Reaction Mechanism of Low-Temperature Selective Catalytic

Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Reaction Mechanism of Low-Temperature Selective Catalytic Reduction of NOx over Fe-Mn Oxides Supported on Fly Ash-Derived SBA-15 Molecular Sieves: Structure-Activity Relationships and In Situ DRIFTs Analysis Ge Li, Baodong Wang, Zhencui Wang, Zenghe Li, Qi Sun, Wayne Qiang Xu, and Yonglong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03135 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 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 57 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 Journal of Physical Chemistry

Reaction Mechanism of Low-Temperature Selective Catalytic Reduction of NOx over Fe-Mn Oxides Supported on Fly Ash-Derived SBA-15 Molecular Sieves: Structure-Activity Relationships and In Situ DRIFTs Analysis Ge Li a,†, Baodong Wang a,* ,†, Zhencui Wangb,*, Zenghe Lic, Qi Sun a,*, Wayne Qiang Xu a,*, Yonglong Lia a b

National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China

Laboratory of Environmental Monitoring, College of Tropical and Laboratory Medicine, Hainan Medical University, Haikou 571101, China c

College of Science, Beijing University of Chemical Technology, Beijing 100029, China

*Corresponding author. E-mail address: [email protected] (B.D.Wang); [email protected] ; [email protected] (Q. Sun). [email protected](W.Q. Xu) † These authors contributed equally to this work.

Abstract: Fly ash emissions caused by coal combustion have been increasing for many years, causing serious environmental pollution. Coal combustion also causes large amounts of NOx to be emitted to the atmosphere, and this has caused environmental problems such as acid rain, which cannot be ignored. The denitrification catalyst V2O5/WO3-TiO2 gives a good denitrification efficiency at a high temperature but the catalyst gives a poor efficiency and is difficult to use at low temperatures(100-300℃). Therefore, in this paper we introduce a new method based on the use of fly ash to control NOx output. We used a two-step alkali hydrothermal method to prepare SBA-15 mesoporous molecular sieves from fly ash obtained from a thermal power plant in Inner Mongolia 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Page 2 of 57

(China). A series of bimetallic iron and manganese oxides were supported on the fly ash-derived SBA-15 catalyst and excellent NO conversion was found for selective catalytic reduction (SCR) of NO with NH3 at low temperatures. The catalysts were characterized by: XRD; XPS; NH3-, O2-, and CO2- TPD; H2-TPR, BET analysis, SEM, TEM, and DRIFT spectroscopy. The denitration activity and denitration mechanism over the catalysts is discussed. The mechanisms of NO reduction and N2O formation over Mn/SBA-15 and Fe-Mn/SBA-15 were investigated through in situ DRIFT studies and a transient reaction study. The strong oxidation, low acidity, and high basicity of the Fe-Mn/SBA-15 catalyst contributed to a large amount of nitrate being produced during the catalysis. The nitrate decomposed to produce N2O, resulting in a decrease in N2 selectivity.

The denitration

mechanism of the Fe-Mn/SBA-15 catalyst in the SCR reaction followed Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen mechanisms.

1．．Introduction The large amount of solid waste discharged from coal-fired power plants, in the form of fly ash, has attracted global attention in terms of addressing its handling and use. As coal consumption increases, fly ash discharged from coal-fired power plants is becoming a major source of industrial solid waste. Every 1 ton of coal combusted produces 250-300 kg of fly ash and 20-30 kg of slag 1.

Annual emissions in China have reached over 600 million tons and it is estimated that the total amount of fly ash accumulated by 2020 will reach more than 3 billion tons. This massive accumulation of fly ash occupies space and poses a serious threat to the environment; thus, it is imperative to find new ways of using fly ash resources. Among various approaches to practically applying fly ash, one is to directly use the raw material in engineering and agriculture, for example, as a precursor for building materials or for soil improvement

2,3

. These approaches do not

effectively utilize high-value resources in fly ash. The main components of SiO2 and Al2O3 in fly ash make up 60%–80% of the total composition. The compositions of fly ash and molecular sieves are very similar, which makes it possible to convert fly ash into molecular sieves. The use of fly ash as a raw material to produce molecular sieves can reduce the reliance on expensive fine-chemical precursors for the production of synthetic molecular sieves and broadens the range of applications of fly ash. To date, fly ash has been used as a raw material for the synthesis of molecular sieves such as: 2


Page 3 of 57 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


3A, 4A, 5A, 13X, Y, P, ZSM-5 and other microporous molecular sieves, and MCM-41, SBA-15 mesoporous molecular sieves

4-6

. SBA-15 features highly ordered two-dimensional hexagonal

channels with a specific surface area in the range of 500–1000 m2/g, has been widely used in catalyst support. However, pure silica-SBA-15 has few lattice defects in its framework and no ion-exchange activity or surface-free acidic sites. Furthermore, its redox properties and weak catalytic selectivity limit the usefulness of pure silica SBA-15 in catalytic fields. Therefore, loading of active components and doping of metal ions into the framework have been used to induce active centers into SBA-15 molecular sieves. The big specific surface area, controllable pore size and rich silanols on surface of SBA-15, enable the active metal oxide to be well dispersed on the surface and in the pores of the molecular sieves. Modified SBA-15 molecular sieves have a thermal stability greater than 900°C and good hydrothermal stability7,8. At present, there have been many reports of the introduction of metals, such as Al, Fe, Ti, Ga, and Mn, into SBA-15 mesoporous materials. However, most of these studies have focused on metal ions supported on SBA-15 molecular sieves that were prepared from pure chemical reagents (sodium silicate or ethyl orthosilicate (TEOS) )9-11. TEOS is expensive, which makes it unsuitable for large-scale industrial production. We have previously reported a successfully low-energy synthesis of fly ash-derived SBA-15 mesoporous molecular sieves 12,13. Coal combustion not only produces fly ash, but also produces nitrogen oxides, carbon oxides, sulfur dioxide and other contaminants. NO and NO2 are collectively referred to as nitrogen oxides (NOx). NOx emissions from coal combustion accounts for 67% of national emissions in China. Nitrogen oxides are the most harmful pollutants in the atmosphere and can cause acid rain, photochemical smog and environmental problems. It is estimated that NOx emissions from coal-fired power plants reached 6.2 million tons in 2016 in China. The control and management of nitrogen oxide pollution is also a topic of interest in terms of global environmental protection. At present, ammonia-selective catalytic reduction (NH3-SCR) technology is mature, as its core SCR catalyst has been extensively studied. The V2O5-WO3/TiO2 system is a mature commercial catalyst, with an optimal activity in the temperature range of 320–450 °C. However, the catalyst cannot achieve ideal denitration performance at low temperature( Cr > Co > Fe > V > Ni, indicating that Mn maintains its high catalytic activity in the presence of water vapor (H2O). In our previous research, Fe and/or Mn/SBA-15 catalysts were prepared by the impregnation method using low-cost fly ash-derived SBA-15 molecular sieve as support and tested towards NH3-SCR. The Fe-Mn/SBA-15 catalyst exhibited significantly high NH3-SCR activities at 150–250 °C 13. In this research, based on the previous studies, we continue to study the mechanism of NOx reduction and N2O production in Fe-Mn/SBA-15 system. To date, differences in the reactivity of different interfaces have been discerned and conclusions have been drawn about the mechanism of reaction. The Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms occur in the majority of MnOx-catalyzed NH3-SCR reactions, as reported in previous studies

24-29

. Moreover,

MnO2-based catalysts exchange with O2 in the gas phase more easily at low temperatures. Kapteijn et al.

24

found that O2 can be oxidized and activate the reduced catalyst to maintain its

stability. This finding indicates that the Mars-van Krevelen mechanism can coexist with the E–R and L–H mechanisms. In existing mechanistic studies, there have been few reports based on the Mn-Fe/SBA-15 catalyst. Because both SBA-15 and MnOx have no Bronsted acidic sites and MnOx is strongly oxidative, there is a need to better understand the influence of the acidity and redox activity of the Mn-Fe/SBA-15 catalyst on the denitrification process. Hence, in this paper, the Fe-Mn/fly ash-derived SBA-15 catalysts were applied to the catalytic denitration of nitrogen oxides in coal-fired flue gas. Through optimization of the 4


Page 5 of 57 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


synthesis, surface treatments, and synergistic effects to the molecular sieves we improved the redox performance, adsorption properties, and selectivity of SBA-15 molecular sieve catalyst. We achieved highly efficient gas adsorption and selective catalysis reduction of nitrogen oxides. Furthermore, a possible redox reaction mechanism and N2O formation pathway in the Fe-Mn/SBA-15 catalysts was proposed based on analysis of: scanning electron microscopy (SEM); transmission electron microscopy (TEM); Brunauer–Emmett–Teller (BET) analysis; x-ray diffraction (XRD); NH3-, O2-, CO2-, and H2- temperature programmed reduction (TPR); x-ray photoelectron spectroscopy (XPS); and in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.

2．．Experimental 2.1. Raw materials and chemicals A high-alumina fly ash formed in the thermal power plant of Inner Mongolia (China) was used as the raw material for SBA-15 and had the following chemical composition: 53.17% SiO2, 36.86% Al2O3, 2.58% Fe2O3, 2.48% TiO2, and 2.27% CaO. Details of the preparation of fly ash-derived SBA-15 mesoporous molecular sieve were previously reported 12. The fly ash-derived SBA-15 had the following final chemical composition: 99.34% SiO2, 0.625% Fe, 0.014% Ca and 0.051% Ti. The chemical reagents of iron nitrate and manganese nitrate (analytical grade) used for the deposition of Fe and Mn, respectively, in SBA-15 were

supplied by Sinopharm Chemical

Reagent Co., Ltd (Beijing, China), where that of ethanol (analytical grade) was provided by Beijing Enoch Technology Co., Ltd (China).

