Methane combustion on some perovskite-like mixed oxides

Methane combustion on some perovskite-like mixed oxides

Davide Ferri, Lucio Forni*

Dipartimento di Chimica Fisica ed Elettrochimica, UniversitaÁ di Milano, Via C. Golgi 19, I-20133 Milan, Italy

Received 18 May 1997; received in revised form 18 July 1997; accepted 18 July 1997

Abstract

Some perovskite-like mixed oxides of general formula La1ÿxA0xBO3 were prepared by the amorphous citrate method and

tested for methane combustion within the 300±6008C temperature range. Substitution at A-site with a bivalent (Eu, Sr) or

tetravalent (Ce) metal cation led to a decrease or increase of catalytic activity, respectively. La0.9Ce0.1CoO3 proved to be the

most active catalyst, showing complete conversion at 5008C. The nature of the metal cation introduced modi®es the oxidation

state of cobalt, which leads to the formation of cationic or anionic vacancies. TPD-MS analysis con®rmed that the catalytic

activity is related to the oxygen storage properties of the catalyst. The substitution at B-site (B � Fe, Co, Ni) allowed to ®nd

interesting correlations between catalytic activity and the temperature Tmax of maximum oxygen desorption rate. # 1998

Elsevier Science B.V.

Keywords: Perovskite as catalyst; Methane combustion; Oxygen mobility

1. Introduction

In the past decades, the interest for catalytic com-

bustion has grown increasingly [1] due to increasing

legislative restrictions on gaseous emissions from

either fossil fuel burners or vehicles. At low tempera-

ture, when the combustion is ¯ameless, the formation

of harmful species such as CO or NOx is limited or

virtually absent. Traditional catalysts for methane

combustion are: supported noble metals, such as

Pd=Al2O3 and Pt=Al2O3 or their combination with

Rh or Ir as in the three-way converters. The main

advantage of noble metals is their ability to activate

both the C±H and O±O bonds. These materials show

high activity even below 3508C [2], but their com-

mercial exploitation is hampered by the relatively high

volatility of oxides, sintering ease above 5008C and

mainly by cost [3]. Hence, much effort has been

devoted to the substitution of noble metal catalysts

with mixtures of selected metal oxides. Perovskite-

like mixed oxides, with general formula ABO3,

showed suitable catalysts for oxidation of light hydro-

carbons and, in particular, of methane [4±7].

Perovskite-like oxides can be tailored to create a

wide family of catalysts, by varying either the A-site

or the B-site metal ion, or both. Indeed, the catalytic

activity of a perovskite can be modi®ed by inserting

proper transition metal ions. Moreover, partial sub-

stitution at A-site with another A0 metal cation to give

A1ÿxA0xBO3�d can strongly affect catalytic activity,

due to stabilisation of unusual oxidation states of the B

Applied Catalysis B: Environmental 16 (1998) 119±126

*Corresponding author. Tel.: +39-2-26603289; Fax: +39-2-

70638129; E-mail: [email protected]

0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 3 3 7 3 ( 9 7 ) 0 0 0 6 5 - 9

component and to simultaneous formation of struc-

tural defects created by such a substitution. Catalytic

activity is admittedly ascribed to the B-site cation, the

lanthanide ion at A-site being considered responsible

for thermal endurance of the catalyst. However, the

size of this cation seems to play a role [7,8], since the

ionic radii of both A and B cations are important for

conferring stability to the perovskitic structure [9].

Structural defects are responsible not only for part

of the catalytic activity, but also for oxygen mobility

within crystal lattice of the solid, due to the non-

stoichiometry created by substituting A0 for A. The

presence of ionic vacancies affects catalytic activity

by favouring or non-favouring the adsorption of reac-

tant from the gas phase.

In the present work, different sets of perovskite-type

mixed oxides, of general formula La1ÿxSrxCoO3

(x�0, 0.2 and 0.4), La0.8Sr0.2BO3 (B�Fe and Ni)

and La0.9A0CoO3 (A0 � Eu and Ce), were prepared,

characterised and tested as catalysts for methane oxi-

dation under ¯ameless conditions. The aim of the

work was to analyse the in¯uence on catalytic activity

of the nature of B-site metal cation and of the substi-

tution of bivalent or tetravalent cations, such as Sr, Eu

or Ce for La. A study of the oxygen storage properties

of these perovskites was also carried out, in order to

correlate these properties with catalytic activity.

