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Synthesis of perovskite LaCoO3 by thermal decomposition of oxalates: Phase evolution and kinetics of the thermal transformation of the precursor
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 5997–6004 www.elsevier.com/locate/ceramint
Synthesis of perovskite LaCoO3 by thermal decomposition of oxalates: Phase evolution and kinetics of the thermal transformation of the precursor Wang Kaituo, Wu Xuehang, Wu Wenwein, Li Yongni, Liao Sen School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, People's Republic of China Received 6 October 2013; received in revised form 10 November 2013; accepted 10 November 2013 Available online 19 November 2013
Abstract LaCoO3 precursor was synthesized by solid-state reaction at low temperatures using La(NO3)3 6H2O, CoSO4 7H2O, and Na2C2O4 as raw materials. LaCoO3 was obtained by calcining a precursor, 0.97/2La2(C2O4)3–CoC2O4 5.3H2O, at 1123 K in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. A high-crystallized LaCoO3 with a rhombohedral structure was obtained when the precursor was calcined at 1123 K in air for 2 h. The thermal transformation of the precursor from ambient temperature to 1150 K in air presented six steps. The values of the activation energies associated with the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O were determined based on the Starink equation. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: LaCoO3; Chemical synthesis; Thermal transformation; Non-isothermal kinetics
1. Introduction Perovskite-type oxides (ABO3; A¼ a rare earth cation, and B¼ a transition metal cation) have many unique properties, such as electrical, mechanical, optical, magnetic, and catalytic properties [1–10]. Thus, perovskite-type oxides have been widely used as electrode materials for solid oxide fuel cells [11–13], chemical sensors [14,15], oxygen-permeating membranes [16–20], thermoelectric devices [21], and catalysts [22–24]. Among the perovskite-type oxides, LaCoO3 and its related materials exhibit interesting electrical and electrocatalytic properties. The structures and properties of these materials highly depend on the synthesis of their precursor powders and/or dopant. Therefore, researchers have paid considerable attention on the improvement of the preparation method of these powders. Various methods of synthesizing LaCoO3 and doped LaCoO3 have been developed, including complex thermal decomposition [3], refluxing method [5], polymerizable complex method [7], templated method [9,22], microwave process n
Corresponding author. Tel./fax: þ 86 771 3233718. E-mail addresses: [email protected], [email protected] (W. Wenwei).
[25], mechanochemical synthesis [26], co-precipitation method [26], molten salt method [27], freeze-drying [28], citrate process [29], sol–gel synthesis [10,30–32], and flame-spray pyrolysis [33]. In the synthesis of LaCoO3, the crystallite diameter, morphology, and crystalline phases of LaCoO3 associated with its performances highly depend on the synthesis method and temperature. Jadhav et al. [5] synthesized rhombohedral LaCoO3 with a rectangular polygon morphology using the refluxing method, followed by calcination at 723 K for 6 h, using La2O3, Co(NO3)2 6H2O, and NaOH as starting materials. Teng et al. [9] obtained rhombohedral LaCoO3 nanowire using the templated method. The LaCoO3 nanowire obtained at 1023 K for 48 h behaved with higher thermal stability and activity of CO oxidation in comparison with spherical nanoparticle. Wu et al. [18] synthesized a series of La0.6Sr0.4Co0.4Fe0.6O3-δ (LSCF) perovskite-type oxides by a modified citrate process under various pH conditions. The LSCF membranes originating from the precursors with lower pH values (pH ¼ 1 and 3) exhibited larger apparent activation energy for oxygen permeation than other samples (pH ¼ 5, 7, and 9) at approximately 1073 K to 1123 K. The material properties of perovskite-type oxides can thus be tailored by controlling the pH values in the synthesis process. Forni et al.
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.048
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W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
[33] synthesized nanosized LaCoO3 powder with an excellent catalytic activity for methane combustion by flame-spray pyrolysis. Most researchers have attempted to obtain perovskite-type LaCoO3 with high performance. However, many methods of synthesizing LaCoO3 are complex processes with high cost, so preparing the product on a large scale is difficult. Therefore, new synthesis methods for perovskite-type LaCoO3 must be studied and innovated. However, the kinetic research of the thermal transformation of LaCoO3 precursor still has fewer reports than the synthesis of LaCoO3. Solid-state reaction at low temperatures is an effective and simple synthetic technique for the synthesis of superfine inorganic materials, with several advantages, such as simplicity, low cost, high output, and little pollution, in addition to superfine compounds that can be easily prepared [34]. This process is simpler than sol–gel synthesis, co-precipitation, polymerizable complex method, and molten salt method. Compared to solid-state reaction at high temperatures using a mixture of CoO and La2O3, the crystallization temperature of LaCoO3 in this study is lower, while the crystallinity of LaCoO3 is higher. This finding is attributed to the fact that direct high temperature solid-state reaction induces difficult penetration between solid particles, resulting in the crystallization of LaCoO3 at higher temperature and in lower degree of crystallinity. However, a mixture of La(NO3)3 6H2O, CoSO4 7H2O, and Na2C2O4 was ground at room temperature in our study. Precursor oxalates can be obtained with molecular-level scale by uniform mixing. Single-phase crystalline LaCoO3 can then be obtained at lower temperature when the precursor is calcined in air. This study aims to prepare a perovskite-type LaCoO3 by calcining a precursor, 0.97/2La2(C2O4)3–CoC2O4 5.3H2O, and study the kinetics of the thermal transformation of the precursor. The latter objective was conducted by thermogravimetry and differential scanning calorimetry (TG/DSC). The non-isothermal kinetics of the thermal transformation of the precursor was interpreted by the Starink equation [35–37]. The kinetic parameters (Ea, A) and mechanism of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O are discussed in this paper for the first time.
