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Structural–chemical transformations of α-Fe2O3 upon transport reduction
Solid State Ionics 133 (2000) 203–210 www.elsevier.com / locate / ssi
Structural–chemical transformations of a-Fe 2 O 3 upon transport reduction a a, b a a I.V. Murin , V.M. Smirnov *, G.P. Voronkov , V.G. Semenov , V.G. Povarov , B.M. Sinel’nikov b a
Department of Chemistry, St. Petersburg State University, Universitetskii Av. 2, St. Petersburg, Stary Petergof 198904, Russia b Stavropol Technical University, Stavropol 355038, Russia Received 16 April 1999; received in revised form 31 March 2000; accepted 5 July 2000
Abstract Transient regions between iron oxide crystalline phases were found in various stages of a-Fe 2 O 3 structural–chemical ¨ transformation into FeO by Mossbauer spectroscopy. Particle sizes in those regions are less than 8 nm. The sequence of phase transformations was established for a-Fe 2 O 3 reduction to iron at temperatures higher and lower than the wustite stability point ( | 5708C). Two phases Fe 12x O and FeO were found to form upon the reduction within the wustite stability region (the stoichiometric phase is formed first). It was shown that reduction of hematite into magnetite occurs within temperature range 570–6008C by the multistage mechanism, while magnetite reduction to wustite occurs by a zonal mechanism, where three phases Fe 3 O 4 , FeO and Fe are present in a sample simultaneously. 2000 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Iron oxide; Metal; Transport; Reduction; Mossbauer PACS: 66; 76; 82.10
1. Introduction Reduction of iron oxide is studied in a great number of works [1,2]. Reduction of oxides during the transport reactions, in particular, in the processes of partial transport, when the reduced substance is only partially transported through the gas phase is not totally investigated. The processes in such systems are called the processes of transport reduction
*Corresponding author. E-mail address: [email protected] (V.M. Smirnov).
(PTR) [3]. PTR allows one to control the degree of oxide reduction and its phase composition precisely and makes it possible to obtain almost the whole spectrum of oxide phases of transition metals and their mixtures, and to control cationic composition of various ferrite and ceramic materials [4]. The process of a-Fe 2 O 3 transport reduction with magnesium as a reducing agent and hydrogen as a carrier-gas can be presented by reactions (1) and (2). Reaction (3) reflects the sum process of magnesium interaction with a-Fe 2 O 3 . Fe 2 O 3 1 3H 2 → 2Fe 1 3H 2 O
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00748-7
(1)
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I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
3H 2 O 1 3Mg → 3MgO 1 3H 2
(2)
Fe 2 O 3 1 3Mg → 3MgO 1 2Fe
(3)
spectroscopy data on structural–chemical transformations of Fe 2 O 3 in the course of PTR.
2. Experimental PTR in the system Fe 2 O 3 –Mg–H 2 O involves reduction of iron(III)–oxygen groups by hydrogen with the parallel magnesium oxidation by water vapors. The process can occur only under the conditions of spatial separation of reagents, that is in the case of magnesium excess, iron will be formed at the surface of a Fe 2 O 3 sample, and MgO at the surface of magnesium. We also should note that unlike the usual oxide reduction with a hydrogen stream of transport reduction (TR) has the lower rate of reduction so that reaction (3) is much more irreversible. At first sight, the offered scheme works properly, it allows varying process duration, temperature, and amount of magnesium to remove oxygen from an initial oxide completely and to obtain the initial mixture with the certain equilibrium composition. Although this method does work correctly for the rough change of Fe 21 / Fe 31 ratio in a mixture, for a precise variation of the oxygen non-stoichiometry such a scheme is unsuitable because a rather small amount of magnesium is required. It results, besides the loss of accuracy, in a chemical heterogeneity of the final oxide mixture. The latter is caused by the low rate of transport redistribution of oxygen inside the oxide phase as compared to oxide reduction. To date the sequence of structural–chemical and phase transformations of Fe 2 O 3 in PTR is not wellstudied. In this paper we present the results of fine control of the composition for the products of a¨ Fe 2 O 3 transport reduction along with Mossbauer
2.1. Sample preparation and characterization Hematite (a-Fe 2 O 3 analytical grade) was used. PTR studies were carried out in a closed system: calculated amounts of oxide and reducing agent were placed in the opposite ends of a sealed Pyrex ampoule. Construction of the ampoule (Fig. 1) prevents reagents from mixing. Transport agent (water) was introduced in amount less than 1 mg either in a sealed capillary tube or as Mg(OH) 2 . The residual pressure in an ampoule before the experiment was 5 mmHg. PTR in such a system (with an excess of iron oxide and deficiency of magnesium) results only in partial oxygen transition from the initial hematite sample to magnesium. Therefore a mixture of oxides with certain ratio of heterovalent iron atoms is expected to be formed. In the system with magnesium excess the final product is a-Fe if chemical equilibrium is achieved. To analyze TR products for Fe 2 O 3 –FeO we used a photocolorimetric method of simultaneous determination of iron(III) and iron(II) with o-phenanthroline [5]. To determine all three iron forms (Fe 31 , Fe 21 , and Fe 0 ) after TR we used the same method: in the first stage the required iron form was isolated from the sample; in the second stage determination of separated iron forms was carried out. For the samples obtained by TR within Fe 2 O 3 – FeO region we have calculated the degree of reduc-
Fig. 1. Reaction ampoule for transport reduction of a-Fe 2 O 3 . (1) Iron oxide; (2) Mg with micro-amount of water; (3) sealing place.
I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
tion X as X5h[Fe 21 ] /([Fe 31 ]1[Fe 21 ])j3100%, according to the data of chemical analysis and assuming that X for FeO is equal to 100%.
