a D/MAX-2200H Rigaku diffractometer with CuK␣ radiation on specimens prepared by packing the sample powder into a glass holder. The diffracted intensity was measured by step scanning in the 2θ range from 5◦ to 90◦ . Specific surface areas, nitrogen adsorption–desorption and pore size distributions were determined using an Autosorp-1 gas sorption system (Quantachrome Corporation) via the BET method. A gaseous mixture of nitrogen and helium was allowed to flow through the analyzer at a constant rate of 30 cm3 /min. Nitrogen was used to calibrate the analyzer and also as the adsorbate at liquid nitrogen temperature. The samples were thoroughly outgassed for 2 h at 150 ◦ C, prior to exposure to the adsorbent gas. Material morphology was observed using a JEOL 5200-2AE (MP15152001) scanning electron microscope. Samples were prepared for SEM analysis by attachment to aluminum stubs, after pyrolyzing at 800 ◦ C. Prior to analysis, the specimens were dried in a vacuum oven at 70 ◦ C for 5 h followed by coating with gold via vapor deposition. Micrographs of the pyrolyzed sample surfaces were obtained at ×7500 magnification.
3. Results and discussion To determine the effect of sol–gel parameters on the formation and properties of ceria gel, a series of gels were formed with different acid and water contents at room temperature. First parameter examined was the effect of acid molar ratio, at constant water to cerium glycolate complex with the molar ratio of 50, the gels were formed at different amounts of hydrochloric acid, as shown in Table 1. At acid:alkoxide molar ratios from 0 to 0.6, the alkoxides are insoluble at 5–7 pH range. When the amount of acid is increased from 0.8 to 1.1 molar ratios, a cloudy yellow solution first occurs fol-
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Table 1 The summary of the gelation time and pH at different acid:alkoxide molar ratio and water:alkoxide molar ratio Acid:alkoxide (mol:mol)
Water:alkoxide (mol:mol)
Gelation time (s)
pH
0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50
0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 0, not soluble 868 931 1183 5206 Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated Precipitated
7 6 6 5 5 5 5 3 3 2 2 2 2 2 2 2 1 1 1 1
lowed by the formation of solid gel and the samples have pH between 2 and 3. A larger acid amount beyond this point drastically increases the gel time. When using HCl:alkoxide molar ratios greater than 1.2, clear yellow solutions followed by precipitation are obtained. The pH of these ratios is in the range of 1–2, resulting in fast hydrolysis and condensation, causing immediate precipitation of ceria. From these results, we conclude that at low acid molar ratios giving higher pH value the hydrolysis rate is slow and becomes faster with increasing amount of acid to a molar ratio of 0.8 to form a polymeric gel. Further increasing the amount of acid, leads to increase gelation time. The addition of more hydrochloric acid did not only speed up the hydrolysis rate by protonating OR ligand, providing a better leaving group, but also increased the gel time by decreasing the rate of condensation. The hydroxy ligands of the metal atom are protonated to form M OH2 + , making the oxygen atom no longer nucleophilic, resulting in a decrease in the driving force for substitution. As more acid is added to the system, more hydroxo ligands are protonated and the condensation reaction becomes slow, increasing the gelation time [20]. For the effect of water alkoxide ratio studied at the ratio between 20 and 40, at the pH value of 2, precipitate was formed (Table 2). The results show the same trend as those using high acid ratios, forming precipitate at pH around 1–2. When the water:alkoxide molar ratio increases from 50 to 55, the gelation time decreases dramatically from 868 to 208 s. The decrease in the amount of water reduces the number of hydroxyl groups for hydrolysis; therefore the condensation is slowed down, resulting in an increase of the gelation time. When increasing the water:alkoxide ratio to 60 and 65, the gelation time is also increased. At larger water:alkoxide molar ratios, 100 and 120, the samples turn to a sol without forming
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Table 2 The summary of the gelation time and pH at different acid:alkoxide and water:alkoxide molar ratio Acid:alkoxide (mol:mol) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
Water:alkoxide (mol:mol) 20 40 50 55 60 65 100 120
Gelation time (s)
pH
Precipitated Precipitated 868 208 774 1494 Sol Sol
2 2 3 3 3 3 4 4
a gel. This is an expecting result since, the higher the water content, the slower the condensation rate, due to the reversed condensation reaction. After the ceria gels were calcined at 500 ◦ C for 1 h, the specific surface area (Fig. 1a) decreased with increasing acid:alkoxide molar ratio from 0.8 to 1.1. This is in agreement with the explanation given above that at higher acid amounts the rate of condensation is greatly slowed down and the particles with little or no bridging are formed. The highest specific surface area at 0.8 molar ratio is 148 m2 /g. The specific surface area of the calcined gel at various water:alkoxide ratios, see Fig. 1b, is almost the same (148–152 m2 /g), and
Fig. 1. The specific surface area (m2 /g) after calcining at 500 ◦ C for 1 h using (a) different HCl:alkoxide molar ratio and (b) different water:alkoxide molar ratio.
