Thermal decomposition kinetics of strontium oxalate

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ones are assigned to the decomposition of the anhydrous strontium oxalate into SrCO3 and the ... alytical reagent strontium chloride. The obtained pre-.

c 2007 Institute of Chemistry, Slovak Academy of Sciences  DOI: 10.2478/s11696-007-0050-3

Thermal Decomposition Kinetics of Strontium Oxalate a

F. A. AL-NEWAISER, a S. A. AL-THABAITI, a A. O. AL-YOUBI, a A. Y. OBAID, and b M. A. GABAL*

a Chemistry

Department, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabia

b Chemistry

Department, Faculty of Science, Benha University, Benha, Egypt e-mail: [email protected]

Received 1 November 2006; Revised 17 February 2007; Accepted 21 February 2007

The thermal decomposition behavior in air of SrC2 O4 · 1.25H2 O was studied up to the formation of SrO using DTA-TG-DTG techniques. The decomposition proceeds through four well-defined steps. The first two steps are attributed to the dehydration of the salt, while the third and fourth ones are assigned to the decomposition of the anhydrous strontium oxalate into SrCO3 and the decomposition of SrCO3 to SrO, respectively. The exothermic DTA peak found at around 300 ◦C is ascribed to the recrystallization of the anhydrous strontium oxalate. On the other hand, the endothermic DTA peak observed at 910 ◦C can be attributed to the transition of orthorhombichexagonal phase of SrCO3 . The kinetics of the thermal decomposition of anhydrous strontium oxalate and strontium carbonate, which are formed as stable intermediates, have been studied using non-isothermal TG technique. Analysis of kinetic data was carried out assuming various solid-state reaction models and applying three different computational methods. The data analysis according to the composite method showed that the anhydrous oxalate decomposition is best described by the two-dimensional diffusion-controlled mechanism (D2 ), while the decomposition of strontium carbonate is best fitted by means of the three-dimensional phase boundary-controlled mechanism (R3 ). The values of activation parameters obtained using different methods were compared and discussed. Keywords: strontium oxalate, DTA-TG, decomposition, kinetics, mechanism

INTRODUCTION Processes involving chemical transformations of solids play an important role in modern technology, as sophisticated and costly solids can be produced by reaction of other, precursory solids [1]. The preparation of strontium oxalate presents a part of the study of oxalate precursor employed in the synthesis of BiSCCO ceramic superconductors [2] usually prepared by the solid-state thermochemical reaction of Bi2 O3 , SrCO3 , CaCO3 , and CuO. Strontium oxalate exists in two different forms [3], the neutral strontium oxalate hydrate, SrC2 O4 · x H2 O, and the acid salt of strontium oxalate, SrC2 O4 · 1/2H2 C2 O4 · x H2 O. Depending on the concentration of oxalic acid and ammonium oxalate as precipitating agents, both forms can be obtained. At ap-

propriate pH, the stoichiometric compound SrC2 O4 · 1/2H2 C2 O4 · H2 O is formed. The extent of hydration depends on the preparation conditions while a part of the crystallization water was reported to be zeolitic [4]. The thermal decomposition of various forms of strontium oxalates was studied in different atmospheres [3] using differential scanning calorimetry (DSC) and thermogravimetry (TG) coupled with Fourier transform infrared spectroscopy (FT-IR) and mass spectroscopy (MS). The results showed that the anhydrous acid oxalate decomposition was accompanied with the release of H2 O, CO, CO2 , and formic acid. The studies on the hydrated salts by Dollimore et al. [4] showed a correlation between their crystal structure and the dehydration mode, where the dehydration of the triclinic single crystal of SrC2 O4 · H2 O

*The author to whom the correspondence should be addressed. Present address: Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah, KSA.

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Chem. Pap. 61 (5) 370—375 (2007)

THERMAL DECOMPOSITION KINETICS OF STRONTIUM OXALATE

Table 1. Composition of the Prepared Strontium Oxalate

wi (calc.)/% wi (found)/%

Sr

C

H

O*

44.22 44.08

12.12 12.15

1.27 1.35

42.39 42.42

*Calculated according to the mass balance.

