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Preparation of Nano-MnFe2O4 and Its Catalytic Performance of Thermal Decomposition of Ammonium Perchlorate
PRODUCT ENGINEERING AND CHEMICAL TECHNOLOGY Chinese Journal of Chemical Engineering, 19(6) 1047ü1051 (2011)
Preparation of Nano-MnFe2O4 and Its Catalytic Performance of Thermal Decomposition of Ammonium Perchlorate* HAN Aijun (**)ࢋ̙ۂ, LIAO Juanjuan (ॷࡹࡹ), YE Mingquan (ྜྷੜ), LI Yan (हཀྵ) and PENG Xinhua ()ܟ School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Abstract Nano-MnFe2O4 particles were synthesized by co-precipitation phase inversion method and low-temperature combustion method respectively, using MnCl2, FeCl3, Mn(NO3)2, Fe(NO3)3, NaOH and C6H8O7. X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) were used to characterize the structure, morphology, thermal stability of MnFe2O4 and its catalytic performance to ammonium perchlorate. Results showed that single-phased and uniform spinel MnFe2O4 was obtained. The average particle size was about 30 and 20 nm. The infrared absorption peaks appeared at about 420 and 574 cm1, and the particles were stable below 524 °C. Using the two prepared catalysts, the higher thermal decomposition temperature of ammonium perchlorate was decreased by 77.3 and 84.9 °C respectively, while the apparent decomposition heat was increased by 482.5 and 574.3 J·g1. The catalytic mechanism could be explained by the favorable electron transfer space provided by outer d orbit of transition metal ions and the high specific surface absorption effect of MnFe2O4 particles. Keywords MnFe2O4, co-precipitation phase inversion method, low-temperature combustion method, ammonium perchlorate, catalysis
1
INTRODUCTION
Ammonium perchlorate (AP), as the important high energy ingredient of solid propellants in the national defense field, occupies a large proportion in the formula. The thermal decomposition of AP can directly affect the burning velocity and energy features of propellants. Therefore, an extensive study on the thermal decomposition of AP was carried out by researchers [1]. Result shows that a small amount of catalyst can reduce the thermal decomposition temperature of AP, especially the higher pyrolytic temperature, increase apparent decomposition heat of AP, so as to improve the burning velocity and efficiency of propellant. Up till now, many scholars have achieved certain results on the catalytic performance of Co3O4, Fe2O3, NiO, PbO, CuO and other metal oxide powders [16]. MnFe2O4 is a composite metal oxide with two transition metal elements in its formula and it is compatible with other components in propellant. However, the catalytic performance of MnFe2O4 to thermal decomposition of AP has not been reported yet. MnFe2O4 is a composite spinel material with cubic and face-centered lattice structure. Transition metal elements Mn and Fe lie in the tetrahedral and octahedral gap, and there are strong ionic bonds with the same intensity, force, thermal conductivity and thermal expansion between the metal ion and oxygen ion. Owing to the particularity of the element nature and the crystal structure, MnFe2O4 has been widely used in coating, magnetic recording media, catalysis and many other fields [7, 8]. Now MnFe2O4 is commonly prepared by soft
chemical methods, such as microemulsion method, hydrothermal method, sol-gel method and co-precipitation method [914]. Due to the low cost, convenient operation, small particle size and potential of industrialization, the co-precipitation phase inversion method and low-temperature combustion method are known to have a good application prospect. Moreover, the reports on MnFe2O4 prepared by these two methods have not been seen as yet. In this paper, co-precipitation phase inversion method and low-temperature combustion method will be adopted to prepare MnFe2O4 samples. The structure and composition, particle size, morphology, infrared characteristics, thermal stability of samples and their catalytic performance to thermal decomposition of AP are studied, and a preliminary analysis of catalytic mechanism is conducted, in order to explore a new catalyst to thermal decomposition of AP. 2 2.1
EXPERIMENTAL Preparation
2.1.1 Sample prepared by co-precipitation phase inversion method MnCl2·4H2O and FeCl3·6H2O were taken in molar ratio of Mn2+ΉFe3+ 1Ή2 to prepare 0.3 mol·L1 metal ion solution of 100 ml containing 0.1 mol·L1 Mn2+ and 0.2 mol·L1 Fe3+, which was then dropped slowly into 100 ml NaOH solution of 3 mol·L1 at the preheated temperature of 95 °C. After aging for 2 h with continuous stirring. the solution was filtered, washed and dried at 60 °C for 12 h to get sample A. All reagents used in this study are of analytical grade.
