Reactivity of La1-xSrxFeO3-y (x = 0-1) perovskites in oxidation reactions

Article abstract of DOI:10.1007/s10975-005-0129-1

Oxygen species and their reactivity in La_1-xSr _xFeO_3-y perovskites prepared using mechanochemical activation were studied by temperature-programmed reduction (TPR) with hydrogen and methane. The experimental data were compared with data on the catalytic activity in oxidation reactions. It was found that the rates of CO and methane oxidation on the perovskites in the presence of gas-phase oxygen correlated (k = 0.8) with the amount of reactive surface oxygen species that were removed by TPR with hydrogen up to 250°C. Maximum amounts of this oxygen species were released from two-phase samples (x = 0.3, 0.4, and 0.8), which exhibited an enhanced activity in the reaction of CO oxidation. In the absence of oxygen in the gas phase, methane is oxidized by lattice oxygen. In this case, the process activity and selectivity depend on the mobility of lattice oxygen, which is determined by the temperature, the degree of substitution, the degree of reduction, and the microstructure of the oxide. Thus, the high mobility of oxygen, which is reached at high concentrations of point defects or interphase/domain boundaries, is of importance for the process of deep oxidation. However, the process of partial oxidation occurs in single-phase samples at low degrees of substitution (x = 0.1-0.2).

Full text of DOI:10.1007/s10975-005-0129-1

Kinetics and Catalysis, Vol. 46, No. 5, 2005, pp. 729–735. Translated from Kinetika i Kataliz, Vol. 46, No. 5, 2005, pp. 773–779.
Original Russian Text Copyright © 2005 by Isupova, Yakovleva, Alikina, Rogov, Sadykov.
Reactivity of La1 – xSrxFeO3 – y (x = 0–1) Perovskites
in Oxidation Reactions
L. A. Isupova, I. S. Yakovleva, G. M. Alikina, V. A. Rogov, and V. A. Sadykov
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received November 10, 2004
Abstract—Oxygen species and their reactivity in La1 – xSrxFeO3 – y perovskites prepared using mechanochem-
ical activation were studied by temperature-programmed reduction (TPR) with hydrogen and methane. The
experimental data were compared with data on the catalytic activity in oxidation reactions. It was found that the
rates of CO and methane oxidation on the perovskites in the presence of gas-phase oxygen correlated (k = 0.8)
with the amount of reactive surface oxygen species that were removed by TPR with hydrogen up to 250°ë.
Maximum amounts of this oxygen species were released from two-phase samples (x = 0.3, 0.4, and 0.8), which
exhibited an enhanced activity in the reaction of CO oxidation. In the absence of oxygen in the gas phase, meth-
ane is oxidized by lattice oxygen. In this case, the process activity and selectivity depend on the mobility of
lattice oxygen, which is determined by the temperature, the degree of substitution, the degree of reduction, and
the microstructure of the oxide. Thus, the high mobility of oxygen, which is reached at high concentrations of
point defects or interphase/domain boundaries, is of importance for the process of deep oxidation. However, the
process of partial oxidation occurs in single-phase samples at low degrees of substitution (x = 0.1–0.2).
INTRODUCTION
over, published data [9] on the oxidation of methane on
homogeneous solid solutions formed in the given sys-
tem at x = 0–0.5, which were synthesized from nitrates
(it was found that the activity of samples in the deep
oxidation of methane was independent of the composi-
tion of these samples), should be taken into account. On
this basis, we can hypothesize that a change in the den-
sity of point defects in perovskites due to substitution
(the formation of vacancies or an increase in the charge
of the 3d-metal cation in homogeneous solid solutions
with perovskite structures) is insignificant for deep oxi-
dation processes. Nevertheless, Li et al. [10] discussed
the participation of perovskite lattice oxygen in the het-
erogeneous reactions of deep oxidation. On the other
hand, because it is well known that the mobility of lat-
tice oxygen depends on the concentration of vacancies,
a change in the point defects has a considerable effect
on the mobility of oxygen in the bulk oxide; this effect
can be of importance for membrane oxidation pro-
cesses.
The high activity and operational stability of oxides
with perovskite structures (ABO3) in various catalytic
reactions have attracted the serious attention of
researchers. It is commonly accepted that the activity of
perovskites in deep oxidation reactions depends on the
nature of the transition metal in the B sublattice and on
the electronic state of this metal, which depends on sub-
stitutional cations in the A and B sublattices. Moreover,
this activity depends on preparation conditions, which
are responsible for the real oxide structure (microstruc-
ture) [1–4]. These factors ultimately determine the
strength of the metal–oxygen bond on the oxide sur-
face; the rates of deep oxidation reactions depend on
this bond strength [5, 6].
