ScaleâActivity Relationship of MnOx-FeOy Nanocage Catalysts

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Scale-Activity Relationship of MnO-FeO Nanocage Catalysts Derived from Prussian Blue Analogues for Lowtemperature NO Reduction: Experimental and DFT Studies Lijun Yan, Yangyang Liu, Kaiwen Zha, Hongrui Li, Liyi Shi, and Dengsong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15527 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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ACS Applied Materials & Interfaces

Scale-Activity Relationship of MnOx-FeOy Nanocage Catalysts Derived from Prussian Blue Analogues for Low-temperature NO Reduction: Experimental and DFT Studies Lijun Yan, Yangyang Liu, Kaiwen Zha, Hongrui Li, Liyi Shi and Dengsong Zhang*

School of Environmental and Chemical Engineering, Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, P. R. China

KEYWORDS: NH3-SCR, Low Temperature, Size-Dependent, NOx, Structure-Activity

ABSTRACT Size eﬀects have been recognized to promote the catalytic activity and selectivity of metal oxide particles. So far, limited works and studies are conducted to investigate the size effect of metal oxide with the tailored shape in the selective catalytic reduction of NOx with NH3 (NH3-SCR). Herein, the MnOx-FeOy nanocage catalysts with varied scales (0.25 µm, 0.5 µm, 1 µm and 2 µm) were synthesized via a Prussian blue analogue (PBA)-derived method and used for NH3-SCR of NO. By preforming a series of the activity tests over the nanocages with different scales, the NH3-SCR activity of 0.5 µm MnOx-FeOy nanocage catalysts exhibit the highest deNOx activity in the temperature range of 80 - 200 oC owing to more preferable physical and chemical properties. It has been demonstrated that there is a strong interaction among Mn and Fe cations in the 0.5 µm MnOx-FeOy nanocages. Moreover, the H2-TPR and XPS analysis prove 0.5 µm nanocages exhibit excellent redox properties, which contribute to the higher conservation of NOx. Through the DFT studies, it is also demonstrated that the 0.5 µm MnOx-FeOy nanocage catalysts could provide more 1

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preferable electronic charge, which give rise to the varied adsorption behavior of the NH3 species and NOx species compared to the nanocages with other scales. The in situ DRIFTs were also employed to evaluate the adsorption status of NH3 with NOx species over MnOx-FeOy nanocage catalysts with varied scales. Finally, the scale-activity relationship of the MnOx-FeOy nanocage catalysts and their corresponding activities are also established. The deep insight into the scale-activity relationship of the PBA-derived MnOx-FeOy nanocage catalyst paves the way for developing and designing highly-efficient Mn-based catalyst at lower temperature.

INTRODUCTION Nitrogen oxides (NOx), emitted from automobile exhaust gas and combustion of fossil fuels, are currently considered as the major pollutants for atmosphere pollution, which has led to the photochemical smog, acid rain, ozone depletion, and greenhouse effects. Therefore, NOx put great threats to the human health and environment sustainability. So far, such technologies as NO decomposition, plasma storage-reduction, selective non-catalytic reduction and selective catalytic reduction (SCR) have been widely utilized for eliminating NOx. Among the current deNOx technologies, the selective catalytic reduction (SCR) of NOx with NH3 is the most widely used technique to eliminate NOx.1-3 V2O5-WO3 (MoO3)/TiO2 catalysts are used worldwide due to the preferable catalytic activity at higher temperature ranging from 300 to 400 °C.4, 5 However, several shortcomings of V-based catalysts are still present, such as the vanadium pentoxide to the environment, narrow operation temperature window, and the rather poor performance at low temperature.

6-8

Hence, developing deNOx catalysts with a desirable low-temperature activity is

quite urgent. 2


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Over the past decades, Mn-based SCR catalysts arouse great interests owing to its inherently environmentally friendly character and excellent low-temperature activity. 9-11 Moreover, it has been demonstrated that bimetallic Mn catalysts such as Mn-Co-O, Mn-Ni-O, Mn-Ce-O, Mn-Fe-O metal oxide catalysts exhibit the enhanced activity and selectivity of NOx to N2. 12-15Among those catalysts, both Mn-Fe-O and Mn-Co-O catalysts display extraordinary activity for eliminating NOx at low temperature. More importantly, it has been demonstrated that strong interactions between Mn cations and Fe (Co) cations give rise to the effective inhibition of particle aggregation of Mn, and easier generation of abundant reactive oxygen species. 12, 16 However, it should be noted that Mn-based catalysts, in most cases, were prepared by the conventional coprecipitation method, giving rise to the low dispersion of active components for achieving high catalytic performance. Catalytically, in addition to the active components in the catalysts, morphology and hierarchical structure of the materials still play an important role in catalytic behaviors. It is known that the hierarchically porous hollow structures would greatly promote the performance of catalysts because of their preferable specific surface area and efficient channels for mass transport and reactant molecular contact. 17-20 Prussian blue analogues (PBAs) containing divalent and trivalent metal ions bridged by cyanide ligands are the prototype materials for this open framework structure, and those materials have been widely investigated to exhibit outstanding physical and chemical properties. simplified formula for these materials is A3B2 (CN)

6

and A

2+

21-23

The

cations are in an ordered

arrangement on the B3+ sites, which could lead to the strong interaction between the active components. Therefore, PBAs could be attractive candidates for designing catalysts with uniform 3



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composition and size, diverse morphologies and architectures as well as open channels that allow rapid insertion of active components. 24 Selecting PBAs as the precursors for synthesizing highly efficient catalysts would give a bright prospect to develop novel catalysts. In our previous research, we prepared and synthetized a kind of uniform MnxCo3−xO4 nanocage with hollow and porous structures, which exhibit significantly improved activity for NOx removal compared to the conventional MnxCo3−xO4 nanoparticles. 25Through the sufficient characterizations and analyses, it revealed that preferable morphology and material structure give rise to the enhanced deNOx activity. Previous literatures on size-dependent effect demonstrate that sizes and scales of the catalysts would bring a bearing effect on their catalytic performance in the variety of catalytic fields. 26-28 So far, further works and efforts on discovering the correlation between the scales and activity of the nanocages have not been ever reported. In this case, Mn-Fe-O nanocages with more preferable low-temperature deNOx activity are used for the study of the scale-activity relationship. In this paper, four different Mn-Fe-O nanocages with gradient scales ranging from 0.25 µm to 2 µm by controlling the ratio of water to ethanol in the precursor solutions were synthesized and examined in the NH3-SCR of NO. More importantly, to fully reveal the scale-activity relationship of the catalysts, both the experimental and theoretical methods are employed, which may provide the guidance on designing and developing the highly-efficient catalyst by tailoring the structures and scales.

