Synthesis, morphology, and activity of La1–xAgxMnO3 ± y catalysts

Article abstract of DOI:10.1134/S0023158416050165

La_1–xAg_xMnO_{3 ± y} (x = 0-0.3) mixed oxides have been synthesized by the pyrolysis of polymer–salt compositions using different organic compounds and different salt: organic compound ratios. The correlation between the reaction medium temperature during pyrolysis, the composition of the resulting oxide, and synthesis conditions has been investigated. The effect of these conditions on the character of the pyrolysis process, on the phase composition and microstructure of the resulting oxide particles and metallic silver, and on their mutual distribution is reported. The catalytic properties of the synthesized oxides in methane and soot oxidation are considered, and a correlation is established between the catalytic activity of the oxides and the synthesis conditions.

Full text of DOI:10.1134/S0023158416050165

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 712–721. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © O.V. Russkikh, D.V. Ivanov, L.A. Isupova, D.S. Chezganov, A.A. Ostroushko, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 5, pp. 719–728.
Synthesis, Morphology, and Activity of La Ag MnO Catalysts
1
– x
x
3 ± y
a
b
b
a
a,
O. V. Russkikh , D. V. Ivanov , L. A. Isupova , D. S. Chezganov , and A. A. Ostroushko *
a
Ural Federal University, Yekaterinburg, 620000 Russia
b
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
*
e-mail: alexander.ostroushko@urfu.ru
Received December 14, 2015
Abstract—La1 Ag MnO (х = 0−0.3) mixed oxides have been synthesized by the pyrolysis of polymer–
3 ± y
– x
x
salt compositions using different organic compounds and different salt : organic compound ratios. The cor-
relation between the reaction medium temperature during pyrolysis, the composition of the resulting oxide,
and synthesis conditions has been investigated. The effect of these conditions on the character of the pyrolysis
process, on the phase composition and microstructure of the resulting oxide particles and metallic silver, and
on their mutual distribution is reported. The catalytic properties of the synthesized oxides in methane and
soot oxidation are considered, and a correlation is established between the catalytic activity of the oxides and
the synthesis conditions.
Keywords: lanthanum manganite, silver, catalytic oxidation of methane and soot, catalytic activity
DOI: 10.1134/S0023158416050165
INTRODUCTION
Mixed oxide materials with a perovskite (ABO )
thanum manganite, as compared to the introduction
of other metals, leads to a greater increase in the cata-
lytic activity of the material in the oxidation of soot,
organic compounds, and carbon monoxide, including
for catalysts supported on nickel foam. It was found [3,
3
structure are finding increasing application owing to
the fact that their properties can be tuned by doping
their A and B sublattices. In some cases, in perovskite
and related structures there can be heterovalent substi-
tution in the oxygen sublattice as well as a result of e.g.,
the introduction of fluoride ions therein [1, 2]. Per-
ovskite-type materials (including lanthanum manga-
nite–based ones [3, 4]) show a high catalytic activity
in redox reactions involving oxygen or oxygen-con-
taining compounds, have a high oxygen ion conduc-
tivity [5], and are capable of transporting protons [6,
14, 15] that part of the silver in the catalyst is in the free
metal state, additionally increasing the catalytic activ-
ity of the system. For this reason, we chose silver-
doped lanthanum manganites with the general for-
mula La Ag MnO to be an object of our further
3 ± y
1
– x
x
studies. These materials seem most promising from
the standpoint of catalytic activity for developing ther-
mocatalytic devices for exhaust gas treatment.
