Spectroscopic Investigation of Iron Substitution in EuCoO3:Related Impact on the Catalytic Properties in theHigh-Temperature N2O DecompositionY. Wu,† C. Dujardin,† P. Granger,†,* C. Tiseanu,‡ S. Sandu,§ V. Kuncser,§ and V.I. Parvulescu‡

†Universite Lille Nord de France, Unite de Catalyse et de Chimie du Solide (UCCS), CNRS UMR 8181, Cite Scientifique,Batiment C3, 59655 Villeneuve d’Ascq, France‡Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, 4-12 Regina Elisabeta Boulevard,Bucharest 030016, Romania§National Institute of Materials Physics, P.O. Box MG 7, Bucharest-Magurele 077125, Romania

*S Supporting Information

ABSTRACT: This study is dedicated to a combined spectroscopic approachfor investigating bulk versus surface reconstructions that originated duringhigh-temperature catalytic applications on EuCo1−xFexO3 and to their relatedimpact on the catalytic properties. By way of illustration, the decomposition ofnitrous oxide (N2O) has been selected as a case study achieved in complexfeed gas composition in the presence of NOx and water vapor, which canspeed up surface and bulk modifications at 900 °C with related impact on thereactivity of N-containing species. Particular attention was paid to the appear-ance of Co2+ species in different stages of the preparation, especially afterreaction. While XPS cannot discriminate the nature of those species withoutany ambiguity, the combination of Mossbauer and photoluminescencespectroscopy can help in understanding changes in the local environmentof europium, iron, and cobalt. The impact of those surface changes on thekinetic parameters of the decomposition of N2O has been examined.

1. INTRODUCTION

Perovskite-based materials can be suitably used for high-temperaturecatalytic applications because of their rather low specific surfaceareas. This must be considered to be a serious obstacle for low-temperature applications.1 Their catalytic performances are oftencurrently related to electronic disturbances and anionic conduc-tivity, which are closely influenced by the composition of theexposed atmospheres.2−4 Recently, it has been found thatLaCoO3 exhibits a rather high activity for N2O decompositionabove 700 °C, especially from exhaust gas of nitric acid plants,which would likely depend on the extent of La segregation intoLa2O3, as suggested from X-ray photoelectron spectroscopy(XPS) analysis.5 It seems obvious that surface reconstructionscan occur at high temperature, with partial segregation oflanthanum oxide, which can dramatically change the catalyticactivity. Significant improvements can be obtained on substoi-chiometric lanthanum solids5 eventually after iron substitutionin the B-site partially replacing Co3+ cations. However, it is usuallydifficult to understand such segregation phenomena for havinga proper description of the real impact of the perovskite struc-ture on the catalytic properties with cooperative effects inducedby partial substitutions on the B or A site of ABO3 perovskitestructures.Previous authors have already reported the potential interest

of europium-based perovskites in the catalytic oxidation of

toluene.6 Partial replacement of Co by Eu induces a rate enhance-ment in methane combustion7,8 ascribed to structural effectsbecause Eu incorporation leads to a decrease in the Goldschmidttolerance factor, increasing the reducibility of the cationslocated in the B site. As a matter of fact, 151Eu as a probephotoluminescent element can be useful for visualizing localstructural changes. By combining Mossbauer spectroscopy forthe characterization of iron species, a more precise approachcould be expected then helping to get more evidence of thecooperative effect of A and B elements in the catalyticdecomposition of N2O and the preservation of this effectaccording to the nature of thermal treatment conditions. In thisstudy, we have attempted to correlate spectral features obtainedon EuCo1−xFexO3 from various techniques (XPS, photo-luminescence (PL), and Mossbauer spectroscopy) to catalyticproperties. Particular attention was paid to the impact ofexposure to isothermal reaction conditions at 900 °C. Indeed,previous investigations by examining aged samples from XPSsuggested partial segregation of single oxides after high-temperature thermal treatment.5 However, such conclusioncould not be univocal because no prediction can be drawn from

Received: March 4, 2013Revised: June 3, 2013Published: June 10, 2013

Article

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XPS analysis on the local chemical environment of Co and Laspecies, especially at high temperature when the perovskitestructure can be destabilized. The extent of such reconstruc-tions on iron-doped LaCoO3 samples is closely related to thedegree of iron substitution. Oxygen mobility and related desorp-tion is strongly enhanced on highly Fe-substituted samples,which locally destabilize the surface structure with subsequentextraction and ultimately partial segregation of Co and La singleoxides. We have examined the impact of those surface recon-structions on EuCo1−xFexO3 with bulk Co and Fe composition,which should not lead to significant extraction of Co preserv-ing the specific properties of perovskite structure and relatedimpact on the catalytic performances.

