Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization

Fahmida Gulshan, Kiyoshi Okada

American Journal of Materials Science and Engineering OPEN ACCESSPEER-REVIEWED

Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization

Fahmida Gulshan1,, Kiyoshi Okada2

1Materials and Metallurgical Engineering Department, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh

2Materials and Structures Laboratory, AMDII Division, Tokyo Institute of Technology, Tokyo, Japan


Fe2O3-Al2O3 compounds with different Fe/Al compositions were synthesized by coprecipitation (CP) method and calcined at 300°-1000°C. The formation of crystalline phases (e.g., maghemite, hematite), transformation temperature, specific surface area, lattice parameter, crystallite size, magnetic properties of iron oxide were all affected by the component ratio and thermal treatment.

Cite this article:

  • Fahmida Gulshan, Kiyoshi Okada. Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization. American Journal of Materials Science and Engineering. Vol. 1, No. 1, 2013, pp 6-11.
  • Gulshan, Fahmida, and Kiyoshi Okada. "Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization." American Journal of Materials Science and Engineering 1.1 (2013): 6-11.
  • Gulshan, F. , & Okada, K. (2013). Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization. American Journal of Materials Science and Engineering, 1(1), 6-11.
  • Gulshan, Fahmida, and Kiyoshi Okada. "Preparation of Alumina-Iron Oxide Compounds by Coprecipitation Method and Its Characterization." American Journal of Materials Science and Engineering 1, no. 1 (2013): 6-11.

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At a glance: Figures

1. Introduction

Mixed oxides, especially binary systems, have been employed successfully in many industrial catalytic processes. The combination of two transitional metal oxides may affect their stoichiometry and their surface, electrical, catalytic, texture and thermal properties. Both Al2O3 and Fe2O3 individually catalyse many reactions. Binary catalysts with the composition Al2O3-Fe2O3 are also used for many purposes [1, 2].

At present, there are some uncertainties concerning the formation of solid solution in the Fe2O3-Al2O3 system. The lattice constants at 25°C, in terms of trigonal axes for α-Al2O3 are: a = 0.4758 and c = 1.2991nm, and those for α-Fe2O3 are a = 0.5034 and c = 1.3752nm. The differences in the a and c constants, 5.80 and 5.86%, respectively, are a consequence of different ionic radii of the metals (for coordination six): 0.054nm for Al3+ and 0.065nm for Fe3+. Brown et al [3] measured the lattice constants and recorded the Mossbauer spectra for α-Al2O3 samples containing Fe3+ ions. A linear increase of the constant a with increase of Fe2O3 up to 10.5 mol% was observed. A series of aluminium substituted hematites, α-(AlxFe1-x)2O3 with x up to 0.32, was prepared by Grave et al [4] by thermal treatment of aluminium substituted goethites, α-FeOOH, at 500°C for 6h. The samples contained particles varying in size from 20 to 100nm, as determined from X-ray diffraction line broadening. Heating of the samples up to 900°C improved the crystallinity, but reduced the maximum obtainable x to ~0.15. Mossbauer spectroscopy was applied for quantitative determination of aluminium substituted goethite-hematite mixtures by Amarasiriwardena et al [5]. The effects of crystallinity and aluminium substitution on the magnetic behaviour and Morin transition of these substituted oxides were also investigated [6, 7] and found that aluminium substituted hematite, prepared under hydrothermal conditions from coprecipitated Al(III)/Fe(III)-hydroxides at temperatures not high enough to remove OH- structural groups, exhibited a substantial deviation from the Vegard rule. The anhydrous aluminium substituted hematites, treated at 900°C, fulfilled the Vegard rule in the solid solution range (x≤ 0.15). The ball milling of equimolar mixtures of goethite and hydrated aluminas, γ-Al(OH)3 (gibbsite), α-Al(OH)3 (bayerite) or γ-AlOOH (boehmite) significantly affected the temperature of formation of solid solution of the calcined products [8].

