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Structural changes in alumina-supported manganese oxides during ozone decomposition
Chemical Physics Letters 408 (2005) 377–380 www.elsevier.com/locate/cplett
Structural changes in alumina-supported manganese oxides during ozone decomposition Hisahiro Einaga *, Masafumi Harada, Shigeru Futamura National Institute of Advanced Industrial Science and Technology, Institute for Environmental Management Technology, AIST Tsukuba West, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan Department of Textile and Apparel Science, Faculty of Human Life and Environment, Nara Womens University, Nara 630-8506, Japan Received 1 March 2005; in final form 9 April 2005 Available online 13 May 2005
Abstract In situ X-ray absorption fine structure spectroscopic studies were carried out to investigate the structural changes in manganese oxides supported on alumina in the catalytic decomposition of ozone at room temperature. In the ozone decomposition with water vapor, Mn atom was oxidized to higher oxidation state with the coordination of water to Mn site, which was caused by the cleavage of Mn–O–Al bond. The used catalyst was regenerated by the heat treatment in an O2 flow at 723 K. Ó 2005 Elsevier B.V. All rights reserved.
O2 ! O2 þ
1. Introduction Manganese oxide catalysts are of interest due to their applicability to catalytic reactions such as selective catalytic reduction of NOx with NH3 [1], CO oxidation [2] and combustion of organic compounds [3] in gaseous phase and selective oxidation of organic compounds [4] in liquid phase. Manganese oxide catalysts are also useful for the decomposition of ozone in gas streams [5–7]. Oyama and his co-workers [8,9] have reported that the mechanism for O3 decomposition on manganese oxide supported on c-Al2O3 (denoted by MnOx/Al2O3), and have reported that the reaction proceeds through two irreversible steps, the adsorption of ozone on the catalyst surface and the desorption of molecular oxygen (Eqs. (1)–(3)), where * denotes the surface site on the catalyst: ! O2 þ O
ð1Þ
O þ O3 ! O2 þ O2
ð2Þ
O3 þ
*
Corresponding author. Fax: +81 29 861 8679. E-mail address: [email protected] (H. Einaga).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.061
ð3Þ
As the intermediate species, atomic oxygen and peroxides have been observed. These reactions are also applicable to the VOCs abatement at low temperatures. The oxygen species formed in this decomposition steps are active for aromatic compounds in gas streams. Naydenov et al. [10] have reported that the use of ozone can lower the reaction temperature for benzene oxidation over un-supported manganese oxides. In the previous papers, we have reported the behavior of benzene oxidation, ozone decomposition, and products formation on the alumina-supported manganese oxide catalysts [11,12]. We herein report the X-ray absorption fine structure (XAFS) spectroscopic studies to investigate the structural changes in alumina-supported manganese oxide catalysts during ozone decomposition reactions. The presence of water, a ubiquitous constituent in effluent gas stream and formed in VOC decomposition, causes the structural changes in Mn oxide structures during the ozone decomposition. The used catalyst can be regenerated by the heat treatment in an O2 flow at 723 K.
378
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
2. Experimental
3. Results and discussion
Alumina-supported manganese oxides were prepared by the impregnation of c-Al2O3 (Catalysis Society of Japan, JRC ALO-4, SBET 170 m2 g1) with the aqueous solution containing an appropriate amount of Mn(CH3COO)2 Æ 4H2O (Wako Pure Chemical, >99.9%). Catalyst samples were dried at 383 K and then calcined at 773 K for 3 h in air. XAFS measurements were carried out on the Photon Factory beam line BL-7C at High Energy Accelerator Research Organization (KEK-PF) with the storage ring operating at an energy of 2.5 GeV. The double crystal monochromator Si(1 1 1) was used. Fig. 1 shows the schematic of the experimental setup. Reaction gases were prepared by N2 (>99.9995%, total hydrocarbon <1 ppm) and O2 (>99.9995%, total hydrocarbon <1 ppm) in cylinders by using the sets of thermal mass flow controllers. Ozone was synthesized from O2 by a silent discharge ozone generator. Ozone concentration was 1000 ± 30 ppm. The flow rate was 250–1000 ml min1. Catalyst samples were pressed into thin self-supporting wafers with 20 mm/ in diameter and set in an in situ cell with polyimide film (Du Pont-Toray Co., Ltd. Kapton 200H), which was connected to the flow-type reaction system described above. The incident and transmitted X-rays were detected by N2-filled ionization chambers. XAFS spectra were recorded at a temperature of 296 K with an equivalent step of 0.25 eV/point for XANES measurements and 1.0 eV/point for EXAFS measurements. Mn foil is used as the reference sample for energy calibration to ascertain that no energy shift was observed during the measurements. Data reduction of experimental absorption spectra was carried out according to the method recommended by the Standards and Criteria Committee of the International XAFS Society [13] using WinXAS v.3.1 [14,15].
