Chemical and physical properties of Al1−xFexPO4 alloys

Chemical and physical properties of Al1−xFexPO4 alloys

Materials Chemistry and Physics 83 (2004) 250–254 Chemical and physical properties of Al1−x Fex PO4 alloys Part I. Thermal stability, magnetic proper...

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Materials Chemistry and Physics 83 (2004) 250–254

Chemical and physical properties of Al1−x Fex PO4 alloys Part I. Thermal stability, magnetic properties and related electrical conductivity M.I. Youssif a , F.Sh. Mohamed b,∗ , M.S. Aziz a a b

Physics Department, Faculty of Science-Demiatta, Mansoura University, New Demiatta, Egypt Chemistry Department, Faculty of Science-Demiatta, Mansoura University, New Demiatta, Egypt

Received 15 October 2002; received in revised form 10 September 2003; accepted 22 September 2003

Abstract A series of samples of the nominal compositions Al1−x Fex PO4 with 0 ≤ x ≤ 0.8 calcined at various temperatures have been synthesized by the co-precipitation method. The structural changes during calcination were investigated by using X-ray powder diffraction and differential thermal analysis. The electrical conductivity and the magnetic properties were studied. Detailed X-ray studies showed that the solid solution formation between FePO4 and AlPO4 , which are not isostructural, takes place by incorporation of Fe3+ ions into the AlPO4 structure for Al0.8 Fe0.2 PO4 and Al0.6 Fe0.4 PO4 . This solid solution formation is accompanied with a phase transformation of AlPO4 from a tridymite to an orthorhombic phase. In Al0.2 Fe0.8 PO4 , the incorporation of AlPO4 into the FePO4 lattice occurred at 800 ◦ C. The electrical properties of uncalcined samples exhibit a metallic behavior. A semiconductor trend has been revealed when the samples were calcined at lower temperatures (up to 600 ◦ C). The magnetic behavior of all uncalcined samples shows a paramagnetic trend. Samples calcined at temperatures ≥600 ◦ C exhibit a ferromagnetic behavior at rich FePO4 content. © 2003 Published by Elsevier B.V. Keywords: Al1−x Fex PO4 alloys; Crystal structure; Thermal stability; Magnetic properties; Electrical conductivity

1. Introduction Iron phosphate catalysts show a unique selectivity for several oxidative dehydrogenation reactions, such as formation of pyruvic acid from lactic acid [1–3], glyoxylic acid from glycolic acid [4], pyruvaldehyde (2-oxopropanal) from hydroxyacetone (acetol) [5], glyoxal from ethylene glycol [6] and methacrylic acid form isobutyric [7]. The iron phosphates tend to be unstable under reaction conditions, and generally convert to the quartz-like FePO4 and other phases, depending on stoichiometry [8,9]. Also the amorphous aluminum phosphate, AlPO4 , has been found application as a solid catalysts in several areas such as alkylation, isomerization, dehydration and catalytic cracking due to their relatively large surface area and thermal stability over broad ranges of temperature and composition [10,11]. The catalytic activity of AlPO4 varies significantly with the method of preparation, thermal treatment, activation temperature, the presence of doping ions and the metal oxide/AlPO4 weight ratio [12–14]. ∗ Corresponding author. Tel.: +20-57-403980; fax: +20-57-403866. E-mail address: [email protected] (F.Sh. Mohamed).

