Promotion of H3PW12O40 Grafting on NOx Abatement over Î³-Fe2O3

Subscriber access provided by Kaohsiung Medical University

Kinetics, Catalysis, and Reaction Engineering

The promotion of H3PW12O40 grafting on NOx abatement over #-Fe2O3: Performance and reaction mechanism Yang Geng, Shangchao Xiong, Bo Li, Yue Peng, and Shijian Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03087 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

The promotion of H3PW12O40 grafting on NOx abatement over γ-Fe2O3: Performance and reaction mechanism Yang Geng, ┼, ╪ Shangchao Xiong, § Bo Li, ╪ Yue Peng, §, * Shijian Yang ┼, ∗ ┼

Jiangsu Key Laboratory of Anaerobic Biotechnology, School of Environment and Civil

Engineering, Jiangnan University, Wuxi, 214122 P. R. China ╪

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of

Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P. R. China §

State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC),

School of Environment, Tsinghua University, Beijing, 100084 P. R. China

∗

Corresponding author phone: 86-18-066068302; E-mail: [email protected] (S. J. Yang);

[email protected] (Y. Peng). 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In this study, γ-Fe2O3 was grafted with tungstophosphoric acid (i.e., HPW) to improve its SCR performance at high temperatures. To investigate the mechanism of HPW grafting on the SCR activity of γ-Fe2O3, the kinetic parameters of NO reduction over γ-Fe2O3 and those of HPW/Fe2O3-500 were compared. Both the recrystallization of γ-Fe2O3 and the phase transition of spinel to hematite were restrained after HPW grafting, resulting in a higher BET surface area of HPW/Fe2O3-500. Meanwhile, the grafted HPW had a high acid strength, so the adsorption of NH3 on γ-Fe2O3 was promoted remarkably. Furthermore, the C-O reaction over γ-Fe2O3 was inhibited notably after HPW grafting as its oxidation ability decreased. Therefore, HPW/Fe2O3-500 exhibited a superior SCR activity at 250-500 oC, which was suitable for the power plants burning lignite. Keywords: HPW grafting; the SCR reaction; the C-O reaction; Fe2O3; reaction mechanism. .

2


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction NOx, discharged from coal-fired power plants, is the major pollutant in the atmosphere, which will present a serious threat to the environment.1, 2 Now, selective catalytic reduction with NH3 (NH3-SCR) using the catalyst of V2O5-WO3(MoO3)/TiO2 is the most effective technology to abate NOx from coal-fired power plants.3-5 Because of the shortage and high cost of high-rank coal, low-rank coal for example lignite has attracted more attention.6 However, the flue gas temperature of the power plants burning lignite is 50-100 oC higher than those burning high-rank coal.7 The deleterious V species in V2O5-WO3/TiO2 can volatilize from the catalytic convertor at high temperatures. Furthermore, the side reaction (i.e., the non-selective catalytic reaction, NSCR) will happen over V2O5-WO3/TiO2 at high temperatures.8 Therefore, it is necessary to develop an environment-friendly SCR catalyst with a superior performance at high temperatures for the power plants burning lignite. Recently, a lot of Fe-based catalysts, such as Fe/TiO2,9,

10

Fe-SSZ-13,11 Fe2O3-PILC,12

Fe2O3/WO3/ZrO2,13 and Fe-Ti spinel 14, were developed as the SCR catalysts due to their low cost, environment-friend and excellent N2 selectivity. Iron oxides often present under oxidizing atmosphere as either γ-Fe2O3 or α-Fe2O3. γ-Fe2O3 shows an excellent SCR performance at 250-350 oC, which is much better than α-Fe2O3. 7, 15 However, γ-Fe2O3 will transform to α-Fe2O3 above 350 oC.16, 17 Furthermore, the catalytic oxidation of NH3 (i.e., the C-O reaction) will happen over γ-Fe2O3 and α-Fe2O3 above 350 oC, resulting in a poor SCR activity at high temperatures. Ti was once incorporated into γ-Fe2O3 to improve its thermal stability and to inhibit the C-O reaction.14 Although Fe-Ti spinel exhibited a superior SCR performance at 300-400 oC, its temperature window did not fit well with the flue gas temperature of the power plants burning lignite. Tungstophosphoric acid (i.e., HPW, H3PW12O40) has excellent thermal-stability and high acidic strength,18, 19 so it has been applied in many acid-catalyzed reactions. In this study, γ-Fe2O3 was grafted with HPW to broaden its temperature window for the SCR reaction. After HPW grafting, the C-O reaction over γ-Fe2O3 was inhibited remarkably because of the weakening of the oxidation ability. Meanwhile, the negative effect of weakening the oxidation ability on the SCR 3



