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SBA-15 catalysts
Catalysis Today 204 (2013) 108–113
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Benzene oxidation with ozone over MnOx /SBA-15 catalysts Mingshi Jin a , Jung Hwan Kim b , Ji Man Kim a , Jong-Ki Jeon c , Jongsoo Jurng d , Gwi-Nam Bae d , Young-Kwon Park b,e,∗ a
Department of Chemistry, BK21 School of Chemical Materials Science and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of Korea Department of Chemical Engineering, Kongju National University, Cheonan 330-717, Republic of Korea d Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea e School of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 11 September 2012 Accepted 20 September 2012 Available online 12 December 2012 Keywords: MnOx /SBA-15 Catalytic oxidation Benzene Ozone Mn acetate Mn nitrate
a b s t r a c t Catalytic oxidation of benzene with ozone has been studied using manganese oxides with two different manganese precursors, Mn(NO3 )2 and Mn(CH3 COO)2 , supported on SBA-15 (MnOx /SBA-15). The catalysts were characterized by X-ray diffraction, N2 adsorption–desorption, Raman spectroscopy, and H2 -temperature programmed reduction. The manganese nitrate (MN) precursor primarily resulted in large particles on the silica support, while the manganese acetate (MA) precursor mainly resulted in a highly dispersed manganese oxide on the silica support. The catalytic activity was dependent upon ozone concentration, reaction times, and the amount of Mn loading. Higher benzene conversion, O3 conversion, and COx yield were observed for MnOx -MA/SBA-15 catalyst over MnOx -MN/SBA-15, due to the highly dispersed manganese oxides on the supports, and the higher oxygen mobility. The 15 wt% MnOx -MA/SBA-15 catalyst shows the highest catalytic activity of all the catalysts considered in this study. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs), whether directly or indirectly, are known to be major contributors to air pollution. Therefore, control of VOCs has been one of the most important issues within the research area of environmental catalysis [1]. There have been numerous reported methods for the removal of VOCs, including catalytic oxidation, thermal oxidation, and adsorption processes. Among these methods, catalytic oxidation is a promising method to control the emission of VOCs. On the other hand, if oxidation is performed at 80 ◦ C rather than at 200 ◦ C, the required energy cost of the oxidation process would be much lower. Therefore, it might be better if oxidation is performed at a lower temperature. To do this, the use of ozone as an alternative oxidant for the catalytic oxidation of VOCs has been reported in the literature [2,3]. Supported, and unsupported, manganese oxides such as Mn3 O4 , Mn2 O3 , and MnO2 are known to exhibit high activity for hydrocarbon and VOC catalytic combustion, producing CO2 upon complete reactant conversion [4]. In particular, they have shown higher
∗ Corresponding author at: Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of Korea. Tel.: +82 2 2210 5623; fax: +82 2 2244 2245. E-mail address: [email protected] (Y.-K. Park). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.09.026
activity for the complete oxidation of benzene and cyclohexane compared to the oxides of Fe, Ni, Co, Cu, and Ag [5]. The catalytic properties of MnOx -based catalysts are attributed to the ability of manganese to form oxides with different oxidation states, and to their high oxygen storage capacity. Their catalytic application is primarily due to their high efficiency in the reduction/oxidation reaction cycles. Redox abilities are strongly enhanced when combined with other elements [6]. In general, manganese oxide catalysts are prepared with a manganese nitrate precursor, probably because of its high solubility and the easy removal of the nitrate anion during calcination. However, the use of manganese acetate as a precursor has been found to give superior performance over manganese nitrate [7,8]. Meanwhile, Oyama and co-workers have studied ozone decomposition on supported manganese oxides in the absence of organic substrates [9–11]. Einaga et al. reported that an alumina-supported manganese oxide catalyst exhibited high reactivity for the oxidation of benzene with ozone [12,13]. In addition, SiO2 -, TiO2 -, and ZSM-5-supported manganese oxide materials were used for the catalytic oxidation of benzene with ozone, with it noted that the surface area of the catalysts is one of the most important factors for the reaction [5]. Recently, mesoporous materials have been considered as promising catalyst supports with well-defined pore size, large surface area, and higher thermal stability. In addition, the application
M. Jin et al. / Catalysis Today 204 (2013) 108–113
of mesoporous materials as catalysts and catalyst supports has been reviewed [14,15]. In these reviews, a range of catalytic processes using mesoporous materials were described. Therefore, the characteristics of mesoporous materials might induce high catalytic activity for the oxidation of benzene with ozone. Siliceous SBA15 is considered a representative mesoporous material [14–17]. To the best of our knowledge, SBA-15 was used for the first time in the catalytic conversion of benzene with ozone. In the present study, benzene was used as a model species for VOCs. Also, manganese oxides supported on highly ordered mesoporous SBA-15 (MnOx /SBA-15) were prepared using two kinds of manganese precursor, namely manganese nitrate and manganese acetate, in order to investigate the effect of manganese precursors on the catalytic oxidation of benzene with ozone for the first time.
2. Experimental 2.1. Preparation of SBA-15 Mesoporous silica SBA-15 was obtained following the procedures described elsewhere [18]. Typically, a triblock polymer, Pluronic P123 (EO20 PO70 EO20 , Mav = 5800, Aldrich), was used as the structure-directing agent and tetraethylorthosilicate (TEOS, SAMCHUN) was used as the silica source for the SBA-15 material. Typically, 30.0 g of P123 was dissolved in a mixture of 721 g of double-distilled water and 182 g of HCl (35%, SAMCHUN). Subsequently, 64 g of TEOS was added to the polymer solution under vigorous stirring at 40 ◦ C for 24 h, and heated at 100 ◦ C for 24 h. The product was filtered, washed with double-distilled water, and dried in an oven at 80 ◦ C. The white powder thus obtained was washed with EtOH, dried at 80 ◦ C for 12 h, and finally calcined at 550 ◦ C in static air for 3 h in order to remove the template.
