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Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and oxidation of Ce37Mn18C45
JOURNAL OF RARE EARTHS, Vol. 31, No. 3, Mar. 2013, P. 271
Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and oxidation of Ce37Mn18C45 DU Yanan (杜亚男), NI Jiansen (倪建森)*, HU Pengfei (胡鹏飞), WANG Jun′an (王均安), HOU Xueling (侯雪玲), XU Hui (徐 晖) (Laboratory for Microstructures, Shanghai University, Shanghai 200072, China) Received 22 November 2012; revised 18 January 2013
Abstract: The Mn-doped CeO2 nanopowders with high catalysis activity were successfully fabricated through a simple hydrolyzed-oxidized approach. Firstly, the alloy Ce37Mn18C45 was prepared in vacuum induction melting furnace. Subsequently, Mn-doped CeO2 nanopowders with 142 m2/g of specific surface area were obtained through a simple hydrolyzed-oxidized procedure of the alloy. Those nanopowders were heat treated at different temperatures. The obtained materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). And the catalytic activity on vinyl chloride (VC) emission combustion was investigated. The results showed that those nanopowders after hydrolyzed-oxidized from Ce37Mn18C45 mainly consisted of CeO2 and Mn3O4. Manganese element increased the thermal stability of CeO2 nanopowders. The Mn-doped CeO2 nanopowders had three morphologies. Small particles were Mn-doped CeO2, square particles were Mn3O4 and the rods were Mn3O4 and Mn2O3. The Mn-doped CeO2 nanopowders had good vinyl chloride (VC) emission catalytic performance. Keywords: cerium manganese carbide; Mn-doped CeO2 nanopowders; hydrolyzed-oxidized; VC catalytic performance; rare earths
The rare earth elements possess special chemical properties owing to its 4f orbital and lanthanide contraction. CeO2 with fluorite structure is interconvertible between Ce3+ and Ce4+ at different oxidation-reduction environment and forms the flowing oxygen vacancy in structure. So CeO2 has good ability for storage and release of oxygen. It is mainly used in catalyst, polishing powder, fuel battery and other aspects[1–3]. The study shows that the pure CeO2 is easily sintered at high temperature. Coulping with increase of crystal size and decrease of specific surface area, the oxygen storage capacity and catalytic activity of CeO2 will be reduced at high temperatures[4,5]. In order to improve the performance of CeO2, the doped modification of CeO2 is researched. The results show that lattice distortion and crystal defect on CeO2 are caused after Ce4+ is partially substituted by metallic cations. Then the redox properties and catalytic activity are improved[6]. With smaller ion radius and rich valence state, Mn is an important component for modification of CeO2. Some literatures reported that the Mn-doped CeO2 has better catalytic activity and a great prospect for catalysis. For example, it can catalyze effectively volatile organic compounds[7]. Besides it can eliminate liquid and gas pollutants[8]. In addition, it increases low-temperature catalytic oxidation of carbon monoxide[9] and oxides of nitrogen oxygen-enriched
condition[10,11]. Moreover, it has good catalytic combustion performance in vinyl chloride (VC) emission[12], etc. The structure of Mn-doped CeO2 is mainly dependent on the preparation method and the proportion of Mn. Up to now, the co-precipitation[13], sol-gel method[14] and hydrothermal process[15] have been introduced to fabricate Mn-doped CeO2 nanostructures. However, these strategies still suffer from the agglomeration, time-/energy-consumption, and low yield. Herein, we reported the mass synthesis of high-performance Mn-doped CeO2 nanopowders through a lowconsumed path at low temperature, which is more convenient and effective[16,17], and is based on hydrolyzed-oxidized procedure of cerium manganese carbide alloy.
