Controlled Synthesis of Spinel-Type Mesoporous MnâCo Rods for

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Kinetics, Catalysis, and Reaction Engineering

Controlled synthesis of Spinel-Type mesoporous MnCo rods for SCR of NOx with NH3 at low temperature Yiran Shi, Xiaolong Tang, Honghong Yi, Fengyu Gao, Shunzheng Zhao, Jiangen Wang, Kun Yang, and Runcao Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05223 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Industrial & Engineering Chemistry Research

Controlled synthesis of Spinel-Type mesoporous Mn-Co rods for SCR of NOx with NH3 at low temperature Yiran Shia, Xiaolong Tanga,b,

, Honghong Yia,b, Fengyu Gaoa,b,

Shunzheng Zhaoa,b, Jiangen Wanga, Kun Yanga, Runcao Zhanga a Department of Environmental Engineering, School of Energy and Environmental Engineering,

University of Science and Technology Beijing, Beijing 100083, PR China b Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, University of

Science and Technology Beijing, Beijing 100083, PR China



Corresponding author: Xiaolong Tang. E-mail: [email protected]

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

ABSTRACT: In this work, a series of Mn-Co rod-like layered catalysts selfassembled from porous rod aggregates were fabricated by a simple solvothermal approach. Compared with Mn-Co particles, Mn-Co rods exhibited a much better SCR performance (achieving >93% NOx conversion at 75℃) and high N2 selectivity. The outstanding catalytic performance was mainly attributed to the large specific surface area and unique rod-like mesoporous structures, which could provide more active sites and reaction centers for the reaction gas to further promote the de-NOx performance of the catalyst. In addition, high atomic ratio of Mn4+/Mn on the catalyst surface, strong acid strength and large acidity amount also had positive effect on its catalytic activity. Moreover, Mn-Co rods possessed a better catalytic cycle performance and more surface acid sites due to the synergistic effect between Co and Mn species.

1. INTRODUCTION Exhaust emissions from vehicles and industrial combustion using fossil fuels are the main pollutant sources of nitrogen oxides. The selective catalytic reduction (SCR) of NOx by ammonia is currently considered as the most feasible method for flue gas denitrification technologies. Among various commercial SCR catalyst, V2O5WO3(MoO3)/TiO2 exhibit particularly high activity and excellent sulfur resistance within the temperature range of 300-400℃. However, several inevitable drawbacks of the toxicity of vanadium species, higher operating temperatures and poor tolerance for dust and alkali metals still exist. Besides, the catalysts could not meet the actual requirement of emerging industries such as boiler and domestic glass furnace in which the temperature of the exhaust gas is usually under 200℃. Hence, it is urgent to develop economical and effective processing catalyst has become a universal problem concerned by the environmental protection industry today. In recent years, Mn-based catalysts promoted by transition metals(Cu, Fe, Ni, Co, Cr) have attracted much attention for low temperature NH3-SCR due to their excellent redox properties1-5. Among these transition metal elements, Co is applied in the field of catalysis because of its excellent redox properties and simple morphology control etc. Moreover, the properties of the catalyst are easily affected by the regulation of the 2 ACS Paragon Plus Environment

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morphology of the material.6 Through previous literature reports, many researchers have synthesized catalysts with special structures or special morphologies by coupling Mn and Co, which has improved some properties of the catalyst and is more conducive to the catalyst reaction. For instance, Zhang et al. used a self-assemble method to prepare hollow porous MnxCo3−xO4 nanocage catalysts derived from metal-organic frameworks, which exhibited better catalytic performance compared to conventional MnxCo3−xO4 nanoparticles and the main reason is attributed to the hollow and porous structures7. Hu et al. synthesized a mesoporous 3D MnxCo3-xO4 nanosphere via a redox reaction between precursors without using templating agent, which showed 80% NOx conversion in the range of 75-325℃. This remarkable catalytic performance was mainly attributed to high specific surface area, abundant acidic sites and metal-metal interaction8. In particular, the rational design and controllable synthesis of Mn-Co bimetal oxide catalysts with different morphologies are very effective ways to improve their catalytic performance, but still many challenges. In this paper, a rod-like hierarchical structure Mn-Co material was synthesized by a simple solvothermal approach to investigate its NH3-SCR activity and water/sulfur resistance at low temperatures. Aiming to observe the difference between the rods structure and the morphology of particles, we also prepared Mn-Co particles with same conditions as Mn-Co rods by co-precipitation method. The rod-like hierarchical structure catalyst exhibited remarkable NOx conversation compared to the particle catalyst. To further illuminate the mechanism of the catalytic performance of Mn-Co catalysts with two different Morphologies, SEM, BET, XRD, Raman spectroscopy, XPS, H2-TPR, NH3-TPD, FTIR are applied to characterize the reaction process.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Activity Test All reagents are analytical grade without further purification. The spinel-type MnCo mesoporous rods were fabricated by a simple solvothermal approach without the assistance of surfactants. In a typical process, 0.25g Mn(COOH)2·4H2O and 0.58g Co(NO3)2·6H2O were dissolved in 60 ml ethylene glycol(EG) and 20 ml deionized 3 ACS Paragon Plus Environment


