Low-Temperature Selective Catalytic Reduction of NO with NH3 over

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Low-Temperature Selective Catalytic Reduction of NO with NH3 over Mn2O3-doped Fe2O3 Hexagonal Microsheets Yi Li, Yuan Wan, Yanping Li, Sihui Zhan, Qingxin Guan, and Yang Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10264 • Publication Date (Web): 08 Feb 2016 Downloaded from http://pubs.acs.org on February 10, 2016

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ACS Applied Materials & Interfaces

Low-Temperature Selective Catalytic Reduction of NO with NH3 over Mn2O3-doped Fe2O3 Hexagonal Microsheets Yi Li*a, Yuan Wana, Yanping Lia, Sihui Zhan*b, Qingxin Guanb, Yang Tianc a

Department of Chemistry, Tianjin University, Tianjin 300072, P.R. China;

b

College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P.R. China;

c

Department of Chemistry, Capital Normal University, Beijing 100875, P.R. China.

ABSTRACT：：Mn2O3-doped Fe2O3 hexagonal microsheets were prepared for the low-temperature selective catalytic reduction (SCR) of NO with NH3. These hexagonal microsheets were characterized by SEM, TEM, XRD, BET, XPS, NH3-TPD, H2-TPR and in situ DRIFT and were shown to exhibit a considerable uniform hexagonal microsheet structure and excellent low temperature SCR efficiency. When doped with different Mn molar ratios, Mn2O3 was detected in the Fe2O3 hexagonal microsheets based on the XRD results without the presence of other MnOX species. In addition, the hexagonal microsheets with a Mn/Fe molar ratio of 0.2 showed the best SCR removal performance among the materials, where a 98% NO conversion ratio at 200 °C at a space velocity of 30 000 h-1 was obtained. Meanwhile, excellent tolerances to H2O and SO2, as well as high thermal stability, were obtained in Mn2O3-doped Fe2O3 hexagonal microsheets. Moreover, based on the XPS and in situ DRIFT results, it can be suggested that coupled Mn2O3 nanocrystals played a key role at low temperatures and produced a possible redox reaction mechanism in the SCR process. KEYWORDS：：Mn2O3-doped Fe2O3, hexagonal microsheets, low temperature，SCR, mechanism.

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INTRODUCTION In general, nitrogen oxides (NOX) from fossil fuels and vehicle exhaust gas are one of the most serious sources of air pollutions, giving rise to photochemical smog, ozone depletion, acid rain, greenhouse effects, etc.1-2 In terms of the many approaches for removing NOX, selective catalytic reduction (SCR) of NOX with NH3 is considered as one of the most efficient and widely used methods.3 Currently, commercial V2O5-WO3/TiO2 is used extensively in industry for removing NOX in a temperature range of 300-400 °C. However, there are a number of disadvantages in this system that need to be addressed, including a narrow operation temperature window, the toxicity of vanadium species, etc.4 Therefore, it necessary for researchers to develop new catalysts to overcome the above issues. Until now, the removal of nitrogen oxides from exhaust gases at temperatures below 250 °C using non-zeolite-based catalysts has been challenging. In contrast to their simple counterparts, nanocomposites combining two different species of metal oxides present some excellent physical and chemical properties and are of great potential for the enhancement of catalytic activity.5-8 Many transition metal supported catalyst formulations have been investigated to verify low-temperature SCR performance through transient isotopic labeling and in situ FT-IR studies.9 Mn-Ti and Mn-Ce-based catalysts were shown to exhibit good low-temperature SCR abilities by many researchers.10-13 As an environmental-friendly and low cost catalyst, iron-based catalysts present very active catalytic and stable abilities in the SCR process.14-16 For example, based on isotopic labeling, iron-based catalysts have shown a stable NH3-SCR performance in the presence of water vapor.17 However, the NOX removal ratio on pure Fe2O3 is far from satisfactory. Despite manganese-based materials showing a higher catalytic performance at low temperatures, they exhibit many drawbacks in practical applications, such as sensitivity to SO2 and lower N2 selectivity.18-20 Therefore, a proper combination of Mn-Fe oxides can exhibit a higher low-temperature SCR removal ratio and a greater practical operability. In addition, hexagonal microsheets with very good composite crystal structures, uniform sizes and large surface areas have been extensively used in many fields such as electrically


