Preparation and Investigation of Iron−Cerium Oxide Compoundsfor NOx ReductionXueyan Hou,† Junning Qian,† Lulu Li,† Fan Wang,† Bin Li,*,†,‡ Fenglang He,† Minguang Fan,†

Zhangfa Tong,† Lihui Dong,*,†,‡ and Lin Dong‡

†Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry andChemical Engineering, Guangxi University, Nanning 530004, P. R. China‡Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing University, Nanjing 210093, P. R. China

*S Supporting Information

ABSTRACT: The work was focused on investigating how Fe-doped CeO2 composites affect thephysicochemical properties in relation to the NO + CO reaction. The coprecipitation method wasadopted to prepare CeO2−FeOx mixed oxides. The NO + CO reaction as well as temperature-programmed reduction, N2 physisorption, oxygen storage capacity, X-ray photoelectron spectroscopy, X-ray diffraction, and in situ Fourier transform infrared spectroscopy were used to characterize the resultingsamples. These results implied that the appropriate doping amount of FeOx would cause an obviouschange on the CeO2 structure and properties. Also, the catalyst with the ratio of Ce:Fe = 1:0.35 showedthe best catalytic properties. In addition, during calcination, adding a few FeOx molecules could benefitthe formation of low-valence Fe to a larger extent. This synergetic effect contributed to the formation ofa larger number of Fe2+ and oxygen vacancies.

1. INTRODUCTION

Recently, with the rapid development of social economy, hazehas drawn worldwide attention, which is mainly composed ofsulfur dioxide (SO2), nitrogen oxides (NOx), and respirableparticulate matter. Among them, NOx is more harmful to theenvironment because it not only has a widespread pollutionsource but greatly facilitates photochemical smog and acidrain.1,2 Generally, it is found that the synchronous eliminationof carbon monoxide (CO) and nitric oxide (NO) caneffectively remove these pollutants.3,4 In the past few years,noble-metal catalysts have been widely reported in the catalyticreduction of NOx because of their redox ability.5,6 However,the high cost restricts their use. Thus, it is imperative todevelop an inexpensive metal to replace the noble metal.Cerium oxide (CeO2) has attracted a great deal of attention

in the reduction−oxidation reaction because of its excellentcompetence in terms of oxygen storage and release,7 whereaspure CeO2 is poor in thermal stability and readily sintered athigh temperatures.8,9 Integrating other kinds of metal ions intoa lattice of ceria is a common way of enhancing its thermalability.10−15 The oxygen exchange ability of an Fe-modifiedCeO2 system improves significantly compared with bareCeO2.

16 Most importantly, iron(III) oxide (Fe2O3) is anunparalleled dopant with rich resources, cheap prices, andnonpolluting features. Yao et al.17 investigated the impact ofvarious preparation methods of CuO−CeO2 catalysts on NOreduction by CO. Their result indicated that the impregnationmethod showed optimal activity because of the largestconcentration of surface oxygen vacancies, the outstanding

ability of reduction, and the largest number of Cu+ species.Wang et al.6 reported that adding FeOx could strengthen theactivity and selectivity of gold/titanium dioxide in NOreduction with CO. Gu et al.18 studied the reducibility andredox stability of Fe2O3 in the presence of CeO2. According tothe study results, the existence of CeO2 improved both ironand cerium oxides in terms of reducibility on the surface and inbulk because of nanosize effects.Despite of the fact that some studies on the catalytic

reduction of NO on cerium mixed oxides had been studied, toour knowledge, there is no systematic study about how loadingiron species impact the cerium mixed oxides during the NO +CO reaction. On that account, many CeO2−Fe2O3 compositesof which the Ce/Fe atomic ratios are different are synthesizedand applied in the NO + CO reaction. Their structures arecharacterized by virtue of X-ray diffraction (XRD), N2-physisorption Brunauer−Emmett−Teller (BET), hydrogentemperature-programmed reduction (H2-TPR), oxygen storagecapacity (OSC), X-ray photoelectron spectroscopy (XPS), andin situ Fourier transform infrared (FT-IR). The catalyticperformances of CeO2−Fe2O3 composites in NO + CO arealso tested. Moreover, how the structure affects the property isalso studied from the point of view of molecules, through theseCeO2−Fe2O3 catalysts.

