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CNTs catalysts by redox co-precipitation and application in low-temperature NO reduction with NH3
Catalysis Communications 62 (2015) 57–61
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Preparation of Mn–FeOx/CNTs catalysts by redox co-precipitation and application in low-temperature NO reduction with NH3 Yanbing Zhang a, Yuying Zheng a,⁎, Xie Wang b, Xiulian Lu a a b
College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China College of Chemistry and Chemical Engineering, AnQing Normal University, Anqing 246000, China
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 23 December 2014 Accepted 24 December 2014 Available online 27 December 2014
a b s t r a c t A novel redox co-precipitation method was firstly adopted to prepare the Mn–FeOx/CNTs catalysts for use in low-temperature NO reduction with NH3. The catalysts were possessed of amorphous structure and exhibited 80–100% NO conversion at 140–180 °C at a high space velocity of 32,000 h−1. © 2014 Elsevier B.V. All rights reserved.
Keywords: Redox co-precipitation Carbon nanotubes Mn–FeOx catalysts Low-temperature SCR
1. Introduction Nitrogen oxide (NOx), as a major air pollutant, is causing a series of environmental issues, such as acid rain, photochemical smog, greenhouse effect and ozone depletion [1,2]. Selective catalytic reduction of NO with NH3 (SCR) is regarded as an efficient technology for abatement of nitrogen oxide from stationary sources [3,4]. However, V2O5 + WO3(MoO3)/TiO2 as a kind of commercial catalyst needs high operating temperature window (300–400 °C) [5,6]. Therefore, development of low-temperature SCR catalysts is necessary due to their potential application in downstream of the desulfurizer and electrostatic precipitator, where the flue gas temperature is usually bellow 200 °C [1,6,7]. Owing to their unique structure property and excellent chemical stability, carbon nanotubes (CNTs) have been widely used as catalyst support material, such as MnOx/CNTs [1,8], Mn-CeOx/CNTs [7,9–11], CeOx/ CNTs [12,13], and VOx/CNTs [14]. The above catalysts have one thing in common: high operating temperature window of 200–300 °C. Therefore, developing low-temperature SCR catalyst (b200 °C) based on CNTs still has some space. It is well-known that preparation method will influence the structure and morphology of the catalyst, and then result in a different catalytic activity [10,15]. Previous studies indicated that Mn–FeOx catalysts were possessed of desirable SCR activity, and the catalysts included NiMn–FeOx [16], Mn–FeOx/TiO2 [17–19], as well as Mn–FeOx [20,21]. ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China. E-mail address: [email protected] (Y. Zheng).
http://dx.doi.org/10.1016/j.catcom.2014.12.023 1566-7367/© 2014 Elsevier B.V. All rights reserved.
However, the preparation methods of these catalysts were either high-temperature calcination treatment or high-pressure hydrothermal reaction. Herein, a novel redox co-precipitation method was employed to fabricate high activity Mn–FeOx/CNTs catalysts, and its reaction mechanism was proposed. As illustrated in Fig. S1, Fe3+ ions were firstly adsorbed on the surface of acid-treated CNTs, and then hydrolyzed to ferric hydroxide [Fe(OH)3] and hydrogen chloride (HCl). After that, HCl was oxidized into Cl2, and Fe(OH)3 was transformed into Fe2O3 by potassium permanganate (KMnO4) solution, respectively. Meanwhile, the KMnO4 was reduced into manganese dioxide (MnO2) on the surface of the CNTs. The as-obtained Mn–FeOx/CNTs catalysts were used for low-temperature SCR of NO with NH3 and presented outstanding SCR activity at 80–180 °C. 2. Experimental 2.1. Catalyst preparation The multiwall CNTs with 40–60 nm in diameter were refluxed with HNO3 (65–68%) under stirring for 4 h at 140 °C in order to remove impurities and introduce oxygen containing functional groups. Then, certain amount of acid-treated CNTs and iron chloride were mixed in 50 ml deionized water and stirred for 8 h. Followed by, 60 ml of potassium permanganate solution was added into the above mixture with vigorous stirring at room temperature for 12 h. Finally, the product was separated by filtration, rinsed with deionized water and ethanol, and dried at 105 °C in air overnight. The as-prepared catalyst via this method is denoted as y Mn–FeOx/CNTs, where y represents the molar ratio of (Mn + Fe)/C.
