Effect of Fe₂O₃ precursors on physicochemical and catalytic properties of CuO/Fe₂O₃ system

2.1 Material and methods

Iron hydroxides were precipitated from both of iron nitrate Fe(NO₃)₃·9H₂O (1 M) solution and iron sulfate Fe₂(SO₄)₃ (1 M) solution using ammonia solution (0.2 M) at 70 °C and pH = 8. The precipitates were carefully washed with bi-distilled water till free from ammonium and nitrate or sulfate ions, then filtered and dried at 110 °C till constant weight. The formula of the prepared hydroxide precipitated from iron nitrate can be expressed as Fe₂O₃·1.5H₂O while that from iron sulfate can be expressed as Fe₂O₃·3H₂O (Pailhé et al., 2008). The catalysts were calcined in air at 400 °C for 4 h.

Nine specimens of CuO/Fe₂O₃ catalysts were prepared by wet impregnation method. A known mass of iron hydroxide was impregnated with solutions containing different amounts of copper nitrate dissolved in the least amount of bi-distilled water sufficient to make pastes which were dried at 110 °C followed by calcination at 400 °C for 4 h. The nominal compositions of the impregnated catalysts from nitrate precursor were 0.05, 0.10, 0.15, 0.20CuO/Fe₂O_3, which correspond to 2.42, 4.73, 6.94, and 9.04 wt% CuO, respectively. The nominal compositions of the impregnated catalysts from sulfate precursor were 0.025, 0.035, 0.050, 0.075, 0.100CuO/Fe₂O₃, which correspond to 1.23, 1.71, 2.42, 3.59, and 4.73 wt% CuO, respectively.

The all catalysts prepared from nitrate and sulfate precursors were nominated as XCuFeO_n and XCuFeO_s (X is the CuO concentration), respectively. All chemicals used in the experiment were of analytical grade.

2.2 Characterization techniques

Thermogravimetric (TG), differential thermogravimetric (DTG), and differential thermal analysis (DTA) curves of the catalysts were obtained using Shimadzu TGA/DTA-50H thermogravimetric analyzer, the rate of heating was kept at 10 °C/min, and the flow rate of nitrogen was 30 ml/min.

X-ray diffractograms of various prepared solids were determined a Brucker diffractometer (Brucker Axs D8 Advance Germany). The patterns were run with CuKα₁ with secondly monochromator (λ = 0.15404 Å) at 40 kV and 40 m A at a scanning rate of 2° in 2θ/min.

The infrared transmission spectra of various solids were determined by FT-IR spectroscopy (Jasco FT-IR 4100 spectrometer (Japan)) employing samples prepared as KBr disks. The IR spectra were measured from 400 to 4000 cm⁻¹. The most important and characteristic absorption bands thus determined were compared with those reported in the literature.

The specific surface area (S_BET) of the prepared catalysts were determined from nitrogen adsorption isotherms measured at −196 °C using a Quantachrome NOVA 2000 automated gas-sorption apparatus model 7.11. All catalysts were degassed at 200 °C for 2 h under a reduced pressure of 10⁻⁵ Torr before undertaking such measurements.

The catalytic activities of the various catalysts were measured by studying the decomposition of H₂O₂ in their presence at 30 °C using 100 mg of a given catalyst sample with 0.5 ml of H₂O₂ of known concentration diluted to 20 ml with distilled water. The reaction kinetics was monitored by measuring the volume of O₂ liberated at different time intervals until no further oxygen was liberated.

3.1 Thermal properties

TG/DTG and DTA-curves of uncalcined 0.05CuFeO_n and 0.05CuFeO_s catalysts are shown in Figs. 1(A, B) and 2(A, B), respectively, and the data are listed in Table 1. It is shown that (i) the TG-curves of 0.05CuFeO_n and 0.05CuFeO_s catalysts were the same and consist of three stages. The first and second steps characterized the departure of physisorbed water and some water of crystallization of iron hydroxide with a complete thermal decomposition of copper nitrate into copper oxide CuO (Deraz, 2008). The last step indicated the complete decomposition of iron hydroxide into iron oxide Fe₂O₃. (ii) Three endothermic peaks were obtained from the DTA-curves of the 0.05CuFeO_n and 0.05CuFeO_s catalysts which correspond to the removal of water of crystallization, decomposition of copper nitrate and iron hydroxide, respectively. Two exothermic peaks were also obtained from DTA-curves, due to the formation of copper ferrite. (iii) It was clear that the thermal evolution of the sulfate precursor toward the formation of different oxides (copper, iron) and spinel phase (CuFe₂O₄) was slower than the nitrate precursor (Martín de Vidalesa et al., 1999). (iv) The difference between the total practical weight loss and theoretical value is attributed to the removal of surface hydroxyl groups which need thermal treatment ⩾ 800 °C (Cao et al., 2008).