2.2 Catalyst preparation Details of the preparation of Fe-Mn/SBA-15 mesoporous molecular sieve catalysts were previously reported 13.

2.3 Characterization methods 5



Page 6 of 57

Powder X-ray diffraction, Scanning Election Microscopy, High-resolution transmission electron microscopy, N2 adsorption-desorption measurements, XPS, H2-TPR, NH3-, CO2- and O2temperature-programmed desorption experiments, In-situ diffuse reflectance infrared Fourier transform spectroscopy techniques were used to investigate the morphology and bulk/surface crystal structure and chemical composition of Fe-Mn/SBA-15. Details of all the above mentioned characterization methods are provided in Supporting Information.

2.4 SCR activity measurements The SCR reaction was evaluated in a fixed-bed reactor. 0.3g sample was put in a reaction tube and placed for 2 h in simulated flue gas. The gas mixture contained 300 ppm NO, 300 ppm NH3, 3% O2, with N2 used as balance gas using a GHSV of ~ 120,000 h−1. Catalyst activity was recorded by measuring the inlet and outlet gas using a flue gas analyzer (MultiGas™ 6030, MKS) at temperatures between 100 and 300 °C. The conversion was calculated according to the following equation(1) and (2): NOx conversion = [([NOx]in-[NOx]out)/ [NOx]in] ×100%

N2 selectivity = 1 −

× 100%

(1) (2)

where NOx is the sum of NO and NO2 concentrations, [NOx]out was the concentration of outlet NOx, [NOx]in was the concentration of inlet NOx, [NH3]out was the concentration of outlet NH3, [NH3]in was the concentration of inlet NH3 and [N2O]out was the concentration of outlet N2O.

3．．Results and discussion 3.1 Characterization of the catalyst support SBA-15 molecular sieve obtained had a highly ordered mesoporous structure, uniform pore size distribution, and regular pores. The BET surface area of the prepared fly ash-derived SBA-15 molecular sieve was 793.59 m2/g, the pore volume was 0.748 cm3/g, and the average pore diameter was 6.11 nm.The SBA-15 molecular sieve were regular and hexagonal, and the skeleton was orderly arranged, the pore diameter was approximately 5 nm, and the thickness of the pore wall was approximately 3 nm. Details of all the characterization of the SBA-15 catalyst support 6


Page 7 of 57 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


are provided in Supporting Information.

3.2 Activity testing for NH3-SCR

Figure 1. NH3-SCR performance of Fe-Mn/SBA-15 catalysts with different molar ratios and different total metal loadings.

7



Page 8 of 57

Figure 2. N2 selectivity of Fe-Mn/SBA-15 catalysts with different molar ratios and different total metal loadings.

The SCR activity results of all the studied catalysts are presented in Figure 1. When the Mn/Fe molar ratio was increased from 1 to 5, the percentage conversion of NO increased (Figure 1a). When the Mn/Fe molar ratio was 1, in general, the conversion of NO increased as the total content of the transition metals was increased (Figure 1b); however, for 13.44Fe13.2Mn/SBA-15, the conversion of NO was low, owing to metal oxides blocking the channels of the molecular sieve channels, which inhibited gas diffusion. In addition, Figure 1a and Figure 1b, show that the NO conversion rate of each sample first increased and then decreased as the reaction temperature was increased. The highest catalyst activity was obtained at 250 °C. This result shows that the 11.2Fe11Mn/SBA-15 catalyst has better catalytic activity in the low temperature range. To study the synergistic effects of Fe and Mn, the SCR activity of 22Mn/SBA-15(denoted as Mn/SBA-15),

22.4Fe/SBA-15(denoted

as

Fe/SBA-15)

and

those

of

commercial

0.6V2O5-WO3/TiO2 catalysts were compared (Figure 1c). As shown in Figure 1c, we found that the SCR activity of the 0.6V2O5-WO3/TiO2 catalyst was less than 20% at temperatures below 250 °C. Commercial V2O5-WO3/TiO2 catalysts are efficient at high operating temperatures 8


Page 9 of 57 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


(300−400 °C), which makes it necessary to locate the SCR unit upstream of the electrostatic precipitator. Such conditions are not suitable for low temperature denitration. Moreover, the Mn/SBA-15 samples had lower NOx removal efficiency over the studied temperature window. The addition of Fe clearly increased the NOx removal efficiency and expanded the operating temperature window. In the low-temperature reaction region (100–350°C), the NOx conversion of 11.2Fe11Mn/SBA-15(denoted as Fe-Mn/SBA-15) was considerably higher than those of Mn/SBA-15 and Fe/SBA-15, indicating the strong synergistic effect of Fe and Mn on the catalytic performance of the 11.2Fe11Mn/SBA-15 catalyst. When the temperature was less than 250°C, NOx conversion of Mn/SBA-15 was higher than that of Fe/SBA-15. However, the opposite trend was observed at temperatures higher than 250°C. These results indicate that MnOx plays a major role in NOx conversion at low temperatures (100–250°C), whereas FexOy played a more important role at high temperatures (>250°C), which is consistent with previous reports 17,19,22. Furthermore, Figure 1c shows that the synergistic effect of Fe and Mn was most pronounced (>90% NOx conversion) over the temperature range of 200–250°C. To understand the N2 selectivity of the catalysts in the SCR reaction, N2-selectivity testing was first performed as shown in Figure 2. The N2 selectivity of all the Fe-Mn/SBA-15 catalysts decreased with increasing temperature. Moreover, when the Mn/Fe molar ratio was 1 all the Fe-Mn/SBA-15 catalysts maintained more than 50% N2 selectivity over the whole temperature window, as shown in Figure 2a. Moreover, the the N2 selectivity of all Fe-Mn/SBA-15 catalysts decreased with increasing Mn/Fe molar ratio, as shown in Figure 2b. Figure 2c illustrates the synergy between Fe and Mn and in terms of the effects on the N2 selectivity of the catalyst. The figure shows that the N2 selectivity of Fe/ SBA-15 was highest, and that the Fe-based catalyst produced almost no N2O. Conversely, Mn/SBA-15 showed the poorest N2 selectivity. The addition of Fe increased the N2 selectivity of Fe-Mn/SBA-15 to be higher than that of Mn/SBA-15. The N2 selectivity of Fe-Mn/SBA-15 considerably decreased from 86% to 63% (Figure 2c). Thus, N2O formation was essentially controlled by the MnOx supported on the molecular sieves. Manganese based catalyst have outstanding low-temperature SCR activity but the significant amount of N2O formation during the reaction remains an issue to be deal with in order for such catalyst to become the preferred low temperature catalyst in a future, N2O regulated, 9



Page 10 of 57

environment . The N2O formation process is as follows (equation(3)): 4NH3+4NO+3O2→4N2O+6H2O (NSCR)

(3)

The activity of Mn-based molecular sieve catalysts and N2 selectivity are mainly decided by the oxidation state of the manganese species, MnOx crystallinity, and the specific surface area. We will discuss the effects of the physicochemical properties of the catalysts on the denitrification efficiency in detail in Section 3.3. The NH3-SCR efficiency of various deNOx Fe-Mn catalysts are summarized in Table S1, together with reported articles.

3.3 Morphology and Characterization of Fe-Mn/SBA-15 3.3.1 XRD

Figure 3. Small angle XRD patterns of Fe-Mn/SBA-15 catalysts with different doping contents.

10


Page 11 of 57 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 small-angle of XRD patterns of the catalysts (Figure 3a) showed three obvious diffraction peaks from θ = 0°–5° in all samples, corresponding to (100), (110), (200) crystal planes of a hexagonal phase with symmetry of P6mm. These results show that all SBA-15 samples had a typical two-dimensional hexagonal pore structure before and after doping of metal ions. However, doping of Fe and Mn decreased the sharpness of the small-angle XRD peaks, and the intensity of the XRD diffraction peaks decreased as doping of Fe-Mn was increased. The diffraction peaks of the molecular sieves (100) shifted to the left, indicating that the degree of ordering of SBA-15 decreased after loading of the metal ions. As a result (table 1), the interplanar distance decreased from 10.27 to 9.82 nm. The diffraction peaks of the (100) crystal plane shifted to a low angle as the transition metal loading was increased, corresponding to an increase of the unit cell size. Table 1 Structural data of Fe-Mn/SBA-15 catalysts with different doping contents. samples

Interplanar spacing

SBA-15

4.48Fe

6.72Fe

8.96Fe

11.2Fe

13.44Fe

22.4

22Mn/SB

4.4Mn/S

6.6Mn/S

8.8Mn/S

11Mn/SB

13.2Mn/S

Fe/SBA

A-15

BA-15

BA-15

BA-15

A-15

BA-15

-15

10.27

10.02

9.97

9.84

9.80

9.80

9.81

9.82

11.86

11.57

11.51

11.36

11.32

11.32

11.33

11.34

d100/nm Unit cell parameter

a0/nm * =

2 ! √3

In the wide-angle XRD patterns of the samples (Figure 3b), the spectra of SBA-15 with different metal doping showed a strong and broad diffraction peak at 23°, which was attributed to a diffraction peak from the SBA-15 pore wall composed of amorphous silica. This observation indicates that introduction of the metal ions did not substantially change the pore wall structure of SBA-15. Most of the characteristic peaks in the XRD patterns of Fe-Mn/SBA-15 belong to β-MnO2 (pyrolusite, PDF 01-0799, a = 4.38Å, c = 2.85 Å), while a small number of the peaks belong to α-Fe2O3 (hematite, PDF01-1035 a = 5.028Å, c = 13.73 Å), indicating that Fe2O3 is highly dispersed on the surface of SBA-15. As the loading was increased, the intensity of the diffraction peaks of β-MnO2 became weaker, indicating that the dispersibility of β-MnO2 is getting better as the loading increases. This may be related to iron, because the load of manganese 11



