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Nanodisperse oxide compounds of iron formed in the FeSO4 - KOH - H2O - H2O2 system (4.0 < pH < 13.0)
D. A. Zherebtsov2, V. Sh. Mirasov2, D.G. Kleschev2, E.V. Polyakov1
1Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia 2South Ural State University (National research university), Chelyabinsk, Russia
zherebtsov_da@yahoo.com, mirasov@gmail.com, dgk1950dgk@mail.ru, rupolyakov@ihim.uran.ru
PACS 68.55.A DOI 10.17586/2220-8054-2015-6-4-593-604
The regularities of phase formation during oxidation of aqueous solutions of FeSO4 and (or) suspensions of Fe(OH)2 at quasi-constant temperature and pH values have been studied for wide intervals of temperature (20 -85 °C) and (4.0 - 13.0) of the reaction medium. The produced nanodisperse materials have been examined by X-ray phase analysis, IR spectroscopy, scanning electron microscopy and X-ray fluorescence analysis, as well as by thermogravimetric analysis combined with thermal analysis and mass spectrometric analysis of released gases. The dependences of the phase, chemical and disperse compositions of the formed precipitates on the synthesis parameters have been revealed.
Keywords: FeSO4 solutions; FeOH2 suspensions; hydrogen peroxide oxidation; oxide compounds of iron(III).
Received: 2 May 2015 Revised: 13 May 2015
1. Introduction
The oxidation of iron(II) salt solutions and (or) iron(II) hydroxide suspensions leads to the formation of poorly soluble nanodisperse iron(III) compounds exhibiting peculiar and in some cases unique ion-exchange, sorption, magnetic, catalytic and other properties [1-5], attracting much interest of specialists in many different fields. It is established [2, 3, 5-12] that the phase and disperse compositions of these compounds depend on numerous synthetic parameters: the temperature and pH of the reaction medium, concentration of iron(II) ions in solution and of Fe(OH)2 in suspension, the nature of salt anion and alkaline agent cation, the presence of uncontrolled or specially introduced impurities in the solution, as well as the feed rate and activity of oxidizing agent. Until now, the question as to which parameters have a determining influence on the phase and disperse compositions of the formed iron(III) compounds remains a point for debate, which restrains the introduction of advanced technologies for the reproducible preparation of products with optimal functional properties.
The aim of this work is to obtain more accurate information about the regularities of phase formation during the oxidation of aqueous solutions of FeSO4 and (or) suspensions of Fe(OH)2 at quasi-constant values of temperature, total iron(II) concentration in solution (suspension), of reaction medium and average oxidation rate in case the solid solutions K and H2O2 are used as alkaline and oxidizing agents, respectively (hereafter the system FeSO4 -H2O - KOH - H2O2).
2. Experimental
The following reagents were used for synthesis: 1.1 M solution of FeSO4 in distilled water, which was preliminarily reduced by iron powder; 3.8 M KOH and 0.9 M H2O2 solutions in distilled water. All the reagents were chemically pure. Oxidation was carried out in a 0.5 dm3 cylindrical reactor furnished with a mixer, electric heating, thermostating, continuous KOH and H2O2 solutions dosing and measuring systems.
In all the experiments, the average oxidation rate of iron(II) ions, 15 ± 1 mmol/(dm3-min), and the total concentration of Fe(II) and Fe(III) in oxidized suspension, 0.51 ± 0.02 mol/dm3, were maintained constant, while the temperature t and pH were varied. The experiments were performed in the following sequence: the calculated amount of FeSO4 solution was poured into the reactor and heated to preset temperature t (20, 40, 60 or 85 °C); the required pH value of the reaction medium1 (4.0; 5.5; 6.5; 8.5; 10.5 or 13.0) was achieved by feeding the KOH solution. After that, the H2O2 solution was added at a constant rate into the reactor, and in the interval 4.0 < < 9.0 the KOH solution was also added to maintain a quasi-constant pH value (± 0.25). Note that over time, the temperature of the reaction medium gradually increased by At ~ 5 °C2 in all the experiments. At the conclusion of oxidation, the precipitate was separated from the mother solution on a Buchner funnel and washed with water until the filtrate contained no sulfate ions; then, it was dried until constant mass at ~ 50 °C and ground.
