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Functional-membrane coated Mn-La-Ce-Ni-Ox catalysts for selective catalytic reduction NO by NH3 at low-temperature
Catalysis Communications 94 (2017) 47–51
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Functional-membrane coated Mn-La-Ce-Ni-Ox catalysts for selective catalytic reduction NO by NH3 at low-temperature Bo Yang a,b, Yuesong Shen a,b, Yun Su b, Peiwen Li c, Yanwei Zeng a, Shubao Shen d, Shemin Zhu a,b,⁎ a
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, Jiangsu 210009, PR China c Department of Aerospace and Mechanical Engineering, The University of Arizona, Tucson, AZ 85721-0119, USA d Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 210009, PR China b
a r t i c l e
i n f o
Article history: Received 29 November 2016 Received in revised form 22 January 2017 Accepted 18 February 2017 Available online 21 February 2017 Keywords: Functional foam coating NH3-SCR Low-temperature Anti-poisoning ability
a b s t r a c t Functional-membrane coated Mn-La-Ce-Ni-Ox catalysts for NH3-SCR at low-temperature were designed and prepared. The catalytic activity for NO removal and the anti-poison ability of catalyst were also investigated and discussed. Result showed that functional-membrane coated Mn-La-Ce-Ni-Ox catalysts had the highest NO removal efficiency of 93% at 180 °C and after injecting 10 vol% H2O and 300 ppm of SO2, the NO removal efficiency still reached about 80%. The functional membrane could not only prevent H2O absorbing on the surface of catalyst and produce sulfur ammonium salt, but also effectively isolate the poison contacting catalyst active sites. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) is one of the major pollutant in the atmosphere. If inhaled NOx directly, it can irritate the lungs, cause respiratory diseases, and lead to permanent changes in the lung as a result. So, it is very harmful to human body. A series of policies and regulations for limiting NOx emissions are being implemented which has driven the development and utilization of a number of NOx control technologies. In these technologies, because of its high efficiency and reliability, selective catalytic reduction (SCR) became the most mature technology for controlling NOx in the industry [1]. The SCR catalyst is significant for SCR technology. At present, the operating temperature of the commercial SCR catalyst is 250–410 °C. However, for those industrial boilers which flue gas temperature is below 250 °C, the commercial SCR catalyst is difficult to meet the needs of such industrial boilers currently and it requires using of an SCR catalyst which can be used at low-temperature [2–4]. Therefore, the key to making low-temperature SCR technology practical application is making SCR catalysts as efficient and stable as possible at low-temperature. Mn-based complex oxides are a hot spot due to its excellent performance for NO removal at low-temperature. Zhihang Chen [5] prepared an Fe3Mn3O8 catalyst using the citric acid method and the molar ratio of ⁎ Corresponding author at: No.5 Xinmofan Road, Nanjing Tech University, College of Materials Science and Engineering, 210009 Nanjing, PR China. E-mail address: [email protected] (S. Zhu).
http://dx.doi.org/10.1016/j.catcom.2017.02.016 1566-7367/© 2017 Elsevier B.V. All rights reserved.
Fe / (Fe + Mn) was 0.4. The results showed that the NO removal efficiency of the Fe3 Mn3 O8 catalyst reached 98.8% at 120 °C. Under these conditions the existence of H2O and SO2 inhibited the removal activity. Roy et al. [6] also prepared a series of Ti0.9Mn0.1O2 − δ and Ti0.9 Mn0.05 Fe 0.05O 2 − δ catalyst by using the solvent combustion method. These catalysts had good performance for NO removal. The NO removal efficiency was over 90% at 140 °C. However, the NO removal efficiency decreased rapidly after injecting SO2 . The SO2 caused the irreversible inactivation on the surface of the catalyst. Haoxi Jiang et al. [7] doped CeO2 into an Mn-based catalyst to improving the stability. The results showed that the MnOx-CeO2 catalyst had a greater rate of diffusion of oxygen and a larger surface area which gives it excellent NO removal performance at low-temperature. However, the MnO x-CeO 2 catalyst is still poisonous under the existence of H2O and SO2. Despite Mn-based catalyst has excellent NO removal performance at low-temperature, but the poisonous effect of SO2 and H2O is a biggest bottleneck for the practical application of low-temperature SCR catalyst. The Polytetrafluoroethene (PTFE) has high strength, toughness and self-lubrication properties. It can be used long-term at a maximum temperature of 250 °C. In addition, it also has excellent performance as a waterproof oil repellent and stain-resistor [8,9]. In our early studies, our research team has doped lanthanum, and nickel oxide in a Mn\\Ce based catalyst and developed the Mn-La-Ce-Ni-Ox catalyst for NH3-SCR of NO at the low-temperature. Our previous work [10] showed that the Mn-La-Ce-Ni-Ox (Mn/La/Ce/Ni = 2.5:2.5:1:1 in mol) catalyst had good
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B. Yang et al. / Catalysis Communications 94 (2017) 47–51