2.2. Synthesis of MnOx /SBA-15 catalysts MnOx /SBA-15 was synthesized by incipient wetness impregnation. Mn(NO3 )2 (Aldrich, 98%) and Mn(CH3 COO)2 (Aldrich, 99% +) were used as the Mn precursors. The catalysts were dried at 110 ◦ C overnight, and then calcined at 550 ◦ C. The impregnated Mn loadings were 5 wt% and 15 wt%. For the synthesis of 5 wt% MA/SBA15 and 5 wt% MN/SBA-15 catalysts, 2.87 mmol of Mn(NO3 )2 ·4H2 O or 2.87 mmol Mn(CH3 COO)2 ·4H2 O, respectively, was dissolved in 3 cm3 of distilled water. The obtained solution was added slowly to 3 g of SBA-15. The Mn loading was determined by ICP. From here on, the SBA-15 catalysts prepared from manganese nitrate and manganese acetate are denoted as MN/SBA-15 and MA/SBA15, respectively. In addition, 15 wt% Mn was impregnated on SiO2 which was purchased from Grace Davison (XPO-2412) using Mn acetate.
2.3. Benzene oxidation Catalytic reactions were carried out with a fixed-bed flow reactor. Ozone was synthesized from O2 by a silent discharge ozone generator. Prior to the catalytic reaction, the sample, under O2 flow, was heated at 450 ◦ C in a Pyrex glass reactor. Then, the catalyst was cooled with its temperature maintained at 80 ◦ C. 0.05 g of catalyst was used. 60 ml min−1 of 200 ppm benzene in N2 was mixed with the 60 ml min−1 of O2 flow. Analysis of the gas sample was performed by gas chromatography for benzene conversion, an indoor gas analyzer for CO and CO2 products, and an ozone analyzer for ozone conversion. In this system, the homogeneous gaseous reaction of benzene with ozone can be neglected.
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2.4. Characterization X-ray powder diffraction (XRD) patterns were collected with a Cu K␣ X-ray source using a Rigaku D/MAX-II instrument. N2 adsorption–desorption isotherms were obtained using a Micromeritics ASAP 2000 at −196 ◦ C (liquid N2 ). Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to estimate the BET surface area and pore size distributions, respectively. Transmission electron microscopy (TEM, JEOL JEM 3010) was performed at an accelerating voltage of 200 kV. Raman spectra were recorded under ambient conditions at room temperature with an Ar ion laser (Renishaw Inc., U.K.). H2 temperature-programmed reduction (H2 -TPR) was performed in a quartz microreactor. 0.06 g samples were first pretreated under an airflow of 30 ml min−1 at 200 ◦ C for 1 h, followed by purging with a He flow of 30 ml min−1 at the same temperature for 30 min before they were cooled to room temperature. Subsequently, 10 vol% H2 in a He flow of 40 ml min−1 was applied, at room temperature, and maintained for 30 min. Finally, the sample was ramped to 700 ◦ C at 10 ◦ C min−1 .
3. Results and discussion Fig. 1 shows the XRD patterns of MnOx /SBA-15 catalysts with two manganese precursors and Mn loadings. As shown in the lowangle XRD patterns, all of the samples exhibit an intense peak and two weaker peaks, corresponding to peaks at 1 0 0, 1 1 0, and 2 0 0, that are characteristic of a 2-dimensional hexagonal mesostructure (plane group, P6 mm) [19]. The high-angle XRD patterns of 5% MA/SBA-15 and 15% MA/SBA-15 show no diffraction intensity, except for the peak that corresponds to amorphous silica, thereby implying that manganese oxides that use manganese acetate are highly dispersed on the support materials. Conversely, manganese oxides using manganese nitrate precursor gave crystalline diffraction peaks for both MnO2 and Mn2 O3 , and as the Mn load increased to 15%, the peak became sharper with increased intensity, thus indicating formation of large-sized particles. This is consistent with the TEM observations (Fig. 2.), which does not show any large manganese oxide clusters for MA/SBA-15 and highly dispersed manganese oxide clusters located in the mesoporous channels of SBA-15. Some large manganese oxides clusters, however, were observed on the external surface of MN/SBA-15. N2 adsorption–desorption isotherms of the SBA-15, 5% MA/SBA15, 15% MA/SBA-15, 5% MN/SBA-15, and 15% MN/SBA-15 are shown in Fig. 3(a). All of these materials exhibit a Type IV isotherm, which, according to the IUPAC nomenclature, is characteristic of a mesoporous material [20]. Furthermore, these catalysts possess uniform mesopores, as can be seen in the corresponding pore-size distribution curves in Fig. 3(b). Textural parameters of all the catalysts are summarized in Table 1. After introducing the manganese precursor, an obvious decrease in the BET surface area, pore volume, and pore size, as well as an increase in the wall thickness were observed. This may be due to partial pore blockage by the introduction of manganese oxide. In particular, the surface area of the MA/SBA-15 catalysts was significantly lower than MN/SBA-15 catalysts. As illustrated by XRD and TEM, MnOx from manganese acetate precursor might form a highly dispersed small particle. Therefore, the particles can be located mainly in the mesopores and are well distributed over the internal surface. This might result in a significant decrease in the surface area of MA/SBA-15. MnOx from the manganese nitrate precursor can form large particles, which are difficult to enter the mesopores and become distributed mainly on the external surface. Therefore, pore blockage by the introduction of MnOx can be prevented somewhat. Furthermore, these materials exhibit a unit cell parameter (a0 ) within the range of 10.6–10.9 nm.
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M. Jin et al. / Catalysis Today 204 (2013) 108–113
Fig. 1. XRD pattern of MA/SBA-15 and MN/SBA-15 catalysts.
Fig. 2. HR-TEM images of (a) 15% MA/SBA-15, (b) 15% MN/SBA-15.
Fig. 3. N2 -isothem and pore size distribution of MA/SBA-15 and MN/SBA-15 catalysts.
