Enhancement of Low-Temperature Catalytic Activity over Highly

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Kinetics, Catalysis, and Reaction Engineering

Enhancement of low-temperature catalytic activity over highly dispersed Fe-Mn/Ti Catalyst for selective catalytic reduction of NOx with NH3 Jincheng Mu, Xinyong Li, Wenbo Sun, Shiying Fan, Xinyang Wang, Liang Wang, Meichun Qin, Guoqiang Gan, Zhifan Yin, and Dongke Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01335 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Industrial & Engineering Chemistry Research

Enhancement of Low-Temperature Catalytic Activity over Highly Dispersed Fe-Mn/Ti Catalyst for Selective Catalytic Reduction of NOx with NH3 Jincheng Mu†, Xinyong Li*†, Wenbo Sun †, Shiying Fan †, Xinyang Wang†, Liang Wang†, Meichun Qin†, Guoqiang Gan†, Zhifan Yin † and Dongke Zhang*‡

†State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Corresponding Author:

*Xinyong Li, Email: [email protected] *Dongke Zhang, E-mail: [email protected]

1

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ABSTRACT A novel Fe2O3-MnO2/TiO2 catalyst was synthesized using a conventional impregnation method assisted with ethylene glycol and used for NH3-SCR. The catalyst exhibited superior low-temperature activity over a broad temperature window (100-325 oC), low apparent activation energy and excellent sulfur-poisoning resistance. The characterization results revealed that the catalyst was greatly dispersed with smaller particles, and the partial doping of Fe into TiO2 lattice thereby the formation of Fe-O-Ti structure could strengthen the electronic inductive effect and increase the ratio of surface chemisorption oxygen, resulting in the enhancement of NO oxidation and favoring the low-temperature SCR activity via “fast SCR” process. The in-situ FTIR analysis showed that the NOx adsorption capacity was significantly improved due to the desired dispersion property, further helping both the SCR activity and reaction rate at low temperatures. The present work confirmed that more active sites can be provided on the catalyst surface by modifying the dispersity.

Keywords:

Highly dispersed catalyst; Fe-Mn/TiO2 catalyst; NH3-SCR; Electronic inductive effect; Fe-O-Ti structure;

2


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1. Introduction Nitrogen oxides (NOx), discharged from stationary and mobile sources, is a kind of major air pollutant which could lead to acid rain, photochemical smog and haze, thus endangering the eco-environment and human health

1-4

. It is well-known that selective catalytic reduction

with NH3 (NH3-SCR) is a highly efficient method to reduce the NOx emission

5-8

. The

TiO2-supported vanadium-based catalysts, such as V2O5-WO3/TiO2 and V2O5-MoO3/TiO2, have been employed commercially in NH3-SCR for several decades and exhibit a good catalytic performance in the process

9-11

. However, the catalysts have been restrained due to

their inevitable disadvantages, such as the toxicity of V2O5, high reaction temperature and the narrow operation temperature window (350-450 oC)

12-14

. Accordingly, many researchers are

trying to develop vanadium-free NH3-SCR catalysts with high deNOx performance at low temperatures in recent years. Actually, Mn-containing catalysts exhibit remarkable SCR activity, which are regarded as potential alternatives for vanadium-based catalysts 16-17

and Fe-Mn/TiO2

18-22

3, 15

. Among the previous works, Mn/TiO2

showed distinguished catalytic activities in the NH3-SCR reaction.

It was reported that Qi and Yang 19 first prepared a Fe-Mn/TiO2 catalyst by the impregnation method, and it was found that the doping of iron oxide to Mn/TiO2 improved not only the NO conversion and N2 selectivity but also the resistance to H2O and SO2. However, the activity 3



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did not increase when the manganese loading was higher than 10%, mainly due to the low dispersion of active species. Afterwards, Wu et al. 16-17 solved the problems to some extent for high loading Mn/Ti catalysts via a sol-gel method, they also found the addition of Fe could enhance the adsorption of ammonia and speed up the reaction rate 18. In addition, Deng et al. 21

explored the function of iron oxides and proposed that iron could improve the SCR activity

by regulating the degree of polymerization and the dispersion of surface Mn oxide species. Furthermore, a Mn-Fe/TiO2 catalysts was prepared by a deposition-precipitation method and used for NH3-SCR reaction

20

. The results showed that the superior Fe-Mn/TiO2 catalyst

possesses high total acidity, high content of chemisorbed oxygen, which result in higher NO conversion. Nevertheless, the catalytic performance over the Fe-Mn/TiO2 catalysts with relatively low loading are still unsatisfied for practical application at low temperatures up till now. It was reported that the catalytic performance could be enhanced by improving the dispersion of catalyst

23-25

. Fang et al.

