Rapid Coprecipitation and Non-Isothermal Decomposition Kinetics of

Ind. Eng. Chem. Res. 2008, 47, 2941-2947

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Rapid Coprecipitation and Non-Isothermal Decomposition Kinetics of New Binary Mn0.5Cu0.5(H2PO4)2‚1.5H2O Banjong Boonchom† and Chanaiporn Danvirutai*,‡ King Mongkut’s Institute of Technology Ladkrabang Chumphon 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

New binary metal dihydrogen phosphate dihydrate Mn0.5Cu0.5(H2PO4)2‚1.5H2O was synthesized by a rapid and simple coprecipitation method using phosphoric acid, manganese metal, and copper oxide at ambient temperature. The thermal stability of Mn0.5Cu0.5(H2PO4)2‚1.5H2O was studied by means of the non-isothermal kinetics (Kissinger method). The synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O shows complex thermal transformations, and its final decomposition product was a binary metal cyclotetraphosphate, MnCuP4O12. The X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis-near-IR, and Fourier transform IR (FTIR) results of the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O and the decomposed MnCuP4O12 appear to be very similar to those of M(H2PO4)2‚2H2O and M2P4O12 (M ) Mn and Cu), which indicate the monoclinic phase with space group P21/n and C2/c, respectively. The dominant features of the synthesized Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O and the decomposition product MnCuP4O12 are compared with those of M(H2PO4)2‚2H2O and M2P4O12 (M ) Cu and Mn), respectively. 1. Introduction Divalent metal phosphates have been reported to be important inorganic compounds and are gaining more interest.1-14 In particular, manganese, iron, cobalt, nickel, and zinc dihydrogen phosphates are components of corrosion-proof compositions.6 Calcium, manganese, and iron dihydrogen phosphates are valuable phosphorus and micronutrient fertilizers due to their solubility in soil.7 Some binary metal dihydrogen phosphates were reported, by thermal analysis (TA) under quasi-isothermal and quasi-isobaric conditions, to follow the mechanism of dehydration.14-19 When calcined, dihydrogen phosphates yield cyclotetraphosphates, which are used as pigments, catalysts, and luminophore-supporting matrixes.20-24 The single-metal dihydrogen phosphates (M(H2PO4)2‚2H2O) and their final decomposition products (M2P4O12) are isostructural with the corresponding binary metal dihydrogen phosphates (M1-xAx(H2PO4)2‚ 2H2O) and their final decomposition products (MAP4O12; when M and A ) Ca, Mg, Mn, Fe, Co, Ni, Cu, or Zn), respectively. They have similar X-ray diffraction patterns and close unit cell parameters, which crystallize in monoclinic space group P21/n (Z ) 2) for the dihydrogen phosphate group and C2/c (Z ) 4) for the cyclotetraphosphate group. So far, single or binary metal dihydrogen phosphates were synthesized from corresponding metal(II) carbonates (or metal oxides) and phosphoric acid at low temperature (40-80 °C) with long time periods.14-27 However, the binary manganese copper dihydrogen phosphate dihydrate has not been reported in the literature, whereas Cu2-xMnP4O12 has been synthesized by mixing copper and manganese cyclotetraphosphates, then melting them together on platinum dishes in an electric furnace at 1000 °C with long time consumption.13 The goal of this work was to synthesize Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O by a rapid coprecipitation method at ambient temperature, which is a simple and cost-effective route. Thermal * 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. ‡ Khon Kaen University.

stability of Mn0.5Cu0.5(H2PO4)2‚1.5H2O was studied by means of the non-isothermal kinetics (Kissinger method). The synthesized powders of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and its decomposition product MnCuP4O12 with particle sizes of ∼50-500 nm were characterized by thermogravimetry-differential thermal gravimetry-differential thermal analysis (TG-DTGDTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform IR (FTIR), and UV-vis-near-IR techniques for the first time. 2. Experimental Section 2.1. Sample Preparation. The binary Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O compound was prepared by a solution coprecipitation method using Mn (c) (99.99%, Merck), CuO (99.99%, Merck), and 86.4 % w/w H3PO4, (Merck) as starting materials. In a typical procedure, 0.5895 g of Mn (c) and 0.7954 g of CuO (a mole ratio corresponding to the nominal composition of Mn/ Cu ratio of 1.0:1.0) were dissolved in 70% H3PO4 (86.4 % w/w H3PO4 dissolved in DI water) with continuous mechanical stirring at ambient temperature. The resulting solution was stirred until H2(g) was completely evolved (5-15 min) and the precipitate was obtained. Then 10 mL of acetone was added to the obtained nearly dry sample to allow highly crystalline product to be developed. The gray solid of Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O product was filtered by suction pump, washed with acetone until free from phosphate ion, and dried in air. Its final decomposition product seemed to occur at temperatures above 350 °C (Figure 1). Then, the dried pale blue solid was calcined in a box-furnace at 400 °C for 2 h in air. The final product was obtained as pale blue solid. 2.2. Sample Characterization. The thermal properties of the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O were investigated on a TG-DTG-DTA Pyris Diamond Perkin-Elmer instrument. The manganese and copper contents of the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O and the decomposed MnCuP4O12 were determined by atomic absorption spectrophotometry (AAS, Perkin-Elmer, Analyst100) after dissolution in 0.0126 M hydrochloric acid. The phosphorus content was determined by colorimetric analysis of the molybdophosphate complex. The