1 Experimental 1.1 Synthesis of nanopowders The alloy Ce37Mn18C45 (at.%) was prepared in vacuum induction melting furnace. During the melting, the graphite crucible was used. Ce, Mn and C melting at high power for a certain period of time was to make carbon saturated in the alloy. After carbon was fully dissolved in the alloy, the alloy melt was cast with fast cooling rate in order to obtain the alloy Ce37Mn18C45. The alloy was crushed into
Foundation item: Project supported by Shanghai Leading Academic Discipline Project (S30107) * Corresponding author: NI Jiansen (E-mail: [email protected]; Tel.: +86-21-56338934) DOI: 10.1016/S1002-0721(12)60271-3
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powders less than 0.25–1.0 mm after the black oxide skin on their surface was sanded out. Then the alloy powder was introduced into deionized water with 1:10–1:40 mass ratios under agitation for 18–30 h at 15–40 °C. After filtration, washing and drying in the cabinet at 100 °C, the brown nanopowders with 110–142 m2/g of specific surface area were obtained. The pure CeO2 nanopowders as a comparison were prepared by the same process. 1.2 Measurement of nanopowders The phases of the synthesized products were analyzed by X-ray powder diffraction (D/max-rc). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations were performed to investigate the morphology of the powders. The specific surface area of the obtained nanopowders was further measured using a BET surface area analyzer (ASAP-2020M+C). 1.3 Catalytic activity testing The catalytic activity testing for the VC combustion was carried out in a quartz reactor. The amount of catalyst was 0.48 ml. The concentration of VC was 0.1% and a space velocity was 15000 h–1. The reactants and products were analyzed online by gas chromatograph equipped with flame ionization detector (FID).
2 Results and discussion 2.1 N2 isothermal adsorption/desorption analysis The alloy was crushed into powders less than 0.25 mm in particle size. Then the alloy powders were introduced into deionized water with 1:10 mass ratio under agitation for 18 h at 20 °C. After filtration, washing and drying in the cabinet at 100 °C, the brown nanopowders were obtained (the following nanopowders were obtained by the same preparation technology and marked Mn-doped CeO2). The nitrogen adsorption-desorption isotherms of nanopowders are shows in Fig. 1. As shown in Fig. 1(a),
Fig. 1 Nitrogen adsorption-desorption isotherms (a) and pore distribution (b) of Mn-doped CeO2 nanopowders
JOURNAL OF RARE EARTHS, Vol. 31, No. 3, Mar. 2013
the nitrogen adsorption/desorption isotherms of the sample are of type IV and have hysteresis loops close at p/p0=0.4–1.0. The pore size distribution of the sample estimated with the BJH method is shown in Fig. 1(b). It can be observed that the maximum peak appears at 3.88 nm and the pore size is about 3–5 nm indicating that the sample has pores in the mesopore region. The surface area of Mn-doped CeO2 is 142 m2/g as estimated using the BET method. 2.2 XRD analysis The XRD pattern of Mn-doped CeO2 nanopowders is shown in Fig. 2(1). The main phase is CeO2 with fluorite structure. The wide diffraction peak indicates that the grains of the sample are very fine. A little of Mn3O4 phase are also detected. The XRD patterns of Mn-doped CeO2 nanopowders and pure CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 2(2) and (5). The diffraction patterns of CeO2 are observed for the two samples. The patterns of Mn3O4 are observed in the Mn-doped CeO2 nanopowders. It is suggested that structure defect is formed after manganese ion displaces cerium ion partially and Mn-doped is beneficial to forming small size of CeO2. And at this temperature, other peaks of non-stoichiometric ratio cerium oxide do not appear[18]. This indicates that the Mn doping increases the thermal stability of CeO2 nanopowders. The XRD patterns of Mn-doped CeO2 nanopowders after heat treatment at different temperatures are shown in Fig. 2(2)–(4). The phase transition can not occur and the crystallinity is fine after heat treatment. At the increasing heat-treatment temperature the FWHM of dif-
Fig. 2 XRD patterns of Mn-doped CeO2 nanopowders at different temperatures (1–4) and pure CeO2 nanopowders at 600 °C (5)
DU Yanan et al., Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and … Table 1 Cell parameter of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders at 600 °C Samples
Cell parameter/nm
CeO2
0.54185
Mn-doped CeO2
0.53915
fraction peaks becomes narrower, especially at 800 °C. It can be attributed to the grain growth at high temperature. The cell parameter of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Table 1. The lattice constants of Mn-doped CeO2 nanopowders show a gradually reduced tendency than pure CeO2 nanopowders. This indicates that the smaller Mn ion (Mn2+ 0.083 nm, Mn3+ 0.065 nm and Mn4+ 0.053 nm) can substitute Ce4+ (0.097 nm) to cause lattice distortion and the shrinkage of crystal cell, which is represented as Ce-Mn solid solution. 2.3 TEM and HRTEM analysis The TEM and HRTEM images of Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 3. It is found that the sample had three morphologies. A is the large particle from small crystals agglomeration (Most area is the morphology). The EDS analysis (Fig.
