Nanoscale

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aResearch Center of Nano Science and Tec

200444, China. E-mail: dszhang@shu.edu.

66136081bSchool of Material Science and Engineering

China

† Electronic supplementary informatioMnCe/CNTs, XPS spectrum of meso-TiO2

catalysts aer stability test. See DOI: 10.1

Cite this: Nanoscale, 2013, 5, 9821

Received 19th June 2013Accepted 28th July 2013

DOI: 10.1039/c3nr03150k

www.rsc.org/nanoscale

This journal is ª The Royal Society of

Design of meso-TiO2@MnOx–CeOx/CNTs with a core–shell structure as DeNOx catalysts: promotion of activity,stability and SO2-tolerance†

Lei Zhang,a Dengsong Zhang,*a Jianping Zhang,a Sixiang Cai,ab Cheng Fang,ab

Lei Huang,a Hongrui Li,a Ruihua Gaoa and Liyi Shiab

Developing low-temperature deNOx catalysts with high catalytic activity, SO2-tolerance and stability is

highly desirable but remains challenging. Herein, by coating the mesoporous TiO2 layers on carbon

nanotubes (CNTs)-supported MnOx and CeOx nanoparticles (NPs), we obtained a core–shell structural

deNOx catalyst with high catalytic activity, good SO2-tolerance and enhanced stability. Transmission

electron microscopy, X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, H2 temperature-

programmed reduction and NH3 temperature-programmed desorption have been used to elucidate the

structure and surface properties of the obtained catalysts. Both the specific surface area and

chemisorbed oxygen species are enhanced by the coating of meso-TiO2 sheaths. The meso-TiO2 sheaths

not only enhance the acid strength but also raise acid amounts. Moreover, there is a strong interaction

among the manganese oxide, cerium oxide and meso-TiO2 sheaths. Based on these favorable

properties, the meso-TiO2 coated catalyst exhibits a higher activity and more extensive operating-

temperature window, compared to the uncoated catalyst. In addition, the meso-TiO2 sheaths can serve

as an effective barrier to prevent the aggregation of metal oxide NPs during stability testing. As a result,

the meso-TiO2 overcoated catalyst exhibits a much better stability than the uncoated one. More

importantly, the meso-TiO2 sheaths can not only prevent the generation of ammonium sulfate species

from blocking the active sites but also inhibit the formation of manganese sulfate, resulting in a higher

SO2-tolerance. These results indicate that the design of a core–shell structure is effective to promote the

performance of deNOx catalysts.

1 Introduction

The emission of nitrogen oxides (NOx) originating from thecombustion of fossil fuels in the industrial and automotivesources has given rise to a variety of environmental and health-related issues, such as photochemical smog, acid rain, ozonedepletion and greenhouse effects.1,2 So far, several well-estab-lished technologies have been employed to eliminate NOx, forinstance, selective catalytic reduction (SCR),1,3–5 catalyticdecomposition6,7 and plasma storage-reduction.8–10 Amongthose methods, the SCR of NOx with NH3 is nowadays consid-ered as the most promising technology for the eliminationof NOx.3–5 V2O5–WO3 (MoO3)/TiO2 catalysts, the typically

hnology, Shanghai University, Shanghai

cn; Fax: +86-21-66136079; Tel: +86-21-

, Shanghai University, Shanghai 200072,

n (ESI) available: EDS analysis of@MnCe/CNTs and TEM images of the039/c3nr03150k

Chemistry 2013

commercial NH3-SCR catalysts, are commonly used with highcatalytic activity at 300–400 �C.11,12 However, some inevitabledisadvantages of the V-based catalysts still remain, such astoxicity of VOx, the narrow operation temperature window andespecially the poor low-temperature catalytic activity.13 There-fore, it is extremely desirable to develop deNOx catalysts withhigh catalytic activity, and high SO2-tolerance within low-temperature regions.14,15

Attracted by their excellent low-temperature activity andinherently environmentally benign characteristics, Mn-basedcatalysts have recently been developed for SCR of NO withNH3.16–22 Unfortunately, Mn-based catalysts are very sensitive tothe presence of SO2 in the feed gas and are severely deactivated bytrace amounts of SO2.23–26 Based on its high oxygen storagecapacity and unique redox properties, ceria has been investigatedextensively,27–29 which is usually introduced as a promoter forenhancing the SO2 resistance of Mn-based catalysts due to theinteraction between MnOx and CeOx, leading to the inhibition offormation of manganese sulfate on the catalyst surface.30,31

Nevertheless, it is still necessary to further improve the resistanceof SO2 for MnOx–CeOx binary metal oxide catalysts.

