Fabrication of Highly Dispersed Cu-based Oxides as Desirable NH3

19 hours ago - In this study, three kinds of CuAl-LDO/CNTs (LDO, layered double oxide) catalysts were prepared by the assembly of CNTs and CuAl-LDH ...
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Energy, Environmental, and Catalysis Applications

Fabrication of Highly Dispersed Cu-based Oxides as Desirable NH3-SCR Catalysts via Employing CNTs to Decorate the CuAl Layered Double Hydroxides Xu Wu, Hao Meng, Yali Du, Jiangning Liu, Benhui Hou, and Xianmei Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08699 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Fabrication of Highly Dispersed Cu-based Oxides as Desirable NH3-SCR Catalysts via Employing CNTs to Decorate the CuAl Layered Double Hydroxides Xu Wu,*,† Hao Meng,*,† Yali Du,‡ Jiangning Liu,† Benhui Hou,† and Xianmei Xie† †College

of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan

030024, PR China. ‡College

of Chemistry and Chemical Engineening, Jinzhong University, Jinzhong 030619, PR

China. KEYWORDS: NH3-SCR, layered double hydroxide, Cu-based active sites, CNTs, high dispersion ABSTRACT: In this study, three kinds of CuAl-LDO/CNTs (LDO, layered double oxide) catalysts were prepared by the assembly of CNTs and CuAl-LDH (LDH, layered double hydroxides) as well as subsequently structural topological transformation. The effects of assembly method on the surface structure property and the DeNOx performance of the prepared samples were systematically investigated. It was found that three CuAl-LDO/CNTs catalysts showed preferable NH3-SCR catalytic performance compared with CuAl-LDO, where the catalyst CuAl-LDO/CNTs(I) exhibited optimum NOx conversion(> 80%) and N2 selectivity (> 90% ) within 180-300 ℃. Such fine catalytic performance can be attributed to the the proper

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surface acidity and redox ability of the catalyst,which might be correlated with the high dispersion of Cu-based active centers caused by the induced nucleation and effective separation action of LDH by carbon nanotubes. In addition, the outstanding H2O and SO2 resistance of the CuAl-LDO/CNTs(I) catalyst was also obtained because of the synergistic effect between CuAlLDO and CNTs which could greatly promote the activation and decomposition of ammonium sulfate at lower temperatures. 1. INTRODUCTION Along with the excessive consumption of fossil fuel, gaseous pollutant from fossil fuel combustion processes has given rise to a variety of environmental issues that to be harmful for ecosystems and human beings, where nitrogen oxides (NOx) are the major contributors.

1-3

In

response to environmental protection requirements, numerous solutions have been proposed to reduce NOx emissions.4,5 Among them, selective catalytic reduction of NOx with NH3 (NH3SCR) is considered as the most promising approach for the removal of NOx.6 In view of the fact that the working temperature window of the mature commercial catalyst (V2O5-WO3 (MoO3)/TiO2) in this technology can't well match the requirement of low temperature DeNOx condition at present. Based on the practical significance of low-temperature flue gas treatment and research, it is of great value to develop low-temperature NH3-SCR catalyst with high efficiency, stability and friendly environment. Recent years, transition metal oxides, particularly, Cu-based oxides catalysts have attracted a wide range of interest because of their environmentally benign characteristics, the excellent redox ability and low cost.7,8 Nevertheless, the Cu-based oxides catalysts are currently in the dilemma of limited DeNOx performance caused by agglomerate of CuO and poor resistance to SO2/H2O.8,9 Among various modified routes, Cu-based mixed metal oxides catalysts derived

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from high temperature topological transformation of layered double hydroxides (LDH, [M2+1xM3+ x(OH)2][An-]x/n·mH2O) with eutectic structure have aroused extensive concern in NH3SCR field.10,11 Upon calcination at intermediate temperatures (400-600 ℃), LDH can undergo laminate collapse and structure topotactic transformation to form layered double oxide (LDO).12,13 Based on the uniform coordinated octahedral ordered structure between metal cations and hydroxyl groups as well as lattice limiting effect between MII and MIII metal ions, the active metal species possess a unique advantage in the dispersion of active components and showed excellent catalytic prospect.14 As recently reported, CuyAlOx mixed metal oxides from the CuAlLDH precursors exhibited higher DeNOx activity than CuO/Al2O3 prepared by traditional impregnation method.15 The results show that this is directly related to the good dispersion of the active center. The (CuxZny)2Al-MMO catalyst derived from Zn-doped CuAl-LDH precursors possessed higher dispersity of CuO compared with Cu2Al-MMO catalyst from CuAl-LDH due to the interspersion between Cu and Zn ions, thus presented better DeNOx performance.16 Despite that, the Cu-based LDHs prepared by traditional co-precipitation method tend to form bundles or layered stacks when the surface is not modified due to the high surface energy, strong hydrogen bonding between laminates and the Jahn-Teller distortion of Cu2+,17-19 where the active components are still prone to agglomeration or sintering and is not conducive to the full exposure of active sites during a long run or calcination treatment at higher temperature. Besides, the SO2/H2O resistance performance is also not satisfactory yet. Thus, searching for an effective way to alleviate the layers stacking of Cu-based LDH is expected to achieve the construction of highly dispersed Cu-based oxides catalysts with excellent DeNOx performance. Based on its one-dimensional tubular structure, large specific surface area and high mechanical strength, the carbon nanotubes (CNTs) have been used as substrates for the formation of nano-