2. Experimental

2.1. Materials

Starting nitrates of metal constituents were pur-

chased from ACROS and MERCK. All materials were

pro-analysi reagents, with >98% purity, except nickel

nitrate (97%). For testing catalytic activity a S.I.A.D.

certi®ed gas mixture of 1.00 vol% CH4 in nitrogen was

used, mixed with S.A.P.I.O. �99.999% pure air. Pure

helium (S.I.A.D. �99.999 vol%) was used both as

carrier and as auxiliary gas for gas chromatographic

(GC) analysis. Ultrapure helium carrier gas (S.I.A.D.

�99.9999%) and oxygen (S.A.P.I.O. �99.999 vol%)

were employed for TPD-MS experiments.

2.2. Catalysts preparation

Catalysts were prepared by the so-called citrate

method [10]. Aqueous solutions of starting nitrates

were mixed with an aqueous solution of citric acid.

The molar ratio between the total amount of nitrates

and citric acid was 1 : 1. The resulting solution was

evaporated at 708C and then completely dried in a

vacuum oven overnight at 708C. The spongy material

thus obtained was crushed and calcined for 2 h in

¯owing air at the temperature determined by thermo-

gravimetric analysis (TGA, vide infra). The main

characteristics of the catalysts prepared are sum-

marised in Table 1.

2.3. Catalysts characterisation

TGA analysis of catalysts precursors was carried

out by means of a Mettler Toledo TA 8000 instrument,

on ca. 10 mg samples, with a temperature ramp of

108C=min from room temperature up to 10008C. XRD

patterns were obtained by a Siemens D-500 diffract-

ometer, using the Cu Ka (�1.5148 AÊ ) radiation, Ni-

®ltered, for all solids, except LaCoO3, for which the

Co Ka (�1.79026 AÊ ) radiation was used. Reference

spectra were compared with the J.C.P.D.S. data [11]

for structure assignment. Surface areas were deter-

mined by nitrogen sorption±desorption (B.E.T.

method), by a Carlo Erba Sorptomatic-1800 instru-

ment.

2.4. Catalytic activity tests

Methane oxidation was carried out in a quartz ®xed-

bed reactor (I.D.�0.6 cm, L�45 cm). The catalyst

powder (ca. 200 mg) was pelleted, crushed, sieved to

60±100 mesh (0.02±0.04 mm) and mixed with ca.

Table 1

Main characteristics of the catalysts employed

Catalyst Code Tc

(8C)

Phase(s) a S.A.

(m2=g)

LaCoO3 Co 640 P (�La2CO5) 11

La0.8Sr0.2CoO3 02-Co 580 P (�SrCO3) 19

La0.6Sr0. 4CoO3 04-Co 580 P (�SrCO3) 8

La0.9Eu0.1CoO3 Eu01-Co 600 P 16

La0.9Ce0.1CoO3 Ce01-Co 580 P (�CeO2) 22

La0.8Sr0.2FeO3 02-Fe 680 P (�SrCO3) 21

La0.8Sr0.2NiO3 02-Ni 800 P (�SrCO3�NiOx) b 5

a P� perovskite. In parentheses, the phases forming in minor

amounts.b NiOx � various Ni oxides.

120 D. Ferri, L. Forni / Applied Catalysis B: Environmental 16 (1998) 119±126

1.3 g of quartz (60±100 mesh). The catalyst bed was

inserted in the reactor tube between quartz beads (20±

60 mesh), to favour gas mixing and heat transfer

between gas and solid. A detailed sketch of the

apparatus was given in a previous work [6].

Catalytic tests were carried out by feeding the

reactor with a total ¯ow of 40 N cm3=min of CH4

in nitrogen gas mixture and air, so as to have 0.5 vol%

of methane. The catalysts were previously activated

overnight in 40 N cm3=min ¯owing air at 508C below

the calcination temperature. After cooling down to

3008C in air, feeding of methane gas mixture was

started (20 N cm3=min), reducing the air ¯ow to

20 N cm3=min. A 28C=min ramp was then begun,

from 3008C up to 6008C, while analysing the ef¯uent

gas every 12 min, by on-line GC.

2.5. TPD-MS analysis

Oxygen TPD-MS analysis was carried out using the

apparatus described in detail in a previous paper [12].