7
]. Manganites are employed as components of elec-
Along with optimizing the perovskite composition,
it is essential to find methods and conditions for the
synthesis of mixed oxide compositions possessing the
best catalytic properties. Synthesis conditions have an
effect on the phase composition, state of the surface,
and particle size and morphology of mixed-oxide sys-
tems, on the mutual distribution of particles of the
main and impurity phases, and on the possibility of
synergistic effects arising from contact between differ-
ent phases in heterogeneous mixed-oxide catalysts.
trodes and solid-oxide membranes in fuel cells, elec-
trochemical sources of oxygen, sensor elements, and
catalysts protecting the atmosphere from toxic emis-
sions [8]. In particular, these catalysts effectively
accelerate the oxidation of carbon monoxide, hydro-
carbons, and various organics and simultaneously
reduce nitrogen oxides [9, 10]. In recent years, there
has been extensive search for catalysts capable of oxi-
dizing soot that is emitted by some industrial processes
and diesel engines [11]. Promising catalysts in this area
are perovskite-type mixed oxides. Depending on their
The base synthetic methods used in this study were
purpose and operating conditions (temperature, gas the pyrolysis of polymer–salt compositions (PSCs)
medium composition, etc.), these systems are know- [16] and its variant involving glycerol as the organic
ingly doped with certain metal ions. The catalytic component added to metal nitrates. Within these
characteristics of the doped systems depend signifi- methods, it is possible to widely vary the synthesis
cantly on the nature and quantity of these ions [12, 13]. conditions (e.g., the proportions of components in the
It was discovered earlier [3, 14, 15] that the introduc- reaction mixture) to tune the properties of the result-
tion of silver ions into the lanthanum sublattice of lan- ing material. One of the main purposes of this study
712

SYNTHESIS, MORPHOLOGY, AND ACTIVITY
713
was to establish a correlation between the synthesis were calculated from low-temperature nitrogen vapor
conditions and the properties of the mixed oxides.
sorption isotherms. The morphology of particles was
studied using an AURIGA CrossBeam scanning elec-
tron microscope (Carl Zeiss NTS, Germany). In
equipment operation and control and data processing,
we used the Analysis Station software package
EXPERIMENTAL
The La1 Ag MnO mixed oxides (х = 0–0.3,
3 ± y
– x
x
(AURIGA series, Version 3.7).
х = 0.1 steps) were synthesized by PSC pyrolysis or by
the glycerol–nitrate method. The starting compo-
The catalytic activity of the mixed oxides was esti-
nents were lanthanum(III) hexahydrate (La(NO ) · mated in methane and soot oxidation reactions.
3
3
6
H O, analytical grade), silver nitrate (AgNO , ana- Methane oxidation was performed in a flow reactor
2 3
lytical grade), manganese(II) nitrate tetrahydrate operating in a near-plug-flow regime. A gas mixture
(
(
(
Mn(NO ) · 4H O, analytical grade), glycerol consisting of 1% CH , 9% O , and helium was fed into
3 2 2
4
2
C H (OH) , analytical grade), and polyvinyl alcohol the reactor. The concentration of the initial reactants
3
5
3
(
methane and oxygen) and reaction products (carbon
PVА, USSR State Standard GOST 10779–78,
oxides, oxygen, and unrelated methane) were deter-
mined on a Tsvet 500 gas chromatograph (Tsvet, Rus-
sia). Catalytic activity was measured on a 0.5–
brand 11/2). The synthesis of mixed oxides by PSC
pyrolysis is based on the introduction of a water-solu-
ble nonionic polymer into a solution of thermally
decomposable metal salts followed by removing the
solvent. The pyrolytic synthesis of the products is due
to the redox interaction of nitrate ions with the poly-
mer component. This reaction is exothermic, leading
to a self-heating of the mixture and, in some cases, to
0
.25 mm particle size fraction while elevating the reac-
tor temperature from 300 to 650°С in steps. The frac-
tion was prepared by pressing the catalyst powder fol-
lowed by sieving it through sieves with opening diam-
eters of 0.25 and 0.5 mm. catalytic tests were carried
3
a spontaneous process similar to self-propagating out on 1 cm samples, whose weight was 1.3–1.5 g.