The aim of the present study was to obtain more detailed knowledge about the formation of solid solutions in an analogous system Fe2O3-Al2O3. The effect of component ratio and thermal treatment conditions on the phase composition and structure were investigated. The samples synthesized were used to test their effectiveness for adsorption of toxic ions in waste-water followed by the magnetic separation by applying a simple magnetic field. The study of this system is of particular interest in connection with the polymorphs of its components and possible phase conversions, which can be followed by X-ray diffraction methods for both components and magnetochemistry of phases containing iron.

2. Experimental

2.1. Synthesis Procedures of Fe2O3-Al2O3 Compound by Coprecipitation Method

The coprecipitation process involves dissolving a salt precursor, usually a chloride, oxychloride or nitrate. The corresponding metal hydroxides precipitate in water upon addition of a base solution such as sodium hydroxide or ammonium hydroxide solution. The resulting salts are then washed and calcined to obtain the final oxide powder. This method is useful in preparing ceramic composites of different oxides by coprecipitation of the corresponding hydroxides in the same solution. One disadvantage of this method resides in the difficulty in controlling the particle size and size distribution.

In this study, ferric nitrate Fe(NO3)3.9H20 and aluminium nitrate Al(NO3)3.9H20 were used as precursors of Fe2O3 and Al2O3 to prepare Fe2O3-Al2O3 compound. Coprecipitated gels were prepared with various ratios of 0.2M Fe(NO3)3•9H2O and Al(NO3)3•9H2O solutions using conc. NH4OH (25 mass%) as the precipitant. Samples containing 0, 20, 40, 60, 80 and 100 mol% Fe2O3 were prepared. For example, to prepare 20 mol% Fe2O3 sample, 4.04g Fe(NO3)3•9H2O was dissolved in 50 ml of distilled water and 15.01g Al(NO3)3•9H2O was dissolved in 200ml of distilled water. Then, these two solutions were mixed with mechanical stirring and 200ml of NH4OH solution was added with continuous stirring. They were dried in an evaporator at 60°C for 4h and in an oven at 110°C for 2 days.

2.2. High Energy Ball Milling

After drying, the powders were dry-ground in a planetary ball mill (LAPO-1, Ito Seisakusho Ltd., Japan) using a Si3N4 pot with 30 Si3N4 balls (5 mm) at 300rpm for 3 hours with a ball: sample mass ratio of 30:1. In this process, known as ‘break down’ or ‘top-down’ approach, high mechanical grinding energy is applied to produce nanoparticles.

2.3. Calcination

After grinding, the samples were calcined at 300°-1000°C in air for 5 hours at a heating rate of 10°C /min. The flowchart for the preparation of the sample is shown in Figure 1.

2.4. Characterization of Samples

The crystalline phases in the samples were determined by x-ray diffraction analysis (XRD-6100, Shimadzu) using monochromated CuKα radiation. The lattice parameters of the resulting crystalline phases were calculated by least-squares method using Si powder as the internal standard. The crystallite sizes of the crystalline phases were calculated using the Scherrer equation for the (104) reflection of α-Fe2O3 and α-Al2O3, and the (400) of γ-Al2O3. The instrumental broadening was corrected using the peak widths measured for 100 mol% Fe2O3 sample heated at 1300°C. The specific surface area (SBET) was obtained by the method using N2 as the adsorbate (Autosorb-1, Quantachrome). The magnetic properties were examined using vibrating sample magnetization (VSM) equipment (BHV-50H, Riken Electronics).