0254-0584/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2003.09.025

Direct-current electrical conductivity, thermal conductivity and Hall effect are some of the common electrontransport properties of solids that characterize the nature of charge carriers. The electronic theory of catalysis of metal oxides is based on the idea of chemisorption of gases on semiconductor oxides is associated with electron transfer [15]. The electronic factor in catalysis refers to the correlation between the bulk electronic properties of the solid and their catalytic activity [16–19]. Magnetic studies of solid catalysts have been used to reveal its structure and to discuss its catalytic behavior. The magnetic behavior often gives evidence for the oxidation state of catalytically active solids that contain elements of the transition series. Magnetic susceptibility measurements were used to evaluate the effective dispersion of an active catalyst component and the thermomagnetic analysis was used to reveal the identity of a ferromagnetic catalyst component [20]. In this paper, we have synthesized different compositions of iron/aluminum phosphate by the co-precipitation method, to improve the thermal stability and hence the catalytic activity and selectivity of iron phosphate. The crystal structures were determined at different temperatures by the

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2. Experimental 2.1. Sample preparation A series of the samples of the nominal compositions Al1−x Fex PO4 with x = 0.0, 0.2, 0.4, 0.6 and 0.8 were prepared by mixing the solutions of AlCl3 ·6H2 O, FeCl3 ·6H2 O and 85% H3 PO4 with a concentrated ammonia solution, following the co-precipitation method. Appropriate amounts of aluminum chloride and/or iron chloride were dissolved in distilled water followed by adding an equivalent amount of phosphoric acid (85%) with vogues stirring at room temperature. Finally, the gel solution was obtained by adding concentrated ammonia solutions until pH 6.5. This was followed by filtration, washing and drying at 120 ◦ C for 12 h. The resulting solids were calcined separately at different temperatures for the same period of time, as described in the text. 2.2. Characterization Differential thermal analysis (DTA) studies of the gel samples were carried out using a Shimadzu thermal analyzer (DTA 50) at a heating rate of 1 K min−1 . The inert reference was ␣-Al2 O3 . The structural properties of all samples were performed at room temperature by X-ray powder diffraction in the angle range 2θ = 10–70◦ using an X’Pert Philips diffractometer with Cu K␣ radiation (wavelength λ = 1.54056 Å) with a graphite monochromator. The electrical conductivity of all samples was achieved in the temperature range 300–600 K. The samples were in pellet form with 13 mm2 area and 3 mm thick. These pellets were hold between nickel electrodes with silver paint in between. The holder was suspended in a glass jacket similar to that used by Jacobs [21]. The cell was evacuated to a pressure of 3 × 10−5 mbar for 24 h using a high-vacuum system, to ensure the complete removal of water. The temperature of the sample was measured with an accuracy of ±0.1 K with the help of a Ni–NiCr thermocouple. The conductivity of all samples was determined by measuring the resistance (R) of the sample using a Keithly programmable electrometer, following a constant current method. This arrangement can be used for measuring the resistance as low as 0.1 k and as high as 200 G with an accuracy of ±0.2%. The conductivity was achieved in case of cooling using the general formula: σ =d/RA, where A is the cross-sectional area of the sample having a thickness d. The formal activation energy E and log σ 0 were calculated from the data by least-square fitting to the equation: σ = σ0 e− E/kT ,

where k is the Boltzmann’s constant, T the absolute temperature, and σ 0 the pre-exponential factor. The magnetic susceptibility of the samples was obtained by measuring the magnetic moment per unit volume induced in the sample by an applied magnetic field, H, following the Gouy method [22]. The magnetic field was generated using an Oxford electromagnet 5200. The strength used varied from about 5–20 G. For all measurements, the samples were in the powder form. A glass tube of 8 mm external diameter and 12 cm long was used for suspensions. The specimen was suspended so that one end was in the maximum field H, while the other end was in a region where the field was negligible. On application of the field, the sample experiences a force (F) that is (1/2)χAH2 , where χ is the susceptibility per unit volume. An electronic balance (ER-120A) measured the force on the tube, and the results have been corrected for the diamagnetism of the glass.

