Exploring the nano - size effect of mordenite zeolites on their

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Kinetics, Catalysis, and Reaction Engineering

Exploring the nano - size effect of mordenite zeolites on their performance in the removal of NOx Jingyan Zhang, Fuyan Liu, Jian Liang, Hui Yu, Wenming Liu, Xiang Wang, Honggen Peng, and Peng Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Industrial & Engineering Chemistry Research

Exploring the nano - size effect of mordenite zeolites on their performance in the removal of NOx Jingyan Zhang,a† Fuyan Liu,a† Jian Liang,a Hui Yu,a Wenming Liu,a Xiang Wang,a Honggen Penga*, Peng Wu b a

Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, College

of Chemistry, Nanchang University, 999 Xuefu Road, Nanchang, Jiangxi 330031, China. b Shanghai

Key Laboratory of Green Chemistry and Chemical Processes, Department

of Chemistry and Molecular Engineering, East China Normal University, North Zhongshan Road 3663, 200062 Shanghai, China

Corresponding authors. E-mail: [email protected] (H. Peng); † These authors contribute equally to this work.

1

ACS Paragon Plus Environment

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

ABSTRACT Herein, the nano-sized MOR zeolites were applied for selective catalytic reduction of NOx with NH3 and the effect of particle size on catalytic performance was studied in detail. Accordingly, three kinds of Cu-MOR with different particle size (0.23, 0.96, and 2.20 µm) were prepared and used for removing NOx from diesel exhaust. Our results demonstrated that Cu-MOR-0.23 with the minimal particle size had the best catalytic performance. Decreasing particle size can effectively shorten the length of the C axis (12-MR channel), which is beneficial for the diffusion of substrates and products. This advantage was also verified by the Weisz–Prater criterion that the Cu-MOR with the smallest particle size has the least hindered diffusion of the mass transportation. Our work provides a new approach for promoting the catalytic performance of conventional cheaper catalysts used in the removal of NOx from diesel exhaust.

Keywords: Cu-Mordenite zeolites; NOx-SCR by NH3; Different particle size; Reaction Mechanism.

INTRODUCTION Nowadays, the harm of nitrogen oxides can not be ignored, which result from fossil fuels combustion processes in industrial boilers, vehicle engines, and marine exhaust15

. In addition, nitrogen oxides contribute to the fine particle pollution, acid rain,

photochemical smog, global warming and water eutrophication. The abatement of NOx remains a challenge, especially in marine exhaust2. Emission regulation of NOx has been perfected. New emission standards for marine exhaust must be introduced, depending on speeds, for engines installed after 2016, thus, the exhaust after-treatment system is needed6. In this after-treatment system, the catalyst is more easily deactivated 2


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compared with conventional vehicle engines, due to the exposure of the catalyst used in marine/diesel exhaust system to high concentrations of particulates (soot), water vapor and SO2, particularly for those marine diesel engines burning low quality fuels7, 8

. Up to present, some advanced technologies have been developed for NOx removal

from various sources of pollution. Commercial ammonia-selective catalytic reduction (NH3-SCR) catalysts and NOx storage-reduction (NSR)9 are the common promising technologies for NOx removal, which signaled the arrival of a new era for the transportation industry. For instance, commercially available vanadium-based SCR catalysts10-13 are among the widely used catalysts in virtue of their better sulfur tolerance. However, they also have some drawbacks, e.g., once the reaction temperature reaches above 450℃, the traditional vanadium-based catalysts cannot remain active due to various reasons, including the phase transformation of the TiO2, oxidation of SO2, the toxicity of V2O5 to the environment and the narrow operating temperature window (300－400 oC)14. Considering the cold start process of the engine, the lowtemperature SCR catalyst is also the most desirable one. Naturally, manganese-based SCR catalysts attracted the attention of the researchers15, 16. They have high activities at 100–200 °C. Nevertheless, the practical applications of MnOx based catalysts have been limited. It’s due to their poor thermal and hydrothermal stability and sulfur resistance. Much attention has been paid to zeolite due to the fact that zeolites has many acid sites, large surface area, uniformed intricate channels, better thermal and hydrothermal stabilities17-19. In the last decades, it has been established that Cu or Fe exchanged zeolites are another type of promising catalysts to remove NOx by NH3-SCR. In the 1980s, Cu-ZSM-520, 21 was extensively studied, and it displayed high NH3- SCR activities. Sultana et al. reported that introduction of copper into Na-ZSM-5 created Lewis and Brønsted acid sites and promoted NH3 activation and oxidation properties of the catalyst, which is beneficial to the NH3-SCR. Recently, Cu ion-exchanged SSZ-13 and SAPO-34 (Cu-CHA) have been commercialized for NOx removal in diesel-powered engines17,