2. Experimental 2.1. Reagent and apparatus All chemicals used are of reagent-grade purity (purity499.9%). The TG/DSC measurements were conducted using a Netzsch Sta 409 PC/PG thermogravimetric analyzer under continuous flow of air (40 mL min 1). The sample mass was approximately 12 mg. X-ray powder diffraction (XRD) was performed using a Rigaku D/Max 2500 V diffractometer equipped with a graphite monochromator and a Cu target. The radiation applied was Cu Kα (λ=0.15406 nm), operated at 40 kV and 50 mA. The XRD scans were conducted from 51 to 701 in 2θ, with a step size of 0.021. The Fourier transform infrared (FT-IR) spectra of the precursor and its calcined products were recorded on a Nexus 470 FT-IR
instrument. The morphologies of the synthesis products were observed using an S-3400 scanning electron microscope (SEM). 2.2. Preparation of LaCoO3 The LaCoO3 precursor samples were prepared by solid-state reaction at low temperatures [1,2]. In a typical synthesis, La (NO3)3 6H2O (39.80 g), CoSO4 7H2O (25.84 g), Na2C2O4 (38.8 g), and surfactant polyethylene glycol-400 (4.0 mL, 50 vol%) were placed in a mortar, and the mixture was thoroughly ground by hand with a rubbing mallet for 40 min. The grinding velocity was about 210 cycles/min, and the strength applied was moderate. The reactant mixture gradually became damp, and a paste was formed immediately. The reaction mixture was kept at 303 K for 1.5 h. The mixture was washed with deionized water to remove soluble inorganic salts until SO24 ion cannot be visually detected with a 0.5 mol L 1 BaCl2 solution. The solid was then washed with a small amount of anhydrous ethanol and dried at 353 K for 5 h. The resulting material was determined to be 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O. Nanocrystalline LaCoO3 was obtained by calcining 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at a heating rate of 5 K min 1 from ambient temperature to 1150 K in air in a muffle furnace, and then kept at 1150 K for 2 h. 3. Method of determining kinetic parameters and mechanism functions 3.1. Determination of activation energy by the Starink equation The activation energy of the thermal transformation of the solid compound can be obtained by the following Starink equation [Eq. (1)]: ! βi Eα ln 1:92 ¼ Const 1:0008 ; ð1Þ RT α T α;i where βi is the heating rate (K min 1), Tα,i is the reaction temperature (K) in TG curve, Eα is the activation energy (kJ mol 1) of thermal transformation corresponding to degree of conversion (α), and R is the gas constant (8.314 10 3 kJ mol 1 K 1). The conversion degree (α) can be expressed as follows: mi mt α¼ ; ð2Þ mi mf where mi, mf, and mt are the initial, final, and current sample masses at moment t, respectively. The dependence of lnðβi =T 1:92 α;i Þ on 1/Tα must lead to a straight line. Thus, the reaction activation energy Eα can be obtained from linear slope [ 1.0008Eα/R, Eq. (1)]. 3.2. Determination of the most probable mechanism functions The following equation was used to estimate the most probable reaction mechanism of the thermal transformation of
W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
0.97/2La2(C2O4)3–CoC2O4 5.3H2O, i.e., g(α) function [37,38]: AE a ex þ ln 2 þ ln hðxÞ ln β; ln gðαÞ ¼ ln ð3Þ R x where x=Ea/(RT), hðxÞ ¼ ðx4 þ 18x3 þ 86x2 þ 96xÞ=ðx4 þ 20x3 þ 120x2 þ 240x þ 120Þ, and β is the heating rate (K min 1). The conversions α corresponding to multiple rates at the same temperature are placed in the left of Eq. (3), combined with 31 types of mechanism functions [39,40]. The slope k and correlation coefficient r2 were obtained from the plot of ln g (α) vs. ln β. The probable function is the one at which the value of the slope k is near 1.00000 and the correlation coefficient r2 is better.
3.3. Calculation of pre-exponential factor A The pre-exponential factor was estimated as follows [36]: βgðαÞEa Ea A¼ exp ; ð4Þ RT max RT 2max where A is the pre-exponential factor (s 1), β is the heating rate (K min 1), g(α) is the most probable mechanism function determined by Eq. (3), Ea is the activation energy (kJ mol 1) of thermal transformation, R is the gas constant (8.314 10 3 kJ mol 1 K 1), and Tmax is the most rapid decomposition temperature [i.e., peak temperature in differential thermogravimetry (DTG ) curve, K].
4. Results and discussion 4.1. Composition analysis of the precursor
The FT-IR spectra of the precursor and its calcined samples are shown in Fig. 2. The precursor exhibited a strong and broad band at about 3381 cm 1 that can be assigned to the symmetric and asymmetric stretching modes of water molecules [41–43]. The bending mode of water expected around 1626 cm 1 overlapped with the intense oxalate band at around 1616 cm 1 [44–47]. The bands at 1320, 1097, 797, and 488 cm 1 can be assigned to
0
80
662 K
-4 -8
70 -12
60 536 K
50
600
-1 -2 -3
-16 -20
450
DSC (mW/mg)
998 K
DTG (mg/min)
0
90
TG (%)
Fig. 1 shows the TG/DTG/DSC curves of the precursor at four heating rates, i.e., 5, 10, 15, and 20 K min 1, from ambient temperature to 1150 K. The TG/DTG/DSC curves show that the thermal transformation of 0.97/2La2(C2O4)3– CoC2O4 5.3H2O below 1150 K occurred in six well-defined steps. At the heating rate of 10 K min 1, the first step started at about 303 K and ended at 325 K, which can be attributed to the dehydration of the 0.3 adsorption water from 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O (mass loss: observed, 1.07%; theoretical, 1.07%). The second transformation step started at 325 K and ended at 385 K, attributed to the dehydration of the 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 5H2O (mass loss: observed, 9.04%; theoretical, 8.91%). The third transformation step started at 385 K and ended at 522 K, attributed to the dehydration of the 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 2.5H2O (mass loss: observed, 8.72%; theoretical, 8.91%). The fourth transformation step started at 522 K and ended at 543 K, attributed to the reaction of CoC2O4 with 2/3O2 into 1/3Co3O4 and the two CO2 molecules (mass loss: observed, 13.92%; theoretical, 13.20%). The fifth transformation step started at 543 K and ended at 895 K, attributed to the reaction of 0.97/2La2(C2O4)3 with 0.7275/2O2 into 0.97/2La2O2CO3 and of 2.425CO2 molecules (mass loss: observed, 15.58%; theoretical, 16.52%). The sixth transformation step started at 895 K and ended at 1163 K, attributed to the reaction of 0.97/2La2O2CO3 and 1/3Co3O4 with 0.5/6O2 into La0.97CoO2.955 and of 0.485CO2 (mass loss: observed, 3.47%; theoretical, 3.70%). The observed weight loss differs from the theoretical mass loss, attributed to the error of the apparatus measurements.
4
372 K 448K
100
40 300
4.2. TG/DTG/DSC analysis of the precursor
4.3. IR spectroscopic analyses of the precursor and its calcined samples
A precursor sample (0.0300 g) was dissolved in 10 mL 50 vol% HCl solution, and then diluted to 100.00 mL with deionized water. The La and Co in the solution were determined by inductively coupled plasma atomic emission spectrometry (Perkin Elmer Optima 5300 DV). The results showed that the La and Co mass percentage were 26.67% and 11.67%, respectively. The molar ratio of La:Co in the precursor was therefore 0.97:1.00.