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Figure 1

Thermograms of uncalcined 0.05CuFeO_n catalysts: (A) TG/DTG and (B) DTA.

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Figure 2

Thermograms of uncalcined 0.05CuFeO_s catalysts: (A) TG/DTG and (B) DTA.

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3.2 Crystal structure and surface properties

3.2.1

3.2.1 XRD analysis

The XRD patterns of 0.05CuFeO_n, 0.20CuFeO_n, 0.05CuFeO_s, and 0.10CuFeO_s catalysts being calcined at 400 °C were determined and illustrated in Fig. 3. It is seen that all of the detectable peaks at 2θ = 33.1, 41.1, 49.5, 54.1, and 64° can be indexed to the Fe₂O₃ hematite phase (JCPDS 33-0664). Also, another peaks at 2θ = 35.6 and 62.6° can be indexed to the CuFe₂O₄ phase (JCPDS 34-0425). No CuO phase was observed in the XRD patterns of the investigated catalysts due to diffusion of CuO into the matrix of α-Fe₂O₃ forming copper ferrite CuFe₂O₄ phase, monolayer dispersion of CuO on the surface of α-Fe₂O₃ and/or complete amorphization of CuO (Jiang et al., 2005).

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Figure 3

XRD patterns of CuFeO catalysts prepared from different Fe₂O₃ precursors calcined at 400 °C. Lines (1) refer to α-Fe₂O₃ and lines (2) refer to CuFe₂O₄ phases.

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The effects of changing the Fe₂O₃ precursors on the degree of ordering and the crystallite sizes of different phases in the investigated catalysts were emerged from the data listed in Table 2. Inspection of Table 2 shows the following: (i) the diffractograms of CuFeO_n and CuFeO_s consist of Fe₂O₃ and CuFe₂O₄ with a moderate degree of ordering. (ii) Increasing of CuO loading on Fe₂O_3n was accompanied by decreasing the degree of ordering and the crystallite sizes of hematite phase while increasing in the case of copper ferrite phase. (iii) Increasing of CuO loading on Fe₂O_3s was accompanied by increasing the degree of ordering and the crystallite sizes of hematite and copper ferrite phases. The smallest degree of ordering was found in CuFeO_s (sulfate precursor). All phases present in the investigated catalysts have nano-crystallite sizes ⩽ 24 nm. (iv) The degree of ordering and the crystallite sizes of Fe₂O₃ phase in 0.05CuFeO_n were higher than those of 0.05CuFeO_s catalysts.

Table 2 Intensity counts of the main diffraction lines, crystallite size of various phases and S_BET for the investigated catalysts calcined at 400 °C.

Catalyst	Intensity count (a.u.)		Crystallite size (nm)		SBET (m2/g)
	Fe2O3	CuFe2O4	Fe2O3	CuFe2O4
Fe2O3n	–	–	–	–	27.5
0.05CuFeOn	8.7	7.9	12.4	16.3	84.6
0.20CuFeOn	8.5	8.5	10.6	23.7	71.2
0.05CuFeOs	4.6	4	6.9	24.3	52.3
0.10CuFeOs	5.6	5.5	7.1	18.3	–

–: not measured.

The explanation of different behavior of 0.05CuFeO_s with comparison to 0.05CuFeO_n catalysts may be due to the difference in the thermal properties as shown in (TG-DTG-DTA) analysis section. This could be attributed to the lower thermal stability of the uncalcined 0.05CuFeO_n catalyst relative to 0.05CuFeO_s catalyst.

3.3 Spectral properties

Fig. 4(A and B) shows the FT-IR spectra of pure α-Fe₂O₃ obtained from thermal decomposition of iron hydroxides precipitated from iron nitrate and iron sulfate (α-Fe₂O_3n, α-Fe₂O_3s) in air at 400 °C, respectively. Sample (A) showed absorption in the regions 3417, 1619, 540, and 455 cm^−l. Sample (B) showed absorption in the regions 3414, 1626, 1121, 537, and 450 cm^−l. The general range of 3600–3100 cm^−l may be assigned to antisymmetrical and symmetrical O–H bonding stretching vibrational modes for water of hydration (Apte et al., 2007). The bonding in the region of 1670–1600 cm^−l also relates to O–H bonding bending vibrational modes (Apte et al., 2007). The bands at 455 and 540 cm^−l observed in two samples can be attributed to metal oxygen stretching vibrational modes (Apte et al., 2007). In addition, the spectrum of α-Fe₂O_3s showed a band at 1121 cm⁻¹ arising from the symmetric stretching mode of SO₄ anion (Zhang et al., 2006). No NO₃ anion band is observed in FT-IR spectrum of α-Fe₂O_3n.

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Figure 4

IR spectra of (A) α-Fe₂O_3n and (B) α-Fe₂O_3s catalysts calcined at 400 °C.

