Iron (III) Oxide Fabrication From Natural Clay With Reference to Phase Transformation γ- → α- Fe2O3

Amorphous iron (III) oxide was obtained from clay, using ammonium hydroxide as a precipitating agent. Influence of freeze drying under vacuum, as a drying method, on particle size, chemical composition, and crystallinity of obtained iron (III) oxide powder was investigated. After freeze drying, precipitate was annealed in air at 500 C and 900 C. X-ray diffraction, particle size analysis, scanning electron microscopy, energy dispersive spectrometry, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis were used to characterize obtained iron (III) oxide powder. All of three powders obtained by freeze drying and annealing, have low crystallinity and particles with irregular layered shape. Narrow particle size distribution was given by an average diameter value of around 50 μm for all observed powders. Iron-bearing materials like α-Fe2O3 and γFe2O3 are obtained. Differential thermal analysis curve of obtained samples showed endothermic reaction at 620 C which could be ascribed to phase transition from cubic form γ→ αFe2O3. Thermal transformations of iron (III) oxide, obtained from clay as a natural source, is suitable to explore in the framework of materials chemistry, and opens the possibility to synthesize materials based on Fe2O3 with specific magnetic behavior.

Near Belgrade there is a huge basin of coal (surface coal mine Kolubara, Serbia) mixed with layers of clay.During exploitation of coal a huge amount of clay is deposited as mullock.This work describes a possibility to obtain iron (III) oxide from clayey material as a raw material by co-precipitation method, and then freeze dried under vacuum.This method does not require any specialized facilities.
The material made of mesoporous material, like SBA-15 and amorphous Fe 2 O 3 , opens possibility to obtain materials of specific magnetic or catalytic behavior, which will be a possible subject of future research.Magnetic behavior of the obtained materials based on Fe 2 O 3 will be also studied, first.

Experimental 2.1. Procedure
Clay as a source of iron (III) oxide was purified using thermal and chemical treatments before processing.Organic impurities have been removed from the material by heat treatment (600 °C, 2 h) in air.Clay was treated in an acidic medium of HCl (aqueous solution of 0.5 M HCl, p.a. 37 %, BDH Prolabo) (wt.% 1:10) in order to leach iron.Suspension was stirred at 60 o C for 6 h, and afterwards at room temperature for 6 day south of mixing.Iron (III) oxide, was obtained from filtrate by precipitation method using ammonium hydroxide as a precipitation agent followed by decanting liquid phase.Obtained precipitate was washed several times with distilled water until the neutral reaction.Powder was prepared by freeze drying using Modulyo Freeze Dryer System Edwards, England, consisting of freeze dryer unit at High Vacuum Pump E 2 M 8 Edwards.The precipitate was pre-frozen at -30 °C and then freeze dried for 24h.Vacuum during 24 h long freeze drying was around 4 mbar.The powders were annealed in air (1 o C/min) at 500 o C and at 900 o C, for 6 h.

Characterizations
Sandy clay from surface coal mine Kolubara, Serbia was used as a raw material.Chemical compositions of starting and thermally and chemically treated clays, obtained by inductively coupled plasma (ICP) spectrometry (Spectro-Flame, Spectro-Analytical Instruments), are listed in Tab.I [38].
Tab.I Chemical composition (percentage weight: wt%) of the starting and thermally and chemically treated clay materials [38].Samples containing iron oxide particles were characterized at room temperature by Xray powder diffraction (XRD) by using Philips PW-1050 diffractometer with Ni-filtered Cu-Kα radiation (1.54178 Å).The range of 2°-80° 2θ was used in a continuous scan mode with a scanning step size of 0.05° at scan rate of 1 o /min.The same analysis was used to characterize clay fraction smaller than 63 μm, corresponding to 37.82 % of the original mass of clay sample with (2-60 o 2θ, continuous scan mode with a scanning step size of 0.02 o at a scan rate of 2 o /min).XRD patterns of the as-received clay and treated clay, are shown in Fig. 1.Quartz (PDF No. 33-1161) is the major phase in starting and treated clays.The X-ray diffractogram (inset in Fig. 1) of oriented sample of the as-received clay showed diffraction peaks that indicate presence of some clay minerals: kaolinite, 7.291 Å (PDF:No.89-6538), mica/illite, 10.054 Å (PDF: No. 88-2198) and 2:1 clay, probably montmorillonite 6.16 Å (PDF: No. 29-1498) [35].There are some differences between the peak intensities of both, as-received and treated clay sample.An ANALYSETTE 22 Micro Tec plus was employed to determine particle size of iron (III) oxide powder.Microstructure analysis is performed by scanning electron microscope (SЕМ, VEGA TS 5130 MM, Tescan).X-ray microanalysis (EDS) was carried out with INCA PentaFET-x3, Oxford Instruments.Functional groups of iron (III) oxidewere studied by using Fourier transform infrared (FTIR) spectroscopy.Infrared spectrum of the powder sample was recorded at ambient conditions between 4000-400 cm -1 (mid-IR region) with a Nicolet IS 50 FTIR.Thermal stability of samples was investigated by simultaneous non-isothermal thermo-gravimetric analysis (TG) by using a SETARAM SETSYS Evolution 1750 instrument.The measurements were conducted at a heating rate of 10 °C/min in air atmosphere (flow rate was 16 cm /min) 3 in the temperature range of 30-1000 °C.Furnace A.D.A.M.E.L equipped with Pt-Pt/Rh thermocouples, together with the digital data acquisition computer system, has been used to obtain differential thermal analysis (DTA).

