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Catalytic combustion of volatile organic compound over spherical-shaped copper–manganese oxide
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JIEC 3224 1–7 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
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Catalytic combustion of volatile organic compound over spherical-shaped copper–manganese oxide Chi-Woong Ahna,b , Young-Woo Youa , Iljeong Heoa , Ji Sook Honga , Jong-Ki Jeonb , Young-Deok Koc , YoHan Kime , Hosik Parkd,**, Jeong-Kwon Suha,* a
Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu 305-600, Daejeon, Republic of Korea Department of Chemical Engineering, KongJu National University, 56 Gongju-Si 314-701, ChungCheongNam-Do, Republic of Korea Puresphere, 105, Shinildong-ro, Daedeok-gu 343-24, Daejeon, Republic of Korea d Advanced Materials Division, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu 305-600, Daejeon, Republic of Korea e School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro, Buk-gu 500-712, Gwangju, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 5 October 2016 Received in revised form 1 December 2016 Accepted 16 December 2016 Available online xxx Keywords: Catalytic combustion Volatile organic compound Spherical-shape Copper–manganese oxide
A B S T R A C T
Amorphous spherical-shaped copper–manganese oxide for the catalytic combustion of benzene were successfully obtained by a wet granulation process under different calcination temperatures. The lowtemperature (400 C) calcined catalysts—which showed a larger BET specific surface area, low temperature reducibility, and an amorphous phase—were more active for the catalytic combustion of benzene than the high-temperature calcined catalyst which underwent the amorphous to crystalline phase transition. The catalytic activity tests, depending on the operating parameters, suggested that amorphous spherical-shaped copper–manganese oxide have potential applications for remedying VOC pollution. © 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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Introduction
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Volatile organic compounds (VOCs) are major contributors to photochemical smog and are dangerous for the human body [1]. Reducing pollutant concentrations, especially in communities and cities with high-density populations, requires the implementation of control technologies to eliminate or transform them [2,3]. Several techniques are available for the reduction of VOCs, such as adsorption, condensation, incineration, catalytic combustion, and bio-filtration [4–8]. Of these, catalytic combustion appears to be the most promising technology when the VOCs are seldom recycled or exist in low concentration. Catalytic combustion also requires a relatively low temperature, typically under 400 C, and generates a smaller amount of by-products when compared to incineration [9]. The catalytic combustion of VOCs has been studied using noble metals and transition metal oxides. The noble metals, such as
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* Corresponding author. Fax: +82 42 860 7533. ** Corresponding author. Fax: +82 42 860 7283. E-mail addresses: [email protected] (H. Park), [email protected] (J.-K. Suh).
platinum, palladium, and rhodium [10–12], are perfectly active at relatively low reaction temperatures, but they are very expensive and undergo poisoning by chlorinated compounds in the gas stream, which limits their actual field application [4,13–16]. On the other hand, transition metal oxides, such as CuOx [17–19], MnOx [9,17,2–25], CeOx [22,26–29], NiOx [20,22,25], and their binary mixtures [17,18,28,30,31], could represent alternative choices as commercially available catalysts for VOC removal due to their low cost, good reducibility, high resistance to poisoning, and requirement for a relatively lower operation temperature compared to noble metal catalysts [1,18]. Among the transition catalysts, a copper–manganese (Cu–Mn) oxide shows high performance in the catalytic combustion of various VOCs [4,20,22,24–26,29]. For example, Chen et al. [32] synthesized mesoporous Cu–Mn hopcalite catalysts and reported good catalytic activity for removing ethylene from a carbon dioxide stream. Morales et al. [33] reported that a Cu–Mn bimetallic oxide prepared by a co-precipitation method had an excellent catalytic performance for eliminating ethanol and propane. In these Cu–Mn catalyst systems, catalytic activity was strongly affected by the transition of the catalyst from an amorphous to a crystalline phase [32].
http://dx.doi.org/10.1016/j.jiec.2016.12.018 1226-086X/© 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: C.-W. Ahn, et al., Catalytic combustion of volatile organic compound over spherical-shaped copper– manganese oxide, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.12.018
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According to the literature, a more effective oxidation reaction is obtained with an amorphous phase Cu–Mn oxide than crystallized Cu–Mn oxide [32,33]. This can be explained by the high catalytic activity in the oxidation reaction that originates from the redox reaction in the form of Cu2+ + Mn3+ fi Cu+ + Mn4+, which results in electron transfer between the copper and manganese cations within the spinel lattice [32,34,35]. However, the crystalline phase has an enrichment of Cu+ and Mn4+ on the surface, which has been suggested to result in deactivation of catalysts [32,34,35]. Buciuman et al. [36] claimed that the amorphous phase of Cu–Mn oxide is more active than the crystallized CuMn2O4 spinel, since the catalytic activity of Cu–Mn oxides was affected by the spillover model, with manganese oxide acting as an oxygen donor and copper oxide as the oxygen acceptor. Therefore, control of the solid structure of a Cu–Mn oxide catalyst becomes very important for VOC removal. The shaping process is another issue that affects the effectiveness of Cu–Mn oxide catalysts for catalytic combustion of VOCs. Powder-type catalysts are difficult to handle and cause large pressure drops as well as a problems with heat transfer in the system [37]. For these reasons, the powder-type materials require an additional shaping step, with a minimal change in physical and chemical properties, to fulfill the requirements of commercial processes. In general, shaping processes are usually conducted using a press or an extrusion method [38]. After these shaping processes, the BET specific surface area decreases dramatically, and some pores are blocked, which leads to a decline in textural characteristics [38]. Previous studies that have compared the shaping processes for spherical-shaped granules and pellets have indicated that better textural properties are obtained with spherical-shaped granule synthesized by a wet granulation process than with pellet shapes obtained by a press or extrusion methods [37,38]. The BET specific surface area of pellet type materials was significantly decreased due to blocked pores, which reduced adsorption capacity, whereas the sphericalshaped granules formed using a wet granulation method avoided this decrease in the ratio of BET specific surface area [37,38]. In the present study, amorphous spherical-shaped copper– manganese oxide granule was prepared using a wet granulation process. The resulting catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction with hydrogen (H2-TPR), and N2 adsorption–desorption measurements. The effect of the Cu–Mn oxide structure, such as amorphous or crystalline structure, was investigated by calcination treatment at various temperatures and the influence of the structure on the catalytic properties was examined for these catalysts. Experimental
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Catalyst preparation and characterization
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Preparation of the spherical-shaped copper–manganese (Cu–Mn) oxide granules The spherical-shaped Cu–Mn oxide granules were prepared using the wet granulation method. An amount (1000 g) of Cu–Mn oxide powder (CuMn(P), PureSphere, Purelyst MD-101) was mixed with a silica sol (Ludox, AS-40 colloidal silica, 40 wt.%) solution and microcrystalline cellulose (Merck, microcrystalline cellulose) using a mixing granulator (Gebrüder Lödige, Maschinenbau GmbH, D-33102). The prepared sample was then dried at room temperature for 6 h to remove water from the granules. This dried sample which contains 15 wt.% of SiO2 as an inorganic binder was denoted as CuMn(G)25-15. Several CuMn(G)25-15 samples were then calcined at various temperatures (i.e., 400, 500, and 600 C) for 5 h; the spherical-shaped Cu–Mn oxide granules calcined at these
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temperatures were designated as CuMn(G)400-15, CuMn(G)50015 and CuMn(G)600-15, respectively. Catalysts denoted as CuMn (G)400-5, CuMn(G)400-10, CuMn(G)400-15, and CuMn(G)40020 were also prepared with SiO2 contents of 5, 10, 15, and 20 wt.%, respectively and calcined these at 400 C for 5 h.