Fig. 3 TEM (a) and HRTEM (b–d) images of Mn-doped CeO2 nanopowders at 600 °C/2 h
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4(a)) shows that the small crystals are ceria-manganese mixed oxide. B is the large particle, with size of about 200 nm. The EDS analysis (Fig. 4(b)) shows that the large particle is manganese oxide. C is the nanorod, with length of several hundreds nanometer. The EDS analysis (Fig. 4(c)) shows that the nanorod is manganese oxide. Besides, Cu is from the copper mesh during the test. Therefore, the products are multiphase mixture. It may be caused by forming incomplete alloy in the condition of this experiment. The HRTEM image of the small crystals from A is shown in Fig. 3(b). The size of most crystals is several nanometers and some interplanar distances are about 0.3100 nm, which correspond to the (111) plane of the CeO2 phase, and this reveals that the small crystals from A are Mn-doped CeO2. Fig. 3(c) and (d) both show the typical HRTEM image of the nanorod. The interplanar distance of the nanorod in Fig. 3(c) is about 0.2877 nm, which corresponds to the (200) plane of the Mn3O4 phase, and the interplanar distance of the nanorod in Fig. 3(d) is about 0.3857 nm, which corresponds to the (211) plane of the Mn2O3 phase. This reveals that nanorods are Mn3O4 and Mn2O3 phase. The TEM image of large square particle in Mn-doped CeO2 nanopowders after heat treatment at 600 °C is shown in Fig. 5(a). The research from Xing et al.[16] showed that square Mn3O4 with size of about 100 nm was obtained through hydrolyzed-oxidized manganese carbide. But the size of large square particle in Fig. 5(a) is about 200 nm. It is due to the grain growth after heat treatment at 600 °C. The HRTEM image of small square particle in Mn-doped CeO2 nanopowders after heat treatment at 600 °C is given in Fig. 5(b). The particle size is about 30 nm and the interplanar distance is about 0.4922 nm, which corresponds to the (101) plane of the Mn3O4 phase. This indicates that the small square particle is Mn3O4 phase, too, which is the same to the result in literature[16]. The HRTEM images of small grains and TEM images of manganese oxides nanorods in Mn-doped CeO2 nanopowders after heat treatment at 700 and 800 °C are shown in Fig. 6. The result shows that the size of small grains is about from 6 to 10 nm and the diameter of nanorod increases with the increasing heat treatment temperature, which is the same to the result in XRD.
Fig. 4 EDS energy spectra of Mn-doped CeO2 nanopowders at 600 °C/2 h (a) Small crystal; (b) Large particle; (c) Nanorod
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Fig. 5 TEM (a) and HRTEM (b) image of large and small square particle in Mn-doped CeO2 nanopowders at 600 °C/2 h
JOURNAL OF RARE EARTHS, Vol. 31, No. 3, Mar. 2013
This indicates that Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method have good catalytic activity of VC combustion because Mn ions are incorporated into the CeO2 lattice to form Ce-Mn solid solution, which improves the activity of oxide on the nanopowders surface. The catalytic activity of Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method is almost the same to the Mn-doped CeO2 nanopowders catalytic activities in Ref. [12] which were prepared by the citric acid sol-gel method after heat treatment at 500 °C.