110
5999
750
900
Temperature (K)
1050
-4 300
450
600
750
900
1050
Temperature (K)
Fig. 1. TG/DTG/DSC curves of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at different heating rates in air.
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the appearance of new M–OC2O3 (M=La, Co) bonds and/or to the combinations of OH group vibration and lattice modes [37,46,47]. The band at 2353 cm 1 was attributed to the asymmetric stretching of vasy (C=O) from CO2 absorption [2,48]. The FT-IR spectra of the precursor and sample obtained at 423 K are similar, implying that the structure of the oxalates remained stable up to 423 K. When the precursor was calcined at 523 K, the bands at about 3381, 1626, 1320, 1097, 797, and 488 cm 1 disappeared and/or became weak. A new band also appeared at about 1437 cm 1, implying that the structure of the oxalates changed. When the precursor was calcined above 973 K, the bands at about 1626 and 1320 cm 1 disappeared, implying that the precursor completed the dehydration and decomposition of C2O2 4 . The spectrum of the calcined sample at 1123 K agrees with that of rhombohedral LaCoO3 from literature [3]. 4.4. XRD analyses of the calcined products Fig. 3 shows the XRD patterns of the precursor and its calcined samples from different calcining temperatures for 2 h. The results showed that the precursor is a mixture containing monoclinic La2(C2O4)3 10H2O [PDF card 49–1255, space group P21/c(14)] and orthorhombic CoC2O4 2H2O [PDF card 1123 K
873 K 723 K 523 K 423 K 353 K
3500
3000
2500
2000
1500
1000
488
797
1320
1626
3381
4000
1097
D ¼ Kλ=ðβ cos θÞ;
2353
Transmittance (%)
973 K
25–0250, space group Cccm(66)] (Fig. 3a). When the precursor was calcined at 523 K for 2 h, the characteristic diffraction peaks of monoclinic La2(C2O4)3 10H2O and orthorhombic CoC2O4 2H2O disappeared, whereas those of cubic Co3O4, hexagonal La2O2CO3, and La2(C2O4)3 [49] appeared. With the increase of the calcination temperature (Fig. 3a), the characteristic diffraction peaks of Co3O4 and La2O2CO3 became weak and/or disappeared. Fig. 3b shows that when the precursor was calcined at 1123 K for 2 h, a new diffraction pattern with strong intensity and smoothed baseline was observed. The calcined product therefore has a high degree of crystallinity. All the diffraction peaks in the pattern agreed with those of rhombohedral LaCoO3 with space group R-3c (167) from PDF card 48–0123. The refined lattice parameters of LaCoO3 obtained at 1123 K were a ¼ b ¼ 0.5427 nm, c¼ 1.3056 nm, α ¼ β ¼ 901, γ ¼ 1201, density ¼ 7.3531 g cm 3. No diffraction peak of Co3O4 was observed, implying that the Co3O4 content is beyond the detection limit of XRD or Co3O4 formed a solid solution with LaCoO3. Compared to other methods of synthesizing rhombohedral LaCoO3, the crystallization temperature and purity of LaCoO3 obtained by thermal decomposition of oxalates in this study have a few differences. For example, Nakayama et al. [26] obtained rhombohedral LaCoO3 by co-precipitation and found that the crystallinity of rhombohedral LaCoO3 depended on the calcination temperature. A single-phase LaCoO3 can be obtained after calcining precursor at 1473 K for 2 h. Ivanova et al. [28] studied the synthesis of rhombohedral LaCoO3 by freeze-drying of metal–citrate precursors, followed by calcination in air. A single-phase LaCoO3 can be obtained over 873 K for 3 h. The crystallite diameter of LaCoO3 was estimated using the following Scherrer formula [37]:
500
−1
Wavenumbers (cm ) Fig. 2. FT-IR spectra of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O and its calcined samples.
ð5Þ
where D is the crystallite diameter, K ¼ 0.89 (the Scherrer constant), λ ¼ 0.15406 nm (wavelength of the X-ray used), β is the width of line at the half-maximum intensity, and θ is the corresponding angle. The crystallite sizes of LaCoO3 from calcining precursor at 873 K, 973 K, and 1123 K in air for 2 h were 31 nm, 36 nm, and 52 nm, respectively. The crystallinity
0
Fig. 3. XRD patterns of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O and its calcined samples at different temperatures for 2 h.
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of LaCoO3 can be calculated by MDI Jade 5.0 software. The crystallinity of LaCoO3 obtained at 1123 K was 99.42%.
6001
follows: 0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5:3H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5H2 O ðsÞþ 0:3H2 O ðgÞ
4.5. SEM analysis of the calcined samples The morphologies of the calcined samples are shown in Fig. 4. Fig. 4a shows that the calcined sample obtained at 873 K is composed of approximately spherical grains and contains particles having a distribution from 100 nm to 300 nm. With the increase of the calcination temperature, the crystallite in the calcined sample aggregated into larger grains. Fig. 4b and c shows SEM images of the samples obtained at 973 K and 1123 K, respectively. The calcined sample at 973 K kept approximately spherical morphology, and the particle sizes were mainly between 100 nm and 300 nm. The calcined sample obtained at 1123 K became platelet grains, with a particle size of between 600 nm and 1250 nm. The average crystallite sizes of the calcined samples determined by X-ray diffraction were significantly smaller than the values determined by SEM. This difference can be attributed to the fact that the values observed by SEM have the size of the secondary particles, which are composed of several or many crystallites by soft reunion. In addition, the X-ray line broadening analysis disclosed only the size of a single crystallite.
4.6. Kinetics of the thermal transformation of the precursor In accordance with the TG/DTG/DSC, IR, and XRD analyses of the precursor and its calcined products mentioned above, the thermal transformation of the precursor below 1150 K consists of six steps, which can be expressed as
ð6Þ
0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U2:5H2 O ðsÞþ 2:5H2 O ðgÞ ð7Þ 0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U2:5H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 ðsÞ þ 2:5H2 O ðgÞ
ð8Þ
CoC2 O4 ðsÞ þ 2=3O2 ðsÞ-1=3Co3 O4 ðsÞþ 2CO2 ðsÞ
ð9Þ
0:97=2La2 ðC2 O4 Þ3 ðsÞþ 0:7275O2 ðgÞ0:97=2La2 O2 CO3 ðsÞ þ 2:425CO2 ðgÞ
ð10Þ
0:97=2La2 O2 CO3 ðsÞþ 1=3Co3 O4 ðsÞþ 0:5=6O2 ðgÞLa0:97 CoO2:955 ðrÞ þ 0:485CO2 ðgÞ
ð11Þ
According to the non-isothermal method, the basic data of α and T were collected from the TG curves of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at four heating rates (5, 10, 15, and 20 K min 1). According to Eq. (1), the isoconversional calculation procedure of the Starink equation was used. The corresponding Starink lines for different transformation steps were obtained at different conversion degrees α and heating rates β. The reaction activation energy Ea can be obtained from linear slope ( 1.0008 Ea/R). The results are shown in Fig. 5. The average values of the activation energies associated with 0.97/2La2(C2O4)3–CoC2O4 5.3H2O thermal transformation were 97.18716.39 kJ mol 1, 84.73713.48 kJ mol 1, 104.95710.26 kJ mol 1, 155.67711.56 kJ mol 1,
Fig. 4. SEM images of the products synthesized at different temperatures for 2 h: (a) 873 K, (b) 973 K, and (c) 1123 K.