2. EXPERIMENTAL2.1. Catalyst Preparation and Characterization. Perovskite-

based catalysts EuCo1−xFexO3 (x = 0, 0.5, 1) were synthesizedaccording to a conventional sol−gel method involving a citrateroute9 using aqueous solutions of Eu(NO3)3·5H2O, Co-(NO3)2·6H2O, and Fe(NO3)3·9H2O with a molar citric acid/(Eu + Co + Fe) ratio of 1. The solid was obtained afterevaporation, drying overnight at 80 °C, thermal decompositionof nitrate precursors at 200 °C, and finally calcination in air at900 °C for 8 h.Specific surface area was calculated from nitrogen physisorp-

tion measurements at −196 °C using a FlowSorb III apparatus.Powder X-ray diffraction (XRD) was performed at room tem-perature (RT) using a Bruker AXS D8 Advance diffractometerworking in Bragg−Brentano geometry and equipped with aLynxEye Super Speed detector. Data were collected with CuKα (λ = 0.154 nm) in the 10−100° 2θ range with a 0.02° 2θstep. The Fullprof Suite program10,11 was used for Rietveldrefinement. The Thompson−Cox−Hastings pseudo-Voigtfunction was chosen for peak profiles. LaB6 was used asstandard to derive the instrument resolution. An isotropic sizeparameter of Gaussian character was refined to take intoaccount the contribution of sample in the broadening of Braggpeaks.XPS experiments were performed on an AXIS Ultra DLD

Kratos spectrometer equipped with a monochromatized aluminumsource for excitation (150 W) and charge compensation. Theanalyzer was operated in a constant pass energy mode (Epass =40 eV). All binding energies were referenced to the C 1s corelevel at 285 eV. Peak area was estimated after subtracting thebackground according to the procedure suggested by Shirley.12

Special attention was paid to the quantification of surface Coand Fe concentration, taking into account the Auger contri-butions from Fe and Co, respectively.PL measurements were carried out at 20 °C using a FluoroMax

4 spectrofluorometer (Horiba) operated in both fluorescenceand phosphorescence mode. The repetition rate of the xenonflash lamp was 25 Hz, the integration window varied between300 ms and 3 s, the delay after flash was fixed at 0.03 ms, andup to 200 flashes were accumulated per data point. The slitswere varied from 1 (emission) to 29 nm (excitation). PL decayswere measured by using the ‘decay by delay’ feature of thephosphorescence mode. 151Eu and 57Fe Mossbauer spectrawere recorded in air and fitted by using the full Hamiltonian forthe hyperfine perturbation of 151Eu nuclear levels and thesimplified perturbation algorithm for 57Fe, as previouslyreported.13

2.2. Catalytic Activity Measurements. Catalytic decom-position of N2O was studied in a fixed-bed flow reactor using

0.7 g of catalyst with a total flow rate of 15 L h−1 within atemperature range 100−900 °C (heating rate 2 °C min−1). Thereaction mixture was composed of 0.1 vol % N2O, 5 vol % NO,6 vol % O2, and 15 vol. % H2O and balanced by He. A firsttemperature-programmed reaction experiment (TPR-1) wassystematically performed on all calcined samples, which wereultimately maintained overnight at 900 °C under reactionmixture. After cooling to 100 °C, a second temperature-programmed reaction experiment (TPR-2) was performedunder similar conditions to evaluate changes in the catalyticperformances. The outlet gas concentrations were quantifiedwith a NOx analyzer (Thermo 42i-HL) using a chemilumi-nescence detector for NO and NO2 and a dispersive IRanalyzer for N2O detection (Emerson XStream). A μGC Varian4900 chromatograph was also used to quantify N2 formation.

3. RESULTS

3.1. Bulk and Surface Characterization of EuCo1−xFexO3Catalysts. 3.1.1. X-ray Diffraction Measurements. As illus-trated in Figure 1A, broad X-ray lines characterize the amorphous

structure of the precursors of EuCo1−xFexO3. Subsequent calcinationat 900 °C enhances the crystallization of EuCo1−xFexO3 phases.XRD patterns in Figure 1B are dominated by characteristic

Figure 1. XRD patterns recorded on EuCo1−xFexO3 before calcination(A), after calcination (B), and after reaction at 900 °C (C) with x = 0(a), x = 0.5 (b), and x = 1 (c).

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X-ray lines ascribed to the orthorhombic perovskite structure.Slight segregation of residual Eu2O3 is detected on all of thecalcined samples at 2θ ≈ 28.5°. A continuous shift of the X-raylines toward lower 2θ angles is observed with an increase iniron composition x currently associated with the expansion ofthe unit cell after iron incorporation. Rietveld refinementresults, summarized in Table 1, corroborate this explanationwith a gradual increase in cell parameters from EuCoO3 toEuFeO3. Indeed, Fe

3+ cations have a relatively larger ionic radii(0.645 Å) compared with Co3+ (0.61 Å).14 Therefore, thesubstitution of cobalt by iron in the B-site of the perovskitestructure would provoke the expansion of the unit cell.The average crystallite size was also determined by Rietveldrefinement, taking into account a Gaussian character. It seemsobvious that iron incorporation can significantly improve theresistance to thermal sintering during the calcination, leading toan average crystallite size of ∼47 nm for EuCo0.5Fe0.5O3, whilethat of EuCoO3 is much larger (85 nm).The samples after catalytic reaction submitted to high tempera-

