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Synthesis and characterization of bismuth molybdate nanoparticles within nanoreactors of reverse micelles
Powder Technology 228 (2012) 228–230
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Synthesis and characterization of bismuth molybdate nanoparticles within nanoreactors of reverse micelles M. Masteri-Farahani ⁎, H.S. Hosseini Faculty of Chemistry, University of Tarbiat Moallem, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 14 April 2012 Accepted 7 May 2012 Available online 12 May 2012 Keywords: bismuth molybdate nanoparticles microemulsion cationic surfactant
a b s t r a c t α-Bi2Mo3O12 nanoparticles were synthesized with water-in-oil (w/o) microemulsion consisted of water/ cetyltrimethylamonium bromide, n-butanol/isooctane. By tuning the Bi/Mo ratio and pH value, controlled formation of α-Bi2Mo3O12 can be achieved. Low pH and high concentration of molybdenum was lead to the formation of α-Bi2Mo3O12. The pure phase of α-Bi2Mo3O12 can be prepared by calcination at 723 K. The resulted material was analyzed with different physicochemical methods which showed nearly spherical and uniform nanoparticles with size of about 100 nm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bismuth molybdate mixed metal oxides form the basis of very important commercial catalysts for the oxidation and ammoxidation of propylene to acrolein and acrylonitrile [1,2]. Thus, several researches have been devoted to prepare and investigate the catalytic properties of this type of materials [3–8]. The most common way to prepare these materials is the precipitation at acidic pH and heating at high temperature to obtain the crystalline materials [7,8]. On the other hand, synthesis of nanoscale materials which exhibit a wide range of electronic, optical and catalytic properties has been of both academic and technological interest. Several approaches are available for the preparation of nanomaterials such as coprecipitation [9], sol–gel processing [10], microemulsion [11], hydrothermal / solvothermal methods [12] and template syntheses [13]. Due to the specific structure of a microemulsion, this method is suitable for producing small nanoparticles of narrow size distribution. Nanostructured bismuth molybdates have been synthesized by solvothermal / hydrothermal [6,14,15], molten salt [16] and ultrasonic assisted syntheses [17]. But to the best of our knowledge there is no report on the investigation of the preparation of this type of material in the microemulsion system. So, herein we present the preparation and characterization of α-Bi2Mo3O12 nanoparticles for the first time with microemulsion method. The advantage of this approach is the better control over
the size and shape of the final nanoparticles in comparison to other methods. 2. Experimental 2.1. Preparation of α-Bi2Mo3O12 nanoparticles Bismuth nitrate, ammonium heptamolybdate, cetyltrimethylammonium bromide (CTAB), isooctane and n-butanol were purchased from Merck chemical company and used without further purification. The quaternary microemulsion system consisted of CTAB/H2O/ isooctane/n-butanol and was prepared as followed: n-butanol (2 g) was added to isooctane (4 g) and then, to this solution was added CTAB (2 g) in water (2 ml). The resulted mixture was stirred for 15 minutes until a clear solution was obtained. Then 2.4 ml of 0.04 M ammonium heptamolybdate aqueous solution and after 20 minutes 2.4 ml of 0.2 M acidic aqueous solution of bismuth nitrate were added to the above solution. The pH of the reaction mixture was adjusted in the range of 1–3 and refluxed for 3 hours. After cooling to room temperature, the resultant yellow product was collected by centrifugation and washed several times with water and ethanol and dried at 363 K for 12 hours. Finally, the product was calcined at 723 K for 8 hours to remove the remained organic residue and form the crystalline α-Bi2Mo3O12. 2.2. Instrumentation
⁎ Corresponding author. Tel./fax: + 98 261 4551023. E-mail address: [email protected] (M. Masteri-Farahani). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.020
Infrared spectra were recorded using Perkin-Elmer Spectrum RXI FT-IR spectrometer, using pellets of the materials diluted with KBr. X-ray diffraction (XRD) patterns of the samples were recorded on a
M. Masteri-Farahani, HS. Hosseini / Powder Technology 228 (2012) 228–230
229
Fig. 2. The XRD pattern of the calcined α-Bi2Mo3O12 product.
Scheme 1. Schematic illustration of preparation of α-Bi2Mo3O12 nanoparticles.