Experimental Section Catalyst Preparation Polyvinyl Pyrrolidone (PVP, K-30) was purchased from the Sigma Aldrich. All other chemical 4


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regents were purchased from Sinopharm Chemical Regent Cooperation. In the experiments, all chemicals are used with analytical grade and without further purification, and the samples are prepared by the similar techniques as previous literatures reported. 29, 30 Typically, 2 g of PVP and 0.42 g of Mn(SO4)2·4H2O were first dissolved into 85 mL of deionized water. Afterwards, 80, 160, 240, 320 ml of ethanol with different volume ratio of ethanol to water to obtain a various scale of Mn3[Fe(CN)6]2·nH2O nanocube precursors due to the varied solution viscosity. Under adequate time for magnetic stirring, the above mentioned solutions form into the transparent solutions except for the solution including 320 ml of ethanol. Subsequently, the required amount of an aqueous solution of K3Fe(CN)6 (0.32 g) was added into the above solution through injection pump under magnetic stirring. Then, the mixed solution was maintained under room temperature for 24 h. After aging, the coffee brown precipitates was collected by centrifugal separation and washed with the absolute ethanol with no less than 5 times, and finally dried at 60 oC in the oven for 12 h. After that, varied scales of Mn3[Fe(CN)6]2·nH2O nanocubes were annealed under 500 oC for 5h with a moderate heating rate of 2 oC/ min under air flow to maintain its well-defined structure and to form into the corresponding metal oxide nanocages. For comparison, the conventional MnOx-FeOy nanoparticles were prepared the method similar to the literature and more detailed procedures could be found in the supporting information. 31

Characterizations The characterization on morphologies and structures of the samples were performed by scanning electron microscopy (SEM, JEOL JEM-200CX), transmission electron microscopy (TEM, JEOL JEM-200CX). The crystallographic structures were characterized by a Rigaku D/MAS-RB X-ray 5



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diffractometer employing Cu Kα (40 kV, 40 mA) radiation with a scanning rate of 8° min−1. BET surface areas of the catalysts were determined from nitrogen physisorption measurements on about 120 mg sample at liquid nitrogen temperature with an Autosorb IQ (Quantachrom) instrµment. The samples were heated to 300 ◦C for 3 h under vacuum prior to the measurement. The temperature-programmed reduction with hydrogen H2-TPR studies were also conducted on an autoadsorption apparatus (tp5080, Tianjin XQ) with a thermal conductivity detector. For the analysis, 60 mg of powdered sample was pretreated in nitrogen at 300 °C for 30 min. After cooling to room temperature, the H2−TPR was recorded in the reducing mixture consisting of 5% H2 and balance N2, and final temperature of 650 °C (heating rate of 10 °C min−1). In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments was carried out on a Nicolet 6700 spectrometer equipped with a Harrick Scientific DRIFT cell and a mercury−cadmium−telluride (MCT) detector cooled by liquid N2. The DRIFT spectra were collected in the range of 1800−1100 cm−1, accumulating 64 scans at 4 cm−1 resolution in Kubelka−Munk format. Prior to each test, all samples were held at 300 °C under N2 flow (50 mL/h) for 30 min and cooled to the desired temperature to get a background spectrum, and this spectrum was then subtracted from the sample spectra for each measurement.

Catalytic Activity Evaluation The NH3-SCR measurements were carried out in a fixed-bed quartz reactor (internal diameter 8 mm) using 0.4 g sample. The following reaction conditions were used: 500 ppm of NO, 500 ppm of NH3, 5 vol. % O2 and N2 balance. The total flow rate was 250 mL/min, corresponding to a gas hourly space velocity (GHSV) of 25, 000 h−1. At each temperature step the concentrations of NOx 6


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was obtained. The concentrations of NOx in the inlet and outlet gases were measured by a 4000VM NOx Analyzer. The NOx conversion percentage and N2 selectivity was calculated using the following expressions: Nܱ௫ conversion ሺ%ሻ =

ሾNைೣ ሿin ି ሾNைೣ ሿout ሾNைೣ ሿin

× 100%

(1)

where the subscripts “in” and “out” represent the inlet and outlet gas concentration of NOx, respectively.

Density Functional Theory (DFT) Calculation In the recent decades, DFT calculation studies were applied using the Material Studio Modeling CASTEP package to figure out the contribution of the active orbits and the distribution of electron over catalyst surface. 32-34 Based on the characterization result (XRD, ICP and H2-TPR), main active components in varied scales of MnOx-FeOy nanocages were investigated by establishing such supercells as Mn2O3, Fe2O3, Fe3O4, Mn doped Fe2O3 (Fe3O4) and the Fe doped Mn2O3.The atomic charges were calculated using the mulliken population approach with the model established above. The effective core potentials (ECP) and spin polarizations were applied.

35, 36

Additionally, a

kinetic energy cut off is set to 420 eV for the plane waves included in the basis set in the whole calculations. The k point parameters for Mn2O3 (Fe doped Mn2O3), Fe2O3 (Mn@Fe2O3) and Fe3O4 (Mn@Fe3O4) were set as 1×1×1. Though comparatively low k point settings were utilized to calculate the mulliken populations of each structure, it could be an acceptable setting parameters due to inherent characters of the crystal structures with higher symmetry and the space groups with comparable length of the lattice parameters. Besides, the field convergence, max force and 7



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max displacement were 1.0e-5 ha, 0.002 ha/Å and 0.005 Å respectively. The models were fully relaxed during the whole calculations. All results of partial electronical charge for each metal oxide phase have been listed in the support information and final results are determined by the average value of above-mentioned results.