reaction was compensated by the improvement of NH3 adsorption. As a result, HPW/Fe2O3-500 showed an excellent SCR activity at 250-500 oC, which was suitable for the power plants burning lignite.

2. Experimental 2.1 Sample preparation HPW-500 was obtained from the calcination of HPW (Sinopharm Chemical Reagent Co., Ltd.) in air at 500 °C for 3 h. Fe3O4 (the precursors of Fe2O3) was synthesized by the co-precipitation method.20 Then, Fe2O3-350 and Fe2O3-500 were obtained after the calcination of Fe3O4 in air for 3 h at 350 and 500 oC, respectively. Furthermore, Fe3O4 was soaked in a HPW solution (25 g L-1) for 12 h with the stirring of 500 rpm. After the centrifugation and washing, it was dried at 105 oC for 12 h (i.e., HPW/Fe2O3). At last, HPW/Fe2O3 was calcined at 500 oC for 3 h in air to obtain HPW/Fe2O3-500. 1% V2O5-WO3/TiO2 and Fe-Ti spinel were synthesized as comparison.14, 21 2.2 Catalytic test The SCR reaction, NH3 oxidation and NO oxidation were all performed on a fixed-bed reactor.8, 14

50-200 mg of the catalyst (40-60 mesh) and 100-200 mL min-1 of the gas flow were used for the

catalytic test, leading to the gas hourly space velocity (i.e., GHSV) of 30000 to 240000 cm3 g-1 h-1. The simulated flue gas generally contained 500 ppm NH3 (when used), 500 ppm NO (when used), 5% O2, 8% H2O (when used) and 100 ppm SO2 (when used). The concentrations of NO2, N2O, NH3 and NO were measured online by an infrared gas analyzer of Thermo IGS Analyzer (its measured deviation Fe2O3-500>Fe2O3-350, indicating that their oxidation abilities increased in the opposite sequence. 3.2.6

NH3 adsorption and NO adsorption

The capacities for NH3 and NO adsorption at 50 oC, obtained from the integration of NH3-TPD and NO-TPD (see Figure 9), were shown in Table 2. HPW-500 exhibited an excellent ability for NH3 adsorption and its capacity was 702 µmol g-1. Fe2O3-500’s ability for NH3 adsorption was very poor and its capacity was only 76 µmol g-1. Fe2O3-350 showed a moderate ability for NH3 adsorption and its capacity was 155 µmol g-1. The capacity of HPW/Fe2O3-500 for NH3 adsorption was 287 µmol g-1, which was 3.8 times that of Fe2O3-500. This indicates that NH3 adsorption on Fe2O3-500 was promoted by HPW grafting. HPW-500, Fe2O3-500, and HPW/Fe2O3-500 all showed poor capacities for NO adsorption (see Table 2). However, Fe2O3-350 showed an excellent capacity for NO adsorption with the capacity of 161 µmol g-1. 3.2.7

NH3 oxidation and NO oxidation

Although HPW-500 had an excellent ability for NH3 adsorption, its oxidation ability was very poor. Therefore, the ability of HPW-500 for NH3 oxidation was poor (see Table 3). Table 3 also shows that both Fe2O3-500 and Fe2O3-350 showed excellent abilities for NH3 oxidation. Although the ability of HPW/Fe2O3-500 for NH3 adsorption was better than those of Fe2O3-500 and Fe2O3-350, the oxidation ability of HPW/Fe2O3-500 was less than those of Fe2O3-500 and Fe2O3-350. Therefore, the ability of HPW/Fe2O3-500 for NH3 oxidation was much less than those of Fe2O3-500 and Fe2O3-350. However, N2 selectivity of HPW/Fe2O3-500 for NH3 oxidation was better than those of Fe2O3-500 and Fe2O3-350. Both Fe2O3-350 and Fe2O3-500 showed excellent activities for NO oxidation (see Figure 10). However, HPW/Fe2O3-500 showed a poor activity for NO oxidation, which may be related to its poor ability for NO adsorption and moderate oxidation ability. NO oxidation to NO2 played a key role in the fast-SCR reaction, which facilitated NO reduction.2,