22-26

. Shen’s group

investigated the methods of preparation of Cu-SAPO-34 for NH3-SCR, and found that catalyst prepared by different methods showed the same apparent activation energy (Ea) 3



and turnover frequency (TOF) values27, 28. Gao et al. reported the structure-activity relationships of Cu-SSZ-13 in the NH3-SCR reaction29. Their results indicated that the size of the zeolite particles have considerable effects on their catalytic performance. Though Cu-CHA zeolites have been commercially used in diesel engine exhaust, the high cost for the preparation of the two CHA zeolites is the largest obstacle for their wide spread application. Mordenite (MOR) is widely used in industrial application due in part that it can be prepared by template-free methods, such as hydroxylation of aromatics reaction30 and removal of volatile organic compounds31. Concerning the MOR zeolite for NOx-SCR, M.F. Ribeiro et al.32 verified that the cocation (H, Na, Cs, Mg and Ba) plays a key role in the CuMOR. The NaCuMOR showed best performance among all catalysts. This phenomenon is due to the present of Na ion hinder migration of Cu2+ species in application process. In-Sik Nam et al.33 proved that the active sties of CuHMOR are both the Brønsted acid site and the metal site. The NH3-SCR reaction on CuHMOR catalyst occurs through the Langmuir-Hinshelwood mechanism. MOR is composed of 12-MR pores of 0.67×0.70 nm, connected by small size 8-MR pores of 0.34×0.48 nm18, 34, 35. As the 8-MR pores in MOR are too small for lots of molecules to enter, the reactants and products are major transported along the long 12 members ring channel. Even molecules with relatively small molecular size like NO (0.32 nm) and N2 (0.36 nm) can hardly enter into or diffuse out the 8-MR pore. This is the main disadvantage for MOR zeolite used in NO reduction with NH3. Therefore, it is significance to study of whether particle size can affect the catalytic performance for MOR. Herein, Cu ion-exchanged MOR with different particle size was prepared and applied for NH3-SCR. Our results revealed that the Cu-MOR-0.23 with the smallest particle size has the highest catalytic performance. XRD, XPS, H2-TPR, and other techniques have been employed. The physicochemical property of the catalysts has been investigated. The mass transportation limitation for the reactants and products which was confirmed by Weisz-Prater criterion. The NH3-SCR reaction mechanism of CuMOR with various particle size was also investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). According to these results, the 4


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effects of the particle size have been discussed in detail. EXPERIMENTAL Catalyst preparation Na-MOR with different particle size was synthesized using colloidal silica, NaOH, NaAlO2 and H3BO3 as precursors and commercial Na-MOR as crystal seeds. The catalysts synthesized according to the method used in our previous study31. The molar composition of start gel was 1.0 SiO2:0.245 Na2O:0.076 Al2O3:22 H2O:0.2 H3BO3:0.0653 TEABr. The mixture was added into stainless steel autoclave with Teflon-lined and maintained 96 h at 180 °C for crystallization. Then, the solid phase was separated by filtration, washed repeatedly with deionized water and dried at 80 °C for 12 h. The product was called as Na-MOR of 0.96 µm and the size of the zeolite can further decrease to 0.23 µm when the amount of TEABr is increased by 30%. The absence of TEABr in the gel leads to the growth of the Na-MOR zeolite particles from 0.96 to 2.20 µm. Then, the samples were calcined at 550 °C for 6 h. The NH4-MOR were given by ion-exchanged with 1.0 M NH4Cl (S/L=50, 24 h at 80 °C). The samples were washed with deionized water, dried at 80 °C for 12 h. Then, they were calcined again, namely H-MOR-2.20, H-MOR-0.96 and H-MOR-0.23. The 4 wt.%Cu-MOR-X (X: 2.20, 0.96, 0.23) catalysts were acquired via exchange impregnation method. Briefly, 0.15 g Cu(NO3)2.3H2O was dissolved in 5 mL deionized H2O. Next, 1 g HMOR-X support was measured and added into this Cu(NO3)2 solution. The mixture was stirred and dried at 25 and 80 °C, respectively, for 24 h and calcined at the same conditions to obtain the 4%Cu-MOR-X (X:2.20, 0.96, 0.23) catalysts. To study catalytic stability, 4%Cu-MOR-X catalysts were calcined at 750 °C containing 5% water vapor for 16 h. These catalysts were called 4%Cu-MOR-HTA-X. These catalysts have similar elemental content. The details of catalyst characterization and activity test were present in “Supplementary Information”.

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RESULTS AND DISCUSSION Physical properties of the catalysts The typical scanning electron microscopy (SEM) images and particle size distribution is shown in Figure. 1. Every catalyst exhibited a relatively uniform morphology, but they differed greatly in morphology and dimension among them. For example, the image in Figure. 1(a) reveals that the support had a uniform length of 2.20 µm. The images in Figure. 1 (b) and (c) show particles of uniform shape, with particle size of 0.96 and 0.23 µm, respectively. Thus, we have successfully synthesized MOR zeolites with uniform morphology and different particle size.

a Mean size： 2.20 um

0.35

Frequency ()

0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.0

0.5

1.0

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4.0

Particle size (um)

b

0.5

Average particle size： 0.96 um

Frequency ()

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c 0.8

Average particle size： 0.23 um

0.7 0.6

Frequency ()