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Characterization of the spherical-shaped copper–manganese (Cu–Mn) oxide granules The XRD patterns of the catalysts were measured on a Rigaku Ultima IV instrument using Cu Ka radiation at a diffraction angle (2u) range of 10–80 . The BET specific surface area (SBET) and total pore volume (Vt) were measured by N2 physisorption at 77 K (TristarII, Micromeritics). The H2-TPR was measured using a chemisorption analyzer (Autochem II, Micromeritics) equipped with a thermal conductivity detector. The H2-TPR analysis was conducted on 10 mg of spherical-shaped Cu–Mn oxide granules that were previously pretreated at 150 C for 1 h in the presence of argon (Ar) before reduction. The reduction gas was a 10% (v/v) mixture of H2 balanced with Ar, with a flow of 10 mL/min, and the catalyst was heated from 50 to 800 C (ramp rate: 5 C/min). The XPS measurements were performed on an AXIS NOVA (KRATOS) probe apparatus using a band-pass energy of 160 eV. The compressive strength was measured using the compressive strength meter shown in Supplementary Fig. S1. One unit of the catalyst was placed on the instrument and the force was vertically applied to the catalyst until the catalyst was broken.
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Catalytic activity tests
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The catalytic activity tests were performed at atmospheric pressure in a fixed-bed reactor (a 50 cm long and 10 mm I.D. stainless steel cylinder), which was loaded with 2 g of sphericalshaped Cu–Mn oxide granules. The reactant gas was a mixed gas containing 50, 100, 150, 300, and 500 ppm benzene in air. The total flow of the reactant mixture was controlled by a mass flow controller. The temperature was decreased from 390 C to 120 C and was maintained for 2 h at each set temperature for stabilization. The concentration of benzene in the effluent gas was analyzed with a gas chromatograph (Agilent J&W, GC Columns 123-503B) equipped with a flame ionization detector. A schematic of the experimental setup is given in Fig. 1. The three samples with different contents of inorganic binder (i.e., CuMn(G)400-10, CuMn(G)400-15, and CuMn(G)400-20) were tested at a constant gas hourly space velocity (GHSV) of 10,000 h1 to verify the effect of inorganic binder contents. The effect of calcination temperature was examined using CuMn(G) 400-15, CuMn(G)500-15, and CuMn(G)600-15 at a space velocity of 10,000 h1. The CuMn(G)400-15 catalyst was also used to test the stability of catalysts and operating conditions such as GHSV (10,000, 25,000, and 50,000 h1). A stability test of the catalysts was carried out using the same equipment as shown in Fig. 1. The catalyst bed was heated to 270 C (a temperature at which complete benzene oxidation was achieved) and maintained constant at this temperature for 30 min, followed by a decrease to room temperature. This procedure was repeated 10 times to evaluate the stability of the granulized catalysts. The conversion of benzene (Xbenzene, %) was calculated as follows:
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X benzene ð%Þ ¼
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C benzeneðoutÞ C benzeneðinÞ
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where Cbenzene (in) and Cbenzene (out) are the concentrations of benzene in the inlet and outlet gases, respectively.
Please cite this article in press as: C.-W. Ahn, et al., Catalytic combustion of volatile organic compound over spherical-shaped copper– manganese oxide, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.12.018
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Fig. 1. Schematic of experimental set up. 171
Results and discussion
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Catalyst characterization
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Material textural properties The main textural properties of all the catalysts, derived from N2 physisorption isotherms, are shown in Table 1 and Fig. S2 in Supplementary data. The BET specific surface area of the dried spherical-shaped Cu–Mn oxide granule (CuMn(G)25-15, 274 m2/g) was slightly lower than that of Cu–Mn oxide powder (CuMn(P), 329 m2/g). This can be explained by the fact that the inorganic binder has a BET specific surface area of approximately 135 m2/g and causes a decrease in the BET specific surface area of the dried spherical-shaped Cu–Mn oxide granules. However, the bulk density of the catalysts was increased after the granulation process (i.e., powder: 512 g/L vs. granule: 735 g/L). Therefore, the BET specific surface area per unit bed volume (BET specific surface area bulk density) was larger for the granules (201,390 m2/L) than for the powder (168,448 m2/L), even though the BET specific surface area was lower for the granules than for the powder. In addition, as the amount of the binder was increased (i.e., CuMn(G) 400-10, CuMn(G)400-15, and CuMn(G)400-20), a small decrease in the BET specific surface area was observed. Since the BET specific surface area was lower for the inorganic binder (135 m2/g) than for
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the Cu–Mn oxide powder, the BET specific surface area would be reduced as the content of the binder increased. On the other hand, the BET specific surface area was decreased with an increase in the calcination temperature, as expected, due to the pore sintering [39]. This decrease of the BET specific surface area could be explained by crystallization of the catalysts, which was confirmed by XRD analysis, as described in Section XRD analysis. The crystallization of amorphous catalysts is well known to cause sintering and then leads to a decrease in the BET specific surface area [39].
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XRD analysis The effect of calcination temperature on the formation of the spherical-shaped Cu–Mn oxide granules was examined by XRD analysis. As shown in Fig. 2, the XRD patterns of CuMn(P), CuMn(G) 25-15, and CuMn(G)400-15 indicated that these catalysts were amorphous or had a poorly crystalline phase. However, sharp diffraction peaks were observed in the catalysts calcined above 400 C (i.e., CuMn(G)500-15 and CuMn(G)600-15). This diffraction pattern indicated the presence of the Cu1.5Mn1.5O4 spinel phase (JCPDS No. 45-0505), together with a minor MnO2 phase (JCPDS No. 44-0141) in these catalysts. The XRD patterns confirmed that the catalysts lost their amorphous phase at calcination temperatures over 400 C [40]. This crystallization of the amorphous phase catalysts at high temperature has a substantial influence on the spillover model of amorphous Cu–Mn oxide [36]. An amorphous Cu–Mn oxide catalyst will have a higher activity in the oxidation of VOCs due to the spillover mechanism, with manganese oxide acting as an oxygen donor and copper oxide as an oxygen acceptor. In agreement with this concept, the CuMn(G)500-15 and CuMn(G) 600-15 catalysts were deactivated by crystallization of their amorphous phase in response to the increase in calcination temperature. In addition, no diffraction peaks of crystalline copper oxide were found in any of the catalysts. This might be explained by the fact that the catalysts synthesized in this study had a low content of copper compared to manganese, so that almost all the copper element would be used to form the spinel phase. Therefore, the spinel phase had a non-stoichiometric formation of Cu1.5Mn1.5O4, widely known as CuxMn3xO4 [41].