Nanoscale, 2013, 5, 9821–9829 | 9821

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In the past decades, due to their remarkable structure-dependent properties, accessible surface and excellent chemicaland physical stabilities, carbon nanotubes (CNTs) are consid-ered as a promising candidate for the catalyst support.32–35 Inparticular, it has been reported that the stability of ammoniumsulfate species signicantly declines on the CNT surface and thedecomposition of ammonium sulfate species becomes easier,which is in favor of enhancing the SO2-tolerance.14,35,36 Previ-ously, we developed several in situ supported methods toachieve the high dispersion of MnOx and CeOx on CNTs, andthese catalysts exhibited desirable low-temperature activity.14,19

Unfortunately, a slight agglomeration was still observed aerthe long-term stability test. Due to the high surface energies andweak adhesion of metal and/or metal oxide nanoparticles (NPs)on the support, the metal and/or metal oxide NPs could migrateand induce a serious agglomeration at high temperature,resulting in the deactivation of catalysts.14,37,38 Despite thesegreat efforts, it is still a challenge to develop low-temperaturedeNOx catalysts with a high SO2-tolerance and stability.

Herein, we demonstrate a core–shell structural deNOx cata-lyst with a high SO2-tolerance and stability, that is, CNTs-sup-ported MnOx and CeOx NPs coated with mesoporous TiO2

(meso-TiO2) sheaths. In this design, the meso-TiO2 sheathsprovide an effective barrier to prevent the migration andagglomeration of metal oxide NPs during the deNOx process.Moreover, the meso-TiO2 sheaths not only prevent the genera-tion of ammonium sulfate species from blocking the active sitesbut also inhibit the formation of manganese sulfate. As illus-trated in Scheme 1, the fabrication process of the meso-TiO2@MnCe/CNTs core–shell structure involves two steps. Therst step involves the oxidation of CNTs by concentrated HNO3

treatment to render their surfaces to show chemical affinity forthe supporting of MnOx and CeOx NPs. Subsequently, the CNTsdecorated with metal oxide NPs are then employed as thesubstrate for the in situ coating of meso-TiO2 shells. The cata-lysts were characterized systematically, and their NH3-SCRactivity, stability and SO2-tolerance were also investigated. The

Scheme 1 Schematic illustration of the preparation process of meso-TiO2@M-nOx–CeOx/CNTs.

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meso-TiO2 shells dramatically promote the activity, stability andSO2-tolerance as compared to the uncoated catalyst.

2 Experimental section2.1 Catalyst preparation

The employed multiwall CNTs supplied by Qinhuangdao TaiChi Ring nano products Co. Ltd (China) were reuxed andoxidized with concentrated HNO3 (68 wt%) under stirring for6 h to remove carbon nanoparticles and metal species. Thetreated CNTs were then washed fully with deionized water andethanol, and dried at 80 �C in air for 12 h. All other chemicalswere of analytical grade, purchased from Sinopharm ChemicalRegent Company and used without further purication.

Preparation of MnCe/CNTs nanocomposites. First, thecatalyst was prepared by a modied impregnation method. In atypical synthesis, the puried CNTs (0.40 g) were dispersed in20 mL of mixed aqueous solution with deionized water andethanol (v/v ¼ 1 : 1) under stirring, followed by ultrasonictreatment for 30 min to obtain a suspension. Subsequently,10 mL of mixed aqueous solution of manganese(II) acetate(0.21 g) and cerium(III) nitrate (0.063 g) was added to thesuspension under continuous ultrasonic treatment for 30 min.Then the solvent was evaporated slowly at 60 �C. Aer that, theresulting solid mixture was dried at 100 �C for 12 h and calcinedat 500 �C for 4 h with a heating rate of 3 �C min�1 in N2

atmosphere with a ow rate of 80 mL min�1. The obtainednanocomposites are denoted as MnCe/CNTs.

