Effect of Ag-doping of nanosized FeAlO system on its structural, surface and catalytic properties

2.1 Materials and methods

Aluminum hydroxide sample was prepared by precipitating Al(NO₃)₃·9H₂O (1 M) solution using 0.2 M ammonia solution at 70 °C and pH = 8. The precipitate was carefully washed with bi-distilled water till free from ammonium and nitrate ions, then filtered and dried at 110 °C till constant weight. Al(OH)₃ sample was calcined in air at 400, 500 and 800 °C for 4 h.

Five specimens of Fe₂O₃/Al₂O₃ solids were prepared by impregnation using known mass of Al(OH)₃ sample with solutions containing different amounts of iron nitrate dissolved in the least amount of bi-distilled water. The obtained pastes were dried at 110 °C till constant weight then calcined in air at temperatures ranged between 400 and 800 °C for 4 h. The nominal compositions of the prepared solids were 0.025, 0.035, 0.045, 0.055, 0.065Fe(NO₃)₂·9H₂O: Al(OH)₃ and the iron oxide content in these specimens was 3.76, 5.19, 6.58, 7.92 and 9.23 wt.%, respectively. The formula of prepared solids was abbreviated as xFeAlO, where x is the concentration of Fe₂O₃.

The Ag₂O-doped 0.045Fe₂O₃/Al₂O₃ system was prepared using known mass of Al(OH)₃ impregnated with solutions containing a fixed amount of iron nitrate and different proportions of silver nitrate. The obtained pastes were dried at 110 °C till constant weight then calcined in air at temperatures ranged between 400 and 800 °C for 4 h. The concentrations of Fe₂O₃ (6.58 wt.%) and of Ag₂O added were 1.5, 2.0, 3.0, 3.5 and 4.0 mol % which corresponded to 3.08, 4.06, 5.97, 6.89 and 7.80 wt.%, respectively. The formula of the prepared samples was abbreviated as yAgFeAlO, where, y is the concentration of Ag₂O. All the chemicals employed were of analytical grade supplied by the BDH Company.

2.2 Characterization techniques

Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) of the catalysts were carried out using Shimadzu TGA-50H thermo-gravimetric analyzer; the rate of heating was kept at 10 °C/min.

X-ray diffractograms of various prepared solids were determined by using 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 surface characteristics, namely specific surface areas (S_BET), total pore volume (V_p), and average pore radius (ŕ) of the various catalysts were determined form 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 at 25–40 °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 kinetics of the catalytic decomposition of H₂O₂ were monitored by measuring the volumes of liberated oxygen at different time intervals till equilibrium was attained at each reaction temperature. The results obtained showed that the reaction follows first-order kinetics in all cases. The relation between ln a/(a − x) and time intervals (t min) plotted gives first-order plots. Where, (a) is the total volume of oxygen collected when the complete decomposition of H₂O₂ takes place and it is a function of the initial concentration (zero time). (a − x) is the collected volume of oxygen at different time intervals and is a function of the concentration of H₂O₂ at different time intervals. The slopes of the first-order plots allow ready determination of the reaction rate constant, k, measured at a given temperature over a given catalyst sample. The catalytic activity was measured as a function of k-value. The reactor used is a batch reactor (continuous stirred reactor). The volume of oxygen is collected by means of buret technique.