Fig. 2 shows the Mn–K edge EXAFS Fourier transforms for 2.5 wt%–MnOx/Al2O3 before the ozone decomposition reaction. The spectra were obtained after heating in O2 flow at 723 K for 2 h and then cooled down to the ambient temperature (296 K). A peak due ˚ , while to the bond of Mn–O is observed at R = 1.5 A no obvious peaks for Mn–Mn bond are observed, indicating that Mn oxides are highly dispersed on the catalyst support. This is consistent with the previous results that Mn-oxides are highly dispersed on alumina support at low loading levels [16,17]. Thus, no size effect has to be taking account in the XANES region of the absorption spectrum [18]. Regarding the incursion of Al in
Fig. 2. Mn–K edge EXAFS Fourier transforms for the aluminasupported manganese oxides. (a) before reaction, (b) after the contact of water vapor, (c) after ozone decomposition with water vapor, and (d) after heat treatment in an O2 flow at 723 K.
Fig. 1. Experimental set-up of in situ XAFS measurements.
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
the environment of Mn, Lopez-Navarrete et al. [19] have underlined the effect of such structural process on the Fourier Transform modulus. It should be noted that the rate for ozone decomposition was not so much influenced by the Mn loadings. Fig. 3 shows the Mn–K edge XANES spectra of the MnOx/Al2O3 catalyst. The presence of water vapor without ozone feed had almost no changes in the XANES spectra and no significant changes in the EXAFS spectrum (Fig. 2), indicating the no significant changes in Mn oxide structure by the physical adsorption of water vapor. On the other hand, the absorption edge of XANES spectra were gradually shifted to higher energy with time on stream and a preedge peak newly appeared at 6540.6 eV (Fig. 3a), when ozone was fed
Fig. 3. Mn–K edge XANES spectra of MnOx/Al2O3 during ozone decomposition reaction. (a) With water vapor; —–: before reaction, - - - - -: after the contact of water vapor, color lines: in situ measurement. (b) In situ measurement without water vapor. The in situ spectra were taken every 21 min after the start of ozone feed.
379
into the cell with water vapor (water concentration: 0.55%). No such spectral changes occurred during ozone decomposition when water vapor was absent (Fig. 3b). In addition, the contact of water vapor in a N2 flow and an O2 flow after the ozone decomposition reaction did not change the Mn–K edge XANES spectra. It has been reported that linear relationship is observed between the energy shift of the Mn–K absorption edge and the oxidation state of the Mn-oxide samples [20,21]. The absorption edge of MnOx/Al2O3 sample is close to that of Mn3O4, which consists of MnII and MnIII species with the average oxidation state of 2.67. This is consistent with the results by other groups that Mn oxidation state estimated from NEXAFS (near-edge X-ray absorption fine structure) studies was 2.4 [22]. The shift of absorption edge to higher energy indicates that Mn oxidation state was changed to higher oxidation state. The appearance of a preedge band at 6540.6 eV, which is characteristic of that for MnO2, implies the formation of MnIV species. The Mn–K edge EXAFS Fourier transforms of the used sample is also shown in Fig. ˚ increased in its intensity after 2. The band at R = 1.5 A the ozone decomposition with water vapor. Comparing the spectrum with that obtained before the ozone feed (Fig. 2), we ascribed the increased band to the coordination of water to Mn sites, which increased the number of Mn–O bond. The coordination of water to Mn site was caused by the bond cleavage of Mn–O–Al bond. Although the bond cleavage of Mn–O–Al was not reflected in XAFS spectra, it is evidenced by the fact that Mn ions were extracted from the catalyst