SCR activity at 100–200 °C. In this study, in order to further improve the anti-poison ability of low-temperature SCR catalyst, a membrane coated Mn-La-Ce-Ni-Ox catalyst for NH3-SCR of NO at low-temperature was designed and prepared. A layer of anti-corrosion, abrasion resistance, and water-repellent functional coating (PTFE-based membrane) will be applied to the surface of the catalyst. The structure schematic of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst was presented in Fig. 1. The catalytic activity for NO removal and the anti-poison ability of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst will also be investigated and discussed. 2. Experimental
2.3. Measurement of catalytic activity The SCR activity of catalysts was measured using a fixed bed reactor made of a quartz reactor tube (the diameter of the reaction tube was 45 mm) and a tube temperature programmed furnace (Nabertherm) composition. The test gas was made of simulate flue gas concentration of each component (NO, O2, SO2 and H2O), according to actual working conditions (N2 as balance gas). The gas flow rate was controlled by mass flow meters (HORIBA METRON, S48 32/HMT). The test condition was presented in Table 1. NO concentration before and after the catalytic reaction was detected by using flue gas analyzer (Ecom, Germany), and then the NO conversion rate was calculated according to equation (Eq. 1).
2.1. Catalyst preparation The functional-membrane was made of a layer of PTFE-based foam coating. In order to insure the gas molecules could contact with the active site through the functional coating, the functional-membrane was prepared of a porous structure by adding to pore former (using ethanol as pore former). The solution of PTFE-based foam coating was mixed using hydroxyethyl cellulose (AR, Sinopharm Chemical Group Co., Ltd.), Methyl alcohol Silicon sodium (AR, Sinopharm Chemical Group Co., Ltd.), ethanol (AR, Sinopharm Chemical Group Co., Ltd.) and 3% of polytetrafluoroethylene (Sigma-Aldrich Co., Ltd.) under continuous stirring for 30 min at 60 °C. The Mn-La-Ce-Ni-Ox catalyst was prepared by the thermal decomposition method. The precursor solution was mixed using Mn(NO3)2(AR, Sinopharm Chemical Group Co., Ltd.), La(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.), Ce(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.), Ni(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.) and deionized water under continuous stirring for 20 min at 60 °C. The precursor solution was dried at 120 °C for 2 h and calcined at 550 °C for 6 h. Then, the Mn-La-Ce-Ni-Ox particles were made about 200 mesh powder. In order to simulate the working of the catalyst, catalyst particles were formed as a monolithic honeycomb (30 mm × 30 mm × 120 mm) catalyst with a wall thickness of 1.1 mm and a channel diameter of 9.0 mm by an extruder. The Mn-La-Ce-Ni-O x monolithic honeycomb catalyst was immersed into the PTFE-based solution for 5 min. Then the catalyst was calcined at 350 °C for 5 min and rapidly cooled to room temperature after calcination. Repeating the above steps until the quantity of sulfur functional coating accounted for 1% of mass of the Mn-La-Ce-Ni-Ox catalyst. 2.2. Characterization The Micro-morphology of membrane coated Mn-La-Ce-Ni-Ox catalysts were analyzed by scanning electron microscope(SEM) using a JEOL model JSM-5900. Fourier infrared spectrum was recorded by a Nexus 670 model FT-IR equipment.
XNO ¼
½NOin −½NOout 100% ½NOin
ð1Þ
The N2 selectivity of catalysts was calculated by detecting the concentrations of NO, NH3, NO2 and N2O. And the concentrations of NO, NH3, NO2 and N2O were analyzed by an FTIR spectrometer (Nexus 670, Nicolet, USA), which equipped with a 2 m multiple-path gas cell. The N2 selectivity of catalysts was expressed by the equation (Eq. 2): η¼
½NOin −½NOout þ ½NH3 in −½NH3 out −½NO2 out −2½N2 O 100% ð2Þ ½NOin −½NOout þ ½NH3 in −½NH3 out
3. Results and discussion 3.1. Micro-morphology of the functional-membrane coated Mn-La-Ce-NiOx catalyst The Micro-morphology of the functional-membrane coated Mn-LaCe-Ni-Ox catalyst was presented in Fig. 2. As shown in Fig. 2, there was a coating on the surface of Mn-La-Ce-Ni-Ox catalyst obviously. This coating can prevent the dust from flue gas impacting the catalyst effectively. In addition, due to the use of a porous former, there were lots of pores and channels (diameter was less than 1 μm) existing in the coating. These pores and channels insured the contact between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalysts. 3.2. SCR activity of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst The NO removal efficiency of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst was investigated at different temperatures and presented in Fig. 3. As shown in Fig. 3, the trend of NO removal efficiency over two catalyst was similar and the high activity temperature window was between 140 and 200 °C. The change of NO removal efficiency with temperature could be divided into three stages. At 80–100 °C, the
Table 1 Test condition of catalytic activity for NO.