2.2. Measurements X-ray analysis was carried out for the samples placed in hermetic cells. X-ray patterns were obtained by a DRF-2 instrument with filtered Cu irradiation using a powder technique. ¨ Mossbauer (transmission) spectra were obtained on a YaGRS-4M spectrometer. Isotope 57 Co in rhodium matrix was used as a source of g-quantum. Values of the chemical shifts (d ) were determined in accordance with d values for a-Fe. All the measurements were done at room temperature.
3. Results and discussion TR between calculated amounts of hematite and magnesium in the presence of a micro-amount of water and with the excess of iron oxide results only in partial transition of the oxygen from initial hematite sample to magnesium. In this case the formation of oxide mixture with certain Fe 21 / Fe 31 ratio should be expected. In Table 1 we present data on mixtures prepared with the given composition within Fe 2 O 3 –FeO region. The degree (X) of Fe 2 O 3 →FeO transformation is also presented in Table 1. Homogeneity of the samples in the row Fe 2 O 3 – Fe 3 O 4 –FeO was assured by the TR process duration (for several hours) and was proved using the chemi-
205
cal analysis of microsamples taken from different parts of the sample (more than five). It is apparent from Table 1 that the regulation of Fe 21 / Fe 31 ratio can be carried out within the whole range of Fe 21 content (from 0 to 100%). The detailed consideration of the final equilibrium state for Fe 2 O 3 –Mg–H 2 O system showed that for calculation of the theoretical degree of reduction (Xtheor ) one should take into account the amount of magnesium consumed during the reduction of water vapors. Thus, we obtained a new theoretical value for the degree of reduction (Xcorr ). Indeed, the wustite phase appearing in the sample when Fe 21 content exceeds 33% can exist only at molar fraction of hydrogen in the gas phase close to 1. Consequently, two subregions can be selected in the whole region of Fe 21 content. The phase mixture Fe 2 O 3 –Fe 3 O 4 can be related to the first region (0–40%), and mixture Fe 3 O 4 –FeO – to the second region (40–100% of 21 Fe ). TR products with various degrees of reduction ¨ (from Fe 2 O 3 to Fe) were studied using Mossbauer spectroscopy (Fig. 2, Tables 2 and 3, where d is the isomer shift, DE is the quadruple splitting, H is the effective superfine field). The data on iron atoms distribution between the TR products obtained for the non-equilibrium conditions with significant excess of reducing agent at temperatures from 570 to 6008C are presented in Table 2. Spectral parameters of iron–oxygen phases are summarized in Table 3. The mechanism of Fe 2 O 3 reduction at different stages was proposed on the basis of the changes in phase ratio of iron–oxygen sample. According to the data of Table 2, Fe 2 O 3 reduction into Fe 3 O 4 occurs
Table 1 Mixture composition, mg in the system Fe 2 O 3 –Mg–H 2 O at 5808C Degree of reduction, a (%)
Mixture composition m(Mg)
m(Fe 2 O 3 )
m(H 2 O)
Xtheor a
Xcorr
Xexp
X1 b
X2 c
6.8 8.0 9.6 11.9 15.3 21.3
156.8 159.6 160.4 160.1 160.3 160.1
0.22 1.0 1.1 1.0 1.1 0.9
28.9 33.3 40.0 49.6 63.8 88.7
28.9 33.3 40.0 46.4 60.3 83.9
29.3 35.1 45.7 46.7 56.0 78.0
0.4 1.8 5.7 2.9 7.8 10.7
0.4 1.8 5.7 0.3 4.3 5.9
a
X5[Fe 21 ] /([Fe 21 ]1[Fe 31 ])3100%2degree of reduction. X1 5Xtheor –Xexp . c X2 5Xexp –Xcorr . b
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21 ¨ Fig. 2. Mossbauer spectra of iron(III) oxide and products of transport reduction depending on the degree of reduction to Fe (X, %) in the system Mg–Fe 2 O 3 –H 2 O (sample nos. 1–14) and in the system Fe–Fe 2 O 3 –H 2 O (sample nos. 15, 16). Samples 1–9 were reduced to Fe 12x O, samples 10–14 and 16 to Fe 0 . Samples: (1) a-Fe 2 O 3 ; (2) partially reduced oxide (X59.6%); (3) the same (X516.2%); (4) the same (X519.9%); (5) the same (X532.6%); (6) the same (X535.1%); (7) the same (X547.3%); (8) the same (X547.6%); (9) the same (X586.7%); (10) oxide reduced to Fe 0 (20.3%); (11) the same (29.6%); (12) the same (49.3%); (13) the same (58.1%); (14) the same (98.6%); (15) Fe 0 oxidation to Fe 12x O; (16) reduction to Fe 12x O.
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Table 2 ¨ Iron distribution in PTR products according to Mossbauer data Sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Xa (%)
9.6 16.2 19.9 32.6 35.1 47.3 47.6 86.7
Fe (%) Fe 2 O 3 100 66.2 46.8 36.0
Fe 3 O 4
Ab
B
Fe 1III
28.9 48.5 59.7 97.7 93.6 76.0 62.2 19.9 74.5 4.9 8.8 10.0
12.4 18.1 24.4 38.2 35.3 26.4 25.3 6.8 31.2 4.5 2.3 3.1
16.5 30.4 35.3 59.5 58.3 49.6 36.9 13.1 43.3 10.4 6.5 6.9
4.9 4.7 4.3 2.3 2.4 1.4
7.9 7.4
2.9 2.8
5.0 4.6
FeO s
3.9 22.5 29.1 47.6
Fe III 2
Fe 12x O
Fe II
Fe 0
8.8 32.5
5.2 26.7 17.7 15.6
25.7 20.1 11.7
24.1 28.3
68.0 58.8
20.3 29.6 49.3 58.1 98.6
3.2 4.1 4.5
1.4
5.5
T (8C)
510 500 600 500 570 575 600 580 510 570 570 580 510 600 600
a
X is a degree of reduction of FeO. b A and B are structural sublattices of Fe 3 O 4 phase; Fe 0 is dispersed metallic iron.