Preparation of meso-TiO2@MnCe/CNTs nanocomposites.For the meso-TiO2 coating of MnCe/CNTs core–shell structures,120 mg of MnCe/CNTs nanocomposites were redispersed in240 mL of mixed aqueous solution with tetrahydrofuran (THF)and dimethylformamide (DMF) (v/v ¼ 5 : 1) under stirring, fol-lowed by ultrasonic treatment for 1 h to obtain a suspension.Then 0.5mL of titanium butoxide was dissolved in 50mL of THF.The THF–DMF solution of MnCe/CNTs was then vigorously stir-red and the THF solution of titanium butoxide was addeddropwise simultaneously. A precipitate was formed gradually,which was taken as evidence that TiO2 had completely coated onthe surface of MnCe/CNTs. Aer 1 h of stirring, a mixture of H2O(0.5 mL) and THF (10 mL) was added to ensure the completehydrolysis of the titanium butoxide precursor, and the mixturewas sonicated for an additional 30 min. The resulting precipitatewas then collected by centrifugation and washed with THF andabsolute ethanol three times, respectively. Finally, the obtainedproduct was dried at 120 �C for 12 h and calcined at 450 �C for 2 hwith a heating rate of 1 �C min�1 in N2 atmosphere with a owrate of 80 mLmin�1. The organic components in the as-preparedTiO2–MnCe/CNTs structure, including THF, DMF and butanolcan be removed by a temperature-programmed calcinationprocess in N2 to generate a mesoporous TiO2 layer. The obtainednanocomposites are denoted as meso-TiO2@MnCe/CNTs.

2.2 Catalyst characterization

The morphological and structural information of the catalystswere characterized by transmission electron microscopy (TEM,

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JEOL JEM-200CX) and high-resolution TEM (HRTEM, JEOLJEM-2100F). X-Ray energy-dispersive spectra (EDS) weremeasured to obtain the components of the samples using anInca Energy 200 TEM system from Oxford Instruments. X-Raydiffraction (XRD) experiments were determined with a RigakuD/MAS-RB X-ray diffractometer employing Cu-Ka (40 kV, 40 mA)radiation. N2 adsorption–desorption isotherms were obtainedat 77 K using an ASAP 2020M Micromeritics apparatus. Prior toN2 adsorption, the samples were degassed at 300 �C for 4 h. Thespecic surface areas were determined using the Brunauer–Emmett–Teller (BET) equation. The pore volumes, average porediameters and pore size distributions were determined by theBarrett–Joyner–Halenda (BJH) method from the desorptionbranches of the isotherms. X-Ray photoelectron spectroscopy(XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system with Mg-Ka radiation. The whole spectra(0–1100 (1200) eV) and the narrow spectra of all the elementswith high resolution were both recorded by using RBD 147interface (RBD Enterprises, USA) through the AugerScan 3.21soware. Binding energies were calibrated by using thecontainment carbon (C 1s ¼ 284.6 eV). H2 temperature-pro-grammed reduction (H2-TPR) was performed on a Tianjin XQTP5080 auto-adsorption apparatus. Prior to H2-TPR experiment,50 mg of samples was treated with N2 with a total ow rate of30 mL min�1 at 300 �C for 30 min, then cooled down to roomtemperature in the N2 atmosphere. Finally, the reactortemperature was raised to 800 �C at a constant heating rate of10 �C min�1 in H2 (5 vol%)/N2 with a ow rate of 30 mL min�1.The H2 consumption during the experiment was monitored by aTCD. NH3 temperature-programmed desorption (NH3-TPD) wasperformed on the same Tianjin XQ TP5080 auto-adsorptionapparatus. Prior to the NH3-TPD experiments, the samples(150 mg) were pre-treated at 300 �C in a ow of He (30 mLmin�1) for 0.5 h and cooled to 100 �C under He ow. Then thesamples were exposed to a ow of NH3 at 100 �C for 1 h, fol-lowed by He purging for 0.5 h. Finally, the temperature wasraised to 900 �C in He ow at a rate of 10 �C min�1.

2.3 Catalytic performance tests

The NH3-SCR activity tests were performed in a xed-bed quartzreactor with an inner diameter of 8 mm under the followingtypical reaction condition: [NO]¼ [NH3]¼ 500 ppm, [O2]¼ 3 vol%, [SO2] ¼ 200 ppm (when used), N2 balance. 0.4 g of catalysts(crushed and sieved to 40–60 mesh) was used for each SCRactivity test. The total ow rate of feed gases was 210 mL min�1

and thus a gas hourly space velocity (GHSV) of 10 000 h�1 wasobtained. The concentration of the feed gases and the effluentstreams were analyzed continuously by a KM9106 ue gasanalyzer. All data were recorded at the chosen temperature from100 to 450 �C, aer the SCR reaction reached a steady state. NOconversion was calculated according to eqn (1):

NO conversion ð%Þ ¼ ½NO�in � ½NO�out½NO�in

� 100% (1)

where [NO]in and [NO]out indicated the inlet and outletconcentration at steady-state, respectively.

