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The effect of Mn substitution on the catalytic properties of ferrites
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2165
The effect of Mn substitution on the catalytic properties of ferrites Luiz C.A. Oliveira, R.M. Lago, R.V.R.A. Rios, R. Augusti, P.P. Sousa, W.N. Mussel and J.D. Fabris Departamento de Quimica, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil Fe3.xMnxO4 (x = 0.00, 0.21; 0.26 and 0.53) spinels have been characterized by MOssbauer, XRD, TG, DSC and TPR experiments. XRD and M6ssbauer data corroborate the Mn incorporation in the spinel lattice. These Mn containing ferrites have been tested as catalysts for two different reactions: the gas phase CO oxidation and the aqueous oxidation of organochloro contaminants with H202. During CO oxidation, it was observed that the presence of Mn strongly affects the phases transformation Fe3Oa--->7-Fe203 (maghemite)--->cz-Fe203 (hematite). The Mn-substituted magnetite series shows the highest catalytic activity for CO oxidation at low temperatures but during the reaction it is oxidised to maghemite which is less active. At higher temperatures, maghemite is converted to hematite, specially the Mncontaining samples, resulting in a strong increase in CO conversion. These ferrites were also active for the oxidation of chlorobenzene in water with H202, being the catalytic activity strongly increased by Mn-containing catalysts. The reaction mechanism apparently involves a Fenton-like radical chemical pathway.
1. INTRODUCTION Spinel ferrites are of scientific and technological interest due to their structural, electronic, magnetic and catalytic properties [1, 2]. An important feature of ferrite is the possibility it presents to replace iron by other metals while maintaining the spinel structure. The physicochemical properties of ferrites are strongly dependent on the nature and amount of foreign cation incorporated into the structure [3, 4]. For example, the introduction of cations with variable oxidation states, e.g. transition metals, specifically Mn [5] and Co [6], in the spinel lattice was found to modify the redox behaviour of the ferrites. This effect is specially important in catalysed oxidation and reduction reactions. In this work, Mn-bearing ferdtes have been prepared, characterised and studied as catalyst in two chemical processes: the gas phase CO oxidation and the aqueous oxidation of organochloro environmental contaminants, with H202. 2. EXPERIMENTAL The sol-gel syntheses were carried out by precipitation of Fe 3+ ions with ammonium hydroxide solution. The precipitate was then washed with ammonium acetate solution, dried and decomposed in a N2 atmosphere at 400~ for 2h.. Room temperature MOssbauer spectra were recorded in a conventional constant acceleration transmission spectrometer and a 57Co/Rh source. Magnetisation measurements were performed with a portable magnetometer
2166 [7]. Fe 2+ and Fe s+ were determined by titration with Na2Cr207 [8]. Thermogravimetric analyses were obtained with a SDT instrument model TA, and the DSC experiments were performed with an equipment Rigaku model 8065 D 1. TPR experiments were performed by using 20 mg of sample, under H2(8%)/N2 with a flow rate of 25 mL minq and a heating rate of 5 ~ minq. The CO oxidation reactions were carried out with 20 mg of the catalyst, with CO and air fluxes of ! 0 and 40 mL minq, respectively, monitored by mass flow controllers. The reaction of chlorobenzene (30 ppm in 100 mL water) with H202 (30%, lmL) and the catalyst (20 mg) was monitored by MIMS (Membrane Introduction Mass Spectrometry) [9]. The hydrogen peroxide decomposition was followed through an O2 volumetric system. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterisation The XRD (patterns not shown) and Mrssbauer analyses (experimental data only, in Figure 1) of the Fe3.xMnxO4 series showed the presence of spinel phases only. The Mn 2+ substitution in the spinel lattice is first confirmed by the linear increase of the lattice parameter (a0, Table 1). The decrease in the saturation magnetisation (0, Table 1) also suggests the manganese incorporation in the lattice. Table 1. Magnetisation values (o), lattice parameters (ao) of the cubic cell, and chemical formulae for the Fes.xMnxO4 samples. o/J kg~T ~ ao/nm Spinel formula* Formula number ~+ | (1 ) 87 0.8380 Fe3_,.]lFe0.83 04 72 0.8389 Fe 3+ 2§ Mn2+ (2) 2.04Fe0.73 0.21 | 0.02 04
*|
60
O.8419
3+ 2+ Mn o.26 2+ @oo7.04 Fez~4Feo..~3
(3)
62
0.8421
Fe32.]aFeZ0.~6Mn'-0.;3|
(4)
04
= cation vacancy.