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conductive applications, luminescence vapochromic properties and so on.21-22 However, novel Mn-Fe hexagonal microsheets for the low-temperature NH3-SCR have not been studied yet. Herein, Mn2O3-doped Fe2O3 hexagonal microsheets, containing very good composite crystal structure, uniform size and large surface area, were fabricated for low-temperature SCR of NO with NH3 using a hydrothermal synthesis method, the microsheets were then characterized using XRD, NH3-TPD, H2-TPR, XPS, TEM, SEM, BET and in situ DRIFT. Compared with pure Fe2O3, Mn2O3-doped Fe2O3 hexagonal microsheets exhibited low temperature SCR removal activities on NOX, fairly good SO2 and H2O resistances and thermal stability. Finally, a possible redox reaction mechanism with the Mn2O3-doped Fe2O3 hexagonal microsheets was proposed. EXPERIMENTAL SECTION Materials. All chemicals used were of analytical grade and used without further purification. Mn(NO3)2·2H2O (99.9 %) was purchased from Sigma-Aldrich. FeCl3·6H2O (99.8%) and, hexadecyl trimethyl ammonium bromide (CTAB 99.5 %) were purchased from Jiangtian Chemical Technology Co. Ltd. (Tianjin, China). Syntheses and characterizations. Mn2O3-doped Fe2O3 hexagonal microsheets were denoted as Mn(y)-FeOx (“y” represented the Mn/Fe molar ratio, where y = 0.1, 0.2 and 0.3). Mn(0.2)-FeOx was prepared via a hydrothermal synthesis method. In a typical process, 1 m mol of FeCl3·6H2O and 0.2 of m mol of Mn(NO3)2·2H2O were dissolved in 0.11 mol of deionized water at room temperature. 0.455 m mol of CTAB and 59 m mol of KOH were dissolved in 0.11 mol of deionized water and stirred under magnetic stirring for 1 h to obtain a clear solution. Then, it was slowly added dropwise into the ferric chloride aqueous solution. After stirring the mixture for 2 h at room temperature, the solution was transferred to a 15 mL Teflon-lined autoclave and then placed in an oven at 240 °C for 2 h. After cooling to room temperature, the samples were filtered with deionized water several times and then dried at 100 °C. As a comparison, the synthesis of samples with other Mn molar ratios was conducted using similar steps as above. Field-emission scanning electron microscopy (FESEM, JSM-6700F with an



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accelerating voltage of 3 kV, Japan) and transmission electron microscopy (TEM, JEOL JEM-2100F with an accelerating voltage of 200 kV, Japan) were used to characterize the morphology and inner structure of the hexagonal microsheets. The X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer using a CuKα radiation source (λ=0.15418 nm) with a voltage of 40 kV and an electric of 40 mA at room temperature (Rigaku D/Max 2200PC, Japan) over a 2θ range of 10° to 80° with a step of 8° min-1. The nitrogen adsorption-desorption isotherms were recorded using Quantachrome AutoSorb iQ-MP operated at -196 °C. The specific surface areas were measured by the Brunauer-Emmett-Teller (BET) method, and the pore size distributions of the samples were recorded by the adsorption branches and a cylindrical pore model (BJH method). Temperature-programmed reduction by hydrogen (H2-TPR) and temperature-programmed desorption of NH3 (NH3-TPD) using about 20 mg of the samples were tested by a Micromeritics Autochem 2920 II instrument with a thermal conductivity detector (TCD). The data of the binding energies of Fe 2p, Mn 2p and O 1s were obtained by XPS (Thermal ESCALAB 250) with an AlKα radiation X-ray source (hν = 1486.6 eV), referencing the C 1s peak at 284.6 eV. A FTIR spectrometer (Thermo Nicolet iS5) equipped with a liquid-nitrogen-cooled MCT detector was used to collect the in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTs) data. Catalytic activity test. The SCR activity measurements were carried out in a fixed-bed quartz reactor (i.d. 10 mm), which was operated under atmospheric pressure from 80-400 °C. Before the SCR activity test, 0.5 g of catalysts were first pressed into blocks and then crushed and sieved into 40-60 meshes. The composition of the mixed reactant gas was 0.5 g catalyst, 500 ppm NO, 500 ppm NH3, 3% O2, 100 ppm SO2 and 10% or 15% H2O. Additionally, the typical reaction conditions were balanced by N2 with a flow rate of 200 mL/min and a gas hourly space velocity (GHSV) of 3.0×104 h-1. In addition, the gas-phase concentrations of all components were measured by a KM-940 flue gas analyzer (Kane International Limited, UK). The NO conversion and N2 selectivity under steady-state reaction condition for 2 h were obtained from the following formula, in which NOin is the concentration of the inlet NO and NOout is the concentration of the outlet NO at steady-state, respectively:


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NO conversion (%) = 100% × (1 N 2 selectivit y (%) = 100% × {1 -

[NOx] out ) [NOx] in

([NO 2]out + 2[N 2O] out ) } ([NOx] in + [NH 3]in - [NH 3]out )

