Catalytic Activity of La1 – xCa xCoO3 – Δ Perovskites (х = 0?1) Prepared by the Pechini Method in the Reaction of Deep Methane Oxidation

Article abstract of DOI:10.1134/S0023158418040031

The catalytic activity of a series of the perovskite-like oxides La_1–xCa_xCoO_3–δ (x = 0–1) prepared by the Pechini method (from polymer–salt compositions) in the reaction of methane oxidation was studied. The dependence of the activity and stability of samples on their composition and reaction temperature was revealed. It was found that an increase in the calcium content of the oxides initially led to an increase in the activity (to the values of x = 0.3) and then to a nonmonotonic decrease with an intermediate maximum at x = 0.6. A decrease in the activity of oxides in the course of testing, which was most pronounced in the samples with x = 0.3, 0.6, and 1, was found. The phase compositions, specific surface areas, and microstructures of the oxides were determined before and after tests. According to X-ray diffraction analysis data, only the samples with x = 0–0.4 were single-phase ones; the samples with x > 0.4 were two-phase samples containing the phases of perovskite and brownmillerite. With the use of high-resolution transmission electron microscopy, it was found that the surface of particles was covered with the nanosized particles of simple calcium and cobalt oxides, and the particles of brownmillerite were present in the samples with x ≥ 0.4. The observed increase in the catalytic activity of the samples in a region to x = 0.3 correlated with an increase in the concentration of weakly bound oxygen in the perovskites, and a decrease at x > 0.3, with the appearance of the less active phase of brownmillerite in the samples and an increase in its concentration. The phase composition and the specific surface area of the samples remained unchanged after tests; however, planar defects were detected in the particles of perovskite and the calcium content on the surface increased. This was caused by the appearance and ordering of cationic and anionic vacancies under the action of a reaction medium, which can explain the observed changes in the activity of the samples.

Full text of DOI:10.1134/S0023158418040031

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 4, pp. 489–497. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © L.A. Isupova, N.A. Kulikovskaya, N.F. Saputina, E.Yu. Gerasimov, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 4, pp. 474–481.
Catalytic Activity of La1 – xCaxCoO3 – δ Perovskites (х = 0−1)
Prepared by the Pechini Method in the Reaction
of Deep Methane Oxidation
L. A. Isupovaa, *, N. A. Kulikovskayaa, N. F. Saputinaa, and E. Yu. Gerasimova
aBoreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
*e-mail: isupova@catalysis.ru
Received October 2, 2017
Abstract—The catalytic activity of a series of the perovskite-like oxides La1 – xCaxCoO3 – δ (x = 0–1) prepared
by the Pechini method (from polymer–salt compositions) in the reaction of methane oxidation was studied.
The dependence of the activity and stability of samples on their composition and reaction temperature was
revealed. It was found that an increase in the calcium content of the oxides initially led to an increase in the
activity (to the values of x = 0.3) and then to a nonmonotonic decrease with an intermediate maximum at
x = 0.6. A decrease in the activity of oxides in the course of testing, which was most pronounced in the sam-
ples with x = 0.3, 0.6, and 1, was found. The phase compositions, specific surface areas, and microstructures
of the oxides were determined before and after tests. According to X-ray diffraction analysis data, only the
samples with x = 0–0.4 were single-phase ones; the samples with x > 0.4 were two-phase samples containing
the phases of perovskite and brownmillerite. With the use of high-resolution transmission electron micros-
copy, it was found that the surface of particles was covered with the nanosized particles of simple calcium and
cobalt oxides, and the particles of brownmillerite were present in the samples with x ≥ 0.4. The observed
increase in the catalytic activity of the samples in a region to x = 0.3 correlated with an increase in the con-
centration of weakly bound oxygen in the perovskites, and a decrease at x > 0.3, with the appearance of the
less active phase of brownmillerite in the samples and an increase in its concentration. The phase composition
and the specific surface area of the samples remained unchanged after tests; however, planar defects were
detected in the particles of perovskite and the calcium content on the surface increased. This was caused by
the appearance and ordering of cationic and anionic vacancies under the action of a reaction medium, which
can explain the observed changes in the activity of the samples.