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Table 1 Physicochemical properties of MA/SBA-15 and MN/SBA-15 catalysts. Samples
SBET (m2 /g)a
Vtotal (cm3 /g)b
DBJH (nm)c
Unit cell (nm)d
Wall thickness (nm)e
SBA-15 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
597 463 371 574 532
0.84 0.81 0.62 0.79 0.74
7.4 7.4 6.3 7.4 6.9
10.8 10.9 10.6 10.9 10.9
3.4 3.5 4.3 3.5 4.0
a b c d e
BET surface areas calculated in the range of p/p0 = 0.05–0.20. Total pore volumes measured at p/p0 = 0.99. BJH pore sizes obtained from the adsorption branches. √ The unit cell parameter a0 = 2d100/ 3. Wall thickness was obtained from wall thickness = unit cell parameter − pore size.
For SBA-15 and SBA-15 impregnated with manganese oxide, there are few changes observed for the lattice parameters of the large unit cell (d100 ). Table S1 lists the surface area and pore volume of SiO2 and MA/SiO2 (supporting information). Fig. 4 shows Raman spectra of the catalysts considered in this work. The 5% MN/SBA-15 and 15% MN/SBA-15 catalysts showed two bands at 528 cm−1 and 649 cm−1 ; and 308 cm−1 and 644 cm−1 , respectively. The MnO2 and Mn2 O3 gave the most intense Raman peak at 654 cm−1 [7]. However, the peak for the MN/SBA-15 catalyst shows a weak Raman shift, due to the large particle size of Mn2 O3 [21]. The MA/SBA-15 catalyst gave only a broad, downshifted peak at approximately 638 cm−1 , assignable to the highly dispersed smaller manganese oxide particles on the support [22]. Analogous results, reported previously, have been attributed to the effect of phonon confinement [23,24]. H2 -TPR experiments were then carried out (Fig. 5). From the acquired data, the MA/SBA-15 showed higher reduction ability, indicating that the lattice oxygen mobility was higher than in MN/SBA-15. The broad peak, and easy reduction peak, may be due to the small, more highly dispersed, Mn particles on the support [22]. The MN/SBA-15 catalysts exhibited one main reduction peak and a small shoulder peak because of the formation of different particle sizes. The reduction peak that appeared in the lower temperature region could be attributed to the reduction of small particles, while the reduction in the high-temperature region could be attributed to the large-sized particles on the support [7,22]. Therefore, we expected that the catalytic activity of MA/SBA-15 would be high due to its high dispersion and high oxygen mobility. Fig. 6 shows typical time progressions for the oxidation of benzene with ozone over manganese oxides, supported on SBA-15.
Remarkably, higher benzene conversion and O3 conversion were observed over 5% MA/SBA-15 and 15% MA/SBA-15 catalysts than were observed for 5% MN/SBA-15 and 15% MN/SBA-15. It is noteworthy that, with the exception of the 5% MN/SBA-15 catalyst, a close to constant rate of benzene conversion by the catalysts was maintained, even after 150 min of reaction. Although the degree of aggregated Mn oxides increased with Mn content, the catalytic activity of 15% MN/SBA-15 was higher than that of 5% MN/SBA-15. These dependencies revealed that not only were the highly dispersed Mn oxides, but also the aggregated Mn oxides were the active sites for benzene oxidation with ozone. The COx (CO + CO2 ) yields from the 5% MA/SBA-15 and 15% MA/SBA-15 catalysts are higher than those of 5% MN/SBA-15 and 15% MN/SBA-15 (Fig. 7). The yields from the 5% MA/SBA-15 and 15% MA/SBA-15 catalysts were 80 and 94%, respectively. On the other hand, the 5% MN/SBA-15 and 15% MN/SBA-15 catalysts gave less than 70%. From the results shown Figs. 5 and 6, it can be concluded that the 15% MA/SBA-15 catalyst shows the best catalytic activity for the reaction. This may be ascribed to the fact that an acetate precursor mainly results in a highly dispersed manganese oxide surface phase, homogeneously distributed throughout the SBA-15, even though there is a higher Mn content. The catalytic activity of MA/SBA-15 was also compared with that of MA/SiO2 to address the efficiency of the mesoporous structure (Fig. 7). The benzene conversion of 15% MA/SBA-15 (98.2%) was slightly higher than that of 15% MA/SiO2 (95.6%). However, the COx yields, the most important parameter for benzene oxidation with ozone, over 15% MA/SBA-15 and 15% MA/SiO2 were 93.9% and 84.1%, respectively. Because adsorption of benzene or conversion
Fig. 4. Raman spectra of MA/SBA-15 and MN/SBA-15 catalysts.
Fig. 5. TPR profiles of MA/SBA-15 and MN/SBA-15 catalysts.
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b
100 80 60 40 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
20 0
0
20
40
60
80
100
120
140
100
O3 Conversion(%)
Benzene Conversion(%)
a
80 60 40 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
20 0
160
0
20
40
60
Time(min)
80
100
120
140
160
Time(min)
Fig. 6. (a) Benzene conversion, (b) O3 conversion of MA/SBA-15 and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.
100
80
80
60
60
40
40
20
20
0 5%
/S MA
5 5 O) O) O2 -15 A-1 A-1 /Si (H2 (H2 /SB /SB MA -15 -15 MN MN BA BA 5% S S 1 % / % % / 5 15 15 MA MN 15% 15%
BA
-15
/S MA
Yield (Carbon wt%)
Conversion(%)
Benzene conversion (%) COx yield (%)
100
0
BA
Fig. 7. Benzene conversion and COx yield of MA/SBA-15 and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.