Where [NOX] =[NO] + [NO2]. RESULTS AND DISCUSSION Characterizations. To examine the Mn2O3-doped Fe2O3 hexagonal microsheets in detail, SEM and TEM observation were carried out to investigate the structure of the hexagonal microsheets. It can be seen that the pure Fe2O3 (Figure 1a) has a smoother surface than Mn(0.2)-FeOx (Figure 1b), which is mainly due to the incorporation of the Mn species. In particular, all hexagonal microsheets have highly uniform hexagonal structures. Also, the Fe2O3 materials possess large surfaces with an inner diameter from 2 µm to 4 µm, which can ensure the adsorption of NH3 on the surface leading to a sufficient SCR removal activity of NO. Based on the HRTEM image, there are two different lattice fringes in Figure 1c. One is approximately 0.367 nm, matching well with Fe2O3 (0.364 nm) in the (011) plane,23-24 and the other is 0.213 nm, corresponding to Mn2O3 in the (404) plane.25-26 These observations show that doped manganese content is able to produce well-regulated crystallographic planes in Fe2O3, which may be beneficial for a high NO removal ratio. In summary, it can be found that the synthesized Fe2O3 has a good composite crystal structure. Meanwhile, energy dispersive X-ray spectroscopy (EDS) was also used to explore the chemical composition of the Mn2O3-doped Fe2O3, as shown in Figure 1d, which clearly indicated that the material contained Fe, Mn and O elements. It is worth noting that, the existence of C and Cu peak in the EDS spectrum may be due to the copper mesh, which was used for TEM.



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Figure 1. SEM images of pure Fe2O3 (a), SEM images (b), HRTEM image (c) and the energy spectrum analysis (EDS) (d) of Mn(0.2)-FeOx.

To obtain structural information on the Mn2O3-doped Fe2O3 hexagonal microsheets, the XRD patterns were obtained as shown in Figure 2. Here, all samples exhibit well-resolved diffraction peaks, which are associated with a uniform hexagonal microsheet structure. Additionally, there are some diffraction peaks at 24.16°, 34.12°, 35.65°, 42.48°, 49.49°, 54.01°, 57.64°, 62.39°, 64.02° and 73.18°, corresponding to the lattice fringes of (0 1 2), (1 0 4), (1 1 0), (2 0 2), (0 2 4), (1 1 6), (1 2 2), (2 1 4), (3 0 0) and (2 2 0), respectively, which belong to pure Fe2O3 (JCPDS 79-1741). With increasing manganese content, minor changes can be observed in the diffraction scan except the diffraction peak of doped Fe2O3 becoming a little weaker than Fe2O3.27 From the additional reflections, it indicates that the presence of other crystalline manganese oxides are not found besides Mn2O3. The diffraction peaks of Mn2O3 can be well indexed to pure Mn2O3 (JCPDS 24-0508),28 corresponding to the lattice planes, such as (2 1 1), (1 2 2), (2 2 2), (4 0 0), (4 0 2), (3 2 3), (4 1 3), (0 4 4), (6 2 2),


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and (4 0 4). Hence, the generation of a stable phase, Mn2O3, is well-coupled into the Fe2O3 matrix and may be indispensably important for the high SCR activities of doped Fe2O3 materials in comparison to pure Fe2O3.29

Figure 2. XRD patterns of Mn2O3-doped Fe2O3 with different doped Mn content hexagonal microsheets.

The N2 adsorption-desorption isotherms measured at liquid nitrogen temperature and the pore diameter distributions of Mn2O3-doped Fe2O3 and pure Fe2O3 are shown in Figure 3. As shown in Figure 3a, the isotherms of the pure Fe2O3 exhibit typical IV curves and H2 type hysteresis loops based on the IUPAC classification,30 which is extraordinarily similar to the adsorption process on macroporous solids.31 However, it can be clearly found that the isotherms of Mn2O3-doped Fe2O3 move from typical IV curves to typical II curves. This phenomenon indicates the existence of macropores (>50 nm) and slit-shaped pore structures, which may be due to the aggregation of oxide particles in Mn2O3-doped Fe2O3.30 Moreover, the hysteresis loops of Mn2O3-doped Fe2O3 change from H2 type to H3 type with the increase of Mn content,32-33 suggesting the existence of slit-shaped pore structures in Mn2O3-doped Fe2O3.30 Meanwhile, the data of pore size distribution, pore volume, and specific surface area are listed in Table 1. As shown in Table 1, Mn2O3-doped Fe2O3 has surface areas of 48.7, 59.3 and 51.1 m2/g and pore volumes of 0.19, 0.21 and 0.21 cm3/g, which are much larger than pure Fe2O3 with a surface area of 28.4 m2/g and a