Received: July 26, 2018Revised: November 11, 2018Accepted: November 15, 2018Published: November 15, 2018

Article

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2. EXPERIMENTAL SECTION

2.1. Catalyst Preparation. CeO2 and iron−cerium mixedoxides of which the content of iron was 30, 35, 40, 45, and 50mol % (labeled as CeFe 30, CeFe 35, CeFe 40, CeFe 45, andCeFe 50, respectively) were prepared by the coprecipitationmethod. A requisite amount of Fe(NO3)3·9H2O together withCe(NO3)3·6H2O was dissolved in water. The mixed solutionwas dropped into an excessive amount of ammonia solution ata slow speed, followed by intense stirring until the pH was10.0. The resulting solution kept stirring and aging for around4.5 and 15 h, accordingly. Then pH 7.0 was reached byfiltration and washing. Subsequently, this solid underwentdrying for around 12 h at a temperature of 60 °C. Later, 4 hwas employed for calcination in a muffle stove at 500 °C inflowing air.2.2. Catalyst Characterization. 2.2.1. XRD Measure-

ment. Measurement of the XRD patterns was conducted by aD/MAX-RB X-ray diffractometer (Rigaku, Japan) by virtue ofCu Kα (λ = 0.15418 nm) radiation. Coverage of the 2θ scanswas 10−80° at a 8° min−1 scan rate. It had a 40 kV acceleratingvoltage and a 40 mA applied current.2.2.2. N2-Physisorption BET Measurement. A Micrometrics

TriStar II 3020 analyzer was employed to record the structuralcharacteristics of samples relying on N2 adsorption at −196 °C,and the methods of BET and Barrett−Joyner−Halenda (BJH)were conducted for the pore distribution of the surface area.Before analysis, about 0.1 g of the catalyst sample underwent 2h of pretreatment in a mixture of N2 and He at 300 °C.2.2.3. XPS Measurement. A Thermo ESCALAB 250

electron spectrometer with high performance was adopted tocarry out XPS analysis by virtue of Al Kα radiation (1,486.6eV) at 150 W accelerating power. Calibration of all bindingenergies (BEs) was performed by adventitious C 1s under284.6 eV.2.2.4. H2-TPR Measurement. A thermal conductivity

detector (TCD) was used for getting H2-TPR results using aU-type quartz tube in a gas atmosphere. Before reduction, 50mg of the sample was heated for 1 h in N2 (50 mL min−1) at110 °C. Then the sample was cooled to room temperatureunder N2 and changed into a stream of 7.03 vol % H2/Ar (10mL min−1) for 30 min. Subsequently, the sample accepted aheat treatment with increasing temperature to 800 °C in N2 ata heating rate of 10 °C min−1. The TCD was then adopted formeasuring the H2 consumption.2.2.5. OSC Measurement. The samples were tested using a

DAS-7000 capacitor with a TCD current of 100 mA. Then, thesamples were warmed from room temperature at a flow rate of30 mL min−1 in an O2 atmosphere at 10 °C min−1 and stablymaintained at 500 °C for 1 h to completely oxidize thesamples. Subsequently, the temperature was lowered to roomtemperature, the mixture was switched to 5% H2/N2 for 1 h atroom temperature, and the program sample was set to becooled from 950 °C to room temperature at 10 °C min−1.Then the TCD signal value was recorded.2.2.6. In Situ FT-IR Adsorption (FT-IR) Measurement. A

Nicole IS50 spectrometer was used to collect FT-IR spectra atroom temperature using a mercury cadmium telluride detectorthat operates in a wavenumber range of 400−4000 cm−1

(number of scans, 32). The collection of empty IR cell spectrawas conducted under NO and/or CO atmosphere(s) underdifferent target temperatures. Ce−O−Fe catalysts wereassembled in a quart IR cell and accepted 1 h of pretreatment

at 300 °C under a N2 atmosphere. With the temperature beingreduced to room temperature, the sample exposed itself to astream of CO−Ar (10% CO) and/or NO−Ar (5% NO) at 5.0mL min−1 for 30 min. Studies on the desorption/reaction werecarried out via heating species. Later, spectra of the samples atvarious target temperatures were recorded. All spectra werefinally obtained by subtracting the corresponding settingreference.