Various oxygen species can occur on the surface of
oxides. However, as found previously [7] for simple
oxides, the most weakly bound species formed at the
outlets of extended defects (for example, dislocations)
are responsible for activity in deep oxidation reac-
tions. An analogous result was obtained previously for
La1 − xCaxFeO3 – y perovskites [8]. This system does not
form a continuous series of homogeneous solid solu-
tions under conditions of ceramic and mechanochemi-
cal syntheses (at 0.17 < x < 1.00, all of the samples are
two-phase microheterogeneous solid solutions with
various microstructures). In this system, we found a
relationship between the microheterogeneity of sam-
ples or maximum concentrations of the most weakly
Homogeneous solid solutions are formed in the
La1 − xSrxFeO3 – y system, and an increase in the stron-
tium content results in the formation of Fe4+ cations
[11]. Therefore, it is likely that a new oxygen species,
which is a constituent of the coordination sphere of a
highly charged cation, appears in this system, and this
species can participate in deep oxidation reactions. In
the series prepared by mechanochemical activation, the
perovskite samples with x = 0.3, 0.4, and 0.8 were
bound surface oxygen species in these samples and the found to be non-single-phase [12]. According to TEM
catalytic activity in deep oxidation reactions [8]. More- data, these samples were microheterogeneous solid
0023-1584/05/4605-0729 © 2005 MAIK “Nauka /Interperiodica”

730
ISUPOVA et al.
solutions, which were formed in low-temperature poly- 2.4 l/h (i.e., 4000 h–1). The rate of reaction was calcu-
lated from the equation
w, [molecule ëç4 m–2 s–1] = kaC0 × 2.69 × 1019, (2)
morphic phase transitions upon cooling the samples in
air (preliminary data). Consequently, by analogy with
the La1 – xCaxFeO3 – y system, these samples can addi-
tionally contain reactive oxygen species inserted into
interphase boundaries.
where ka is the apparent rate constant of first-order reac-
tion, which was calculated by the equation ka =
−2.3log( 1 – X)/(tmSsp) for a plug-flow reactor (X is the
conversion of ëç4, m is the sample weight, t is the con-
tact time, and C0 = 0.5 is the initial concentration of
methane). The error of the chromatographic determina-
tion of gas mixture components was no higher
than 10%.
The TPR study of samples with hydrogen was per-
formed in a flow system with a thermal-conductivity
detector using a fraction of samples with a particle size
of 0.25–0.5 mm. Before the reduction, the samples
were pretreated in O2 at 500°C for 0.5 h and cooled to
room temperature. The sample weight was 50 mg; the
flow rate of the reducing mixture (10% H2 in Ar) was
40 cm3/min. The samples were heated at a rate of
10 K/min to 900°C. The peak areas of the TPR of
samples corresponding to hydrogen consumption
(mmol/(g sample)) were calculated with the use of an
asymmetric Gaussian function profile.
The TPR with methane was performed using a flow
of 1% ëç4 + He (10 l/h) at a heating rate of 5 K/min.
The sample weight (q) (particle size of 0.5–1 mm) was
0.2–0.3 g. After reaching a temperature of 880°ë, the
samples were additionally kept in a flow of the reaction
mixture for 70 min. The reaction products (ëé2, ëé,
H2, and H2é) were analyzed using a PEM-2M gas ana-
lyzer. The amount of removed oxygen was found from
the graphical representation of the amount of a product
as a function of temperature (time). The rates of oxygen
consumption were calculated from the rates of product
formation using the equations
Thus, various oxygen species can occur on the sur-
face of perovskites in the La1 – xSrxFeO3 – y system pre-
pared with the use of mechanochemical activation.
These are surface species, species inserted into inter-
faces, and lattice species, including those bound to Fe4+
and Fe3+ cations. Temperature-programmed reduction
(TPR) is one of the techniques that can distinguish var-
ious oxygen species according to their reactivity.
Therefore, the aim of this work was to study the reac-
tivity of various oxygen species in perovskite oxides
from the La1 – xSrxFeO3 – y series by TPR and to relate
these species to the activity of oxides in the reactions of
CO and ëç4 oxidation.
