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5PHASE TRANSITIONS OF y-FeOOH DURING HEAT TREATMENT
IN NaOH AQUEOUS SOLUTIONS
D. G. Kleschev, R. R. Klescheva, A. V. Tolchev*, R. N. Pletnev**
Department of Common and Experimental Physics, Southern Urals State University 76, Lenina av., 454080, Chelyabinsk, Russia
* Department of Engineering Science, Chelyabinsk State Pedagogical University
454080, Chelyabinsk, Russia
** Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences
620219, Ekaterinburg, Russia Phone: +7-351-267-93-07; fax: +7-351-265-59-50; e-mail: dgk@susu.ac.ru
The phase (into a-FeOOH) and chemical (into a-Fe2O3) transitions of the nonequilibrium phase
y-FeOOH subjected to hydrothermal treatment in NaOH aqueous solutions with concentration 0 < CNaOH
< 5
molel-1 at temperatures from 110 to 230 °C have been studied. The regions of formation of these phases have been refined. Possible compositions of crystal-forming complexes, appearing in solutions, and reactions occurring when they are built into different atomically smooth faces of a-FeOOH and a-Fe2O3 crystals have been considered. Experimentally established dependences of the crystal habit, phase and disperse composition of synthesized products on hydrothermal treatment parameters have been interpreted on the basis of a proposed model.
1. Introduction
Nonequilibrium iron(III) oxyhydroxides of y- and 8-modifications, formed on oxidation of aqueous solutions of iron(II) salts or iron(II) hydroxide suspensions under high oversaturation conditions [1-4], undergo phase (PT) or chemical (CT) transitions into equilibrium phases a-FeOOH or a-Fe2O3 respectively when heat treated in alkaline solutions. These phases have found extensive application as inorganic pigments, starting materials for magnetic powders, ceramics, etc. The synthesis technique proposed allows a utilizing of the alkali used for hydrogen energetics needs, among them when producing hydrogen by the water-alkali or melt-alkali electrolysis. According to [2, 5-11], the kinetics and type of transition of the nonequilibrium FeOOH, as well as the crystal size and habit of the arising phases depend on numerous parameters, the main of which are the temperature of heat treatment, the concentration of alkali and FeOOH in suspension, the structure and mean crystal size of initial iron(III) oxyhydroxides. The aim of this work is to establish possible reasons for multi-parameter dependence of the phase composition and crystal habit of iron(III) oxide compounds formed during hydrothermal treatment of y-FeOOH in NaOH solutions.
2. Experimetal
Single-phase (as evident from XRD and TEM data) sample of y-FeOOH with lamellar crystals (the mean size of ~15 nm) was obtained after the technique described elsewhere [4] by oxidation of
aqueous solution of iron(II) sulfate (chemical purity grade) by air at temperature 25 ± 2 °C and pH of the reaction medium 7.3 ± 0.5 maintained by continuous feed of aqueous solution of sodium hydroxide (chemical purity grade) into the reactor. The concentrations of FeSO4 and NaOH solutions were 1.1 and 3.9 molel-1 respectively. The oxidation took place in a reactor with a volume of 3 dm3 equipped with a mixing device. The average oxidation rate of Fe2+ ^ Fe3+ was approximately 1.5 mole l-1h-1. The precipitate formed was separated from the initial solution using a Buchner funnel and water-washed until no sulfate-ions were present in the filtrate. Then it was repulped in distilled water and aqueous solutions of NaON with concentration (CNaOH) from 0.1 to 5.0 molel-1.
Hydrothermal treatment of y-FeOOH suspensions was carried out in 0.07 dm3 capacity autoclaves (coefficient of admission k = 0.8) at temperatures (/) 110, 135, 170, 200, and 230 °C for 2.5 h. The time of autoclave heating to a pre-assigned temperature was not more than 0.5 h, the accuracy of temperature maintenance was ±5 °C.
XRD patterns were taken from samples using a wide-angle X-ray powder diffractometer (model DRON-3, filtered FeKa-radiation). The crystalline phases were identified using the ASTM X-ray powder diffraction data files (y-FeOOH: 8-098; a-FeOOH: 17-536; a-Fe2O3: 13-534). The mass fraction (W) of a-Fe2O3 phase in heat-treated samples was determined by quantitative X-ray phase analysis [12]. The mean crystal size of initial and heat-treated products was evaluated from intrin-
Статья поступила в редакцию 16.03.2007 г.
