MnO2 NanowireâCeO2 Nanoparticle Composite Catalysts for the

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Energy, Environmental, and Catalysis Applications 2

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MnO Nanowire-CeO Nanoparticle Composite Catalysts for the Selective Catalytic Reduction of NO with NH x

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Su Hyo Kim, Bum Chul Park, Yoo Sang Jeon, and Young Keun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09605 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

MnO2 Nanowire-CeO2 Nanoparticle Composite Catalysts for the Selective Catalytic Reduction of NOx with NH3 Su Hyo Kim, Bum Chul Park, Yoo Sang Jeon, and Young Keun Kim* Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea KEYWORDS: MnO2 nanowires, CeO2 nanoparticles, composite nanocatalyst, electrodeposition, selective catalytic reduction

ABSTRACT: MnOx-based catalysts have been applied to the selective catalytic reduction (SCR) of NOx with ammonia (NH3) owing to their high NOx removal efficiency and catalytic stability. In general, the fabrication of a variety of nanomaterials in a complex structure requires complicated processes, including heat treatment and a series of cleaning steps. In addition, MnO2 which has diverse polymorphs, exhibits different catalytic effects depending on its crystalline structure. Among them, synthesizing the ε-MnO2 phase, which functions as a nanocatalyst, has been the most difficult and has hardly been reported. Here, we report the synthesis of heterostructured composite nanocatalysts consisting of ε-MnO2 nanowires (NWs) and CeO2 nanoparticles by applying pulsed currents sequentially. This method drastically

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simplifies the overall process compared to the conventional techniques. Through x-ray diffraction and transmission electron microscopy, it was confirmed that 2–3 nm of CeO2 NPs were formed on the surfaces of the ε-MnO2 NWs. The de-NOx efficiency of the nanocatalysts was analyzed in terms of content variation, specific surface area, and the elemental chemical state of the surface. A ceramic filter containing the nanocatalysts shows a high catalytic activity over the broad operating temperature range 100–400 °C. In the low-temperature region, ε-MnO2 plays a major role in determining the catalytic property, which is consistent with the Brunauer-Emmett-Teller, H2 temperatureprogrammed reduction (TPR), and x-ray photoelectron spectroscopy (XPS) results. On the other hand, in the high-temperature region, the efficiency increases gradually as the content of CeO2 increases. The H2 TPR, NH3-temperature-programmed desorption, and XPS patterns reveal why the composite exhibits such superior characteristics in the temperature range mentioned above.

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1. INTRODUCTION Nitrogen oxides (NOx), one of the major environmental pollutants, are generated by the combustion of coke and fossil fuels in power plants, automobiles, and vessels. With the gradual implementation of international policies for NOx regulation, like EURO 6, it has become necessary to limit NOx emissions by using catalytic materials with high deNOx efficiencies.1,2 Although NOx is thermodynamically unstable, it does not readily decompose because of its high activation energy. Therefore, the use of catalysts is necessary to decompose NOx by reducing the activation energy.3,4 Among the various types of catalytic NOx-removal technologies used, such as three-way catalysts, NOx storage–reduction catalysts, and SCR, the SCR of NOx with NH3 (NH3 SCR) is the most advantageous method, as it exhibits high NOx removal efficiency and can withstand sulfur poisoning.5,6 Metal oxide nanomaterials have attracted great research interest owing to their novel catalytic effects and relatively low costs compared to those of noble metals. Moreover, the catalytic operating temperature of metal oxide nanomaterials for de-NOx can be controlled using appropriate compositions7 and shapes.8 Thus, many researchers have focused on synthesizing various kinds of nanocomposites of different compositions for extending the operating temperature range from low to high temperatures. Several synthetic methods, including impregnation,9 co-precipitation,10 and sol-gel synthesis,11 have been introduced; however, most of them have the disadvantages of being complicated processes, difficulty in forming heterostructures with good stability, and the requirement of heat treatments.