Keywords: La1 – xCaxCоO3 – δ, perovskites, methane oxidation, stability
DOI: 10.1134/S0023158418040031
INTRODUCTION
Thus, it was found that the region of formation of
homogeneous solid solutions upon substitution in the
La1 – xCaxСoO3 – δ system on ceramic synthesis was
extended as the heat treatment temperature was
increased. The heat treatment of initial reagents at a
temperature of 885°C made it possible to obtain sin-
gle-phase samples only in a range of x = 0–0.25 [8] or
to the values of x < 0.4 at 1100°C [9, 10]; this is consis-
tent with the statement of Mastin et al. [11] on the
impossibility of preparing solutions with x > 0.3 by a
ceramic method (1100°C). According to Kononyuk et al.
[12], an increase in the temperature of annealing to
1200°C increased the boundary of solubility to x = 0.5.
The synthesis of samples from solutions makes it
possible to obtain homogeneous solid solutions with a
higher degree of substitution at lower calcination tem-
peratures [11]. The following single-phase samples
were studied: compositions with x = 0–0.3 prepared
by a ceramic method (Tcalc = 1100°C) and composi-
The La1 – xMxCоO3 – δ oxides (M = Ca, Sr, and Ba)
with the structure of perovskite are promising materi-
als to be used in a number of high-temperature processes.
For example, they can be used in high-temperature elec-
trochemical devices, gas sensors, oxygen-permeable
membranes, etc. [1–3]. The La1 – xMxCоO3 – δ solid
solutions also acquired a good reputation as catalysts
for the deep oxidation of hydrocarbons, ammonia,
and CO and other processes [4–7].
It is well known that the physicochemical proper-
ties of substituted perovskites considerably depend on
both the degree of substitution and the preparation
conditions, which are responsible for the phase com-
position, microstructure, and structure imperfection
of oxides. This can be a reason for differences in pub-
lished data on not only the properties but also the
phase composition of the oxides [8–18].
489

490
ISUPOVA et al.
tions with x = 0.4 and 0.5 prepared by a polymer–salt vacancies upon the introduction of calcium into the
precursor method (Tcalc = 900°C). All of the substi- perovskites and a nonmonotonic change in activity in
the oxidation reaction of propane at temperatures of
140–420°C. At x = 0.2, the activity of perovskite
(La0.8Ca0.2CoO3) with respect to lanthanum cobaltite
tuted samples contained an impurity of CaO. The
samples with x = 0 and 0.5 at room temperature were
identified as a rhombohedral modification, whereas
the samples with x = 0.2–0.4, as an orthorhombic decreased or increased at a larger quantity of calcium.
modification. At a temperature higher than 200°C, the The found order of activity—La0.6Ca0.4CoO3
>
LaCoO3 >La0.8Ca0.2CoO3—did not correlate with an
increase in the concentration of vacancies in the
oxides. The authors of the above publications reported
the high stability of the samples in a reaction medium.
Unfortunately, other data on the catalytic oxidation of
hydrocarbons on the above oxides are absent from the
literature.
samples with x = 0.2–0.4 became rhombohedral and
the sample with x = 0.5 became cubic. It was shown
that the temperature of a polymorphic transition to the
cubic modification decreased with the calcium con-
tent of oxides from 1600°C for the sample with x = 0 to
20°C for the sample with x = 0.5. Mastin et al. [11] also
established that the concentration of oxygen vacancies
in the oxides increased with the quantity of calcium,
and the stability on heating in nitrogen decreased. The
latter led to a decrease in the calcium content of the
perovskite phase and the appearance of layered per-
ovskite phases of (La,Ca)2CoO4 and CoO.
Discordance between published data on the phase
composition of the La1 – xCaxCoO3 – δ perovskites pre-
pared by different methods can be caused by different
conditions of their synthesis (the degree of homogeni-
zation of the components at the stage of mixing and
the duration and atmosphere of the subsequent heat
treatment) and by a possible decrease in the solubility
Melo et al. [13] used a polymer–salt precursor
method to synthesize the La1 – xCaxCoO3 – δ per-
ovskites with x = 0–0.4. They found that the heat of calcium with the temperature of heat treatment
treatment of the samples at 700°C in air for 4 h facili- because of the ordering of oxygen vacancies by anal-
tated the formation of almost single-phase perovskites ogy with the La1 – xCаxFeO3 – δ system studied previ-
with a rhombohedral structure. They demonstrated
that, in contrast to the data reported by Mastin et al.
[11], an increase in the calcination temperature to 800
and 900°C led to the formation of cubic perovskite in
the sample with x = 0.4.
