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Applied Catalysis A: General 457 (2013) 42– 51

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l ho me pag e: www.elsev ier .com/ locate /apcata

ffect of surface decoration with LaSrFeO4 on oxygen mobility andatalytic activity of La0.4Sr0.6FeO3−ı in high-temperature N2Oecomposition, methane combustion and ammonia oxidation

.V. Ivanov ∗, L.G. Pinaeva, L.A. Isupova, E.M. Sadovskaya, I.P. Prosvirin, E.Yu. Gerasimov,.S. Yakovlevaoreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, pr. Acad. Lavrentieva 5, Novosibirsk 630090, Russia

r t i c l e i n f o

rticle history:eceived 28 November 2012eceived in revised form 26 February 2013ccepted 4 March 2013vailable online xxx

eywords:anthanum–strontium–ferrite mixed oxideayer-structured perovskitesitrous oxide decomposition

a b s t r a c t

This article is an attempt to elucidate the role of lattice oxygen mobility in catalytic activity of Sr-substituted ferrites at high temperatures. For this goal, three catalysts with close element (La, Sr,Fe) content but different phases and surface compositions: LaSrFeO4(surface)–La0.4Sr0.6FeO3 (LSF-N),La0.15Sr0.85FeO3–La0.7Sr0.3FeO3 (LSF-C) and LaSrFeO4 have been studied in high temperature reactionsof N2O decomposition, ammonia oxidation and methane combustion. The kinetics of 18O/16O oxygenexchange have been analyzed at 800 ◦C and 0.005 atm oxygen partial pressure, which is closely corre-sponding to the reaction conditions, and the rates of surface oxygen exchange and the coefficient of latticeoxygen diffusion have been evaluated. It allowed us to reveal the direct correlation between the surfaceexchange rate constant and the rate of N2O decomposition. For NH3 oxidation, evidence of the same order

ethane oxidationmmonia oxidationxygen mobility

of the samples activity both in surface oxygen exchange and ammonia oxidation reaction was shown. Inmethane oxidation it was found that activity correlates with the rate of exchange for 20 monolayers ofoxygen atoms indicating the growing influence of lattice oxygen mobility. The results show that improve-ment of catalytic properties can be achieved when synthesizing “LaSrFeO4(surface)–La0.4Sr0.6FeO3”composites. Such composites exhibit increased rate of surface oxygen exchange on retention of highlattice oxygen mobility, which can be attributed to formation of heterostructured interfaces.

. Introduction

Mixed oxides with perovskite structure are of great inter-st for application as catalysts for high-temperature processes,athodes for solid oxides fuel cells, oxygen conducting membranes.ne of the promising group of the catalysts for oxidation reac-

ions is based on La1−xSrxMO3±ı perovskites (M = Mn, Fe, Co, Ni,u) because of flexible electronic properties of transition metal

n octahedral environment, high level of oxygen and electronicobility, as well as good structural and chemical stability. Since

he 70th, from the work of Voorhoeve [1], systematic studies of

he relationship between perovskite composition and its reac-ivity in high-temperature hydrocarbon oxidation were initiated2–5]. It was found that starting from 700 ◦C oxidation processes

Abbreviations: ICDD PDF, International Center for Diffraction Data Powderiffraction File; DDPA, differential dissolution phase analysis; XRD, X-ray diffrac-

ion; XPS, X-ray photoelectron spectroscopy; ASF, atomic sensitivity factor.∗ Corresponding author. Tel.: +7 383 3269515; fax: +7 383 3308056.

E-mail addresses: ivanov@catalysis.ru, dmitryivanov12@yahoo.comD.V. Ivanov).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.03.007

© 2013 Elsevier B.V. All rights reserved.

predominantly obey Mars–van Krevelen model, which includesthe two step redox reactions: the reaction between the activesurface oxygen and the oxidizable reactant and then the reoxi-dation of the catalyst active site by gas phase oxygen [6–9]. Formany perovskite catalysts it is considered that adsorption of themolecular oxygen seems to be the rate controlling step of the reac-tion. However, it has been proposed that at high temperatures amodified Mars–van Krevelen model, so called Ionic Redox model,more accurately represents the oxidation reaction mechanism onLa1−xSrxMnO3 [10,11]. The difference between these two mecha-nisms is that the latter one assumes the oxidation and reductionmay occur at different surface sites, with the oxygen being trans-ported to reduction site though the lattice. In this case one cansuppose that reactivity of perovskites may depend on the rate ofbulk oxygen diffusion. To elucidate this question it is importantto study bulk oxygen diffusion in the reaction conditions. Oth-erwise controversies may arise due to the different state of thecatalyst during the reaction conditions and in the experiments of

oxygen mobility measuring. For example, Вorovskikh et al. stud-ied Sr-substituted cobaltites in methane combustion and foundthat catalytic activity could depend on the rate of bulk oxygen dif-fusion estimated by temperature-programmed oxygen exchange

D.V. Ivanov et al. / Applied Catalysis

Nomenclature

b ratio between surface oxygen atom concentration(1019 atom/m2) and concentration of oxygen atomsin the gas phase (cm−1)

CO2 concentration of oxygen the gas phase (atoms/cm3)C18O2 concentration of 18O2 (mol/l)D* self-diffusion coefficient of 18O in the oxide bulk

(cm2/s)D*fast self-diffusion coefficient of fast-exchangeable 18O in

the bulk (cm2/s)D*/r2 total value of effective rate constant of oxygen self-

diffusion (s−1)D*fast/r2 effective rate constant of fast exchangeable oxygen

self-diffusion in the bulk (s−1)Ei activation energy of the rate of the ith oxygen form

desorption (J/mol)f34 fraction of 16O18O in the gas phasek0

ipre-exponential factor of the rate of the ith oxygenform desorption (cm2 atom−1 s−1)

K coefficient of oxygen mass transfer (cm/s)m sample weight (g)NA Avogadro constantNO concentration of oxygen atoms in one monolayer

