AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Applied Catalysis B: Environmental 29 (2001) 239–250

AFeO3 (A = La, Nd, Sm) and LaFe1−xMgxO3 perovskites asmethane combustion and CO oxidation catalysts:

structural, redox and catalytic properties

P. Ciambellia, S. Ciminob, S. De Rossic, L. Lisi d, G. Minelli c, P. Portac,∗, G. Russoba Dipartimento di Ingegneria Chimica e Alimentare, Università di Salerno, Salerno, Italy

b Dipartimento di Ingegneria Chimica, Università Federico II, Napoli, Italyc Centro di Studio del CNR su Struttura e Attività Catalitica di Sistemi di Ossidi (SACSO),

c/o Dipartimento di Chimica, Università La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italyd Istituto di Ricerche sulla Combustione, CNR, c/o Dipartimento di Ingegneria Chimica, Piazzale V. Tecchio 80, 80125 Napoli, Italy

Received 20 February 2000; received in revised form 21 May 2000; accepted 15 June 2000

Abstract

Catalytic methane combustion and CO oxidation were investigated over AFeO3 (A = La, Nd, Sm) and LaFe1−xMgxO3

(x = 0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterizedby X-ray diffraction (XRD). Redox properties and the content of Fe4+ were derived from temperature programmed reduction(TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m2 g−1 range. XRD analysis showed that LaFeO3, NdFeO3,SmFeO3 and LaFe1−xMgxO3 (x · 0.3) are single phase perovskite-type oxides. Traces of La2O3, in addition to the perovskitephase, were detected in the LaFe1−xMgxO3 catalysts withx = 0.4 and 0.5. TPR gave evidence of the presence in AFeO3

of a very small fraction of Fe4+ which reduces to Fe3+. The fraction of Fe4+ in the LaFe1−xMgxO3 samples increasedwith increasing magnesium content up tox = 0.2, then it remained nearly constant. Catalytic activity tests showed that allsamples gave methane and CO complete conversion with 100% selectivity to CO2 below 973 and 773 K, respectively. Forthe AFeO3 materials the order of activity towards methane combustion is La> Nd > Sm, whereas the activity, per unit SA,of the LaFe1−xMgxO3 catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase(x = 0.3). Concerning the CO oxidation, the order of activity for the AFeO3 materials is Nd> La > Sm, while the activity(per unit SA) of the LaFe1−xMgxO3 catalysts decreases at high magnesium content. © 2001 Elsevier Science B.V. All rightsreserved.

Keywords:Catalytic methane combustion; Catalytic CO oxidation; Perovskite solid solutions; Orthoferrites

1. Introduction

Catalytic combustion processes have been drawingan increasing interest during the last decades as theyrepresent a convenient mean for both emissions pre-vention (control of nitrogen oxides NOx and UHC in

∗ Corresponding author. Fax:+39-6-490324.E-mail address:[email protected] (P. Porta).

heat and power generation), and clean-up (VOC re-moval, automobile exhaust converters) [1].

Due to their high activity and thermal stability,much attention has been paid to perovskite-type ox-ides, of general formula ABO3 (where A and B areusually rare earth and transition metal cations, re-spectively), as catalysts for complete oxidation of hy-drocarbons and CO, in substitution of the very activenoble metals Pt and Pd, which are more expensive

0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-3373(00)00215-0

240 P. Ciambelli et al. / Applied Catalysis B: Environmental 29 (2001) 239–250

and do not resist to operating temperatures exceeding850 K [1–8].

Many metallic elements are stable in the ABO3perovskite structure provided that their cationicradii fit well the sizes of the 12-coordinated Aand 6-coordinated B sites, e.g.rA > 0.90 Å andrB > 0.51 Å. Moreover, the high stability of the per-ovskite structure allows the partial substitution of ei-ther or both A and B site cations by other metals withdifferent oxidation state and consequent creation ofstructural defects such as anionic or cationic vacancies[8–10]. A widely variable oxygen nonstoichiometry(reductive or oxidative, ABO3±δ) of the perovskitestructure can thus be achieved with a high ease ofremoving oxygen but still preserving the originalframework. The mechanisms of redox processes onthese materials (both on the surface and in the bulk)account for the reversible loss and uptake of oxygenand/or for the creation and filling up of vacancies.

The effect of the nature of the B cation on thecatalytic properties of lanthanum based perovskiteshas been widely studied, perovskites containing man-ganese, cobalt and iron having been found the mostactive in methane combustion [8,11], whereas the ef-fect of rare earth has been less investigated, only fewstudies concerning ACoO3 and AMnO3 perovskiteshaving been reported [12–14].

