Effect of substitution by cerium on the activity of LaMnO3 perovskite in methane combustion

Applied Catalysis A: General 245 (2003) 231–243

Effect of substitution by cerium on the activity of LaMnO3 perovskite in methane combustion M. Alifanti a , J. Kirchnerova b , B. Delmon a,∗ a

b

Unité de Catalyse et de Chimie des Matériaux Divises, Université Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain la Neuve, Belgium Département de génie chimique, École Polytechnique, C.P. 6079 Succ Centre-Ville, P.Q., Montréal, Canada H3C 2A7 Received 29 July 2002; received in revised form 4 December 2002; accepted 4 December 2002

Abstract This study concerns the effect of lanthanum substitution by cerium on the catalytic activity of La1−x Cex MnO3 catalysts and its relation to their physico-chemical characteristics. Samples of pure and cerium-substituted lanthanum manganese perovskites, La1−x Cex MnO3 with x = 0.1–0.5 and LaCex MnO3 with x = 0.1, 0.2 and 0.3, were prepared by the citrate method and calcined 5 h at 973 or 1073 K. All samples were characterized by XRD, XPS and oxygen TPD and had their specific surface area (SSA) determined by nitrogen adsorption. The catalytic activity was determined, using 0.1 g catalyst, 1% methane in air at a flow rate of 75 ml/min (GHSV = 45,000 ml/gcat h). Substitution with cerium affects significantly the physico-chemical properties of individual compositions. It slows the rate of perovskite phase formation, increases the SSA, has an effect on thermal stability and modifies the oxygen desorption characteristics. However, these changes do not correlate in the expected way with changes in activity for methane combustion. Substitution with cerium or addition of cerium over the formal stoichiometry were positive only for x = 0.1 in samples calcined 5 h at 973 K. Higher x values resulted in lower activity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cerium substituted lanthanum manganese perovskites; Catalytic methane combustion; Specific surface area in relation to activity in methane combustion

1. Introduction Over the last 30 years, the catalytic combustion of methane and other light hydrocarbons has been the subject of numerous studies [1–4]. The objective was to find suitable materials allowing the reaction at low temperature and resisting both to the high temperatures attained during the combustion and the corrosive atmosphere consisting of certain combustion products. These include precious metals [5], ∗ Corresponding author. Tel.: +32-10-473591; fax: +32-10-473649. E-mail address: [email protected] (B. Delmon).

single base-metal oxides [6,7], as well as a whole range of both single phase and multi-phase mixed oxide compositions [8–11]. Lanthanum-transition metal perovskites (LaMO3 ) have enjoyed a lot of attention [12–16]. Considering single oxides with high specific surface area (SSA), the activity in total methane oxidation (combustion) decreases approximately in the following order: PdO  Co3 O4 > Mn2 O3 > MnO2 > CuO > NiO > Fe2 O3 > Cr 2 O3  CeO2 [7]. Ceria is not a very good catalyst by itself, but can dramatically enhance the activity of some of the transition metal oxides, in particular, PdO, CuO, MnOx or Co3 O4 , or vice versa [5,8,17,18]. However, the high activity of these materials is limited to relatively low

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00644-0

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working temperatures for two reasons: oxide instability and loss of specific surface area after operation at high temperatures. On the other hand, LaMO3±λ perovskites are characterized by substantially higher thermal stability and the corresponding stabilization of unusual oxidation states of the active metal ions. High oxygen mobility observed in many perovskites is generally considered as an additional advantage. This suggests that the association of LaMO3 perovskites with cerium might bring about interesting effects. The catalytic activity of LaMO3 perovskites tends to decrease in an order similar to that of the corresponding single base metal oxides MOx and their thermal stability also correlates with that of individual parent oxides. For example, among the stable binary perovskites, the oxygen stoichiometric LaCoO3 has in several cases been mentioned as the most active [13,16]. But the oxygen over-stoichiometric LaMnO3+λ more frequently exhibits the highest activity [12,16]. This is undoubtedly due in part to its higher thermal stability and inherently larger SSA [19–25]. Nevertheless, its defective and cation deficient lattice, and the presence of manganese in two oxidation states (Mn3+ /Mn4+ ), resulting in a relatively stable and constant oxygen excess [26], have been considered as being the main origin of the high activity of LaMnO3+λ (which, for convenience, will be simply represented in this paper by LaMnO3 , except in specific situations). Catalytic activity of binary perovskites can further be enhanced by partial substitution of lanthanum. This can induce oxygen non-stoichiometry. This might have two consequences, namely (i) the formation of a larger proportion of metal ions in unstable oxidation states, and (ii) an overall enhancement of oxygen mobility. For example, La1−x Srx MO3 perovskites are typically more active than LaMO3 , even in the case of La1−x Srx MnO3 [12,23]. Interestingly, improvement in activity can also be obtained in some cases by lanthanum substitution with cerium, such as in La1−x Cex CoO3 , La1−x Cex FeO3 or La1−x Cex MnO3 [20,21,27–31], but the effect is quite different according to cases. For instance, while a significant activity enhancement in methane combustion by cerium substitution was observed in the case of La1−x Cex CoO3 [17,28–30], the improvement remained rather small in La1−x Cex FeO3 [29]. Likewise, in propane combustion, a high activity increase due to cerium substitution up to x = 0.6 was observed for