(1 × 1019 atoms m−2)r particle radius in spherical approach (m)R total rate constant of heteroexchange (s−1)Ra absolute gas constant (J mol−1 K−1)S surface area (m2)SBET BET surface area (m2/g)T temperature (K)ts time of isotopic replacement (s)U reaction mixture flow rate (mol/s)WR total rate of heteroexchange (atom m−2 s−1)X N2O conversionxfast

bulk fraction of fast exchangeable oxygen in the bulkxslow

bulk fraction of slow exchangeable oxygen in the bulk˛g atomic fraction of 18O in the gas phase oxygen

rate of heating (K/s)ˇslow

bulk coefficient of exchange for slow exchangeable oxy-gen (s−1)

�i (i = 1, . . ., N) concentration of the different form of oxygen(atoms/cm2)

� contact time (s/m)�20 time of isotope replacement for 20 monolayers of

oxygen atoms (s)

[ug[soet

sbgoti

i = −2k0e− i a�2,

�s time of isotope replacement (s)

12]. On the other hand, in the work of Martinez-Ortega et al.sing kinetic relaxation method it was shown that lattice oxy-en mobility did not influence the rate of methane oxidation13]. That is why we used SSITKA (Steady State Isotopic Tran-ient Kinetic Analysis), which allows us to evaluate the latticexygen self-diffusion coefficient and the rate of the surface oxygenxchange under the conditions corresponding to those of catalyticests.

In our previous articles we have shown that in case of Sr-ubstituted manganites, when the rate of oxygen surface exchangeeing substantially high in comparison to the rate of bulk oxy-en self-diffusion, catalytic activity in high-temperature reactions

f N2O decomposition and methane combustion correlates withhe coefficient of lattice oxygen self-diffusion. That is why, choos-ng La0.4Sr0.6FeO3 composition, which is known for its higher bulk

A: General 457 (2013) 42– 51 43

oxygen mobility in comparison to manganites, as the object ofthis study, we aimed at finding the relationship between the oxy-gen exchange properties and reactivity of perovskites. To do this,three high-temperature catalytic reactions of practical use, namely,nitrous oxide decomposition, ammonia oxidation and methanecombustion, have been chosen. For each of these reactions cat-alytic activity of perovskites to a greater (methane combustion)or lesser (N2O decomposition) extent may depend on both rate ofsurface oxygen exchange and lattice oxygen mobility. This papershows that when synthesizing Sr-substituted ferrites by mechano-chemical method using different starting compounds, Sr(NO3)2 orSrCO3, one can obtain composites of the different phase and surfacecomposition, which allowed us to demonstrate the positive effectof surface decoration with LaSrFeO4 on surface oxygen exchangeproperties and catalytic activity.

2. Experimental

2.1. Catalyst preparation

Two samples of La0.4Sr0.6FeO3 were prepared by mechano-chemical method [14] using different starting compounds: LSF-N– La2O3, Fe2O3 and Sr(NO3)2, and LSF-C – La2O3, Fe2O3 and SrCO3(all of chemical pure grade or pure for analysis grade). LaSrFeO4was synthesized from La2O3, Fe2O3 and SrCO3. Stoichiometricamounts of these compounds were mixed and intensively milledfor 3 min in a planetary mill APF-5 under the following conditions:air atmosphere, steel drums of 25 cm3 volume, steel ball of 5 mmin diameter, mass of the power 15 g, balls to power weight equal to4:1, basic disc rotation speed 850 rpm, acceleration 40 g. To obtainthe final catalyst the powder was calcined in air at 1100 ◦C for 4 h.

2.2. Catalyst characterization

The crystalline phases in the samples were identified byX-ray diffraction with Cu K� monochromatic radiation. High-temperature diffraction experiments were performed on a BrukerD8 diffractometer using Anton Paar high-temperature X-ray cham-ber. Surface cation composition of the samples was investigatedby XPS using Specs spectrometer (Germany) Al K� irradiation(h� = 1486.6 eV). Binding energy scale was preliminarily calibratedby the position of the peaks of Au 4f7/2 (84.0 eV) core levels.The method of differential dissolution phase analysis (DDPA)was used to analyze the phase composition of the catalysts(including poorly crystallized phases), the cation ratios in therevealed phases and possible screening of one phase by another[15]. TPD O2 experiments were carried out for the powder with0.25–0.5 mm size fraction in a plug-flow reactor coupled to QMS-200 mass-spectrometer. Before each experiment, the samples wereconditioned in the mixture of 20% O2 in He at 900 ◦C for 0.5 h andthen were cooled down to room temperature. A quantity of 200 mgwas used for each test. During the heating from room temperatureto 900 ◦C at a rate 10 ◦C/min pure He passed through the reactor ata flow rate 16 cm3/min. The calculation of the apparent activationenergy of oxygen desorption was made based on the equations ofkinetics of the second order:

1ˇ

dCO2

dT= −1

�CO2 + b

N∑1

k0i e−Ei/RaT

�2i

1 d� E /R T

ˇ dT i i

where is the rate of heating (K/s); CO2 is the concentrationof oxygen in the gas phase (atoms/cm3); Ra is the absolute gas

44 D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51

RD pro

ct(pftt(

ilaAcftsw1

cqtgtewfltic

2

p6qsGic�tg(

Fig. 1. Experimental and calculated X

onstant (J mol−1 K−1); T is the temperature (K); � is the contactime (s); b is the ratio between surface oxygen atom concentration1019 atom/m2) and concentration of oxygen atoms in the gashase (cm−1); �i (i = 1,. . ., N) is the concentration of the differentorm of oxygen (atoms/cm2); k0

iis the pre-exponential factor of

he rate of the ith oxygen form desorption (cm2 atom−1 s−1); Ei ishe activation energy of the rate of the ith oxygen form desorptionJ/mol).

