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Rapid synthesis of cryptomelane-type manganese oxide under ultrasonic process
Materials Letters 65 (2011) 3184–3186
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Rapid synthesis of cryptomelane-type manganese oxide under ultrasonic process Ming Sun a, Lin Yu a,⁎, Fei Ye a, Guiqiang Diao a, Qian Yu a, Yuying Zheng a, Jean-Yves Piquemal b a b
Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China ITODYS, UMR CNRS 7086, Université Paris Diderot-Paris 7, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France
a r t i c l e
i n f o
Article history: Received 23 April 2011 Accepted 29 June 2011 Available online 3 July 2011 Keywords: Manganese oxide Ultrasonic irradiation Nanosize Oxidation
a b s t r a c t Cryptomelane-type manganese oxide (OMS-2) was rapidly prepared under ultrasonic irradiation in short time. Characterization results using X-ray powder diffraction (XRD), surface area analyzer, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O2-TPD) revealed that ultrasonic irradiation has tremendous effect on the surface area, morphology, surface defects, and redox properties of the OMS-2 materials. The OMS-2 prepared via ultrasonic irradiation shows nanoneedle morphology with smaller crystallite size, larger surface area (120.4 m 2/g), more surface defects, and higher oxygen mobility, thus it demonstrates excellent activity in the catalytic combustion of dimethyl ether with a start-off temperature of 160 °C and a complete combustion temperature of 172 °C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cryptomelane-type manganese oxide, also called manganese oxide octahedral molecular sieve (OMS-2), has wide application in adsorption, separation, supercapacitors and catalysis [1,2]. OMS-2 can be produced through several routes such as the reflux method, the sol–gel method, and the hydrothermal method [3]. However, several hours up to several days are necessary to synthesize OMS-2 materials. Moreover, OMS-2 materials have a relatively small surface area after calcination, typically in the range of 40–60 m 2/g based on our experiment results. It is well known that the surface area of nanomaterials plays a key role in their activities. Therefore, to produce a high-surface-area OMS-2 in a short time is important. In recent years, ultrasonic irradiation has been used in the production of nanomaterials because of its special cavitation effect [4]. To the best of our knowledge, few studies have reported the preparation of OMS-2 materials using ultrasonic irradiation. In this communication, we propose to prepare the OMS-2 materials under ultrasonic irradiation in a short time, and study the effects of ultrasonic irradiation on its physical characteristics and the activity in dimethyl ether (DME) combustion.
2. Experimental The conventional method for the synthesis of OMS-2 is the same as previously reported [5]. In the ultrasonic synthesis process, all the
⁎ Corresponding author. Tel.: + 86 20 39322202; fax: + 86 20 39322231. E-mail address: [email protected] (L. Yu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.108
conditions were the same as in the conventional process except for the reaction, which was carried out under ultrasonic irradiation at a power of 240 W and a frequency of 40 kHz for 4 h. The sample was dried at 80 °C for 12 h and then calcined at 500 °C for 2 h. The material prepared via ultrasonic irradiation was denoted OMS-2 (UL). X-ray powder diffractions (XRD) were obtained from a MSAL-XD2 X-ray diffractometer with CuKa radiation. N2 physisorption experiments were realized at 77 K using a Gemini V 2380 apparatus from Micromeritics. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were obtained using a JEM-2010HR instrument. X-ray photoelectron spectroscopy (XPS) was recorded on a VG MultiLab 2000 instrument equipped with an Mg Kα X-ray source. Oxygen temperature-programmed desorption (O2-TPD) was measured in a TP5000 apparatus. The catalyst (50 mg) was pretreated in a helium stream at 350 °C for 1.5 h and subsequently cooled to room temperature. The material was then exposed to an oxygen atmosphere. Finally, the sample was heated in flowing helium, from 50 to 850 °C at a rate of 10 °C min − 1. The DME combustion test was conducted in a fixed bed quartz reactor. Two reactant gases, DME and 20 vol%O2/He, were co-fed into the reactor with a mole ratio of 1:50. The gas hourly space velocity was about 1 L (gcat·h) − 1. The reactor effluent was analyzed by an online gas chromatograph (Agilent 6820). 3. Results and discussion Fig. 1 illustrates the XRD patterns of the OMS-2 and the OMS-2 (UL) materials. Both have the same XRD patterns equivalent to natural cryptomelane (KMn8O16, JCPDS 290–1020) with a tetragonal structure. The crystallite size of the peak (210) was calculated based on the Scherrer Formula. The OMS-2 (UL) has a smaller crystallite size
M. Sun et al. / Materials Letters 65 (2011) 3184–3186
Fig. 1. XRD patterns of the OMS-2 materials.