the system La1−x Cex MnO3 [31]. On the other hand, although LaMnO3 and La1−x Srx MnO3 are possibly the most studied perovskites in methane combustion [12,16,19–25,27], only two publications provide information concerning La1−x Cex MnO3 [21,22]. In a series of La0.9 A0.1 MnO3 , the A-site substitution brought basically no improvement of activity of La0.9 Ce0.1 MnO3 in comparison with LaMnO3 [21]. Another publication concerning A subsitituents in La1−x Ax MnO3 included La0.7 Ce0.3 MnO3 . In methane and carbon monoxide combustion, La0.7 Ce0.3 MnO3 was found to be substantially more active for CO combustion than LaMnO3 , but was less active in methane combustion [22]. Our previous study indicated that a significant increase of catalytic activity in methane combustion was caused by cerium substitution, even beyond its solubility limit in the La1−x Cex CoO3 perovskite phase [17]. This effect could in part be interpreted by the existence of a cooperation between the different phases detected in the system. The aim of the present study was to understand the factors controlling the properties of catalysts of comparable activity, namely those corresponding to the La1−x Cex MnO3 composition, and especially to detect the possible existence of similar phenomena. Individual samples prepared by the citrate method were systematically characterized by XRD, XPS, TPD-O2 and activity measurements. For an objective comparison and an assessment of the role of the perovskite structure in the activity, samples with cerium in excess over formal stoichiometry, namely LaCex MnO3 , with x = 0.1, 0.2 and 0.3, as well as Mn2 O3 and CeO2 prepared by the same method as the other catalysts, were included in the study. 2. Experimental 2.1. Catalyst preparation All La1−x Cex MnO3 and LaCex MnO3 samples were prepared by the citrate method, which allows the formation of amorphous citrates of metals with a wide flexibility of compositions [17,32]. Thermal decomposition of these precursors leads to mixed oxides or solid solutions of high homogeneity. In our preparation procedure, the corresponding nitrates (La(NO3 )3 ·6H2 O (Aldrich), Ce(NO3 )3 ·6H2 O (Fluka),

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and Mn(NO3 )2 ·4H2 O (Merck)) in the appropriate quantities were dissolved in deionized water to give 0.1 M solutions. Citric acid monohydrate (Merck 244) was added in 10 wt.% excess over the stoichiometric quantity to insure full complexation of the metal ions. Water was removed on a rotary evaporator at 313 K until the appearance of a gel. The obtained viscous material was dried overnight in a vacuum oven at 343 K. During this treatment an intense production of nitrogen oxides occurred. The resulting spongy, highly hygroscopic, amorphous material was then crushed and calcined in air for 5 h at 973 or 1073 K to obtain the desired phases. 2.2. Catalyst characterization BET specific surface areas (SSA) were determined by nitrogen adsorption at 77 K on a Micromeritics ASAP 2000 instrument with samples of 0.15 g. Prior to each analysis the powders were degassed 2 h at 423 K under a pressure of 0.1 Pa. XRD patterns were collected by means of a Kristalloflex Siemens D5000 diffractometer using the Cu K␣ radiation at λ = 1.5418 Å. The powders were mounted on silicon monocrystal sample holders. Data acquisition was realized in the 2θ range 2–65◦ with a scan step size of 0.03◦ . XPS spectra were recorded at room temperature under a vacuum of 10−7 Pa on a SSX-100 Model 206 Surface Science Instrument spectrometer with monochromatized Al K␣ radiation (hν = 1486.6 eV). The charge neutralization was achieved using an electron flood-gun adjusted at 10 eV and placing a Ni grid 3 mm above the sample. Atomic composition of the surface was calculated using the sensitivity factors (Scofield) provided by the instrument software. For deconvolution, peaks were considered to be Gaussian for 85% and Lorentzian for 15%. The baseline was considered linear and tangent to the peak wings. The charge correction was made considering that the C1s signal of contaminating carbon (C–C or C–H bonds) was centered at 284.8 eV. 2.3. Thermo-programmed desorption of oxygen (TPD-O2 ) TPD-O2 measurements were performed at atmospheric pressure in a conventional installation.

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Usually, 0.12 g of pelletized catalyst powder (40– 100 ␮m particle size) was loaded in a commercial U-shape quartz microreactor having a reaction zone of 1 cm diameter with inserted frit to hold the catalyst and a thermocouple well pointing to the center of the catalyst bed. The pretreatment procedure comprised heating the catalyst from room temperature to 973 K in a flow of 5% O2 (He as balance) at a rate of 10 K/min, maintaining 30 min at 923 K, followed by cooling to room temperature. The gas flow (45 ml/min) was then switched to pure He and the system was thoroughly flushed for 2 h. Finally, for the measurements, the temperature was raised to 973 K at a heating rate of 10 K/min while the oxygen evolution was followed by a Hewlett–Packard G1800A gas chromatograph equipped with a quadrupole mass-spectrometer. Calibration was made by injecting four different volumes of 5% oxygen in helium. It was indirectly confirmed by measurement on the LaMnO3.123 perovskite and compared with other works [26,31]. 2.4. Catalytic activity measurements Catalytic activity in the combustion of 1% methane in air was determined using 0.10 g pelletized catalyst broken to 40–100 ␮m particle size, loaded in a U-shape quartz microreactor having the same design as the one used for TPD measurements. The reactor was operated in a down-flow mode at atmospheric pressure. Prior to each evaluation, the catalyst was activated 2 h at 973 K under flow of air and then cooled down to 573 K. This period of time was chosen after a preliminary study in which the reactor outlet flow was monitored by a mass spectrometer. After two hours of activation, the relative ion intensities of the masses 18 and 44 (H2 O and CO2 ) reached a plateau and the surface was considered clean. For activity determination, 1 vol.% methane in air flowing at 75 ml/min was admitted and the reactor was heated up to 973 K at a rate of 2 K/min, namely we made a measurement leading to a so-called “light-off” curve. The total gas hourly space velocity (GHSV) was fixed at 45,000 ml/gcat h. The outlet and inlet gas compositions were followed using an on-line Delsi 2000 gas-chromatograph, equipped with a Carbosphere packed column and a thermal conductivity detector (TCD). Helium was used as a carrier gas at a flow of 25 ml/min and the analysis was conducted