water (3:1 v/v ratio) to form a transparent solution under magnetic stirring for half an hour. Then, 0.38g H2C2O4 was added to this solution slowly under continuous stirring for about 1 h at room temperature (RT). The final pink solution was transferred into an autoclave (Teflon-lined stainless steel) with a volume of 100 mL and hydrothermally treated at 140°C for 24 h in an oven. The products were collected by centrifugation, washed several times with deionized water and absolute ethanol, and dried in air at 80°C for 12 h. The Mn2O3/Co3O4 rods were prepared by using 3mmol Mn(CH3COO)2·4H2O or Co(NO3)2·6H2O, while the other steps were consistent with the preparation of Mn-Co rods. As a comparison, the Mn-Co particles were synthesized by the co-precipitation method. Briefly, 0.25g Mn(COOH)2·4H2O and 0.58g Co(NO3)2·6H2O were dissolved in 60 ml ethylene glycol(EG) and 20 ml deionized water (3:1 v/v ratio) at room temperature. Then, ammonia solution (28%) were added dropwise to the above solution under continuously stirred until the pH of the solution was maintained at 9. After aging for 24h, the product was collected by filtration, dried at 80 °C for 12 h. Finally, all products were calcined in air at 450°C for 2 h with a heating rate of 2°C min−1. The NH3-SCR activity tests were carried out in a fixed-bed quartz reactor(i.d.=8mm) using 0.35 g catalyst with 40-60 mesh at atmospheric pressure. The typical gas mixture used here was made up of 2000 ppm NO, 2000 ppm NH3, 8 vol% O2, 10 vol% H2O (while necessary), 100 ppm SO2 (while necessary) and N2 acted as balance gas, and the total flow rate of feed gas was 100 mL/min, corresponding to the GHSV of 32000 h-1. The inlet and outlet concentrations of NOx was measured using Thermo Fisher Scientific Nicolet iS50 FT-IR spectrometer. NOx conversion and N2 selectivity were calculated as follows: NOx Conversion (%) = (1- [NOx]outlet/[NOx]inlet)×100% with [NOx]=[NO]+[NO2] N2 Selectivity (%) = (1 -

2[𝑁2𝑂]𝑜𝑢𝑡

)×100%

[𝑁𝑂𝑥]𝑖𝑛 + [𝑁𝐻3]𝑖𝑛 - [𝑁𝑂𝑥]𝑜𝑢𝑡 ― [𝑁𝐻3]𝑜𝑢𝑡

2.2. Characterization The morphology and structure of the prepared Mn-Co series samples were characterized by scanning electron microscopy (SEM, SU8000), transmission electron 4 ACS Paragon Plus Environment

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microscopy (TEM, JEM-2010) and a high-resolution transmission electron microscope (HRTEM, JEM1200EX). N2 adsorption-desorption isotherms were obtained after pretreatment of samples at 300℃ for 2 h using a surface area analyzer (ASAP 2020, USA). Surface areas were determined by Brunauer-Emmett-Teller(BET) method and Barrett-Joyner-Hallend (BJH) method, respectively. X-ray diffraction(XRD) patterns were performed on a Bruker:D8 Advance Diffractometer with Cu Kα radiation running at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo Fisher Scientific Escalab 250XI XPS System with Al K a radiation. All binding energies were calibrated using the C 1s peak (BE = 286.6 eV). The Raman spectra of the catalysts were performed at room temperature by a Thermo Fisher Scientific DXR2 at 532 nm with the exposure time of 50 s. H2-TPR experiments were performed on a Quanta chrome Instruments. the samples (100 mg) were pretreated at 350℃ in N2 for 1 h and cooled down to room temperature (30℃). Then the temperature was raised to 800℃ at a rate of 10℃ min−1. NH3-TPD was characterized by Builder Chemisorption Analyzer. Prior to TPD experiments, the samples were pretreated at 350°C in an N2 flow for 1 h. Subsequently, the samples were cooled down to the room temperature (30℃) and saturated with NH3 until adsorption equilibrium was reached, then followed by flushing with N2 at the same temperature to avoid physisorption of NH3. Finally, the temperature was raised to 800°C at a rate of 10℃ min−1 in flowing N2. In-situ DRIFTS experiments were carried out on an Nicolet IS50 spectrometer equipped with an MCT/A detector cooled by liquid nitrogen and a ZnSe window. Prior to each experiment, the sample was pretreated at 400°C for 1 h under N2 purging at a total flow rate of 100 mL/min. The background spectrum was collected in flowing N2 and automatically subtracted from the sample spectrum. All spectra were recorded from 600 to 4000 cm-1 by collecting 100 scans with a resolution of 4cm−1.