Pathak et al. [14] established that the samples with
x = 0.4 and 0.55 contained the impurities of calcium
and cobalt oxides in addition to the rhombohedral
phase of perovskite; this fact is indicative of the forma-
tion of limited solid solutions in the system to the val-
ues of x = 0.4.
ously [19–21]. Furthermore, with consideration for
data on the stability of the La1 – xCaxCoO3 – δ per-
ovskites on heating in nitrogen [11], it is reasonable to
study their stability in a reaction medium containing
reducing agents at higher catalytic process tempera-
tures than those used by Merino with coauthors [15,
16]. Thus, the degradation of perovskites with x >
0.3 (predominantly in the near-surface layers of
oxides) under the action of a methane-containing
reaction atmosphere was observed earlier even in
the La1 – xCаxMnO3 system [22–24].
The polymer–salt precursor method is widely used
for the synthesis of multicomponent solid solutions.
Because of the good homogenization of components
at the stage of precursor preparation, the complete
degree of interaction is reached at sufficiently low
temperatures and short calcination times in compari-
son with those in the ceramic method. The advantages
of the method over a precipitation method include the
absence of wastewater to be utilized.
The aim of this work was to prepare a wide range of
La1 – xCaxCoO3 – δ (х = 0–1) homogeneous solid solu-
tions by the polymer–salt precursor method, to deter-
mine their physicochemical and catalytic properties in
a reaction of methane oxidation, and to study their
stability in the reaction medium.
At the same time, Kumar et al. [17] stated that sin-
gle-phase samples with the values of x = 0–0.8 were
obtained from the polymer–salt precursors after their
heat treatment at 900°C for 12 h. The samples with
x = 0 and 0.2 were identified as a rhombohedral mod-
ification, while the samples with x ≥ 0.4, as a cubic
modification. Kumar et al. [17] noted a decrease in
particle sizes with the calcium content of the oxides
and a decrease in electron density on oxygen ions. The
single-phase samples with x = 0–0.8 were also synthe-
sized by a citrate method after heat treatment at 700–
750°C; it was shown that cobalt in the oxides had the
oxidation state close to +3, and vacancies were formed
in the oxides upon substitution [18].
Merino with coauthors [15, 16] studied the cata-
lytic properties of single-phase perovskites with x = 0–
0.5 prepared by the citrate method (heat treatment at
700°C for 2 h). The oxides were characterized as a
rhombohedral modification. The presence of a lan-
EXPERIMENTAL
The samples of La1 – xCaxCoO3 – δ were synthe-
thanum oxide impurity, which cannot be detected by sized by the polymer–salt composition method (the
X-ray diffraction analysis, on the surfaces of all sam- Pechini method). For this purpose, the aqueous solu-
ples was found. The authors of the cited publications tions of lanthanum, calcium, and cobalt nitrates were
revealed an increase in the concentration of oxygen mixed in a required proportion, and citric acid and
KINETICS AND CATALYSIS
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CATALYTIC ACTIVITY
491
ethylene glycol were added; the contents were evapo-
XCH , %
4
600°C
rated at 70–80°C until the formation of a resinous
polymer (polymer–salt composition). After the oxida-
tive destruction of the polymer, the resulting product
was calcined at 900°C for 4 h.
100
90
80
70
60
50
40
30
20
10
0
550°C
500°C
The X-ray diffraction patterns of the samples were
measured on an HZG 4-C diffractometer (Freiberger
Präzisionsmechanik, Germany) with the use of CuKα
radiation on scanning with a step of 2θ = 0.05° point
by point and an accumulation time of 3 s at each point
in a range of the angles 2θ = 10°–75°.
The high resolution (to 1.4 Å) electron microscopic
images were obtained on a JEM-2010 instrument
(JEOL, Japan). The X-ray microanalysis (EDX) of the
elemental composition of the samples was carried out
with the use of a Phoenix energy-dispersive spectrom-
eter (EDAX, the United States) with a Si(Li) detector
at an energy resolution of 130 eV.
*500°C
450°C
400°C
350°C
0.2
0.4
0.6
0.8
1.0
0
x, La1 – xCaxCoO3 – δ
The temperature-programmed reduction with
hydrogen (TPR-H2) was carried out in a flow system
with a thermal conductivity detector using a sample
fraction of 0.25–0.5 mm. Before the reduction, the
samples were trained in O2 for 0.5 h at 500°C and then
cooled in O2 to room temperature. The sample weight
was 10 mg, and the flow rate of a reducing mixture
(10% H2 in Ar) was 40 cm3/min. The samples were
heated to 900°C at a rate of 10 K/min. The peak areas
under the TPR curves of the samples, which corre-
sponded to the consumption of hydrogen (mole per
gram of the sample), was calculated using the Origin
6.0 software.