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

0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

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6


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Figure. 1. SEM images of H-MOR-X with different particle size. (a) 2.20 µm, (b) 0.96 µm and (c) 0.23 µm. To determine the crystal structure of the MOR, Cu-MOR and Cu-MOR-HTA-X catalysts with different particle sizes, the powder X-ray diffraction (XRD) technique was used. The results are displayed in Figure. 2 and Figure. S1. The diffraction peaks of all samples are in line with the inorganic crystal structure database (PDF No. 43– 0171) of the MOR zeolite crystal structure. The enlarge map on the right side of the Figure. 2 reveals the presence of two peaks at 2θ of 35.4° and 38.8° compared with the patterns of support zeolites (Figure. S1(a)), which are attributed to some highly dispersed CuO species14, 36, indicating that Cu species have a high exchange degree or are highly dispersed on these supports, especially for 4%Cu-MOR-0.23. In addition, all the samples maintained well MOR framework and crystal structure after exchange impregnation and hydrothermal aging (Figure. S1(b)). 

CuO 

4Cu-MOR -2.20

4Cu-MOR-2.20

4Cu-MOR-0.96

Intensity (a.u.)



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




4Cu-MOR -0.96



 

4Cu-MOR -0.23

4Cu-MOR-0.23

5

10

15

20

25

35

30

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45

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32

34

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42

2 ()

2 ()

Figure. 2. XRD patterns of 4%Cu-MOR-X catalysts. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. To further understand the copper species of these catalysts, the catalysts were analyzed by the UV-Vis technique, and the spectra are displayed in Figure. 3. The results of all catalysts is similar with each other.

7



210 nm Isolated Cu

Absorbance (a.u.)

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2+

Dimeric Cu/CuOx cluster 780 nm Crystalline CuOx

4%Cu-MOR-0.23 4%Cu-MOR-0.96 4%Cu-MOR-2.20

200

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Wavelength (nm)

Figure. 3. The UV-Vis spectra of H-MOR-X catalysts. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. The band at about 210 nm was attributed to the zeolite structure20. The band about 240 nm was assigned to charge-transfer of O → Cu2+. While the band about 780 nm hint to the Cu2+ d-d transition. In addition, the bands about 380 and 310 nm attribute to crystalline CuOx and oligonuclear [Cu-O-Cu]n species, respectively. It should be mentioned that the band at 240 nm give high intensity compared with other bands37. The UV-Vis results indicated that the isolated Cu2+ occupies the largest proportion over the three catalysts, which is in line with XRD result. The N2 adsorption-desorption isotherms and the pore size curves of H-MOR-X and 4%Cu-MOR-X are presented in Figure. S2. All catalysts exhibited high uptake steps in the P/P0 < 0.01, which belong to the typical type I,14, 38 indicating the presence of micropores. The pore size distribution curves of the catalyst showed a primary peak at around 0.70 nm consistent with the 12-MR of MOR39. Interestingly, compared with MOR-2.20, the other catalysts have large amount of external surface areas and the supports show a broader peak in the ranges from 1–3 nm and 10–100 nm, which were inter-particle assembled mesopores. The textural parameters of the three MOR zeolite catalysts are listed in Table 1. The BET surface area is inversely proportional to the particle size. The catalysts with the largest particle size showed the smallest BET surface area of 391 m2 g-1. 8


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Table 1 Texture structure of the H-MOR-X and 4%Cu-MOR-X. Catalyst

Specific surface area (m2 g-1)a

Pore volume (cm3 g-1)b

SBET

Smicro

Sext

Vtot

Vmeso

H-MOR-2.20

391

361

30

0.23

0.04

H-MOR-0.96

404

350

54

0.28

0.10

H-MOR-0.23

414

377

37

0.27

0.07

4%Cu-MOR-2.20

339

305

34

0.20

0.04

4%Cu-MOR-0.96

334

294

40

0.23

0.08

4%Cu-MOR-0.23

346

309

38

0.22

0.06

[a] Calculated by the BET method. [b] Determined by the BJH method. In addition, the surface area of all the supports consisted of micropores and external surface area. The supports have similar micropore area and different external surface area, which indicate that the diminished particle size does not damage the interior structure of the MOR, and just increase the external surface area40. In general, smaller particle size with larger external surface area not only shortens the inner pores, but also improves the utilization ratio of the inner pores, which is beneficial for the diffusion of the reactant and product molecules, as well as the dispersion of Cu species. The addition of Cu species, compared with the supports, lead to surface areas and pore volume of Cu-MOR-X slightly decrease, proving that the Cu species have been very well dispersed on these supports. This finding is consistent with the XRD analysis result. Highly dispersed Cu species play a positive role on the improvement of their catalytic performance. Chemical properties of the catalysts To determine the redox behavior of the catalysts and the oxidation state of copper, H2-TPR analysis of all the catalysts were conducted and the profiles are displayed in Figure. 4. For comparison, the quantitative H2 consumption results are also listed in Table S1.