Table 3 a ¨ Mossbauer parameters of the PTR products
d (mm / s) DE (mm / s) H (kOe) a
Fe 2 O 3
Fe 1III
A
B
FeO s
Fe 2III
Fe 12x O
Fe II
Fe 0
0.37 0.22 517.5
0.13 0.55
0.37 0.02 491.2
0.64 0.01 460.5
1.06 0.29
0.02 0.99
0.86 0.74
0.44 1.20
0.01 0.02 330.1
All symbols are the same as in Table 2.
by the one-step mechanism, while the mechanism of Fe 3 O 4 reduction into FeO is zonal, when three nonequilibrium phases Fe 3 O 4 , FeO and Fe are present in the sample simultaneously. Moreover the obtained data prove the fact that iron atoms in the sample can be found not only in the phases Fe 2 O 3 , Fe 3 O 4 , FeO and Fe but also in the transient regions (TRg) ¨ between these phases. Mossbauer characteristics of these TRg differ from those of the bulk phases. ¨ These TRg can be found in Mossbauer spectra as doublets. Spectral characteristics allow us to relate two of them to three-valent iron (we marked them as III Fe III 1 and Fe 2 ). The third doublet may be related to two-valent iron (Fe II ). Considering samples obtained at the temperatures higher than 5708C and taking into account the data of Table 2, we suppose that Fe III is a TRg between 1 Fe 2 O 3 and Fe 3 O 4 phases and Fe II is a TRg between
¨ FeO and Fe phases. Using Mossbauer spectroscopy data we also note that iron atoms in Fe 1III TRg are three-valent. However, comparing these atoms with the atoms in Fe 2 O 3 one can observe the reduced value of the isomer shift (d ), and the consequently increased s-electron density for the atom nuclei. At the same time the higher value of the quadruple splitting (DE) indicates the growth of the covalent component of the chemical bond and the asymmetry of the iron atoms environment. Besides, iron is in a low-spin state (S51 / 2). Such a shift in the spectral parameters may be caused by the higher Fe 21 content in Fe III when compared with Fe 2 O 3 . Re1 duced IS and increased QS values are also observed for iron atoms in the other TRg but they refer to FeO phase. Thus, the total spin of iron atoms in FeO is equal to 2, while for iron atoms in TRg it is equal to 1. Such a state is not typical for two-valent iron [6].
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With the further increase of metallic Fe content IS decreases (from 0.45 at 30% Fe content to 0.4 at 60%), approaching the values characterising a singlevalent iron state (S51 / 2). Considering samples No. 5 and 6 from Table 2, corresponding to the process of Fe 3 O 4 reduction into FeO, we observe a reduced amount of Fe III 1 due to the complete disappearance of Fe 2 O 3 phase. In this case the FeO s phase has appeared, the spectral parameters of it (Table 3) are close to those of the stoichiometric FeO [7]. Further decrease of amount of Fe III and the simultaneous changes in doublet 1 parameters (d reduction and DE increase) have been found for sample No. 7 along with considerable content of FeO s phase (22.5%). Nevertheless we can state that in this case Fe III 1 phase plays a role of TRg too. Considering the changes in d and DE we suggest transformation of Fe III into the next TRg Fe III 1 2 (sample No. 8). This TRg is characterized by d values close to 0 and DE values close to 1 (Table 2). This TRg is associated with the formation of a new phase Fe 12x O, which differs from FeO s phase by the increased oxygen / iron ratio (sample No. 9). There is no evidence for simultaneous existence of Fe 12x O and Fe 2III phases that proves the stability of a new TRg only at the threshold of the new phase Fe 12x O formation. However, starting with the spectral data of Fe III and Fe 12x O one can find the Fe III →Fe II 2 transition as a result of reduction. As it has already been mentioned, both decrease in d and the growth of DE values show the increase the s-electron density, more covalent character of the chemical bond and the asymmetry in iron atoms environment. Probably these effects in Fe III 2 are close to the critical ones that result in transition of iron atoms from three into two-valent state accompanied by the drastic rise in d and decrease in DE values for Fe 12x O phase. Thus, we can write the following consecutive reactions for Fe 2 O 3 reduction at temperatures higher than 5708C: Fe 2 O 3 → Fe 1III → Fe 3 O 4
(4)
Fe 3 O 4 → Fe 1III → FeO s
(5)
Fe 3 O 4 → Fe 2III → Fe 12x O
(6)
Fe 12x O → FeO s
(7)
FeO s → Fe
(8)
Iron oxide reduction within the temperature range from 500 to 5708C (i.e. at temperatures lower than the wustite stability point) may be presented by the following reactions: Fe 2 O 3 → Fe 1III → Fe 3 O 4 Fe 3 O 4 → Fe 1III → Fe
(9) (10)
It is well-known that paramagnetic properties are peculiar not only for wustite but also for hematite in a finely dispersed state (when particle size is less than 10 nm [8–11]). According to Ref. [12], ¨ Mossbauer spectra of hematite with particle size 4 nm contain two doublets, with parameters practically III the same as the parameters of Fe III 1 and Fe 2 . This allows us to assume that in the course of phase transformations iron oxide undergoes chemical dispersion and that so-called transient regions are finely dispersed phases with particle size less than 8 nm. Therefore according to the obtained and reference data [11,12], where the decrease in the particle size is associated with DE value growth (greater than 1 mm / s), we note the decrease in the particle size upon phase transformation Fe 3 O 4 →Fe 12x O in comparison to Fe 3 O 4 →FeO s transformation. Table 2 presents the data of PTR products investigations (sample nos. 15 and 16) in the system Fe 2 O 3 –Fe–H 2 O (i.e. Fe 0 (carbonyl iron) instead of Mg behaves as a reducing agent) at 6008C and at conditions close to equilibrium (duration of the experiment was |500 min). In this system it is possible to observe simultaneously both reduction of Fe 2 O 3 and oxidation of Fe. In this particular case PTR products are two iron–oxygen species of Fe 12x O: reduced Fe 2 O 3 and oxidized Fe (sample nos. 15 and 16, respectively). Bearing in mind that the total degree of reduction for sample nos. 10 and 16 are almost the same (|85%), we found redistribution of iron between two phases near the equilibrium state resulting in the increase of Fe 12x O phase amount. This means that this phase is stable, and it can be the only phase of the equilibrium conditions. From