Fig. 1. Dependence of the conversion of methane on the
calcium content (x) of the La
Ca CоO
oxides at
1 – x
x
3 – δ
testing temperatures of 350–600°C. * The experiment was
carried out after sample testing at 600°C and the subse-
quent decrease of temperature to 500°C.
RESULTS AND DISCUSSION
Figures 1 and 2 illustrate data on the conversion
and the rate of oxidation of methane for the test sam-
ples. It is evident that the activity of all of the samples
increased with the test temperature, and the conver-
sion of methane reached 100% at 600°C for all of the
oxides other than those with x = 1.
The catalytic activity of the samples in a reaction of
methane oxidation was determined in a flow system at
temperatures of 350–600°C. A 1-g portion of a cata-
lyst fraction of 0.25–0.5 mm was mixed with 1 cm3 of
quartz and placed in a U-shaped quartz reactor with
an inside diameter of 4.5 mm. The feed rate of a reac-
tion mixture of 0.9% CH4 + 9% O2 (and the balance
N2) was 2.4 L/h. Before the measurements, the sample
In a temperature range of 350–550°C, an increase
in the calcium content of the samples was accompa-
nied by a nonmonotonic change in conversion with
maximums at x = 0.3 and 0.6, which were most pro-
nounced at T = 450–500°C. The dependence of the
reaction rate of methane oxidation on the composition
of oxides had an analogous form (Fig. 2). The repeated
determination of the activity of samples after a tem-
was kept in the reaction mixture for ~30 min at a spec- perature decrease from 600 to 500°C revealed a
ified temperature. After testing at 600°C, the sample noticeable decrease in the activity of samples with x =
was cooled in the reaction mixture to 500°C and its 0.3 and x = 0.6 (the samples with maximum initial
activity was determined once again. Only carbon diox- activities) and also a sample with x = 1.
ide and water were the oxidation products of methane.
The determination of the specific surface areas of
the samples before and after tests did not reveal
changes within the limits of measurement error (Table 1).
The specific surface area of all of the substituted sam-
ples before and after tests was 3 0.5 m2/g. The spe-
cific surface area of unsubstituted lanthanum cobaltite
(x = 0) was higher (5.1–5.8 m2/g).
The reaction rate w was calculated under the
assumption that the reaction occurred in a plug-flow
regime using the formula
w = 2.69 ×1019kC0, (CH4 molecule)m−2 s−1,
where C0 is the initial concentration of methane, %;
The experimental data that are indicative of the
influence of the calcium content of the samples on the
activity and a decrease in the activity of some catalysts
in the course of tests cannot be explained by the spe-
cific surface areas of the catalysts. The substituted
k is the reaction rate constant
k = ln(1 − XCH τSspm),
m–2 s–1
(
is the degree of methane co4nversion;
X
CH4
τ is the contact time, s; m is the sample weight, g; and
Ssp is the specific surface area of the sample, m2/g).
The specific surface area was measured by the BET samples, which were characterized by a higher degree
method based on the thermal desorption of argon.
of methane conversion than that on unsubstituted lan-
KINETICS AND CATALYSIS Vol. 59 No. 4 2018

492
W × 1016, (molecule CH4) m–2 s–1
ISUPOVA et al.
and 36-1388, respectively), and the sample with x =
0.4, to the cubic Pm3-m symmetry (JCPDS 36-1391).
The sample with x = 0.3 exhibited the peaks of per-
ovskite phases with the orthorhombic (R-3m) and
cubic (Pm-3m) symmetry. Starting with x = 0.4, a
Ca2Co2O5 phase impurity with the structure of brown-
millerite appeared in the samples, and its amount
increased with x; in this case, the composition of the
perovskite phase remained unchanged, and its param-
eter corresponded to La0.6Ca0.4CoO3 – δ. The phases of
Ca3Co2O6 and CaO were also detected among the
impurities. The sample of CaCoO3 – δ predominantly
consisted of the two phases: Ca3Co4O9 and Ca3Co2O6;
the presence of a CaO impurity phase also cannot be
excluded. Figure 4 shows changes in the volume of a
unit cell upon the introduction of calcium. A decrease
in the unit cell volume observed in fresh samples
(Fig. 4) indicates that calcium entered into the lattice
of perovskite with the formation of a limited number of
solid solutions in the system, in contrast to published
data [17, 18].