Fig. 1. Structure schematic of the membrane coated Mn-La-Ce-Ni-Ox catalyst.
Items
Condition
Sample size
Monolithic honeycomb catalyst 30 mm × 30 mm × 120 mm, Wall thickness = 1.1 mm, channel diameter = 9.0 mm 1 5000 h−1 600 ppm 6% 80–240 °C 300 ppm 10 vol%
NH3/NO molar ratio GHSV Inlet NO gas concentration O2 concentration Reaction temperature SO2 concentration H2O concentration
B. Yang et al. / Catalysis Communications 94 (2017) 47–51
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3.3. Effect of H2O and SO2 on the SCR activity over functional-membrane coated Mn-La-Ce-Ni-Ox catalyst
Fig. 2. Micro-morphology of the membrane coated Mn-La-Ce-Ni-Ox catalyst.
NO removal efficiency of the catalyst increased rapidly because catalytic components of catalyst were gradually activated as the reaction temperature increased. The surface energy and chemisorption reaction capacity of the catalyst was increased gradually as well [11]. When at 100– 200 °C, the NO removal efficiency of catalyst was relatively stable. In this stage, the chemical reaction including adsorption and desorption was balanced for NO removal. When the reaction temperature was above 200 °C, the NH3 adsorption capacity of the catalyst gradually decreased and oxidation of NH3 also occurred. Therefore, the NO removal efficiency of catalyst decreased gradually as the temperature was increased. The structure, acidity, redox property and reaction mechanism of Mn-LaCe-Ni-Ox catalyst were discussed in Supplementary Information. Moreover, the highest NO removal efficiency of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst reached 99.3% and 93% respectively. This suggests that the effect of the functional membrane on the catalytic activity of Mn-La-Ce-Ni-Ox was slight. It also confirmed that the pores and channels (presented in Fig. 2) were useful for making a contact between gas molecules and the active site of Mn-LaCe-Ni-Ox catalyst. Fig. 3 also shows the N2 selectivity of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst. As shown in Fig. 3, the effect of the functional membrane on the N2 selectivity was almost none. And the membrane coated Mn-La-Ce-Ni-Ox catalyst exhibited high N2 selectivity. When in the temperature range of 80–110 °C, the N2 selectivity reached 99%. However, with the increase of the temperature, the N2 selectivity decreased slightly to 91% because of the formation of N2O and NO2.
A lot of early studies all showed that the presence of H2O and SO2 would severely affect the catalytic activity of SCR catalysts for NO removal at low-temperature, especially Mn-based catalyst. Fig. 4 shows the effect of H2O and SO2 on the SCR activity over Mn-Ce-Ox, Mn-La-Ce-Ni-Ox and functional-membrane coated Mn-La-Ce-Ni-Ox catalyst. The catalyst was tested at 180 °C and the filtration velocity was 1 m/min and NH3/NO = 1. As shown in Fig. 4, after injecting H2O and SO2, the NO removal efficiency of the Mn-Ce-Ox catalyst decreased significantly. The NO removal efficiency rapidly decreased to 40%. The main reason could be divided into two aspects: on the one hand, the SO2 and H2O could react with NH3 producing a large amount of sulfur ammonium salt (NH4HSO4, (NH3)2SO4), which covered some active sites on Mn-Ce-Ox catalyst surface, resulting in a reduction of the active sites; On the other hand, the H2O, SO2 and NH3 occurred competitive adsorption [12,13], the H2O and SO2 occupied some active sites on the catalyst and hindered the partial catalytic reaction [14,15]. The mechanism was showed in Fig. 5. As it can be seen from the Fourier infrared spectrum of Mn-Ce-Ox catalyst after injecting SO2 and H2O, the −1 vibration spectrums of NH+ ) and SO24 −(1127 cm− 1) 4 (1403 cm were detected [15–17]. It indicated that there was a certain amount of sulfur ammonium salt formed in the surface of Mn-Ce-Ox catalyst. It also proved that sulfur ammonium salt which covered on catalyst surface was the main reason for decreased activity. Compared to Mn-Ce-Ox catalyst, after injecting SO2 and H2O, the NO removal efficiency of the Mn-La-Ce-Ni-Ox catalyst decreased by about 20% to 70%. It was due to the doping of lanthanum and nickel, which could improve the anti-poisoning ability of the Mn-Ce-Ox catalyst to a certain extent by forming special