23

obtained CeO2 on carbon nanotubes with high

dispersion via a pyridine-thermal route, and found that the interaction between the highly dispersed CeO2 particles and the CNTs caused a large amount of strong acid sites and high surface chemisorbed oxygen, which result in the excellent NH3-SCR performance. Additionally, Qu et al.

24

prepared a highly dispersed carbon nanotubes supported Fe2O3 4


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catalyst by an ethanol-assisted impregnation method, and more than 90% NO conversion was achieved in the range of 200-325 oC due to the well dispersion of Fe2O3 and the strong interaction between Fe2O3 particles and the support. Considering that NH3-SCR performance could be facilitated by regulating the dispersion degree of catalyst. Herein, a novel Fe-Mn/Ti catalyst with fine dispersion was synthesized by an ethylene glycol-assisted impregnation method in the present work. By using comprehensive characterizations like XRD, Raman, TEM, EPR, XPS, H2-TPR, and in-situ FTIR, the prepared catalyst was identified to be an efficient NH3-SCR catalyst, which exhibited excellent low-temperature catalytic performance and a broad temperature window (100-325 oC).

2. Experimental Section 2.1. Preparation of Catalysts Anatase TiO2, C2H6O2 and precursor salts (Mn(CH3COO)2·4H2O and Fe(NO3)3·9H2O) were analytical reagent and used without any further purification. The Fe-Mn/Ti catalyst was prepared by an impregnation method, and the Fe-Mn/Ti(M) catalyst was prepared by the same method assisted with ethylene glycol (EG). In a typical process, 0.506 g Fe(NO3)3·9H2O and 0.564 g Mn(CH3COO)2·4H2O were dissolved in 5 mL 5



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deionized water (for Fe-Mn/Ti) or the mixed solvent of 3 mL deionized water and 2 mL EG (for Fe-Mn/Ti(M)) at ambient temperature. Then, the mixture were added to 2.0 g anatase TiO2 dropwise, respectively. After placed overnight, the precursors were dried at 100 oC for 12 h and calcined at 400 oC for 4 h in an air condition with a heating rate of 10 oC·min-1. For comparison, the Fe-Mn/Ti(S) catalyst was synthesized with the same process mentioned above except for the addition of 5 mL EG as solvent. The iron oxides and manganese oxides in three catalysts were 10 wt %. All the samples were crushed and sieved to 20-40 mesh for test. 2.2. Characterization of Catalysts The catalysts were characterized by X-ray diffraction (XRD), Raman, transmission electron microscopy (TEM), N2 adsorption-desorption, Electron paramagnetic resonance (EPR), X-ray photoelectron spectra (XPS), Temperature-programmed reduction by hydrogen (H2-TPR), and in-situ Fourier Transform Infrared Spectroscopy (FTIR). This selection was described in greater detail in the Supporting Information (SI). 2.3. Active test The catalytic activity test for NH3-SCR was carried out in a fixed-bed U-shaped quartz reactor (inner diameter = 4 mm) with a thermocouple under atmospheric conditions. The model reactant gas consist of 1000 ppm NO, 1000 ppm NH3, 10 vol % O2, 100 ppm SO2 6


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(when used), and balance He with a total flow rate of 100 mL·min-1. Different GHSV (30 000 h-1, 60 000 h-1 and 90 000 h-1) were obtained by changing the amount (200 mg, 100 mg and 67 mg) of the catalyst. The concentrations of NO and NO2 in the outlet gas were measured online with a gas analyzer (KANE 9506). The N2 concentration in outlet gas was measured by a gas chromatography equipped with a 5A molecular sieve column (Techcomp 7890II). The NOx conversion (%) X(NOx) and N2 selectivity were calculated as equation (1) and (2), respectively: XNOx =

[NOx ]in -[NOx ]out [NOx ]in

N2 selectivity % =

×100%

(1)

2[N2 ]out [NOx ] -[NOx ] in

out

+[NH3 ] -[NH3 ] in

×100%

(2)

out

Where NOx represents NO + NO2 and the subscripts “in” and “out” denote the inlet and outlet gas concentration of the reactant, respectively. The reaction rate k (mol·s-1·m-2) was normalized on a per specific surface area basis under 100 oC via the equation (3) 26: k=-

Q·ln(1-X) S·m

(3)

where Q is the flow of gaseous molecules (mol·s−1), m is the weight of the catalysts (g), X is the fractional conversion, and S is the specific surface area (m2·g−1). The apparent activation energies (Ea) were calculated from the slope of the linear plot of ln(k) versus 1000/T through the Arrhenius law (equation (4)): 7



k = A·exp(

-Ea RT )

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(4)

The catalytic performance of NO oxidation to NO2 was also measured in the same quartz reactor with 200 mg catalyst. The reaction gas composition was as follows: 1000 ppm NO, 10 vol % O2 and balance He with a total flow rate of 100 mL·min−1, and the GHSV was about 30 000 h−1. The NO conversion was calculated by using the following equation (5): NO conversion =