2.3. Catalytic Activity. Measurement of these samplecatalytic properties for reducing NO by CO was performed atsteady state, which involved a feed stream equipped with asettled element, 10% CO, 5% NO, and 85% He. A total of 50mg of the sample went through filtration in a quartz tubefollowed by 1 h of pretreatment in a N2 stream under 110 °C,aimed at eliminating impurities. These samples were cooled toambient temperature with mixed gases, and they wereperformed under various temperatures at 37000 mL g−1 h−1.The results of NO conversion and N2 selectivity were recordedfor each catalyst, accordingly.

3. RESULTS AND DISCUSSION3.1. XRD. Figure 1 presents XRD patterns in relation to

pure CeO2, Fe2O3, and cerium−iron samples. Pure CeO2 and

Fe2O3 samples show fluorite CeO2 and hematite phases,respectively. The diffractogram patterns of CeO2 at 28.6°,33.1°, 47.5°, and 56.4° correspond to the (111), (200), (220),and (311) crystallographic planes, respectively.19 According toFigure 1, all samples are determined to be in the fluorite CeO2phase (JCPDS 34-0394), and no diffraction peaks exhibit theFe2O3 structure, revealing the absence of crystallized ironspecies. Besides, the diffraction peak presents a slight shift tohigher angle compared to the pure CeO2 sample, revealing theformation of smaller-sized CeO2.

17 Iron species are able to leadto shrinking of the lattice of CeO2 because the ionic radii of theCe4+ (∼0.97 Å) and Fe3+ (∼0.68 Å) ions are different.20

Because of the absence of Fe2O3 peaks and no shift of CeO2peaks, it can be inferred that a portion of iron is integrated inCeO2 for forming cerium−oxygen−iron species, and the otherportions are presented in a dispersed state.

Figure 1. XRD patterns in relation to CeO2, Fe2O3, and differentcerium−iron mixed oxides.

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3.2. Structural Characterization (N2-Physisorption).As can be seen from Figure 2a, the CeFe 35 and CeFe 40

samples present IV-type isotherms coupled with H3-typehysteresis loops. These have narrow, slit, or platelike particlesassociated with the mesopores. That is, these samples containmesopores (2−50 nm).21 Pure CeO2 and other samplespresent IV-type isotherms coupled with H2-type hysteresisloops, which possesses a mesoporous-structure-like wormholeas well as an interstitial mesopore by IUPAC. Furthermore,these samples have a uniform mesopore size distribution (ascan be seen from Figure 2b).Moreover, Table 1 displays the structural properties of these

samples. In contrast with pure CeO2, doping Fe3+ into theCeO2 lattice contributes to the rise of specific surface areasexcept CeFe 50. Also, with the addition of iron, the specificsurface area gradually decreases because of the FeOx

agglomerate (the particles are small, not detected by XRD).In addition, the pore volume increases with increasing dopingof iron, which benefits interaction with the reactants as well asfurther enhances the catalytic properties, which is greatlyassociated with the crystallite size. This also means thatincorporating Fe3+ into the CeO2 lattice can result in aneffective improvement on the structural properties. Especially,CeFe 35 possesses the largest pore volume.

3.3. XPS Analysis. XPS was carried out for furtherinvestigation on these catalysts in terms of their surfacecompositions and elementary oxidation states. Given that XPSanalysis suffers from changing effects, adventitious carbon(284.6 eV) was used to calibrate the BE scale. Figure 3presents the XPS spectra of Ce 3d and Fe 2p for thesecatalysts. Obviously, Ce 3d of a CeO2-based material has acomplicated XPS such as Ce 4f hybrids with ligand orbitals; inthe meantime, valence 4f orbitals are fractionally occupied.22,23

Thus, this is necessary to help the peaks experience multipletsplitting into doublets, and considering the final state effects,each doublet is characterized by a further structure.23

According to Figure 3a, eight elements equipped withassignments are fitted. Two span-orbital mutiplet groups,namely, 3d3/2 and 3d5/2, correspond to u and v, respectively,which expand in the BE range of 878−920 eV. Bands that getthe labels u′ and v′ stand for the 3d104f1 primary electronicstate and are associated with Ce3+. Bands that get the labels u‴and v‴, u′′ and v′′, and u and v are associated with Ce4+.24

