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Decomposition kinetics study of AlOOH coated calcium carbonate
Materials Chemistry and Physics 115 (2009) 418–422
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Decomposition kinetics study of AlOOH coated calcium carbonate Dalai Jin a,∗ , Xiaojing Yu a , Linhai Yue b,∗∗ , Lina Wang c a
Center of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China c Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, PR China b
a r t i c l e
i n f o
Article history: Received 6 September 2008 Received in revised form 14 November 2008 Accepted 14 December 2008 Keywords: Calcium carbonate Coatings TG–DTA Decomposition kinetics
a b s t r a c t AlOOH coated calcium carbonate was synthesized by a two-step chemical solution reaction. The phase and microstructure analysis were carried out by X-ray diffraction and scanning electron microscopy. Nonisothermal decomposition kinetics of the AlOOH coated calcium carbonate has been investigated on the basis of the TG–DTA analysis. The model-free and model fitting approaches were applied to the data for non-isothermal decomposition kinetics of AlOOH surface-coated calcium carbonate as well as the reference. Master-plot method was used to determine the decomposition mechanisms based on the kinetic parameter Ea achieved by model-free Kissinger equation. The reference sample decomposed according to the contracting area mechanism R2 and the AlOOH surface-coated calcium carbonate decomposed according to one-dimensional mechanism D1 . The kinetic parameters Ea verified by both integral method and differential method showed about 90 kJ mol−1 increase in the decomposition process after being coated. This is possibly due to the as-shell structure of surface AlOOH, which hinders the diffusion of the dissociated CO2 into the outer atmosphere. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Calcium carbonate is one of the most widely used fillers utilized in fields as diverse as rubber, plastics, paper, paint, medicine and so on. It was found that high filler loading may adversely affect processability, ductility and strength of the compound. It was of great interests to functionalize the surface of the calcium carbonate [1–3]. Most research works proved that surface modification may improve the dispersion of the calcium carbonate as filler and strengthen the interaction between fillers and substrate [4]. The behavior of coated calcium carbonate under heating was of great value for its industrial applications. The surface modifier selected in this paper, AlOOH, is one of the ceramic materials with high thermal stability [5]. It is of great significance to investigate the thermal decomposition process of the AlOOH surface-coated calcium carbonate. Non-isothermal method was being extensively used in studying the decomposition kinetics of solid system [6–10]. Although recurring to the theory and equations of isothermal method and achieving doubt for its applicability and reliability, non-isothermal method has been still widely used for its outstanding convenience and excellence. Multiple techniques have been established for determining the reaction mechanism and deducing kinetic param-
∗ Corresponding author. Tel.: +86 571 86843265; fax: +86 571 86843265. ∗∗ Corresponding author. Tel.: +86 571 87952403; fax: +86 571 87951895. E-mail addresses: [email protected] (D. Jin), zjchem [email protected] (L. Yue). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.12.013
eters [11–18]. Some improvements on the theory, calculation and data treating techniques have been subsequently reported. Masterplot method [19] is a popularly used method of data treating for the non-isothermal kinetics analysis due to its high resolving power and being less influenced by experimental conditions [20,21]. It has been successfully used in studying the thermal decomposition kinetics of nano-scaled calcium carbonate [22] as well as magnesium hydroxide [23]. Contrary to the homogeneous reaction, the mechanism and kinetics of solid decomposition will vary with many factors such as the particle size, the crystal form, and the reaction conditions [24]. And the case of coated solid system is more complex. Thermal decompositions kinetics of surface SiO2 coated calcium carbonate [25] and stearic acid modified calcium carbonate [26] have been investigated. In this paper, an improved Masterplot method with numerical regression and optimization algorithm was suggested and the thermal decomposition kinetic behavior of AlOOH surface-coated calcium carbonate was analyzed.
