A bifunctional MnOx@PTFE catalytic membrane for efficient low temperature NOx-SCR and dust removal

Journal Pre-proof A Bifunctional MnOx@PTFE Catalytic Membrane for Efficient Low Temperature NOx-SCR and Dust Removal

Shasha Feng, Mengdi Zhou, Feng Han, Zhaoxiang Zhong, Weihong Xing PII:

S1004-9541(19)30935-8

DOI:

https://doi.org/10.1016/j.cjche.2019.11.014

Reference:

CJCHE 1597

To appear in:

Chinese Journal of Chemical Engineering

Received date:

23 October 2019

Revised date:

11 November 2019

Accepted date:

21 November 2019

Please cite this article as: S. Feng, M. Zhou, F. Han, et al., A Bifunctional MnOx@PTFE Catalytic Membrane for Efficient Low Temperature NOx-SCR and Dust Removal, Chinese Journal of Chemical Engineering(2019), https://doi.org/10.1016/ j.cjche.2019.11.014

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© 2019 Published by Elsevier.

Journal Pre-proof A Bifunctional MnOx@PTFE Catalytic Membrane for Efficient Low Temperature NOx-SCR and Dust Removal Shasha Fenga, Mengdi Zhoua, Feng Hanb, Zhaoxiang Zhonga,*, Weihong Xinga a

State Key Laboratory of Materials-Oriented Chemical Engineering, National Engineering Research

Center for Special Separation Membrane, Nanjing Tech University, Nanjing 210009, China b

Nanjing Industrial Technology Research Institute of Membrane Material Co. Ltd., Nanjing 211100,

China

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ABSTRCT

Low-temperature selective catalytic reduction of NOx combined with dust removal technique due

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to its energy conservation characteristic has been attracted much attention for fume purification. In this work, the MnOx wrapped PTFE membrane with efficient dust removal and low-temperature

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NH3-SCR has been prepared with a facile route. MnOx with different crystal structure were

MnOx@PTFE(O-MnOx@PTFE)

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uniformly grown around the PTFE fibrils through water bath. The flower-sphere-like and

lamellar-interlaced

ripple-like

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MnOx@PTFE(W-MnOx@PTFE) have large specific surface area which is favor for enhancing

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catalytic performance. Also, the uniformly wrapped W-MnOx around the PTFE fibrils optimized the pore structure for ultrafine dust capture. The membrane can almost 100% reject particles that

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are smaller than 1.0 μm with a low filtration resistance. Meanwhile, W-MnOx@PTFE with more surface chemisorbed oxygen has the best NO conversion efficiency of 100% at a comparative low and wide activity temperature window of 160~210 ºC, which is far to the thermal limitation of the PTFE. Therefore, this efficient and energy conservation membrane has a bright application prospects for tail gas treatment compared to the original treatment process. Keywords: MnOx, PTFE, catalytic membrane, low-temperature, SCR

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1.INTRODUCTION

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Fossil fuels as the most crucial energy has play a vital role in supplying the power for

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industries. Inevitably, it also exhausts many kinds of contaminates in the burning process. Among the pollutants, particulate matter, nitrogen oxides (NO x), and sulfur dioxide are the main

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injurant.[1, 2] Naturally, the treatment of industrial flue gas usually consists of three parts, , dust removal, and desulphurization.[3-5] Aiming to comprehensive utilize the energy of high temperature fume gas and realize the lower discharge of pollutants. The three parts usually planted of denitrification, dust removal, and desulphurization, separately. The divided devices usually need more occupation area, equipment cost, and energy consumption. Catalytic membrane coupling with the function of denitrification and dust interception is now a very promising technology for industrial fume purification. Dust can be intercepted on the surface of the separation layer, while gas pollutants can be degraded in the support layer with suitable conditions.[6, 7] Currently, the research of catalytic membrane is almost focus on ceramics due to the thermostability requirement of the catalyst preparation process and its SCR activity temperature window. Commonly, to prepare the SCR catalyst, the raw material need to be 2