Partial substitution of lanthanum with divalent ions,in particular Sr2+, increases the average oxidationstate of B cation in the case of Co and Fe based per-ovskites, without significant formation of anion vacan-cies [15]. Due to the relative ease of the redox processbetween B4+ and B3+, larger quantities of oxygenare available at low temperature and the overall oxi-dation activity is enhanced [5]. Also La1−xSrxMnO3are more active in methane combustion than the corre-sponding unsubstituted LaMnO3 sample, principallybecause of the higher content of Mn4+ in the per-ovskite structure [8,13].

It has also been reported that partial substitution oftransition metal at B site with other trivalent cationcan be effective to enhance oxidation activity of per-ovskites [16–18]. In particular, Zhong et al. [16] stud-ied LaFe1−xMxO3 (M = Al, Mn, Co) perovskites andclaimed a synergistic effect due to the presence oftwo types of B cations, which causes an increase intheir average oxidation state, resulting in better per-formances for methane oxidation. Nevertheless, the

complexity of perovskite structures containing twocations, which can potentially assume different oxida-tion states, makes very difficult to understand the realcause of enhanced activity.

This study reports on the preparation and char-acterization of AFeO3 (A = La, Nd, Sm) andLaFe1−xMgxO3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) per-ovskites, and on their catalytic activity in methanecombustion and CO oxidation, with the aim to under-stand the effect of the rare earth A and of the Fe4+content induced by partial substitution with Mg2+.

2. Experimental

AFeO3 (A = La, Nd, Sm) and LaFe1−xMgxO3 (x =0.1, 0.2, 0.3, 0.4, 0.5) perovskite catalysts were pre-pared according to the citrate method [19], which al-lows a very homogeneous dispersion of the precursorsalts and in turn lowers the calcination temperatureneeded to form the perovskite structure, thus avoidingsintering phenomena and producing higher surface ar-eas (SA). A concentrated solution of metal nitrates wasmixed with an aqueous solution of citric acid. The mo-lar ratio of citric acid to total metal cations was fixedat unity. Water was evaporated from the mixed solu-tion at 353 K until a viscous gel was obtained. The gelwas then heated overnight at 383 K to yield a brownglassy material which was ground, fired at 423 K for1 h and then slowly calcined to attain the temperatureof 1073 K for 5 h.

The content of rare earth metals was determinedby inductively coupled plasma (ICP) emission spec-troscopy. Atomic absorption was employed to deter-mine the iron and magnesium content.

Phase analysis, lattice parameters and particlesizes determination were performed by X-ray powderdiffraction using a Philips PW 1029 diffractometerwith Ni-filtered Cu Ka radiation. Lattice parameterswere calculated by means of the UNITCELL pro-gram [20]. Particle sizes were evaluated by means ofthe Scherrer equationD = Kλ/β cosθ after Warren’scorrection for instrumental broadening [21].K is aconstant equal to 0.9,λ the wavelength of the X-rayused,β the effective line width of the X-ray reflec-tion under observation calculated by the expressionβ2 = B2 − b2 (where B is the full width at halfmaximum (FWHM), b the instrumental broadening

P. Ciambelli et al. / Applied Catalysis B: Environmental 29 (2001) 239–250 241

determined through the FWHM of the X-ray reflec-tion at θ = 14◦ of SiO2 having particles larger than1000 Å), θ the diffraction angle of the (1 0 1) X-rayreflection (θ = 11.4◦, d = 3.89 Å) for samarium andneodymium compounds and of the (1 2 1) X-ray re-flection (θ = 16.1◦, d = 2.779 Å) for the lanthanumperovskites. BET SA of the materials were measuredby krypton adsorption at 77 K using a volumetric allglass apparatus.

Temperature programmed reduction (TPR) ex-periments were performed using a MicromeriticsTPD/TPR 2900 analyzer equipped with a TC detec-tor and coupled with a Hiden HPR 20 mass spec-trometer. Samples (100 mg) were preheated in flow-ing air at 1073 K for 2 h and then, after cooling atroom temperature, reduced with a 2% H2/Ar mixture(25 cm3 min−1) heating 10 K min−1 up to 1073 K.Water produced by the sample reduction was con-densed in a cold trap before reaching the detectors.Only H2 was detected in the outlet gas confirming theeffectiveness of the cold trap.