I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
our point of view iron oxidation may be represented as follows. FeO s phase is formed on the surface of iron, then Fe 3 O 4 phase appears on the surface of FeO s , and only then the new phase Fe 12x O is formed between FeO s and Fe 3 O 4 . Fe 12x O is the final product of oxidation at the given conditions. Discussing all results mentioned above we can write the following sequence of phase formation for the direct and reverse reaction Fe 3 O 4 53Fe12O 2 , respectively: Fe 3 O 4 → FeO s → Fe 12x O → Fe sample reduction (11) Fe → FeO s → Fe 3 O 4
sample oxidation
(12)
Taking into account this sequence and phase arrangement one can also judge about the structural changes taking place during Fe 3 O 4 reduction and Fe oxidation. For example, during iron oxidation its crystal lattice transforms into the lattice of NaCl-type similar to stoichiometric FeO s phase. Further FeO s conversion into Fe 3 O 4 is probably accompanied by the following structural changes: rapid oxygen introduction into wustite lattice resulting in the appearance of a huge number of cation vacancies and the shift of one-third of iron atoms into the tetrahedral position. The remaining two-thirds retain their octahedral coordination, but half of them are also threevalent (depending on experimental conditions we obtained Fe 3 O 4 more or less distinguished from the stoichiometric oxide; we should discuss this later). As FeO s and Fe 3 O 4 penetrate into the sample, the
209
rate of oxidation falls down, and vacancies formed in FeO s upon oxygen atoms introduction have time to be distributed in the bulk. As a result, there is no transformation into a spinel-type structure, but wustite structure becomes more defective, and a new phase, Fe 12x O, is formed at the boundary between FeO s and Fe 3 O 4 phases. We obtained additional interesting information by analyzing the iron atoms distribution between two structural sublattices (A and B) of Fe 3 O 4 phase (Table 3). In the case of an ideal Fe 3 O 4 structure, the amount of two-valent iron must be 33.3% of the total iron content. In Table 4 summarized data are presented on iron atom content in positions A and B, B /A ratio (which for an ideal structure must be equal to 2) and Fe II atom fraction in the total iron content in Fe 3 O 4 assuming that half of B-positions is occupied by Fe II .
4. Conclusions Various processes (at equilibrium and non-equilibrium conditions) of TR of a-Fe 2 O 3 were studied. It is possible to control iron reduction from Fe 31 to Fe 21 at equilibrium conditions and to obtain the mixture of oxides Fe 2 O 3 –Fe 3 O 4 –FeO with the certain ratio of iron atoms of various valence. TR process carried out at non-equilibrium conditions is accompanied by the formation of a number of intermediate iron oxides of a non-stoichiometric ¨ composition. Using Mossbauer spectroscopy transient regions were found to exist between the crystalline phases of iron oxides. These TRg consist of finely dispersed (less than 8 nm) particles of iron oxide.
Table 4 a ¨ Iron atoms distribution in Fe 3 O 4 according to Mossbauer data Sample no.
A (%) B (%) B/A Fe II (%) a
2
3
4
5
6
7
8
9
15
16
10
11
12
13
12.4 16.5 1.33 28.761.0
18.1 30.4 1.68 31.361.6
24.4 35.3 1.45 29.660.5
38.2 59.5 1.56 30.560.7
35.3 58.3 1.65 31.160.5
26.4 49.6 1.88 32.660.1
25.3 36.9 1.46 29.760.6
6.8 13.1 1.93 32.961.3
2.9 5.0 1.72 31.662.5
2.8 4.6 1.64 31.160.4
31.2 43.3 1.39 29.160.4
4.5 10.4 2.31 34.961.3
2.3 6.5 2.83 36.665.3
3.1 6.9 2.23 34.365.7
Samples numbers (2–16) are the same as in Fig. 2.
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I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
Acknowledgements This work was supported by Russian Foundation of Basic Researches under Grant No. 99-03-32010 and ‘Integration’ program under Grant No. A-0146.
References [1] A.F. Belov (Ed.), Iron Reduction and Refinement, Nauka, Moscow, 1968, p. 158. [2] V.A. Gorbachev, S.V. Shavrin, in: Nuclei Formation in the Processes of Oxides Reduction, Nauka, Moscow, 1985. [3] V.B. Aleskowskii, V.G. Povarov, V.M. Smirnov, S.S. Pershin, Dokl. AN SSSR 303 (1988) 1393. [4] V.G. Povarov, G.P. Voronkov, V.M. Smirnov, Zh. Prikl. Khim. 66 (1993) 1972.
[5] G. Sharlo, in: Methods in Analytical Chemistry. Quantitative Analysis of Inorganic Compounds, Khimiya, Moscow, 1978, p. 271. [6] L.V. Vilkov, Yu.A. Pentin, Physical Methods in Chemistry. Resonance and Electrooptical Studies, Moscow, Khimij, 1989, p. 189. [7] N.N. Grenwood, A.T. Howe, J. Chem. Soc., Dalton Trans. 1 (1972) 116. [8] S. Morup, H. Topsoe, Appl. Phys 11 (1976) 63. [9] T.K. McNab, R.A. Fox, A.J.F. Boyle, J. Appl. Phys. 39 (1968) 5703. [10] C. Janot, H. Gibert, C. Tobias, Bull. Soc. Fr. Mineral. Crystallogr. 96 (1973) 281. [11] W. Kindig, H. Bummel, G. Constabaris, R.H. Lindquist, Phys. Rev. 142 (1966) 327. [12] A.M. van der Kraan, Phys. Status Solidity A 18 (1973) 215.