The quartz micro-reactor was loaded with ca. 100 mg

of catalyst. The catalyst was activated in ultrapure

helium (20 N cm3=min) overnight at proper tempera-

ture, 508C below the calcination temperature. The

¯owing gas was then swept to 15 N cm3=min oxygen

for 15 min and the sample cooled down to 508C in

¯owing oxygen. At 508C, O2 was swept to He and,

after attaining the steady state conditions, a 108C=min

temperature ramp was started from 508C up to 8008C,

while continuously monitoring by the quadrupolar MS

the fragments with m=z � 18, 28, 32 and 44, corre-

sponding to water, CO, oxygen and CO2, respectively.

3. Results

3.1. Catalysts characterisation

TGA analysis of citrates showed three decomposi-

tion steps, as also described by other authors [13]. The

temperature chosen for calcining the catalysts was ca.

208C higher than that corresponding to the third step

of the TGA thermogram, i.e. ca. 208C above the lowest

temperature needed for the formation of perovskite-

like structure. This temperature is well below that

usually needed for the preparation of perovskites by

the traditional route, so that B.E.T. surface area of the

present ®nished samples is up to one order of magni-

tude higher.

XRD patterns (Fig. 1) con®rm the formation of the

pure perovskite-like structure for the Eu-containing

system. In Ce01±Co, calcined both at 5808C and

8008C, CeO2 was hardly detected, since very weak

re¯ections attributable to that phase were observable

(Fig. 1g and h, respectively). In a previous work [12],

it was shown that the solubility of cerium in

La1ÿxCexCoO3 is limited to x�0.05, probably due

to that preparation method and to the high calcination

temperature needed for forming the perovskite-like

structure. Tabata et al. [14] and Nitadori et al. [15]

found CeO2 in their La1ÿxCexCoO3 samples; they

used also high calcination temperature for synthesis.

As for the Sr-containing catalysts (Fig. 1b,c) XRD

analysis showed a small amount of SrCO3, besides the

perovskite-like structure. Co, the only sample ana-

lysed by the Co Ka radiation, showed a small amount

of La2CO5 phase and probably also a trace of Co3O4

(Fig. 1a), as already observed by Daturi et al. [16] for

a similar catalyst. The 02±Ni (Fig. 1e) and 04±Co

(Fig. 1c) samples showed more complex diffracto-

grams, the SrCO3 phase seeming more abundant than

in other catalysts. Some of the re¯ections could be

attributed also to other oxides. In fact, Zhao et al. [17]

reported the presence of mixed oxides such as the

La2NiO4 of K2NiF4 structure or LaNiO3 phases for the

La1ÿxSrxNiO3 series. Finally, for 04±Co the SrCO3

phase seems to be the most abundant secondary

compound.

3.2. Catalytic activity and TPD results

Catalytic activity data for methane combustion are

presented in Fig. 2. They are in accordance with

literature data [14,18]. By expressing the activity

through the light-off temperature (T50) at which

50% of methane conversion is reached, Ce01±Co

appears the most active catalyst.

In the series La1ÿxSrxCoO3 (x�0, 0.2 and 0.4),

the maximum activity is for x�0.2 and the order is

02±Co>Co>04±Co, as it was also found by Tabata et

al. [19]. On the other hand, in the series La0.8Sr0.2BO3

(B�Fe, Co and Ni) the Co-based system proved more

active than the other two catalysts, for which T50

showed practically the same value. At T>T50 02±Ni

appeared more active than 02±Fe and 100% conver-

D. Ferri, L. Forni / Applied Catalysis B: Environmental 16 (1998) 119±126 121

sion was attained around 5508C. Ce01±Co was

more active than the corresponding Eu-containing

system.

CO was never detected in the products for any

catalyst, and for every catalyst methane combustion

took place within the 300±6008C temperature range,

Fig. 1. XRD patterns. (a) Co; (b) 02-Co; (c) 04-Co; (d) 02-Fe; (e) 02-Ni; (f) Eu01±Co; (g) Ce01±Co; and (h) Ce01±Co calcined at 8008C. Cu

Ka radiation, Ni filtered, except for sample (a), for which the Co Ka radiation was used. (*) Perovskite; (*) SrCO3; and (5) CeO2.


whereas for the Ce-containing cobaltate catalyst, full

conversion was achieved at 5008C.

TPD spectra (Fig. 3) show that all the present

catalysts release more oxygen than the corresponding

Mn-based perovskites [6]. The spectra present two or

three desorption peaks within the 200±8008C tem-

perature range. The high-temperature peak is, how-

ever, less important for methane combustion, since the

reaction takes place at much lower temperature. The

low-temperature peak has been attributed to supraf-

acial adsorbed oxygen, while the oxygen released

around 5008C may be considered as coming from

the bulk of the solid. This oxygen species occupy

the inner vacancies created by substitution of Sr for La

or by segregation of La-containing phases (as in the

present Co sample) [20].