[NO]in -[NO]out [NO]in

×100%

(5)

3. Results and discussion 3.1. Catalytic performance The catalytic activities over the three catalysts (Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S)) are shown in Figure 1a. The results showed that the light-off temperature (where the NOx conversion achieves 50%) for Fe-Mn/Ti catalyst was ~90 oC, and reached 100% NOx conversion at 175 oC over a operation temperature window with above 90% NOx conversion from 150 to 300 oC. While the Fe-Mn/Ti(M) catalyst exhibited superior catalytic performance, especially the low-temperature activity, for which the light-off temperature decreased to ~70 o

C and achieved full NOx conversion at 125 oC with a wide temperature window from 100 to

325 °C. The results suggested that the active sites of Fe-Mn/Ti(M) catalyst were greatly modified. The low-temperature NOx conversion as well as temperature window over the 8


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Fe-Mn/Ti(S) catalyst, however, were lower than that over the Fe-Mn/Ti(M) catalyst. Meanwhile, the N2 selectivity for each sample was investigated and the results are presented in Figure S1. As shown in Figure S1, the Fe-Mn/Ti catalyst exhibited inferior N2 selectivity and rapid decrease with the increasing of reaction temperature due to the formation of unwanted N2O, while slight enhanced N2 selectivities were achieved over Fe-Mn/Ti(M) and Fe-Mn/Ti(M) catalysts. The results indicated that both the catalytic activity and N2 selectivity were boosted by adding appropriate EG as solvent. Moreover, the SCR performance of the Fe-Mn/Ti(M) catalyst has also been compared with that of other Fe-Mn/Ti catalysts in the previous work (Table S1). Obviously, the outperformed low-temperature catalytic performance of Fe-Mn/Ti(M) catalyst has been achieved in this work as compared with the previous SCR catalysts. The reaction rates were also normalized to further evaluate the catalytic performance. As shown in Figure 1b, the normalized reaction rates for NOx conversion increased from 50 to 100 oC over three samples. For the Fe-Mn/Ti(M) catalyst, the reaction rates were about 15×10−8 mol·s−1·m−2 at 50 oC and 25×10−8 mol·s−1·m−2 at 75 oC, which were twice higher than that of the Fe-Mn/Ti catalyst. This result demonstrated again that the Fe-Mn/Ti(M) catalyst possessed excellent catalytic activity in NH3-SCR reaction. 3.2. XRD and Raman 9



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Figure 2 shows the XRD and Raman patterns of the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts. In the XRD patterns (Figure 2a), the peaks at 2θ = 25.3o, 36.9o, 37.9o, 38.7o, 48.0o, 54.0o, 55.0o, 62.7o, 68.9o, 70.3o and 75.1o were attributed to anatase TiO2 (PDF#21-1272). There were just the diffraction peaks of TiO2, and the diffraction peaks of Fe and Mn oxides could not be detected in the three patterns, which indicated that Fe and Mn oxides were dispersed with amorphous phase or crystallite phase of very small particle size on the anatase TiO2 support

27-28

. Similarly, the Raman spectra in Figure 2b only showed

characteristic peaks of anatase TiO2. Eg peaks (144 and 636 cm−1), B1g peak (394 and 514 cm−1), and A1g peak (514 cm−1) represented the symmetric stretching vibration, the symmetric bending vibrations, and anti-symmetric bending vibrations of O-Ti-O, respectively

29-30

. It

could be observed, however, that the characteristic Raman peaks derived from Fe-MnTi(M) catalyst possessed of the features of the lowest intensities, manifesting that the introduction of appropriate amount of EG could markedly weaken the crystallization of TiO2 surface, thus probably leading to surface modification of the catalyst and generation of more active species 31

.

3.3. TEM and BET TEM was performed to characterize the morphologies of the catalysts, and the statistical size distribution size for each catalyst was estimated accordingly. Figure 3 shows the TEM 10


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images of the three samples. Compared with the Fe-Mn/Ti catalyst, the Fe-Mn/Ti(M) catalyst was observed to be fine dispersed with smaller particles (Figure S2), and the Fe-Mn/Ti(S) catalyst showed bulk-like structure with serious aggregation configuration. The results certified that the dispersion of the catalyst could be well modulated by introducing a suitable amount of EG, which was consistent well with XRD and Raman results. Therefore, more active sites would form on the catalyst surface in consequence of the changes of the chemical innards with highly dispersed property 32. The N2 adsorption-desorption isotherms of the catalysts are shown in Figure S3. According to IUPAC classification, the three similar adsorption isotherms the catalysts were type IV, suggesting that the prepared catalysts were typical mesoporous materials