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Figure 1. TG-DTG-DTA curves of Mn0.5Cu0.5(H2PO4)2‚2H2O at a heating rate of 10 °C min-1 in static air.

structure and crystalline size of the prepared product and its decomposition product were studied by X-ray powder diffraction using a D8 Advanced powder diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu KR radiation (λ ) 0.1546 nm). The Scherrer method was used to evaluate the crystalline size (i.e., D ) Kλ/(β cos θ), 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)).28,29 The morphology of the selected resulting samples was examined by SEM using a LEO SEM VP1450 after gold coating. The roomtemperature FTIR spectra were recorded in the range of 4000370 cm-1 with eight scans on a Perkin-Elmer Spectrum GX FTIR/FT-Raman spectrometer with the resolution of 4 cm-1 using KBr pellets (KBr, spectroscopy grade, Merck). The diffuse reflectance spectra (solid-state UV-vis-near-IR) were obtained by a Shimadzu UV-3100 spectrophotometer using an integrating sphere and BaSO4 as the reference blank. 2.3. Activation Energy by the Kissinger Method. Thermal analysis measurements of about 8.0 ( 0.3 mg of sample mass were carried out by a Pyris Diamond Perkin-Elmer apparatus with an alumina crucible at heating rates of 5, 10, 15, and 20 °C min-1 over the 30-400 °C temperature range and at an O2 flow rate of 100 mL min-1. The activation energies for the thermal phase transformation steps of Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O were calculated from four endothermic peaks on the DTA curves using the Kissinger equation,30,31

ln

()

Ea β )+ const 2 RTp T

(1)

Here, β is the DTA scan rate (°C min-1), Ea is the activation energy for the phase transformation (kJ mol-1), Tp is the endothermic temperature (K) peak in the DTA curve, and R is the gas constant (8.314 kJ mol-1 K-1). The activation energy is obtained from the slope of the plot of ln(β/T2) versus 1/Tp. The shape factor (n) of the endothermic peak is represented by the following equation,32

n)

2 2.5 Tp ∆T Ea/R

(2)

where n is the Avrami constant and ∆T is the fwhm of the endothermic peak. Following a theoretical treatment developed by Vlase and co-workers,33,34 the relation between the isokinetic temperature (Ti) and the wave number of the activated bond is given as follows:

kb ω ) Ti ) 0.695Ti hc

(3)

Figure 2. XRD patterns of (a) Mn0.5Cu0.5(H2PO4)2‚2H2O and (b) MnCuP4O12.

where kb and h are, respectively, the Boltzmann and Planck constants, and c is the light velocity. Because the breaking bond has an anharmonic behavior, the specific activation is also possible due to more than one quanta, or by a higher harmonic: ωsp ) qωcalc, q ∈ N, where ωsp is the assigned spectroscopic wave number for the bond supposed to break, which relates to the evolved gas in the thermal decomposition step. 3. Results and Discussion 3.1. Chemical Analysis. The chemical analysis of the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O gives 9.58 wt % Mntotal, 11.2 wt % Cutotal, and 20.86 wt % Ptotal, suggesting the molar ratio Mntotal/Cutotal/Ptotal ) 1.00:1.01:3.86. The water content was analyzed by TG data and was about of 20.02 wt % (3.22 mol) H2O. These indicate that the precipitate could be Mn0.5Cu0.5(H2PO4)2‚1.5H2O, whereas the chemical analysis of the decomposed MnCuP4O12 gives 12.33 wt % Mntotal, 14.45 wt % Cutotal, and 26.78 wt % Ptotal, suggesting the molar ratio Mntotal/Cutotal/Ptotal ) 1.00:1.01:3.85. These indicate that the general formula would be MnCuP4O12. 3.2. Thermal Analysis. The TG-DTG-DTA curves of Mn0.5Cu0.5(H2PO4)2‚1.5H2O are shown in Figure 1. The TG curve of Mn0.5Cu0.5(H2PO4)2‚1.5H2O shows two stages of the weight loss in the range of 50-400 °C. These two stages appear in the respective DTG and DTA as two peaks (115 and 235 °C). Two weight loss steps in the TG curve were observed over the ranges of 97-150 and 150-300 °C. The corresponding observed weight losses were 9.19% and 10.83% by mass, which correspond to 1.48 and 1.74 mol of water, respectively. The thermal decomposition of Mn0.5Cu0.5(H2PO4)2‚1.5H2O is a complex process, which involves the dehydration of the coordinated water molecules (1.5 mol of H2O) and an intramolecular dehydration of the protonated phosphate groups (2 mol of H2O); these processes could be formally presented as