3.1 Thermal properties

Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) of uncalcined Al(OH)₃, 0.045FeAlO and 3.5% AgFeAlO solids were determined and shown in Fig. 1(A–C), respectively. Inspection of Fig. 1: (i) The TG curve of Al(OH)₃ solid consists of three stages. The first and second thermal processes are indicative to desorption of physisorbed water and starting the decomposition of aluminum hydroxide to aluminum oxyhydroxide Al(OOH) (Shaheen and Hong, 2002). The last step corresponds to the complete decomposition of aluminum hydroxide into the corresponding oxide Al₂O₃. (ii) TG curve of uncalcined 0.045FeAlO solid consists of three stages. The first step represents desorption of physisorbed water and water of crystallization of iron nitrate. The second step is indicative to start the decomposition of aluminum hydroxide to aluminum oxyhydroxide compound Al(OOH) (Shaheen and Hong, 2002). The last step corresponds to the complete decomposition of iron nitrate and aluminum oxyhydroxide into Fe₂O₃ and Al₂O₃, respectively. (iii) TG curve of uncalcined 3.5% AgFeAlO solid consists of three stages. The first step represents desorption of physisorbed water and water of crystallization of iron nitrate. The second step represents the complete decomposition of silver nitrate and aluminum hydroxide into Ag₂O and Al₂O₃, respectively. The last step corresponds to the complete decomposition of iron nitrate yielding the corresponding oxide Fe₂O₃. It can be concluded that doping Fe₂O₃/Al₂O₃ system with Ag₂O enhanced the thermal decomposition of aluminum hydroxide and ferric nitrate to Al₂O₃ and Fe₂O_3, respectively. This result is confirmed by the previous published data (Shaheen, 2006), which indicated the presence of Ag₂O enhanced the thermal decomposition of aluminum hydroxide and ammonium vanadate to Al₂O₃ and V₂O₅, respectively.

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Figure 1

TG and DTG curves of uncalcined (A) Al(OH)₃, (B) 0.045FeAlO and (C) 3.5% AgFeAlO solids.

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3.2 XRD investigation of the prepared solids

3.2.1

3.2.1 XRD of Fe₂O₃/Al₂O₃ system

The X-ray diffractograms of γ-Al₂O₃, 0.045FeAlO and 0.065FeAlO solids being calcined at 500 °C were determined and illustrated in Fig. 2. The effect of Fe₂O₃ loading on the degree of ordering of γ-Al₂O₃ phase was investigated as shown in Table 1. Inspection of Fig. 2 and Table 1: (i) the diffractograms contain diffraction lines at d-spacing = 2.78, 2.39, 1.98, and 1.396 Ǻ due to γ-Al₂O₃ phase which is amorphous in nature (JCPDS 10-425). The diffraction lines related to Fe₂O₃ phase in the investigated solids calcined at 500 °C are absent. The absence of any XRD peaks attributable to iron oxide as separate phase in Fe₂O₃/Al₂O₃ system confirms their high dispersion and its small size to be detected by XRD tool (Wu et al., 2010a,b; Shaheen, 2006). So, γ-Al₂O₃ acted as a convenient support for hematite. On the other hand, the crystallization temperature of pure α-Fe₂O₃ is usually lower than 500 °C, while in our samples the nucleation and growth of α-Fe₂O₃ grains were restrained by the amorphous structure of alumina matrix (Liu et al., 2005). (ii) Increasing the Fe₂O₃ content from 4.5 to 6.5 mol % decreases the degree of ordering of γ-Al₂O₃ phase which has nano size ⩽ 6 nm.

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Figure 2

XRD diffractograms of pure γ-Al₂O₃ and FeAlO solids with various Fe₂O₃ loading calcined at 500 °C. Lines (1) refer to γ-Al₂O₃ phase.

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Table 1 Intensity counts of the main diffraction lines of XRD 0f γ-Al₂O₃ phase for FeAlO sample’s calcined at 500 and 800 °C.