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172.32729.29 kJ mol 1, and 381.897260.84 kJ mol 1 for steps 1, 2, 3, 4, 5, and 6, respectively. The activation energy changes in steps 1, 2, 5, and 6 with α are higher than 10%, whereas those in steps 3 and 4 with α are lower than 10%. Hence, the thermal transformations of 0.97/2La2(C2O4)3– CoC2O4 5.3H2O for steps 1, 2, 5, and 6 could be multi-step reaction mechanisms. By contrast, the thermal transformations for steps 3 and 4 are the dehydration of 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 2.5H2O and the reaction of CoC2O4 with 2/3 O2 into 1/3Co3O4, respectively, which are simple reaction mechanisms [37,38,50,51]. The activation energy of step 6 is higher than those of steps 1 to 5. Step 6 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O may thus be interpreted as a “slow” stage, whereas the other steps may be interpreted as “fast” stages. Fig. 6 shows the curves of α vs. t and dα/dt vs. t for steps 3 and 4. In Fig. 6a, step 3 consists of two models, i.e., an accelerating model between 0.0 min and about 7.06 min and a decelerating model between about 7.06 min and 13.69 min [36]. In the accelerating model, rate increased continuously with increasing extent of conversion and reached its maximum at the end of the process (i.e., at 6.05 min). In the decelerating model, the process rate had maximum at the beginning of the process, and then decreased continuously as the extent of conversion increased. The result shows that the rate of the
decelerating stage reached its maximum at about 10.04 min (Fig. 6a). Step 4 is a sigmoidal model (sometimes called autocatalytic) that represents the process of step 4 [36], whose initial and final stages demonstrate the accelerating and decelerating behaviors, respectively. Thus, the process rate reached its maximum at some values of the extent of conversion. The rate for step 4 reached its maximum at 1.71 min (Fig. 6b). We randomly chose several temperatures corresponding to conversions 0.10oαo0.90. The conversions corresponding to the temperature for β¼ 5, 10, 15, and 20 K min 1 were assigned to 31 types of mechanism functions [38,39]. The slope k, correlation coefficient r2, and intercept B of the linear regression of ln g(α) vs. ln β were obtained. Two probable mechanism functions with better correlation coefficient r2 were determined. Several temperatures were randomly chosen to calculate the slope k, correlation coefficient r2, and intercept B of the two probable mechanism functions using the same method. The mechanism function in which the value of k is closest to 1.00000 and the correlation coefficient r2 is higher was chosen as the mechanism function of the thermal transformation of 0.97/2La2(C2O4)3– 1.0
0.8
step 4
step 3
500
5 K min
450
0.6
−1
Ea (kJ mol )
step 1 step 2 step 3 step 4 step 5 step 6
350 300 250
10 K min−1 15 K min−1
α
400
−1
20 K min−1
0.4
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200 150
0.0
100
390
420
450
50 0.2
0.3
0.4
0.5
0.6
0.7
480
0.8
α Fig. 5. The dependence of Ea on α at different thermal transformation steps.
15 1.0
0.6
0.8
step 4
12
0.8
9
0.6
0.6 0.4
6.05 min
6
0.4
0.4
1.71min
0.2
0.2 0.0
10.04 min
0.2
3
0.0
0
0.0 0
2
4
6
8
t (min)
10
12
14
540
Fig. 7. Comparison of model results (solid line) with the experimental data (dash line) of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 3 and 4 at different heating rates.
0.8 step 3
1.0
510
T (K)
0.0
0.4
0.8
1.2 t (min)
Fig. 6. Curves of α vs. t and dα/dt vs. dt at heating rate of 10 K min 1.
1.6
2.0
W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
CoC2O4 5.3H2O. The probable mechanism function integral forms of the thermal transformation of 1/2La2(C2O4)3– CoC2O4 5.3H2O for steps 3 and 4 were g(α)=1–(1–α)1/2 and g (α)=[(1þ α)1/3–1]2, respectively. The rate-determining mechanisms for steps 3 and 4 were contracting cylinder and threedimensional diffusion, respectively. The pre-exponential factor A was obtained using Eq. (4), inserting the most probable mechanism function g(α), β, Ea, R, and Tmax values. The pre-exponential factors ln A(s 1) of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 3 and 4 were determined to be 23.90 and 28.20, respectively. The experimental data and the results of the kinetic mechanism for every heating rate were compared to prove the validity of the kinetic mechanism for steps 3 and 4 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O. The results are shown in Fig. 7. The model-predicted plots agree with the experimental plots. Therefore, the mechanism functions for steps 3 and 4 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O are reliable. 5. Conclusions We have successfully synthesized a rhombohedral LaCoO3 by calcining 0.97/2La2(C2O4)3–CoC2O4 5.3H2O in air. The XRD analysis suggests that a rhombohedral LaCoO3 with a crystallite size of 52 nm can be obtained by calcining 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O at 1123 K in air for 2 h. The thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O from ambient temperature to 1150 K experienced six steps, namely, the dehydration of 0.3 adsorption water, the dehydration of 2.5 crystal waters, the dehydration of 2.5 crystal waters, the reaction of CoC2O4 with 2/3O2 into 1/3Co3O4, the reaction of 0.97/2La2(C2O4)3 with 0.7275O2 into 0.97/2La2O2CO3, and the reaction of 0.97/2La2O2CO3 and 1/3Co3O4 with 0.5/6O2 into the rhombohedral LaCoO3. The thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 1, 2, 5, and 6 can be multi-step reaction mechanisms, whereas those for steps 3 and 4 are simple reaction mechanisms.
[4]
[5] [6] [7]
[8] [9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19] [20]
Acknowledgments This study was financially supported by the National Nature Science Foundation of China (Grant no. 21161002) and the Guangxi Nature Science Foundation of China (Grant no. 2011GXNSFA018036).