Mn0.5Cu0.5(H2PO4)2‚1.5H2O f Mn0.5Cu0.5(H2PO4)2 + ∼1.5H2O (I) Mn0.5Cu0.5(H2PO4)2 f 0.5MnCuP4O12 + ∼2H2O (II) An intermediate compound has been registered such as acid polyphosphate Mn0.5Cu0.5(H2PO4)2. The binary manganese copper cyclotetraphosphate, MnCuP4O12, is found to be the final

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 2943 Table 1. Average Particle Sizes and Lattice Parameters of CuO‚2(H3PO4)‚2H2O, Mn(H2PO4)2‚2H2O, Mn0.5Cu0.5(H2PO4)2‚2H2O, and Cu2P4O12, Mn2P4O12, MnCuP4O12 Calculated from XRD Data compound

method

a/Å

b/Å

c/Å

β/deg

CuO‚2(H3PO4)‚2H2O Mn0.5Cu0.5(H2PO4)2‚1.5H2O Mn(H2PO4)2‚2H2O

this work PDF no.350010

7.32(3) 7.31

10.15(1) 10.08

5.39(1) 5.37

94.75(1) 94.75

Cu2P4O12 MnCuP4O12 Mn2P4O12

PDF no. 731280 this work PDF no. 380314

12.56 12.03(4) 11.88

8.088 8.42(9) 8.59

9.574 9.96(6) 10.14

118.58 118.13(7) 119.21

av particle sizes/nm 82 ( 17

55 ( 14

product of the thermal decomposition at T > 350 °C. The total mass loss is 20.02% (3.22 mol of H2O), which is close to theoretical value (22.5% (3.5 mol of H2O)). However, this result is also in agreement with other reported isostructural binary dihydrogen phosphate dihydrates in the literature that reported the moles of water in the range of 1-4.6,26,27 The thermal stability, mechanism, and phase transition temperature of Mn0.5Cu0.5(H2PO4)2‚1.5H2O show the difference from that of the individual Mn(H2PO4)2‚2H2O26 and CuO‚2(H3PO4)‚2H2O.20 On the basis of these results, we can conclude that the different thermal behavior is caused by the incorporation between Mn and Cu metals in the skeleton. This result supports the formation of new binary Mn0.5Cu0.5(H2PO4)2‚1.5H2O, which is confirmed by XRD (Figure 2). 3.3. X-ray Powder Diffraction. The XRD patterns of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are similar to those of M(H2PO4)2‚1.5H2O and M2P4O12 (M ) Mn and Cu),35 respectively, but intensities are slightly different (Figure 2). These results confirm that new binary Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are isostructural to the type series of M(H2PO4)2‚ 2H2O and M2P4O12 (M ) Mg, Mn, Co, Ni, Fe, Zn), respectively. All reflections can be distinctly indexed based on a pure monoclinic phase with space group P21/n (Z ) 2) for Mn0.5Cu0.5(H2PO4)2‚1.5H2O and C2/c (Z ) 4) for MnCuP4O12, which note to be similar to those of the standard XRD patterns of Mn(H2PO4)2‚2H2O (PDF no. 350010) and M2P4O12 (PDF no. 380314 for Mn and PDF no. 731280 for Cu), respectively. The average crystallite sizes and lattice parameters of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 samples were calculated from XRD patterns and are tabulated in Table 1. The lattice parameters of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are comparable to those of the standard data as M(H2PO4)2‚2H2O