high-temperature synthesis. The resulting mixed
Soot was oxidized in air in an open reactor while
oxide exerts an additional catalytic effect on redox
raising the reactor temperature from 200 to 600°С in
reactions involved in the synthesis [10, 16, 17]. In
5
0°С steps. Weighed, thermally pretreated alumina
order to investigate the effect of the synthesis method
and conditions on the formation of catalytic composi-
tions and their properties, we varied the proportions of
the salt and polymer components of PSC. In addition,
as was mentioned above, glycerol was used as the
organic component of the reaction mixture for the
sake of comparison.
boats containing a soot–catalyst mixture were placed
in a preheated furnace and were heat-treated for
3
0 min. Thereafter, the boats were weighed again. The
reaction zone temperature was controlled with an
Oven TRM10 temperature regulator (Oven, Russia)
and a KhA68 thermocouple. The model soot Printex
(Degussa) was examined, which is commonly used to
The PSCs were prepared and pyrolyzed in the fol- compare results obtained in different laboratories with
lowing way. Weighed samples of nitrates of appropri- different catalysts. A soot sample was combined with a
ate metals were dissolved in distilled water in a water fourfold weight excess of a powder catalyst, and the
bath. A solution containing 5% PVA was separately mixture was thoroughly ground in order to obtain tight
prepared. Combining these solutions or adding glycerol contact between particles.
to the nitrate solution yielded a PSC, which was then
poured into a porcelain dish and heated on a hot plate.
Initially, water removal took place, which was followed
RESULTS AND DISCUSSION
The La Ag MnO (х = 0–0.3) samples were
(
in the case of PVA used as the organic component) by
1
– x
x
3 ± y
the formation of a film. After the film dried completely,
it underwent pyrolysis. The resulting powder was
finally heat-treated at 650°С for 48 h. The pyrolysis
temperature was measured with a Testo 835-T2
black powders. Table 1 presents the pyrolysis tempera-
ture data obtained using the infrared pyrometer. Vari-
ation of the salt : polymer affects the course of pyroly-
sis. For the samples prepared from PSC at a stoichio-
metric polymer-to-nitrate ratio (reaction designed for
the formation of water, carbon dioxide, and molecular
nitrogen as gaseous products [10, 16]), pyrolysis has a
specific stepwise character. The process does not take
place simultaneously throughout the reaction mix-
ture, but it occurs sequentially in different reaction
zones of the polymer–salt film. The following pyroly-
sis stages are observed, irrespective of the composition
of the mixed oxide being synthesized. As water is pro-
gressively removed from the PSC, a film forms, whose
(
Testo, Germany) infrared high-temperature ther-
mometer (pyrometer).
Identification of the synthesized mixed oxides and
determination of their phase composition were carried
out by X-ray powder diffraction (XRD) on a DRON-
6
2
automated diffractometer (Russia, CuK radiation,
α
θ = 20°–90°). Phases were identified using ICDD
files [18]. Crystallographic parameters were calculated
using the Fullprof software. Specific surface areas
(
S
) were measured on a TriStar 3020 (Micromerit-
BET
ics, United States) automated sorption analyzer using temperature is initially no higher than 100°С. Next,
the volumetric variant of the BET method. S values pyrolysis commences at the edges of the film at a tem-
BET
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

7
14
RUSSKIKH et al.
Salt : polymer
Table 1. Characteristics of mixed oxide synthesis
Temperature, °C
Sample
no.
Organic
component
Formula
Т1
Т2
Тmax
Тfin
1
2
3
4
5
6
7
LaMnO3 ± y
La Ag MnO
3 ± y
PVA
PVA
PVA
PVA
Glycerol
PVA
PVA
1 : 1
1 :1
108
102
110
300
−*
100
93
96
190
−*
−*
183
314
395
446
516
482
406
520
145
243
310
330
190
210
284
0.9
0.1
La Ag MnO
1 : 1
1 : 1
1 : 1
1 : 2
1 : 4
0.8
0.2
3 ± y
3 ± y
3 ± y
3 ± y
3 ± y
La Ag MnO
0.7
0.3
La Ag MnO
0.8
0.2
La Ag MnO
146
156
0.8
0.2
La Ag MnO
0.8
0.2
*
No film formation takes place in this system.