Received 2011-05-12, accepted 2011-11-03. * Supported by the National Natural Science Foundation of China (90305008, 51077072). ** To whom correspondence should be addressed. E-mail: [email protected]
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2.1.2 Sample prepared by low-temperature combustion method Mn(NO3)2 and Fe(NO3)3ǜ9H2O were taken in molar ratio of Mn2+ΉFe3+ 1 : 2 to form 50 ml nitrate solution (0.3 mol·L1), which was then dropped slowly into the prepared citric acid (0.3 mol·L1) of pH 5 with continuous stirring. Molar ratio of total metal ions and citric acid was kept 1Ή1, chelating for 2 h at 60 °C. Next, water was evaporated to form a dry gel, then it was treated in a muffle furnace at 450 °C for 1 h to get sample B. All reagents used in this study are of analytical grade. 2.2
Characterization
The crystal structure and composition of the samples were characterized by Bruker’s D8 Advance X-ray diffraction (using Cu-target, KĮ, 1.5406×1010 m, 2ș, 15°70°). The morphological analysis was performed with Tecnai-12 transmission electron microscope (Philips). The thermal stability was tested by TG-DTA-50 thermo gravimetric differential thermal analysis (Shimadzu, Japan, heating rate 20 °C·min1 and temperature from 50 °C to 1000 °C) and the FT-IR features was characterized by FTIR-8400S Fourier transform infrared spectrometer (Shimadzu, Japan). 2.3 Catalytic performance to thermal decomposition of AP AP and the prepared samples were taken in mass ratio of 97Ή3, and then blended and grinded in a certain amount of ethanol. After the volatilization of ethanol, the thermal decomposition analysis was carried out by DSC-823e (Mettler Toledo, Swiss). The temperature ranges from 50 to 500 °C with the heating rate 20 °C·min1 and the flow rate of N2 20 ml·min1, the catalytic activity of samples was measured by the changes of decomposition temperature and the apparent decomposition heat of AP. 3 3.1
RESULTS AND DISCUSSION Phase analysis
XRD patterns of samples A and B are presented as curves 1 and 2 in Fig. 1. It can be found that diffraction peaks appear at (111), (220), (311), (400), (511) and (440) planes in both curves 1 and 2. Compared with the standard PDF (Portable Document Format) card, they are recognized as the diffraction peaks of spinel MnFe2O4 (Joint Committee on Powder Diffraction Standards No. 10-0319), indicating that single phase spinel MnFe2O4 can be prepared by the above two methods. Using the co-precipitation phase inversion method, Fe(OH)3 was first formed, then į-FeOOH was generated [15], and part of Fe3+ in į-FeOOH was easy to be replaced by Mn2+ to get [(Mn2+)
Figure 1 XRD patterns of nano-MnFe2O4 1üsample A ; 2üsample B
(Fe3+)2(OH)4(O2)2], Finally MnFe2O4 was generated by aging [16, 17] in aqueous solution. The crystal nucleus grew and the lattice improved gradually under certain conditions, which results a high crystallinity and regular arrangement of the internal particles. So the intensity of crystal diffraction peak in curve 1 is higher and the peak shape is sharp and symmetrical. Homogeneous dry gel was first formed using citric acid and metal nitrate by the low temperature combustion method, then the dry gel was ignited and combusted quickly to get MnFe2O4. The particle size of the synthesized sample is small and in narrow distribution because of the good homogeneity of mixture of raw materials. However, the sample prepared by low temperature combustion method has a low crystallinity, and crystal dislocations and defects exist in the crystal of the sample, because of the relatively short reaction time. Therefore, the intensity of crystal diffraction peak in curve 2 is weaker and the peak shape is relatively broad. 3.2
Morphology analysis
The TEM images of sample A and B are presented in Fig. 2. The particles are cube-shaped corresponding to the cubic crystal system and they have regular morphology, uniform size and good dispersion. The average particle size of samples A and B is about 30 and 20 nm respectively. 3.3
FT-IR analysis
The FT-IR spectra of sample A and B correspond respectively to curves 1 and 2 as presented in Fig. 3. It is clear that there is no large difference in the infrared spectra between two samples. According to the structure analysis of spinel MnFe2O4, M2+ (such as Mg2+, Mn2+, Ni2+) occupies the Td symmetrical position, Fe3+ and O2 occupy the molecular position of D3d and C3v respectively. There are a total of 13 vibration freedom degrees in the molecule, of which four are infrared active and all of them are represented as F1u. Moreover, two high-frequency (ȣ1, ȣ2) IR absorption peaks appear at 600400 cm1 [18]. Hence the absorption peak about 420 and 574 cm1 of the two samples can