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2.2. Catalyst characterization X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250 spectrometer equipped with a dual Al/Mg anode with about 0.6 eV resolution at the C1s peak. Transmission electron microscopy (TEM) images were observed using a JEOL model JEM 2010 EX instrument. Field emission scanning electron microscopy (FESEM) images were obtained by ZEISS SUPRA 55. The powder X-ray diffraction (XRD) was performed on an X'Pert Pro MPD X-ray diffractometer using Cu-Kα radiation (λ = 0.15406 nm). The data were recorded for scattering angles (2θ) ranging from 10 to 80° with a 0.02° step size. H2 temperature-programmed reduction (H2-TPR) was performed on a custom-made TCD apparatus. Prior to H2-TPR experiment, 50 mg of catalyst needed to be pretreated in N2 at 200 °C for 1 h. H2-TPR tests were performed in pure N2 containing 6% H2 at a flow rate of 30 ml/min and a heating rate of 10 °C/min. 2.3. Catalytic activity tests The SCR activity tests were carried out in a fixed-bed quartz reactor. 200 mg of catalyst (ca. 1.3 ml) was used in each test. The gas composition consisted of 500 ppm NH3, 500 ppm NO, 5% O2 balanced by N2. The total flow rate was 700 ml/min corresponding to a space velocity (GHSV) of 32,000 h−1. The gas concentration was monitored by a flue gas analyzer (Kane International Limited, KM940). All data were recorded after 30 min when the SCR reaction reached a steady state. 3. Results and discussion 3.1. Catalytic activity NO conversion as a function of temperature for Mn–FeOx/CNTs catalysts fabricated via redox co-precipitation method with different molar ratio of (Mn + Fe)/C (ratio of KMnO4 and FeCl3 to acid-treated CNTs) was shown in Fig. 1. NO conversion of the catalysts was excellent at low temperature and reached 60–70% at 80 °C at a space velocity of 32,000 h−1. With the temperature increased, NO conversion improved significantly and achieved 92–100% at 140 °C. The SCR activity at low temperatures decreased in the following order: 12% Mn–FeOx/ CNTs N 10% Mn–FeOx/CNTs N 8% Mn–FeOx/CNTs N 14% Mn–FeOx/CNTs. It was worth noting that 12% Mn–FeOx/CNTs catalyst exhibited optimal SCR activity in the tested temperature range. Additionally, the catalyst also showed outstanding cyclic stability, and the detailed illustration could be found in electronic supplementary information (ESI).
Fig. 1. NO conversion as a function of temperature for Mn–FeOx/CNTs catalysts fabricated via redox co-precipitation method with different (Mn + Fe)/C molar ratios. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, N2 as balance gas, GHSV = 32,000 h−1, 200 mg sample.