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Here, we suggest a new method to synthesize dimensionally integrated nanostructures consisting of ε-MnO2 NWs and CeO2 NPs prepared by sequential electrodeposition and apply them as NH3-SCR catalysts for NOx removal. The MnO2–CeO2 nanocatalyst could afford high de-NOx efficiencies over a broad range of operating temperatures because of its high surface area and heterostructure. In recent years, MnOx-based catalysts have been studied comprehensively owing to their high de-NOx efficiency at low temperatures that results from their various valence states and high specific surface area.12,13 The catalytic properties of MnO2 are considerably influenced by its polymorphs, including hollandite (α-MnO2), pyrolusite (β-MnO2), ramsdelllite (γMnO2), birnessite (δ-MnO2), and akhtenskite (ε-MnO2).14 The catalytic properties of α-, β-, γ-, and δ-MnO2 phases have been reported; however, ε-MnO2 has rarely been mentioned.15-17 Since it is difficult to synthesize pure ε-MnO2, even the reports that focused on ε-MnO2 revealed that the other polymorphs coexisted. ε-MnO2 could be a potential candidate for a de-NOx catalyst because it has more structural defects than the other polymorphs that serve as active sites for the catalytic reaction.18 Additionally, CeO2 NPs not only have high de-NOx efficiencies at high temperatures, but also act as co-catalysts and enhance the de-NOx efficiency of MnOx owing to the high mobility of Ce3+ and Ce4+ ions with O2- and high oxygen storage capacity.19 Both MnO2 and CeO2 are known to be environmentally compatible and offer long-term catalytic stability, excellent resistance to sulfur and water, and high N2 selectivity, all of which are essential requirements for designing de-NOx catalysts.20-23 In this study, we successfully synthesized MnO2 NW–CeO2 NP composite catalysts via one-bath electrodeposition method. We have investigated the electrodeposition method for fabricating multisegment nanowires consisting of more than two materials 4


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that have different standard reduction potentials.24 The MnO2 NWs were first synthesized on a supporting material such as cordierite (3(2MgO·2Al2O3·5SiO2)) ceramic filters by applying a low current. Subsequently, the CeO2 NPs were formed on the surface of the MnO2 NWs at a high applied current. The overall catalyst-preparation process consists of only two steps: electrodeposition and drying. The present method is simple and straightforward compared to the conventional processes such as dip coating and wash coating, where the catalyst synthesis, coating, and heat treatment steps are carried out separately. Moreover, we determined the de-NOx properties of the MnO2 NW–CeO2 NP composite as a function of the relative contents of MnO2 and CeO2. We demonstrated the catalystic properties of the composties in relation to their specific area and surface properties, which were measured by BET, XPS, and TPR/NH3 TPD. In brief, the schematics of the synthesis strategy and the NOx removal processes are illustrated in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Synthesis of cordierite ceramic filters. A ceramic filter disc of cordierite (3(2MgO·2Al2O3·5SiO2)) powder with a mean particle size of 150 µm was prepared by first aging the powder for 24 h in a thermo-hygrostat and then adding a binder (methyl cellulose), a plasticizer (glycerin), a lubricant (Lu-6418), and water. The ceramic filters were fabricated by the pressing method using a disc-type mold (25 mm diameter). The specimens were first dried at room temperature and then at 100 °C for 24 h. The dried specimens were then sintered at 1410 °C. The fabricated filters should exhibit a porosity of 40%, a strength of more than 10 MPa, and a face velocity of less than 5 cm/s.