X-ray diffraction (XRD)
Chemical composition of the as-received and treated clay is mainly composed of SiO 2 and Al 2 O 3 , also oxides like Fe 2 O 3 , Na 2 O and K 2 O are present (Tab.I) [8].Fig. 2 shows XRD patterns of the freeze dried powder and powders annealed at 500 °C and at 900 o C with line broadening caused by reduced size of crystallites and low crystallinity.It was hard to identify the exact positions of diffraction peaks.[36] and Zhong and Zhang [37] the intermediate crystalline phase, such as α-or γ-Fe 2 O 3 , are often derived in the sol-gel and co-precipitate precursors when they are calcined, as in our case.There are some differences in peak intensities between the samples sintered at different temperatures (500 o C and 900 o C (Fig. 2)).These observations are in good agreement with the results obtained by TG/DTA analyses (Part 3.5., Fig. 6.).

Analysis of Particle Size
The specimen annealed at 900 o C (Fig. 3a) has slightly narrowed particle size distribution in comparison with powders obtained by freeze drying and annealed at 500 o C. Particle size distribution was given by an average diameter value of 47.9 μm for the powder obtained by the freeze drying method.For samples annealed at 500 o C and 900 o C the average particle diameters are 53.9 and 49.8 μm, respectively.All of these observations are in good agreement with results obtained by the SEM analyses (Fig. 3b).

Scanning electron microscope and energy dispersive spectrometry (SEM/EDS)
Inherent layered structure of the clay seems to be retained in both samples, obtained at 500 °C and 900 o C. According to Zboril et al. [34] in some cases, Fe 2 O 3 particles showed tendency to maintain structural or morphological features of the original compound as in our case.Energy dispersive spectrometry (EDS) analysis showed the obvious signals for Fe and O elements (Fig. 4).Other elements were detected as well; owing to the fact that iron was precipitated from natural clay (Tab.I) [38] and Cl used in the chemical treatment of clay.This is probably due to the fact that the fine fraction of clay is likely to lag behind in the preparation process, which is confirmed by XRD analysis (Fig. 2).

Fourier transformation infrared (FTIR) spectroscopy
Measurements were performed on freeze dried sample, and samples annealed at 500 o C, and at 900 o C (Fig. 5).As one can notice from FTIR, spectra of the freeze dried sample and sample annealed at 500 o C are rather similar.Bands in high wave number region, as well as bend at 1630 cm -1 , are attributed to -OH groups vibration of interlayer water molecules present in the clay [39].Vibrations at 1398 cm -1 and 1040 cm -1 indicate appearance of SiO 2 [40,41], while band at 850 cm -1 is attributed to Al-Mg-OH band vibration [39].The sample annealed at 900 o C showed absence of characteristic bands (in accordance with XRD results) pointing to thermal degradation of the clay at higher temperatures.Band at 1086 cm -1 is ascribed to Si-0-Si symmetric stretching vibration [42].Fig. 5 shows enlarged low wave number region which is characteristic for Fe-O vibrations.As ICP spectrometry showed a significantly lower content of iron (III) oxide compared to SiO 2 .The bands originated from iron oxides are weak and broad (Tab.I) [38].Shoulder at 621 cm -1 in freeze dried sample and sample annealed at 500 o C, attributed to the Fe-O bond, appeared as a consequence of decrease in crystal symmetry, characteristic for γ-Fe 2 O 3 phase [43].Spectrum of the sample annealed at 900 o C presented two bands at similar positions as in the spectra of the sample annealed at 500 o C. As XRD investigation confirmed the absence of clay at 900 o C. The observed bands in this region are attributed to Fe-O bands vibration.Band at 428 cm -1 pointed at the presence of Fe-O vibration, while the band at 540 cm -1 is characteristic for α-Fe 2 O 3 phase [40].Absence of band at 621 cm -1 (characteristic for γ-Fe 2 O 3 phase) and, simultaneously, appearance of the band at 520 cm -1 , could be indicative of phase transformation γ-→ α-Fe 2 O 3 phase.These observations are also confirmed by TG/DTA measurements (Part 3.5., Fig. 6).