proceeded in a single step whereas the dehydration of strontium oxalate polyhydrate took place in stages depending on the crystal symmetry. The authors also showed that the anhydrous SrC2 O4 exists in a polycrystalline form and its decomposition to carbonate is complicated by the disproportionation of the CO produced. Abd El-Khalik et al. [5] measured the surface area of strontium oxalate monohydrate and its thermal decomposition products obtained in the temperature range 400—510 ◦C in the presence of air or water vapor at various pressures, as well as in the atmosphere of nitrogen, hydrogen, or carbon dioxide. The surface area of SrC2 O4 · H2 O depended on the texture variation of the solid exposed to the chosen atmosphere. In the present work, the thermal decomposition behavior of SrC2 O4 · 1.25H2 O in air was followed using differential thermal analysis (DTA), thermogravimetry (TG), and differential thermogravimetry (DTG) techniques. Kinetics analysis of the non-isothermal TG data was performed with reference to the various heterogeneous solid-state reaction models and computational methods. EXPERIMENTAL Strontium oxalate was prepared by the direct precipitation, using oxalic acid solution added dropwise to a solution containing stoichiometric amount of analytical reagent strontium chloride. The obtained precipitate was filtered off, washed several times with distilled water until it was chloride ion-free, and then dried in air. The results of elemental analysis of the obtained precipitate were consistent with the composition of SrC2 O4 · 1.25H2 O (Table 1). Behavior of the strontium compounds under DTATG-DTG experimental conditions was investigated simultaneously using a Shimadzu DT-40 thermal analyzer. The measurements were carried out over a wide range of temperatures up to 1050 ◦C at a heating rate of 20 ◦C min−1 in flowing dry air at a flow rate of 30 mL min−1 . The kinetic experiments were carried out in dry air (30 mL min−1 ) up to 1000 ◦C under dynamic conditions using different heating rates of 1 ◦C min−1 , 2 ◦C min−1 , 3 ◦C min−1 , 5 ◦C min−1 , and 10 ◦C min−1 . In all experiments, the sample mass in the Pt crucible was about 11 mg in order to assure linear heating rate and accurate temperature measurements.

Chem. Pap. 61 (5) 370—375 (2007)

Fig. 1. TG, DTG, and DTA curves for the thermal decomposition of SrC2 O4 · 1.25H2 O in air at the heating rate of 20 ◦C min−1 .

RESULTS AND DISCUSSION Thermal Analysis Fig. 1 shows the process of SrC2 O4 · 1.25H2 O thermal decomposition in dry air at the heating rate of 20 ◦C min−1 . One can see that the salt decomposes in four well-defined steps to SrO. The first step of decomposition was observed in the temperature range 127—180 ◦C with the maximum at 150 ◦C (endothermic DTA peak). Mass loss in this decomposition step amounted to 8.9 %, the amount corresponding to the loss of one water molecule (calculated mass loss: 9.1 %). The second decomposition step started at 260 ◦C showing a mass loss of about 2.4 % at 320 ◦C that can be attributed to the loss of the remaining 0.25 molecules of water (calculated mass loss: 2.3 %) with the formation of anhydrous strontium oxalate. The temperature range at which the water molecules were eliminated corresponded to their coordination in the salt [3, 6]. Nagase et al. [7] reported an exothermic DTA peak for SrC2 O4 · 2H2 O at 290 ◦C without any mass loss at the heating rate of 5 ◦C min−1 in nitrogen. This behavior was found to be irreversible (on cooling) indicating that the lower-temperature form of strontium oxalate dihydrate is metastable with respect to the higher-temperature one. Using X-ray diffraction, Dollimore et al. [4] attributed an exothermic DTA peak found at around 315 ◦C for the decomposition of SrC2 O4 · H2 O to recrystallization phenomena. They also showed that the recrystallization could take place at the same time with the dehydration process, so a net endothermic change is only observed. Thus, taking into account all the above information, the appearance of a very weak exothermic peak at the temperature of 317 ◦C in DTA curve could be attributed to the exothermic effect of recrystallization overwhelm-

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F. A. AL-NEWAISER, S. A. AL-THABAITI, A. O. AL-YOUBI, A. Y. OBAID, M. A. GABAL

ing the endothermic impact of the sample dehydration. The third step, decomposition of anhydrous strontium oxalate to strontium carbonate was accompanied with a mass loss of 14.3 % due to the evolution of CO (calculated mass loss: 14.1 %). The oxidation of CO to CO2 [4] is obviously exothermic reaction in nature the one responsible for the appearance of an exothermic DTA peak at the temperature of 497 ◦C (Fig. 1). At room temperature SrCO3 crystals are orthorhombic undergoing transformation to the hexagonal system at 912 ◦C [8]. A small endothermic peak found at 910 ◦C was attributed to the orthorhombichexagonal phase transition of SrCO3 , while another broad endothermic DTA peak corresponded to the decomposition of SrCO3 . The mass loss accompanying this decomposition amounted to 22.1 % (calculated mass loss: 22.2 %). No further changes were observed by raising the temperature. Kinetic Study Kinetic analysis of the thermal decomposition of anhydrous strontium oxalate and strontium carbonate intermediates was carried out under dynamic conditions, using heating rates of 1 ◦C min−1 , 2 ◦C min−1 , 3 ◦C min−1 , 5 ◦C min−1 , and 10 ◦C min−1 assuming various solid-state reaction equations [6]. Three integral computational methods, namely the Coats—Redfern [9], Ozawa [10], and Diefallah Composite [6, 11, 12] method were used for this purpose. Assuming the Composite method of analysis [6], the values of activation parameters obtained not only at different heating rates (β) but also taking into account different fractions decomposed (α) are superimposed on one master curve. This can be achieved either by the use of the modified Coats—Redfern equation (Composite I) 