RESULTS AND DISCUSSION Morphologies and Structure of the Catalysts Through the observations of the SEM and scale statics on the images of various catalysts, the as prepared Mn3[Fe(CN)6]2·nH2O precursor nanocubes have well-defined shapes and uniform size distributions ranging from 0.25 - 2 µm, indicating the a highly morphological uniformity with specific scale for each sample (Figure 1). Furthermore, the XRD was also performed to check the chemical composition and phase purity of the Mn3[Fe(CN)6]2·nH2O. Except for the 0.25 µm nanocube precursor, it could be clearly observed that XRD patterns of all samples were well consistent with the characteristic reflections of face-center-cubic Mn3[Fe(CN)6]2·nH2O with an Fm3m system. 37 Moreover, the appearance of sharp and strong characteristic diffraction peaks of precursor nanocubes indicate each sample exhibits a high purity and crystallinity. It has been demonstrated that Mn3[Fe(CN)6]2·nH2O precursor displayed less thermal stability. Under higher annealing temperature (above 400 oC), they can decompose and transform into the corresponding metal oxide in the presence of air condition. To investigate the morphologies and detailed geometrical structure of the samples after annealing, both SEM and TEM images were shown in Figure 2. Given the SEM images of the samples after calcination, all samples retain their cubic morphologies and scales with higher uniformity owing to the moderate heating rate during the 8


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calcination process. In addition, it could be observed that each sample display the rough porous structure because of the release of gas by decomposing the CN bonding to metals. Small scale of the nanocube precursor is more preferable to form the porous structure due to the sufficient and effective heat transfer and diffusion with thinner nanocube wall. From the TEM images, nanocube samples have been fully transformed into the nanocages with the hollow structure by observing the sharp contrast between the inner center and outer edges. More importantly, a smaller scale of the nanocube precursor (0.5 µm and 0.25 µm) tend to exhibit corresponding thinner thickness (100 nm and 50 nm, respectively) of the cage wall, which would be benefit to the reactant diffusion through the nanocages. Noting that based on the Kirkendall effect, there would be also an enhanced synergistic effect between the active components, giving rise to the elevated deNOx performance. 38, 39 In view of the XRD patterns demonstrated in Figure 3, the characteristic reflections of samples with the scales of 0.5 to 2 µm correspond very well to the standard card of Mn2O3 (PDF#76-0150), Fe2O3 (PDF#33-0664) and Fe3O4 (PDF#65-3107). However, great discrepancies could be found for the phase of 0.25 µm nanocage sample, where the K2Mn2(SO4)3 (PDF#84-0124) instead of Mn2O3 appeared, which could be ascribed to the existence of inevitable residual K and S components during the synthesis process of the 0.25 µm nanocube under higher ratio of ethanol to water. To further compare the contribution of active components in varied-scale catalysts, RIR method was conducted with the results shown in Figure 3. 40, 41 In Figure 4, it could be clearly observed that the main components and corresponding proportion change with downsizing the nanocages. For 2 µm nanocage, the main active phrases are Mn2O3, Fe3O4 and Fe2O3 with the proportion of 13.5, 5.2 and 81.3 %, respectively. At the process of the transformation from 2 µm to 9



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1 µm scale of the nanocage, there is a significant change on the proportion of Fe3O4. In addition, when the scale of nanocage reduced to the 0.5 µm, only the Mn2O3 and Fe2O3 phrase could be detected and the former one occupied around 74.9 %. In view of the 0.25 µm nanocage, Fe2O3 phrase continuously decrease and its proportion has been reduced to the 13.5 %. Therefore, it is demonstrated that with reducing the scale of the nanocages, the type and contents of Fe species have been greatly changed. However, it seems to be a contradiction that the ratio of Mn/Fe is nearly not a constant while the ratio of Mn/Fe should be the same under the strict control in the synthesis process. Accordingly, the ICP analysis was performed to detect the accurate content in each samples, and the results demonstrate that the nanocages with the scale ranging from 0.5 µm to 2 µm exhibit almost the same weight percentages of Mn and Fe, indicating that the doping of Fe and Mn into the lattice of M2O3 and Fe2O3 (Fe3O4) probing from the Kirkendall effect (Table 1). Additionally, it could be observed (See in Figure S1) that in the XRD patterns the characteristic peaks for both Mn2O3 and Fe2O3 have a slight shift when the scale of the nanocages are reduced, proving the successful insertion of Mn (Fe) cations into the Fe2O3 (Mn2O3). For the 0.25 µm nanocages, the reduced contents of Fe species could be ascribed to the competition between the combination of the Mn cations with Fe (CN)6- and the substitution of K cations with Mn cations in Mn2SO4 (note: in the solution with higher viscosity, less Mn2SO4 could be dissolved and less chance would be available for the combination of Fe(CN)6- with Mn2+). In that case, K contents in the 0.25 µm nanocage is nearly 3-4 folder higher than that of other samples. The previous reports on catalyst poisoning have demonstrated that higher contents of the K cations could easily block the acid sites of the catalysts, greatly reducing the catalytic activity. 42, 43 The specific surface areas for each catalyst are obtained through N2 physisorption technique and the results are summarized 10


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in Table 2. It could be clearly found that with the reduced scale of the nanocages, an apparent decrease of the surface area occurred and the 0.25 µm displayed the smallest surface area of 53.6 m2g-1. It shows that XRD patterns of nanocages with the larger scale (2 µm) displayed comparatively weaken intensity of the characteristic peaks for each phrase, indicating the lower crystalline degree. In previous studies, it is proved that the crystals with lower crystalline possess the surface defects and large area of contact interface, giving rise to higher surface area.