Methane catalytic combustion experiments wereperformed in the experimental apparatus and accord-ing to the procedures already described [14,22]. Thespace velocity was 40,000 N cm3 g−1 h−1 in all tests(0.4 g catalyst powder), and the feed gas composi-tion was 0.4% CH4, 10% O2, N2 as balance. Theexperiments were carried out with a downflow quartzannular reactor electrically heated in a three zone tubefurnace. Catalyst particles in 180–250mm range werediluted 1:10 in quartz powder of the same dimensionand placed on a porous quartz disk. The specific grav-ity was about 1 g cm−3. For each test the methane

Table 1SmFeO3, NdFeO3, LaFeO3 and LaFe1−xMgxO3 perovskites calcined at 1073 Ka

Catalyst Phases a (Å) b (Å) c (Å) V (Å3) SA (m2 g−1) D (Å)

SmFeO3 P 5.605 7.720 5.412 234.2 4.3 300NdFeO3 P 5.584 7.758 5.458 236.4 2.3 610LaFeO3 P 5.571 7.866 5.564 243.9 2.9 570LaFe0.9Mg0.1O3 P 5.575 7.872 5.563 244.1 4.3 500LaFe0.8Mg0.2O3 P 5.573 7.869 5.564 243.9 5.5 380LaFe0.7Mg0.3O3 P 5.575 7.869 5.563 244.1 7.9 235LaFe0.6Mg0.4O3 P+ L 5.598 7.879 5.561 245.3 9.7 200LaFe0.5Mg0.5O3 P+ L 5.600 7.881 5.561 245.4 5.3 170

a Phases detected by XRD (P, perovskite; L, La2O3). Perovskite lattice parameters from ASTM cards [23]. SmFeO3, a = 5.598,b = 7.709,c = 5.398 Å, V = 232.95 Å3; NdFeO3, a = 5.583,b = 7.757,c = 5.453 Å, V = 236.16 Å3 and LaFeO3, a = 5.567,b = 7.855,c = 5.553 Å, V = 242.8 Å3.

conversion was calculated as the average of at leastthree measurements. Carbon balance was closed towithin ±5% in all catalytic tests.

CO oxidation tests were carried out in a fixed bedof catalyst particles (0.5 g) supported on a silica fitteddisk inside a quartz tube. The reactor was placed in atubular PID-regulated oven and the temperature wasmonitored with a Ni-NiCr thermocouple positionedin correspondence to the catalyst bed. The gaseousflow rates were measured by MKS 1259 mass flowmeters and mixed at atmospheric pressure to obtaininlet concentrations of 1% CO, 20% O2, He as bal-ance, with a space velocity of 12,000 N cm3 g−1 h−1.Reaction temperature was raised at 1 K min−1 from473 to 873 K, and product stream was analyzed bygas-chromatography, using an Alltech CTR 1 col-umn (made of coaxial Porapak and Molecular Sievecolumns) and a TCD detector. Carbon balance wasclosed to within±5% in all experiments, CO2 beingthe only reaction product detected.

3. Results and discussion

3.1. Materials characterization

Table 1 reports the observed values of the latticeparameters, SA and particle sizes for each catalyst. Thephases detected by X-ray diffraction (XRD) analysisare also shown.

The nominal metal contents agree within 5%with the experimental ones. The followings valueswere obtained for the experimental and nominal (in


parentheses) values: LaFe1−xMgxO3 samples, forx = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, respectively; La=57.8 (57.2), 56.1 (57.9), 58.4 (58.7), 57.3 (59.5), 57.8(60.3), 58.1 (61.2); Mg= (0.0), 1.06 (1.01), 2.07(2.05), 2.96 (3.13), 3.95 (4.23), 5.09 (5.35); Fe= 21.9(23.0), 20.2 (20.9), 18.4 (18.9), 16.0 (16.8), 13.9(14.5), 12.0 (12.3).

• NdFeO3: Nd = 56.7 (58.1), Fe= 21.8 (22.5).• SmFeO3: Sm= 56.9 (58.1), Fe= 21.8 (22.5).

Fig. 1 shows that NdFeO3, SmFeO3, LaFeO3and LaFe1−xMgxO3 samples are single phaseperovskite-type oxides. However, the XRD pattern ofthe LaFe1−xMgxO3 catalyst withx = 0.4 reveals, inaddition to perovskite, the presence of a very weakpeak at 2θ = 29.9◦ (d = 2.98 Å) which correspondsto the strongest line of La2O3 ([23],a). For thex = 0.5sample, in addition to the above peak, a very weakpeak (2θ = 26.1◦, d = 3.41 Å), also belonging toLa2O3, is visible. NdFeO3 ([23],b), SmFeO3 ([23],c),

Fig. 1. XRD spectra for the LaFeO3, LaFe1−xMgxO3, SmFeO3 andNdFeO3 samples. Asterisks indicate the strongest lines of La2O3

([23],a). Reference X-ray lines belonging to NdFeO3 ([23],b) andLaFeO3 ([23],d) are given at the top and bottom, respectively.

LaFeO3 ([23],d) and LaFe1−xMgxO3 exhibit the or-thorhombic perovskitePnmastructure. Note that thecrystallinity of the samples is very high for NdFeO3and LaFeO3, lower for SmFeO3, and is continuouslydecreasing at the increase of magnesium content inthe LaFe1−xMgxO3 samples.