The article has entered in publishing office 16.03.2007.
sic broadening A2© of diffraction peaks by the Scherrer formula [12] and TEM or SEM data. The dispersion composition and the habit of individual crystals were controlled by TEM (model UEMV-100K) and SEM (model JEOL JSM-6460LV). Volumetric analysis was used to determine the chemical composition of the solution, initial and heat-treated samples.
3. Results and Discussion
It follows from the experimental data (Fig. 1) that during hydrothermal treatment the y-FeOOH phase undergoes a phase or chemical transition depending on CNaOH and t, these processes being capable of running concurrently. Note that at a fixed concentration of NaOH in solution the PT y—> a-FeOOH takes place at lower temperatures as compared to the CT y-FeOOH ^a-Fe2O3. The dependence of the phase formation sequence on C
NaOH at t = const in the interval 135 < t < 200 (°C) is not monotonous: at CNaOH<0.5 molel-1 and
CNaOH ^ 3 m°le-l-1
CT into a-Fe2O3
CNaOH = 3 mole-l-:L,
whereas for a-Fe2O3 at CNaOH >
> 3 molel-1 the isometric crystal habit changes for a lamellar one (Fig. 2). It was established that when the temperature or NaOH concentration in solution rises (the other parameter being constant), the mean crystal size of a-FeOOH and a-Fe2O3 monotonously increases in the direction of their preferred growth (Fig. 3).
Considerable changes in the crystal habit and size of the initial phase (y-FeOOH) and those formed during hydrothermal treatment (a-Fe2O3 and a-FeOOH) indicate that the FT and CT of the nonequilibrium FeOOH in alkaline and weak alkaline solutions proceed by the "dissolution-precipitation" mechanism (DPM) [7], which involves (as elementary stages) dissolution of crystals of the nonequilibrium phase, formation of crystal-forming complexes (CFC) in solution, transfer of CFC through solution and building them into the formed crystals of the phase that is equilibrium for given thermodynamic conditions. It is commonly recognized [13] that the limiting stage of the layer-by-layer mechanism of crystal growth is the formation of two-dimensional nuclei on an atomically smooth face. Since for difficultly soluble compounds including a-FeOOH and a-Fe2O3, the size of the two-dimensional nucleus is comparable with that of CFC in solution [7], the problem of its genesis
Fig. 1. Mass fraction (W) of the a-Fe2O3 phase in sumples obtained during heat treatment at 110—230 °C as a function of NaOH concentration in solution
the y-FeOOH phase undergoes a whereas at the intermediate concentrations it transforms into a-FeOOH.
The mean crystal size and habit of the phases formed also depend on heat treatment parameters. In particular, a-FeOOH and a-Fe2O3, appearing during hydrothermal treatment of y-FeOOH in distilled water, are characterized by a columnar and isometric crystal shape respectively. As CNaOH in solution increases at t = const, the crystal habit of these phases changes regularly. For a-FeOOH, this shows up in the increased formfactor f of columnar crystals from f = 2-4 at CNaOH = 0 to f = 6-8 at
Fig. 2. The TEM (a) and SEM (b) micrographs of a-Fe2O3 crystals, formed during y-FeOOH heat-treatment at 230 °C in NaOH solution with concentration 0.5 (a) and 5.0 (b) mole-l-1
CNaOH' mol/l
Fig. 3. The average size of a-Fe2O3 phase crystals (d) as a function of NaOH concentration in solution
reduces to incorporation of CFC into the surface layer of these crystals. Let us consider the structure of a-FeOOH and a-Fe2O3 faces, the composition and configuration of CFC in solutions of different compositions.