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

 4Cu-MOR-2.20

H2 Consumption (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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4Cu-MOR-0.96

4Cu-MOR-0.23

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600

o

Temperature ( C)

Figure. 4. H2-TPR profiles of 4%Cu-MOR-X catalysts. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. As reported previously, the reduction pathway for the copper species of all 4%CuMOR is as follows. The reduction of isolated Cu2+ species occurs through a two-step process: Cu2+ + 1/2H2 → Cu+ + H+ and Cu+ + 1/2H2 → Cu0 + H+. However, the reduction of CuO species occurs through a one-step: CuO + H2 → Cu0 + H2O.5, 41 The central temperature of the reduction peak moves slightly to a higher temperature with the decrease of the particle size, and the reduction peak of the 4%Cu-MOR-0.23 catalyst at low temperature is sharp, which may be due to 4%Cu-MOR-0.23 have more Cu2+.4 All catalysts exhibit two distinct peaks with different area. It is worth noting that the area of the peak under 350 oC, which was ascribed to the reduction of isolated Cu2+ to Cu+ ions and CuO to Cu, is larger than that of the peak in the high temperature, which was ascribed to the reduction of Cu+ to Cu,4, 41 indicating that the copper species of all catalysts included CuO and Cu2+ species. The data in Table S1 reveals that 4%CuMOR-0.23(130) has a larger area of the β peak than the 4%Cu-MOR-0.96(124) and 4%Cu-MOR-2.20(106). In short, the β peak area increases with the decrease of the particle size, which proves that each catalyst has a different amount of Cu2+( See details in Table S1). The H2-TPR results are consistent with those of the XPS analysis. It is 10


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well known that the Cu2+ of the catalysts is the most active sites for NH3-SCR reaction28, due to its capability to facilitate the NO oxidation into NO2, a key step in this reaction. To further examine the surface copper species of the catalysts, the catalysts were analyzed by the X-ray photoelectron spectroscopy (XPS) technique, and the results are displayed in Figure. 5. The XPS fingerprint was deconvoluted in two peaks at about 933.5 eV and 936.0 eV, which are related to different copper species on zeolite framework42, 43. Concretely, the peak located at 933.5 eV is ascribed to CuO particles present on the surface and the other peak located at 936.0 eV is ascribed to isolated Cu2+ species44. The relative amount of isolated Cu2+ species of 4%Cu-MOR-0.23 is 100%, but for 4%Cu-MOR-0.96 and 4%Cu-MOR-2.20, is 82% and 73% (See details in Table S2), respectively. In addition, the presence of additional satellite bands (942.0946.0 eV) on the Cu 2p spectra is assigned to the electronic configuration of unsaturated Cu, which is associated to the Cu(II) component.

Cu 2p

933.5 936.0

Cu 2p3/2

4%Cu-MOR-2.20 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


4%Cu-MOR-0.96

4%Cu-MOR-0.23

946

944

942

940

938

936

934

932

930

Binding energy (eV)

Figure. 5. XPS spectra of the 4%Cu-MOR-X catalysts. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. The data shown in Figure. 5, allow us to draw the following conclusion：compared with the 4%Cu-MOR-2.20 and 4%Cu-MOR-0.96, the 4%Cu-MOR-0.23 has more 11



isolated Cu2+ species, which is beneficial to improve the NH3-SCR reaction performance. To investigate the acidic properties of the catalysts, NH3-TPD technique was used with the deconvolution results displayed in Figure. 6.



 4Cu-MOR-2.20

4Cu-MOR-0.96

4Cu-MOR-0.23

200

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Semi-Quantification (a.u.)

a

 Intensity (a.u.)

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B D 4Cu-MOR-2.20 F 4Cu-MOR-0.96 H 4Cu-MOR-0.23 J L N P R T V X Z AB AD

150

700

Temperature (C)

250 o Centre temperature ( C)

b

550

Figure. 6 (a) NH3-TPD profiles of the 4%Cu-MOR-X catalysts (b) Semi-quantification. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. The peaks located at 150 °C and 250 °C are consistent with physically adsorbed ammonia species and captured ammonia species by Lewis acid sites, while the peaks at about 550 °C are related to the ammonia species adsorbed on Brønsted acid sites45-47. As previously reported, the Cu exchange impregnation process results in a decrease of Brønsted acidity and an increase of Lewis acid sites48. The data presented in Figure. 6 (b) and Table S3, reveal that the amount of reactive NH3 species could be increased with the decrease of the particle size. 4%Cu-MOR-0.23 has the largest amount of Brønsted and Lewis acid sites, which is beneficial to adsorb of the NH3 species. Therefore, the reactive NH3 species is attributed to promote the catalytic performance, which is in line with their in situ DRIFTS analysis results. Catalytic performance Figure. 7. shows the catalytic performance of NOx reduction over the 4%Cu-MORX catalysts with different particle size at a weight hourly space velocity (WHSV) of 60,000 mL h-1 gcat-1. The results reveal that the 4%Cu-MOR-0.23 catalyst not only has 12


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a broad temperature range, but also a high NOx conversion at each temperature compared with other catalysts. 500