44

However, it is found that with the enhancement of the scale, higher crystalline degree occurs due to easier heat transfer through thinner cage wall, which would also greatly eliminate the rough surface and reduce the surface area. Luckily, only the slight decrease could be observed for nanocage of 0.5 µm scale thanks to preferable porous and hollow interior structure, compensating the great loss of the surface area. When the scale further decreases to the 0.25 µm, higher content of the K residual would severely block the active sites and give rise to the sharp decrease of surface area. In addition, the pore volume starts to decrease at initial when the scale reduced from 2 µm to 1µm, which could be ascribed to the decreasing amount of the pores in the transformation. Interestingly, an obvious enhancement of the pore volume could be found at the smaller scale of the nanocages (0.5 µm) owing to the enhanced pore size and larger area of hollow structure. However, further decreasing the scale of the nanocages lead to a significant reduction of the pore volume and this phenomenon could be explained by the block of active sites in the presence of excessive amounts of the k cations, which are in the good accordance with results shown in the XRD patterns. Overall, it could be deduced that the nanocages with smaller scales possess such preferable physical properties as big pores and large hollow structure for efficient heat diffusion and 11



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sufficient reactant contact. However, downsizing the scale of the sample leads to the un-satisfied chemical component, giving rising to the decreased activity of the catalysts. As a consequence, there should be a compromise between the physical properties and chemical properties in designing highly efficient PBA-derived catalysts through determining the scale.

Redox Behavior Characterizations The redox behavior of the catalyst has a bearing effect on heterogeneous catalysis system. Consequently, both H2-TPR and XPS are employed to investigate the redox behavior of MnOx-FeOy nanocage catalysts. From the results of H2-TPR profiles (Figure 5), the catalysts with the scale ranging from the 0.5 µm to 2 µm show two broaden distinct peaks, where the former one located at approximately 310 °C around and the latter one located at region from 450 -500 oC. It is demonstrated that the H2 consumption peaks closer to 300 oC could be ascribed to the reduction of MnO2 to Mn2O3 and Fe2O3 to Fe3O4. 16 Meanwhile, an obvious trend could be observed that the peak corresponding to the reduction of Mn2O3 to MnO and Fe3O4 to FeO have shifted backward for almost 30 oC with reducing the scale. Additionally, with decreasing the scale, the sharp enhancement of H2-consumption also could be found for the reduction of Mn3+ and Fe3+ due to the elevated synergistic effect between Mn3+ and Fe3+ at preferable scale. In the view of H2-TPR profile of 0.25 µm nanocage sample, a new peak that located at 440 oC appeared due to the reduction of the K cations. At the same time, apparent decreasing amounts of H2-consumption could also be found, which could be attributed to the reduced redox capability of the MnOx-FeOy nanocage in the presence of K+. This corresponds well to the above-mentioned results that the considerable 12


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contents of K cations would block active sites to react with hydrogen and weaken redox capability of the catalysts. Therefore, the 0.5 µm nanocage exhibits the most preferable redox behavior among the catalysts, which might contribute to the promising deNOx activity. Oxidation state of the metal elements and surface element concentration play an important role of the redox properties over catalyst surface. Accordingly, the XPS spectra of Mn, Fe, and O demonstrated in Figure 6, and relative atomic concentrations are summarized in Table S1. As shown in Figure 6a, two distinct peaks centered at 642.6 and 653.6 eV would be observed, which are ascribed to Mn 2p3/2 and Mn 2p1/2, respectively.

45, 46

By performing peak-fitting

deconvolutions, the Mn 2p3/2 XPS peaks were divided into three well-defined peaks assigned to Mn2+ (640.5 eV), Mn3+ (642.3 eV), and Mn4+ (644.7 eV). Over 0.5 µm nanocage sample, its Mn 2p3/2 peak possesses the highest ratio of Mn4+ (24.6%) due to the easier insert of Mn4+ (effective radius of Mn4+: 0.39- 0.53 Å) into the Fe2O3 lattice (effective radius of Fe3+: 0.49- 0.55 Å) probing from preferable scale of nanocages. However, the highest ratio of Mn2+ cations could be observed for 0.25 µm nanocage and this could be explained by the formation of K2Mn2(SO4)2. As a consequence, the average oxidative ability of manganese species in the 0.5 µm nanocage should be the strongest among all catalysts, which favors the oxidation of NO to NO2. Meanwhile, the larger amount of NO2 readily triggers the fast SCR reaction on the basis of the overall reaction as “NO + NO2 + 2NH3→2N2 + 3H2O”, contributing to the enhanced deNOx activity. 47, 48 In Figure 6b, the Fe 2p3/2 XPS spectra demonstrate two broaden peaks centered at 709.8 and 711.2 eV ascribed to FeII and FeIII, respectively. 49, 50 After the peak-fitting deconvolution, the percentage of FeIII species in 0.5 µm nanocage sample is much higher than that of other samples, which could be attributed to reluctant incorporation of Fe3+ (effective radius of Fe3+ : 0.49- 0.55 Å) into the 13



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M2O3 lattice (effective radius of Mn3+: 0.58- 0.65 Å) under the strong synergistic effect between the cations over 0.5 µm nanocages surface. However, it seems to be different from the proportion of crystal phases demonstrated in XRD patterns. It should be noted that XPS spectra mainly illustrates the surface status of cations over the catalysts, and the enhanced interaction between the Mn3+ and Fe3+ could be realized owning to sufficient mass transfer under the preferable scale (0.5 µm) during the calcination. On the other hand, it has been demonstrated that the ratios of Mn/Fe for 0.5- 2 µm nanocages are almost the same despite great discrepancies of the proportion of active phrase in each sample, indicating the occurrence of easier incorporation in catalyst with preferable nanoscale. Previous studies have suggested redox cycles Mn4+ +Fe2+→Mn3+ +Fe3+ and Fe3++ Mn2+→Mn3++Fe2+ guaranteed the improved redox capabilities of the catalysts. Therefore, the 0.5 µm nanocages possess highest percentage of Fe3+ (78.7 %) tends to exhibit more promising activity for NOx removal. In the case of O1S spectra for each sample (Figure 6c), they were also performed and fitted into three groups of sub-peaks according to the previous literature. The O 1s binding energies centered at 529.2–530.2,530.3–531.7 and 533.1–533.5 eV could be assigned to lattice oxygen species (O2−, denoted as Oβ) and chemisorbed oxygen species (O2 &−, containing O− and O2−, denoted as Oα and Oγ, respectively).