The obtained samples were examined by X-ray phase analysis (XPA; DRON-3 X-ray unit with digital data recording attachment; filtered CKa radiation; the powder diffraction electronic database ICSD was used). The average size of the precipitate crystals, d, was calculated by the Debye formula from the physical broadening of reflections3. The samples obtained at fixed values of t = 40 °C (4.0 < pH < 13.0) and pH = 6.5 (20 °C < t < 85 °C) were also studied on a scanning electron microscope JEOL JSM-7001F with an energy-dispersion spectrometer Oxford INCA X-max 80 to determine their elemental (X-ray fluorescence spectral analysis -XSA; random error below 5 %) and disperse compositions (scanning electron microscopy -SEM), as well as by thermogravimetric and thermal analysis methods (synchronous analyzer Netzsch STA 449C Jupiter combined with a mass spectrometer Netzsch QMS 403C Aeolos; the thermograms were recorded over 20 - 900 °C at a heating rate of 10 K/min in air flow of 20 ml/min) and IR spectroscopy (Fourier IR spectrometer Bruker Tensor-27; transmission survey in the 400 - 4000 cm-1 range in a KBr matrix).
3. Results and discussion
When H2O2 is fed into the reactor, a yellow precipitate is formed (experiments at pH = 4.0) or the Fe(OH)2 suspension changes its color (pH > 5.5). We note a typical dark-green color of the intermediate samples for suspensions produced at pH 5.5 - 8.5, which changes to yellow, orange or brown at the end of oxidation, depending on the temperature and pH. According to [2, 8], in this pH interval corresponding to partial precipitation of iron(II) ions from the solution, an intermediate compound is formed during oxidation, which is known in the literature as "green rust II". It is this compound which determines the suspension's color.
The XPA data show that the following nanodisperse phases can be formed in the examined system depending on temperature and pH: iron(III) oxyhydroxides of a, y and 5 modifications and iron(III) oxide Y-Fe2O3. The phase and disperse compositions of the precipitates,
1The time required to attain a preset pH value was less than 3 min.
2Hereafter the value of initial temperature t is indicated.
3Non-overlapping reflections of phases with crystallographic indices (hkl): a -FeOOH - (110); 7 -FeOOH -(120); S -FeOOH - (101); 7-Fe2O3 - (220) were chosen as analytical reflections.
Table 1. Phase composition and grain dimension of precipitates as a function of temperature and pH in the system FeSO4 - H2O - KOH - H2O2
pH Tem perature t, °C
20 40 60 85
4.0 a(100) 9 a(100) 12 a(100) 19 a(100) 40
5.5 a(40) + Y (60) 8 7 a(80) + Y (20) 12 9 a(95) + Y(5) 18 12 a(5) + Y*(95) - 23
6.5 a(50) + Y (50) 8 7 a(100) 10 a(40) + Y*(60) 12 20 Y*(100) 27
8.5 a(80) + Y (20) 9 10 Y*(100) 20 Y*(100) 23 Y*(100) 30
10.5 a(10) + Y *(90) 10 20 Y*(100) 23 Y*(100) 25 Y*(100) 40
13.0 5(100) 8 a(40) + 5(60) 10 10 a(40) +5(60) 15 16 a(45) +y *(45) + 5(10) 15 50 -
1. a - phase a-FeOOH; y - phaseY-FeOOH; ô - phase ô-FeOOH; Y* - phase y-Fe2O3.