Page 12 of 57

increases and the load of iron also increase according to Mn/Fe molar ratio was 1. The doping of Fe greatly influences the dispersibility of β-MnO2. To study the synergy between Fe and Mn, we also analyzed the phase compositions of Fe/SBA-15 and Mn/SBA-15, respectively. Figure 3c shows XRD patterns of Fe/SBA-15, Mn/SBA-15, and Fe-Mn/SBA-15. The characteristic peaks of α-Fe2O3 (hematite, PDF01-1035 a = 5.028 Å, c = 13.73 Å) appeared at 30°, 36°, 45° in Fe/SBA-15. When only manganese was loaded, the characteristic peaks of the three crystal types of MnOx were present, namely β-MnO2 (pyrolusite, PDF 01-0799), Mn2O3 (PDF 24-0508), and Mn5O8 (PDF 39-1218). In the XRD pattern of Fe-Mn/SBA-15, only characteristic peaks from β-MnO2 were present and no peaks were observed from α-Fe2O3. This result indicates that the Fe2O3 was highly dispersed on the surface of SBA-15 and suggests that the addition of MnOx improved the dispersibility of Fe2O3 on the surface of the molecular sieves. The presence of Fe also appeared to improve the crystallinity of MnOx. The denitrification activity of MnOx over the temperature range of 110–300 °C decreased in the order: MnO2 > Mn5O8 > Mn2O3 > Mn3O4 > MnO. 24 Hence, the addition of Fe improved crystallinity of MnOx and showed a tendency to increase the content of the high oxidation state MnO2. The higher oxidation state of MnOx, explains the enhanced denitrification activity, and hence the denitrification activity of Fe-Mn/SBA-15 is much higher than that of Mn/SBA-15. In addition, Tang et al.

30

found that β-MnO2 showed better NO conversion activity and lower N2

selectivity than those characteristics of α-MnO2 at the same unit specific surface area. The high N2O formation rate of β-MnO2 is attributed to the lower number of Mn-O bonds, which promoted the formation of gaseous active NH3 species. The formation rate of N2O and the selectivity of N2O increased considerably with increasing NH3 concentration. This effect also explains the low N2 selectivity, shown in Figure 2.

3.3.2 XPS analysis

12


Page 13 of 57 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


13



Page 14 of 57

Figure 4. XPS patterns of Fe-Mn/SBA-15 catalysts with different doping amounts

The surface properties of the Fe-Mn/SBA-15 samples with different doping amounts were characterized by and high resolution XPS, spectra of the Fe2p, Mn2p and O1s regions, as shown in Figure 4. These results show that the Fe-Mn/SBA-15 samples with different doping contents contained Fe and Mn in the same form, regardless of the loading content. After peak fitting, the main peak of the Fe2p spectrum was assigned to ferric species (Figure 4a). The XPS peak of 710.1 eV was attributed to the 2p3/2 peak of Fe2O3. Hence, the impregnation process did not change the oxidation state of Fe. The Mn 2p energy spectrum was dominated by MnO2 species with a small amount of Mn2O3(Figure 4b). The XPS peaks at 642.83 and 641.67 eV belonged to the 2p3/2 peaks of MnO2 and Mn2O3, respectively. Table 2 shows the binding energies and surface atomic concentrations of Mn, Fe, and O from the catalyst. These data indicate that the value of Mn4+/Mn3+ increased as the total metal loading was increased. Hence, as total metal loading increased, the MnOx tended to form in a higher oxidation state of MnO2. Previous studies have shown that a higher Mn4+/Mn3+ value is related to higher catalytic activity. Through the 14


Page 15 of 57 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


above analysis of the XPS data, based on the Fe3+/Fe2+ ratio of Fe-Mn/SBA-15 higher than that of Fe/SBA-15, and the value of Mn4+/Mn3+ ratio of Fe-Mn/SBA-15 also different from that of Mn/SBA-15 in Table 2, we inferred that a redox reaction Fe2+ + Mn4+↔ Fe3+ + Mn2+ might take place between the Fe and Mn ions. The ratios of Mn4+/Mn3+ and Fe3+/Fe2+ were related to the microscopic synergistic effects as discussed in other research works.

18

The higher the ratios of metallic ions, the greater the

production of intermediates, which may be because the faster redox reaction happens over the catalysts as well as the faster electron transfer. Then, the synergistic effect between iron ions and manganese ions has been proven to be feasible and rational utilizing the solid evidence that the E0 of the propounded synergistic effects (Fe2+ + Mn4+ ↔ Fe3+ + Mn3+) was equal to 0.18 V, that is, the reaction(Fe2+ + Mn4+ ↔ Fe3+ + Mn3+) is indeed thermodynamically favorable.

18

It could be

speculated that the process of Fe2++ Mn4+ ↔ Fe3+ + Mn3+ played a role as a multifunctional transfer-electron-bridge (denoted as METB) to transfer electrons. The proposed activation of absorbed NH3 utilizing the METB over Fe-Mn/SBA-15 catalyst was shown as eqn (4)-(8). NH3(gas)→NH3(ads)

(4)

1/4O2(gas) →1/4O2(ads)

(5)

Fe3+ + NH3(ads) →Fe2+ -NH2 + H+

(6)

Mn3++1/4 O2(ads) →Mn4++1/2 O (act)

(7)

Fe2+ -NH2 + H++ Mn4++1/2 O (act)→Fe3+ + Mn3+ + -NH2 +1/2 H2O

(8)

According to the Mars-van Krevelen mechanism

24

, O2 plays an important role in the SCR

reaction. The presence of lattice oxygen, adsorbed oxygen and gas-phase oxygen introduced during the SCR reaction are maintained in a dynamic equilibrium. This dynamic process preserves the denitrification activity of the catalyst at low temperatures. It has been reported that the peaks at low binding energies (529.0–530.5 eV) can be attributed to lattice oxygen (Oα), whereas those observed at higher binding energies (531.0–533 eV) are related to chemisorbed oxygen (Oβ), which is mainly in the form of O22− or O−, belonging to oxide defects or hydroxyl-like groups. 31-36 Table 2 shows that the Oα/Oβ ratio of Mn/SBA-15 is 12.26, and this ratio becomes 65.10 after Fe doping. Thus, the surface of the Fe-Mn/SBA-15 catalyst is more oxygen rich than that of the Mn/SBA-15 catalyst, which implies that addition of the auxiliary Fe results in the formation of more oxides on the catalyst surface. Figure 4c shows that the addition of Fe considerably increases 15



Page 16 of 57

the chemisorbed oxygen content of the catalyst and promotes oxygen storage and exchange on the catalyst at low temperature. Various studies have shown that more oxide forms on the surface of the catalyst and the NH3 reducing gas is easily adsorbed to active sites of the catalyst. 31-36 At the same time, oxygen on the catalyst surface is oxidized to active oxygen, which can effectively promote the SCR reaction. Owing to consumption of surface oxygen, many oxygen vacancies are left on the surface of the catalyst, which are conducive to the adsorption of oxygen from the gas phase. Electrons are continuously exchanged through interactions between the adsorbed oxygen and metal ions in the active surface components. This exchange maintains the balance of reactive oxygen species involved in the reaction on the catalyst surface. The continuous transfer of electrons makes it possible for Mn on the catalyst surface to coexist in a multivalent state, which is advantageous for SCR reactions. In addition, more chemisorbed oxygen greatly promotes oxidation of NH3, resulting in more by-product N2O, which is a main factor contributing to reduced N2 selectivity. Table 2 Binding energies and surface atomic concentrations of Mn, Fe, and O of the Fe and/or Mn /SBA-15 catalysts. Samples

Surface concentration(%) Mn2p

4.48Fe

Fe2p

O1s

BE(eV) Fe 2p3/2 Fe3+

Fe2+

O 1s

Mn 2p

Mn4+/Mn3+

Fe3+/Fe2+

Oβ/Oα

Mn 2p3/2 Mn4+

Mn3+

3.14

1.03

95.83

712.20

710.66

533.03

642.34

642.83

641.67

4.14

5.25

30.41

1.45

2.56

95.98

711.85

710.48

532.78

642.38

642.70

641.46

15.75

8.06

25.26

3.43

1.91

94.67

711.60

710.35

532.96

642.12

642.97

642.08

3.73

9.84

19.12

2.42

1.38

96.20

711.80

710.55

532.92

642.43

642.60

641.74

39.55

8.39

65.10

2.92

3.06

94.02

711.80

710.57

532.98

642.38

642.51

641.62

40.08

8.21

19.87

Only Fe

——

2.3

97.7

712.20

710.75

533

——

——

——

——

5.10

2.61

Only Mn

7.08

——

92.92

——

——

533

641.85

642.40

641.40

6.20

——

12.26

4.4Mn 6.72Fe 6.6Mn 8.96Fe 8.8Mn 11.2Fe 11Mn 13.44Fe 13.2Mn

3.3.3 N2 adsorption/desorption

16


Page 17 of 57 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


Figure 5. N2 adsorption-desorption isotherms of Fe-, Mn-, and Fe-Mn-loaded SBA-15 catalysts.