[21]
[22]
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 5997–6004 www.elsevier.com/locate/ceramint
Synthesis of perovskite LaCoO3 by thermal decomposition of oxalates: Phase evolution and kinetics of the thermal transformation of the precursor Wang Kaituo, Wu Xuehang, Wu Wenwein, Li Yongni, Liao Sen School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, People's Republic of China Received 6 October 2013; received in revised form 10 November 2013; accepted 10 November 2013 Available online 19 November 2013
Abstract LaCoO3 precursor was synthesized by solid-state reaction at low temperatures using La(NO3)3 6H2O, CoSO4 7H2O, and Na2C2O4 as raw materials. LaCoO3 was obtained by calcining a precursor, 0.97/2La2(C2O4)3–CoC2O4 5.3H2O, at 1123 K in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. A high-crystallized LaCoO3 with a rhombohedral structure was obtained when the precursor was calcined at 1123 K in air for 2 h. The thermal transformation of the precursor from ambient temperature to 1150 K in air presented six steps. The values of the activation energies associated with the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O were determined based on the Starink equation. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: LaCoO3; Chemical synthesis; Thermal transformation; Non-isothermal kinetics
1. Introduction Perovskite-type oxides (ABO3; A¼ a rare earth cation, and B¼ a transition metal cation) have many unique properties, such as electrical, mechanical, optical, magnetic, and catalytic properties [1–10]. Thus, perovskite-type oxides have been widely used as electrode materials for solid oxide fuel cells [11–13], chemical sensors [14,15], oxygen-permeating membranes [16–20], thermoelectric devices [21], and catalysts [22–24]. Among the perovskite-type oxides, LaCoO3 and its related materials exhibit interesting electrical and electrocatalytic properties. The structures and properties of these materials highly depend on the synthesis of their precursor powders and/or dopant. Therefore, researchers have paid considerable attention on the improvement of the preparation method of these powders. Various methods of synthesizing LaCoO3 and doped LaCoO3 have been developed, including complex thermal decomposition [3], refluxing method [5], polymerizable complex method [7], templated method [9,22], microwave process n
Corresponding author. Tel./fax: þ 86 771 3233718. E-mail addresses: [email protected], [email protected] (W. Wenwei).
[25], mechanochemical synthesis [26], co-precipitation method [26], molten salt method [27], freeze-drying [28], citrate process [29], sol–gel synthesis [10,30–32], and flame-spray pyrolysis [33]. In the synthesis of LaCoO3, the crystallite diameter, morphology, and crystalline phases of LaCoO3 associated with its performances highly depend on the synthesis method and temperature. Jadhav et al. [5] synthesized rhombohedral LaCoO3 with a rectangular polygon morphology using the refluxing method, followed by calcination at 723 K for 6 h, using La2O3, Co(NO3)2 6H2O, and NaOH as starting materials. Teng et al. [9] obtained rhombohedral LaCoO3 nanowire using the templated method. The LaCoO3 nanowire obtained at 1023 K for 48 h behaved with higher thermal stability and activity of CO oxidation in comparison with spherical nanoparticle. Wu et al. [18] synthesized a series of La0.6Sr0.4Co0.4Fe0.6O3-δ (LSCF) perovskite-type oxides by a modified citrate process under various pH conditions. The LSCF membranes originating from the precursors with lower pH values (pH ¼ 1 and 3) exhibited larger apparent activation energy for oxygen permeation than other samples (pH ¼ 5, 7, and 9) at approximately 1073 K to 1123 K. The material properties of perovskite-type oxides can thus be tailored by controlling the pH values in the synthesis process. Forni et al.
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.048
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[33] synthesized nanosized LaCoO3 powder with an excellent catalytic activity for methane combustion by flame-spray pyrolysis. Most researchers have attempted to obtain perovskite-type LaCoO3 with high performance. However, many methods of synthesizing LaCoO3 are complex processes with high cost, so preparing the product on a large scale is difficult. Therefore, new synthesis methods for perovskite-type LaCoO3 must be studied and innovated. However, the kinetic research of the thermal transformation of LaCoO3 precursor still has fewer reports than the synthesis of LaCoO3. Solid-state reaction at low temperatures is an effective and simple synthetic technique for the synthesis of superfine inorganic materials, with several advantages, such as simplicity, low cost, high output, and little pollution, in addition to superfine compounds that can be easily prepared [34]. This process is simpler than sol–gel synthesis, co-precipitation, polymerizable complex method, and molten salt method. Compared to solid-state reaction at high temperatures using a mixture of CoO and La2O3, the crystallization temperature of LaCoO3 in this study is lower, while the crystallinity of LaCoO3 is higher. This finding is attributed to the fact that direct high temperature solid-state reaction induces difficult penetration between solid particles, resulting in the crystallization of LaCoO3 at higher temperature and in lower degree of crystallinity. However, a mixture of La(NO3)3 6H2O, CoSO4 7H2O, and Na2C2O4 was ground at room temperature in our study. Precursor oxalates can be obtained with molecular-level scale by uniform mixing. Single-phase crystalline LaCoO3 can then be obtained at lower temperature when the precursor is calcined in air. This study aims to prepare a perovskite-type LaCoO3 by calcining a precursor, 0.97/2La2(C2O4)3–CoC2O4 5.3H2O, and study the kinetics of the thermal transformation of the precursor. The latter objective was conducted by thermogravimetry and differential scanning calorimetry (TG/DSC). The non-isothermal kinetics of the thermal transformation of the precursor was interpreted by the Starink equation [35–37]. The kinetic parameters (Ea, A) and mechanism of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O are discussed in this paper for the first time.