Structural–chemical transformations of a-Fe 2 O 3 upon transport reduction a a, b a a I.V. Murin , V.M. Smirnov *, G.P. Voronkov , V.G. Semenov , V.G. Povarov , B.M. Sinel’nikov b a
Department of Chemistry, St. Petersburg State University, Universitetskii Av. 2, St. Petersburg, Stary Petergof 198904, Russia b Stavropol Technical University, Stavropol 355038, Russia Received 16 April 1999; received in revised form 31 March 2000; accepted 5 July 2000
Abstract Transient regions between iron oxide crystalline phases were found in various stages of a-Fe 2 O 3 structural–chemical ¨ transformation into FeO by Mossbauer spectroscopy. Particle sizes in those regions are less than 8 nm. The sequence of phase transformations was established for a-Fe 2 O 3 reduction to iron at temperatures higher and lower than the wustite stability point ( | 5708C). Two phases Fe 12x O and FeO were found to form upon the reduction within the wustite stability region (the stoichiometric phase is formed first). It was shown that reduction of hematite into magnetite occurs within temperature range 570–6008C by the multistage mechanism, while magnetite reduction to wustite occurs by a zonal mechanism, where three phases Fe 3 O 4 , FeO and Fe are present in a sample simultaneously. 2000 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Iron oxide; Metal; Transport; Reduction; Mossbauer PACS: 66; 76; 82.10
1. Introduction Reduction of iron oxide is studied in a great number of works [1,2]. Reduction of oxides during the transport reactions, in particular, in the processes of partial transport, when the reduced substance is only partially transported through the gas phase is not totally investigated. The processes in such systems are called the processes of transport reduction
*Corresponding author. E-mail address: [email protected] (V.M. Smirnov).
(PTR) [3]. PTR allows one to control the degree of oxide reduction and its phase composition precisely and makes it possible to obtain almost the whole spectrum of oxide phases of transition metals and their mixtures, and to control cationic composition of various ferrite and ceramic materials [4]. The process of a-Fe 2 O 3 transport reduction with magnesium as a reducing agent and hydrogen as a carrier-gas can be presented by reactions (1) and (2). Reaction (3) reflects the sum process of magnesium interaction with a-Fe 2 O 3 . Fe 2 O 3 1 3H 2 → 2Fe 1 3H 2 O
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00748-7
(1)
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I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
3H 2 O 1 3Mg → 3MgO 1 3H 2
(2)
Fe 2 O 3 1 3Mg → 3MgO 1 2Fe
(3)
spectroscopy data on structural–chemical transformations of Fe 2 O 3 in the course of PTR.
2. Experimental PTR in the system Fe 2 O 3 –Mg–H 2 O involves reduction of iron(III)–oxygen groups by hydrogen with the parallel magnesium oxidation by water vapors. The process can occur only under the conditions of spatial separation of reagents, that is in the case of magnesium excess, iron will be formed at the surface of a Fe 2 O 3 sample, and MgO at the surface of magnesium. We also should note that unlike the usual oxide reduction with a hydrogen stream of transport reduction (TR) has the lower rate of reduction so that reaction (3) is much more irreversible. At first sight, the offered scheme works properly, it allows varying process duration, temperature, and amount of magnesium to remove oxygen from an initial oxide completely and to obtain the initial mixture with the certain equilibrium composition. Although this method does work correctly for the rough change of Fe 21 / Fe 31 ratio in a mixture, for a precise variation of the oxygen non-stoichiometry such a scheme is unsuitable because a rather small amount of magnesium is required. It results, besides the loss of accuracy, in a chemical heterogeneity of the final oxide mixture. The latter is caused by the low rate of transport redistribution of oxygen inside the oxide phase as compared to oxide reduction. To date the sequence of structural–chemical and phase transformations of Fe 2 O 3 in PTR is not wellstudied. In this paper we present the results of fine control of the composition for the products of a¨ Fe 2 O 3 transport reduction along with Mossbauer
2.1. Sample preparation and characterization Hematite (a-Fe 2 O 3 analytical grade) was used. PTR studies were carried out in a closed system: calculated amounts of oxide and reducing agent were placed in the opposite ends of a sealed Pyrex ampoule. Construction of the ampoule (Fig. 1) prevents reagents from mixing. Transport agent (water) was introduced in amount less than 1 mg either in a sealed capillary tube or as Mg(OH) 2 . The residual pressure in an ampoule before the experiment was 5 mmHg. PTR in such a system (with an excess of iron oxide and deficiency of magnesium) results only in partial oxygen transition from the initial hematite sample to magnesium. Therefore a mixture of oxides with certain ratio of heterovalent iron atoms is expected to be formed. In the system with magnesium excess the final product is a-Fe if chemical equilibrium is achieved. To analyze TR products for Fe 2 O 3 –FeO we used a photocolorimetric method of simultaneous determination of iron(III) and iron(II) with o-phenanthroline [5]. To determine all three iron forms (Fe 31 , Fe 21 , and Fe 0 ) after TR we used the same method: in the first stage the required iron form was isolated from the sample; in the second stage determination of separated iron forms was carried out. For the samples obtained by TR within Fe 2 O 3 – FeO region we have calculated the degree of reduc-
Fig. 1. Reaction ampoule for transport reduction of a-Fe 2 O 3 . (1) Iron oxide; (2) Mg with micro-amount of water; (3) sealing place.
I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
tion X as X5h[Fe 21 ] /([Fe 31 ]1[Fe 21 ])j3100%, according to the data of chemical analysis and assuming that X for FeO is equal to 100%.