Journal Pre-proof sintered at the temperature above 400 °C. [8, 9] The optimum temperature for the catalyst usually around 350 °C.[10, 11] This means the fabrication and application temperature of the catalytic membrane may significantly increase the energy consumption. Also, the comparative lower gas permeation (~80 m3·m-2·h-1·kPa -1)[12] and higher fabrication and running cost of ceramic membranes are inevitable problems for practical application. Therefore, excellent dust removal efficiency combine with high gas permeation and NOx catalytic activation at a comparative low temperature are the most essential ingredients for catalytic membrane. Bidirectional stretching PTFE membrane with the fibrils and nodes connected net-like

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structure can effectively intercept the particulate matters[13]. Besides, the chemical stability and thermal resistance of the PTFE membrane made it an excellent material for flue gas

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purification[14]. Endowing the PTFE membrane with catalytic function can simultaneously

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removal solid particles and NOx of the fume gas, which can significantly solve the above-mentioned problems. However, to maintain the uniform net-like pore structure of the PTFE

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membrane, common catalyst loading procedure including precipitation and high temperature sintering should be avoided. Considering to the thermal resistance restriction of PTFE membrane,

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the catalyst loading process and its activation temperature should lower than 300 ºC[15, 16].

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In recent years, metal oxides (VOx, NiOx, CuOx, MnOx, CeOx, FeOx, and CoOx) based catalyst have been reported as the low temperature NH 3-SCR activation[17, 18]. Among them,

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MnOx has attracted much attention since its high activity on NH3-SCR at a comparative low temperature (~300 ºC)[19, 20]. The main preparation methods of MnOx catalyst including hydrothermal[21, 22], impregnation[23, 24] precipitation[25, 26], and thermolysis of manganese compounds[27], etc. In this work, to prepare the catalytic membrane without reduce the filtration performance, the in-situ growth in solvent atmosphere under water bath has been carried out to loading the MnOx on the PTFE membrane. By control the reaction conditions of the experiment, the high activity MnOx with several kind crystal structures were successfully loaded on PTFE membrane. The comprehensive performance of the MnOx@PTFE membrane was systematically characterized and evaluated. This catalytic membrane shows an excellent dust filtration and low temperature NO degradation performance.

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Journal Pre-proof 2. EXPERIMENTAL 2.1 Materials. Porous PTFE membranes with a mean pore diameter of 5 µm in the form of circular disc (diameter: 47 mm; thickness: 100 µm) were purchased from Sartorius (Goettingen, Germany) and used as received. Potassium permanganate (KMnO4) and nitric acid (HNO3) which are all analytically pure were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. 2.2 Preparation of MnOx@PTFE Membrane PTFE was first adhered on the glass slide with the dual adhesive tape, which can maintain the

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membrane with a flat structure when dipping in the reaction solution. To grow the MnOx on the membrane uniformly, the membranes were stuck on the glass slides and then dipped in the ethanol

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to wet the membrane pore channels. 8.0 g KMnO4 powder was first dissolved in 200 mL distilled

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water and magnetic stirring for 0.5 h. 15 mL of nitric acid was then added to the solution dropwise with magnetic stirring. The ethanol wetted PTFE were then immersed in the solution and then

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water bath for 1-5 h at the temperature of 80 C. The treated samples were washed by deionized water and then dried naturally.

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2.3 Catalytic Activity Test. The low-temperature SCR catalytic activities of the samples were

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recorded in a catalytic device (as shown in Fig. 1) with the reactant gas composition of NO (500 ppm), NH3(500 ppm) and O2(5%). Additionally, the typical reaction conditions were balanced by

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N2 with a flow rate of 80-240 mL/min and a gas hourly space velocity (GHSV) of 60,000−183,000 h−1. The catalytic temperature was ranged from 40 to 250 C with the holding time of 10 min in each experiment condition. The outlet NO and NO 2 were continually monitored by the flue gas analyzer (Testo-350, Germany Testo Company). The NOx conversion was calculated by the following equation: NOx conversion (%) =

[𝑁𝑂𝑥]𝑖𝑛 −[𝑁𝑂𝑥]𝑜𝑢𝑡 [𝑁𝑂𝑥]𝑖𝑛

where NOx was the sum of NO and NO2, and the subscripts ‘‘in” and “out” denoted the inlet and outlet gas concentrations of NOx, respectively.

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Fig. 1 The NH3-SCR test device and the membrane module.