23, 32

. The BET

specific surface area, pore volume, and pore size are displayed in Table S2. The results illustrated that the physical properties of the as-samples catalysts were almost the same, implying that the BET specific surface area was not the main factor affecting NH3-SCR activity 33. 3.4. EPR The EPR was applied to detect the defect and geometry of transition metal ions. As shown in Figure 4, the EPR spectra of the Fe-Mn/Ti(M) catalyst showed three signals located at the g value of 1.984, 2.504, and 3.959, respectively. According to previous studies, the signal of g 11



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value around 1.99 was assigned to Ti vacancies (Ti3+ defect and Ti4+ defect) 34, which could be attributed to the partial substitution of Fe and/or Mn for Ti in the TiO2 lattice and the further generation of O-rich environment between Ti-O-Ti parallel lattice chains

35

. And the

signal of g = 3.959 could be ascribed to Fe3+ substituted in the lattice adjacent to a charge-compensating oxide anion vacancy

36

. Besides, the broad shoulder peak around g =

2.504 was assigned to Mn2+ species, suggesting that a distorted octahedral coordination in TiO2 was occupied, or there should be more than one Mn2+ species 37. On the contrary, only a very weak signal with the g value of 1.993 showed in the Fe-Mn/Ti catalyst, as the negligible doping of Fe or/and Mn to TiO2. For the Fe-Mn/Ti(S) catalyst, the EPR signal exhibited a isotropic single line at g = 2.321 with Lorentzian shape, which conformed to the Mn2+ species 37

. Since the typical Mn4+ ion within a cubic structure generally showed an EPR signal around

g value of 2.0, and this signal further splits into six hyperfine lines on account of the interaction between the unpaired electrons and the Mn nucleus

38

. The absence of six

hyperfine lines in Fe-Mn/Ti(S) catalyst due to the strong electron dipole-dipole interaction among the dense MnOx species on the TiO2 surface

39

, which might be caused by the

aggregation of the MnOx species. 3.5. XPS Analysis To investigate the surface atomic concentrations and valence states of the catalysts, Fe, Mn, 12


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Ti, and O elements over the three catalysts were measured by XPS (Table 1 and Figure 5). As listed in Table 1, the mole ratio of Fe/Ti and Mn/Ti were obviously higher than the corresponding bulk ratio, and the ratio of Mn/Ti increased obviously, whereas the ratio of Fe/Ti decreased slightly with the increase of EG, indicating the improvement of surface Mn species and the decline of surface Fe species. The XPS results demonstrated that the amount of EG could greatly affect the surface active species. The XPS spectra of Fe 2p are shown in Figure 5a. The two distinct peaks around 712.0 eV and 725.5 eV were ascribed to Fe 2p3/2 and Fe 2p1/2, respectively, which attributed to the characteristic Fe3+ 25, 28, 40-41. For Fe-Mn/Ti(M) catalyst, both of the Fe 2p3/2 and Fe 2p1/2 peaks located at higher binding energies, which revealed the strongest average oxidative ability of iron species in the Fe-Mn/Ti(M) catalyst due to the existence of electronic inductive effect 32. Figure 5b shows the XPS results of Mn 2p over the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts. According to the previous literatures, two characteristic peaks located at 642.5 and 653.7 eV were observed and assigned to Mn 2p3/2 and Mn 2p1/2, respectively 14, 28, 42

. The sub-bands at the binding energies of 641.2 and 642.6 eV were corresponding to Mn3+

and Mn4+, respectively

28

, and the peak at 644.7 eV represented Mn nitrate

43-44

. It is

well-known that Mn4+ species is more active than Mn3+ species, so that the ratio of Mn4+/Mn3+ is very significant for Mn-containing catalysts. As listed in Table 1, the 13



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Mn4+/Mn3+ ratio for Fe-Mn/Ti(M) was higher than the other two catalysts, which indicated that both surface concentration and oxidation state of Mn species were changed in the high dispersed Fe-Mn/Ti(M) catalyst. Ti 2p XPS spectra are displayed in Figure 5c. The two peaks ~458.5 eV and ~464.3 eV could be ascribed to Ti 2p3/2 and Ti 2p1/2, respectively, which were corresponding to Ti4+. Compared with Fe-Mn/Ti catalyst, both of the Ti 2p3/2 and Ti 2p1/2 peaks in the Fe-Mn/Ti(M) catalyst located at the lower binding energies. The previous studies proposed that the lower binding energies of Ti 2p and the corresponding higher binding energies of Fe 2p was attributed to the formation of Fe-O-Ti bond

32, 45

. Coupled with the EPR and XPS results, it

could be concluded that the interaction between Fe and Ti in the high dispersed Fe-Mn/Ti(M) catalyst was enhanced through the formation of Fe-O-Ti structure, which played a key role in the improvement of SCR activity 32. Figure 5d shows the O 1s XPS spectra of the prepared catalysts. The sub-bands at binding energy of ~529.8 eV were assigned to the lattice oxygen O2- (denoted as Oβ) and the sub-bands at binding energy of ~531.2 eV were assigned to the surface-adsorbed oxygen (denoted as Oα), including defect oxide and hydroxyl-like groups