The oxygen exchange experiments were carried out using annstallation consisting of reactant-dosing system, reactor and ana-ytic system. The reactant-dosing system was constructed in such

way as to enable an appropriate reaction mixture to be obtained. quartz reactor (i.d. 5 mm, L = 250 mm) with a thin bed of theatalyst resting on a layer of non-porous quartz was placed in aurnace. The temperature of the catalyst was determined using ahermocouple placed outside the catalyst bed. When the steady-tate was achieved under the 0.5% 16O2 in He flow, the gas mixtureas replaced by the same one, but containing 18O2 instead of

6O2 (18O2 enrichment = 95%). Transient changes in the isotopic gasomposition (18O2, 18O16O, 16O2) were continuously monitored byuadrupole mass spectrometer SRS RGA 200. The 18O2 isotope con-ained small admixture of argon as an inert tracer of the hold-up ofases on their way from the valve switching between oxygen iso-opes to the spectrometer’s detector. The measurement of oxygenxchange was performed under the following conditions: a sampleeight was 0.1 g (0.25–0.5 mm), the temperature was 800 ◦C, theow rate of the reaction mixture was 16.7 cm3/s, corresponding tohe contact time 0.001 s in the reaction conditions. More detailednformation about the numerical modeling of isotopic responsesan be found in [11].

.3. Catalytic tests

The catalytic activity in methane combustion and N2O decom-osition was measured in a fixed-bed U-shaped reactor at00–900 ◦C and ambient pressure. The reactor was a 5 mm i.d.uartz tube located in a furnace. A quantity of 3–20 mg of theample (particles of 0.25–0.5 mm in size) was used for each test.as mixtures consisted of 1% CH4+ 10% O2 in He and 0.15% N2O

n He passed through the reactor with a flow rate 16.7 cm3/s,orresponding to the contact time per unit surface area

= S[m2]/U[m3/s] = 4 × 103 − 6 × 103[s/m] in normal condi-ions. Outlet mixture composition was analyzed by on-lineas chromatograph with HeyeSep (i.d. = 3 mm, l = 3 m) and NaXi.d. = 3 mm, l = 2 m) columns to separate CO2 – (O2 + CH4 + CO) or

file for the samples LSF-N and LSF-C.

N2O – (N2 + O2). The absence of the reaction limitation by diffusionin the pores was additionally verified. The rate of N2O decompo-sition was calculated based on the first order kinetics [16,17]:

WN2O = U · NA

m · SBETln

11 − X

[molecules

m2 · s

]

where U is the flow rate at normal conditions (mol/s), m is thesample weight (g), SBET is the BET surface area (m2/g), X is the N2Oconversion, and NA is the Avogadro constant.

Ammonia oxidation reaction was carried out in a quartz fixed-bed tubular reactor with inner diameter of 3 mm at 300–900 ◦Cand atmospheric pressure. A quantity of 3–20 mg of the samples(particles of 0.25–0.5 mm in size) was tested. The catalyst temper-atures were measured with chromel–alumel thermocouples. Thereaction mixture of 1% ammonia in air passed through the catalystbed at 0.3 l/min flow rate corresponding to the contact time per unitsurface area � = S[m2]/U[m3/s] = 4 × 103[s/m] in normal condition.Ammonia, NO, N2O and NO2 concentrations were determined usingan on-line IR-spectrometer by the procedure described elsewhere[18].

3. Results

3.1. Catalyst characterization

3.1.1. Phase composition and its stabilityFig. 1 shows the XRD patterns of two La0.4Sr0.6FeO3−ı samples

prepared from the different starting compounds and calcined inair at 1100 ◦C. Although both samples have the same stoichiome-try, their phase composition was significantly different. The sampleobtained using SrCO3 as an initial compound (referred to as LSF-C) contained two solid solution of perovskite La1−xSrxFeO3−ı withthe different lattice parameters, indicating the different amount ofSr therein. Quantitative analysis of the phase composition by theRietveld refinement showed that the Sr stoichiometry correspondsto x = 0.85 for the first perovskite phase and x = 0.3 for the secondone (Table 1).

The sample prepared using Sr(NO3)2 (referred to as LSF-N) con-tained the solid solution of perovskite with x = 0.6 and admixture oftetragonal layer-structured perovskite LaSrFeO4±� . The calculatedvalues of the unit cell parameters and quantitative composition ofthe phases are listed in Table 1.

Phase diagrams of La–Fe–Sr–O system in air obtained byGavrilova et al. [19] and Fossdal et al. [20] show that homogeneoussolid solutions of the layered perovskite phase (La1−ySry)2FeO4±�

can be synthesized in the narrow range of composition 0.5 ≤ y ≤ 0.6

D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51 45

Table 1Phase composition, structural parameters, BET surface area and crystallite size estimated from XRD (dXRD) and SBET (dBET) of the samples La0.4Sr0.6FeO3−ı (LSF-C and LSF-N)and LaSrFeO4±� .

Sample Phase composition SBET

(m2/g)a c Space

groupdXRD dBET

a V/Z(Å3)

La0.4Sr0.6FeO3 (LSF-N) La0.4Sr0.6FeO3−ı (∼90 wt.%)La1.0–0.9Sr1.0–1.1FeO4±� (∼10 wt.%)

1 5.4923.861

13.41212.752

R-3cI4/mmm

90 430 58.495.1

La0.4Sr0.6FeO3 (LSF-C) La0.15Sr0.85FeO3−ı (∼60 wt.%)La0.7Sr0.3FeO3−ı (∼40 wt.%)

0.8 3.8685.527 13.499

Pm3mR-3c

9090

450 57.959.5

LaSrFeO4 LaSrFeO4±� (∼88 wt.%)La0.8–0.7Sr0.2–0.3FeO3−ı (∼10 wt.%)La2O3 (∼2 wt.%)

2.1 3.8675.521

12.73813.461

I4/mmmR-3c

90 210 95.359.2

a Estimated for the spherical particles with the density of 6.67 g/cm3.

wtcavtXifb

hrst

the associated satellite of the main Fe 2p peak (710.2 eV) has

Fig. 2. High-temperature XRD data for the sample LSF-C.

hen sintering at 1100 ◦C in air. As a consequence LaSrFeO4 syn-hesized by mechanochemical method (our case) predominantlyonsisted of the tetragonal layered-perovskite phase LaSrFeO4±�

nd small admixtures of La1−xSrxFeO3−ı and La2O3. The calculatedalues of the unit cell parameters and quantitative phase composi-ion of the phases are listed in Table 1. Both, for LSF-N and LSF-C, theRD crystallite size estimated by the Scherrer equation (Table 1)

s much less than the particle size estimated from the BET sur-ace area (Table 1), which can evidence the formation of intergrainoundaries.