2. Theory Mechanism-free equation is firstly introduced to estimate the activation energy as a prerequisite for determining the reaction mechanism. After the reaction mechanism is deduced, the corresponding kinetic equations are then used to further confirm the kinetic parameters obtained previously. The steps of this algorithm can be shortly described as follows:
D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
419
1. Kissinger equation [27], which is independent on the decomposition mechanism, is applied to estimate the activation energy: ln(A) = ln
E a
R
− ln
Tp2
+
ˇ
Ea RTp
(1)
where Tp represents the peak temperature of a DTA curve, ˇ is the heating rate. Changing the heating rate ˇ and plotting (ln(Tp2 /ˇ)) against 1/Tp , the activation energy Ea could be derived from the line slope. 2. An improved Master-plot method [23], which is more accurate and less influenced by experimental conditions, is used to determine the reaction mechanism. Here, a numerical integral is applied instead of the approximate formula to compute the experimental value of function z*(˛): d˛ Ea /RT z ∗ (˛) = e dT
T
e−Ea /RT dT
(2)
T0
Meanwhile, the theoretical value of z(˛) can be easily calculated for a pair of chosen mechanism function by z(˛) = f(˛)*g(˛). The most commonly used mechanism functions, f(˛) and g(˛), can be found in the literature [28]. Try all possible mechanism function pair f(˛) and g(˛) (we use total 29 functions) to calculate
Fig. 1. XRD patterns of the CaCO3 (a) before and (b) after being coated.
2 (z(˛) − z ∗ (˛)) . The one that minimizes value repre-
=
˛
sents the most suitable mechanism. In addition, for more direct observation, z*(˛) can also be plotted together with a series of standard z(˛) curves to see how well z*(˛) fit the corresponding mechanism curve as in the normal Master-plot case. 3. Afterward, Coats and Redfern equation [29], referred to as integral method: log
g(a) T2
= log
AR Eˇ
−
Ea 2.303RT
(3)
and Achar, Brindley and Sharp equation [30], referred to as differential method: ln
da dT
ˇ
= ln (Af (a)) −
Ea RT
(4)
are further used to confirm the obtained activation energy. The more detailed introduction of the theory used here may be referred to Ref. [23]. 3. Experimental Purified CO2 was bubbled through and carbonized with 5–10% Ca(OH)2 slurry at 30 ◦ C until the pH level was 7–8. Then the slurry was heated to 60 ◦ C and the pH level was adjusted up to 9 by 5% NaOH. 6% Al2 (SO4 )3 solution was dripped into the CaCO3 slurry slowly under strong stirring. The Al2 (SO4 )3 required to produce Al2 O3 was 4 wt.% relative to CaCO3 . The slurry was kept at 60 ◦ C for 2 h more aging time. Then the precipitate was filtrated, washed with deionized water until the pH value of the filtrate equals to about 7, and desiccated at 110 ◦ C to constant weight and then shifted through 120 mesh. Reference sample was also prepared without the addition of Al2 (SO4 )3 solution. X-ray powder diffraction patterns were obtained at room temperature by a Philips diffractometer model X’Pert MPD using Cu K␣ radiation. The accelerating voltage was 40 kV with 45 mA flux at a scanning rate of 1◦ min−1 and scanning step 0.02◦ . The non-isothermal kinetic experiments were carried out by a WCT-1 analyzer at the heating rates of 5, 10, 15, and 20 ◦ C min−1 in air atmosphere with the flow rate of 20–30 mL min−1 .
4. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the sample before and after being coated. Peaks appeared at 2 = 29.4◦ , 35.9◦ , 39.5◦ , 43.1◦ are in good agreement with the values from the standard card (JCPDS no. 05-0586) of well-crystallized calcite. SEM images in Fig. 2 present short fibers on the surface of the spindle shaped calcium
Fig. 2. SEM images of the CaCO3 (a) before and (b) after being coated.
carbonate after being coated. TG–DTA curves in Fig. 3 show that the weight loss of the calcium carbonate before 550 ◦ C increases after being coated, which is in agreement with that reported by Li et al. [31]. For further investigation, the coated powder was treated with buffer solution of pH ≈ 4 to get rid of the calcite. The XRD pattern of the coating layer is shown in Fig. 4. It proves that the coated layer was AlOOH, one of the metastable alumina phase, almost amorphous.
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D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
Fig. 3. TG–DTA curves of the CaCO3 before and after being coated.
Fig. 4. XRD pattern of the coating layer.
(ln(Tp2 /ˇ)) against 1/Tp (Eq. (1)) plots have been shown in Figs. 5 and 6 for each heating rate of both reference CaCO3 and coated CaCO3 , respectively, on the basis of the TG–DTA curves. Both Figs. 5 and 6 show a straight line which fits the data points best. The activation energy Ea values obtained from the line slope are 212.3 and 303.2 kJ mol−1 , respectively. As described in Section 2, a numerical optimization method was used to search the best fitting decomposition mechanism. It can be seen directly from Figs. 7 and 8, the contracting area mechanism R2 fits the experimental data best for the reference calcium carbonate
Fig. 5. (ln(Tp2 /ˇ)) vs. 1/Tp plot of reference calcium carbonate.