80

60

b 400 300 200

4Cu-MOR-2.20 4Cu-MOR-0.96 4Cu-MOR-0.23

100

40 0

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4Cu-MOR-2.20 4Cu-MOR-0.96 4Cu-MOR-0.23 150

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(ppm)

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N2O Concentration

N2 Concentration (ppm)

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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


o

Temperature ( C)

o

Temperature ( C)

Figure. 7. NH3-SCR on 4%Cu-MOR-X catalysts. (a) NOx conversion, (b) N2 and N2O concentration. X represents different particle sizes: 0.23, 0.96 and 2.20 µm. Conditions: 500 ppm NO, 500 ppm NH3, 5%O2, balance Ar, WHSV = 60,000 mL h-1 gcat-1 For the 4%Cu-MOR-2.20 with an average particle size of 2.20 µm, an NOx conversion of 73% and 61% were obtained at 200 and 400 °C, respectively. However, the 4%Cu-MOR-0.23 catalyst with NOx conversion of 90% and 81% at the same temperatures, respectively. It is important to highlight that, as displayed in Figure. 7 (b), N2 is the predominant product, and the concentration of by-products (N2O) is lower than that of 30 ppm in the process of catalysts tested. The results proved that 4%CuMOR-X catalysts are highly selective to N2, although with different particle size. To further illustrate the catalytic performance of 4%Cu-MOR materials, we compared it with catalytic materials previously reported (See Table S4). The results proved that 4%Cu-MOR is weak than commercialized chabazite (Cu-CHA) catalysts, but compared with other catalysts, it's still good. Thus, the catalyst with the smallest particle size has the best catalytic performance. This finding should contribute to facilitate mass transportation of reactants and products using small particle size catalysts. Remarkably, the NH3-SCR catalytic performance order is also consistent with the isolated Cu2+ content calculated by TPR. To further understand the contribution of 13



isolated Cu2+ or Cu mass to the NH3-SCR reaction, the TOF was quantified relative to the isolated Cu2+ content and the results are listed in Table S1. The results showed that the highest TOF is achieved on 4%Cu-MOR-0.23(1.15×10-3 or 1.00×10-3) and the lowest on 4%Cu-MOR-2.20 (1.00×10-3 or 0.78×10-3), which also indicated that the better catalytic performance is closely related with the smaller particle size. The 12-MR channel of MOR is effectively shortened by decreasing its size, which could effectively facilitate the mass transfer of all gases and promote the accessibility of the reactants to the active sites. It is generally known that zeolite materials should have superior hydrothermal stability due to the existence of water, which may cause the dealumination of the zeolite. Sulphur and soot tolerance are also an important standard to evaluate the application potential of catalysts, especially in marine deNOx application (NOx, sulfur oxide and soot co-exist).3, 7, 8, 10, 49-52 To further investigate the hydrothermal stability and the sulfur and soot tolerance of the catalyst, the 4%Cu-MOR-X catalysts were also evaluated in co-presence of 50 ppm SO2, 5% H2O or 10 wt.% soot in the reaction feed (10 wt.% soot was physically mixed with the catalysts), and the results are displayed in Figure. 8. 100

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50 ppm SO2 add

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50 ppm SO2 and 5%H2O add

80 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

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and soot add 60

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Figure. 8. NH3-SCR on 4%Cu-MOR-X catalysts. NOx conversion (a) 50 ppm SO2 add, (b) 50 ppm SO2 and 5%H2O add, (c) 50 ppm SO2, 5%H2O and 10% soot add, (d) after hydrothermally aged, at different temperatures. The results displayed in Figure. 8(a) indicate that the addition of 50 ppm SO2 in the dry feed induces little deactivation of the 4%Cu-MOR-X catalysts at different temperatures. The results shown in Figure. 8(b), reveal that the co-existence of both 5% H2O and 50 ppm SO2 in the reaction feed leads to further deactivation of the 4%CuMOR-X catalysts. The results are owing to competitive adsorbing or poisoning of SO2 and H2O at active sites. To further understand the effect of soot dopants on the catalytic activity, the catalyst of 10% soot mixture was also tested in the co-presence of both 50 ppm SO2 and 5% H2O in reaction feed. The results are displayed in Figure. 8(c). It was observed that all catalysts of the 10% soot mixture had better performance than the pure catalysts (Figure. 8 (b)) and catalysts after hydrothermally aged (Figure. 8 (d)). According to previous reports, the presence of soot had a negative impact on NOx removal by catalysts with very basic supports but had no effect or a positive influence if the support is not basic. The supports (H-MOR) with acid character for all catalysts shown a positive influence, which may be due to not only the adsorption of the reactants on the face of catalyst is not affected but also the reduction of NOx by soot contributes to NOx removal53-56. It is also worth mentioning that the effect of the particle size of the support is more significant to the performance of the catalyst. For example, in Figure. 8 (c), on the 4%Cu-MOR-0.23, 4%Cu-MOR-0.96 and 4%Cu-MOR-2.20 catalysts, the NOx conversion of 86, 76 and 53% were obtained at 200 °C, respectively. The results 15