3. RESULTS AND DISCUSSION 3.1 Materials characterization 5 ACS Paragon Plus Environment


The morphologies of series Mn-Co catalysts with different molar ratios observed by SEM are shown in Figure 1. For Mn-Co(1:2) catalyst (Figure 1 a-c), rod-like hierarchical structure was observed and built by self-assembled bundles of porous rods aggregates. For comparison, Mn-Co(1:1) catalyst also presented needlelike rods structure, but have a better long stick shape with smooth edges (Figure 1 d-f). However, as the ratio of Mn/Co is 2:1, the morphology of the rods and the particles coexist in the catalyst, which showed in Figure 1 g-i. To summarize, the morphology of the catalyst had an obvious difference with the increase of the molar ratios of Mn/Co, and MnCo(1:1) exhibited perfect rod-like hierarchical structures compared to others. TEM and HRTEM images were obtained as shown in Figure 2 a and b to further observe the microstructure of the Mn-Co(1:1) rods. It clearly reveals that the formation of hierarchical rod-like structures is self-assembled from very small nanoparticles. In addition, the representative HRTEM image of Figure 2b exhibited the lattice spacings are 0.205 nm, and 0.458 nm, corresponding to the (440) and (111) crystal facets of MnCo spinel phase7, 9. The lattice spacing of 0.235 nm probably ascribed to the (400) facet of formation of Mn2O310. This result can be further confirmed in subsequent XRD and Raman analysis.


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Figure 1. SEM images of materials: (a-c) Mn-Co(1:2), (d-f) Mn-Co(1:1), (g-i) Mn-Co(2:1)

Figure 2. TEM images (a) and high resolution TEM images (b) of the Mn-Co (1:1) rod catalyst

In order to investigated the crystal structures of different molar ratio/ hydrothermal synthesis temperature of Mn-Co catalysts, XRD technology was applied in this paper. All of the diffraction peaks in the XRD patterns are shown in Figure 3. As illuminated in Figure 3 a, the pure MnOx rods and CoOx rods presented the anastomotic diffraction peaks with Mn2O3 (PDF#71-0636) and Co3O4 (PDF#42-1467). Similar diffraction 7 ACS Paragon Plus Environment


peaks of Co3O4 or MnCo2O4.5(PDF card NO.32-0297)

11

were detected in all Mn-Co

rods catalysts from Figure 3 b. According to previous research reports10, in the MnaCobOx series of catalysts, Co3O4 exhibited at very similar diffraction peaks to MnCo2O4.5 under the condition that the molar ratio of Mn/Co is different from MnCo2O4.5. It can be inferred that the XRD diffraction peak of the Mn-Co rod catalysts may overlap with the peak of Co3O4 and MnCo2O4.5. While Mn-Co(1:1) particles exhibited (Co, Mn)(Co, Mn)2O4 crystal phase with different crystal structure from MnCo rods, It is worth noting that the diffraction peaks of Mn-Co rods catalyst has no obvious change with the increase of the hydrothermal reaction temperature, which indicated that the hydrothermal temperature has little effect on the crystal structure. In contrast, the intensity of diffraction peaks of Mn-Co rods becomes stronger with the increase in Co content. Notably, the diffraction peak of impurity Mn2O3 phase was not detected in all the catalysts, indicating that part of the Mn icons may be incorporate into Co3O4 crystal lattice or highly dispersed on the surface of the catalyst12. The Raman spectra was also applied to the characterization of a series of Mn-Co catalysts, and the results are shown in Figure 3 c. Mn rods showed two bands at approximately 319 and 651 cm-1. The strongest peak was assigned to the one reported for Mn2O313, 14. The weaker peak was ascribed to the Mn-O bending modes5. Co rods exhibited five peaks located at 207, 487, 530, 624, 693 cm-1 corresponding to F2g, Eg, F2g, F2g and A1g modes. The Raman spectra of Mn-Co(1:1) catalysts with two morphologies are greatly different. Mn-Co(1:1) particles also exhibited five peaks that shifted to the low-frequency Raman bands compared to Co rods. This phenomenon might indicated that part of Mn icons incorporate into Co3O4 lattice15. In contrast, MnCo(1:1) rods displayed two bands at 482 and 638 cm-1 with lower intensity and broader feature as compared to Mn-Co(1:1) particles. The Raman bands at 482 and 638 cm-1 correspond to Co3O416, 17 and Mn(III, IV) oxides17, 18, respectively. The lower and width Raman spectra of Mn-Co(1:1) rods indicated that the crystallinity of the catalysts was reduced due to the lattice distortion caused by the introduction of manganese into the Co3O4 lattice, which is conducive to the formation of lattice defects and suppressed the speed of crystal growth10, 12, 19, 20. This finding was consistent with XRD results. 8 ACS Paragon Plus Environment

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a

b

(311)

(220)

(111)

(511)

(400)

Mn-Co(1:1)-180 rods

Mn-Co(1:1)-100 rods

(440)

Co rods

Standard Co3O4 (222)

Intensity (a.u.)

Mn-Co(1:2) rods

Intensity (a.u.)

Mn-Co(1:1) rods (220)

(111)

(311)

(400)

(511) (440)

Mn-Co(2:1) rods

Standard MnCo2O4.5

(440) (400)

(211)

(622)

(332) (431)

(202) (113)

Mn rods

(404)

Standard Mn2O3

10

20

30

40

50

60

70

80

Mn-Co(1:1) particles

Standard (Co,Mn)(Co,Mn)2O4

90

10

2θ (degree)

20

30

40

50

60

70

80

90

2θ (degree) 656

c Mn-Co(1:1) particles Mn-Co(1:1) rods Mn rods Co rods

Intensity (a.u.)