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composites in various fields.20,21 Importantly, the carbon nanotubes can promote the decomposition of ammonium sulfate, and meanwhile the hydrophobicity of graphite carbon materials can also inhibit the formation of ammonium sulfate to some extent.22,23 Thus, the introduction of carbon nanotubes will be helpful to the improvement of sulfur resistance of catalysts. Inspired by those mentioned points, the simple binary CuAl-LDH was selected as the main research subject, and a series of CuAl-LDH/CNTs nanohybrids were synthesized via three different assembly methods (including in-situ, stir and mechanically assembly). It is expected that the effective separation of CuAl-LDH layers can be realized by means of the spatial barrier action of carbon nanotubes (CNTs), and subsequently prepare highly dispersed Cu-based oxide catalysts via structural topology transformation. The multiple characterizations were employed to investigate the effects of assembly methods on the structures, morphology and surface property correlation of the catalysts. As the results proposed, in comparison to CuAl-LDO, the three synthetic CuAl-LDO/CNTs exhibited higher DeNOx activity as expected. Owing to the loose stacking structure of CuAl-LDH/CNTs(I) precursor during the in-situ assembly process, the high dispersion of Cu-based active components after calcination was realized. Therefore, the CuAlLDO/CNTs(I) catalyst prepared by in-situ assembly exhibited optimal DeNOx performance. This work may serve as an important reference for Cu-based LDH precursors to be tailored based on laminate structure control for potential catalytic applications. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The multiwalled CNTs (Chengdu Institute of Organic Chemistry, Chinese) with a diameter of 10-20 nm and a length of 1-10 um were treated by refluxing in 68% nitric acid at 120 °C for 8 h, then filtered and washing with distilled water until neutral. Finally, the collected deposition was dried at 60 °C for 12 h to obtain the modified carbon nanotubes

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(modified CNTs). All the other chemicals were purchased from Sinopharm Chemical Reagent Company and used without further purification. The CuAl-LDH/CNTs(I) catalyst precursor was synthesized through in-situ assembly method. Briefly, take 0.5 g of the purified CNTs and add them to 25 mL deionized water to form dispersion liquid. Then, the 50 mL mixed nitrate salt solution (c(Cu2+)=0.6 mol L-1, c(Al3+)=0.2 mol L-1) and 100 mL NaOH solution (1.0 mol L-1) were slowly dripped into the CNTs dispersions solution at the same time by double strand titration under the action of magnetic stirring. The resulting slurry was transferred into a 200 mL PTFE autoclave and then placed in an oven at 110 °C for 12 h. After hydrothermal crystallization, the precipitate was filtered, washed several times with deionized water until neutral. The CuAl-LDH/CNTs(I) was obtained by drying at 80 °C for 12 h in an oven. Subsequently, the CuAl-LDO/CNTs(I) mixed oxides were obtained after calcined at 500 °C for 5 h in nitrogen atmosphere with a flowing rate of 80 mL/min. The mass percent of CNTs in this catalyst was calculated by carbon sulfur analyzer being about 13.49%. Meanwhile, according the same method as above, the single CuAl-LDH was also prepared without the introduction of CNTs. The compositions of the CuAl-LDO was measured by the ICP-OES technique and the average molar ratio of Cu to Al is 2.8 : 1. About the catalysts prepared by stir assembly and mechanically assembly. The feed ratio of both samples is consist with CuAl-LDH/CNTs(I). The stir assembly procedure is as follows: firstly, the purified CNTs were addition into CuAl-LDH slurry and vigorously stirring for 8 h. Then the product (denoted as CuAl-LDH/CNTs(S)) was obtained through filtered, washed with deionized water and drying at 80 °C for 12 h. In the mechanically assembly process, the purified CNTs and CuAl-LDH powder were physically mixed and fully grinded, the obtained sample was labelled as CuAl-LDH/CNTs(M). Subsequently, both the catalyst precursors were calcined at

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500 °C for 5 h in nitrogen atmosphere. The samples after calcination were denoted as CuAlLDO/CNTs(S) and CuAl-LDO/CNTs(M), respectively, in where the content of CNTs were also calculated by carbon sulfur analyser are 13.67% and 13.78%. 2.2. Catalyst Characterization. The thermogravimetric (TG) and the derivative thermogravimetric (DTG) analyses were run in a thermal analyzer (METTLER TOLEDO, TGA/SDTA851). The samples were loaded onto the sample holder, and the temperature of the TG furnace was increased from 50 ℃ to 800 ℃ in a flowing argon (60 mL/min) with a rate of 5 ℃ min-1. The quadrupole mass spectrometer (QMS) was used to analyse the evolved gases. The powder XRD (X-ray diffraction) patterns were recorded on a Phillips Xpert diffractometer using nickel-filtered Cu Kα (wavelength 1.54056 Å) radiation source and a scintillation counter detector. An aluminum or glass holder was used to support the catalyst samples. Intensity data were collected over a 2θ range of 5°-85° with a scanning rate of 8°/min and a step size of 0.03°. Crystalline phases were identified by comparison with the reference data from International Center for Diffraction Data (ICDD) files. The Raman spectra were recorded on a Renishaw Micro-Raman System 2000 spectrometer with a 532 nm laser of Ar+ ion laser as excitation source at room temperature. The power to the sample was filtered down to 0.3 mW. H2-temperature programed reduction (H2-TPR) experiment was carried out through FINESORB-3010 chemisorption analyzer (FINETEC INSTRUMENTS, China). Typically, the samples were pretreated at 250 °C for 1 h in a flow of ultrahigh purity helium (30 mL/min) stream and cooled down to room temperature. After the pretreatment, H2-Ar mixture (10 vol% H2 by volume) was switched on and the samples were heated from 50 to 800 °C with a linear

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heating rate of 10 °C/min. The amount of consumed hydrogen was measured quantitatively by a thermal conductivity detector. NH3-temperature programmed desorption (NH3-TPD) experiments were performed through FINESORB-3010 chemisorption analyzer (FINETEC INSTRUMENTS, China) to determine NH3 adsorption property of the samples. Prior to each measurement, the catalyst was pretreated in a He stream at 250 °C for 1h. Then the reactor was cooled to 50 °C in the same stream. Subsequently, the samples were saturated by a flow of NH3-He mixture (3 vol.% NH3 by volume) for 1 h, and flushed with purged He to remove weakly adsorbed NH3 for a further 1h. The temperature was linearly increased to 800 °C at 10 °C/min. The NH3-TPD data were recorded in an on-line gas chromatographer using a TCD detector. (NH4)2SO4-temperature programmed decomposition ((NH4)2SO4-TPDC) was carried out in FINESORB-3010 chemisorption analyzer (FINETEC INSTRUMENTS, China). The (NH4)2SO4 was pre-deposited on the catalysts by impregnation method, followed by overnight drying at 80 °C. The 0.1 g catalysts with 0.01 g (NH4)2SO4 were pre-treated in Ar stream (50 mL/min) at 200 °C for 1 h, and then cooled to 50 °C. Subsequently, the samples were heated from 50 °C to 500 °C with the rate of 10 °C/min. NO-(NH4)2SO4-temperature

programmed

surface

reaction

(NO-(NH4)2SO4-TPSR)

of

experiments was performed by a laboratory self-made apparatus (fixed bed reactor). The 0.5g (NH4)2SO4-deposited catalysts was exposed to a mixture stream containing about 500 ppm NO, 5 vol.% O2 in N2 balance and with programmed heating from 30 to 500 ℃ at heating rate of 10 ℃/min. Exiting NO was continuously analyzed using the flue gas on-line analyzer, and the amount of NO removed was used to estimate the reactivity of (NH4)2SO4 and NO. For comparison, the pure (NH4)2SO4 was performed in the same experimental process.