2. In the parentheses are depicted mass % of the phase, the numbers under the parentheses mean crystal size, calculated from the X-ray diffraction data, nm.
as functions of oxidation parameters, are presented in Table 1 and Fig. 1, in which the regions of formation of these phases are demonstrated in temperature - coordinates. From Fig. 1, it follows that a-FeOOH is formed over the whole considered pH interval; Y-FeOOH - at 5.5 < pH < 9.0; 5-FeOOH - at pH > 10.5; and y-Fe2Os - at 5.5 - 13.0. When the temperature is raised, the pH range in which these phases are formed expand for Y-Fe2O3 and a-FeOOH and narrow for y- and 5-FeOOH. With increased temperature, the reflection half-width A26 of the corresponding phases decreases, which is indicative of an increase in the average size d of their crystals (Fig. 2).
It follows from the SEM results that each of the synthesized phases has a characteristic crystal morphology, the average size of which depends on the oxidation parameters. In particular, the Y-Fe2O3 and 5-FeOOH phases have an isometric (nearly-spherical) crystal shape. The a- and y-FeOOH crystals are characterized respectively by needle- and plate-like growth forms. Note that the average crystal size determined from the SEM data agrees reasonably well with X-ray analysis data.
The XSA data show that all the synthesized samples consist mainly of iron and oxygen4, however, they also contain much smaller amounts of potassium and sulfur atoms. The relative contents of Fe/O and S/O in the most typical samples are given in Table 1.
The data for the phase and chemical compositions of the samples are confirmed by the results of IR spectroscopic and thermogravimetric studies. In particular, the IR spectrum of sample 1 (a-FeOOH), which is monophase according to XPA data, contains (Fig. 3, curve a): narrow absorption bands of Fe-O bonds with wave numbers v 407, 462 and 631 cm-1; vibrations of groups (797 and 893 1787 cm-1); deformation vibrations of H2O (1645 cm-1), as well
4The employed XSA method allows determining the elements with atomic number z > 5. Therefore it should be assumed that along with the above mentioned elements, the examined samples contain also hydrogen.
Fig. 1. The phase composition of the precipitate obtained in the system FeSO4 -H2O - KOH - H2O2 as a function of temperature and pH. Designations of phases contained in the precipitate: 1 - a-FeOOH; 2 - 7-FeOOH; 3 - Y-Fe2O3; 4 - 5-FeOOH. Phase formation regions: I - a-FeOOH; II - 7-FeOOH; III - Y-Fe2O3; IV - 5-FeOOH.
Fig. 2. The average crystal size d (nm) of phases formed in the system FeSO4 -KOH - H2O - H2O2 at a fixed pH value as a function of temperature (t, °C): a-FeOOH (pH - 4.0); 7-FeOOH (pH - 5.5); 5-FeOOH (pH - 13.0); Y-Fe2O3 (pH - 10.5).
as a wide absorption band with a maximum at 3158 stretching vibrations of H-O bonds [13]. Additionally, the spectrum of this sample contains weaker absorption bands with v 1053, 1137 and 1160 cm-1, which can be attributed to the stretching vibrations of (SO4)2- ions [14].
The IR spectra of the binary mixture samples a- + 7-FeOOH and a- + 5-FeOOH (samples 2 and 4, respectively), along with the absorption bands typical of a-FeOOH, also contain additional absorption bands (Fig. 3, curves b and c) with wave numbers 1022 cm-1 (sample 2) and 1050, 1530 cm-1 (sample 4). These bands should be attributed to vibrations of groups in 7- and 5-FeOOH, respectively [15].
Fig. 3. The IR spectra of samples: - a-FeOOH; b - a- +7-FeOOH; c - a-+#-FeOOH; d - 7-Fe2O3.
The IR spectra of monophase samples of Y-Fe2O3 (Fig. 3, curve d) contain absorption bands of Fe-O bonds with wave numbers 447, 581, 626 cm-1, typical of Fe2O3 [16], deformation vibrations of H2O (1625 cm-1), a broad absorption band with a maximum at 3441 stretching vibrations of H-O bonds and weak absorption bands of stretching vibrations of (SO4)2- ions.