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XPS analysis Information about the chemical state of the catalyst components was obtained by X-ray photoelectron spectroscopy analysis.
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Table 1 Specific BET surface area and total pore volume of the catalysts, as derived from the N2 isotherms at 77 K. Catalyst
SBET (m2 g1)
Vta (cm3 g1)
CuMn(P) CuMn(G)25-15 CuMn(G)400-10 CuMn(G)400-15 CuMn(G)400-20 CuMn(G)500-15 CuMn(G)600-15
329 274 170 165 162 68 42
0.44 0.37 0.41 0.41 0.41 0.35 0.24
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Vt = total pore volume, measured at P/P0 = 0.99.
Fig. 2. XRD patterns of the (a) CuMn(P), (b) CuMn(G)25-15, (c) CuMn(G)400-15, (d) CuMn(G)500-15 and (e) CuMn(G)600-15 samples.
Please cite this article in press as: C.-W. Ahn, et al., Catalytic combustion of volatile organic compound over spherical-shaped copper– manganese oxide, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.12.018
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As shown in Fig. 3, Mn4+, Mn3+, Cu2+, and Cu+ species were found to exist on the surface of the catalyst. The peak at 933.8 eV in Fig. 3A is Cu2+ species on the surface of the catalysts. The CuMn(G)50015 and CuMn(G)600-15 catalysts showed a negative shift of the Cu 2p 3/2 peak at the binding energy of 930.9 eV, which represents the Cu+ species occupying the tetrahedral sites of the spinel structure [34]. The intensity of these peaks increased further at the higher calcination temperature of 600 C. Previous reports in the literature [32,34,35] have explained that the Cu–Mn oxide catalysts are deactivated with dissolution of their amorphous phase at the crystallization temperature due to the chemical/electrical properties of the Cu1.5Mn1.5O4 spinel. The redox couple, in the form of Cu2 + + Mn3+ Ð Cu+ + Mn4+ in the Cu–Mn spinel lattice, promotes its catalytic activity. As shown in Fig. 3A, the Cu+ species are generated to a greater extent on catalyst surfaces as the calcination temperature increases. The deactivation of CuMn(G)500-15 and CuMn(G)600-15, as a function of increasing calcination temperature, could be explained by this result. According to previous work [32,34,35], this redox couple apparently operates in both the amorphous phase and the crystalline spinel, but it is shifted predominantly toward the more stable Cu+ and Mn4+ species in the spinel.
Fig. 3. XPS spectra in the Cu 2p region (A) and Mn 2p region (B) of the sphericalshaped Cu–Mn oxide catalysts; (a) CuMn(G)25-15; (b) CuMn(G)400-15; (c) CuMn (G)500-15; (d) CuMn(G)600-15 catalysts.
The Mn 2p spectrum in Fig. 3B could be decomposed into two major components, with binding energies of 642.1 eV (Mn 2p3/2) and 653.8 eV (Mn 2p1/2). The binding energies of the Mn 2p3/2 of curves are very close, at 641.6 0.3 eV. Mn4+ is known as a deactivated species for catalytic oxidation because the surface oxygen of catalysts is dissociatively chemisorbed by MnO2 [35]. The shift in the Mn electron binding energies on the sphericalshaped Cu–Mn oxide granules was not determined from the chemical shifts due to the slight differences in the corresponding binding energies of the main peaks. The broader peaks of the catalysts appear to be coordinated with Mn3+ and Mn4+ [42].
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H2-TPR curves Fig. 4 shows a comparison of the H2-TPR profiles of the catalysts prepared at different calcination temperatures in this work. The H2-TPR profiles of the Cu–Mn oxide show a reduction process that was shifted to higher temperatures with increasing calcination temperatures. At low temperature, the reduction peaks of CuMn (G)25-15 and CuMn(G)400-15 were decomposed into two parts that were attributable to the reduction of copper and manganese oxide species. The first peak for CuMn(G)25-15 and for CuMn(G) 400-15 was detected at a low temperature (170 C and 194 C, respectively), which is the reduction peak of the species Cu2+/Cu+ to Cu0. This is explained by the fact that the copper oxide is more reducible than MnOx species in the mixed Cu–Mn oxide [43,44]. The second peak at higher temperature (215 C and 242 C, respectively) can be attributed to the reduction of Mn4+ to Mn3+. The main peak of CuMn(G)500-15 and of CuMn(G)600-15 was confirmed as the reduction peak of Cu1.5Mn1.5O4 spinel and manganese oxides. The CuMn(G)500-15 and CuMn(G)60015 showed no dissociated peak for manganese, which seemed to have a strong interaction with the Cu1.5Mn1.5O4 spinel state [44,45]. Therefore, the reduction temperature was lower for the amorphous Cu–Mn oxide catalysts than for the other catalysts that were crystallized.
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Evaluation of the catalytic behavior
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Influence of the content of inorganic binder The binder content is one of the main parameters to affect the properties of granules as well as to maintain the mechanical stability of granulized catalyst in the system; therefore, the granulation experiments were conducted by varying the ratio of
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Fig. 4. H2-TPR profiles of the (a) CuMn(G)25-15, (b) CuMn(G)400-15, (c) CuMn(G) 500-15 and (d) CuMn(G)600-15 catalysts.
Please cite this article in press as: C.-W. Ahn, et al., Catalytic combustion of volatile organic compound over spherical-shaped copper– manganese oxide, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.12.018
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the inorganic binder content at 10, 15, and 20 wt.%, with a constant microcrystalline cellulose content as an organic binder to simplify the granulation process. To verify the mechanical stability of granulized catalysts, the compressive strength of each catalysts, which is an important factor for the granulation process was evaluated. Because higher mechanical stability could increase the tolerance of the granules to use in diverse processes in the field [38]. Supplementary Fig. S3 shows the compressive strength of spherical-shaped Cu–Mn oxide with different inorganic binder content. As shown in Supplementary Fig. S3, CuMn(G)40020 granules which contains 20 wt.% of inorganic binder showed the highest compressive strength compared to less inorganic binder containing spherical-shaped catalyst granules (i.e., CuMn (G)400-5, CuMn(G)400-10 and CuMn(G)400-15). However, the CuMn(G)400-15 (1.14 kgf/unit) was chosen for the catalytic activity test in this study due to its enough mechanical strength. Even though CuMn(G)400-10 granules (0.6 kgf/unit) which showed the best catalytic activity, this was excluded for further experiments, since they were easily broken during the catalytic combustion experiments. In addition, the CuMn(G)400-20 granules, which showed the highest compressive strength (2.65 kgf/unit) than the other catalysts, were also excluded to minimize the amount of inorganic binder content in the spherical Cu–Mn oxide granules. The catalytic activities of the spherical-shaped Cu–Mn oxide granules with different contents of inorganic binder are shown in Fig. 5 and the BET specific surface areas are shown in Table 1. Granulation followed by calcination at 400 C resulted in no significant difference in the BET specific surface area in terms of the inorganic binder contents (i.e., 10–20 wt.%). As shown in Fig. 5, the CuMn(G)400-10 catalyst with the lowest inorganic binder content showed better catalytic combustion properties than CuMn(G)40015 and CuMn(G)400-20, as confirmed by the T90 determinations (the temperature at which 90% conversion is achieved). The T90 of spherical-shaped catalysts which denoted as CuMn(G)400-10, CuMn(G)400-15 and CuMn(G)400-20 depending on the inorganic binder contents (i.e., 10–20 wt.%) were 228, 231, and 240 C, respectively. Influence of the calcination temperature The effect of the calcination temperature during the granulation process was examined by performing a catalytic activity test for removal benzene with the granulized catalysts treated at different calcination temperatures (i.e., 400, 500, and 600 C). As shown in Fig. 6, CuMn(G)400-15, CuMn(G)500-15, and CuMn(G)600-
Fig. 5. Effect of inorganic binder contents on the catalytic performance of the spherical-shaped Cu–Mn oxide catalysts (GHSV: 10,000 h1).
