Powder Technology 214 (2011) Contents lists available at SciVerse ScienceDirect. Powder Technology

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1 Powder Technology 214 (2011) Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: Short communication Effect of calcination temperature on the oxidation of benzene with ozone at low temperature over mesoporous α-mn 2 Mingshi Jin a, Jeong Wook Kim b, Ji Man Kim a, Jongsoo Jurng c, Gwi-Nam Bae c, Jong-Ki Jeon d, Young-Kwon Park b,e, a Department of Chemistry, BK21 School of Chemical Materials Science and Department of Energy Science, Sungkyunkwan University, Suwon , Republic of Korea b Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul , Republic of Korea c Environmental Sensor System Research Center, Korea Institute of Science and Technology, Seoul , Republic of Korea d Department of Chemical Engineering, Kongju National University, Cheonan , Republic of Korea e School of Environmental Engineering, University of Seoul, Seoul , Republic of Korea article info abstract Article history: Received 1 March 2011 Received in revised form 27 August 2011 Accepted 31 August 2011 Available online 3 September 2011 Keywords: Mesoporous α-mn 2 Benzene Ozone TPR Highly ordered mesoporous α-mn 2 was synthesized from cubic mesoporous silica (KIT-6) via nano-casting method. The mesoporous α-mn 2 thus obtained was calcined at C, and characterized using XRD, N 2 sorption and temperature-programmed reduction (TPR). The calcination temperature did not significantly affect the BET surface areas, mesopore sizes, pore structures and crystallinities of the mesoporous α-mn 2 materials. The mesoporous α-mn 2 calcined at 300 C showed the highest catalytic activity due to its high reduction ability revealed from the TPR analysis. However, the catalytic activity was negligible without ozone. In addition, the selectivity to CO 2 was about 90% and this seems to be an advantage of mesoporous α-mn 2 for removing benzene using ozone Elsevier B.V. All rights reserved. 1. Introduction Removal of volatile organic compounds (VOCs) has been an important subject in atmospheric pollution studies. Among the various methods used to eliminate the VOCs, catalytic oxidation is one of the most promising technologies for the control of hazardous atmospheric pollutants emitted from gas streams. Furthermore, the catalytic oxidation can proceed at much lower temperatures compared with thermal oxidation alone [1 3]. However,it is still necessary to reduce the reaction temperature because it is typically higher than 200 C. Therefore, with the use of ozone it is possible to reduce the operating removal temperature of VOCs to below 100 C. In most cases, ozone is added to polluted flue gas streams before they are passed through a catalyst bed. In terms of low pollutant concentrations and reaction temperatures, catalytic oxidation using ozone has been recognized as a useful method for the treatment of atmospheric pollutants [2]. Manganese oxide catalysts have frequently been studied for the catalytic oxidation of VOCs [3 6], both with and without ozone. However, most work has concentrated on supported or unsupported MnO x catalyst, but the catalytic oxidation using ozone over mesoporous α-mn 2 has never been reported. Corresponding author at: Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul , Republic of Korea. address: catalica@uos.ac.kr (Y.-K. Park). Mesoporous materials have recently attracted much attention due to their well defined pore structures and large pore sizes [7 9]. Especially, ordered mesoporous structures may exhibit enhanced catalytic activity, with continuous pore channels, which are beneficial to the diffusion and transport of reactant and product molecules [10,11]. Mesoporous α-mn 2 can be synthesized by the replication route, using mesoporous silica as a hard template, and utilized in many catalytic reactions. Ren et al. and Park et al. reported mesoporous α-mn 2 exhibited higher catalytic performance compared to the corresponding bulk Mn 2 for the oxidation of CO and decomposition of H 2 O 2, respectively, due to its high surface area as well as the number of active sites [12,13]. In this work, mesoporous α-mn 2 was synthesized and applied to the oxidation of benzene, using ozone, at low temperature. Also, the catalytic activity of mesoporous α-mn 2 was compared with that of the commercial bulk Mn Experimental 2.1. Catalyst preparation Mesoporous α-mn 2 was prepared using a nano-replication method [14]. The mesoporous silica (KIT-6) was used as the template, with manganese (III) nitrate hexahydrate (Mn(N ) 2 6H 2 O Aldrich) as the precursor. The molar ratio of KIT-6, Mn(N ) 2 6H 2 O and H 2 O were , and In the typical synthesis, the precursor /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.powtec