The thermograms for FeOOH samples are similar in many ways, therefore it is convenient to consider them on the example of thermal transformations of monophase a-FeOOH (sample 1). According to the TG data (Fig. 4), the relative mass variation in the interval 20 -900 °C, Am2o-9oo/m0, for this sample is 16.49 %. On the differential thermal analysis (DTA)
curves, there are three endothermic effects with maxima at 80, 270 and 750 °C which are accompanied by reduction in mass. Mass-spectrometry studies of the gaseous phase composition revealed (Fig. 4) that the first and the second endothermic effects are due to dehydration and the third effect is caused by the decomposition of sulfate ions. According to the XPA data, at temperatures above 300 °C the a-FeOOH sample undergoes a transformation into a-Fe2O3. From comparison of the obtained findings with the literature data [3, 17], it can be concluded that physically adsorbed water and water of hydration (OH groups) are removed from the sample during the first and second endothermic effects, respectively. Then, the empirical structural formula FeOOH can be represented as FeOOH(H2O)n(SO3)y and the thermal transformation of a-FeOOH during heatingt occurs as follows:
a-FeOOH(H2O)n(SO3)y 20~18° a-FeOOH(SO3)y 180-320°C)
180-320°C „ „ s >620° „ „
-> a-Fe2O3(SO3)y-> a-Fe2O3,
where n and y is the content of adsorbed water and (SO4) terms of FeO.
2
(1)
ions in the sample expressed in
Fig. 4. The data of thermal (TG curve - heavy line, DTA curve - thin line) and mass spectrometric (H2O, SO2 curves - dash lines) analysis of a-FeOOH sample
From the TG data on mass variation for FeOOH samples (No. 1-4) at each stage of thermal transformation (Table 2), we calculated the values of n and y and, with allowance for the structural formula, the ratios Fe/O and S/O in the initial samples (Table 2). From Table 2, it follows that the data on the chemical composition of FeOOH samples obtained by two independent methods (XSA and TG) coincide within experiment error.
Table 2. Phase and chemical composition of the samples
No. Conditions of synthesis Phase composition Chemical composition
pH t, ◦C Method PCA Method TG
Fe/O, r.u. S/O, r.u. Ami/moa, r.u. Am2/fflob, r.u. Am3/m0c, r.u. n, r.u. m, r.u. y. r.u. Fe/O d, r.u. S/O, r.u.
1 6.5 40 a-FeOOHe 0.47 0.008 0.0474 0.1011 0.0164 0.25 - 0.020 0.49 0.009
2 5.5 40 a- + y-FeOOH 0.47 0.010 0.0444 0.1012 0.0204 0.26 - 0.024 0.48 0.010
3 6.5 20 a- + y-FeOOH 0.46 0.014 0.0738 0.1011 0.0297 0.24 - 0.038 0.47 0.015
4 13.0 40 a- + £-FeOOH 0.52 0.001 0.0407 0.1011 - 0.21 - < 0.001 0.50 -
5 6.5 85 Y-Fe2O3f 0.57 0.004 0.021 0.0168 0.0042 0.195 0.156 0.009 0.63 0.003
6 10.5 40 Y -Fe2O3 0.50 0.002 0.0194 0.0155 0.0021 0.179 0.143 0.0044 0.63 0.0015
r
s
o g
S'
TO
S
TO g
TO
o
o s s
S
o
s
a)
b)
c)
d)
e)
f)
Am1/m0 - relative mass loss of the sample at the temperature(20 - 180 °C, 1st end of thermic effect); Am2/m0 - relative mass loss of the sample at the temperature (200 - 500 °C, 2nd end of thermic effect); Am3/m0 - relative mass loss of the sample at the temperature 500 - 900°C; we didn't tack into consideration amount of oxygen, participated in the samples in the form of adsorbed water due to total desorption of this form of water molecules in the high vacuum chamber of the PCA analytical installation used.
empirical formula of the a-, y-, i-FeOOH contained samples is FeOOHnH2OSO3. empirical formula of the Fe2O3 contained samples is Fe2O3-(n + m)H2OySO.