Evidences of manganese substitution in the spinel lattice are finally given by room temperature MOssbauer spectroscopy (Figure 1). Fitted MOssbauer parameters (not presented) for the sample of Formula (1) (cf. Table 1) are nearly those reported for pure magnetite (e.g. ref. [10]). From Figure 1, it can be observed that, at lower Mn content, a more pronounced broadening and decrease of the resonant signals due to signal B-site suggest that manganese enters preferentially in the octahedral site. For higher Mn contents a substitution in the tetrahedral site can not be discarded, but further measurements under externally applied magnetic field must be done to confirm this possibility. Thermogravimetric profiles of the Fe3. xMnxO4 samples in air for pure Fe304 shows a weight gain from 130 up to 300 ~ due to oxidation of Fe 2§ producing cubic 7-Fe203 (maghemite). This behaviour was confirmed by the powder XRD and MOssbauer spectroscopy analyses of the reddish-brown magnetic produced material. The increase of the cubic lattice parameter (a0) for the oxidised Fe3.xMnxO4 ferrites suggests that during the phase transition Mn remains in the structure of the maghemite.
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Fig. 1. Room temperature MOssbauer spectra for Fe3.xMnxO4 samples (x = 0.00; 0.21; 0.26 and 0.53)
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Fig. 2. DSC analyses in air of the samples Fe304 and Fe2.47Mno.5304
.... 0
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Fig. 3. TPR profiles of the Fe3.xMnxO4 oxides (x = 0.00; 0.21; 0.26 and 0.53)
For Mn-containing samples, a second increase in weight is observed at temperatures higher than c a . 320 ~ which is probably due to the oxidation of manganese. DSC analyses of the samples corresponding to Formulas (1) and (4) (Table 1) in air showed the oxidation to maghemite at approximately 230 ~ for both samples. However, the conversion temperature of maghemite to hematite is strongly lowered by the presence of
2168
reactive Fe 3+ surface species [11]. The sample Fe304 was reduced at 400 ~ and analysed by powder XRD showed the presence of a high purity magnetite phase as suggested by the increase of the a lattice parameter of the cubic structure to 0.8457 nm. A second reduction peak is observed in the temperature range of 500-600 ~ XRD analysis of the sample Fe2.47Mn0.5304 after the second TPR peak showed that at 540 ~ the ferrite is reduced to produce FeO and MnO phases. After the reduction at 800 ~ the phases observed by XRD were Fe ~ and MnO. It can be observed a shift of the TPR peaks to lower temperatures as the manganese is substituted in the structure. This shift suggests that the presence of manganese facilitates the reduction o f F ( a, transforming Fe304 to FeO. 3.3. O x i d a t i o n of C a r b o n M o n o x i d e
The Fe3.xMnxO4 oxides were tested as catalysts for the oxidation of carbon monoxide (Figure 4). It can be observed that up to 200 ~ all the catalysts showed similar activity for CO oxidation. For temperatures higher than 200 ~ the catalyst corresponding to Formula (4) (Table 1) shows a remarkable increase in CO conversion followed by the ferrite corresponding to Formula (3) which increases the catalytic activity for temperatures superior to 250~ The catalysts of Formulas (2) and (1) did not have any significant increase of CO conversion up to 450 ~ A strong hysteresis effect was observed when the temperature was decreased, suggesting that the catalyst changes its nature during the reaction. XRD and MOssbauer analyses of the catalyst of Formula (4) used at 160 ~ showed the presence of a mixture of the magnetite and maghemite phases. This is in agreement with the TG experiments in air, which shows that Fe 2§ is oxidised at temperatures of 150~ When the reaction was carried out at temperatures higher than 300 ~ the catalyst was completely converted to ~-Fe,O3 hematite. These results show that under the reaction conditions the magnetite is consecutively converted to maghemite and hematite phases as the temperature increases. Figure 5 shows the carbon monoxide oxidation in the presence of ferrite, maghemite and hematite. It can be observed that up to temperatures of 200 ~ the ferrite is significantly more active than maghemite and hematite. At this temperature the magnetite is oxidised to maghemite and a decrease in CO conversion is observed. Maghemite shows low activity up to 350 ~ when it is converted to hematite and an increase in CO conversion is observed.