1, 46-47

. It was reported that

Oα was beneficial for SCR reaction via ‘‘fast SCR” process on account of its higher mobility 47

. For the prepared catalysts, the Oα/(Oα + Oβ) ratios were calculated and are listed in Table 1. 14


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It could be seen that the Oα/(Oα + Oβ) ratios decreased as follows: Fe-Mn/Ti(M) (24.5%) > Fe-Mn/Ti(S) (20.7%) > Fe-Mn/Ti (19.8%), which was in accordance with the low-temperature SCR performance (Figure 1). 3.6. H2-TPR H2-TPR was applied to investigate the reduction properties of the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts, and the results are shown in Figure 6. For each prepared catalyst, three broad peaks α, β, and γ of H2 consumption located at ~366, 467, and 591 oC were observed, which could be assigned to the reduction of MnOx, FeOx and TiO2 species, respectively 21, 48. Compared with Fe-Mn/Ti catalyst, the peaks of MnOx species, FeOx species and TiO2 reduction in Fe-Mn/Ti(M) catalyst shift to higher temperature, which implied the stronger inner interaction between active species and support as well as its lower reducibility, and demonstrated that the Fe-Mn/Ti(M) catalyst was high dispersed

21

. What’s more, it was

reported that the Mn4+-O-Mn4+ oligomers was one of the primary active species in the high dispersed Fe-Mn/Ti catalyst

21, 48

. According to the XPS results, therefore, it could be

concluded that the Mn4+-O-Mn4+ oligomer species and the Fe-O-Ti structures with strong interaction were the main species in the high dispersed Fe-Mn/Ti(M) catalyst, which led to higher reduction temperature. For the Fe-Mn/Ti(S) catalyst, however, it was opposite as the formation of large amounts of aggregated active species. 15



Page 16 of 50

According to the processed results, the reduction peak areas of α, β, and γ of Fe-Mn/Ti(M) were relatively larger than that of Fe-Mn/Ti, indicating more H2 consumption over the Fe-Mn/Ti(M) catalyst. One reason was that the more active manganese species with +4 valence migrated from the bulk to the surface leading to more consumption of reductant 28. Another was the formation of abundant Fe-O-Ti structure with strong interaction. 3.7. NO oxidation It is commonly accepted that the low-temperature SCR catalytic performance could be promoted via “fast SCR” process (2NO + 2NO2 + 4NH3 → 4N2 + 6H2O) by enhancing the procedure of NO oxidation

42, 46, 49

. Therefore, the activities of NO oxidation to NO2 over the

Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts were measured in this work. As shown in Figure 7, the NO conversions were sensitive to reaction temperature and improved with the rise of temperature over the three samples. For Fe-Mn/Ti(M) catalyst, the catalytic oxidation activity was higher than that over the other two catalysts in the range of 50-150 oC. Some previous studies have proved that NO could be easily oxidized to NO2 over Fe3+ species 49-50. Liu et al.

32

have proposed that gaseous NO could react with surface reactive oxygen on the

Fe3+ sites with enhanced oxidative ability due to the inductive effect of Ti4+ to generate adsorbed NO2 or nitrate species, further participate in the following reactions with NH3. Therefore, it could be affirmed that the highly dispersed catalyst with more chemisorbed 16


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oxygen induced by novel Fe-O-Ti structure could strengthen NO oxidation, thereby enhancing the low-temperature SCR activity 50-51. 3.8. Kinetic parameters The Arrhenius plots of normalized reaction rate over Fe-Mn/Ti and Fe-Mn/Ti(M) catalysts are shown in Figure 8. The apparent activation energies (Ea) for SCR reaction (Table 1) were determined by the slope in the temperature range of 50-100 oC 26. Compared with Fe-Mn/Ti catalyst (Ea = 28.14 kJ·mol-1), the apparent activation energy over Fe-Mn/Ti(M) (Ea = 18.55 kJ·mol-1) and Fe-Mn/Ti(S) (Ea = 18.86 kJ·mol-1) catalysts markedly decreased, and the Fe-Mn/Ti(M) catalyst exhibited higher reaction rate. The results demonstrated that the modification of catalyst not only decreased the energy barriers but also increased the reaction rate of NH3-SCR, which resulted from the boosted and/or increased active sites on the highly dispersed Fe-Mn/Ti(M) catalyst. 3.9. Effect of GHSV and SO2 The NOx conversions over Fe-Mn/Ti(M) catalyst under different GHSV were tested by changing the amount of catalyst to meet the variation of reaction conditions. As shown in Figure 9, the NOx conversions decreased in some degree with the increase of GHSV (from 30 000 h−1 to 90 000 h−1), especially in the range of 50-125 oC. When the GHSV increased to 60 000 h−1, the Fe-Mn/Ti(M) catalyst still maintained more than 90% NOx conversion in the 17