EXPERIMENTAL
The samples of La1 – xSrxFeO3 – y were synthesized by
the calcination of the mechanically activated mixtures
of parent oxides taken in required ratios at 1100°ë for
4 h. The time of mechanical treatment was 3 min; the
procedure of the mechanochemical synthesis and the
results of phase analysis were described elsewhere
[12]. With the use of X-ray diffraction analysis and dif-
ferentiating phase dissolution, it was found that single-
phase perovskites were formed in the system under the
specified conditions of the synthesis except for samples
with x = 0.3, 0.4, and 0.8, which consisted of two
phases: orthorhombic and rhombohedral (or cubic) per-
ovskites.
The catalytic activity of samples (particle size frac-
tions of 0.5–1 and 1–2 mm) in the reaction of CO oxi-
dation was determined in a circulation-flow reactor at
250–450°ë with the use of chromatographic analysis.
The sample weight was 1 g; the circulation rate was
1200 l/h; the flow rate of the reaction mixture (1% CO +
1% é2 in He) was 10 l/h. The specific rate of reaction at
a 1% concentration of CO was calculated from the
equation
wCO , [atom O m–2 s–1]
= 29.876 × 1017[CO2]/(qSsp),
2
(3)
wCO, [atom O m–2 s–1] = 7.469 × 1017[CO]/(qSsp). (4)
RESULTS AND DISCUSSION
w, [molecule ëé m–2 s–1]
Catalytic Activity of Substituted Perovskites
from the La1 – xSrxFeO3 – y Series in the Deep Oxidation
Reactions of CO and Methane
= X/(1 – X)(7.47 × 1017/Ssp),
(1)
where X is the conversion of CO. The error of the chro-
matographic determination of gas mixture components
was no higher than 10%.
The specific catalytic activity of perovskites from
the La1 – xSrxFeO3 – y series in the reaction of carbon
monoxide oxidation nonmonotonically changed with x.
Two activity maximums were observed at x = 0.3–0.4
and 0.8 (Fig. 1).
The catalytic activity of samples (particle size of
0.5–1 mm) in the reaction of ëç4 oxidation was deter-
mined in a flow reactor at 500–600°ë using GC for the
In the catalytic reaction of the deep oxidation of
analysis of reactants and products. The sample weight methane, which occurred at higher temperatures (500–
was 1 g; the catalyst volume was 0.6 cm3; the flow rate 600°ë) (Fig. 2), the reaction selectivity for ëé2 was
of the reaction mixture (0.5% ëç4 + 9% é2 in He) was 100% for all of the samples regardless of test tempera-
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

REACTIVITY OF La1 – xSrxFeO3 – y (x = 0–1) PEROVSKITES
731
w × 10–19, molecule CO m–2 s–1
w × 10–16, molecule CH4 m–2 s–1
1
2
3
2.0
1.5
1.0
0.5
0
1
2
3
4
5
10
8
6
4
2
0
0.2
0.4
0.6
0.8
x
0
0.2
0.4
0.6
0.8
1.0
x
Fig. 1. Dependence of the rate of CO oxidation on the com-
position (x) of La Sr FeO at (1) 250, (2) 300, (3)
350, (4) 400, and (5) 450°C.
Fig. 2. Dependence of the rate of methane oxidation on the
composition (x) of La
and (3) 600°C.
Sr FeO
at (1) 500, (2) 550,
1 – x
x
3 – y
1 – x
x
3 – y
8
3.0
2.5
7
6
5
1.0
0.8
0.6
0.4
4
2.0
2
1
3
2
1
1.5
1.0
0.5
0.2
0
0
0.2
0.4
0.6
0.8
1.0
200
400
600
800
T, °C
x
Fig. 4. Dependence of (1) the rate of CO oxidation at 400°ë
and (2) the amount of hydrogen consumed up to 250°ë on
Fig. 3. Curves of the TPR of perovskites from the
La Sr FeO series with hydrogen: x = (1) 0, (2) 0.2,
(3) 0.3, (4) 0.4, (5) 0.6, (6) 0.7, (7) 0.8, and (8) 1.
1 − x
x
3 – y
the composition (x) of La Sr FeO .
