Ind. Eng. Chem. Res. 2007, 46, 9071-9076
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Thermal Decomposition Kinetics of FePO4‚3H2O Precursor To Synthetize Spherical Nanoparticles FePO4 Banjong Boonchom†,‡ and Chanaiporn Danvirutai*,‡ King Mongkut’s Institute of Technology Ladkrabang Chumpon Campus, 17/1 M. 6 Pha Thiew District, Chumphon 86160, Thailand, and Department of Chemistry, Faculty of Science, Khon Kaen UniVersity, Khon Kaen 40002, Thailand
The thermal decomposition of iron phosphate trihydrate FePO4‚3H2O was investigated in air using TG-DTG/ DTA. The FePO4‚3H2O decomposes in two steps, and the final decomposition product (FePO4) was studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The activation energies of the second dehydration reaction of FePO4‚3H2O were calculated through the isoconversional methods of Ozawa and Kissinger-Akahira-Sunose (KAS), and the possible conversion functions have been estimated through the Coats-Redfern method. The specificity of thermal decomposition was characterized by identification of the bonds to be selectively activated due to energy absorption at the vibrational level, which was assigned by comparing the calculated wavenumbers with the observed wavenumbers in FTIR spectra. The kinetic model that better describes the second reaction of dehydration for FePO4‚3H2O is the Fn model as a simple n-order reaction, and the corresponding function is ×c4(R) ) (1-R)2.50 and g(R) ) -[1-(1-R)-1.50/(1.50)]. 1. Introduction Iron(III) phosphate is recently gaining more and more interest as a material to be used in the field of catalysts,1,2 wastewater purification systems,3 ferroelectrics,4 and lithium batteries.5,6 The existence of several crystalline iron phosphate phases was reported in the literature: the orthorhombic heterosite FePO4, obtained from the delithiated LiFePO4,7,8 the monoclinic FePO4, and the orthorhombic FePO49 hydrated phases include the phosphosiderite (or metastrengite) FePO4‚2H2O monoclinic and the FePO4‚2H2O orthorhombic form.10,11 The preparation methods need to obtain the well-defined chemical microstructure, which depends mainly on the conditions of synthesis. Different synthetic routes have so far been reported for synthesizing FePO4, LiFePO4, and some of the synthesis conditions reported in the literature.12,13 Nevertheless, the reported preparations were of generally high temperature (550-800 °C) and long time consuming (0.5 and 24 h) with the ability to induce the sintering and aggregation of particles, which are deleterious for their electrochemical and catalytic performances.7,14 Thermal treatment of iron phosphate hydrates has a great synthetic potential, which relates to the hydrate in the conventional crystal form. The presence of the water molecules influences the intermolecular interactions (affecting the internal energy and enthalpy) as well as the crystalline disorder (entropy) and, hence, influences the free energy, thermodynamic activity, solubility, stability, electrochemical, and catalytic activity. To control the state of hydration of the active ingredient, it is, therefore, important and necessary to understand the kinetics and mechanisms of hydration and dehydration processes under the appropriate conditions.15-17 In this respect, amorphous FePO4 has attracted the interest of many researchers due to the realization that FePO4 was used for the preparation of the delithiated LiFePO4, which is a candidate material for recharge* To whom correspondence should be addressed. Tel.: +66-43202222 to 9 ext 12243. Fax: +66-43-202373. E-mail:
[email protected]. † King Mongkut’s Institute of Technology Ladkrabang Chumpon Campus. ‡ Khon Kaen University.
