B ENVIRONMENTAL

ELSEVIER Applied Catalysis B: Environmental 15 (1998) 179-187

Catalytic combustion of methane over perovskites

Luca Marchetti, Lucia Forni* Dipartimento di Chimica Fisica ed Elettrochimica, Universitri di Milano, via Golgi 19, I-20133 Milan, Italy

Received 1 5 March 1997; received in revised form 29 May 1997; accepted 4 June 1997

Abstract

Perovskite-type oxides of the series Lar_,AXMn03 (A = Sr, Eu and Ce) were prepared by the amorphous citrate process, leading to high surface area catalysts (up to 45 m* g-r). They were tested in a flow reactor for the total combustion of methane. Complete conversion was obtained over all of the catalysts between 500 and 600°C and catalyst performance did not change significantly after 100 h on-stream. Specific activity was found to decrease monotonically with increasing the temperature of the O2 TPD desorption peak maximum. The rate of methane combustion was low below 5OO”C, then grew very fast, showing that two kinds of oxygen are active in these catalysts: an adsorbed oxygen species, that reacts at low temperature, and a lattice oxygen species, that becomes available at high temperature, boosting the catalytic activity. 0 1998 Elsevier Science B.V.

Keywords: Perovskite; Methane combustion; Oxygen exchange reaction; Structure-activity correlation

1. Introduction

A very pressing environmental problem is the NO, removal from the combustion exhaust gases. NO, formation is related to high combustion tempera- ture: in excess of 1600 K nitrogen reacts with oxygen to give NO, through a radical mechanism [l]. In addition, N-compounds in fuel produce NO, during combustion. These processes are very fast due to the high temperature of the flame (up to 2000 K). There are many ways to control NO, emission: some of them reduce the NO, formation during the reaction (primary methods); others eliminate them by treat- ing the exhaust gas (secondary methods). Primary

*Corresponding author: Tel: +39 2 26603289; fax: +39 2 70638129; e-mail: fom@rs6.csrsrc.mi.cnr.it

methods rely essentially on a proper control of the combustion, e.g. by using more efficient burners or by employing the so-called catalytic flameless combustion.

On both noble metals (Pd, Pt) and perovskite-type catalysts the combustion can be carried out much below the temperature of the flame, so reducing substantially the formation of NO, [2]. However, for natural-gas-fueled power plants, noble metals, though very active, remain unsatisfactory, because of sublimation and sinterisation problems, while perovskite oxides present better thermal stability [3-61.

The properties of perovskites, of general formula ABOs, depend tightly on the nature of A and B ions and on the valence state of the transition metal ion B, that can be altered, at least in part, by choosing a proper A ion. Moreover, by replacing part of A or B

0926-860x/98/$19.00 f!! 1998 Elsevier Science B.V. All rights reserved. PIZ SO926-3373(97)00045-3

180 L. Marchetti. L. Forni/Applied Catalysis B: Environmental 15 (1998) 179-187

ions, with A’ and B’ ions, respectively, it is possible to create or suppress oxygen vacancies on the catalyst.

Manganese-containing perovskites are among the most active catalysts for CO and hydrocarbon oxida- tion, their activity being comparable to that of Pt and Pd-based catalysts [7-121. The surface area of these catalysts depend noticeably on the method of prepara- tion. The usual methods, e.g. from metal oxides or acetates, require high temperature of calcination, often in excess of lOOO”C, to give the perovskitic structure. This leads to a rather small surface area, typically < 5 m2 g-l. To increase the surface area the so-called ‘amorphous citrate process’ has been suggested [ 13,141: by calcining an amorphous citrate precursor, the catalyst can be synthesised in a well-crystallised perovskite structure at 600-700°C i.e. 200-300°C below the usual temperature needed in other pro- cesses.

The aim of the present work was to test the effect of changing the nature and concentration of metal ion A in a series of perovskites of general formula Lal_,A,MnOa, with A = Sr, Eu, Ce, on catalytic activity for low-temperature total combustion of methane, diluted in a large excess of inert gas.

2. Experimental

2.1. Catalysts preparation

The catalysts were prepared by adding an equimolar amount of citric acid to a solution of the metal nitrates. After evaporation to dryness in rotavapor, the viscous liquid so obtained was dehydrated in vacua (ca. 10 mbar) overnight at 60°C to give a light spongy solid, which was then finely crushed and calcined for 2 h in flowing air at the desired temperature, deter- mined by TGA.