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Fig. 6. Effect of calcination temperature on the catalytic performance of the spherical-shaped Cu–Mn oxide catalysts (GHSV: 10,000 h1).
15 showed different catalytic activities. CuMn(G)400-15 was the most active for benzene oxidation under 300 C, whereas the catalytic activities of CuMn(G)500-15 and CuMn(G)600-15 were lower. This agreed with the XRD results, which showed that CuMn (G)400-15 maintained its amorphous phase, in contrast to the other catalysts. It also agreed with the data presented in Table 1, where the BET specific surface area of Cu–Mn oxide granules was dramatically decreased with increasing calcination temperature at 500 C. The sintering process induced the crystallization of the amorphous Cu–Mn oxide above 400 C, which led to a decrease in the catalyst activation that accompanied the BET specific surface area [35,46]. The XPS data confirmed a higher concentration of Cu+ and Mn4+ in CuMn(G)500-15 and CuMn(G)600-15, indicating that the catalysts decreased the catalytic activity of the Cu–Mn oxide granule for the catalytic oxidation of benzene. This result also agreed with the TPR curves (Fig. 4) that showed CuMn(G)400-15 to be more reducible than CuMn(G)500-15 and CuMn(G)600-15. The results from the characterization of spherical-shaped Cu– Mn oxide granules and the catalytic combustion experiments confirm that the catalytic activity of amorphous Cu–Mn oxide granules was decreased with changes in their solid structure from amorphous to crystalline. The formation of a Cu1.5Mn1.5O4 spinel phase and the existence of Cu2+ species in the prepared Cu–Mn oxide catalysts play important roles in increasing the activity of Cu –Mn oxide catalysts for benzene oxidation. Therefore, elimination of VOCs will be more effectively achieved using an amorphous state of Cu–Mn oxide, as this is a more active phase than the crystallized Cu–Mn oxide with a CuMn2O4 spinel structure.
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Optimization of the operating conditions and stability tests
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The catalytic behavior of the spherical-shaped Cu–Mn oxide granules was tested for the catalytic combustion of the benzene using three different GHSV values. As shown in Fig. 7, the CuMn(G) 400-15 showed the best catalytic performance for benzene removal at a lower GHSV (10,000 h1). The CuMn(G)400-15 catalyst reached a T90 at 240 C, since the reactant molecules (O2 and benzene) were retained to a greater extent within the pores, thereby favoring oxidation reactions going to completion. On the other hand, at higher GHSV (50,000 h1), the catalyst was unable to achieve 90% conversion, as a consequence of the lower residence time of the molecules within the catalyst framework [9]. As shown in Supplementary Fig. S4, the effect of shaping process was investigated over the CuMn(P), CuMn(G)400-15,
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Please cite this article in press as: C.-W. Ahn, et al., Catalytic combustion of volatile organic compound over spherical-shaped copper– manganese oxide, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.12.018
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Fig. 7. Effect of GHSV on the catalytic performance of the CuMn(G)400-15 catalyst (GHSV: 10,000, 25,000, 50,000 h1). 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405
CuMn(G)500-15, and CuMn(G)600-15. As it is expected, the powder type of Cu–Mn oxide are more active than sphericalshaped Cu–Mn oxides. It can be explained that the decrease of BET specific surface area obtained by the sintering of catalysts with granulation and calcination step reduced the catalytic activity of spherical-shaped Cu–Mn oxides. Additionally, the existence of void in the reactor is effected on the catalytic activity of sphericalshaped Cu–Mn oxides. The effect of benzene concentration was further investigated over the CuMn(G)400-15 catalyst at 210 C. Supplementary Fig. S5 shows the benzene conversions over the CuMn(G)40015 catalyst with different benzene concentrations (i.e., 50, 100, 150, 300, and 500 ppm) in the feed gas at fixed temperature of 210 C. 100% conversion of benzene was achieved when benzene concentration was 50 ppm, and benzene conversion rate was decreased as benzene concentration was increased. These results indicate that increasing benzene concentration results in decreasing catalytic activity of CuMn(G)400-15. It can be explained that the site competition between oxygen and hydrocarbon on the catalyst surface effects on the catalytic oxidation of VOCs [47]. Similar results have been extracted by prior research for a variety of catalysts and VOCs [20,48]. Fig. 8 illustrates the results of the stability measurements for the CuMn(G)400-15. The catalytic oxidation of Cu–Mn oxide was
Fig. 8. Stability test for catalytic oxidation of benzene at 270 C over sphericalshaped CuMn(G)400-15 catalyst (benzene conconcentration: 150 ppm, GHSV: 10,000 h1).
repeated 10 times at a benzene concentration of 150 ppm at 270 C for 30 min for each test. The bed height of the catalysts was visually almost unchanged after the reaction. The conversion rate of benzene was maintained at 100% for each test and no decrease in the catalytic activity had occurred after the 10th reaction. Some samples that were reacted with benzene for the 5th and 10th runs underwent BET analysis, which revealed no significant changes in the BET specific surface area (i.e., 5th run: 134 m2/g, 10th run: 134 m2/g). Thus, the amorphous spherical-shaped Cu–Mn oxide granule showed considerable durability under time-on-stream.
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Conclusions
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Spherical-shaped Cu–Mn oxide granules were successfully synthesized by a wet granulation process that only minimal effects on the properties of the Cu–Mn oxide powder. The catalytic activity of the spherical-shaped Cu–Mn oxide granules confirmed that a sample calcined at 400 C was suitable for use as a catalyst for benzene combustion. Spherical-shaped Cu–Mn oxide granules calcined at higher temperatures had lower catalytic activities due to increasing crystallization of MnO2 and the spinel Cu1.5Mn1.5O4, in response to temperature. The spherical-shaped Cu–Mn granules with relatively high concentrations of Cu+ and Mn4+, known to be deactivated species from their redox couples during crystallization, readily lost their catalytic properties with increasing calcination temperatures. In conclusion, control of the BET specific surface area and the amorphous phase without Cu+ and Mn4+ is a key parameter for eliminating benzene by catalytic combustion using Cu–Mn oxide granules. The amorphous Cu–Mn oxide granules also maintained a favorably high conversion during a stability test.