   βg(α) E AR ln − = ln 2 T E RT

(1)

or Doyle equation (Composite II)   AE E log g(α)β = log − 2.315 − 0.4567 R RT

(2)

where g(α) is the kinetic model function, A frequency factor, E activation energy, and R gas constant. Thus, the variation of ln [βg(α)/T 2 ] or log [g(α)β] with 1/T calculated for different values of α at their respective values of β should present a single master straight line for the correct form of g(α), and hence a single activation energy (E ) and frequency factor (A) could be readily calculated. The Coats—Redfern method [9] calculates the values of activation energy assuming the best fit of the 372

Fig. 2. Thermal decomposition of anhydrous SrC2 O4 to SrCO3 (a) and SrCO3 to SrO (b) at the heating rate of 1 ◦C min−1 (1), 2 ◦C min−1 (2), 3 ◦C min−1 (3), 5 ◦C min−1 (4), and 10 ◦C min−1 (5).

linearized kinetic model obtained from the Composite method     2RT E AR g (α) 1− + (3) = − ln − ln 2 T βE E RT In the Ozawa method [10], a master curve was derived from the TG data obtained at different heating rates using the following expression − log β = 0.4567(E/RT ) + constant

(4)

The frequency factor is calculated from the equation    E E log A = log g(α) P (5) βR RT A computer program has been written to perform the data analysis according to the Composite method [13]. The kinetic mechanism discrimination was based on the best fit of experimental data assuming the highest correlation coefficient and the lowest standard deviation. Vyazovkin and Wight [14] pointed out that the use of the model-free approach is a trustworthy way of obtaining a reliable result from both non-isothermal and isothermal data. Although, the Composite method involves a model-fitting kinetic approach, it does not assume a particular reaction model, but it allows us to choose the model function that gives the best representation of all (α, T, β) values obtained during the experiments with different heating rates. Deviation from a straight line relationship was interpreted in terms of multistep reaction mechanism [15, 16]. Fig. 2 shows representative sample mass changes as a function of temperature obtained from the measurements of the thermal decomposition of anhydrous strontium oxalate and strontium carbonate. The activation energy and the frequency factor for these two decomposition steps were calculated according to the Composite method using the Doyle equation and assuming different kinetic models [6, 11]. The results reported in Table 2 show that under the experimental conditions applied the decomposition of anhydrous

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Chem. Pap. 61 (5) 370—375 (2007)

THERMAL DECOMPOSITION KINETICS OF STRONTIUM OXALATE

Table 2. Activation Parameters of the Non-Isothermal Decomposition of Anhydrous SrC2 O4 and SrCO3 in Air Calculated According to the Composite Method II Assuming Different Kinetic Models Decomposition of anhydrous SrC2 O4 Model