1 – x
x
3 – y
Temperature-Programmed Reduction of Samples
ture. Close values of specific catalytic activity (no max-
imums) in methane oxidation were obtained for all
samples. Thus, the behaviors of the La1 – xCaxFeO3 and
La1 – xSrxFeO3 – y systems were different in CO and
methane oxidation processes. In the Ca-containing sys-
tem [8], the activity–composition relationship was the
from the La1 – xSrxFeO3 – y Series with Hydrogen
As follows from the experimental data (Figs. 3–5,
Table 1), the reduction of samples with hydrogen
occurred in different temperature regions. This can be
due to both a variety of surface and lattice oxygen spe-
same in both of the deep oxidation reactions and exhib- cies in perovskites and a non-single-phase character of
samples from this series [12]. Three main peaks in the
TPR curves of the samples can be recognized (Fig. 3):
low-temperature hydrogen consumption, which quanti-
tatively corresponds to the reduction of no more than a
monolayer surface coverage with oxygen; consumption
with maximums in the temperature region 400–600°ë,
which quantitatively corresponds to the reduction of
Fe4+ cations to Fe3+ (or the removal of oxygen from the
ited maximums for samples with intermediate compo-
sitions (which also corresponded to a change in the
amount of surface oxygen species). In the Sr-contain-
ing system there were maximums in CO oxidation and
no maximums in methane oxidation [12]; this suggests
the participation of different oxygen species in reac-
tions that occur at different temperatures.
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

732
H2 consumption, mmol/g
ISUPOVA et al.
Amount of O2 × 105, mol/g
3.0
2
0.005
0.004
0.003
0.002
0.001
2.5
2.0
1.5
1.0
0.5
1
0
0.2
0.4
0.6
0.8
1.0
x
Fig. 5. Dependence of the amounts of hydrogen consumed
to (1) 600 and (2) 900°C on the composition (x) of
La
Sr FeO
.
1 − x
x
3 – y
0
0.2
0.4
0.6
0.8
1.0
coordination sphere of highly charged cations with the
formation of oxygen vacancies); and high-temperature
consumption at temperatures higher than 600°ë, which
quantitatively corresponds to the deeper steps of the
reduction of iron cations. It can be seen that the low-
temperature consumption (to 250°ë) nonmonotonically
changed with increasing strontium content (Fig. 4),
whereas the consumption in the region to 600°ë and the
total amount of oxygen removed in the course of TPR
to 900°ë (Fig. 5) or the reducibility of samples
increased. Thus, the bulk reduction of lanthanum ferrite
began at ~900°ë, whereas the bulk reduction of stron-
tium ferrite (Fe3+ to Fe0) occurred by more than 80% at
900°ë (Fig. 3). Let us consider the experimental data in
accordance with the above subdivision into three
groups and compare them with data on the catalytic
activity.
w400 × 10–19, molecule CO m–2 s–1
Fig. 6. Linear correlation (with a coefficient of 0.8) between
the amount of weakly bound oxygen and the catalytic activ-
ity of La
Sr FeO
in CO oxidation.
1 – x
x
3 – y
(1) The lowest temperature (to 250°ë) consumption
of hydrogen quantitatively corresponded to the removal
of no more than a monolayer of oxygen. Because the
phase reduction of oxides does not occur in this temper-
ature region, the consumption of hydrogen can charac-
terize the most weakly bound reactive surface oxygen
species (Table 1). We calculated the amount of hydro-
gen consumed in this temperature region as a function
of sample composition. We found that this function
exhibited maximums at x = 0.3 and 0.8 (Fig. 4, curve 1);
this is consistent with changes in the catalytic activity
Table 1. Amount of hydrogen consumed in the TPR of samples with hydrogen
H2 consumption, mol/g
Sample
Amount of Fe4+, %*
(calculated from species 2)
to 250°C
to 600°C
to 900°C
composition, x
(species 1) × 105
(species 2) × 103
(total) × 103
0
1.98
1.57
2.77
0.69
0.56
1.07
1.13
1.14
0.71
0.49
0.82
0.96
1.5
1.24
1.44
2.37
2.94
3.90
4.04
4.44
4.82
–
0.2
0.3
0.4
0.6
0.7
0.8
1.0
22.7
37.2
42.6
63.6
76.2
79.5
74.4
1.75
1.97
1.96
* Percentage with respect to a theoretically possible value.