(PDF no. 350010 for Mn and CuO‚2(H3PO4)‚2H2O has no PDF data available) and M2P4O12 (PDF no. 380314 for Mn and PDF no. 731280 for Cu), respectively. The crystallite sizes of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are found in the range between that of individual metals of M(H2PO4)2‚2H2O and M2P4O12. In the systems of binary manganese copper solid solutions and individual metal dihydrogen phosphate (or metal cyclotetraphosphate), the electric charges of cations are equivalent, and the radii of cations are close to each other, so the spectrum peaks are quite similar.10-19 On the basis of the above analysis, we can draw a conclusion that the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O and its decomposition product MnCuP4O12 are solid solutions and not a mixture of the individual phases of the Mn and Cu compounds. 3.4. FTIR Spectroscopy. The FTIR spectra of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are shown in Figure 3, which are very similar to those of Mn(H2PO4)2‚2H2O and MnP4O12. Vibrational bands are identified in relation to the crystal structure

Figure 3. FTIR spectra of Mn0.5Cu0.5(H2PO4)2‚2H2O (a) and MnCuP4O12 (b).

Figure 4. SEM micrographs of Mn0.5Cu0.5(H2PO4)2‚2H2O (a) and MnCuP4O12(b).

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Figure 5. Diffuse reflectance spectra of (a) CuO‚(H3PO4)‚2H2O, Mn(H2PO4)2‚2H2O, Mn0.5Cu0.5(H2PO4)2‚2H2O and (b) MnCuP4O12, Mn2P4O12, Cu2P4O12.

in terms of the fundamental vibrating units, namely, H2PO4and H2O for Mn0.5Cu0.5(H2PO4)2‚1.5H2O and [P4O12]4- ion for MnCuP4O12, which are assigned according to the literature.35-39 Vibrational bands of H2PO4- ion are observed in the regions of 300-500, 700-900, 1160-900, 840-930, 1000-1200, and 2400-3300 cm-1. These bands are assigned to the δ(O2PO2), γ(POH), δ(POH), ν(PO2(H2)), ν(PO2), and ν(OH), respectively. The observed bands in the 1600-1700 and 3000-3500 cm-1 regions are attributed to the water bending/C band and stretching vibrations/A band, respectively. Vibrational bands of [P4O12]4ion are observed in the ranges of 1350-1220, 1150-1100, 1080-950, and 780-400 cm-1. These bands can be assigned to νasOPO-, νsOPO-, νasPOP, and νsPOP vibrations, respectively.38,39 The observation of a strong νsPOP band is known to be the most striking feature of cyclotetraphosphate spectra, along with the presence of the νasOPO- band.40

3.5. Scanning Electron Microscopy. The scanning electron micrographs of Mn0.5Cu0.5(H2PO4)2‚1.5H2O and MnCuP4O12 are shown in Figure 4. The SEM micrograph of Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O illustrates many small and some large rodlike particles, which were about 0.50-0.80 µm in length and 0.20-0.40 µm in width and about 1.00-4.00 µm in length and 0.80-1.20 µm in width, respectively. The SEM micrograph of MnCuP4O12 appears to have more aggregate of particles with homogeneous and heterogeneous compositions, which are caused primarily by the process of dissolution, a rapid precipitation, and decomposition process, respectively. 3.6. UV-Vis-Near-IR Spectroscopy. The UV-vis-nearIR absorption spectra of the Mn(H2PO4)2‚2H2O, CuO‚2(H3PO4)‚ 2H2O, and Mn0.5Cu0.5(H2PO4)2‚1.5H2O powders are shown in Figure 5a. The UV-vis spectrum of the binary compound shows a dominant feature of Cu2+ in octahedral sites; Mn2+ is

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Figure 6. DTA curves of Mn0.5Cu0.5(H2PO4)2‚2H2O at four heating rates (5, 10, 15, and 20 °C min-1).

Figure 7. Decomposition kinetics study of Mn0.5Cu0.5(H2PO4)2‚2H2O by the Kissinger model.

too weak to be observed as it involves spin-forbidden transitions. The spectrum of Mn0.5Cu0.5(H2PO4)2‚1.5H2O powder shows two sets of the bands, one in the short-wavelength (UV-vis) region and another in the near-IR region, which correspond to gray white color. The deconvolution of these bands shows five peaks situated at 284, 457, 670, 910, and 1349 nm (Figure 5a). The band around 284 nm is too strong to be d-d transitions, lying in the UV region; it is assigned to a charge-transfer band. The band at 457 nm is characteristic of Cu(II) in an octahedral site and is attributed to the Cu2+ f Mn2+ electronic charge transfer.41 The bands observed in the visible and near-IR regions at 670, 910, and 1349 nm correspond to the crystal field transitions between the full occupied states 2Eg(dxz;yz), 2B2g(dxy), and 2A1g(dz2) and the partially occupied level 2B1g(dx2-y2) of Cu2+ ion (3d9) in an octahedral site (Figure 5a). The UV-vis-nearIR absorption spectra of M2P4O12 (M ) Mn and Cu) and