2.4 Characterization

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The surface morphology and the microstructure of the membranes were recorded by the field

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emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). A thin layer of Au-Pd alloy was sputter-coated on all the membrane samples for 20 s prior to SEM observation. The

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TEM results of the samples were collected by the JEOL 2010, Japan. Samples were first scraped using a scraper, and then ultrasonicated in ethyl alcohol for 3 h with the power of 300 W. The

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dispersion contains of MnOx in the ethyl alcohol was subsequently placed on the copper wire

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mesh for TEM imaging. In order to investigate the loading of MnOx on the membrane, the samples were weighted before and after modification. Fourier transform infrared (FT-IR) spectra

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were obtained with Nicolet 8700 in the total refection method from 4000cm-1 to 500cm-1. The X-ray photoelectron spectra (XPS) were acquired with Thermo Escalab 250. Al K radiation (1486.6 eV) was used as the source and the C 1s peak was used as a reference. The crystallization structure of the MnOx and the PTFE was characterized by XRD (MiniFlex 600, Japan) with the Cu Kα radiation (λ = 0.154 nm) at a generator voltage of 40 kV and a generator current of 15 mA. The scanning speed and the step were 5 °/min and 0.02 °/min, respectively. The gas permeation of the membranes was measured by the pore-size distribution analyzer (PSDA-20, Gaoqian, China). the testing pressure of the apparatus was ranged from 0 to 10 kPa. The test area was 0.79 cm2 (φ=1 cm).

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Journal Pre-proof The gas flux was determined using the equation: J=Q/A. Where J is the gas flux(m3·m-2·h-1), Q is the gas flow rate which is collected by the PSDA-20 with its unit of m3·h-1, A is the effective test area with the unit of m2. The equation for gas permeation was described as: P=Qp/(∆P∙A). Where the gas permeation P with the unit of m3·m-2·h-1·kPa-1 is interpreted as the function of gas flux Q p (m3·h-1), the pressure drop ΔP (kPa), and the test area A (m2). The pore size distribution of the membrane was tested according the bubble point method with the instrument of PSDA-20. The membrane was first wetted with the isopropyl alcohol prior to test. The inverse pressure - pore size relationship for a cylindrical pore can be expressed by the equation: (D=4γcosθ/ΔP)[28]

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where D (m) is pore diameter, γ (N/m) is the surface tension of wetting fluid, ΔP (Pa) is the pressure difference between gas and liquid and θ is the contact angle of wetting fluid on

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membrane surface. The dust capture efficiency of the samples was carried out on the filtration performance test apparatus (LZC-K1, Suzhou Huada instrument and equipment Co, Ltd.) with the

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filtration area of 12.5 cm2, gas flux of 4 L/min. The rejection ratio for NaCl particles at the dimeter

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3 RESULTS AND DISCUSSION

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of 0.3 μm, 0.5 μm, 1.0 μm, 2.5 μm, 5.0 μm, and 10 μm were tested.

3.1 Growth of MnOx on the PTFE Fiber

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Fig. 2(a) shows the surface morphology of pristine PTFE membrane. The continuously

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connected fibers of original PTFE membrane were connected by nodes through bidirectional stretching. The length of fibers is ranged from 10-50 μm. Fig. 2(b) shows the uniform distributed PTFE fibers possess a smooth surface with its diameter of 0.1-1 μm. After growing the MnOx by water-bath heating, the prepared MnOx were wrapped around the PTFE fibers as shown in Fig. 2(c). The MnOx enwrapped PTFE fibers still keep its original pore structure with contiguous fibers separated. From the magnified image of sample MnOx@PTFE (Fig. 2(e)), the MnOx was well-proportioned around the PTFE fibers with a lamellar and interlaced ripple-like structure(W-MnOx@PTFE). Compared to the solid structure of pristine PTFE membrane, this ripple-like structure MnOx have a large amount of ~50 nm open-type fold, which would significantly improve the special surface of the material. Besides the lamellar and interlaced ripple-like structure, flower sphere structure(O-MnOx@PTFE) (Fig. 2(d)), and rod like structure(X-MnOx@PTFE) (Fig. 2(f)) of MnOx were grown on PTFE membrane by control the 6

Journal Pre-proof reaction conditions, respectively. The O-MnOx@PTFE, W-MnOx@PTFE, and X-MnOx@PTFE were prepared with the water bath of 80 C, and reaction duration of 1 h, 3 h, and 5 h respectively. The MnOx loading amounts of O-MnOx@PTFE, W-MnOx@PTFE, and X-MnOx@PTFE were 47.53%, 51.29%, 53.77%, respectively (as shown in Fig. 3). Which means MnOx loading amounts have an increase tendency with the prolongation of the reaction time. The ~3% comparative

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increase amounts of the samples demonstrated the reaction time has little effect on loading MnOx.