Fig. 2. XRD patterns of ceria oxides prepared using the HCl:alkoxide:water ratio of 0.8:1:55, and calcined for 1 h at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C and (f) 800 ◦ C.
starts to decrease as the ratio increases to 65 (118 m2 /g) and 70 (65 m2 /g), due to higher water amount driving the condensation reaction to go backward, resulting in a longer gelation time. The short chain network was thus formed, giving lower surface area after the gels were calcined. The effect of calcination temperature and time of ceria gels at various temperatures (400–800 ◦ C) and times (1, 3, 5 and 7 h) were studied on the samples prepared with the HCl:alkoxide:water molar ratio of 0.8:1:55. The crystallinity of CeO2 was changed with increasing calcination temperature and time. The XRD patterns in Fig. 2 show an amorphous solid at 300 ◦ C whereas at 400 ◦ C the cubic pattern of CeO2 [21] is identifiable, but it exhibits broad and low intensity peaks. The intensity and the sharpness of the XRD peaks increases dramatically for the samples calcined at the temperature of 700 and 800 ◦ C, representing the higher crystallinity of CeO2 . Similarly, the XRD patterns in Fig. 3 show the presence of cubic phase of CeO2 for all calcination times,
Fig. 3. XRD patterns of ceria oxides prepared using the HCl:alkoxide:water ratio of 0.8:1:55, and calcined at 500 ◦ C for (a) 1 h, (b) 3 h, (c) 5 h and (d) 7 h.
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Fig. 4. The specific surface area of the product formed with 0.8:1:55 acid:alkoxide:water molar ratio as a function of calcination temperature (400–800 ◦ C) and time (1, 3, 5, 7 h).
65
Fig. 6. The nitrogen adsorption–desorption isoterm of mesoporous ceria obtained from 0.8 HCl:alkoxide molar ratio and calcined at 400 ◦ C for 3 h.
and indicate the same trend as the study of calcinations temperature in which the longer the calcination time, the sharper the peaks with higher intensity. Fig. 4 shows the decrease of specific surface area as a function of calcinations, temperature and time. The highest specific surface area (180 m2 /g) is obtained at the lowest calcinations time and temperature (400 ◦ C, 1 h). This temperature is confirmed by the presence of organic residue using TGA (Fig. 5), and it shows 100% ceramic yield, implying that pure CeO2 is produced at 400 ◦ C calcinations temperature. The surface area decreases to 71 m2 /g when the calcinations temperature reaches 800 ◦ C. The nitrogen adsorption–desorption isotherm of the material obtained at 0.8 HCl:alkoxide molar ratio and 400 ◦ C calcinations temperature for 3 h, shown in Fig. 6, is of type V isoterm with H3-type hysteresis loop. Its morphology is observed as aggregates of plate-like particles [22]. Examples of the SEM micrographs of ceria taken after calcining at 500 ◦ C (Fig. 7a) and 800 ◦ C (Fig. 7b) support the specific surface area results. At higher calcination tempera-
Fig. 5. The TGA thermogram of cerium gel calcined at 400 ◦ C for 1 h.
Fig. 7. Effect of the heat treatment on ceria prepared using the acid:alkoxide:water ratio of 0.8:1:55, and calcined for 1 h: (a) 500 ◦ C and (b) 800 ◦ C.
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Fig. 9. The plots of tan δ with time (s) at HCl:alkoxide molar ratio of 0.8.
nents n and n , G (ω) ∝ G (ω) ∝ ωn
Fig. 8. Effect of the heat treatment on ceria prepared using the acid:alkoxide:water ratio of 0.8:1:55, and calcined at 500 ◦ C for (a) 1 h and (b) 7 h.