perature T , while the temperature of the film itself on the amount of silver introduced into the lanthanum
1
decreases to T . Thereafter, the film gradually burns sublattice. The incorporation of silver ions in the per-
2
ovskite structure (Table 3, samples 2, 3) increases the
unit cell parameters and volume, which can be
explained by the difference between the ionic radii of
away, with its temperature exceeding T . As the film
1
burns, its temperature grows progressively, and the
film residue burns at the highest temperature (T ).
max
3+
+
La and Ag (1.03 and 1.15 Å, respectively). However,
as the silver content is further increased (Table 3, sam-
ples 3, 4), the unit cell parameters decrease, which can
be due to the fact that the introduction of the differ-
ently charged dopant generates an additional amount
The pyrolysis process yields a mixed oxide, whose
final temperature (T ) at the combustion cessation
fin
point is below the maximum pyrolysis temperature.
Note that increasing the amount of silver introduced
into the manganite structure leads to a monotonic
4+
of Mn , which has a smaller ionic radius. The nonlin-
ear dependence of the unit cell parameters in the
amount of silver introduced correlates well with our
earlier results [22].
increase in Т
.
max
At smaller salt : polymer ratios, the process does
not proceed stepwise. After the onset of pyrolysis at
the film edges, a dramatic heating takes place, which is
not observed in the pyrolysis of the stoichiometric
composition, and then the entire surface burns out
rapidly. With an increasing polymer content, the max-
imum pyrolysis temperature and the onset tempera-
ture of pyrolysis increase.
The effect of synthesis conditions on the morphol-
ogy of La Ag MnO samples (Figs. 2, 3) was
3 ± y
0.8
0.2
studied by scanning electron microscopy. Perovskite
particles form large, stable, porous aggregates.
Increasing the proportion of polymer in the initial
PSC leads to an increase in the particle size of both
phases—perovskite and metallic silver located on the
surface of the aggregated particles (Fig. 3). This may
be a consequence of the higher temperature reached in
the pyrolysis process.
With glycerol in place of the polymer component
(
La Ag MnO , sample 2), pyrolysis begins imme-
0.8 0.2 3± y
diately after the removal of the liquid (no film forma-
tion is observed).
According to XRD data, all La1 Ag MnO sam-
3 ± y
– x
x
In La Ag MnO synthesized from the PSC
3 ± y
0.8
0.2
ples contain, aside from the perovskite phase, the
metallic silver phase. The introduction of silver changes
the shape of diffraction peaks from the perovskite
with a stoichiometric salt : polymer ratio (Table 1,
sample 3), the size of perovskite particles is 40 to 80
nm (Fig. 2a), which is in agreement with the particle
size derived from the width of diffraction peaks from
the perovskite phase (Table 2). These nanoparticles
form high-porosity, branched, submicron- and
micron-sized agglomerates. Figures 2b and 2c show
formations typical of this mixed oxide synthesized by
pyrolysis. The synthesis yields a porous, gas-perme-
able catalytic composition based on aggregated of
mixed oxide nanoparticles that are in rather intimate
contact (Figs. 2a, 2b). As a rule, the inter-aggregate
phase, shifts them relative to the peaks of LaMnO
and changes the space group of the phase (Fig. 1).
,
± y
3
Table 2 lists the silver-to-perovskite ratios in the
samples. These data suggest that part of the silver in
the La
Ag MnO
samples is incorporated in the
1
– x
x
3 ± y
perovskite structure. This finding is in agreement with
our earlier data [19] and with data reported by
other researchers [20, 21]. Kucharczyk and Tylus
[
20] believe that La Ag MnO
is a single-
0
.9
0.1
3± y
phase material. This is possibly explained by the spe- space is filled by extensive particle ensembles with
cific features of their synthetic procedure. Table 3 strong point contacts creating the specific cellular
presents the calculated crystallographic parameters of structure of the mixed oxide (Fig. 2c). In some cases,
La
Ag MnO
3 ± y
solid solutions (х = 0.1–0.3). The a denser matrix can form cells with a cavity accommo-
1
– x
x
unit cell parameters of this phase depend nonlinearly dating the above-mentioned network of nanoparticles.