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Acknowledgment
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This subject is supported by the Korean Ministry of Environment (MOE) as “The advancement of scientific research and technological development in environmental science (#2014000110005) program”.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jiec.2016.12.018.
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Contents lists available at ScienceDirect
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Catalytic combustion of volatile organic compound over spherical-shaped copper–manganese oxide Chi-Woong Ahna,b , Young-Woo Youa , Iljeong Heoa , Ji Sook Honga , Jong-Ki Jeonb , Young-Deok Koc , YoHan Kime , Hosik Parkd,**, Jeong-Kwon Suha,* a
Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu 305-600, Daejeon, Republic of Korea Department of Chemical Engineering, KongJu National University, 56 Gongju-Si 314-701, ChungCheongNam-Do, Republic of Korea Puresphere, 105, Shinildong-ro, Daedeok-gu 343-24, Daejeon, Republic of Korea d Advanced Materials Division, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu 305-600, Daejeon, Republic of Korea e School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro, Buk-gu 500-712, Gwangju, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 5 October 2016 Received in revised form 1 December 2016 Accepted 16 December 2016 Available online xxx Keywords: Catalytic combustion Volatile organic compound Spherical-shape Copper–manganese oxide
A B S T R A C T
Amorphous spherical-shaped copper–manganese oxide for the catalytic combustion of benzene were successfully obtained by a wet granulation process under different calcination temperatures. The lowtemperature (400 C) calcined catalysts—which showed a larger BET specific surface area, low temperature reducibility, and an amorphous phase—were more active for the catalytic combustion of benzene than the high-temperature calcined catalyst which underwent the amorphous to crystalline phase transition. The catalytic activity tests, depending on the operating parameters, suggested that amorphous spherical-shaped copper–manganese oxide have potential applications for remedying VOC pollution. © 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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Introduction
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Volatile organic compounds (VOCs) are major contributors to photochemical smog and are dangerous for the human body [1]. Reducing pollutant concentrations, especially in communities and cities with high-density populations, requires the implementation of control technologies to eliminate or transform them [2,3]. Several techniques are available for the reduction of VOCs, such as adsorption, condensation, incineration, catalytic combustion, and bio-filtration [4–8]. Of these, catalytic combustion appears to be the most promising technology when the VOCs are seldom recycled or exist in low concentration. Catalytic combustion also requires a relatively low temperature, typically under 400 C, and generates a smaller amount of by-products when compared to incineration [9]. The catalytic combustion of VOCs has been studied using noble metals and transition metal oxides. The noble metals, such as
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* Corresponding author. Fax: +82 42 860 7533. ** Corresponding author. Fax: +82 42 860 7283. E-mail addresses: [email protected] (H. Park), [email protected] (J.-K. Suh).
platinum, palladium, and rhodium [10–12], are perfectly active at relatively low reaction temperatures, but they are very expensive and undergo poisoning by chlorinated compounds in the gas stream, which limits their actual field application [4,13–16]. On the other hand, transition metal oxides, such as CuOx [17–19], MnOx [9,17,2–25], CeOx [22,26–29], NiOx [20,22,25], and their binary mixtures [17,18,28,30,31], could represent alternative choices as commercially available catalysts for VOC removal due to their low cost, good reducibility, high resistance to poisoning, and requirement for a relatively lower operation temperature compared to noble metal catalysts [1,18]. Among the transition catalysts, a copper–manganese (Cu–Mn) oxide shows high performance in the catalytic combustion of various VOCs [4,20,22,24–26,29]. For example, Chen et al. [32] synthesized mesoporous Cu–Mn hopcalite catalysts and reported good catalytic activity for removing ethylene from a carbon dioxide stream. Morales et al. [33] reported that a Cu–Mn bimetallic oxide prepared by a co-precipitation method had an excellent catalytic performance for eliminating ethanol and propane. In these Cu–Mn catalyst systems, catalytic activity was strongly affected by the transition of the catalyst from an amorphous to a crystalline phase [32].
http://dx.doi.org/10.1016/j.jiec.2016.12.018 1226-086X/© 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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According to the literature, a more effective oxidation reaction is obtained with an amorphous phase Cu–Mn oxide than crystallized Cu–Mn oxide [32,33]. This can be explained by the high catalytic activity in the oxidation reaction that originates from the redox reaction in the form of Cu2+ + Mn3+ fi Cu+ + Mn4+, which results in electron transfer between the copper and manganese cations within the spinel lattice [32,34,35]. However, the crystalline phase has an enrichment of Cu+ and Mn4+ on the surface, which has been suggested to result in deactivation of catalysts [32,34,35]. Buciuman et al. [36] claimed that the amorphous phase of Cu–Mn oxide is more active than the crystallized CuMn2O4 spinel, since the catalytic activity of Cu–Mn oxides was affected by the spillover model, with manganese oxide acting as an oxygen donor and copper oxide as the oxygen acceptor. Therefore, control of the solid structure of a Cu–Mn oxide catalyst becomes very important for VOC removal. The shaping process is another issue that affects the effectiveness of Cu–Mn oxide catalysts for catalytic combustion of VOCs. Powder-type catalysts are difficult to handle and cause large pressure drops as well as a problems with heat transfer in the system [37]. For these reasons, the powder-type materials require an additional shaping step, with a minimal change in physical and chemical properties, to fulfill the requirements of commercial processes. In general, shaping processes are usually conducted using a press or an extrusion method [38]. After these shaping processes, the BET specific surface area decreases dramatically, and some pores are blocked, which leads to a decline in textural characteristics [38]. Previous studies that have compared the shaping processes for spherical-shaped granules and pellets have indicated that better textural properties are obtained with spherical-shaped granule synthesized by a wet granulation process than with pellet shapes obtained by a press or extrusion methods [37,38]. The BET specific surface area of pellet type materials was significantly decreased due to blocked pores, which reduced adsorption capacity, whereas the sphericalshaped granules formed using a wet granulation method avoided this decrease in the ratio of BET specific surface area [37,38]. In the present study, amorphous spherical-shaped copper– manganese oxide granule was prepared using a wet granulation process. The resulting catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction with hydrogen (H2-TPR), and N2 adsorption–desorption measurements. The effect of the Cu–Mn oxide structure, such as amorphous or crystalline structure, was investigated by calcination treatment at various temperatures and the influence of the structure on the catalytic properties was examined for these catalysts. Experimental
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Catalyst preparation and characterization
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Preparation of the spherical-shaped copper–manganese (Cu–Mn) oxide granules The spherical-shaped Cu–Mn oxide granules were prepared using the wet granulation method. An amount (1000 g) of Cu–Mn oxide powder (CuMn(P), PureSphere, Purelyst MD-101) was mixed with a silica sol (Ludox, AS-40 colloidal silica, 40 wt.%) solution and microcrystalline cellulose (Merck, microcrystalline cellulose) using a mixing granulator (Gebrüder Lödige, Maschinenbau GmbH, D-33102). The prepared sample was then dried at room temperature for 6 h to remove water from the granules. This dried sample which contains 15 wt.% of SiO2 as an inorganic binder was denoted as CuMn(G)25-15. Several CuMn(G)25-15 samples were then calcined at various temperatures (i.e., 400, 500, and 600 C) for 5 h; the spherical-shaped Cu–Mn oxide granules calcined at these
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temperatures were designated as CuMn(G)400-15, CuMn(G)50015 and CuMn(G)600-15, respectively. Catalysts denoted as CuMn (G)400-5, CuMn(G)400-10, CuMn(G)400-15, and CuMn(G)40020 were also prepared with SiO2 contents of 5, 10, 15, and 20 wt.%, respectively and calcined these at 400 C for 5 h.