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(a) Sample A
(b) Sample B
Figure 2 TEM images of nano-MnFe2O4
Figure 3 IR spectra of nano-MnFe2O4 1üsample A; 2üsample B
be identified as the infrared absorption peaks of MnFe2O4. But the attribution of each characteristic absorption peak is still controversial. As each O2 is shared by a tetrahedral cation and three octahedral cations, the vibration of O2 can not only belong to tetrahedral or octahedral. Bujoreanu and Segal’s study 1 [19] shows that the absorption peak at 600400 cm of MnFe2O4 mainly depends on the vibration of the octahedral group. In addition, there is a wider peak at 574 cm1 of curve 2 with low absorption intensity, which belongs to the decreased molecular vibrational probability and vibrational dipole moment caused by the low crystallinity, crystal dislocations and defects of MnFe2O4 prepared by low temperature combustion method. 3.4
TG-DTA analysis
The TG-DTA curves of samples A and B are presented in Fig. 4, which can be used to characterize the thermal stability of the samples. The thermal stability of two samples is almost the same. It can be seen from TG that there is no mass loss below 792 °C. Small endothermic peaks appear at 524 °C and 527 °C corresponding to samples A and B in DTA curves while intensive endothermic peaks appear at 792 °C and 793 °C, respectively. Chen et al. [20] show that the
(a) Sample A
(b) Sample B Figure 4
TG-DTA curves of nano-MnFe2O4
endothermic peak located at about 300 °C is caused by the crystal structure change of MnFe2O4, which is driven by the distribution change of manganese cation in tetrahedral and octahedral. However, the temperature is not high enough to generate new materials. According to the study of Bonsdorf et al. [21], at this point the MnFe2O4 lattice is metastable. In Ref. [22], it can be seen from the XRD patterns of samples calcined in muffle furnace at different temperatures that the intensity of diffraction peak of MnFe2O4 becomes weaker and the crystallinity is decreased with increasing
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temperature. The characteristic peaks of Į-Fe2O3 and Į-Mn2O3 appear until 800 °C, indicating that the intensive endothermic peak at about 793 °C is caused by the partial decomposition of MnFe2O4 to Į-Mn2O3 and Į-Fe2O3. Therefore, it can be known that the prepared MnFe2O4 shows a good thermal stability below 524 °C and it can be used as the catalyst to thermal decomposition of AP below that temperature. 3.5
Evaluation of catalytic performance
3.5.1 Characterization of the catalytic performance of the samples to AP The thermal decomposition of pure AP can be divided into three stages [23]. The first stage corresponds to the phase transition endothermic process of AP, which transforms from orthorhombic to cubic crystal with the temperature ranging from 240 to 250 °C. The second stage corresponds to the low-temperature decomposition exothermic process of AP with the temperature ranging from 300 to 330 °C. The third stage corresponds to the higher temperature decomposition exothermic process of AP with the temperature ranging from 420 to 480 °C. Curves 1, 2 and 3 in Fig. 5 correspond to the differential scanning calorimetry (DSC) curves for pure AP, AP added sample A, and AP added sample B. It can be seen from the graph that the higher thermal decomposition temperature in curves 2 and 3 are 352.9 and 345.3 °C, respectively, decreased by 77.3 and 84.9 °C compared with pure AP (430.2 °C). The low-temperature exothermic decomposition peaks ( 289.2 and 284.6 °C) overlap with the higher temperature exothermic decomposition peak, and are not obvious in Fig. 5 owing to the large decreased temperature of the higher temperature decomposition peak. Moreover, the crystal transition temperatures locate at near 247 °C with no significant change. The DSC results also show that the apparent decomposition heat of AP added samples A or B are 1071.9 or 1163.7 J·g1 respectively, increasing greatly by 482.5 or 574.3 J·g1 compared with pure AP (589.4 J·g1).