2. Experimental 2.1. Reagent and apparatus All chemicals used are of reagent-grade purity (purity499.9%). The TG/DSC measurements were conducted using a Netzsch Sta 409 PC/PG thermogravimetric analyzer under continuous flow of air (40 mL min 1). The sample mass was approximately 12 mg. X-ray powder diffraction (XRD) was performed using a Rigaku D/Max 2500 V diffractometer equipped with a graphite monochromator and a Cu target. The radiation applied was Cu Kα (λ=0.15406 nm), operated at 40 kV and 50 mA. The XRD scans were conducted from 51 to 701 in 2θ, with a step size of 0.021. The Fourier transform infrared (FT-IR) spectra of the precursor and its calcined products were recorded on a Nexus 470 FT-IR
instrument. The morphologies of the synthesis products were observed using an S-3400 scanning electron microscope (SEM). 2.2. Preparation of LaCoO3 The LaCoO3 precursor samples were prepared by solid-state reaction at low temperatures [1,2]. In a typical synthesis, La (NO3)3 6H2O (39.80 g), CoSO4 7H2O (25.84 g), Na2C2O4 (38.8 g), and surfactant polyethylene glycol-400 (4.0 mL, 50 vol%) were placed in a mortar, and the mixture was thoroughly ground by hand with a rubbing mallet for 40 min. The grinding velocity was about 210 cycles/min, and the strength applied was moderate. The reactant mixture gradually became damp, and a paste was formed immediately. The reaction mixture was kept at 303 K for 1.5 h. The mixture was washed with deionized water to remove soluble inorganic salts until SO24 ion cannot be visually detected with a 0.5 mol L 1 BaCl2 solution. The solid was then washed with a small amount of anhydrous ethanol and dried at 353 K for 5 h. The resulting material was determined to be 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O. Nanocrystalline LaCoO3 was obtained by calcining 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at a heating rate of 5 K min 1 from ambient temperature to 1150 K in air in a muffle furnace, and then kept at 1150 K for 2 h. 3. Method of determining kinetic parameters and mechanism functions 3.1. Determination of activation energy by the Starink equation The activation energy of the thermal transformation of the solid compound can be obtained by the following Starink equation [Eq. (1)]: ! βi Eα ln 1:92 ¼ Const 1:0008 ; ð1Þ RT α T α;i where βi is the heating rate (K min 1), Tα,i is the reaction temperature (K) in TG curve, Eα is the activation energy (kJ mol 1) of thermal transformation corresponding to degree of conversion (α), and R is the gas constant (8.314 10 3 kJ mol 1 K 1). The conversion degree (α) can be expressed as follows: mi mt α¼ ; ð2Þ mi mf where mi, mf, and mt are the initial, final, and current sample masses at moment t, respectively. The dependence of lnðβi =T 1:92 α;i Þ on 1/Tα must lead to a straight line. Thus, the reaction activation energy Eα can be obtained from linear slope [ 1.0008Eα/R, Eq. (1)]. 3.2. Determination of the most probable mechanism functions The following equation was used to estimate the most probable reaction mechanism of the thermal transformation of
W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
0.97/2La2(C2O4)3–CoC2O4 5.3H2O, i.e., g(α) function [37,38]: AE a ex þ ln 2 þ ln hðxÞ ln β; ln gðαÞ ¼ ln ð3Þ R x where x=Ea/(RT), hðxÞ ¼ ðx4 þ 18x3 þ 86x2 þ 96xÞ=ðx4 þ 20x3 þ 120x2 þ 240x þ 120Þ, and β is the heating rate (K min 1). The conversions α corresponding to multiple rates at the same temperature are placed in the left of Eq. (3), combined with 31 types of mechanism functions [39,40]. The slope k and correlation coefficient r2 were obtained from the plot of ln g (α) vs. ln β. The probable function is the one at which the value of the slope k is near 1.00000 and the correlation coefficient r2 is better.
3.3. Calculation of pre-exponential factor A The pre-exponential factor was estimated as follows [36]: βgðαÞEa Ea A¼ exp ; ð4Þ RT max RT 2max where A is the pre-exponential factor (s 1), β is the heating rate (K min 1), g(α) is the most probable mechanism function determined by Eq. (3), Ea is the activation energy (kJ mol 1) of thermal transformation, R is the gas constant (8.314 10 3 kJ mol 1 K 1), and Tmax is the most rapid decomposition temperature [i.e., peak temperature in differential thermogravimetry (DTG ) curve, K].
4. Results and discussion 4.1. Composition analysis of the precursor
The FT-IR spectra of the precursor and its calcined samples are shown in Fig. 2. The precursor exhibited a strong and broad band at about 3381 cm 1 that can be assigned to the symmetric and asymmetric stretching modes of water molecules [41–43]. The bending mode of water expected around 1626 cm 1 overlapped with the intense oxalate band at around 1616 cm 1 [44–47]. The bands at 1320, 1097, 797, and 488 cm 1 can be assigned to
0
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Fig. 1 shows the TG/DTG/DSC curves of the precursor at four heating rates, i.e., 5, 10, 15, and 20 K min 1, from ambient temperature to 1150 K. The TG/DTG/DSC curves show that the thermal transformation of 0.97/2La2(C2O4)3– CoC2O4 5.3H2O below 1150 K occurred in six well-defined steps. At the heating rate of 10 K min 1, the first step started at about 303 K and ended at 325 K, which can be attributed to the dehydration of the 0.3 adsorption water from 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O (mass loss: observed, 1.07%; theoretical, 1.07%). The second transformation step started at 325 K and ended at 385 K, attributed to the dehydration of the 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 5H2O (mass loss: observed, 9.04%; theoretical, 8.91%). The third transformation step started at 385 K and ended at 522 K, attributed to the dehydration of the 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 2.5H2O (mass loss: observed, 8.72%; theoretical, 8.91%). The fourth transformation step started at 522 K and ended at 543 K, attributed to the reaction of CoC2O4 with 2/3O2 into 1/3Co3O4 and the two CO2 molecules (mass loss: observed, 13.92%; theoretical, 13.20%). The fifth transformation step started at 543 K and ended at 895 K, attributed to the reaction of 0.97/2La2(C2O4)3 with 0.7275/2O2 into 0.97/2La2O2CO3 and of 2.425CO2 molecules (mass loss: observed, 15.58%; theoretical, 16.52%). The sixth transformation step started at 895 K and ended at 1163 K, attributed to the reaction of 0.97/2La2O2CO3 and 1/3Co3O4 with 0.5/6O2 into La0.97CoO2.955 and of 0.485CO2 (mass loss: observed, 3.47%; theoretical, 3.70%). The observed weight loss differs from the theoretical mass loss, attributed to the error of the apparatus measurements.
4
372 K 448K
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4.2. TG/DTG/DSC analysis of the precursor
4.3. IR spectroscopic analyses of the precursor and its calcined samples
A precursor sample (0.0300 g) was dissolved in 10 mL 50 vol% HCl solution, and then diluted to 100.00 mL with deionized water. The La and Co in the solution were determined by inductively coupled plasma atomic emission spectrometry (Perkin Elmer Optima 5300 DV). The results showed that the La and Co mass percentage were 26.67% and 11.67%, respectively. The molar ratio of La:Co in the precursor was therefore 0.97:1.00.
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Fig. 1. TG/DTG/DSC curves of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at different heating rates in air.