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isothermally at 423 K. The only reaction products were CO2 and H2 O. Some measurements were made at random during sample cooling. They gave values corresponding to those measured in the “light off curve”. We did not detect changes. 3. Results 3.1. Physico-chemical characteristics 3.1.1. XRD and specific surface area The XRD patterns of the La1−x Cex MnO3 samples of this study, calcined 5 h at 973 K for one set and at 1073 K for the other, are shown in Fig. 1a and b, respectively. Among the samples calcined at 973 K, only those of compositions LaMnO3 and La0.9 Ce0.1 MnO3 formed a well crystallized perovskite phase, the patterns showing no other peaks. The formation of single phase LaMnO3 under such mild calcination conditions has been repeatedly reported in literature [22,33]. In fact, well crystallized LaMnO3 perovskite can be obtained by calcination at as low a temperature as 873 K [22,24]. The patterns of LaMnO3 correspond clearly to the rhombohedral lattice geometry, actually to that of the non-stoichiometric LaMnO3.15 perovskite (JCPDS data file 86-1234). The same pattern was apparently observed in other laboratories [22], even when slightly different conditions of preparation were used [18,21]. Compositions with higher Ce content (x = 0.2, 0.3, 0.4 and 0.5) calcined 5 h at 973 K were amorphous, the patterns barely suggesting the onset of the perovskite phase formation or the incipient presence of ceria. Some retardation in the formation of the perovskite phase was also observed in the La1−x Cex CoO3 compositions with x > 0.2 calcined at 973 K, although in a somehow different way [17]. On the other hand, all samples calcined 5 h at 1073 K were crystalline, but the width of the main peaks indicated progressively smaller crystallite size as the ceria content increased. No peaks of phases other than perovskite and ceria (reflections at 2θ = 28.6 and 56.4◦ corresponding to cerianite (JCPDS file 43-1002)) were detected in any of the patterns. This contrasts with the patterns of La1−x Cex CoO3 compositions, in which the presence of ceria was detected already in compositions with x = 0.1, and that of segregated Co3 O4 appeared in samples

Fig. 1. XRD patterns of La1−x Cex MnO3 samples calcined 5 h (a) at 923 K, and (b) 1073 K (c) XRD patterns of LaCex MnO3 samples containing “over-stoichiometric” cerium, calcined 5 h at 923 K. In all cases, the main peaks characteristic of the only phase other than of perovskite structure, namely CeO2 , are marked by symbol asterisks (∗).

with x = 0.2 [17], in agreement with other literature reports. In the case of the La1−x Cex MnO3 samples prepared by calcination 10 h at 1123 K, Nitadori et al. [31] also observed only peaks of perovskite and segregated ceria, up to x = 0.6, but the patterns were not published. Yet, their conclusions concerning the

M. Alifanti et al. / Applied Catalysis A: General 245 (2003) 231–243

phase composition were unequivocal. The patterns in Fig. 1a and b suggest that LaMnO3 can accommodate a higher degree of cerium substitution than LaCoO3 [17]. Another important fact is that for samples calcined at high temperature the rhombohedral perovskite structure was observed also for La0.9 MnO3 (Nitadori et al. [31]). This shows that the perovskite lattice can accommodate a surprisingly high proportion of cation vacancies. But in the pattern for La0.85 MnO3 , peaks of Mn3 O4 appeared in addition to those of the perovskite [34]. However, much higher cation deficiency, especially above 33%, is very unlikely, even in a metastable state. It seems more plausible that the part of manganese oxide unable to enter the perovskite structure dissolved in ceria [18]. As such it could not be detected by XRD. As reported in Table 1, the SSA varied from about 10–36 m2 /g. Among the samples calcined 5 h at 973 K, the lowest SSA was exhibited by Mn2 O3 , followed by LaMnO3 and ceria. Let us consider first the formally stoichiometric compositions (first part of Table 1). Lanthanum substitution by 10% cerium caused a

235

twofold increase of SSA, to 32.3 m2 /g, although the SSA of pure ceria was only 21 m2 /g. For most of the La1−x Cex MnO3 samples prepared by calcination 5 h at 1073 K, the SSA was significantly lower, in a few cases by more than 50%, although those of La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 retained values of 32.2 and 28.6 m2 /g, respectively. The LaMnO3 perovskite prepared by calcination 5 h at 1073 K exhibited the lowest SSA (9.2 m2 /g), in good agreement with data reported in literature for samples prepared under similar conditions. For comparison, some relevant literature data are collected in Table 2. The SSA values of our samples correlate well with the crystallite size d012 (considering the 012 plane), estimated by the Scherrer’s approach to be between 20 and 38 nm. For example, knowing that the crystal density of LaMnO3.15 is 7.2 g/ml, the calculated SSA for cubic crystallites of 25 nm is 33 m2 /g. The values of crystallite size of our samples are also in a good agreement with those obtained by Lee et al. [35], or by Ciambelli et al. [36], using similar preparation methods.