2 M. Jin et al. / Powder Technology 214 (2011) Table 1 Physical properties of mesoporous α-mn 2 materials at various calcination temperatures and bulk Mn 2. Sample S BET /m 2 g 1 V tot /cm 3 g 1 Mesoporous α-mn 2 (200 C a ) Mesoporous α-mn 2 (300 C a ) Mesoporous α-mn 2 (400 C a ) Mesoporous α-mn 2 (500 C a ) Commercial Bulk Mn a Calcination temperature. Fig. 1. XRD patterns of a) mesoporous α-mn C calcination, b) mesoporous α-mn C calcination c) mesoporous α-mn C calcination, d) mesoporous α-mn C calcination. was dissolved in distilled water, and then impregnated into the silica template. For the spontaneous infiltration of the manganese precursor within the mesopores of the silica template, the sample was dried overnight at 80 C. Subsequently, the sample was heated at 550 C under ambient conditions for 3 h. The silica template was almost removed using a 2 M NaOH aqueous solution twice. Eventually, the mesoporous α-mn 2 material was obtained by washing several times with distilled water and acetone, and then dried at 80 C. To investigate the effect of calcination temperature, the mesoporous α-mn 2 material was calcined at 200, 300, 400 and 500 C before the catalytic reaction. The commercial bulk Mn 2 was purchased from Aldrich Characterization of catalyst The powder XRD pattern was determined by X-ray diffractometer (Rigaku D/MAX-III) using Cu-Kα radiation. N 2 adsorption desorption isotherms were obtained using a Micromeritics ASAP 2000 at 196 C (liquid N 2 ). Brunauer Emmett Teller (BET) and Barrett Joyner Halenda (BJH) methods were used to estimate the BET surface area and pore size distributions, respectively. Temperature programmed reduction (TPR) was carried out with 0.06 g of calcined catalyst, from room temperature to 700 C, at a heating rate of 10 C min 1 in flow of 10 wt.% H 2 /He gas. The hydrogen consumption was monitored via a TCD Catalytic oxidation of benzene using ozone Catalytic reactions were carried out in a fixed bed flow reactor. Ozone was synthesized from O 2 using a silent discharge ozone generator. Prior to the catalytic reaction, a 0.05 g sample was heated in a Pyrex glass reactor at 300 C, with an O 2 flow rate of 120 ml/min. The catalyst was cooled and then maintained at 80 C. The inlet flow rate of O 2 and ozone was 60 ml/min and the ozone concentration was 2000 ppm. The inlet flow rate of mixed gas (benzene+nitrogen) was 60 ml/min and concentration of benzene was 200 ppm. After ozone/o 2 flow and benzene/n 2 flow were combined, total 120 ml/min of /benzene/o 2 /N 2 was fed to the catalytic reactor. The product gas sample was passed through a GC/FID (Young Lin 6000 Series) with HP-5 column to analyze the benzene conversion, with an indoor gas analyzer (Woori Systems ISR-401) used for the CO and CO 2 products, and an ozone analyzer (Ozonetech LAB-S) for the ozone conversion. In this system, the homogeneous gaseous reaction of benzene with ozone can be neglected. 3. Results and discussion 3.1. Characterization of catalyst Fig. 1 shows the XRD patterns of the mesoporous α-mn 2 prepared using different calcination temperatures. The low-angle XRD patterns indicate that the mesostructure of α-mn 2 replica materials was highly ordered and similar to those of the silica template of KIT-6 [15]. The as-synthesized mesostructure was maintained under the various calcination temperatures. However, the mesostructure of mesoporous α-mn 2 calcination at 400 C and 500 C were slightly collapsed. The high-angle XRD patterns of the mesoporous α-mn 2 were indicative of a well-crystallized framework. The N 2 adsorption desorption isotherms and corresponding BJH pore size distribution curves also indicate that all of the α-mn 2 materials exhibited well-ordered mesostructures. As shown in Fig. 2, the N 2 sorption isotherm was a typical type IV isotherm with a hysteresis loop which is characteristic of mesoporous materials. A defined step appeared in the adsorption desorption curves around a relative pressure, p/p 0, of The narrow pore size distributions indicated homogeneity of the pores of the mesoporous α-mn 2 materials [16,17]. The physical properties of the mesoporous α-mn 2 obtained using various calcination temperatures are summarized in Table 1. The surface areas and total pore volumes of the samples prepared Fig. 2. (1) N 2 adsorption desorption isotherms and corresponding (2) BJH pore size distribution curve of a) mesoporous α-mn C calcination, b) mesoporous α-mn C calcination, c) mesoporous α-mn C calcination, d) mesoporous α-mn C calcination.