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the appearance of new M–OC2O3 (M=La, Co) bonds and/or to the combinations of OH group vibration and lattice modes [37,46,47]. The band at 2353 cm 1 was attributed to the asymmetric stretching of vasy (C=O) from CO2 absorption [2,48]. The FT-IR spectra of the precursor and sample obtained at 423 K are similar, implying that the structure of the oxalates remained stable up to 423 K. When the precursor was calcined at 523 K, the bands at about 3381, 1626, 1320, 1097, 797, and 488 cm 1 disappeared and/or became weak. A new band also appeared at about 1437 cm 1, implying that the structure of the oxalates changed. When the precursor was calcined above 973 K, the bands at about 1626 and 1320 cm 1 disappeared, implying that the precursor completed the dehydration and decomposition of C2O2 4 . The spectrum of the calcined sample at 1123 K agrees with that of rhombohedral LaCoO3 from literature [3]. 4.4. XRD analyses of the calcined products Fig. 3 shows the XRD patterns of the precursor and its calcined samples from different calcining temperatures for 2 h. The results showed that the precursor is a mixture containing monoclinic La2(C2O4)3 10H2O [PDF card 49–1255, space group P21/c(14)] and orthorhombic CoC2O4 2H2O [PDF card 1123 K
873 K 723 K 523 K 423 K 353 K
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D ¼ Kλ=ðβ cos θÞ;
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Transmittance (%)
973 K
25–0250, space group Cccm(66)] (Fig. 3a). When the precursor was calcined at 523 K for 2 h, the characteristic diffraction peaks of monoclinic La2(C2O4)3 10H2O and orthorhombic CoC2O4 2H2O disappeared, whereas those of cubic Co3O4, hexagonal La2O2CO3, and La2(C2O4)3 [49] appeared. With the increase of the calcination temperature (Fig. 3a), the characteristic diffraction peaks of Co3O4 and La2O2CO3 became weak and/or disappeared. Fig. 3b shows that when the precursor was calcined at 1123 K for 2 h, a new diffraction pattern with strong intensity and smoothed baseline was observed. The calcined product therefore has a high degree of crystallinity. All the diffraction peaks in the pattern agreed with those of rhombohedral LaCoO3 with space group R-3c (167) from PDF card 48–0123. The refined lattice parameters of LaCoO3 obtained at 1123 K were a ¼ b ¼ 0.5427 nm, c¼ 1.3056 nm, α ¼ β ¼ 901, γ ¼ 1201, density ¼ 7.3531 g cm 3. No diffraction peak of Co3O4 was observed, implying that the Co3O4 content is beyond the detection limit of XRD or Co3O4 formed a solid solution with LaCoO3. Compared to other methods of synthesizing rhombohedral LaCoO3, the crystallization temperature and purity of LaCoO3 obtained by thermal decomposition of oxalates in this study have a few differences. For example, Nakayama et al. [26] obtained rhombohedral LaCoO3 by co-precipitation and found that the crystallinity of rhombohedral LaCoO3 depended on the calcination temperature. A single-phase LaCoO3 can be obtained after calcining precursor at 1473 K for 2 h. Ivanova et al. [28] studied the synthesis of rhombohedral LaCoO3 by freeze-drying of metal–citrate precursors, followed by calcination in air. A single-phase LaCoO3 can be obtained over 873 K for 3 h. The crystallite diameter of LaCoO3 was estimated using the following Scherrer formula [37]:
500
−1
Wavenumbers (cm ) Fig. 2. FT-IR spectra of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O and its calcined samples.
ð5Þ
where D is the crystallite diameter, K ¼ 0.89 (the Scherrer constant), λ ¼ 0.15406 nm (wavelength of the X-ray used), β is the width of line at the half-maximum intensity, and θ is the corresponding angle. The crystallite sizes of LaCoO3 from calcining precursor at 873 K, 973 K, and 1123 K in air for 2 h were 31 nm, 36 nm, and 52 nm, respectively. The crystallinity
0
Fig. 3. XRD patterns of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O and its calcined samples at different temperatures for 2 h.
W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
of LaCoO3 can be calculated by MDI Jade 5.0 software. The crystallinity of LaCoO3 obtained at 1123 K was 99.42%.
6001
follows: 0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5:3H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5H2 O ðsÞþ 0:3H2 O ðgÞ
4.5. SEM analysis of the calcined samples The morphologies of the calcined samples are shown in Fig. 4. Fig. 4a shows that the calcined sample obtained at 873 K is composed of approximately spherical grains and contains particles having a distribution from 100 nm to 300 nm. With the increase of the calcination temperature, the crystallite in the calcined sample aggregated into larger grains. Fig. 4b and c shows SEM images of the samples obtained at 973 K and 1123 K, respectively. The calcined sample at 973 K kept approximately spherical morphology, and the particle sizes were mainly between 100 nm and 300 nm. The calcined sample obtained at 1123 K became platelet grains, with a particle size of between 600 nm and 1250 nm. The average crystallite sizes of the calcined samples determined by X-ray diffraction were significantly smaller than the values determined by SEM. This difference can be attributed to the fact that the values observed by SEM have the size of the secondary particles, which are composed of several or many crystallites by soft reunion. In addition, the X-ray line broadening analysis disclosed only the size of a single crystallite.
4.6. Kinetics of the thermal transformation of the precursor In accordance with the TG/DTG/DSC, IR, and XRD analyses of the precursor and its calcined products mentioned above, the thermal transformation of the precursor below 1150 K consists of six steps, which can be expressed as
ð6Þ
0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U5H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U2:5H2 O ðsÞþ 2:5H2 O ðgÞ ð7Þ 0:97=2La2 ðC2 O4 Þ3 CoC2 O4 U2:5H2 O ðsÞ0:97=2La2 ðC2 O4 Þ3 CoC2 O4 ðsÞ þ 2:5H2 O ðgÞ
ð8Þ
CoC2 O4 ðsÞ þ 2=3O2 ðsÞ-1=3Co3 O4 ðsÞþ 2CO2 ðsÞ
ð9Þ
0:97=2La2 ðC2 O4 Þ3 ðsÞþ 0:7275O2 ðgÞ0:97=2La2 O2 CO3 ðsÞ þ 2:425CO2 ðgÞ
ð10Þ
0:97=2La2 O2 CO3 ðsÞþ 1=3Co3 O4 ðsÞþ 0:5=6O2 ðgÞLa0:97 CoO2:955 ðrÞ þ 0:485CO2 ðgÞ
ð11Þ
According to the non-isothermal method, the basic data of α and T were collected from the TG curves of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O at four heating rates (5, 10, 15, and 20 K min 1). According to Eq. (1), the isoconversional calculation procedure of the Starink equation was used. The corresponding Starink lines for different transformation steps were obtained at different conversion degrees α and heating rates β. The reaction activation energy Ea can be obtained from linear slope ( 1.0008 Ea/R). The results are shown in Fig. 5. The average values of the activation energies associated with 0.97/2La2(C2O4)3–CoC2O4 5.3H2O thermal transformation were 97.18716.39 kJ mol 1, 84.73713.48 kJ mol 1, 104.95710.26 kJ mol 1, 155.67711.56 kJ mol 1,
Fig. 4. SEM images of the products synthesized at different temperatures for 2 h: (a) 873 K, (b) 973 K, and (c) 1123 K.