ture (T = 900 °C) were also characterized by XRD. (See Figure 1C.)A similar orthorhombic perovskite structure is observed for allsamples. The cell parameters obtained from Rietveld refine-ment remain constant, underlining a relative good structuralstability under reaction conditions at 900 °C. It is worthwhileto mention that the segregated Eu2O3 previously evidenced oncalcined catalysts is no longer detectable after reaction, thussuggesting a continuation of the reaction of residual Eu2O3 withvery dispersed CoO. An increase in crystallite size associatedwith a decrease in specific surface area detected for all samplesafter reaction seems to support this hypothesis. The improvedcrystallinity is more remarkable on EuCoO3. Its average crys-tallite size increases from 85 to 120 nm during the reaction.Thermal sintering is greatly limited on Fe-containing samples.3.1.2. X-ray Photoelectron Spectroscopy Analysis. Surface

properties of EuCo1−xFexO3 samples at different stages of thepreparation and after aging were studied by XPS analysis. Spectrafor EuCoO3 recorded before/after calcination and after exposureto 900 °C under reactive conditions are compared in Figure 2.For all of the samples, spectral features of Eu 3d photo-peaks reveal the stabilization of Eu3+, as reported elsewhere.15

However, it is obvious that binding energy (B.E.) values of Eu3d5/2 core level vary according to the nature of the thermaltreatment. Indeed, the Eu 3d5/2 photopeak initially located at1135 eV for the precursor shifts after calcination to lower B.E.at 1133.2 eV, which reflects the incorporation of Eu3+ cations,initially stabilized into amorphous Eu2O3 phase, inside theperovskite structure. Let us notice that these spectral featuresremain quasi-unchanged after reaction at 900 °C. The examina-tion of the Co 2p photopeak in Figure 2B provides comple-mentary information. The typical satellite structure at 786.7 eVcharacterizes the presence of Co2+ species in the precursor,

which could be ascribed to the formation of Co(OH)2 accord-ing to the B.E. value of Co 2p3/2 core level at 780.9 eV.16

Subsequent calcination at 900 °C leads to the formation of apure perovskite phase with the characteristic shakeup structureat 790.1 eV (Figure 2B(b)) for Co3+ cations. After reaction at900 °C, a slight growth of the contribution at ∼786.7 eV wouldsuggest a partial formation Co2+. Now regarding O 1s photo-peaks in Figure 2C, different types of surface oxygen can bedistinguished and the spectral fitting data are summarized inTable 2. For EuCoO3 precursor, an intense peak is found at531.8 eV with a shoulder at 529.4 eV. These spectral featuresare quite similar with those reported on Eu2O3.

17 Nevertheless,we cannot rule out a possible contribution from carbonatespecies to the O 1s photopeak because it is also located at∼531.5 eV18 and the surface C/Eu atomic ratio is rather high(Table 3). The weak contribution initially observed at 529.4 eVon the precursor intensifies after calcination at 900 °C andbecomes predominant.This contribution can be ascribed to lattice oxygen O2− of

the perovskite as reported elsewhere.19 The stabilization ofad-NOx species is highlighted after catalytic reaction by anadditional contribution above 532.0 eV.20 The overall surfacecomposition summarized in Table 3 emphasizes the accumu-lation of N-containing species as well as a surface Eu enrich-ment by comparing, respectively, the atomic N/Eu and Co/Euratios irrespective of the thermal treatment procedure, asreported elsewhere.6

XPS analyses on EuFeO3 are collected in Figure 3. Fe 2p3/2photopeak on EuFeO3 located at 710.2 eV characterizes thepresence of Fe2+ cations, with satellite structure at 714.5 eV,likely stabilized as Fe3O4.

21 After calcination at 900 °C, theperovskite structure develops associated with the typical Fe3+

spectral features, as illustrated in Figure 3B(b). Similar Fe 2pphotopeak is observed after catalytic reaction at 900 °C, under-lining the greater thermal stability of iron species comparedwith that of cobalt species. Regarding EuCo0.5Fe0.5O3, nosignificant change seems to occur on the Fe 2p photopeak.(See Figure 4.) Similar electronic disturbances as those pre-viously described on EuCoO3 seem to characterize the Co 2pcore level. Interestingly, it is noticeable that the B.E. value ofthe Co 2p3/2 photopeak remains unchanged after reaction. Theshakeup structure at 786.7 eV intensifies. Semiquantitative analysisreveals the same surface Eu enrichment on EuCo0.5Fe0.5O3 asthat described on the reference EuFeO3 and EuCoO3 samples.More surprisingly is the deviation observed on the atomic Co/Eu and Fe/Eu, which emphasizes a relative Fe enrichment. Thislatter observation could be in contradiction with partial extrac-tion and segregation of Co at the surface. A sharp increase inthe relative intensity of contribution at 532.6 eV is observableparallel to the development of the contribution at 407 eVon the N 1s photopeak. These observations suggest the

Table 1. Structural and Textural Properties of EuCo1−xFexO3 Solids before and after Catalytic Reaction at 900°C.

cell parameters (Å)a

samples thermal treatment a b c crystallite size (nm) SBET (m2/g)

EuCoO3 calcined 5.256 5.371 7.477 85 3.0after reaction 5.256 5.372 7.478 120 1.7

EuCo0.5Fe0.5O3 calcined 5.313 5.486 7.575 47 3.5after reaction 5.315 5.485 7.577 56 2.1

EuFeO3 calcined 5.375 5.595 7.687 45 3.8after reaction 5.375 5.598 7.688 60 2.1

aOrthorhombic structure a ≠ b ≠ c, α = β = γ = 90°

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stabilization of N-containing species as nitrates (see Figure 5),which form during the cooling to RT after TPR-2.22 Anincrease in the atomic N/Eu ratio is clearly discernible.