3 460 M. Jin et al. / Powder Technology 214 (2011) Intensity (a.u) (200 C) (300 C) (400 C) (500 C) bulk-mn 2 at 400 CNcalcined at 500 C calcined at 200 C. This implied that the highest lattice oxygen mobility or oxygen vacancies on the Mn 2 surface were in the mesoporous α-mn 2 calcined at 300 C. Therefore, the catalytic oxidation activity was expected to be improved due to the high oxygen mobility. Fig. 4 shows the XPS spectra of mesoporous α-mn 2 calcined at various temperatures. The peak at ev is assigned to crystal lattice oxygen (O L ), while the peaks located at ev could be attributed to surface oxygen and surface hydroxyl groups (O H ) [18,19]. Thefitting area of O L on the mesoporous α- Mn 2 materials were in the order: calcined at 300 CNcalcined at 400 CNcalcined at 500 CNcalcined at 200 C. Tian et al. also reported that the lattice oxygen contributed to the catalytic activity [20] Temperature ( C) Fig. 3. H 2 -TPR profile of mesoporous α-mn 2 materials at various calcination temperatures and bulk Mn 2. at the various calcination temperatures were similar. Fig. 3 shows the TPR profiles of the mesoporous α-mn 2 calcined at various temperatures. The reduction temperatures of all the mesoporous α-mn 2 materials were significantly lower than that of commercial bulk Mn 2. Furthermore, the reduction ability of the mesoporous α-mn 2 materials was in the order: calcined at 300 C Ncalcined 3.2. Catalytic oxidation of benzene using ozone Fig. 5 shows the effect of calcination at various temperatures on the benzene conversion using ozone. The catalytic activities of mesoporous α-mn 2 were much higher than that of the commercial bulk Mn 2. The conversions of benzene over the various mesoporous α-mn 2 materials were in the order: calcined at 300 C Ncalcined at 200 CNcalcined at 400 C Ncalcined at 500 C. Fig. 5 also shows that the ozone conversion was very well matched to that of benzene, with higher ozone conversion also showing higher benzene conversion. In addition, the order of the catalytic activities can be explained by the TPR and XPS results. The high activity of the mesoporous α-mn 2 calcined at 300 C might be due to its high oxygen mobility or oxygen vacancies and lattice oxygen. The Fig. 4. The O1s XPS spectra of mesoporous α-mn 2 materials at various calcination temperatures ev-lattice oxygen (O L ); ev-surface oxygen and surface hydroxyl groups (O H ).