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SYNTHESIS, MORPHOLOGY, AND ACTIVITY
It was possible to visualize metallic silver particles
715
(
а)
only using the backscattered electron technique. This
technique provides information about the average
atomic number of the elements constituting the mate-
rial examined, thus making it possible to distinguish
phases differing in their composition. Figures 2b and
Intensity, arb. units
I
9
000
000
30000
Ag
7
2
5000
0000
5000
000
000
3
2
d show images of an area of the sample surface that
were obtained at a fixed magnification but in different
secondary and backscattered electron imaging)
2
1
6
6 67 68 69 70 71
(
15000
10000
2θ
modes. The metallic silver particles (Figs. 2d, 3a) are
lighter colored and are more uniformly distributed in
the surface of the agglomerates. Their size is approxi-
mately 50 nm and is comparable with the size of the
primary perovskite particles in the agglomerates.
4
3
2
1
5000
0
1
0
20 30 40 50 60 70 80
In the La Ag MnO sample prepared using a
3 ± y
0.8
0.2
2θ
twofold excess of PVA in the initial PSC (Table 1,
sample 6), the size of metallic silver particles is 300–
(b)
Intensity, arb. units
5
00 nm. Electron micrographs allow the shape of sep-
I
arate silver crystals to be distibuished, which have a
cubic structure (Fig. 3b). in this case, the silver parti-
cles are larger than the primary perovskite particles,
whose size is at most 100 nm.
9
000
3
2
2
0000
Ag
7000
5000
3
5000
0000
000
1000
65 67 69 71
2θ
In the La Ag MnO obtained using a fourfold
3 ± y
0.8
0.2
excess of PVA (Table 1, sample 7), the size of metallic
silver particles is up to 1 μm (Fig. 3c) and is compara-
ble with the size of the perovskite particle aggregates.
The primary particles of perovskite in this sample are
also larger (100−200 nm). However, despite this parti-
cle coarsening, the micron-sized aggregates retain
their high porosity.
1
5000
0000
1
4
3
2
5000
1
0
1
0
20 30 40 50 60 70 80
Figure 3d presents an image of La Ag MnO
3 ± y
0.8
0.2
2θ
synthesized using glycerol in place of PVA as the
organic component (Table 1, sample 5). This sample
also contains micron-sized porous aggregates of pri-
mary, 30- 40-nm perovskite particles. The size of
metallic silver particles is 30−50 nm, and they can be
distinguished only using the backscattered electron
technique.
Fig. 1. (a) XRD patterns of (1) LaMnO3± y
,
(
2) La Ag0.1MnO , (3) La Ag MnO
, and
0
.9
3± y
3± y
0.8 0.2
3
±
y
(
4) La Ag MnO . (b) XRD patterns of (1) LaMnO
0.7 0.3
3± y
and (2–4) La Ag MnO
samples prepared from
0.8 0.2
3 ±
y
PSCs with different salt : polymer ratios: (2) sample 3,
(3) sample 6, and (4) sample 7.
These data indicate that the size of the primary per-
ovskite particles and silver particles depends on the
polymer content of the PSC. It would be expected that
the highest catalytic activity will be shown by
Figures 4−6 plot the dependences of the methane
and soot conversions (α) on the reaction temperature
and on the amount of silver (х) in La Ag MnO .
1
– x
x
3 ± y
La Ag MnO
samples synthesized at a stoichio-
0.8
0.2
3 ± y
Table 4 presents data characterizing the specific cata-
lytic activity of the mixed oxides in methane oxidation
and data concerning soot conversion and soot ignition
and complete oxidation temperatures. For all of the
samples, 100% methane conversion is observed at
metric salt : organic component ratio, since more
finely dispersed heterogeneous nanosystems form in
this case. As was mentioned above, in all samples the
metallic silver phase is uniformly distributed with
respect to the perovskite phase.