2.2. Catalysts characterisation

BET surface area was measured by means of a Carlo Erba Sorptomatic 1800 apparatus. XRD analy- sis was made by means of a Rigaku III D Max powder diffractometer, by employing the Cu Kcr radiation, Ni filtered. Some relevant characteristics of the catalysts prepared and employed in the present work are given in Table 1.

Table 1 BET surface area and calcination temperature of the catalysts

Catalyst BET surface area (m* g-‘)

Calcination temperature (“C)

LaMnO (5) 5.0 750 LaMnO:, (20) 20.0 670 Lao.&o.2Mn@ 45.4 740 Lao 6Sro.4Mn03 18.7 590 La&u0 MO3 26.4 740 Lao&co.lMn03 32.0 750

2.3. Activity data

Catalytic activity tests have been carried out by means of a 0.9 cm id. tubular reactor assembly, by feeding a gas mixture of CH4 (0.5%), air (10%) and N2 (balance) at total flow rate 40 N cm3 mini’ through a ca. 4 cm long bed, packed with 0.2 g of catalyst, throughly mixed with 1.3 g of quartz beads. Before reaction, the catalyst was activated by flushing with 40 N cm3 mini’ of air (SAP10 SP-grade, 299.995 vol % pure) at 500°C for one hour. The gases were analysed by gaschromatography on a Porapak Q column (for CO:! or H20) followed by a MS-5A column (for Nz, CH4 and CO). Data were collected and integrated by means of a Kontron PC Pack 3.0 software.

2.4. Analysis of catalyst and of reaction mechanism

The samples for TPD-MS measurements were first activated in oxygen (SAP10 5.0-grade, 299.999 vol % pure) at 550°C for one hour to clean up the surface and to saturate any surface oxygen vacancies, then cooled down to 50°C. The flowing gas was then switched to helium (SIAD 6.0-grade, 299.9999 vol % pure) to eliminate physically adsorbed oxygen. Temperature was then raised (10°C mini) up to 820°C and then maintained at this temperature, while continuously monitoring the m/z = 32 signal by means of a quadrupolar UT1 100 C mass spectrometer (MS).

Analysis of the behaviour of the catalyst during the reaction was carried out by TPR-MS only on the most active sample, Lae,gEua.,MnOj. The sample (100 mg) was first activated in oxygen at 550 “C for one hour,

L. Marchetti, L. Fomi/Applied Catalysis B: Environmental I5 (1998) 179-187 181

then cooled to 50°C and, after switching the carrier gas from oxygen to helium, a 10°C min-’ temperature ramp was started, up to 820°C. Pulses (0.13 cm3) of the mentioned methane gas mixture (1.00% in N2) were then injected into the carrier gas after every 20°C temperature increase, while monitoring the composi- tion of the outcoming gas by means of the MS.

XPS analysis was carried out on the LaMnOs sample at different temperatures, after the following pretreatments:

A = sample heated for 15 min at lo-’ mbar; B = sample heated for 15 min in O2 atmosphere; C = B after outgassing. TDS-MS analysis was made with all of the cata-

lysts, by monitoring the reaction products obtained by reacting methane and “02 in a batch quartz reactor system. The catalyst was first pretreated in vacua (<1O-6 mbar) at 500°C to remove moisture and other impurities adsorbed on it. 1 mbar of 1602 was then introduced to activate the sample. After outgassing at room temperature, 1 mbar of CH4 and 1 mbar of ‘so2 were introduced and the temperature raised from 50-550°C at 5°C min-‘. Reaction products were analysed by the MS after every 20°C increase of temperature.

. z s \o 0

3 0

100

80

60

40

20

0

3. Results and discussion

Catalyst calcination temperature and BET surface area (Table 1) show that the latter exceeds by ca. one order of magnitude the values reported for the same samples prepared by the usual routes. Futhermore, the influence of calcination temperature on BET area is clearly noticed. For instance, the same LaMnO citrate precursor calcined at 760 or 670°C gave 5.0 and 20.0 m* g-‘, respectively, and this reflects on a higher catalytic activity for the sample calcined at lower temperature (Fig. 1).