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

REACTIVITY OF La1 – xSrxFeO3 – y (x = 0–1) PEROVSKITES
733
of samples in the reaction of CO oxidation (Fig. 4, polymorphic phase transition (by analogy with the
curve 2). Moreover, as follows from Fig. 6, a linear La1 − xSrxëÓO3 – y system [13, 14]) and, consequently,
relation between the amount of oxygen removed in this the desorption of a portion of reactive surface species
temperature region, normalized to the unit surface area related to interfaces.
of samples, and the specific catalytic activity (per
The experimental data allowed us to conclude that
square meter) in the reaction of CO oxidation can be
the lattice oxygen species of perovskites, including spe-
established with a correlation coefficient of 0.8. This
cies in the coordination sphere of a highly charged cat-
relation indicates that the catalytic activity of perovs-
ion, did not participate in the heterogeneous catalytic
kites in the reaction of CO oxidation depends on the
deep oxidation reactions of not only carbon monoxide
amount of this oxygen species. By analogy with pub-
but also methane.
lished data [8], it is likely that the observed maximums
The observed correlation between changes in the
of activity are due to oxygen adsorbed at interfaces
catalytic activity of samples and their surface reduction
because these samples are non-single-phase, as found
(to 250°ë) is consistent with previous conclusions [1, 2,
4] on the stepwise reaction mechanism of CO oxidation
previously [12].
(2) The consumption of hydrogen at temperatures
up to 600°ë monotonically increased with strontium
content. Moreover, the addition of strontium decreased
the temperature of reduction. The amount of this oxy-
gen species corresponds to the removal of one to a few
tens of oxygen monolayers. This species can be associ-
ated with oxygen in the coordination sphere of the
highly charged Fe4+ cation. These low temperatures of
reduction (much lower than the reduction temperatures
of α-Fe2O3) and the amount of removed oxygen sug-
gest the removal of this oxygen species from perovs-
kites in this temperature region. The amount of Fe4+ cat-
ions calculated from data on the consumption of hydro-
gen at temperatures lower than 600°ë indicates that
their concentration is close to the theoretically possible
value (Table 1); this can also be indicative of the correct
assignment of this peak. However, it can be seen that
the amount of oxygen removed in this temperature
region does not correlate with data on the catalytic
activity of the samples in both of the catalytic oxidation
reactions.
on perovskites. The occurrence of reactive oxygen spe-
cies on the surface near interfaces can be explained by
the fact that clusters of coordinatively unsaturated Fe2+
cations can be formed at the sites of the outlets of these
interfaces. These clusters can adsorb the most weakly
bound oxygen species [15].
The independence of the activity of samples in the
reaction of methane oxidation from the composition of
samples could also be explained by a smaller degree of
surface reduction in a methane-containing reaction
atmosphere than that in an atmosphere containing CO.
In addition, this could be explained by greater surface
coverages with water formed in the course of reaction.
It is well known that surface hydroxylation decreases
the concentration of coordinatively unsaturated surface
centers (in the case under consideration, iron cations).
On the other hand, the microstructure of some samples
could be changed with increasing test temperature
because polymorphic phase transitions can occur in the
system [16].
Thus, lattice oxygen did not participate in the deep
oxidation reactions of methane and CO in an excess of
oxygen. At the same time, the participation of lattice
oxygen species is most important for membrane oxida-
tion processes, the activity and selectivity of which
depends on the mobility of oxygen [17, 18]. For this
reason, we consider the effect of strontium substitution
for lanthanum in perovskite on TPR with methane.
(3) The consumption of hydrogen at higher temper-
atures (up to 900°ë) corresponds to the deeper reduc-
tion of oxides (Table 1). Therefore, it can be ascribed to
the bulk reduction of perovskites, for example, the
reduction of Fe3+ to Fe2+ and then to Fe0 in the perovs-
kite structure. The amount of hydrogen consumed in
this region and the total consumption monotonically
increased with x; this is inconsistent with the character
of changes in the activity of the samples.
Temperature-Programmed Reduction of Samples
from the La1 – xSrxFeO3 – y Series with Methane
Thus, in the given system, which is electrically neu-
tral because of the formation of highly charged Fe4+
cations, it is most likely that oxygen in the coordination
Methane was oxidized in the TPR of the specified
sphere of these cations does not participate in the heter- catalysts with methane, and the products of deep and
ogeneous reactions of CO and CH4 oxidation. It is partial oxidation (ëé2, ëé, H2, and H2é) were
likely that the correlation between the amount of sur- released into the gas phase. Moreover, samples with x =
face oxygen species and the activity in the reaction of 0.8 and 1 released oxygen into the gas phase (Figs. 7, 8,
CO oxidation suggests that reactive surface oxygen Table 2). The experimental results suggest the occur-
species primarily participated in the reactions of deep rence of three lattice oxygen species. One of these spe-
oxidation. In our opinion, the absence of such a corre- cies is responsible for the process of deep oxidation,
lation for the reaction of methane oxidation can be whose products are ëé2 and H2é. The second species
explained by a change in the sample microstructure at is responsible for selective oxidation, which results in
the test temperatures specified due to a low-temperature the formation of ç2 and CO. It is likely that the release
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

734
Product concentration, %
ISUPOVA et al.