Figure 5 shows that the nitrogen adsorption isotherm for the different samples followed Langmuir type IV behavior (IUPAC classification standard), which is a typical characteristic of mesoporous materials. The adsorption branch of the SBA-15 isotherm is located at a relative partial pressure of 0.45–0.8. Two obvious inflection points appeared, which indicate the distribution range of the mesopores. The sharp capillary condensation in the middle of the inflection point indicates that the distribution of mesopores is concentrated. The mesopore distribution of molecular sieve catalysts was broad for loading of both one and two metals. After loading of one metal, the hysteresis loop shape and mesopore distribution did not change notably. The degree of the abrupt change of the capillary polymerization curve of the double-loaded molecular sieve catalyst gradually decreased. There was no obvious saturated adsorption platform, indicating that the pore structure was irregular. The shape of the hysteresis loop of the Fe/Mn-loaded molecular sieves changed, which indicated a change of the pore shape. In SBA-15 and molecular sieve catalysts loaded with one metal type, the shape of the hysteresis loop reflected a uniformly distributed cylindrical hole with two openings, whereas the hysteresis loop of the Fe/Mn-loaded molecular sieves indicated a flat plate with narrow seams, cracks or wedges. The N2 adsorption in the low specific pressure range (P/P0 Fe-Mn/SBA-15>Fe/SBA-15. The trend of the NO-TPD desorption curve of three catalysts are similar to those of the CO2-TPD desorption curve, indicating that there 25



Page 26 of 57

is one type of basic center on the catalyst surface. Figure 7e is a NO+O2 desorption curve for each catalyst. The adsorption of NO on the active site of the catalyst forms a NOx (x≥2) species, which is the primary step for the selective catalytic reduction of NO by NH3. According to literature reports23, NO is derived from the thermal decomposition of NO2 during NOx-TPD. At the same time, it can be seen in Figure 7e that the NOx desorbed below 200 °C is mainly derived from the physical adsorption or weakly chemisorbed NxOy species (NO- or NO2- species) on the catalyst surface. NOx desorption at 250°C-500 °C is attributed to the decomposition of nitrate species stored at the active site, and the synergistic effect between Fe and Mn reduces the ability of the catalyst to store nitrate species. Combined with the NH3-TPD results, it can be seen that the synergistic effect between Fe and Mn increases the acidity of the catalyst, thereby inhibiting the adsorption of NOx by the catalyst. The synergistic effect also increases the content of surface active oxygen, which promotes the conversion process of NO-species to nitrate species. By integrating the desorption peak of NO+O2-TPD, the storage NOx of each catalyst can be calculated and decreased in the following order: Mn/SBA-15>Fe /SBA-15>Fe-Mn/SBA-15. It is shown that the synergistic effect between Fe and Mn reduces the ability of the catalyst to store NOx.

3.3.6 SEM a

b

26


Page 27 of 57 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


c

d

e

f

Figure 8. Scanning electron microscopy images of (a) SBA-15, (b) Fe/SBA-15, (c)

Mn/SBA-15, and (d) Fe-Mn/SBA-15. Energy-dispersive X-ray spectroscopy mapping of Fe-Mn/SBA-15catalyst showing the (e) Fe species and (f) Mn species distributions. .

To further investigate the dispersion of the active components on the catalyst surface, high magnification SEM analysis of the catalysts was conducted, as shown in Figure 8. The appearance of SBA-15 (Figure 8a) was a worm-shaped accumulation of rods, consistent with literature reports. 8,9

Figure 8b and Figure 8c respectively show that small amounts of small bright particles, formed

from agglomerations of Fe2O3 and MnOx, were present on the surfaces of the Fe/SBA-15 and Mn/SBA-15 catalysts. The particle sizes of Fe2O3 and MnOx were approximately 50–90 and 80– 120 nm, respectively. Figure 8d, Figure 8e, and Figure 8f shows the morphology of the Fe-Mn/SBA-15 catalyst and elemental mapping of Fe-Mn on its surface. The two active particle types, i.e., MnOx and Fe2O3, were uniformly dispersed on the catalyst surface. The high degree of 27



Page 28 of 57

dispersion of the active components favored the catalytic reaction and contributed to the high SCR activity of the Fe-Mn/SBA-15 catalyst.

3.3.7 HR-TEM

Figure 9. TEM images of Fe-Mn/SBA-15 catalyst.

Figure 9 shows TEM images of Fe-Mn/SBA-15 catalyst. The loading of the transition metal oxide did not change the original pore structure of the SBA-15, and the transition metal oxide existed both inside and outside the pores of the molecular sieve. The diameters of pores featuring transition metal oxide outside the pores were approximately 30–90 nm, which was consistent with 28


Page 29 of 57 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 SEM results. The crystal planes appearing in Figure 9 were all related to SBA-15, and crystal planes of MnO2 and Fe2O3 were not observed. Therefore, the transition metals were well distributed in the pores and outside the pores of the SBA-15 molecular sieve. To study the distribution of the catalyst particles and to determine the distribution of the transition metal elements, TEM-EDS-mapping was also performed. Figure 10 shows the distribution of Mn and Fe elements in the catalyst pores. The TEM-mapping images show that the distribution of Fe in the molecular sieve channels was more uniform than that of Mn. The TEM-EDS results show that the Fe/Mn ratio in the pores exceeded 1(Fe 1.78wt%, Mn 3wt%), indicating that Fe2O3 particles were more abundant in the pores. Combined with SEM-EDS analysis of the catalysts, we deduced that many Fe2O3 grains entered into the pores of the molecular sieve; however, MnO2 grains remained outside the pores.

29



Page 30 of 57

Figure 10. TEM-EDS-mapping images of the Fe-Mn /SBA-15 catalysts.

3.4 Reaction mechanism 30


Page 31 of 57 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


3.4.1 Adsorption and Reactions on Fe-Mn/SBA-15 Catalyst Surface (1) Adsorption of NO and O2 onto the catalyst surface Figure 11a shows in-situ IR spectrum of the Fe-Mn/SBA-15 sample after passage of 800-ppm NO and 3%-O2/N2 for different times. When NO was adsorbed onto the catalyst surface it became oxidized to NO2 and then formed a series of nitrates and nitrites. Two strong peaks appeared at 1627 and 1594 cm−1, respectively, when treated with NO and O2 for 5 min; these peaks are related to bridged bidentate nitrate and chelate bidentate nitrate, respectively. Bidentate nitrates were converted from less stable nitrates and nitrites. The order of thermal stability of these materials increases in the order: linear monodentate nitrite < bridged nitrite < monodentate complex nitrite < monodentate nitrate < bridged bidentate nitrate < chelated bridged nitrate. These adsorbed nitrites and nitrates were highly active on the surface of the catalyst and reacted with NH3 to produce NH4NO3 and finally decomposed to N2O and N2, and thus make an important contribution to the activity of the catalyst. Two weak peaks appeared at 1355 and 1425 cm−1, which were both attributed to monodentate complex nitrite peaks. Hence, NO and O2 were adsorbed on the surface of the molecular sieves at 200 °C, and nitrate and nitrite were formed. NO adsorbed through chemical adsorption rather than weak physical adsorption. Under the continuous NO and O2 flows the intensities of these peaks gradually increased. After the He-purge, the peaks at 1627 and 1594 cm−1 decreased; however, those at 1355 and 1425 cm−1 showed no notable changes, indicating that the nitrate formed on the catalyst surface was unstable and that nitrite more stable.

31



Page 32 of 57

32


Page 33 of 57 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


33



Page 34 of 57

Figure 11. DRIFT spectra of Fe-Mn/SBA-15 catalyst exposed to NO and O2 for various times(a), preabsorbed with NO + O2 and then treated with NH3(b), exposed to NH3 for various times(c), preabsorbed with NH3 and then treated with NO + O2(d), exposed to NH3 + NO + O2 for 30 min(e), with temperature gradient in NH3 + NO + O2 atmosphere (f).

(2) Transient study of NH3 adsorption after NO + O2 adsorption The Fe-Mn/SBA-15 catalyst was treated with a flow of 800-ppm NO and 3%-O2 for 30 min at 200 °C, and then purged with He gas to remove NOx in the gas phase. Then, 800-ppm NH3 was introduced into the reaction cell. The changes of adsorbed species over time are shown in Figure 11b. After NH3 was introduced for 10 min, peaks emerged relating to -NH2 species (1554 cm−1) and gas phase or weakly adsorbed NH3 (1057 cm−1), and a symmetrical stretching vibration (1190 cm−1) of NH3 at L acidic sites gradually developed. The NO + O2 adsorption species formed on the catalyst surface as follows: chelating bidentate nitrate at 1593 cm−1 gradually decreased and bridged bidentate nitrate at 1629 cm−1 also gradually decreased. The monodentate nitrite at 1360 cm−1 showed little change, indicating that the monodentate nitrite was more stable on the surface of the molecular sieve under a flow of NH3 and could note easily react with NH3. However, the peak intensities of the chelate bidentate nitrate and bridged bidentate nitrate markedly decreased, indicating that these species were unstable on the catalyst surface and participated in the SCR reaction. Thus, nitrate is the main active species in the reaction at 200°C. In addition, according to previous reports, nitrite decomposition can produce N2O, and nitrite decomposition produced N2 34


Page 35 of 57 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


and H2O. These decomposition pathways further suggest that a large amount of N2O was produced in the denitration experiment, contributing to a decrease of N2 selectivity (Figure 2). Because the actual denitrification experiment was conducted with NH3 adsorbed after NO and O2 gases for 1 h, the results of DRIFT analysis suggested that the adsorbed NO reacted with adsorbed NH3 at 200 °C, which is consistent with the L-H mechanism. (3) Adsorption of NH3 on the surface of the Fe-Mn/SBA-15 catalyst Figure 11c shows in-situ IR spectra of the Fe-Mn/SBA-15 sample under a flow of 800-ppm NH3 at 200 °C. After a 5-min gas flow, peaks from many species appeared including a bending vibration at 1595 cm−1 and stretching vibrations at 3500–3800 cm−1 corresponding to -OH groups. The vibration peak at 1020 cm−1 corresponds to weakly adsorbed or gas phase NH3 peaks. The vibrational peaks at 1280 and 1583 cm−1 were related to asymmetric and symmetric stretching vibrations of NH3 at L acid sites. Peaks at 1121, 1377, and 1538 cm−1 are related to surface-adsorbed -NH2 species. These species derived from NH3 molecules adsorbed at the L acid sites deprotonated by surface active oxygen. The vibrational peaks at 1454 and 1649 cm−1 are related to NH4+ ions at B acid sites, which mainly include contributions from transition metal oxides. With prolonged flow times, the peak intensity of each species increased, indicating that all species were stable on the catalyst surface. After an He purge, the vibrational peak at 1020 cm−1 disappeared, which indicated that weakly adsorbed and gas phase NH3 was unstable and all other species were stable on the catalyst surface. (4) Transient study of NO + O2 adsorption after NH3 adsorption After the Fe-Mn/SBA-15 catalyst was passed through 800-ppm NH3 for 30 min at 200 °C, the NH3 gas was purged with He and then 800 ppm NO and 3% O2 were passed into reaction vessel. The changes of surface adsorption species over time are shown in Figure 11d. After the NO and O2 flows, the NH4+ peak (1451 cm−1) at B acid sites, the NH3 peak at L acid sites (1207 cm−1), and the peak of -NH2 species (1374 cm−1) remained. However, the peak at 1583 cm−1 became weaker. After the introduction of NO and O2 for 10 min, the peaks of bridging nitrates and bidentate nitric appeared at 1623 and 1598 cm−1 respectively, with intensities much higher than those of NH4+ at B- acid sites and NH3 at L-acid sites. Under a continuous flow of NO and O2, the 35