Moreover, evaluation of the relative content of Ce3+ is basedon the area ratio among u′ and v′ and others, as in thefollowing equation:25

S SS S

Ce (%)( )

1003 u v

u v= ′ + ′

∑ +×+

According to Table 2, on the surface of the samples, cerium’schemical valence primarily presents the Ce3+ oxidation state,and there is a small part of Ce4+ as well. The strong Fe 2p3/2peak at ∼710.8 eV belongs to Fe2+ species. Yet, there is a smallpart of Fe3+ because the shape appears to be asymmetric.6,26

The calculation of the relative content of Fe2+ is based on thearea ratio of Fe2+ to Fe2+ + Fe3+.17 In Table 2, it is surprising tosee that the relative content of Ce3+ presents the same orderwith that of Fe2+ in terms of these cerium−iron catalysts.Obviously, as the relative contents of Ce3+ and Fe2+ increase,the surface oxygen vacancy will come into being.A major band at 529.2 eV (O′) is formed, relying on a

typical lattice oxygen, which is bound to metal cations. Theshoulder of the BE as high as 531.1 eV (O′′) is treated aschemisorbed oxygen.27 Because of the difference in theelectronegativity between iron (1.83) and cerium (1.12), thelattice oxygen in the sample is shifted to low energy comparedwith pure CeO2, which indicates that the lattice oxygen afterdoping is unstable and easily escapes to the surface, easilyforming defects and oxygen vacancies.28 As shown in Table 2,notably, chemisorbed oxygen, which accepts quantification by

Figure 2. (a) N2 adsorption/desorption isotherm. (b) Distributioncurves of the pore size.

Table 1. Sample Structural Properties BJH of Cerium−IronMixed Oxides and OSC

sampleBET surface area

(m2 g−1)pore volume(cm3 g−1)

mean porediameter (nm)

OSC(mmolg−1)

CeFe30

85.4 0.177 8.304 0.239

CeFe35

73.8 0.381 20.639 0.383

CeFe40

71.3 0.274 15.398 0.310

CeFe45

67.8 0.167 9.904 0.279

CeFe50

61.9 0.168 10.882 0.245

CeO2 64.9 0.139 8.595 0.137Fe2O3 15.9 0.149 37.544

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combining the area ratio of O′′/(O′ + O′′) in the CeFe 35catalyst, shows the highest ratio value, proving the positiveeffect of a molar ratio of Ce:Fe = 1:0.35 for the generation ofoxygen vacancies on oxide surfaces. Typically, lattice oxygen isnot chemisorbed by oxygen actively.29 Therefore, high

chemisorbed oxygen is also considered to be beneficial forincreasing the OSC.

3.4. H2-TPR Analysis. The TPR profiles of these samplesshow similar shape (Figure 4). For pure Fe2O3, the α and β

peaks show the two-step reduction of Fe2O3 → Fe3O4 andFe3O4 → Fe0 at about 428 and 724 °C, respectively.30

Moreover, the reduction peak of CeO2 is at 514 °C. However,the CeO−FeOx sample shows reduction peaks at approximate377, 530, and 660 °C. When pure CeO2 and Fe2O3 arecompared, it can be seen that the second weak peak at 514 °Cis caused by the surface Ce4+ reduction.5 The FeOx species ischaracterized step by step with the first and third one. Clearly,the rise of the iron content will cause a transfer of thereduction peaks to lower temperatures in a comparison of pureFe2O3, suggesting that the reducing ability of the sample showsa rising trend compared with that of pure Fe2O3. Cerium andiron species strongly interact with each other because theentrance of iron ions into the CeO2 lattice can lead to a chargeimbalance together with a lattice distortion within the structureof CeO2, which facilitates the generation of oxygen vacancies.

15

Interestingly, the position of the first peak of CeFe 35 is thelowest in all catalysts, indicating that its reductive property isthe best among all of the samples.