This comparison shows a stronger accumulation of nitrites/nitrates species on EuCo0.5Fe0.5O3 compared with EuCoO3.

3.1.3. Photoluminescence Spectroscopy. PL confirmed thewell-organized structure of the calcined perovskites. Theemission spectrum of EuFeO3 before calcination was similarbut with a weaker intensity than the spectrum after calcination(Figure S1 of the Supporting Information, SI). The low quan-tum yield of europium emission is related to strong non-radiative Eu3+−Eu3+ interactions, nonradiative energy transferto iron or cobalt, or host absorption in the spectral range ofeuropium emission. Differences were observed, however,between EuFeO3 and EuCoO3 samples before calcination(Figure S2 of the SI). The europium (Eu3+) emission after calcina-tion is very weak and narrower, with a double-peak structure at615 and 619 nm (Figure S3 of the SI), which may be related toa low distorted symmetry at Eu3+ sites (C2v or lower).

3.1.4. Mossbauer Spectroscopy. The compound EuFeO3belongs to the orthorhombic space group Pbnm with a = 0.537 nm,b = 0.560 nm, and c = 0.768 nm (also indicated in Table 1).23

From the magnetic point of view, it is essentially antiferromagnetic(with a Neel temperature of over 427 °C) but shows a weakferromagnetism due to a slight spin canting at the Fe sites.The usual perovskite structure presents a unique iron position(Fe3+ in octahedral configuration) that gives rise to a Mossbauersextet component with a hyperfine magnetic field Bhf of ∼50.8 Tat RT and ∼55 T at low temperatures (below −193 °C).The isomer shift (IS) is ∼0.4 mm/s, whereas the quadrupolecorrection, ε (ε = QS/2 with QS the quadrupole splitting), isnegligible (some 0.01 mm/s). The next TM (transition metal)neighbors around a central Fe are in the second-coordinationsphere (after O), the structure presenting four Fe neighbors at adistance of ∼0.7a and the other two Fe neighbors at a much largerdistance. In EuCo0.5Fe0.5O3, Co may occupy iron positions. Adiscrete distribution of iron sites, with corresponding hyperfineparameters and the probability of occurrence given by the Poissondistribution function, is expected. In the present case, the pro-babilities of having 0 and 1, 2, and 3 and 4 Co neighbors are 31,38, and 31%, respectively. Therefore, the much broader Mossbauersextet belonging to the sample EuCo0.5Fe0.5O3 can be fitted inthe magnetic frozen regime via three sextet components ofrelative spectral areas equating the above-mentioned proba-bilities.Figure 6 presents Mossbauer spectra collected on EuFeO3

samples, as follows: (a) at RT, before calcination; (b) at −268 °C,before calcination; (c) at RT after calcination; and (d) at RTafter reaction. The Mossbauer spectrum of EuFeO3 precursorcollected at RT shows two central paramagnetic components,for example, a doublet with IS of ∼0.35 mm/s and QS of1.00 mm/s, specific to Fe3+ ions in high spin state (S = 5/2)and in very distorted configurations, and a second doublet withIS of ∼1.03 mm/s and quadrupole splitting of 2.6 mm/s,specific to Fe2+ ions, in high spin state (S = 2). The amount ofthe last phase is ∼20 at %. This result seems to be in goodagreement with previous XPS analysis, revealing also Fe2+

species in the EuFeO3 precursor. The lack of the magneticorder at RT for the analyzed precursor could originate either inthe superparamagnetic behavior, in connection with a nano-meter size of nanoparticles, or in a strong decrease in the Neeltemperature, in relation to structural aspects. It is known thatthe Neel temperature TN is very sensitive to the Fe3+−O−Fe3+bond angle, and in the case of distorted configurations (as inthe present case) it could drop consistently from the typicalvalue of more than 427 °C specific to the perovskite structure.24

Figure 2. XPS spectra of Eu 3d (A), Co 2p (B), and O 1s (C) corelevels recorded on EuCoO3 before calcination (a), after calcination(b), and after reaction at 900 °C (c).