Table 1 The BET surface area and pore volume for raw CNTs, acid-treated CNTs and Mn–FeOx/CNTs catalysts. Sample
SBET (m2 g−1)
Pore volume (cm3 g−1)
Raw CNTs Acid-treated CNTs 8% Mn–FeOx/CNTs 10% Mn–FeOx/CNTs 12% Mn–FeOx/CNTs 12% Mn–FeOx/CNTs (run 3) 14% Mn–FeOx/CNTs
64.9 89.6 136.8 134.4 133.6 138.2 130.3
0.1962 0.2146 0.2467 0.2652 0.2741 0.2863 0.2743
3.2. BET and size distribution The BET surface area and pore volume for raw CNTs, acid-treated CNTs and Mn–FeOx/CNTs catalysts were shown in Table 1. Acidtreated CNTs presented higher surface area and pore volume than that of raw CNTs due to the oxidization of HNO3. After being loaded with the Mn–Fe mixed oxides, the surface area and pore volume of all Mn– FeOx/CNTs catalysts became larger. It should be noted that 12% Mn– FeOx/CNTs was possessed of optimum SCR activity (Fig. 2) but relatively small surface area, implying that surface area was not a decisive factor for SCR activity [22]. The nitrogen adsorption/desorption isotherms of 12% Mn–FeOx/CNTs and 12% Mn–FeOx/CNTs (run 3) as well as the size distribution (inset) plots were illustrated in Fig. S2. Based on the IUPAC classification [23], the adsorption isotherms of the two samples displayed typical type-IV curves with obvious type-H3 hysteresis loops, suggesting the existence of mesopore. The size distribution (inset of Fig. S2) calculated by BJH formula showed that the size of mesopore was between 2.5–5.6 nm.
3.3. XRD, FSEM and TEM XRD patterns of CNTs-based catalysts were displayed in Fig. 2. Only the diffraction of CNTs could be observed from XRD patterns, whereas the crystalline phase corresponding to manganese oxides or iron oxides could not be found. The above results implied that Mn–Fe mixed oxides highly dispersed on the surface of CNTs and presented amorphous
Fig. 2. XRD patterns of CNTs-based catalysts: (a) acid-treated CNTs; (b) 8% Mn–FeOx/ CNTs; (c) 10% Mn–FeOx/CNTs; (d) 12% of Mn–FeOx/CNTs; (e) 12% Mn–FeOx/CNTs (run 3); and (f) 14% Mn–FeOx/CNTs.
Y. Zhang et al. / Catalysis Communications 62 (2015) 57–61
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Fig. 3. FSEM images for (a) acid-treated CNTs and (b) 12% Mn–FeOx/CNTs, and element mapping of 12% Mn–FeOx/CNTs for C (c), O (d), Fe (e) and Mn (f).
phase. In addition, the peak intensity of graphite declined with the increase of the loading of metal oxides, suggesting that the structure of CNTs had been influenced by metal oxides catalysts. FSEM images for acid treated CNTs and 12% Mn–FeOx/CNTs were illustrated in Fig. 3. The surface of acid-treated CNTs (Fig. 3a) was clear.
After being supported by the Mn–Fe mixed oxides, the surface of acidtreated CNTs became rough and was covered by catalysts (Fig. 3b). Besides, the element mapping (Fig. 3c, d, e and f) demonstrated that Mn–Fe mixed oxides catalysts were successfully decorated and well dispersed on the surface of CNTs.
Fig. 4. TEM and HRTEM for acid-treated CNTs and 12% Mn–FeOx/CNTs: (a) acid-treated CNTs; (b) and (c) 12% Mn–FeOx/CNTs; and (d) EDX of 12% Mn–FeOx/CNTs from the red circle region of panel b. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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TEM was applied to further investigate the morphologies of acidtreated CNTs and 12% Mn–FeO x/CNTs. Acid-treated CNTs were possessed of clean external surface (Fig. 4a), while their surface was specked and covered by nanoflake-like metal species after introduction of the Mn–Fe mixed oxide catalysts (Fig. 4b). Besides, the obvious crystal lattice was not found from HRTEM (Fig. 4c), indicating that Mn–Fe mixed oxides with amorphous structure existed on CNTs. This conclusion was in line with the XRD results (Fig. 2), and the amorphous oxides presented higher SCR activity than that of crystal oxides at low temperature [15,24]. Furthermore, the presence of the Mn and Fe elements in EDX spectrum from the red region of Fig. 4b further demonstrated the formation of 12% Mn–FeOx/CNTs catalyst. 3.4. XPS XPS was adopted to obtain the information on the chemical states of the elements and the content in the near surface region. The full spectrum of XPS (Fig. 5a) showed the signals of C, O, Mn and Fe, suggesting the existence of C, O, Mn and Fe elements. The Mn2p core level spectrum (Fig. 5b) of 12% Mn–FeOx/CNTs exhibited two peaks indexing to Mn2p1/2 and Mn2p 3/2, which were centered at the binding energies of 653.85 eV and 642.05 eV, respectively. Additionally, the energy separation was 11.8 eV between Mn2p1/2 and Mn2p 3/2, which was consistent with the previous research data for MnO2 [25,26]. Fe2p spectrum (Fig. 5c) revealed that the binding energies of Fe2p1/2 and Fe2p3/2 for