2.2. Measurements X-ray analysis was carried out for the samples placed in hermetic cells. X-ray patterns were obtained by a DRF-2 instrument with filtered Cu irradiation using a powder technique. ¨ Mossbauer (transmission) spectra were obtained on a YaGRS-4M spectrometer. Isotope 57 Co in rhodium matrix was used as a source of g-quantum. Values of the chemical shifts (d ) were determined in accordance with d values for a-Fe. All the measurements were done at room temperature.
3. Results and discussion TR between calculated amounts of hematite and magnesium in the presence of a micro-amount of water and with the excess of iron oxide results only in partial transition of the oxygen from initial hematite sample to magnesium. In this case the formation of oxide mixture with certain Fe 21 / Fe 31 ratio should be expected. In Table 1 we present data on mixtures prepared with the given composition within Fe 2 O 3 –FeO region. The degree (X) of Fe 2 O 3 →FeO transformation is also presented in Table 1. Homogeneity of the samples in the row Fe 2 O 3 – Fe 3 O 4 –FeO was assured by the TR process duration (for several hours) and was proved using the chemi-
205
cal analysis of microsamples taken from different parts of the sample (more than five). It is apparent from Table 1 that the regulation of Fe 21 / Fe 31 ratio can be carried out within the whole range of Fe 21 content (from 0 to 100%). The detailed consideration of the final equilibrium state for Fe 2 O 3 –Mg–H 2 O system showed that for calculation of the theoretical degree of reduction (Xtheor ) one should take into account the amount of magnesium consumed during the reduction of water vapors. Thus, we obtained a new theoretical value for the degree of reduction (Xcorr ). Indeed, the wustite phase appearing in the sample when Fe 21 content exceeds 33% can exist only at molar fraction of hydrogen in the gas phase close to 1. Consequently, two subregions can be selected in the whole region of Fe 21 content. The phase mixture Fe 2 O 3 –Fe 3 O 4 can be related to the first region (0–40%), and mixture Fe 3 O 4 –FeO – to the second region (40–100% of 21 Fe ). TR products with various degrees of reduction ¨ (from Fe 2 O 3 to Fe) were studied using Mossbauer spectroscopy (Fig. 2, Tables 2 and 3, where d is the isomer shift, DE is the quadruple splitting, H is the effective superfine field). The data on iron atoms distribution between the TR products obtained for the non-equilibrium conditions with significant excess of reducing agent at temperatures from 570 to 6008C are presented in Table 2. Spectral parameters of iron–oxygen phases are summarized in Table 3. The mechanism of Fe 2 O 3 reduction at different stages was proposed on the basis of the changes in phase ratio of iron–oxygen sample. According to the data of Table 2, Fe 2 O 3 reduction into Fe 3 O 4 occurs
Table 1 Mixture composition, mg in the system Fe 2 O 3 –Mg–H 2 O at 5808C Degree of reduction, a (%)
Mixture composition m(Mg)
m(Fe 2 O 3 )
m(H 2 O)
Xtheor a
Xcorr
Xexp
X1 b
X2 c
6.8 8.0 9.6 11.9 15.3 21.3
156.8 159.6 160.4 160.1 160.3 160.1
0.22 1.0 1.1 1.0 1.1 0.9
28.9 33.3 40.0 49.6 63.8 88.7
28.9 33.3 40.0 46.4 60.3 83.9
29.3 35.1 45.7 46.7 56.0 78.0
0.4 1.8 5.7 2.9 7.8 10.7
0.4 1.8 5.7 0.3 4.3 5.9
a
X5[Fe 21 ] /([Fe 21 ]1[Fe 31 ])3100%2degree of reduction. X1 5Xtheor –Xexp . c X2 5Xexp –Xcorr . b
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21 ¨ Fig. 2. Mossbauer spectra of iron(III) oxide and products of transport reduction depending on the degree of reduction to Fe (X, %) in the system Mg–Fe 2 O 3 –H 2 O (sample nos. 1–14) and in the system Fe–Fe 2 O 3 –H 2 O (sample nos. 15, 16). Samples 1–9 were reduced to Fe 12x O, samples 10–14 and 16 to Fe 0 . Samples: (1) a-Fe 2 O 3 ; (2) partially reduced oxide (X59.6%); (3) the same (X516.2%); (4) the same (X519.9%); (5) the same (X532.6%); (6) the same (X535.1%); (7) the same (X547.3%); (8) the same (X547.6%); (9) the same (X586.7%); (10) oxide reduced to Fe 0 (20.3%); (11) the same (29.6%); (12) the same (49.3%); (13) the same (58.1%); (14) the same (98.6%); (15) Fe 0 oxidation to Fe 12x O; (16) reduction to Fe 12x O.
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Table 2 ¨ Iron distribution in PTR products according to Mossbauer data Sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Xa (%)
9.6 16.2 19.9 32.6 35.1 47.3 47.6 86.7
Fe (%) Fe 2 O 3 100 66.2 46.8 36.0
Fe 3 O 4
Ab
B
Fe 1III
28.9 48.5 59.7 97.7 93.6 76.0 62.2 19.9 74.5 4.9 8.8 10.0
12.4 18.1 24.4 38.2 35.3 26.4 25.3 6.8 31.2 4.5 2.3 3.1
16.5 30.4 35.3 59.5 58.3 49.6 36.9 13.1 43.3 10.4 6.5 6.9
4.9 4.7 4.3 2.3 2.4 1.4
7.9 7.4
2.9 2.8
5.0 4.6
FeO s
3.9 22.5 29.1 47.6
Fe III 2
Fe 12x O
Fe II
Fe 0
8.8 32.5
5.2 26.7 17.7 15.6
25.7 20.1 11.7
24.1 28.3
68.0 58.8
20.3 29.6 49.3 58.1 98.6
3.2 4.1 4.5
1.4
5.5
T (8C)
510 500 600 500 570 575 600 580 510 570 570 580 510 600 600
a
X is a degree of reduction of FeO. b A and B are structural sublattices of Fe 3 O 4 phase; Fe 0 is dispersed metallic iron.
