Preparation and characterization of nanometer perovskite-type complex oxides LaMnO3.15 and their application in catalytic oxidation

Preparation and characterization of nanometer perovskite-type complex oxides LaMnO3.15 and their application in catalytic oxidation

Materials Letters 59 (2005) 1373 – 1377 www.elsevier.com/locate/matlet Preparation and characterization of nanometer perovskite-type complex oxides L...

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Materials Letters 59 (2005) 1373 – 1377 www.elsevier.com/locate/matlet

Preparation and characterization of nanometer perovskite-type complex oxides LaMnO3.15 and their application in catalytic oxidation Zhongai Hu*, Yuying Yang, Xiuli Shang, Hailong Pang Key Lab. of Polymer, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Received 22 September 2004; accepted 1 December 2004 Available online 12 January 2005

Abstract LaMnO3+d nanocrystalline powders were prepared using a new method in which the insoluble carboxyl-containing grafted starch copolymer (ISC) with a 3D tangled network was used to adsorb metal ions by carboxyl and form precursors. The structural components in the ISC and the precursors were identified from Fourier transform infrared spectra (FT-IR) of the samples. Thermogravimetric-differential thermal analysis (TG-DTA) for the precursors gave the detail of their decomposition in the dry air. The crystalline phase of LaMnO3.15 was characterized by X-ray diffraction (XRD). The shape and size distribution of particles were observed by transmission electron microscopy (TEM). The resultant nanoparticles were firstly used as catalysts for selective oxidation in solution and exhibited evidently catalytic activity for oxidation of cinnamaldehyde at moderate temperatures. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Perovskite; Preparation; Grafted starch; Catalytic oxidation

1. Introduction Perovskite type complex oxides are promising materials for application as electrode materials in solid oxide fuel cells [1–3], exhaust gas sensors in automobiles, membranes for separation processes and as catalysts [4–6]. Intensive research is devoted to the perovskite type oxides based on rare earth and 3d transition element properties with formula LaMeO3 (Me: Mn, Co, Fe). Among the various materials, LaMnO3 and related compounds have especially attracted much attention due to its excellent catalytic activity. However, these materials usually have low specific surface areas (generallyb10 m2/g), so the potential applications of these materials as catalysts are limited. In order to solve this problem, the best way is to process materials into nanoconstructures. It is well known that nanosized materials exhibit unique physical and chemical properties compared with those of conventional bulk materials [7–10] because of its smaller size and higher specific surface areas. * Corresponding author. 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.12.047

Many methods have been reported recently, which include physical techniques [11], chemical vapor deposition [12,13], microemulsion [14], solid-state reaction [15], and other chemical solution methods such as sol-gel [16], polymeric gel [17], amorphous citrate [18] or coprecipitation [19]. However the solid-state reaction method has some drawbacks of high reaction temperature, large size of particles, deficiency of chemical homogeneity and low specific surface areas (generally below 10 m2/g). The chemical solution methods can provide products of fine and homogeneous particles with high specific surface areas, but the processes are generally complicated and the agents used are very expensive. The need still exists for an easy, inexpensive and effective method for synthesizing fine and uniform LaMnO3 nanoparticles. Many research activities have devoted themselves to find the development of a synthesis method with low-cost and high-volume production [20]. In this paper we presented a new method of preparing perovskites LaMnO3+d nanoparticles. The insoluble carboxyl-containing grafted starch copolymer (ISC) with a 3D tangled network was used to adsorb metal ions in stoichiometric and form precursors, that were heated at high

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temperature to prepare well-controlled nanoparticles, which were used as a heterogeneous phase catalyst for conversion of cinnamaldehyde to its acid.