The particle sizes,D, are in 170–610 Å range. Theirvalues reflect the trend of the crystallinity shown bythe XRD patterns. For the LaFe1−xMgxO3 solid so-lutions the continuous decrease ofD with increasingMg content agrees with the opposite trend observedfor the SA values (apart from the sample withx = 0.5,where La2O3 is present in higher amount) increasingfrom 2.9 to 9.7 m2 g−1 for x = 0 and 0.4, respectively.

The evaluation of the unit cell parameters showsthat NdFeO3, SmFeO3 and LaFeO3 have a, b, andc values similar to those quoted in the literature([23],b–d). Moreover, the trend ofV (reported inFig. 2), i.e. SmFeO3 < NdFeO3 < LaFeO3, agreeswith expectations based on the ionic radii values ofthe involved rare earth metals in 12-coordination sym-metry (rSm

3+ = 1.24, rNd3+ = 1.27, rLa

3+ = 1.36 Å[24]).

Lattice symmetry changes from orthorhombic tonearly cubic in the series SmFeO3 → NdFeO3 →LaFeO3. In fact, as shown in Table 1, the lattice pa-rametera decreases slightly, whereas bothb andc in-crease in such a way that, in LaFeO3, c approachesthe value ofa, andb (7.866 Å) is equal to the doublelength of the cubic pseudo-cell dimension (3.932 Å).

The average A–O and Fe–O bond distances wereevaluated by taking into account the half value ofbas the cell dimension of the idealized cubic unit cell

Fig. 2. Cell volume as a function of the substitutional parameterx. (d) LaFe1−xMgxO3, (m) NdFeO3, (j) SmFeO3.


(A–O =b√

2/4, Fe–O= b/4), and gave the followingvalues.

• In SmFeO3, Sm–O= 2.73, Fe–O= 1.93.• In NdFeO3, Nd–O= 2.74, Fe–O= 1.94.• In LaFeO3, La–O= 2.78, Fe–O= 1.97.

If the distances expected from the sum of ionicradii (Sm–O= 2.64, Nd–O= 2.67, La–O= 2.76,HS–Fe3+–O = 2.045 Å [24]) are considered, wefind that the above estimated A–O bond distancesare higher (especially for SmFeO3 and NdFeO3) andthe Fe–O ones are lower than the expected ones. Itmight be thus deduced that a polarization of oxy-gens towards iron occurs in the AFeO3 catalystswith the effect of some degree of covalence in theFe–O bonds, whose nature may also depend on thetype of the partner A rare-earth ion. It seems, more-over, that, among the AFeO3 perovskites, the Fe–Obond is slightly stronger in SmFeO3 than in NdFeO3and LaFeO3. The increasing Fe–O bond strength(SmFeO3 > NdFeO3 > LaFeO3) may be relevant fora correlation between structure and catalytic activityin methane combustion, where lattice oxygen is likelyinvolved.

Concerning the LaFe1−xMgxO3 solid solutions aslight increase of the unit cell volume,V, is observedwith increasing magnesium content (Fig. 2). This isthe result of the combined effect of the substitutionof Fe3+ ions by larger Mg2+ and smaller Fe4+ ionsin the octahedralB sublattice of the perovskite struc-ture (high-spin,rFe

3+ = 0.645 Å, rFe4+ = 0.585 Å,

rMg2+ = 0.72 Å [24]).

Table 2 and Fig. 3a and b describe the main featuresof the TPR experiments. TPR profiles of LaFeO3 andSmFeO3 show the presence of a peak at low temper-ature with the maximum at about 560 K and two sig-nals at higher temperature. The TPR curve of SmFeO3also shows a signal in the isothermal region due to adeeper reduction of this sample at high temperature.For NdFeO3 a larger peak is present after the firstone at lower temperature. The TPR patterns of theLaFe1−xMgxO3 catalysts show that the low tempera-ture peak increases with increasing magnesium con-tent up tox = 0.2, then slightly decreases for furtherx increase. In Table 2 the total H2 uptake and that re-ferred to the first signal are reported. The total amountof H2 consumed in all experiments give evidence ofthe low extent of iron reduction compared with other

transition metal cations, namely manganese and cobaltin lanthanum based perovskites [22]. Values of theH2/Fe ratio far from 0.5 indicate that we cannot as-sume the reduction of all Fe3+ present in the samplesto Fe2+. Note that the reduction of Fe3+ to Fe2+ andfurther to metallic iron in LaFeO3 has been reported[11].