25
20
15
10
5
550°C
500°C
450°C
400°C
350°C
0
0.2
0.4
0.6
0.8
1.0
0
x, La1 – xCaxCoO3 – δ
Fig. 2. Dependence of the reaction rate of methane oxida-
tion on the calcium content (x) of the La Ca CоO
3 – δ
1 – x
x
oxides at testing temperatures of 350–550°C.
Thus, in accordance with the XRD analysis data,
almost single-phase solid solutions were formed on
thanum cobaltite (the samples with x = 0.2, 0.3, 0.6, the introduction of calcium only to x = 0.4; in this
and 0.8), possessed a smaller specific surface area, and case, a change in their structural modification was
their surface was not decreased after tests. This fact observed; therefore, the sample with x = 0.3 can be
indicates that a change in the chemical composition of attributed to the region of a morphotropic phase tran-
the surface of oxides in the course of tests can be the sition.
reason for the observed decrease in the activity of the
samples.
The phase composition of single-phase samples
with x = 0–0.4 remained unchanged after catalytic
tests in the reaction of methane oxidation (Fig. 5),
although it was possible to note an insignificant
increase in the unit cell volume (Fig. 4), which was
likely caused by the appearance of oxygen vacancies in
the samples and, possibly, by a decrease in the calcium
content.
According to the X-ray diffraction (XRD) analysis
data (Fig. 3), the samples of La1 – xCaxCоO3 – δ to x = 0.4
before the catalytic tests were almost single-phase
homogeneous solid solutions. The introduction of cal-
cium leads to a decrease in unit cell parameters (Table 1),
which is consistent with published data [17]. The sam-
ples with x = 0–0.2 belong to the rhombohedral sym-
Figure 6 shows the TPR-H2 curves. The reduction
metry R-3c (x = 0) and R-3m (x = 0.2) (JCPDS 48-123 of samples by hydrogen with an increase in the tem-
Table 1. Specific surface areas and unit cell parameters of the samples of La1 – xCaxCоO3 – δ before and after catalytic tests
in the reactions of methane oxidation
Unit cell parameters of the samples
after the oxidation of СН4
Unit cell parameters
of the initial samples
Ssp, m2/g
Catalyst composition
after the
oxidation of СН4
initial
a, Å
c, Å
a, Å
c, Å
LaСоO3 – δ
5.6
3.0
3.0
3.0
2.2
3.0
2.6
5.1
3.5
–
5.4402
5.4168
5.4154
3.787
–
13.0807
13.0836
13.0946
3.787
–
5.4471
5.4336
–
13.0996
La0.8Ca0.2СоO3 – δ
La0.7Ca0.3СоO3 – δ
La0.6Ca0.4СоO3 – δ
La0.4Ca0.6СоO3 – δ
La0.2Ca0.8СоO3 – δ
CaСоO3 – δ
13.1205
–
3.816
–
1.9
–
3.816
–
–
–
–
–
–
–
–
–
–
–
Dashes denote that the corresponding parameters were not determined.
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CATALYTIC ACTIVITY
493
La1 – xCaxCoO3
CaO
*
Ca2Co2O5
Ca3Co2O6
*
x = 1
x = 0.8
x = 0.6
x = 0.4
x = 0.2
x = 0
10
20
30
40
2θ, deg
50
60
70
80
Fig. 3. X-ray diffraction patterns of the samples of La
Ca CоO
before catalytic tests.
1 – x
x
3 – δ
perature occurred gradually: the first low-temperature improved reducibility of the samples with increasing x.
According to Futai and Yonghua [25], the reduction of
cations Co3+ → Co2+, Co4+ → Co2+, and Co4+ → Co0
occurred at temperatures to 500°C, and the reduction
Co2+ → Co0 occurred above 500°C. We calculated the
absorption of hydrogen in the regions corresponding
to different steps of the reduction of samples by anal-
ogy with previously published data [26].
peak appeared at ~400°C, and the second high-tem-
perature peak appeared at about 600°C. With increas-
ing x, the former peak was split and its intensity
increased, and the temperature of the onset of reduc-
tion decreased. These changes were also characteristic
of the high-temperature peak; this fact indicated the
The data given in Fig. 7 are indicative of an increase
in the absorption in regions of both of peaks and in the
V/N, Å3
56.2
56.0
110
55.8
55.6
55.4
after tests
55.2
55.0
54.8
54.6
024
214
012
202
220
54.4
54.2
x = 0.4
x = 0.2
x = 0
fresh
0.1
0.2
0.3
0.4
0
x, La1 – xCaxCoO3 – δ
20
30
40
2θ, deg
50
60
70
Fig. 4. Changes in the unit cell volume of the samples
before and after tests depending on the calcium content (x)
Fig. 5. X-ray diffraction patterns of the samples of La
-
1 – x
of the La
Ca CоO
oxides.