According to in situ high-temperature XRD analysis (Fig. 2),eating of the sample LSF-C in air or in vacuum at 900 ◦C results in

eversible polymorphic phase transition of rhombohedric perov-kite (x = 0.3) to the cubic one, finally yielding to the mixture ofhe two cubic perovskites (x = 0.3 and x = 0.85). Phase composition

Fig. 4. XPS spectra of Fe2p, La3d and Sr3

Fig. 3. Phase composition of LSF-N after (1) milling, (2) sintering at 700 ◦C in airduring 4 h, (3) sintering at 1200 ◦C in air during 4 h.

of LaSrFeO4 and LSF-N samples remained unaltered when heatedto 900 ◦C. As follows from the phase analysis of the LSF-N sampleprecursor (Fig. 3), after the milling and following calcination at 700,900 and 1200 ◦C temperatures, the admixture of the layered perov-skite phase LaSrFeO4±� is formed at 700 ◦C and retained after thefollowing sintering.

3.1.2. Surface composition and microstructureXPS was used to determine the surface composition of the

samples. The Fe 2p spectra of all the samples showed that Fe3+

is the predominant state of iron cation on the surface because

3/2

the binding energy at 718.5 eV [21] (Fig. 4). The Sr 3d5/2 peaksobserved for all the samples at 131.7, 133.5 and 135–135.4 eV canbe assigned to the Sr2+ in perovskite, surface oxide and carbonate,

d for LSF-C, LSF-N and LaSrFeO4.

46 D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51

Table 2Surface concentration of La, Sr, Fe based on XPS data for the samples La0.6Sr0.4FeO3−ı (LSF-N and LSF-C) and LaSrFeO4.

Sample Fe Laper Lacarb Srper Srcarb Sroxide Srper//(Srper + Laper) (Srper + Laper)//Fe

LSF-N 0.26 0.086 0.079 0.23 0.29 0.05 0.73 1.20LSF-C 0.33 0.110 0.023 0.23 0.27 0.04 0.67 1.04LaSrFeO4 0.12 0.119 0.086 0.26 0

rsarosSrMetsxxeiis

rT

Fig. 5. Differential dissolution phase analysis of LSF-N.

espectively [22] (Fig. 4). The La 3d5/2 peak also includes twotate with the binding energy 833.1 and 834.7 eV, which can bettributed to the La3+ in perovskite and surface carbonate or oxide,espectively [22,23] (Fig. 5). The calculated surface compositionsf perovskite phase obtained from XPS analysis deviate from thetoichiometric values to some extent (Table 2). The atomic ratios ofrper/(Laper + Srper) = 0.73–0.67 found for LSF-C and LSF-N sampleseveal a slight surface enrichment of Sr in perovskite phase.oreover, the atomic ratio of (Laper + Srper)/Fe = 1.20 for LSF-N

xceeds the stoichiometry value (Laper + Srper)/Fe = 1.0 evidencinghe segregation of the layered perovskite phase LaSrFeO4±� on theurface. As to LSF-C, it can be concluded that perovskite phase with

= 0.85 is presented on the surface to a greater extent than that of = 0.3. The (Laper + Srper)/Fe = 3.29 for LaSrFeO4 exceeds stoichiom-try value (Laper + Srper)/Fe = 2.0, which can be due to the deviationn the ratio between Srper, Srcarb, Sroxide or Laper, Lacarb and isndicative of the formation of concreted “layered perovskite-SrO”

tructures.

To define exactly the phase composition and stoichiometicatio of cations, LSF-N sample was additionally analyzed by DDPA.he data obtained showed that, at first, quick-dissolving phase

Fig. 6. HREM image and the Fourier transform image of the ed

.31 0.10 0.69 3.29

corresponding to the stoichiometry of La0.9Sr1.1FeO4 in amounts of7–8 wt.% was removed in the flow of H2O:HCl = 1:10 and then thephase corresponding to La0.4Sr0.6FeO3 in amounts of 93–92 wt.%was dissolved in the flow of H2O:HCl = 1:1 (Fig. 5). This result is ingood agreement with XRD and XPS data.

The microstructure of the samples was studied by HREM withEDX analysis. Fig. 6 shows some examples of the high-resolutionimages and the calculated fast Fourier transform image for LSF-Nand LaSrFeO4. As follows from HREM analysis, the LSF-N sam-ple consists of the particles of 1 �m in diameter comprised ofthe concreted crystallites of 50–100 nm, which is in agreementwith XRD crystallite size (Table 1). Calculated Fourier transformimage from the edge of the particles (Fig. 6a) displayed the order-ing with d-spacing 2.23 A corresponding to d006 of La1−xSrxFeO3−ı

and 2.65 and 2.95 A corresponding to d110 and d103 of theLaSrFeO4±� phase. Along with XPS and DDPA data this result sug-gests that both tetragonal phase of LaSrFeO4±� and La1−xSrxFeO3−ı

are located on the surface region of the particles. In support ofthis assumption the EDX analysis of that region showed that thecation ratio of (La + Sr):Fe = 1.5:1 exceeded the stoichiometric value(La + Sr):Fe = 1:1. It can be supposed as well that perovskite andLaSrFeO4 formed the intergrowth domains.

HREM images of LaSrFeO4 sample show good agreement withthe data obtained by the Rietveld refinement (Table 1). The mostof the sample contains well crystallized phase of layered perov-skite LaSrFeO4±� as well as small admixture of La1−xSrxFeO3 andSrO. The LaSrFeO4 particles have uniform cation composition andmorphology, and formed of concreted crystallites 200–400 nm inaverage size. Calculated Fourier transform image from the edge ofthe particles (Fig. 6b) showed the ordering with d-spacing 2.09,2.66 and 2.17 A corresponding to d114, d105, d110 of LaSrFeO4±�

and 3.81 A corresponding to d012 of La1−xSrxFeO3−ı. The ratio of(La + Sr):Fe = 2.5 calculated from EDX spectrum agrees with XPSdata.