Solid	Calcination temperature °C	Intensity count (a.u) γ-AI2O3
Al2O3	500	17.1
0.045FeAlO	500	10.5
0.065FeAlO	500	8.4
3.50% Ag-0.045FeAlO	500	9.0
Al2O3	800	20.1
0.045FeAlO	800	14.9
3.50% Ag-0.045FeAlO	800	13.5
4.00% Ag-0.045FeAlO	800	12.5

3.2.2

3.2.2 XRD of Ag₂O-doped Fe₂O₃/Al₂O₃ system

The X-ray diffractograms of pure and Ag₂O-doped 0.045FeAlO solids being calcined at 500 and 800 °C were determined and illustrated in Figs. 3A and B, respectively. Inspection of Figs. 3A and B and Table 1: (i) the diffractograms of pure and doped solids calcined at 500 °C consist of diffraction peaks due to γ-Al₂O₃ phase which is amorphous in nature and there are no diffraction lines due to Fe₂O₃ phase or silver species (Ag₂O or Ag metal). (ii) The diffractograms of pure and doped solids precalcined at 800 °C consist of diffraction lines related to poorly crystalline γ-Al₂O₃ phase and the absence of any lines related to Fe₂O₃ phase, iron aluminate spinel or silver species. (iii) Increasing the calcination temperature of pure and doped solids from 500 to 800 °C increases the degree of ordering of γ-Al₂O₃ phase which has nano size (5–10 nm). (iv) Doping of 0.045FeAlO sample with Ag₂O followed by calcination at 500 and 800 °C did not much affect the degree of ordering of γ-Al₂O₃ phase.

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Figure 3A

XRD diffractograms of pure and Ag₂O treated 0.045FeAlO solids calcined at 500 °C. Lines (1) refer to γ-Al₂O₃ phase.

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Figure 3B

XRD diffractograms of pure and treated 0.045FeAlO solids with various amounts of Ag₂O calcined at 800 °C. Lines (1) refer to γ- Al₂O₃ phase.

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The above results can be explained on the light of the following: (i) the absence of diffraction lines due to Ag-species (Ag₂O or Ag metal) in doped solids calcined at 500 or 800 °C was expected because the small amounts of silver oxide added were below the detection limits of the employed X-ray diffractometer (El-Shobaky et al., 2003). It has been reported that heating of Ag₂O at a temperature above 500 °C gives metallic Ag-species (Turky, 2003; Imamura et al., 2000) which are not detected by XRD technique because of their minute amounts. (ii) The absence of any solid–solid interaction between Fe₂O₃ and γ-Al₂O₃ yielding aluminate spinel can be attributed to the reaction between the transition metal oxides and Al₂O₃ to produce metal aluminate which is strongly dependent upon the nature of the transition metal element. The rate of reaction between the metal oxide and Al₂O₃ deceases in the following order: Cu > Co > Ni>>Fe (Gardener et al., 1991; Fagal et al., 2001). (iii) The slight decrease in the degree of ordering of γ-Al₂O₃ phase precalcined at 500 and 800 °C due to Ag₂O-doping could be attributed to a possible coating of the γ-Al₂O₃ crystallites with Ag₂O film which hinders the particle adhesion process, thus limiting their grain growth during the course of heat treatment (El-Shobaky et al., 2001). It has been reported that the presence of Ag effected the dispersion of Fe₂O₃ particles in the catalyst surface (Brinen et al., 1975). (iv) The increase in the degree of ordering of γ-Al₂O₃ phase for pure and doped solids due to increasing the calcination temperature from 500 to 800 °C could be explained in the light of the grain growth mechanism or sintering processes (Shaheen, 2007a,b).

The observed changes in the degree of ordering of γ-Al₂O₃ present in the investigated catalysts as a result of Ag₂O-doping are expected to induce changes in the specific surface areas of FeAlO system.