Page 18 of 50

range of 150 to 350 oC, and further raised the GHSV to 90 000 h−1, the temperature window decreased seriously, suggesting that the GHSV has a significant influence on NOx conversion of the Fe-Mn/Ti(M) catalyst. Generally, a certain amount of SO2 and other toxic components exist in the real flue gas 13, 52-53

, more importantly, SO2 makes some considerable influences due to the possible

formation of stable metal sulfates and ammonium sulfate species on the surface of catalyst 33. Hence, the effect of SO2 on catalytic performance over the Fe-Mn/Ti(M) catalyst was further investigated. As shown in Figure S4, the NOx conversions just decreased slightly in the whole temperature range with 100 ppm SO2. Moreover, it could be found that the temperature window did not change and more than 95% NOx conversion was obtained from 125 to 275 oC, which indicated that the Fe-Mn/Ti(M) catalyst possessed of excellent sulfur-poisoning resistance. Additionally, durability experiments of Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) were also performed at 125 oC and the results are shown in Figure 10. Obviously, all of the prepared catalysts showed well stabilities with reaction time in the absence of poison. When 100 ppm SO2 was added into the reaction system, the Fe-Mn/Ti and Fe-Mn/Ti(S) catalysts showed serious activity decline, while NOx conversion over the Fe-Mn/Ti(M) catalyst just exhibited slight decrease and still kept more than 90% in the following operation. After SO2 was switched off, the NOx conversion could promptly recovery to the original level, 18


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demonstrating the superior durability against SO2 over the Fe-Mn/Ti(M) catalyst. 3.10. In-situ FTIR 3.10.1. In-situ FTIR of NH3/NO+O2 adsorption In order to investigate the adsorption behavior of reactants in the NH3-SCR reaction, especially in the low temperature range, both NOx and NH3 adsorption were measured by in-situ FTIR spectroscopy at 100 °C. After pretreatment of each sample, 1000 ppm NH3 or 1000 ppm NO + 10 vol % O2 was introduced and IR spectra were recorded. To compare NOx and NH3 adsorption performance between the two samples at low temperature, the FTIR spectra over Fe-Mn/Ti and Fe-Mn/Ti(M) catalysts are shown in Figure 11. Figure 11a shows the FTIR spectra of NH3 adsorption over the two samples. Several bands were observed in the range of 1000-1700 and 3000-4000 cm−1. The strong absorbed bands at 1161, 1222 and 1595 cm−1 could be assigned to symmetric and asymmetric bending vibrations of the N-H bonds in NH3 coordinately linked to Lewis acid sites 18, 42, 54-55, while the relatively weak absorbed bands at 1441 cm−1 and around 1675 cm-1 were also observed, which could be attributed to asymmetric and symmetric bending vibrations of NH4+ species on Brønsted acid sites

18, 56-58

. In the stretching region, some bands at 3257, 3352 cm−1 and several negative

bands around 3661 cm−1 could be assigned to the surface N-H stretching and O-H stretching, respectively

18, 56-57

. The intensity of all bands over Fe-Mn/Ti(M) catalyst was slightly lower 19



Page 20 of 50

than that over Fe-Mn/Ti catalyst, manifesting the decrease of NH3 adsorption capacity as well as surface acidity

55

. Although the decline of NH3 adsorption capacity, the apparent NOx

conversions over the Fe-Mn/Ti(M) catalyst still increased obviously in the low temperature range below 125 °C (Figure 1), suggesting that in which NH3 adsorption was not the rate-determining step of NH3-SCR. Base on it, the amounts of adsorbed NH3 species over the Fe-Mn/Ti(M) catalyst should be totally enough for the typical SCR reaction, so that the amounts of adsorbed NOx species should be the key factor for the apparent SCR activities 32. In contrast to NH3 adsorption behavior, great changes occurred on NOx adsorption performance. As shown in Figure 11b, several distinct bands at 1610, 1584, 1488, 1285, 1256, and 1007 cm−1 were discovered in the range of 2000-1000 cm-1. The bands at 1007 and 1584 cm-1 might be assigned to bidentate nitrate

58-59

. The bands at 1285 and 1488 cm−1 could be

ascribed to the monodentate nitrate 18, 60, and the shoulder band at 1256 and the band at 1610 cm−1 could be attributed to bridged nitrate and gaseous NO2 molecules, respectively

18, 50, 58

.

Notably, the intensity of all bands over Fe-Mn/Ti(M) catalyst was obviously higher than that over Fe-Mn/Ti catalyst, which demonstrated that more nitrate species and gaseous NO2 adsorbed on the surface of the Fe-Mn/Ti(M) catalyst. It has been reported that the formation of monodentate nitrate species Fe-O-NO2 was ascribed to the oxidation of gaseous NO by O2 over Fe3+

50, 61

, and Wu et al proposed that bridged nitrate was formed on Fe2O3

18

.