4
(‡)
0.075
0.050
0.025
5
2
5
4
1
3
3
2
1
70 min
0
400
800 880
880
T, °C
70 min
0
400
800 880
880
T, °C
Fig. 7. Products released into a gas phase in the course of
the TPR reduction of a SrFeO sample with methane:
3 – y
(b)
(1) CO , (2) CO, (3) H , (4) H O, and (5) O .
2
2
2
2
5
of oxygen in the course of TPR with methane is due to
the high mobility of oxygen at low temperatures, which
are insufficient for methane activation. The overall
degree of sample reduction at the end of the TPR pro-
cess changed only slightly as the strontium content of
the samples was increased (Table 2). However, the
maximum rate of reduction increased; this fact suggests
an increase in the mobility of oxygen in substituted
samples. Moreover, the ratio between gaseous products
of the reaction (process selectivity) and the temperature
at which the rate of reduction reached a maximum
changed. For example, note that the temperature and
the maximum rate increased with strontium content.
This reflects a relationship between the amount of
vacancies formed and the mobility of oxygen. In the
samples that released oxygen, the selectivity for hydro-
gen decreased without a decrease in the selectivity for
CO; this may be indicative of the oxidation of the
resulting hydrogen. The experimental data allowed us
to recommend various perovskite compositions for
deep or partial oxidation processes. Thus, samples with
x = 0.1–0.2 can be used for the production of synthesis gas.
4
3
2
1
70 min
0
400
800 880
880
T, °C
Fig. 8. Formation of (a) CO and (b) ëé in the TPR of per-
2
ovskites from the La
x = (1) 0.2, (2) 0.3, (3) 0.6, (4) 0.8, and (5) 1.
Sr FeO
series with methane:
1 – x
x
3 – y
[18], who also suggested a periodic (alternating-feed) pro-
cess for producing synthesis gas. It is likely that single-
phase samples with x = 0.4–0.8 can be used in the mem-
Similar recommendations were made by Li Ranjia et al. brane or periodic processes of deep methane oxidation.
Table 2. Reactivity of perovskites according to data on TPR with methane
Rates of reduction at 800°C, as calculated
Number of oxygen monolayers
based on various products ×1016,
Sample
Ssp,
m2/g
Overall degree
of reduction, %
removed at temperatures (°C) to
atom O m–2 s–1
CO2
composi-
tion, x
CO
H2
400
700
880
0.2
0.3
0.6
0.8
1
3.1
2.6
1.3
0.9
0.1
2.38
2.79
10.56
4.03
2.84
–
0
14.84
17.72
29.67
49.64
362.64
71.6
64.0
61.9
45.3
66.3
58.2
68.3
0.96
8.20
1.20
8.50
4.53
19.13
18.15
289.6
6.60
0.90
33.13
204.1
224.2
10.37
74.10
2170
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REACTIVITY OF La1 – xSrxFeO3 – y (x = 0–1) PEROVSKITES
735
2. Yamazoe, N. and Teraoka, V., Catal. Today, 1990, vol. 8,
CONCLUSION
no. 2, p. 175.
The reactivity of different oxygen species in
La1 − xSrxëÓO3 – y perovskites prepared by mecha-
nochemical activation has been studied by hydrogen
and methane TPR methods.The data obtained have
been compared with catalytic activity data for the deep
oxidation of methane and carbon monoxide. The reduc-
tion of the perovskites with hydrogen proceeds in three
temperature regions: the reactive surface oxygen is
removed below 250°C, Fe4+ cations are reduced to Fe3+
up to 600°C, and the formation of iron metal occurs
above 600°C. The reactivity of the perovskites in CO
oxidation at 250–450°C depends linearly, with a corre-
lation coefficient of 0.8, on the amount of the most reac-
tive surface oxygen species, removable by hydrogen
TPR below 250°C. The highest concentration of this
oxygen is observed in the microheterogeneous speci-
mens with x = 0.3–0.4, which result from low-tempera-
ture polymorphic transitions. The absence of a linear
dependence between methane oxidation activity and
the amount of surface oxygen species in the tempera-
ture range 500−600°C may be due to the fact that the
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KINETICS AND CATALYSIS Vol. 46 No. 5 2005