51, 52

Generally, chemisorbed oxygen species are thought to be more active

species in the oxidation reactions due to its higher mobility.

53

As a result, more active oxygen

species could largely facilitate the highly-efficient catalytic activity at lower temperature for maintaining higher oxidation status of metal cations through the reactions as follows: Fe2+ + 0.5 O2→Fe3++ 0.5 O2- and Mn3+ +0.5 O2→Mn4+ + 0.5 O2-. The relative concentration ratios of Oα+γ/Oβ for catalysts with varied scales were also calculated and the 0.5 µm nanocages exhibited the 14


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highest ratios of the active oxygen species, which benefit the higher deNOx activity.

Adsorption Behavior of the Reactant Species over Catalyst Surface In a heterogeneous catalysis process, the adsorption behavior is considered as a crucial step. The adsorption status of the reactant on the catalyst surface will have a bearing effect on the degree of the catalytic process. As a consequence, the adsorption of NH3 and NOx on each catalyst with varied nanoscale (0.25 - 2 µm) was investigated. Both the DFT studies and in situ DRIFTs are employed to investigate the amounts and categories of the reactant species anchoring over catalyst surface and to give a deep insight to the promotional effect of scale on the adsorption behavior of the catalysts surface. Partial Electronic Charge Difference The partial electronic charge difference (PECD) between catalyst sites and reactants determined the adsorption behavior of the reactant species over the catalyst surface. 54-56 In Figure 7a, it could be clearly observed that Fe2O3 and Mn@Fe2O3 contacting with ammonia species possess higher PECD values, where PECD between Fe cation of Fe2O3 and N atom of NH3 (1.96 e) is higher than that between Mn cation of Mn2O3 and N atom of NH3 (1.898 e). At the same time, the PECD would be enlarged after the doping of Mn into the Fe2O3 lattice and be decreased after doping of the Fe into Mn2O3 lattice. In view of the PECD derived from N atom of NH4+ and Fe2O3 (Mn2O3), similar trend that the incorporation of Mn into the Fe2O3 lattice favors higher PECD value could be observed. Additionally, the PECD derived from Fe3O4 and NH3 (NH4+) are higher than that of H3N (+H4N) ···Mn2O3 but lower than that of H3N (+H4N) ···Fe2O3. It has been demonstrated that the more positive value of partial electronic charge difference, more easily the reactant species 15



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would adsorb on the catalyst surface. In consequence, NH3 and NH4+ tend to adsorb over Fe2O3 and Mn@Fe2O3 more easily than over Fe3O4 and Mn2O3. However, NH4+ would act a varied adsorption behavior, which is different from the NH3 adsorbing over the surfaces owing to an extra flexible H atom when it gets closer to the surface. As illustrated in Figure 7b, the partial electronic charge of H atom in NH3 (0.446 e) is much higher than that of H atom in NH4+ (0.313 e), strong prohibiting the NH4+ to anchor over the active metal sites with higher positive charge. As a result, the preference for NH4+ species adsorbing over each metal oxide components would follow the order of Mn2O3(Fe@Mn2O3), Fe3O4 (Mn@Fe3O4) and Fe2O3 (Mn@Fe2O3), which is totally reverse to the sequence of the NH3 adsorption over each metal oxide phase. Above all, the MnOx-FeOy nanocages would go through a varied adsorption behavior for ammonia species with the reduced scales from 2 to 0.5 µm due to the increasing proportion of the Mn2O3 (Fe@Mn2O3), where the preferential adsorption for NH3 would transform to the adsorption for NH4+. When the scale was further reduced to 0.25 µm, both NH4+ and NH3 could hardly adsorb over the catalyst surface owing to the blocking effect of the K2Mn2(SO4)3. For the NOx adsorption over each metal oxide surface, two main dominant nitrate species (bridged nitrate and chelating nitrate) were investigated on the basis of the in situ DRIFTs study results. In Figure 7, Fe cations in Fe2O3 and Mn@Fe2O3 contacting with O atom in bridged nitrate are demonstrated to have PECD values of 1.558 and 1.568 e, which is much higher than the PECD values in bridged nitrate···Fe3O4 (1.499 e) and bridged nitrate species···Mn2O3 (1.496 e), suggesting an easier bridged nitrate adsorption over Fe3O4 and Mn2O3 components. Meanwhile, varied adsorption behavior would occur in the adsorption systems of chelating nitrate···metal oxides surface and both the N and O atoms in chelating nitrate should be taken into consideration 16


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as well. Apparently, O atom in chelating nitrate is more likely to attach the Fe2O3 and Mn@Fe2O3 owing to the higher PECD values compared to the Fe3O4 and Mn2O3. However, the partial electronic charge of N atom in chelating nitrate is 0.614 e, repelling the metal oxide with more positive electronic charge to adsorb the chelating nitrate. Therefore, Mn2O3 and Fe@Mn2O3 possess smaller PECD values compared to the other metal oxide components, probably giving rising to the changed adsorption behavior for nitrate species. In that case, with the decreasing scale of MnOx-FeOy nanoscale and increasing proportion of the Mn2O3 and Fe@Mn2O3, the chelating nitrate instead of bridged nitrate species would be likely to adsorb over the MnOx-FeOy nanocages of 0.5 µm. When the scale was reduced to 0.25 µm cage, the K2Mn2 (SO4)3 phase in nanocages greatly changed the NOx adsorption behavior. Given the above discussions, the scale of nanocages would have a bearing effect on the adsorption behavior of the ammonia and NOx species, giving rising to the varied catalytic activity for NOx removal. A direct observation for the adsorption behavior of reactants over each catalyst with different scales is investigated by preforming the in situ DRIFTs studies. Ammonia Species. The in situ DRIFTs of NH3 adsorption over nanocages with the scales ranging from 0.25 - 2 µm are shown in Figure 8 a-d. For the 2 µm nanocage catalyst, the bands at 1593 and 1309 cm-1 are assigned to the asymmetric and symmetric bending vibrations of NH3, respectively. The asymmetric and symmetric bending vibration at bands at 1631 and 1410 cm-1 was also detected, regarding as the N-H bond in NH4+ linked to Brønsted acid site. 57, 58 For the 1 µm nanocage catalyst, the symmetric bending vibration at 1410 cm-1 was also detected, regarding symmetric bending vibration of NH4+. Meanwhile, the broad peaks located at 1593 and 1280 cm−1 were assigned to the asymmetric and symmetric bending vibrations of NH3 bounded to Lewis acid 17