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2.2. Synthesis of MnO2-CeO2 nanocatalyst. For preparing the MnO2–CeO2 nanocatalysts via the one-bath electrodeposition method, 10 mmol of manganese(II) sulfate monohydrate (MnSO4·H2O) and 50 mmol of cerium(III) nitrate-6-hydrate ((Ce(NO3)3·6H2O, crown) were added to obtain the precursor, and sulfuric acid (H2SO4) was then used to adjust the solution pH to 2–2.5. A Pt plate was used as both the anode and the cathode. The porous cordierite filter was positioned at the cathode. The inner pressure of the bath was decreased to below 0.133 Pa by using a vacuum pump to facilitate the penetration of the inner pores by the mixed solution. The difference between the standard potentials and the diffusion rates of Mn2+ and Ce3+ ions can cause separated formation of the ε-MnO2 NWs and CeO2 NPs. Pulsed electrodeposition of MnO2 and CeO2 was carried out with pulses of 5 mA/cm2 and 20 mA/cm2 for 2, 4, and 6 h that were produced using a Keithley 2400 power station. First, the ε-MnO2 NWs were deposited below 5 mA/cm2 on the cordierite filter positioned on the Pt plate. Subsequently, the CeO2 NPs were deposited on the MnO2 NWs below 20 mA/cm2 (see Figure S1 of Supporting Information). 2.3. Structural and compositional characterization. The microstructures of the synthesized samples were determined by FE-SEM (Hitachi S-4300) and TEM (JEOL JEM-2100F). Chemical quantitative analysis was carried out by XRF (Rigaku ZSK). Crystal structure analysis was performed by XRD (PANalytical X’Pert Pro) using Cu Kα radiation in the diffraction spectrum of 20° < 2θ < 70° with a scanning speed of 2°/min. The specific surface area of the nanocatalysts was estimated using the BET (Micromeritics ASAP 2420) method. The surface structure of the nanocatalysts was analyzed by XPS (ULVAC-PHI X-TOOL). H2 TPR was performed on a TPR/TPD analyzer (BEL-CAT, BEL Japan Inc.) with an auto-adsorption apparatus. Prior to the H2 6


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TPR experiment, 50 mg of the catalysts was pretreated with N2 at the total flow rate of 30 mL min−1 at 300 °C for 0.5 h, and then, cooled to room temperature in the N2 atmosphere. Finally, the temperature was raised to 800 °C at the constant heating rate of 10 °C min−1 in a flow of H2 (5 vol.%)/N2 (30 mL min−1). 2.4. Characterization of catalytic activity. The inlet gas consisted of NO (1000 ppm), NH3 (1000 ppm), O2 (5%), and balance N2. The flow rate was controlled by a mass flow controller. The GHSV was 10000 h-1. The NO concentrations of the inlet and outlet streams were measured using a NOx analyzer (Thermo, 42C). The NOx removal efficiency was calculated as follows: NO removal efficiency =

, [, ] [, ]

×

100, where [NOx, in] and [NOx, out] are the inlet and outlet concentrations of NOx.

3. RESULTS AND DISCUSSION The morphological and microstructural properties of the nanocatalyst on the cordierite filter were analyzed by HR-TEM and FESEM. As shown in Figure 2(a) and (b), the MnO2 NW–CeO2 NP composites were successfully deposited on the surface of the cordierite filter. We confirmed that the MnO2 NWs with a length of 109 nm and diameter of 22 nm were formed on the surface of the cordierite filter before the formation of small-sized CeO2 NPs. The CeO2 NPs were frequently observed on the MnO2 NWs, and filled the vacant surface of the cordierite filter. The CeO2 NPs formed as aggregated clusters consisting of several NPs. In more detail, single-crystalline CeO2 NPs of size 2–3 nm were confirmed to be present on the MnO2 NWs based on the HR-TEM and FFT images shown in Figure 2(c). The CeO2 NPs in the yellow circled area of Figure 2(c) reveal the fringes of the (200) planes,