12% Mn–FeOx/CNTs centered at 724.3 eV and 710.6 eV, respectively. And the binding energy of Fe2p3/2 for 12% Mn–FeOx/CNTs was in good agreement with those reports for the Fe3 + state [27,28]. Besides, the weak shake-up satellite offset from the basic photoelectron lines of Fe2p3/2 by 8.1 eV towards higher binding energy, indicating main formation of Fe3+ ions [28,29]. By performing peak-fitting deconvolutions, the O1s could be divided into three characteristic peaks (Fig. 5d). The low binding energy (529.7 eV) could be indexed to the lattice oxygen (OL) species based on Mn, and the high binding energies (530–534 eV) were assigned to the chemisorbed oxygen species (OS) [10,24] bound to Mn. The atomic ratio of OS/(OS + OL) for 12% Mn–FeOx/CNTs reached 65.7%. Previous studies reported that the chemisorbed oxygen species exhibited higher activity than that of lattice oxygen species due to their higher mobility. Therefore, high content of chemisorbed oxygen species was helpful for NO oxidation to NO2, and then, accelerated the SCR reaction [10]. 3.5. The formation mechanism of the catalyst To further elucidate the novel redox co-precipitation method, the reaction mechanism was proposed. According to the XPS results as well as the formation of chlorine (Cl2) in the preparation procedure, the reaction mechanism could be inferred as follows: the iron chloride (FeCl3) was firstly partially hydrolyzed into Fe(OH)3 and HCl via reversible hydrolysis reaction, and then HCl was oxidized into Cl2 by KMnO4, thus the hydrolysis reaction was accelerated. Meanwhile, the Fe(OH)3
Fig. 5. XPS spectra for 12% Mn–FeOx/CNTs: (a) wide range spectrum; (b) Mn 2p spectrum; (c) Fe 2p spectrum; and (d) O 1s spectrum.
Y. Zhang et al. / Catalysis Communications 62 (2015) 57–61
was transformed into Fe2O3 by KMnO4 and KMnO4 was reduced into MnO2. The reaction equations were listed below: 8 < FeCl3 þ 3H2 O⇌FeðOHÞ3 ↓ þ 3HCl 2KMnO4 þ 2FeðOHÞ3 þ 6HCl ¼ Fe2 O3 ↓ þ 2MnO2 ↓ þ 3Cl2 ↑ : þ 2KOH þ 5H2 O Total reaction : 2KMnO4 þ 2FeCl3 þ H2 O ¼ Fe2 O3 ↓ þ 2MnO2 ↓ þ 3Cl2 ↑ þ 2KOH:
Step reaction :
4. Conclusions In summary, a series of Mn–FeOx/CNTs catalysts were firstly prepared through a novel redox co-precipitation method. The as-obtained catalysts were used for selective catalytic reduction of NO with NH3 and exhibited outstanding low-temperature SCR activity at 80–180 °C at a high space velocity of 32,000 h−1. The 12% Mn–FeOx/CNTs catalyst presented firstrate low-temperature SCR activity and still possessed more than 70% NO conversion at 80 °C after three cycle tests. The amorphous structure, stability of morphology, high content of chemisorbed oxygen and strong reducibility could endow the Mn–FeOx/CNTs catalyst with excellent lowtemperature SCR activity. Acknowledgments This work was supported by the Scientific and Technological Innovation Project of Fujian Province (Grant no. 2012H6008) and the Scientific and Technological Innovation Project of Fuzhou City (Grant no. 2013-G-92). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.12.023.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Preparation of Mn–FeOx/CNTs catalysts by redox co-precipitation and application in low-temperature NO reduction with NH3 Yanbing Zhang a, Yuying Zheng a,⁎, Xie Wang b, Xiulian Lu a a b
College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China College of Chemistry and Chemical Engineering, AnQing Normal University, Anqing 246000, China
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 23 December 2014 Accepted 24 December 2014 Available online 27 December 2014
a b s t r a c t A novel redox co-precipitation method was firstly adopted to prepare the Mn–FeOx/CNTs catalysts for use in low-temperature NO reduction with NH3. The catalysts were possessed of amorphous structure and exhibited 80–100% NO conversion at 140–180 °C at a high space velocity of 32,000 h−1. © 2014 Elsevier B.V. All rights reserved.