able lithium ion batteries.5,6,18-20 In addition, iron is environmentally friendly and cheap. Thus, in the past few years many works have undertaken a series of research studies on the synthesis, characterization, and electrochemical performance of different FePO4.12,13,21 The key factor in obtaining FePO4 with different properties has been the use of different preparative methods. In the literature, FePO4 has been prepared by thermal decomposition of many phases of FePO4‚2H2O precursors.1,6,7,9 In the present study, the formation of FePO4 from FePO4‚ 3H2O was followed using thermogravimetry-differential thermal analysis (TG-DTG/DTA), X-ray powder diffraction (XRD), scanning electron microscopy, and Fourier Transform-infrared (FT-IR) spectroscopy. In the literature, there is no report on the thermal decomposition kinetics of FePO4‚3H2O. Hence, the nonisothermal kinetics analysis for the second decomposition step of the iron phosphate was carried out using the isoconversional methods of Ozawa22 and Kissinger-Akahira-Sunose (KAS).23 The possible conversion functions had been estimated using the Coats-Redfern method which gives the best description of the studied dehydration process and allows the calculation of reliable values of the kinetic parameters (E and A). A correlation between the temperature peak in DTA and the assigned wavenumbers are discussed for first time. 2. Experimental Section FePO4‚3H2O crystalline powder (analytical grade) was commercially obtained from Fluka Chemical Industries Co. Ltd. and was used without further purification. Thermal analysis measurements (thermogravimetry, TG; differential thermogravimetry, DTG; and differential thermal analysis, DTA) were carried out on a Pyris Diamond PerkinElmer apparatus by increasing the temperature from 30 to 250 °C with calcined R-Al2O3 powder as the standard reference. The experiments were performed in static air, at heating rates of 5, 10, 20, and 30 oC min-1. The sample mass was kept at about 6.0-10.0 mg in an aluminum crucible without pressing. The structure and crystalline size of FePO4‚3H2O and its final decomposition products were studied by X-ray powder diffrac-
10.1021/ie071107z CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2007
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Figure 1. TG/DTG/DTA curves of FePO4‚3H2O in air at a heating rate of 10 °C min-1.
Figure 2. FT-IR spectra of FePO4‚3H2O (a) and its dehydration product (FePO4) (b).
tion using an X-ray diffractometer (Phillips PW3040, The Netherland) with Cu KR radiation (λ ) 0.1546 nm). The Scherrer method was used to evaluate the crystalline size (i.e., D ) Kλ/βcosθ, where λ is the wavelength of X-ray radiation, K is a constant taken as 0.89, θ is the diffraction angle, and β is the full width at half-maximum (fwhm)).24-26 The morphology of the selected resulting samples was examined by a scanning electron microscope (SEM) using LEO SEM VP1450 after gold coating. The room-temperature FTIR spectra were recorded in the range of 4000-370 cm-1 with 8 scans on a Perkin-Elmer Spectrum GX FT-IR/FT-Raman spectrometer with the resolution of 4 cm-1 using KBr pellets (KBr, Merck, spectroscopy grade).
temperature needed for the calcinations process. Thus, the FePO4‚3H2O sample was calcined at 200 °C for 2 h in the furnace, which is the lower temperature compatible with the other hydrate precursors.1,6,7,9 The specificity of the thermal decomposition was characterized by identification of the bonds to be selectively activated due to energy absorption at the vibrational level.27 These bonds were assigned by comparing the calculated wavenumbers with the observed wavenumbers in the FT-IR spectra. This breaking of the bond is assimilated with a Morse oscillators28 coupled nonlinear27 with the harmonic oscillators of the thermic field. Following, a theoretical treatment developed by Vlase et al.,29 the relation between the average maximum temperature peak Tp (DTA) at four heating rates (Figure 2) and the wavenumber of the activated bond is given as follows
3. Results and Discussion 3.1. Thermal Analysis. TG/DTG/DTA curves of the thermal decomposition of FePO4‚3H2O at a heating rate of 10 °C min-1 are shown in Figure 1. The TG curve shows that the two thermal decomposition steps occur at temperatures below 200 °C. The eliminations of water are observed in two areas: 50-75 and 75-200 °C. The corresponding observed weight losses are 7.70 and 16.00% by mass, which correspond to 0.88 and 1.82 mol of water, respectively. Two endothermic effects on DTA curves are observed at 54 and 105 °C, which correspond to DTG peaks at 54 and 103 °C, respectively. The first and second decomposition steps are the loss of one and two molecules of water in crystallization, respectively. Total mass loss is 23.70% (2.70 H2O), which is close to the theoretical value for FePO4‚3H2O (26.35%, 3.00 H2O). The mass retained of about 76.40% is compatible with the value expected for the formation of FePO4, which is verified by XRD and FTIR measurement. The overall reaction is
FePO4‚3H2O f FePO4‚2H2O + H2O
(1)
FePO4‚2H2O f FePO4 + 2H2O
(2)