When the interaction between the crystal surface and dispersion medium is negligibly small, which is true of neutral solutions, distilled water among them, the atomically smooth (001) face of a-FeOOH consists of alternating rows of water molecules (layer I) and hydroxyl groups (layer II), while the (100) and (010) faces are made up of three and two adjacent rows of hydroxy l groups respectively, which are separated by one vacancy row (Fig. 4,a). For a-Fe2O3, the (100) face includes alternating rows of water molecules (layer I) and oxygen anions (layer II), whereas the (001) face consists only of hydroxyl groups forming a hexagonal net on it (Fig. 4,b). According to [14],
CNaOH' mol/l
Fig. 4. Fragments of structures of the a-FeOOH (a) and a-Fe2O3 (b) phases in polyhedral interpretation and the structure
of the crystal-forming complexes [fe2 (oh) • 4h2o] (c), [fe2 (oh)7 • 3h2o]- (d), and [fe2 (oh)8 • 2h2o]2 (e)
the crystal-forming complex existing in these solutions includes two octahedrons coupled along the bi-pyramid base edge (Fig. 4,c) and has a composition of [Fe2 (OH)6 • 4H2o]0 . When CFC is built into a-FeOOH crystals, on the (001) face the reaction of olation proceeds in points 1-4; on the (100) face the oxolation reaction takes place in points 5, 7 and the olation reaction in point 6; on the (010) face the oxolation reaction runs in points 8, 10 and the olation reaction occurs in point 9. For a-Fe2O3, the olation reaction takes place in points 1-3 and the dehydratation reaction in points 4, 5 on the (100) face; the oxolation reaction runs in points 6-8 and the olation reaction in points 9, 10 on the (001) face.
In alkaline solutions, especially at high heat treatment temperatures, sodium hydroxide may react with ligands both located on the surface of crystals and incorporated into crystal-forming complexes. Therefore, on the one hand, in low-concentration solutions of NaOH, reactions of complete or partial substitution of water molecules for hydroxyl groups are expected to proceed on the (001) face in a-FeOOH and the (100) face in a-Fe2O3, whereas in high-concentration solutions, along with substitution reactions one will also an-
ticipate oxolation reactions between hydroxyl groups of NaOH and the surface layer of (001), (010), and (100) faces in a-FeOOH or the (001) face in a-Fe2O3. As a result of these interactions, the above faces acquire a negative charge, which is compensated by Na+-ions located in the adsorption layer near the crystal. This is likely to be the reason for increased solubility of oxide compounds of iron(III) in alkaline solutions as compared to neutral or weak alkaline media, which manifests itself in higher equilibrium concentration of iron(III) hydrox-oaquacomplexes (HAC) [15].
On the other hand, according to the data [16], the interaction between alkali and electroneutral HAC [Fe(OH)3]aq in alkaline solutions gives rise
also to negatively charged HAC
concentration of which changes symbately as C"NaOH increases. Therefore it is possible to assume that
besides CFC of the composition [Fe2 (OH)6 • 4H2O3 J° ,
negatively charged complexes [Fe2 (OH) • 3H2O3 ]
(CFC', Fig. 4,d) and [Fe2(OH)8 • 2H2O3]2_ (CFC",
Fig. 4,e) can also be present in NaOH solutions, the maximum concentration of CFC' and CFC" being achieved in low- and high-concentration solutions of NaOH respectively.
In this connection, in alkaline media, as compared to neutral media, the type of reactions occurring when crystal-forming complexes are built into the surface layer of crystals changes. In particular, in low-concentration solutions of NaOH, when CFC' are incorporated into the structure of a-FeOOH, on the (001) face the olation reaction runs in points 1-3 and the oxolation reaction in point 4, whereas on the (100) and (010) faces in points 5, 7 and 8, 10, respectively, the oxolation reaction takes place, and in points 6 and 9 the dehydroxylation reaction, as a result of which OH--groups of CFC' pass into solution:
Or- + OH
CFC '
► Or- + OH,
sol-
For a-Fe2O3,
on the (100) face the oxolation reaction proceeds in points 1-3 and the dehydration reaction in points 4, 5, whereas on the (001) face in points 6-8 there occurs the oxolation reaction and in points 9, 10 the olation reaction.
In high-concentration solutions of NaOH, when CFC" are built into a-FeOOH, on the (001) face the olation reaction runs in point 1, the dehydroxylation reaction in points 2, 3, and the oxolation reaction in point 4; on the (100) and (010) faces the dehydroxylation reaction takes place in all points 5-10. For a-Fe2O3, on the (100) face the type of the reactions does not change, whereas on the (001) face the de-hydroxylation reaction proceeds in points 6-8 and the dehydratation reaction in points 9, 10.