Keywords: Redox co-precipitation Carbon nanotubes Mn–FeOx catalysts Low-temperature SCR
1. Introduction Nitrogen oxide (NOx), as a major air pollutant, is causing a series of environmental issues, such as acid rain, photochemical smog, greenhouse effect and ozone depletion [1,2]. Selective catalytic reduction of NO with NH3 (SCR) is regarded as an efficient technology for abatement of nitrogen oxide from stationary sources [3,4]. However, V2O5 + WO3(MoO3)/TiO2 as a kind of commercial catalyst needs high operating temperature window (300–400 °C) [5,6]. Therefore, development of low-temperature SCR catalysts is necessary due to their potential application in downstream of the desulfurizer and electrostatic precipitator, where the flue gas temperature is usually bellow 200 °C [1,6,7]. Owing to their unique structure property and excellent chemical stability, carbon nanotubes (CNTs) have been widely used as catalyst support material, such as MnOx/CNTs [1,8], Mn-CeOx/CNTs [7,9–11], CeOx/ CNTs [12,13], and VOx/CNTs [14]. The above catalysts have one thing in common: high operating temperature window of 200–300 °C. Therefore, developing low-temperature SCR catalyst (b200 °C) based on CNTs still has some space. It is well-known that preparation method will influence the structure and morphology of the catalyst, and then result in a different catalytic activity [10,15]. Previous studies indicated that Mn–FeOx catalysts were possessed of desirable SCR activity, and the catalysts included NiMn–FeOx [16], Mn–FeOx/TiO2 [17–19], as well as Mn–FeOx [20,21]. ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China. E-mail address: [email protected] (Y. Zheng).
http://dx.doi.org/10.1016/j.catcom.2014.12.023 1566-7367/© 2014 Elsevier B.V. All rights reserved.
However, the preparation methods of these catalysts were either high-temperature calcination treatment or high-pressure hydrothermal reaction. Herein, a novel redox co-precipitation method was employed to fabricate high activity Mn–FeOx/CNTs catalysts, and its reaction mechanism was proposed. As illustrated in Fig. S1, Fe3+ ions were firstly adsorbed on the surface of acid-treated CNTs, and then hydrolyzed to ferric hydroxide [Fe(OH)3] and hydrogen chloride (HCl). After that, HCl was oxidized into Cl2, and Fe(OH)3 was transformed into Fe2O3 by potassium permanganate (KMnO4) solution, respectively. Meanwhile, the KMnO4 was reduced into manganese dioxide (MnO2) on the surface of the CNTs. The as-obtained Mn–FeOx/CNTs catalysts were used for low-temperature SCR of NO with NH3 and presented outstanding SCR activity at 80–180 °C. 2. Experimental 2.1. Catalyst preparation The multiwall CNTs with 40–60 nm in diameter were refluxed with HNO3 (65–68%) under stirring for 4 h at 140 °C in order to remove impurities and introduce oxygen containing functional groups. Then, certain amount of acid-treated CNTs and iron chloride were mixed in 50 ml deionized water and stirred for 8 h. Followed by, 60 ml of potassium permanganate solution was added into the above mixture with vigorous stirring at room temperature for 12 h. Finally, the product was separated by filtration, rinsed with deionized water and ethanol, and dried at 105 °C in air overnight. The as-prepared catalyst via this method is denoted as y Mn–FeOx/CNTs, where y represents the molar ratio of (Mn + Fe)/C.