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Fig. 2 Surface morphology of (a, b) pristine PTFE membrane and the magnified image; (c) MnOx@PTFE membrane; d-f list three configurations of MnOx modified PTFE membranes, d-f were attributed to O-MnOx@PTFE, W-MnOx@PTFE, X-MnOx@PTFE, respectively.

Fig. 3 MnOx loading amounts of different samples.

3.2 Characterization of MnOX@PTFE Membranes 7

Journal Pre-proof The MnOx@PTFE with different configurations were characterized by FT-IR and XRD respectively. As shown in Fig. 4 (a), characteristic peaks appeared at 1208 cm-1 and 1152 cm-1 on FTIR spectrum are assigned to symmetric C−F stretch and asymmetric C−F stretch of pristine PTFE membrane[29]. At the position of 3300 cm-1 and 1640 cm-1, the peaks are corresponded to – OH, which are come from the crystal water[30, 31]. The intensity for C-F of W-MnOx@PTFE (Fig. 4(a(3)) are lower than that of other samples. From Fig. 2 (c)and (e), the MnOx has wrapped tightly around the PTFE nanofibers without any space, which may lead to a weak intensity of PTFE related peaks. Also, the lower –OH related peaks in W-MnOx@PTFE (Fig. 4(a(3))

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represent that the W-MnOx possesses less crystal water than O-MnOx@PTFE and X-MnOx@PTFE. The crystalline phases of the MnOx were characterized by XRD analysis. Due

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to the high crystallization of PTFE, the intensity of the peak PTFE was extremely strong than that

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of MnOx. The XRD patterns was manually magnified to investigate the peak characteristics of the samples. As shown in Fig. 4(b, c), the peak centered at 18.06° was assigned to the crystalline

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PTFE[32]. Other additional weak peaks were observed at 31.8°, 36.9°, and 49.3°[33]. Compared with the peaks of PTFE, MnOx has a comparative weak peak intensity at 2θ of 12.2° and 36.9° (as

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shown in Fig. 4(c(2,3))). Corresponding to the (003) and (101) lattice planes of δ-MnO2

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respectively[21]. The interchain d-spacings were calculated to be 7.2 Å and 2.4 Å by using Bragg equation. For the O-MnOx@PTFE(Fig. 4(c(2))), peak lied in 12.2° was weak than that of

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W-MnOx@PTFE(Fig. 4(c(3))), which means the crystallization degree of O-MnOx@PTFE is lower than that of W-MnOx@PTFE. The results were consistent with the crystal morphology as observed in FESEM (Fig. 2(d)). Peaks in Fig. 4c (4) are almost exactly consistent with pristine PTFE. Peaks of X-MnOx may be covered by the strong PTFE peaks. According to other research work, the nanorods MnOx may be α-MnO2[34].

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Journal Pre-proof Fig. 4 (a) FT-IR spectra and (b)XRD patterns of (1) pristine PTFE, (2) O-MnOx@PTFE,(3) W-MnOx@PTFE, (4) X-MnOx@PTFE, respectively. Panel (c) is a high-magnification image of panel (b). To further investigate the composition and proportion of MnOx, energy dispersive spectrometer (EDS) and XPS analysis were performed. The element dispersion on the membrane was shown in Fig. 5. The elements “C”, “F”, “Mn” and “O” were uniformly distributed along with the MnOx@PTFE fibers. Element “K” from the reactant potassium permanganate was also detected, which means there are some K doped components on the membrane. The XPS results of Mn 2p and O 1s peaks were deconvoluted into several peaks through PeakFit. As shown in Fig.