This journal is ª The Royal Society of Chemistry 2013

3 Results and discussion3.1 Characterizations of the catalysts

The morphological structure of the samples and the dispersionof metal oxides on the CNTs before and aer the meso-TiO2

coating were investigated by TEM and HRTEM. As illustrated inFig. 1A and B, the TEM images of MnCe/CNTs clearly exhibit auniform dispersion of discrete metal oxide nanoparticles on thesurface of CNTs. Likewise, the EDS spectrum (Fig. S1, ESI†) ofMnCe/CNTs also conrms the presence of Mn and Ce elements,which suggests Mn and Ce species effectively anchor on thesurface of CNTs. The inset of Fig. 1A reveals that the discretemetal oxide nanoparticles have an average particles size of4.4 nm. Fig. 1C and D show typical TEM images of the meso-TiO2@MnCe/CNTs. The images clearly indicate that theuniform TiO2 sheaths cover both the CNTs andmetal oxide NPs,and no extra TiO2 particles are agglomerated and scatteredaround the MnCe/CNTs. The EDS spectrum (Fig. 1E) of meso-TiO2@MnCe/CNTs also conrms that the MnCe/CNTs havebeen coated by the TiO2 layers. The thickness of TiO2 layersobtained from HRTEM (Fig. 1F) is about 7 nm. Additionally,almost no lattice fringes of TiO2 layers are visible in the HRTEMimage (Fig. 1F), which indicates that the TiO2 layers areconstituted by the amorphous phase. The HRTEM image alsoreveals that the metal oxide nanoparticles have no obviouschange aer being coated with the TiO2 sheaths.

XRD was performed to determine the chemical compositionsand phases of all samples, and the obtained patterns of thesamples are shown in Fig. 2A. The XRD pattern of meso-TiO2@MnCe/CNTs is very similar to that of MnCe/CNTs. Asshown in Fig. 2A, the diffraction peaks at 26.5, 43.16, 54.2 and77.6� are ascribed to the characteristic reections of CNTs. Thesharp and strong characteristic diffraction peaks of CeO2

around 2q 28.4 and 32.8� are clearly observed. The diffractionpeak around 34.8� is ascribed to the characteristic diffractionpeaks of MnO; and moreover, the diffraction peak of Mn2O3

(58.7�) is also present. Those results are in good consistencywith our previous studies.14,19 The diffraction peaks of titaniumoxide species have not been observed, which may be attributedto the overlap of diffraction peaks of anatase and CNTs orsuggest the TiO2 layers are completely amorphous. On the basisof the aforementioned HRTEM results, it can be induced thatthe TiO2 layers are completely amorphous.

To compare the change of specic surface area and porestructure before and aer coating with the TiO2 layers, N2

adsorption–desorption was performed. Fig. 2B demonstratesthe N2 adsorption–desorption isotherms and pore size distri-bution (inset) of MnCe/CNTs and meso-TiO2@MnCe/CNTs. Asillustrated in Fig. 2B, all the adsorption isotherms of thesamples show typical type-IV curves with distinct type-H3hysteresis loops according to the denition of IUPAC, indicatingthe presence of mesopores. For MnCe/CNTs, the mesoporescould be attributed to the internal cavities of CNTs and thespaces in between the bundles of nanotubes. Aer coating bymeso-TiO2 layers, the mesopores could be correlated to both theinternal cavities of the CNTs and meso-TiO2 sheaths. Table 1summarizes the specic surface areas, pore volumes and pore

Nanoscale, 2013, 5, 9821–9829 | 9823

Fig. 1 (A and B) TEM images and size distribution of metal oxide nanoparticles (inset) of MnCe/CNTs; (C and D) TEM images, (E) EDX spectrum and (F) HRTEM image ofmeso-TiO2@MnCe/CNTs.

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sizes of the samples. The BET surface area ofmeso-TiO2@MnCe/CNTs is 156.9 m2 g�1, which is higher than that of MnCe/CNTs(136.0 m2 g�1). Moreover, meso-TiO2@MnCe/CNTs presents alarger pore volume (0.447 cm3 g�1) than that of MnCe/CNTs(0.273 cm3 g�1). Both of the BJH pore size distributions of thecatalysts reveal a narrow distribution centered at 5 nm, corre-sponding to the internal cavities of CNTs. However, somemesopores at small pore size (<5 nm) are observed in the poresize distribution of meso-TiO2@MnCe/CNTs, and these smallpores make a contribution to the greater surface area. Themesoporous feature of TiO2 sheaths is benecial to theadsorption of reactants and necessary to allow the reactants toaccess the catalyst, which is evidenced by the catalytic perfor-mance of meso-TiO2@MnCe/CNTs.