Table 1 SSA and oxygen desorption from different manganese containing perovskites Composition

SSA (m2 /g)

Eapp (kJ/mol)

k823 (␮mol/g s bar)

k823 (␮mol/m2 s bar)

O2 TPD

␮mol O2 /g

Tmax (K)

T (<683 K) 8.3 15.3 38 48 30.8 14.2

Calcined 5 h at 973 K LaMnO3␭ La0.9 Ce0.1 MnO3 La0.8 Ce0.2 MnO3 La0.7 Ce0.3 MnO3 La0.6 Ce0.4 MnO3 La0.5 Ce0.5 MnO3

16.5 32.3 32.6 27.7 33.1 36.0

95 88 78 94 82 76

608 805 439 363 289 316

37 25 12.9 13.1 8.7 8.8

486/551 498 543/601 509/569 542/548 553

Calcined 5 h at 973 K LaCe0.1 MnO3 LaCe0.2 MnO3 LaCe0.3 MnO3 Mn2 O3 CeO2

27.2 26.9 35.4 15.1 21

85 93 90 78 128

847 725 602 350 51

31 27 17 23.2 2.4

– – – 498 663

Calcined 5 h at 1073 K LaMnO3 La0.9 Ce0.1 MnO3 La0.8 Ce0.2 MnO3 La0.7 Ce0.3 MnO3 La0.6 Ce0.4 MnO3 La0.5 Ce0.5 MnO3

9.2 22.5 32.2 28.6 18.6 14.8

95 96 86 72 74 87

424 444 304 303 270 220

46.1 18.7 9.6 10.6 14.5 14.9

468/553 503 493/523 508 533 583

CeO2

14.1

109

38

2.7

–

– – – 9 0.35

T (683–973 K) 240 210 130 130 123 72 – – – >111 1.5

2.3 16.3 38.4 33 41 47

265 243 234 230 219 213

–

–

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Table 2 Literature data for the activity in methane combustion over LaMnO3+λ perovskite catalysts References

Preparation method

Calcination SSA (m2 /g) (K/h)

λ

Qa (ml/min)

GHSV (ml/h g)

k823 (␮mol/g s bar)

Eapp (kJ/mol)

[19] [12] [36] This work [23] [22] [21] [35] [24] This work [22] [21]

OH co-precipitation Solution evaporation Citrate Citrate Citrate Spray decomposition Citrate Spray-freezing/freeze-drying Citrate Citrate Spray decomposition Citrate

1170/12 1123/5 1073/5c 1073/5 1073/4 1073/4 1023/2 973/12 973/5 973/5 973/4 943/2

8 4.6 20 9.2 8 8.8 5 13 8.8 16 12.2 20

0.14 – 0.18d 0.123 >0.1e – – 0.15 – – – –

200 400 200 75 72 200 40

200b 350 700 424 220 520 450 – 700 602 620 780

82 91 97 95 – –

240 75 200 40

120000 45000 40000 45000 4320 12000 12000 – 144000 45000 12000 12000

1023/2

32

–

40

12000

520

–

18.7

–

500

12000

375

–

La0.9 Ce0.1 MnO3 [21] Citrate La0.7 Ce0.3 MnO3 [22] Spray decomposition

973/4

– – 95 – –

a

Total flowrate. 1% CH4 /4% oxygen/helium. c Pretreatment 5 h at 823 K, grinding before calcination. d Based on the proportion of Mn4+ determined by redox titration. e Evolved oxygen not quantified. b

Another part of the results deals with samples containing an amount of Ce in excess to formal stoichiometry. Two of the three LaCex MnO3 compositions (x = 0.1, 0.2), prepared by calcination 5 h at 973 K, were fully crystalline, their XRD pattern showing only perovskite and ceria (Fig. 1c). The XRD pattern of LaCe0.3 MnO3 indicated that segregated ceria was not well crystallized and the SSA of this composition was about 30% higher than that of the other two LaCex MnO3 catalysts (second section of Table 1). 3.1.2. Surface composition XPS analysis of the freshly prepared La1−x Cex MnO3 samples indicated that surface composition closely reflected that of the bulk, with only a slight enrichment in lanthanum and cerium (Table 3). This, however, was lower than for the La1−x Cex CoO3 compositions [17]. The binding energies of Mn 2p3 (641.7–642.1 eV) showed that manganese was to a large extent in the 3+ oxidation state. Presence of the Mn4+ ions could not be well established because of the proximity of its peak to that of the Mn3+ ion [37]. No Mn2+ ions were detected.