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pore volume of 0.04 cm3/g. Furthermore, the surface area improved from 48.7 to 59.3 m2/g upon increasing the Mn mole ratio from 0.1 to 0.2, while the surface area decreased from 59.3 to 51.1 m2/g when doped with 0.3 mol of the Mn species. Beyond this value, a further increase of manganese doping would lead to the collapse or sintering of the hexagonal structure, which may result in the decline of the catalytic activity. In other words, the low-temperature reduction activity of NO is interrelated to the apparent surface properties to a certain degree. Based on the data shown in Figure 3b, it can be concluded that the pore size distribution of all Mn2O3-doped Fe2O3 samples are indeed larger than that of pure Fe2O3. In addition, there exists a peak value near 4 nm in Mn2O3-doped Fe2O3. However, excess iron species in the pure Fe2O3 causes a left-shift to near 2.3 nm. In conclusion, it is suggested that the reduction activity of NO in NH3-SCR is related to the apparent surface properties to some extent.


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Figure 3. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of Mn2O3-doped Fe2O3 and pure.Fe2O3 hexagonal microsheets。

Table 1. Summary of the textural parameters of the samples Materials

Specific surface area (m2/g)

Pore volume (cm3/g)

Fe2O3

28.4

0.04

Mn(0.1)-FeOx

48.7

0.19

Mn(0.2)-FeOx

59.3

0.21

Mn(0.3)-FeOx

51.1

0.21

To find out the surface chemical states of the most active part and understand the nature of the interaction between the two metal oxide species, XPS measurements of Fe 2p, Mn 2p, and O 1s in fresh were conducted. As shown in Figure 4a, the centers of the electron-binding energies of Fe 2p3/2 and Fe 2p1/2 are 710.7 and 725.1 eV, respectively, which are assigned to Fe (III).34 The satellite peak located at 719.2 eV is accorded well with that in Fe2O3 compound.23 Moreover, there is no obvious difference between Mn(0.2)-FeOx and pure Fe2O3 in the electron-binding energy of Fe 2p. The Mn 2p core level spectrum in Figure 4b display two peaks at 641.6 and 653.1 eV, assigned to the Mn 2p3/2 and Mn 2p1/2 spin-orbit states of Mn2O3, respectively.35-36 Furthermore, the surface atom concentrations of Mn, Fe and O of Mn2O3-doped Fe2O3 and pure Fe2O3 are summarized in Table 2, the percent of O on Mn2O3-doped Fe2O3 hexagonal microsheets clearly increases from 62.9% to 70.3%



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with increasing Mn content, which enhances the removel activity of NO. Moreover, as shown in Table 2, a further increase of manganese doping leads to an oxygen reduction, which agrees with the results of the specific area analysis.37-38 Additionally, the peaks of Mn(0.2)-FeOx and pure Fe2O3 are observed in the XPS spectrum of O 1s , as shown in Figures 4c and 4d. The O 1s spectrum can be fitted into two peaks, which correspond to various oxygen containing chemical bonds. According to previous research,39 the binding energy at 529.5-529.8 eV corresponds to that of the lattice oxygen O2−, which is denoted as Oβ and O-, which has a higher binding energy at 531.8-532.5 eV, is denoted as Oα. With increasing Mn content, the chemisorbed oxygen Oα content is gradually increased. The ratio of Oα over Mn(0.2)-FeOx is 32.59%, and the ratio of Oα over pure Fe2O3 is 24.74%. It is suggested that the chemisorbed oxygen content is greatly increased after the introduction of Mn. As we all know, surface chemisorbed oxygen acts as the most active oxygen and plays an indispensable role in oxidation reactions. That is to say, the Mn2O3-doped Fe2O3may have better activity for the oxidation of NO to NO2 than pure Fe2O3 in the NH3-SCR process, which can be verified in the SCR test below.


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Figure 4. XPS spectra of Fe 2p (a), Mn 2p (b) and O 1s (c-d). Table 2. XPS results of Mn2O3-doped Fe2O3 and pure Fe2O3 Surface Atomic Concentration (%) Materials Fe

Mn

Oα

Oβ

Ototal

Fe2O3

37.1%

0

15.56%

47.34%

62.9%

Mn(0.1)-FeOx

26%

7.72%

20.4%

45.88%

66.28%

Mn(0.2)-FeOx

21%

8.97%

21.6%

48.43%

70.03%

Mn(0.3)-FeOx

24%

8.02%

20.9%

47.08%

67.98%

To obtain additional insights into the NO conversion of Mn2O3-doped Fe2O3, the H2-TPR of pure Fe2O3 and Mn(0.2)-FeOx hexagonal microsheets were carried out to evaluate the low temperature SCR reaction. The H2 consumption peaks over the two samples are generally located at 250-500 and 500-700 °C, which are respectively attributed to the surface lattice oxygen and bulk lattice oxygen.40 As shown in Figure 5a, the pure Fe2O3 is characterized by three different reduction peaks at 327 °C, 487 °C and 601 °C, which correspond to the surface lattice oxygen promoting the reduction of Fe2O3-Fe3O4 (327 °C, 487 °C) and the bulk lattice oxygen triggering the reduction process of Fe3O4-FeO (601 °C), respectively.23,