implied that a small particle size promote the diffusion of all gases, as well as the dispersion of Cu2+ species. According to previous reports57-59, we selected the condition of calcined at 750 °C containing 5% water vapor for 16 h to investigate the hydrothermal stability of all catalysts. For zeolite catalytic materials, this hydrothermal condition is belong to moderate level. Such as, in order to investigate the effect of promoter (Y or P) on the stability of zeolite catalysts, hydrothermal conditions are stricter (800 °C for 16 h or 900 °C for 8 h )59, 60. For most catalysts, the hydrothermal conditions are between 650 °C to 800 °C58, 61 .The catalytic performance of catalysts after hydrothermally aged displayed in Figure. 8 (d). Obviously, the catalytic capability of all catalysts did not significant decrease, even if they go through strict treatment under high temperature steaming, which proved 4%Cu-MOR catalysts has good hydrothermal stability. In addition, it can contribute to further understand the relationship between the particle size and the catalytic performance. However, it should consider the influence of the internal and external diffusion22. The activity test of the catalysts was conducted at high speed conditions (WHSV = 300,000 mL h-1 gcat-1) and the NOx conversion below 20%. The results, which are displayed in Figure. 9 (a), reveal that the order of the catalysts according to their activity is 4%Cu-MOR-0.23 > 4%CuMOR-0.96 > 4%Cu-MOR-2.20, which is in line with the results presented in Figure. 7. The Arrhenius plots of the NH3-SCR reaction rates on the 4%Cu-MOR-X are depicted in Figure. 9(b). The 4%Cu-MOR-0.23 catalyst shows higher SCR reaction rate than the other catalysts, in the whole temperature range (130–190 °C), which further indicates that the method of reducing the support particle size could be another synthesis strategy for high-performance SCR catalysts. The apparent activation energy of all catalysts is similar and the value is around 85 kJ mol-1, which proves that the particle size do not influence the NH3-SCR mechanism28. According to previous reported studies, the active center for zeolite catalysts is isolated Cu2+. We calculated the TOF of isolated Cu2+ by the above equation28, which not only explores the effect of the particle size on the diffusion of reactants and products, but also quantifies the contribution of isolated Cu2+ to the NH3-SCR reaction, and the results are displayed in Figure. 9 (c). 16


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o

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Figure. 9. NH3-SCR on 4%Cu-MOR-X catalysts, (a) NOx conversion, (b) Arrhenius plots, and (c) the turnover frequency (TOF). X represents different particle sizes: 0.23, 17



0.96 and 2.20 µm. Conditions: 500 ppm NO, 500 ppm NH3, 5%O2, balance Ar, WHSV = 300,000 mL h-1gcat-1. The data presented in Figure. 9(c) show that the TOF value of all catalysts is slightly increased with the decrease of the size of the particles at a certain temperature. These results clearly proved that small particle size is important to eliminate transport limitations and improve the catalytic performance of 4%Cu-MOR-0.23 for NH3-SCR to remove NOx. Internal mass transfer limitations in 4%Cu-MOR-X All catalysts have a similar composition (MOR particle phase, one-dimensional pore, Si/Al=6, 4%Cu), except for the particle size of the support. From the above experiments, it was found that the order of the catalyst according to their activity was 4%Cu-MOR2.20 < 4%Cu-MOR-0.96 < 4%Cu-MOR-0.23, which proved that transport limitations should not be ignored in the case of zeolite-supported catalyst with large particle size. The 4%Cu-MOR-0.23 catalyst has the shortest micropore channels (12 MR channel) and the best performance among all catalysts, which is in line with their negligible diffusion resistance. To further investigate the effect of the internal diffusion limitations on zeolites with different particle size, the mass transportation was calculated on all catalysts by the Weisz-Prater criterion39, 62. -rAR2

CWP =

DeCAS

where -rA = (--) mol m-3 cat s-1, is the observed reaction rate of NOx; R = (--) m, is the radius of zeolites; CAS = 0.016 mol m-3, is the gas concentration of NOx at the catalyst surface; De = 7.30×10-12 m2s-1, is the diffusion coefficient obtained from previous reports in the literature62-65. Compared with the 4%Cu-MOR-0.96 (7.7 > 1) and 4%Cu-MOR-2.20 (34.8 > 1) catalysts, the 4%Cu-MOR-0.23 (0.5 < 1) catalyst has the smallest Weisz-Prater parameter and it is low than 1 (Table S5). Thus, the limitation in internal mass transfer is negligibly low for 4%Cu-MOR-0.23 catalyst, which agree with its reaction 18