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The surface element composition, element content and oxidation state of the catalyst were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Japan). Using Al Kα (1486.6 eV) as radiation source and C 1s binding energy (284.6 eV) to correct the measured spectra in 300W ultra-high vacuum (6.7×10-8 Pa), the charge effect in the samples was eliminated. The estimation error of ±0.2 eV can be taken into account in all measurement results. The morphology and surface structures of the samples were characterized using a scanning electron microscope (SEM, Hitachi S4700 apparatus) equipped with energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) analyses were performed on a Tecnai G2 F20 (FEI, USA) with an accelerating voltage of 200 kV. 2.3. Activity Measurements. The NH3-SCR activity was evaluated in a fixed-bed reactor device. Before the experiment, all the catalysts were crushed and sieved (40-60 mesh) and filled in a quartz tube with the inner diameter of 6 mm. The NOx conversion, N2 selectivity, and H2O & SO2 resistance stability of the catalysts were investigated. The reaction gas group including 600 ppm NO, 600 ppm NH3, 5.0 vol.% O2, N2 as equilibrium gas with a total gas flow rate of 375 mL/min, and corresponding space velocity (GHSV) was 45000 h-1. During the evaluation of SO2 and H2O resistance, 100 ppm SO2 and 10 vol. % H2O were added to the original gas composition, and the N2 flow rate was adjusted to keep the space velocity constant. The reaction temperature was carried out from 150 °C to 330 °C. At each test temperature, the data were recorded when the SCR reaction reached steady state after 20 min. The composition of the product gas was analyzed by Thermofisher IS10 FTIR for multiple species of molecule. The NOx conversion and N2 selectivity were calculated using the follow Eqs.

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NOx conversion =

N2 selectivity =

[ NOx]in - [NOx]out × 100% [NOx]in

[NOx]in + [ NH3]in - [NOx]out - [NH3]out - 2[N2O]out × 100% [NOx]in + [ NH3]in - [NOx]out - [NH3]out

Where NOx includes both NO and NO2, with the subscript “in” and “out” represent the inlet and outlet gas concentrations of the reactant, respectively. 3. RESULTS AND DISCUSSION

Figure. 1 XRD patterns of (a) CuAl-LDH and CuAl-LDH/CNTs precursors; (b) CuAl-LDO and CuAl-LDO/CNTs catalysts. 3.1 XRD The XRD patterns of the catalyst precursors prepared via different assembly methods are shown in Figure. 1(a). All precursors exhibited characteristic diffraction peaks at 10.22°, 20.36°, 34.94° and 59.72°, which can correspond to the typical structure of layered double hydroxide ((003), (006), (012) and (110)), indicative of R3m symmetry.24,25 The reflection peak at 26.12° can be indexed to the (002) plane of graphitic carbon (JCPDS No.41-1487). In addition, the (003) diffraction peaks of LDH structure in CuAl-LDH/CNTs nano-hybrid became broad and the crystallinity was reduced in comparison to that of single CuAl-LDH, which may be related to the smaller sizes of crystals in (00l) direction.26 According to the Scherrer’s equation, the

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average crystallite sizes of CuAl-LDH, CuAl-LDH/CNTs(M), CuAl-LDH/CNTs(S) and CuAlLDH/CNTs(I) were calculated to be 10.51, 9.89, 8.76 and 6.58 nm, respectively. The mixed metal oxides derived from CuAl-LDH and CuAl-LDH/CNTs precursors were also analyzed by XRD and the results were shown in Figure. 1(b). All the catalysts exhibited characteristic diffraction peaks of CuO (JCPDS: no.45-0937) and Cu2O (JCPDS: no. 05-0667). In addition, the absence of the characteristic diffraction peak of Al2O3 implied its amorphous state.27 Notably, compared with CuAl-LDO, the diffraction peaks for the CuAl-LDO/CNTs broadened considerably and the lowest crystallinity of CuOx phase in CuAl-LDO/CNTs(I) was presented, indicating high dispersion of active components.

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Figure. 2 SEM (left) and (right) TEM images of (a, e) CuAl-LDH, (b, f) CuAl-LDH/CNTs(M) (c, g) CuAl-LDH/CNTs(S) and (d, h) CuAl-LDH/CNTs(I) precursors. TEM images of (i) CuAlLDO, (j) CuAl-LDO/CNTs(M), (k) CuAl-LDO/CNTs(S) and (l) CuAl-LDO/CNTs(I) catalysts. 3.2 SEM and TEM. The SEM and TEM are the effective analytic technologies to reveal the real micro-morphologies of the CuAl-LDH and CuAl-LDH/CNTs hybrids. As can be seen from the Figure. 2(a), the CuAl-LDH presents a hierarchical morphology corresponding typical welldefined hydrotalcite. Due to the influence of high surface energy and strong hydrogen bonding of the laminate, where severely stacked gauze-like LDH sheets can be further observed by TEM image (Figure. 2(e)). Meanwhile, the element scanning analysis of CuAl-LDH surface was carried out by means of EDX characterization. As shown in Figure. S1, O, Cu and Al elements were uniformLy distributed on the surface of the CuAl-LDH, wherein the molar ratio of Cu: Al being 2.65:1 was slightly smaller than the original feed ratio, which might be related to the incomplete precipitation of the metal ions. For the CuAl-LDH/CNTs(M) (Figure. 2(b, f)), the stacked hierarchical structure is still visible, which indicating the original stacking morphology can't be effectively broken through the mechanically assembly process. In contrast, the nanosheets in CuAl-LDH/CNTs(S) composite (Figure. 2(c)) presented irregular morphology and obviously diminished dimension in comparison to the original CuAl-LDH crystallites (Figure. 2(g)). Clearly, significantly different from the compact stacking of other two CuAl-LDH/CNT nanohybrids, the CuAl-LDH/CNTs(I) (Figure. 2(d)) precursor presented a anisotropic thin slice