surface by washing the used catalyst with water. This finding indicates that Mn was isolated from catalyst support to become a free Mn ion to which water was coordinated. The catalyst color was also changed slightly from brown to brownish pink. No such bond cleavage was observed without ozone contact: Mn ions were not extracted from the catalyst by the wash treatment. Oyama and his co-workers [22] have reported that ozone decomposition on MnOx/Al2O3 catalyst proceeds by electron transfer from Mn site to ozone and Mn is reduced back in the desorption of oxygen species: O3 + Mnnþ ! O2 + Mnðnþ2Þþ + O2
ð4Þ
ðnþ2Þþ þ O2 O3 þ O2 þ Mnðnþ2Þþ ! O2 2 þ Mn
ð5Þ
ðnþ2Þþ O2 ! Mnnþ þ O2 2 þ Mn
ð6Þ
As described above, the presence of water vapor is essential for the structural changes in Mn sites. Although the role of water vapor is under investigation, the presence of water vapor may accelerate the oxidation of Mn and/or inhibits its reduction. Water vapor is generally contained in effluent gas streams and is formed in the oxidation of organic
380
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
compounds. Our previous paper shows that the feed of water vapor gives rise to the decrease in the rate for ozone decomposition with MnOx/Al2O3 catalyst [12]. Ozone adsorption on the Mn sites is inhibited by the presence of water vapor, leading to the decrease in the decomposition rate. However, the structural change of Mn site can also have influence on the rate for ozone decomposition. Heat treatment in an O2 flow leads to the regeneration of MnOx/Al2O3 catalyst. Fig. 4 shows the changes in Mn–K edge XANES spectra of the used MnOx/ Al2O3 catalyst obtained when the catalyst was heated at 423, 573, and 723 K compared with the preheated catalyst. The catalyst was heated at the desired temperature for a few minutes and then cooled down to room temperature (296 K) before XAFS spectroscopic measurements. The used catalyst heated at 423 K had almost no changes in the XANES spectra, and that at 573 K slightly shifted the Mn–K absorption edge to lower energy with the disappearance of preedge peak at 6540.6 eV. Further heat treatment at 723 K gave the same spectrum with that for fresh catalyst sample. This finding shows that the oxidation state of Mn site was reduced back to that of the fresh catalyst sample by the oxidation of catalyst at 723 K. The Mn–K edge EXAFS Fourier transforms for the regenerated sample show the decrease of the band intensity of Mn–O band to the level of fresh sample (Fig. 2), showing that the coordinated water to Mn site was desorbed from the catalyst sample. At this time, Mn ions were not extracted from the catalyst surface by washing the catalyst with water, confirming the complete regeneration of the catalyst.
Fig. 4. Mn–K edge XANES spectra of MnOx/Al2O3 after heat treatment in an O2 flow. (a) Before heat treatment, (b) at 423 K, (c) 573 K, and (d) 723 K.
4. Conclusion In this study, we reported that manganese oxides supported on alumina suffered the structural changes during ozone decomposition reaction in the presence of water vapor. Mn was oxidized to higher oxidation state, along with the coordination of water to Mn. The presence of water vapor in the ozone decomposition was essential for the structural changes. The catalyst was completely regenerated by the heat treatment in an O2 flow at 723 K, which gave the same XAFS spectra with that for fresh sample. These findings provided significant information on the catalytic properties of supported manganese oxides for the oxidation reactions using ozone.
Acknowledgements We are grateful to the approval of Photon Factory Advisory Committee (PAC) (Proposal No. 2003G071) at High Energy Accelerator Research Organization (KEK) for the XAFS measurements.