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Characterization of the spherical-shaped copper–manganese (Cu–Mn) oxide granules The XRD patterns of the catalysts were measured on a Rigaku Ultima IV instrument using Cu Ka radiation at a diffraction angle (2u) range of 10–80 . The BET specific surface area (SBET) and total pore volume (Vt) were measured by N2 physisorption at 77 K (TristarII, Micromeritics). The H2-TPR was measured using a chemisorption analyzer (Autochem II, Micromeritics) equipped with a thermal conductivity detector. The H2-TPR analysis was conducted on 10 mg of spherical-shaped Cu–Mn oxide granules that were previously pretreated at 150 C for 1 h in the presence of argon (Ar) before reduction. The reduction gas was a 10% (v/v) mixture of H2 balanced with Ar, with a flow of 10 mL/min, and the catalyst was heated from 50 to 800 C (ramp rate: 5 C/min). The XPS measurements were performed on an AXIS NOVA (KRATOS) probe apparatus using a band-pass energy of 160 eV. The compressive strength was measured using the compressive strength meter shown in Supplementary Fig. S1. One unit of the catalyst was placed on the instrument and the force was vertically applied to the catalyst until the catalyst was broken.
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Catalytic activity tests
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The catalytic activity tests were performed at atmospheric pressure in a fixed-bed reactor (a 50 cm long and 10 mm I.D. stainless steel cylinder), which was loaded with 2 g of sphericalshaped Cu–Mn oxide granules. The reactant gas was a mixed gas containing 50, 100, 150, 300, and 500 ppm benzene in air. The total flow of the reactant mixture was controlled by a mass flow controller. The temperature was decreased from 390 C to 120 C and was maintained for 2 h at each set temperature for stabilization. The concentration of benzene in the effluent gas was analyzed with a gas chromatograph (Agilent J&W, GC Columns 123-503B) equipped with a flame ionization detector. A schematic of the experimental setup is given in Fig. 1. The three samples with different contents of inorganic binder (i.e., CuMn(G)400-10, CuMn(G)400-15, and CuMn(G)400-20) were tested at a constant gas hourly space velocity (GHSV) of 10,000 h1 to verify the effect of inorganic binder contents. The effect of calcination temperature was examined using CuMn(G) 400-15, CuMn(G)500-15, and CuMn(G)600-15 at a space velocity of 10,000 h1. The CuMn(G)400-15 catalyst was also used to test the stability of catalysts and operating conditions such as GHSV (10,000, 25,000, and 50,000 h1). A stability test of the catalysts was carried out using the same equipment as shown in Fig. 1. The catalyst bed was heated to 270 C (a temperature at which complete benzene oxidation was achieved) and maintained constant at this temperature for 30 min, followed by a decrease to room temperature. This procedure was repeated 10 times to evaluate the stability of the granulized catalysts. The conversion of benzene (Xbenzene, %) was calculated as follows:
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X benzene ð%Þ ¼
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C benzeneðoutÞ C benzeneðinÞ
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where Cbenzene (in) and Cbenzene (out) are the concentrations of benzene in the inlet and outlet gases, respectively.
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Fig. 1. Schematic of experimental set up. 171
Results and discussion
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Catalyst characterization
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Material textural properties The main textural properties of all the catalysts, derived from N2 physisorption isotherms, are shown in Table 1 and Fig. S2 in Supplementary data. The BET specific surface area of the dried spherical-shaped Cu–Mn oxide granule (CuMn(G)25-15, 274 m2/g) was slightly lower than that of Cu–Mn oxide powder (CuMn(P), 329 m2/g). This can be explained by the fact that the inorganic binder has a BET specific surface area of approximately 135 m2/g and causes a decrease in the BET specific surface area of the dried spherical-shaped Cu–Mn oxide granules. However, the bulk density of the catalysts was increased after the granulation process (i.e., powder: 512 g/L vs. granule: 735 g/L). Therefore, the BET specific surface area per unit bed volume (BET specific surface area bulk density) was larger for the granules (201,390 m2/L) than for the powder (168,448 m2/L), even though the BET specific surface area was lower for the granules than for the powder. In addition, as the amount of the binder was increased (i.e., CuMn(G) 400-10, CuMn(G)400-15, and CuMn(G)400-20), a small decrease in the BET specific surface area was observed. Since the BET specific surface area was lower for the inorganic binder (135 m2/g) than for
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the Cu–Mn oxide powder, the BET specific surface area would be reduced as the content of the binder increased. On the other hand, the BET specific surface area was decreased with an increase in the calcination temperature, as expected, due to the pore sintering [39]. This decrease of the BET specific surface area could be explained by crystallization of the catalysts, which was confirmed by XRD analysis, as described in Section XRD analysis. The crystallization of amorphous catalysts is well known to cause sintering and then leads to a decrease in the BET specific surface area [39].
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XRD analysis The effect of calcination temperature on the formation of the spherical-shaped Cu–Mn oxide granules was examined by XRD analysis. As shown in Fig. 2, the XRD patterns of CuMn(P), CuMn(G) 25-15, and CuMn(G)400-15 indicated that these catalysts were amorphous or had a poorly crystalline phase. However, sharp diffraction peaks were observed in the catalysts calcined above 400 C (i.e., CuMn(G)500-15 and CuMn(G)600-15). This diffraction pattern indicated the presence of the Cu1.5Mn1.5O4 spinel phase (JCPDS No. 45-0505), together with a minor MnO2 phase (JCPDS No. 44-0141) in these catalysts. The XRD patterns confirmed that the catalysts lost their amorphous phase at calcination temperatures over 400 C [40]. This crystallization of the amorphous phase catalysts at high temperature has a substantial influence on the spillover model of amorphous Cu–Mn oxide [36]. An amorphous Cu–Mn oxide catalyst will have a higher activity in the oxidation of VOCs due to the spillover mechanism, with manganese oxide acting as an oxygen donor and copper oxide as an oxygen acceptor. In agreement with this concept, the CuMn(G)500-15 and CuMn(G) 600-15 catalysts were deactivated by crystallization of their amorphous phase in response to the increase in calcination temperature. In addition, no diffraction peaks of crystalline copper oxide were found in any of the catalysts. This might be explained by the fact that the catalysts synthesized in this study had a low content of copper compared to manganese, so that almost all the copper element would be used to form the spinel phase. Therefore, the spinel phase had a non-stoichiometric formation of Cu1.5Mn1.5O4, widely known as CuxMn3xO4 [41].