XPS measurements were employed to investigate the atomicconcentrations and element chemical state of the layers close tothe surface of catalysts. Fig. 3 illustrates the obtained XPSspectra of Mn 2p, Ce 3d and O 1s, respectively, and the corre-sponding surface atomic concentrations and the relativeconcentration ratios are presented in Table 2. For meso-TiO2@MnCe/CNTs, the surface concentration of Mn and Ce arefound to be lower than that of the uncoated sample, which

9824 | Nanoscale, 2013, 5, 9821–9829

indicates that Mn and Ce are within the metal oxide layers, sothat the corresponding intensity of photoelectrons are lower.32

In general, the mean-free path of photoelectrons in solids isonly 1–2 nm, suggesting that metal oxide particles on CNTs arecovered bymeso-TiO2 layers. This result is in good conformity tothe TEM analysis. In the case of Mn 2p3/2 spectra (Fig. 3A), theMn 2p3/2 can be divided into three characteristic peaks, whichcan be attributed to Mn2+ (640.4 eV), Mn3+ (642.0 eV) and Mn4+

(644.1 eV) by performing peak-tting deconvolutions, respec-tively.14,25 From Table 2, it is clear that the molar concentrationof Mn4+ on meso-TiO2@MnCe/CNTs (43.3%) is higher than thaton the MnCe/CNTs (39.7%). Previous studies have demon-strated that manganese species with higher oxidation state aremore active for redox reactions over manganese-based cata-lysts.39,40 It has been found that Mn4+ could promote theoxidation of NO to NO2, which is benecial to enhancing thelow-temperature SCR activity.41 In addition, the Mn 2p spectraof meso-TiO2@MnCe/CNTs shi slightly to higher bindingenergy, indicating an interaction of manganese oxide speciesand TiO2 layers.

As shown in Fig. 3B, the Ce 3d XPS spectra of the two cata-lysts could be tted into ten peaks: vo (880.5 eV), v (882.5 eV), v0

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Fig. 2 (A) XRD patterns, (B) nitrogen adsorption–desorption isotherms and thesize distribution curves (inset) of the catalysts: (a)meso-TiO2@MnCe/CNTs and (b)MnCe/CNTs.

Fig. 3 XPS spectra of (A) Mn 2p, (B) Ce 3d and (C) O 1s of the catalysts: (a)meso-TiO2@MnCe/CNTs and (b) MnCe/CNTs.

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(884.9 eV), v0 0 (888.8 eV), v0 0 0 (898.3 eV), uo (899.0 eV), u(901.1 eV), u0 (903.5 eV), u00 (907.5 eV) and u0 0 0 (916.6 eV), cor-responding to ve pairs of spin–orbit doublets.19,20,42 Theseseries of peaks labelled v and u refer to the Ce 3d5/2 and Ce 3d3/2spin–orbit components, respectively. The spin–orbit splitting is18.4 eV, which is well in accordance with the previous study.32

The peaks denoted by vo, v0, uo and u0 are associated with thecharacteristic peaks of Ce3+ species, whereas those denoted by v,v0 0, v0 00, u, u0 0 and u0 0 0 are assigned to Ce4+ species.20 The Ce 3dXPS spectra reveal the coexistence of Ce3+ and Ce4+ species.Aer coating by meso-TiO2 layers, the molar concentration ofCe4+ is enhanced. Besides, a slight shi in the Ce 3d spectratowards higher binding energies can be observed in the coatedsample. These results indicate that the TiO2 shell has a stronginteraction with cerium oxide species, and thus leads to theenhancement of catalytic activity.

Fig. 3C presents the O 1s XPS spectra of the two catalysts.The O 1s XPS spectra are tted into two peaks assigned to the

Table 1 Surface area and pore characteristics of catalysts

CatalystBET surfacearea/m2 g�1

meso-TiO2@MnCe/CNTs 156.9MnCe/CNTs 136.0

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lattice oxygen species at low binding energy (529.6–530.1 eV)and the chemisorbed oxygen species (O2

2� or O� belonging todefect-oxide or hydroxyl-like group) at high binding energy(531.3–532.0 eV), donated as Ob and Oa, respectively.41 As shownin Fig. 3C, a distinct increase of Oa has been observed aercoating by meso-TiO2. From Table 2, it can be seen that the

Pore volume/cm3 g�1 Pore size/nm

0.447 7.60.273 11.7

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Table 2 The surface atomic concentrations of Mn, Ce, O, Ti and the relative concentration ratios

Catalyst Mn (%) Ce (%) O (%) Ti (%) Mn4+ (%) Ce4+ (%) Oa (%)

meso-TiO2@MnCe/CNTs 0.7 0.3 16.6 2.3 43.3 69.9 70.1MnCe/CNTs 1.2 0.5 9.8 — 39.7 62.5 68.7

Fig. 4 (A) H2-TPR and (B) NH3-TPD profiles of the catalysts: (a) meso-TiO2@MnCe/CNTs and (b) MnCe/CNTs.