AP and are better than nano metal Ni [24], nano alloy NiB [25] and nano metal oxide Fe2O3 [26]. MnFe2O4 prepared by low-temperature combustion method shows better catalytic activity. 3.5.2 Catalytic mechanism analysis A great deal of research on the thermal decomposition mechanism of AP has been done in recent years [2729]. The catalytic mechanism of MnFe2O4 to the thermal decomposition of AP lies in the following two aspects: Firstly, both transition metal ions Mn2+ and Fe3+ in the structure of MnFe2O4 have outer d orbital with 3d5 electronic configurations, and the d orbitals are not filled with electrons and have hole conductivity. By accepting electrons transferred from the degradation process of ClO4, their own hole annihilation is achieved, which objectively provides a useful space to the electron transfer in the thermal decomposition of AP, serves a bridge role and promotes the degradation of ClO4. Secondly, owing to the high specific surface area and large amount of surface active sites of nano-MnFe2O4 particle, it is easy to adsorb gas phase redox reaction molecules to their surface, which promotes redox reactions between them. Nano-MnFe2O4 crystal shows good catalytic effects by promoting the two key steps of AP thermal decomposition. Nano-MnFe2O4 prepared by low temperature combustion method shows better catalytic activity due to its smaller particle size. 4
CONCLUSIONS
Regular cube-shaped and single-phase spinel MnFe2O4 samples were synthesized by co-precipitation phase inversion method and low temperature combustion method respectively, with the average particle size of about 30 and 20 nm. In FTIR spectra, vibrational absorption peaks appear at about 420 and 574 cm1. The synthesized MnFe2O4 was thermally stable below 524 °C. Nano-MnFe2O4 particles synthesized by the two methods decreased the higher thermal decomposition temperature of AP by 77.3 and 84.9 °C and increased the exothermic quantity of decomposition by 482.5 and 574.3 J·g1 respectively, showing good catalytic effect on AP decomposition. MnFe2O4 synthesized by low temperature combustion method showed better catalytic activity due to its smaller particle size. REFERENCES 1
Figure 5 DSC curves of AP catalyzed by nano-MnFe2O4 1üAP; 2üAP + sample A; 3üAP + sample B
Therefore, both MnFe2O4 samples show good catalytic performance to the thermal decomposition of
2
3
Chen, W.F., Li, F.S., Liu, J.X., “Preparation of nanocrystalline Co3O4 and its catalytic performance for thermal decomposition of ammonium perchlorate”, J. Catal., 26 (12), 10731077 (2005). (in Chinese) Ma, Z.Y., Li, F.S., Cui, P., Bai, H.P., “Preparation of nanocrystalline Fe3O4 and its catalytic performance for thermal decomposition of ammonium perchlorate”, J. Catal., 24 (10), 795798 (2003). (in Chinese) Zhu, J.W., Chen, H.Q., Xie, B., Yang, X.J., Lu, L.D., Wang, X., “Preparation of nano crystalline Cu2O and its catalytic performance for thermal decomposition of ammonium perchlorate”, J. Catal., 25
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and Co substitution į-FeOOH and its decomposition products”, J. Mater. Sci., 25, 52075214 (1990). Shi, X., Wang, G., Li, C., “Preparation of ferrite ultrafine particle catalysts by sol-gelmethod”, Chem. Res. Appl., 14 (5), 531533 (2002). (in Chinese) Bujoreanu, V., Segal, E., Brezeanu, M., “On the formation of manganese ferrite from MnO2 and FeSO4·7H2O aqueous solutions through coprecipitation”, Thermochim. Acta, 288, 221237 (1996). Chen, J., Sorensen, C., Klabunde, J., “Size-dependent magnetic properties of MnFe2O4 fine particles synthesized by coprecipitation”, Phys. Rev. B, 54 (13), 92889296 (1996). Bonsdorf, G., Deneckeb, M., Christen, S., “X-ray absorption spectroscopic and Mössbauer studies of redox and cation-ordering processes in manganese ferrite”, Solid State Ionics., 101, 351357 (1997). Zhang, W., “Synthesis and properties research of spinel iron acid manganese materials”, M.A. Thesis, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing (2010). (in Chinese) Boldyrev, V., “Thermal decomposition of ammonium perchlorate”, Thermochim. Acta, 443, 136 (2006). Li, R., Liu, X.X., Wang, X.J., “Preparation of Ni and Ni-P