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performance.39, 63 In summary, the kinetic data calculated above proved that particle size can affect the internal diffusion, which leads to remarkable differences in the activity of the different catalysts. The zeolite catalyst with smallest particle size exhibited the best catalytic performance. In Situ DRIFTS First, the catalyst was cleaned at 300 °C in N2 for 1 h to obtain a clean surface. Second, the catalyst was cooled to 200 °C. Third, 500 ppm NO + 5% O2/N2 was introduced. Simultaneously, in situ DRIFTS spectra were collected at different times, which was similar as previously reported elsewhere66, 67. The DRIFTS spectra of the 4%Cu-MOR0.23 catalyst in a feed of 500 ppm NO + 5% O2/N2 are displayed in Figure. 10 (a).The bands at 3587 and 3454 cm-1 were assigned to H2O adspecies48. The bands at 3274 cm1

was assigned to the disappearance of OH groups(2NO + (1/2)O2 + 2O–H+ = H2O +

2O–NO+)48. The bands at 1619 and 1386 cm-1 were assigned to (nitrite) nitrate species68. Interestingly, the bands at 3587, 3454 cm-1 and 3274 cm-1 were the first to appear and increase. These findings proved NOx species may be captured on the surface of the 4%Cu-MOR-0.23 catalyst. After the 500 ppm NO + 5% O2/N2 adsorption experiments were finished, the mixture gas was replaced by flowing N2 on the catalyst for 1 h, and then followed by 500 ppm NH3/N2 instead of the N2 gas. The spectra recorded at different times, are displayed in Figure. S3. The spectra shown in Figure. S3 indicate that with 500 ppm NH3/N2 introduced, the bands at 3587 and 3454 cm-1 gradually decreased. In addition, the bands at 3720, 3633, 3361, 3268, 3181, 1622 and 1513 cm1

appeared68. The bands at 3720 and 3633 cm-1 were ascribed to the coordinated NH3

on Si-OH. The bands at 3361 and 3268 cm-1 were attributed to the NH4+ stretching vibration modes, the bands at 3181 cm-1 was ascribed to the coordinated NH3 on Cu ions69. The peaks of 1622 and 1513 cm-1 were ascribed to the coordinated NH3 on the Lewis and Brønsted acid sites.

19



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Figure. 10. In situ DRIFTS spectra of the 4%Cu-MOR-X catalyst under an atmosphere of 500 ppm NO+5%O2/N2 at 200°C. X represents different particle sizes: (a) 0.23 µm (b) 2.20 µm. All the results indicated that the reaction between the absorbed NOx species and NH3 occurred easily. Compared with the 4%Cu-MOR-2.20 catalyst (Figure. 10(b)), the 4%Cu-MOR-0.23 exhibited extra peaks at 3339 and 3191 cm-1, which is consistent with the presence of NOx adspecies. These results implied that the small size catalyst (4%CuMOR-0.23) with more Cu2+ has more adsorption sites for NOx. The reactive NOx species is advantageous to improve the NH3-SCR performance.33 Similar to the above operation process. The reaction between nitrogen oxides and ammonia adspecies was also studied. First, the catalyst was cleaned at 300 °C in N2 for 1 h to obtain a clean surface. Second, the catalyst was cooled to 200 °C. Third, 500 ppm NH3/N2 was introduced. At the same time, in situ DRIFTS spectra were recorded at different times. The spectra of the 4%Cu-MOR-0.23 catalyst in a feed of 500 ppm NH3/N2 are displayed in Figure. 11 (a). The spectra of the 4%Cu-MOR-0.23 is similar to that of the 4%Cu-MOR-2.20 catalyst in the gas containing 500 ppm NH3/N2(Figure. 11(b)). It is worth noting that the 4% Cu-MOR-0.23 catalyst has a stronger band at 3181 cm-1 compared with the 4%Cu-MOR-2.20 catalyst69. These results implied that the catalyst with the support with smaller particle size (4%Cu-MOR-0.23) has more isolated Cu2+, which is beneficial to NH3-SCR. The data displayed in Figure. S4 indicate that the intensity of the bands at 3720, 3633, 3361, 3268, 3181, 1617 and 1460 cm-1 decreased with mixture gases of 500 ppm NO + 5%O2/N2 was introduced. 20


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Figure. 11. In situ DRIFTS spectra of the 4%Cu-MOR-X catalyst under an atmosphere containing 500 ppm NH3/N2 at 200°C. X represents different particle sizes: (a) 0.23 µm (b) 2.20 µm. The spectra after 40 min reaction is very similar to that shown in Figure. 10 (a), which indicates that the reaction between the absorbed NH3 species and NOx occurred easily. Overall, not only NH3 reacts with species of activated NOx, but also NOx reacts with species of activated NH3 for the 4%Cu-MOR-X. These results proved that 4%CuMOR-X catalysts are both the Brønsted acid site and the metal site, and they can activate NH3 and NO, respectively.33 Thus, it can be concluded that the NH3-SCR reaction on the 4%Cu-MOR catalyst occurs through the Langmuir-Hinshelwood mechanism. Conclusions In summary, Cu-MOR catalysts with different particle size (0.23, 0.96 and 2.20 µm) were prepared and used to investigate the effect of the internal mass transportation on the NH3-SCR in detail. The results revealed that Cu-MOR-0.23 with the smallest particle size had the best catalytic performance even when SO2, H2O and soot are introduced in the reaction feed (modelling the diesel exhaust contents). Various kinds of characterizations, for instance, XRD, H2-TPR, NH3-TPD and XPS analysis, demonstrated that the different particle size of the supports affected the distribution of Cu species (both CuO and Cu2+). Moreover, the 4%Cu-MOR-0.23 catalyst has more isolated Cu2+ located on the ion-exchange sites and higher number of acid sites than the 21