The intensity of the first peak, very weak for the un-substituted perovskites but increasing with Mg substi-tution for Fe in LaFe1−xMgxO3 samples is due to thereduction to Fe3+ of the fraction of Fe4+ compensatingthe lower charge of Mg2+. XRD analysis of reducedsamples, performed (i) after stopping TPR run at theend temperature of the first peak and (ii) after the over-all TPR experiment, confirmed the supposed reduc-tion of Fe4+ to Fe3+. In fact, the perovskite structureis always preserved with the absence of extra phases,but a definite shifting towards lower 2θ angles (cor-responding to lattice expansion) appears in the XRDpatterns of the reduced LaFe1−xMgxO3 samples witha higher fraction of Fe4+. By taking into account theionic radii of the involved cations, the observed unitcell expansion is likely due, as suggested by TPR, tothe reduction to Fe3+ of the fraction of Fe4+ inside theperovskite structure, which in order to preserve chargeneutrality, also becomes more oxygen deficient. A par-tial reduction of Fe3+ to Fe2+ could likely occur athigher temperature even if it does not lead to the lossof the perovskite structure. The peak appearing in theTPR curve of LaFe0.5Mg0.5O3 at about 723 K couldbe attributed to the reduction of some more stronglybonded iron within the perovskite.

On the basis of the TPR hydrogen consumption,the amount of Fe4+ in the LaFe1−xMgxO3 samplesis reported in Fig. 4 in terms of molar ratio of Fe4+to perovskite and to Mg2+ ions, as a function of theMg substitution. The figure shows that the content ofFe4+, present in small amounts also in the unsubsti-tuted LaFeO3 perovskite, reaches a maximum value incorrespondence tox = 0.2. If the charge defectivitydue to Mg substitution were totally balanced by Fe4+a constant Fe4+/Mg2+ ratio = 1 should be expectedfor each LaFe1−xMgxO3 sample. However, only sam-ples withx = 0.2 show a ratio close to 1, while thisvalue strongly decreases forx > 0.2, indicating thatthe amount of Fe4+ is much lower than that neces-sary to the charge balance. Therefore, oxygen vacan-cies should compensate the positive charge defectivity

244P.

Cia

mb

elli

et

al./A

pp

lied

Ca

talysis

B:

Enviro

nm

en

tal

29

(20

01

)2

39

–2

50


Fig. 3. (a) TPR profiles of AFeO3 perovskites. The length of thevertical bar corresponds to an H2 consumption of 1mmol g−1 K−1;(b) TPR profiles of LaFe1−xMgxO3 perovskites. The length of thevertical bar corresponds to an H2 consumption of 1mmol g−1 K−1.

[11,15,18]. According to this assumption, in the samefigure the nonstoichiometryδ of the perovskite (oxy-gen excess or defect necessary to close the charge bal-ance) is reported as a function of the Mg substitution.Its value is very close to 0 forx ≤ 0.2 but increasesup to−0.2 for LaFe0.5M0.5O3.

Fig. 4. Effect of magnesium substitution on nonstoichiometry ofLaFe1−xMgxO3 perovskites.

In conclusion, AFeO3 perovskites show a very lowoxidative nonstoichiometry (+δ), also reported byTejuca et al. [11], whereas the LaFe1−xMgxO3 solidsolutions exhibit a weak reductive nonstoichiometry(−δ) for x ≤ 0.2 that becomes very significant forsamples with a higher Mg content. The chemicalformulae, reported in Table 2 for each sample, werededuced accordingly to the above considerations.

3.2. Methane combustion

In the experimental conditions investigated all cat-alysts exhibited good activity in the total oxidationof methane. The contribution of homogeneous reac-tions to methane conversion up to the temperature of1023 K was verified to be negligible by test runs car-ried out with the reactor filled only with quartz parti-cles. Fig. 5a, reporting the CH4 conversion as functionof the reaction temperature, shows that complete con-version of methane is attained with all AFeO3 samplesat temperatures below 973 K with total selectivity toCO2 over the whole range of temperatures.

The comparison of the temperature values for 10,50 and 90% methane conversion reported in Table 2indicates that best catalytic performances among theAFeO3 perovskites are shown by the lanthanum ferritesample. Nd and Sm perovskites have similar overallactivities, but lower with respect to LaFeO3, and their


Fig. 5. (a) CH4 conversion as a function of temperature for AFeO3 perovskites, homogeneous reaction; (b) Arrhenius plots of CH4

combustion for AFeO3 perovskites; (c) CH4 conversion as a function of temperature for LaFe1−xMgxO3 perovskites; (d) Arrhenius plotsof CH4 combustion for LaFe1−xMgxO3 perovskites.

relevant conversion curves intersect at about 843 K,NdFeO3 becoming less active than SmFeO3 after thattemperature. Arrhenius plots (Fig. 5b) obtained assum-ing isothermal plug flow conditions and methane firstorder reaction rate expression (zero order for oxygen)show a linear trend over the entire temperature range,suggesting the presence of a single (kinetic) regime.The calculated values of the apparent activation energyare reported in Table 2 together with the correspond-ing pre-exponential factors of the Arrhenius expres-sion. LaFeO3 and NdFeO3 have the same activationenergy (about 21 kcal mol−1), suggesting that the typeof active sites involved in the reaction is nearly inde-pendent of the nature of rare earth cation. A highervalue of Ea (23.7 kcal mol−1) is shown by SmFeO3,with a SA larger than both LaFeO3 and NdFeO3.