SIEFERT XRD 3003 PTS diffractometer using a Cu Kα radiation (λ = 0.1542 nm). Scanning electron micrographs (SEM) of the samples were taken with ZEISS-DSM 960A microscope with attached camera. Transmission electron microscopy (TEM) observations were carried out using a Philips EM 208 S instrument with an accelerating voltage of 100 kV. 3. Results and discussion As shown in Scheme 1, reaction between Bi3 + and MoO42- ions in quaternary microemulsion comprised of CTAB/water/isooctane/n-butanol resulted in the preparation of α-Bi2Mo3O12 nanoparticles. With addition of bismuth nitrate solution to the molybdate containing microemulsion system, bismuth molybdate nucleation occurs in the nanoreactors of
Fig. 1. The FT-IR spectra of the products before (inside) and after calcinations.
Fig. 3. The SEM image of the as-prepared (top) and calcined (middle and bottom) bismuth molybdate nanoparticles.
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M. Masteri-Farahani, HS. Hosseini / Powder Technology 228 (2012) 228–230
Fig. 4. The TEM image of the calcined material.
the water droplets and nanoparticles with surfactant coatings are formed in a very short time. The sizes of the nanoparticles are restricted to the size of the water droplets acting as nanoreactors. In order to investigate the effect of pH on the crystal phase of resulted material, we conducted the reaction at pH = 2 to pH = 6. According to results obtained from X-ray diffraction analyses the pure crystalline phase of α-Bi2Mo3O12 was formed at pH = 2, which was in agreement with earlier reports [5].On the Other hand, different Bi/Mo ratios (from Bi/Mo = 0.5 to 1.0) was studied to investigate the effect of Bi/Mo ratio on the crystal phase of the final material. The pure crystalline phase of α-Bi2Mo3O12 was formed at Bi/Mo = 0.7. FT-IR spectroscopy provides good evidence for the formation of the CTAB-bismuth molybdate composite material. In the FT-IR spectrum of the as-prepared material (Fig. 1) the stretching vibrations of C-H groups of CTAB species were observed in 2849 and 2918 cm - 1. On the other hand, the bands at 1468 and 1486 cm - 1 were assigned to the bending vibrations of CH2 groups. Some bands in the range of 700–1000 cm - 1 (not shown here) are due to Mo-O and Bi-O vibrations in noncalcined material. Thus FT-IR spectrum of the noncalcined material showed the presence of the bismuth molybdate material associated with CTAB coating. In the FT-IR spectrum of the product after calcination (Fig. 1) the bands due to the presence of the C-H groups were disappeared as a result of the combustion and removal of CTAB species. In the FT-IR spectrum of the calcined α-Bi2Mo3O12, the bands in the range of 700–950 cm - 1 were assigned to the stretching vibrations of Mo-O groups and those of 400–600 cm - 1 are due to Bi-O stretching vibrations in the calcined and crystalline material. X-ray diffraction analysis was used to examine the crystal structure of the products. In the XRD pattern of the as-prepared product (not shown here), the observation of very broad peaks indicated the amorphous structure of the material. On the other hand, the presence of the CTAB inhibits the observation of the peaks of bismuth molybdate species. The XRD pattern of the calcined α-Bi2Mo3O12 is shown in Fig. 2. All the diffraction peaks can be indexed to the tetragonal
structure of the α-Bi2Mo3O12 with monoclinic phase and lattice constant a= 7.72 Å, b= 11.51 Å, c =11.98 Å (JCPDS file number 21–0103). The crystallinity of the product is reflected by the presence of distinct diffraction peaks in the calcined material and no impurity was detected in the X-ray diffraction pattern. The morphology and structure of the as-prepared and calcined products were further investigated with SEM analysis. Typical SEM images of the as-prepared and calcined bismuth molybdate nanoparticles are shown in Fig. 3. The SEM image of the as-prepared material indicates that the obtained product is composed of spherical nanoparticles with diameters about 30 nm. On the other hand, the SEM image of the calcined material shows that calcination increases the size of the nanoparticles to about 100 nm. In the calcination step, some agglomeration of nanoparticles occurred because of the removal of the CTAB species which act as protecting groups against the growth of the nanoparticles. The morphology and nanostructure of the calcined material was further investigated by TEM analysis. Fig. 4 shows the TEM image of the calcined material. This image shows that the nearly spherical nanoparticles have an average diameter of about 100 nm, which is in agreement with the SEM observation after calcination of the asprepared material. 