3. Results and discussion 3.1. Structural properties The XRD studies of AlPO4 samples calcined at various temperatures revealed that the structure remained in the amorphous form up to a calcination temperature of 1000 ◦ C, Fig. 1a. On calcination at around 1100 ◦ C, a tridymite form having a pseudo-hexagonal structure was observed. No decomposition of AlPO4 into Al2 O3 and P2 O5 was found during the calcination as shown in Fig. 1b. Fig. 2a–c shows the diffraction of Al0.8 Fe0.2 PO4 calcined at 600, 800 and 1000 ◦ C, respectively. From these figures, we can conclude that the structure remains in its amorphous phase up to the calcination temperature of ≈600 ◦ C, Fig. 2a. By rising up the calcination temperature to 800 ◦ C, the XRD pattern (Fig. 2b) shows only diffraction lines corresponding to the tridymite phase of AlPO4 , and there is no indication of FePO4 as a separated phase. The phase transformation of the tridymite form to the orthorhombic structure is observed upon calcination of this sample to 1000 ◦ C, as revealed by XRD in Fig. 2c. Based on this result, we can conclude that the temperature

Relative intensity

X-ray diffraction (XRD). In addition, the electrical conductivity and the magnetic behavior of all samples have been investigated.

251

(b) (a)

10

20

30

40

50

60

70

2 Θ (deg) Fig. 1. XRD pattern of AlPO4 calcined at (a) 900 ◦ C and (b) 1100 ◦ C.

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Relative intensity

252

(c)

(b) (a) 10

20

30

40

50

60

70

2 Θ (deg) Fig. 2. XRD pattern of Al0.8 Fe0.2 PO4 calcined at (a) 600 ◦ C, (b) 800 ◦ C, and (c) 1000 ◦ C.

Relative intensity

of the phase transformation of AlPO4 is affected by the solid solution formation of FePO4 . In addition, the decomposition of FePO4 has not been taking place at this temperature, due to the incorporation of FePO4 in the AlPO4 structure. The same behavior has been found for the Al0.6 Fe0.4 PO4 compound. The calculated lattice parameters of the orthorhombic phase in this mixed phosphate are a = 7.6170 Å, b = 8.5042 Å, c = 7.6738 Å with volume V = 497.0816 Å3 , compared [23] to orthorhombic AlPO4 with a = 7.082 Å, b = 7.098 Å, c = 6.933 Å, and V = 306.298 Å3 . This increase in the lattice parameters could be explained in terms of the higher ionic radius of Fe3+ (Fe3+ : 0.64 Å and Al3+ : 0.54 Å). Fig. 3 reveals the structural behavior of rich iron phosphate Al0.2 Fe0.8 PO4 as a function of calcination temperature. In Fig. 3a, a poor crystalline structure has been observed for the sample calcined at 400 ◦ C. With raising up the calcination temperature to around 600 ◦ C, FePO4 has partially decomposed to Fe3 PO4 while the tridymite phase of AlPO4 remains as a separate phase, as shown in Fig. 3b. A complete incorporation of AlPO4 into the FePO4 lattice has occurred at 800 ◦ C with a trigonal crystal structure as shown in Fig. 3c, due to the formation of a solid solution between the two phosphates. Moreover, the decomposition of FePO4 to Fe3 PO7 and ␣-Fe2 O3 has been observed. The

Fig. 4. DTA curves for Al1−x Fex PO4 , where (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, and (e) x = 0.8.