Thus, when synthesizing the La0.4Sr0.6FeO3 by mechano-

chemical method (activation during 3 min in a planetary mill withthe ratio ball/powder = 4 followed by calcination in air at 1100 ◦C)using SrCO3 or Sr(NO3)2 as starting compound one can two typesof composites: in the former case, La0.15Sr0.85FeO3 (to a greater

ge region of the particle for (a) LSF-N and (b) LaSrFeO4.

D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51 47

r LSF-

ec

3

po4iaot6

Fa

Fig. 7. TPD O2 profile fo

xtend presented on the surface)–La0.7Sr0.3FeO3 and, in the latterase, LaSrFeO4 (on the surface)–La0.4Sr0.6FeO3.

.2. TPD O2

Fig. 7 represents the obtained TPD O2 profiles for all the sam-les. As can be seen, each profile contains two peaks: the firstne is in the medium-temperature interval with maximum at50–460 ◦C and activation energy about 90 kJ/mol, the second one

s in the high-temperature interval with the maximum at >700 ◦C

nd activation energy about 50 kJ/mol. The total amount of des-rbed oxygen in the first and in the second peaks correspondso 14.5 × 1019 and 45.5 × 1019 atoms/g for LSF-N, 18.8 × 1019 and1.2 × 1019 atoms/g for LSF-C and 1.9 × 1019 and 7.1 × 1019 atoms/g

ig. 8. Experimental and calculated responses of ˛g(t) and f34(t) over LSF-N, LSF-Cnd LaSrFeO4 samples at 800 ◦C and p(O2) = 0.005 atm.

N, LSF-C and LaSrFeO4.

for LaSrFeO4, correspondingly. The low activation energy for oxy-gen desorption in the second peak can be attributed to the influenceof bulk oxygen diffusion. The activation energy of the bulk oxy-gen diffusion for Sr-substituted ferrites closely corresponds tothe oxygen vacancy migration enthalpy. The values of oxygenvacancy migration enthalpy quoted in the literature are around60–80 kJ/mol [24,25], which is in agreement with our data. Themaximum of the second peak in LaSrFeO4 is shifted to the highertemperature in comparison with the LSF-N and LSF-C indicat-ing the higher binding strength of the lattice oxygen in layeredperovskite. Based on the quantity of the desorbed oxygen in theboth peaks and assuming the fully oxidation state at room tem-perature, the following values of oxygen non-stoichiometry at900 ◦C in helium were calculated: LaSrFeO3.95 (0.9 × 1020 atoms/gof desorbed oxygen) < LSF-N − La0.4Sr0.6FeO2.78 (6 × 1020 atoms/gof desorbed oxygen) < LSF-C − La0.4Sr0.6FeO2.71 (8 × 1020 atoms/g ofdesorbed oxygen).

3.3. Oxygen mobility

The results of switches from 16O2 + He to 18O2 + Ar + He for LSF-C, LSF-N and LaSrFeO4 at 800 ◦C are presentenced in Fig. 8 as thechanges of 18O fraction in the gas phase – ˛g(t)

˛g(t) =16O18O + 218O2

2∑

iOjO

and 18O16O fraction in the gas phase – f34(t) versus time

f34(t) =16O18O∑

iOjO.

The total amount of exchanged oxygen calculated as N18O =∫ ts (0.95 − ˛ (t))dt · C0 · 2 · U · N /m (where U is the flow rate

0 g 18O2

A

(l/s), C018O2

is the concentration of 18O2 (mol/l), NA is the

6.02 × 1023 atoms/mol, m is the sample weight (g), ts is the time ofisotopic replacement (∼350 s)) accounts for 5.7–8.5 × 1021 atoms/g

48 D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51

Table 3Calculated kinetics parameters of oxygen exchange for La0.4Sr0.6FeO3−ı (LSF-C и LSF-N) and LaSrFeO4±� at T = 800 ◦C.

Sample Surface exchange Bulk exchange

R (s−1) WR (atom/m2 s) D*/r2 (s−1) D*a (cm2/s)

One type of oxygen in the bulkLSF-C 3.5 3.5 × 1019 0.025 5.0 × 10−11

LaSrFeO4 8 8 × 1019 0.005 0.2 × 10−11

Sample Surface exchange Bulk exchange

R (s−1) WR (atom/m2 s) xfastbulk

D*fast/r2, s−1 xslowbulk

ˇslowbulk

, s−1 D*/r2 (s−1) D* (cm2/s)

Two types of oxygen in the bulkLSF-N 13 13 × 1019 0.4 >0.03 0.6 0.01 >0.018 >3.7 × 10−11

a Calculated for the rBET (r = 210–450 nm) (Table 1).x onstae eable

c

cemsthtofwcdiigoooo

3

r6

Fr

fastbulk

– fraction of fast exchangeable oxygen in the bulk; D*fast/r2 – effective rate cxchangeable oxygen in the bulk; ˇslow

bulk– coefficient of exchange for slow exchang

oefficient of oxygen self-diffusion.

orresponding to the participation of the lattice oxygen in oxygenxchange. Thus, the numerical analysis of isotope responses wasade based on the simple scheme of oxygen exchange considering

urface exchange reaction and bulk diffusion. More detail descrip-ion of this scheme of exchange can be found in [11]. The relativelyigh parameter sensitivity of the model allowed us to evaluate bothhe surface exchange constant (R, s−1) and the coefficient of bulkxygen self-diffusion (D*, cm2/s) (Table 3). It was found that the sur-ace exchange constant for the layered perovskite phase LaSrFeO4as higher than that for LSF-C, but the highest one belonged to the

omposite LSF-N. The values of the coefficient of bulk oxygen self-iffusion are varied through a 0.2 × 10−11–5 × 10−11 cm2/s and are

ncreased in the following order LaSrFeO4 < LSF-N ≈ LSF-C, which isn agreement with the values of the total amount of desorbed oxy-en at 900 ◦C in He estimated from TPD O2. Notice that for LSF-N webtained the satisfactory result only when distinguishing two typesf oxygen in the bulk – slow and fast exchangeable. The appearancef the fast exchangeable oxygen could be assigned to the formationf weakly bound O− species, which was found earlier by XPS [26].