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morphology with looser stacking structure. The TEM image further demonstrates that the CNTs can act as “spacers” and effectively separate LDH layers (Figure. 2(h)), thus inhibit the selfagglomeration of CuAl-LDH nanoplates. The calcined products of CuAl-LDO and CuAl-LDO/CNTs were also characterized by TEM technology. As shown in Figure. 2(i, j, k, l), the dispersion state of active components on the catalysts followed the order of CuAl-LDO ≈ CuAl-LDO/CNTs(M) < CuAl-LDO/CNTs(S) < CuAl-LDO/CNTs(I), which is consistent with XRD results. Comprehensive analysis of the above results shows that the dispersion of CuAl-LDO/CNTs is closely related to the stacking degree of precursors. Thus, for the CuAl-LDO/CNTs(I), the aggregation or reunite of Cu-based active centers after the roasting process was effectively avoided due to the loose stacking of CuAl-LDH caused by the good spatial barrier effect of CNTs in CuAl-LDO/CNTs(I).

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Figure. 3 Raman spectra of the (a) CuAl-LDH, (b) CuAl-LDH/CNTs precursors and (c) CuAlLDO, (d) CuAl-LDO/CNTs(I), (e) CuAl-LDO/CNTs(M) and (f) CuAl-LDO/CNTs(S) product of roasting. 3.3 Raman spectra. The Raman spectroscopy was used as a powerful technique to study the properties of carbon nano-materials.28 In order to further probe the interface interaction between CNTs and CuAl-LDH, Raman analysis on each sample was carried out. As seen in Figure 3(a), the vibration peaks at 257 cm-1 and 513 cm-1 can be attributed to the stretching vibration modes of metal oxygen (M-O) or metal hydroxyl group (M-OH) in CuAl-LDH.24,29 The Raman peaks at 712 cm-1,1054 cm-1 and 1322 cm-1 can be ascribed to the ν4 、ν1 and ν3 vibrational modes of the interlayer free NO3-, respectively,24,30 showing the spatial symmetry structure of D3h, which is consistent with the XRD results. Meanwhile, the absent Raman vibration peaks of LDH compounds for the assembled CuAl-LDH/CNTs nanohybrid (Figure. 3(b)) may be related to the good dispersion of hydrotalcite-like compounds or the masking of carbon materials. In addition, the Raman band around 1348 cm-1 in each sample was the D mode of CNTs, which can be associated to the A1g vibration mode of carbon atoms with dangling bonds in plane terminations of disordered graphite structure. The band in the 1580 cm-1 was the G mode resulting from the E2g vibration mode of sp2 bonded carbon atoms in a two-dimensional hexagonal lattice.31 For the

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CuAl-LDH/CNT hybrids, the D band and G band presented a slight blue shift of 3-5 cm-1 compared with single CNTs. This can attribute to the LDH layers with positive charge attract electrons from the surface of carbon nanotubes and could increase the necessary energy for vibrations to occur. Generally speaking, the value of R=ID/IR (the intensities ratio of D and G bands) is considered for evaluating the surface defects density in CNTs.32 For the modified carbon nanotubes, the R=ID/IR value of 1.66 indicates a higher degree of defect density. In contrast, the intensity ratio in all CuAl-LDH/CNTs appeared decline in sequence of 1.59, 1.48 and 1.01 for CuAl-LDH/CNTs(M), CuAl-LDH/CNTs(S) and CuAl-LDH/CNTs(I), respectively, which indicating the decrease of defect density. Such “repairable” defect sites can be related to the interface contact between CNTs and LDH.33 The defect density in CuAl-LDH/CNTs(I) decreased dramatically, indicating the existence of a stronger interfacial effect. This strong interaction may be associated with the induction effect for LDH nucleation by the electron-rich defect sites. That is to say, the defect sites of carbon nanotubes can be used as favorable nucleation centers of CuAl-LDH, which makes CNTs can act as the role of “spacers” and effectively separate the stacking of LDH laminates as shown in electron microscope analysis showed. The Raman spectra of the calcined products (CuAl-LDO and CuAl-LDO/CNTs) were shown in Figure. 3(c)-(f). Compared with the nano-hybrids before calcination, the value of R=ID/IR for the three CuAl-LDO/CNTs increased to varying degrees. Among them, the maximum growth of defect density can be found in the topological transition process of CuAl-LDH/CNTs(I) that the value of R=ID/IR increased from 1.01 in CuAl-LDH/CNTs(I) to 1.42 in CuAl-LDO/CNTs(I). As some defects can be introduced to the carbon nanotubes surface during the calcination process, the Raman spectrum of the modified carbon nanotube after calcination was further implemented

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and the result was presented in Figure. S2. As the results showed, the defect density of CNTs only increased slightly with the value of R=ID/IR increasing from 1.66 to 1.73 after calcination in nitrogen atmosphere. Thus, the obvious increase of defect density in CuAl-LDO/CNTs can be attributed to the formation of stable chemical bonds (M-O-C) with sp3 hybrid form between π electrons from the surface of CNTs and metal oxides during the high temperature topological transformation.34 In addition, the low wavenumber at 225 cm-1 and 634 cm-1 can be assigned to 2Γ-12 mode of Cu2O and B2 g mode of CuO, respectively.35 Compared with CuAl-LDO, the weaker vibration intensity presented on CuAl-LDO/CNTs may be related to the better dispersion of active components.36