4. Conclusion These results show that the microemulsion system provide efficient nanoreactors for the reaction between Bi 3 + and MoO42- ions and nucleation and growth of the bismuth molybdate nanoparticles. As the TEM and SEM images show, the resulted nanoparticles are nearly spherical and uniformly sized about 100 nm. References [1] N.I. Inescu, M. Caldararu, Heterogeneous selective oxidation of lower olefins, Edirura Acad. Romane, Bucharest, 1993, p. 278. [2] A. Bielanski, J. Haber, Oxygen in catalysis, Marcell Dekker, New York, 1991, p. 320. [3] J.D. Burington, C.T. Kartisek, R.K. Grasseli, Journal of Catalysis 63 (1980) 235–240. [4] R.K. Grasseli, Topics in Catalysis 21 (2002) 79–88. [5] A.M. Beale, G. Sankar, Chemistry of Materials 15 (2003) 146–153. [6] H.H. Li, K.W. Li, H. Wang, Materials Chemistry and Physics 116 (2009) 134–139. [7] G.W. Keulks, J.L. Hall, C. Daniel, K. Suzuki, Journal of Catalysis 34 (1974) 79–85. [8] F. Trifiro, H. Hoser, R.D. Scarle, Journal of Catalysis 25 (1972) 12–18. [9] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Journal of Colloid and Interface Science 146 (1991) 38–52. [10] B. Fegley, P. White, H.K. Bowen, American Ceramic Society Bulletin 64 (1985) 1115–1120. [11] M.P. Pileni, Journal of Physical Chemistry 97 (1993) 6961–6973. [12] L. Zhen, W.S. Wang, C.Y. Yu, W.Z. Zhao, L.C. Qin, Materials Letters 62 (2008) 1169–1172. [13] Z.T. Zhao, A.J. Rondinone, J.X. Ma, J. Shen, S. Dai, Advanced Materials 17 (2005) 1415–1419. [14] H.H. Li, K.W. Li, H. Wang, C.Y. Liu, Journal of Materials Science 43 (2008) 7026–7034. [15] C. Xu, D. Zou, L. Wang, H. Luo, T. Ying, Ceramics International 35 (2009) 2099–2102. [16] L.J. Xie, J.F. Ma, G.J. Xu, Materials Chemistry and Physics 110 (2008) 197–200. [17] L. Zhou, W.Z. Wang, L.S. Zhang, Journal of Molecular Catalysis A 268 (2007) 195–200.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Synthesis and characterization of bismuth molybdate nanoparticles within nanoreactors of reverse micelles M. Masteri-Farahani ⁎, H.S. Hosseini Faculty of Chemistry, University of Tarbiat Moallem, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2012 Received in revised form 14 April 2012 Accepted 7 May 2012 Available online 12 May 2012 Keywords: bismuth molybdate nanoparticles microemulsion cationic surfactant
a b s t r a c t α-Bi2Mo3O12 nanoparticles were synthesized with water-in-oil (w/o) microemulsion consisted of water/ cetyltrimethylamonium bromide, n-butanol/isooctane. By tuning the Bi/Mo ratio and pH value, controlled formation of α-Bi2Mo3O12 can be achieved. Low pH and high concentration of molybdenum was lead to the formation of α-Bi2Mo3O12. The pure phase of α-Bi2Mo3O12 can be prepared by calcination at 723 K. The resulted material was analyzed with different physicochemical methods which showed nearly spherical and uniform nanoparticles with size of about 100 nm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bismuth molybdate mixed metal oxides form the basis of very important commercial catalysts for the oxidation and ammoxidation of propylene to acrolein and acrylonitrile [1,2]. Thus, several researches have been devoted to prepare and investigate the catalytic properties of this type of materials [3–8]. The most common way to prepare these materials is the precipitation at acidic pH and heating at high temperature to obtain the crystalline materials [7,8]. On the other hand, synthesis of nanoscale materials which exhibit a wide range of electronic, optical and catalytic properties has been of both academic and technological interest. Several approaches are available for the preparation of nanomaterials such as coprecipitation [9], sol–gel processing [10], microemulsion [11], hydrothermal / solvothermal methods [12] and template syntheses [13]. Due to the specific structure of a microemulsion, this method is suitable for producing small nanoparticles of narrow size distribution. Nanostructured bismuth molybdates have been synthesized by solvothermal / hydrothermal [6,14,15], molten salt [16] and ultrasonic assisted syntheses [17]. But to the best of our knowledge there is no report on the investigation of the preparation of this type of material in the microemulsion system. So, herein we present the preparation and characterization of α-Bi2Mo3O12 nanoparticles for the first time with microemulsion method. The advantage of this approach is the better control over