3.5. OSC. On the basis of the quantitative calculation of H2consumption, the OSC is analyzed. As shown in Figure 5 andTable 1, the OSC of doped CeO2 is higher than that of pureCeO2, and the OSC increases slowly with an increase of thecontent of iron. Up to a ratio of Ce:Fe = 1:0.35, the OSC is thestrongest. In order to maintain the electronegativity of thecrystal, more Ce4+ is converted into Ce3+ after Fe2O3 is dopedinto CeO2, because of the doping of low-valent ions into high-

Figure 3. XPS spectra of (a) Ce 3d, (b) Fe 2p, and (c) O 1s.

Table 2. Surface Atomic Ratios by XPS

sample Ce3+/(Ce3+ + Ce4+) Fe2+/(Fe2+ + Fe3+) O′′/(O′ + O′′)CeFe 30 16.43 45.01 26.12CeFe 35 34.33 67.75 35.35CeFe 40 17.25 56.67 32.60CeFe 45 10.04 46.65 30.38CeFe 50 9.20 38.84 31.07CeO2 17.88 51.77Fe2O3 80.21 17.58

Figure 4. H2-TPR profiles in relation to cerium−iron mixed oxides.

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valent ions, and oxygen vacancies are formed because ofoxygen desaturation, and then the bulk oxygen rapidly diffusesto the catalyst surface. Therefore, the increase of the oxygenmobility in the crystal enhances the reduction of CeO2

31,32 andoxygen storage and release capacities. Thus, CeFe 35 has thestrongest OSC.3.6. Catalytic Property (NO + CO Reaction). Figure 6

shows NO conversion and N2 selectivity for these catalystsbased on different temperatures. As for all catalysts, NOconversion rises sharply from 150 to 200 °C, which is due tothe intense synergy function in relation to the oxygen and Fe2+

vacancies. However, N2 selectivity rapidly declines as NOmainly forms N2O at 175−200 °C. While the reactiontemperature is continually increased, the N2 selectivity of thecatalysts remains slightly down except for CeFe 35 and CeFe40. When the temperature is between 200 and 240 °C, the N2selectivity rises sharply, indicating that the amount of N2O(byproduct) is decreasing and gradually transforming into N2.Finally, when the temperature further increases to 250 °C,both the NO conversion and N2 selectivity of all samples areclose to 100%. That finding is superior to other research in theelimination of NOx.

33−35 It should be noted that the CeFe 35sample has the best NO conversion and N2 selectivity amongall catalysts. Its NO conversion reaches 100% at only 175 °C,while the N2 selectivity reaches 100% at 240 °C. That isattributed to the increased reducibility, enhanced BET surface,and sufficient oxygen vacancies for the CeFe 35 catalyst, whichis also confirmed by the XPS and OSC results.3.7. NO and/or CO Interaction with the CeFe 35

Catalyst. In situ FT-IR adsorption simulated under varioustemperatures is illustrated in Figure 7, which clearly explainsthe adsorption and reduction properties of CeFe 35. As can beseen in Figure 7a, the band at 1014 cm−1 is caused by thebridging bidentate nitrate exhibiting an obvious NO2symmetric vibration. The band at 1600 cm−1 is associatedwith the bridging bidentate nitrate presenting a NOstretching model.24 The two bands result from the chelatingbidentate nitrates appearing at 1218 and 1540 cm−1,respectively.21,36,37 The band at 1238 cm−1 is associated withthe linear nitrite, which presents NO2 asymmetric vibration.These two bands observed respectively at 1282 and 1501 cm−1

belong to the monodentate nitrate. The peak that refers to thebridging monodentate nitrate can be seen at 1574 cm−1.35

Also, in the course of heat treatment, it is interesting to findthat the bridging bidentate nitrate, linear nitrite, andmonodentate nitrate decrease gradually, indicating that thesenitrate species gradually decompose under high temperature.Meanwhile, two peaks that correspond to the chelatingbidentate nitrate can be observed at 1218 and 1540 cm−1,and the trend increases at first and then decreases as thetemperature increases to 275 °C, causing difficulty in completedesorption/transformation/decomposition. Above all, it ex-hibits that these species are rearranged instead of beingdesorbed or decomposed. Also, the chelating bidentate nitrateundergoes a relatively stable adsorption at a high temperatureunder a NO atmosphere.21 Similarly, Figure 7b shows theinteraction of CO with the representative catalyst. Accordingly,bands resulting from various vibration modes in relation tomonodentate carbonates, namely, ν(C−O), νs(CO3

2−), andνas(CO3

2−), can be observed at approximately 1033, 1298, and1435 cm−1, respectively. Bands that belong to hydrogencarbonates presenting the δ(C−O···H) vibration mode[νs(COO−) and νas(COO−)] happen at approximately1391, 1569, and 1595 cm−1.24,37,38 The band at 1778 cm−1

belongs to carbonate. What is more, thermal desorption andreduction due to rising temperature lead to the disappearance

Figure 5. H2 consumption of a series of cerium−iron mixed oxides.