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


693 313

362 482

188

638 651 487 530

207

624 319

100

200

300

400

500

600

700

800

Raman shift (cm-1)

Figure 3. XRD patterns of (a) pure CoOx and MnOx (b) different molar ratio/hydrothermal synthesis temperature of Mn-Co catalysts (c) Raman spectra of Mn-Co catalysts

The series of different Mn/Co molar ratios of Mn-Co rods and Mn-Co(1:1) particles were measured by BET analysis. The N2 adsorption-desorption isotherms are shown in Figure 4. Pore diameter, pore volumes and specific surface areas were listed in Table 1. According to the IUPAC classifications, the isotherms of Mn-Co rod samples with different molar ratios corresponding to type IV with a type H1 hysteresis loop, indicating the existence of ordered mesoporous structure. For Mn-Co particles, the isotherm has a type IV curve of the H3 hysteresis loop, indicating the characteristics of the mesoporous, which might be due to the accumulation of particles. According to the information from Table 1, It is obvious that the surface area decreased in the order of Mn-Co(1:1)＞Mn-Co(2:1)＞Mn-Co(1:2)＞Mn rods＞Mn-Co particles＞Co rods, which is highly consistent with the results of catalytic performance. Moreover, the pore volume of the Mn-Co(1:1) rods is larger than that of the Mn-Co(1:1) particles, which results from an mesoporous structure formed by self-assembled particle aggregates. In 9 ACS Paragon Plus Environment


addition, large specific surface area and unique mesoporous structures of Mn-Co(1:1) rods could provide more active sites and reaction centers for the reaction gas, which promotes the de-NOx performance of the catalyst. Quantity Adsorbed Quantity Ddsorption

Quantity Adsorbed (cm3/g)

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

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MnCo(1:2) rods

MnCo(1:1) rods MnCo(2:1) rods MnCo(1:1) particles 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

Figure 4. N2 adsorption−desorption isotherms of the series of Mn-Co catalysts

Table 1 Summary of textural parameters of the samples Pore diameter(nm) Pore volume(cm3/g) Surface area(m2g-1) Mn rods 9.1 0.36 66 Co rods 35.7 0.31 22 Mn-Co(1:1) particles 30.6 0.32 30 Mn-Co(1:2) rods 5.8 0.42 83 Mn-Co(1:1) rods 5.8 0.44 108 Mn-Co(2:1) rods 6.8 0.38 94 In order to explore the surface chemical states in Mn-Co(1:1) rods and Mn-Co(1:1) particles, X-ray photoelectron spectra(XPS) was performed and presented in Figure 5, surface atomic concentration and relative atomic percentages of different valence elements are listed in Table 2. In the Mn 2p XPS spectra, three characteristic peaks divided by the Mn 2p3/2 spectrum can be attributed to Mn2+(641.5 eV), Mn3+(642.6 eV), and Mn4+(643.8 eV) by performing peak-fitting deconvolutions. As shown in Table 2, the percentage of Mn4+/Mn on Mn-Co(1:1) rods is 35.7%, which is much higher than that over Mn-Co(1:1) particles (27.0%), suggesting that more Mn4+ species are presented on the surface of the Mn-Co(1:1) rods compared to the Mn-Co(1:1) particles. It has been reported that Mn4+ could improve the ability of NO oxidation to NO2 by 10 ACS Paragon Plus Environment

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participating in a reversible redox reaction cycle, which is conducive to the promotion of the rapid SCR reaction7, 21, 22. In summary, abundant Mn4+ is present on the surface of the Mn-Co(1:1) rods catalyst, which is considered to be one of the factors for improving the activity of the catalyst. This result is also highly consistent with the activity of the SCR at low temperatures. As can be seen from the Co 2p XPS spectrum, the Co 2p could be fitted into two peaks. The peaks centered around 781.9 and 780.6 eV are assigned to Co2+ and Co3+, respectively23. The peak around 787.1 ± 0.3 eV could be assigned to the satellite peak of Co 2p. As shown in Table 2, the relative atomic percentages concentration of Co3+/Co

were higher than that of Mn-Co(1:1) particles. Generally, Co3+ is much more

reactive than Co2+ because of its stronger redox ability, and therefore, it plays an crucial role in catalyst redox properties7, 24. As demonstrated in Figure 5 c, the O 1s spectra of the catalyst can be deconvoluted into two peaks. The peak at higher binding energy(531.7 eV) was attributed to surface adsorbed oxygen (denoted as Oα), which mainly consists of O22− or O− belonging to defect-oxide or hydroxyl-like group25-27. While the second peak located at lower binding energy (530.4 eV) corresponding to the lattice oxygen such as O2-(denoted as Oβ). The relative atomic percentages concentration of Oα/(Oα+Oβ) were 29.4% and 27.2% for Mn-Co(1:1) particles and Mn-Co(1:1) rods, respectively. According to previous literature reports, Oα can be considered as a electrophilic oxygen, which is beneficial to the NH3-SCR reaction at low temperatures due to its high mobility28. However, by comparing the two catalysts, the Mn-Co(1:1) particles with a higher proportion of surface-adsorbed oxygen showed poor catalytic activity, from which it can be inferred that the active surface oxygen species are not the only factors affecting the activity of the catalyst. Moreover, Shi et al. investigated the synergistic effect between two components on catalytic performance by synthesizing NiyCo1-yMn2Ox three-component metal oxide microspheres and found that the catalyst has excellent activity which was influenced by positive factors such as large SBET, high Ni2+, Co3+ and Mn4+ percentages, appropriate ratio of Ni/Co and crystal structures, etc29. Based on the above XPS analysis, it can be concluded that the main reason for the excellent catalytic performance of Mn11 ACS Paragon Plus Environment