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6(a), Mn 2p region contained a spin–orbit doublet with Mn 2p1/2 ranging a binding energies (BE) of 649.6–659.6 eV and Mn 2p3/2 ranging a BE of 637–649.6 eV. The Mn 2p3/2 can be split into

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three peaks at around 640.6, 642.5, and 644.3 eV, corresponding to Mn2+, Mn3+, and Mn4+ cations,

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respectively[35, 36]. The value of Mn4+/(Mn3++Mn2+) calculated by the integral area of the XPS spectra show that the ratio of (X-MnOx)-(W-MnOx)-(O-MnOx) is 48.22%, 36.96%, and

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32.05%(Table 1), respectively, presenting more Mn4+ in X-MnOx than that in W-MnOx and O-MnOx. The results revealed the longer reaction duration is good for generating more Mn4+. The

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spectra of O 1s can be fitted into three peaks as shown in Fig. 6(b), which correspond to various

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oxygen containing chemical bonds. The binding energy at 533.4 eV corresponds to the surface hydroxyl groups(-OH), while the binding energy at 531.4 eV can be assigned to the Oα, such as

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CO32-, which are adsorbed under chemical effect. The binding energy of 530.0 eV correspond to the lattice oxygen O2-, which is denoted as Oβ[37, 38]. The Oα/(Ototal) ratio in as-prepared MnOx@PTFE follows the trend of W-MnOx@PTFE (33.57%) > X-MnOx@PTFE (31.57%) > O-MnOx@PTFE (30.75%) (Table 1). As is known,

surface chemisorbed oxygen is the most

active oxygen for catalytic reactions[39, 40]. That means the W-MnOx@PTFE may have better catalytic performance for denitrification than that of O-MnOx@PTFE and X-MnOx@PTFE. Table 1 Percentage of Elements Existing on the Surface of the Samples Samples\%

Mnatomic

Mn4+/Mntotal

Oatomic

Oα/Ototal

O/Mn

O-MnO@PTFE

28.12

32.05

64.67

30.75

2.30

W-MnO@PTFE

25.88

36.96

60.88

33.57

2.35

X-MnO@PTFE

28.76

48.23

64.21

31.57

2.23

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Fig. 5 The elemental mapping images of the W-MnOx@PTFE membrane.

Fig. 6 (a)XPS spectra of (a) Mn2p and (b) O1s of O-MnOx@PTFE, W-MnOx@PTFE and X-MnOx@PTFE membrane. To investigate the microstructure, assembly form and lattice type of MnOx, high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) were performed as shown in Fig. 7(a-e). According to Fig. 7(a,b) the O-MnOx and W-MnOx possess a 10

Journal Pre-proof petal like structure with its diameter ~250 nm. The “flower” seems to be assembled by many nanosheets, which have some wrinkles. The loose structure of the “flower” may significantly improve the specific surface area of the membrane. As for the X-MnOx (Fig. 7(c)), the nanorods with its length of ~100 nm was dispersed out of order. The diameter of the nanorods were about 5 nm. The HRTEM of petal like MnOx as shown in Fig. 7(d) revealed the lattice space between adjacent lattice planes is 2.40 Å, 2.15 Å and 1.83 Å. While the lattice space of X-MnOx is 3.10 Å (Fig. 7(e)). The lattice space of the samples can be verified in the XRD analysis result. The inset SAED results shows the petal like MnOx is a polycrystalline material while X-MnOx is a single

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crystal material. The proposed mechanism for the formation of MnOx on the PTFE surface was shown in Fig. 7(f). In the initial stage, the amorphous state of MnO2 begin to form nanoparticles as

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shown in Fig. 7(f-1). According to Ostwald ripening process[41, 42], When the nucleus was formed, the comparative bigger particles would growing bigger and bigger by sacrificing litter

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particles. Also, under the effect of K+ and water molecular, the particles will grow with

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preferential orientation. In addition, in the effect of perturbation condition, the nucleus with disordered dispersion on the PTFE fiber may lead to a flower sphere structure of the MnOx (Fig.

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7(f-2)), while the uniform dispersion of nucleus on the PTFE fiber may grow to the lamellar and

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interlaced ripple-like structure (Fig. 7(f-3)). With the increase of the reaction time, the preferential orientation plays a vital role in transfer the MnOx to rod like structure by consuming the litter

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particles near the major MnOx structure (Fig. 7(f-4)).