Ca CоO
after catalytic tests.
1 – x
x
3 – δ
x
3 – δ
KINETICS AND CATALYSIS Vol. 59 No. 4 2018

494
Absorption ×105, mol H2 g–1 s–1
ISUPOVA et al.
According to the transmission electron microscopy
data described in detail by Gerasimov et al. [27], the
particles of perovskite with x = 0–0.2 (LaCoO3 – δ and
La0.8Ca0.2CoO3 – δ) were well-crystallized, round-
shaped, and micron-sized. Starting with x = 0.3
(Fig. 8a), the particles of calcium oxide with sizes 5–
20 nm were detected on the surface of the perovskite
particles. It is likely that the small particle size of CaO
did not make it possible to detect this phase by XRD
analysis. The sample of La0.6Ca0.4CoO3 – δ consisted of
polycrystalline lamellar particles with a smaller size of
crystallites than that in the above samples; this can be
related to a changed symmetry group. The particles of
this composition were also characterized by coating
with a layer of CaO on the surface of a perovskite phase
(Fig. 8b).
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
x = 1
x = 0.8
x = 0.6
x = 0.4
x = 0.2
x = 0
200
400
600
800
1000
Temperature, °C
0
After tests in the reaction of methane oxidation, the
particles of perovskite in the samples with x = 0–0.2
(LaCoO3 – δ and La0.8Ca0.2CoO3 – δ) retained rounded
well-crystallized shapes; however, structurally differ-
ent locally ordered regions appeared in them: regions
with increased interplanar spacing typical of orthor-
hombic oxide (a high-temperature modification) in
the sample with x = 0; planar defects in the sample
with x = 0.2 (Fig. 9a); and superstructurally ordered
regions (structural fragments of brownmillerite Ca2-
Co2O5 and perovskite–brownmillerite phase bound-
aries) in the particles of perovskite with x = 0.4
(Fig. 9b). In this case, the surfaces of all calcium-con-
taining perovskite particles after tests were covered
with a CaO layer to 20 nm in thickness. The particles
of Co3O4 with sizes of 5–10 nm were also present [27].
Fig. 6. TPR-H curves of the samples of La
Ca СоO
.
2
1 – x
x
3 – δ
Absorption ×103, mol H2 g–1
3
2
10
8
6
1
4
2
The electron microscopic data indicated the insta-
bility of the La1 – xCaxCоO3 – δ solid solutions obtained
in a methane-containing reaction atmosphere at tem-
peratures to 600°C and the formation of planar defects
in the perovskite structure, apparently, as a result of
the ordering of both cationic and anionic vacancies,
which were formed because of the partial degradation
of oxides and the outcrops of calcium and cobalt
oxides on the surface of the particles. Note that, obvi-
ously, a rearrangement occurred in the relatively thin
near-surface layers of the perovskite phase because the
X-ray diffraction patterns did not exhibit considerable
changes to be indicative of the presence of new phases or
planar defects in the bulk of perovskite phase particles.
The formation of planar defects in the structures after
catalytic tests was observed earlier in the near-surface lay-
ers of the perovskite particles of La1 – xCaxMnO3 – δ [10]
and La1 – xCaxFeO3 – δ [20, 24] prepared by the
Pechini method.
0
0.2
0.4
0.6
0.8
1.0
x, La1 – xCaxCoO3 – δ
Fig. 7. Dependence of the amount of absorbed hydrogen
on the composition of samples for the TPR-H regions of
2
(1) 20–500, (2) 500–900, and (3) 20–900°C.
total absorption of hydrogen by the samples and hence
of an increase in the oxygen content of them. In accor-
dance with the electroneutrality principle, this should
lead to an increase in the charge on a portion of cobalt
cations; that is, to an increase in the quantity of Co+4
cations with an increase in the calcium content (x) or
to a decrease in the charge on oxygen ions.