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with d(200) = 0.267 nm obtained from the FFT (inset) image. Figure 2(d) displays the SAED patterns of the heteronanostructure measured by using an aperture of 170 nm. The diffraction spots of the MnO2 NWs and the CeO2 NPs reveal a hexagonal closed packed (hcp) structure and a fluorite structure with fcc unit cells, respectively. The dspacings estimated from the (200) and (400) SAED spots of the CeO2 NPs were 0.267 nm and 0.136 nm, respectively, while those of the (100), (101), and (102) SAED spots of the MnO2 nanowires were 0.242 nm, 0.215 nm, and 0.168 nm, respectively. The XRD patterns shown in Figure 3 clearly verify the structures of MnO2 and CeO2, as represented by the SAED pattern. MnO2 NWs, CeO2 NPs, and MnO2–CeO2 nanocatalysts with different weight ratios were synthesized using a Pt plate as the cathode and were investigated by XRD. Before performing XRD, we confirmed that the morphology and crystal structure of the nanocatalysts electrodeposited on the Pt plate and the cordierite filter were analogous with each other. More images of the MnO2 NWs and CeO2 NPs synthesized using the Pt plate and the cordierite filter can be found in Figure S2. In all the samples, the MnO2 NWs and CeO2 NPs were indexed to akhtenskite (εMnO2, JCPDS no. 30-0820) and ceria (CeO2, JCPDS no. 34-0394), respectively. No other polymorphs were observed, except for the ε-MnO2 phase. The peaks of the CeO2 phase are broader than those of MnO2, indicating that the CeO2 NPs had a smaller crystalline size than the MnO2 NWs. The crystallite size was estimated from the XRD main peak broadening of ε-MnO2 (100) and CeO2 (111) by using the Scherrer equation. The average crystalline size was 11.5 nm for ε-MnO2 NWs and 4.9 nm for CeO2 NPs. The results of the TEM and XRD analyses presented several highlights. MnO2 NWs and CeO2 NPs could be prepared with any anode and the cordierite filter or Pt plate as the 8


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cathode. MnO2 NWs and CeO2 NPs were confirmed to be successfully synthesized on both the cordierite filter and the Pt plate with no difference in their crystal structure and morphology (Fig. S2). Several studies have suggested the mechanism of nucleation and growth of MnO225-27 and CeO228-30 on the electrode. Based on the abovementioned reports and our analyses, we propose the following main reactions for the synthesis of MnO2–CeO2 nanocatalysts. Hydroxyl (OH-) anions could be formed near the cathode through nitrate reduction and the electrolysis of water. Subsequently, Mn2+ and Ce3+ (cations) could react with OH- (anion). The Mn(OH)2 and Ce(OH)3 thus formed were then transformed to MnO2 and CeO2, respectively, because both the hydroxides are relatively unstable. 2$% & + 2( → 2&$ + $% ↑

(1)

N&+ + 7$% & + 8( → .$/0 + 10&$

(2)

12%0 2&$ → 12(&$)%

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212(&$)% + &% → 212&% + 2$% &

(4)

5( +0 + 3&$ → 5((&$)+

(5)

Ce(&$)+ → 5(&% + $+ &0 + (

(6)

The relative proportions of MnO2 NWs and CeO2 NPs could be controlled by varying the duration of current application for each component. As shown in Figure 3, the diffraction intensity of the nanocatalysts was changed according to the weight ratio of MnO2 NWs to CeO2 NPs. The specific contents of the MnO2 NWs and CeO2 NPs in the nanocatalysts were estimated by XRF, and the details are given in Table S1. The XRF results of the MnO2–CeO2 nanocatalysts with content ratios of 7:3, 5:5, and 3:7 were 7.18:3.13, 4.82:5.09, and 2.92:6.99 (wt.%).