3 Conclusions
Fig. 6 HRTEM (a, b) and TEM (c, d) images of small grains and nanorods in Mn-doped CeO2 nanopowders at 700 °C/2 h (a, c) and 800 °C/2 h (b, d)
2.4 Catalytic performance testing for VC emission The VC catalytic activities of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 7. The catalyst activity is characterized by T10, T50 and T90 (the reaction temperature with 10%, 50% and 90% vinyl chloride (VC) conversion, respectively). The T10, T50 and T90 of Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method are all lower than the pure CeO2 nanopowders.
Fig. 7 VC catalytic activities of pure CeO2 and Mn-doped CeO2 nanopowders at 600 °C/2 h
(1) The alloy Ce37Mn18C45 was prepared in vacuum induction melting furnace. Subsequently, the Mn-doped CeO2 nanopowders with specific surface area of 142 m2/g were obtained through hydrolyzed-oxidized method. Mn doping increased the thermal stability of CeO2 nanopowders. (2) The Mn-doped CeO2 nanopowders with three morphologies were consisted of Mn-doped CeO2 nanoparticles, square Mn3O4 phase and Mn3O4 and Mn2O3 nanorods. (3) The Mn-doped CeO2 nanopowders through hydrolyzed-oxidized method had good VC emission catalytic performance. (4) The procedure was simple and suitable for industrial production. As no other chemicals such as catalyst, acid and base were used, it was a green technique. Acknowledgements: The authors wish to acknowledge Li Qiang, Peng Jianchao, Yu Weijun, Zhu Yuliang and Lou Yanyan, the Key Laboratory for Advanced Micro-analysis, Shanghai University, for their help for these experiments.
References: [1] Guo Y, Lu G Z. Current status and some perspectives of rare earth catalytic materials. J. Chin. Soc. Rare Earths (in Chin.), 2007, 25(1): 1. [2] Murugan B, Ramaswamy A V, Srinivas D, Gopinath C S, Ramaswamy V. Effect of fuel and its concentration on the nature of Mn in Mn/CeO2 solid solutions prepared by solution combustion synthesis. Acta Mater., 2008, 56: 1461. [3] Feng X J, Shi Y L, Zhou H J. Electrocatalytic enhancement of methanol oxidation by adding CeO2 nanoparticle on porous electrode. J. Rare Earths, 2012, 30(1): 29. [4] Yuan W H, Zhou C C, Li L. Study on catalytic properties of nano-CeO2-ZrO2 mixed oxides prepared by modified sol-gel method. J. Inorg. Mater., 2010, 25(8): 820. [5] Yao H C, Yao Yu Y F. Ceria in automotive exhaust catalysts I. Oxygen storage. J. Catal., 1984, 86: 254. [6] Ran R, Weng D, Wu X D, Fan J, Wang L, Wu X D. Structure and oxygen storage capacity of Pr-doped Ce0.26Zr0.74O2 mixed oxides. J. Rare Earths, 2011, 29(11):
DU Yanan et al., Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and … 1053. [7] Delimaris D, Ioannides T. VOC oxidation over MnOxCeO2 catalysts prepared by a combustion method. Appl. Catal., B, 2008, 84: 303. [8] Zeng C Y, Ma R H, Liu X S, Luo M F, Lu J Q. Catalytic oxidation of dichlorom ethane over Ce1–xMnxO2 catalysts. J. Chin. Soc. Rare Earths (in Chin.), 2010, 28(3): 306. [9] Imamura S, Shono M, Okamoto N, Hamada A, Ishida S. Effect of cerium on the mobility of oxygen on manganese oxides. Appl. Catal., A, 1996, 142: 279. [10] Wu X D, Liang Q, Weng D, Fan J, Ran R. Synthesis of CeO2-MnOx mixed oxides and catalytic performance under oxygen-rich condition. Catal. Today, 2007, 126: 430. [11] Wu X D, Lin F, Xu H B, Weng D. Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx-CeO2 mixed oxides. Appl. Catal., B, 2010, 96: 101. [12] Wan Y L, Zhang C H, Guo Y L, Guo Y, Lu G Z. Catalytic combustion of vinyl chloride emission over CeO2-MnOx catalyst. J. Catal., 2012, 33(3): 557. [13] Liu X S, Lu J Q, Qian K, Huang W X, Luo M F. A comparative study of formaldehyde and carbon monoxide