Co(1:1) rods was attributed to high proportion of Mn4+ and Co3+ which were more easy reduction, high percentage active of Mn4+ and Co3+ cationic species on the surface of the catalyst. A)

Mn 2p3/2

Mn 2p1/2

Intensity (a.u.)

a

Mn3+ Mn4+

b

658

656

654

652

650

648

646

Mn2+

644

642

640

Binding Energy (eV)

B)

Co 2p3/2 Co 2p1/2

Intensity (a.u.)

a

Co3+

Co2+

b

810

Sat.

805

800

795

790

785

780

775

Binding Energy (eV)

C)

O O

a

Intensity (a.u.)

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

b 536

534

532

530

528

Binding Energy (eV)

Figure 5. XPS spectra for Mn 2p (A), Co 2p (B), and O 1s (C) of the catalysts: (a) Mn-Co(1:1) rods and (b) Mn-Co(1:1) particles.


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1 2 3 Table 2. Binding energy and relative atomic percentages of different valence 4 elements 5 6 Percent of valence state % (binding energy, eV) 7 catalysts 8 Mn2+ Mn3+ Mn4+ Co2+ Co3+ Oα Oβ Oα/ Oβ Mn4+/Mn3+ 9 10 11 21.9 51.1 27.0 54.8 45.2 29.4 70.6 12 MnCo(1:1) particles 0.42 0.52 13 (641.4) (642.5) (644.0) (781.8,797.0) (780.5,795.6) (531.5) (530.2) 14 15 19.3 45.0 35.7 53.2 46.8 27.2 72.8 16 MnCo(1:1) rods 0.37 0.79 (641.5) (642.6) (643.8) (781.9,797.0) (780.6,795.6) (531.7) (530.4) 17 18 19 20 In order to investigated the series of Mn-Co catalysts on redox properties, H2-TPR 21 22 technique was applied in this experiment and the result were shown in Figure 6. It can 23 24 be seen that Mn rods exhibited two reduction peaks at 286 and 392℃, which could be 25 26 attributed to the reduction of MnO2 to Mn3O4 and Mn3O4 to MnO, respectively10, 30, 31. 27 28 For Co rods, the low temperature reduction peak at 285℃ is assigned to the reduction 29 30 of Co3O4→CoO, and the high temperature peak at 368℃ is ascribed to the 31 32 transformation of CoO→Co11, 32. Obviously, three main H2 consumption peaks were 33 34 observed in all Mn-Co bimetal catalysts. For Mn-Co catalysts, the three main reduction 35 36 reactions are carried out at temperatures ranging from 222 to 245℃, 310-378℃ and 37 38 439-530℃, respectively. The peak around 222-245℃ is assigned to the reduction of 39 40 Mn4+ to Mn2O3, whereas the high temperature peaks around 310-378℃ is due to further 41 42 reduction of Mn2O3 towards Mn3O4 and the transformation of Co3II/IIIO4→CoIIO. The 43 reduction peak around 439-530℃ belongs to the simultaneous reduction of 44 45 Mn3O4→MnO and Co2+→Co. Notably, the reduction peak of Mn-Co(1:1) rods 46 47 transferred to low temperature regions compared with MnOx, CoOx and Mn-Co(1:1) 48 49 particles, implying oxygen mobility was greatly improved due to strong synergetic 50 51 between Mn and Co species. It is well known that synergy can cause severe deformation 52 53 of the structure while producing abundant oxygen defects, and oxygen deficiency 54 55 promotes the diffusion of oxygen in the surface of the catalyst and further penetrates 56 57 into the bulk33-37. All of the above features promote the performance of NH3-SCR. 58 59 60


Co3+/Co2+

0.82

0.88


368 285

Intensity (a.u.)

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

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Co rods

392

286

Mn rods 530

378

MnCo(1:1) particles

232

222 238

100

200

458

321

245

310 317

447

MnCo(2:1) rods

439

MnCo(1:1) rods MnCo(1:2) rods

300

400

500

600

700

800

Temperature (℃ ) Figure 6. H2-TPR profiles over series of Mn-Co catalysts

Adsorption of ammonia on acidic sites of catalyst surface is considered to be an important step in SCR reaction. The NH3-TPD profiles of Mn-Co(1:1) particles and Mn-Co(1:1) rods were shown in Figure 7. Mn-Co(1:1) rods catalyst exhibited four main desorption peaks at 133, 221, 310 and 494℃. As for Mn-Co(1:1) particles, several medium-high temperature NH3 desorption peaks at 207, 328, 374 and 404℃ were observed. According to the literature, for Mn-based catalysts, the peak located at lowmedium temperatures (100~300℃) is assigned to weak and medium adsorbed ammonia, and the desorption peaks at high temperature (above 300℃) are assigned to strongly adsorbed ammonia, respectively38,

39.