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molar concentration of Oa on the meso-TiO2@MnCe/CNTs(70.1%) is higher than that on the MnCe/CNTs (68.7%). It isdemonstrated that the chemisorbed oxygen species are moreactive than the lattice oxygen species, attributed to their highermobility than that of lattice oxygen.43 Previous reports havefound that the presence of NO2 in the fumes is benecial to theSCR reaction on the catalysts.44–46 Therefore, the higher relativeconcentration ratio of Oa/(Oa + Ob) is favorable to the NH3-SCRreaction, resulting from the promotion of the oxidation of NO toNO2 and the subsequent facilitation of the “fast SCR” reac-tion.41,47 In addition, larger amounts of surface hydroxyl-likegroups are created by TiO2 sheaths, which act as Brønsted acidsites to adsorb NH3 and form NH4

+. During the NH3-SCRprocess, the formed NH4

+ can react with the NO2 adsorbednearby to produce N2 and H2O.48

As shown in Fig. S2 (ESI†), the Ti 2p XPS spectrum consists ofdouble peaks (Ti 2p1/2 464.1 eV and Ti 2p3/2 458.3 eV).Compared to the pristine TiO2 reported by the previous studies,the Ti 2p1/2 and Ti 2p3/2 peaks of coated TiO2 layers shi tolower binding energies, indicating that the bonding environ-ment of the coated TiO2 layers has changed.43,49 The bindingenergy values reveal the coexistence of Ti(III) and Ti(IV). Thisresult indicates that the TiO2 shell has a strong interaction withthe MnCe/CNTs core, which may play a synergistic role in theNH3-SCR reaction.

As is well known, the redox property of catalysts has a crucialrole in the catalytic cycle of NH3-SCR of NO. H2-TPR was appliedto investigate the reducibility of the catalysts before and aercoating bymeso-TiO2 shells. Fig. 4A presents the H2-TPR prolesof uncoated MnCe/CNTs and meso-TiO2@MnCe/CNTs. Asillustrated in Fig. 4A, the H2-TPR prole of MnCe/CNTs presentstwo reduction peaks around 345 and 431 �C, corresponding tothe reduction of highly dispersed manganese oxide and ceriumoxide species, respectively.20,50 By contrast, aer coating bymeso-TiO2 sheaths, the reduction peaks of manganese oxide(366 �C) and cerium oxide (442 �C) species move slightly to ahigher temperature region, which probably results from aninteraction of TiO2 sheaths and manganese oxide and ceriumoxide species. This result is in good conformity to the XPSanalysis. However, the area of the reduction peak for meso-TiO2@MnCe/CNTs is larger than that of MnCe/CNTs. It isgenerally accepted that the area of the reduction peak is posi-tively correlated with the consumption of H2. Previous studieshave demonstrated that a broad reduction peak of Ti4+ to Ti3+

for pristine TiO2 was observed in the range of 400 to 600 �C, butin the MnOx/TiO2 composites, the reduction peak of Ti4+ to Ti3+

for TiO2 shied to lower temperature due to the interaction oftitania and manganese oxide.17,18 Therefore, for meso-TiO2@MnCe/CNTs, the larger reduction peaks are attributed tothe overlap of the peaks for TiO2 reduction and those for

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manganese oxide and cerium oxide species. The above resultssuggest the interaction between manganese oxide, cerium oxideandmeso-TiO2 sheaths plays a synergistic role in the reducibilityof the catalysts, which could enhance the activity of the samplesin the NH3-SCR reaction.

Based on the mechanisms of NH3-SCR of NO, the adsorp-tion and activation of NH3 on the surface acid sites of thecatalysts are generally viewed as a primary process in the NH3-SCR of NO. To investigate the inuence of the TiO2 layer on thesurface acid amount and strength of the catalysts, NH3-TPDwas performed and the obtained NH3-TPD proles of themeso-TiO2@MnCe/CNTs and MnCe/CNTs are shown in Fig. 4B. TheTPD prole of MnCe/CNTs shows two desorption peaks: theNH3 desorption peak centered at 144 �C assigned to weak acidsites and that centered at 800 �C attributed to strong acidsites.14,51–53 Whereas, for meso-TiO2@MnCe/CNTs, threedistinct desorption peaks are observed: the NH3 desorptionpeak centered at 176 �C assigned to weak acid sites, onecentered at 355 �C originating from medium acid sites, and a