CO CO 2
100
Selectivity in COx (%)
into intermediates may be included in benzene conversion, it can be regarded that the catalytic activity of 15% MA/SBA-15 which has higher COx yield may be higher than that of 15% MA/SiO2 . As shown in Fig. S1 (Supporting Information), Mn peaks were observed in the XRD pattern of 15% MA/SiO2 . In addition, TEM showed that some large manganese oxides were observed on MA/SiO2 (Fig. S2). This suggests that larger MnOx is formed on the SiO2 . From these results, SBA-15 appears to have the potential to be a better support for Mn oxide for benzene oxidation with ozone than SiO2 . Fig. 8 shows the distributions of CO and CO2 in the total COx over 15% Mn/SBA-15. For the MA/SBA-15, a high selectivity to CO2 was observed and no gaseous byproducts were detected. In addition, as shown in Fig. 7, the carbon balance of 15% MA/SBA-15 reached 96%. The CO2 selectivity was lower on the 15% MN/SBA15 catalyst than on the 15% MA/SBA-15 catalyst, and the carbon balance of 15% MN/SBA-15 was much lower than 100%. The poor carbon balance of 15% MN/SBA-15 might be due to the buildup of intermediates on the catalyst surface because no gaseous byproducts were detected when the 15% MN/SBA-15 catalyst was used. To identify the intermediates on the spent catalyst surface, the temperature programmed desorption (TPD) of spent 15% MN/SBA-15 was performed under a He atmosphere. The products desorbed were analyzed directly by gas chromatography/mass spectrometry. For 15% MN/SBA-15, high amounts of intermediates, such as formic acid, acetic acid, acetaldehyde, 2(5H)-furanone, etc. were detected (data not shown). Among them, formic acid was
80
60
40
20
0
) ) -15 -15 2O 2O BA BA 5(H 5(H A/S N/S A-1 A-1 M M B B /S /S 15% 15% MA MN 15% 15%
Fig. 8. Selectivity of CO and CO2 in COx .
detected as a major by-product. On the other hand, very small amount of formic acid was detected when the 15% MA/SBA-15 catalyst was used, which might explain the high carbon balance of 15% MA/SBA-15.
Benzene Conversion (%) COx Yield (Carbon wt%)
100
100
80
80
60
60
40
40
20
20
0
COx Yield (Carbon wt%)
Benzene Conversion (%)
M. Jin et al. / Catalysis Today 204 (2013) 108–113
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to be higher with the MA/SBA-15 catalyst, compared to MN/SBA-15 catalysts. Among the catalysts considered herein, 15% MA/SBA15 showed the best catalytic activity, due to highly dispersed manganese oxides on the SBA-15 supports, and the high Mn content. Acknowledgements This research was supported by the Converging Research Center Program, funded by the Ministry of Education, Science and Technology (no. 2012K001372). Young-Kwon Park acknowledges Prof. Ryong Ryoo’s valuable discussion and comments.
0
0
200
400
600
800
1000
O3 Concentration (ppm) Fig. 9. Effect of ozone concentation on benzene conversion over 15% MA/SBA-15 catalyst.
In addition, the effect of water vapor (0.7 vol%) on benzene oxidation with ozone was investigated (Figs. 7 and 8). As water was added, the CO2 selectivity and carbon balance increased for the 15% MN/SBA-15 catalyst. When the TPD of MN/SBA-15 with water vapor was carried out, the amount of intermediates of the by-products decreased significantly (data not shown). This is consistent with the result reported by Einaga and Ogata [25], who performed benzene oxidation with ozone on Mn/SiO2 . They suggested that water vapor added might promote the decomposition of intermediate compounds on the catalyst surface. Therefore, for 15% MN/SBA-15, water vapor might facilitate the oxidation of intermediate compounds. For MA/SBA-15, the selectivity to CO2 and carbon balance was similar irrespective of the addition of water. Furthermore the amount of byproducts on spent MA/SBA-15 with water vapor was similar to that on spent MA/SBA-15 (data not shown). The effect of ozone concentration on benzene conversion and COx yield by the 15% MA/SBA-15 catalyst is illustrated in Fig. 9. Benzene conversion and COx yield increased with an increase in ozone concentration. When the concentration of ozone reached 1000 ppm, the highest conversion of benzene and COx yield were exhibited. This indicates that the behavior of benzene oxidation and COx yield strongly depend on the concentration of ozone. 4. Conclusions For the purpose of catalytic benzene oxidation, manganese oxide, using a manganese acetate precursor, was highly dispersed on a SBA-15 support, whereas manganese oxide with manganese nitrate precursor formed large particles on SBA-15. MA/SBA-15 catalysts showed higher catalytic activity and stability than those of MN/SBA-15 catalysts, due to the highly dispersed manganese oxide and high oxygen mobility. The COx yield was also observed
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2012.09.026. References [1] S.W. Baek, J.R. Kim, S.K. Ihm, Catalysis Today 93–95 (2004) 575. [2] A. Gervasini, G.C. vezzoli, V. Ragaini, Catalysis Today 29 (1996) 449. [3] J.H. Kim, J.S. Jurng, G.N. Bae, J.K. Jeon, K.Y. Jung, J.H. Yim, Y.K. Park, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects; http://dx.doi.org/10.1080/15567036.2010.547923 [4] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, Applied Catalysis B: Environmental 22 (1999) 249. [5] H. Einaga, S. Futamura, Journal of Catalysis 227 (2004) 304. [6] R. Lin, W.P. Liu, Y.-J. Zhong, M.-F. Luo, Applied Catalysis A: General 220 (2001) 165. [7] F. Kapteijn, A. Dick van Langeveld, J.A. Moulijn, A. Andreini, M.A. Vuurman, A.M. Turek, J.-M. Jehng, I.E. Wachs, Journal of Catalysis 150 (1994) 94. [8] J.H. Ko, S.H. Park, J.K. Jeon, S.S. Kim, S.C. Kim, J.M. Kim, D. Chang, Y.K. Park, Catalysis Today 185 (2012) 290. [9] B. Dhandapani, S.T. Oyama, Applied Catalysis B: Environmental 11 (1997) 129. [10] S.T. Oyama, Catalysis Reviews: Science and Engineering 42 (2000) 279. [11] W. Li, G.V. Gibbs, S.T. Oyama, Journal of the American Chemical Society 120 (1998) 9041. [12] H. Einaga, M. Harada, A. Ogata, Catalysis Letters 129 (2009) 422. [13] H. Einaga, S. Futamura, Applied Catalysis B: Environmental 60 (2005) 49. ˇ [14] R.M. Martín-Aranda, J. Cejka, Topics in Catalysis 53 (2010) 141. [15] A. Taguchi, F. Schüth, Microporous Mesoporous Materials 77 (2005) 1. [16] H.W. Lee, H.J. Cho, J.H. Yim, J.M. Kim, J.K. Jeon, J.M. Sohn, K.S. Yoo, S.S. Kim, Y.K. Park, Journal of Industrial and Engineering Chemistry 17 (2011) 504. [17] J.H. Yim, D.I. Kim, J.A. Bae, Y.K. Park, J.H. Park, J.K. Jeon, S.H. Park, J. Song, S.S. Kim, Journal for Nanoscience and Nanotechnology 11 (2011) 1714. [18] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.E. Chmelka, G.D. Stucky, Journal of the American Chemical Society 120 (1998) 6024. [19] J. Bae, K.C. Song, J.K. Jeon, Y.S. Ko, Y.K. Park, J.H. Yim, Microporous Mesoporous Materials 123 (2009) 289. [20] H.J. Shin, R. Ryoo, Z. Liu, O. Terasaki, Journal of the American Chemical Society 123 (2001) 1246. [21] J. Zi, C. Falter, W. Ludwig, K. Zhang, X. Xie, Applied Physics Letters 69 (1992) 200. [22] Y.-F. Han, F.X. Chen, Z.Y. Zhong, K. Ramesh, L. Chen, E. Widjaja, Journal of Physical Chemistry B 110 (2006) 24450. [23] J. Zuo, C. Xu, Y. Liu, Y. Qian, Nanostructured Materials 10 (1998) 1331. [24] M. Ludvigsson, J. Lindgren, J. Tegenfeldt, Journal of Materials Chemistry 11 (2001) 1269. [25] H. Einaga, A. Ogata, Journal of Hazardous Materials 164 (2009) 1236.