At almost the same temperature Ce01±Co released

a larger amount of oxygen with respect to Eu01±Co,

but its high-temperature peak is shifted to lower

temperature (<6008C). In the La1ÿxSrxCoO3 series

the amount of oxygen released is larger for x�0.4,

while for x�0 and 0.2 the catalysts behaved compar-

ably from this point of view. For the La0.8Sr0.2BO3

series the order for oxygen desorption was 02±Ni>02±

Fe�02±Co. The Fe- and Ni-perovskites showed only

two desorption peaks, while all the Co-perovskites

showed three, at ca. 250, 600 and 8008C, respectively.

This means that oxygen mobility is higher than in the

corresponding manganites [6] which released oxygen

only at 6008C.

These results lead to the conclusion that activation

of the oxygen molecule is possible at low temperature

Fig. 2. Methane conversion for the series (a) La0:9A00:1CoO3

(A0 � Ce, Eu); (b) La1ÿxSrxCoO3 (x�0, 0.2, 0.4); and (c) ±

La0.8Sr0.2BO3 (B�Fe, Co, Ni).

Fig. 3. TPD spectra after O2 saturation. (a) Ce01±Co; (b) Eu01±

Co; (c) 04±Co; (d) 02±Co; (e) Co; (f) 02±Ni; and (g) 02±Fe.


(ca. 2008C), whereas activation of the C±H bond

appears to be more dif®cult, the reaction starting in

any case only above 3008C.

4. Discussion

It is well known that in perovskite-like mixed

oxides substitution of the trivalent A-site metal ion

with a bivalent or tetravalent metal cation (A0) is

accompanied by a modi®cation of the oxidation

state of B-site metal cation, thus modifying catalytic

activity. Moreover, modi®cation of the oxidation

state of B-site metal cation by insertion of A0 may

be accompanied by the formation of structural

defects, thus leading to non-stoichiometry. In the

Co-containing perovskites this usually means

oxygen defects, while in Mn-perovskites it results

in oxygen excess [9]. Fe [18] and Ni-based perovskites

[21] were shown to have an intermediate behaviour. In

the latter, formation of anionic vacancies seems to be

easier than oxidation of Ni3� to Ni4�, since the ionic

radius of oxidised species does not ®t the perovskitic

structure.

In Ce01±Co and in Eu01±Co the low calcination

temperature leads to pure perovskite, and CeO2 was

hardly detected. The presence of a small amount of

this phase means that not all of Ce4� has been

incorporated within the perovskitic structure. This

results in either partial reduction and oxidation of

Co, Co3� being the average oxidation state in these

perovskites. In fact, cation vacancies are created at A-

site by the insertion of Eu2�, resulting in the oxidation

of Co3� to Co4�, while the insertion of Ce4� brings

about the reduction to Co2�.

The case of Sr2�-catalysts is similar to that of Eu-

containing ones. Sr substitution for La leads to higher

oxidation states for Co, so that the higher the amount

of Sr, the higher is the concentration of Co4�. How-

ever, since Co4� is particularly unstable, then some

oxygen release can take place, ending in the formation

of oxygen vacancies. Furthermore, XRD patterns of

this series show the formation of SrCO3, very likely

accompanied by the formation of cation vacancies.

This is in line with TPD data of La1ÿxSrxCoO3 sam-

ples (Fig. 3) showing that by increasing x the con-

centration of Co4� increases, as the amount of

desorbing oxygen increases accordingly.

By contrast, as previously shown [6], Mn-based

perovskites released oxygen in the opposite order,

i.e. by increasing x the amount of released oxygen

decreases, due to a decrease of cation vacancies con-

centration. It can be concluded that the substitution of

a bivalent cation for La3� is accompanied by oxidation

of the B cation. On the contrary, insertion of Ce4�

leads to partial reduction of the B-site metal cation.

According to Wu [18] and Tabata [19], by increas-

ing x in La1ÿxSrxCoO3 more oxygen is released. In the

present case, the maximum activity falls at 20%

substitution of Sr for La. 04±Co is less active than

02±Co, though it releases a larger amount of oxygen,

as shown by TPD measurements (Fig. 3). Indeed,

oxygen vacancies are directly connected with oxygen

mobility. The higher the amount of anion defects (the

higher the value of x), the higher is the oxygen

mobility. However, a too high oxygen release is

accompanied by a dif®cult surface reoxidability

[22], so leading to a less active catalyst.