35

Although the ability of

Fe2O3-500 for NO oxidation was generally better than that of HPW/Fe2O3-500, its SCR activity 9



was much worse than that of HPW/Fe2O3-500 (see Figure 1a). These suggest that the fast SCR reaction route may not work during NO reduction over these catalysts. 3.3 Transient reaction study Three characteristic vibrations at 1606, 1420, and 1221 cm-1 appeared on HPW/Fe2O3-500 after NH3 was introduced at 250 oC (see Figure 11a). The band at 1420 cm-1 was attributed to NH4+ on the Brønsted acid sites, while the bands at 1606 and 1221 cm-1 were assigned to coordinated NH3 on the Lewis acid sites.7 Adsorbed NH3 species including both ionic NH4+ and coordinated NH3 gradually diminished as NO+O2 were introduced. This indicates that NO reduction over HPW/Fe2O3-500 may involve the Eley-Rideal mechanism (i.e., gaseous NO reacted with adsorbed NH3 species). At last, a characteristic vibration at 1611 cm-1 corresponding to monodentate nitrite appeared on HPW/Fe2O3-500.7 After NO+O2 were introduced at 250 oC, HPW/Fe2O3-500 was covered by monodentate nitrite (at 1611 cm-1). After NH3 was further introduced, monodentate nitrite rapidly disappeared. This indicates that NO reduction over HPW/Fe2O3-500 may involve the Langmuir-Hinshelwood mechanism (i.e., adsorbed NH3 species reacted with adsorbed monodentate nitrite).

4. Discussion 4.1 The inhibition on both phase transition and re-crystallization The oxidation of magnetite to maghemite involved a reduction of Fe atoms per unit cell from 24 to 64/3.31 Therefore, the outward migration of ferrous cations toward the surface happened during the oxidation of magnetite. To maintain the spinel structure, cation vacancies were created together with the oxidation of Fe2+ to Fe3+ (see Figure 12). However, the structure strain gradually arose with the increase of cation vacancies, which caused to the phase transition of maghemite to hematite at high temperatures.36 During the calcination of HPW/Fe2O3, ferrous cations on magnetite were oxidized by O2 to form a rim of maghemite. Therefore, Fe2+ cannot be observed on HPW/Fe2O3-500. However, XPS analysis shows that there were a lot of Fe2+ cations in HPW/Fe2O3-500. This suggests that the migration of ferrous cations toward the surface during the calcination at 500 oC was inhibited by the grafted HPW. As iron oxides presented in HPW/Fe2O3-500 mainly as Fe3O4 (see Figure 12), 10