structure NiMnO3 (XRD analysis in Supplementary Information) [10,18,19]. However, the adsorption of NH3 was a significant step for NO conversion on Mn-La-Ce-Ni-Ox catalyst (reaction analysis in Supplementary information), the producing of sulfur ammonium salt and consuming of NH3 also made the decreasing of NO removal efficiency. As shown in Fig. 4, after injecting SO2 and H2O, the NO removal efficiency of the membrane coated Mn-La-Ce-Ni-Ox catalyst was attractive. The NO removal efficiency still reached about 80%. This suggests that the functional membrane coated Mn-La-Ce-Ni-Ox catalyst had excellent anti-posing ability which was due to the excellent waterproof of the functional-membrane (PTFE-based foam coating). The functionalmembrane could prevent the absorption of H2O on the surface of catalyst and the production of sulfur ammonium salt (the SO2 and H2O could easily react with NH3 producing sulfur ammonium salt) effectively. In addition, the functional-membrane could also effectively isolate
Fig. 3. NO removal efficiency and N2-selectivity of Mn-La-Ce-Ni-Ox catalysts and membrane coated Mn-La-Ce-Ni-Ox catalysts.
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B. Yang et al. / Catalysis Communications 94 (2017) 47–51
Fig. 4. Effect of SO2 and H2O on NO removal efficiency and Micro-morphology over Mn-Ce-Ox, Mn-La-Ce-Ni-Ox and membrane coated Mn-La-Ce-Ni-Ox catalysts.
the sulfur ammonium salt (NH4HSO4, (NH3)2SO4) making a contact with the catalyst active sites, thereby slowing these poison substances deposited on the surface of catalyst. However, after injecting H2O and SO2, due to the competitive adsorption of H2O, SO2, and NH3, the NO removal efficiency of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst still decreased about 10%. The mechanism was showed in Fig. 5. The Micro-morphology and EDS analysis of the membrane coated Mn-La-Ce-Ni-Ox catalyst after injecting H2O and SO2 for 10 h were shown in Fig. 4. As shown in SEM micrograph, there was a small amount of poisonous substances deposited on the surface of the functional membrane. As shown in EDS analysis, the poisonous substances were mainly sulfur ammonium salts. It indicated that the sulfur ammonium salt was isolated by functional membrane. Moreover, as it can be seen SEM micrograph, there were still lots of pores and channels existing on the surface of the functional membrane, which could insure the
contact and reaction of gas molecules with the active site of the MnLa-Ce-Ni-Ox catalyst. 4. Conclusion In this study, a membrane coated Mn-La-Ce-Ni-Ox catalyst for selective catalytic reduction NO by NH3 at low-temperature was designed and prepared. The highest NO removal efficiency of the catalyst reached was 93% at 180 °C. Moreover, the membrane coated Mn-La-Ce-Ni-Ox catalyst also had excellent anti-poison ability at low-temperature. The NO removal efficiency still reached about 80% after injecting 10 vol% H2O and 300 ppm SO2 at 180 °C. The excellent anti-poison ability was due to a layer of anti-corrosion, abrasion resistant, and hydrophobic functional-membrane (PTFE-based foam coating) which coated on the surface of catalyst. On the one hand,
Fig. 5. Mechanism of reaction gas action on the surface of catalyst after injecting SO2 and H2O.
B. Yang et al. / Catalysis Communications 94 (2017) 47–51
the functional-membrane had pores and channels, which insured the contact and reaction between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalyst. On the other hand, the functional membrane could not only effectively prevent the absorption of H2O on the surface of the catalyst and the production of sulfur ammonium salt, but also isolate poison such as sulfur ammonium salt and dust contacting with the catalyst active sites, making sure the reaction between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalyst.
References
Acknowledgments
[10]
This project was supported by the National Natural Science Foundation of China (No. 51272105), National Key Research and Development Program of China (2016YFC0205500), China Scholarship Council (CSC, 201608320158). We also thank the project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
[11] [12] [13] [14] [15] [16]
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.02.016.