The case of cerium is the subject of controversies with respect to band attribution, since the signal of the 3d level has a very complicated structure. It is formed of two series of peaks: 3d5/2 and two very pronounced “shake-up” satellites, and 3d3/2 with the same characteristics, respectively. Very often these peaks are asymmetric due to the coexistence of Ce3+ and Ce4+ ions. However, specific Ce3+ features are present around 884.9 and 903.7 eV. These signals are considered as fingerprints for the existence of some reduced ions. Thermodynamics predict the exclusive formation of Ce(IV) by simple decomposition of the precursors in air. Ce3+ is not stable in these conditions. Even though its transient formation during the surface reaction with hydrocarbons is highly plausible, it could hardly be observed under the conditions of our XPS analyses. The overwhelming presence of Ce4+ in all investigated catalysts was shown by the Ce 3d3/2 peak around 916.3 eV observed in all samples. Its area was in the right ratio with the second “shake-up” satellite of Ce 3d5/2 (signal centered at 882.1–882.3 eV), namely 1.5, a feature characteristic of Ce4+ . For our catalysts and in the conditions of analysis, partial oxidation could

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237

Table 3 XPS surface composition of La1−x Cex MnO3 perovskites Composition

[Mn]/[La + Ce + Mn]

[La]/[La + Ce + Mn]

Olat /Oads

Theoretical

Fresh

Theoretical

Fresh

Fresh

Calcined 5 h at 973 K LaMnO3 La0.9 Ce0.1 MnO3 La0.8 Ce0.2 MnO3 La0.7 Ce0.3 MnO3 La0.6 Ce0.4 MnO3 La0.5 Ce0.5 MnO3

0.5 0.5 0.5 0.5 0.5 0.5

0.42 0.41 0.44 0.42 0.41 0.45

0.50 0.45 0.40 0.35 0.30 0.25

0.57 0.49 0.45 0.39 0.34 0.26

1.33 1.22 1.11 1.42 1.2 1.16

Calcined 5 h at 1083 K LaMnO3 La0.9 Ce0.1 MnO3 La0.8 Ce0.2 MnO3 La0.7 Ce0.3 MnO3 La0.6 Ce0.4 MnO3 La0.5 Ce0.5 MnO3

0.5 0.5 0.5 0.5 0.5 0.5

0.40 0.41 0.45 0.47 0.47 0.51

0.50 0.45 0.40 0.35 0.30 0.25

0.60 0.47 0.39 0.33 0.27 0.20

1.24 1.46 1.16 1.25 1.1 1.6

occur only if the exposure of samples to the exciting X-ray beam was quite long, causing local heating. It was verified that this was not the case. The oxygen signal showed two peaks, centered on 529 and 531 eV, in all catalysts. The low binding energy peak was assigned to lattice oxygen (Olatt ), while the broader high binding energy one can be associated with adsorbed oxygen or surface hydroxyl groups (Oads ) [37]. The broadness of the second peak varied with the composition, actually indicating the existence of several types of surface oxygen species (adsorbed and/or in the form of hydroxyls).

and accurately indicate trends, but perhaps cannot be interpreted in a full quantitative way. The TPD-O2 trace for LaMnO3 , prepared by calcination at 1073 K, showing mainly one large peak at 973 K, as well as the amount of oxygen desorbed (267.3 ␮mol O2 /g), are in a very good agreement with literature. For example, Nitadori et al. [31] obtained 267 ␮mol O2 /g, Rosso et al., 263 ␮mol O2 /g [25] and

3.2. Temperature programmed oxygen desorption Relatively large quantity of oxygen desorbed from all catalyst samples. The desorption pattern and the total amount depended on cerium substitution and the temperature of calcination. Characteristic traces recorded for the oxygen evolution from samples calcined at 1073 K during the temperature ramp are shown in Fig. 2. Note, however, that the oxygen evolution was followed after the temperature of 973 K had been reached and subsequently, at this same temperature, until the evolution ceased, typically for an additional period of 30 min. To be considered are therefore the desorbed amounts indicated in Table 1 (last column), that include desorption at constant 973 K. These data approach the real values

Fig. 2. Typical traces representing the rate of oxygen evolution during TPD (10 K/min) from La1−x Cex MnO3 samples calcined at 1073 K and saturated with oxygen (5% in helium) at 923 K. The traces are arbitrarily displaced vertically, but the scale of rate variation is identical.

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Marti et al., 283 ␮mol/g [19]. However, the latter value by Marti et al. [19] was obtained by desorption up to 1300 K on a sample calcined 12 h at 1170 K. This quantity corresponds to the well known excess of oxygen, about 4%, which is often reflected by the formula used, namely LaMnO3.123 . Indeed, this formula is so reproducible that LaMnO3.123 could be used as a calibration standard [17]. Alternatively, this excess can be considered as determined by the equilibrium amount (∼25%) of Mn4+ to Mn3 , corresponding to the thermal decomposition (reduction) [38]. About the same total amount of oxygen (266 ± 6 ␮mol O2 /g) evolved also from all other samples calcined at 1073 K, regardless the cerium substitution. However, as the cerium substitution increased, the amount of oxygen desorbed at around 973 K gradually decreased, while that desorbed at lower temperature (more labile oxygen) increased (Fig. 2). Low temperature oxygen desorption from segregated CeO2 might perhaps partly explain this fact. Unfortunately, this cannot be quantified. A comparison with pure CeO2 is probably meaningless in principle because neither the amount nor the dispersion of segregated CeO2 were determined. At any rate, desorption from pure CeO2 is very low [17] (Table 1 of the present article, end of the second section). Slightly less oxygen evolved (248.3 ␮mol O2 /g) from LaMnO3 prepared by calcination 5 h at 973 K, this corresponding to a formula LaMnO3.12 , again mainly in a single peak centered at 973 K. Possibly, the milder conditions of preparation were not sufficient for manganese to reach the equilibrium oxidation state Mn3+ /Mn4+ . From the well crystallized La0.9 Ce0.1 MnO3 prepared at 973 K the total amount of evolved oxygen was only 225.3 ␮mol O2 /g, but the amount desorbed below 683 K was twice as large as that from LaMnO3.12 . All other Ce substituted oxides (x ≥ 0.2) prepared at 973 K, which were not crystalline, evolved significantly less of oxygen in comparison with the corresponding samples calcined at 1073 K and this amount gradually decreased as the degree of substitution x increased. Nevertheless, even from these samples, the amount of oxygen desorbed at lower temperature (<683 K) significantly increased as x increased, the largest amount (48 ␮mol O2 /g) being desorbed from La0.7 Ce0.3 MnO3 . Interestingly though, for the first three amorphous samples of the series (x = 0.2, 0.3, 0.4), the peaks of the more labile oxygen desorption were shifted to higher tempera-