Fig. 6. (ln(Tp2 /ˇ)) vs. 1/Tp plot of coated calcium carbonate.
Fig. 7. Master-plot curves of z(˛) and experimental data for reference calcium carbonate at heating rate 20 K min−1 .
and one-dimensional diffusion mechanism D1 seems the suitable mechanism for the coated calcium carbonate. For further confirmation of the decomposition kinetic mechanism, experimental data was used to plot ln(g(˛)/T2 ) against 1/T according to Eq. (3). It is found that the curves corresponding to as previously deduced mechanisms at each heating rates are more close to a straight line than other mechanisms as shown in
Fig. 8. Master-plot curves of z(˛) and experimental data for coated calcium carbonate at heating rate 20 K min−1 .
D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
421
Table 1 Determination of reaction mechanism and activation energy at different heating rate. Sample
Reaction mechanism
Heating rate (K min−1 )
Ea (kJ mol−1 ) obtained by Eq. (4) differential method
Ea (kJ mol−1 ) obtained by Eq. (3) integral method
Reference calcium carbonate
R2
20
206.2
205.4
15 10 5
208.5 209.7 216.8
204.6 209.8 214.5
210.3
208.6
20
299.2
305.9
15 10 5
298.4 303.8 304.1
308.2 310.1 309.6
301.4
308.5
Ea AlOOH coated calcium carbonate
D1
Ea
Fig. 9. ln(g(˛)/T2 ) vs. 1/T for reference calcium carbonate at heating rate 20 K min−1 .
90 kJ mol−1 increase of Ea value in the decomposition process after being coated. The thermal decomposition of solids is a very complex process even in the simple case expressed by the stoichiometric equation A(s) → B(s) + C(g). This process takes place in several stages, e.g. the chemical act of breaking of bonds, destruction of the initial crystal lattice, formation of the crystal lattice of the solid product B, consisting of the formation of crystallization centers and the growth of these centers, adsorption–desorption of the gaseous product C, diffusion of C and heat transfer. The rate of the thermal decomposition is determined by the rate of one or more of these stages. It was argued by Mulokozi and Lugwisha [32] that the thermal decomposition of calcium carbonate mainly consists of two stages. The initial stage is the loss of carbon dioxide from calcium carbonate lattice occurring at the interface between calcium carbonate and active calcium oxide. With the progress of the decomposition, the calcium oxide has recrystallized to its stable crystal habit and acts as a cage preventing the diffusion of CO2 . Igraham and Marier [33] have reported that the thermal decomposition of calcium carbonate was inhibited with the increase of the fractional pressure of the CO2 in the system resulting in the increase of the CO2 concentration around the reaction interface. In the case of AlOOH surface-coated calcium carbonate, the coating layer is a fine ceramic material with high thermal stability. It acts well as compact shell around the calcium carbonate core. During the thermal decomposition process of calcium carbonate, the released CO2 was considerably prevented from diffusing outside. So the diffusion stage appears to be the rate determining step. And the decomposition mechanism changes to D1 mechanism. 5. Conclusions
Fig. 10. ln(g(˛)/T2 ) vs. 1/T for coated calcium carbonate at heating rate 20 K min−1 .