References [1] W.S. Kijlstra, D.S. Brands, E.K. Poels, A. Bliek, J. Catal. 171 (1997) 208. [2] A.B. Lamb, W.C. Bray, J.C. Frazer, J. Ind. Eng. Chem. 12 (1920). [3] L.S. Puckhaber, H. Cheung, D.L. Cocke, A. Clearfield, Solid State Ionics 32/33 (1989) 206. [4] R. Ghosh, Y.-C. Son, V.D. Makwana, S.L. Suib, J. Catal. 224 (2004) 288. [5] P. Hunter, S.T. Oyama, Control of Volatile Organic Compound Emissions: Conventional and Emerging Technologies, Wiley, New York, 2000. [6] B. Dhandapani, S.T. Oyama, Appl. Catal. B 11 (1997) 129. [7] S.T. Oyama, Catal. Rev. – Sci. Eng. 42 (2000) 179. [8] W. Li, G.V. Gibbs, S.T. Oyama, J. Am. Chem. Soc. 120 (1998) 9041. [9] W. Li, S.T. Oyama, J. Am. Chem. Soc. 120 (1998) 9047. [10] A. Naydenov, D. Mehandjiev, Appl. Catal. A 97 (1993) 17. [11] H. Einaga, S. Futamuara, React. Kinet. Catal. Lett. 81 (2004) 121. [12] H. Einaga, S. Futamuara, J. Catal. 227 (2004) 304. [13] D.C. Koningsberger, Jpn. J. Appl. Phys. 32 (Suppl. 2) (1993) 877. [14] T. Ressler, J. Synchrotron Radiat. 5 (1998) 118. [15] T. Ressler, J. Phys. IV 7 (C2) (1997) 269. [16] W.S. Kijlstra, E.K. Poels, A. Bliek, B.M. Weckhuysen, R.A. Schoonheydt, J. Phys. Chem. B 101 (1997) 309. [17] R. Radhakrishnan, S.T. Oyama, Y. Ohminami, K. Asakura, J. Phys. Chem. B 105 (2001) 9067. [18] D. Bazin, J.J. Rehr, J. Phys. Chem. B 107 (2003) 12398. [19] E. Lopez-Navarrete, A. Caballero, A.R. Gonzalez-Elipe, M. Ocan, J. Eur. Ceram. Soc. 24 (2004) 3057. [20] T. Ressler, J. Wong, J. Roots, I.L. Smith, Environ. Sci. Technol. 34 (2000) 950. [21] J.M. Ramallo-Lo´pez, E.J. Lede, F.G. Requejo, J.A. Rodriguez, J.-Y. Kim, R. Rosas-Salas, J.M. Domı´nguez, J. Phys. Chem. B 108 (2004) 20005. [22] R. Radhakrishnan, S.T. Oyama, J.G. Chen, K. Asakura, J. Phys. Chem. B 105 (2001) 4245.
Structural changes in alumina-supported manganese oxides during ozone decomposition Hisahiro Einaga *, Masafumi Harada, Shigeru Futamura National Institute of Advanced Industrial Science and Technology, Institute for Environmental Management Technology, AIST Tsukuba West, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan Department of Textile and Apparel Science, Faculty of Human Life and Environment, Nara Womens University, Nara 630-8506, Japan Received 1 March 2005; in final form 9 April 2005 Available online 13 May 2005
Abstract In situ X-ray absorption fine structure spectroscopic studies were carried out to investigate the structural changes in manganese oxides supported on alumina in the catalytic decomposition of ozone at room temperature. In the ozone decomposition with water vapor, Mn atom was oxidized to higher oxidation state with the coordination of water to Mn site, which was caused by the cleavage of Mn–O–Al bond. The used catalyst was regenerated by the heat treatment in an O2 flow at 723 K. Ó 2005 Elsevier B.V. All rights reserved.
O2 ! O2 þ
1. Introduction Manganese oxide catalysts are of interest due to their applicability to catalytic reactions such as selective catalytic reduction of NOx with NH3 [1], CO oxidation [2] and combustion of organic compounds [3] in gaseous phase and selective oxidation of organic compounds [4] in liquid phase. Manganese oxide catalysts are also useful for the decomposition of ozone in gas streams [5–7]. Oyama and his co-workers [8,9] have reported that the mechanism for O3 decomposition on manganese oxide supported on c-Al2O3 (denoted by MnOx/Al2O3), and have reported that the reaction proceeds through two irreversible steps, the adsorption of ozone on the catalyst surface and the desorption of molecular oxygen (Eqs. (1)–(3)), where * denotes the surface site on the catalyst: ! O2 þ O
ð1Þ
O þ O3 ! O2 þ O2
ð2Þ
O3 þ
*
Corresponding author. Fax: +81 29 861 8679. E-mail address: [email protected] (H. Einaga).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.061
ð3Þ
As the intermediate species, atomic oxygen and peroxides have been observed. These reactions are also applicable to the VOCs abatement at low temperatures. The oxygen species formed in this decomposition steps are active for aromatic compounds in gas streams. Naydenov et al. [10] have reported that the use of ozone can lower the reaction temperature for benzene oxidation over un-supported manganese oxides. In the previous papers, we have reported the behavior of benzene oxidation, ozone decomposition, and products formation on the alumina-supported manganese oxide catalysts [11,12]. We herein report the X-ray absorption fine structure (XAFS) spectroscopic studies to investigate the structural changes in alumina-supported manganese oxide catalysts during ozone decomposition reactions. The presence of water, a ubiquitous constituent in effluent gas stream and formed in VOC decomposition, causes the structural changes in Mn oxide structures during the ozone decomposition. The used catalyst can be regenerated by the heat treatment in an O2 flow at 723 K.
