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Materials Research

Print version ISSN 1516-1439On-line version ISSN 1980-5373

Mat. Res. vol.11 no.3 São Carlos July/Sept. 2008

http://dx.doi.org/10.1590/S1516-14392008000300013 

REGULAR ARTICLES

 

LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 perovskites: synthesis, characterization and catalytic activity in H2O2 reactions

 

 

Fabiano MagalhãesI; Flavia Cristina Camilo MouraII; José Domingos ArdissonIII; Rochel Montero LagoI, *

IDepartamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte - MG, Brazil
IIDepartamento de Química, ICEB, Universidade Federal de Ouro Preto, 35400-000 Ouro Preto - MG, Brazil
IIILaboratório de Física Aplicada, Centro de Desenvolvimento de Tecnologia Nuclear, CDTN-CNEN, Av. Antônio Carlos, 6627, Belo Horizonte - MG, Brazil

 

 


ABSTRACT

In this work two perovskites were prepared: LaMn1-xFexO3, and LaMn0.1-x Fe0.90MoxO3. XRD and Mössbauer spectroscopy suggest the formation of pure phase perovskite with the incorporation of Fe and Mo in the structure. The catalytic activity of these materials was studied in two reactions with H2O2: the decomposition to O2, and the oxidation of the model organic contaminant methylene blue. The perovskite composition strongly affects the catalytic activity, while Fe decreases the H2O2 decomposition Mo strongly improves dye oxidation.

Keywords: perovskites, oxidation, catalysis


 

 

1. Introduction

The reaction of organic compounds with Fenton reagent is one of the most efficient methods for the destruction of organic contaminants in wastewaters. The classical Fenton system, a mixture of H2O2 and a Fe(II) salt, generates in situ free hydroxyl radicals according to the Haber-Weiss mechanism (Equation 1).

The strong oxidizing hydroxyl radicals react with organic compounds in water leading to their mineralization to CO2 and H2O as the final harmless products.

Increasingly efficient heterogeneous Fenton-like systems have been investigated where soluble Fe2+ species is replaced by different iron oxides such as goethite, hematite and ferrihydrite.

Perovskites type oxides, ABO3, have been extensively investigated as catalysts for several processes including fuel cells1, water dissociation2, hydrogenation, hydrogenolysis3, ammonia oxidation4 and NOx reduction5. Several reviews covering these fields can be found in the literature6,7. Perovskites, especially LaMnO3, have also been used in environmental applications, e.g. the oxidation of hydrocarbons8-10, chlorinated organics11 and H2O2 reactions6,12. LaMnO3 shows good stability, flexible oxygen stoichiometry (d) and the different Mn oxidation states i.e. Mn2+, Mn3+, Mn4+, which strongly affects the catalytic behavior. Also, isomorphic substitution of metals in the perovskite structure allows some control the catalytic properties of the material. Several LaMnO3 derivatives, i.e. La1-xAxMn1-yMyO3 (where A is a lanthanide, actinide, alkaline or earth alkaline metal and M is another transition metal such as Co, Ni, etc.) have been previously investigated13,14.

In this work, we studied the isomorphic substitution of Mn in the LaMnO3 structure by different amounts of Fe and Mo to produce LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3. The Fe and Mo can also vary their oxidation states and this property can improve the catalytic activity in oxidation processes. These perovskites were characterized and the catalytic activity was investigated using two H2O2 reactions, i.e. the decomposition to O2 and the oxidation of the dye methylene blue used as model contaminant.

 

2. Experimental

The perovskites were prepared by the reaction of 0.5 mol of citric acid (CA) dissolved in 2 mol of water at 60 °C, followed by the addition of 1 mmol of La(NO3)3.6H2O and different proportions of the other metals such as x mmol Fe(NO3)3.9H2O, y mmol Mn(NO3)2.4H2O and z mmol Mo(acac)2O2 in order to produce the desired stoichiometry LaMnyFexMozO3 . The mixture was stirred for about 2 hours, until a clear orange solution of the stable metal-CA complexes was obtained. After complete dissolution, 400 mmol of ethyleneglycol (EG) was added and the solution was continuously stirred while the temperature was slowly increased to 90 °C. This step removes the excess of water and allows the polyesterification reaction between CA and EG to be further activated. The heating at 90 °C over 7 hours resulted in a viscous orange resin.12 This resin was then treated at 400-450 °C in air over 2 hours for the carbonization. The final product, a dark brown powder, was ground and then calcined at 800 °C in air for 6 hours.