41

In addition, after the

introduction of Mn2O3, it can be easily seen that the peaks in Mn(0.2)-FeOX at 327 °C and 487 °C shift to lower temperatures (278 °C and 461 °C) but the other peak shifts


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to a higher temperature (614 °C). Because the position of the reduction peaks directly reflects the reduction properties of the material,42 it suggests that an adjunction of Mn2O3 improves the Fe3O4-FeO process and weakens the process Fe2O3-Fe3O4, indicating an improvement in reducibility of the sample. The total adsorption capabilities of NH3 over pure Fe2O3 and Mn(0.2)-FeOx were measured by NH3-TPD. From the NH3-TPD results in Figure 5b, a large peak, including two main ammonia desorption peaks (380 °C and 463 °C) is found in pure Fe2O3 over the entire desorption temperature range. It has been suggested that the NH3 molecules are attributed to the Lewis acid sites, which are higher than the thermal stability of NH4+ restricted in the Brønsted acid sites.43 Therefore, it can be concluded that the adsorption peaks at a low temperature can be assigned to the Brønsted acid sites and those at a high temperature to the the Lewis acid sites. Thus, the adsorption peak at 380 °C belongs to a Brønsted acid, while the other peak is attributed to a Lewis acid. Moreover, with the increase of Mn content, the two peaks of Mn2O3-doped Fe2O3 obviously shift to higher temperatures (431 °C and 553 °C), and a stronger absorption intensity is observed , as shown in Figure 5b. Consequently, we can we can conclude that the incorporation of Mn can increase the adsorption of ammonia species, leading to the enhancement of catalytic activities on Mn2O3-doped Fe2O3 in the NH3-SCR reaction. The result is confirmed in the situ DRIFT studies to certain degrees. Figure 5c shows the thermogravimetric analysis (TGA) curves for the pure Fe2O3 and Mn(y)-FeOx hexagonal microsheets. The weight change profiles of the four samples show similar weight loss steps in the whole temperature window, suggesting that the Mn-Fe metallic oxides without any impurities are thermally stable up to 800 °C.




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Figure 5. H2-TPR patterns (a), NH3-TPD patterns (b) and TGA patterns (c) of Mn2O3-doped Fe2O3 and pure Fe2O3 hexagonal microsheets。

Low-temperature SCR performance. The low-temperature SCR removal activity of NOX with NH3 over different samples is shown in Figure 6, including Mn(0.1)-FeOx, Mn(0.2)-FeOx, Mn(0.3)-FeOx and pure Fe2O3 hexagonal microsheets. As illustrated in Figure 6a, the Mn2O3-doped Fe2O3 materials exhibit well-defined catalytic activities in that nearly 95% conversion can be reached in a large temperature window between 150-300 °C at a relatively high space velocity of 30 000 h-1. For Mn(0.2)-FeOx, the low-temperature SCR activity is significantly enhanced to 98% NO conversion at 200 °C. The best NO conversion for pure Fe2O3 is only 80.42%, indicating that NO conversion with Mn2O3-doped Fe2O3 hexagonal microsheets is much higher than that of pure Fe2O3 in the whole temperature window. In addition, the SCR removal activity of Mn(0.2)-FeOx is slightly higher than that of Mn(0.1)-FeOx and Mn(0.3)-FeOx, which may be because there is a maximum limit of the synergistic effect between Mn-Fe.This outcome obtained is in accordance with the surface chemisorbed oxygen presented in the XPS results. As shown in Figure 6b, Fe2O3 immersed in 0.2 of mol Mn(NO3)2 and a conventional vanadium-based material (V2O5-WO3/TiO2) are tested for comparison.1 In terms of the pure Fe2O3, there is a little improvement over Fe2O3 immersed with 0.2 mol Mn(NO3)2. On the contrary, Mn2O3-doped Fe2O3 hexagonal microsheets exhibit a better low-temperature SCR ability than Fe2O3 immersed in Mn(NO3)2, on account of the synergistic effect of the inner structures of Mn and Fe. The increase of the Mn content is embedded in the Fe2O3 hexagonal microsheet lattice, which really plays an important role in low temperature NH3-SCR. Furthermore, it is clearly observed that the SCR activity on the VWTi catalyst is only less than 20% below 150 °C and can increase quickly to 78% at 275 °C. Moreover, the NO conversion ratio over Mn2O3-doped Fe2O3 is much better than that over the commercial VWTi in the temperature window of 80-300 °C. However, the commercial VWTi has a higher NO conversion ratio than Mn2O3-doped Fe2O3 when the temperature increases to 350 °C, indicating that Mn2O3-doped Fe2O3 has a better low-temperature practical operation.44-45 As is well-known, GHSV determined by