Table 3 a ¨ Mossbauer parameters of the PTR products
d (mm / s) DE (mm / s) H (kOe) a
Fe 2 O 3
Fe 1III
A
B
FeO s
Fe 2III
Fe 12x O
Fe II
Fe 0
0.37 0.22 517.5
0.13 0.55
0.37 0.02 491.2
0.64 0.01 460.5
1.06 0.29
0.02 0.99
0.86 0.74
0.44 1.20
0.01 0.02 330.1
All symbols are the same as in Table 2.
by the one-step mechanism, while the mechanism of Fe 3 O 4 reduction into FeO is zonal, when three nonequilibrium phases Fe 3 O 4 , FeO and Fe are present in the sample simultaneously. Moreover the obtained data prove the fact that iron atoms in the sample can be found not only in the phases Fe 2 O 3 , Fe 3 O 4 , FeO and Fe but also in the transient regions (TRg) ¨ between these phases. Mossbauer characteristics of these TRg differ from those of the bulk phases. ¨ These TRg can be found in Mossbauer spectra as doublets. Spectral characteristics allow us to relate two of them to three-valent iron (we marked them as III Fe III 1 and Fe 2 ). The third doublet may be related to two-valent iron (Fe II ). Considering samples obtained at the temperatures higher than 5708C and taking into account the data of Table 2, we suppose that Fe III is a TRg between 1 Fe 2 O 3 and Fe 3 O 4 phases and Fe II is a TRg between
¨ FeO and Fe phases. Using Mossbauer spectroscopy data we also note that iron atoms in Fe 1III TRg are three-valent. However, comparing these atoms with the atoms in Fe 2 O 3 one can observe the reduced value of the isomer shift (d ), and the consequently increased s-electron density for the atom nuclei. At the same time the higher value of the quadruple splitting (DE) indicates the growth of the covalent component of the chemical bond and the asymmetry of the iron atoms environment. Besides, iron is in a low-spin state (S51 / 2). Such a shift in the spectral parameters may be caused by the higher Fe 21 content in Fe III when compared with Fe 2 O 3 . Re1 duced IS and increased QS values are also observed for iron atoms in the other TRg but they refer to FeO phase. Thus, the total spin of iron atoms in FeO is equal to 2, while for iron atoms in TRg it is equal to 1. Such a state is not typical for two-valent iron [6].
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With the further increase of metallic Fe content IS decreases (from 0.45 at 30% Fe content to 0.4 at 60%), approaching the values characterising a singlevalent iron state (S51 / 2). Considering samples No. 5 and 6 from Table 2, corresponding to the process of Fe 3 O 4 reduction into FeO, we observe a reduced amount of Fe III 1 due to the complete disappearance of Fe 2 O 3 phase. In this case the FeO s phase has appeared, the spectral parameters of it (Table 3) are close to those of the stoichiometric FeO [7]. Further decrease of amount of Fe III and the simultaneous changes in doublet 1 parameters (d reduction and DE increase) have been found for sample No. 7 along with considerable content of FeO s phase (22.5%). Nevertheless we can state that in this case Fe III 1 phase plays a role of TRg too. Considering the changes in d and DE we suggest transformation of Fe III into the next TRg Fe III 1 2 (sample No. 8). This TRg is characterized by d values close to 0 and DE values close to 1 (Table 2). This TRg is associated with the formation of a new phase Fe 12x O, which differs from FeO s phase by the increased oxygen / iron ratio (sample No. 9). There is no evidence for simultaneous existence of Fe 12x O and Fe 2III phases that proves the stability of a new TRg only at the threshold of the new phase Fe 12x O formation. However, starting with the spectral data of Fe III and Fe 12x O one can find the Fe III →Fe II 2 transition as a result of reduction. As it has already been mentioned, both decrease in d and the growth of DE values show the increase the s-electron density, more covalent character of the chemical bond and the asymmetry in iron atoms environment. Probably these effects in Fe III 2 are close to the critical ones that result in transition of iron atoms from three into two-valent state accompanied by the drastic rise in d and decrease in DE values for Fe 12x O phase. Thus, we can write the following consecutive reactions for Fe 2 O 3 reduction at temperatures higher than 5708C: Fe 2 O 3 → Fe 1III → Fe 3 O 4
(4)
Fe 3 O 4 → Fe 1III → FeO s
(5)
Fe 3 O 4 → Fe 2III → Fe 12x O
(6)
Fe 12x O → FeO s
(7)
FeO s → Fe
(8)
Iron oxide reduction within the temperature range from 500 to 5708C (i.e. at temperatures lower than the wustite stability point) may be presented by the following reactions: Fe 2 O 3 → Fe 1III → Fe 3 O 4 Fe 3 O 4 → Fe 1III → Fe
(9) (10)
It is well-known that paramagnetic properties are peculiar not only for wustite but also for hematite in a finely dispersed state (when particle size is less than 10 nm [8–11]). According to Ref. [12], ¨ Mossbauer spectra of hematite with particle size 4 nm contain two doublets, with parameters practically III the same as the parameters of Fe III 1 and Fe 2 . This allows us to assume that in the course of phase transformations iron oxide undergoes chemical dispersion and that so-called transient regions are finely dispersed phases with particle size less than 8 nm. Therefore according to the obtained and reference data [11,12], where the decrease in the particle size is associated with DE value growth (greater than 1 mm / s), we note the decrease in the particle size upon phase transformation Fe 3 O 4 →Fe 12x O in comparison to Fe 3 O 4 →FeO s transformation. Table 