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Fig. 4. CO conversion over Mncontaining ferrites, Fe3mxMnxO4 (x = 0.00; 0.21; 0.26 and 0.53)
rol'C
Fig. 5. Oxidation of CO in the presence ofFe304, t~-Fe203 and 7-Fe203
2169 3.4. Chiorobenzene Oxidation with H202 The oxidation of organics with H202 has been extensively studied in the last years for decontamination of effluents [ 10] The most active system is the Fenton's reagent, a mixture of 1-1202 and Fe 2§ which generates in situ the radical oxidising species HO [ 11 ] A drawback of the Fenton's system is the large stoichiometric amount of ferrous salt necessary for the reaction, which is not recovered after the process Therefore, a heterogeneous system for the catalytic activation of H202 for the oxidation of organics in water is of great interest The presence of both Fe 3+ and specially Fe 2§ make the ferrites potential catalysts Preliminary results on the oxidation of chlorobenzene in water are shown in Table 2. In the absence of ferrite a low chlorobenzene conversion has been observed, probably due to the slow homolysis of H202 to OH'. When the reaction was carried out in the presence of the ferrites the following order of conversion was obtained (according to formula number in Table 1): (4) > (3) > (2) > (1), showing that the presence of manganese strongly increases the catalytic activity for chlorobenzene oxidation. It was also studied the H202 decomposition in the presence of the Mn-containing ferrites (Figure 6). Kinetic data showed a first order dependence on the HzO2 concentration with the calculated rate constant displayed in Table 2. It can be observed that the presence of manganese also enhances the hydrogen peroxide decomposition. Table 2. Chlorobenzene oxidation and H202 decomposition (Kd~omp) in the presence of Mn substituted ferrites (conversion after 60 min reaction at 25 ~
Catalyst Formula* None (1) (2)
(3) (4) *According to Table 1.
Chlorobenzene conversion/% <1 2 5
5 9
H202- kd~r
.4 min"1
0.4 2.4 7.6 19.9
No iron or manganese have been detected in the solution, by atomic absorption, excluding the possibility of a homogeneously catalysed reaction by solubilized cations. A radical mechanism is generally proposed for the 1-1202 decomposition by ions Fe § in solution [12, 13]. To investigate the radical participation in the reactions with ferrites, the H202 decomposition was studied in the presence of 30 ppm chlorobenzene or 50-500 ppm phenol (Figure 6). It was observed a strong decrease in H202 decomposition in the presence of chlorobenzene and phenol. These results suggest that radicals are participating at least partially in the reactions. A simple reported [14] mechanism for the 1-1202 decomposition is shown schematically in Figure 7. Therefore, apparently both Fe 2§ and specially Mn 2+ in the surface of the fen-ite can activate H202 to produce radicals probably HO or HOO, leading either to the formation of 02 or to the oxidation of organics present in solution.