3.3 Surface properties

The nitrogen adsorption isotherms were measured at −196 °C for pure and Ag₂O doped samples preheated at 500 and 800 °C and illustrated in Fig. 4. The isotherms obtained, not given here, are of type II of Brunauer’s classification (Brunauer et al., 1938) showing closed hysteresis loops. The specific surface areas were calculated from these adsorption isotherms by applying the BET equation (Brunauer et al., 1938) the data obtained are given in Table 2 and Figs. 5A and B, respectively. The total pore volumes (V_p) were taken at P/P° = 0.95 and are expressed in ml/gm, where, P is the actual gas pressure and P° is the saturated vapor pressure of the adsorbing gas. The average pore radius, ŕ (Å), was calculated from the above-mentioned textural properties, applying the relationship: ŕ = (2V_p/S_BET) × 10⁴ Å (El-Hakam, 1999). Another series of specific surface areas S_t were computed from the V_L-t plots of the various investigated adsorbents. These plots were constructed using the de Boer-t plot (Lippens and deBoer, 1965). The computed S_t values are also given in Table 2. Representative V_L-t plot curves of investigated samples calcined at 500 and 800 °C are shown in Figs. 6A and B, respectively.

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Figure 4

The representative nitrogen adsorption–desorption isotherms on pure and Ag₂O-doped FeAlO samples calcined at 500 and 800 °C.

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Figure 5A

Linear BET plot of Fe₂O₃, Al₂O₃, pure FeAlO and Ag₂O-doped samples calcined at 500 °C.

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Figure 5B

Linear BET plot of Al₂O₃, pure 0.045FeAlO and Ag₂O-doped samples calcined at 800 °C.

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Figure 6A

V_L-t plots of Fe₂O₃, Al₂O₃, pure FeAlO and Ag₂O-doped samples calcined at 500 °C.

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Figure 6B

V_L-t plots of Al₂O₃, pure 0.045FeAlO and Ag₂O-doped samples calcined at 800 °C.

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Inspection of Figs. 5A and B and Table 2 shows the following: (i) addition of different amounts of Fe₂O₃ (4.5 & 6.5 mol %) to γ-Al₂O₃ support followed by calcination at 500 °C resulted in a limited decrease in S_BET of γ-Al₂O₃ attained about 2% and 10%, respectively. (ii) The S_BET of Fe₂O₃/Al₂O₃ system is bigger than that of Fe₂O₃ single oxide precalcined at 500 °C with about 204%. Increasing Fe₂O₃ content to 6.5 mol % is accompanied by a decrease in S_BET of FeAlO system with about 8%. (iii) Doping of 0.045FeAlO sample with 3.5 mol % Ag₂O followed by calcination at 500 °C led to a slight increase in its S_BET with about 3%. (iv) The average pore radius ŕ of the investigated samples precalcined at 500 °C decreases by loading Fe₂O₃ on γ-Al₂O₃. (v) The rise in the calcination temperature of pure γ-Al₂O₃, 0.045FeAlO and 3.5% Ag-0.045FeAlO samples from 500 to 800 °C brought about a significant decrease in their specific surface areas and an increase in the average pore radius ŕ. The decrease in S_BET values, due to increasing the calcination temperature, attained about 41%, 39% and 42%, respectively. (vi) Doping of 0.045FeAlO sample with 3.5 and 4.0 mol % Ag₂O followed by calcination at 800 °C led to a slight decrease in its S_BET which attained about 3% and 7%, respectively. (vii) Doping of 0.045FeAlO sample with Ag₂O followed by calcination at 500 and 800 °C does not much affect the values of average pore radius ŕ of the investigated system.

According to V_L-t plot curves as shown in Fig. 6A and B of investigated samples calcined at 500 and 800 °C show the following: (i) Fe₂O₃ is a mesoporous material. This behavior is indicated by the upward deviation following an initial linear region by demonstrating the existence of mesopores. (ii) The V_L-t plots of γ-Al₂O₃, 0.045FeAlO, 0.065FeAlO, 3.5% Ag-0.045FeAlO samples calcined at 500 °C and 4.0% Ag-0.045FeAlO sample calcined at 800 °C reveal the microporosity character, as indicated by downward deviation from the initial straight line which passes through the origin. (iii) The V_L-t plot of γ-Al₂O₃, 0.045FeAlO and 3.5% Ag-0.045FeAlO samples calcined at 800 °C, the initial linear region is followed by an upward deviation which is limited and a decrease in its slope is noted. This indicates the filling of some of the pores present by both multilayer formation and capillary condensation and the rest solely by multilayer formation. This indicates that these samples actually constitute of a mixture of meso- and micropores.