Figs. 9 and 10. The obtained values of Ea are listed in Table 1. The results verified by differential method are also listed in Table 1. The two results fit well with each other and as well with that obtained from mechanism-free equation (1). It shows that there is about
Non-isothermal decomposition kinetics of AlOOH surfacecoated calcium carbonate as well as the reference has been investigated. It showed that reference sample decomposes according to the contracting area mechanism R2 . D1 mechanism seemed to be the most suitable mechanism for the decomposition process of AlOOH coated calcium carbonate. The activation energy obtained from mechanism-free equation was consistent with that obtained from mechanism equation. It manifested the validity of both the reaction mechanism and the activation energy value. The activation energy of the AlOOH surfacecoated calcium carbonate obtained from our experiments was approximately 300 kJ mol−1 , about 90 kJ mol−1 more than that of the reference sample. It was considered that the surface AlOOH layer acted as a shell hindering the diffusion of the dissociated product CO2 into the
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D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
outer atmosphere, which resulted in the thermal decomposition mechanism changing from R2 to D1 after being coated. Acknowledgements This work is supported by the scientific research project of Science and Technology Department of Zhejiang Province, PR China (No. 2007F70025). This work is also supported by Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 111383A4Y06054. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
E.A. Fekete, B. Pukanszkyb, R. Toth, J. Colloid Interf. Sci. 135 (1) (1990) 200. M. Bob, H.W. Walker, Colloid Surf. A: Physicochem. Eng. Aspects 191 (2001) 17. A. Tabtiang, R. Venables, Eur. Polym. J. 36 (2000) 137. I. Demjen, R. Pukanszky, E. Foldes, et al., J. Colloid Interf. Sci. 170 (1997) 427. W.O. Milligan, J.L. McAtee, J. Phys. Chem. 60 (1956) 273. I. Halikia, A. Economacou, Int. J. Chem. Kinet. 25 (1993) 609. B. Boonchom, C. Danvirutai, S. Maensiri, Mater. Chem. Phys. 109 (2008) 404–410. H. Barkia, L. Belkbir, S.A.A. Jayaweera, J. Therm. Anal. Calorim. 76 (2) (2004) 623. M.A. Gabal, A.A. El-Bellihi, H.H. El-Bahnasawy, Mater. Chem. Phys. 81 (2003) 174–182.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
T. Vlase, G. Vlase, M. Doca, et al., J. Therm. Anal. Calorim. 72 (2) (2003) 597. C.D. Doyle, J. Appl. Polym. Sci. 5 (1961) 285. J. Zsako, J. Phys. Chem. 72 (1968) 2406. R.T. Rajeswara, Chem. Eng. Technol. 19 (1996) 373. J.M. Criado, J. Malek, A. Ortega, Thermochim. Acta 147 (1989) 377. J. Malek, Thermochim. Acta 200 (1992) 257. E. Urbanovici, C. Popescu, E. Segal, J. Therm. Anal. Calorim. 60 (2000) 581. A.M. Gadalla, Thermochim. Acta 74 (1984) 255. A.R. Silva, S. Marisa, C.A. Crespi, Ribeiro, et al., J. Therm. Anal. Calorim. 75 (2004) 401. L.A. Perez-Maqueda, et al., Thermochim. Acta 277 (1996) 165. Z.R. Lu, Chin. J. Inorg. Chem. 14 (2) (1998) 119. L.H. Yue, M. Shui, Z.D. Xu, Chin. J. Inorg. Chem. 15 (2) (1999) 225. L.H. Yue, M. Shui, Z.D. Xu, Thermochim. Acta 335 (1999) 121. L.H. Yue, D.L. Jin, D.Y. Lv, Z.D. Xu, Acta Phys.-Chim. Sin. 21 (2005) 752. A.M. Ghozza, J. Therm. Anal. Calorim. 75 (2004) 135. M. Shui, L.H. Yue, Y.M. Hua, Z.D. Xu, Thermochim. Acta 386 (2002) 43. X.J. Li, J.F. Liao, Chem. Res. Appl. 9 (5) (1997) 478 (Chinese). H.E. Kissinger, J. Res. Natl. Bur. Stand. 57 (1956) 712. D. Dollimore, Ping Tong, K. Alexander, Thermochim. Acta 282/283 (1996) 13. A.W. Coats, J.P. Redfern, Nature 201 (1964) 68. J.H. Sharp, S.A. Wendworth, Anal. Chem. 41 (1969) 206. Y.Y. Li, J.P. Liu, Z.J. Jia, Mater. Lett. 60 (2006) 3586. A.M. Mulokozi, E. Lugwisha, Thermochim. Acta 194 (1992) 375. T.R. Igraham, P.J. Marier, Chem. Eng. 8 (1963) 170.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Decomposition kinetics study of AlOOH coated calcium carbonate Dalai Jin a,∗ , Xiaojing Yu a , Linhai Yue b,∗∗ , Lina Wang c a
Center of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China c Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, PR China b
a r t i c l e
i n f o
Article history: Received 6 September 2008 Received in revised form 14 November 2008 Accepted 14 December 2008 Keywords: Calcium carbonate Coatings TG–DTA Decomposition kinetics