Figure 1. Flowchart for the preparation of Al2O3-Fe2O3 compounds by coprecipitation method

3. Results and Discussion

3.1. Characterization of Samples

The x-ray diffraction patterns of the samples calcined at various temperatures are shown in Figure 2 (a-e). From these figures it is clear that the as-synthesized samples were amorphous, but crystallized at 500-700°C in all samples. The crystallization temperature increased with increasing Al2O3 content in the Fe-rich compositions, being highest at about 60 mol% Fe2O3 composition. In the Fe-rich compositions, the crystalline phase was α-Fe2O3 and was thought to incorporate Al2O3. The increase of iron content in the CP samples is associated with an increase of intensities of the α-Fe2O3 reflections, therefore with higher crystallinity of the phase. In the Al-rich compositions, the crystalline phases changed from γ-Al2O3 to θ-Al2O3 and then to α-Al2O3 at higher temperatures

In the 20 mol% Fe2O3 sample (Figure 3), the only detectable phase at temperature 600°C is γ-Al2O3, located at 2θ= 36.95, 45.46, 66.36o. At temperature 800°C, the XRD pattern shows the appearance of new XRD reflections at 2θ = 24.15, 33.25, 35.72, 49.65, 54.30, 62.68 and 64.280 are attributed to α-Fe2O3. The intensity of the latter XRD reflections strongly increased with the increase of calcination temperature indicating the enhancement of crystallinity of α-Fe2O3. A phase transition of γ-Al2O3 to α-Al2O3 is clearly observed upon elevating the calcination temperatures to 1000°C.

A field map of the product phases as functions of composition and heating temperature is shown in Figure 4. From this field map, it can be seen that the formation of α-Fe2O3 takes place at 600°C in 80 mol% Fe2O3 samples and 700°C in 60 mol% Fe2O3 whereas α-Fe2O3 is formed at 500°C in 90 mol% Fe2O3 sample. The crystallization of α-Fe2O3 shifted gradually to higher temperature by increasing the alumina content, indicating the effect of alumina.

Figure 2(a). The XRD pattern of the samples heated at 500°C for different Fe/(Fe+Al) ratio. Key: G = γ-Al2O3, H = α-Fe2O3
Figure 2(b). The XRD pattern of the samples heated at 600°C for different Fe/(Fe+Al) ratio. Key: G = γ-Al2O3, H = α-Fe2O3
Figure 2(c). The XRD pattern of the samples heated at 700°C for different Fe/(Fe+Al) ratio. Key: G = γ-Al2O3, H = α-Fe2O3
Figure 2(d). The XRD pattern of the samples heated at 800°C for different Fe/(Fe+Al) ratio. Key: G = γ-Al2O3, H = α-Fe2O3
Figure 2(e). The XRD pattern of the samples heated at 1000°C for different Fe/(Fe+Al) ratio. Key: C = α-Al2O3, H = α-Fe2O3,T=θ-Al2O3
Figure 3. The XRD patterns of the 20 mol% Fe2O3 CP samples heated at different calcination temperatures. Key: C = α-Al2O3, G = γ-Al2O3, H = α-Fe2O3
Figure 4. A field map of the phases formed in the samples as functions of composition and calcined temperature

Table 1. Lattice constants (nm) of α-Fe2O3 in the various samples

The lattice parameters of the Fe-rich samples (Table 1) decreased with decreasing Fe2O3 content up to 40 mol% Fe2O3 in samples heated at ≤800°C. The amount of Al2O3 estimated from the resulting lattice parameters was about 20 mol% in these samples. By contrast, the lattice parameters of the samples heated at 1000°C decreased in samples containing up to 90 mol% but became constant in the samples containing ≤90 mol% Fe2O3. The limit of Al2O3 incorporation in these samples is found to be ≤10 mol%, in fair agreement with Pownceby et al. [9]. Thus, the resulting data observed at ≤800°C appear to correspond to a transient state that attains equilibrium on heating at ≥1000°C.

The lattice parameters determined for Al-rich samples suggest that 10-20 mol% of Fe2O3 is incorporated in the γ-Al2O3 and α-Al2O3 phases.