It is seen from Table 2 that the values of S_BET and S_t are close to each other which justify the correct choice of standard t-curves used in the analysis.

The changes in the specific surface areas of the prepared and calcined samples can be explained as follows: (a) the observed increase in the S_BET value of hematite due to loading on γ-Al₂O₃ support sample precalcined at 500 °C can be discussed in the light of fine dispersion Fe₂O₃ particles on the surface of γ-Al₂O₃ (Wu et al., 2010a,b). Indeed, XRD peaks due to iron oxide were not detected in the investigated samples in the present work indicating that the iron oxide phase exists in a highly divided or amorphous state in these specimens. The change from the mesoporous structure to microporous structure as a result of supporting iron oxide on γ-Al₂O₃ as shown in Fig. 5A is another factor. (b) The induced decrease in the surface area of 0.045FeAlO sample due to increasing the amount of Fe₂O₃ loading from 4.5 to 6.5 mol % may be ascribed to the aggregation of small iron species into larger bulk particles of iron in preparation process (Wang et al., 2002). (c) The slight increase in S_BET value due to Ag₂O-doping precalcined at 500 °C can be attributed to creation of new pores due to liberation of nitrogen oxides gases during the thermal decomposition of AgNO₃ dopant added (El-Molla et al., 2004). (d) The observed significant decrease in S_BET of the investigated samples as result of increasing the calcination temperature from 500 to 800 °C could be attributed to the sintering process. The sintering process might take place according to the collapse of the pore structure, pore widening (El-Shobaky and Deraz, 2001) and/or the particle adhesion (grain growth) process together with possible phase transformation (Deraz, 2003, 2008; Radwan et al., 2005).

The observed changes in textural properties of the investigated solids as a result of increasing the extent of iron oxide loading, Ag-doping and increasing the calcination temperature should modify the concentration of catalytically active constituents taking part in the catalyzed reaction.

3.4 Catalytic properties of the prepared solids

3.4.1

3.4.1 Effect of extent of Fe₂O₃ loading on the catalytic activity of FeAlO system

The catalytic decomposition of H₂O₂ is a model reaction chosen to study the redox properties of the prepared catalysts. Fig. 7 shows the first-order plots of H₂O₂ decomposition conducted at 30 °C using FeAlO catalysts at different Fe₂O₃ loadings calcined at 400 °C. Fig. 7 shows that γ-Al₂O₃ support solid exhibits no catalytic activity toward H₂O₂ decomposition reaction. The catalytic activity of FeAlO catalyst is higher than that of pure Fe₂O₃ catalyst. The increase in amount of Fe₂O₃ content from 2.5 to 6.5 mol % is accompanied with increasing the catalytic activity of FeAlO system precalcined at 400 °C. The maximum increase in the catalytic activity attained about 34% for 0.065FeAlO catalyst at k_30°C with respect to pure Fe₂O₃.

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Figure 7

First-order plots of H₂O₂ decomposition conducted at 30 °C over pure and various extents of Fe₂O₃/Al₂O₃ catalysts calcined at 400 °C.

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XRD and S_BET measurements showed that the high dispersion of Fe₂O₃ on γ-Al₂O₃ and the significant increase in the S_BET may be responsible for the higher catalytic activity of FeAlO than the pure Fe₂O₃. The absence of any XRD patterns detected for Fe₂O₃ as separate phase reflected the decrease in the crystallite size of detected phase which becomes so small to be detected by the employed XRD technique and hence increasing the surface area of investigated solids. Other factor, we cannot overlook, is the creation of bivalent catalytic centers (Shaheen and Hong, 2002; Mucka and Tabacik, 1991) such as Fe³⁺–Fe²⁺ ion pairs that are involved in H₂O₂-decomposition reaction. It has been reported that a favorable redox couple of Fe²⁺–Fe³⁺ is essential for the catalytic decomposition of H₂O₂ through electron exchange (Parida et al., 2005).