a b s t r a c t AlOOH coated calcium carbonate was synthesized by a two-step chemical solution reaction. The phase and microstructure analysis were carried out by X-ray diffraction and scanning electron microscopy. Nonisothermal decomposition kinetics of the AlOOH coated calcium carbonate has been investigated on the basis of the TG–DTA analysis. The model-free and model fitting approaches were applied to the data for non-isothermal decomposition kinetics of AlOOH surface-coated calcium carbonate as well as the reference. Master-plot method was used to determine the decomposition mechanisms based on the kinetic parameter Ea achieved by model-free Kissinger equation. The reference sample decomposed according to the contracting area mechanism R2 and the AlOOH surface-coated calcium carbonate decomposed according to one-dimensional mechanism D1 . The kinetic parameters Ea verified by both integral method and differential method showed about 90 kJ mol−1 increase in the decomposition process after being coated. This is possibly due to the as-shell structure of surface AlOOH, which hinders the diffusion of the dissociated CO2 into the outer atmosphere. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Calcium carbonate is one of the most widely used fillers utilized in fields as diverse as rubber, plastics, paper, paint, medicine and so on. It was found that high filler loading may adversely affect processability, ductility and strength of the compound. It was of great interests to functionalize the surface of the calcium carbonate [1–3]. Most research works proved that surface modification may improve the dispersion of the calcium carbonate as filler and strengthen the interaction between fillers and substrate [4]. The behavior of coated calcium carbonate under heating was of great value for its industrial applications. The surface modifier selected in this paper, AlOOH, is one of the ceramic materials with high thermal stability [5]. It is of great significance to investigate the thermal decomposition process of the AlOOH surface-coated calcium carbonate. Non-isothermal method was being extensively used in studying the decomposition kinetics of solid system [6–10]. Although recurring to the theory and equations of isothermal method and achieving doubt for its applicability and reliability, non-isothermal method has been still widely used for its outstanding convenience and excellence. Multiple techniques have been established for determining the reaction mechanism and deducing kinetic param-
∗ Corresponding author. Tel.: +86 571 86843265; fax: +86 571 86843265. ∗∗ Corresponding author. Tel.: +86 571 87952403; fax: +86 571 87951895. E-mail addresses: [email protected] (D. Jin), zjchem [email protected] (L. Yue). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.12.013
eters [11–18]. Some improvements on the theory, calculation and data treating techniques have been subsequently reported. Masterplot method [19] is a popularly used method of data treating for the non-isothermal kinetics analysis due to its high resolving power and being less influenced by experimental conditions [20,21]. It has been successfully used in studying the thermal decomposition kinetics of nano-scaled calcium carbonate [22] as well as magnesium hydroxide [23]. Contrary to the homogeneous reaction, the mechanism and kinetics of solid decomposition will vary with many factors such as the particle size, the crystal form, and the reaction conditions [24]. And the case of coated solid system is more complex. Thermal decompositions kinetics of surface SiO2 coated calcium carbonate [25] and stearic acid modified calcium carbonate [26] have been investigated. In this paper, an improved Masterplot method with numerical regression and optimization algorithm was suggested and the thermal decomposition kinetic behavior of AlOOH surface-coated calcium carbonate was analyzed.
2. Theory Mechanism-free equation is firstly introduced to estimate the activation energy as a prerequisite for determining the reaction mechanism. After the reaction mechanism is deduced, the corresponding kinetic equations are then used to further confirm the kinetic parameters obtained previously. The steps of this algorithm can be shortly described as follows:
D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
419
1. Kissinger equation [27], which is independent on the decomposition mechanism, is applied to estimate the activation energy: ln(A) = ln
E a
R
− ln
Tp2
+
ˇ
Ea RTp
(1)
where Tp represents the peak temperature of a DTA curve, ˇ is the heating rate. Changing the heating rate ˇ and plotting (ln(Tp2 /ˇ)) against 1/Tp , the activation energy Ea could be derived from the line slope. 2. An improved Master-plot method [23], which is more accurate and less influenced by experimental conditions, is used to determine the reaction mechanism. Here, a numerical integral is applied instead of the approximate formula to compute the experimental value of function z*(˛): d˛ Ea /RT z ∗ (˛) = e dT
T
e−Ea /RT dT
(2)
T0
Meanwhile, the theoretical value of z(˛) can be easily calculated for a pair of chosen mechanism function by z(˛) = f(˛)*g(˛). The most commonly used mechanism functions, f(˛) and g(˛), can be found in the literature [28]. Try all possible mechanism function pair f(˛) and g(˛) (we use total 29 functions) to calculate
Fig. 1. XRD patterns of the CaCO3 (a) before and (b) after being coated.