20


Page 21 of 50 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


Furthermore, bidentate nitrate was considered to be formed on the MnOx species

18

. Hence,

according to the XPS and H2-TPR results, it could be concluded that the more monodentate nitrate and bridged nitrate species were attributed to the large number of Fe3+ in the Fe-O-Ti structures. And the increase of NO2 species and bidentate nitrate on the surface of the catalyst mainly resulted from the increase of chemisorbed oxygen and Mn species, respectively. As a consequence, the increased nitrate species formed on the highly dispersed catalyst were responsible for the enhancement of catalytic performance. 3.10.2. In-situ FTIR of SCR reaction To further investigate the reaction process of NOx and NH3 species over the Fe-Mn/Ti and Fe-Mn/Ti(M) catalysts in SCR reaction, the in-situ FTIR spectra of reaction between NO + O2 and pre-adsorbed NH3 species at 100 °C were recorded, and the results are shown in Figure 12. For the Fe-Mn/Ti catalyst (Figure 12 (a)-(b)), several bands attributed to ionic NH4+ (1444 and 1665 cm−1) and coordinate NH3 (1161, 1222, and 1599 cm−1) were detected after NH3 pre-adsorption and He purge. When NO+O2 was added, the intensity of the shoulder peak in 1222 cm−1 increased first, which should be caused by the adsorption of NOx species. Then, all bands of NH3 species weakened gradually and almost totally disappeared after 40 min. Meanwhile, new bands ascribed to H2O (3523 cm−1) and NOx species (1010, 1255, 1286, 1479, 1583 and 1608 cm−1) began to form on the catalyst surface. After 60 min, NH3 species 21



Page 22 of 50

totally disappeared and the catalyst was covered by nitrate species. This results indicated that both coordinate NH3 and ionic NH4+ participated in the SCR reaction. The process for the Fe-Mn/Ti(M) catalyst (Figure 12 (c)-(d)) was almost the same with the Fe-Mn/Ti catalyst expect for the less NH3 species adsorption. Certainly, NOx species also played a very important role in NH3-SCR reaction, especially in the low temperature range. Thus, the in-situ FTIR experiment between NH3 and pre-adsorbed NOx species over Fe-Mn/Ti and Fe-Mn/Ti(M) surface were carried out at 100 °C (Figure 13). As shown in Figure 13 (a)-(b), after NOx adsorption and He purge, the Fe-Mn/Ti catalyst surface was mainly covered by monodentate nitrate (1286 and 1491 cm−1), bidentate nitrate (1009 and 1582 cm−1), bridged nitrate (1261 cm−1), and gaseous NO2 (1610 cm−1) species. When NH3 was introduced, the intensity of the band around 1610 cm−1 decreased gradually, suggesting the consumption of surface nitrate species. Meanwhile, the bands around 1491 and 1286 cm−1 were shifted to 1443 and 1297 cm−1, respectively, which was caused by the overlap of bands ascribed to monodentate nitrate and adsorbed NH3 species 50. Besides, a new band at 1201 cm−1 attributed to coordinated NH3 appeared after 35 min. However, the bands at 1582 and 1009 cm−1 and the shoulder band at 1261 cm−1 almost reminded unchanged until reaction equilibrium. Finally, both adsorbed NH3 and unreacted NOx species could be observed on the catalyst surface. Apparently, ammonia species only reacted with gaseous NO2 species, 22


Page 23 of 50 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


indicating gaseous NO2 species was the active one in this process. Thus, the more gaseous NO2 species on the Fe-Mn/Ti(M) catalyst (Figure 13 (c)-(d)) corresponding to the better low-temperature catalytic performance for the SCR reaction. According to the above in-situ FTIR results, it could be confirmed that the Langmuir-Hinshelwood (L-H) mechanism was the main pathway for both Fe-Mn/Ti and Fe-Mn/Ti(M) catalysts at 100 oC, since NH3 only reacted with adsorbed NOx species (NO2) 35, 42, 50

. For the Fe-Mn/Ti(M) catalyst, large amount of adsorbed gaseous NO2 made the

enhancement of catalytic performance via “fast SCR” process. In addition, considering the reaction time and the quantity of adsorbed NOx species, it could be potentially concluded that the reaction rate for the Fe-Mn/Ti(M) catalyst was relatively faster in the low-temperature SCR process.