The reaction rate both per unit weight and unit SA(RR and SRR in Table 2) clearly shows that the or-der of catalytic activity for the AFeO3 compounds isLa > Nd > Sm. The above behavior reflects the in-creasing Fe–O bond strength (LaFeO3 < NdFeO3 <

SmFeO3) deduced from bond distance considerations,

confirming the major role of the lattice oxygen speciesin methane oxidation.

Partial substitution of magnesium for iron in theLaFe1−xMgxO3 perovskites, leading to the formationof a higher fraction of Fe4+ (evidenced by TPR), re-sults in a progressive reduction of the overall activityat least at low temperature (conversion plot in Fig. 5c,and reaction rates per unit SA, SRR, in Table 2). Sam-ples with magnesium content up tox = 0.3, which canbe regarded as pure perovskite phases, all show acti-vation energies of about 23.4 kcal mol−1, higher thanthat found for the LaFeO3 catalyst (20.8 kcal mol−1).Samples with higher Mg2+ substitution (x = 0.4 and0.5) show a still increased activation energy value(25.6 kcal mol−1), which is probably related to thepresence of different segregated phases (revealed byXRD analysis) and/or to even higher oxygen defi-ciencies in the perovskite structure. In fact, as clearlyshown by TPR experiments, the introduction of Mg2+is balanced not only by an induced oxidation state ofiron higher than 3+, but also by the formation of an-ion vacancies (see formulae in Table 2). Moreover,


the presence of more ionic antagonistic Mg–O bondmay cause a polarization of oxygen towards iron, withthe effect of stronger Fe–O bonds. All these circum-stances lead (at the increase ofx in the LaFe1−xMgxO3perovskites) to more strongly bonded oxygen species[18,25], which require higher temperatures to start par-ticipating a redox mechanism with methane.

A preliminary kinetic study has been performed formethane combustion on the three most active cata-lysts investigated, i.e. LaFe1−xMgxO3 with x = 0.0,0.1 and 0.2, in order to further elucidate the effect ofmagnesium substitution for iron. Several catalytic ac-tivity measurements have been carried out at differenttemperatures by varying both CH4 and O2 concen-trations in the feed. Resulting conversion data havebeen modeled by the power law rate equation:r =k × pn

CH4× pm

O2, assuming isothermal plug flow con-

ditions. Fig. 6 shows a reduction of methane conver-sion to CO2 with increasing inlet CH4 concentrationfor the three samples. Since it is possible to assumeconstant O2 concentration throughout the reactor inthese runs (oxygen being fed in large excess with re-spect to methane oxidation stoichiometry), this behav-ior is due to a less than linear dependence of the re-action kinetics on CH4 concentration. The simplifiedkinetic model used to evaluate the apparent reactionorder for methane:r = k′ × pn

CH4givesn values for

LaFeO3 almost constant with temperature (n = 0.77).

Fig. 6. Effect of inlet methane partial pressure on overall conversionat 783, 833 and 873 K over LaFe1−xMgxO3 perovskites (squarex = 0.0, trianglex = 0.1, circle x = 0.2).

This result is slightly different from the first orderdependence generally reported for total oxidation ofmethane over most perovskite-type oxides (at higherCH4 partial pressure) [8].

Even in the case of Mg substituted samples we haveevaluated a dependence of reaction rate on methanepartial pressure lower than the first order reportedby Saracco et al. [26,27] for LaCr1−xMgxO3 andLaMn1−xMgxO3 perovskites. With LaFe1−xMgxO3(x = 0.1 and 0.2)n is equal to 0.71 atT = 783 K,but it increases with increasing temperature up to thesame value calculated for the unsubstituted orthofer-rite (n = 0.77). This behavior could be likely relatedto an inhibiting effect caused by products adsorption(CO2 and/or water) on the catalysts surface, enhancedby the presence of basic Mg sites. Such adsorptioneffect is expected to be more significant at lower tem-peratures, while it tends to disappear with increasingtemperature. Otherwise, the mobility of more stronglybonded oxygen species, whose presence was assumedfor LaFe1−xMgxO3 perovskites, could increase athigher temperature enhancing the reaction order forMg substituted samples. As a consequence, at highertemperature both substituted and unsubstituted per-ovskites should exhibit the same catalytic behavior.