Page 36 of 57

intensities of these two peaks became stronger, indicating greater stability of the nitrate produced on the catalyst surface. No nitrite peaks were observed, and the oxidation of the catalyst was strong. The -NH2 at L-acidic sites changed considerably and the peaks related to species at B-acidic sites remained almost unchanged during the reaction. Together with the SCR activity results, we infer that NH3 was involved in the SCR reaction mainly in the form of species coordinated at L-acid sites, with -NH2 being the main active species.

(5) Transient study of NH3 + NO + O2 Figure 11e shows in situ IR spectra of the Fe-Mn/SBA-15 catalyst under the simultaneous introduction of NO, O2, and NH3 at 200 °C. In the figure, the three gases were introduced for 5 min to generate NH4+ at B- acid sites (1434 cm−1), chelate bidentate nitrate (1600 cm−1), bridge nitrate (1631 cm−1), linear nitrite (1482 cm−1), and N2O4 (1724 cm−1). Under the simultaneous flow of these three gases, the peak intensities of the chelated bidentate nitrate and bridged nitrate increased. However, after 30 min of gas flow, the peaks of the chelate bidentate nitrate and bridge nitrate weakened, and the peak of NH4+ at B-acidic sites also became relatively weak, indicating that nitrate and NH4+ adsorbed on the surface of the molecular sieves were partially desorbed. Neither NO nor -NH2 species were detected during the reaction either in the gas phase or weakly adsorbed. Therefore, we conclude that gas phase and weakly adsorbed NO, chelate bidentate nitrate, -NH2 species, NH4+ at B acid sites, and bridging bidentate nitrate participated in the SCR reaction. The NO + O2 adsorption and NH3 adsorption on the catalyst surface could be inferred from the amount of -NH2 species, which are the main intermediate species. Adsorbed NO species are mainly involved in the reaction in the form of gaseous NO and nitrate. (6) Steady state study of NH3 + NO + O2 To study the variation of different species formed on the surface of the catalyst and determine the temperature at which the activity of the catalyst reached a steady state, we conducted a steady-state DRIFTS experiment. Figure 11f shows the steady-state IR spectra of the SCR reaction of NH3 + NO + O2 on the surface of Fe-Mn/SBA-15 catalyst at different temperatures. At 100 °C, a series of infrared absorption peaks from NOx adsorbed species were observed on the catalyst 36


Page 37 of 57 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


surface. Peaks at 1629 and 1598 cm−1 were attributed to bridged bidentate nitrate and chelate bidentate nitrate, respectively. The peaks at 1404 and 1319 cm−1 related to monodentate nitrite peaks, and the peaks at 1422 and 1684 cm−1 to NH4+ peaks at B-acidic sites. The peak at 1726 cm−1 derived from N2O4 and those at 1513 and 1544 cm−1 from monodentate nitrate. The peak at 1556 cm−1 was attributed to -NH2 species and the peak at 1066 cm−1 to NH3 at L-acidic sites. As the temperature was gradually increased, almost no peaks from -NH2 species were observed, and the intensities of the peaks from monodentate nitrite and NH4+ at B-acidic sites also decreased. These results indicate that -NH2 species, monodentate nitrite, and NH4+ at B-acidic sites were active on the catalyst surface, with almost all of the catalyst surface being occupied by nitrate species. The E-R and L-H mechanisms generally take place for MnOx-catalyzed NH3-SCR reactions, as suggested by many previous studies.

40,41

Manganese-based catalysts show more facile

exchange with O2 in the gas phase at low temperatures. Kapteijn et al 24 studied the role of O2 in the NH3-SCR reaction with MnOx/Al2O3 as a catalyst. It was found that O2 can oxidatively activate the reduced catalyst to maintain the stability of the catalyst, indicating that the Mars-van Krevelen mechanism can occur simultaneously with the E-R and L-H mechanisms. In this paper, the good oxygen storage ability, high oxidation of Fe-Mn/SBA-15 (Figure 7) and abundance of chemisorbed oxygen (Figure 4) promoted smooth progress of the SCR reaction. Hence, the Mars-van Krevelen mechanism was found to occur simultaneously with the E-R and L-H mechanisms in our study.

3.4.2 Adsorption reaction on Mn/SBA-15 catalyst surface (1) Adsorption of NO and O2 on the catalyst surface

37



Page 38 of 57

38


Page 39 of 57 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


39



Page 40 of 57

Figure 12. DRIFT spectra of Mn/SBA-15 catalyst exposed to NO and O2 for various times(a), preabsorbed with NO + O2 and then with NH3(b), exposed to NH3 for various times(c), preabsorbed with NH3 and then NO + O2(d), exposed to NH3 + NO + O2 for 30 min(e), with temperature gradient in NH3 + NO + O2 atmosphere(f).

For the Mn/SBA-15 catalyst, Figure 12a shows that the peaks of bridged bidentate nitrate (1629 cm−1), chelate bidentate nitrate (1599 cm−1) and N2O4 (1726 cm−1) appeared after NO and 40


Page 41 of 57 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


O2 were introduced into the Mn/SBA-15 catalyst. A monodentate nitrite peak did not appear, reflecting the oxidative strength of the Mn/SBA-15 catalyst. The peak at 1726 cm−1 is associated with a vibration of N2O4 adsorbed to MnOx or Fe ions in the molecular sieve, which indicates that NO and O2 adsorbed on the molecular sieve surface can react to form N2O4, and the amount of N2O4 produced increased over time. No NH4+ was present at B-acidic sites of the Mn/SBA-15 catalyst, as indicated by the absence of a peak at 1412 cm−1; however, this peak was present for Fe-Mn/SBA-15(Figure11a). Hence, there were fewer B acidic sites present in the Mn/SBA-15 catalyst than in the Fe-Mn/SBA-15 catalyst. After He purging, the peaks at 1629 and 1599 cm−1 decreased markedly, indicating that nitrate formed on the surface of the Mn/SBA-15 catalyst was unstable. However, the peak intensity of nitrate in the Fe-Mn/SBA-15 catalyst after the He purge remained stable, indicating that nitrate on the Fe-Mn/SBA-15 catalyst was more stable. The decomposition of nitrates tends to produce N2O; however, nitrite decomposition tends to produce N2 and H2O

40,41

, which also

explains the lower N2 selectivity of the Mn/SBA-15 catalyst than that of Fe-Mn/SBA-15 catalyst (Figure 2). (2) Transient study of NH3 adsorption after NO + O2 adsorption The Mn/SBA-15 catalyst was treated with a flow of 800-ppm NO and 3%-O2 for 30 min at 200 °C, and then purged with He gas to remove NOx in the gas phase. Then, 800-ppm NH3 was introduced into the reaction cell. The changes of surface adsorption species over time are shown in Figure 12b. After NH3 was introduced into the reaction the peaks of nitrate at 1629 and 1596 cm−1 decreased immediately and the peak of N2O4 at 1731 cm−1 became more intense. This result indicates that when nitrate was desorbed from the surface of the molecular sieve, a large amount of N2O4 was continuously generated and adsorbed on the surface of the catalyst. At the same time, the peak of -NH2 species at 1542 cm−1 also became stronger, indicating that more NH3 was oxidized to -NH2 and that -NH2 becomes more stable on the catalyst surface. Because -NH2 is the main active species in the SCR reaction this result also explains why the denitrification activity of the Mn/SBA-15 catalyst was lower than that of the Fe-Mn/SBA-15 catalyst. (3) Adsorption of NH3 on the catalyst surface 41



Page 42 of 57

Figure 12c shows an in-situ IR spectrum of Mn/SBA-15 sample treated by a gas flow of 800-ppm NH3 at 200 °C. The figure shows that many peaks appeared after 5 min of gas flow, including peaks at 1597 cm−1 and 3500–3800 cm−1 related to vibrations of -OH groups and a peak at 1020 cm−1 corresponded to weakly adsorbed or gas phase NH3. The vibrational peak at 1104 cm−1 was related to a symmetric stretching vibration of NH3 at L-acidic sites. Peaks at 1136, 1300, and 1556 cm−1 related to surface -NH2 species peaks, were generated after proton deoxidation of NH3 molecules adsorbed at L-acidic sites by surface active oxygen. The vibrational peaks at 1418 and 1663 cm−1 were related to NH4+ ions at B acid sites. With extended time under the gas flow, the peak intensities of all species became stronger. After He purging, all species remained stable on the catalyst surface. Figure 12c shows that the Mn/SBA-15 catalyst featured more NH3 species (1061–1269 cm−1) at the L-acidic sites and weaker NH4+ ions (1663 and 1418 cm−1) at B-acidic sites, compared with those features of the Fe-Mn/SBA-15 catalyst(Figure11c). This result indicates that MnO2 contained fewer B-acidic sites, which is consistent with previous reports. 40 We can infer that NH4+ at B-acidic sites on the Fe-Mn/SBA-15 catalysts was mainly produced by NH3 adsorption onto Fe2O3. It has been previously reported