(9.1 nm), compared with that of the conventionally synthesized OMS2 (12.2 nm). The surface area of OMS-2 (UL) is 120.4 m 2/g, larger than that of the OMS-2 material (50.2 m 2/g). Considering ultrasonic irradiation, several reaction parameters can be modified: i) the irradiation time, ii) the ultrasonic power and iii) the ultrasonic frequency. Prolonged irradiation times led to higher specific surface areas. Similar
3185
tendencies were observed with the ultrasonic frequency and power (see supplementary materials, Table S1). The microstructure of the OMS-2 materials was further characterized by TEM, as shown in Fig. 2. The conventionally synthesized OMS-2 shows a rod-like morphology, with a length of about 200 nm and a diameter of 20 nm. However, the OMS-2 catalysts prepared under various ultrasonic power and frequency show fine needlelike morphology (supplementary data, Fig. S1). In Fig. 2(c), the needlelike OMS-2 (UL) has a length of about 100 nm and a diameter of 10 nm. The HRTEM image in Fig. 2(b) demonstrates that the OMS-2 nanorod has well-defined lattice fringes and that the d-spacing is about 0.69 nm, corresponding to the (110) plane of OMS-2. However, the lattice fringes of the OMS-2 (UL) are not well formed, with a fringe spacing of 0.49 nm, corresponding to the (200) planes. Fig. 2(d) shows some defects exiting in the OMS-2 (UL) catalyst (marked by arrows), especially around the boundary of the nanoneedle. In considering the preparation process, these defects should be produced by the ultrasonic irradiation based on the hot spot mechanism [4]. XPS measurements were applied to investigate the surface properties of the OMS-2 materials. The O1s XPS results of the samples are shown in Fig. 3. For the two OMS-2 samples, the main O 1s peaks were observed at 529–530 eV and at 531–533 eV, which can be assigned to the lattice oxygen (Oα) and the defect-oxide or surface oxygen ions (Oβ), respectively [6,7]. However, the ratio of Oβ/Oα for the OMS-2 (UL) is 0.26, a little bigger than that of the OMS-2 (0.22), which indicates that the OMS-2 synthesized under ultrasonic
Fig. 2. TEM and HRTEM images of the OMS-2 materials [(a, b), OMS-2; (c, d) OMS-2 (UL)].
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M. Sun et al. / Materials Letters 65 (2011) 3184–3186
Fig. 4. DME combustion activity of the OMS-2 materials.
Fig. 3. O 1s XPS spectra of the OMS-2 materials.
irradiation might have more defective oxide ions. The O 1s XPS results are in good agreement with the HRTEM results. Generally three types of oxygen desorption peaks are detected from the O2-TPD results (supplementary data, Fig. S2), namely the weakly bound physisorbed oxygen at around 200 °C, the surface oxygen at around 600 °C, and the bulk lattice oxygen at around 800 °C. The results are similar to those reported in [8,9]. The O2 profile for the OMS-2 differs from those for the OMS-2 (UL). Physisorbed oxygen begins to evolve at 145 °C, surface and lattice oxygen appear at 590 and 805 °C, respectively. In contrast, the OMS-2 (UL) shows physisorbed oxygen release at 188 °C, surface and lattice oxygen emerge at 579 and 779 °C, respectively. These results show that ultrasonic irradiation has a great impact on the oxygen desorption temperature. The mobility of surface and lattice oxygen is increased. Considering that the OMS-2 (UL) has a smaller crystallite size and more defects, the higher diffusion mobility of oxygen might be caused by such physical characteristics. The DME conversion for OMS-2 and OMS-2 (UL) materials were plotted as a function of the reaction temperature and the results are given in Fig. 4. The data show that for temperatures higher than 160 °C, the combustion activity of OMS-2 (UL) is superior to that of OMS-2. Indeed, T10 and T90 were respectively 160 °C and 172 °C for OMS-2 (UL), and 170 °C and 180 °C for OMS-2. The activities of the OMS-2 materials are related to their physical and chemical properties. Liu et al. [10] reported that the activity of OMS-2 materials was related to their surface areas. Schurz et al. [11] also proved that the activity of OMS-2 is correlated to its surface area in the oxidation of benzyl alcohol. This is also true in our study. OMS-2 (UL), which has a larger surface area (120.4 m 2/g), possesses more catalytic active sites per unit mass and exhibits better performance than the conventionally synthesized OMS-2, which has a smaller surface area (50.2 m 2/g). Apart from the surface area, the surface defects of the OMS-2 also play a key role in reactivity [5]. The HRTEM and the O 1s XPS results show that the surface defects of OMS-2 were enhanced after