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Fig. 7. Radical mechanism for H202 decomposition
ACKNOWLEDGMENT: The authors thank FAPEMIG and CNPq (Brazil) for the financial support. REFERENCE 1. B. Viswanathan and V.R.K. Murty (eds.), Ferrites: Its Science and Technology, Narosa Publishers, Dehli, 1990. 2. Hoshino, S. Iida and M. Sugimoo (eds.), International Conference on Ferrites, Japan, 1970. 3. J. R. Goldstein and A.C.C. Tseung, J. Catal., 32 (1974) 452. 4. A.I. Onushukuwu, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1447. 5. P. Lahiri and K. Sengupta, J. Chem. Sot., Far. Trans., 91 (1995) 3489. 6. R.M. Persons, E.D. Grave, P.M.A. Bakker and R.E Vandenerghe, Phys. Rev B, 47 (1993) 5894. 7. J. M. D. Coey, O. Cugat, J. McCauley, J. D. Fabris, Revista de Fisica Aplicada e Instrumenta~.o 7 (1992) 25. 8. Jeffery P.G., Hutchison D., Chemical Methods of Rock Analysis, Pergamon Press, Oxford, 1981. 9. R. Augusti, A.O. Dias, L.L. Rocha and R.M. Lago, J. Phys. Chem. A, 102 (1998) 10723. 10. E. Murad and J. H. Johnston. Iron oxides and oxyhidroxides. In: G. J. Long (Ed.) MOssbauer spectroscopy applied to inorganic chemistry, vol. 2, Plenum Press, New York, 1987, pp. 507- 582. 11. M. Shimokawabe, R. Fiuruiche and T. Ishii, Thermochim. Acta, 28 (1979) 287 12. Peyton, G. A Comparison of Advanced Oxidation Processes, Symposium on Advanced Oxidation Process, Toronto, 1990. 13. D.W Tedder and F.G Pohland (eds), Emerging Technologies in Hazardous Waste management III, Am.Chem.Soc.Pub., Washington, 1993. 14. G. Strukul (ed.), Catalytic Oxidations with Hydrogen Peroxide, Kluwer Academic Pub, Dordrecht, 1992, p. 107.
2165
The effect of Mn substitution on the catalytic properties of ferrites Luiz C.A. Oliveira, R.M. Lago, R.V.R.A. Rios, R. Augusti, P.P. Sousa, W.N. Mussel and J.D. Fabris Departamento de Quimica, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil Fe3.xMnxO4 (x = 0.00, 0.21; 0.26 and 0.53) spinels have been characterized by MOssbauer, XRD, TG, DSC and TPR experiments. XRD and M6ssbauer data corroborate the Mn incorporation in the spinel lattice. These Mn containing ferrites have been tested as catalysts for two different reactions: the gas phase CO oxidation and the aqueous oxidation of organochloro contaminants with H202. During CO oxidation, it was observed that the presence of Mn strongly affects the phases transformation Fe3Oa--->7-Fe203 (maghemite)--->cz-Fe203 (hematite). The Mn-substituted magnetite series shows the highest catalytic activity for CO oxidation at low temperatures but during the reaction it is oxidised to maghemite which is less active. At higher temperatures, maghemite is converted to hematite, specially the Mncontaining samples, resulting in a strong increase in CO conversion. These ferrites were also active for the oxidation of chlorobenzene in water with H202, being the catalytic activity strongly increased by Mn-containing catalysts. The reaction mechanism apparently involves a Fenton-like radical chemical pathway.