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sites. Only three peaks centered at 1621, 1420 and 1198 cm-1 could be observed for the 0.5 µm nanocages, where the former two bands could be attributed to asymmetric and symmetric bending vibrations of NH4+, and the latter one could be assigned to the symmetric bending vibrations of NH3. With further decreasing the scale of nanocages, great difference could be found over the 0.25 µm nanocages for their adsorption behavior of NH3 species. There are only two distinct bands with weaken intensity located at 1615 and 1210 cm-1, assigning to the asymmetric bending vibrations of NH4+ and symmetric bending vibrations of NH3, respectively. Additionally, when the scale was reduced to 0.25 µm, the intensity for the adsorbed ammonia species is less 1/10 folder than that of other catalysts, indicating the decreasing amount anchoring over the 0.25 µm nanocages. This could be explained by the blocking effect of the larger amount of K cations in MnOx-FeOy nanocages of 0.25 µm. On the basis of the adsorption behavior over varied catalysts with different scales, there is clear trend that the ratio of the NH4+/NH3 increased with reduced scale. It has been demonstrated that Brønsted acid sites could efficiently transform NH4+ into NH2 species with assistance of the metal oxide of higher oxidation status. Afterwards, NH2 could be an effective intermediate to contact with the gaseous NO to convert the toxic nitride oxide in to environmentally friendly nitrogen through this faster path. 59 As a consequence, the 0.25 µm nanocages possess the highest ratio of NH4+/NH3, giving rise to more preferable performance for removing the NOx.

NOx species. Figure 9 a-d shows the in situ DRIFT spectra of NOx adsorption over the catalyst surfaces below 300 oC. For 2 µm and 1 µm nanocage catalysts (Figure 9c and d), only one band at 1217 cm-1 could be detected, ascribing to the bridged nitrate species. When the size of nanocages 18


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switch to 0.5 µm (Figure 9b), one band located at 1252 cm-1 could be observed, which could be attributed to the chelating nitrate species. 60 Meanwhile, similar to the great variations found at in situ DRIFTs for NH3 adsorption over 0.25 µm nanocage (Figure 9a), several new bands at 1634, 1510, 1394 and 1307 cm-1 could be found in 0.25 µm nanocage catalyst, which could be assigned to gaseous NO2, bidentate nitrate species, linear nitrites and chelating nitrate, respectively. 61-63 In addition, along with the increase in temperature, only the chelating nitrate quickly desorbed from the catalyst surface and totally disappeared when the temperature rise to 200 oC. This phenomena strongly imply the adsorbed chelating nitrate species would favour the NOx to be decomposed and removed from the catalyst surface. However, such nitrate species as bridged nitrate and bidentate nitrate have been demonstrated to be so stable even at higher temperature (300 oC). Among the catalysts, the 0.5 µm nanocage adsorbed the nitrate species as chelating nitrate, contributing to the higher deNOx efficiency. Overall, it could be deduced that there is an increasing amount of the active NH4+ species would adsorb over the nanocages with downsizing the scale, and less active nitrate species (bridged nitrates) would be greatly reduced through decreasing the nanoscale by 0.5 µm, which is also in a good agreement of DFT studies. Consequently, 0.5 µm nanocages exhibit the most preferable adsorption behavior for NH3 and NOx species, giving rising to highly efficient conversion of nitride oxide.

Catalytic Performance As depicted in Figure 10, the corresponding catalytic activities of MnOx-FeOy nanocages with different scales are plotted as a function of temperature. In view of the 2 µm nanocages, only 50 % 19



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conversion of NOx could be observed at 80 o C. When the scale is reduced to 1 µm, there is an obvious enhancement of deNOx activity, which could be attributed to the elevated chemical and physical properties in the process of reduced scale. Further decreasing the scale of nanocages by 0.5 µm leads to the extraordinary performance for reduction of the NOx (Conversion = 87.8 % at 80 oC) due to the preferable redox capabilities and enhanced synergistic effect between the components. In addition, it could also be found that the MnOx-FeOy nanocages exhibit a more extensive operating temperature window for eliminating the NOx when their scale decreases from 2 µm to 0.5 µm. However, when the scale of nanocages has been further reduced to the 0.25 µm, a sharp decrease of corresponding deNOx activity could be found owing to the blocking effect of K cations on catalyst surface. Overall, the 0.5 µm nanocages exhibit the most promising activity for NOx removal among the catalysts, which corresponds well to the characterizations discussed above. Accordingly, to design and develop the novel and highly efficient PBA-derived NH3-SCR catalysts, the relationship between the catalytic activity and corresponding scale has been established and investigated, which would be expounded later. Additionally, the sample with 0.5 µm is selected as a target sample to compare with the nanoparticle made with conventional method (See in Figure S4). Apparently, the 0.5 µm nanocages under tailor design exhibit a more preferable activity for eliminating NOx owing to its well-dispersed active components and enhanced synergistic effect between the cations, which has been demonstrated by the previous study.