.4. Catalytic activity

Fig. 9a presents data on activity of studied samples in theeaction of N2O decomposition in the temperature interval00–900 ◦C. Catalytic activity was analyzed based on the values

ig. 9. Catalytic activity of LSF-N, LSF-C and LaSrFeO4 in (a) N2O decomposition (0.15% Nate of N2O decomposition and the constant of surface oxygen exchange at 800 ◦C.

nt of fast exchangeable oxygen self-diffusion in the bulk; xslowbulk

– fraction of slowoxygen; D*/r2 – total value of effective rate constant of oxygen self-diffusion; D* –

of N2O conversion measured at the same contact time per unitof surface area � = S[m2]/U[m3/s] = 4 × 103[s/m]. As can be seenthe N2O conversion is increased in the following order of thesamples: LSF-C < LaSrFeO4 < LSF-N and does not correlate withthe coefficient of bulk oxygen diffusion (Table 3). On the otherhand, there is the correspondence between the rate of N2O decom-position measured at 800 ◦C and the constant of surface oxygenexchange (Fig. 9b). It means that the limiting step of the reactionis either N2O adsorption or oxygen desorption from the surface.Since the estimated rate of surface oxygen exchange at 800 ◦CWR = R · NO (NO = 1 × 1019 atoms/m2) (Table 3) and the rate of thereaction (Fig. 9b) are about of the same order, it gives us reasonto believe that the rate-determining step of the reaction is oxygendesorption from the surface. Thus, it seems that the diffusion ofadsorbed oxygen through the bulk does not influence the rate ofN2O decomposition because either the sites of N2O decompositionand oxygen desorption are nearby or the rate of bulk oxygendiffusion is high and oxygen transfer through the bulk is not arate-controlling step of the reaction.

Fig. 11a shows the ammonia conversion as a function of thetemperature as measured at the same contact time per unit of

surface area � = S[m2]/U[m3/s] = 4 × 103[s/m]. One can see that atleast up to 800 ◦C the order of conversion over samples – LSF-N > LaSrFeO4 > LSF-C is the same. Since there is no way to calculatethe rate of ammonia oxidation at 800 ◦C due to non-differential

2O in He, T = 600–900 ◦C, P = 1 atm, � = 4 × 103 s/m) and (b) correlation between the

D.V. Ivanov et al. / Applied Catalysis A: General 457 (2013) 42– 51 49

FN

roNqrbtNo

tottFLosntatoos

and La2O3, corresponding to the stoichiometry La0.4Sr0.6FeO3, with

Fb

ig. 10. Catalytic activity of LSF-N, LSF-C and LaSrFeO4 in ammonia oxidation (1%H3 in air, T = 300–900 ◦C, P = 1 atm, � = 4 × 103 s/m).

eactor operating conditions, catalytic activity was analyzed basedn the values of T50 corresponding to the temperature of the halfH3 conversion (Fig. 10). The values of T50 cannot serve as auantitative characteristic of catalytic activity, nevertheless, theyeveals the true order of the samples activity. It is therefore cane concluded that the T−1

50 values change in the same order ashe samples efficiency toward surface oxygen exchange – LSF-

> LaSrFeO4 > LSF-C (Fig. 10 and Table 3) and does not match therder of the samples for bulk oxygen diffusion (Table 3).

Methane oxidation on these samples proceeds with the forma-ion of only CO2 and H2O. Catalytic activity was analyzed basedn the values of methane conversion measured at the same con-act time per unit surface area � = S[m2]/U[m3/s] = 6 × 103[s/m] inhe temperature interval 750–900 ◦C (Fig. 11a). As illustrated inig. 12a, methane conversion is increased in the following order:SF-C ∼ LaSrFeO4 < LSF-N, and depends on neither the coefficientf bulk oxygen diffusion (Fig. 11a and Table 3) nor the constant ofurface oxygen exchange. Supposing that four oxygen atoms areecessary to oxidize CH4 molecule to CO2 and H2O, participation ofhe near subsurface lattice oxygen in this reaction is quite reason-ble. Indeed, it was found that methane conversion correlates withhe characteristic time of the exchange of about 20 monolayers of

xygen atoms (�20, Fig. 11b). This result may indicate that bothxygen diffusion through the sub-surface layers from oxidationites to the reduction sites and the efficiency of oxygen dissociative

ig. 11. (a) Catalytic activity of LSF-N, LSF-C and LaSrFeO4 in methane combustion (1%etween methane conversion and the characteristic time of the exchange of 20 monolaye

Fig. 12. Crystal structure of LaSrFeO4, comprising two regular oxygen sites – apicaland equatorial oxygen.

adsorption on the surface can influence the catalytic activity offerrites in methane combustion.

4. Discussion

Our results show that surface composition and microstructureof Sr-substituted ferrites do influence the rate of surface oxygenexchange and catalytic properties in high-temperature reactions ofmethane combustion, N2O decomposition and ammonia oxidation.The formation of composites, containing the admixture of LaSrFeO4on the surface of the particles, is an example how it is possibleto increase the rate of surface oxygen exchange on retention ofhigh bulk oxygen diffusion. To obtain these composites we appliedmechanochemical method using the mixture of Sr(NO3)2, Fe2O3

the following calcination at 1100 ◦C in air. However, according tothe phase diagram of La1−xSrxFeO3−ı system, homogeneous solidsolutions of La1−xSrxFeO3−ı can be obtained up to x = 0.8 when

CH4 + 10% O2 in He, T = 750–900 ◦C, P = 1 atm, � = 6 × 103 s/m) and (b) correlationrs of oxygen atoms �20 at 800 ◦C.