The results obtained indicate that the samples were
almost completely oxidized at room temperature (the
vacancies were occupied with oxygen), and the
removal of a portion of weakly bound oxygen and an
increase of the quantity of vacancies in the oxides can
occur in the reaction atmosphere with increasing the
test temperature.
The above results make it possible to relate the pro-
nounced decrease in the activity of a sample with x =
0.3 observed in the course of tests to the appearance of
planar defects in the structure of perovskite and to the
formation of a calcium oxide layer, which is not active
in a deep oxidation reaction, on the surface of the par-
ticles. Because planar defects were also formed in the
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CATALYTIC ACTIVITY
(a)
495
(b)
La0.7Ca0.3O3
La0.6Ca0.4O3
CaO
CaO
[111]
10 nm
10 nm
Fig. 8. Micrographs of the samples of (a) La Ca CоO
and (b) La Ca CоO
before testing in the reaction of methane
0.7 0.3
3 – δ
0.6 0.4
3 – δ
oxidation.
(b)
(a)
2 nm
0.38 nm
0.22 nm
0.45 nm
0.11 nm
0.27 nm
[101]hex
10 nm
10 nm
Fig. 9. Micrographs of the samples after testing in the reaction of methane oxidation: (a) the microstructure of La Ca CоO
0.8 0.2
3 – δ
(black arrows show planar defects in the direction of the crystal faces (101)); (b) the microstructure of La Ca CоO
(inserts
3 – δ
0.6 0.4
show the filtered image and the Fourier transform of the chosen region to illustrate the presence of perovskite–brownmillerite
phase boundaries.
particles of a sample with x = 0.4, but a layer of cal- quantity of weakly bound oxygen (Fig. 7), and the
lowered activity of the samples with x ≥ 0.4 was, prob-
ably, related to the presence of a less active vacancy-
ordered phase of Ca2Co2O5 on the surface.
cium oxide was present on its surface before the tests in
contrast to the sample with x = 0.3, it is likely that an
increase in the quantity of calcium oxide did not qual-
itatively change the surface composition and, corre-
spondingly, did not exert a noticeable effect on its
activity. Thus, the decrease in the activity of the sam-
ple with x = 0.3 observed in the course of reaction was
mainly caused by the formation of calcium oxide on
the surface layer, whereas the lower and stable activity
of the sample with x = 0.4 was explained by the pres-
ence of a calcium oxide layer and a brownmillerite
impurity phase in the initial oxide.
Thus, the higher stability of samples with low (x =
0–0.2) or high (x = 0.4–0.8) degrees of substitution in
a reaction atmosphere becomes clear because either
few vacancies were formed in them or they already
contained less active phases of Ca2Co2O5 and CaO.
With increasing the degree of substitution accompa-
nied by an increase in the concentration of oxygen
vacancies in the oxides after increasing the catalytic
process temperature, their ordering occurred and
manifested itself in the formation of vacancy-ordered
structures (planar defects) in the structure of per-
The observed increase in the initial catalytic activ-
ity of the samples upon the introduction of calcium to
a value of x = 0.3 can be caused by an increase in the ovskite and the release of calcium oxide onto the sur-
KINETICS AND CATALYSIS Vol. 59 No. 4 2018

496
ISUPOVA et al.
face. Indeed, the degree of deactivation of the sample decrease in the activity of the samples in the course of
with x = 0.3 was higher than that of the samples with
x < 0.3 (Fig. 2). Taking into account this tendency, we
can also assume that the lower activity of the samples
with x = 0.4–0.8, which initially contained the impu-
rity phases of Ca2Co2O5 and CaO, was due to the fact
that new phases did not appear under the action of a
reaction atmosphere. It is possible that a certain
decrease in the activity of these samples in the course
of tests was related to changes in both the phase of per-
ovskite and the impurity phase of Ca2Co2O5 because
the activity of the sample with x = 1 also decreased
under the action of the reaction atmosphere.
tests in the reaction of methane oxidation.
ACKNOWLEDGMENTS
We are grateful to V.A. Rogov for the acquisition of
the TPR-H2 data.
This work was performed within the framework of
a state contract at the Boreskov Institute of Catalysis,
Siberian Branch, Russian Academy of Sciences (pro-
ject no. AAAA-A17-1170 41710090-3).
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KINETICS AND CATALYSIS Vol. 59 No. 4 2018