Figure 6. Change trends of the (a) NO conversion (%) and (b) N2selectivity (%). Feed composition: 5% NO, 10% CO, and 85% He byvolume; GV = 37000 mL g−1 h−1.

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of the band at 1033 cm−1. Furthermore, the increasingtemperature enhances the intensity of the bands belonging tothose carbonates, which is because CO and CO2 prefer easyadsorption on reduced-state Ce3+ sites instead of oxidized-stateCe4+ sites.38,39 The peak belonging to the bidentate formateoccurs at 1356 cm−1, which is formed mainly because CO

molecules interact with surface hydroxyls.40 In addition, theasymmetrical peak about 2100 cm−1 is associated with thesurface physical adsorption of CO species at a lowertemperature (which can be seen in the inset). When thetemperature reaches 150 °C, peaks appear symmetrical,revealing the appearance and overlap of a new peak. Thepeak that is associated with CO molecule chemical adsorptionon Fe2+ sites can be observed at 2112 cm−1, revealing thepresence of some Fe2+ species in the samples at ambienttemperatures. This is in agreement with the XPS results, whichreveals that Fe2+ is present on the surface of the samples. Onthe one hand, gaseous CO2 at 2112 cm−1 increases with anappearance of Fe2+−CO species at 150 °C. On the other hand,a new band that is formed by coordinating COx into reducedceria can be seen at 1056 cm−1. These indicate that, by thesynergistic interaction between iron oxide and oxygenvacancies, ceria can promote iron oxide to be reduced fromFe3+ to Fe2+ and ceria to provide oxygen at the interface.17 Ingeneral, CO molecules adsorbed on Fe0, Fe2+, and Fe3+ willrespectively lead to the formation of peaks that exhibitcharacteristic vibrational frequencies below 2130, 2080−2160,and 2150−2220 cm−1, and Fe2+−CO species present the moststable adsorption at ambient temperature.24,36

According to Figure 7c, CO + NO coadsorption wasconducted in situ FT-IR spectra under simulated conditions ofreaction for an in-depth study on the reaction mechanism.There are several kinds of adsorbed NO species on the surface,which is similar to single NO adsorption results. There is nocarbonate species below 150 °C, indicating that the surface isabsorbed by NO molecules completely, and they cover theactive sites. Because of its unpaired electron, the adsorption ofCO species is hindered.17 Therefore, the CO gas onlyundergoes a slow reaction with adsorbed NO species. Whenthe temperature gradually rises to 150 °C, the adsorbed NOspecies disappear, except for the chelating bidentate nitrate,after being desorbed, arranged, and reacted with free gas CO.Consequently, CO adsorption occurs as a result of exposure ofthe catalyst’s active sites.21 In this case, carbon dioxide isgenerated as the temperature rises, and Fe3+ is reduced to Fe2+

by CO, contributing to the generation of a larger number ofoxygen vacancies. Also, the inner reaction (Ce3+ + Fe3+ ↔ Ce4+

+ Fe2+) can produce more Fe2+, which is consistent with XPS.Interestingly, with rising temperature up to 275 °C, the peak ofFe2+−CO becomes weak, which indicates that Fe0 has beenformed. Thus, this infers that Fe3+ is reduced to Fe2+ at firstand then further reduced to Fe0 by CO, which is consistentwith the H2-TPR result. In addition, the color of the samplehas changed from brown to gray-black, further illustrating thatFe0 is generated. It is reported that N2 is a diatomichomonuclear molecule and thus does not have any IR-activemode.17 At the same time, N2O is able to oxidize Fe2+ to Fe3+,suggesting the role of Fe2+ in facilitating N2O transformationinto N2.