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complete oxidation on MnOx-CeO2 catalysts. J. Rare Earths, 2009, 27(3): 418. [14] Wang X Y, Kang Q, Li D. Catalytic combustion of chlorobenzene over MnOx-CeO2 mixed oxide catalysts. Appl. Catal., B, 2009, 86: 166. [15] Li H F, Lu G Z, Dai Q G, Wang Y Q, Guo Y, Guo Y L. Efficient low-temperature catalytic combustion of trichloroethylene over flower-like mesoporous Mn-doped CeO2 microspheres. Appl. Catal., B, 2011, 102: 475. [16] Xing B Y, Ni J S, Ding W Z, Geng S H, Li Q T. Preparation of Mn3O4 nanopowders by hydrolysis and oxidation of manganese carbides. China’s Manganese Industry, 2010, 28(1): 17. [17] Wu Y Q, Ni J S, Du Y N, Hu P F, Ding W Z, Geng S H. Preparation of CeO2 nanopowders by hydrolysis and oxidation of cerium carbide. J. Inorg. Mater., 2012, 27(5): 489. [18] Gao X F, Shen C L, Ren D Y, Zhang J, Su D S. Structural effects of cerium oxides on their thermal stability and catalytic performance in propane oxidation dehydrogenation. J. Catal., 2012, 33(7): 1069.
Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and oxidation of Ce37Mn18C45 DU Yanan (杜亚男), NI Jiansen (倪建森)*, HU Pengfei (胡鹏飞), WANG Jun′an (王均安), HOU Xueling (侯雪玲), XU Hui (徐 晖) (Laboratory for Microstructures, Shanghai University, Shanghai 200072, China) Received 22 November 2012; revised 18 January 2013
Abstract: The Mn-doped CeO2 nanopowders with high catalysis activity were successfully fabricated through a simple hydrolyzed-oxidized approach. Firstly, the alloy Ce37Mn18C45 was prepared in vacuum induction melting furnace. Subsequently, Mn-doped CeO2 nanopowders with 142 m2/g of specific surface area were obtained through a simple hydrolyzed-oxidized procedure of the alloy. Those nanopowders were heat treated at different temperatures. The obtained materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). And the catalytic activity on vinyl chloride (VC) emission combustion was investigated. The results showed that those nanopowders after hydrolyzed-oxidized from Ce37Mn18C45 mainly consisted of CeO2 and Mn3O4. Manganese element increased the thermal stability of CeO2 nanopowders. The Mn-doped CeO2 nanopowders had three morphologies. Small particles were Mn-doped CeO2, square particles were Mn3O4 and the rods were Mn3O4 and Mn2O3. The Mn-doped CeO2 nanopowders had good vinyl chloride (VC) emission catalytic performance. Keywords: cerium manganese carbide; Mn-doped CeO2 nanopowders; hydrolyzed-oxidized; VC catalytic performance; rare earths
The rare earth elements possess special chemical properties owing to its 4f orbital and lanthanide contraction. CeO2 with fluorite structure is interconvertible between Ce3+ and Ce4+ at different oxidation-reduction environment and forms the flowing oxygen vacancy in structure. So CeO2 has good ability for storage and release of oxygen. It is mainly used in catalyst, polishing powder, fuel battery and other aspects[1–3]. The study shows that the pure CeO2 is easily sintered at high temperature. Coulping with increase of crystal size and decrease of specific surface area, the oxygen storage