Scale-Activity Relationship According to the activity test and series characterizations for each catalyst, nanocages with the 20


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reduced scale exhibit the more preferable deNOx performance due to the advantageous physical and chemical properties (shown in the Scheme 1). For the physical part, with reducing the scale, more complete hollow porous structures could be readily achieved by the thin wall of the nanocube precursor under the sufficient transfer, which provide abundant channels for shortening distance for molecular adsorption and sites to contact with the reactants. As a result, such reactant molecular NH3 and NOx could more easily adsorb or desorb form the catalyst surface, or react with each other fiercely inside catalyst with porous and hollow structure. Additionally, the enlarged pore size of nanocages in smaller scale could benefit the gas to get attached to the surface of the catalysts, and the easier heat diffusion and mass transfer. It seems that the nanocages with greatly reduced scale would exhibit more promising deNOx activity. However, chemical properties would also play an important role in affecting the SCR activity of NOx. When the size of nanocages decreases to 0.25 µm, the K cations would attach to catalyst surface and block the active sites to be exposed to the reactant molecular, which are in good agreement with the XRD patterns and XPS result. In this case, though nanocages with the scale of 0.25 µm exhibit more preferable physical properties for gas diffusion, its deNOx activity has been greatly reduced by its chemical components. For the chemical part, nanocages with the specific scale exhibit totally different kinds and proportion of active phrase well as the redox capabilities owing to the different synergistic effect between the cations under the varied scale. By preforming the H2-TPR and XPS, it could be deduced that when the nanocages scale decreases from 2 µm to 0.5 µm, the higher contents of Mn4+, Fe3+ and active oxygen species could be achieved owing to the more sufficient heat and mass transfer in the nanocages with smaller scale. Probing from Kirkendall effect, metal cations would move closer to each other or incorporation into the other’s lattice, giving rise to the 21



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enhanced synergistic effect between the cations. To prove the analysis through the experimental results, DFT studies have been employed on the basis of the XRD patterns and RIR analysis method. We investigate the PECD values for the components (Mn2O3, Fe2O3 and Fe3O4) in each sample as well as the modified metal oxide (Mn doped Fe2O3, Mn doped Fe3O4 and Fe doped Mn2O3) when considering the synergistic effect between the Mn and Fe cations. It has been demonstrated that the active phrase in 0.5 µm nanocages exhibit more preferable PECD values for active NH4+ and chelating nitrate species to adsorb compared to those in the nanocages with other scales. Accordingly, the 0.5 µm nanocages with highest ratios of the Fe doped Mn2O3 phrase would contribute to the accumulation of more active components, giving to the extremely high conservation of the nitride oxide even at lower temperature.

CONCLUSIONS In summary, a series of MnOx-FeOy nanocage catalysts were prepared by a PBA-derived method for the SCR of NO with NH3. Compared with the nanocage catalyst with other scales (2 µm, 1µm and 0.25 µm), the nanocage with the scale of 0.5 µm demonstrate evident enhancement of NOx conversion in the range of 80 - 200 oC. On the basis of the results and discussions, it could be deduced that the scale plays an important role in designing and developing highly-efficient the PBA-derived catalysts for NOx removal at the low temperature. It could be explained by the preferable structure for reactant diffusion and heat transfer, efficient redox capability, as well as the favorable adsorption behavior for reactant species that the 0.5 µm MnOx-FeOy nanocages could provide, which is supported by experimental studies and DFT calculations. The investigation of the effect of catalyst scales on their performance would have a fundamental 22


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impact on the development of high performance catalysts.

ASSOCIATED CONTENT Supporting Information The 2D in situ DRIFTs of NH3 and NOx adsorption and surface atomic concentration of each prepared MnOx-FeOy nanocages with varied scales; The enlarged image of XRD patterns of various samples; Mulliken charge populations of the main active phase. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel: +86-21-66137152; E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the support of the National Natural Science Foundation of China (U1462110).

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mechanistic study of the NH3NO/NO2 “Fast SCR” reaction over a commercial Fe-ZSM-5 catalyst Catal. Today 2012, 184, 107-114. 48. Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M., The chemistry of the NO/NO2– NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis J. Catal. 2008, 256, 312-322. 49. Wang, L.; Cheng, X.; Wang, Z.; Ma, C.; Qin, Y., Investigation on Fe-Co binary metal oxides supported on activated semi-coke for NO reduction by CO Appl. Catal., B 2017, 201, 636-651. 50. Zhang, R.; Yang, W.; Luo, N.; Li, P.; Lei, Z.; Chen, B., Low-temperature NH3-SCR of NO by lanthanum manganite perovskites: Effect of A-/B-site substitution and TiO2/CeO2 support Appl. Catal., B 2014, 146, 94-104. 51. Zhang, T.; Qu, R.; Su, W.; Li, J., A novel Ce–Ta mixed oxide catalyst for the selective catalytic reduction of NOx with NH3 Appl. Catal., B 2015, 176, 338-346. 52. Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C., A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3 Appl. Catal., B 2012, 115–116, 446-468. 53. Cai, S.; Zhang, D.; Zhang, L.; Huang, L.; Li, H.; Gao, R.; Shi, L.; Zhang, J., Comparative study of 3D ordered macroporous Ce0.75Zr0.2M0.05O2−δ(M = Fe, Cu, Mn, Co) for selective catalytic reduction of NO with NH3 Catal. Sci. Technol. 2014, 4, 93-101. 54. Wang, D.; Zhang, L.; Kamasamudram, K.; Epling, W. S., In situ-DRIFTS Study of Selective Catalytic Reduction of NOx by NH3 over Cu-Exchanged SAPO-34 ACS Catal. 2013, 3, 871-881. 55. Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A., Mechanism of the Selective Catalytic Reduction of NO by NH3 over MnOx /Al2O3 J. Catal. 1997, 171, 219-230. 56. Chen, L.; Li, J.; Ge, M., DRIFT study on cerium-tungsten/titania catalyst for selective catalytic reduction of NOx with NH3 Environ Sci Technol 2010, 44, 9590-9596. 57. Hu, H.; Cai, S.; Li, H.; Huang, L.; Shi, L.; Zhang, D., In situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism Over Mn-Doped Co3O4 for the Selective Catalytic Reduction of NOx With NH3 J. Phys. Chem. C 2015, 119, 22924-22933. 58. Lian, Z.; Liu, F.; He, H.; Shi, X.; Mo, J.; Wu, Z., Manganese–niobium mixed oxide catalyst for the selective catalytic reduction of NOx with NH3 at low temperatures Chem. Eng. J. 2014, 250, 390-398. 59. Ma, L.; Cheng, Y.; Cavataio, G.; Mccabe, R. W.; Fu, L.; Li, J., In situ DRIFTS and 28