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The high QS associated with the Fe3+ ions may give argumentsfor highly distorted Fe configurations in the precursor, leadingto a lower magnetic transition temperature. Mossbauer measure-ments on EuFeO3 precursor at −268 °C (Figure 6b), evidencethe starting magnetic regime with a magnetic transition tem-perature very close to −268 °C, as resulted from the collapsingmagnetic component assigned to Fe3+ species. On the contrary,the calcination induces the right perovskite structure, evidencedin the Mossbauer spectrum (Figure 6c) by the typical RT sextetwith a magnetic hyperfine field of 50.3 T, IS of ∼0.35 mm/s,and negligible quadrupole shift. Also, the Fe2+ speciesdisappears completely by calcination. The same sample afterreaction at 900 °C shows at RT the typical sextet pattern with amagnetic hyperfine field of 50.7 T. The IS and QS values aresimilar to the case of the sample before reaction. No significantelectronic and configurational changes around Fe can bementioned, but the absorption line width becomes evennarrower (0.28 mm/s as compared with 0.34 mm/s). Boththe larger hyperfine field and narrower line width suggest thatthe magnetic transition temperature of the perovskite structureafter reaction is slightly increased as compared with the case ofcalcined sample. This gives evidence of an even better crys-talline structure after reaction at 900 °C, as evidenced fromXRD measurements. The magnetization measurements per-formed on sample EuFeO3 after calcination (with a well-formedperovskite structure) are not suitable to provide information onthe saturation field and spontaneous magnetization due to thelack of saturation even in the highest induction field of theSQUID magnetometer of 7 T (Figure 7). The coercive fieldsobserved from the hysteresis loops are of 950 Oe at −263 °Cand 500 Oe at RT. (See Figure 7.) The last one is consistentwith the presence of the magnetic order above RT, as alsoevidenced by Mossbauer spectroscopy. It is worth mentioningthat calcined EuFeO3 sample consists of nanoparticles withperovskite structure and average size of ∼45 nm. In the casethat they will show a superparamagnetic transition with ablocking temperature TB above RT, the coercive field shoulddecrease versus temperature according to a typical lawof type: Hc = Hc

0(1 − T/TB)1/2. Taking Hc

0 = 950 Oe, a

coercive field of 500 Oe can be reached at RT just with ablocking temperature much higher than 427 °C, namely,higher than the Neel temperature of the perovskite phase.Therefore, we consider that such nanoparticles do not sup-port superparamagnetic transitions at all, extending thisfinding also to the Co containing perovskites of even muchlarger crystallite size.The Mossbauer spectra of different samples of EuCo0.5Fe0.5O3

are shown in Figure 8. The prepared sample EuCo0.5Fe0.5O3 isparamagnetic at RT, similar to the case of EuFeO3 precursor.The Fe2+ ions are present in a much reduced amount (roughly 7%),as compared with EuFeO3 (Figure 8a). The spectrum collectedat −268 °C (Figure 8b) clearly shows the beginning of themagnetic relaxation at a much higher temperature as comparedwith the EuFeO3 precursor. While the Fe substitution by Coions in such orthoferrites generally reduces the Neel temper-ature,23,25 the higher TN observed for the EuCo0.5Fe0.5O3precursor as compared with the EuFeO3 precursor has to berelated to a better crystalline structure of the former. The RTMossbauer spectrum of calcined EuCo0.5Fe0.5O3 sample (Figure 8c)clearly evidences the sole perovskite phase (no trace of Fe2+

phase is observed) reflected in a central doublet with an IS of∼0.34 mm/s and QS of 0.43 mm/s (QS is ∼0.97 mm/s in theas-prepared sample). The low value of QS is a first indicationfor a better crystalline structure of calcined sample as comparedwith the precursor. Such affirmation is also supported by the−268 °C spectrum (Figure 8d), presenting the specific sextet ofthe perovskite structure with much narrow absorption lines,proving the complete magnetic frozen regime. This magneticpattern in the magnetic static regime was successfully fitted bythree magnetic sextets of fitted relative areas, in goodagreement with the theoretically estimated Poisson distribu-tions. However, the corresponding hyperfine magnetic fields of50.5, 46.2, and 40.6 T are slightly lower than those reported forsimilar configurations, proving that the structure of calcinedsample is still not perfect. After reaction at 900 °C, theMossbauer spectra look at first view to be very similar to thecase of calcined sample (Figures 8e,f). However, a carefulquantitative analysis of the hyperfine parameters shows a

Table 2. Semi-Quantitative Analysis of the O 1s Photopeak of EuCo1−xFexO3 Submitted to Various Thermal Treatmentsa

sample thermal treatment spectral fitting of O1s photopeak

EuCoO3 before calcination 529.4 (15.2%) 531.8 (84.8%)calcined 529.0 (54.1%) 531.2 (39.7%) 533.1 (6.2%)after reaction 529.0 (35.9%) 530.9 (24.3%) 532.8 (39.8%)

EuCo0.5Fe0.5O3 before calcination 529.4 (18.5%) 531.7 (81.5%)calcined 529.0 (54.9%) 531.3 (41.6%) 533.4 (3.5%)after reaction 529.0 (26.6%) 530.7 (15.3%) 532.6 (58.1%)

EuFeO3 before calcination 531.4 (100%)calcined 529.2 (54.5%) 531.3 (42.6%) 533.6 (2.9%)after reaction 529.2 (48.1%) 531.0 (27.1%) 532.7 (24.8%)

aIn parentheses: Relative contributions on the overall O1s signal.

Table 3. XPS Analysis of EuCo1−xFexO3 Solids before/after Calcination and after Reaction at 900 °C

before calcination calcined after reaction

Co/Eu Fe/Eu O/Eua C/Eu N/Eu Co/Eu Fe/Eua O/Eu C/Eu Co/Eu Fe/Eu O/Eua C/Eu N/Eu

EuCoO3 0.28 1.08 1.18 0.07 0.23 0.83 0.36 0.24 1.16 0.38 0.17EuCo0.5Fe0.5O3 0.18 0.11 1.47 1.72 0.12 0.13 0.08 0.92 0.39 0.11 0.10 1.68 0.59 0.37EuFeO3 0.21 1.57 1.74 0.05 0.16 0.88 0.35 0.23 1.19 0.37 0.11

aAtomic O/Eu ratio has been calculated taking into account the overall intensity of the O 1s photopeak.