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[17] [18] [19]
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Functional-membrane coated Mn-La-Ce-Ni-Ox catalysts for selective catalytic reduction NO by NH3 at low-temperature Bo Yang a,b, Yuesong Shen a,b, Yun Su b, Peiwen Li c, Yanwei Zeng a, Shubao Shen d, Shemin Zhu a,b,⁎ a
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, Jiangsu 210009, PR China c Department of Aerospace and Mechanical Engineering, The University of Arizona, Tucson, AZ 85721-0119, USA d Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 210009, PR China b
a r t i c l e
i n f o
Article history: Received 29 November 2016 Received in revised form 22 January 2017 Accepted 18 February 2017 Available online 21 February 2017 Keywords: Functional foam coating NH3-SCR Low-temperature Anti-poisoning ability
a b s t r a c t Functional-membrane coated Mn-La-Ce-Ni-Ox catalysts for NH3-SCR at low-temperature were designed and prepared. The catalytic activity for NO removal and the anti-poison ability of catalyst were also investigated and discussed. Result showed that functional-membrane coated Mn-La-Ce-Ni-Ox catalysts had the highest NO removal efficiency of 93% at 180 °C and after injecting 10 vol% H2O and 300 ppm of SO2, the NO removal efficiency still reached about 80%. The functional membrane could not only prevent H2O absorbing on the surface of catalyst and produce sulfur ammonium salt, but also effectively isolate the poison contacting catalyst active sites. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx) is one of the major pollutant in the atmosphere. If inhaled NOx directly, it can irritate the lungs, cause respiratory diseases, and lead to permanent changes in the lung as a result. So, it is very harmful to human body. A series of policies and regulations for limiting NOx emissions are being implemented which has driven the development and utilization of a number of NOx control technologies. In these technologies, because of its high efficiency and reliability, selective catalytic reduction (SCR) became the most mature technology for controlling NOx in the industry [1]. The SCR catalyst is significant for SCR technology. At present, the operating temperature of the commercial SCR catalyst is 250–410 °C. However, for those industrial boilers which flue gas temperature is below 250 °C, the commercial SCR catalyst is difficult to meet the needs of such industrial boilers currently and it requires using of an SCR catalyst which can be used at low-temperature [2–4]. Therefore, the key to making low-temperature SCR technology practical application is making SCR catalysts as efficient and stable as possible at low-temperature. Mn-based complex oxides are a hot spot due to its excellent performance for NO removal at low-temperature. Zhihang Chen [5] prepared an Fe3Mn3O8 catalyst using the citric acid method and the molar ratio of ⁎ Corresponding author at: No.5 Xinmofan Road, Nanjing Tech University, College of Materials Science and Engineering, 210009 Nanjing, PR China. E-mail address: [email protected] (S. Zhu).
http://dx.doi.org/10.1016/j.catcom.2017.02.016 1566-7367/© 2017 Elsevier B.V. All rights reserved.
Fe / (Fe + Mn) was 0.4. The results showed that the NO removal efficiency of the Fe3 Mn3 O8 catalyst reached 98.8% at 120 °C. Under these conditions the existence of H2O and SO2 inhibited the removal activity. Roy et al. [6] also prepared a series of Ti0.9Mn0.1O2 − δ and Ti0.9 Mn0.05 Fe 0.05O 2 − δ catalyst by using the solvent combustion method. These catalysts had good performance for NO removal. The NO removal efficiency was over 90% at 140 °C. However, the NO removal efficiency decreased rapidly after injecting SO2 . The SO2 caused the irreversible inactivation on the surface of the catalyst. Haoxi Jiang et al. [7] doped CeO2 into an Mn-based catalyst to improving the stability. The results showed that the MnOx-CeO2 catalyst had a greater rate of diffusion of oxygen and a larger surface area which gives it excellent NO removal performance at low-temperature. However, the MnO x-CeO 2 catalyst is still poisonous under the existence of H2O and SO2. Despite Mn-based catalyst has excellent NO removal performance at low-temperature, but the poisonous effect of SO2 and H2O is a biggest bottleneck for the practical application of low-temperature SCR catalyst. The Polytetrafluoroethene (PTFE) has high strength, toughness and self-lubrication properties. It can be used long-term at a maximum temperature of 250 °C. In addition, it also has excellent performance as a waterproof oil repellent and stain-resistor [8,9]. In our early studies, our research team has doped lanthanum, and nickel oxide in a Mn\\Ce based catalyst and developed the Mn-La-Ce-Ni-Ox catalyst for NH3-SCR of NO at the low-temperature. Our previous work [10] showed that the Mn-La-Ce-Ni-Ox (Mn/La/Ce/Ni = 2.5:2.5:1:1 in mol) catalyst had good
48
B. Yang et al. / Catalysis Communications 94 (2017) 47–51