378
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
2. Experimental
3. Results and discussion
Alumina-supported manganese oxides were prepared by the impregnation of c-Al2O3 (Catalysis Society of Japan, JRC ALO-4, SBET 170 m2 g1) with the aqueous solution containing an appropriate amount of Mn(CH3COO)2 Æ 4H2O (Wako Pure Chemical, >99.9%). Catalyst samples were dried at 383 K and then calcined at 773 K for 3 h in air. XAFS measurements were carried out on the Photon Factory beam line BL-7C at High Energy Accelerator Research Organization (KEK-PF) with the storage ring operating at an energy of 2.5 GeV. The double crystal monochromator Si(1 1 1) was used. Fig. 1 shows the schematic of the experimental setup. Reaction gases were prepared by N2 (>99.9995%, total hydrocarbon <1 ppm) and O2 (>99.9995%, total hydrocarbon <1 ppm) in cylinders by using the sets of thermal mass flow controllers. Ozone was synthesized from O2 by a silent discharge ozone generator. Ozone concentration was 1000 ± 30 ppm. The flow rate was 250–1000 ml min1. Catalyst samples were pressed into thin self-supporting wafers with 20 mm/ in diameter and set in an in situ cell with polyimide film (Du Pont-Toray Co., Ltd. Kapton 200H), which was connected to the flow-type reaction system described above. The incident and transmitted X-rays were detected by N2-filled ionization chambers. XAFS spectra were recorded at a temperature of 296 K with an equivalent step of 0.25 eV/point for XANES measurements and 1.0 eV/point for EXAFS measurements. Mn foil is used as the reference sample for energy calibration to ascertain that no energy shift was observed during the measurements. Data reduction of experimental absorption spectra was carried out according to the method recommended by the Standards and Criteria Committee of the International XAFS Society [13] using WinXAS v.3.1 [14,15].
Fig. 2 shows the Mn–K edge EXAFS Fourier transforms for 2.5 wt%–MnOx/Al2O3 before the ozone decomposition reaction. The spectra were obtained after heating in O2 flow at 723 K for 2 h and then cooled down to the ambient temperature (296 K). A peak due ˚ , while to the bond of Mn–O is observed at R = 1.5 A no obvious peaks for Mn–Mn bond are observed, indicating that Mn oxides are highly dispersed on the catalyst support. This is consistent with the previous results that Mn-oxides are highly dispersed on alumina support at low loading levels [16,17]. Thus, no size effect has to be taking account in the XANES region of the absorption spectrum [18]. Regarding the incursion of Al in
Fig. 2. Mn–K edge EXAFS Fourier transforms for the aluminasupported manganese oxides. (a) before reaction, (b) after the contact of water vapor, (c) after ozone decomposition with water vapor, and (d) after heat treatment in an O2 flow at 723 K.
Fig. 1. Experimental set-up of in situ XAFS measurements.
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
the environment of Mn, Lopez-Navarrete et al. [19] have underlined the effect of such structural process on the Fourier Transform modulus. It should be noted that the rate for ozone decomposition was not so much influenced by the Mn loadings. Fig. 3 shows the Mn–K edge XANES spectra of the MnOx/Al2O3 catalyst. The presence of water vapor without ozone feed had almost no changes in the XANES spectra and no significant changes in the EXAFS spectrum (Fig. 2), indicating the no significant changes in Mn oxide structure by the physical adsorption of water vapor. On the other hand, the absorption edge of XANES spectra were gradually shifted to higher energy with time on stream and a preedge peak newly appeared at 6540.6 eV (Fig. 3a), when ozone was fed
Fig. 3. Mn–K edge XANES spectra of MnOx/Al2O3 during ozone decomposition reaction. (a) With water vapor; —–: before reaction, - - - - -: after the contact of water vapor, color lines: in situ measurement. (b) In situ measurement without water vapor. The in situ spectra were taken every 21 min after the start of ozone feed.