capacity and catalytic activity of CeO2 will be reduced at high temperatures[4,5]. In order to improve the performance of CeO2, the doped modification of CeO2 is researched. The results show that lattice distortion and crystal defect on CeO2 are caused after Ce4+ is partially substituted by metallic cations. Then the redox properties and catalytic activity are improved[6]. With smaller ion radius and rich valence state, Mn is an important component for modification of CeO2. Some literatures reported that the Mn-doped CeO2 has better catalytic activity and a great prospect for catalysis. For example, it can catalyze effectively volatile organic compounds[7]. Besides it can eliminate liquid and gas pollutants[8]. In addition, it increases low-temperature catalytic oxidation of carbon monoxide[9] and oxides of nitrogen oxygen-enriched
condition[10,11]. Moreover, it has good catalytic combustion performance in vinyl chloride (VC) emission[12], etc. The structure of Mn-doped CeO2 is mainly dependent on the preparation method and the proportion of Mn. Up to now, the co-precipitation[13], sol-gel method[14] and hydrothermal process[15] have been introduced to fabricate Mn-doped CeO2 nanostructures. However, these strategies still suffer from the agglomeration, time-/energy-consumption, and low yield. Herein, we reported the mass synthesis of high-performance Mn-doped CeO2 nanopowders through a lowconsumed path at low temperature, which is more convenient and effective[16,17], and is based on hydrolyzed-oxidized procedure of cerium manganese carbide alloy.
1 Experimental 1.1 Synthesis of nanopowders The alloy Ce37Mn18C45 (at.%) was prepared in vacuum induction melting furnace. During the melting, the graphite crucible was used. Ce, Mn and C melting at high power for a certain period of time was to make carbon saturated in the alloy. After carbon was fully dissolved in the alloy, the alloy melt was cast with fast cooling rate in order to obtain the alloy Ce37Mn18C45. The alloy was crushed into
Foundation item: Project supported by Shanghai Leading Academic Discipline Project (S30107) * Corresponding author: NI Jiansen (E-mail: [email protected]; Tel.: +86-21-56338934) DOI: 10.1016/S1002-0721(12)60271-3
272
powders less than 0.25–1.0 mm after the black oxide skin on their surface was sanded out. Then the alloy powder was introduced into deionized water with 1:10–1:40 mass ratios under agitation for 18–30 h at 15–40 °C. After filtration, washing and drying in the cabinet at 100 °C, the brown nanopowders with 110–142 m2/g of specific surface area were obtained. The pure CeO2 nanopowders as a comparison were prepared by the same process. 1.2 Measurement of nanopowders The phases of the synthesized products were analyzed by X-ray powder diffraction (D/max-rc). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations were performed to investigate the morphology of the powders. The specific surface area of the obtained nanopowders was further measured using a BET surface area analyzer (ASAP-2020M+C). 1.3 Catalytic activity testing The catalytic activity testing for the VC combustion was carried out in a quartz reactor. The amount of catalyst was 0.48 ml. The concentration of VC was 0.1% and a space velocity was 15000 h–1. The reactants and products were analyzed online by gas chromatograph equipped with flame ionization detector (FID).