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the sample’ weight and the gas flow rate is another influential factor in the SCR process. As shown in Figure 7c, to further investigate the SCR activity under different GHSVs, the NO conversion ratio has been determined with GHSVs of 30000, 90000 and 180000 h-1. It can be clearly observed that the NO conversion ratio decreases with increasing GHSV from 30000 to 180000 h-1, owing to a shorter contact time between the catalyst and the mixed gases.46 In general, the GHSV has little influence on the SCR activity in the entire temperature window of 50-400 °C. The Mn(0.2)-FeOx hexagonal microsheet still achieves more than 91% at low temperatures ranging from 175 to 275 °C with a GHSV of 90000 h-1. Even at a high GHSV of 180000 h-1, the NO conversion can reach more than 87% at a temperature window of 150-275 °C. Compared to other low-temperature catalysts such as Mn-Ti, Mn-Ce and Ce-Ti, Mn2O3-doped Fe2O3 hexagonal microsheets can maintain a stable NO conversion ratio of

more than 88% at a wide temperature window of 150-250 °C with elevated

GHSVs from 30000 to 180000 h-1.10,11 Therefore, Mn2O3-doped Fe2O3 hexagonal microsheet is a potential candidate for low temperature NH3-SCR in the future.


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Figure 6. NO conversion ratios of Mn2O3-doped Fe2O3 (a); Mn(0.2)-FeOx, Fe2O3 immersed in 0.2 mol Mn(NO3)2 and commercial VWTi (b); Effect of GHSV on NO conversion for Mn(0.2)-FeOx (c) under the conditions of 500 ppm NO, 500 ppm NH3, 3% O2, GHSV of 30,000 h-1 (a, b) and N2 balance gas.

The resistance of H2O and SO2, thermal stability and N2 selectivity tests. For practical purposes, many scientists are devoting themselves to develop good resistance materials against H2O and SO2.35As is well-known, the existence of H2O and SO2 may restrain the performance of the NH3-SCR reaction and lower the life



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cycle of the material. The resistances of H2O and SO2 over Mn2O3-doped Fe2O3 at the reaction temperature of 200 °C are shown in Figure 7a. Mn2O3-doped Fe2O3 is investigated in the presence of 10% and 15% H2O using a 120 h test under a GHSV of 30 000 h-1. As seen, in the absence of 10% H2O, the NO conversion ratio of Mn2O3-doped Fe2O3 hexagonal microsheets decreases from 98% to 94% at 200 °C and then maintains this state for the next 100 h. After cutting off the inlet H2O, the original level of 98% NO conversion is rapidly recovered. In addition, Mn2O3-doped Fe2O3 hexagonal microsheets retain a 92% NO conversion ratio when at 15% H2O, which also show a recovery to its original level when cutting off the inlet H2O. This indicates that the presence of water vapor has a slight influence on the active sites of the sample. More importantly, Mn2O3-doped Fe2O3 has a reversible inhibition effect of H2O in the NH3-SCR reaction.36 Furthermore, the effect of SO2 over Mn2O3-doped Fe2O3 is exhibited in the inset of Figure 7a. With the presence of SO2, the NO conversion ratio quickly decreases from 98% to 85%. Thus, SO2 poisoning occurs in the Mn2O3 doped Fe2O3 materials, causing a decline in the conversion ratio to a certain extent and indicating that some sites have lost activities on the surface. However, after cutting off the SO2, the NO conversion recovers to 90% after 100 hours and remains at 92% with increasing time. This proves that the Mn2O3-doped Fe2O3 has a better reversible inhibition effect on SO2. Moreover, the Mn2O3-doped Fe2O3 maintains a 98% NO removal efficiency during the continuous 60 h duration at the reaction temperature of 200 °C (Figure 7b). Thus, the Mn2O3-doped Fe2O3 not only shows a wide operating temperature window but also exhibits good thermal stability, suggesting that the Mn2O3-doped Fe2O3 is a good candidate for the low temperature NH3-SCR reaction. The N2 selectivity, which can promote the excellent NH3 oxidation ability of SCR over Mn2O3-doped Fe2O3, was tested as shown in Figure 7b (inset). It is apparent that the N2 selectivity is approximately 100% at 80 °C. However, Mn2O3-doped Fe2O3 preserves more than 93% N2 selectivity in the low temperature window between 80-250 °C. As is well-known, N2O emerges when the N2 selectivity is decreased, suggesting an excellent ability to convert NO to N2 in the low-temperature window.