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The catalytic properties of MnO2 have been reported to be closely correlated to their polymorphs. Generally, the structure of MnO2 is determined by the arrangement of the [MnO6] octahedron; ε-MnO2 crystallizes in the hcp structure and half of the octahedral sites of oxygen are occupied by Mn4+ ions. Although ε-MnO2 is known to show similar electrochemical activity to γ-MnO2, the de-NOx properties of ε-MnO2 have not been reported so far.31,32 Furthermore, most of the ε-MnO2 reported exhibited non-uniform morphologies or coexisted with other polymorphs.18,33 The ε-MnO2 NWs investigated in this study have uniform morphology and unmixed crystal structure, and were obtained by controlling the various conditions of cathodic electrodeposition in the form of low current density, low concentration of Mn2+ precursor, and adopting a vacuum system. Thus, it is meaningful to analyse the de-NOx properties of ε-MnO2-CeO2 nanocatalysts with a well-defined morphology. However, a precise electrochemical reaction for determining the shape and crystal structure of MnO2 should be studied further. The NH3-SCR activity and the relevant characteristics of the MnO2-CeO2 nanocatalysts with content ratios of 10:0, 7:3, 5:5, 3:7, and 0:10 were estimated in this study. It is well known that the redox properties of catalysts in the NH3 SCR of NO are highly related to the catalytic cycle.7 H2 TPR measurements were used to evaluate the reducibility of the catalysts, and the obtained profiles are illustrated in Figure 4(a) and (c). For the MnO2 NWs, the H2 TPR profile presents two well-defined reduction peaks at around 323 and 426 °C. The first peak represents the reduction of Mn4+ to Mn3+, while the second one refers to further reduction of Mn3+ to Mn2+.34 In the case of the CeO2 NPs, a peak at 400 °C was observed, which was attributed to the reduction of Ce4+ to Ce3+.35 The analysis of the H2 reducing capability of the catalysts showed that the MnO2 NWs exhibited the highest NOx removal efficiency at 300 °C, while the CeO2 10


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NPs showed the best NOx removal efficiency at 400 °C (Figure 4(b)). In addition, we evaluated the NOx removal efficiency of the nanocatalysts based on the content ratio of MnO2 to CeO2. As shown in Figure 4 (d), the catalytic activity of all the samples is the highest (90%) at around 250–300 °C. Considering that the NOx conversion efficiency of the MnO2 NW-CeO2 NP composites was determined by combining the catalytic properties of the MnO2 NWs and the CeO2 NPs, it is accepted that the optimal reaction window is 250–300 °C for all the samples. Furthermore, we could identify the distinct changes according to the relative proportions of MnO2 NWs and CeO2 NPs, especially in the relatively low- (150–200 °C) and high-temperature (350–500 °C) regions. The H2 TPR results are shown in Figure 4(c); the area of the peak centered at 327 °C indicated the amount of reducible species that is related to the transition of Mn4+ to Mn3+. The MnO2–CeO2 (7:3) nanocatalysts reveal the largest area of the peak corresponding to the reduction of Mn4+ to Mn3+, which suggests that the amount of the reducible species increased as the content of the CeO2 NPs increased from 3 to 5 wt.%. When the content of the CeO2 NPs further increased to 7 wt.%, we could confirm that the de-NOx efficiency decreased abruptly at low temperatures. The reduction peak of the MnO2–CeO2 (7:3) nanocatalysts at 324 °C decreased in intensity as the content of CeO2 NPs increased beyond the critical point (Figure 4(c)). Although the samples with the higher MnO2 contents were expected to show higher removal efficiencies at low temperatures, the MnO2-CeO2 (7:3) nanocatalyst, containing the highest amount of MnO2, exhibited a lower efficiency than MnO2-CeO2 (5:5). We could confirm that the MnO2-CeO2 (5:5) sample showed a superior reduction capacity at 200 °C. In the temperature range 350–500 °C, the de-NOx efficiency increased gradually as the content of the CeO2 NPs increased. It is also consistent with the results obtained from 11



the H2 TPR profiles. We confirmed through the H2 TPR data that the catalysts displayed better reducing ability at higher temperatures (Figure 4(c)) when the content of the CeO2 NPs was increased from 3 to 7 wt.%. The area of the peak centered at 461 °C increased in the case of the MnO2-CeO2 (7:3) nanocatalyst. To further demonstrate the relationship between the results of H2 TPR and NOx removal, we examined the surface properties, including NH3-TPD, the surface elemental composition and specific area. The Surface properties are closely related to the catalytic reactions between nanocatalysts and external gas molecules. The result of NH3 TPD provides information on the acid sites of the catalyst. In general, the absorption and activation of NH3 on the acid site of a catalyst surface plays a key role in NH3 SCR. It was reported that the NH3 adsorbed on the Brønsted acid sites was more easily desorbed than that adsorbed on the Lewis acid sites.36 In other words, desorption in the low-temperature region indicates the presence of the weak Brønsted acid sites, whereas desorption in the high-temperature region suggests the existence of the strong Lewis acid sites. As shown in Figure 5, the desorption peaks at 150–300 °C increased with the increase in the amount of CeO2 loaded on the Mn-Ce catalyst. This indicates that the number of Brønsted acid sites increased with increasing content of CeO2, which was found to be a critical factor for increasing the catalytic activity. Regarding the surface area, low-dimension MnO2–CeO2 nanocatalysts with heteronanostructures could provide several reaction sites for the NOx species, which may be beneficial for the NH3 SCR of NOx. The specific surface areas of the MnO2– CeO2 nanocatalysts were determined by BET, and the results are listed in Table 1. The MnO2-CeO2 (5:5) sample exhibited the highest specific surface area and pore volume, resulting in efficient de-NOx reaction. 12