The water in crystalline hydrate may be considered either as crystal water or as coordinated water. The strength of the binding of these molecules in the crystal lattice is different, hence, resulting in different dehydration temperatures. The water eliminated at 150 °C and below can be considered as crystal water, whereas water eliminated at 200 °C and above indicates its coordination by the metal atom. Water molecules eliminated at intermediate temperatures can be coordinately linked water as well as crystal water.38,39 The dehydration temperatures obtained in this work suggest that the water in hydrated iron phosphate can be considered as crystal water. The temperature at which theoretical mass loss is achieved can be also determined from the TG curve and considered to be the minimum
ω)
kb T ) 0.695Tp hc p
(3)
where kb and h are respectively the Boltzmann and Planck constants, and c is the light velocity. Because the breaking of the bond has an anharmonic behavior, the specific activation is possible also due to more than one quanta or by a higher harmonic, ωsp ) qωcalc, q ∈ N, where ωsp is the assigned spectroscopic number for the bond supposed to break. In order to corroborate the calculated data with the spectroscopic ones, we drew up the FT-IR spectra of the studied compound. According to the above-mentioned equation, the average Tp (DTA) at four heating rates (5, 10, 20, and 30 °C/min) for the first and second decomposition steps is 337.15 and 394.60 K, respectively. The calculated harmonic energy (ωcalc) values of the first decomposition step were 1640, 3280, and 3514 cm-1, which correspond to 7, 14, and 15 quanta numbers, respectively. Similarly, the ωcalc values of the second decomposition step were 1645, 3290, and 3565 cm-1, which correspond to 6, 12, and 13 quanta numbers, respectively. These wavenumbers are close to the water vibration of the water of crystallization reported in the literature.27,28 The studied compound exhibited a very good agreement between the calculated wavenumbers from average Tp (DTA) and the observed wavenumbers from FT-IR spectra for the bonds suggested being broken, which confirm two thermal decomposition steps correspond to the loss of water of crystallization. 3.2. Vibrational Spectroscopy. FT-IR spectra of FePO4‚ 3H2O and its dehydration product (FePO4) are shown in Figure 2. Vibrational bands are identified in relation to the crystal structure in terms of the fundamental vibrating units namely PO43-, H2O, for FePO4‚3H2O and FePO4.20,31 FTIR spectra of PO43- in FePO4‚3H2O and FePO4 show the antisymmetric
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Figure 3. The XRD patterns of FePO4‚3H2O (a) and its dehydration product (FePO4) (b).
stretching mode (ν3) in the 1000-1200 cm-1 region and the ν4 mode in the 400-560 cm-1 region. The observed bands in the 1600-1660 cm-1 and 3000-3500 cm-1 regions are attributed to the water bending and stretching vibrations. These water bands disappear in FT-IR spectra of its dehydration product (FePO4), which support a very good agreement with the thermal analysis results. The XRD data along with FTIR spectra confirm that the calcined FePO4‚3H2O at 200 °C for 2 h transforms to FePO4. 3.3. X-ray Powder Diffraction. The XRD patterns of FePO4‚ 3H2O and its dehydration product (FePO4) are shown in Figure 3. All detectable peaks are indexed as FePO4 with structure in standard data as PDF#840876. This result indicates that the crystal structure is in a hexagonal system with space group P3121 for FePO4. The average crystallite size of 33 ( 13 nm for the FePO4 sample was calculated from X-ray line broadening of the reflections of (012), (110), (104), and (114), using the Scherrer equation (i.e., D ) 0.89λ/βcosθ), where λ is the wavelength of X-ray radiation, θ is the diffraction angle, and β is the full width at half-maximum (fwhm)).24-26 The lattice parameters calculated from the XRD spectra are a ) 5.024(1) and c ) 11.241(0) Å for FePO4 and are close to the reported ones in standard data as PDF#840876 (a ) 5.027 and c ) 11.234). 3.4. Scanning Electron Microscopy. The SEM micrographs of FePO4‚3H2O and its final decomposition product FePO4 are shown in Figure 4. The particle shape and size are changed throughout the whole decomposition product. The SEM micrograph of FePO4‚3H2O (Figure 4a) illustrates many spherical nanoparticles of different sizes in the range of 80-200 nm. The SEM micrograph of FePO4 (Figure 4b) shows coalescence in aggregates of spherical nanoparticles of different sizes in the range of 30-100 nm. The morphology of FePO4 shows a smaller size than that of FePO4‚3H2O, which is the effect of the thermal dehydration process. 3.5. Kinetics Studies. 3.5.1. Calculation of the Activation Energy. Dehydration of crystal hydrates is a solid-state process of the following type:32-35 A (solid) f B (solid) + C (gas). The kinetics of such reactions is described by various equations taking into account the special features of their mechanisms. This is a model-free method, which involves measuring the temperatures corresponding to the fixed values of R (extent of conversion) from experiments at different heating rates (β). The activation energy (ER) can be calculated according to the isoconversional methods. In the kinetic study of FePO4‚3H2O, Ozawa22 and KAS23 equations were used to determine the activation energy of the dehydration reaction in only the second step (Scheme 2), because the first dehydration step is the very fast mass loss in the short temperature range (50-75 °C). The
Figure 4. SEM micrographs of FePO4‚3H2O (a) and its dehydration product (FePO4) (b).