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Figure 7. Tests of H2O resistance (a), SO2 resistance (inset), stability (b) and N2 selectivity (inset) over Mn(0.2)-FeOx. Reaction conditions: Raction temperature at 200°C, [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, [H2O] =10% or 15% when used, [SO2] = 100 ppm when used, N2 balance and GHSV = 30000 h−1.

In situ DRIFT tests. To study the detailed reaction mechanism of NO conversion and the gas molecule adsorption of NH3 on the surface at different temperature, in situ DRIFT spectra of Mn2O3-doped Fe2O3 and pure Fe2O3 hexagonal microsheets are



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obtained, as shown in Figure 8. To eliminate CO2 and H2O before gas adsorption, two samples are treated initially with N2 at 300 °C for 2 hours. It is worth noting that the background spectra of the two samples need to be measured under the same conditions. Moreover, 500 ppm NH3 is passed into the system for 30 min when the two samples are cooled to the goal temperature of 50 °C. Last but not least, the DRIFT spectra are measured with increasing temperature from 50 °C to 400 °C. After the adsorption of NH3 on pure Fe2O3 at a temperature window of 50-400 °C, some significant bands at 1610, 1500, 1430, 1380, 1260 and 1210 cm−1 appear (as shown in Figure 8a). The bands at 1610 and 1210 cm−1 (δs) are assigned to coordinated NH3 bound to the Lewis acid sites,47 and the bands at 1260 and 1430 cm−1 (δas) are attributed to ionic NH4+ bound to the Brønsted acid sites. 42 The band at approximately 1380 cm−1 (ν(as)) may be attributed to the oxidization or deformation species of adsorbed NH3. The band at 1500 cm−1 may be assigned to the -NH2 species.47 In addition, it is obvious that there are some large differences for the NH3 adsorption over Mn2O3-doped Fe2O3, which may be crucial for its excellent ability in low-temperature SCR. As shown in Figure 8c, there exist obvious bands at 1610, 1500, 1460, 1430, 1260 and 1180 cm−1. The bands at 1610 and 1180 cm−1 (δs) can be related to coordinated NH3 bound to the Lewis acid sites, and the bands at 1260 and 1460 cm−1 (δas) are typical of NH4+ bound to the Brønsted acid sites.37, 47 The band at approximately 1430 cm−1 (ν(as)) may be attributed to the oxidization or deformation species of adsorbed NH3. The bands at 1500 cm−1 may be assigned to the -NH2 species.42, 47 Moreover, the incorporation of manganese oxide to iron oxide leads to bands corresponding to the increasing adsorbed ammonia species on Fe2O3, which may be due to the synergistic effect of Fe and Mn. This result is consistent with the result of NH3-TPD. Significantly, when the temperature rises from 50 to 250 °C, NH3 coordinated on the Lewis acid sites (in Figure 8c, 1610 and 1180 cm−1) still exists, while the Lewis acid sites diminish after the temperature rises from 250 to 400 °C. These results indicate that NH3 coordinated on the Lewis acid sites is more stable at a low-temperature window, which may be a reason why Mn2O3-doped Fe2O3 shows a better SCR performance in a low-temperature window not a high-temperature


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window. Accordingly, after the adsorption of NO+O2 on pure Fe2O3 at the same temperature window as above, five bands at 1546, 1448, 1360 and 1267 cm−1 appear in Figure 8b. The bands are assigned to bidentate nitrates, linear nitrites, monodentate nitrite, and nitro compound, respectively.47 With the doping of Mn, the bands at 1630, 1610, 1340 and 1280 cm−1 are assigned to gaseous NO2, bidentate nitrates, linear nitrites, monodentate nitrite and nitro compound, respectively (Figure 8d). As can be seen, the band of Mn2O3-doped Fe2O3 corresponding to bidentate nitrate is much stronger than pure that of Fe2O3. Previous researches has suggested that bidentate nitrate could result from the further oxidization of monodentate nitrite,24 which could improve NO adsorption, indicating that the adsorption of NO+O2 on Mn2O3-doped Fe2O3 is obviously promoted due to the incorporation of Mn. As seen, the adsorption of NO+O2 on Mn2O3-doped Fe2O3 obviously decreases, indicating a decline in the NO conversion ratio when the adsorption temperature increases to 300 °C. Therefore, low-temperature SCR activity can be reasonably achieved over Mn2O3-doped Fe2O3, in line with the previous characterizations.