Page 10 of 38

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the structure strain, which caused to the phase transition, obviously decreased. Therefore, the phase transition shifted approximately 180 oC to high temperature after HPW grafting. As the outward migration of ferrous cations was blocked by HPW grafting, the re-crystallization of Fe2O3 was inhibited. Therefore, the crystal size of HPW/Fe2O3-500 was much less than that of Fe2O3-500 (see Figure 4), resulting in a higher BET surface of HPW/Fe2O3-500. 4.2 Mechanism of the improvement of the SCR performance of Fe2O3 by HPW grafting According to the reaction kinetic study (see the supporting information), the kinetic parameters of NO reduction over Fe2O3-350, Fe2O3-500, and HPW/Fe2O3-500 were obtained (see Table 4). Equation S31 indicates that kSCR-ER was related to k1 and NH2 concentration. k1 was the reaction constant of Reaction S3, so k1 of HPW/Fe2O3-500 was approximately equal to those of Fe2O3-350 and Fe2O3-500. Therefore, kSCR-ER was mainly dependent on NH2 concentration. Equation S15 indicates that NH2 concentration was related to k3, the concentrations of Fe3+ and NH3 adsorbed on the surface. k3 reflected the catalyst’s ability for NH3 oxidation, so it was mainly dependent on the catalyst’s oxidation ability. H2-TPR analysis hints that the catalysts’ oxidation abilities decreased in the sequence of Fe2O3-350>Fe2O3-500>HPW/Fe2O3-500. This suggests that k3 decreased in the same sequence. Table 1 shows that Fe3+ concentration on HPW/Fe2O3-500 was 35.8%, which was slightly less than those on Fe2O3-350 and Fe2O3-500 (40%). Although the ability of HPW/Fe2O3-500 for NH3 adsorption was better than that of Fe2O3-350 (indicated by NH3-TPD analysis), k3 of Fe2O3-350 was much higher than that of HPW/Fe2O3-500 and Fe3+ concentration on Fe2O3-350 was slightly higher than that on HPW/Fe2O3-500. According to Equation S15, NH2 concentration (i.e., kSCR-ER) on Fe2O3-350 was much higher than that on HPW/Fe2O3-500 (see Table 4). Although k3 of Fe2O3-500 was higher than that of HPW/Fe2O3-500 and Fe3+ concentration on Fe2O3-500 was slightly higher than that on HPW/Fe2O3-500, the amount of NH3 adsorbed on HPW/Fe2O3-500 was at least 3.7 times that on Fe2O3-500 (indicated by NH3-TPD analysis). According to Equation S15, NH2 concentration (i.e., kSCR-ER) on HPW/Fe2O3-500 was higher than that on Fe2O3-500 (see Table 4). Equation S32 suggests that kSCR-LH was related to k2 and NH4NO2 concentration. k2 was the rate constant of NH4NO2 decomposition, so k2 of HPW/Fe2O3-500 was approximately equal to those of 11



Fe2O3-350 and Fe2O3-500. Therefore, kSCR-LH mainly depended on NH4NO2 concentration. Equation S25 indicates that NH4NO2 concentration was mainly related to the concentrations of NH3 and NOx adsorbed. NO-TPD analysis shows that Fe2O3-500 and HPW/Fe2O3-500 both showed poor abilities for NOx adsorption. However, the amount of NH3 adsorbed on HPW/Fe2O3-500 was much larger than that on Fe2O3-500 (see Table 4). Indicated by Equations S25 and S32, kSCR-LH of HPW/Fe2O3-500 was much higher than that of Fe2O3-500 (see Table 4). Although the concentration of NH3 adsorbed on HPW/Fe2O3-500 was higher than that on Fe2O3-350, the concentration of NOx adsorbed on Fe2O3-350 was much higher than that on HPW/Fe2O3-500 (indicated by NO-TPD analysis). According to Equations S25 and S32, kSCR-LH of HPW/Fe2O3-500 was less than that of Fe2O3-350 (see Table 4). Equation S33 suggests that kside was mainly dependent on k4 and the concentrations of NH2 and Fe3+ on the surface. Indicated by Equation S16, k4 reflected the catalyst’s ability for NH2 oxidation. Therefore, k4 may mainly depend on the catalyst’s oxidation ability. As k4, NH2 concentration, and Fe3+ concentration of/on HPW/Fe2O3-500 were all less than those of/on Fe2O3-350, kside of Fe2O3-350 was much higher than that of HPW/Fe2O3-500 (see Table 4). Although NH2 concentration on HPW/Fe2O3-500 was higher than that on Fe2O3-500, k4 of Fe2O3-500 was much higher than that of HPW/Fe2O3-500 and Fe3+ concentration on Fe2O3-500 was slightly higher than that on HPW/Fe2O3-500. Therefore, kside of Fe2O3-500 was higher than that of HPW/Fe2O3-500 (see Table 4). k5 was the reaction constant of Reaction S10, so k5 of HPW/Fe2O3-500 was approximately equal to those of Fe2O3-350 and Fe2O3-500. k6 reflected the catalyst’s ability for NH oxidation (indicated by Equation S18), so it may mainly depend on the catalyst’s oxidation ability. Therefore, the values of k6/k5 increased in the sequence of HPW/Fe2O3-500

Promotion of H3PW12O40 Grafting on NOx Abatement over Î³-Fe2O3

Recommend Documents