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2.2. Catalyst characterization X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250 spectrometer equipped with a dual Al/Mg anode with about 0.6 eV resolution at the C1s peak. Transmission electron microscopy (TEM) images were observed using a JEOL model JEM 2010 EX instrument. Field emission scanning electron microscopy (FESEM) images were obtained by ZEISS SUPRA 55. The powder X-ray diffraction (XRD) was performed on an X'Pert Pro MPD X-ray diffractometer using Cu-Kα radiation (λ = 0.15406 nm). The data were recorded for scattering angles (2θ) ranging from 10 to 80° with a 0.02° step size. H2 temperature-programmed reduction (H2-TPR) was performed on a custom-made TCD apparatus. Prior to H2-TPR experiment, 50 mg of catalyst needed to be pretreated in N2 at 200 °C for 1 h. H2-TPR tests were performed in pure N2 containing 6% H2 at a flow rate of 30 ml/min and a heating rate of 10 °C/min. 2.3. Catalytic activity tests The SCR activity tests were carried out in a fixed-bed quartz reactor. 200 mg of catalyst (ca. 1.3 ml) was used in each test. The gas composition consisted of 500 ppm NH3, 500 ppm NO, 5% O2 balanced by N2. The total flow rate was 700 ml/min corresponding to a space velocity (GHSV) of 32,000 h−1. The gas concentration was monitored by a flue gas analyzer (Kane International Limited, KM940). All data were recorded after 30 min when the SCR reaction reached a steady state. 3. Results and discussion 3.1. Catalytic activity NO conversion as a function of temperature for Mn–FeOx/CNTs catalysts fabricated via redox co-precipitation method with different molar ratio of (Mn + Fe)/C (ratio of KMnO4 and FeCl3 to acid-treated CNTs) was shown in Fig. 1. NO conversion of the catalysts was excellent at low temperature and reached 60–70% at 80 °C at a space velocity of 32,000 h−1. With the temperature increased, NO conversion improved significantly and achieved 92–100% at 140 °C. The SCR activity at low temperatures decreased in the following order: 12% Mn–FeOx/ CNTs N 10% Mn–FeOx/CNTs N 8% Mn–FeOx/CNTs N 14% Mn–FeOx/CNTs. It was worth noting that 12% Mn–FeOx/CNTs catalyst exhibited optimal SCR activity in the tested temperature range. Additionally, the catalyst also showed outstanding cyclic stability, and the detailed illustration could be found in electronic supplementary information (ESI).
Fig. 1. NO conversion as a function of temperature for Mn–FeOx/CNTs catalysts fabricated via redox co-precipitation method with different (Mn + Fe)/C molar ratios. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, N2 as balance gas, GHSV = 32,000 h−1, 200 mg sample.
Table 1 The BET surface area and pore volume for raw CNTs, acid-treated CNTs and Mn–FeOx/CNTs catalysts. Sample
SBET (m2 g−1)
Pore volume (cm3 g−1)
Raw CNTs Acid-treated CNTs 8% Mn–FeOx/CNTs 10% Mn–FeOx/CNTs 12% Mn–FeOx/CNTs 12% Mn–FeOx/CNTs (run 3) 14% Mn–FeOx/CNTs
64.9 89.6 136.8 134.4 133.6 138.2 130.3
0.1962 0.2146 0.2467 0.2652 0.2741 0.2863 0.2743
3.2. BET and size distribution The BET surface area and pore volume for raw CNTs, acid-treated CNTs and Mn–FeOx/CNTs catalysts were shown in Table 1. Acidtreated CNTs presented higher surface area and pore volume than that of raw CNTs due to the oxidization of HNO3. After being loaded with the Mn–Fe mixed oxides, the surface area and pore volume of all Mn– FeOx/CNTs catalysts became larger. It should be noted that 12% Mn– FeOx/CNTs was possessed of optimum SCR activity (Fig. 2) but relatively small surface area, implying that surface area was not a decisive factor for SCR activity [22]. The nitrogen adsorption/desorption isotherms of 12% Mn–FeOx/CNTs and 12% Mn–FeOx/CNTs (run 3) as well as the size distribution (inset) plots were illustrated in Fig. S2. Based on the IUPAC classification [23], the adsorption isotherms of the two samples displayed typical type-IV curves with obvious type-H3 hysteresis loops, suggesting the existence of mesopore. The size distribution (inset of Fig. S2) calculated by BJH formula showed that the size of mesopore was between 2.5–5.6 nm.