ultrasonic treatment. Therefore, the excellent performance of OMS-2 (UL) may be attributed to the large amount of defects and distortions in the material structure. The redox properties of the OMS-2 are other key factors that influence their activities. The O2-TPD studies confirm that the OMS-2 (UL) has abundant available lattice oxygen, and that the oxygen mobility is higher than with the OMS-2. Based on these facts, we conclude that the ultrasonic treatment affects the morphology of the solid and hence the mobility of oxygen on the catalyst surface. 4. Conclusions Manganese oxide octahedral molecular sieve, denoted OMS-2 (UL), was rapidly prepared via ultrasonic irradiation. The ultrasonic irradiation had a marked effect on the surface area (the specific surface area was doubled compared to the non irradiated sample), morphology, surface defects, and redox properties of the OMS-2 (UL) material. This OMS-2 (UL) showed remarkable catalytic activity in the DME combustion. Acknowledgements This work was supported by Natural Science Foundation of Guangdong Province (10251009001000003) and the 211 Project of Guangdong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.06.108. References [1] Qi Feng, Hirofumi Kanoh, Kenta Ooi. J Mater Chem 1999;9:319. [2] Suib SL. J Mater Chem 2008;18:1623–31. [3] Brock SL, Duan NG, Tian ZR, Giraldo O, Zhnou H, Suib SL. Chem Mater 1998;10: 2619–28. [4] Gedanken A. Ultrason Sonochem 2004;11:47–55. [5] Jothiramalingam R, Viswanathan B, Varadarajan TK. Catal Commun 2005;6:41–5. [6] Tang XF, Li JH, Hao JM. Catal Commun 2010;11:871–5. [7] Larachi F, Pierre J, Adnot A, Bernis A. Appl Surf Sci 2002;195:236–50. [8] Yin YG, Xu WQ, Shen YF, Suib SL, O'Young CL. Chem Mater 1994;6:1803–8. [9] Yin YG, Xu WQ, Suib SL, O'Young CL. Inorg Chem 1995;34:4187–93. [10] Liu J, Makwana V, Cai J, Suib SL, Aindow M. J Phys Chem B 2003;107:9185–94. [11] Schurz F, Bauchert JM, Merker T, Schleid T, Hasse H, Glaser R. Appl Catal A 2009;355:42.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Rapid synthesis of cryptomelane-type manganese oxide under ultrasonic process Ming Sun a, Lin Yu a,⁎, Fei Ye a, Guiqiang Diao a, Qian Yu a, Yuying Zheng a, Jean-Yves Piquemal b a b
Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China ITODYS, UMR CNRS 7086, Université Paris Diderot-Paris 7, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France
a r t i c l e
i n f o
Article history: Received 23 April 2011 Accepted 29 June 2011 Available online 3 July 2011 Keywords: Manganese oxide Ultrasonic irradiation Nanosize Oxidation
a b s t r a c t Cryptomelane-type manganese oxide (OMS-2) was rapidly prepared under ultrasonic irradiation in short time. Characterization results using X-ray powder diffraction (XRD), surface area analyzer, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O2-TPD) revealed that ultrasonic irradiation has tremendous effect on the surface area, morphology, surface defects, and redox properties of the OMS-2 materials. The OMS-2 prepared via ultrasonic irradiation shows nanoneedle morphology with smaller crystallite size, larger surface area (120.4 m 2/g), more surface defects, and higher oxygen mobility, thus it demonstrates excellent activity in the catalytic combustion of dimethyl ether with a start-off temperature of 160 °C and a complete combustion temperature of 172 °C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cryptomelane-type manganese oxide, also called manganese oxide octahedral molecular sieve (OMS-2), has wide application in adsorption, separation, supercapacitors and catalysis [1,2]. OMS-2 can be produced through several routes such as the reflux method, the sol–gel method, and the hydrothermal method [3]. However, several hours up to several days are necessary to synthesize OMS-2 materials. Moreover, OMS-2 materials have a relatively small surface area after calcination, typically in the range of 40–60 m 2/g based on our experiment results. It is well known that the surface area of nanomaterials plays a key role in their activities. Therefore, to produce a high-surface-area OMS-2 in a short time is important. In recent years, ultrasonic irradiation has been used in the production of nanomaterials because of its special cavitation effect [4]. To the best of our knowledge, few studies have reported the preparation of OMS-2 materials using ultrasonic irradiation. In this communication, we propose to prepare the OMS-2 materials under ultrasonic irradiation in a short time, and study the effects of ultrasonic irradiation on its physical characteristics and the activity in dimethyl ether (DME) combustion.