2 presents the data of PTR products investigations (sample nos. 15 and 16) in the system Fe 2 O 3 –Fe–H 2 O (i.e. Fe 0 (carbonyl iron) instead of Mg behaves as a reducing agent) at 6008C and at conditions close to equilibrium (duration of the experiment was |500 min). In this system it is possible to observe simultaneously both reduction of Fe 2 O 3 and oxidation of Fe. In this particular case PTR products are two iron–oxygen species of Fe 12x O: reduced Fe 2 O 3 and oxidized Fe (sample nos. 15 and 16, respectively). Bearing in mind that the total degree of reduction for sample nos. 10 and 16 are almost the same (|85%), we found redistribution of iron between two phases near the equilibrium state resulting in the increase of Fe 12x O phase amount. This means that this phase is stable, and it can be the only phase of the equilibrium conditions. From
I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
our point of view iron oxidation may be represented as follows. FeO s phase is formed on the surface of iron, then Fe 3 O 4 phase appears on the surface of FeO s , and only then the new phase Fe 12x O is formed between FeO s and Fe 3 O 4 . Fe 12x O is the final product of oxidation at the given conditions. Discussing all results mentioned above we can write the following sequence of phase formation for the direct and reverse reaction Fe 3 O 4 53Fe12O 2 , respectively: Fe 3 O 4 → FeO s → Fe 12x O → Fe sample reduction (11) Fe → FeO s → Fe 3 O 4
sample oxidation
(12)
Taking into account this sequence and phase arrangement one can also judge about the structural changes taking place during Fe 3 O 4 reduction and Fe oxidation. For example, during iron oxidation its crystal lattice transforms into the lattice of NaCl-type similar to stoichiometric FeO s phase. Further FeO s conversion into Fe 3 O 4 is probably accompanied by the following structural changes: rapid oxygen introduction into wustite lattice resulting in the appearance of a huge number of cation vacancies and the shift of one-third of iron atoms into the tetrahedral position. The remaining two-thirds retain their octahedral coordination, but half of them are also threevalent (depending on experimental conditions we obtained Fe 3 O 4 more or less distinguished from the stoichiometric oxide; we should discuss this later). As FeO s and Fe 3 O 4 penetrate into the sample, the
209
rate of oxidation falls down, and vacancies formed in FeO s upon oxygen atoms introduction have time to be distributed in the bulk. As a result, there is no transformation into a spinel-type structure, but wustite structure becomes more defective, and a new phase, Fe 12x O, is formed at the boundary between FeO s and Fe 3 O 4 phases. We obtained additional interesting information by analyzing the iron atoms distribution between two structural sublattices (A and B) of Fe 3 O 4 phase (Table 3). In the case of an ideal Fe 3 O 4 structure, the amount of two-valent iron must be 33.3% of the total iron content. In Table 4 summarized data are presented on iron atom content in positions A and B, B /A ratio (which for an ideal structure must be equal to 2) and Fe II atom fraction in the total iron content in Fe 3 O 4 assuming that half of B-positions is occupied by Fe II .
4. Conclusions Various processes (at equilibrium and non-equilibrium conditions) of TR of a-Fe 2 O 3 were studied. It is possible to control iron reduction from Fe 31 to Fe 21 at equilibrium conditions and to obtain the mixture of oxides Fe 2 O 3 –Fe 3 O 4 –FeO with the certain ratio of iron atoms of various valence. TR process carried out at non-equilibrium conditions is accompanied by the formation of a number of intermediate iron oxides of a non-stoichiometric ¨ composition. Using Mossbauer spectroscopy transient regions were found to exist between the crystalline phases of iron oxides. These TRg consist of finely dispersed (less than 8 nm) particles of iron oxide.
Table 4 a ¨ Iron atoms distribution in Fe 3 O 4 according to Mossbauer data Sample no.
A (%) B (%) B/A Fe II (%) a
2
3
4
5
6
7
8
9
15
16
10
11
12
13
12.4 16.5 1.33 28.761.0
18.1 30.4 1.68 31.361.6
24.4 35.3 1.45 29.660.5
38.2 59.5 1.56 30.560.7
35.3 58.3 1.65 31.160.5
26.4 49.6 1.88 32.660.1
25.3 36.9 1.46 29.760.6
6.8 13.1 1.93 32.961.3
2.9 5.0 1.72 31.662.5
2.8 4.6 1.64 31.160.4
31.2 43.3 1.39 29.160.4
4.5 10.4 2.31 34.961.3
2.3 6.5 2.83 36.665.3
3.1 6.9 2.23 34.365.7
Samples numbers (2–16) are the same as in Fig. 2.
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I.V. Murin et al. / Solid State Ionics 133 (2000) 203 – 210
Acknowledgements This work was supported by Russian Foundation of Basic Researches under Grant No. 99-03-32010 and ‘Integration’ program under Grant No. A-0146.
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[5] G. Sharlo, in: Methods in Analytical Chemistry. Quantitative Analysis of Inorganic Compounds, Khimiya, Moscow, 1978, p. 271. [6] L.V. Vilkov, Yu.A. Pentin, Physical Methods in Chemistry. Resonance and Electrooptical Studies, Moscow, Khimij, 1989, p. 189. [7] N.N. Grenwood, A.T. Howe, J. Chem. Soc., Dalton Trans. 1 (1972) 116. [8] S. Morup, H. Topsoe, Appl. Phys 11 (1976) 63. [9] T.K. McNab, R.A. Fox, A.J.F. Boyle, J. Appl. Phys. 39 (1968) 5703. [10] C. Janot, H. Gibert, C. Tobias, Bull. Soc. Fr. Mineral. Crystallogr. 96 (1973) 281. [11] W. Kindig, H. Bummel, G. Constabaris, R.H. Lindquist, Phys. Rev. 142 (1966) 327. [12] A.M. van der Kraan, Phys. Status Solidity A 18 (1973) 215.