(2)
the gel point is observed at a crossover point where n = n = n [16], as shown in Fig. 10 (acid:alkoxide molar ratio 0.8). The crossover of n and n is at 868 s, which is equal to the gelation time from the plot of tan δ. The different stages of the critical moments near the gelation are observed in a plot of G , G (Pa) with ω (rad/s) at pregel stage, gel point and postgel stage as seen in Fig. 11. For all investigations, the G (ω) is greater than G (ω) because of the colloidal nature of the system in which solid-like behavior becomes predominant at an initial stage. Both G and G could exhibit a power-law dependence on applied frequency, at the gel point (tg ) n = n = ngel , and the power-law plots for G and G were parallel [24]. Table 3 shows the viscoelastic exponent n at different acid:alkoxide molar ratios, the n values increase from ∼0.14 to 0.24 with increasing acid ratio. The results suggest that the viscoelastic exponent depends
tures the structures are agglomerated and the pores collapse, causing the decrease of the specific surface area. The same trend (Fig. 8) is also seen with increasing calcination times from 1 h (Fig. 8a) to 7 h (Fig. 8b). From the rheological point of view, for the case of different acid:alkoxide molar ratio 0.8, 0.9, 1.0 and 1.1, the gelation time (tg ) is determined by employing the well-established criterion of gelation, proposed by Winter and Chambon [23] tan δ =
nπ G = tan G 2
or δ =
nπ 2
(1)
at a frequency-independent value of tan δ obtained from a multi-frequency plot of tan δ versus gelation time (Fig. 9) in which G and G are storage and loss modulus, respectively, and n is viscoelastic exponent. The result indicates that tg values increase as a function of acid amount which is corresponding to the result in Table 1. An alternative method to determine the gel point is by plotting the viscoelastic expo-
Fig. 10. The plots of the apparent exponents, the storage moduli (n ) and the loss moduli (n ) during the course of gelation for the 0.8 HCl:alkoxide molar ratio.
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Fig. 11. The frequency dependence curves of G (ω) and G (ω) at pregel stage (B = −3), gel point (B = 0) and postgel stage (B = 3) of (a) 0.8, (b) 0.9, (c) 1.0 and (d) 1.1 HCl:alkoxide ratio.
on the acid concentrations and higher crosslink density will probably reduce the value of n. The different viscoelastic exponents reflect different structures at the sol–gel transition point. The experimentally viscoelastic exponent, n, can be further related to the mass fractal dimension, df , and the structure may be described by a fractal dimension as Eq. (3) M = Rdf
(3)
where M is the mass of molecular cluster and R is the radius of gyration [16]. The connection between the fractal dimensions and the viscoelastic exponents is given in Eq. (4) by
Table 3 The viscoelastic exponent, fractal dimension and gelation time at different HCl:alkoxide molar ratios HCl:alkoxide:water (molar ratio)
n
df
Gelation time (s)
0.8:1:50 0.9:1:50 1.0:1:50 1.1:1:50
0.1389 0.1411 0.1560 0.2431
2.3786 2.3766 2.3628 2.2795
868 931 1182 5206
Muthukumar [24] n=
d (d + 2 − 2df ) 2 (d + 2 − df )
(4)
where, n = viscoelastic exponent, d = space dimension and df = fractal dimension, shows that the value of n also depends on the fractal dimension, i.e. also on cluster structure. The estimation of fractal dimensions is calculated because the scaling exponents do not have a direct physical meaning. The connection between the viscoelastic exponents and fractal dimensions is justified because the value for viscoelastic exponent comes from the spanning cluster under stress that is dependent on the polymer structure, meaning that for an aggregate structure having a lower fractal dimension, the molecular weight grows slower with radius than the one having a higher fractal dimension. The aggregated structure having lower fractal dimension can thus be described as a more open structure [25]. The values of fractal dimensions in Table 3 decrease with increasing n value and increasing acid ratio, indicating that the incipient gels with high values of the relaxation exponent have an open structure and gels with lower n values have a tight structure [15,16]. These results are corresponding to the explanation of Tables 1 and 2 such that the
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perature and time affect the specific surface area and the crystallinity. The highest specific surface area was obtained at 0.8:1:55 HCl:alkoxide:water molar ratio gel calcined at 400 ◦ C for 1 h (180 m2 /g). The viscoelastic properties of the incipient ceria gels at different HCl:alkoxide molar ratios indicate that the gelation time increased as a function of the amount of HCl. The fractal dimension determined from the frequency-scaling exponent of the modulus at the gel point indicated a dense critical gel structure at high HCl ratio and tight structure at low amount of acid. The gel strength increased as a function of acid:alkoxide molar ratio, 0.8 molar ratio gave the highest gel strength. Acknowledgements Fig. 12. The plots of gel strength parameter S at the gel point as a function of HCl:alkoxide molar ratio 0.8, 0.9, 1.0 and 1.1.