obtained data in this step will be highly sensitive to nonisothermal kinetics analysis errors.38,39 The equations used for ER calculation are
Ozawa equation log β ) log
( ) AER
Rg(R)
- 2.315 - 0.4567
( ) ER RT
(4)
KAS equation: ln
() ( ) ( ) AER ER β ) ln 2 RT Rg(R) T
(5)
where A (the pre-exponential factor) and E (the activation energy) are the Arrhenius parameters, and R is the gas constant (8.314 J mol-1‚K-1). The Arrhenius parameters, together with the reaction model, are sometimes called the kinetic triplet. g(R) ) ∫R0 dR/f(R) is the integral form of the ×c4(R), which is the reaction model that depends on the reaction mechanism. According to the isoconversional method, the basic data of R and T collected from the TG curves of the second dehydration of FePO4‚3H2O at various heating rates (5, 10, 20, and 30 °C min-1) are illustrated in Table 1. According to the abovementioned equations, the plots of log β versus 1000/T (Ozawa) and ln β/T2 versus 1000/T (KAS) corresponding to different conversions of R can be obtained by a linear regression of the least-squares method, respectively. The Ozawa and KAS
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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 Table 3. Kinetic Functions ×c4(r) and g(r) Used for Present Analysis no.
×c4(R)
g(R)
kinetic function name
A2 A3 D1 F0 F1/2 F1 F2/3 F2 Fn
(1-R)[-ln(1-R)]1/2 (1-R)[-ln(1-R)]2/3 1/R (1-R)0 (1-R)1/2 (1-R) (1-R)2/3 (1-R)2 (1-R)n
2[-ln(1-R)]1/2 3[-ln(1-R)]1/3 R2/2 R 2[1-(1-R)1/2 ] -ln(1-R) 3[1-ln(1-R)]1/3 R/(1-R) [1-(1-R)1-n]/ (1-n)
Avrami-Erofeyev (n ) 2) Avrami-Erofeyev (n ) 3) one-dimensional (R ) kt1/2) power law (R )kt4) reaction order (n ) 1/2) Avrami-Erofeyev (n ) 1) two-thirds order kinetics second-order kinetics nth-order kinetics (n * 1)
Table 4. Kinetics Parameters Obtained from the Differential Method and the Integral Method at Different Heating Rates (β ) 5, 10, 20, and 30 oC min-1) Coats-Redfern method model A2 A3 D1 F0 F1/2 F1 F2/3 F2 Fn (n ) 2.50) R3 Figure 5. Ozawa analysis (a) and KAS analysis (b) for the second dehydration reaction of FePO4‚3H2O at four different heating rates in TG measurements below 200 °C. Table 1. r-Τ Data at Different Heating Rates, β (oC min-1), for the Second Dehydration Reaction of FePO4‚3H2O temperature/K R
β)5
β ) 10
β ) 20
β ) 30
0.2 0.3 0.4 0.5 0.6 0.7 0.8
365.99 372.21 377.80 383.79 391.09 400.23 413.29
370.61 378.30 385.11 391.47 399.07 407.90 420.01
375.23 383.79 391.45 398.29 406.41 414.83 426.67
378.02 388.01 395.15 403.84 411.48 419.35 430.64 (r2)
Table 2. Activation Energies (Er) vs Correlation Coefficient Calculated by the Ozawa and KAS Methods for the Dehydration of FePO4‚3H2O Ozawa method
KAS method
R
ER/kJ mol-1
r2
ER/kJ mol-1
r2
0.2 0.3 0.4 0.5 0.6 0.7 0.8 av
163.14 131.25 122.11 111.30 112.85 125.18 145.45 130.18 ( 18
0.99999 0.99924 0.99882 0.99888 0.99966 0.99958 0.99994 0.99944
165.37 131.71 121.98 110.50 112.00 124.83 145.94 130.33 ( 19
0.99999 0.99916 0.9987 0.99874 0.99962 0.99954 0.99994 0.99938