VO VO
The DTA curves of monophase samples of Y-Fe2O3 (Fig. 5) have two endothermic effects with maxima at 80 and 260 °C which are due to removal of physically adsorbed water and water of hydration and are accompanied by mass loss. Also present is a pronounced exothermic effect at 560 °C, occurring without any noticeable variation in the sample mass (the mass reduction from 500 - 900 °C is ~ 0.2 %). The XPA data show that the exothermic effect is brought about by the phase transition of Y-Fe2O3 into a-Fe2O3. Assuming that the variation of the mass for the Fe2O3 samples at temperatures above 500 °C, as in the case of FeOOH, is caused by the process of desulfation, the structural formula of Fe2O3 can be represented as Fe2O3(H2O)n(SO3)y and its thermal transformation during heating occurs in the following manner:
Y-Fe2O3(H2O)(n + m)(SO3 )y
180—500°C
20-180°C
> Y-Fe2O3(H2O)m(SO3)y
180—500°C
T^ A /OA N 500-900°C „ > Y-Fe2O3(SO3)y-> a-Fe2O3,
(2)
where n, m and y are the contents of adsorbed water and water of hydration and (SO4)2 ions in the sample expressed in terms of Fe2O3.
-8
-10
2,0
1,9
1,8
1,7
1,6
0 100
200
300 400 500 600 700 800
900
1,5
1,4
1,3
1,2
1,1
t°C
Fig. 5. The data of thermal (TG curve - heavy line, DTA curve - thin line) and mass spectrometric (H2O curve - dash line) analysis of Y-Fe2O3 sample
As in the case of thermal transformations of FeOOH, values for m, n and y were calculated from the TG data, and the ratios Fe/O and S/O in the initial samples were determined taking into account the structural formula (Table 2). From Table 2, it follows that the XSA method gives a slightly underestimated Fe/O ratio in the Y-Fe2O3 samples as compared to TG analysis.
The established dependence of the phase and disperse compositions of the precipitates formed in the FeSO4 - H2O - KOH - H2O2 system on the oxidation parameters agrees on the whole with the results obtained from the examination of analogous systems [2, 3, 6-12] and provides evidence that crystal growth during the oxidation of aqueous solutions of iron(II) salts and (or) suspensions of iron(II) hydroxide, especially at low temperatures, takes place under conditions of high supersaturation. As a result, the average size of crystals decreases, their imperfections increase, and phases with disordered crystal structure are formed, in particular, i-FeOOH. At the same time, it should be noted that our data for Y-Fe2O3 phase formation do not agree with previous results [7, 18] for the formation of the Fe3O4 phase during the oxidation of Fe(OH)2 suspensions by atmospheric air under analogous conditions (pH « 9). This contradiction is eliminated if we assume that in our experiments the application of a more active oxidizer (H2O2) originally leads to the formation of the Fe3O4 phase in a nanodisperse state, which is subsequently oxidized by hydrogen peroxide to Y-Fe2O3 by a topotactical solidstate reaction mechanism. The possibility of Fe3O4 ^ Y-Fe2O3 transformation occurring in oxygen-containing aqueous media has been reported previously [3, 19].