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XPS analysis Information about the chemical state of the catalyst components was obtained by X-ray photoelectron spectroscopy analysis.
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Table 1 Specific BET surface area and total pore volume of the catalysts, as derived from the N2 isotherms at 77 K. Catalyst
SBET (m2 g1)
Vta (cm3 g1)
CuMn(P) CuMn(G)25-15 CuMn(G)400-10 CuMn(G)400-15 CuMn(G)400-20 CuMn(G)500-15 CuMn(G)600-15
329 274 170 165 162 68 42
0.44 0.37 0.41 0.41 0.41 0.35 0.24
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Vt = total pore volume, measured at P/P0 = 0.99.
Fig. 2. XRD patterns of the (a) CuMn(P), (b) CuMn(G)25-15, (c) CuMn(G)400-15, (d) CuMn(G)500-15 and (e) CuMn(G)600-15 samples.
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As shown in Fig. 3, Mn4+, Mn3+, Cu2+, and Cu+ species were found to exist on the surface of the catalyst. The peak at 933.8 eV in Fig. 3A is Cu2+ species on the surface of the catalysts. The CuMn(G)50015 and CuMn(G)600-15 catalysts showed a negative shift of the Cu 2p 3/2 peak at the binding energy of 930.9 eV, which represents the Cu+ species occupying the tetrahedral sites of the spinel structure [34]. The intensity of these peaks increased further at the higher calcination temperature of 600 C. Previous reports in the literature [32,34,35] have explained that the Cu–Mn oxide catalysts are deactivated with dissolution of their amorphous phase at the crystallization temperature due to the chemical/electrical properties of the Cu1.5Mn1.5O4 spinel. The redox couple, in the form of Cu2 + + Mn3+ Ð Cu+ + Mn4+ in the Cu–Mn spinel lattice, promotes its catalytic activity. As shown in Fig. 3A, the Cu+ species are generated to a greater extent on catalyst surfaces as the calcination temperature increases. The deactivation of CuMn(G)500-15 and CuMn(G)600-15, as a function of increasing calcination temperature, could be explained by this result. According to previous work [32,34,35], this redox couple apparently operates in both the amorphous phase and the crystalline spinel, but it is shifted predominantly toward the more stable Cu+ and Mn4+ species in the spinel.
Fig. 3. XPS spectra in the Cu 2p region (A) and Mn 2p region (B) of the sphericalshaped Cu–Mn oxide catalysts; (a) CuMn(G)25-15; (b) CuMn(G)400-15; (c) CuMn (G)500-15; (d) CuMn(G)600-15 catalysts.
The Mn 2p spectrum in Fig. 3B could be decomposed into two major components, with binding energies of 642.1 eV (Mn 2p3/2) and 653.8 eV (Mn 2p1/2). The binding energies of the Mn 2p3/2 of curves are very close, at 641.6 0.3 eV. Mn4+ is known as a deactivated species for catalytic oxidation because the surface oxygen of catalysts is dissociatively chemisorbed by MnO2 [35]. The shift in the Mn electron binding energies on the sphericalshaped Cu–Mn oxide granules was not determined from the chemical shifts due to the slight differences in the corresponding binding energies of the main peaks. The broader peaks of the catalysts appear to be coordinated with Mn3+ and Mn4+ [42].
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H2-TPR curves Fig. 4 shows a comparison of the H2-TPR profiles of the catalysts prepared at different calcination temperatures in this work. The H2-TPR profiles of the Cu–Mn oxide show a reduction process that was shifted to higher temperatures with increasing calcination temperatures. At low temperature, the reduction peaks of CuMn (G)25-15 and CuMn(G)400-15 were decomposed into two parts that were attributable to the reduction of copper and manganese oxide species. The first peak for CuMn(G)25-15 and for CuMn(G) 400-15 was detected at a low temperature (170 C and 194 C, respectively), which is the reduction peak of the species Cu2+/Cu+ to Cu0. This is explained by the fact that the copper oxide is more reducible than MnOx species in the mixed Cu–Mn oxide [43,44]. The second peak at higher temperature (215 C and 242 C, respectively) can be attributed to the reduction of Mn4+ to Mn3+. The main peak of CuMn(G)500-15 and of CuMn(G)600-15 was confirmed as the reduction peak of Cu1.5Mn1.5O4 spinel and manganese oxides. The CuMn(G)500-15 and CuMn(G)60015 showed no dissociated peak for manganese, which seemed to have a strong interaction with the Cu1.5Mn1.5O4 spinel state [44,45]. Therefore, the reduction temperature was lower for the amorphous Cu–Mn oxide catalysts than for the other catalysts that were crystallized.
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Evaluation of the catalytic behavior
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Influence of the content of inorganic binder The binder content is one of the main parameters to affect the properties of granules as well as to maintain the mechanical stability of granulized catalyst in the system; therefore, the granulation experiments were conducted by varying the ratio of
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Fig. 4. H2-TPR profiles of the (a) CuMn(G)25-15, (b) CuMn(G)400-15, (c) CuMn(G) 500-15 and (d) CuMn(G)600-15 catalysts.
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the inorganic binder content at 10, 15, and 20 wt.%, with a constant microcrystalline cellulose content as an organic binder to simplify the granulation process. To verify the mechanical stability of granulized catalysts, the compressive strength of each catalysts, which is an important factor for the granulation process was evaluated. Because higher mechanical stability could increase the tolerance of the granules to use in diverse processes in the field [38]. Supplementary Fig. S3 shows the compressive strength of spherical-shaped Cu–Mn oxide with different inorganic binder content. As shown in Supplementary Fig. S3, CuMn(G)40020 granules which contains 20 wt.% of inorganic binder showed the highest compressive strength compared to less inorganic binder containing spherical-shaped catalyst granules (i.e., CuMn (G)400-5, CuMn(G)400-10 and CuMn(G)400-15). However, the CuMn(G)400-15 (1.14 kgf/unit) was chosen for the catalytic activity test in this study due to its enough mechanical strength. Even though CuMn(G)400-10 granules (0.6 kgf/unit) which showed the best catalytic activity, this was excluded for further experiments, since they were easily broken during the catalytic combustion experiments. In addition, the CuMn(G)400-20 granules, which showed the highest compressive strength (2.65 kgf/unit) than the other catalysts, were also excluded to minimize the amount of inorganic binder content in the spherical Cu–Mn oxide granules. The catalytic activities of the spherical-shaped Cu–Mn oxide granules with different contents of inorganic binder are shown in Fig. 5 and the BET specific surface areas are shown in Table 1. Granulation followed by calcination at 400 C resulted in no significant difference in the BET specific surface area in terms of the inorganic binder contents (i.e., 10–20 wt.%). As shown in Fig. 5, the CuMn(G)400-10 catalyst with the lowest inorganic binder content showed better catalytic combustion properties than CuMn(G)40015 and CuMn(G)400-20, as confirmed by the T90 determinations (the temperature at which 90% conversion is achieved). The T90 of spherical-shaped catalysts which denoted as CuMn(G)400-10, CuMn(G)400-15 and CuMn(G)400-20 depending on the inorganic binder contents (i.e., 10–20 wt.%) were 228, 231, and 240 C, respectively. Influence of the calcination temperature The effect of the calcination temperature during the granulation process was examined by performing a catalytic activity test for removal benzene with the granulized catalysts treated at different calcination temperatures (i.e., 400, 500, and 600 C). As shown in Fig. 6, CuMn(G)400-15, CuMn(G)500-15, and CuMn(G)600-
Fig. 5. Effect of inorganic binder contents on the catalytic performance of the spherical-shaped Cu–Mn oxide catalysts (GHSV: 10,000 h1).