calculated unit cell parameters for the trigonal structure are a = 5.010 Å and c = 11.130 Å. The same behavior of a sample of the chemical formula Al0.4 Fe0.6 PO4 has been found. Based on these results, we can concluded that the increase in both of thermal stability and phase transition temperature of iron/aluminum mixed phosphate compared to pure iron phosphate [8,9] is due to the solid solution and the incorporation between the two phosphates. The DTA data support the idea that the species present in the uncalcined samples of Al1−x Fex PO4 (where x = 0.0, 0.2, 0.4, 0.6, 0.8) and the calcined samples are different. The DTA curves of uncalcined Al1−x Fex PO4 are shown in Fig. 4. The endothermic and exothermic peaks are shown downward and upward to the base line, respectively. Based on these results, one can conclude that the endothermic effect for all of the samples with a minimum around 100 ◦ C could be attributed to the loss of physically adsorbed water molecules. These endothermic peaks are shifted to lower temperatures, and their intensity increases with increasing iron phosphate content. The strong exothermic effect shown in Fig. 4d for Al0.4 Fe0.6 PO4 with a maximum at ≈600 ◦ C has been shifted to 500 ◦ C. This behavior can be understood as a consequence to the processes of phase transition and sample crystallization. As a part from the above described effects, the DTA curves reveal another endothermic peak with close lying minima above 700 ◦ C, which are related to the partial decomposition of the FePO4 contents in A10.4 Fe0.6 PO4 and A10.2 Fe0.8 PO4 into Fe3 PO7 and ␣-Fe2 O3 as indicated by a small loss of their masses. The behavior of pure AlPO4 in Fig. 4a shows only endothermic peak at ≈100 ◦ C corresponding to the removal of adsorbed water. This has been confirmed by XRD (Fig. 1a).

(c)

3.2. Conductivity (b) (a) 10

20

30

40

50

60

70

2 Θ (deg)

Fig. 3. XRD pattern of Al0.2 Fe0.8 PO4 calcined at (a) 400 ◦ C, (b) 600 ◦ C, and (c) 800 ◦ C.

The electrical conductivity of all samples was determined by measuring the resistance of the sample following a constant current method. The Arrhenius plot of the electrical conductivity of uncalcined Al1−x Fex PO4 samples over the temperature range 300–600 K is shown in Fig. 5. This figure shows a shift in the critical temperature (TC ) to the lower temperature with increasing FePO4 content in

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Table 1 Physical properties of uncalcined Al1−x Fex PO4 samples measured at 308 K

470 416 400 381 333

T < TC

T > TC

6.9 17.9 23.0 27.6 51.8

−96.6 −22.5 −80.5 −138.0 −103.0

the compound. A strong dependence of the conductivity on temperature is observed. At T < TC , all samples show a semiconductor behavior, i.e., the conductivity increases with increasing temperature. At temperatures T > TC , a metallic trend is characterizes the behavior of the uncalcined samples with the conductivities as the temperature increases. Table 1 summarizes the calculated values of the conductivity at 308 K, the critical temperature, the activation energies, the number of charge carrier, and the mobility, µ, for the uncalcined Al1−x Fex PO4 samples. It is clear that the conductivity increases with increasing x, typically from 2.2 × 10−7 to 7.5 × 10−5 ( m)−1 for Al0.8 Fe0.2 PO4 and Al0.2 Fe0.8 PO4 , respectively. For T < TC and T > TC , the activation energies calculated from the slopes of the ln σ vs. 1/T plots show different signs, which could be attributed to the change of the conduction mechanism [24]. The ln σ vs. 1/T plots for Al1−x Fex PO4 samples calcined at 600 ◦ C is shown in Fig. 6. Surprisingly, the critical temperature TC is almost constant (≈400 K). This behavior can be understood as a consequence to the incorporation of the two phosphates. For 0 ≤ x ≤ 0.6 of the calcined samples the conduction mechanism is characterized by metallic behavior, i.e., decreasing conductivity with increasing temperature. For x = 0.8, a semiconductor trend has been found for T > 450 K. This behavior could be explained in terms of the phase transformation of iron phosphate from the tridymite to the trigonal structure confirmed by XRD. Table 2 summarizes the calculated values of σ, TC , E, N and µ for Al1−x Fex PO4 samples calcined at 600 ◦ C.