nanoparticles and their catalytic effect on the decomposition of ammonium perchlorate”, Initiators Pyro. Techn., 4, 1518 (2008). (in Chinese) Yang, Y., Pan, Z.H., Li, L.X., Li, Y.B., Cao, X.F., “Preparation of nanometer NiB/Al composite and its thermal catalysis effect on AP decomposition”, Chinese Journal of Energetic Materials, 17 (4), 446450 (2009). (in Chinese) Lin, B.L., Bu, J.J., Zheng, H.Y., Zhao, W.Z., “Preparation and catalytic performance on ammonium perchlorate decomposition of Fe2O3 nanoparticles”, Chemical Defence on Ships, 4, 1215 (2009). (in Chinese) Duan, H.Z., Lin, X.Y., Liu, G.P., Xu, L., Li, F.S., “Synthesis of Co nanoparticles and their catalytic effect on the decomposition of ammonium perchlorate”, Chin. J. Chem. Eng., 16 (2), 325328 (2008). Sergey, V., Charles, A.W., “Kinetics of thermal decomposition of cubic ammonium perchlorate”, Chem. Mater., 11 (11), 33863393 (1999). Fan, X.Z., Li, J.Z., Fu, X.L., Wang, H., “Thermal decompositions of ammonium perchlorate of various granularities”, Acta Chim. Sinica, 67 (1), 3944 (2009).
Preparation of Nano-MnFe2O4 and Its Catalytic Performance of Thermal Decomposition of Ammonium Perchlorate* HAN Aijun (**)ࢋ̙ۂ, LIAO Juanjuan (ॷࡹࡹ), YE Mingquan (ྜྷੜ), LI Yan (हཀྵ) and PENG Xinhua ()ܟ School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Abstract Nano-MnFe2O4 particles were synthesized by co-precipitation phase inversion method and low-temperature combustion method respectively, using MnCl2, FeCl3, Mn(NO3)2, Fe(NO3)3, NaOH and C6H8O7. X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) were used to characterize the structure, morphology, thermal stability of MnFe2O4 and its catalytic performance to ammonium perchlorate. Results showed that single-phased and uniform spinel MnFe2O4 was obtained. The average particle size was about 30 and 20 nm. The infrared absorption peaks appeared at about 420 and 574 cm1, and the particles were stable below 524 °C. Using the two prepared catalysts, the higher thermal decomposition temperature of ammonium perchlorate was decreased by 77.3 and 84.9 °C respectively, while the apparent decomposition heat was increased by 482.5 and 574.3 J·g1. The catalytic mechanism could be explained by the favorable electron transfer space provided by outer d orbit of transition metal ions and the high specific surface absorption effect of MnFe2O4 particles. Keywords MnFe2O4, co-precipitation phase inversion method, low-temperature combustion method, ammonium perchlorate, catalysis
1
INTRODUCTION
Ammonium perchlorate (AP), as the important high energy ingredient of solid propellants in the national defense field, occupies a large proportion in the formula. The thermal decomposition of AP can directly affect the burning velocity and energy features of propellants. Therefore, an extensive study on the thermal decomposition of AP was carried out by researchers [1]. Result shows that a small amount of catalyst can reduce the thermal decomposition temperature of AP, especially the higher pyrolytic temperature, increase apparent decomposition heat of AP, so as to improve the burning velocity and efficiency of propellant. Up till now, many scholars have achieved certain results on the catalytic performance of Co3O4, Fe2O3, NiO, PbO, CuO and other metal oxide powders [16]. MnFe2O4 is a composite metal oxide with two transition metal elements in its formula and it is compatible with other components in propellant. However, the catalytic performance of MnFe2O4 to thermal decomposition of AP has not been reported yet. MnFe2O4 is a composite spinel material with cubic and face-centered lattice structure. Transition metal elements Mn and Fe lie in the tetrahedral and octahedral gap, and there are strong ionic bonds with the same intensity, force, thermal conductivity and thermal expansion between the metal ion and oxygen ion. Owing to the particularity of the element nature and the crystal structure, MnFe2O4 has been widely used in coating, magnetic recording media, catalysis and many other fields [7, 8]. Now MnFe2O4 is commonly prepared by soft