the size and shape of the final nanoparticles in comparison to other methods. 2. Experimental 2.1. Preparation of α-Bi2Mo3O12 nanoparticles Bismuth nitrate, ammonium heptamolybdate, cetyltrimethylammonium bromide (CTAB), isooctane and n-butanol were purchased from Merck chemical company and used without further purification. The quaternary microemulsion system consisted of CTAB/H2O/ isooctane/n-butanol and was prepared as followed: n-butanol (2 g) was added to isooctane (4 g) and then, to this solution was added CTAB (2 g) in water (2 ml). The resulted mixture was stirred for 15 minutes until a clear solution was obtained. Then 2.4 ml of 0.04 M ammonium heptamolybdate aqueous solution and after 20 minutes 2.4 ml of 0.2 M acidic aqueous solution of bismuth nitrate were added to the above solution. The pH of the reaction mixture was adjusted in the range of 1–3 and refluxed for 3 hours. After cooling to room temperature, the resultant yellow product was collected by centrifugation and washed several times with water and ethanol and dried at 363 K for 12 hours. Finally, the product was calcined at 723 K for 8 hours to remove the remained organic residue and form the crystalline α-Bi2Mo3O12. 2.2. Instrumentation
⁎ Corresponding author. Tel./fax: + 98 261 4551023. E-mail address: [email protected] (M. Masteri-Farahani). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.020
Infrared spectra were recorded using Perkin-Elmer Spectrum RXI FT-IR spectrometer, using pellets of the materials diluted with KBr. X-ray diffraction (XRD) patterns of the samples were recorded on a
M. Masteri-Farahani, HS. Hosseini / Powder Technology 228 (2012) 228–230
229
Fig. 2. The XRD pattern of the calcined α-Bi2Mo3O12 product.
Scheme 1. Schematic illustration of preparation of α-Bi2Mo3O12 nanoparticles.
SIEFERT XRD 3003 PTS diffractometer using a Cu Kα radiation (λ = 0.1542 nm). Scanning electron micrographs (SEM) of the samples were taken with ZEISS-DSM 960A microscope with attached camera. Transmission electron microscopy (TEM) observations were carried out using a Philips EM 208 S instrument with an accelerating voltage of 100 kV. 3. Results and discussion As shown in Scheme 1, reaction between Bi3 + and MoO42- ions in quaternary microemulsion comprised of CTAB/water/isooctane/n-butanol resulted in the preparation of α-Bi2Mo3O12 nanoparticles. With addition of bismuth nitrate solution to the molybdate containing microemulsion system, bismuth molybdate nucleation occurs in the nanoreactors of
Fig. 1. The FT-IR spectra of the products before (inside) and after calcinations.
Fig. 3. The SEM image of the as-prepared (top) and calcined (middle and bottom) bismuth molybdate nanoparticles.
230
M. Masteri-Farahani, HS. Hosseini / Powder Technology 228 (2012) 228–230
Fig. 4. The TEM image of the calcined material.
the water droplets and nanoparticles with surfactant coatings are formed in a very short time. The sizes of the nanoparticles are restricted to the size of the water droplets acting as nanoreactors. In order to investigate the effect of pH on the crystal phase of resulted material, we conducted the reaction at pH = 2 to pH = 6. According to results obtained from X-ray diffraction analyses the pure crystalline phase of α-Bi2Mo3O12 was formed at pH = 2, which was in agreement with earlier reports [5].On the Other hand, different Bi/Mo ratios (from Bi/Mo = 0.5 to 1.0) was studied to investigate the effect of Bi/Mo ratio on the crystal phase of the final material. The pure crystalline phase of α-Bi2Mo3O12 was formed at Bi/Mo = 0.7. FT-IR spectroscopy provides good evidence for the formation of the CTAB-bismuth molybdate composite material. In the FT-IR spectrum of the as-prepared material (Fig. 1) the stretching vibrations of C-H groups of CTAB species were observed in 2849 and 2918 cm - 1. On the other hand, the bands at 1468 and 1486 cm - 1 were assigned to the bending vibrations of CH2 groups. Some