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decreased QS value at RT (0.41 mm/s) and relatively increasedhyperfine fields at −268 °C (51.6, 49.9 and 47.8 T, respectively,both aspects providing evidence of a better crystallineperovskite structure of the sample after reaction as comparedwith the calcined one. In fact, the evolution of the averagehyperfine magnetic field versus temperature (not shown) alsoproves a slightly higher magnetic transition temperature (in therange from −73 °C to RT) in the tested sample. It is tomention that the lower TN in the case of the well-crystallizedEuCo0.5Fe0.5O3 sample (lower than RT), as compared with the

well-crystallized EuFeO3 sample (much larger than RT), is justthe result of the Co substitution and agrees with the mentionedliterature. As a last observation on the EuCo0.5Fe0.5O3 sample,we can underline the higher relative spectral area of the mostexternal sextet (belonging to the Fe configurations with 0 and 1Co neighbors) in the sample after reaction as compared withthe calcined one. (See Figure 8d,f.) This is a direct prooffor partial Co extraction during the test reaction of theFe-containing perovskite.

3.2. Catalytic Performances in the Decompositionof N2O. Similar methodology as described elsewhere5 wasimplemented for the evaluation of the catalytic properties ofcalcined EuCo1−xFexO3 catalysts (TPR-1) and after exposure tothe reaction conditions overnight at 900 °C (TPR-2).Temperature-programmed reaction curves are reported inFigure 9. They demonstrate that calcined EuCoO3 is muchmore active than EuFeO3 with intermediate activity ofEuCo1−xFexO3 in the conversion of N2O. The deviationsobserved on the light-off temperatures recorded on calcinedsamples and after exposure overnight at 900 °C to the reactionmixture also reveal that EuCoO3 is more sensitive todeactivation. As a general trend, a shift of the conversionprofiles to higher temperatures is usually observed from TPR-2experiments, which highlights deactivation phenomena but inlesser extent after iron incorporation to EuCoO3. It isremarkable that the activity is not directly related to thespecific surface area, which also varies to different extents onthe used samples after reaction but more likely on the densityof active cobalt sites and the nature of their local chemicalenvironment. For better clarification on the role of ironincorporation to EuCoO3, rate constants were calculated asdescribed elsewhere,5 which account for the verification of theabsence of significant external and internal diffusion limitationsand a first-order kinetic in the decomposition of N2O. Apparentactivation energies and pre-exponential factors were estimatedfrom the slopes and the intercepts of the Arrhenius plotsrepresented in Figure 10. The estimates reported in Table 4highlight a compensation effect with a sharp decrease in theapparent activation after iron incorporation in conjunction witha lowering of the pre-exponential factor. Interestingly, bothparameters converge toward comparable values on EuCoO3and EuCo0.5Fe0.5O3 exposed overnight to reaction conditions at900 °C. Specific and intrinsic rates have been tentativelycalculated at 575 °C. At this temperature, EuFeO3 exhibits arather low activity that has been neglected for the estimation ofthe intrinsic rate taking also a homogeneous distribution of Coand Fe species in the solid. Hence, further comparisons showthat the value remains unchanged on calcined EuCo0.5Fe0.5O3and after exposition at 900 °C to reaction conditions. Asignificant decrease is noticeable on EuCoO3.

4. DISCUSSIONThe objective of this study was initially to get a more precisedescription of the nature and extent of surface modifications ofperovskite-based materials occurring in the course of high-temperature catalytic reactions. One of the objectives was to getmore relevant correlations with catalytic properties on the basisof estimated kinetic parameters. Such comparisons in thispresent study have been achieved on EuCo1−xFexO3 catalyti-cally active for the decomposition of nitrous oxide up to900 °C.Previous investigations on stoichiometric and nonstoichio-

metric LaCoyFe1−yO3 (y = 1 − x) materials underlined that the

Figure 3. XPS spectra of Eu 3d (A), Fe 2p (C), and O 1s (C) corelevels recorded on EuFeO3 before calcination (a), after calcination (b),and after reaction at 900 °C (c).

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optimization and stabilization of surface properties is a difficulttask, especially at high temperatures, for example, at 900 °C,when the samples are submitted to wet atmospheres.5

A volcano-type behavior was clearly observed versus iron con-tent. Iron incorporation promotes the oxygen mobility, whichoriginates a significant rate enhancement26 due to a fasterpathway for oxygen transfer through the creation of oxygenvacancies in the perovskite lattice. Generally speaking, oxygendesorption is always accompanied by the reduction of B-siteelements.27 Such trend was clearly evidenced from XPS

measurements and still observed in this present study onEuCo1−xFexO3. However, an extensive iron substitution canhave a detrimental effect. Indeed, for solids exhibiting high anionicconductivity, structural reconstructions might take place abovea critical concentration of defects, providing more orderedstructures less active toward the decomposition of N2O.