5 talysis

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iaiornSsLLbsosasrftiaooLa[boipma

etatgirdtchabogtir

0 D.V. Ivanov et al. / Applied Ca

intering at 1100 ◦C in air [19,27]. It indicates that phase com-osition of LSF-N is nonequilibrium, but stable at 900 ◦C in air.

t seems that the phase inhomogenity can arise from the loweratio of the weights of milling balls and oxides powder (4:1) wesed in this work than that which is usually used to synthesizeerovskites (10:1) [23]. When comparing phase composition ofSF-N to LSF-C sample, obtained using SrCO3 and consisted of twoerovskite phases La1−xSrxFeO3 (x = 0.85 and x = 0.3), we can con-lude that in case of LSF-N the lower decomposition temperature ofr(NO3)2 and softness when grinding as compared to SrCO3 allowss to achieve the greater extent of interaction between componentshen milling and calcining and to synthesize almost homoge-eous sample containing only 10 wt.% of admixture of LaSrFeO4hase.

The rates of 18O substitution in Sr-substituted ferrites increasedn the following order: LSF-C (La1−xSrxFeO3 composite with x = 0.3nd 0.85) < LaSrFeO4 < LSF-N (La0.4Sr0.6FeO3 + LaSrFeO4 compos-te). Elshof et al. found the correlation between the rate of surfacexygen exchange in La1−xSrxFeO3−ı and the level of surface seg-egation of SrO and SrCO3 [28]. However, in our study we didot find any correspondence between the surface segregation ofr as oxides and/or carbonates or even La and the constant ofurface exchange (Tables 2 and 3). The amount of the surfacea carbonate estimated by XPS increased in the following orderSF-C < LaSrFeO4 < LSF-N, whereas the content of the surface Sr car-onates and oxide is approximately the same for all samples. Iteems that the microstructure of the particles and the formationf LaSrFeO4±� phase have the more profound effect on the rate ofurface oxygen exchange. Tetragonal LaSrFeO4 phase consist of thelternate layers of perovskite and (La, Sr)O slabs with the rock-salttructure and has two crystallographic sites of oxygen – O1 (equato-ial position) and O2 (apical position) (Fig. 12). From the structuraleatures of layer-structured perovskites, it can be supposed thathe higher values of the surface exchange constant for LaSrFeO4±�

n comparison to LSF-C could be attributed to the formation of thedditional surface oxygen vacancies due to the possible movementf the apical oxygen toward the interstitial sites. Recent studiesf oxygen migration paths in layered-perovskites LaSrCoO4±� anda2NiO4±� found the possibility of interstitial oxygen conductionlong a “wave-like” 2D path between apical and interstitial sites29,30]. In the review devoted to Ln2MO4 as cathode materials doney Zhao and LiPing [31] it is also reported that the (1 1 4) plane of Fe-r Cu-substituted La2NiO4±� , which contains O-apical oxygen andnterstitial oxygen atoms, can be involved in the oxygen reductionrocess [32]. However, more detail investigation of surface oxygenobility and oxygen reduction kinetics on Sr-substituted ferrites

re needed to proof this assumption.In case of composite sample LSF-N it seems that the high-

st rate of surface oxygen exchange could also be attributed tohe formation of intergrown boundaries between the perovskitend layered-perovskite phases, which favors the incorporation ofhe surface oxygen in the lattice. The enhancement of the oxy-en reduction kinetics were also found by Crumlin et al. [33]n the heterogeneous LaSrCoO4/La1−xSrxCoO3 thin films. Theiresults indicate that disordered interfacial La1−xSrxCoO3/LaSrCoO4omains may be responsible for this effect. Sase et al. also foundhat surface exchange coefficient for heterogeneous material thatan be considered as LaSrCoO4 supported onto La1−xSrxCoO3 isigher than that single phase La1−xSrxCoO3 sample due to appear-nce of the fast oxygen-incorporation paths along the heterophaseoundary [34]. However, more detail investigation of the influencef perovskite-“layered” perovskite domains on the surface oxy-

en mobility is required. In addition, it is not inconceivable thathe higher rate constant of the surface oxygen exchange for LSF-Nn comparison to LSF-C may be attributed to the lower degree ofeduction (lower amount of desorbed oxygen at the same level of

A: General 457 (2013) 42– 51

Sr content) and, therefore, lower binding strength of the surfacelattice oxygen as follows from TPD O2 data.

To analyze the correspondence of the results obtained in thiswork by SSITKA with those found in the literature we calcu-lated the coefficient of oxygen mass transfer K = D*/r2 · r (cm/s)(where D* is the coefficient of oxygen self-diffusion (cm2/s), r isthe particle radius in spherical approach calculated from SBET (cm))and compare it with the k* is the constant of the isotopic sur-face oxygen exchange (cm/s) (literature data). To compare thevalues of k* obtained in a wide range of pO2 , we assumed thatk* follows a pressure dependence given approximately by p0.5

O2found for rich electronic conductors [35]. Thus, Geffroy et al. [36]using Isotopic Exchange Depth Profile Method (IEDP) reportedthe values of k* = 10−6 cm/s (La0.6Sr0.4Fe0.6Ga0.4O3−ı) – 10−8 cm/s(La0.9Sr0.1Fe0.9Ga0.1O3−ı) at 800 ◦C and pO2 = 21 kPa, which cor-responds to k* = 1.5 × 10−7–1.5 × 10−9 cm/s at pO2 = 0.5 kPa (ourconditions). Elshof et al. [37] found k* ≈ 3 × 10−7 cm/s forLa0.6Sr0.4FeO3−ı and La0.9Sr0.1FeO3−ı at 800 ◦C and pO2 = 0.25 kPausing electrical conductivity relaxation. In this work we calculatedthe following values of k* = 5.6·10−7, 0.5·10−7and 6.5·10−7 cm·s−1

at 800 ◦C and pO2 = 0.5 kPa corresponding to LSF-C, LaSrFeO4 andLSF-N, which is the same order of magnitude with values obtainedin the literature. As to the coefficient of oxygen self-diffusion, thevalues of D* obtained in this work by SSITKA (see Table 3) are inagreement with those found by IEDP in the work of Kim et al. [38]– D* = 10−11 cm2/s for La0.6Sr0.4FeO3−ı and D* = 2.5 × 10−10 cm2/sfor La0.4Sr0.6FeO3−ı at 1000 ◦C. However, the values of D* obtainedfrom electrical conductivity relaxation experiments in the workof Søgaard for La0.4Sr0.6FeO3 [39] were three orders of magnitudegreater, which could be due to the differences in microstructure ofthe samples.