The synthetic conditions for oxyhydroxide phases in the examined system are characterized by high supersaturaion and lead to the formation of nanodisperse primary particles, which, owing to developed specific surface area, are apt to form compact aggregates, whose average size (D) increases as d decreases [3]. In this connection, it is interesting to consider the empirical dependence between the size of primary crystals of new phases and the temperature of synthesis in the framework of the familiar nucleation model [20]. From this dependence, it follows that the relation between the average size of primary crystals d (nm) of phases formed in the FeSO4 - KOH - H2O - H2O2 system at a fixed value and temperature (Fig. 2 and Table 1) obeys the Kelvin equation:
RT ln(a/a0) = 4YV/d, (3)
in which the average size of primary crystals (d) correlates with their relative solubility; a, a0 is the solubility of crystals of size d under synthetic conditions and of crystals with infinitely large size under equilibrium conditions; y, V is the interfacial tension determining the solubility and molar volume of crystal; R is the universal gaseous constant; and T is the temperature of formation for crystals of a given size [20]. Then, we transform equation (3) to a form convenient for the analysis of experimental dependences (4) by the least square method:
1/d = b[0] + b[1]T, (4)
where b[0] is the empirical constant and b[1] = Rln(a/a0)/4YV. It is seen that all the empirical data on the sizes of primary crystals and the synthesis temperature for the FeSO4 - KOH - H2O -H2O2 system obey equation (4). In Fig. 6, these experimental dependences are represented in the coordinates of equation (4) and are characterized by the correlation coefficient in the range (0.94 - 0.99) and by the Fisher F-criterion parameter for the nucleation model (50 - 120). A peculiar feature of Fig. 6 is a negative value for the parameter b[1] in eq. (4). This may imply that ln(a/a0) < 0 or the relative solubility of primary crystals during synthesis is (a/a0) < 1. This dependence contradicts the model of critical nucleus formation both during the condensation of liquid drops from vapor and during primary particle crystallization from aqueous solutions of electrolytes [20] and can be explained by the fact that the majority of primary particles inside the compact aggregates are isolated from the mother solution and do not affect the value of a0, while the contribution to the solubility of new phase is made mainly by the small crystals located on the surface of aggregates. Indeed, from Fig. 7, it is seen that the growth of primary crystal sizes and consequently the reduction of the average aggregate size D
results in the disappearance of anomalous relative solubility when the particle size approaches 25 - 35 nm, whereas further increase in the primary particle sizes with the growth of d may lead to a traditional character of temperature dependence for therelative solubility, (a/a0) > 1.
Fig. 6. The average crystal size d (nm) of phases formed in the system FeSO4 -KOH - H2O - H2O2 at a fixed pH value as a function of temperature represented in the coordinates of equation (4). Inset: the synthesis pH values are designated by figures.
In conclusion, let us consider the nature of the water of hdration in y-Fe2O3. According to [9, 19, 21], the nanodisperse samples of Y-Fe2O3, as distinct from coarse crystalline ones, have a spinel-type defect structure and regular defects in the cationic sublattice. The structural formula of nanodisperse Y-Fe2O3 is Fe8^2.67Fe13.33O32, where □ is a vacancy in the octahedral cationic positions. In the topochemical oxidation reaction, water molecules of the solution can occupy the vacant positions, forming hydrogen bonds with oxygen anions and thereby stabilize the y-Fe2O3 lattice. This explains the higher temperatures for removing the water of hydration on the thermograms of Y-Fe2O3 (above 200 °C) as compared to adsorbed water.
4. Conclusion
For wide temperature (20 - 85 °C) and (4.0 - 13.0) intervals of the reaction medium, we have established the regularities for the formation of nanodisperse iron(III) oxide compounds during the oxidation of aqueous FeSO4 solutions and (or) Fe(OH)2 suspensions by hydrogen peroxide under quasi-stationary conditions with the use of KOH as an alkaline agent. The dependences of the phase, chemical and disperse compositions of the resulting products upon the synthetic conditions have been determined.
Fig. 7. The variation of the inclination angle b[ 1] in the empirical equation (4) as a function of the minimal size of primary crystals registered in each series of experiments at constant pH in the system FeSO4 - KOH - H2O - H2O2. The line in the Figure exhibits a tendency towards b[1] variation with the growth of the size of primary particles.
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