LaMn1-xFe xO3 and LaMn0.1-xFe0.90Mo x O3 perovskites: synthesis, characterization and catalytic activity in H2O2 reactions

REGULAR ARTICLES

LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x O₃ perovskites: synthesis, characterization and catalytic activity in H₂O₂ reactions

Fabiano Magalhães^I; Flavia Cristina Camilo Moura^II; José Domingos Ardisson^III; 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: LaMn_1-xFe_xO₃, and LaMn_0.1-x Fe_0.90Mo_xO₃. 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 NO_x reduction⁵. Several reviews covering these fields can be found in the literature^6,7. Perovskites, especially LaMnO₃, have also been used in environmental applications, e.g. the oxidation of hydrocarbons^8-10, chlorinated organics¹¹ and H₂O₂ reactions^6,12. 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. La_1-xA_xMn_1-yM_yO₃ (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 investigated^13,14.

In this work, we studied the isomorphic substitution of Mn in the LaMnO₃ structure by different amounts of Fe and Mo to produce LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x 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 LaMn_yFe_xMo_zO₃ . 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 LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x 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 arrangement^16,17. 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 LaFe_0.90Mn_0.1-xMo_x 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 LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x O₃ varying between 120-144 nm and 11-15 m² g^-1.

Mössbauer spectra of the LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x O₃ perovskites are shown in Figure 3, with the hyperfine parameters in Table 1.

The spectra of LaMn_0.73Fe_0.27O₃ and LaMn_0.46Fe_0.54O₃ 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 LaMn_0.10Fe_0.90O₃ perovskite, for example, shows a signal with hyperfine parameters at d 0.37 mm/s, ε -0.05 mm/s and magnetic hyperfine field (H_hf) 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 H_hf 49.2 T is also observed with a relative area of 53%. This is identified as the poorly crystallized LaFeO₃^17,18 probably due to small particle size or to the presence of Mn.

The results for the Mo containing LaMn_0.08Fe_0.90Mo_0.02 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 H_hf 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 H_hf 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 LaMn_0.46Fe_0.54O₃ 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 literature^18-19. 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 LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x 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 LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.90Mo_x O₃. It is interesting to observe that the reactions without catalyst and the reactions in the presence of LaMn_0.10Fe_0.90O₃ and LaMn_0.1-xFe_0.90Mo_x 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. LaMn_0.46Fe_0.54O₃ LaMn_0.73Fe_0.27O₃, 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 (Fe_0.27) led to an increase on the H₂O₂ decomposition rate (k_dec). On the other hand, it is observed a significant decrease of the H₂O₂ decomposition rate for the perovskite with higher Fe concentration (Fe_0.57 e Fe_0.90). Figure 6b shows that the rates of the H₂O₂ decomposition in the presence of LaMn_0.1-xFe_0.90Mo_x 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 LaMn_0.1-xFe_0.90Mo_x 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 (LaMn_0.1-xFe_0.90Mo_x 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 (k_discol) are shown in the Figure 7b.

From Figure 7b it is clear that the k_discol 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 (Mo_0.05). 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 LaMn_1-xFe_xO₃ caused a decrease on the H₂O₂ decomposition activity. The perovskites LaMn_0.1-xFe_0.90Mo_x 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.