2 Results and discussion 2.1 N2 isothermal adsorption/desorption analysis The alloy was crushed into powders less than 0.25 mm in particle size. Then the alloy powders were introduced into deionized water with 1:10 mass ratio under agitation for 18 h at 20 °C. After filtration, washing and drying in the cabinet at 100 °C, the brown nanopowders were obtained (the following nanopowders were obtained by the same preparation technology and marked Mn-doped CeO2). The nitrogen adsorption-desorption isotherms of nanopowders are shows in Fig. 1. As shown in Fig. 1(a),
Fig. 1 Nitrogen adsorption-desorption isotherms (a) and pore distribution (b) of Mn-doped CeO2 nanopowders
JOURNAL OF RARE EARTHS, Vol. 31, No. 3, Mar. 2013
the nitrogen adsorption/desorption isotherms of the sample are of type IV and have hysteresis loops close at p/p0=0.4–1.0. The pore size distribution of the sample estimated with the BJH method is shown in Fig. 1(b). It can be observed that the maximum peak appears at 3.88 nm and the pore size is about 3–5 nm indicating that the sample has pores in the mesopore region. The surface area of Mn-doped CeO2 is 142 m2/g as estimated using the BET method. 2.2 XRD analysis The XRD pattern of Mn-doped CeO2 nanopowders is shown in Fig. 2(1). The main phase is CeO2 with fluorite structure. The wide diffraction peak indicates that the grains of the sample are very fine. A little of Mn3O4 phase are also detected. The XRD patterns of Mn-doped CeO2 nanopowders and pure CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 2(2) and (5). The diffraction patterns of CeO2 are observed for the two samples. The patterns of Mn3O4 are observed in the Mn-doped CeO2 nanopowders. It is suggested that structure defect is formed after manganese ion displaces cerium ion partially and Mn-doped is beneficial to forming small size of CeO2. And at this temperature, other peaks of non-stoichiometric ratio cerium oxide do not appear[18]. This indicates that the Mn doping increases the thermal stability of CeO2 nanopowders. The XRD patterns of Mn-doped CeO2 nanopowders after heat treatment at different temperatures are shown in Fig. 2(2)–(4). The phase transition can not occur and the crystallinity is fine after heat treatment. At the increasing heat-treatment temperature the FWHM of dif-
Fig. 2 XRD patterns of Mn-doped CeO2 nanopowders at different temperatures (1–4) and pure CeO2 nanopowders at 600 °C (5)
DU Yanan et al., Synthesis and characterization of manganese doped CeO2 nanopowders from hydrolysis and … Table 1 Cell parameter of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders at 600 °C Samples
Cell parameter/nm
CeO2
0.54185
Mn-doped CeO2
0.53915
fraction peaks becomes narrower, especially at 800 °C. It can be attributed to the grain growth at high temperature. The cell parameter of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Table 1. The lattice constants of Mn-doped CeO2 nanopowders show a gradually reduced tendency than pure CeO2 nanopowders. This indicates that the smaller Mn ion (Mn2+ 0.083 nm, Mn3+ 0.065 nm and Mn4+ 0.053 nm) can substitute Ce4+ (0.097 nm) to cause lattice distortion and the shrinkage of crystal cell, which is represented as Ce-Mn solid solution. 2.3 TEM and HRTEM analysis The TEM and HRTEM images of Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 3. It is found that the sample had three morphologies. A is the large particle from small crystals agglomeration (Most area is the morphology). The EDS analysis (Fig.