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Figure. 4 TG-MS curves of (a) CuAl-LDH, (b) CuAl-LDH/CNTs(M), (c) CuAl-LDH/CNTs(S), (d) CuAl-LDH/CNTs(I) and (e) Modified CNT. 3.4 TG-MS results. The thermal stability and thermal decomposition behavior of various precursor samples were investigated by TG-MS analysis (Figure. 4). As shown in Figure. 4(a), the weight loss stage at lower temperature (100-200 °C) can be corresponded to removal of physically adsorbed and intercalated water molecules in LDH. Another mass loss stage at higher temperatures (200-450 °C) involved dehydroxylation and decomposition of interlayer anion in LDH interlamination, which indicated the gradual collapse of LDH layered structures as well as the transformation to composite metal oxides.37 For the as-synthesized CuAl-LDH/CNTs composites, it is worth paying attention to the appearance of CO2 gas signal in mass spectrum, which might be attributed to the decomposition of oxygen functional group (COO-) from modified CNTs, in agreement with the results of thermo gravimetric analysis of single modified carbon nanotubes (Figure. 4(e)). Among them, the lowest decomposition temperature of LDH structure in CuAl-LDH/CNTs(I) composites was presented, which further proved the existence of strong surface interaction between LDH plates and modified CNTs, which can weaken the binding force between LDH layers and interlayer anions, and resultantly promote the collapse of

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the LDH structure. According to the thermal decomposition performance, it is reasonable to choose 500 ℃ as roasting temperature, which can ensure CuAl-LDH be completely decomposed into stable composite metal oxides. Based on the above analysis results, a simple synthesis mechanism can be proposed as shown in Scheme 1. Scheme. 1 The schematic formation procedure of CuAl-LDH/CNTs(I) and CuAl-LDO/CNTs(I).

Figure. 5 (a) NH3-SCR activity and (b) N2 selectivity as a function of temperature from 150 °C to 330 °C; Reaction condition: 600 ppm NH3, 600 ppm NO, 5.0 vol% O2, and N2 as the balance gas with a GHSVs was 45000 h-1. 3.5 Catalytic performance in NH3-SCR. The NH3-SCR catalytic activity was examined in the temperature range of 150 ℃ to 330 ℃. As displayed in Figure. 5(a), the single CuAl-LDO exhibited the lower catalytic activity that NOx conversion at 240 ℃ is only 77%. The

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introduction of carbon nanotubes can effectively improve the NOx conversion, which promoting effect might be closely associated with the assembly methods. Among them, the CuAlLDO/CNTs(I) catalyst presented optimum catalytic activity with NOx conversion exceeding 80% in the region of 180-300 ℃, and the maximum NOx conversion (95.3%) can be obtained at 240 ℃. In addition, the CuAl-LDO/CNTs (I) catalyst was also tested in the wide GHSVs (Figure. S3), where the catalyst can maintain a higher catalytic activity even at 12000 h-1 GHSVs. This is an excellent result ever achieved for the other recent employed Cu-based oxides catalysts for NH3-SCR reaction (Table S1). Meanwhile, the CuAl-LDO/CNTs(I) also exhibited N2 selectivity beyond 90% throughout the testing period (Figure. 5(b)). In order to get a better understanding of the catalytic performance, the CuAl-LDO and CuAl-LDO/CNTs(I) catalysts were selected to further test the catalytic oxidation performance of NH3 and the results were shown in Figure. S4. The CuAl-LDO presented better oxidizing ability for NH3 and the NH3 conversion could exceed 80% at 330 ℃. In addition, the selectivity of N2O in CuAl-LDO was obviously elevated with the temperature increasing. In contrast, the ability of CuAl-LDO/CNTs(I) catalyst to oxidize NH3 was significantly inhibited, and only about 46% NH3 conversion at 330 ℃ was obtained. Meanwhile a small amount of NOx and N2O were produced at higher temperature. Therefore, N2O produced in the non-selective catalytic oxidation of NH3 may be the main reason for the low N2 selectivity of CuAl-LDO catalyst in NH3-SCR reaction. According to the literature, the inferior dispersion of active component and the formation of crystal grains on the CuAl-LDO catalyst surface would endow the catalyst with strong oxidation ability for the unselective consumption of reducing agent NH3 in SCR reaction, which can cause the poor N2 selectivity.38-40

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According to the above results, the distinctive catalytic behaviors of the prepared catalysts, which may be closely related to the dispersion of the active components. Here, the NH3-TPD, H2-TPR and XPS characterization technologies would be adopted to further reveal the essential reasons for the difference of catalytic activity.

Figure. 6 (a) NH3-TPD profiles and (b) integral area of the different catalysts. 3.6 NH3-TPD. The adsorption of the NH3 is considered as a crucial step in the NH3-SCR catalytic reaction.5 Therefore, the NH3-TPD was performed to determine the strength and amount of surface acid sites on these catalysts. As shown in Figure. 6(a), both the CuAl-LDO/CNTs(I) and CuAl-LDO/CNTs(S) catalysts presented the desorption peaks at 100-200 °C, 300-400 °C and about 600 °C, which can be assigned to the weak, medium and strong acid sites, respectively.41 For the CuAl-LDO, there is only contain two desorption peaks are attributed to weak and medium acid sites. The CuAl-LDO/CNTs(M) displayed the same desorption peaks position as CuAl-LDO just a significant increase in acid amount. Besides, the integral areas of NH3 desorption were also determined (Figure. 6(b)), where the CuAl-LDO/CNTs(I) exhibited the strongest acidic, which can be associated with the high dispersion of active components for promoting full exposure of acidic sites. Therefore, the strong adsorption ability of NH3 can contribute to high DeNOx activity on the CuAl-LDO/CNTs(I) catalyst.