The catalysts were characterized by XRD, TPR (temperature programmed reduction), Mössbauer spectroscopy, BET surface area and thermal analysis. The surface area was determined by the BET method using a 22 cycles of N2 adsorption/desorption in an Autosorb 1 Quantachrome instrument. The Mössbauer spectroscopy experiments were carried out in the transmission geometry on a constant-acceleration conventional spectrometer with a 57Co/Rh source at room temperature (RT) using α-Fe as a reference. The powder XRD data were obtained in a Philips X'Pert equipment using Cu Kα or Co Kα radiation scanning from 2 to 80º at a scan rate of 4º per minute. The TPR (temperature programmed reduction) analysis was performed in a CHEM BET 3000 TPR using H2 (8% in N2) with heating rate of 10 °C per minute. The H2 consumption was obtained after calibration of the TPR system using a CuO standard. The thermal analysis was carried out in a SHIMADZU DTG-60, with a constant heating rate of 10 °C per minute under air flow (50 mL/min). The hydrogen peroxide (Synth) decomposition was carried out with 7 mL H2O2 (2.9 mol.L-1) and 60 mg of catalyst. The formation of gaseous O2 was measured in a volumetric glass system. All the reactions were carried out using magnetic stirring in a recirculating temperature controlled bath kept at 25 ± 1 °C. The oxidation of the methylene blue at the concentration of 0.05 g.L-1 with H2O2 was monitored by UV/Vis at 663 nm. During the reaction the pH varied from 5.5 up to 6.0 which does not change significantly the absorptivity of the dye. All the reactions were carried out using 30 mg of the catalyst in a recirculating temperature controlled bath at 25 ± 1 °C.

 

3. Results and Discussion

3.1. Characterization of the perovskites

The study of the crystalline phases present in the perovskites LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 was carried out by XRD analyses (Figures 1 and 2).

The main peaks in Figure 1a are related to the perovskite phase, e.g. LaMnO3 2θ at 22.9, 32.6, 40.2, 46.9, 52.7, 58.1 and 68.2º 15,with crystal structures adjusted to a pseudo-cubic arrangement16,17. It can be observed, however, a gradual shift of the XRD peaks to lower diffraction angles as the Fe content increased with cell parameters increasing from 3.885(2) to 3.944(1) Å (a = b = c).

XRD results for the LaFe0.90Mn0.1-xMox O3 series (Figure 2) were very similar, showing a perovskite phase with the peak shift to lower difractons angles as the Mo content increaased (Figure 2b) suggesting the incorporation of Mo into the structure. Both crystallite size and BET surface area did not change significantly for the series LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 varying between 120-144 nm and 11-15 m2 g-1.

Mössbauer spectra of the LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 perovskites are shown in Figure 3, with the hyperfine parameters in Table 1.

 

 

The spectra of LaMn0.73Fe0.27O3 and LaMn0.46Fe0.54O3 perovskites showed only doublets with isomer shift (δ) 0.33 mm/s, quadrupolar splitting (Δ) 0.52 mm/s and δ 0.33 mm/s, Δ 0.62 mm/s, likely related to octahedric Fe3+ dispersed in the perovskite structure. As seen in Figure 3a, the spectrum splits into sextets for higher concentration of Fe. The LaMn0.10Fe0.90O3 perovskite, for example, shows a signal with hyperfine parameters at d 0.37 mm/s, ε -0.05 mm/s and magnetic hyperfine field (Hhf) 51.3 T with a relative area of 47%, assigned to the well crystallized LaFeO3 structure. A second sextet at δ 0.37 mm/s, ε 0.05 mm/s and Hhf 49.2 T is also observed with a relative area of 53%. This is identified as the poorly crystallized LaFeO317,18 probably due to small particle size or to the presence of Mn.

The results for the Mo containing LaMn0.08Fe0.90Mo0.02 O3 perovskite are very similar (Figure 3b), but the relative intensity of the more crystalline phase is lowered to 36% (δ 0.36 mm/s, ε -0.05 mm/s and Hhf 50.9 T) with a slight increase to 64% of the poorly crystallized phase (δ 0.37 mm/s, ε 0.09 mm/s and Hhf 48.4 T). These results suggest that the introduction of Mo into the perovskite structure induces a loss of crystallinity. The presence of higher Mo concentration apparently induces a more significant disorder in the perovskite structure producing a segregation of Fe3+ species detected as an additional signal at δ 0.23 mm/s, Δ 0.62 mm/s with a small relative spectral area of 7%.