chemical methods, such as microemulsion method, hydrothermal method, sol-gel method and co-precipitation method [914]. Due to the low cost, convenient operation, small particle size and potential of industrialization, the co-precipitation phase inversion method and low-temperature combustion method are known to have a good application prospect. Moreover, the reports on MnFe2O4 prepared by these two methods have not been seen as yet. In this paper, co-precipitation phase inversion method and low-temperature combustion method will be adopted to prepare MnFe2O4 samples. The structure and composition, particle size, morphology, infrared characteristics, thermal stability of samples and their catalytic performance to thermal decomposition of AP are studied, and a preliminary analysis of catalytic mechanism is conducted, in order to explore a new catalyst to thermal decomposition of AP. 2 2.1
EXPERIMENTAL Preparation
2.1.1 Sample prepared by co-precipitation phase inversion method MnCl2·4H2O and FeCl3·6H2O were taken in molar ratio of Mn2+ΉFe3+ 1Ή2 to prepare 0.3 mol·L1 metal ion solution of 100 ml containing 0.1 mol·L1 Mn2+ and 0.2 mol·L1 Fe3+, which was then dropped slowly into 100 ml NaOH solution of 3 mol·L1 at the preheated temperature of 95 °C. After aging for 2 h with continuous stirring. the solution was filtered, washed and dried at 60 °C for 12 h to get sample A. All reagents used in this study are of analytical grade.
Received 2011-05-12, accepted 2011-11-03. * Supported by the National Natural Science Foundation of China (90305008, 51077072). ** To whom correspondence should be addressed. E-mail: [email protected]
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2.1.2 Sample prepared by low-temperature combustion method Mn(NO3)2 and Fe(NO3)3ǜ9H2O were taken in molar ratio of Mn2+ΉFe3+ 1 : 2 to form 50 ml nitrate solution (0.3 mol·L1), which was then dropped slowly into the prepared citric acid (0.3 mol·L1) of pH 5 with continuous stirring. Molar ratio of total metal ions and citric acid was kept 1Ή1, chelating for 2 h at 60 °C. Next, water was evaporated to form a dry gel, then it was treated in a muffle furnace at 450 °C for 1 h to get sample B. All reagents used in this study are of analytical grade. 2.2
Characterization
The crystal structure and composition of the samples were characterized by Bruker’s D8 Advance X-ray diffraction (using Cu-target, KĮ, 1.5406×1010 m, 2ș, 15°70°). The morphological analysis was performed with Tecnai-12 transmission electron microscope (Philips). The thermal stability was tested by TG-DTA-50 thermo gravimetric differential thermal analysis (Shimadzu, Japan, heating rate 20 °C·min1 and temperature from 50 °C to 1000 °C) and the FT-IR features was characterized by FTIR-8400S Fourier transform infrared spectrometer (Shimadzu, Japan). 2.3 Catalytic performance to thermal decomposition of AP AP and the prepared samples were taken in mass ratio of 97Ή3, and then blended and grinded in a certain amount of ethanol. After the volatilization of ethanol, the thermal decomposition analysis was carried out by DSC-823e (Mettler Toledo, Swiss). The temperature ranges from 50 to 500 °C with the heating rate 20 °C·min1 and the flow rate of N2 20 ml·min1, the catalytic activity of samples was measured by the changes of decomposition temperature and the apparent decomposition heat of AP. 3 3.1
RESULTS AND DISCUSSION Phase analysis
XRD patterns of samples A and B are presented as curves 1 and 2 in Fig. 1. It can be found that diffraction peaks appear at (111), (220), (311), (400), (511) and (440) planes in both curves 1 and 2. Compared with the standard PDF (Portable Document Format) card, they are recognized as the diffraction peaks of spinel MnFe2O4 (Joint Committee on Powder Diffraction Standards No. 10-0319), indicating that single phase spinel MnFe2O4 can be prepared by the above two methods. Using the co-precipitation phase inversion method, Fe(OH)3 was first formed, then į-FeOOH was generated [15], and part of Fe3+ in į-FeOOH was easy to be replaced by Mn2+ to get [(Mn2+)
Figure 1 XRD patterns of nano-MnFe2O4 1üsample A ; 2üsample B