4. Conclusions A novel Fe-Mn/Ti catalyst for the NH3-SCR reaction was prepared using an EG-assisted impregnation method. The XRD, Raman, and TEM results confirmed that the Fe-Mn/Ti(M) catalyst was finely dispersed with regular nanoparticles. The Fe-Mn/Ti(M) catalyst exhibited superior efficacy for NO removal below 150 oC over a broad temperature window between 100 and 325 oC. Meanwhile, the as-prepared catalyst showed excellent durability against the 23



Page 24 of 50

SO2 poisoning and more than 90% NO conversion was achieved even in the presence of 100 ppm SO2. The EPR, XPS and H2-TPR results showed that the formation of Fe-O-Ti structure with strong interaction strengthened the electronic inductive effect and increased the ratio of surface chemisorption oxygen. Therefore, the NOx adsorption capacity as well as the NO oxidation performance was greatly enhanced as a result of the improvement of dispersion even though the slight decrease of surface acidity, and NH3-SCR activity was consequently boosted at low temperatures.

Acknowledgements This work was supported financially by the Key Project of the National Ministry of Science and Technology (No. 2016YFC0204204), the National Natural Science Foundation of China (No 21577012), the Major Program of the National Natural Science Foundation of China (No. 21590813), the Program of Introducing Talents of Discipline to Universities (B13012), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

Associated Content Supporting Information 24


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The section of catalyst characterization; Comparison of catalytic activity (Table S1); Physical properties of the catalysts (Table S2); N2 selectivity of the catalysts (Figure S1); Particle size distribution of the catalysts (Figure S2); N2 adsorption-desorption isotherms and BJH pore size distributions curves (Figure S3); NOx conversion under the presence of SO2 (Figure S4).

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35



Page 36 of 50

Table 1. Chemical composition and properties of the catalysts analysed using XPS

Mole ratio Samples

Mn4+/Mn3+

Oα ratio (%)

Ea (kJ·mol-1)

Mn/Ti

Fe/Ti

Fe-Mn/Ti

0.123(0.09)a

0.153(0.05)a

3.17

19.8

28.14

Fe-Mn/Ti(M)

0.182(0.09)

0.108(0.05)

3.44

24.5

18.55

Fe-Mn/Ti(S)

0.188(0.09)

0.097(0.05)

2.03

20.7

18.86

a

The number in the brackets is denoted as the corresponding bulk ratio.

36


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Figure 1.

NOx conversions (a) and normalized reaction rates (b) of the Fe-Mn/Ti, Fe-Mn/Ti(M) and

Fe-Mn/Ti(S) catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 10 vol %, He balance and GHSV = 30 000 h−1.

37



Figure 2.

Page 38 of 50

XRD (a) and Raman (b) patterns of the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts.

38


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Figure 3.

Typical TEM images of the catalysts: (a) and (d) Fe-Mn/Ti, (b) and (e) Fe-Mn/Ti(M), (c) and

(f) Fe-Mn/Ti(S).

39



Figure 4.

Page 40 of 50

EPR spectra of the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts. Inset: the magnified

EPR spectra of Fe-Mn/Ti catalyst in the range of 3000-4000 G.

40


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Figure 5.

XPS spectra of Fe 2p (a); Mn 2p (b); Ti 2p (c); and O 1s (d) over the Fe-Mn/Ti, Fe-Mn/Ti(M)

and Fe-Mn/Ti(S) catalysts.

41



Figure 6.

Page 42 of 50

H2-TPR profiles of the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts.

42


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Figure 7.

NO conversions in oxidation of NO to NO2 over the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S)

catalysts. Reaction conditions: [NO] = 1000 ppm, [O2] = 10 vol %, He balance, and GHSV = 30 000 h−1.

43



Figure. 8.

Page 44 of 50

The Arrhenius plots of the Napierian logarithm of the reaction rates with respect to the

catalyst surface over the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 10 vol %, He balance and GHSV = 30 000 h−1.

44


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Figure 9.

NOx conversions under different GHSV over the Fe-Mn/Ti(M) catalyst. Reaction conditions:

[NO] = [NH3] = 1000 ppm, [O2] = 10 vol %, and He balance.

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Figure 10.

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Stabilities and effect of SO2 over the Fe-Mn/Ti, Fe-Mn/Ti(M) and Fe-Mn/Ti(S) catalysts at 125 oC. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 10 vol %, [SO2] = 100 ppm (when used), He balance and GHSV = 30 000 h−1.

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Figure 11.

In-situ FTIR spectra of NH3 adsorption (a) and NO + O2 adsorption (b) over the Fe-Mn/Ti and

Fe-Mn/Ti(M) catalysts at 100 °C.

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Figure 12.

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In-situ FTIR spectra of NO + O2 adsorption after per-adsorption of NH3 over the Fe-Mn/Ti

(a)-(b) and Fe-Mn/Ti(M) (c)-(d) catalysts at 100 °C.

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Figure 13.

In-situ FTIR spectra of NH3 adsorption after per-adsorption of NO + O2 over the Fe-Mn/Ti

(a)-(b) and Fe-Mn/Ti(M) (c)-(d) catalysts at 100 °C.

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Enhancement of Low-Temperature Catalytic Activity over Highly

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