Fig. 3. Characteristic light-off curves for samples of La1−x Cex MnO3 ; 0.1 g catalyst, 1% CH4 in air, 75 ml/min (GHSV = 45,000 ml/gcat h).

tures (Table 1) compared to the case of crystalline samples of the same composition prepared at 1073 K. 3.3. Catalytic activity All La1−x Cex MnO3 and particularly the LaCex MnO3 samples showed a very good (specific, i.e. per gram) activity in combustion of 1% methane in air (45,000 ml/gcat h) with full conversion obtained at temperatures below 1000 K. An example of the effect of cerium substitution and calcination temperature on the catalytic activity (in the form of a light-off curve) is given in Fig. 3. Activity data presented in terms of apparent first order specific (per gram) k823 and area (per m2 ) k823 kinetic constants at 823 K and corresponding apparent activation energies are collected in Table 1. These data were determined as previously [17] from the experimental conversions by applying the first order model in an integral mode for a plug flow reactor. Linear Arrhenius plots were obtained for the whole range of conversions up to 90%. The uncertainty on the values of Eapp determined by linear regression did not exceed 3 kJ/mol and was lower than 10% on the k823 values. Table 1 includes data for Mn2 O3 determined during this work as well as those for CeO2 obtained previously [17]. The values of apparent activation energies fall well within the usual range of

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Fig. 4. Rate of combustion as a function of cerium substitution (i) at 823 K over La1−x Cex MnO3 perovskites calcined 5 h at 973 and (ii) at 1073 K over LaCex MnO3 perovskites calcined 5 h at 973 K; 0.10 g, 1% CH4 in air, 75 ml/min (GHSV = 45,000 ml/gcat h).

70–120 kJ/mol [12–14], and are in a particularly good agreement with those obtained for La1−x Cex CoO3 compositions, which varied from 78 to 95 kJ/mol [17]. The variation of activity (k823 ) with the degree of cerium substitution x is illustrated in Fig. 4. Considering the whole set of results, the apparent (specific) activity of both sets of La1−x Cex MnO3 samples was comparable to that of La1−x Cex CoO3 catalysts [17] within an order of magnitude, although some very important differences between the two systems were observed. First of all, the LaMnO3 perovskite (x = 0) was more active than Mn2 O3 (sample also prepared by calcination at 973 K). It is worth to mention that Co3 O4 calcined at 973 K was about 50% more active (per gram) than Mn2 O3 , in spite of a substantially lower SSA (3.6 versus 15.1 m2 /g). Activities of both LaMnO3 samples, one calcined at 973 K, the other at 1073 K, were significantly higher than those of corresponding LaCoO3 perovskites, for which the k823 were 166 and 118 ␮mol/g s bar, respectively. The difference was more than triple for the samples calcined at 973 K and nearly quadruple for the samples calcined at 1073 K, which actually had comparable SSA (10.6 and 9.2 m2 /g for LaCoO3 and LaMnO3 , respectively). On the other hand, among all La1−x Cex MnO3 compositions, only La0.9 Ce0.1 MnO3 was more active than LaMnO3 , when comparing spe-

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cific activities or the temperatures of a given conversion. However, the difference was significant (about 30%) only for the samples calcined at 973 K whereas it was barely perceivable for the samples calcined at 1073 K (see Figs. 3 and 4). Similarly, Marchetti and Forni observed [21] that La0.9 Ce0.1 MnO3 was only slightly more active than LaMnO3 (Table 2). Activity of all other cerium substituted manganese containing samples slowly decreased as the degree of substitution x increased. Song et al. [22] have also reported that La0.7 Ce0.3 MnO3 was much less active in methane combustion than LaMnO3 (Table 2). In terms of area activities, all La1−x Cex MnO3 samples were substantially less active than LaMnO3 , regardless of the calcination temperature. Among the samples calcined at 973 K, areal activities of all non-crystalline compositions (x ≥ 0.2) were even lower than that of Mn2 O3 . In contrast, regardless of the calcination temperature, both specific and areal activities of all La1−x Cex CoO3 samples, as well as the other cobalt based perovskite compositions with cation non-stoichiometry (La1−x CoO3 ), were significantly higher than that of LaCoO3 , which was also substantially less active than Co3 O4 . Perhaps the most interesting result of this study is the very high specific activity (per gram) of the LaCex MnO3 samples. Actually, LaCe0.1 MnO3 was the most active of all compositions, although, within the uncertainty of determination, its activity was not very different from that of La0.9 Ce0.1 MnO3 (Table 1). For the sake of comparison, while LaCex CoO3 catalysts were more active than LaCoO3 , they were less active than La1−x Cex CoO3 . In the present case, it is surprising to note that the same proportion of cerium has approximately the same effect as a substituent to lanthanum or added in excess over formal stoichiometry, LaCex MnO3 . 4. Discussion Let us first remark that these samples do not exhibit any special physico-chemical characteristics in the range of Ce contents between 0.1 and 0.3. They behave qualitatively like La1−x Cex MnO3 . For this reason, we shall not specifically discuss the corresponding results. The present results clearly confirm the unique character of the LaMnO3 perovskite as a highly