(Fe3+)2(OH)4(O2)2], Finally MnFe2O4 was generated by aging [16, 17] in aqueous solution. The crystal nucleus grew and the lattice improved gradually under certain conditions, which results a high crystallinity and regular arrangement of the internal particles. So the intensity of crystal diffraction peak in curve 1 is higher and the peak shape is sharp and symmetrical. Homogeneous dry gel was first formed using citric acid and metal nitrate by the low temperature combustion method, then the dry gel was ignited and combusted quickly to get MnFe2O4. The particle size of the synthesized sample is small and in narrow distribution because of the good homogeneity of mixture of raw materials. However, the sample prepared by low temperature combustion method has a low crystallinity, and crystal dislocations and defects exist in the crystal of the sample, because of the relatively short reaction time. Therefore, the intensity of crystal diffraction peak in curve 2 is weaker and the peak shape is relatively broad. 3.2
Morphology analysis
The TEM images of sample A and B are presented in Fig. 2. The particles are cube-shaped corresponding to the cubic crystal system and they have regular morphology, uniform size and good dispersion. The average particle size of samples A and B is about 30 and 20 nm respectively. 3.3
FT-IR analysis
The FT-IR spectra of sample A and B correspond respectively to curves 1 and 2 as presented in Fig. 3. It is clear that there is no large difference in the infrared spectra between two samples. According to the structure analysis of spinel MnFe2O4, M2+ (such as Mg2+, Mn2+, Ni2+) occupies the Td symmetrical position, Fe3+ and O2 occupy the molecular position of D3d and C3v respectively. There are a total of 13 vibration freedom degrees in the molecule, of which four are infrared active and all of them are represented as F1u. Moreover, two high-frequency (ȣ1, ȣ2) IR absorption peaks appear at 600400 cm1 [18]. Hence the absorption peak about 420 and 574 cm1 of the two samples can
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(a) Sample A
(b) Sample B
Figure 2 TEM images of nano-MnFe2O4
Figure 3 IR spectra of nano-MnFe2O4 1üsample A; 2üsample B
be identified as the infrared absorption peaks of MnFe2O4. But the attribution of each characteristic absorption peak is still controversial. As each O2 is shared by a tetrahedral cation and three octahedral cations, the vibration of O2 can not only belong to tetrahedral or octahedral. Bujoreanu and Segal’s study 1 [19] shows that the absorption peak at 600400 cm of MnFe2O4 mainly depends on the vibration of the octahedral group. In addition, there is a wider peak at 574 cm1 of curve 2 with low absorption intensity, which belongs to the decreased molecular vibrational probability and vibrational dipole moment caused by the low crystallinity, crystal dislocations and defects of MnFe2O4 prepared by low temperature combustion method. 3.4
TG-DTA analysis
The TG-DTA curves of samples A and B are presented in Fig. 4, which can be used to characterize the thermal stability of the samples. The thermal stability of two samples is almost the same. It can be seen from TG that there is no mass loss below 792 °C. Small endothermic peaks appear at 524 °C and 527 °C corresponding to samples A and B in DTA curves while intensive endothermic peaks appear at 792 °C and 793 °C, respectively. Chen et al. [20] show that the
(a) Sample A
(b) Sample B Figure 4
TG-DTA curves of nano-MnFe2O4
endothermic peak located at about 300 °C is caused by the crystal structure change of MnFe2O4, which is driven by the distribution change of manganese cation in tetrahedral and octahedral. However, the temperature is not high enough to generate new materials. According to the study of Bonsdorf et al. [21], at this point the MnFe2O4 lattice is metastable. In Ref. [22], it can be seen from the XRD patterns of samples calcined in muffle furnace at different temperatures that the intensity of diffraction peak of MnFe2O4 becomes weaker and the crystallinity is decreased with increasing
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temperature. The characteristic peaks of Į-Fe2O3 and Į-Mn2O3 appear until 800 °C, indicating that the intensive endothermic peak at about 793 °C is caused by the partial decomposition of MnFe2O4 to Į-Mn2O3 and Į-Fe2O3. Therefore, it can be known that the prepared MnFe2O4 shows a good thermal stability below 524 °C and it can be used as the catalyst to thermal decomposition of AP below that temperature. 3.5