2. Experimental 2.1. Synthesis of ISC The insoluble carboxyl-containing grafted starch copolymer (ISC) was prepared following the procedure adopted by Zhang et al. [21]. The reactions were carried out in a 250-ml three-neck flask equipped with a stirrer and condenser, and the flask was immersed into a thermostat water bath. The N2 gas purged into the flask to remove the oxygen during the reaction. The starch slurry was prepared from 2 g of commercial crosslinked starch and 100 ml deionized water, and it was preheated at 85 8C for about 30 min with stirring. After it was gelatinized, the flask content was cooled to a 45 8C temperature. Then an 2.0103 mold L 1 kalium hydroxide was added to the flask to allow the starch to be preoxidized for 10 min, and then the required amount of acrylonitrile and an 5.0102 mold L 1 concentration of sulphuric acid were introduced. After the flask was kept in a thermostat water bath at 45 8C for 5 h, the grafting reaction was terminated. The grafted-copolymer was transferred to a sodium hydroxide solution and allowed to saponification react at approximately 90 8C for 2 h. Then the reaction mixture was added to excess methanol/water cosolvents (volume ration 7:3) to remove the unreacted residue and the possibly formed homopolymer. The specimen was vacuum dried at 60 8C until the weight of the specimen was constant. 2.2. Preparation of LaMnO3.15 Analytical grade Mn(Ac)2d 4H2O, La(NO3)3d 6H2O and NH3d H2O were used in the experiment. Mn(Ac)2d 4H2O and La(NO3)3d 6H2O were firstly dissolved in water in stoichiometric ratio, and then excessive amount of ISC was added to the solution. Ammonia water was slowly added to adjust the pH to 6–7. The mixtures were magnetically stirred at room temperature in order to obtain the stable ISC-metal complexes. After 3 h the mixtures were filtered and repeatedly washed with distilled water and absolute ethanol, and then dried under vacuum at 80 8C to obtain ISC-metal precursor. The precursor was poured into a tray and heated slowly to 475 8C, then the intermediate products were crushed in a agate mortar and were calcined again for 4 h at 600 8C, 700 8C, 800 8C, respectively, to form nanoparticles of LaMnO3+d .

bath at 50 8C. The mixture was stirred under O2 for 15 min. Then 10 g cinnamaldehyde was added drop by drop and the mixture was stirred at 50 8C for a certain time. After the catalyst was separated from the mixture by filtration, 10% HCl was added to adjust the pH to 6–7. Subsequently, some active carbon were added, and the resulting mixture was stirred at 30 8C for 30 min. The acicular crystal grew in the aqueous phase separated and acidified by 10% HCl. The resulting product was washed with distilled water and dried under vacuum. 2.4. Characterization Infrared spectroscopy was carried out on a Nicolet Nexus 670 FT-IR to measure the structural components in ISC and ISC-metal ions. Thermogravimetric-differential thermal analysis(TG-DTA)of ISC-La/Mn was carried out to understand the detail of their decomposition process in the dry air with a heating rate of 10 8C/min on the Shimadzu TA50WSITG and DTA instruments. X-ray diffraction measurement was performed by a Rigaku D/MAX2400 diffractomter with Cu-Ka radiation to investigate the crystalline phase of LaMnO3+y. The TEM (JEM-1200EX, Japan) was used to investigate the morphology and the size distribution of nanoparticles.

3. Result and discussion 3.1. IR analysis The curves (a) and (b) are respectively the IR spectra of pure ISC and the ISC -metal ions in Fig. 1. The bands in 1021 cm 1–1156 cm 1 and 3410 cm 1 contributed to –C– O–C– and –OH groups on the starch. The absorption peak at 1410 cm 1 shows that –CONH2 has not saponified

2.3. Measurement of catalytic activity Catalytic oxidation experiments for cinnamaldehyde were performed in a three-neck flask with 130 ml distilled water, 100 mg catalyst and 1.4 g sodium hydroxide in water

Fig. 1. The infrared spectrum of ISC (a) and ISC-La/Mn (b).