other catalysts. In combination with the lowest mass transportation limit, the 4%CuMOR-0.23 displayed the highest catalytic performances. In addition, with the decrease of the particle size, the TOF value increased and the Weisz-Prater parameter decreased. Thus, it is demonstrated that the smaller particle size could facilitate the mass transfer of all gases and promote the accessibility of the reactants to the active sites. The in situ DRIFTS analysis further showed that the catalyst (4%Cu-MOR-0.23) with small particle size has more adsorption sites for NOx and NH3, and confirmed that the reaction occurs through the Langmuir-Hinshelwood mechanism. This work provided a novel insight into the design of high performance and cheaper deNOx catalysts for diesel exhaust. ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Supporting Information. Experimental details of Cu-MOR-X characterization, performance evaluation. XRD patterns of H-MOR-X, N2 adsorption-desorption profiles, a part In situ DRIFT spectra, the measured results by H2-TPR, XPS, NH3-TPD, the Weisz–Prater parameter and the performance of representative materials for DeNOx. (PDF) AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] (H. Peng) Notes 22


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The authors declare no competing financial interest. Acknowledgements This work was supported by the National Key R&D Program of China (2016YFC0205900), the National Natural Science Foundation of China (21503106, 215666022 and 21773106), and the Natural Science Foundation of Jiangxi Province (20171BCB23016,

20171BAB203024

and

20181BCD4004).

We

greatly

acknowledged their financial supports. References (1).Zheng, L.; Zhou, M.; Huang, Z.; Chen, Y.; Gao, J.; Ma, Z.; Chen, J.; Tang, X. Self-Protection Mechanism of Hexagonal WO3-Based DeNOx Catalysts against Alkali Poisoning. Environ Sci Technol. 2016, 50, 1195111956. (2).Cimino, S.; Lisi, L.; Tortorelli, M. Low temperature SCR on supported MnOx catalysts for marine exhaust gas cleaning: Effect of KCl poisoning. Chem Eng J. 2016, 283, 223-230. (3).Peng, Y.; Peng, H.; Liu, W.; Xu, X.; Liu, Y.; Wang, C.; Hao, M.; Ren, F.; Li, Y.; Wang, X. Sn-MFI as active, sulphur and water tolerant catalysts for selective reduction of NOx. RSC Advances. 2015, 5, 42789-42797. (4).Torre-Abreu, C. R., M.F.; Henriques, C.; Delahay, G. NO TPD and H 2-TPR studies for characterisation of CuMOR catalysts The role of Si/Al ratio, copper content and cocation. Appl Catal B. 1997, 14, 261272. (5).Delahay, G. C., B.; Broussous, L. . Selective catalytic reduction of nitrogen monoxide by decane on copper-exchanged beta zeolites. Appl Catal B. 1997, 12, 49-59. (6).Magnusson, M.; Fridell, E.; Ingelsten, H. H. The influence of sulfur dioxide and water on the performance of a marine SCR catalyst. Appl Catal B. 2012, 111-112, 20-26. (7).Hoek, G.; Beelen, R.; de Hoogh, K.; Vienneau, D.; Gulliver, J.; Fischer, P.; Briggs, D. A review of landuse regression models to assess spatial variation of outdoor air pollution. Atmos Environ. 2008, 42, 7561-7578. (8).Kim, D. S., Lee, C.S. Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 695-704 (9).De La Torre, U.; Pereda-Ayo, B.; González-Velasco, J. R. Cu-zeolite NH3 -SCR catalysts for NOx removal in the combined NSR–SCR technology. Chem Eng J. 2012, 207-208, 10-17. (10).Huang, Z.; Li, H.; Gao, J.; Gu, X.; Zheng, L.; Hu, P.; Xin, Y.; Chen, J.; Chen, Y.; Zhang, Z.; Chen, J.; Tang, X. Alkali- and Sulfur-Resistant Tungsten-Based Catalysts for NOx Emissions Control. Environ Sci Technol. 2015, 49, 14460-5. (11).Chen, J.; Chen, Y.; Zhou, M.; Huang, Z.; Gao, J.; Ma, Z.; Chen, J.; Tang, X. Enhanced Performance of Ceria-Based NOx Reduction Catalysts by Optimal Support Effect. Environ Sci Technol. 2017, 51, 473-478. (12).Liu, Z.; Li, Y.; Zhu, T.; Su, H.; Zhu, J. Selective Catalytic Reduction of NOxby NH3over Mn-Promoted V2O5/TiO2Catalyst. Ind Eng Chem Res. 2014, 53, 12964-12970. (13).Zhang, Y.-s.; Li, C.; Wang, C.; Yu, J.; Xu, G.; Zhang, Z.-g.; Yang, Y. Pilot-Scale Test of a V2O5–WO3/TiO223



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Exploring the nano - size effect of mordenite zeolites on their

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