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peak higher than 800 �C attributed to strong acid sites. Ourprevious work has demonstrated that nitric acid-treated CNTscould supply strong acid sites above 800 �C.35 Thus, thedesorption peak higher than 800 �C can be attributed to strongacid sites on the CNTs. It has been demonstrated that NH3

molecules coordinated to Lewis acid sites are more thermallystable than NH4

+ ions bound to Brønsted acid sites, so it canbe deduced that the desorption peak at low temperature isassigned to NH4

+ ions bound to Brønsted acid sites and thedesorption peaks at high temperature is associated withcoordinated NH3 molecules originating from the Lewis acidsites.54,55 As is well known, the position and area of desorptionpeaks are correlated with the acid strength and acid amount,respectively.14 As compared to the MnCe/CNTs, the low-temperature desorption peak of meso-TiO2@MnCe/CNTsshis to higher temperature by about 32 �C, suggesting thatthe strength of the Brønsted acid sites is enhanced by coatingmeso-TiO2 sheaths. In addition, a new and strong desorptionpeak appears at 355 �C, indicating the creation of medium acidsites aer the meso-TiO2 coating. The NH3-TPD resultsdemonstrate that the coated meso-TiO2 sheaths not onlyenhance the acid strength but also raise acid amount, whichmay be benecial to the NH3-SCR reaction.

3.2 Catalytic performance

The conversion of NO as a function of reaction temperature inthe NH3-SCR reaction on the two catalysts is depicted in Fig. 5.As illustrated clearly in Fig. 5, the meso-TiO2@MnCe/CNTsdemonstrates a higher NO conversion and a more extensiveoperating temperature window than that of MnCe/CNTs underidentical operating conditions. The uncoated MnCe/CNTsshows a narrow operating temperature window for the NH3-SCRreaction and the maximum NO conversion is 92%. It isremarkable to note that the NO conversion is dramaticallydecreased when the operating temperature is beyond 325 �C,indicating that the MnCe/CNTs has a poor thermal stability.Due to the weak adhesion of metal oxide NPs on the CNTssupport, the metal oxide NPs can migrate and induce theagglomeration at high temperature, resulting in the

Fig. 5 NH3-SCR performance of the catalysts. Reaction conditions: [NO] ¼ [NH3]¼ 500 ppm, [O2] ¼ 3 vol%, N2 balance, and GHSV ¼ 10 000 h�1.

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deactivation of catalysts. Aer coating with meso-TiO2 layers, bycontrast, the temperature window is signicantly broadenedand the maximum NO conversion is enhanced distinctly,especially at the high temperature region. The temperaturewindow for >90% NO conversion ranges from 225 to 400 �C andthe maximum NO conversion is enhanced from 92 to 99%.Based on above results, it can be concluded that the meso-TiO2

sheaths have effectively prevented the migration and aggrega-tion of the metal oxide NPs in the high-temperature region,resulting in enhancement of the thermal stability of the catalyst.The diversity of catalytic activity over the catalysts can beattributed to the core–shell structure of meso-TiO2 coatedMnCe/CNTs. The N2 sorption analysis indicates that the meso-TiO2@MnCe/CNTs has a larger specic surface area, which is infavor of the adsorption of reactants. The NH3-TPD resultsdemonstrate that medium acid sites around 355 �C have beengenerated and the acid strength has been enhanced aer themeso-TiO2 coating, which can facilitate the adsorption andactivation of NH3, resulting in better performance in the SCRreaction. From the XPS analyses, it is clearly observed that thereis an interaction of themeso-TiO2 layers, MnOx and CeOx, whichmay be responsible for the catalytic cycle in the NH3-SCR reac-tion. Moreover, the enhanced chemisorbed oxygen species canpromote the oxidation of NO to NO2 and thus the SCR activity ofmeso-TiO2@MnCe/CNTs is greatly enhanced by facilitating the“fast SCR” reaction. Based on these favorable properties of thecore–shell structure of meso-TiO2 coating, the meso-TiO2@MnCe/CNTs catalyst exhibits excellent performance inthe NH3-SCR reaction.