Better agreement with the results reported in the lit-erature [8,26] has been found with respect to the ef-fect of O2 concentration on CH4 conversion to CO2.Fig. 7 shows that higher conversions are obtained by

Fig. 7. Effect of inlet oxygen partial pressure on overall methaneconversion at 783, 833 and 873 K over LaFe1−xMgxO3 perovskites(squarex = 0.0, trianglex = 0.1, circle x = 0.2).


Fig. 8. (a) CO oxidation as a function of temperature for AFeO3 perovskites; (b) Arrhenius plots of CO oxidation for AFeO3 perovskites;(c) CH4 conversion as a function of temperature for LaFe1−xMgxO3 perovskites, homogeneous reaction; (d) Arrhenius plots of COoxidation for LaFe1−xMgxO3 perovskites.

increasing oxygen inlet partial pressure in a range ofvalues where it is still in large excess. This effect, asmodeled by the power law rate equation, reveals a re-action order for O2 in 0.12–0.16 range for all samples.

3.3. CO oxidation

As far as the catalytic CO oxidation is concerned itshould be pointed out that we have investigated only

Table 3SmFeO3, NdFeO3, LaFeO3 and LaFe1−xMgxO3 perovskites calcined at 1073 Ka

Catalyst T10 T50 T90 Ea

(kcal mol−1)A(L m−2 h−1×10−7)

RR(mmol g−1 h−1)

SRR(mmol m−2 h−1)

SmFeO3 553 639 715 15.9 0.11 0.86 0.20NdFeO3 529 594 661 18.3 3.36 1.95 0.85LaFeO3 545 621 673 18.1 1.44 1.11 0.38LaFe0.9Mg0.1O3 530 598 669 17.3 0.82 1.59 0.37LaFe0.8Mg0.2O3 535 606 655 18.1 0.87 1.51 0.28LaFe0.7Mg0.3O3 544 615 667 18.1 0.52 1.28 0.16

a Data for CO oxidation: temperature values for 10%,T10; 50%, T50 and 90%,T90, conversion to CO2; activation energy (kcal mol−1),Ea; pre-exponential factor (L m−2 h−1 × 107), A; reaction rate (mmol g−1 h−1), RR, at 573 K; surface reaction rate (mmol m−2 h−1), SRR,at 573 K.Y CO = 0.01, Y oxygen = 0.2.

the catalysts showing a single perovskite phase. Theconversion plots reported in Fig. 8a and c show thatthe overall activities are very similar for all samples,in a temperature range in which the contribution of ho-mogeneous reaction is negligible. The correspondingArrhenius plots reported in Fig. 8b and d, obtained as-suming first order oxygen rate equation (large oxygenexcess) and isothermal plug flow reactor, are linearand suggest that a single kinetic regime is operative.


Concerning the activity scale per unit SA (SRRin Table 3), NdFeO3 is the most active catalyst, andamong the unsubstituted AFeO3 samples the order isNd > La > Sm. LaFeO3 and NdFeO3 catalysts showvery similar apparent activation energies of about18 kcal mol−1 (Table 3), whereasEa = 15.9 kcalmol−1 for SmFeO3. The different activities could belikely related to the difference in the total number ofactive sites, which is not directly related to the exten-sion of total exposed surface. Indeed, NdFeO3, whichhas the lowest SA, shows a surface reaction rate 2.3times larger than LaFeO3, due to a higher density ofactive sites.

The introduction of magnesium in LaFeO3 does notseem to affect the activity at the first stage of substitu-tion (x = 0.1), whereas a clear decrease in activity isobserved at higher Mg content, as shown in Table 3 bythe surface reaction rates, SRR. The lower activity ofthe LaFe1−xMgxO3 samples with higher magnesiumcontent may be related, as previously suggested for themethane combustion reaction, to the presence of morestrongly bonded oxygen species, which require highertemperatures to start participating a redox mechanismalso with CO.

4. Conclusions

The main points put forward in this study can besummarized as follows.

1. All the examined samples, except LaFe1−xMgxO3with x = 0.4 and 0.5, are single perovskite phases.

2. A small fraction of Fe4+ is present, in addition toFe3+, in all the AFeO3 catalysts, the content ofFe4+ being compensated by cation defectivity.

3. The fraction of Fe4+ increases withx (up to x =0.2) in the LaFe1−xMgxO3 catalysts. However, theFe4+/Mg2+ ratio is lower than that necessary tocharge compensation which is achieved by an in-crease of anion vacancies concentration.