analysis results of four TG measurements below 200 °C are presented in Figure 5 (parts a and b, respectively). The activation energies ER can be calculated from the slopes of the straight lines with a better linear correlation coefficient (r2). The slopes change depending on the degree of conversion (R) for the dehydration reaction of FePO4‚3H2O. The activation energies are calculated at heating rates of 5, 10, 20, and 30 °C min-1 via the Ozawa and KAS methods in the R range of 0.2-0.8. The activation energies calculated by the Ozawa and KAS methods are close to each other, which are shown in Table 2, so the results are credible. The activation energy values
ER/kJ
mol-1)
19.14 ( 0.26 10.56 ( 0.21 65.12 ( 0.22 29.27 ( 0.14 36.42 ( 0.25 44.87 ( 0.42 54.64 ( 0.62 65.93 ( 1.23 77.95 ( 1.18 39.09 ( 0.30
ln A /s-1
r2
-5.5101 ( 0.24 -8.64 ( 0.12 8.36 ( 0.45 -2.58 ( 0.24 -5.46 × 10-3 ( 0.32 2.96 ( 0.43 6.34 ( 0.56 10.13 ( 0.82 14.26 ( 0.87 0.93 ( 0.36
0.98192 0.97344 0.96570 0.95762 0.97534 0.98706 0.99398 0.98634 0.99850 0.97985
calculated by the KAS method are close to those obtained by the Ozawa method The activation energies change little with R, so we draw a conclusion that the dehydration reaction of FePO4‚3H2O could be a single kinetic mechanism, which corresponds to an endothermic peak at 105 °C in the DTA curve. 3.5.2. Estimation of the Conversion Function and the PreExponential Factor. For a one-step reaction, the estimation of kinetic parameters can be turned into a multiple linear regression problem through the Coats-Redfern equation.36
Coats-Redfern equation: ln
( ) ( ) ( ) g(R) T2
) ln
AER
Rg(R)
-
ER RT
(6)
Hence, ln(g(R)/T2) calculated for the different R values at the single β value on 1/T must give rise to a single master straight line, so the activation energy and the pre-exponential factor can be calculated from the slope and intercept through ordinary least-squares estimation. The activation energy, preexponential factor, and the correlation coefficient can be calculated from the equation of the Coats-Redfern combined with 9 conversion functions (Table 3).32,37 Generally speaking, the one with the highest correlation coefficient (>0.99) is the best-fit kinetic model. The optimized values from the CoatsRedfern method are the data of activation energy and ln A, those which were calculated with the best equation. Experimental results are shown in Table 4. The values of activation parameters calculated using the Coats-Redfern method for the FePO4‚3H2O decomposition reaction showed a smaller value compared with those obtained from the Ozawa and KAS methods. The CoatsRedfern analysis of data showed higher correlation and less standard deviation in the calculated experimental parameters. Hence, the results suggest that the Fn model gives a much less satisfactory fit to the data. From the above analysis, the value of r2 according to the model F2/3 is highly close to the one of F2.5, 0.99398 and 0.99850, respectively. However, the strengths of binding of water molecules in the crystal lattice are different
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and, hence, result in different dehydration temperatures and kinetic parameters. The activation energy for the losing of crystal water lies in the range of 60-80 kJ mol-1, while the value for the coordinately bounded one is within the range of 130-160 kJ mol-1.38,39 The energy of activation found in the F2.5 model for the dehydration reaction (Table 4) suggests that the water molecules are coordinately linked to water as well as to a crystal one. This result is inconsistent with the dehydration temperature obtained from the DTA-TG experiments. Therefore, we can draw a conclusion that the obtained possible conversion function is the Fn model for the dehydration of FePO4‚3H2O, and the corresponding function is ×c4(R) ) (1-R)n and g(R) ) [1-(1R)1-n/(1-n). The calculated kinetic parameters of CoatsRedfern are ER ) 77.95 ( 1.18 kJ mol-1, ln A ) 14.26 ( 0.87 s-1, and n ) 2.50. 