28 Thisbehavior has been highlighted on Fe-rich LaCoyFe1−yO3 (y = 1 − x).For high Fe content (y < 0.4, or equivalently, x > 0.6), a weakdestabilization of the perovskite structure likely takes place thatprovokes a partial extraction of Co3O4 at the surface furtherinhibiting the decomposition of N2O.

5 In such configuration,XPS is unable to discriminate accurately the existence of Co2+

stabilized in different chemical environments inside the perov-skite structure or as Co3O4 phase.Returning to the most important information reported in this

study, XRD, PL, and Mossbauer spectroscopy measurementsrevealed a greater crystallization after reaction compared withcalcined samples. Interestingly, the presence of bulk Eu2O3

impurities detected on calcined samples disappears during thereaction at 900 °C, suggesting an incomplete intercalation ofCo3+, Fe3+, and Eu3+ cations during calcination to get ultimatelya pure orthorhombic structure. Those impurities are no longerdetected after reaction. Rietveld refinements on EuCo0.5Fe0.5O3

show that the cell parameters remain unchanged aftercalcination and reaction, which also seems to be consistent

Figure 4. XPS spectra of Eu 3d (A), Co 2p (B), Fe 2p (C), and O 1s (D) core levels recorded on EuCo0.5Fe0.5O3 before calcination (a), aftercalcination (b), and after reaction at 900 °C (c).

Figure 5. XPS spectra of N 1s recorded on EuCo0.5Fe0.5O3 (a),EuFeO3 (b), and EuCoO3 (c) after reaction at 900 °C.

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with the absence of significant extraction and subsequentsegregation of single oxides during the reaction, as reportedelsewhere for a high degree of iron substitution.5 This is also inagreement with a lower surface Co concentration estimatedfrom XPS analysis, especially on EuCo0.5Fe0.5O3, for which thestoichiometric Co/Fe ratio is similar to that given by theelemental analysis, suggesting homogeneous distribution of Coand Fe on aged samples. Further comparisons of the profiles forthe Co 2p photopeak can be discussed more accurately bytaking into account the above-mentioned information. As seen,the B.E. of the Co 2p3/2 core level value recorded on theprecursor centered at ∼781 eV with an intense shakeupstructure at ∼786 eV characteristic of CoO strongly differs fromthat recorded on calcined and aged samples after reaction.Indeed, the Co 2p3/2 B.E. at 780 eV and a shift of the shakeupstructure at ∼790 eV typically characterize the predominantformation of Fe3+ cations. However, Mossbauer observationsemphasize the fact that a partial extraction of Co cannot beruled out.The loss of specific surface area after reaction is rather

limited and not related to significant growth of crystallite ofEuCo0.5Fe0.5O3, contrarily to EuCoO3. As compared, the

crystallite size is much lower on the former sample, whichshould correspond to a greater oxygen mobility29 because

Figure 6.Mossbauer spectra of EuFeO3 sample recorded at RT, beforecalcination (a); at −268 °C, before calcination (b); at RT, aftercalcination (c); and at RT, after reaction at 900 °C (d).

Figure 7. Hysteresis loops obtained at −263 °C and RT on calcinedEuFeO3 sample.

Figure 8. Mossbauer spectra of EuCo0.5Fe0.5O3 samples, as follows: atRT, before calcination (a); at −268 °C, before calcination (b); at RT,after calcination (c); at −268 °C, after calcination (d); at RT afterreaction (e); and at −268 °C after reaction (f).

Figure 9. Catalytic decomposition of N2O during temperature-programmed experiments in the presence of NO, O2, and H2O oncalcined catalysts (TPR-1) (full symbol) and after exposition overnightto the reaction mixture at 900 °C (TPR-2) (open symbol): EuCoO3(▲), EuCo0.5Fe0.5O3 (■), and EuFeO3 (◆).

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Royer et al. found a higher resistance to oxygen diffusion onlarger crystallite.30 This could finally explain a more extensivedeactivation on EuCoO3.Regarding the estimates of the kinetic parameters listed in

Table 4, it seems obvious that Co is responsible for the catalyticactivity. The lower apparent activation energies after ironsubstitution especially on calcined EuCo0.5Fe0.5O3 might reflecta greater oxygen mobility and reactivity with subsequentdesorption and connected formation of anionic vacancies.Indirect information can be provided by XPS, especially via theexamination of the N 1s photopeak. Interestingly, a significantcontribution arises at high B.E. values on EuCo0.5Fe0.5O3, muchgreater than that observed on EuCoO3 and EuFeO3,corresponding to nitrates species. The formation of thosespecies is likely due to a significant oxygen species available atthe surface related to reactive oxygen species from N2Odissociation or O2 adsorption interacting with anionicvacancies. Differences also arise in the pre-exponential factoron calcined EuCoO3 and EuCo0.5Fe0.5O3, which might reflect ahigher density of active sites on the former catalyst. While thepre-exponential factor remains unchanged on EuCo0.5Fe0.5O3,after reaction at 900 °C, a significant lowering affects EuCoO3.Such an observation can be compared with previous findingsdemonstrating the dependence on the distance between twoCu−O centers to accelerate the recombination of atomicoxygen species versus the copper content.31 Hence, a lower