XRD patterns show a well-crystallised perovskite structure for anyone of the catalysts. Some Sr carbo- nate is also present in Sr-containing samples (Fig. 2).

All of the catalysts proved very active, leading to total conversion of methane within the 500-600°C temperature range. Examples are given in Fig. 3. Laa9Eua.rMn0s showed the most active. Moreover, catalyst performance did not change significantly after 100 h on-stream, so showing a fairly good stability of our samples.

Two trends are observable as for the effect of Sr substitution for La (series Lal_,Sr,Mn03). At lower temperature (<4OO”C) the activity order is

300 400 500 600

V”C) Fig. 1. Effect of specific surface area (in parentheses) on catalyst activity under standard reaction conditions (see text)

182 L. Marchetti, L. Fork/Applied Catalysis B: Environmental 15 (1998) 179-187

LaMnO (20) &-#!_dL 15 25 35 45 55 65 15 25 35 45 55 65

28 20

La0.6Sr0.2Mn03 L 15 25 35 45 55 65

28

1 La0.9EuO.lMn03

15 25 35 45 55 65 15 25 35 45 55 65

26 28

I La0.6SrO.4Mn03

Llljs, IS 25 35 45 55 65

28

La0.9CeO.lMn03

Fig. 2. XRD patterns of the catalysts (specific surface area in parentheses).

Laa6Sra.4Mn03 > LaosSro.2Mn03 > LaMnOs, while a non-stoichiometric compound [18], in which, in at higher temperature the order is LaMnO > order to reduce the static Jahn-Teller distortion of LaosSrc,2Mn03 > Lac6Sra4Mn0s. Several authors Mn3+, cationic vacancies are created, accompanied [ 15-171 reported the presence of two TPD oxygen by the formation of Mn4’ ions and of an oxygen desorption peaks, referred to as o and ,& respectively. excess. As a consequence, with LaMnOs the p peak The former, obtained at lower temperature, is due to is very large and no o peak is observed. By adding a oxygen adsorbed on the catalyst surface. The amount relatively small amount of Sr (20%), cationic vacan- of desorbing oxygen, measured by the area of this (Y cies and oxygen excess decrease and p peak becomes peak, can be referred to surface oxygen vacancies. The smaller. By substituting 40% Sr for La, cationic /3 peak, obtained at higher temperature, can be referred vacancies are suppressed and anionic vacancies are to the reduction of B ion. The effect of Sr substitution created, so that the p peak decreases noticeably and for La on the change of the Q: and /3 peak area has also the Q! peak appears. It is possible to conclude that been reported. In fact, it is well known that LaMnOs is Laa6Sr,,4Mn03 appears as the most active catalyst at

L. Marchetti, L. Fork/Applied Catalysis B: Environmental 15 (1998) 179-187 183

m T(X) m” Fig. 3. Activity comparison data under standard reaction condi- tions.

Fig. 4. TPD-MS thermograms of oxygen (m/z = 32): (a) LaMnO (20.0 rr? g-l); (b) Lao.8Sr0.2Mn03; (c) Lao.sCeo 1Mn03; (d) La,, 9Euo ,Mn03. Temperature ramp: 10°C min-‘, from 50- 82O”C, followed by an isotherm at the latter temperature.

low temperature, because it possesses more adsorbed oxygen on its surface. By contrast, at higher tempera- ture, when the lattice oxygen becomes available, LaMnOa shows the most active catalyst, because of the oxygen excess in its structure.

0.014

0.002

LaMn(20)

600 640 680

Lax (“Cl

Fig. 5. Specific activity data, calculated at 45o”C, vs. T,,,,,.

By comparing the temperatures of oxygen desorp- tion peak maximum (T,,,), taken from the TPD-MS thermograms (Fig. 4), with specific activity measured at 450°C one may see that the lower Tmax, the higher is the specific activity of the catalyst (Fig. 5). This can be explained in terms of ability of the catalyst in providing active oxygen at its surface. Indeed, the oxidation power of the catalyst increases with the mobility of lattice oxygen, T,,, being an index of such a mobility.