4. Conclusions FePO4‚3H2O decomposes in two steps by starting after 50 °C, and the final product is spherical nanoparticles FePO4. The dehydration of FePO4‚3H2O is important for its further treatments. The final product is confirmed by XRD data, scanning electron microscopy, and FTIR measurements. Kinetic analysis from nonisothermal TG applying the model-fitting method results in a single value of E on the different R which can be assigned to a simple reaction. The studied compound exhibits a very good agreement between the calculated wavenumbers from average Tp (DTA), and the observed wavenumbers from the FT-IR spectra for the bonds suggested being broken, which confirms two thermal decomposition steps correspond to the loss of water of crystallization. The kinetic model that better describes the second dehydration reaction for FePO4‚3H2O is the Fn model as a simple n-order reaction (n ) 2.50). The corresponding function is ×c4(R) ) (1-R)2.50 and g(R) ) -[1(1-R)-1.50/(1.50)] and is reported for the first time. Acknowledgment The authors would like to thank the Chemistry and Physics Departments, Khon Kaen University for providing research facilities. This work is financially supported by King Mongkut’s Institute of Technology Ladkrabang (KMITL) and the Center for Innovation in Chemistry: Postgraduate Education and Research Program in Chemistry (PERCH-CIC). Literature Cited (1) Scaccia, S.; Carewska, M.; Prosini, P. P. Thermoanalytical Study of Iron(III) Phosphate obtained by Homogeneous Precipitation from Different Media. Thermochim. Acta 2004, 41, 381. (2) Ai, M.; Ohdam, K. Effects of The Method of Preparing Iron Orthophosphate Catalyst on The Structure and The Catalytic Activity. Appl. Catal., A 1999, 180, 47. (3) Perri, E.; Tsamouras, D.; Dalas, E. Ferric Phosphate Precipitation in Aqueous Media. J. Cryst. Growth 2000, 213, 93. (4) Benmokhtar, S.; Belmal, H.; El Jazouli, A.; Chaminade, J. P.; Gravereau, P.; Pe´chev, S.; Grenier, J. C.; Villeneuve, G.; DeWaal, D. Synthesis, Structure, and Physicochemical Investigations of The New R Cu0.50TiO(PO4) Oxyphosphate. J. Solid State Chem. 2007, 180, 772. (5) Masquelier, C.; Reale, P.; Wurm, C.; Morerette, M.; Dupont, L.; Larcher, D. Hydrated Iron Phosphates FePO4‚nH2O and Fe4(P2O7)3‚nH2O as 3 V Positive Electrodes in Rechargeable Lithium Batteries. J. Electrochem. Soc. 2002, 149, A1037. (6) Hong, Y. S.; Park, Y. J.; Ryu, K. S.; Chang, S. H. Crystalline Fe3PO7 as An Electrode Material for Lithium Secondary Batteries. Solid State Ionics 2003, 156, 27. (7) Chang, H. H.; Chang, C. C.; Wu, H. C.; Guo, Z. Z.; Yang, M. H.; Chiang, Y. P.; Sheu, H. S.; Wu, N. L. Kinetic Study on Low-Temperature Synthesis of LiFePO4 via Solid-State Reaction. J. Power Sources 2006, 158, 550.
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ReceiVed for reView August 14, 2007 ReVised manuscript receiVed September 19, 2007 Accepted September 30, 2007 IE071107Z