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Benzene oxidation with ozone over MnOx /SBA-15 catalysts Mingshi Jin a , Jung Hwan Kim b , Ji Man Kim a , Jong-Ki Jeon c , Jongsoo Jurng d , Gwi-Nam Bae d , Young-Kwon Park b,e,∗ a
Department of Chemistry, BK21 School of Chemical Materials Science and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of Korea Department of Chemical Engineering, Kongju National University, Cheonan 330-717, Republic of Korea d Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea e School of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 11 September 2012 Accepted 20 September 2012 Available online 12 December 2012 Keywords: MnOx /SBA-15 Catalytic oxidation Benzene Ozone Mn acetate Mn nitrate
a b s t r a c t Catalytic oxidation of benzene with ozone has been studied using manganese oxides with two different manganese precursors, Mn(NO3 )2 and Mn(CH3 COO)2 , supported on SBA-15 (MnOx /SBA-15). The catalysts were characterized by X-ray diffraction, N2 adsorption–desorption, Raman spectroscopy, and H2 -temperature programmed reduction. The manganese nitrate (MN) precursor primarily resulted in large particles on the silica support, while the manganese acetate (MA) precursor mainly resulted in a highly dispersed manganese oxide on the silica support. The catalytic activity was dependent upon ozone concentration, reaction times, and the amount of Mn loading. Higher benzene conversion, O3 conversion, and COx yield were observed for MnOx -MA/SBA-15 catalyst over MnOx -MN/SBA-15, due to the highly dispersed manganese oxides on the supports, and the higher oxygen mobility. The 15 wt% MnOx -MA/SBA-15 catalyst shows the highest catalytic activity of all the catalysts considered in this study. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs), whether directly or indirectly, are known to be major contributors to air pollution. Therefore, control of VOCs has been one of the most important issues within the research area of environmental catalysis [1]. There have been numerous reported methods for the removal of VOCs, including catalytic oxidation, thermal oxidation, and adsorption processes. Among these methods, catalytic oxidation is a promising method to control the emission of VOCs. On the other hand, if oxidation is performed at 80 ◦ C rather than at 200 ◦ C, the required energy cost of the oxidation process would be much lower. Therefore, it might be better if oxidation is performed at a lower temperature. To do this, the use of ozone as an alternative oxidant for the catalytic oxidation of VOCs has been reported in the literature [2,3]. Supported, and unsupported, manganese oxides such as Mn3 O4 , Mn2 O3 , and MnO2 are known to exhibit high activity for hydrocarbon and VOC catalytic combustion, producing CO2 upon complete reactant conversion [4]. In particular, they have shown higher
∗ Corresponding author at: Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of Korea. Tel.: +82 2 2210 5623; fax: +82 2 2244 2245. E-mail address: [email protected] (Y.-K. Park). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.09.026
activity for the complete oxidation of benzene and cyclohexane compared to the oxides of Fe, Ni, Co, Cu, and Ag [5]. The catalytic properties of MnOx -based catalysts are attributed to the ability of manganese to form oxides with different oxidation states, and to their high oxygen storage capacity. Their catalytic application is primarily due to their high efficiency in the reduction/oxidation reaction cycles. Redox abilities are strongly enhanced when combined with other elements [6]. In general, manganese oxide catalysts are prepared with a manganese nitrate precursor, probably because of its high solubility and the easy removal of the nitrate anion during calcination. However, the use of manganese acetate as a precursor has been found to give superior performance over manganese nitrate [7,8]. Meanwhile, Oyama and co-workers have studied ozone decomposition on supported manganese oxides in the absence of organic substrates [9–11]. Einaga et al. reported that an alumina-supported manganese oxide catalyst exhibited high reactivity for the oxidation of benzene with ozone [12,13]. In addition, SiO2 -, TiO2 -, and ZSM-5-supported manganese oxide materials were used for the catalytic oxidation of benzene with ozone, with it noted that the surface area of the catalysts is one of the most important factors for the reaction [5]. Recently, mesoporous materials have been considered as promising catalyst supports with well-defined pore size, large surface area, and higher thermal stability. In addition, the application
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of mesoporous materials as catalysts and catalyst supports has been reviewed [14,15]. In these reviews, a range of catalytic processes using mesoporous materials were described. Therefore, the characteristics of mesoporous materials might induce high catalytic activity for the oxidation of benzene with ozone. Siliceous SBA15 is considered a representative mesoporous material [14–17]. To the best of our knowledge, SBA-15 was used for the first time in the catalytic conversion of benzene with ozone. In the present study, benzene was used as a model species for VOCs. Also, manganese oxides supported on highly ordered mesoporous SBA-15 (MnOx /SBA-15) were prepared using two kinds of manganese precursor, namely manganese nitrate and manganese acetate, in order to investigate the effect of manganese precursors on the catalytic oxidation of benzene with ozone for the first time.