D1 D2 D3 D4 R2 R3 F1 F2 F3 A2 A3 A4 E1

E/(kJ mol−1 ) 263 297 299 286 156 161 172 91 160 98 73 60 89

± ± ± ± ± ± ± ± ± ± ± ± ±

4 4 5 4 5 5 5 9 12 7 8 8 15

log A

r

± ± ± ± ± ± ± ± ± ± ± ± ±

0.986 0.985 0.989 0.989 0.952 0.956 0.962 0.699 0.789 0.814 0.682 0.591 0.521

17.4 18.4 19.3 18.3 9.8 10.0 11.3 6.1 11.4 6.1 4.4 3.5 8.1

0.3 0.3 0.4 0.3 0.4 0.4 0.4 0.7 1.0 0.5 0.6 0.6 1.1

strontium oxalate to strontium carbonate is best described by the diffusion-controlled reaction models (D2 , D3 , and D4 ) when the reaction rate is governed by the diffusion of gaseous product through a continuous product layer. The diffusion and subsequent oxidation of the volatilized CO produced by the thermal decomposition of SrC2 O4 to CO2 occurs at the surface of the sample grains (model D2 ). The other models, i.e. the phase boundary-controlled reaction models (R2 and R3 ) and the first-order kinetics model (F1 ) were less satisfactory in describing the observed result. On the other hand, the data reported in Table 2 show that the decomposition of carbonate to strontium oxide is best described by the phase boundarycontrolled reaction models (R2 and R3 ) assuming that the reaction is controlled by the movement of an interface at a constant velocity and the nucleation occurs virtually instantaneously so that the surface of each particle is covered with a layer of product. This would probably indicate that the rate-controlling step in the decomposition reaction is the bond rupture at the reaction interface and the rate is determined by the geometry and the available interfacial area [17]. The first-order kinetics model (F1 ) was less potent in fitting the experimental data. For both decomposition steps, it is evident that the exponential law model (E1 ) gave the least satisfactory fit of the experimental data. Figs. 3 and 4 show typical composite plots of the non-isothermal data for both decomposition steps. Analysis of the non-isothermal data was also carried out using the Coats—Redfern and Ozawa methods, assuming the reaction model best fitting the experiment using the Composite method. Table 3 summarizes the results of the activation parameters obtained using different computational methods. The values reported in the case of the Coats—Redfern and Ozawa methods represent an average calculated for

Chem. Pap. 61 (5) 370—375 (2007)

Decomposition of SrCO3 E/(kJ mol−1 ) 382 399 421 406 232 231 249 126 199 151 119 103 77

± ± ± ± ± ± ± ± ± ± ± ± ±

9 9 11 10 1 2 3 9 13 5 6 7 18

log A

r

± ± ± ± ± ± ± ± ± ± ± ± ±

0.968 0.965 0.959 0.963 0.997 0.996 0.993 0.785 0.806 0.939 0.847 0.771 0.338

15.7 16.3 16.7 16.0 8.9 9.0 10.1 5.2 8.8 5.8 4.4 3.7 4.8

0.4 0.5 0.5 0.5 0.1 0.1 0.1 0.4 0.6 0.2 0.3 0.4 0.9

Fig. 3. Composite analysis of TG data for the non-isothermal decomposition of anhydrous SrC2 O4 to SrCO3 in air employing D2 (a) and E1 (b) models.

Fig. 4. Composite analysis of TG data for the non-isothermal decomposition of SrCO3 to SrO in air employing R3 (a) and E1 (b) models.

different heating rates and fractional reaction values, respectively. Generally, the results computed according to the different methods are similar (within experimental errors) except those obtained for the decomposition of anhydrous strontium oxalate using the Ozawa method, which showed slightly lower values than the other methods. Fig. 5 presents the variation of the activation energies estimated by the Ozawa method for both decomposition steps with α. Approximately constant value

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F. A. AL-NEWAISER, S. A. AL-THABAITI, A. O. AL-YOUBI, A. Y. OBAID, M. A. GABAL

Table 3. Activation Parameters of the Non-Isothermal Decomposition of Anhydrous SrC2 O4 and SrCO3 Calculated Using Different Methods Decomposition of anhydrous SrC2 O4 * Method of analysis

Decomposition of SrCO3 **

E/(kJ mol−1 )

log A

E/(kJ mol−1 )

log A

297 ± 4 306 ± 11 277 ± 3

18.4 ± 0.3 19.8 ± 0.8 19.5 ± 0.3

231 ± 2 234 ± 10 229 ± 3

9.0 ± 0.1 8.8 ± 0.5 8.5 ± 0.3

Composite II Coats—Redfern Ozawa

*Assuming the two-dimensional diffusion-controlled reaction model D2 . **Assuming the three-dimensional phase boundary-controlled reaction model R3 .

300

E/(kJ mol-1)

280 260 240 220 0

0.2

0.4

0.6

0.8

1

α

Fig. 5. Activation energy of the thermal decomposition reaction of anhydrous SrC2 O4 (diamonds) and SrCO3 (squares) calculated by the Ozawa method at different values of α.