30

that catalysts with lower acidity are more likely to

generate N2O as a by-product, which is one reason for the lower N2 selectivity of the Mn/SBA-15 catalyst than that of the Fe-Mn/SBA-15 catalyst (Figure 2). The Mn/SBA-15 catalyst adsorbed fewer NH3 active groups onto its surface, compared with the NH3 adsorption of the Fe-Mn/SBA-15, indicating that the introduction of Fe can increase the acidity of the catalyst, which is consistent with the results of NH3-TPD. (4) Transient study of NO + O2 adsorption after NH3 adsorption After the Mn/SBA-15 catalyst was treated by a gas flow of 800-ppm NH3 for 30 min at 200 °C, the NH3 gas phase was purged with He gas and then 800-ppm NO and 3%-O2 were flowed into the reaction cell. The changes of surface adsorbed species over time were measured, as shown in Figure 12d. Under a flow of NO and O2 for 10 min, peaks of bridged bidentate nitrate and chelate bidentate nitrate appeared at 1631 and 1598 cm−1, and under a continuous flow of NO and O2, the intensity of the two peaks increased, indicating that nitrate generated on the catalyst became more stable. However, under a continuous flow of NO and O2, a peak from N2O4 appeared 42


Page 43 of 57 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


at 1742 cm−1, indicating that the introduced NO was continuously oxidized to form NO2 and adsorbed on the surface of the catalyst. The peak of NH3 at L acidic sites (1066 cm−1) became weaker, indicating that a reaction of NH3, NO, and O2 occurred. Figure 12d shows that the nitrate formation rate was faster for the Mn/SBA-15 catalyst than for the Fe-Mn/SBA-15 catalyst (Figure11d), indicating that the oxidation of the Mn/SBA-15 catalyst was higher and that the introduction of Fe reduced the catalyst oxidation potential.

(5) Transient study of NH3 + NO + O2 adsorption Figure 12e shows in situ IR spectra of the three kinds of NO, O2, and NH3 gas simultaneously introduced into the Mn/SBA-15 catalyst at 200 °C. The three gases were flowed for 5 min to form species including: monodentate nitrate (1522 cm−1), chelate bidentate nitrate (1596 cm−1), bridged bidentate nitrate (1631 cm−1), linear nitrite (1452 cm−1), N2O4 (1724 cm−1), and NH3 at L-acidic sites (1080 cm−1). Under the simultaneous flow of the three gases, the peak intensities of the chelated bidentate nitrate and bridged nitrate increased. However, under the gas flow for 30 min, the peaks of chelate bidentate nitrate and bridge nitrate weakened, and the peak from linear nitrite also decreased, indicating that nitrate and nitrite adsorbed on the molecular sieve surface partially desorbed. Neither gas phase NO nor weakly adsorbed -NH2 species were detected during the reaction; hence, the gas phase and weakly adsorbed NO, chelate bidentate nitrate, -NH2 species, nitrite and bridged bidentate nitrate likely participated in the SCR reaction. The adsorption of NO and O2 on the catalyst surface and adsorption of NH3 to the catalyst surface, suggested that -NH2 species were the main intermediates and that NO-adsorbed species, mainly in the form of NO, nitrite, and nitrate participated in the reaction. Figure 12e shows that more monodentate nitrate, N2O4, and NH3 at L-acidic sites were present for the Mn/SBA-15 catalyst than for the Fe-Mn/SBA-15 catalyst(Figure11e). These are the main active species of the SCR reaction. Thus, this result further confirms that the denitration activity of the Mn/SBA-15 catalyst was lower than that of the Fe-Mn/SBA-15 catalyst (Figure 1).

(6) Steady state study of NH3 + NO + O2 adsorption 43



Page 44 of 57

Figure 12f shows steady-state IR spectra of the SCR reaction of NH3 + NO + O2 on Mn/SBA-15 catalyst at different temperatures. Figure 12f shows that in addition to peaks from bridged bidentate nitrate (1629 cm−1) and chelate bidentate nitrate (1598 cm−1), a large superimposed peak was found at 1438–1558 cm−1 overlaid on the monodentate nitrate peak at 1529 cm−1 and -NH2 peak at 1545 cm−1. As the temperature was increased, when the temperature reached 150 °C, this superposed peak disappeared, indicating that monodentate nitrate and -NH2 species desorbed and participated in the SCR reaction. As temperature was increased, a peak for N2O4 appeared at 1724 cm−1. In addition, the intensity of peaks from bridged bidentate nitrate and chelated bidentate nitrate decreased, indicating that nitrate on catalyst surface was partially desorbed. Nitrate decomposition produced N2O and H2O, which explains the decrease in N2 selectivity of the Mn/SBA-15 catalyst with increasing temperature (Figure 2).

3.4.3 N2O production pathway To explore the pathway of N2O generation, we recorded changes of NO, NO2, NH3, and inlet and outlet NOx concentrations at different temperatures using Fe-Mn/SBA-15 as a catalyst. These results are shown in Figure 13.

Figure 13. Outlet gas concentration of Fe-Mn/SBA-15 catalyst at different temperatures in NO + O2 + NH3 atmosphere.

44


Page 45 of 57 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


Figure 13 shows that as the temperature increased, NO and NH3 concentrations declined rapidly, NO2 concentration slightly increased, and the N2O concentration gradually increased.

Considering the different atomic ratios, the outlet nitrogen content in the NOx can be expressed as NO + NO2 + 2N2O, and the inlet nitrogen concentration can be expressed by the inlet NO. The NO concentration reached its lowest value of 30.7 ppm at 200 °C, and then slightly increased with increasing temperature. At 150 °C, the concentration of NO and NH3 decreased rapidly and the concentration of N2O gradually increased, indicating that NH4NO3 generated at this time decomposed to form N2O. At temperatures above 200 °C, the NH3 concentration decreased slowly. The concentration of by-product N2O increased gradually as the temperature was increased beyond 100 °C, reaching the highest value of 79.8 ppm at 250 °C, indicating that decomposition of NH4NO3 or oxidation of NH3 was more obvious and a large amount of N2O was formed. However, at 250 °C, the concentration of NO2 increased and content of nitrogen in the

outlet exceeded that of the nitrogen provided by inlet NO. According to the principle of elemental conservation, a portion of the NH3 participated in the reaction to produce N2O. However, at 300 °C, the concentration of N2O decreased again, indicating that the NH4NO2 generated at this time decomposed to form N2 and H2O, such that the concentration of N2O decreased over this time.

Figure 14. Outlet N2O gas concentration of Fe-Mn/SBA-15 catalyst at different temperatures at 200 °C.

According to previous reports 30-35, N2O mainly comes from the direct oxidation of NH3 in O2 45



Page 46 of 57

[reaction (9)] and the reaction of NO or NO2 with adsorbed NH3 6-7 [reactions (10) and (11)]: 2 NH3 +1/2O2→ N2O+ 3H2,

(9)

4NO + 4NH3 + 3O2→ 4N2O + 6H2O,

(10)

2 NO2 + 2 NH3 →N2+ N2O+ 3H2 O,

(11)

3NO→ N2O+ NO2.

(12)

The presence of NO and the reaction of NO2 and adsorbed NH3 to produce N2 is consistent with the fast SCR model and the reaction rate and extent are much larger than those of reaction (11). 38 Therefore, the effect of reaction (11) on N2O production is negligible and can be ignored. Hence, no NH3 was present in the reactants, and almost no N2O was detected at the outlet in the presence of only NO and O2 (as shown in Figure 14). Therefore, reaction (12) can also be neglected. The main reactions (9) and (10) produce N2O. The formation mechanism of N2O is similar to that of N2. Hence, gaseous NO might react with activated NH3 (-NH2) to form N2O (Eley–Rideal mechanism), or adsorbed NO3− might react with NH3 in a nearby location to form transition state substances, such as NH4NO3, and then decompose to form N2O (Langmuir– Hinshelwood mechanism). According to analysis of the DRIFT spectra, a large amount of nitrate was present throughout the SCR process; hence, we infer that more N2O was produced by the L–H mechanism. The NO reduction reaction by the L–H mechanism (including the SCR reaction and NSCR reaction) can be represented by the following reaction equations 39: NH3(g) → NH3(ad)

(13)

NO(g) → NO(ad)

(14)

≡Mn4+= O + NO(ad)→≡Mn3+- O-NO

(15)

≡Mn3+- O-NO + 1/2 O2→≡Mn3+- O-NO2

(16)

≡Mn3+- O-NO + NH3(ad) → ≡Mn3+- O-NO-NH3 → ≡Mn3+- OH + N2 + H2O

(17)

≡Mn3+- O-NO2 + NH3(ad) → ≡Mn3+- O-NO2-NH3 → ≡Mn3+- OH + N2O+ H2O

(18)

≡Mn3+- OH +1/4O2→≡Mn4+= O +1/2H2O

(19)

46


Page 47 of 57 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


Because Mn4+ plays a key role in low temperature catalysis (Figure 1), we only list reactions in which Mn4+ participates, and those involving Fe3+ are not listed. The actual reactions are more complicated that presented here, and these have been simplified for the convenience of discussion. Reaction (13) is the process of adsorption of gaseous NH3 onto the catalyst acidic sites (Bronsted acid site and Lewis acid site) to form adsorbed ammonia species (NH4+, bound NH3). Reaction (14) reflects the physical adsorption of gaseous NO on the Fe-Mn/SBA-15 catalyst, and the adsorbed NO is oxidized by Mn4+ to form Mn3+- O-NO in the adsorbed state, as shown in reaction (15). Reaction (16) is the process by which adsorbed Mn3+-O-NO is oxidized by O2 to form adsorbed Mn3+-O-NO2. The adsorbed Mn3+- O-NO and Mn3+- O-NO2 then rapidly react with adjacent adsorbed ammonia species to form Mn3+- O-NO- NH3 and Mn3+- O-NO2- NH3, as shown by reactions (17) and (18). Finally, Mn3+- O-NO- NH3 and Mn3+- O-NO2- NH3 decompose to form N2 and N2O, respectively. Reaction (19) shows the Mn4+ regeneration of the Fe-Mn/SBA-15 catalyst. The NO reduction reaction by the Eley–Rideal mechanism can be represented by the following reaction equations (20)-(25): NH3(g) → NH3(ad)