2 (z(˛) − z ∗ (˛)) . The one that minimizes value repre-
=
˛
sents the most suitable mechanism. In addition, for more direct observation, z*(˛) can also be plotted together with a series of standard z(˛) curves to see how well z*(˛) fit the corresponding mechanism curve as in the normal Master-plot case. 3. Afterward, Coats and Redfern equation [29], referred to as integral method: log
g(a) T2
= log
AR Eˇ
−
Ea 2.303RT
(3)
and Achar, Brindley and Sharp equation [30], referred to as differential method: ln
da dT
ˇ
= ln (Af (a)) −
Ea RT
(4)
are further used to confirm the obtained activation energy. The more detailed introduction of the theory used here may be referred to Ref. [23]. 3. Experimental Purified CO2 was bubbled through and carbonized with 5–10% Ca(OH)2 slurry at 30 ◦ C until the pH level was 7–8. Then the slurry was heated to 60 ◦ C and the pH level was adjusted up to 9 by 5% NaOH. 6% Al2 (SO4 )3 solution was dripped into the CaCO3 slurry slowly under strong stirring. The Al2 (SO4 )3 required to produce Al2 O3 was 4 wt.% relative to CaCO3 . The slurry was kept at 60 ◦ C for 2 h more aging time. Then the precipitate was filtrated, washed with deionized water until the pH value of the filtrate equals to about 7, and desiccated at 110 ◦ C to constant weight and then shifted through 120 mesh. Reference sample was also prepared without the addition of Al2 (SO4 )3 solution. X-ray powder diffraction patterns were obtained at room temperature by a Philips diffractometer model X’Pert MPD using Cu K␣ radiation. The accelerating voltage was 40 kV with 45 mA flux at a scanning rate of 1◦ min−1 and scanning step 0.02◦ . The non-isothermal kinetic experiments were carried out by a WCT-1 analyzer at the heating rates of 5, 10, 15, and 20 ◦ C min−1 in air atmosphere with the flow rate of 20–30 mL min−1 .
4. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the sample before and after being coated. Peaks appeared at 2 = 29.4◦ , 35.9◦ , 39.5◦ , 43.1◦ are in good agreement with the values from the standard card (JCPDS no. 05-0586) of well-crystallized calcite. SEM images in Fig. 2 present short fibers on the surface of the spindle shaped calcium
Fig. 2. SEM images of the CaCO3 (a) before and (b) after being coated.
carbonate after being coated. TG–DTA curves in Fig. 3 show that the weight loss of the calcium carbonate before 550 ◦ C increases after being coated, which is in agreement with that reported by Li et al. [31]. For further investigation, the coated powder was treated with buffer solution of pH ≈ 4 to get rid of the calcite. The XRD pattern of the coating layer is shown in Fig. 4. It proves that the coated layer was AlOOH, one of the metastable alumina phase, almost amorphous.
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D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
Fig. 3. TG–DTA curves of the CaCO3 before and after being coated.
Fig. 4. XRD pattern of the coating layer.
(ln(Tp2 /ˇ)) against 1/Tp (Eq. (1)) plots have been shown in Figs. 5 and 6 for each heating rate of both reference CaCO3 and coated CaCO3 , respectively, on the basis of the TG–DTA curves. Both Figs. 5 and 6 show a straight line which fits the data points best. The activation energy Ea values obtained from the line slope are 212.3 and 303.2 kJ mol−1 , respectively. As described in Section 2, a numerical optimization method was used to search the best fitting decomposition mechanism. It can be seen directly from Figs. 7 and 8, the contracting area mechanism R2 fits the experimental data best for the reference calcium carbonate
Fig. 5. (ln(Tp2 /ˇ)) vs. 1/Tp plot of reference calcium carbonate.
Fig. 6. (ln(Tp2 /ˇ)) vs. 1/Tp plot of coated calcium carbonate.