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Fig. 6. Effect of calcination temperature on the catalytic performance of the spherical-shaped Cu–Mn oxide catalysts (GHSV: 10,000 h1).
15 showed different catalytic activities. CuMn(G)400-15 was the most active for benzene oxidation under 300 C, whereas the catalytic activities of CuMn(G)500-15 and CuMn(G)600-15 were lower. This agreed with the XRD results, which showed that CuMn (G)400-15 maintained its amorphous phase, in contrast to the other catalysts. It also agreed with the data presented in Table 1, where the BET specific surface area of Cu–Mn oxide granules was dramatically decreased with increasing calcination temperature at 500 C. The sintering process induced the crystallization of the amorphous Cu–Mn oxide above 400 C, which led to a decrease in the catalyst activation that accompanied the BET specific surface area [35,46]. The XPS data confirmed a higher concentration of Cu+ and Mn4+ in CuMn(G)500-15 and CuMn(G)600-15, indicating that the catalysts decreased the catalytic activity of the Cu–Mn oxide granule for the catalytic oxidation of benzene. This result also agreed with the TPR curves (Fig. 4) that showed CuMn(G)400-15 to be more reducible than CuMn(G)500-15 and CuMn(G)600-15. The results from the characterization of spherical-shaped Cu– Mn oxide granules and the catalytic combustion experiments confirm that the catalytic activity of amorphous Cu–Mn oxide granules was decreased with changes in their solid structure from amorphous to crystalline. The formation of a Cu1.5Mn1.5O4 spinel phase and the existence of Cu2+ species in the prepared Cu–Mn oxide catalysts play important roles in increasing the activity of Cu –Mn oxide catalysts for benzene oxidation. Therefore, elimination of VOCs will be more effectively achieved using an amorphous state of Cu–Mn oxide, as this is a more active phase than the crystallized Cu–Mn oxide with a CuMn2O4 spinel structure.
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The catalytic behavior of the spherical-shaped Cu–Mn oxide granules was tested for the catalytic combustion of the benzene using three different GHSV values. As shown in Fig. 7, the CuMn(G) 400-15 showed the best catalytic performance for benzene removal at a lower GHSV (10,000 h1). The CuMn(G)400-15 catalyst reached a T90 at 240 C, since the reactant molecules (O2 and benzene) were retained to a greater extent within the pores, thereby favoring oxidation reactions going to completion. On the other hand, at higher GHSV (50,000 h1), the catalyst was unable to achieve 90% conversion, as a consequence of the lower residence time of the molecules within the catalyst framework [9]. As shown in Supplementary Fig. S4, the effect of shaping process was investigated over the CuMn(P), CuMn(G)400-15,
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Fig. 7. Effect of GHSV on the catalytic performance of the CuMn(G)400-15 catalyst (GHSV: 10,000, 25,000, 50,000 h1). 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405
CuMn(G)500-15, and CuMn(G)600-15. As it is expected, the powder type of Cu–Mn oxide are more active than sphericalshaped Cu–Mn oxides. It can be explained that the decrease of BET specific surface area obtained by the sintering of catalysts with granulation and calcination step reduced the catalytic activity of spherical-shaped Cu–Mn oxides. Additionally, the existence of void in the reactor is effected on the catalytic activity of sphericalshaped Cu–Mn oxides. The effect of benzene concentration was further investigated over the CuMn(G)400-15 catalyst at 210 C. Supplementary Fig. S5 shows the benzene conversions over the CuMn(G)40015 catalyst with different benzene concentrations (i.e., 50, 100, 150, 300, and 500 ppm) in the feed gas at fixed temperature of 210 C. 100% conversion of benzene was achieved when benzene concentration was 50 ppm, and benzene conversion rate was decreased as benzene concentration was increased. These results indicate that increasing benzene concentration results in decreasing catalytic activity of CuMn(G)400-15. It can be explained that the site competition between oxygen and hydrocarbon on the catalyst surface effects on the catalytic oxidation of VOCs [47]. Similar results have been extracted by prior research for a variety of catalysts and VOCs [20,48]. Fig. 8 illustrates the results of the stability measurements for the CuMn(G)400-15. The catalytic oxidation of Cu–Mn oxide was
Fig. 8. Stability test for catalytic oxidation of benzene at 270 C over sphericalshaped CuMn(G)400-15 catalyst (benzene conconcentration: 150 ppm, GHSV: 10,000 h1).
repeated 10 times at a benzene concentration of 150 ppm at 270 C for 30 min for each test. The bed height of the catalysts was visually almost unchanged after the reaction. The conversion rate of benzene was maintained at 100% for each test and no decrease in the catalytic activity had occurred after the 10th reaction. Some samples that were reacted with benzene for the 5th and 10th runs underwent BET analysis, which revealed no significant changes in the BET specific surface area (i.e., 5th run: 134 m2/g, 10th run: 134 m2/g). Thus, the amorphous spherical-shaped Cu–Mn oxide granule showed considerable durability under time-on-stream.
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Conclusions
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Spherical-shaped Cu–Mn oxide granules were successfully synthesized by a wet granulation process that only minimal effects on the properties of the Cu–Mn oxide powder. The catalytic activity of the spherical-shaped Cu–Mn oxide granules confirmed that a sample calcined at 400 C was suitable for use as a catalyst for benzene combustion. Spherical-shaped Cu–Mn oxide granules calcined at higher temperatures had lower catalytic activities due to increasing crystallization of MnO2 and the spinel Cu1.5Mn1.5O4, in response to temperature. The spherical-shaped Cu–Mn granules with relatively high concentrations of Cu+ and Mn4+, known to be deactivated species from their redox couples during crystallization, readily lost their catalytic properties with increasing calcination temperatures. In conclusion, control of the BET specific surface area and the amorphous phase without Cu+ and Mn4+ is a key parameter for eliminating benzene by catalytic combustion using Cu–Mn oxide granules. The amorphous Cu–Mn oxide granules also maintained a favorably high conversion during a stability test.
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Acknowledgment
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This subject is supported by the Korean Ministry of Environment (MOE) as “The advancement of scientific research and technological development in environmental science (#2014000110005) program”.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jiec.2016.12.018.
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