As an important indicator to evaluate the performance ofcatalysis, the catalytic stability of the catalysts before and aermeso-TiO2 coating was investigated. Fig. 6A illustrates thestability tests of the catalysts as a function of time at a typicaltemperature of 300 �C. The NO conversion on meso-TiO2@MnCe/CNTs is maintained during the test period. Incontrast, the activity of MnCe/CNTs gradually decreases withtime. As compared to the uncoated catalyst, the meso-TiO2@MnCe/CNTs presents a better stability under the iden-tical reaction conditions. In order to clarify the differentbehavior of the catalysts during the stability test, the TEMimages of catalysts undergoing the stability test are shown inFig. S3 (ESI†). It is worth noting that the meso-TiO2@MnCe/CNTs does not exhibit a morphological change aer the stabilitytest at 300 �C for 25 h (Fig. S3A, ESI†). However, in sharpcontrast, the metal oxide NPs are observed to aggregate in theabsence of TiO2 sheaths aer the stability test at 300 �C for only20 h (Fig. S3B, ESI†). These results indicate that the meso-TiO2

sheaths can serve as an effective barrier to prevent the aggre-gation of metal oxide NPs during the stability test, resulting inenhancement of the stability of catalyst.

In practical applications, even though passing through thedesulfurization device, the exhaust fumes still contain traceamounts of SO2 (about 100 ppm),56 which can induce catalystpoisoning and deactivation.57 Therefore, the impact of SO2 onthe SCR activity of the catalysts was investigated. Fig. 6Bdepicts the catalytic performance of the catalysts, as a functionof time in the presence of 200 ppm SO2 at a typical temperature

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Fig. 6 (A) Stability test and (B) SO2-tolerance study of the catalysts at 300 �C.Reaction conditions: [NO] ¼ [NH3] ¼ 500 ppm, [O2] ¼ 3 vol%, [SO2] ¼ 200 ppm(when used), N2 balance, and GHSV ¼ 10 000 h�1.

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of 300 �C. As shown in Fig. 6B, in the absence of SO2, the NOconversion over meso-TiO2@MnCe/CNTs is 98%, anddecreases slightly to 95% aer introducing 200 ppm SO2, thengradually recovers to the initial value and remains unchangedduring the test period. By contrast, the presence of SO2 in thefeed gas induces a dramatic decrease of NO conversion overthe MnCe/CNTs by 11%. Aer eliminating SO2 from the feedgas, the conversion of NO over the MnCe/CNTs is graduallyrestored to a certain extent, but is less than the initial value,and nally returned to 87%. According to previous reports, SO2

leads to catalyst poisoning and deactivation, which is usuallyassociated with the following two aspects.57,58 Firstly, ammo-nium sulfate species are generated by the reaction of SO2 andNH3 and deposited on the catalyst surface, blocking the activesites of the catalyst surface. Meanwhile, as the reductant in theNH3-SCR process, NH3 is consumed by SO2, instead of reactingwith NO. The deactivation of the catalyst caused by thisprocess is reversible. Secondly, active phases of the catalystcan be sulfated to form stable sulfated species, which leads toan irreversible deactivation. As compared to the uncoatedcatalysts, the meso-TiO2@MnCe/CNTs shows a better resis-tance to SO2. These results indicate that the effective resis-tance to SO2 appears to be achieved by meso-TiO2 overcoating.Therefore, it can be concluded that the meso-TiO2 sheaths notonly prevent the generation of ammonium sulfate speciesfrom blocking the active sites but also inhibit the formation ofmanganese sulfate.

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4 Conclusions

In summary, we have successfully fabricatedmeso-TiO2@MnCe/CNTs with a core–shell structure as a high performance deNOx

catalyst. The meso-TiO2@MnCe/CNTs displays better NH3-SCRactivity, a more extensive operating-temperature window,higher stability and SO2-tolerance than that of the uncoatedcatalyst. The excellent catalytic performance of meso-TiO2@MnCe/CNTs can be attributed to the uniform core–shellstructure. The porous feature provides a larger surface area toadsorb reagents, resulting in a higher catalytic activity; whilethemeso-TiO2 sheaths serve as an effective barrier to prevent theaggregation of metal oxide NPs and thus enhance the thermalstability. Moreover, the interaction among the manganeseoxide, cerium oxide, CNTs and TiO2 sheaths not only preventthe generation of ammonium sulfate species from blocking theactive sites but also inhibit the formation of manganese sulfate,resulting in a high SO2-tolerance. The present study demon-strates that the ne design of special morphology and structureat the nanoscale level offers a general strategy to promote theperformance of catalysts.

Acknowledgements

The authors acknowledge the support of the National NaturalScience Foundation of China (51108258); the Science andTechnology Commission of Shanghai Municipality(13NM1401200 & 11NM0502200), the Doctoral Fund of Ministryof Education of China (20123108120018) and the ShanghaiFirst-Class Discipline Construction in Colleges and Universities.We thank Mr W. J. Yu and Mr Y. L. Chu from the Analysis andTest Center of SHU for help with the TEM and SEM measure-ments. The authors would like to thank Dr K. Zhang from theAnalysis and Test Center of NERCN for help with the HRTEMmeasurements.

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