4. All the samples, after reduction, preserve the per-ovskite structure with expansion of the unit cellvolume as a consequence of the higher content oflarger Fe3+ ions.

5. All the catalysts give complete methane and COconversion with 100% selectivity to CO2 below 973and 773 K, respectively.

6. For methane combustion the order of activ-ity per unit SA among the AFeO3 catalysts isLaFeO3 > NdFeO3 > SmFeO3, whereas the ac-tivity decreases with the increase of Mg in theLaFe1−xMgxO3. The difference in activity hasbeen correlated with the structural characteris-tic of the catalysts which in SmFeO3 and in theLaFe1−xMgxO3 solid solutions have more stronglybonded lattice oxygen, so requiring slightly highertemperatures to start reacting and thus limiting tosome extent the overall activity.

7. Concerning the CO oxidation, NdFeO3 is the mostactive among the AFeO3 catalysts, while the activ-ity per unit SA decreases in LaFe1−xMgxO3 solidsolutions at higher Mg content, a behavior whichis also related to more strongly bonded latticeoxygen.

References

[1] R.L. Garten, R.A. Dalla Betta, J.C. Schlatter, in: G. Ertl, H.Knozinger, J. Weitkamp (Eds.), Handbook of HeterogeneousCatalysis, Vol. 4, VHC Weinheim, Germany, 1998, p. 1668.

[2] L.G. Tejuca, J.L.G. Fierro (Eds.), Properties and Applicationsof Perovskite-Type Oxides, Marcel Dekker, New York, 1993.

[3] R.E. Newman, in: A. Navrotsky, D.J.Weidner (Eds.), Structure–Property Relationship in PerovskiteElectroceramics: Perovskite: A Structure of Great Interestto Geophysics and Material Science, American GeophysicalUnion, Washington, DC, 1989, p. 66.

[4] R.J.H. Voorhoeve, Advanced Materials in Catalysis, AcademicPress, New York, 1977, p. 129.

[5] M.F. Zwinkels, S.G. Järås, P.G. Menon, Catal. Rev. Sci. Eng.35 (1993) 319.

[6] K. Eguchi, H. Arai, Catal. Today 29 (1996) 379.[7] J.G. Mc Carty, H. Wise, Catal. Today 8 (1990) 231.[8] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26

(1986) 265.[9] L.G. Tejuca, J. Less Common Met. 146 (1988) 261.

[10] R.J.H. Voorhoeve, D.W. Johnson Jr., J.P. Remeika, P.K.Gallagher, Science 195 (1977) 827.

[11] L.G. Tejuca, J.L.G. Fierro, J.M.D. Tascón, Adv. Catal. 36(1989) 237.

[12] A. Baiker, P.E. Marti, P. Keusch, E. Fritsch, A. Reller, J.Catal. 146 (1994) 268.

[13] M. Futai, C. Yonghua, Louhui, React. Kinet. Catal. Lett. 31(1986) 47.

[14] P. Ciambelli, S. Cimino, S. De Rossi, M. Faticanti, L. Lisi,G. Minelli, I. Pettiti, P. Porta, G. Russo, M. Turco, Appl.Catal. B 24 (2000) 243.

[15] T. Seiyama, Catal. Rev. Sci. Eng. 34 (1992) 281.[16] Z. Zhong, K. Chen, Y. Li, D. Yan, Appl. Catal. A 156 (1997)

29.


[17] P. Salomonsson, T. Griffin, B. Kasemo, Appl. Catal. A 104(1993) 175.

[18] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura, N. Yamazoe,J. Catal. 121 (1990) 432.

[19] (a) B. Delmon, J. Droguest, in: W.H. Fuhn (Ed.), Proceedingsof the 2nd International Conference on Fine Particles, TheElectrochemical Society, 1973, p. 242; (b) Belg. Patent No.735, 476 (1069).

[20] T.J.B. Holland, S.A.T. Redfern, J. Appl. Crystallogr. 30 (1997)84.

[21] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures forPolycrystalline and Amorphous Materials, Wiley, London,1962, p. 491.

[22] L. Lisi, G. Bagnasco, P. Ciambelli, S. De Rossi, P. Porta,G. Russo, M. Turco, J. Solid State Chem. 146 (1999)176.

[23] X-ray Powder Data File, ASTM cards: (a) 5-602 for La2O3;(b) 25-1149 for NdFeO3; (c) 39-1490 for SmFeO3; (d)37-1493 for LaFeO3.

[24] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751.[25] J.E. France, A. Shamsi, M.Q. Ashan, Energy Fuels 2 (1988)

235.[26] G. Saracco, G. Scibilia, A. Iannibello, G. Baldi, Appl. Catal.

B 8 (1996) 229.[27] G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B 20 (1999)

277.

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Documents