3.3. XRD, FSEM and TEM XRD patterns of CNTs-based catalysts were displayed in Fig. 2. Only the diffraction of CNTs could be observed from XRD patterns, whereas the crystalline phase corresponding to manganese oxides or iron oxides could not be found. The above results implied that Mn–Fe mixed oxides highly dispersed on the surface of CNTs and presented amorphous
Fig. 2. XRD patterns of CNTs-based catalysts: (a) acid-treated CNTs; (b) 8% Mn–FeOx/ CNTs; (c) 10% Mn–FeOx/CNTs; (d) 12% of Mn–FeOx/CNTs; (e) 12% Mn–FeOx/CNTs (run 3); and (f) 14% Mn–FeOx/CNTs.
Y. Zhang et al. / Catalysis Communications 62 (2015) 57–61
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Fig. 3. FSEM images for (a) acid-treated CNTs and (b) 12% Mn–FeOx/CNTs, and element mapping of 12% Mn–FeOx/CNTs for C (c), O (d), Fe (e) and Mn (f).
phase. In addition, the peak intensity of graphite declined with the increase of the loading of metal oxides, suggesting that the structure of CNTs had been influenced by metal oxides catalysts. FSEM images for acid treated CNTs and 12% Mn–FeOx/CNTs were illustrated in Fig. 3. The surface of acid-treated CNTs (Fig. 3a) was clear.
After being supported by the Mn–Fe mixed oxides, the surface of acidtreated CNTs became rough and was covered by catalysts (Fig. 3b). Besides, the element mapping (Fig. 3c, d, e and f) demonstrated that Mn–Fe mixed oxides catalysts were successfully decorated and well dispersed on the surface of CNTs.
Fig. 4. TEM and HRTEM for acid-treated CNTs and 12% Mn–FeOx/CNTs: (a) acid-treated CNTs; (b) and (c) 12% Mn–FeOx/CNTs; and (d) EDX of 12% Mn–FeOx/CNTs from the red circle region of panel b. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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TEM was applied to further investigate the morphologies of acidtreated CNTs and 12% Mn–FeO x/CNTs. Acid-treated CNTs were possessed of clean external surface (Fig. 4a), while their surface was specked and covered by nanoflake-like metal species after introduction of the Mn–Fe mixed oxide catalysts (Fig. 4b). Besides, the obvious crystal lattice was not found from HRTEM (Fig. 4c), indicating that Mn–Fe mixed oxides with amorphous structure existed on CNTs. This conclusion was in line with the XRD results (Fig. 2), and the amorphous oxides presented higher SCR activity than that of crystal oxides at low temperature [15,24]. Furthermore, the presence of the Mn and Fe elements in EDX spectrum from the red region of Fig. 4b further demonstrated the formation of 12% Mn–FeOx/CNTs catalyst. 3.4. XPS XPS was adopted to obtain the information on the chemical states of the elements and the content in the near surface region. The full spectrum of XPS (Fig. 5a) showed the signals of C, O, Mn and Fe, suggesting the existence of C, O, Mn and Fe elements. The Mn2p core level spectrum (Fig. 5b) of 12% Mn–FeOx/CNTs exhibited two peaks indexing to Mn2p1/2 and Mn2p 3/2, which were centered at the binding energies of 653.85 eV and 642.05 eV, respectively. Additionally, the energy separation was 11.8 eV between Mn2p1/2 and Mn2p 3/2, which was consistent with the previous research data for MnO2 [25,26]. Fe2p spectrum (Fig. 5c) revealed that the binding energies of Fe2p1/2 and Fe2p3/2 for