value on EuCoO3 could be related to a lower concentration ofCo entities. This seems to be in relative agreement with thelessening of the apparent activation energy value estimatedfrom TPR-2 experiments that could be ascribed to thesubsequent migration of Co impurities segregated at thesurface after reaction overnight at 900 °C.Iron incorporation in the framework of the perovskite lattice

of EuCoO3 induces an expansion of the unit cell volume and asubsequent weakening of the B−O bond.29 Hence, suchproperties seem directly related to catalytic properties basedon the calculation of intrinsic rates expressed by per surfaceCo atom and can correctly explain deactivation phenomenamore accentuated on EuCoO3. A greater crystallizationtaking place during reaction on this last catalyst comparedwith EuCo0.5Fe0.5O3, for which the crystallites size does notvary significantly, would be mostly responsible for a decreasein the intrinsic activity. Hence, while Mossbauer spectro-scopic measurements demonstrate on EuCo0.5Fe0.5O3 apartial Co extraction from the perovskite lattice, it seemsobvious that this process would not depend on the cry-stallite size and does not seems correlated to an extensivedeactivation.

5. CONCLUSIONSThis work deals with a combined spectroscopic approach forstudying bulk and surface reconstructions that might occur inthe course of the catalytic decomposition of nitrous oxide athigh temperature on the perovskite−structured catalystEuCoxFe1−xO3 (with x = 0, 0.5, and 1). Photolumiscence andMossbauer spectroscopies were useful for characterizingeuropium and iron segregated as single oxides in the precursorstate or incorporated as trivalent cations inside the perovskitelattice. XPS was profitably used to get more insight intochanges in the chemical environment of cobalt. Typically, morecrystallized samples were evidenced by XRD, PL, andMossbauer spectroscopy after reaction at 900 °C under a wetatmosphere, accompanied by a loss of specific surface area andadditional incorporation of cobalt species inside the perovskitelattice. Mossbauer spectroscopy data gave direct evidence of theCo extraction after Fe incorporation in EuCoO3, which couldrelate the loss of specific surface area to densification processesafter aging. It is also remarkable that Fe addition improves theresistance to deactivation with a strong attenuation of thethermal sintering. Further comparisons with the kineticparameters (activation energy, pre-exponential factor of therate constant) emphasize the fact that Co species are mainlyinvolved as active sites. A compensation effect is observed onEuCo0.5Fe0.5O3 explained by an enhancement of oxygenmobility due to Fe incorporation and in parallel a lowering of

Figure 10. Arrhenius plots ln k versus 1/T over the N2O conversionrange from 5 to 20% during temperature-programmed experiments inthe presence of NO, O2 and H2O on calcined catalyst (TPR-1) (fullsymbol) and after exposition overnight to the reaction mixture at900 °C (TPR-2) (open symbol): EuCoO3 (▲), EuCo0.5Fe0.5O3 (■),and EuFeO3 (◆).

Table 4. Catalytic Performances in N2O Decomposition on EuCoxFe1−xO3 in the Presence of 5 vol % NO, 6 vol % O2, and15 vol % H2O

T50 (°C)a

activation energy(kJ/mol) pre-exponential factor

specific rate(× 10−5 mol·h−1·g−1)b

intrinsic rate(× 10−2 molec·h−1·surf.

Co at−1)c

calcined after reactiond ΔT50 (°C) calcined after reactiond calcined after reactiond calcined after reactiond calcined after reactiond

EuCoO3 607 654 47 216 167 1.0 × 1014 4.0 × 1010 21.0 9.4 9.8 4.9

EuCo0.5Fe0.5O3 646 672 26 159 165 1.5 × 1010 2.3 × 1010 10.7 6.2 9.0 8.4

EuFeO3 705 750 45 150 170 1.5 × 109 8.1 × 109 3.8 1.9

aT50: temperature corresponding to 50% conversion of N2O.bSpecific reaction rate at 575 °C was normalized by mass of catalyst. cIntrinsic reaction

rate at 575 °C was normalized by mol of cobalt accessible on the surface according to XPS analysis. dAfter exposure to reaction conditions overnightat 900 °C.

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the density of Co sites. Subsequently, thermal aging has nosignificant impact, highlighting EuCo0.5Fe0.5O3 as the moststable and active catalyst likely related to the oxygen mobilityaccompanied by a faster desorption promoted on smallcrystallites.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional photoluminescence (PL) measurements on LaCoO3and LaFeO3. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Tel: +33 320 434 938. Fax: +33 320 436 561. E-mail: pascal.granger@univ-lille1.fr

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The UCCS participates in the Institut de Recherche enENvironnement Industriel (IRENI), which is financed by theCommunaute Urbaine de Dunkerque, the Region Nord Pas-de-Calais, the Ministere de l′Enseignement Superieur et de laRecherche, the CNRS, and European Fund for RegionalDevelopment (FEDER). We gratefully acknowledge the IRENIand the ADEME for financial support through a Ph.D.fellowship (Y. Wu). The support from the Romanian projectsPNII IDEI 275/2011 and 75/2011 is also gratefully acknowl-edged.

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