SCR activity at 100–200 °C. In this study, in order to further improve the anti-poison ability of low-temperature SCR catalyst, a membrane coated Mn-La-Ce-Ni-Ox catalyst for NH3-SCR of NO at low-temperature was designed and prepared. A layer of anti-corrosion, abrasion resistance, and water-repellent functional coating (PTFE-based membrane) will be applied to the surface of the catalyst. The structure schematic of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst was presented in Fig. 1. The catalytic activity for NO removal and the anti-poison ability of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst will also be investigated and discussed. 2. Experimental
2.3. Measurement of catalytic activity The SCR activity of catalysts was measured using a fixed bed reactor made of a quartz reactor tube (the diameter of the reaction tube was 45 mm) and a tube temperature programmed furnace (Nabertherm) composition. The test gas was made of simulate flue gas concentration of each component (NO, O2, SO2 and H2O), according to actual working conditions (N2 as balance gas). The gas flow rate was controlled by mass flow meters (HORIBA METRON, S48 32/HMT). The test condition was presented in Table 1. NO concentration before and after the catalytic reaction was detected by using flue gas analyzer (Ecom, Germany), and then the NO conversion rate was calculated according to equation (Eq. 1).
2.1. Catalyst preparation The functional-membrane was made of a layer of PTFE-based foam coating. In order to insure the gas molecules could contact with the active site through the functional coating, the functional-membrane was prepared of a porous structure by adding to pore former (using ethanol as pore former). The solution of PTFE-based foam coating was mixed using hydroxyethyl cellulose (AR, Sinopharm Chemical Group Co., Ltd.), Methyl alcohol Silicon sodium (AR, Sinopharm Chemical Group Co., Ltd.), ethanol (AR, Sinopharm Chemical Group Co., Ltd.) and 3% of polytetrafluoroethylene (Sigma-Aldrich Co., Ltd.) under continuous stirring for 30 min at 60 °C. The Mn-La-Ce-Ni-Ox catalyst was prepared by the thermal decomposition method. The precursor solution was mixed using Mn(NO3)2(AR, Sinopharm Chemical Group Co., Ltd.), La(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.), Ce(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.), Ni(NO3)2·6H2O (AR, Sinopharm Chemical Group Co., Ltd.) and deionized water under continuous stirring for 20 min at 60 °C. The precursor solution was dried at 120 °C for 2 h and calcined at 550 °C for 6 h. Then, the Mn-La-Ce-Ni-Ox particles were made about 200 mesh powder. In order to simulate the working of the catalyst, catalyst particles were formed as a monolithic honeycomb (30 mm × 30 mm × 120 mm) catalyst with a wall thickness of 1.1 mm and a channel diameter of 9.0 mm by an extruder. The Mn-La-Ce-Ni-O x monolithic honeycomb catalyst was immersed into the PTFE-based solution for 5 min. Then the catalyst was calcined at 350 °C for 5 min and rapidly cooled to room temperature after calcination. Repeating the above steps until the quantity of sulfur functional coating accounted for 1% of mass of the Mn-La-Ce-Ni-Ox catalyst. 2.2. Characterization The Micro-morphology of membrane coated Mn-La-Ce-Ni-Ox catalysts were analyzed by scanning electron microscope(SEM) using a JEOL model JSM-5900. Fourier infrared spectrum was recorded by a Nexus 670 model FT-IR equipment.
XNO ¼
½NOin −½NOout 100% ½NOin
ð1Þ
The N2 selectivity of catalysts was calculated by detecting the concentrations of NO, NH3, NO2 and N2O. And the concentrations of NO, NH3, NO2 and N2O were analyzed by an FTIR spectrometer (Nexus 670, Nicolet, USA), which equipped with a 2 m multiple-path gas cell. The N2 selectivity of catalysts was expressed by the equation (Eq. 2): η¼
½NOin −½NOout þ ½NH3 in −½NH3 out −½NO2 out −2½N2 O 100% ð2Þ ½NOin −½NOout þ ½NH3 in −½NH3 out
3. Results and discussion 3.1. Micro-morphology of the functional-membrane coated Mn-La-Ce-NiOx catalyst The Micro-morphology of the functional-membrane coated Mn-LaCe-Ni-Ox catalyst was presented in Fig. 2. As shown in Fig. 2, there was a coating on the surface of Mn-La-Ce-Ni-Ox catalyst obviously. This coating can prevent the dust from flue gas impacting the catalyst effectively. In addition, due to the use of a porous former, there were lots of pores and channels (diameter was less than 1 μm) existing in the coating. These pores and channels insured the contact between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalysts. 3.2. SCR activity of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst The NO removal efficiency of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst was investigated at different temperatures and presented in Fig. 3. As shown in Fig. 3, the trend of NO removal efficiency over two catalyst was similar and the high activity temperature window was between 140 and 200 °C. The change of NO removal efficiency with temperature could be divided into three stages. At 80–100 °C, the
Table 1 Test condition of catalytic activity for NO.