3.4.2

3.4.2 Effect of calcination temperature on the catalytic activity of 0.045FeAlO system

Variation of the catalytic activity expressed as reaction rate constant (k min⁻¹) as a function of precalcination temperature of 0.045FeAlO system in the range of 400–800 °C toward H₂O₂ decomposition conducted at 25–40 °C was investigated and determined as shown in Fig. 8. Inspection of Fig. 8 (i) Increasing the calcination temperature from 400 to 500 °C increases the catalytic activity of 0.045FeAlO system; the increase in the k_30°C value attained about 14%. (ii) Increasing the calcination temperature from 500 to 800 °C was accompanied by a progressive decrease in the catalytic activity of 0.045FeAlO system; the decrease in the k_30°C value attained about 45%. (iii) The catalytic activity of 0.045FeAlO system increased with increasing the reaction temperature from 25 to 40 °C.

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Figure 8

Variation of reaction rate constant (k) as a function of calcination temperatures for the catalytic decomposition of H₂O₂ conducted at 25–40 °C over 0.045FeAlO catalysts.

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Increasing the catalytic activity of 0.045FeAlO system as a result of increasing the calcination temperature from 400 to 500 °C may be attributed to increase in the concentration of catalytically active constituents of Fe³⁺–Fe²⁺ ion pairs taking part in the catalysis of H₂O₂-decomposition reaction. The progressive decrease in the catalytic activity of 0.045FeAlO system as a result of increasing the calcination temperature from 500 to 800 °C may be due to (i) increasing the degree of ordering of γ-Al₂O₃ phase (Table 1). (ii) The sintering process of catalytically active sites with subsequent decrease in its specific surface area from 211.7 to 130.3 m² g⁻¹ (Table 2) (Shaheen, 2002; Shaheen, 2007a,b). (iii) The effective removal of surface OH groups which act as active sites for the H₂O₂ decomposition reaction (El-Shobaky et al., 2001).

3.4.3

3.4.3 Effect of Ag₂O-doping on the catalytic activity of 0.045FeAlO system calcined at different calcination temperatures

Variation of the catalytic activity expressed as reaction rate constant (k min⁻¹) of H₂O₂ decomposition conducted at 25–40 °C as a function of wt.% of Ag₂O for the solids precalcined at 500 and 800 °C was investigated and graphically represented in Figs. 9A and B, respectively. The variation of k (min⁻¹) of H₂O₂ decomposition over 3.5% AgFeAlO conducted at 30 °C as a function of calcination temperature is graphically represented in Fig. 10. It is seen from these Figs. 9A, B and 10 that: (i) the catalytic activity of 0.045FeAlO catalysts increases progressively by increasing the amounts of dopant up to certain extent reaching to a maximum at 3.5 mol % Ag₂O, the increase in the k_30°C value of 3.5% AgFeAlO calcined at 500 °C attained about 502%. (ii) Increasing Ag₂O content to 4.0 mol % is accompanied by a sharp decrease in the catalytic activity falling to values greater than those of pure catalysts precalcined at the same temperature. The maximum decrease in the k_30°C value on increasing the amount of dopant from 3.5 to 4.0 mol % attained about 40% and 30% for the catalysts precalcined at 500 and 800 °C, respectively. (iii) The catalytic activity of pure and doped 0.045FeAlO system increased with increasing the reaction temperature from 25 to 40 °C. (iv) The catalytic activity of doped 0.045FeAlO solid with 3.5 mol % Ag₂O increases progressively by increasing the calcination temperature from 400 to 800 °C. The increase in the k_30°C value due to doping with 3.5 mol % Ag₂O attained about 383%, 502%, 635% and 1511% for the catalysts precalcined at 400, 500, 600 and 800 °C, respectively.