As compared with Mn-Co(1:1) particles, the

medium-high temperature desorption peaks of the Mn-Co(1:1) rods transferred to the high temperature region, indicating that the intensity of the acid sites on the Mn-Co(1:1) rods is stronger than that of the Mn-Co(1:1) particles. Moreover, the peak area of the Mn-Co(1:1) rods in the high temperature region is larger than Mn-Co(1:1) particles, indicating that a large amount of the total acid sites are present on the catalyst surface. Meanwhile, its large specific surface area and rod-like structure formed by selfassembly of particles could provide more adsorption sites for the reaction gas, and the acid strength is enhanced by the synergistic effect of manganese and cobalt, all of the above features lead to the improvement of catalytic activity. 14 ACS Paragon Plus Environment

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494

308

133 221

Intensity (a.u.)

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


Mn-Co(1:1) rods

328

207

374

404

Mn-Co(1:1) particles

100

200

300

400

500

Temperature (ºC)

Figure 7. NH3 -TPD profiles of two catalysts

In order to further reveal the change of acid sites on the catalyst surface, in situ DRIFT technology was applied to the experiment. The DRIFT spectra of the different NH3 and NO+O2 adsorption times over two catalysts at 100℃ are shown in Figure 8 and 9, respectively. As shown in Figure 8 a and b, the bands between 3000-3400 cm1(3347,

3295, 3238) were ascribed to the N-H stretching vibration of coordinated NH3

species40-42. The bands located at 1604, 1197 and 1135 cm-1 can be assigned to the NH3 molecule coordinated Lewis acid sites. The bands at 1640 and 1444 cm-1 were assigned to ionic NH4+ species bound to the Brönsted acid sites43-45. The bands at 930 and 809 cm-1 were ascribed to gas-phase NH3 or weakly adsorbed NH3 species46. With the increase of adsorption time, the intensity of the band attributed to the Lewis acid sites and Brönsted acid sites of the two catalysts increased gradually, indicating an increase in the number of ammonia species adsorbed on the catalyst surface. Most obviously, the strength of Lewis acid and Brönsted acid sites on Mn-Co(1:1) rods were much stronger than that of Mn-Co(1:1) particles during the whole NH3 adsorption process (Figure 8 a, b and 9 a). This result exhibited that more Lewis and Brönsted acid sites were formed and take part in NH3-SCR reaction on Mn-Co(1:1) rods catalyst, the proper ratio of acid sites may be the main reason for its higher catalytic performance of the 15 ACS Paragon Plus Environment


Mn-Co(1:1) rods catalyst at low temperature. This analysis is also consistent with the NH3-TPD results. FTIR spectra over the two catalysts were treated by 2000 ppm NO+8vol% O2 (100 ml/min) co-adsorption were recorded at 100℃, which are shown in Figure 8 c and d. the band at 1564, 1035 and 988 cm-1 can be assigned to bidentate nitrate29, 47, 48, and the bands at 1374 and 1352 cm-1 were ascribed to M-NO2 nitro compounds7, 43. The band at 1637 and 1236 cm-1 can be attributed to the bridged nitrates49, 50, and the monodentate nitrite species were also observed at 1532, 1524 and 1273 cm-1 51. After introducing the NO+O2, all of the nitrate species adsorbed on the Mn-Co(1:1) rods are slightly different to those on the Mn-Co(1:1) particles, and all nitrate species gradually strengthened with the increasing adsorption time, which indicated that abundant NOx species were adsorbed or produced on the surface of both two catalysts. However, the band at 1524 cm-1 assigned to monodentate nitrite species over two catalysts appeared at the beginning of adsorption and then decreased or disappeared with increasing time, indicating that the species are unstable and eventually oxidizes to a more stable species. In addition, two differences can be observed from the contrast figure (Figure 9 b) that the intensity of the NO+O2 adsorption peaks of the two catalysts is significantly different in the early stage, but the adsorption capacity tends to be consistent with the increase of time. In order to further confirm the adsorption of NH3 on the surface of the two catalysts at different temperatures, the in-situ DRIFT spectra were collected from 50℃ to 200℃, and the results were shown in Figure 10 a, b. When the reaction temperature rises from 50℃ to 200℃, the intensity of the bands bounding to NH3(L) and NH4+(B) are gradually decreased on both catalysts. However, The NH4+ adsorbed on the Bronsted acid sites (1640 cm-1) gradually disappears before 150℃ while the NH3 on the Lewis acid sites still exists at 200℃. All these phenomena indicate that the NH3 species present at the Lewis acid sites are more stable, which is consistent with the analysis of NH3-TPD. The DRIFTS spectra of NO + O2 co-adsorption over two catalysts at different temperatures are presented in Figure 10 c and d. The intensities of several bands attributed to bidentate nitrate (164, 1035 and 988 cm-1), monodentate nitrite (1289 cm-1) in both 16 ACS Paragon Plus Environment