2. Experimental 2.1. Preparation of SBA-15 Mesoporous silica SBA-15 was obtained following the procedures described elsewhere [18]. Typically, a triblock polymer, Pluronic P123 (EO20 PO70 EO20 , Mav = 5800, Aldrich), was used as the structure-directing agent and tetraethylorthosilicate (TEOS, SAMCHUN) was used as the silica source for the SBA-15 material. Typically, 30.0 g of P123 was dissolved in a mixture of 721 g of double-distilled water and 182 g of HCl (35%, SAMCHUN). Subsequently, 64 g of TEOS was added to the polymer solution under vigorous stirring at 40 ◦ C for 24 h, and heated at 100 ◦ C for 24 h. The product was filtered, washed with double-distilled water, and dried in an oven at 80 ◦ C. The white powder thus obtained was washed with EtOH, dried at 80 ◦ C for 12 h, and finally calcined at 550 ◦ C in static air for 3 h in order to remove the template.
2.2. Synthesis of MnOx /SBA-15 catalysts MnOx /SBA-15 was synthesized by incipient wetness impregnation. Mn(NO3 )2 (Aldrich, 98%) and Mn(CH3 COO)2 (Aldrich, 99% +) were used as the Mn precursors. The catalysts were dried at 110 ◦ C overnight, and then calcined at 550 ◦ C. The impregnated Mn loadings were 5 wt% and 15 wt%. For the synthesis of 5 wt% MA/SBA15 and 5 wt% MN/SBA-15 catalysts, 2.87 mmol of Mn(NO3 )2 ·4H2 O or 2.87 mmol Mn(CH3 COO)2 ·4H2 O, respectively, was dissolved in 3 cm3 of distilled water. The obtained solution was added slowly to 3 g of SBA-15. The Mn loading was determined by ICP. From here on, the SBA-15 catalysts prepared from manganese nitrate and manganese acetate are denoted as MN/SBA-15 and MA/SBA15, respectively. In addition, 15 wt% Mn was impregnated on SiO2 which was purchased from Grace Davison (XPO-2412) using Mn acetate.
2.3. Benzene oxidation Catalytic reactions were carried out with a fixed-bed flow reactor. Ozone was synthesized from O2 by a silent discharge ozone generator. Prior to the catalytic reaction, the sample, under O2 flow, was heated at 450 ◦ C in a Pyrex glass reactor. Then, the catalyst was cooled with its temperature maintained at 80 ◦ C. 0.05 g of catalyst was used. 60 ml min−1 of 200 ppm benzene in N2 was mixed with the 60 ml min−1 of O2 flow. Analysis of the gas sample was performed by gas chromatography for benzene conversion, an indoor gas analyzer for CO and CO2 products, and an ozone analyzer for ozone conversion. In this system, the homogeneous gaseous reaction of benzene with ozone can be neglected.
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2.4. Characterization X-ray powder diffraction (XRD) patterns were collected with a Cu K␣ X-ray source using a Rigaku D/MAX-II instrument. N2 adsorption–desorption isotherms were obtained using a Micromeritics ASAP 2000 at −196 ◦ C (liquid N2 ). Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to estimate the BET surface area and pore size distributions, respectively. Transmission electron microscopy (TEM, JEOL JEM 3010) was performed at an accelerating voltage of 200 kV. Raman spectra were recorded under ambient conditions at room temperature with an Ar ion laser (Renishaw Inc., U.K.). H2 temperature-programmed reduction (H2 -TPR) was performed in a quartz microreactor. 0.06 g samples were first pretreated under an airflow of 30 ml min−1 at 200 ◦ C for 1 h, followed by purging with a He flow of 30 ml min−1 at the same temperature for 30 min before they were cooled to room temperature. Subsequently, 10 vol% H2 in a He flow of 40 ml min−1 was applied, at room temperature, and maintained for 30 min. Finally, the sample was ramped to 700 ◦ C at 10 ◦ C min−1 .
3. Results and discussion Fig. 1 shows the XRD patterns of MnOx /SBA-15 catalysts with two manganese precursors and Mn loadings. As shown in the lowangle XRD patterns, all of the samples exhibit an intense peak and two weaker peaks, corresponding to peaks at 1 0 0, 1 1 0, and 2 0 0, that are characteristic of a 2-dimensional hexagonal mesostructure (plane group, P6 mm) [19]. The high-angle XRD patterns of 5% MA/SBA-15 and 15% MA/SBA-15 show no diffraction intensity, except for the peak that corresponds to amorphous silica, thereby implying that manganese oxides that use manganese acetate are highly dispersed on the support materials. Conversely, manganese oxides using manganese nitrate precursor gave crystalline diffraction peaks for both MnO2 and Mn2 O3 , and as the Mn load increased to 15%, the peak became sharper with increased intensity, thus indicating formation of large-sized particles. This is consistent with the TEM observations (Fig. 2.), which does not show any large manganese oxide clusters for MA/SBA-15 and highly dispersed manganese oxide clusters located in the mesoporous channels of SBA-15. Some large manganese oxides clusters, however, were observed on the external surface of MN/SBA-15. N2 adsorption–desorption isotherms of the SBA-15, 5% MA/SBA15, 15% MA/SBA-15, 5% MN/SBA-15, and 15% MN/SBA-15 are shown in Fig. 3(a). All of these materials exhibit a Type IV isotherm, which, according to the IUPAC nomenclature, is characteristic of a mesoporous material [20]. Furthermore, these catalysts possess uniform mesopores, as can be seen in the corresponding pore-size distribution curves in Fig. 3(b). Textural parameters of all the catalysts are summarized in Table 1. After introducing the manganese precursor, an obvious decrease in the BET surface area, pore volume, and pore size, as well as an increase in the wall thickness were observed. This may be due to partial pore blockage by the introduction of manganese oxide. In particular, the surface area of the MA/SBA-15 catalysts was significantly lower than MN/SBA-15 catalysts. As illustrated by XRD and TEM, MnOx from manganese acetate precursor might form a highly dispersed small particle. Therefore, the particles can be located mainly in the mesopores and are well distributed over the internal surface. This might result in a significant decrease in the surface area of MA/SBA-15. MnOx from the manganese nitrate precursor can form large particles, which are difficult to enter the mesopores and become distributed mainly on the external surface. Therefore, pore blockage by the introduction of MnOx can be prevented somewhat. Furthermore, these materials exhibit a unit cell parameter (a0 ) within the range of 10.6–10.9 nm.