Evaluation of catalytic performance
3.5.1 Characterization of the catalytic performance of the samples to AP The thermal decomposition of pure AP can be divided into three stages [23]. The first stage corresponds to the phase transition endothermic process of AP, which transforms from orthorhombic to cubic crystal with the temperature ranging from 240 to 250 °C. The second stage corresponds to the low-temperature decomposition exothermic process of AP with the temperature ranging from 300 to 330 °C. The third stage corresponds to the higher temperature decomposition exothermic process of AP with the temperature ranging from 420 to 480 °C. Curves 1, 2 and 3 in Fig. 5 correspond to the differential scanning calorimetry (DSC) curves for pure AP, AP added sample A, and AP added sample B. It can be seen from the graph that the higher thermal decomposition temperature in curves 2 and 3 are 352.9 and 345.3 °C, respectively, decreased by 77.3 and 84.9 °C compared with pure AP (430.2 °C). The low-temperature exothermic decomposition peaks ( 289.2 and 284.6 °C) overlap with the higher temperature exothermic decomposition peak, and are not obvious in Fig. 5 owing to the large decreased temperature of the higher temperature decomposition peak. Moreover, the crystal transition temperatures locate at near 247 °C with no significant change. The DSC results also show that the apparent decomposition heat of AP added samples A or B are 1071.9 or 1163.7 J·g1 respectively, increasing greatly by 482.5 or 574.3 J·g1 compared with pure AP (589.4 J·g1).
AP and are better than nano metal Ni [24], nano alloy NiB [25] and nano metal oxide Fe2O3 [26]. MnFe2O4 prepared by low-temperature combustion method shows better catalytic activity. 3.5.2 Catalytic mechanism analysis A great deal of research on the thermal decomposition mechanism of AP has been done in recent years [2729]. The catalytic mechanism of MnFe2O4 to the thermal decomposition of AP lies in the following two aspects: Firstly, both transition metal ions Mn2+ and Fe3+ in the structure of MnFe2O4 have outer d orbital with 3d5 electronic configurations, and the d orbitals are not filled with electrons and have hole conductivity. By accepting electrons transferred from the degradation process of ClO4, their own hole annihilation is achieved, which objectively provides a useful space to the electron transfer in the thermal decomposition of AP, serves a bridge role and promotes the degradation of ClO4. Secondly, owing to the high specific surface area and large amount of surface active sites of nano-MnFe2O4 particle, it is easy to adsorb gas phase redox reaction molecules to their surface, which promotes redox reactions between them. Nano-MnFe2O4 crystal shows good catalytic effects by promoting the two key steps of AP thermal decomposition. Nano-MnFe2O4 prepared by low temperature combustion method shows better catalytic activity due to its smaller particle size. 4
CONCLUSIONS
Regular cube-shaped and single-phase spinel MnFe2O4 samples were synthesized by co-precipitation phase inversion method and low temperature combustion method respectively, with the average particle size of about 30 and 20 nm. In FTIR spectra, vibrational absorption peaks appear at about 420 and 574 cm1. The synthesized MnFe2O4 was thermally stable below 524 °C. Nano-MnFe2O4 particles synthesized by the two methods decreased the higher thermal decomposition temperature of AP by 77.3 and 84.9 °C and increased the exothermic quantity of decomposition by 482.5 and 574.3 J·g1 respectively, showing good catalytic effect on AP decomposition. MnFe2O4 synthesized by low temperature combustion method showed better catalytic activity due to its smaller particle size. REFERENCES 1
Figure 5 DSC curves of AP catalyzed by nano-MnFe2O4 1üAP; 2üAP + sample A; 3üAP + sample B
Therefore, both MnFe2O4 samples show good catalytic performance to the thermal decomposition of
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