Fig. 1. Structure schematic of the membrane coated Mn-La-Ce-Ni-Ox catalyst.
Items
Condition
Sample size
Monolithic honeycomb catalyst 30 mm × 30 mm × 120 mm, Wall thickness = 1.1 mm, channel diameter = 9.0 mm 1 5000 h−1 600 ppm 6% 80–240 °C 300 ppm 10 vol%
NH3/NO molar ratio GHSV Inlet NO gas concentration O2 concentration Reaction temperature SO2 concentration H2O concentration
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3.3. Effect of H2O and SO2 on the SCR activity over functional-membrane coated Mn-La-Ce-Ni-Ox catalyst
Fig. 2. Micro-morphology of the membrane coated Mn-La-Ce-Ni-Ox catalyst.
NO removal efficiency of the catalyst increased rapidly because catalytic components of catalyst were gradually activated as the reaction temperature increased. The surface energy and chemisorption reaction capacity of the catalyst was increased gradually as well [11]. When at 100– 200 °C, the NO removal efficiency of catalyst was relatively stable. In this stage, the chemical reaction including adsorption and desorption was balanced for NO removal. When the reaction temperature was above 200 °C, the NH3 adsorption capacity of the catalyst gradually decreased and oxidation of NH3 also occurred. Therefore, the NO removal efficiency of catalyst decreased gradually as the temperature was increased. The structure, acidity, redox property and reaction mechanism of Mn-LaCe-Ni-Ox catalyst were discussed in Supplementary Information. Moreover, the highest NO removal efficiency of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst reached 99.3% and 93% respectively. This suggests that the effect of the functional membrane on the catalytic activity of Mn-La-Ce-Ni-Ox was slight. It also confirmed that the pores and channels (presented in Fig. 2) were useful for making a contact between gas molecules and the active site of Mn-LaCe-Ni-Ox catalyst. Fig. 3 also shows the N2 selectivity of Mn-La-Ce-Ni-Ox catalyst and membrane coated Mn-La-Ce-Ni-Ox catalyst. As shown in Fig. 3, the effect of the functional membrane on the N2 selectivity was almost none. And the membrane coated Mn-La-Ce-Ni-Ox catalyst exhibited high N2 selectivity. When in the temperature range of 80–110 °C, the N2 selectivity reached 99%. However, with the increase of the temperature, the N2 selectivity decreased slightly to 91% because of the formation of N2O and NO2.
A lot of early studies all showed that the presence of H2O and SO2 would severely affect the catalytic activity of SCR catalysts for NO removal at low-temperature, especially Mn-based catalyst. Fig. 4 shows the effect of H2O and SO2 on the SCR activity over Mn-Ce-Ox, Mn-La-Ce-Ni-Ox and functional-membrane coated Mn-La-Ce-Ni-Ox catalyst. The catalyst was tested at 180 °C and the filtration velocity was 1 m/min and NH3/NO = 1. As shown in Fig. 4, after injecting H2O and SO2, the NO removal efficiency of the Mn-Ce-Ox catalyst decreased significantly. The NO removal efficiency rapidly decreased to 40%. The main reason could be divided into two aspects: on the one hand, the SO2 and H2O could react with NH3 producing a large amount of sulfur ammonium salt (NH4HSO4, (NH3)2SO4), which covered some active sites on Mn-Ce-Ox catalyst surface, resulting in a reduction of the active sites; On the other hand, the H2O, SO2 and NH3 occurred competitive adsorption [12,13], the H2O and SO2 occupied some active sites on the catalyst and hindered the partial catalytic reaction [14,15]. The mechanism was showed in Fig. 5. As it can be seen from the Fourier infrared spectrum of Mn-Ce-Ox catalyst after injecting SO2 and H2O, the −1 vibration spectrums of NH+ ) and SO24 −(1127 cm− 1) 4 (1403 cm were detected [15–17]. It indicated that there was a certain amount of sulfur ammonium salt formed in the surface of Mn-Ce-Ox catalyst. It also proved that sulfur ammonium salt which covered on catalyst surface was the main reason for decreased activity. Compared to Mn-Ce-Ox catalyst, after injecting SO2 and H2O, the NO removal efficiency of the Mn-La-Ce-Ni-Ox catalyst decreased by about 20% to 70%. It was due to the doping of lanthanum and nickel, which could improve the anti-poisoning ability of the Mn-Ce-Ox catalyst to a certain extent by forming special structure NiMnO3 (XRD analysis in Supplementary Information) [10,18,19]. However, the