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Figure. 7 H2-TPR profiles of the catalysts. 3.7 H2-TPR. Figure. 7 shows the H2-TPR of CuAl-LDO and CuAl-LDO/CNTs catalysts prepared by three assembly methods. All the samples exhibited two reduction peaks throughout the testing. The lower temperature reduction peak at about 200 ℃ can be attributed to the reduction of well dispersed CuO on the catalyst surface (denoted as α), and the reduction peak at higher temperature can be attributed to the reduction of CuO with crystallinity (denoted as β).42 In addition, no reduction peak of single Cu+ was found at higher temperature, which means that the reduction of Cu+ was accompanied by the reduction of CuO due to the influence of H2 overflow effect.43,44 Compared with CuAl-LDO, the reduction temperature of CuAl-LDO/CNTs moved to low temperature with different extent, which indicates an improvement in redox potential due to the enhanced dispersion of CuO.9,16 The H2 consumption of each sample was calculated through peak-fitting deconvolution using CuO as the standard sample (Figure. S5). According to the calculation results, the CuAl-LDO catalyst was mainly composed of the crystallinity CuO. The introduction of CNTs into the CuAl-LDO catalysts resulted in an increased reducibility of the CuO species as the higher consumption ratio of α/(α+β), and such ratio in CuAl-LDO/CNTs(I) was the maximum (0.48), which provides a favorable basis for the

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good dispersion of active copper species on the surface of the catalyst.42 Therefore, the catalyst has good redox ability, which is another important factor for its good NH3-SCR catalytic performance. 3.8 XPS. The surface components concentrations and oxidation state of copper species in different catalysts was examined by X-ray photoelectron spectrometer (XPS) (Figure. S6 and Table S2). By performing peak-fitting deconvolutions, the peak with higher binding energy (933.9~935.4eV) and accompanied by the characteristic shake-up peaks at 938.7-946.9 eV can be assigned to Cu2+.45 The fitting peak in the lower binding energy (931.1-938.2 eV) can be attributed to Cu+ or Cu0.46 The Cu LMM XAES was further implemented and the auger spectrum could be deconvolved into two peaks by fitting Gaussian peaks (Figure. S6(b)), where the peaks at 912.2-913.7 eV and 916.3-916.8 eV can be assigned to Cu+ and Cu2+, respectively.47,48 The Cu0 species with higher kinetic energy (> 918 eV) were not found. The results witness that the Cu2+ species were the main presence in the CuAl-LDO catalyst system. For the three CuAlLDO/CNTs, the transformation of Cu2+ to Cu+ was promoted and a similar valence distribution were obtained due to the carbothermal reduction action of CNTs. Therefore, the subtle difference in valence distribution of Cu species on these CuAl-LDO/CNTs catalysts may not be the main reason for the difference of catalytic activity. In addition, the three CuAl-LDO/CNTs can also provide higher content of surface active oxygen than CuAl-LDO (Figure. S6(c)), which may be related to the strong adsorption and activation ability of Cu+ active sites to oxygen.49,50 The results of XPS indicated that the assembly way not have a significant effect on the valence distribution of CuOx species and surface active oxygen content with the same content of CNTs.

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Figure. 8 SO2 & H2O resistance tests at 240 °C. Reaction condition: 600 ppm NH3, 600 ppm NO, 5.0 vol% O2, 100 ppm SO2 (when used), 10 vol% H2O (when used) and N2 as the balance gas with a GHSVs was 45000 h-1. 3.9 Stability and SO2 resistance tests. To meet industrial application, the long-term stability as well as the durability in the presence of H2O & SO2 were tested at 240 °C, and the results were shown in Figure. 8. The CuAl-LDO/CNTs(I) catalyst showed excellent catalytic stability and the NOx conversion only decreased from 95.5% at the beginning to 93.4% after 56 h of continuous evaluation without SO2 & H2O. Comparatively, the DeNOx activity of CuAl-LDO catalyst was significantly different, and the NOx conversion dropped from 77.1% to 65.7%. The used samples after the test of stability evaluation (denoted as CuAl-LDO/CNTs(I)-used and CuAl-LDO-used) were collected, and the structures were characterized by XRD and TEM analysis (Figure. S7). It can be seen from the XRD results (Figure. S7(a)) that there is no obvious change in the crystal phase structure compared with the results before the test. However, the TEM images (Figure. S7(b, c)) indicated that the aggregation degree of active components on the surface of used CuAl-LDO catalyst showed a tendency of enhancement. According to the Ostwald ripening (OR) and particle migration and coalescence (PMC) mechanism, this can be

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attributed to the sintering of active species after long-term process at high temperature.51 For the used CuAl-LDO/CNTs(I), the surface active components of the catalyst remained highly dispersed after long-term activity evaluation, which further proved its excellent stability.

Figure. 9 FT-IR spectra of (a) CuAl-LDO and CuAl-LDO-S; (b) CuAl-LDO/CNTs(I) and CuAlLDO/CNTs(I)-S. When the vapour was introduced, the NOx conversion slightly declined on both catalysts. After cutting off H2O, the NOx conversion could gradually recover to its balance activity, indicating the competitive adsorption between NH3 and H2O is the main reason for the activity decrease.52 When both SO2 & H2O were added in reaction flow, the CuAl-LDO and CuAl-LDO/CNTs(I) catalysts start to show different resistivity abilities. The NOx conversion of CuAl-LDO presented a decline from 76.8% to 50.1%. For Cu-Al-LDO/CNTs(I) catalyst, there is only a smaller drop and the NOx conversion finally maintained at around 83.3%. While turning SO2 and H2O off, the NOx conversion for CuAl-LDO/CNTs(I) could restore to 88.5% in 10 h, suggesting that the deactivation was reversible to some extent. The used samples after the test of SO2 and H2O resistance (denoted as CuAl-LDO/CNTs(I)-S and CuAl-LDO-S) were collected. As the FT-IR results (Figure. 9) showed, three new adsorption peaks of ammonium sulfate were observed on the CuAl-LDO-S and CuAl-LDO/CNTs(I)-S catalysts, which can be attributed to the asymmetric

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bending vibration of N-H bond, asymmetric stretch vibration of S=O bond and anti-symmetrical tensile vibration of S=O=S bond, respectively.53-56 In comparison to CuAl-LDO/CNTs(I)-S, the stronger adsorption peak intensity on the CuAl-LDO-S catalyst indicates the more ammonium sulfate deposition on the catalyst surface. For the low temperature NH3-SCR reaction, deactivation of the catalysts caused by SO2 and H2O was mainly attributed to the formation of sulfate species and the resultant blockage of the pores as well as coverage of the surface active sites.57,58 Thus, the outstanding sulfur resistance for CuAl-LDO/CNTs(I) catalyst may be associated with ammonium sulfate decomposition, which needs to be explored deeply.