Oxygen TPD signals for perovskite-like oxides are

usually attributed to release of oxygen species situated

at structural defects, i.e. essentially oxygen from the

bulk [23]. However, since a relatively large amount of

structural defects is present in these catalysts, a part of

desorbing oxygen can be due to surface species too.

The high-temperature peak (Fig. 3) is surely attribu-

table to more tightly bound oxygen, i.e. to structural

bulk oxygen. However, the low-temperature peaks

(200±6008C) correspond to oxygen originated by

the reduction process involving the B-site metal

[24] and the formation of anionic vacancies. At low

temperatures, methane combustion on these catalysts

is a suprafacial reaction [25] involving oxygen coming

from the gas phase or sitting in the anion vacancies of

the catalyst [20]. In fact, it has been shown [4] that

methane combustion rate depends on CH4 partial

pressure only at low temperature, whereas at high

temperature CH4 reacts with the oxygen supplied

by the catalyst and coming from the bulk (intrafacial

mechanism [25]). Our data con®rm these ®ndings,

showing that for all the catalysts (Fig. 2) the rate of

methane oxidation boosts when bulk oxygen becomes

available.

Fig. 2a data show also that Ce01±Co is more active

than Eu01±Co. Besides the larger amount of oxygen

available for methane oxidation, as indicated by the

TPD patterns (Fig. 3), XPS data (not reported here) for


Ce01±Co indicate the presence of Co2�, generated by

the insertion of Ce4� for La3�. This leads to the

presence of a larger amount of active sites for oxygen

adsorption from gas phase, i.e. an easier reoxidability

of the catalyst.

In our La0.8Sr0.2BO3 series, where B is Fe, Co or Ni,

Sr substitution for La is accompanied by oxidation of

part of B species up to Fe4�, Co4� or Ni4�, respec-

tively. 02±Co proved more active than 02±Fe and 02±

Ni. It released oxygen at ca. 2508C, while 02±Fe and

02±Ni at ca. 3608C and 4008C, respectively (Fig. 3).

Formation of oxygen defects promotes oxygen

adsorption from gas phase. Therefore, in the 02±Co

sample the formation of defects due to cobalt reduc-

tion to Co3� is enhanced, resulting in a higher activity

since that temperature. The difference in activity of

these catalysts can also be ascribed to the different

stability of the mentioned uncommon oxidation states

created by substituting Sr2� for La3�.

Finally, the activity data here obtained allow us to

compare the two families of cobaltates and manga-

nites, which have been taken as reference catalysts for

a wide number of reactions [2,20,26±30]. It is clear

that Co- and Mn-based perovskites behave in an

opposite way and that, by substituting A0 (Sr, Eu or

Ce) for A, different situations are encountered. Co-

based perovskites are oxygen de®cient and charac-

terised by the presence of anionic vacancies, favouring

oxidation catalysis. On the other hand, in Mn-based

catalysts an excess of oxygen and cationic vacancies

are present and anionic vacancies seem to appear only

beyond a given value of x [6,31]. As a consequence,

substitution of Eu or Ce for La brings about a different

behaviour towards methane combustion. Ce01±Co is

more active than Eu01-Co while Eu01-Mn is more

active than Ce01-Mn [6]. The Co-based catalysts

release more oxygen, while the manganites release

less oxygen and only at ca. 6008C. Oxygen mobility is

thus higher for the former and the activity for methane

oxidation increases accordingly from Mn- to Co-based

perovskites. However, it must be kept in mind that also

the stability of the B-site metal cation affects the

activity. In manganites the introduction of Sr2� leads

to Mn4� and to cationic vacancies, since Mn4� is more

stable than Mn3� [22]. In the cobaltates Co4� is less

stable than Co3�, thus leading to reduction, oxygen

release and creation of oxygen vacancies. Hence, the

different stability of the oxidation state of B-site cation

may reduce the difference in activity between the two

series of catalysts.

Acknowledgements

We are grateful to M. Catti and A. Sironi for XRD

analysis and to A. Lunghi of the Stazione Sperimen-

tale per i Combustibili, S. Donato Milanese, for TGA

analysis. Thanks are due also to R. Revilla for his help

in experimental work.

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Methane combustion on some perovskite-like mixed oxides