Fig. 7. Master-plot curves of z(˛) and experimental data for reference calcium carbonate at heating rate 20 K min−1 .
and one-dimensional diffusion mechanism D1 seems the suitable mechanism for the coated calcium carbonate. For further confirmation of the decomposition kinetic mechanism, experimental data was used to plot ln(g(˛)/T2 ) against 1/T according to Eq. (3). It is found that the curves corresponding to as previously deduced mechanisms at each heating rates are more close to a straight line than other mechanisms as shown in
Fig. 8. Master-plot curves of z(˛) and experimental data for coated calcium carbonate at heating rate 20 K min−1 .
D. Jin et al. / Materials Chemistry and Physics 115 (2009) 418–422
421
Table 1 Determination of reaction mechanism and activation energy at different heating rate. Sample
Reaction mechanism
Heating rate (K min−1 )
Ea (kJ mol−1 ) obtained by Eq. (4) differential method
Ea (kJ mol−1 ) obtained by Eq. (3) integral method
Reference calcium carbonate
R2
20
206.2
205.4
15 10 5
208.5 209.7 216.8
204.6 209.8 214.5
210.3
208.6
20
299.2
305.9
15 10 5
298.4 303.8 304.1
308.2 310.1 309.6
301.4
308.5
Ea AlOOH coated calcium carbonate
D1
Ea
Fig. 9. ln(g(˛)/T2 ) vs. 1/T for reference calcium carbonate at heating rate 20 K min−1 .
90 kJ mol−1 increase of Ea value in the decomposition process after being coated. The thermal decomposition of solids is a very complex process even in the simple case expressed by the stoichiometric equation A(s) → B(s) + C(g). This process takes place in several stages, e.g. the chemical act of breaking of bonds, destruction of the initial crystal lattice, formation of the crystal lattice of the solid product B, consisting of the formation of crystallization centers and the growth of these centers, adsorption–desorption of the gaseous product C, diffusion of C and heat transfer. The rate of the thermal decomposition is determined by the rate of one or more of these stages. It was argued by Mulokozi and Lugwisha [32] that the thermal decomposition of calcium carbonate mainly consists of two stages. The initial stage is the loss of carbon dioxide from calcium carbonate lattice occurring at the interface between calcium carbonate and active calcium oxide. With the progress of the decomposition, the calcium oxide has recrystallized to its stable crystal habit and acts as a cage preventing the diffusion of CO2 . Igraham and Marier [33] have reported that the thermal decomposition of calcium carbonate was inhibited with the increase of the fractional pressure of the CO2 in the system resulting in the increase of the CO2 concentration around the reaction interface. In the case of AlOOH surface-coated calcium carbonate, the coating layer is a fine ceramic material with high thermal stability. It acts well as compact shell around the calcium carbonate core. During the thermal decomposition process of calcium carbonate, the released CO2 was considerably prevented from diffusing outside. So the diffusion stage appears to be the rate determining step. And the decomposition mechanism changes to D1 mechanism. 5. Conclusions
Fig. 10. ln(g(˛)/T2 ) vs. 1/T for coated calcium carbonate at heating rate 20 K min−1 .
Figs. 9 and 10. The obtained values of Ea are listed in Table 1. The results verified by differential method are also listed in Table 1. The two results fit well with each other and as well with that obtained from mechanism-free equation (1). It shows that there is about
Non-isothermal decomposition kinetics of AlOOH surfacecoated calcium carbonate as well as the reference has been investigated. It showed that reference sample decomposes according to the contracting area mechanism R2 . D1 mechanism seemed to be the most suitable mechanism for the decomposition process of AlOOH coated calcium carbonate. The activation energy obtained from mechanism-free equation was consistent with that obtained from mechanism equation. It manifested the validity of both the reaction mechanism and the activation energy value. The activation energy of the AlOOH surfacecoated calcium carbonate obtained from our experiments was approximately 300 kJ mol−1 , about 90 kJ mol−1 more than that of the reference sample. It was considered that the surface AlOOH layer acted as a shell hindering the diffusion of the dissociated product CO2 into the
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outer atmosphere, which resulted in the thermal decomposition mechanism changing from R2 to D1 after being coated. Acknowledgements This work is supported by the scientific research project of Science and Technology Department of Zhejiang Province, PR China (No. 2007F70025). This work is also supported by Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 111383A4Y06054. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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