Thermo-gravimetric analysis and differential thermal analysis (TG/DTA)
TG/DTA curves of the samples obtained by freeze drying and annealed at 500 o C are shown in Fig. 6.TG curve of sample obtained by freeze drying shows two-stage weight loss at the temperature ranges about of 30-150 o C and 150-540 o C (Fig. 6a).Weight loss at 30-150 o C can be attributed to evaporation of free adsorbed water is evidenced from an endothermic peak at 110 o C in the DTA curve (Fig. 6b).A gradual weight loss from 150 o C to 290 o C (Fig. 6a) is due to dehydroxylation of chemically bonded water.Two broad exothermic peaks in the DTA curve around 190 and 285 o C, also emphasize the same (Fig. 6b).These two peaks are distinctive of iron, iron oxy hydroxides and oxides.The Process of dehydroxylation of the chemically bonded water ends at around 400 o C with formation of γ-Fe 2 O 3 phase (maghemite) [44][45][46].Exothermic reaction, around 400 o C could probably be ascribed to further rearrangement of the crystal structure to cubic form of γ-Fe 2 O 3 (maghemite).The endothermic reaction at 620 o C could be ascribed to further rearrangement of crystal structure to phase transition from cubic form γ-to α-Fe 2 O 3 (trigonal).The temperature range of the endothermic peaks at 720 o C and 860 o C seems to correspond well with literature data [44][45][46].Peak from the exothermic reaction at 400 o C has lower intensity in the sample obtained at 500 o C relative to the sample obtained by freeze drying, thus indicating that the reaction of rearrangement of the crystal lattice was occurred at lower temperatures.Also, there are no endothermic reactions at 720 o C and 860 o C in the sample annealed at 500 o C.This means that the sample has already undergone thermal treatment which can be inferred from a much smaller amount of moisture and constitutional water (Fig. 6b) [44][45][46].These results are in accordance with the FTIR investigation.

Conclusions
Iron oxides were obtained from clay as a raw material by co-precipitation as a simple and inexpensive method.Benefit of this method is that it does not require any specialized facilities.Synthesized particles of iron oxides by the freeze drying and annealed at 500 o C and 900 o C have size around 50 μm.The obtained Fe 2 O 3 particles could be used as catalysts or catalyst support.45. S. Bharathi, D. Nataraj, D. Mangalaraj, Y. Masuda, K. Senthil, K. Yong, J. Phys.D: Appl.Phys., 43 (2010) 1. 46.X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, C. Tang, T. Shen, Z. Zhang, J. Mater.Chem., 14 (2004)

Fig. 2 .
Fig. 2. XRD patterns of freeze dried sample and samples, anniled at 500 o C/900 o C: Q-quartz (SiO 2 ), F-α-Fe 2 O 3 , A-σ-Al 2 O 3 .The XRD pattern of samples annealed at 500 o C, exhibited a remarkable broadening in 2θ region from 20-33 o which is associated with the presence of amorphous silica.In the case of sample annealed at 900 o C, quartz SiO 2 (PDF: No. 33-1161) and σ-Al 2 O 3 (PDF: No. 88-1609) are the main reaction products, in addition toγ-and α-Fe 2 O 3 (PDF: No. 89-8104, PDF: No. 89-5892).According to Park et al.[36] and Zhong and Zhang[37] the intermediate crystalline phase, such as α-or γ-Fe 2 O 3 , are often derived in the sol-gel and co-precipitate precursors when they are calcined, as in our case.There are some differences in peak intensities between the samples sintered at different temperatures (500 o C and 900 o C (Fig.2)).These observations are in good agreement with the results obtained by TG/DTA analyses (Part 3.5., Fig.6.).

Fig. 5 .
Fig. 5. FT-IR spectra of the freeze dried sample and samples annealed at 500 o C and 900 o C.