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The chemical state of the nanocatalysts in terms of the elements present were analysed by XPS. Figure 6(a) shows the XPS pattern of Mn 2p for the MnO2–CeO2 samples. The MnO2–CeO2 (5:5) sample appeared to have the highest Mn4+/Mn3+ ratio. The binding energies of 643.3 and 641.1 eV could be attributed to the presence of the Mn4+ and Mn3+ species, respectively, in the MnO2–CeO2 (5:5) sample. Mn4+ and Mn3+ were regarded as the dominant valence states that affected the redox properties of the catalysts. In particular, it is well known that Mn4+, the higher oxidation state of manganese, plays a crucial role in the fast SCR of NOx. Thus, the MnO2–CeO2 (5:5) nanocatalyst exhibits the highest reducing capability and NOx removal efficiency because of its high Mn4+ content, which could enhance the oxidation of NO to NO2. The corresponding XPS patterns of O 1s for the MnO2–CeO2 samples are shown in Figure 6(b); two surface oxygen species can be clearly observed based on the deconvoluted O 1s spectra. The binding energy of 529–530 eV is characteristic of the lattice oxygen (O2) (hereafter denoted as Oβ), and the binding energy of 531–533 eV can be assigned to defect oxides or the surface oxygen ions with a low coordination (hereafter denoted as Oα).37 The abundance of Oα, which has a higher mobility than Oβ in the MnO2–CeO2 nanocatalysts, was important for the occurrence of the SCR reaction. The elemental states of the MnO2–CeO2 catalysts, as determined from the XPS patterns, are as listed in Table 2. All the MnO2–CeO2 nanocatalysts had a high Oα /(Oα + Oβ) ratio, and MnO2–CeO2 (3:7) revealed a relatively high value of 61.45%. This might be due to the defective structure of the 2–3 nm CeO2 NPs, which provided abundant Oα.38

4. CONCLUSIONS 13



MnO2-CeO2 composite nanocatalysts were grown on the pore surfaces of cordierite filters by electrodeposition for application in de-NOx. MnO2 NWs and CeO2 NPs were directly electrodeposited on the internal surface of the cordierite filters by employing different current densities. The content ratio of the MnO2 NWs to the CeO2 NPs was controlled by regulating the duration of the applied current. The MnO2 NWs and CeO2 NPs were confirmed to have well-defined morphologies, compositions, and microstructures. This facile synthesis method could replace the conventional process for preparing deNOx catalysts. The nanocatalysts exhibited a high catalytic activity over a broad operating temperature range (100 to 400 °C) for the SCR of NOx with NH3. In particular, the best de-NOx performance (over 90% efficiency) was observed at 250 °C when the GHSV was 10000 h-1. The de-NOx efficiency was analysed in terms of the variation in the relative contents, the chemical state of the surface in terms of the elements present, and specific surface area. The MnO2–CeO2 (5:5) nanocatalyst with the highest Mn4+/Mn3+ ratio and specific surface area showed the best activity.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: A schematic illustration of the synthesis procedure used to prepare the nanocatalysts, their morphology, and additional images of the microstructures of the nanocatalysts, along with a table showing the compositional variation based on the duration of applied current are included (PDF). 14