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catalysts decrease with increasing temperature, accompanied by a strengthening of the peak at 1637cm-1. Moreover, the band at 1289/1281 cm-1 gradually shifts to 1229/1234 cm-1 as the temperature rises from 50℃ to 200℃, which indicated that the transition from monodentate nitrate to more thermally stable bridged nitrate52, 53. Remarkably, the intensities of bidentate nitrate on Mn-Co (1:1) rods are much higher than that of particles especially in low temperature regions. All the in-situ DRIFT analysis demonstrate that the reactant molecules have excellent adsorption and activation capabilities on the Mn-Co (1:1) rods, resulting in better low temperature catalytic performance.

a

NH3 1min 3min 5min 10min 20min 30min 60min

1135 1604 1444 1640

3295 3238

Intensity (a.u.)

Intensity (a.u.)

b

1197 NH3 1min 3min 5min 10min 20min 30min 60min

3347

930 809

3500

3000 2000

1500

1000

1197 1640 1604 1291 1444

3295 3228

3500

3000 2000

Wavenumber(cm-1)

930

1500

809

1000

Wavenumber (cm-1)

c

d NO 1min 3min 5min 10min 20min 30min 60min

1564 1532 1352

1637

1273 1236

Intensity (a.u.)

NO 1min 3min 5min 10min 20min 30min 60min

Intensity (a.u.)

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


1035 988

1564 1637

1352

1273 1236 1035

988

1524 1524

1374

2000

1750

1500

1250

1000

750

2000

1750

Wavenumber (cm-1)

1500

1250

1000

750

Wavenumber (cm-1)

Figure 8. In situ DRIFT spectra of different adsorption times (a) NH3 adsorption on Mn-Co (1:1) rods (b) NH3 adsorption on Mn-Co (1:1) particles(c) NO + O2 adsorption on Mn-Co (1:1) rods (d) NO + O2 adsorption on Mn-Co (1:1) particles at 100℃.



a)

b)

MnCo(1:1) particles MnCo(1:1) rods

NH3 adsorption 1min

MnCo(1:1) particles MnCo(1:1) rods

NO+O2 adsorption 1min


Intensity (a.u.)

Intensity (a.u.)

NH3 adsorption 3min

NH3 adsorption 5min

NH3 adsorption 10min








2000


1750

1500

1250

1000

2000

750

1750

1500

1250

1000

750

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 9. Comparison of (a) NH3 adsorption and (b) NO+O2 adsorption in situ DRIFT spectra of two catalysts at 100℃

a

b

1197 1444

1640

1135

1604

Intensity (a.u.)

930 809

50ºC 75ºC

Intensity (a.u.)

1640 3295 3360 3238

75ºC

125ºC

125ºC

150ºC

150ºC

200ºC

200ºC

3500

3000 2000

1500

1000

930 809

50ºC

100ºC

4000

4000

3500

3000 2000

Wavenumber (cm-1)

1000

d

1564 1637

1229 1349

1035 988

1564

Intensity (a.u.)

200℃

150℃ 125℃

1637

125℃

100℃ 75℃

50℃

50℃

1250

1035 988

150℃

75℃

1500

1234 1349

200℃

1289

100℃

1750

1500

Wavenumber (cm-1)

c

2000

1604 1197 1444 1135

3295 3360 3238

100ºC

Intensity (a.u.)

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

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1000

2000

1281

1750

Wavenumber (cm-1)

1500

1250

1000

Wavenumber (cm-1)

Figure 10. In situ DRIFTS of (a) NH3 adsorption on Mn-Co (1:1) rods (b) NH3 adsorption on MnCo (1:1) particles (c) NO + O2 adsorption on Mn-Co (1:1) rods (d) NO + O2 adsorption on Mn-Co (1:1) particles from 50 to 200 °C.

3.2 NH3-SCR Activity The performance of a series of catalysts with different Mn/Co molar ratios and Mn-Co particles was shown in Figure 11 a. Co rods exhibited the lowest SCR 18 ACS Paragon Plus Environment

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performance, which removal efficiency did not exceed 40% in the whole reaction process. Mn rods presented the best SCR removal efficiency with the NOx conversion above 90% at 175℃ and showed higher catalytic performance at low temperature compared with Co rods. However, When Mn and Co form mixed oxide particles, the SCR performance was even lower than that of Mn rods. Besides, both of the catalytic activity of Mn-Co rods with different molar ratios were higher than Mn-Co particles This result might be attributed to its unique rod morphology, which was one of the main factors affecting SCR activity. It is clear that the removal rate of NOx of all Mn-Co rods at 75℃ was more than 80% under space velocity of 32,000h-1, which showed excellent NH3-SCR activity at low temperature. Mn-Co(1:1) exhibited the best SCR activity(exceeded 92%) from 75-200℃ compared with the catalysts with Mn/Co molar ratios of 1:2 and 2:1. 100

100

a

b Mn-Co(1:1) rods

80

80

Mn-Co(1:2) rods Mn-Co(2:1) rods

N2 selectivity (%)

NOx Conversion (%)

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


Mn-Co particles

60

Mn rods Co rods

40

60

Mn-Co(1:1) rods

40

Mn-Co(1:2) rods Mn-Co(2:1) rods Mn-Co(1:1) particles

20

20

Mn rods Co rods

0

0 50

75

100

125

150

175

200

50

75

100

125

150

175

200

Temperature (ºC)

Temperature (ºC)

Figure 11. NOx conversion (a) and N2 selectivity (b) in the NH3-SCR reaction as a function of the temperature over Mn-Co catalysts.