adsorption of NH3 was a significant step for NO conversion on Mn-La-Ce-Ni-Ox catalyst (reaction analysis in Supplementary information), the producing of sulfur ammonium salt and consuming of NH3 also made the decreasing of NO removal efficiency. As shown in Fig. 4, after injecting SO2 and H2O, the NO removal efficiency of the membrane coated Mn-La-Ce-Ni-Ox catalyst was attractive. The NO removal efficiency still reached about 80%. This suggests that the functional membrane coated Mn-La-Ce-Ni-Ox catalyst had excellent anti-posing ability which was due to the excellent waterproof of the functional-membrane (PTFE-based foam coating). The functionalmembrane could prevent the absorption of H2O on the surface of catalyst and the production of sulfur ammonium salt (the SO2 and H2O could easily react with NH3 producing sulfur ammonium salt) effectively. In addition, the functional-membrane could also effectively isolate
Fig. 3. NO removal efficiency and N2-selectivity of Mn-La-Ce-Ni-Ox catalysts and membrane coated Mn-La-Ce-Ni-Ox catalysts.
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B. Yang et al. / Catalysis Communications 94 (2017) 47–51
Fig. 4. Effect of SO2 and H2O on NO removal efficiency and Micro-morphology over Mn-Ce-Ox, Mn-La-Ce-Ni-Ox and membrane coated Mn-La-Ce-Ni-Ox catalysts.
the sulfur ammonium salt (NH4HSO4, (NH3)2SO4) making a contact with the catalyst active sites, thereby slowing these poison substances deposited on the surface of catalyst. However, after injecting H2O and SO2, due to the competitive adsorption of H2O, SO2, and NH3, the NO removal efficiency of the functional-membrane coated Mn-La-Ce-Ni-Ox catalyst still decreased about 10%. The mechanism was showed in Fig. 5. The Micro-morphology and EDS analysis of the membrane coated Mn-La-Ce-Ni-Ox catalyst after injecting H2O and SO2 for 10 h were shown in Fig. 4. As shown in SEM micrograph, there was a small amount of poisonous substances deposited on the surface of the functional membrane. As shown in EDS analysis, the poisonous substances were mainly sulfur ammonium salts. It indicated that the sulfur ammonium salt was isolated by functional membrane. Moreover, as it can be seen SEM micrograph, there were still lots of pores and channels existing on the surface of the functional membrane, which could insure the
contact and reaction of gas molecules with the active site of the MnLa-Ce-Ni-Ox catalyst. 4. Conclusion In this study, a membrane coated Mn-La-Ce-Ni-Ox catalyst for selective catalytic reduction NO by NH3 at low-temperature was designed and prepared. The highest NO removal efficiency of the catalyst reached was 93% at 180 °C. Moreover, the membrane coated Mn-La-Ce-Ni-Ox catalyst also had excellent anti-poison ability at low-temperature. The NO removal efficiency still reached about 80% after injecting 10 vol% H2O and 300 ppm SO2 at 180 °C. The excellent anti-poison ability was due to a layer of anti-corrosion, abrasion resistant, and hydrophobic functional-membrane (PTFE-based foam coating) which coated on the surface of catalyst. On the one hand,
Fig. 5. Mechanism of reaction gas action on the surface of catalyst after injecting SO2 and H2O.
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the functional-membrane had pores and channels, which insured the contact and reaction between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalyst. On the other hand, the functional membrane could not only effectively prevent the absorption of H2O on the surface of the catalyst and the production of sulfur ammonium salt, but also isolate poison such as sulfur ammonium salt and dust contacting with the catalyst active sites, making sure the reaction between gas molecules with the active site of Mn-La-Ce-Ni-Ox catalyst.
References
Acknowledgments
[10]
This project was supported by the National Natural Science Foundation of China (No. 51272105), National Key Research and Development Program of China (2016YFC0205500), China Scholarship Council (CSC, 201608320158). We also thank the project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
[11] [12] [13] [14] [15] [16]
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.02.016.
[1] [2] [3] [4] [5] [6] [7] [8] [9]
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