of E for the whole investigated range of α suggests that the rate-limiting step of both decomposition reactions is a single reaction step [12]. In agreement with this result, a single master straight line for all (α, T, β) values was obtained assuming the Composite method at different heating rate experiments as shown in Fig. 3. Dollimore et al. [4] studied the kinetics of the thermal decomposition of anhydrous strontium oxalate using TG experiments operated isothermally and in rising temperature mode. In typical isothermal experiments, for the values of α lower than 0.20 the decomposition kinetics was described by the first-order decay law and the calculated values of E and A were (317 ± 3) kJ mol−1 and 1016.55 s−1 , respectively. For the values of α ranging between 0.2 and 0.60 the values of activation energy and frequency factor calculated on the basis of the Avrami—Erofeev equation were E = (305 ± 3) kJ mol−1 and A = 1015.75 s−1 . The value of E calculated employing the data from the dynamic experiments was different except that obtained for lower values of α. The values of activation energy quoted by Freeberg et al. [18] and Boldyrev et al. [19] were 272.09—284.65 kJ mol−1 and 167.44— 175.81 kJ mol−1 , respectively. The value calculated in the present study according to the Ozawa method agrees well with that published by Freeberg et al. [18]. A physical approach to the interpretation of the mechanism and kinetics of the thermal decomposition of solids has been applied by Lvov [20] to investigate the decomposition mechanism of SrCO3 . Ex374

perimental value of the activation energy of 290 kJ mol−1 was correlated with the theoretical one of 261 kJ mol−1 , calculated on the basis of thermodynamic functions. The suggested reason explaining the deviation of the experimental value from the theoretical one was the Avrami—Erofeev model with the variable n factor (An ) derived for sigmoid α—time curve used in the processing of experimental data. This model takes into account nucleation and growth processes, which are typical for the decomposition of majority of carbonates, however, not giving true information about the steady state rate of decomposition at the deceleratory stage. The value of activation energy for the decomposition of SrCO3 obtained under present experimental conditions (232 ± 2) kJ mol−1 is lower than the theoretical one (261 kJ mol−1 ), which can be attributed to the use of the solid-state reaction mechanism (R3 ) for the data interpretation. REFERENCES 1. Boldyrev, V. V., Bulens, M., and Delmon, B., The Control of the Reactivity of Solids. Elsevier, Amsterdam, 1979. 2. Marta, L., Zaharescu, M., Ciontea, L., and Petrisor, T., Appl. Supercond. 1, 677 (1993). 3. Knaepen, E., Mullens, J., Yperman, J., and Van Poucke, L. C., Thermochim. Acta 284, 213 (1996). 4. Dollimore, D., Heal, G. R., and Passalis, N. P., Thermochim. Acta 92, 543 (1985). 5. Abd El-Khalik, M., Hanafi, S., and Selim, S. A., Surf. Technol. 25, 349 (1985). 6. Diefallah, El-H. M., Thermochim. Acta 202, 1 (1992). 7. Nagase, K., Sato, K., and Tanaka, N., Bull. Chem. Soc. Jpn. 48, 439 (1975). 8. Basahel, S. N. and Diefallah, El-H. M., Can. J. Chem. 70, 888 (1992). 9. Urbanovici, E., Popescu, C., and Segal, E., J. Therm. Anal. Calorim. 58, 683 (1999). 10. Flynn, J. H., Thermochim. Acta 283, 35 (1996). 11. Gabal, M. A., Thermochim. Acta 412, 55 (2004). 12. Gabal, M. A., Thermochim. Acta 402, 199 (2003). 13. Diefallah, El-H. M., Obaid, A. Y., Qusti, A. H., ElBellihi, A. A., Wahab, M. A., and Moustafa, M. M., Thermochim. Acta 274, 165 (1996). 14. Vyazovkin, S. and Wight, C. A., Thermochim. Acta 341, 53 (1999). 15. Brown, M. E., Maciejewski, M., Vyazovkin, S., Nomen,

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Chem. Pap. 61 (5) 370—375 (2007)

THERMAL DECOMPOSITION KINETICS OF STRONTIUM OXALATE

R., Sempere, J., Burnham, A., Opfermann, J., Strey, R., Anderson, H. L., Kemmler, A., Keuleers, R., Janssens, J., Desseyn, H. O., Li, C. R., Tang, T. B., Roduit, B., Malek, J., and Mitsuhashi, T., Thermochim. Acta 355, 125 (2000). 16. Gabal, M. A., El-Bellihi, A. A., and El-Bahnasawy, H. H., Mater. Chem. Phys. 81, 174 (2003). 17. Galwey, A. K., J. Therm. Anal. Calorim. 41, 267 (1994).

Chem. Pap. 61 (5) 370—375 (2007)

18. Freeberg, F. E., Hartman, K. O., Hisatsune, I. C., and Schempf, J. M., J. Phys. Chem. 71, 397 (1967). 19. Boldyrev, V. V., Eroshkim, V. I., Pismenko, U. T., Pyzhak, J. A., Medvinsy, A. A., Schmidt, I. V., and Kefeli, L. M., Kinet. Katal. 9, 260 (1968). 20. Lvov, B. V., Thermochim. Acta 386, 1 (2002).

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