Finally, by comparing the catalytic activity of the Laa.sMa.iMnOa samples (M = Eu or Ce) with LaMnOs (Fig. 6) one may note the effect of the valence state of M. In fact, the substitution of a bivalent ion (Et?+) for La3+ creates anionic vacancies that improve the catalytic activity. On the opposite, by replacing a tetravalent ion (Ce4+) for La3+, the anionic vacancies are suppressed and the catalyst activity decreases.

The TPR-MS study on Lac,9Eu0,1Mn03 shows that methane oxidation takes place between 400 and 500°C (see CO2 signal, m/z = 44, in Fig. 7). Negative peaks in the oxygen signal (m/z = 32) confirm that oxygen is consumed during reaction. The TDS-MS analysis shows that CO2 formation starts at ca. 100°C (Fig. 8(d)) but, as indicated also by the TPR-MS data (Fig. 7), the reaction boots between 400 and 500°C. This behaviour confirms once again the presence of two types of oxygen. The first type (surface adsorbed oxygen) is active at low temperature. By increasing

184 L. Marchetti, L. Foni/Applied Catalysis B: Environmental I5 (1998) 179-187

loo

80

60

40

2.0

0

+Lao9&01NhCX3

300 400 500 Tee,

Fig. 6. Activity comparison data under identical reaction conditions for Lq.~M,,lMn03 (M = Eu or Ce) and LaMnO,.

400 600 800 +

T (“C) Fig. 7. TPR-MS thermogram of methane pulses over La,,9Eu0.1Mn03 preactivated in flowing oxygen at 550°C for 1 h.

temperature, its coverage decreases, while the second and by the oxygen signal (m/z = 32) of our TPR-MS type of oxygen (lattice oxygen) becomes available, so analysis. Indeed, the XPS 01s signal (Fig. 9) splits increasing the catalytic activity, as observed also by into two peaks: by increasing temperature, the lower- others [ 191. This is confirmed both by our XPS spectra binding-energy peak, related to lattice oxygen [20,21],

L. Marchetti, L. Fomi/Applied Catalysis B: Environmental 15 (1998) 179-187 185

8000 c 3

ti 6000

.&

g 92 4000

,c

5 2000 .a co

0

a)

0 200 400 600 T WA

8000 8000

3. 4 ” 7 d)

46000 s 6000

.a 0

c” s 4000 i! Q) 4000

C E

s 2000 z 2000 9 .a v) co

0 0

0 200 400 600 0 200 400 600 T (“Cl -I- (“C)

Fig. 8. CH4 + “02 TDS analysis on (a) LaMnO (19.9 m2 pm’); (b) LaosSr0,2Mn03; (c) La&ro4Mn03; (d) Lao.sEuo ,Mn03. Pretreatment by evacuation (~10~~ mbar) at 55o”C, followed by introduction of 1 mbar of 1602, outgassing at room temperature and introduction of reactants (1 mbar each)

0 2ooT (“C)mo 60’

becomes more intense than the higher-binding-energy peak, connected with adsorbed oxygen. The m/z = 32 signal (Fig. 7) shows two peaks, at 200-300°C and at 500-7OO”C, corresponding to cy and p peak, respec- tively (vide supra).

The reaction here studied occurs between CI-I, and the oxygen present on the catalyst surface. Indeed, in the TDS experiments, carried out by introducing gas- eous 1802, the COz. formed has m/z = 44 (C1602) (Fig. 8). This means that gas phase ‘*Oz does not react directly with methane, otherwise we should observe the formation of C”O,, with m/z = 48. However, the m/z = 36 (‘*O*) and m/z = 32 ( 1602) signals indicate that gas phase “02 replaces the oxygen on the catalyst surface (160,), (m/z = 36 decreases and m/z = 32 and 34 increase). This means that an oxygen exchange reaction [22] occurs before the increase of catalytic activity. This effect can be clearly observed when considering the series Lal_XSrXMnOs (X = 0, 0.2 and 0.4). An immediate evidence is that, along the

series, by progressively increasing the value of x, the decrease of the gas phase oxygen (1802) begins at lower and lower temperature and, simultaneously, the signal of 1602, related to the catalyst oxygen, becomes stronger and broader (Fig. 8(a-c). One may conclude that this occurs because, by increasing the amount of Sr in the perovskite structure, more anionic vacancies are created, so that the oxygen exchange reaction

180w + 2160[,) = 21*o(,) + 160qg)

18o2(a) + l60(,) = nQ,) + 1601*0~,)

becomes easier. Although oxygen is available at lower temperature,

for any value of x the reaction boots within the 400- 500°C temperature range (Fig. 8). This means that the most difficult step in methane oxidation is the cleavage of the C-H bond, as expected.