Fig. 3 TEM (a) and HRTEM (b–d) images of Mn-doped CeO2 nanopowders at 600 °C/2 h
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4(a)) shows that the small crystals are ceria-manganese mixed oxide. B is the large particle, with size of about 200 nm. The EDS analysis (Fig. 4(b)) shows that the large particle is manganese oxide. C is the nanorod, with length of several hundreds nanometer. The EDS analysis (Fig. 4(c)) shows that the nanorod is manganese oxide. Besides, Cu is from the copper mesh during the test. Therefore, the products are multiphase mixture. It may be caused by forming incomplete alloy in the condition of this experiment. The HRTEM image of the small crystals from A is shown in Fig. 3(b). The size of most crystals is several nanometers and some interplanar distances are about 0.3100 nm, which correspond to the (111) plane of the CeO2 phase, and this reveals that the small crystals from A are Mn-doped CeO2. Fig. 3(c) and (d) both show the typical HRTEM image of the nanorod. The interplanar distance of the nanorod in Fig. 3(c) is about 0.2877 nm, which corresponds to the (200) plane of the Mn3O4 phase, and the interplanar distance of the nanorod in Fig. 3(d) is about 0.3857 nm, which corresponds to the (211) plane of the Mn2O3 phase. This reveals that nanorods are Mn3O4 and Mn2O3 phase. The TEM image of large square particle in Mn-doped CeO2 nanopowders after heat treatment at 600 °C is shown in Fig. 5(a). The research from Xing et al.[16] showed that square Mn3O4 with size of about 100 nm was obtained through hydrolyzed-oxidized manganese carbide. But the size of large square particle in Fig. 5(a) is about 200 nm. It is due to the grain growth after heat treatment at 600 °C. The HRTEM image of small square particle in Mn-doped CeO2 nanopowders after heat treatment at 600 °C is given in Fig. 5(b). The particle size is about 30 nm and the interplanar distance is about 0.4922 nm, which corresponds to the (101) plane of the Mn3O4 phase. This indicates that the small square particle is Mn3O4 phase, too, which is the same to the result in literature[16]. The HRTEM images of small grains and TEM images of manganese oxides nanorods in Mn-doped CeO2 nanopowders after heat treatment at 700 and 800 °C are shown in Fig. 6. The result shows that the size of small grains is about from 6 to 10 nm and the diameter of nanorod increases with the increasing heat treatment temperature, which is the same to the result in XRD.
Fig. 4 EDS energy spectra of Mn-doped CeO2 nanopowders at 600 °C/2 h (a) Small crystal; (b) Large particle; (c) Nanorod
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Fig. 5 TEM (a) and HRTEM (b) image of large and small square particle in Mn-doped CeO2 nanopowders at 600 °C/2 h
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This indicates that Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method have good catalytic activity of VC combustion because Mn ions are incorporated into the CeO2 lattice to form Ce-Mn solid solution, which improves the activity of oxide on the nanopowders surface. The catalytic activity of Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method is almost the same to the Mn-doped CeO2 nanopowders catalytic activities in Ref. [12] which were prepared by the citric acid sol-gel method after heat treatment at 500 °C.
3 Conclusions
Fig. 6 HRTEM (a, b) and TEM (c, d) images of small grains and nanorods in Mn-doped CeO2 nanopowders at 700 °C/2 h (a, c) and 800 °C/2 h (b, d)
2.4 Catalytic performance testing for VC emission The VC catalytic activities of pure CeO2 nanopowders and Mn-doped CeO2 nanopowders after heat treatment at 600 °C are shown in Fig. 7. The catalyst activity is characterized by T10, T50 and T90 (the reaction temperature with 10%, 50% and 90% vinyl chloride (VC) conversion, respectively). The T10, T50 and T90 of Mn-doped CeO2 nanopowders prepared through hydrolyzed-oxidized method are all lower than the pure CeO2 nanopowders.
Fig. 7 VC catalytic activities of pure CeO2 and Mn-doped CeO2 nanopowders at 600 °C/2 h
(1) The alloy Ce37Mn18C45 was prepared in vacuum induction melting furnace. Subsequently, the Mn-doped CeO2 nanopowders with specific surface area of 142 m2/g were obtained through hydrolyzed-oxidized method. Mn doping increased the thermal stability of CeO2 nanopowders. (2) The Mn-doped CeO2 nanopowders with three morphologies were consisted of Mn-doped CeO2 nanoparticles, square Mn3O4 phase and Mn3O4 and Mn2O3 nanorods. (3) The Mn-doped CeO2 nanopowders through hydrolyzed-oxidized method had good VC emission catalytic performance. (4) The procedure was simple and suitable for industrial production. As no other chemicals such as catalyst, acid and base were used, it was a green technique. Acknowledgements: The authors wish to acknowledge Li Qiang, Peng Jianchao, Yu Weijun, Zhu Yuliang and Lou Yanyan, the Key Laboratory for Advanced Micro-analysis, Shanghai University, for their help for these experiments.
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