MnCuP4O12 compounds (Figure 5b) show broad absorptions from 600 and 1200 nm, which can be attributed to a ligandmetal charge transfer from a framework electron to M(II). The spectral feature of the octahedral site in MnCuP4O12 is very similar to that observed in Cu2P4O12. The electronic spectra indicate that octahedrally coordinated Cu(II) cations are present in the blue and pale blue color of Cu2P4O12 and MnCuP4O12, respectively. M(II) in octahedral sites are present in these structures.41 The color changes observed in each sample are usually associated with changes in the electronic environment (e.g., ligand field effects) or with changes of the incorporated metal.42 The electronic spectra indicate that Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O and MnCuP4O12 are the solid solutions, not the mixture of the two individual solids. 3.7. Calculation of Activation Energy. According to the Kissinger method, the basic data of T are collected from the

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DTA curves of the decomposition of Mn0.5Cu0.5(H2PO4)2‚ 1.5H2O at various heating rates (5, 10, 15, and 20 °C min-1) (Figure 6). The non-isothermal DTA method is desirable to analyze the reaction mechanism and calculate the activation energy of the solid state.30-32 Several non-isothermal techniques have been proposed which are quicker and less sensitive to the previous and next transformations. From the slopes of the curves (Figure 7), the activation energy values in two decomposition steps of the synthesized Mn0.5Cu0.5(H2PO4)2‚1.5H2O were determined as 99.75 ( 1.84 (R2 ) 0.9967) and 202.84 ( 20.12 (R2 ) 0.9903) kJ mol-1, respectively. These activation energies are consistent with the former hypothesis that the intermediate nucleates and crystallizes as a metastable phase with adequate growth kinetics before the stable phase MnCuP4O12. The value of the Avrami exponent provides information regarding the morphology of the growing crystal.32 The value of n reflects the mechanism dominating crystallization. Here, smaller n values indicate that the crystallization is dominated by a surface crystallization and/or that the crystallization dimension is low. On the other hand, larger n values are expected only in case of increasing nucleation rates. For Mn0.5Cu0.5(H2PO4)2‚1.5H2O, the n values are 1.34 for the first and 1.93 for the second decomposition steps, which are random nucleation and growth of nuclei for both decomposition steps. In order to corroborate the calculated data with the spectroscopic ones, we drew up the FTIR spectra of the studied compound (Figure 5). The average Tp (DTA) at four heating rates (5, 10, 20, and 30 °C/min) for the first and second decomposition steps are 272.11 and 353.45 K, respectively. The calculated harmonic energy (ωcalc) values by using eq 3 were found to be 1632 (n ) 6), 3265 (12), and 3537 (13) cm-1 in the first step and 706 (2), 1060 (3), 1767 (5), 2474 (7), 2827 (8), and 3181 (9) cm-1 in the second step. These results confirm that the loss of the water of crystallization in the first step is followed by a continuous intermolecular polycondensation and elimination of water for the second step.33-38 The studied compound exhibits a very good agreement between the calculated wavenumbers from average Tp (DTA) and the observed wavenumbers from FTIR spectra for the bonds suggested as being broken, which confirms the two thermal decomposition steps correspond to the loss of the water of crystallization and the water of constituent. 4. Conclusions New binary Mn0.5Cu0.5(H2PO4)2‚1.5H2O was successfully synthesized by a rapid and simple precipitation at ambient temperature. Non-isothermal kinetic analysis applying the Kissinger (KAS) method results exhibit the activation energies (Ea) for the random nucleation and growth of nuclei mechanism, which correspond to the loss of water of crystallization in the first step, subsequent to a continuous intermolecular polycondensation and elimination of water of constituent in anion (the second step). Thermal analysis, XRD, FTIR, UV-visnear-IR, and SEM results suggest the formation of new binary Mn0.5Cu0.5(H2PO4)2‚1.5H2O and its decomposition product (MnCuP4O12). Acknowledgment The authors thank the Chemistry and Physics Departments, Khon Kaen University, for facilities. This work is financially supported by the Center for Innovation in Chemistry: Postgraduate Education and Research Program in Chemistry (PERCHCIC) and King Mongkut’s Institute of Technology Ladkrabang (KMITL), Ministry of Education, Thailand.

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ReceiVed for reView October 4, 2007 ReVised manuscript receiVed February 14, 2008 Accepted February 18, 2008 IE071342H