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temperature-programmed technology study on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts Appl. Catal., B 2014, 156–157, 428-437. 60. Peng, Y.; Liu, Z.; Niu, X.; Zhou, L.; Fu, C.; Zhang, H.; Li, J.; Han, W., Manganese doped CeO2– WO3 catalysts for the selective catalytic reduction of NOx with NH3: An experimental and theoretical study Catal. Commun 2012, 19, 127-131. 61. Yan, L.; Liu, Y.; Hu, H.; Li, H.; Shi, L.; Zhang, D., Investigations on the Antimony Promotional Effect on CeO2-WO3/TiO2 for Selective Catalytic Reduction of NOx with NH3 ChemCatChem 2016, 8, 2267-2278. 62. Maitarad, P.; Zhang, D.; Gao, R.; Shi, L.; Li, H.; Huang, L.; Rungrotmongkol, T.; Zhang, J., Combination of Experimental and Theoretical Investigations of MnOx/Ce0.9Zr0.1O2Nanorods for Selective Catalytic Reduction of NO with Ammonia J. Phys. Chem. C 2013, 117, 9999-10006. 63. Maitarad, P.; Han, J.; Zhang, D.; Shi, L.; Namuangruk, S.; Rungrotmongkol, T., Structure– Activity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies J. Phys. Chem. C 2014, 118, 9612-9620.

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Figure 1. SEM images of Mn3[Fe(CN)6]2·nH2O precursor nanocubes with the scales (0.25, 0.5, 1 and 2 µm); Their corresponding sample images and scale statics results are displayed as well.

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Figure 2. TEM images of MnOx-FeOy (a) nanocages with 2 µm scale, (b) nanocages with 1 µm, (c) nanocages with 0.5 µm scale and (d) nanocages with 0.25 µm scale derived from Mn3[Fe(CN)6]2·nH2O precursor nanocubes with the scales (0.25, 0.5, 1 and 2 µm); corresponding SEM images of the nanocages with varied scales (e-h).

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Figure 3. XRD patterns of MnOx-FeOy (a) nanocages with 2 µm scale, (b) nanocages with 1 µm, (c) nanocages with 0.5 µm scale and (d) nanocages with 0.25 µm scale.

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Figure 4. Charts for illustrating the proportion of the active phases (Mn2O3, Fe2O3 and Fe3O4) in the nanocages with the varied scales (0.25, 0.5, 1 and 2 µm) on the basis of the “RIR” analysis method; the corresponding diagram for each catalyst’s morphologies and chemical structures are also well displayed. (Noting: to visualize the doping structures, the colors (green for Fe and orange for Mn) and positions for doped cations are in special selections, which are different from the models in the DFT calculations)

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Figure 5. H2-TPR profiles of the catalysts: MnOx-FeOy nanocages with 2 µm scale (red line), nanocages with 1 µm (green line), nanocages with 0.5 µm scale (blue line) and nanocages with 0.25 µm scale (pink line).

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Figure 6. XPS spectra for (a) Mn 2p, (b) Fe 2p, and (c) O 1s for each of the catalysts: MnOx-FeOy nanocages with varied scales (0.25, 0.5, 1 and 2 µm).

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Figure 7. Mulliken atomic charges of (a) the active phrase of the MnOx-FeOy with varied scales and (b) the mainly adsorbed nitrate species (bridged nitrate and chelating nitrate) and ammonia species (NH3 and NH4+) over the catalyst surface. 36


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Figure 8. In situ DRIFT spectra of NH3 adsorption and desorption on (a) nanocages with 2 µm scale, (b) nanocages with 1 µm, (c) nanocages with 0.5 µm scale and (d) nanocages with 0.25 µm scale as a function of temperature after the catalysts were exposed to a flow of 500 ppm of NH3 for 60 min.

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Figure 9. In situ DRIFT spectra of NOx desorption on (a) nanocages with 2 µm scale, (b) nanocages with 1 µm, (c) nanocages with 0.5 µm scale and (d) nanocages with 0.25 µm scale as a function of temperature after the catalysts were exposed to a flow of 500 ppm of NO + 5% O2 for 60 min.

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Figure 10. NOx conversion during the NH3−SCR reaction over MnOx-FeOy nanocages with varied scales (0.25, 0.5, 1 and 2 µm) Reaction conditions: 500 ppm of NO, 500 ppm of NH3, 5 vol % O2, N2 as balance gas, total flow rate 250 mL/min, and GHSV = 25 000 h−1.

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Scheme 1. Proposed Scale-Activity Relationship of NH3−SCR of NOx over MnOx-FeOy nanocage catalysts with varied scales (0.25, 0.5,1 and 2 µm).

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Table 1. The Metal Contents for Nanocage Catalysts with varied scales (0.25, 0.5, 1 and 2 µm) Determined by ICP Measurement

Sample

Mn(w.t. %)

Fe (w.t. %)

K (w.t. %)

0.25 µm

28.43

13.91

15.26

0.5 µm

38.67

27.44

3.86

1 µm

33.21

24.23

6.57

2 µm

37.77

27.12

2.57

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Table 2. Specific Surface Area and Pore Volume for Various Catalysts Determined by N2 Adsorption and Desorption Measurement.

Specific Surface Area

Catalyst

Pore Volume (cc/g)

Pore Diameter (nm)

(m2g-1)

2 µm NC

0.362

19.13

93.7

1 µm NC

0.285

19.12

95.2

0.5 µm NC

0.347

3.83

80.3

0.25 µm NC

0.162

19.16

53.6

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TOC

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ScaleâActivity Relationship of MnOx-FeOy Nanocage Catalysts

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ScaleâActivity Relationship of MnOx-FeOy Nanocage Catalysts