2.23 2.70 1.60 4.50 4.66

× × × × ×

µ (m2 (V s)−1 )

0.336 × 103 1.600 × 103 0.722 × 103 2.22 × 105 1.005 × 108

1010 109 109 108 106

x= 0.8

-8 -1

1.20 × 2.20 × 10−7 1.85 × 10−7 1.6 × 10−5 7.5 × 10−5

N (m−3 )

-12

(Ωm)

10−6

0.0 0.2 0.4 0.6 0.8

E (×10−2 eV)

TC (K)

0.6 -16

ln σ

σ ( m)−1

x

0.0

0.2 0.4

-20 20

24

28

32

4

10 / T (1/K)

Fig. 6. Arrhenius plot of the electrical conductivity of Al1−x Fex PO4 samples calcined at 600 ◦ C. Table 2 Physical properties measured at 308 K for Al1−x Fex PO4 samples calcined at 600 ◦ C x

σ ( m)−1

0.0 0.2 0.4 0.6 0.8

1.05 2.38 4.13 2.20 3.30

× × × × ×

10−9 10−9 10−8 10−6 10−4

TC (K)

E (eV), T > TC

N (m−3 )

400 339 333 400 333

1.55 0.20 0.52 0.52 0.48

1.63 1.88 4.50 4.50 9.50

× × × × ×

µ (m2 (V s)−1 )

109 109 106 106 106

1.66 × 104 3.90 × 104 4.80 × 104 3.00 × 106 2.1 × 108

3.3. Magnetic properties The magnetic susceptibility is obtained by measuring the magnetic moment per unit volume induced in the sample by an applied magnetic field, following the Gouy method [22]. Fig. 7 shows the absolute paramagnetic susceptibility of Al1−x Fex PO4 where 0 ≤ x ≤ 0.8 at room temperature for uncalcined sample and for samples calcined in air for 6 h at

TC

-1

ln σ (Ωm)

Susceptibility χ (SI)

x= 0.8 0.6

-8

-12 0.0 0.2 0.4

-16

20

24

28

32

36

4

10 / T (1/K)

Fig. 5. Arrhenius plot of the electrical conductivity of Al1−x Fex PO4 uncalcined samples.

(b)

20 15 10

(a)

5 0 0.0

0.2 0.4 0.6 Fe content (mole)

0.8

Fig. 7. The magnetic susceptibility of Al1−x Fex PO4 as a function of Fe content: (a) uncalcined and (b) calcined at 600 ◦ C.

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600 ◦ C. This figure shows that the volume susceptibility of uncalcined samples increases linearly with x, this indicates that no crystalline phase has been formed and the apparent susceptibility is due to the mixed phosphate compound. This behavior has been confirmed by the amorphous structure found in the XRD pattern (Fig. 2a). On the other hand, samples calcined in air at 600 ◦ C for 6 h were found to behave totally different. When 0 ≤ x ≤ 0.4, the susceptibility increases linearly with the Fe content and the susceptibility is a little bit higher than that of the uncalcined samples. This could be interpreted in same way that the decomposition of FePO4 has not been taken place at this temperature, due to the incorporation of FePO4 in the AlPO4 structure (Fig. 2a). Surprisingly, a sudden increase of the magnetic susceptibility is found at x = 0.6, and reaches a saturation value at x = 0.8. This could be due to the decomposition of FePO4 to different phases such as Fe3 PO7 and ␣-Fe2 O3 . These samples show also a crystalline phase transition at this temperature, as confirmed by XRD (Fig. 3).

References [1] [2] [3] [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14] [15]

4. Conclusions The XRD studies showed that the solid solution formation between FePO4 and AlPO4 takes place by the incorporation of Fe3+ ions into the AlPO4 structure for x < 0.5. For x > 0.5 the incorporation of Al3+ ions into FePO4 takes place. The electrical conductivity of all samples shows a strong dependence on temperature. The uncalcined samples exhibit a metallic behavior. A semiconductor trend has been revealed for the calcined samples. The magnetic behavior of all uncalcined samples shows a paramagnetic trend. Samples calcined at temperatures ≥600 ◦ C exhibit a ferromagnetic behavior at rich FePO4 content.

[16] [17] [18] [19]

[20] [21] [22] [23] [24]

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