12% Mn–FeOx/CNTs centered at 724.3 eV and 710.6 eV, respectively. And the binding energy of Fe2p3/2 for 12% Mn–FeOx/CNTs was in good agreement with those reports for the Fe3 + state [27,28]. Besides, the weak shake-up satellite offset from the basic photoelectron lines of Fe2p3/2 by 8.1 eV towards higher binding energy, indicating main formation of Fe3+ ions [28,29]. By performing peak-fitting deconvolutions, the O1s could be divided into three characteristic peaks (Fig. 5d). The low binding energy (529.7 eV) could be indexed to the lattice oxygen (OL) species based on Mn, and the high binding energies (530–534 eV) were assigned to the chemisorbed oxygen species (OS) [10,24] bound to Mn. The atomic ratio of OS/(OS + OL) for 12% Mn–FeOx/CNTs reached 65.7%. Previous studies reported that the chemisorbed oxygen species exhibited higher activity than that of lattice oxygen species due to their higher mobility. Therefore, high content of chemisorbed oxygen species was helpful for NO oxidation to NO2, and then, accelerated the SCR reaction [10]. 3.5. The formation mechanism of the catalyst To further elucidate the novel redox co-precipitation method, the reaction mechanism was proposed. According to the XPS results as well as the formation of chlorine (Cl2) in the preparation procedure, the reaction mechanism could be inferred as follows: the iron chloride (FeCl3) was firstly partially hydrolyzed into Fe(OH)3 and HCl via reversible hydrolysis reaction, and then HCl was oxidized into Cl2 by KMnO4, thus the hydrolysis reaction was accelerated. Meanwhile, the Fe(OH)3
Fig. 5. XPS spectra for 12% Mn–FeOx/CNTs: (a) wide range spectrum; (b) Mn 2p spectrum; (c) Fe 2p spectrum; and (d) O 1s spectrum.
Y. Zhang et al. / Catalysis Communications 62 (2015) 57–61
was transformed into Fe2O3 by KMnO4 and KMnO4 was reduced into MnO2. The reaction equations were listed below: 8 < FeCl3 þ 3H2 O⇌FeðOHÞ3 ↓ þ 3HCl 2KMnO4 þ 2FeðOHÞ3 þ 6HCl ¼ Fe2 O3 ↓ þ 2MnO2 ↓ þ 3Cl2 ↑ : þ 2KOH þ 5H2 O Total reaction : 2KMnO4 þ 2FeCl3 þ H2 O ¼ Fe2 O3 ↓ þ 2MnO2 ↓ þ 3Cl2 ↑ þ 2KOH:
Step reaction :
4. Conclusions In summary, a series of Mn–FeOx/CNTs catalysts were firstly prepared through a novel redox co-precipitation method. The as-obtained catalysts were used for selective catalytic reduction of NO with NH3 and exhibited outstanding low-temperature SCR activity at 80–180 °C at a high space velocity of 32,000 h−1. The 12% Mn–FeOx/CNTs catalyst presented firstrate low-temperature SCR activity and still possessed more than 70% NO conversion at 80 °C after three cycle tests. The amorphous structure, stability of morphology, high content of chemisorbed oxygen and strong reducibility could endow the Mn–FeOx/CNTs catalyst with excellent lowtemperature SCR activity. Acknowledgments This work was supported by the Scientific and Technological Innovation Project of Fujian Province (Grant no. 2012H6008) and the Scientific and Technological Innovation Project of Fuzhou City (Grant no. 2013-G-92). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.12.023.
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