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completely. The molar percentage of –CONH2 group decreases with the saponification time, but the conversion cannot take place quantificationally. The ISC has absorption bands in 1570 cm 1 and 1680 cm 1 due to stretching vibration of carboxylate. In the IR spectra of ISC-metal ions the carboxylate vibration peaks at 1570 cm 1 and 1680 cm 1 disappear, but a new peak at 1668 cm 1 occurs. It can be concluded that chelate bonds between ISC and metal ions form. 3.2. Thermal analysis The thermal analysis results of TG and DTA for ISC-La/ Mn precursor were shown in Fig. 2. The change of residual weight with temperature was shown with an integral curve. There were mainly three-weight loss region in the TG curves. The weight loss in each region was corresponding with the thermal decomposition processes of the ISC-La/Mn. The weight loss region from 28 to 205 8C resulted from the loss of adsorbed water. The strong weight loss region from 205 to 398 8C was attributed to the oxidation and carbonization of the ISC. The region of weight loss from 398 to 531 8C came of the further decomposition of carboxyl manganese/lanthanum and the combustion of the residual of carbonization. No further peak or weight loss appeared after the sample was calcined above 550 8C, indicating that all organic components were decomposed completely and the perovskite oxide of LaMnO3+d began to form. On the DTA curve there were three exothermic peaks observably. The exothermic peak at 466 8C was very strong, corresponding with the combustion of the residual organic components. 3.3. XRD analysis The XRD patterns of samples calcined at different temperatures are shown in Fig. 3. As seen, peaks from

Fig. 3. X-ray diffraction pattern of the samples calcined at different temperature (a) 600 8C, (b) 700 8C, (c) 800 8C.

La2O3 and Mn2O3 were so weak to be hardly observed for samples prepared at 600 8C. When samples were heated at 800 8C for 4 h, the pure LaMnO3+d phase formed. The XRD date for samples is near to the oxygen overstoichiometric structure LaMnO3.15 referenced in the JCPDS 32-0484 than to the stoichiometric LaMnO3 perovskite (JCPDS 33-0713), which present an orthorhombic symmetry. The peaks of the LaMnO3+d perovskite phase were intensified with increasing calcinations temperature, indicating that the crystalline phase of LaMnO3.15 became well with higher calcinations temperature. From the X-ray linewidth, one can estimate the average particle size using the Scherrer formation. The calculated results showed that the particle size increased significantly with the increase of calcinations temperature. When the samples were calcined at 600 8C for 4 h, the diameter of crystal particles was about 17 nm. However, crystal particles grew to 20 nm in diameter at 800 8C. 3.4. TEM analysis Fig. 4 is TEM image of LaMnO3.15 nanoparticles. As seen from the photographs, smooth, well-separated and approximately spherical LaMnO3+d particles were obtained. The average diameter of the nanoparticles is about 25 nm, which is well in agreement with the XRD analysis. It is pointed out that excessive amount of ISC is necessary in this process to obtain the homogeneous products. In addition, the excessive ISC is favorable for producing smaller and well-separated particles. 3.5. The catalytic activity of LaMnO3.15

Fig. 2. TG-DTA curves of ISC-La/Mn precursor.

The prepared nanoparticles was used as catalysts for oxidation of cinnamaldehyde in solution at moderate

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temperatures. In the catalytic reaction, oxygen was used as oxidants. The resultant cinnamonic acid was analysed by IR spectroscopy. In the IR spectrum (Fig. 5), the –COOH absorption frequencies are at 1630 cm 1 and 1680 cm 1, which is consistent with the standard spectrum of cinnamonic acid. In addition the melting point of the product is 132–135 8C, which is near to that of the cinnamonic acid. Thus it can be concluded that the product is cinnamonic acid. A 20% conversion of cinnamaldehyde has been achieved for 1.5 h with 20 mg catalysts. After 1.5 h the reaction was near to equilibrium and the yields has no noticeable change. The catalytic performance of the materials strongly depends on their treatment temperature. The samples treated at 800 8C exhibited better activity for the oxidation of cinnamaldehyde. However, the catalytic activity is relatively poor for the LaMnO3+d obtained at 600 8C. The crystal structure of LaMnO3.15 is a main reason resulting from this phenomenon. At 600 8C the crystalline phase of LaMnO3.15 have not formed completely, but the pure crystalline phase of LaMnO3.15 was obtained at 800 8C according to Fig. 3. It should be point out that the conversion of reactions without catalyst was very low (b2% conversion for 2 h). The catalytic reactions are supposed to occur on the perovskite LaMnO3+d surface, on which its oxygen species are partly consumed by cinnamaldehyde and then regenerated through absorptions from gaseous phase in a cycle, and the presence of vacancies is believed to promote the lattice oxygen mobility during the reaction. On the other hand, the transition metal Mn in perovskite can be particularly active in oxidative catalysis because it can fluctuate between two oxidation states (Mn3+ and Mn4+). By this way it is possible to balance electrically: (1) the insertion of O2 ions in the lattice from the gas-phase O2, and the related capture of electrons (O2+4e 2O2 ), and (2) the formation of O radicals which can be bound to the