Figure. 10 (a) TPDC of (NH4)2SO4 and (b) TPSR of NO-(NH4)2SO4 profiles on the different samples. 3.10 TPDC and TPSR results. Given the above, keeping the equilibrium between the formation and decomposition of ammonium sulfates might be a feasible way for prolonging the longevity of the catalysts. Thus, temperature programmed decomposition (TPDC) experiments were carried out to explore the decomposition behaviors of the ammonium sulfate salts in detail. As shown in Figure. 10(a), the pure (NH4)2SO4 presented the higher decomposition temperature. When (NH4)2SO4 was loaded on the CuAl-LDO surface, the decomposition become easier and reach the highest decomposition rate at about 300 ℃. Comparatively, the decomposition

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temperature greatly decreased when (NH4)2SO4 was supported on CuAl-LDO/CNTs(I), on which the decomposition started at about 200 ℃ and finished at 330 ℃. The (NH4)2SO4 loading on pure CNTs also showed a similar decomposition process, indicating that the addition of CNTs could effectively reduce the stability of ammonium sulfate and promote its decomposition at lower temperatures. However, as ammonium sulfate cannot decompose sufficiently on the surface of CuAl-LDO/CNTs(I) catalyst at 240 ℃, thus, there might be other factors contributing to such excellent sulfur resistance in low temperature. As the low deposition amount of ammonium sulfate on the surface of CuAl-LDO/CNTs(I) catalyst, another consumption way might be through the reaction of ammonium sulfate with NO.59 Given that, the temperature programmed surface reaction (TPSR) of NO and (NH4)2SO4 was further explored, in which the change of NO concentration is used to express the reaction process and the reactivity. As shown in Figure. 10(b), the reaction of NO and (NH4)2SO4 could not proceed in the absence of a catalyst. Attributed to the synergistic action between CNTs and CuAl-LDO, the reaction of (NH4)2SO4 and NO on CuAl-LDO/CNTs(I) can take place at lower temperatures. The formation and decomposition of ammonium sulfate can reach a dynamic equilibrium, and the excessive deposition of ammonium sulfate on the surface of catalyst could be avoided. 4. CONCLUSIONS In summary, three kinds of CuAl-LDO/CNTs catalysts were prepared from efficient assembly of CuAl-LDH and CNTs as precursor templates and were applied in NH3-SCR reaction. Compared with the single CuAl-LDO, the introduction of carbon nanotubes significantly promoted the dispersion of Cu-based active components, and increased the low-temperature DeNOx activity. Among all the synthesized catalysts, the CuAl-LDO/CNTs(I) prepared by in-

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situ assembly method presented the best DeNOx performance in a wide reaction temperature window from 150 ℃ to 330 ℃. The characterization analysis indicated that CNTs could fully exert their steric barrier during the in-situ assembly process, and effectively alleviated the stacking of CuAl-LDH precursors and realized the high dispersion of Cu-based metal oxides after roasting. Consequently, the surface acidity and redox ability were effectively enhanced, which was responsible for the excellent performance of NH3-SCR reaction. Better yet, the outstanding SO2 & H2O resistance of the CuAl-LDO/CNTs(I) catalyst was also obtained, which could be related to the synergistic action between CNTs and CuAl-LDO that the reduction of the stability of ammonium sulfate by the CNTs and the promotion of the reactivity of ammonium sulfate by the Cu-based active centers. Therefore, the activation and decomposition of ammonium sulfate could occur at lower temperature, where the formation and accumulation of excess ammonium sulfate salts on the CuAl-LDO/CNTs(I) catalyst surface could be avoided. ASSOCIATED CONTENT EDX element mapping of CuAl-LDH precursor; Raman spectra of the modified CNTs after and before calcination; NOx conversion over CuAl-LDO/CNTs(I) catalyst under different GHSVs; NH3 oxidation activities, products selectivity of CuAl-LDO/CNTs(I) and CuAl-LDO catalysts; H2-TPR profiles of the CuO standard sample; XPS spectrum of Cu 2p, Cu LMM and O 1s for the different catalysts; XRD and HR-TEM of CuAl-LDO-used and CuAl-LDO/CNTs(I)used catalysts; Review of Cu-based oxides catalysts and the catalytic performance in NH3-SCR; The surface components content of the catalysts obtained by XPS analyses. AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected]. Fax: +86-351-6018528. * E-mail: [email protected]. Fax: +86-351-6018528. ORCID: Xu Wu: 0000-0002-4997-2148 Hao Meng: 0000-0001-6494-0825 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. X.W. and H.M. contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The project was supported by National Natural Science Foundation of China (21073131), the Natural Science Foundation of Shanxi Province, China (201601D102007). REFERENCES (1) Gao, F.; Tang, X.; Yi, H.; Li, J.; Zhao, S.; Wang, J.; Chu, C.; Li, C. Promotional Mechanisms of Activity and SO2 Tolerance of Co- or Ni-doped MnOx-CeO2 Catalysts for SCR of NOx with NH3 at Low Temperature. Chem. Eng. J. 2017, 317, 20-31. (2) Sun, C.; Liu, H.; Chen, W.; Chen, D.; Yu, S.; Liu, A.; Dong, L.; Feng, S. Insights into the Sm/Zr co-doping Effects on N2 Selectivity and SO2 Resistance of a MnOx-TiO2 Catalyst for the NH3-SCR Reaction. Chem. Eng. J. 2018, 347, 27-40.