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

• 1. Tao S, Irvine JTS, Kilner JA. An Efficient Solid Oxide Fuel Cell Based upon Single-Phase Perovskites. Advanced Materials. 2005; 17(14):1734-1737.
• 2. Kim J, Hwang DW, Kim HG, Bae SW, Lee JS, Li W, Oh SH. Highly Efficient Overall Water Splitting Through Optimization of Preparation and Operation Conditions of Layered Perovskite Photocatalysts. Topics in Catalysis. 2005; 35(3-4): 295-303.
• 3. Ichimura K, Inoue Y, Yasumori I. Hydrogenation and hydrogenolysis of hydrocarbons on perovskite oxides. Catalysis Reviews - Science and Engineering 1992; 34(4):301-20.
• 4. Isupova LA, Sutormina EF, Kulikovskaya NA, Plyasova LM, Rudina NA, Ovsyannikova IA, Zolotarskii IA, Sadykov VA. Honeycomb supported perovskite catalysts for ammonia oxidation processes. Catalysis Today. 2005; 105(3-4):429-435.
• 5. Liu ZM, Hao JM, Fu LX, Zhu TL. Study of Ag/La_0.6Ce_0.4CoO₃ catalysts for direct decomposition and reduction of nitrogen oxides with propene in the presence of oxygen. Applied Catalysis B: Environmental. 2003; 44(4):355-370.
• 6. Keane, MA. Ceramics for catalysis. Journal Of Materials Science 2003; 38(23):4661-4675.
• 7. Pena MA, Fierro JLG. Chemical Structures and Performance of Perovskite Oxides. Chemical Reviews. 2001; 101(7):1981-2018.
• 8. Yi N, Cao Y, Su Y, Dai WL, He HY, Fan KN. Nanocrystalline LaCoO₃ perovskite particles confined in SBA-15 silica as a new efficient catalyst for hydrocarbon oxidation. Journal of Catalysis 2005; 230(1):249-253.
• 9. Stephan K, Hackenberger M, Kiessling D, Wendt G. Total Oxidation of Methane and Chlorinated Hydrocarbons on Zirconia Supported A_1-xSr_xMnO₃ Catalysts. Chemical Engineering & Technology. 2004; 27(6):687-693.
• 10. Lee YN, Lago RM, Fierro JLG, Cortes V, Sapina F, Martinez E. Surface properties and catalytic performance for ethane combustion on La_1-xK_xMnO₃₊δ perovskites. Applied Catalysis, A: General 2001; 207(1,2):17 24.
• 11. Lago RM, Moura FCC, Araujo MH, Ardisson JD, Macedo WA, Albuquerque AS. Investigation of the solid state reaction of LaMnO₃ with Fe° and its effect on the catalytic reactions with H₂O₂ Journal of the Brazilian Chemical Society. 2007; 18(2):322-329.
• 12. Popa M, Frantti J, Kakihana M. Lanthanum ferrite LaFeO₃₊δ nanopowders obtained by the polymerizable complex method. Solid State Ionics 2002; 154-155;437-445.
• 13. Lee YN, Lago RM, Fierro JLG, Gonzalez J. Hydrogen peroxide decomposition over Ln_1-xA_xMnO₃ (Ln = La or Nd and A = K or Sr) perovskites. Applied Catalysis, A: General. 2001; 215(1-2):245-256.
• 14. Lee YN, Lago RM, Fierro JLG. Surface properties and catalytic performance for ethane combustion on La_1-xK_xMnO₃₊δ perovskites. Applied Catalysis, A: General. 2001; 207(1,2):17-24.
• 15. Wandekar RV, Wani BN, Bharadwaj SR. High temperature phase transition in Sm_0.95MnO_2.925 Journal of Alloys and Compounds. 2007; 437(1-2):53-57.
• 16. Naray-Szabo S, The structural type of perovskite (CaTiO₃). Naturwissenschaften. 1943; 31:202-3.
• 17. Islam MS, Cherry M, Catlow CRA. Oxygen diffusion in LaMnO₃ and LaCoO₃ perovskite-type oxides: a molecular dynamics study. Journal of Solid State Chemistry. 1996; 124(2):230-237.
• 18. Berry FJ, Ren X, Marco JF. Reduction properties of perovskite-related rare earth orthoferrites. Czechoslovak Journal of Physics. 2005; 55(7):771-780.
• 19. Zhong Z, Chen K, Ji Y, Yan Q. Methane combustion over B-site partially substituted perovskite-type LaFeO₃ prepared by sol-gel method. Applied Catalysis, A: General. 1997; 156(1):29-41.
• 20. Sinquin G, Petit C, Hindermann JP, Kiennemann A. Study of the formation of LaMO₃ (M = Co, Mn) perovskites by propionates precursors: application to the catalytic destruction of chlorinated VOCs. Catalysis Today. 2001; 70(1-3):183-196.
• 21. Poplawski K, Lichtenberger J, Keil FJ, Schnitzlein K, Amiridis MD. Catalytic oxidation of 1,2-dichlorobenzene over ABO₃-type perovskites. Catalysis Today. 2000; 62(4):329-336.
• 22. Sinquin G, Petit C, Libs S, Hindermann JP, Kiennemann A. Catalytic destruction of chlorinated C2 compounds on a LaMnO₃₊ perovskite catalyst. Applied Catalysis, B: Environmental. 2001; 32(1-2):37-47
• 23. Spinicci R, Tofanari A, Delmastro A, Mazza D, Ronchetti S. Atalytic properties of stoichiometric and non-stoichiometric LaFeO₃ perovskite for total oxidation of methane. Materials Chemistry and Physics. 2002; 76(1):20-25.
• 24. Gutiérrez JLG, Fuentes GA, Terán MEH, García P, Guevara FM, Cruz FJ. Ultra-deep oxidative desulfurization of diesel fuel by the Mo/Al₂O₃-H₂O² system: The effect of system parameters on catalytic activity. Applied Catalysis, A: General 2008; 334(1-2):366-373.

Publication Dates

• Publication in this collection
  29 Oct 2008
• Date of issue
  Sept 2008

History

• Reviewed
  08 Aug 2008
• Received
  07 May 2008