network formation of ceria gel prepared at higher acid ratio is shorter than that prepared at lower acid ratio due to the faster hydrolysis and condensation rates, as discussed previously. Moreover, the calcined product shows the lower specific surface area at higher acid ratio meaning lower porosity. The gel-strength parameter (S), which is depended on the cross-linking density and the molecular chain flexibility, is shown in Eq. (5). S=
G ωn Γ (1 − n) cos
δ
(5)
where G is the storage modulus, ω is the frequency (rad/s), Γ (1−n) is the gamma function, n is the viscoelastic exponent and δ is the strain being independent of frequency but proportional to the relaxation exponent (Eq. (6)). δ=
nπ 2
(6)
The effect of viscoelastic exponent on the strength of gel in Fig. 12 shows that the strength slightly increases from 0.8 to 1.0 molar ratio and then dramatically decreases at 1.1 molar ratio. From the relationship between the strength value and the cross-linking density and the molecular chain flexibility, it is indicated that 1.1 molar ratio has the lowest cross-linking density of the cerium dioxide network which is again corresponding to the explanation of Table 1. Thus, at low acid ratio, the critical gel cluster is relatively stronger than the gel formed, at high acid ratio.
4. Conclusions High surface area cerium dioxide was successfully synthesized via the sol–gel process using inexpensive and moisturestable cerium glycolate precursor. The gel can be formed at 0.8–1.1 HCl:alkoxide and 50–65 water:alkoxide molar ratios. Gelation time increases and the specific surface area decreases with increasing amount of HCl. Calcination tem-
This research work is supported by the Postgraduate Education and Research Program in Petroleum and Petrochemical Technology (ADB) Fund, Ratchadapisakesompoch Fund, Chulalongkorn University and the Thailand Research Fund (TRF). References [1] R. Craciun, W. Daniell, H. Knozinger, Appl. Catal. A-Gen. 230 (2002) 153. [2] D. Terribile, A. Trovarelli, C.D. Leitenburg, A. Primavera, G. Dolcetti, Catal. Today 47 (1999) 133. [3] E.S. Putna, X.L. Bunluesin, X.L. Fan, R.J. Gorte, J.M. Vohs, R.E. Lakis, T. Egami, Catal. Today 50 (1999) 343. [4] A. Trovarelli, C.D. Leitenburg, M. Boaro, G. Dolcetti, Catal. Today 50 (1999) 353. [5] Y. Konishi, T. Murai, S. Asai, Ind. Eng. Chem. Res. 36 (1997) 2641. [6] Y. Izumi, Y. Iwata, K.I. Aika, J. Phys. Chem. 100 (1996) 9421. [7] W. Liu, M.F. Stephanopoulos, J. Catal. 153 (1995) 317. [8] H. Cordatos, T. Bunluesin, J. Stubenrauch, J.M. Vohs, R.J. Gorte, J. Phys. Chem. 100 (1996) 785. [9] A. Turkovic, Z.C. Orel, Solid State Ionics 89 (1996) 255. [10] Z.C. Orel, B. Orel, Sol. Energy Mat. Sol. C. 40 (1996) 205. [11] A. Turkovic, Z.C. Orel, Sol. Energy Mat. Sol. C. 45 (1997) 275. [12] W.P. Hsu, L. Ronnquist, E. Matijevic, Langmuir 4 (1988) 31. [13] D. Terribile, A. Trovarelli, J. Llorca, C.D. Leitenburg, G. Dolcetti, J. Catal. 178 (1998) 299. [14] X.Z. Ding, X.H. Liu, Mater. Sci. Eng. A-Struct. 224 (1997) 210. [15] N. Kao, S.N. Bhattacharya, J. Rheol. 42 (1998) 493. [16] A.L. Kjoniksen, B. Nystrom, Macromolecules 29 (1996) 5215. [17] N. Phonthammachai, T. Chairassameewong, E. Gulari, A.M. Jamieson, S. Wongkasemjit, J. Met. Mat. Min. 12 (2002) 23. [18] P. Piboonchaisit, S. Wongkasemjit, R. Laine, Science-Asia, J. Sci. Soc. Thailand 25 (1999) 113–119. [19] B. Ksapabutr, E. Gulari, S. Wongkasemjit, Mater. Chem. Phys. 1 (2004) 34. [20] D.A. Ward, E.I. Ko, Chem. Mater. 5 (1993) 956. [21] A.A. Antonova, O.V. Zhilina, G.G. Kagramanov, K.I. Kienskaya, V.V. Nazarov, I.A. Petropavlovskii, I.E. Fanasyutkina, Colloid J. 63 (2001) 662. [22] K.S.W. Singh, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, I. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [23] H.H. Winter, F. Chambon, J. Rheol. 30 (1986) 367. [24] M. Muthukumar, Macromocules 22 (1989) 4656. [25] M. Jokinen, E. Gyorvary, J.B. Rosenholm, Colloid Surf. A 141 (1998) 205.