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(3) Yan, L.; Gu, Y.; Han, L.; Wang, P.; Li, H.; Yan, T.; Kuboon, S.; Shi, L.; Zhang, D. Dual Promotional Effects of TiO2-Decorated Acid-Treated MnOx Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NOx. ACS Appl. Mater. Interfaces 2019, 11, 1150711517. (4) Han, Y.; Mu, J.; Li, X.; Gao, J.; Fan, S.; Tan, F.; Zhao, Q. Triple-Shelled NiMn2O4 Hollow Spheres as an Efficient Catalyst for Low-Temperature Selective Catalytic Reduction of NOx with NH3. Chem. Commun. 2018, 54, 9797-9800. (5) Geng, Y.; Chen, X.; Yang, S.; Liu, F.; Shan, W. Promotional Effects of Ti on a CeO2MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3. ACS Appl. Mater. Interfaces 2017, 9, 16951-16958. (6) Cai, S.; Liu, J.; Zha, K.; Li, H.; Shi, L.; Zhang, D. A General Strategy for the In Situ Decoration of Porous Mn-Co bi-Metal Oxides on Metal Mesh/Foam for High Performance DeNOx Monolith Catalysts. Nanoscale 2017, 9, 5648-5657. (7) Yu, Y.; Chen, C.; He, C.; Miao, J.; Chen, J. In situ Growth Synthesis of CuO@Cu-MOFs Core-Shell Materials as Novel Low-Temperature NH3-SCR Catalysts. ChemCatChem 2019, 11, 979-984. (8) Chen, Z.; Fan, C.; Pang, L.; Ming, S.; Guo, W.; Liu, P.; Chen, H.; Li, T. One-Pot Synthesis of High Performance Cu-SAPO-18 Catalyst for NO Reduction by NH3-SCR: Influence of Silicon Content on the Catalytic Properties of Cu-SAPO-18. Chem. Eng. J. 2018, 348, 608-617. (9) Chen, L.; Si, Z.; Wu, X.; Weng, D. DRIFT Study of CuO-CeO2-TiO2 Mixed Oxides for NOx Reduction with NH3 at Low Temperatures. ACS Appl. Mater. Interfaces 2014, 6, 8134-8145.

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(10) Montanari, B.; Vaccari, A.; Gazzano, M.; Käßner, P.; Papp, H.; Pasel, J.; Dziembaj, R.; Makowski, W.; Lojewski, T. Characterization and Activity of Novel Copper-Containing Catalysts for Selective Catalytic Reduction of NO with NH3. Appl. Catal., B 1997, 13, 205-217. (11) Chmielarz, L.; Kuśtrowski, P.; Rafalska-Łasocha, A.; Majda, D.; Dziembaj, R. Catalytic Activity of Co-Mg-Al, Cu-Mg-Al and Cu-Co-Mg-Al Mixed Oxides Derived from Hydrotalcites in SCR of NO with Ammonia. Appl. Catal., B 2002, 35, 195-210. (12) Xu, Y.; Wang, Z.; Tan, L.; Yan, H.; Zhao, Y.; Duan, H.; Song, Y.-F. Interface Engineering of High-Energy Faceted Co3O4/ZnO Heterostructured Catalysts Derived from Layered Double Hydroxide Nanosheets. Ind. Eng. Chem. Res. 2018, 57, 5259-5267. (13) Bai, D.; Wang, F.; Lv, J.; Zhang, F.; Xu, S. Triple-Confined Well-Dispersed Biactive NiCo2S4/Ni0.96S on Graphene Aerogel for High-Efficiency Lithium Storage. ACS Appl. Mater. Interfaces 2016, 8, 32853-32861. (14) Xu, Y.; Wang, Z.; Tan, L.; Zhao, Y.; Duan, H.; Song, Y.-F. Fine Tuning the Heterostructured Interfaces by Topological Transformation of Layered Double Hydroxide Nanosheets. Ind. Eng. Chem. Res. 2018, 57, 10411-10420. (15) Yan, Q.; Nie, Y.; Yang, R.; Cui, Y.; O’Hare, D.; Wang, Q. Highly Dispersed CuyAlOx Mixed Oxides as Superior Low-Temperature Alkali Metal and SO2 Resistant NH3-SCR Catalysts. Appl. Catal., A 2017, 538, 37-50. (16) Zhang, Y.-s.; Li, C.; Yu, C.; Tran, T.; Guo, F.; Yang, Y.; Yu, J.; Xu, G. Synthesis, Characterization and Activity Evaluation of Cu-based Catalysts Derived from Layered Double Hydroxides (LDHs) for DeNOx Reaction. Chem. Eng. J. 2017, 330, 1082-1090.

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(23) Xiao, X.; Sheng, Z.; Yang, L.; Dong, F. Low-Temperature Selective Catalytic Reduction of NOx with NH3 over a Manganese and Cerium Oxide/Graphene Composite Prepared by a Hydrothermal Method. Catal. Sci. Technol. 2016, 6, 1507-1514. (24) Al-Jaberi, M.; Naille, S.; Dossot, M.; Ruby, C. Interlayer Interaction in Ca-Fe Layered Double Hydroxides Intercalated with Nitrate and Chloride Species. J. Mol. Struct. 2015, 1102, 253-260. (25) Sanati, S.; Rezvani, Z. Ultrasound-Assisted Synthesis of NiFe-Layered Double Hydroxides as Efficient Electrode Materials in Supercapacitors. Ultrason. Sonochem. 2018, 48, 199-206. (26) Dou, L.; Zhang, H. Facile Assembly of Nanosheet Array-Like CuMgAl-Layered Double Hydroxide/rGO Nanohybrids for Highly Efficient Reduction of 4-Nitrophenol. J. Mater. Chem. A 2016, 4, 18990-19002. (27) Yan, Q.; Chen, S.; Zhang, C.; O'Hare, D.; Wang, Q. Synthesis of Cu0.5Mg1.5Mn0.5Al0.5Ox Mixed Oxide from Layered Double Hydroxide Precursor as Highly Efficient Catalyst for LowTemperature Selective Catalytic Reduction of NOx with NH3. J. Colloid Interface Sci. 2018, 526, 63-74. (28) Reddy, S.; Xiao, Q.; Liu, H.; Li, C.; Chen, S.; Wang, C.; Chiu, K.; Chen, N.; Tu, Y.; Ramakrishna, S.; He, L. Bionanotube/Poly (3,4-Ethylenedioxythiophene) Nanohybrid as an Electrode for Neural Interface and Dopamine Sensor. ACS Appl. Mater. Interfaces 2019, 11, 18254-18267.

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An excellent NH3-SCR catalyst of CuAl-LDO/CNTs(I) nanohybrids with highly dispersed active component was prepared derived from topological transformation of CuAl-LDH/CNTs(I). 200x157mm (96 x 96 DPI)

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