1. INTRODUCTION Spinel ferrites are of scientific and technological interest due to their structural, electronic, magnetic and catalytic properties [1, 2]. An important feature of ferrite is the possibility it presents to replace iron by other metals while maintaining the spinel structure. The physicochemical properties of ferrites are strongly dependent on the nature and amount of foreign cation incorporated into the structure [3, 4]. For example, the introduction of cations with variable oxidation states, e.g. transition metals, specifically Mn [5] and Co [6], in the spinel lattice was found to modify the redox behaviour of the ferrites. This effect is specially important in catalysed oxidation and reduction reactions. In this work, Mn-bearing ferdtes have been prepared, characterised and studied as catalyst in two chemical processes: the gas phase CO oxidation and the aqueous oxidation of organochloro environmental contaminants, with H202. 2. EXPERIMENTAL The sol-gel syntheses were carried out by precipitation of Fe 3+ ions with ammonium hydroxide solution. The precipitate was then washed with ammonium acetate solution, dried and decomposed in a N2 atmosphere at 400~ for 2h.. Room temperature MOssbauer spectra were recorded in a conventional constant acceleration transmission spectrometer and a 57Co/Rh source. Magnetisation measurements were performed with a portable magnetometer
2166 [7]. Fe 2+ and Fe s+ were determined by titration with Na2Cr207 [8]. Thermogravimetric analyses were obtained with a SDT instrument model TA, and the DSC experiments were performed with an equipment Rigaku model 8065 D 1. TPR experiments were performed by using 20 mg of sample, under H2(8%)/N2 with a flow rate of 25 mL minq and a heating rate of 5 ~ minq. The CO oxidation reactions were carried out with 20 mg of the catalyst, with CO and air fluxes of ! 0 and 40 mL minq, respectively, monitored by mass flow controllers. The reaction of chlorobenzene (30 ppm in 100 mL water) with H202 (30%, lmL) and the catalyst (20 mg) was monitored by MIMS (Membrane Introduction Mass Spectrometry) [9]. The hydrogen peroxide decomposition was followed through an O2 volumetric system. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterisation The XRD (patterns not shown) and Mrssbauer analyses (experimental data only, in Figure 1) of the Fe3.xMnxO4 series showed the presence of spinel phases only. The Mn 2+ substitution in the spinel lattice is first confirmed by the linear increase of the lattice parameter (a0, Table 1). The decrease in the saturation magnetisation (0, Table 1) also suggests the manganese incorporation in the lattice. Table 1. Magnetisation values (o), lattice parameters (ao) of the cubic cell, and chemical formulae for the Fes.xMnxO4 samples. o/J kg~T ~ ao/nm Spinel formula* Formula number ~+ | (1 ) 87 0.8380 Fe3_,.]lFe0.83 04 72 0.8389 Fe 3+ 2§ Mn2+ (2) 2.04Fe0.73 0.21 | 0.02 04
*|
60
O.8419
3+ 2+ Mn o.26 2+ @oo7.04 Fez~4Feo..~3
(3)
62
0.8421
Fe32.]aFeZ0.~6Mn'-0.;3|
(4)
04
= cation vacancy.
Evidences of manganese substitution in the spinel lattice are finally given by room temperature MOssbauer spectroscopy (Figure 1). Fitted MOssbauer parameters (not presented) for the sample of Formula (1) (cf. Table 1) are nearly those reported for pure magnetite (e.g. ref. [10]). From Figure 1, it can be observed that, at lower Mn content, a more pronounced broadening and decrease of the resonant signals due to signal B-site suggest that manganese enters preferentially in the octahedral site. For higher Mn contents a substitution in the tetrahedral site can not be discarded, but further measurements under externally applied magnetic field must be done to confirm this possibility. Thermogravimetric profiles of the Fe3. xMnxO4 samples in air for pure Fe304 shows a weight gain from 130 up to 300 ~ due to oxidation of Fe 2§ producing cubic 7-Fe203 (maghemite). This behaviour was confirmed by the powder XRD and MOssbauer spectroscopy analyses of the reddish-brown magnetic produced material. The increase of the cubic lattice parameter (a0) for the oxidised Fe3.xMnxO4 ferrites suggests that during the phase transition Mn remains in the structure of the maghemite.