Fig. 4. TEM image of the LaMnO3+d nanoparticles calcined at 800 8C.

Fig. 5. The infrared spectrum of cinnamonic acid.

cinnamaldehyde from lattice O2 (2O2 2O+4e ) [17]. Thus, the ease of interconversion of Mn(III) and Mn(IV) ions should be critical for the activity of the catalyst. In conclusion LaMnO3+d exhibits catalytic activity for conversion of cinnamaldehyde to cinnamonic acid because of the fluctuation between two oxidation states (Mn3+ and Mn4+) and the formation of O radicals. Its application in heterogeneous catalysis represents technological and environmental advantages. Moderate conditions of reactions make operations simple, and to use oxygen as oxidants does not pollute environment. In addition, the separation between catalyst and reaction components is relatively easy.

4. Conclusion In this work, a new method was developed for the preparation of perovskites nanoparticles. In the method the insoluble carboxyl-containing grafted starch copolymer (ISC) with a 3D tangled network was used to adsorb metal ions and form precursors, which were used to prepare pure single-phase nanoparticles of LaMnO3.15 with homogeneous microstructure. The size of the obtained particles was around 20 nm in diameter. The overall process is easy, simple, and low-cost because of using cheap reagents. The ISC not only can absorb but can also complex with metal ions in the solutions by carboxyl groups, helping the homogeneous incorporation of metal ions in the polymer network and preventing the flocculation. Besides, the evolution of carbon dioxide and nitrogen dioxide from ISC tend to cause the expansion and produce well-separated nanoparticles during the calcining process. So the ISC is a good candidate for the formation of metal oxides nanoparticles. LaMnO3.15 can be used as catalysts for selective oxidation of cinnamaldehyde in the solutions. In the catalytic reaction with a solid–liquid–gas process, the significantly catalytic activity was observed at moderate temperatures. In addition, the conditions of the reaction are easily controlled, and the catalyst can be recycled without any lose in its capacity and efficiency.

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Acknowledgements This work was supported by Project of KJCXGC-01 of Northwest Normal University, and major project of Gansu Science and Technology Committee (2GS035-A52-026). We would like to express our sincere thanks to Chief Engineer Da-Kang Song of the Material Department of Lanzhou University for measurement and analysis of XRD data. References [1] L. Kindermann, D. Das, H. Nickel, K. Hilpert, J. Electrochem. Soc. 144 (1997) 717. [2] M.A. Pena, J.L.G. Fierro, J. Chem. Rev. 101 (2001) 1981. [3] E.P. Vyshatko, V. Kharton, A.L. Shaula, E.N. Naumovich, F.M.B. Marqnes, J. Mater. Res. Bull. 38 (2003) 185. [4] M. Alifanti, J. Kirchnerova, B. Delmon, J. Appl. Catal. A: Gen. 245 (2003) 231. [5] R. Spinicci, M. Faticanti, P. Marini, S. De Rossi, P. Porta, J. Mol. Catal., A Chem. 197 (2003) 147. [6] R. Spinicci, A. Tofanari, A. Delmastro, D. Mazza, S. Ronchetti, J. Mater. Chem. Phys. 78 (2002) 393.

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