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Fig. 1. XRD pattern of MA/SBA-15 and MN/SBA-15 catalysts.
Fig. 2. HR-TEM images of (a) 15% MA/SBA-15, (b) 15% MN/SBA-15.
Fig. 3. N2 -isothem and pore size distribution of MA/SBA-15 and MN/SBA-15 catalysts.
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Table 1 Physicochemical properties of MA/SBA-15 and MN/SBA-15 catalysts. Samples
SBET (m2 /g)a
Vtotal (cm3 /g)b
DBJH (nm)c
Unit cell (nm)d
Wall thickness (nm)e
SBA-15 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
597 463 371 574 532
0.84 0.81 0.62 0.79 0.74
7.4 7.4 6.3 7.4 6.9
10.8 10.9 10.6 10.9 10.9
3.4 3.5 4.3 3.5 4.0
a b c d e
BET surface areas calculated in the range of p/p0 = 0.05–0.20. Total pore volumes measured at p/p0 = 0.99. BJH pore sizes obtained from the adsorption branches. √ The unit cell parameter a0 = 2d100/ 3. Wall thickness was obtained from wall thickness = unit cell parameter − pore size.
For SBA-15 and SBA-15 impregnated with manganese oxide, there are few changes observed for the lattice parameters of the large unit cell (d100 ). Table S1 lists the surface area and pore volume of SiO2 and MA/SiO2 (supporting information). Fig. 4 shows Raman spectra of the catalysts considered in this work. The 5% MN/SBA-15 and 15% MN/SBA-15 catalysts showed two bands at 528 cm−1 and 649 cm−1 ; and 308 cm−1 and 644 cm−1 , respectively. The MnO2 and Mn2 O3 gave the most intense Raman peak at 654 cm−1 [7]. However, the peak for the MN/SBA-15 catalyst shows a weak Raman shift, due to the large particle size of Mn2 O3 [21]. The MA/SBA-15 catalyst gave only a broad, downshifted peak at approximately 638 cm−1 , assignable to the highly dispersed smaller manganese oxide particles on the support [22]. Analogous results, reported previously, have been attributed to the effect of phonon confinement [23,24]. H2 -TPR experiments were then carried out (Fig. 5). From the acquired data, the MA/SBA-15 showed higher reduction ability, indicating that the lattice oxygen mobility was higher than in MN/SBA-15. The broad peak, and easy reduction peak, may be due to the small, more highly dispersed, Mn particles on the support [22]. The MN/SBA-15 catalysts exhibited one main reduction peak and a small shoulder peak because of the formation of different particle sizes. The reduction peak that appeared in the lower temperature region could be attributed to the reduction of small particles, while the reduction in the high-temperature region could be attributed to the large-sized particles on the support [7,22]. Therefore, we expected that the catalytic activity of MA/SBA-15 would be high due to its high dispersion and high oxygen mobility. Fig. 6 shows typical time progressions for the oxidation of benzene with ozone over manganese oxides, supported on SBA-15.
Remarkably, higher benzene conversion and O3 conversion were observed over 5% MA/SBA-15 and 15% MA/SBA-15 catalysts than were observed for 5% MN/SBA-15 and 15% MN/SBA-15. It is noteworthy that, with the exception of the 5% MN/SBA-15 catalyst, a close to constant rate of benzene conversion by the catalysts was maintained, even after 150 min of reaction. Although the degree of aggregated Mn oxides increased with Mn content, the catalytic activity of 15% MN/SBA-15 was higher than that of 5% MN/SBA-15. These dependencies revealed that not only were the highly dispersed Mn oxides, but also the aggregated Mn oxides were the active sites for benzene oxidation with ozone. The COx (CO + CO2 ) yields from the 5% MA/SBA-15 and 15% MA/SBA-15 catalysts are higher than those of 5% MN/SBA-15 and 15% MN/SBA-15 (Fig. 7). The yields from the 5% MA/SBA-15 and 15% MA/SBA-15 catalysts were 80 and 94%, respectively. On the other hand, the 5% MN/SBA-15 and 15% MN/SBA-15 catalysts gave less than 70%. From the results shown Figs. 5 and 6, it can be concluded that the 15% MA/SBA-15 catalyst shows the best catalytic activity for the reaction. This may be ascribed to the fact that an acetate precursor mainly results in a highly dispersed manganese oxide surface phase, homogeneously distributed throughout the SBA-15, even though there is a higher Mn content. The catalytic activity of MA/SBA-15 was also compared with that of MA/SiO2 to address the efficiency of the mesoporous structure (Fig. 7). The benzene conversion of 15% MA/SBA-15 (98.2%) was slightly higher than that of 15% MA/SiO2 (95.6%). However, the COx yields, the most important parameter for benzene oxidation with ozone, over 15% MA/SBA-15 and 15% MA/SiO2 were 93.9% and 84.1%, respectively. Because adsorption of benzene or conversion
Fig. 4. Raman spectra of MA/SBA-15 and MN/SBA-15 catalysts.