17 In other words, the NO + CO reaction relying onthese catalysts can happen between gaseous molecules andadsorbed species in the whole reaction temperature range,which indicates that the Langmuir−Hinshelwood and Eley−Rideal (E−R) mechanisms are both observed in the gas-phasereaction. However, it is mainly the E−R mechanism thatcauses most of the CO to exist in the form of a gas phase.35,36

3.8. Possible Mechanism for CO + NO Reaction. For allof the above results, the catalyst exposed to an atmosphere ofmixed CO and NO gases follows the E−R mechanism. Forfurther verification, the effect of different sequences of reactive

Figure 7. In situ FT-IR spectra about the adsorption of NO (5%)and/or CO (10%) on the CeFe 35 catalyst from 50 to 275 °C: (a)NO; (b) CO and the peak of pure CO adsorbed (inset); (c) CO +NO.

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gas flowing into the catalyst is explored. Detailed informationcan be found in the Supporting Information, pp S1−S3. It canbe observed that the activity of the catalyst is increased bypretreatment with CO, whereas the activity is reduced bypretreatment with NO. That is because the oxygen vacanciesare the main active sites, especially for the decomposition ofNO.41

Figure 8 shows that NO molecules preferentially adsorb onthe catalyst surface for the formation of many kinds of nitriteand nitrate species relying on the unpaired electron, which

limits CO species adsorption.21 According to reports in theliterature, the N−O band can be divided into N and O atomsby surface oxygen vacancies.41−43 Thus, adsorbed NO speciescan be dissociated, which exposes active sites with risingtemperature to a small amount of CO species. Meanwhile, Fe3+

can be reduced by CO, producing more Fe2+ species. COmolecules can react with the O atoms produced by thedissociated of NO species on the surface oxygen vacancies toform CO2. Also, residual N radicals by the dissociated of NOspecies are either recombined with NO to form N2O orcombined with themselves to generate N2. Eventually, part ofthe Fe2+ species are reduced to Fe0 by CO, and the other partof the Fe2+ species are oxidized to Fe3+ by N2O, forming aportion of N2. The rest of N2O is reduced by CO completely.Thus, all of accessory substance N2O is further reduced to N2.In conclusion, the intense synergistic effect between Fe2+ andoxygen vacancies can accelerate the reaction of NO and COand improve the selectivity and activity of the CeO2−FeOxcatalyst. Therefore, it is confirmed that the synergy of the Fe2+

and surface oxygen vacancies promotes CeFe 35 in terms ofthe best catalytic activity.

4. CONCLUSIONS

In the study, these Ce−Fe catalysts exhibit outstanding NOreduction by CO by virtue of a simple coprecipitation method,which features high economy. Through various character-izations, it is revealed that iron can positively affect the CeO2structure and redox properties. Some conclusions have beendrawn based on the analysis above:(1) The best catalytic activity is exhibited when the iron

molar percentage is 35%. CeFe 35 has the lowest reductiontemperature, maximum H2 consumption, and strongest OSC,which can generate more Ce3+ and oxygen vacancies.(2) The synergistic effect, which results from Fe2+ and

surface oxygen vacancies over these CeO2−FeOx catalysts,plays an important role in the NO + CO model reaction.(3) NO + CO reaction over a CeFe 35 composite follows

the E−R mechanism.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.iecr.8b03472.

Digital plots of the catalytic properties for CO and NOpretreatment in a CeFe35 catalyst, separately (DOC)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: binli@gxu.edu.cn (B.L.). Tel.: +86 0771-3233718.Fax: +86 0771-3233718.*E-mail: donglihui2005@126.com (L.D.). Tel.: +86 0771-3233718. Fax: +86 0771-3233718.

ORCID

Bin Li: 0000-0002-4080-3268Lihui Dong: 0000-0002-1184-7317Lin Dong: 0000-0002-8393-6669NotesThe authors declare no competing financial interest.

Figure 8. Mechanism of the NO + CO reaction over the CeFe 35sample.

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■ ACKNOWLEDGMENTS

This work was supported by the National Nature ScienceFoundation of China (Grants 21507014, 21663006, and21763003) and Program for Science and TechnologyDevelopment Plan of Nanning (Grant 20163146).

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