379
into the cell with water vapor (water concentration: 0.55%). No such spectral changes occurred during ozone decomposition when water vapor was absent (Fig. 3b). In addition, the contact of water vapor in a N2 flow and an O2 flow after the ozone decomposition reaction did not change the Mn–K edge XANES spectra. It has been reported that linear relationship is observed between the energy shift of the Mn–K absorption edge and the oxidation state of the Mn-oxide samples [20,21]. The absorption edge of MnOx/Al2O3 sample is close to that of Mn3O4, which consists of MnII and MnIII species with the average oxidation state of 2.67. This is consistent with the results by other groups that Mn oxidation state estimated from NEXAFS (near-edge X-ray absorption fine structure) studies was 2.4 [22]. The shift of absorption edge to higher energy indicates that Mn oxidation state was changed to higher oxidation state. The appearance of a preedge band at 6540.6 eV, which is characteristic of that for MnO2, implies the formation of MnIV species. The Mn–K edge EXAFS Fourier transforms of the used sample is also shown in Fig. ˚ increased in its intensity after 2. The band at R = 1.5 A the ozone decomposition with water vapor. Comparing the spectrum with that obtained before the ozone feed (Fig. 2), we ascribed the increased band to the coordination of water to Mn sites, which increased the number of Mn–O bond. The coordination of water to Mn site was caused by the bond cleavage of Mn–O–Al bond. Although the bond cleavage of Mn–O–Al was not reflected in XAFS spectra, it is evidenced by the fact that Mn ions were extracted from the catalyst surface by washing the used catalyst with water. This finding indicates that Mn was isolated from catalyst support to become a free Mn ion to which water was coordinated. The catalyst color was also changed slightly from brown to brownish pink. No such bond cleavage was observed without ozone contact: Mn ions were not extracted from the catalyst by the wash treatment. Oyama and his co-workers [22] have reported that ozone decomposition on MnOx/Al2O3 catalyst proceeds by electron transfer from Mn site to ozone and Mn is reduced back in the desorption of oxygen species: O3 + Mnnþ ! O2 + Mnðnþ2Þþ + O2
ð4Þ
ðnþ2Þþ þ O2 O3 þ O2 þ Mnðnþ2Þþ ! O2 2 þ Mn
ð5Þ
ðnþ2Þþ O2 ! Mnnþ þ O2 2 þ Mn
ð6Þ
As described above, the presence of water vapor is essential for the structural changes in Mn sites. Although the role of water vapor is under investigation, the presence of water vapor may accelerate the oxidation of Mn and/or inhibits its reduction. Water vapor is generally contained in effluent gas streams and is formed in the oxidation of organic
380
H. Einaga et al. / Chemical Physics Letters 408 (2005) 377–380
compounds. Our previous paper shows that the feed of water vapor gives rise to the decrease in the rate for ozone decomposition with MnOx/Al2O3 catalyst [12]. Ozone adsorption on the Mn sites is inhibited by the presence of water vapor, leading to the decrease in the decomposition rate. However, the structural change of Mn site can also have influence on the rate for ozone decomposition. Heat treatment in an O2 flow leads to the regeneration of MnOx/Al2O3 catalyst. Fig. 4 shows the changes in Mn–K edge XANES spectra of the used MnOx/ Al2O3 catalyst obtained when the catalyst was heated at 423, 573, and 723 K compared with the preheated catalyst. The catalyst was heated at the desired temperature for a few minutes and then cooled down to room temperature (296 K) before XAFS spectroscopic measurements. The used catalyst heated at 423 K had almost no changes in the XANES spectra, and that at 573 K slightly shifted the Mn–K absorption edge to lower energy with the disappearance of preedge peak at 6540.6 eV. Further heat treatment at 723 K gave the same spectrum with that for fresh catalyst sample. This finding shows that the oxidation state of Mn site was reduced back to that of the fresh catalyst sample by the oxidation of catalyst at 723 K. The Mn–K edge EXAFS Fourier transforms for the regenerated sample show the decrease of the band intensity of Mn–O band to the level of fresh sample (Fig. 2), showing that the coordinated water to Mn site was desorbed from the catalyst sample. At this time, Mn ions were not extracted from the catalyst surface by washing the catalyst with water, confirming the complete regeneration of the catalyst.