Fig. 5. TPR profiles of MA/SBA-15 and MN/SBA-15 catalysts.
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b
100 80 60 40 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
20 0
0
20
40
60
80
100
120
140
100
O3 Conversion(%)
Benzene Conversion(%)
a
80 60 40 5% MA/SBA-15 15% MA/SBA-15 5% MN/SBA-15 15% MN/SBA-15
20 0
160
0
20
40
60
Time(min)
80
100
120
140
160
Time(min)
Fig. 6. (a) Benzene conversion, (b) O3 conversion of MA/SBA-15 and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.
100
80
80
60
60
40
40
20
20
0 5%
/S MA
5 5 O) O) O2 -15 A-1 A-1 /Si (H2 (H2 /SB /SB MA -15 -15 MN MN BA BA 5% S S 1 % / % % / 5 15 15 MA MN 15% 15%
BA
-15
/S MA
Yield (Carbon wt%)
Conversion(%)
Benzene conversion (%) COx yield (%)
100
0
BA
Fig. 7. Benzene conversion and COx yield of MA/SBA-15 and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.
CO CO 2
100
Selectivity in COx (%)
into intermediates may be included in benzene conversion, it can be regarded that the catalytic activity of 15% MA/SBA-15 which has higher COx yield may be higher than that of 15% MA/SiO2 . As shown in Fig. S1 (Supporting Information), Mn peaks were observed in the XRD pattern of 15% MA/SiO2 . In addition, TEM showed that some large manganese oxides were observed on MA/SiO2 (Fig. S2). This suggests that larger MnOx is formed on the SiO2 . From these results, SBA-15 appears to have the potential to be a better support for Mn oxide for benzene oxidation with ozone than SiO2 . Fig. 8 shows the distributions of CO and CO2 in the total COx over 15% Mn/SBA-15. For the MA/SBA-15, a high selectivity to CO2 was observed and no gaseous byproducts were detected. In addition, as shown in Fig. 7, the carbon balance of 15% MA/SBA-15 reached 96%. The CO2 selectivity was lower on the 15% MN/SBA15 catalyst than on the 15% MA/SBA-15 catalyst, and the carbon balance of 15% MN/SBA-15 was much lower than 100%. The poor carbon balance of 15% MN/SBA-15 might be due to the buildup of intermediates on the catalyst surface because no gaseous byproducts were detected when the 15% MN/SBA-15 catalyst was used. To identify the intermediates on the spent catalyst surface, the temperature programmed desorption (TPD) of spent 15% MN/SBA-15 was performed under a He atmosphere. The products desorbed were analyzed directly by gas chromatography/mass spectrometry. For 15% MN/SBA-15, high amounts of intermediates, such as formic acid, acetic acid, acetaldehyde, 2(5H)-furanone, etc. were detected (data not shown). Among them, formic acid was
80
60
40
20
0
) ) -15 -15 2O 2O BA BA 5(H 5(H A/S N/S A-1 A-1 M M B B /S /S 15% 15% MA MN 15% 15%
Fig. 8. Selectivity of CO and CO2 in COx .
detected as a major by-product. On the other hand, very small amount of formic acid was detected when the 15% MA/SBA-15 catalyst was used, which might explain the high carbon balance of 15% MA/SBA-15.
Benzene Conversion (%) COx Yield (Carbon wt%)
100
100
80
80
60
60
40
40
20
20
0
COx Yield (Carbon wt%)
Benzene Conversion (%)
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to be higher with the MA/SBA-15 catalyst, compared to MN/SBA-15 catalysts. Among the catalysts considered herein, 15% MA/SBA15 showed the best catalytic activity, due to highly dispersed manganese oxides on the SBA-15 supports, and the high Mn content. Acknowledgements This research was supported by the Converging Research Center Program, funded by the Ministry of Education, Science and Technology (no. 2012K001372). Young-Kwon Park acknowledges Prof. Ryong Ryoo’s valuable discussion and comments.
0
0
200
400
600
800
1000
O3 Concentration (ppm) Fig. 9. Effect of ozone concentation on benzene conversion over 15% MA/SBA-15 catalyst.
In addition, the effect of water vapor (0.7 vol%) on benzene oxidation with ozone was investigated (Figs. 7 and 8). As water was added, the CO2 selectivity and carbon balance increased for the 15% MN/SBA-15 catalyst. When the TPD of MN/SBA-15 with water vapor was carried out, the amount of intermediates of the by-products decreased significantly (data not shown). This is consistent with the result reported by Einaga and Ogata [25], who performed benzene oxidation with ozone on Mn/SiO2 . They suggested that water vapor added might promote the decomposition of intermediate compounds on the catalyst surface. Therefore, for 15% MN/SBA-15, water vapor might facilitate the oxidation of intermediate compounds. For MA/SBA-15, the selectivity to CO2 and carbon balance was similar irrespective of the addition of water. Furthermore the amount of byproducts on spent MA/SBA-15 with water vapor was similar to that on spent MA/SBA-15 (data not shown). The effect of ozone concentration on benzene conversion and COx yield by the 15% MA/SBA-15 catalyst is illustrated in Fig. 9. Benzene conversion and COx yield increased with an increase in ozone concentration. When the concentration of ozone reached 1000 ppm, the highest conversion of benzene and COx yield were exhibited. This indicates that the behavior of benzene oxidation and COx yield strongly depend on the concentration of ozone. 4. Conclusions For the purpose of catalytic benzene oxidation, manganese oxide, using a manganese acetate precursor, was highly dispersed on a SBA-15 support, whereas manganese oxide with manganese nitrate precursor formed large particles on SBA-15. MA/SBA-15 catalysts showed higher catalytic activity and stability than those of MN/SBA-15 catalysts, due to the highly dispersed manganese oxide and high oxygen mobility. The COx yield was also observed
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