Thus, the crystalline phases in the samples of intermediate compositions are solid solutions of Fe2O3 and Al2O3. The incorporation of Al or Fe ions in these phases causes broadening of their XRD reflections, suggesting smaller crystallite sizes. The crystallite sizes of the samples [showed in Figure 5] were estimated using the Scherrer’s equation, D= Kλ/βcosθ, where D is the crystallite size, λ is the X-ray wavelength (0.15418nm), K is a numerical constant (0.9), β is the full width at half maximum for the reflection and θ is the Bragg angle [10].The crystallite sizes of the α-Fe2O3 in 100 mol% Fe2O3 samples varied from 73nm at 500°C to 460nm at 1000°C while those of the 40 mol% Fe2O3 samples ranged from 9.1nm at 500°C to 140nm at 1000°C. Thus, the crystallite sizes of the pure Fe2O3 samples are several times larger than those of the Al2O3-containing samples. The crystallite sizes of the α-Al2O3 samples decreased from 150nm in the 20 mol% Fe2O3 sample to 71nm in the 40 mol% Fe2O3 sample. The crystallite sizes of γ-Al2O3 also decreased with higher Fe2O3 content, from 5.8nm at 500°C to 6.1nm at 800°C in the 0 mol% Fe2O3 and from 3.4nm at 500°C to 5.3nm at 800°C in the 40 mol% Fe2O3 sample.

3.2. Magnetic Properties

The magnetic properties of the samples were measured to determine their potential usefulness for magnetic separation after adsorption. The magnetization was determined as a function of magnetic field at room temperature by VSM. It employs an electromagnet which provides the magnetizing field (DC), a vibrator mechanism to vibrate the sample in the magnetic field, and detection coils which generate the signal voltage due to the changing flux emanating from the vibrating sample. The output measurement displays the magnetic moment (M) as a function of the magnetic field (H). This applied magnetic field was varied from -15000 Oe to 15000 Oe. All the samples showed very weak saturation magnetization, <1emu/g.

Figure 5. Crystallite size vs. temperature as a function of Fe/(Fe+Al)

The samples containing α-Fe2O3 solid solutions showed very weak hysteresis loops in the magnetization curves [Figure 6] corresponding to the antiferromagnetic property of α-Fe2O3 and the magnetization was insufficient to attract these powders to a domestic magnet.

Figure 6. Magnetization curves of 80 mol% Fe2O3 CP sample calcined at 800°C [blue line] and that at 1000°C [pink line]
3.3. Specific Surface Area

Changes in the SBET values of the samples of various compositions [values listed in Table 2] are shown in Figure 7 as a function of heating temperature and Fe2O3 content, together with the values reported by Li et al. [11]. The SBET values differed greatly, ranging from 330m2/g in the sample with 0 mol% Fe2O3 heated at 500°C to 3m2/g in the sample containing 100 mol% Fe2O3 heated at 800°C. The SBET values decreased slightly with higher heating temperature to 1000°C. On the other hand, the effect of the Fe2O3 content on SBET was different in the samples with Fe2O3≤80 mol% and Fe2O3=100 mol%. The SBET values were almost constant or slightly less, up to Fe2O3≤80 mol% contents, but decreased very steeply from 80 to 100 mol%. Figure 6 suggests that mixtures of crystalline phases are effective in maintaining high SBET values. The SBET values of the present samples (solid lines) are higher than those reported by Li et al. [11] (dashed lines).

Table 2. Specific surface area values [m2/g]

Figure 7. Changes in the SBET values of the samples of various compositions as a function of heating temperature and Fe2O3 content, together with the values reported by Li et al

4. Conclusion

The as-synthesized samples were amorphous but crystallized on heating at 500-700°C. The crystalline phases formed were only α-Fe2O3 in the Fe-rich compositions, while they ranged from γ-Al2O3 to θ-Al2O3 to α-Al2O3 with increasing heating temperature in the Al-rich compositions. About 10-20mol% of Al2O3 was incorporated in α-Fe2O3 and a similar amount of Fe2O3 was incorporated in γ-Al2O3 and α-Al2O3. The crystallite sizes decreased significantly by incorporation of these ions. The SBET values differed greatly from 330m2/g to 3m2/g . The samples showed very weak magnetization.


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