Fig. 4. Mn–K edge XANES spectra of MnOx/Al2O3 after heat treatment in an O2 flow. (a) Before heat treatment, (b) at 423 K, (c) 573 K, and (d) 723 K.
4. Conclusion In this study, we reported that manganese oxides supported on alumina suffered the structural changes during ozone decomposition reaction in the presence of water vapor. Mn was oxidized to higher oxidation state, along with the coordination of water to Mn. The presence of water vapor in the ozone decomposition was essential for the structural changes. The catalyst was completely regenerated by the heat treatment in an O2 flow at 723 K, which gave the same XAFS spectra with that for fresh sample. These findings provided significant information on the catalytic properties of supported manganese oxides for the oxidation reactions using ozone.
Acknowledgements We are grateful to the approval of Photon Factory Advisory Committee (PAC) (Proposal No. 2003G071) at High Energy Accelerator Research Organization (KEK) for the XAFS measurements.
References [1] W.S. Kijlstra, D.S. Brands, E.K. Poels, A. Bliek, J. Catal. 171 (1997) 208. [2] A.B. Lamb, W.C. Bray, J.C. Frazer, J. Ind. Eng. Chem. 12 (1920). [3] L.S. Puckhaber, H. Cheung, D.L. Cocke, A. Clearfield, Solid State Ionics 32/33 (1989) 206. [4] R. Ghosh, Y.-C. Son, V.D. Makwana, S.L. Suib, J. Catal. 224 (2004) 288. [5] P. Hunter, S.T. Oyama, Control of Volatile Organic Compound Emissions: Conventional and Emerging Technologies, Wiley, New York, 2000. [6] B. Dhandapani, S.T. Oyama, Appl. Catal. B 11 (1997) 129. [7] S.T. Oyama, Catal. Rev. – Sci. Eng. 42 (2000) 179. [8] W. Li, G.V. Gibbs, S.T. Oyama, J. Am. Chem. Soc. 120 (1998) 9041. [9] W. Li, S.T. Oyama, J. Am. Chem. Soc. 120 (1998) 9047. [10] A. Naydenov, D. Mehandjiev, Appl. Catal. A 97 (1993) 17. [11] H. Einaga, S. Futamuara, React. Kinet. Catal. Lett. 81 (2004) 121. [12] H. Einaga, S. Futamuara, J. Catal. 227 (2004) 304. [13] D.C. Koningsberger, Jpn. J. Appl. Phys. 32 (Suppl. 2) (1993) 877. [14] T. Ressler, J. Synchrotron Radiat. 5 (1998) 118. [15] T. Ressler, J. Phys. IV 7 (C2) (1997) 269. [16] W.S. Kijlstra, E.K. Poels, A. Bliek, B.M. Weckhuysen, R.A. Schoonheydt, J. Phys. Chem. B 101 (1997) 309. [17] R. Radhakrishnan, S.T. Oyama, Y. Ohminami, K. Asakura, J. Phys. Chem. B 105 (2001) 9067. [18] D. Bazin, J.J. Rehr, J. Phys. Chem. B 107 (2003) 12398. [19] E. Lopez-Navarrete, A. Caballero, A.R. Gonzalez-Elipe, M. Ocan, J. Eur. Ceram. Soc. 24 (2004) 3057. [20] T. Ressler, J. Wong, J. Roots, I.L. Smith, Environ. Sci. Technol. 34 (2000) 950. [21] J.M. Ramallo-Lo´pez, E.J. Lede, F.G. Requejo, J.A. Rodriguez, J.-Y. Kim, R. Rosas-Salas, J.M. Domı´nguez, J. Phys. Chem. B 108 (2004) 20005. [22] R. Radhakrishnan, S.T. Oyama, J.G. Chen, K. Asakura, J. Phys. Chem. B 105 (2001) 4245.