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Fig. 7 TEM images of as prepared (a)O-MnOx, (b)W-MnOx and (c)X-MnOx; (d)(e) shows the HRTEM of O-MnOx and X- MnOx, inset are the selected area electron diffraction (SAED) results of O-MnOx and X-

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MnOx. (f) Proposed mechanism for the formation of MnOx@PTFE in the solution. (1) the initial stage of the reaction, (2) flower sphere like structure, lamellar and interlaced ripple-like structure, (3) rod like structure MnOx on the surface of the PTFE fiber.

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3.3 Gas Permeation Property of MnOx@PTFE Membranes

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To investigate the growing conditions of MnOx on PTFE membrane and the influence on pore characteristics of the membrane, gas permeation and pore size distribution of the samples

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were measured (Fig. 8). The clean air permeation of pristine PTFE and MnOx@PTFE were conducted by pure N2. The range of the operating pressure drop was from 0 to 2 kPa. The error bars represent a standard deviation of gas flux within five measurements. Fig. 8 shows that the gas permeation of pristine PTFE is 550 m3m-2h-1kPa-1. After growing of MnOx on PTFE membrane, the gas permeation of the samples has a decline around 12%28%. The accurate value of the gas permeation for O-MnOx@PTFE, W-MnOx@PTFE, and X-MnOx@PTFE were 441.4 m3m-2h-1kPa-1, 483.5 m3m-2h-1kPa-1, and 394.3 m3m-2h-1kPa -1, respectively. This result revealed that the W-MnOx@PTFE has a highest gas permeation than that of other samples. While X-MnOx@PTFE has a minimum gas permeation. That is because the sphere like MnOx may grow in the space between the filters which may block the open channels (Fig. 2(d)). For the X-MnOx@PTFE, the nanorods bridged the neighbored fibers or covered on the surface of the 12

Journal Pre-proof membrane (Fig. 2(f)) may substantially reduce the gas permeation of the membrane. The W-MnOx were grown around the fibers which can enhance the fibers and keep a steady pore structure of the membrane (Fig. 2(e)). The MnOx narrowed nanofibers have few effects on decreasing the pore size of the membrane, leading a comparative high gas permeation that other simples. Pore size distribution of the samples were conducted to further verified the above analyzed conclusion. As shown in Fig. 8(b), the original pore size of the pristine PTFE membrane was about 5.1 μm. There is no doubt the modification process may decrease the pore size of the samples. The decline tendency of pore size and gas permeation is consistent. W-MnOx@PTFE

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possesses the largest pore size (~3.5 μm) than that of other O-MnOx@PTFE(~2.4 μm) and

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X-MnOx@PTFE (~2.2 μm) membranes.

Fig. 8 (a) gas permeation and (b)pore size distribution of (1) pristine PTFE, (2) O-MnOx@PTFE, (3)

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W-MnOx@PTFE, and X-MnOx@PTFE.

3.4 Low Temperature SCR Performance of the Membranes The low-temperature SRC removal activity of NOx with NH 3 over different samples are shown in Fig. 9(a). The NO conversion at the temperature of 60-90 C was about 50%. The catalytic activity of the samples was improved with the increase of the reaction temperature from 90 to 160 C. At the range of 160 to 210 C, the catalytic membrane possesses the best activity efficiency for SCR reaction. For different samples, W-MnOx@PTFE has the best NO conversion efficiency than that of O-MnOx@PTFE and X-MnOx@PTFE. That is because the W-MnOx@PTFE possesses more surface adsorbed oxygen (33.57%) and larger specific surface area (131.87 m2/g), which is favor for denitrification. Automatically, the X-MnOx@PTFE and O-MnOx@PTFE with comparative smaller specific surface area (17.65 m2/g, 92.74 m2/g) and fewer of adsorbed oxygen (31.57%, 30.75%) has a lower NO removal efficiency. To evaluate the 13