Temperature Programmed Reduction (TPR) experiments were performed to investigate the reducibility of the different perovskites. TPR profile for LaMnO3 (Figure 4) showed two sets of peaks: Peak 1 at 300-530 °C assigned to the reduction of Mn4+ and some Mn3+ and Peak 2 at temperatures higher than 600 °C due to the reduction of Mn3+ to produce MnO according to the Equation 2:10

 

 

This TPR profile is similar to the previously published results for LaMnO3.8 TPR of the LaMn0.46Fe0.54O3 sample (Figure 4) exhibit the same Peak 1 reduction features, but two important characteristics have changed: Peak 2 shifted to lower temperature (~700 °C) with a significant decrease in the peak area and a new reduction feature appeared starting at temperatures near 800 °C. The latter is likely related to the reduction of the Fe species. Thus, the reduction of iron and the formation of Feº seem to occur only at temperature above 800 °C, which is when the perovskite structure collapses. Such remarkable thermal stability of iron in the perovskite structure has already been documented in the literature18-19. The smaller peaks for TPR spectra of D, E and F samples (Figure 4) in the temperature range 300-600 °C are likely related to the reduction of the molybdenum and manganese species present only in small amount.

3.2. Hydrogen peroxide decomposition and oxidation of the dye methylene blue

The catalytic activity of the perovskites LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 was studied using two reactions: the H2O2 decomposition to O2 (H2O2 H2O + 0.5O2); and the oxidation of the dye methylene blue a model contaminant with H2O2 in aqueous medium.

Figure 5 shows the H2O2 decomposition in the presence of different perovskites LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3. It is interesting to observe that the reactions without catalyst and the reactions in the presence of LaMn0.10Fe0.90O3 and LaMn0.1-xFe0.90Mox O3 perovskites did not show catalytic activity for the H2O2 decomposition. On the other hand, when the Fe concentration in the perovskite decreased, i.e. LaMn0.46Fe0.54O3 LaMn0.73Fe0.27O3, and finally LaMnO3, the activity increased significantly. The linear behavior of the H2O2 decomposition plots suggests a pseudo zeroth order kinetics under the reaction conditions studied. The reaction rates were calculated by the slope of the decomposition curves (Figure 6).

From Figure 6a it is possible to observe that the presence of small amounts of iron in the perovskite structure (Fe0.27) led to an increase on the H2O2 decomposition rate (kdec). On the other hand, it is observed a significant decrease of the H2O2 decomposition rate for the perovskite with higher Fe concentration (Fe0.57 e Fe0.90). Figure 6b shows that the rates of the H2O2 decomposition in the presence of LaMn0.1-xFe0.90Mox O3 were very low and did not vary significantly.

Although the mechanism of this reaction is not clear, the results suggested that the presence of manganese in the perovskite structure plays an important role in the catalytic activity, once their substitution by Fe and/or Mo induced a significant decrease on the H2O2 decomposition rate.

For the oxidation studies it was used the dye methylene blue as probe molecule. Methylene blue shows several interesting features as a probe molecule for oxidation reactions, such as: i) high solubility in water, ii) the oxidation can be monitored simply by spectrophotometric measurements, and iii) it simulates the behavior of textile dyes which are an important class of contaminant.

The reaction was monitored by the discoloration which is related to the first oxidation steps to produce non-colored intermediates:

Dye + H2O2 non-colored intermediates CO2/H2O

The discoloration activities in the presence of the perovskites LaMnO3 and LaMn0.1-xFe0.90Mox O3 are shown in Figure 7. Preliminary tests of adsorption showed that these perovskites does not adsorb significantly the dye methylene blue and the adsorption process did not interfere the discoloration results.

It is interesting to observe that the perovskite LaMnO3 did not present catalytic activity for the methylene blue discoloration. However, the presence of Mo in the perovskite (LaMn0.1-xFe0.90Mox O3) led to an increase on the activity. The linear behavior of the methylene blue discoloration plots again suggested a pseudo zeroth order kinetics. The methylene blue discoloration rates (kdiscol) are shown in the Figure 7b.

From Figure 7b it is clear that the kdiscol increases with the Mo content in the perovskite structure. It is interesting to observe from Figure 7b, that this increase is linear up to Mo content of 0.05 (Mo0.05). Although, the role of Mo to improve dye oxidation is not clear, one can envisage that surface Mo can activate H2O2 to form surface peroxomolybdenium complexes active for oxidation processes. These surface complexes can react directly with the dye molecule or can react with H2O to form highly reactive HO* radicals24.

 

4. Conclusion

The results obtained by XRD and Mössbauer spectroscopy showed that Fe and Mo can be incorporated in the perovskite structure. The addition of Fe to the series LaMn1-xFexO3 caused a decrease on the H2O2 decomposition activity. The perovskites LaMn0.1-xFe0.90Mox O3 did not show any significant catalytic activity for the H2O2 decomposition. On the other hand, for the heterogeneous Fenton reactions, the reaction rate increased with the Mo concentration.

 

Acknowledgements

The authors are grateful to CNPq, FAPEMIG, CAPES for financial support.

 

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Received: May 7, 2008; Revised: August 8, 2008

 

 

* e-mail: rochel@ufmg.br

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