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel: +82 2 3290 3899, Fax: +82 2 928 3584. ORCHID Young Keun Kim: 0000-0002-0868-4625 Author Contributions The manuscript was written based on the contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (No. 2014M3A7B4052193). ABBREVIATIONS SCR, selective catalytic reduction; NWs, nanowires; NPs, nanoparticles; BET, Brunauer-Emmett-Teller; XPS, x-ray photoelectron spectroscopy; TPR, temperatureprogrammed reduction; TPD, temperature-programmed desorption; FESEM, field emission scanning electron microscopy; TEM, transmission electron microscopy; XRF, x-ray fluorescence; XRD, x-ray diffraction; GHSV, gaseous hourly space velocity;

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HRTEM, high-resolution TEM; FFT, fast Fourier transform; SAED, selected area electron diffraction; fcc, face centered cubic.

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Tables and Figures

Table 1. Specific area, pore volume, and pore diameter of MnO2-CeO2 nanocatalysts with different weight percentage ratios

Samples (wt% ratio)

CeO2

MnO2

MnO2CeO2 (7:3)

MnO2CeO2 (5:5)

MnO2CeO2 (3:7)

SBET (m2/g)

0.91

1.51

3.46

3.59

2.26

Table 2. XPS results for MnO2-CeO2 nanocatalysts with different weight percentage ratios BE (eV) Sample

Mn4+

Mn3+

MnO2-CeO2 (7:3)

643.3

641.7

MnO2-CeO2 (5:5)

643.2

MnO2-CeO2 (3:7)

643.2

Mn4+/Mn3+

BE (eV) Oα/(Oa+Oβ) Oα

Oβ

0.67

530.9

529.0

58.81

641.5

0.74

531.0

529.1

58.79

641.4

0.57

531.4

529.2

61.45

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Figure 1. Schematic diagram for (a) experimental preparation procedure of MnO2-CeO2 nanocatalysts and (b) selective catalytic reaction of NOx with NH3. MnO2 NWs were electrodeposited on the surface of the cordierite ceramic filter, and CeO2 NPs were subsequently synthesized on the MnO2 NWs and vacant surface of the cordierite filter. MnO2-CeO2 nanocatalysts could operate in a broad temperature of 100–400 °C for the de-NOx reaction.

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Figure 2. Morphological and microstructural analyses of MnO2-CeO2 composite nanocatalysts. (a) Low-magnification and (b) high-magnification FESEM images of MnO2-CeO2 composite nanocatalysts on the cordierite filters. (c) HRTEM image of MnO2-CeO2 composite nanocatalysts. The inset in (c) is the fast-Fourier-transform (FFT) image of the yellow circled region. (d) SAED pattern of the ε-MnO2-CeO2 nanocatalyst. Diffraction spots of MnO2 NWs and CeO2 NPs are marked by a ring and triangle, respectively, by indexing the plane.

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Figure 3. XRD patterns of the MnO2 NWs, CeO2 NPs, and MnO2-CeO2 nanocatalysts with different weight percentage ratios. Intensities of MnO2 and CeO2 peaks were changed by changing the corresponding weight percentage ratios. JCPDS indices are also displayed. 26


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Figure 4. Catalytic activity of MnO2 NWs and CeO2 NPs. (a) H2-TPR profiles, and (b) NOx conversion for MnO2 NWs and CeO2 NPs. Catalytic activity of MnO2-CeO2 nanocatalysts with different content ratios. (c) H2-TPR profiles, and (d) NOx conversion for MnO2-CeO2 nanocatalysts. Reaction conditions: [NO] = 1000 ppm, [NH3] = 1000 ppm, [O2] = 5%, balance = N2, and the gas hourly space velocity (GHSV) = 10000 h-1.

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Figure 5. NH3-TPD profiles of MnO2 and CeO2 catalysts.

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Figure 6. Surface properties of MnO2-CeO2 nanocatalysts with different content ratios. XPS spectra for (a) Mn 2p and (b) O 1s. Peak fitting of the XPS spectra was carried out for Mn 2p3/2 and O 1s.

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TOC/Graphical Abstract

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