2. Experimental The conventional method for the synthesis of OMS-2 is the same as previously reported [5]. In the ultrasonic synthesis process, all the
⁎ Corresponding author. Tel.: + 86 20 39322202; fax: + 86 20 39322231. E-mail address: [email protected] (L. Yu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.108
conditions were the same as in the conventional process except for the reaction, which was carried out under ultrasonic irradiation at a power of 240 W and a frequency of 40 kHz for 4 h. The sample was dried at 80 °C for 12 h and then calcined at 500 °C for 2 h. The material prepared via ultrasonic irradiation was denoted OMS-2 (UL). X-ray powder diffractions (XRD) were obtained from a MSAL-XD2 X-ray diffractometer with CuKa radiation. N2 physisorption experiments were realized at 77 K using a Gemini V 2380 apparatus from Micromeritics. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were obtained using a JEM-2010HR instrument. X-ray photoelectron spectroscopy (XPS) was recorded on a VG MultiLab 2000 instrument equipped with an Mg Kα X-ray source. Oxygen temperature-programmed desorption (O2-TPD) was measured in a TP5000 apparatus. The catalyst (50 mg) was pretreated in a helium stream at 350 °C for 1.5 h and subsequently cooled to room temperature. The material was then exposed to an oxygen atmosphere. Finally, the sample was heated in flowing helium, from 50 to 850 °C at a rate of 10 °C min − 1. The DME combustion test was conducted in a fixed bed quartz reactor. Two reactant gases, DME and 20 vol%O2/He, were co-fed into the reactor with a mole ratio of 1:50. The gas hourly space velocity was about 1 L (gcat·h) − 1. The reactor effluent was analyzed by an online gas chromatograph (Agilent 6820). 3. Results and discussion Fig. 1 illustrates the XRD patterns of the OMS-2 and the OMS-2 (UL) materials. Both have the same XRD patterns equivalent to natural cryptomelane (KMn8O16, JCPDS 290–1020) with a tetragonal structure. The crystallite size of the peak (210) was calculated based on the Scherrer Formula. The OMS-2 (UL) has a smaller crystallite size
M. Sun et al. / Materials Letters 65 (2011) 3184–3186
Fig. 1. XRD patterns of the OMS-2 materials.
(9.1 nm), compared with that of the conventionally synthesized OMS2 (12.2 nm). The surface area of OMS-2 (UL) is 120.4 m 2/g, larger than that of the OMS-2 material (50.2 m 2/g). Considering ultrasonic irradiation, several reaction parameters can be modified: i) the irradiation time, ii) the ultrasonic power and iii) the ultrasonic frequency. Prolonged irradiation times led to higher specific surface areas. Similar
3185
tendencies were observed with the ultrasonic frequency and power (see supplementary materials, Table S1). The microstructure of the OMS-2 materials was further characterized by TEM, as shown in Fig. 2. The conventionally synthesized OMS-2 shows a rod-like morphology, with a length of about 200 nm and a diameter of 20 nm. However, the OMS-2 catalysts prepared under various ultrasonic power and frequency show fine needlelike morphology (supplementary data, Fig. S1). In Fig. 2(c), the needlelike OMS-2 (UL) has a length of about 100 nm and a diameter of 10 nm. The HRTEM image in Fig. 2(b) demonstrates that the OMS-2 nanorod has well-defined lattice fringes and that the d-spacing is about 0.69 nm, corresponding to the (110) plane of OMS-2. However, the lattice fringes of the OMS-2 (UL) are not well formed, with a fringe spacing of 0.49 nm, corresponding to the (200) planes. Fig. 2(d) shows some defects exiting in the OMS-2 (UL) catalyst (marked by arrows), especially around the boundary of the nanoneedle. In considering the preparation process, these defects should be produced by the ultrasonic irradiation based on the hot spot mechanism [4]. XPS measurements were applied to investigate the surface properties of the OMS-2 materials. The O1s XPS results of the samples are shown in Fig. 3. For the two OMS-2 samples, the main O 1s peaks were observed at 529–530 eV and at 531–533 eV, which can be assigned to the lattice oxygen (Oα) and the defect-oxide or surface oxygen ions (Oβ), respectively [6,7]. However, the ratio of Oβ/Oα for the OMS-2 (UL) is 0.26, a little bigger than that of the OMS-2 (0.22), which indicates that the OMS-2 synthesized under ultrasonic
Fig. 2. TEM and HRTEM images of the OMS-2 materials [(a, b), OMS-2; (c, d) OMS-2 (UL)].