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~
-10.0-7.5-5.0-2.5
0.0 2.5 5.0 7.5
Velocity/ram
10.0
s "1
Fig. 1. Room temperature MOssbauer spectra for Fe3.xMnxO4 samples (x = 0.00; 0.21; 0.26 and 0.53)
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360
,
400
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9
440 480
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Fig. 2. DSC analyses in air of the samples Fe304 and Fe2.47Mno.5304
.... 0
0.00
, .... | .... , .... | .... i .... ! .... , .... i .... | .... 100 200 300 400 500 000 700 800 900 1000 Tem
pe ratu re/~
Fig. 3. TPR profiles of the Fe3.xMnxO4 oxides (x = 0.00; 0.21; 0.26 and 0.53)
For Mn-containing samples, a second increase in weight is observed at temperatures higher than c a . 320 ~ which is probably due to the oxidation of manganese. DSC analyses of the samples corresponding to Formulas (1) and (4) (Table 1) in air showed the oxidation to maghemite at approximately 230 ~ for both samples. However, the conversion temperature of maghemite to hematite is strongly lowered by the presence of
2168
reactive Fe 3+ surface species [11]. The sample Fe304 was reduced at 400 ~ and analysed by powder XRD showed the presence of a high purity magnetite phase as suggested by the increase of the a lattice parameter of the cubic structure to 0.8457 nm. A second reduction peak is observed in the temperature range of 500-600 ~ XRD analysis of the sample Fe2.47Mn0.5304 after the second TPR peak showed that at 540 ~ the ferrite is reduced to produce FeO and MnO phases. After the reduction at 800 ~ the phases observed by XRD were Fe ~ and MnO. It can be observed a shift of the TPR peaks to lower temperatures as the manganese is substituted in the structure. This shift suggests that the presence of manganese facilitates the reduction o f F ( a, transforming Fe304 to FeO. 3.3. O x i d a t i o n of C a r b o n M o n o x i d e
The Fe3.xMnxO4 oxides were tested as catalysts for the oxidation of carbon monoxide (Figure 4). It can be observed that up to 200 ~ all the catalysts showed similar activity for CO oxidation. For temperatures higher than 200 ~ the catalyst corresponding to Formula (4) (Table 1) shows a remarkable increase in CO conversion followed by the ferrite corresponding to Formula (3) which increases the catalytic activity for temperatures superior to 250~ The catalysts of Formulas (2) and (1) did not have any significant increase of CO conversion up to 450 ~ A strong hysteresis effect was observed when the temperature was decreased, suggesting that the catalyst changes its nature during the reaction. XRD and MOssbauer analyses of the catalyst of Formula (4) used at 160 ~ showed the presence of a mixture of the magnetite and maghemite phases. This is in agreement with the TG experiments in air, which shows that Fe 2§ is oxidised at temperatures of 150~ When the reaction was carried out at temperatures higher than 300 ~ the catalyst was completely converted to ~-Fe,O3 hematite. These results show that under the reaction conditions the magnetite is consecutively converted to maghemite and hematite phases as the temperature increases. Figure 5 shows the carbon monoxide oxidation in the presence of ferrite, maghemite and hematite. It can be observed that up to temperatures of 200 ~ the ferrite is significantly more active than maghemite and hematite. At this temperature the magnetite is oxidised to maghemite and a decrease in CO conversion is observed. Maghemite shows low activity up to 350 ~ when it is converted to hematite and an increase in CO conversion is observed.
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0 50
100 150 200 250 300 350 400 450 500 550 TO m p o r a t u
Fig. 4. CO conversion over Mncontaining ferrites, Fe3mxMnxO4 (x = 0.00; 0.21; 0.26 and 0.53)
rol'C
Fig. 5. Oxidation of CO in the presence ofFe304, t~-Fe203 and 7-Fe203
2169 3.4. Chiorobenzene Oxidation with H202 The oxidation of organics with H202 has been extensively studied in the last years for decontamination of effluents [ 10] The most active system is the Fenton's reagent, a mixture of 1-1202 and Fe 2§ which generates in situ the radical oxidising species HO [ 11 ] A drawback of the Fenton's system is the large stoichiometric amount of ferrous salt necessary for the reaction, which is not recovered after the process Therefore, a heterogeneous system for the catalytic activation of H202 for the oxidation of organics in water is of great interest The presence of both Fe 3+ and specially Fe 2§ make the ferrites potential catalysts Preliminary results on the oxidation of chlorobenzene in water are shown in Table 2. In the absence of ferrite a low chlorobenzene conversion has been observed, probably due to the slow homolysis of H202 to OH'. When the reaction was carried out in the presence of the ferrites the following order of conversion was obtained (according to formula number in Table 1): (4) > (3) > (2) > (1), showing that the presence of manganese strongly increases the catalytic activity for chlorobenzene oxidation. It was also studied the H202 decomposition in the presence of the Mn-containing ferrites (Figure 6). Kinetic data showed a first order dependence on the HzO2 concentration with the calculated rate constant displayed in Table 2. It can be observed that the presence of manganese also enhances the hydrogen peroxide decomposition. Table 2. Chlorobenzene oxidation and H202 decomposition (Kd~omp) in the presence of Mn substituted ferrites (conversion after 60 min reaction at 25 ~
Catalyst Formula* None (1) (2)
(3) (4) *According to Table 1.