(20)

≡Mn4+= O + NH3(ad)→≡Mn3+- OH + -NH2

(21)

≡Mn4+= O + -NH2→≡Mn3+- OH + -NH

(22)

-NH2 +NO(g)→N2 +H2O

(23)

≡Mn4+= O + -NH + NO(g)→≡Mn3+- OH +N2O

(24)

≡Mn3+- OH+1/4O2→≡Mn4+= O + 1/2H2O

(25)

Reaction (21) reflects the adsorption of ammonia species activated by Mn4+ onto the Fe-Mn/SBA-15 catalyst to form the amide species (-NH2). The so-formed -NH2 can then be further oxidized to -NH (reaction 22), followed by a reaction of gaseous NO with -NH2 and -NH, to form N2 and N2O, respectively (reactions 23 and 24). Reaction 25 shows the regeneration of the Mn4+ of the Fe-Mn/SBA-15 catalyst. Previous transient reaction studies have shown that the Fe-Mn/SBA-15 catalyst mainly follows the Langmuir–Hinshelwood mechanism at low temperatures (200 °C)

39-41

. However, the 47



Page 48 of 57

Eley–Rideal mechanism also contributes to the reduction of NO at low temperatures. As the temperature increases, the Eley–Rideal mechanism is promoted and dominates at high temperatures (250 °C), where the SCR reaction competes with NSCR. The entire denitration mechanism of the Fe-Mn/SBA-15 catalyst in the SCR reaction is illustrated in Figure 15. Since the Fe-Mn/SBA-15 is a strong basic catalyst, NH3 adsorbed in the molecular sieve is weak, while NO and O2 is strongly adsorbed on the surface of the molecular sieve. These adsorbed species produce the intermediate NH4NO2 or NH4NO3, in which NH4NO2 decomposes to N2O and H2O. This process follows the L-H mechanism, as shown in equations (26)-(29). The generated intermediate product NH4NO3 may react with gaseous NO to produce NH4NO2 and NO2. And NH4NO3 can also be decomposed directly to generate N2O and H2O. The entire process follows the E-R mechanism, as shown in equations (30)-(31). NH 3 (g) + H + → NH 4 + (a)

(26)

NH4+ (a) + N2O4(a)→NH4N2O4+(a)→…→N2(g) + H2O + O(a)

(27)

NH4+ (a) + NO2-→NH4NO2(a)→…→N2(g) + H2O

(28)

NH4+ (a) + NO3-→NH4NO3(a)→…→N2(g) + H2O+ O(a)

(29)

NO(g) + NH4NO3(a)→NH4NO2(a) + NO2(g)

(30)

NH4NO3(a)→N2O(g) + 2H2O

(31)

Figure 15. Low temperature NH3-SCR reaction mechanism on Fe-Mn/SBA-15 catalyst.

48


Page 49 of 57 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


In summary, the strong oxidative properties, low acidity, and high basicity of the Fe-Mn/SBA-15 catalyst are indicated by NH3-, CO2-, O2-, NO- and NO+O2-TPD characterization. During the catalytic process, a large amount of nitrate is produced, which decomposes to produce N2O, resulting in a decrease in N2 selectivity. To improve the N2 selectivity, we are considering adding alumina from fly ash to the molecular sieve to increase the acidity and inhibit the formation of N2O.

4．．Conclusions Fe and/or Mn/SBA-15 catalysts were prepared by an impregnation method with the use of fly ash-derived SBA-15 molecular sieves as a support. The low-temperature SCR activity of these catalysts was investigated. Owing to synergistic effects of Fe and Mn, the SCR activity of the Fe-Mn/SBA-15 catalyst was much higher than that of the catalysts loaded with only one type of metal. The NOx conversion efficiency surpassed 90% at 200–250°C. Metals loaded on the molecular sieves were present in the form of oxides on the surface and in pores of the molecular sieve. The original hexagonal pore structure of SBA-15 was retained after metal loading. The active components MnOx and Fe2O3 were uniformly dispersed on the surface and pores of the molecular sieve. The Fe-Mn/SBA-15 catalyst had the largest reduction peak area, which coincided with its strong reduction ability at low temperatures. The atomic ratio of Mn4+/Mn3+ and the concentration of the adsorbed oxygen species in the Fe-Mn/SBA-15 catalyst were highest, thereby resulting in high NOx low-temperature denitrification activity. The reaction mechanisms of Mn/SBA-15 and Fe-Mn/SBA-15 catalysts at 200°C were determined by studying the adsorption state of the reaction gas in SCR reaction and the transient and steady state reaction processes, with the use of in situ FTIR. The mechanism of N2O generation was also discussed. These results show that the reaction mainly occurs between NH3 and NO on the Mn/SBA-15 molecular sieve denitration catalyst. When a bidentate nitrate is formed on the Mn/SBA-15 molecular sieve, coordination of NH3 on the catalyst is inhibited, resulting in partial obstruction of the SCR reaction. The Fe-Mn/SBA-15 catalyst features both B-acidic sites and L-acidic sites, and the NH3 adsorbed at L-acidic sites can dehydrogenate to 49



Page 50 of 57

-NH2. The NO and O2 adsorbed on the catalyst surface can form nitrates and nitrites. Nitrite present on the catalyst surface is relatively stable and does not participate in the SCR reaction. However, the nitrate species are unstable and decompose to form N2O and H2O, which explains the relatively low N2 selectivity. When a bidentate coordination nitrate is formed on the surface of the Fe-Mn/SBA-15 molecular sieve, the coordinated NH3 can still form on Fe2O3. The bidentate coordination nitrate can be transformed into new acidic sites on the catalyst, which promote the reaction. Therefore, NO degradation has two reaction channels (SCR and NSCR) on Fe-Mn/SBA-15. In situ FTIR analysis also showed that the addition of Fe reduced the oxidation potential of the Fe-Mn/SBA-15 catalyst, by decreasing the formation of nitrate and thereby limiting the formation of N2O. The addition of Fe increased the amounts of L-acidic and B-acidic sites on the catalyst surface, which promoted adsorption of NH3 to form more active intermediates and further improved the catalytic performance at low temperatures. The B-acidic sites on the Fe-Mn/SBA-15 catalyst were mainly provided by Fe2O3. The denitration mechanism of the Fe-Mn/SBA-15 catalyst in the SCR reaction followed the Mars-van Krevelen mechanism, E-R, and L-H mechanisms. Through analysis of NH3-, CO2-, and O2-TPD, we infer that the strong oxidation, low acidity, and high alkalinity of the Fe-Mn/SBA-15 catalyst led to a large amount of nitrate produced during the catalysis. The nitrate decomposed to produce N2O, resulting in a decrease in N2 selectivity. In summary, power plant fly ash was used to prepare molecular sieves, which were in turn applied as a support for metal catalysts used in the selective catalytic reduction of flue gas. Our approach offers a method of waste gas treatment with solid waste, which has a number of economic and social benefits. The results of this paper provide a new way of applying fly ash and controlling nitrogen oxide emissions from coal-fired flue gas. Hence, we offer a new way of achieving clean and efficient utilization of coal energy, which has a broad range of prospective applications.

ASSOCIATED CONTENT 50


Page 51 of 57 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


*Supporting Information Supporting Information Available: (Full description of the material.) This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment The authors gratefully acknowledge support by the National High Technology Research and Development Program (“863” program) of China (2012AA06A115), China Postdoctoral Science Foundation (2017M610723) and Hainan Medical University (Hy2018-17). We thank Andrew Jackson, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

References (1) Yao Z.T.; Xia M.S.; Sarker P.K.; Chen T. A review of the alumina recovery from coal fly ash, with a focus in China, Fuel 2014, 120, 74-85. (2) Fan F.H.; Liu Z.; Xu G.J.; Peng H.; Cai C.S. Mechanical and thermal properties of fly ash based geopolymers. Constr. Build. Mater., 2018, 160, 66-81. (3) Moffatt E. G.; Thomas M. D.A.; Fahim A. Performance of high-volume fly ash concrete in marine environment. Cement Concrete Res., 2017, 102, 127-135. (4) Amrhein C.; Haghnia G. H.; Kim T.S.; Mosher P. A.; Gagajena R. C.; Amanios T.; Torre L. Synthesis and properties of zeolites from coal fly ash. Environ. Sci. Technol., 1996, 30, 735-742. (5) Li D.D.; Min H.Y.; Jiang X.; Ran X.Q.; Zou L.Y.; Fan J.W. One-pot synthesis of aluminum-containing ordered mesoporous silica MCM-41 using coal fly ash for phosphate 51



Page 52 of 57

adsorption. J. Colloid Interface Sci. 2013, 404, 42-48. (6) Rayalu S.S.; Udhoji J.S.; Munshi K.N.; Hasan M.Z. Highly crystalline zeolite - a from flyash of bituminous and lignite coal combustion. J. Hazard. Mater. B 2001, 88, 107-121. (7) Impe´ror-Clerc M.; Davidson P.; Davidson A. Existence of a microporous corona around the mesopores of silica-based SBA-15 materials templated by triblock copolymers. J. Am. Chem.