bands in the range of 700–1000 cm - 1 (not shown here) are due to Mo-O and Bi-O vibrations in noncalcined material. Thus FT-IR spectrum of the noncalcined material showed the presence of the bismuth molybdate material associated with CTAB coating. In the FT-IR spectrum of the product after calcination (Fig. 1) the bands due to the presence of the C-H groups were disappeared as a result of the combustion and removal of CTAB species. In the FT-IR spectrum of the calcined α-Bi2Mo3O12, the bands in the range of 700–950 cm - 1 were assigned to the stretching vibrations of Mo-O groups and those of 400–600 cm - 1 are due to Bi-O stretching vibrations in the calcined and crystalline material. X-ray diffraction analysis was used to examine the crystal structure of the products. In the XRD pattern of the as-prepared product (not shown here), the observation of very broad peaks indicated the amorphous structure of the material. On the other hand, the presence of the CTAB inhibits the observation of the peaks of bismuth molybdate species. The XRD pattern of the calcined α-Bi2Mo3O12 is shown in Fig. 2. All the diffraction peaks can be indexed to the tetragonal
structure of the α-Bi2Mo3O12 with monoclinic phase and lattice constant a= 7.72 Å, b= 11.51 Å, c =11.98 Å (JCPDS file number 21–0103). The crystallinity of the product is reflected by the presence of distinct diffraction peaks in the calcined material and no impurity was detected in the X-ray diffraction pattern. The morphology and structure of the as-prepared and calcined products were further investigated with SEM analysis. Typical SEM images of the as-prepared and calcined bismuth molybdate nanoparticles are shown in Fig. 3. The SEM image of the as-prepared material indicates that the obtained product is composed of spherical nanoparticles with diameters about 30 nm. On the other hand, the SEM image of the calcined material shows that calcination increases the size of the nanoparticles to about 100 nm. In the calcination step, some agglomeration of nanoparticles occurred because of the removal of the CTAB species which act as protecting groups against the growth of the nanoparticles. The morphology and nanostructure of the calcined material was further investigated by TEM analysis. Fig. 4 shows the TEM image of the calcined material. This image shows that the nearly spherical nanoparticles have an average diameter of about 100 nm, which is in agreement with the SEM observation after calcination of the asprepared material. 4. Conclusion These results show that the microemulsion system provide efficient nanoreactors for the reaction between Bi 3 + and MoO42- ions and nucleation and growth of the bismuth molybdate nanoparticles. As the TEM and SEM images show, the resulted nanoparticles are nearly spherical and uniformly sized about 100 nm. References [1] N.I. Inescu, M. Caldararu, Heterogeneous selective oxidation of lower olefins, Edirura Acad. Romane, Bucharest, 1993, p. 278. [2] A. Bielanski, J. Haber, Oxygen in catalysis, Marcell Dekker, New York, 1991, p. 320. [3] J.D. Burington, C.T. Kartisek, R.K. Grasseli, Journal of Catalysis 63 (1980) 235–240. [4] R.K. Grasseli, Topics in Catalysis 21 (2002) 79–88. [5] A.M. Beale, G. Sankar, Chemistry of Materials 15 (2003) 146–153. [6] H.H. Li, K.W. Li, H. Wang, Materials Chemistry and Physics 116 (2009) 134–139. [7] G.W. Keulks, J.L. Hall, C. Daniel, K. Suzuki, Journal of Catalysis 34 (1974) 79–85. [8] F. Trifiro, H. Hoser, R.D. Scarle, Journal of Catalysis 25 (1972) 12–18. [9] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Journal of Colloid and Interface Science 146 (1991) 38–52. [10] B. Fegley, P. White, H.K. Bowen, American Ceramic Society Bulletin 64 (1985) 1115–1120. [11] M.P. Pileni, Journal of Physical Chemistry 97 (1993) 6961–6973. [12] L. Zhen, W.S. Wang, C.Y. Yu, W.Z. Zhao, L.C. Qin, Materials Letters 62 (2008) 1169–1172. [13] Z.T. Zhao, A.J. Rondinone, J.X. Ma, J. Shen, S. Dai, Advanced Materials 17 (2005) 1415–1419. [14] H.H. Li, K.W. Li, H. Wang, C.Y. Liu, Journal of Materials Science 43 (2008) 7026–7034. [15] C. Xu, D. Zou, L. Wang, H. Luo, T. Ying, Ceramics International 35 (2009) 2099–2102. [16] L.J. Xie, J.F. Ma, G.J. Xu, Materials Chemistry and Physics 110 (2008) 197–200. [17] L. Zhou, W.Z. Wang, L.S. Zhang, Journal of Molecular Catalysis A 268 (2007) 195–200.