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172.32729.29 kJ mol 1, and 381.897260.84 kJ mol 1 for steps 1, 2, 3, 4, 5, and 6, respectively. The activation energy changes in steps 1, 2, 5, and 6 with α are higher than 10%, whereas those in steps 3 and 4 with α are lower than 10%. Hence, the thermal transformations of 0.97/2La2(C2O4)3– CoC2O4 5.3H2O for steps 1, 2, 5, and 6 could be multi-step reaction mechanisms. By contrast, the thermal transformations for steps 3 and 4 are the dehydration of 2.5 crystal waters from 0.97/2La2(C2O4)3–CoC2O4 2.5H2O and the reaction of CoC2O4 with 2/3 O2 into 1/3Co3O4, respectively, which are simple reaction mechanisms [37,38,50,51]. The activation energy of step 6 is higher than those of steps 1 to 5. Step 6 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O may thus be interpreted as a “slow” stage, whereas the other steps may be interpreted as “fast” stages. Fig. 6 shows the curves of α vs. t and dα/dt vs. t for steps 3 and 4. In Fig. 6a, step 3 consists of two models, i.e., an accelerating model between 0.0 min and about 7.06 min and a decelerating model between about 7.06 min and 13.69 min [36]. In the accelerating model, rate increased continuously with increasing extent of conversion and reached its maximum at the end of the process (i.e., at 6.05 min). In the decelerating model, the process rate had maximum at the beginning of the process, and then decreased continuously as the extent of conversion increased. The result shows that the rate of the
decelerating stage reached its maximum at about 10.04 min (Fig. 6a). Step 4 is a sigmoidal model (sometimes called autocatalytic) that represents the process of step 4 [36], whose initial and final stages demonstrate the accelerating and decelerating behaviors, respectively. Thus, the process rate reached its maximum at some values of the extent of conversion. The rate for step 4 reached its maximum at 1.71 min (Fig. 6b). We randomly chose several temperatures corresponding to conversions 0.10oαo0.90. The conversions corresponding to the temperature for β¼ 5, 10, 15, and 20 K min 1 were assigned to 31 types of mechanism functions [38,39]. The slope k, correlation coefficient r2, and intercept B of the linear regression of ln g(α) vs. ln β were obtained. Two probable mechanism functions with better correlation coefficient r2 were determined. Several temperatures were randomly chosen to calculate the slope k, correlation coefficient r2, and intercept B of the two probable mechanism functions using the same method. The mechanism function in which the value of k is closest to 1.00000 and the correlation coefficient r2 is higher was chosen as the mechanism function of the thermal transformation of 0.97/2La2(C2O4)3– 1.0
0.8
step 4
step 3
500
5 K min
450
0.6
−1
Ea (kJ mol )
step 1 step 2 step 3 step 4 step 5 step 6
350 300 250
10 K min−1 15 K min−1
α
400
−1
20 K min−1
0.4
0.2
200 150
0.0
100
390
420
450
50 0.2
0.3
0.4
0.5
0.6
0.7
480
0.8
α Fig. 5. The dependence of Ea on α at different thermal transformation steps.
15 1.0
0.6
0.8
step 4
12
0.8
9
0.6
0.6 0.4
6.05 min
6
0.4
0.4
1.71min
0.2
0.2 0.0
10.04 min
0.2
3
0.0
0
0.0 0
2
4
6
8
t (min)
10
12
14
540
Fig. 7. Comparison of model results (solid line) with the experimental data (dash line) of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 3 and 4 at different heating rates.
0.8 step 3
1.0
510
T (K)
0.0
0.4
0.8
1.2 t (min)
Fig. 6. Curves of α vs. t and dα/dt vs. dt at heating rate of 10 K min 1.
1.6
2.0
W. Kaituo et al. / Ceramics International 40 (2014) 5997–6004
CoC2O4 5.3H2O. The probable mechanism function integral forms of the thermal transformation of 1/2La2(C2O4)3– CoC2O4 5.3H2O for steps 3 and 4 were g(α)=1–(1–α)1/2 and g (α)=[(1þ α)1/3–1]2, respectively. The rate-determining mechanisms for steps 3 and 4 were contracting cylinder and threedimensional diffusion, respectively. The pre-exponential factor A was obtained using Eq. (4), inserting the most probable mechanism function g(α), β, Ea, R, and Tmax values. The pre-exponential factors ln A(s 1) of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 3 and 4 were determined to be 23.90 and 28.20, respectively. The experimental data and the results of the kinetic mechanism for every heating rate were compared to prove the validity of the kinetic mechanism for steps 3 and 4 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O. The results are shown in Fig. 7. The model-predicted plots agree with the experimental plots. Therefore, the mechanism functions for steps 3 and 4 of the thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O are reliable. 5. Conclusions We have successfully synthesized a rhombohedral LaCoO3 by calcining 0.97/2La2(C2O4)3–CoC2O4 5.3H2O in air. The XRD analysis suggests that a rhombohedral LaCoO3 with a crystallite size of 52 nm can be obtained by calcining 0.97/ 2La2(C2O4)3–CoC2O4 5.3H2O at 1123 K in air for 2 h. The thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O from ambient temperature to 1150 K experienced six steps, namely, the dehydration of 0.3 adsorption water, the dehydration of 2.5 crystal waters, the dehydration of 2.5 crystal waters, the reaction of CoC2O4 with 2/3O2 into 1/3Co3O4, the reaction of 0.97/2La2(C2O4)3 with 0.7275O2 into 0.97/2La2O2CO3, and the reaction of 0.97/2La2O2CO3 and 1/3Co3O4 with 0.5/6O2 into the rhombohedral LaCoO3. The thermal transformation of 0.97/2La2(C2O4)3–CoC2O4 5.3H2O for steps 1, 2, 5, and 6 can be multi-step reaction mechanisms, whereas those for steps 3 and 4 are simple reaction mechanisms.
[4]
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[10]
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[16]
[17]
[18]
[19] [20]
Acknowledgments This study was financially supported by the National Nature Science Foundation of China (Grant no. 21161002) and the Guangxi Nature Science Foundation of China (Grant no. 2011GXNSFA018036).
[21]
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