3186
M. Sun et al. / Materials Letters 65 (2011) 3184–3186
Fig. 4. DME combustion activity of the OMS-2 materials.
Fig. 3. O 1s XPS spectra of the OMS-2 materials.
irradiation might have more defective oxide ions. The O 1s XPS results are in good agreement with the HRTEM results. Generally three types of oxygen desorption peaks are detected from the O2-TPD results (supplementary data, Fig. S2), namely the weakly bound physisorbed oxygen at around 200 °C, the surface oxygen at around 600 °C, and the bulk lattice oxygen at around 800 °C. The results are similar to those reported in [8,9]. The O2 profile for the OMS-2 differs from those for the OMS-2 (UL). Physisorbed oxygen begins to evolve at 145 °C, surface and lattice oxygen appear at 590 and 805 °C, respectively. In contrast, the OMS-2 (UL) shows physisorbed oxygen release at 188 °C, surface and lattice oxygen emerge at 579 and 779 °C, respectively. These results show that ultrasonic irradiation has a great impact on the oxygen desorption temperature. The mobility of surface and lattice oxygen is increased. Considering that the OMS-2 (UL) has a smaller crystallite size and more defects, the higher diffusion mobility of oxygen might be caused by such physical characteristics. The DME conversion for OMS-2 and OMS-2 (UL) materials were plotted as a function of the reaction temperature and the results are given in Fig. 4. The data show that for temperatures higher than 160 °C, the combustion activity of OMS-2 (UL) is superior to that of OMS-2. Indeed, T10 and T90 were respectively 160 °C and 172 °C for OMS-2 (UL), and 170 °C and 180 °C for OMS-2. The activities of the OMS-2 materials are related to their physical and chemical properties. Liu et al. [10] reported that the activity of OMS-2 materials was related to their surface areas. Schurz et al. [11] also proved that the activity of OMS-2 is correlated to its surface area in the oxidation of benzyl alcohol. This is also true in our study. OMS-2 (UL), which has a larger surface area (120.4 m 2/g), possesses more catalytic active sites per unit mass and exhibits better performance than the conventionally synthesized OMS-2, which has a smaller surface area (50.2 m 2/g). Apart from the surface area, the surface defects of the OMS-2 also play a key role in reactivity [5]. The HRTEM and the O 1s XPS results show that the surface defects of OMS-2 were enhanced after
ultrasonic treatment. Therefore, the excellent performance of OMS-2 (UL) may be attributed to the large amount of defects and distortions in the material structure. The redox properties of the OMS-2 are other key factors that influence their activities. The O2-TPD studies confirm that the OMS-2 (UL) has abundant available lattice oxygen, and that the oxygen mobility is higher than with the OMS-2. Based on these facts, we conclude that the ultrasonic treatment affects the morphology of the solid and hence the mobility of oxygen on the catalyst surface. 4. Conclusions Manganese oxide octahedral molecular sieve, denoted OMS-2 (UL), was rapidly prepared via ultrasonic irradiation. The ultrasonic irradiation had a marked effect on the surface area (the specific surface area was doubled compared to the non irradiated sample), morphology, surface defects, and redox properties of the OMS-2 (UL) material. This OMS-2 (UL) showed remarkable catalytic activity in the DME combustion. Acknowledgements This work was supported by Natural Science Foundation of Guangdong Province (10251009001000003) and the 211 Project of Guangdong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.06.108. References [1] Qi Feng, Hirofumi Kanoh, Kenta Ooi. J Mater Chem 1999;9:319. [2] Suib SL. J Mater Chem 2008;18:1623–31. [3] Brock SL, Duan NG, Tian ZR, Giraldo O, Zhnou H, Suib SL. Chem Mater 1998;10: 2619–28. [4] Gedanken A. Ultrason Sonochem 2004;11:47–55. [5] Jothiramalingam R, Viswanathan B, Varadarajan TK. Catal Commun 2005;6:41–5. [6] Tang XF, Li JH, Hao JM. Catal Commun 2010;11:871–5. [7] Larachi F, Pierre J, Adnot A, Bernis A. Appl Surf Sci 2002;195:236–50. [8] Yin YG, Xu WQ, Shen YF, Suib SL, O'Young CL. Chem Mater 1994;6:1803–8. [9] Yin YG, Xu WQ, Suib SL, O'Young CL. Inorg Chem 1995;34:4187–93. [10] Liu J, Makwana V, Cai J, Suib SL, Aindow M. J Phys Chem B 2003;107:9185–94. [11] Schurz F, Bauchert JM, Merker T, Schleid T, Hasse H, Glaser R. Appl Catal A 2009;355:42.