Thus, the incorporation of crystal-forming complexes into a-FeOOH and a-Fe2O3 crystals may be accompanied by the competing reactions of dehydratation, olation, oxolation, and dehydroxyla-
a
b
d
c
e
tion depending on the concentration of NaOH in solution. In terms of thermodynamics, the most advantageous reactions are the former two reactions, during which instead of weak Van der Waals bonds (Fe(III) - H2O), strong ol-bonds or oxo-bonds between iron(III) ions of the crystal surface and the crystal-forming complex are formed leading to a decrease in the free energy F of the "crystal -CFC" system. The reaction of oxolation
OH-r + OHCFC
-Or- + H2O,
2 sol ■
accompanied by deprotonization of hydroxyl group with a strong covalent bonding is characterized by a higher activation energy in comparison with the olation and dehydratation reactions and, as a consequence, takes place at higher temperatures of thermal treatment [17]. For the reaction of dehy-droxylation AF a 0, as its activation requires an energy for disruption of Fe(III) - OH- bonds in the CFC, which is comparable with the energy of ol-bonds and oxo-bonds in crystal. Therefore this reaction cannot compete with those discussed above.
Based on the number and type of reactions occurring when CFC are built into the structure of crystals, let us qualitatively evaluate the probability of nucleation on different crystal faces and on this basis analyze the experimental data. In particular, the decrease in the number of bonds formed when CFC are incorporated into the (100) and (010) faces (three bonds), as compared to the (001) face (four bonds), is considered in [14] to be responsible for preferred growth of a-FeOOH crystals in the crystallographic direction c, and, consequently, for their columnar habit. An increase in hydrothermal treatment temperature at C"NaOH = = const results in activation of all reactions including oxolation reactions on the (100) and (010) faces. This manifests itself both in the increased mean length and decreased formfactor of a-FeOOH crystals. In contrast, as the concentration of NaOH in solution grows, the probability of nucleation on the (100) and (010) faces lowers due to dehy-droxylation reactions, which is the reason for an increase in the formfactor of a-FeOOH crystals.
When complexes are built into the (100) and (001) faces of a-Fe2O3, a similar number (five) of bonds is formed. This results in the isometric shape of a-Fe2O3 crystals arising during CT of y-FeOOH in neutral and low-concentration alkaline solutions. In strongly alkaline media, the probability of nu-cleation on the (001) face decreases because of the dehydroxylation reactions, and lamellar a-Fe2O3 crystals are formed, in which the plane of plates, as evidenced by X-ray studies [8, 9], is perpendicular to the c direction.
The model of crystal-forming complexes also permits interpreting the data on the effect of hydrothermal treatment parameters on the type of y-FeOOH transition in alkaline solutions. So, since twice as much oxo-bonds arise on the average during a-Fe2O3 crystal growth as compared to a-FeOOH,
at CNaOH = const the CT y-FeOOH -
■ a-Fe2O3 takes
place at higher temperatures than the PT y—> ^ a-FeOOH. For neutral, low- (CNaOH < 1 mole- l-1), and high-concentration (CNaOH > 3 mole - l-1) solutions of NaOH, the type and the number of reactions occurring when CFC, CFC', and CFC" are built into the (001) face of a-FeOOH or the (100) face of a-Fe2O3, change in the following sequence: 4 olation reactions ^ 3 olation reactions + 1 oxala-tion reaction ^ 1 olation reaction + 1 dehydratation reaction + 2 dehydroxylation reactions and 3 olation reactions + 2 dehydratation reaction ^ 3 oxolation reactions + 2 dehydratation reactions (for low- and high-concentration solutions of NaOH), respectively. Therefore at t = const the probability of a-Fe2O3 nucleation in comparison with a-FeOOH is greater in neutral and high-concentration alkaline media and smaller for NaOH solutions with intermediate concentrations, which is in agreement with the experimental data on phase formation in the y-FeOOH - H2O - NaOH system.
4. Conclusion
1. We have studied the phase (into a-FeOOH) and chemical (into a-Fe2O3) transitions of the non-equilibrium phase y-FeOOH during hydrothermal treatment in NaOH solutions with concentration 0 < CNaOH < 5 mole - l-1 at temperatures from 110 to 230 °C and refined the regions of formation of these phases.
2. It was shown that the transitions of y-FeOOH in alkaline solutions take place by the "dissolution-precipitation" mechanism. Possible composition of crystal-forming complexes appearing in solutions and reactions running when they are built into different atomically smooth faces of a-FeOOH and a-Fe2O3 have been considered. The data obtained were used to interpret the established dependences between the phase and dispersion composition of the products formed and the hydrothermal treatment parameters.
Acknowledgments
This work was financially supported by the Russian Fund of the Fundamental Investigations (RFFI) under grants № 05-08-01356 and 06-0332517, which is gratefully acknowledged.
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