Catalytic tests of Sr-substituted ferrites in high-temperatureN2O decomposition and revealed the correlation between the rateof surface oxygen exchange and activity (Fig. 9b). For ammonia oxi-dation, order of the samples activity (Fig. 10) also corresponded tothe order of samples efficiency toward surface oxygen exchange(Table 3). Based on the results it can be concluded that ammoniaoxidation follow Mars–van Krevelen model, which includes twoindependent steps:

(1) The R + [O]lat → P + [ ]lat(2) 2[ ]lat + O2 → 2[O]lat,

where substrate R (R = NH3) is activated by lattice oxygen at the sur-face resulting in product P (P = NO, etc.) and oxygen lattice vacancy.The second step is oxygen vacancy reoxidation by gas phase oxy-gen. In case of N2O, the first step of the reaction can be consideredto be dissociative adsorption of N2O on the surface oxygen vacancyfollowed by the formation N2 and lattice oxygen:

(1) N2O → N2 + [O]latThe second step is oxygen desorption from the surface:

(2) 2[O]lat → O2 + 2[ ]lat

For both reactions, the relationship between catalytic activityand bulk oxygen diffusion were absent. It indicates that eitheroxidation and reduction take place approximately on the sameactive sites or diffusion of oxygen from the oxidation site to thereduction site is high and it is not a rate-controlling step of thereaction. However, catalytic activity of Sr-substituted ferrites inmethane combustion reveal the correspondence between the rate

of exchange of 20 oxygen monolayers and methane conversion at800 ◦C. This implies that for methane combustion the transitionfrom the Mars–van Krevelen to Ionic Redox mechanism takes place.The limiting step of the reaction is the oxygen vacancy reoxidation,

talysis

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D.V. Ivanov et al. / Applied Ca

hich can proceed due to the oxygen diffusion from the oxidationo the reduction site through the sub-surface layers.

In case of Sr-substituted manganites we have shown earlier11,40] that when the rate of surface oxygen exchange is notice-bly higher in comparison to the rate of bulk oxygen diffusionR/(D*/r2) ≈ 103 [40]) catalytic activity in methane combustion and2O decomposition depends on oxygen self-diffusion coefficient

D*), evidencing the participation of bulk oxygen in reoxidation ofurface oxygen vacancies. In case of ferrites, the ratio between theates of surface oxygen exchange and bulk oxygen diffusion is onerder of magnitude lower (R/(D*/r2) ≈ 102) in comparison to man-anites and catalytic properties correlate with the rate of surfacexygen exchange. It may indicate that in ferrites the concentrationf active oxygen on the surface is high and the reaction takes placen the sites nearby to the oxidation ones. On the other hand, it isot inconceivable that oxidation and reduction sites are separated,ut oxygen diffusion through the bulk is fast and do not influencehe rate of the reaction. Therefore synthesizing perovskite-“layerederovskite” composites in Sr-substituted ferrites it is possible to

ncrease the rate of surface oxygen exchange on retention of highulk oxygen diffusion and to improve the catalytic properties.

. Conclusions

In conclusion, we have shown that in La1−xSrxFeO3−ı system at = 0.6 two types of composites can be synthesized using mecha-ochemical method (sintering at 1100 ◦C in air for 4 h). The firstomposite can be obtained starting from SrCO3 as initial com-ound and contains two solid solutions of perovskite with cubic andhombohedral lattice. The second sample can be synthesized usingr(NO3)2 and constitute the composite LaSrFeO4±�/La1−xSrxFeO3−ı.bove 800 ◦C in air, in the first sample, transition of rhombohedral

o cubic perovskite occurs, whereas the composition of the secondample remains unaltered. The difference in composition of thesewo samples allowed us to find the relationship between surfaceegregation of LaSrFeO4 phase, parameters of oxygen exchange andatalytic activity in high-temperature reactions of N2O decompo-ition, methane combustion and NH3 oxidation. It was noticed thaturface decoration of the particles with phase of layer-structurederovskite LaSrFeO4±� resulted in increased rate of surface oxy-en exchange and catalytic activity. It seems that formation ofntergrown perovskite – layered perovskite phases results in theppearance of interphase boundaries, which can serve as the fastxygen diffusion paths accelerating oxygen incorporation in theattice. Moreover, it was found that layered perovskite LaSrFeO4hase exhibits higher surface oxygen exchange properties in com-arison to La1−xSrxFeO3−ı, which may be due to possible interstitialxygen diffusion resulting in the formation of the vacant apicalite for oxygen adsorption. Catalytic tests reveal that activity of theamples in N2O decomposition directly correlates with the oxygenurface exchange properties and do not depend on bulk oxygenobility. For ammonia oxidation, the correspondence between the

rder of the samples activity and the order of the samples efficiencyoward surface oxygen exchange. This result allowed us to make aonclusion that the processes NH3 oxidation and N2O decomposi-ion involve participation of surface lattice oxygen and reductionnd oxidation reactions proceed either on the same active sites ors a result of high oxygen diffusion through the lattice. In methane

ombustion the correlation between the rate of exchange for 20onolayers of oxygen atoms and methane conversion at 800 ◦C was

ound. This implies that the transition from Mars–van Krevelen toonic Redox model occurs and the sites of oxidation and reduction

[

[

A: General 457 (2013) 42– 51 51

can be separated. In this case lattice oxygen mobility can influencethe reactivity of perovskites. The obtained data illustrate the poten-tial of utilizing the composites based on the intergrown perovskitestructures to develop highly active catalysts for high temperatureprocesses.

Acknowledgements

We should like to thank Dr. Alexander Nadeev and Prof. SergeyTsybulya for their help in interpretation of XRD spectra and usefuldiscussions.

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