Furthermore, it may be also noticed that the most active catalyst for the oxygen exchange reaction is the

186 L. Marchetti, L. Fomi/Applied Catalysis B: Environmental 15 (1998) 179-187

200 I 525 530 535 540 545

B.E. (ev)

-A -6

200 I 525 530 535 540 545

B.E. (ev)

-A -B_____.C

200 I 525 530 535 540 545

BE. (ev)

-A _B _____. c

T 800

d

c 600

a 0 400

525 530 535 540 545 B.E. (ev)

-A _B_____.C

1000 2 d 800

c 0’ 600 0

400

200 I 525 530 535 540 545

B.E. (ev)

-A-_B.__...C

Fig. 9. XPS spectra of 01s level of LaMn03: A = sample heated for 15 min at lo-’ mbar; B = sample heated for 15 min in O2 atmosphere; G = B after outgassing; (a) 25°C; (b) 100°C; (c) 200°C; (d) 300°C; (e) 400°C.

least active for methane oxidation. In fact, the oxygen exchange reaction takes place in a temperature range (200-300°C) where adsorbed oxygen, related to the anionic vacancies, controls the perovskite reactivity. This confirms that the oxygen exchange reaction is a suprafacial reaction, tightly connected with the pre- sence of anionic vacancies on the catalyst surface.

Acknowledgements

We are indebted to Dr. B.E. Nieuwenhuys and to C. Doomkamp for helping L.M. in carrying out some experiments at Leiden University (Netherlands) within the framework of the EU Erasmus project.

L. Marchetti, L. Fork/Applied Catalysis B: Environmental 15 (1998) 179-187 187

References

[l] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev. Sci. Eng. 32 (1990) 51.

[2] SM. Decorso, Power 25 (1981) 60. [3] W.F. Libby, Science 171 (1971) 499. [4] R.J.H. Voorhoeve, J.P. Remeika, Ann. N.Y. Acad. Sci. 272

(1976) 3. [5] H.C. Yao, M. Shelef, J. Catal. 31 (1973) 377. [6] P.K. Gallagher, D.W. Johnson Jr., F. Schrey, Mat. Res. Bull. 9

(1974) 1345. [7] L.G. Tejuca, J.I.G. Fierro, J.M.D. Tascon, Adv. Catal. 36

(1989) 237. [8] J.J. Carberry, S. Rajadurai, C.B. Alcock, B. Li, Catal. Lett. 4

(1990) 43. [9] K. Tabata, M. Misono, Catal. Today 8 (1990) 249.

[ 101 T. Seiyama, Catal. Rev. Sci. Eng. 34 (1992) 281.

[ 1 l] T. Seiyama, in: L.G. Tejuca, J.I.G. Fierro (Eds.), Properties and Application of Perovskite-Type Oxyde, Marcel Dekker, New York, 1993, p. 215.

[12] P.E. Marti, A. Baiker, Catal. Lett. 26 (1994) 71. [13] M.S.G. Baython, F.R. Sale, J. Mater. Sci. 17 (1982) 2757. [14] H.M. Zhang, Chem. Lett. (1987) 665-668. [15] T. Nitadori, S. Kurihara, M. Misono, J. Catal. 98 (1986) 221. [16] Y. Teraoka, M. Yoshimatsu, N. Yamazoe, T. Seiyama, Chem.

Lett. (1984) 893. [17] N. Yamazoe, Y. Teraoka, Catal. Today 8 (1990) 175. [18] B.C. Tofield, W.R. Scott, J. Solid State Chem. 10 (1974) 183. [19] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26

(1986) 265. [20] Z. Zhong, L. Chen, Q. Yan, X. Fu, Chinese Sci. Bull. 40

(1995) 1703. [21] N. Gunasekaran, S. Rajadurai, J.J. Carberry, N. Bakshi, C.B.

Alcock, Solid State Ionics 81 (1995) 243. [22] K. Klier, J. Novakova, P. Jim, J. Catal. 2 (1963) 479.