4 M. Jin et al. / Powder Technology 214 (2011) mesoporous α-mn 2 calcined at 400 C and 500 C showed high oxygen mobility or oxygen vacancies and lattice oxygen compared to mesoporous α-mn 2 calcined at 200 C. However, the catalytic activity was lower than that of mesoporous α-mn 2 calcined at 200 C. The lower catalytic activities of mesoporous α-mn 2 calcined at 400 C and 500 C may be due to the slightly collapsed mesostructures as shown in XRD data. The lowest activity of commercial bulk Mn 2 seemed to be ascribed to its lower oxygen mobility and surface area. Fig. 6 shows the effect of ozone on the oxidation of benzene over the mesoporous α-mn 2 at calcined at 300 C. Without ozone, the catalytic activity was negligible, but increasing ozone consumption increased the conversion of benzene. When the ozone consumption is above 750 ppm, the slope of benzene/ozone consumption was rapidly decreased compared to the ozone consumption below 750 ppm. Therefore, more ozone molecules are needed to convert benzene above 750 ppm. This implied that the optimal ratio of amount of ozone consumption to that of converted benzene in this study was estimated to be around 11. It has been reported that ozone decomposes to O 2 over supported manganese oxide catalysts, as expressed by the following equations, where * denotes the active sites on the catalyst [4,21]. þ T O 2 þ OT OT þ O 2 þ O 2 T O 2 T O 2 þ T: CO x Yield (Carbon wt%) Fig. 5. Effect of calcination temperature on benzene conversion and CO x yield CO x Yield Benzene Conversion Consumption (ppm) Fig. 6. Effect of ozone consumption on benzene conversion and CO x yield over mesoporous α-mn 2 calcined at 300 C ð1þ ð2þ ð3þ Benzene Conversion (%) The oxygen species O* formed during the decomposition of ozone will oxidize benzene to form oxygen-containing byproducts on the catalyst. These byproducts are further oxidized to CO 2 and CO (CO x ). The fact that ozone did not react with benzene in the homogeneous gas phase indicates that ozone itself is not the active species that decomposed these compounds. Following the mechanism described above, ozone may be decomposed to O 2 +O* on the mesoporous α-mn 2 ; especially the mesoporous α-mn 2 calcined at 300 C, which had greater oxygen mobility, may accelerate the decomposition of ozone. As shown in Fig. 6, the ozone consumption was in good agreement with the benzene conversion; the higher the ozone consumption, the higher the benzene conversion. The stoichiometric reaction process of benzene with ozone was expressed as the following equation: C 6 H 6 þ 15 ¼ 6CO 2 þ 3H 2 O þ 15O 2 : While the stoichiometric ratio of benzene over ozone is 15, the ratio of amount of ozone consumption to that of converted benzene in this study was estimated to be around 11, which was lower than the stoichiometric value of 15. This implied that oxygen may also react with benzene as following equation. R þ O 2 RO 2 CO 2 ; CO: Here R refers to organic radical intermediates formed in benzene oxidation. This result was also reported in literature [5]. In addition, all of benzene was converted to CO 2 and CO (CO x ) over mesoporous α-mn 2 and the selectivity to CO 2 was about 90% (data not shown). This may be another advantage of mesoporous α-mn 2 because the CO 2 selectivities of other Mn based catalysts were below ca. 80% [5]. Thus mesoporous α-mn 2 catalyst is an effective catalyst for benzene oxidation with ozone having high selectivity to CO Conclusion The mesoporous α-mn 2 calcination at different calcination temperatures showed different catalytic activities and all of calcined mesoporous α-mn 2 exhibited higher activity compared to the commercial bulk Mn 2. The mesoporous α-mn 2 calcined at 300 C showed highest catalytic activity probably due to its higher surface area as well as the large number of oxygen vacant sites (highest oxygen mobility) and lattice oxygen. These characteristics of mesoporous α-mn 2 may play an important role to oxidize benzene with high selectivity to CO 2. Acknowledgment This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (No. 2011K000752). References [1] C.L. Chang, Y.C. Lin, H. Bai, Y.H. Liu, Applying spray pyrolysis to synthesize MnO x for decomposing isopropyl alcohol in ozone- and thermal-catalytic oxidation, Korean Journal Chemistry Engineering 26 (2009) [2] J.H. Kim, J.S. Jurng, G.N. Bae, J.K. Jeon, K.Y. Jung, J.H. Yim, Y.K. Park. Benzene oxidation with ozone at low temperature over MnO x nanoparticle synthesized by spray pyrolysis, Energy Sources Part A: Recovery, Utilization, and Environmental Effects. in press. [3] W. 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