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active oxidation catalyst, especially in comparison with LaCoO3 . This distinctive character is even more evident considering the effects of lanthanum substitution by cerium in lanthanum manganese perovskites (La1−x Cex MnO3 ), which seem remarkably different from those observed in the La1−x Cex CoO3 system. The interpretation of the results suggests new speculations about the factors determining the nature of activity in catalytic combustion in methane. This will be discussed later. Overall, the physico-chemical characteristics of the two samples of LaMnO3 perovskite prepared for this study as well as their activity (data in Table 1) compare reasonably well with those available in literature and summarized in Table 2. Note that these data (k823 ) were in most cases calculated from reported conversions for a given GHSV, using the same method as for the data in Table 1. As can be noted, most of these selected literature data correspond to samples prepared under conditions similar to those of this work. Nevertheless, for an objective assessment of various effects and their interpretation, data for two samples prepared at higher temperatures [12,19] are included. Particularly good agreement is seen for values of SSA, which appear to correlate well with the temperature of calcination. Several of the discrepancies in SSA can most likely be related to the method of preparation. On the other hand, the activity data seem to be less uniform, even when considering the potentially higher uncertainty in the corresponding measurement. For instance, the reported specific (per gram) rate constants for samples calcined at 1073 K and having SSA approximately 9 m2 /g vary from as low as 220 ␮mol/g s bar [23] to a high 520 ␮mol/g s bar [22]. These differences must in part reflect the state of the catalyst surface resulting from a given preparation method. However, in addition, these differences may also reflect the conditions of activity determination. When determined at low flow rates (<100 ml/min), the apparent activity may be lower than when determined under high flow rates (>100 ml/min) because of the inhibition by combustion products [39]. The exceptionally large SSA (20 m2 /g) and high activity of the sample prepared by Ciambelli et al. [36] by the citrate method and final calcination 5 h at 1073 K may most likely be due to the pretreatment 5 h at 823 K and regrinding before final calcination, this allowing higher homogeneity and the stabilization of the surface mor-

phology. On the other hand, all data by Marchetti and Forni [21] have to be taken with caution. These authors used the citrate method, but calcined their samples only 2 h at the indicated temperature. The SSA they report for the LaMnO3 composition calcined 2 h at 1023 K (only 5 m2 /g), seems much too low, although the activity compares well with that of other samples. Judging from their published XRD patterns [21], the described samples had not the perovskite structure. In view of the above discussion concerning these variations, our samples may be considered as well representative, justifying further discussion of the effect of ceria substitution. In spite of the obvious sensitivity to the process of preparation, which is most likely similar to that of compositions based on other transition metals, the lanthanum–manganese perovskites seem more active (in methane combustion) than corresponding lanthanum–cobalt perovskites prepared in a similar way. The high catalytic activity of the LaMnO3 perovskite and its variation with substitution by cerium in La1−x Cex MnO3 is undoubtedly related not only to the specificities of the perovskite structure, but also to the redox properties of manganese. As is well known, manganese can form several oxides, depending on temperature and oxygen partial pressure [40]. Of these, in air, Mn2 O3 is the most stable, decomposing to Mn3 O4 at temperatures above 1173 K, while MnO2 is stable only up to about 800 K [40,41]. Furthermore, the sinterability or the SSA after calcination of individual manganese oxides is different. While Mn2 O3 is easily prepared with large SSA, MnO2 is generally highly crystalline, with low SSA (<1 m2 /g) [41], although high SSA polymorphs of MnO2 are known [42]. Incorporation of manganese oxide in the perovskite lattice by reaction with lanthanum oxide stabilizes higher oxidation state of manganese. Depending on the conditions of reaction or annealing, about 25% or more of manganese forms Mn4+ ions, in equilibrium with Mn3+ , even at temperatures as high as 1173 K [26,38]. Since the perovskite lattice cannot accommodate interstitial oxygen ions, electroneutrality of the lattice is maintained by formation of cation vacancies [26,38]. Actually, the facility of manganese based perovskites to completely fill the oxygen sites seems to cause the formation of a relatively large range of either A or B-site cation vacancies. When substituting La3+ ion by a smaller, higher valence Ce4+ ion in proportions