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3.4 Catalytic activity

Catalytic decomposition of H₂O₂ is a model reaction chosen to study the redox properties of the prepared catalysts. Fig. 5(A and B) shows the first-order plots of catalytic decomposition of H₂O₂ conducted at 30 °C over CuFeO_n and CuFeO_s catalysts at different CuO loading calcined at 400 °C, respectively. The variation of reaction rate constant (k) as a function of wt% of CuO for the CuFeO_n and CuFeO_s catalysts precalcined at 400 °C is graphically represented in Fig. 6(A and B), respectively. It is shown from Fig. 5(A and B) that (i) the catalytic activity of all CuFeO_n and CuFeO_s catalysts is higher than the pure Fe₂O₃ catalyst. (ii) The catalytic activity of CuFeO_n catalysts is higher than CuFeO_s catalysts at all CuO loadings. Inspection of Fig. 6(A and B) revealed that (i) the increase in amount of CuO content from 2.42 to 9.04 wt% is accompanied with a progressive increasing of the catalytic activity of CuFeO_n catalysts calcined at 400 °C. (ii) The catalytic activity of CuFeO_s catalysts increases progressively by increasing the amounts of CuO up to a certain extent reaching to a maximum at 2.42 wt% of CuO. Further increase in the amount of CuO 2.42 wt% was followed by a decrease in the catalytic activity.

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Figure 5

First-order plots of H₂O₂ decomposition conducted at 30 °C over (A) CuFeO_n and (B) CuFeO_s catalysts at different CuO loading calcined at 400 °C.

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Figure 6

Variation of reaction rate constant (k) as a function of the wt% of CuO for the catalytic decomposition of H₂O₂ conducted at 30 °C over (A) CuFeO_n and (B) CuFeO_s catalysts calcined at 400 °C.

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The observed increase in the catalytic activity of all CuFeO catalysts calcined at 400 °C in comparison to pure Fe₂O₃ can be interpreted in terms of the concept of bivalent catalytic centers (Shaheen, 2007) such as Cu²⁺–Fe³⁺ and/or Cu¹⁺–Fe²⁺ ion pairs beside the one component sites Cu²⁺–Cu⁺¹, Fe³⁺–Fe²⁺. The presence of Cu²⁺–Fe³⁺ and/or Cu¹⁺–Fe²⁺ ion pairs is a result of mutual charge interaction that is involved in H₂O₂-decomposition reaction (Shaheen, 2008). From the electronic theory of catalysis and the principle of bivalent catalytic sites there are two kinds of catalytic sites in equilibrium on the catalyst surface, i.e. donor and acceptor sites, which may be formed by metal catalyst ions in various valence states (depending on oxidation or reduction potential of each cation relative to that of H₂O₂) or by charge defects stabilized on the catalyst surface. Therefore, the catalytic reactions proceed on the catalytic sites constituted from the ion pairs (Fe³⁺–Fe²⁺) and/or (Cu²⁺–Cu⁺¹) where one of the ions occurs in a lower and the second ion in a higher oxidation state (Radwan, 2004; Hasan et al., 1999; Radwan, 2006).

Alternatively, the modification of the surface area of the catalysts might be another critical reason for promoting the catalytic activity (see Table 2). Also, this behavior was due to greater lattice oxygen mobility in the mixed oxide catalysts than single one. The presence of Cu decreases the strength of nearly Fe–O bonds, thus increasing the mobility of lattice oxygen and facilitating redox reactions on the catalyst surface (Litt and Almquist, 2009).

Changing the precursor of Fe₂O₃ as shown in CuFeO_n increased the catalytic activity. The catalytic activity of CuFeO_n is higher than CuFeO_s catalysts at all CuO loadings. This could be attributed to the presence of traces of sulfate anions in CuFeO_s catalysts as shown from FT-IR section. The presence of ${\text{SO}}_4^{-2}$ ions leads to increase of the reaction solution acidity and then decrease of the H₂O₂-decomposition rate (Zhang et al., 2006). These results agree with the results of Schumb et al., (1955) which reported that the hydrogen peroxide is more stable at low pH. In addition, the ${\text{SO}}_4^{-2}$ ions can adsorb strongly on the iron oxide surface instead of surface hydroxyl groups, especially at low ${\text{SO}}_4^{-2}$ concentrations (Martin and Smart, 1987). This leads to the formation of a surface complex between ${\text{SO}}_4^{-2}$ and α-FeOOH. This surface complex inhibits the catalytic decomposition of H₂O₂, possibly by affecting the surface charge or redox potential at the catalyst surface by decreasing H₂O₂ adsorption on decomposition sites (Watts et al., 1999; Sigg and Stumm, 1981; Choudhary et al., 2006; El-Shobaky et al., 2001).