In order to investigated the effect of different hydrothermal time on activity of MnCo(1:1) catalyst, three different temperatures were selected for experiments, and the results are shown in Figure 12 a. The NOx conversion of Mn-Co(1:1) has greatly improved with the increase of the hydrothermal temperature reached to 140℃ and then decreased with temperature increased to 180℃. We can deduced from the results that the catalytic performance of the catalyst is greatly influenced by the temperature of hydrothermal reaction. N2 selectivity of the catalyst has also been investigated, the results are shown in the Figure 11 and 12 b. The N2 selectivity of all catalysts decreased with increasing of reaction temperature while the temperature of hydrothermal 19 ACS Paragon Plus Environment


synthesis has little effect on it. However, the N2 selectivity of series of Mn-Co rods was higher than that of Mn-Co particles. The results also showed a similar trend of the NOx conversion. 100

100

a

b 80

N2 selectivity (%)

80

NOx Conversition (%)

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

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60 Mn-Co(1:1)-100

40

Mn-Co(1:1)-140 Mn-Co(1:1)-180

60 Mn-Co(1:1)-100

40

Mn-Co(1:1)-140 Mn-Co(1:1)-180

20

20

0

0 50

75

100

125

150

175

200

50

75

Temperature (ºC)

100

125

150

175

200

Temperature (ºC)

Figure 12. Different hydrothermal temperatures(a) and N2 selectivity (b) over series of Mn-Co catalysts

3.3 Effect of SO2 and H2O In practical engineering applications, water vapor and SO2 exist in the flue gas, which has an adverse effect on the activity of the catalyst. Figure 13 showed the H2O and SO2 resistance of Mn-Co(1:1) rods catalyst at 150℃ under a GHSV of 32000h-1 in the whole process. When introduced only 10% H2O into flue gas, the NOx conversion of the catalyst decreased slightly in the first 2h and then recovered to 100%. It suggested that the competitive adsorption between large amounts of water vapor and NO/NH3 over Mn-Co(1:1) catalyst. The active sites belonged to NO/NH3 for adsorption and activation were occupied by water, leading to a decrease in catalyst activity. However, with the presence of 100 ppm SO2 at 150℃, the NOx conversion over Mn-Co(1:1) rods gradually decrease from 100% to 80%, and then reaches a steady state. After stopping the pump of SO2, the activity gradually returned to the initial level and remained unchanged. Considering the practical application, 10 vol % H2O and 100 ppm SO2 are simultaneously pumped into the reactor, the de-NOx activity of the catalyst gradually decreased and stabilized during the test period, but its catalytic activity was still maintained at about 80%. the NOx conversion could be restored to near the initial level after switching H2O and SO2 off. According to previous studies, it is proved that the synergistic inhibition between H2O and SO2 was the main factor affecting the catalytic 20 ACS Paragon Plus Environment

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activity of the catalyst. When H2O and SO2 coexist, ammonium sulfate will form and further deposit on the surface of the catalyst to block the active sites on it, resulting in a decrease in catalyst activity. It can be deduced that the Mn-Co(1:1) rods has excellent water and sulfur resistance, which is mainly attributed to its unique ordered porous rodlike structure, the uniform distribution of active sites and strong interaction between manganese and cobalt.

100 90

NOx Conversion (%)

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


on

80

off

70 10% H2O

60

100 ppmSO2

50

10 vol% H2O+100 ppmSO2

40 30 20

0

2

4

6

8

10

12

14

16

18

Reaction Time (h)

Figure 13. Effect of 10 vol %H2O, 100 ppm SO2 and 10 vol % H2O+100 ppm SO2 on the catalytic performance of Mn-Co(1:1) rods catalyst.

Conclusions In summary, we synthesized a series of Mn-Co rod-like hierarchical structure catalysts built by self-assembled bundles of porous rods aggregates. Mn-Co(1:1) rods with the molar ratio of Mn/Co is 1:1 exhibits optimal catalytic performance, high N2 selectivity at low temperature compared to Mn-Co(1:1) particles. The above excellent features can be attributed to large specific surface area, unique rod-like mesoporous structures, strong interaction between manganese and cobalt oxide species, high atomic ratio of Mn4+/Mn on the catalyst surface, strong acid strength and large acidity amount. The combination of these properties affects the de-NOx mechanism and play an 21 ACS Paragon Plus Environment


important role in promoting the NH3-SCR reactions at low temperature.

Acknowledgments This work was financially supported by the National Key R&D Program of China (2017YFC0210303), National Natural Science Foundation of China (U1660109, 21806009), Project funded by China Postdoctoral Science Foundation (2018M631344), and Fundamental Research Funds for the Central Universities (FRF-TP-18-019A1).


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