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above its solubility limit (x ∼ 0.08), the excess segregates as ceria. Segregation of ceria would a priori be expected to be accompanied by segregation of some manganese oxide (predominantly Mn2 O3 ). This, however, was not detected by XRD. At low cerium substitution, possibly up to about x = 0.2, formation of an A-site deficient perovskite, say La0.8 Ce0.07 MnO3 , could be assumed. Nevertheless, when the proportion of segregated ceria exceeds a certain value, some manganese oxide should segregate in parallel. Alternatively, the excess of manganese oxide could dissolve in the segregated ceria [18]. This may be a process retarding, at the lower temperature, the formation of the apparently non-stoichiometric La0.9+x Ce0.1−x MnO3 perovskite phase and to some degree interfering in the oxidation of manganese to reach the Mn3+ /Mn4+ equilibrium state. Nevertheless, at the same time, by the dissolution in ceria, manganese oxide may be also stabilized in the higher oxidation state (MnO2 ), although at a lower degree of stability [18]. This would make that the apparent overall Mn3+ /Mn4+ ratio remains quasi-constant for all La1−x Cex MnO3 compositions, as suggested by the high quasi-constant amount of desorbed oxygen. On the other hand, the retardation in the formation of the perovskite phase observed in the La1−x Cex CoO3 compositions with x > 0.2 calcined at 973 K [17], is apparently due to competitive formation of LaCo2 O4 resulting from a relatively slow oxidation of divalent cobalt. In addition to the described effects, lanthanum substitution by cerium is accompanied by a large increase of SSA, regardless the calcination temperature, and by a substantial increase of the amount of a rather labile oxygen species, desorbing below 683 K. The first factor is very important and the second generally assumed to be so, in determining the activity of a given material in catalytic combustion [12,13,16,17]. Nevertheless, in the present case of activity in methane combustion of La1−x Cex MnO3 and LaCex MnO3 perovskites, these factors seem to play a secondary role. On the other hand, they seem to have a positive effect on the catalytic combustion of carbon monoxide [22]. Combustion of the easily activated carbon monoxide is apparently controlled by suprafacial catalysis. Considering the second factor, a work concerning easily oxidizable propane suggests the same conclusions as ours. The areal rates of combustion at 500 K over all La1−x Cex MnO3 with x = 0.1–0.6, were dramat-

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ically higher than that over LaMnO3 [31]. The latter catalysts, prepared by calcination at 1123 K, showed oxygen desorption patterns similar to those of our samples calcined at 1073 K, both as shape and amount of desorbed oxygen are concerned, in spite of significantly lower SSA (2.4–4 m2 /g). This work suggests a conclusion similar to ours in the case of methane combustion, namely that the low temperature oxygen mobility reflected in easy oxygen desorption is not a critical factor determining the activity, although it may perhaps play some subtle role in cases where other factors have some influence [44,45]. The strong influence of variations of SSA, vacancy formation and redox processes make impossible to decide whether CeO2 segregation has an impact on catalytic activity, as it actually has in the case of La1−x Cex CoO3 . Very little is known about the effect of crystallinity on the activity of metal oxides. In the case of our La1−x Cex MnO3 (x ≥ 0.2) calcined at 973 K, low crystallinity appears to be somehow detrimental. In contrast, Oliva et al. [43] reported that an amorphous sample of LaMnO3 nominal composition was more active than crystalline LaMnO3 -perovskite, simply because of higher SSA. However, they also showed that the structure (morphology) of the sample prepared by calcination at lower temperature was not stable. Furthermore, Oliva et al. observed that in crystalline samples with a well defined perovskite structure but prepared differently, the catalytic activity was dependent on oxygen availability [43]. Our work does not permit to clarify this point.

5. Summary and conclusions This work confirms that formation of lanthanum manganites with well defined oxygen excess (LaMnO3.123 ), physical morphology as well as catalytic activity in methane combustion, is fairly reproducible, provided the catalyst preparation is well controlled. As generally accepted, the excess oxygen is apparently stabilized in the perovskite lattice by the presence of cation vacancies and the presence of ∼25% of manganese in the 4+ oxidation state. This 4+ state, probably corresponding to the presence of a properly coordinated (octahedrally) oxygen species, determines the high specific catalytic activity

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in methane combustion. This characteristic, inherent to manganese, makes LaMnO3 unique among the other LaMO3 perovskites and contrasts particularly with LaCoO3 . The substitution of lanthanum by 10% cerium corresponds to a level only slightly above the cerium solubility limit in the LaMnO3 perovskite lattice. This results in an important increase of the SSA and of the quantity of oxygen easily desorbing below 680 K, i.e. more labile, although the total amount of oxygen desorbing below temperatures as high as 973 K remains constant. These changes are accompanied by a small increase of specific activity (per gram), while areal activity (per m2 ) decreases dramatically. Further substitution up to 50% (x = 0.5) by cerium results in a two-phase system consisting of a cation (lanthanum) deficient perovskite and ceria. With increasing the cerium content, the catalysts progressively exhibit lower activity, both specific and, more considerably, areal, in methane combustion, although the amount of oxygen available for desorption at temperatures below 973 K remains constant and the amount of the labile oxygen increases. In this La1−x Cex MnO3 perovskite based system, there is no evidence of cooperation between the phases, in contrast to the case of La1−x Cex CoO3 . An interesting new finding is the very high specific activity (per gram) of the LaCex MnO3 samples. The present work does not allow to distinguish a specific effect of cerium in excess to formal stoichiometry. The characteristics are very similar to those of cerium at the same concentrations, but in substitution for lanthanum. This contrasts with the activity of LaCex CoO3 catalysts, which were less active than La1−x Cex CoO3 , a positive influence that was important in the case of cobalt containing catalysts.

Acknowledgements The authors thank to the European Community for the financial support of the research under contract ENV4-CT97-0599.

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