Soc. 2000,122, 11925-11933. (8) Nozaki C.; Lugmair C. G.; Bell A. T; Tilley T. D. Synthesis, characterization, and catalytic performance of single-site iron (III) centers on the surface of SBA-15 silica. J. Am. Chem.

Soc. 2002, 124, 13194-13203. (9) Sabbaghi A.; Lam F. L.Y.; Hu X.J. Zr-SBA-15 supported Ni catalysts for lean NOx reduction.

J. Mol. Catal. A: Chem. 2015, 409, 69-78. (10) Zhang H.L.; Tang C.J.; Sun C.Z.; Qi L.; Gao F.; Dong L.; Chen Y. Direct synthesis, characterization and catalytic performance of bimetallic Fe–Mo-SBA-15 materials in selective catalytic reduction of NO with NH3. Micropor. Mesopor. Mat. 2012, 151, 44-55. (11) Li J.; Yang C.W.; Zhang Q.; Li Z.; Huang W. Effects of Fe addition on the structure and catalytic performance of mesoporous Mn/Al-SBA-15 catalysts for the reduction of NO with ammonia. Catal. Commun. 2015, 62, 24-28. (12) Li G.; Wang B.D.; Sun Q.; Xu W.Q.; Han Y.F. Adsorption of lead ion on amino-functionalized fly-ash-based SBA-15 mesoporous molecular sieves prepared via two-step hydrothermal method. Micropor. Mesopor. Mat. 2017, 252, 105-115. (13) Li G.; Wang B.D.; Wang H.Y.; Ma J.; Xu W. Q.; Li Y. L.; Han Y.F.; Sun Q. Fe and/or Mn oxides supported on fly ash-derived SBA-15 for low temperature NH3-SCR. Catal. Commun. 2018,108, 82–87. (14) Dahlin S.; Nilsson M.; Bäckström D.; Bergman S. L.; Bengtssond E.; Bernasek S. L.; Pettersson L. J. Multivariate analysis of the effect of biodiesel-derived contaminants on V2O5-WO3/TiO2 SCR catalysts. Appl. Catal. B: Environ. 2016, 183, 377-385.

52


Page 53 of 57 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


(15) Usberti N.; Jablonska M.; Blasi M. D.; Forzatti P.; Lietti L.; Beretta A. Design of a “high-efficiency” NH3-SCR reactor for stationary applications. A kinetic study of NH3 oxidation and NH3-SCR over V-based catalysts. Appl. Catal. B: Environ. 2015, 179, 185-195. (16) Chapman D. M. Behavior of titania-supported vanadia and tungsta SCR catalysts at high temperatures in reactant streams: tungsten and vanadium oxide and hydroxide vapor pressure reduction by surficial stabilization. Appl. Catal. A: Gen. 2011,392, 143-150. (17) Li J.H.; Chang H.Z.; Ma L.; Hao J.M.; Yang R. T. Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts-A review .Cataly. Today, 2011, 175, 147-156. (18) Mu W.T.; Zhu J.; Zhang S.; Guo Y.Y.; Su L.Q.; Li X.Y.; Li Z. Novel proposition on mechanism aspects over Fe-Mn/ZSM-5 catalyst for NH3-SCR of NOx at low temperature: rate and direction of multifunctional electron-transfer-bridge and in situ DRIFTs analysis.

Catal. Sci. Technol., 2016, 6, 7532-7548. (19) Karthik M.; Lin L.Y.; Bai H.L. Bifunctional mesoporous Cu-Al-MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs. Micropor. Mesopor. Mat. 2009, 117, 153-160. (20) Liu N.; Zhang R.D.; Chen B.H.; Li Y.P.; Li Y. X. Comparative study on the direct decomposition of nitrous oxide over M (Fe, Co, Cu)-BEA zeolites. J. Catal. 2012, 294, 99-112. (21) Wang C.; Wang J.; Wang J.Q.; Yu T.; Shen M.Q.; Wang W.L.; Li W. The effect of sulfate species on the activity of NH3-SCR over Cu/SAPO-34. Appl. Catal. B: Environ. 2017, 204, 239-249. (22) Beale A. M.; Gao F.; Lezcano-Gonzalez I.; Pedenc C. H. F.; Szanyi J. Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials. Chem.

Soc. Rev., 2015,44,7371 -7405. (23) Huang J.H.; Tong Z.Q.; Huang Y.; Zhang J.F. Selective catalytic reduction of NO with NH3 at low temperatures over iron and manganese oxides supported on mesoporous silica. Appl. 53



Page 54 of 57

Catal.B: Environ. 2008, 78, 309-314. (24) Pena D.A.; Uphade B.S.; Reddy E.P.; Smirniotis P.G. Identification of surface species on titania-supported manganese, ehromium and copper oxide low-temperature SCR catalysts. J.

Phys. Chem. B, 2004,108, 9927-9936. (25) Kapteijn F.; Singoredjo L.; Vandriel M.; Andreini A.; Moulijn J. A.; Ramis G.; Busca G. Alumina-supported

manganese oxide catalysts:Ⅱ. surface characterization and adsorption

of ammonia and nitric oxide. J. Catal.,1994, 150,105-116. (26) Qi G.; Yang R.T. Characterization and FTIR studies of MnOx-CeO2 catalyst for low-temperature selective catalytic reduction of NO with NH3. J. Phys. Chem. B, 2004, 108, 5738-5747. (27) Li Y.; Wan Y.; Li Y.P.; Zhan S.H.; Guan Q.X.; Tian Y. Low-temperature selective catalytic reduction of NO with NH3 over Mn2O3-doped Fe2O3 hexagonal microsheets. ACS Appl.

Mater. Interfaces, 2016, 8, 5224-5233. (28) Gao F.Y.; Tang X.L.; Yi H.H.; Zhao S.Z.; Li C.L.; Li J.Y.; Shi Y.R.; Meng X.M. A review on selective catalytic reduction of NOx by NH3 over Mn-based catalysts at low temperatures: catalysts, mechanisms, kinetics and DFT calculations. Catalysts 2017, 7, 199-231. (29) Yao X.J.; Kong T.T.; Yu S.Y.; Li L.L.; Dong L. Influence of different supports on the physicochemical properties and denitration performance of the supported Mn-based catalysts for NH3-SCR at low temperature. Appl. Surf. Sci. 2017, 402, 208-217. (30) Xu H.M.; Yan N.Q.; Qu Z.; Liu W.; Mei J.; Huang W.J.; Zhao S.J. Gaseous heterogeneous catalytic reactions over Mn-based oxides for environmental applications: A critical review.

Environ. Sci. Technol., 2017, 51, 8879-8892. (31) Tang W.X.; Yao M.S.; Deng Y.Z.; Li X.F.; Chen Y.F. Decoration of one-dimensional MnO2 with Co3O4 nanoparticles: A heterogeneous interface for remarkably promoting catalytic oxidation activity. Chem. Eng. J., 2016, 306, 709-718. (32) France L.J.; Yang Q.; Li W.; Chen Z.H.; Li X.H. Ceria modified FeMnOx-enhanced performance and sulphur resistance for low-temperature SCR of NOx. Appl. Catal. B: 54


Page 55 of 57 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


Environ. 2017, 206 , 203-215. (33) Chang H.Z.; Chen X.Y.; Li J.H.; Ma L.; Wang C.Z.; Liu C.X.; Schwank J. W.; Hao J.M. Improvement of activity and SO2 tolerance of Sn-modified MnOx-CeO2 catalysts for NH3-SCR at low temperatures. Environ. Sci. Technol. 2013, 47, 5294-5301. (34) Wu S.G.; Yao X.J.; Zhang L.; Cao Y.; Zou W.X.; Li L.L.; Ma K.L.; Tang C.J.; Gao F.; Dong L. Improved low temperature NH3-SCR performance of FeMnTiOx mixed oxide with CTAB-assisted synthesis. Chem. Commun., 2015, 51, 3470-3473. (35) Zhu Y.W.; Zhang Y.P.; Xiao R.; Huang T.J.; Shen K. Novel holmium-modified Fe-Mn/TiO2 catalysts with a broad temperature window and high sulfur dioxide tolerance for low-temperature SCR. Catal. Commun. 2017, 88, 64-67. (36) Long R. Q.; Yang R. T.; Chang R. Low temperature selective catalytic reduction (SCR) of NO with NH3 over Fe-Mn based catalysts. Chem. Commun., 2002, 5, 452-453. (37) Li L.L.; Sun B.W.; Sun J.F.; Yu S.H.; Dong L. Novel MnOx-CeO2 nanosphere catalyst for low-temperature NH3-SCR. Catal. Commun. 2017, 100, 98-102. (38) Liu F. D.; He H.; Ding Y.; Zhang C. B. Effect of manganese substitution on the structure and activity of iron

titanate

catalyst

for

the

selective

catalytic

reduction

of

NO

with NH3. Appl. Catal. B: Environ. 2009, 93, 194-204. (39) Kijlstra W. S.; Brands D.S.; Poels E. K.; Bliek A. Mechanism of the selective catalytic reduction of NO by NH3 over MnOx/Al2O3. J. Catal.,1997, 171, 208-218. (40) Putluru S. S. R.; Schill L.; Jensen A. D.; Siret B.; Tabaries F.; Fehrmann R. Mn/TiO2 and Mn–Fe/TiO2 catalysts synthesized by deposition precipitation-promising for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal.B: Environ. 2015, 165, 628-635. (41) Hadjiivanov K. I. Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev., 2000, 42, 71-144. (42) Jiang B.Q.; Li Z.G.; Lee S.C. Mechanism study of the promotional effect of O2 on 55



Page 56 of 57

low-temperature SCR reaction on Fe-Mn/TiO2 by DRIFT. Chem. Eng. J. 2013,225, 52-58.

56


Page 57 of 57 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


TOC Graphic

57


Reaction Mechanism of Low-Temperature Selective Catalytic

Recommend Documents