Chlorobenzene conversion/% <1 2 5
5 9
H202- kd~r
.4 min"1
0.4 2.4 7.6 19.9
No iron or manganese have been detected in the solution, by atomic absorption, excluding the possibility of a homogeneously catalysed reaction by solubilized cations. A radical mechanism is generally proposed for the 1-1202 decomposition by ions Fe § in solution [12, 13]. To investigate the radical participation in the reactions with ferrites, the H202 decomposition was studied in the presence of 30 ppm chlorobenzene or 50-500 ppm phenol (Figure 6). It was observed a strong decrease in H202 decomposition in the presence of chlorobenzene and phenol. These results suggest that radicals are participating at least partially in the reactions. A simple reported [14] mechanism for the 1-1202 decomposition is shown schematically in Figure 7. Therefore, apparently both Fe 2§ and specially Mn 2+ in the surface of the fen-ite can activate H202 to produce radicals probably HO or HOO, leading either to the formation of 02 or to the oxidation of organics present in solution.
2170 m
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40.
tr
==" pure water J
l9
nnni 9 9
mm=~m9 9 9
9
F9 ~
M~
9
phenol 50ppm
Fer"~ H O "
+ "OH
15 . .
10.
9
5
j
9 m
r -
P9
9
9
9
9
phenol 500ppm
O,v Time I rain
Fig.
6.
H202
decomposition
3+ 2+ Fe2.14Fe~ ~2+ ~)0.07 0 4 at 50 and 500 ppm phenol
with
catalyst with
Fig. 7. Radical mechanism for H202 decomposition
ACKNOWLEDGMENT: The authors thank FAPEMIG and CNPq (Brazil) for the financial support. REFERENCE 1. B. Viswanathan and V.R.K. Murty (eds.), Ferrites: Its Science and Technology, Narosa Publishers, Dehli, 1990. 2. Hoshino, S. Iida and M. Sugimoo (eds.), International Conference on Ferrites, Japan, 1970. 3. J. R. Goldstein and A.C.C. Tseung, J. Catal., 32 (1974) 452. 4. A.I. Onushukuwu, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1447. 5. P. Lahiri and K. Sengupta, J. Chem. Sot., Far. Trans., 91 (1995) 3489. 6. R.M. Persons, E.D. Grave, P.M.A. Bakker and R.E Vandenerghe, Phys. Rev B, 47 (1993) 5894. 7. J. M. D. Coey, O. Cugat, J. McCauley, J. D. Fabris, Revista de Fisica Aplicada e Instrumenta~.o 7 (1992) 25. 8. Jeffery P.G., Hutchison D., Chemical Methods of Rock Analysis, Pergamon Press, Oxford, 1981. 9. R. Augusti, A.O. Dias, L.L. Rocha and R.M. Lago, J. Phys. Chem. A, 102 (1998) 10723. 10. E. Murad and J. H. Johnston. Iron oxides and oxyhidroxides. In: G. J. Long (Ed.) MOssbauer spectroscopy applied to inorganic chemistry, vol. 2, Plenum Press, New York, 1987, pp. 507- 582. 11. M. Shimokawabe, R. Fiuruiche and T. Ishii, Thermochim. Acta, 28 (1979) 287 12. Peyton, G. A Comparison of Advanced Oxidation Processes, Symposium on Advanced Oxidation Process, Toronto, 1990. 13. D.W Tedder and F.G Pohland (eds), Emerging Technologies in Hazardous Waste management III, Am.Chem.Soc.Pub., Washington, 1993. 14. G. Strukul (ed.), Catalytic Oxidations with Hydrogen Peroxide, Kluwer Academic Pub, Dordrecht, 1992, p. 107.