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Figure 9A

Variation of reaction rate constant (k) as a function of wt.% of Ag₂O for the catalytic decomposition of H₂O₂ conducted at 25–40 °C over pure 0.045FeAlO catalysts calcined at 500 °C.

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Figure 9B

Variation of reaction rate constant (k) as a function of wt.% of Ag₂O for the catalytic decomposition of H₂O₂ conducted at 25–40 °C over pure 0.045FeAlO catalysts calcined at 800 °C.

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Figure 10

Variation of reaction rate constant (k) as a function of calcination temperatures for the catalytic decomposition of H₂O₂ conducted at 30 °C over 3.5% AgFeAlO catalysts.

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The significant enhancement in the catalytic activity of 0.045FeAlO solids in the investigated redox reaction as a result of Ag₂O-doping could be discussed in terms of: (i) the slight decrease in the degree of ordering of γ-Al₂O₃ phase in 3.5% AgFeAlO sample calcined at 500 °C (as shown in XRD section). This effect could result from a possible coating of the γ-Al₂O₃ support with an Ag₂O film which acts as an energy barrier opposing their particles adhesion. This reflected the role of Ag₂O treatment in increasing the degree of dispersion of Fe₂O₃ and consequently increasing the catalytic activity of H₂O₂ decomposition. (ii) The possible changes in the concentration of ion pairs acting as active sites for the catalyzed reaction present in the outermost surface layers of the treated solids (Shaheen, 2006; Shaheen and Ali, 2001; Shaheen et al., 2003). The created ion pairs due to Ag₂O-doping may be Ag⁺–Fe²⁺, Ag–Fe³⁺, and Ag–Ag⁺ (Turky et al., 2001).

However, the observed significant decrease in the k_30°C value on increasing the amount of dopant from 3.5 to 4.0 mol % can be attributed to decrease in the concentration of catalytically active species involved in the catalyzed reaction. This could result from the location of dopant species on the surface layers of the treated catalysts thus blocking some of the active constituents by dopant cation and/or metallic silver (Radwan et al., 2002).

The observed increase in the catalytic activity of Ag-doped FeAlO solids by increasing the calcination temperature from 400 to 800 °C (Fig. 10) could be attributed to the possible presence of metallic Ag-species and increase in the concentration of surface excess oxygen as a result of heating at a temperature above 450 °C (Turky, 2003; Imamura et al., 2000) according to the following reaction (Anand and Srivastava, 2002). $$2{\text{AgNO}}_3\stackrel{>450°\text{C}}{⟶}2\text{Ag}+2{\text{NO}}_2+{\text{O}}_2\uparrow$$ In spite of increasing the calcination temperature decreases the surface areas of the Ag-doped solid catalysts. The observed increase in catalytic activity clearly reflects the minor role played by the surface area in determining the catalytic activity of doped solids.

It has been reported that (Fu et al., 2010) the catalytic decomposition of H₂O₂ on iron oxide surfaces at low pH occurs by oxidation and its reduction via the classical Fenton reaction:

(1)

$${\text{Fe}}^{3+}+{\text{H}}_2{\text{O}}_2\to {\text{Fe}}^{2+}+{\text{O}}_2+2{\text{H}}^{+}(\text{oxidation})$$

(2a)

$${\text{Fe}}^{2+}+{\text{H}}_2{\text{O}}_2\to {\text{Fe}}^{3+}+\text{OH}+{\text{OH}}^{-}(\text{reduction})$$

(2b)

$${\text{Fe}}^{2+}+\text{OH}\to {\text{Fe}}^{3+}+{\text{OH}}^{-}(\text{reduction})$$

These reactions occur primarily on the solid oxide surface, concentrations of dissolved Fe²⁺ and Fe³⁺ species are very low.