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Figure 8. In situ DRIFTs spectra of (a) NH3 adsorption and (b) NO + O2 adsorptions on pure Fe2O3 and (c) NH3 adsorption and (d) NO + O2 adsorptions on Mn2O3-doped Fe2O3.

Reaction mechanism. As well-known, both Fe2O3 and Mn2O3 oxides have the catalytic activity in NH3-SCR process.14-15,18-19 A proper combination of Mn-Fe oxides can not only exhibit a higher low-temperature SCR removal ratio but also enhance the practical operability such as SO2 resistance and lower N2 selectivity.20 It has been reported by Zhang et al. that the doping of manganese can produce the excess oxygen in catalyst and higher the content of active metal ions involved in the whole redox circle so that enhance the catalytic ability.42 As described in paper, XPS, H2-TPR, NH3-TPD and in situ DRIFTs results suggest an adjunction of Mn2O3 in Fe2O3 similarly enhances the amount of active surface chemisorbed oxygen, lattice oxygen and the adsorption of ammonia species to join the catalytic reaction, respectively. In addition, the trace Mn2O3 doping in Fe2O3 still maintains the hexagonal microsheets with very good composite crystal structure, uniform size and large surface area, which are also propitious to a good NH3-SCR ability. So the synergistical effects of Mn-Fe hexagonal microsheets contribute to the good NH3-SCR performance. Moreover, as depicted in Figure 9, the possible reaction pathway for the selective catalytic reduction of NO with NH3 over Mn2O3-doped Fe2O3 catalyst is proposed. It can be



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concluded from the in situ DRIFT spectra that the NOX, which is adsorbed on the catalyst surface, have four types of species, including gaseous NO2, bidentate nitrates, linear nitrites and monodentate nitrites.47-50 NH4+ ions are adsorbed on the surfaces of the Brønsted acid sites and the Lewis acid sites, which combined with adsorbed monodentate NO2− to generate NH4NO2. NH4NO2 decomposes to N2 and H2O.47 Trace NH4+ connects with the adsorbed monodentate NO3− to generate NH4NO3, which can decompose to N2O. N2O is regarded as an intermediate product during the SCR reaction. Moreover, adsorbed NO and O can be obtained from N2O decomposition, which can react with each other to form adsorbed NO2 and eventually connect with NH4+ to decompose to N2 in a secondary reaction.44 The reactions of NO with NH3 follow a typical SCR mechanism (NO + NH3 + (1/4)O2 → N2 + (3/2)H2O). Moreover, as shown in Figure 7b (inset), more than 93% N2 selectivity ratio was obtained in a wide temperature window of 80-250 °C, suggesting an excellent ability for converting NO to N2 in the low-temperature window. Therefore, Mn2O3-doped Fe2O3 hexagonal microsheets can be a potential candidate for low temperature NH3-SCR.

Figure 9. The mechanism of NH3-SCR on Mn2O3-doped Fe2O3 hexagonal microsheets in low temperatures.

CONCLUSIONS


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In this paper, Mn2O3-doped Fe2O3 hexagonal microsheets were prepared for the low temperature selective catalytic reduction (SCR) of NO with NH3. This material presented a considerably uniform hexagonal structure, leading to a good catalytic activity. With different Mn doping

molar ratios, a 0.2 mol proportion Mn doping in

Fe2O3 showed the best SCR ability with a 98% NO conversion ratio at 200 °C at a space velocity of 30 000 h-1. Meanwhile, Mn2O3-doped Fe2O3 hexagonal microsheets exhibited excellent H2O resistance, SO2 resistance and thermal stability. Moreover, based on the intensive analyses of XPS and in situ DRIFT, it was suggested that Mn species played a key role in the low temperature SCR and produced a possible redox reaction mechanism. In conclusion, Mn2O3-doped Fe2O3 presented a very good comprehensive property, making it a potential candidate for NH3-SCR in the future.

AUTHOR INFORMATION Corresponding Author ∗ E-mail: [email protected] or [email protected] Phone: +86-022-23502756; Fax: +86-022-23502756. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the national natural science foundation of China (Grant NO. 21377061, 81270041), Independent innovation fund of Tianjin University (2015XRG0020), Key Laboratory of Colloid and Interface Chemistry (Shandong University, Ministry of Education) (201401) and by

Natural

Science Foundation of Tianjin (Grant No.

13ZCZDSF00300,

No.

15JCYBJC48400). Thanks for the revision of ACS ChemWorx Authoring Services (QCJL45R7).

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Low-Temperature Selective Catalytic Reduction of NO with NH3 over

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