Journal Pre-proof catalytic performance of the W-MnOx@PTFE in accurate, the gas hourly space velocity (GHSV) was adjusted from 60,000 h-1 to 183,000 h-1. As shown in Fig. 9(b), with the increase of the GHSV, the NO conversion has a decline tendency. When the GHSV was 60,000 h-1, the NO can be totally degraded at the temperature of 160210 C. With the GHSV increased to 183,000 h-1, the NO conversion was decreased from 100% to 77% at the best temperature window. The catalytic activity of the W-MnOx@PTFE at the low gas velocity is better than that in high gas velocity. As shown in Fig. 9(c), no matter at which temperature region, the catalytic activity was decline with the increase of the gas flux. That means the residence time of the NO has significant effect on the

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SCR efficiency. The low temperature SCR performance of the samples compared to others were shown in table 2. As can be seen in table 2, the MnOx catalyst demonstrated the best low

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temperature NOx removal ability.

Fig. 9 (a) NO conversion on MnOx@PTFE membranes with the GHSV of 60,000 h-1, (b) NO conversion on

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W-MnOx@PTFE membrane at different GHSV, (c) NO conversion on W-MnOx@PTFE membrane at two reaction temperature regions under the different GHSV.

Table 2 Comparison of activation temperature and GHSV of the catalyst with other reported work Catalyst

Activation temperature/°C

GHSV

Reference

Cu2Mn0.5Al0.5Ox

150-200

-

[43]

CeVO4

200-350

26,000 h-1

[44]

MnOx/ZrO2‒TiO2

˃200

-

[45]

Mn/CeO2

300

60,000 ml·g-1·h-1

[22]

Mn2CoO4@rGO

160-200

30,000 h-1

[46]

MnOx

160-210

60,000 h-1

This work

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Journal Pre-proof 3.5 Particulate Matter Filtration Performance of the Membranes Dust interception performance of the samples were performed with NaCl aerosol particles. As shown in Fig. 10, PTFE membrane can almost completely intercept the particles with their diameter more than 1.0 μm. For the most penetrating particle size (MPPS), which is typically around 0.3 μm or smaller, pristine PTFE membrane can intercept more than 97.03% of them, 98.94% for the particles size ranged to 0.5 μm. For the MnOx modified PTFE membrane, the MnOx constructed new structure has decreased the pore size of the membrane which is favor for dust interception. Logically, the particles removal efficiency of the MnOx@PTFE membranes was

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much higher than that of pristine PTFE. The (0.3 μm and 0.5 μm) particles interception for O-MnOx@PTFE, W-MnOx@PTFE, and X-MnOx@PTFE were (97.91%, 98.97%), (98.21%,

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99.13%), and (99.77%, 99.88%), respectively. The X-MnOx@PTFE due to its comparative

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smaller pore size and nanorods constructed hierarchical structure has the best particle removal

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efficiency.

Fig. 10 Dust interception efficiency of the samples. 4 CONCLUSIONS In this paper, MnOx@PTFE membrane with different morphologies (including flower sphere structure, lamellar and interlaced ripple-like structure, and rod-like structure) were prepared by a facile water bath growth method. The obtained materials such as W-MnOx@PTFE has an 15

Journal Pre-proof excellent low temperature SCR activity. The reaction time has the significant effect on the morphologies of the MnOx. With the increase of the reaction time, the δ-MnO2 dominated MnOx tend to convert to α-MnO2 dominated MnOx. The δ-MnO2 with extremely uniform structure possesses a large specific surface area and more surface adsorbed oxygen, which are critical for improving the catalytic activity for low temperature SCR (100%, GHSV=60,000 h-1, 160-210C). Meanwhile, the MnOx homogeneously packaged around the PTFE fibers play a positive effect on keeping the pore structure steady, leading to a comparative high gas permeation (483.5 m3m-2h-1kPa-1). This MnOx wrapped PTFE membrane shows an excellent application

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perspective for low temperature air purification to remove NOx and ultrafine dust simultaneously.

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AUTHOR INFORMATION

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Corresponding Author

*Phone: +86 83172163 fax: +86 83172292 e-mail: [email protected]

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Notes

ACKNOWLEDGEMENTS

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The authors declare no competing financial interest.

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Financial support was provided by the National Key R&D Program (2016YFC0204000), the

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National Natural Science Foundation of China (21878148, and U1510202), the Jiangsu Province Scientific Supporting Project (BK20170046), and the Natural Science Foundation of Jiangsu Province (BK20180164).

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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