5
00°C (Fig. 4a); 100% soot conversion, at 700°C
Variation of the salt : polymer ratio and the conse- (Fig. 5a). The introduction of silver into lanthanum
quent variation of the temperature reached in the manganite generally enhances its catalytic activity at
pyrolysis process have no significant effect on the spe- low temperatures (350–400°С in methane oxidation
cific surface area of the product. The resulting samples and 350–500°С in soot oxidation); however, in both
2
differ in S
by at most 1 m /g (Table 4). This can be reactions the increase in catalytic activity is not propor-
BET
explained by the fact that, in spite of the coarsening of tional to the amount of silver introduced (Figs. 4b, 5b).
the perovskite and metallic silver particles, the porous Among samples 1–4, which were prepared by the
structure of the agglomerates remains invariable.
same method, sample 2 displays the highest catalytic
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

7
16
RUSSKIKH et al.
Table 2. Phase composition of La1 – xAg MnO
x
3 ± y
Proportions of phases,
mol %
Total Ag
content
in mole
Formula
of perovskite
from experimental
data***
Sample
no.
Identified
phases
Formula
LaMnO3
Ag
fractions**
1
2
3
4
LaMnO3 ± y
La Ag MnO
LaMnO3 ± y
100
93
89
0
7
11
16
0
LaMnO3 ± y
LaMnO3 ± y*, Ag
LaMnO3 ± y*, Ag
LaMnO3 ± y*, Ag
0.099
0.17
0.26
La Ag0.029Mn3 ± y
0.9
0.1
3 ± y
3 ± y
3 ± y
0.9
La Ag MnO
La Ag0.07Mn3 ± y
0.8
0.2
0.8
La Ag MnO
84
La Ag Mn
0.7
0.3
0.7
0.1
3 ± y
*
Solid solution based on LaMnO3 ± y
.
*
* Determined by atomic absorption spectroscopy and converted to the perovskite formula.
*
** Calculated from XRD atomic absorption spectroscopy data.
Table 3. Unite cell parameters of La1 – хAg MnO
х
3 ± y
Unite cell parameters
Sample
no.
Identified
phases
Space
group
Formula
V, Å3
a**, Å
b, Å
c, Å
1
2
LaMnO3 ± y
La Ag MnO
LaMnO3 ± y
5.497
5.502
5.497
13.321
348.6
R3c
LaMnO3 ± y*, Ag Pbnm
5.449
–
7.745
–
232.2
68.3
0.9
0.1
3 ± y
3 ± y
3 ± y
4
.088
5.450
.085
5.494
.089
Fm3m
3
4
La Ag MnO
LaMnO3 ± y*, Ag Pbnm
5.452
–
7.728
–
231.7
68.2
0.8
0.2
4
Fm3m
La Ag MnO
LaMnO3 ± y*, Ag Pbnm
5.450
–
7.742
–
231.8
68.4
0.7
0.3
4
Fm3m
*
Solid solution based on LaMnO3 ± y
.
*
* The measurement error is no larger than 0.005.
Table 4. Catalytic properties of La1 – х Ag MnO
x
3 ± y
Soot oxidation
Methane oxidation rate,
μmol m–
2
s
–1
Sample
no.
2
Formula
SBET, m /g
conversion, %
Т , °C
Т100%, °C
ign
300°C
400°C
300°C
500°C
1
2
3
4
6
LaMnO3 ± y
La Ag MnO
8.5
7.8
0.0011
0.0017
0.0010
0.0011
0.003*
0.0008
0.0100
0.0153
0.0149
0.0112
0.0104
0.0029
6.0
72
393
365
324
325
311
265
750
500
600
550
450
400
1.8
1.7
100
95
0.9
0.1
3 ± y
3 ± y
3 ± y
3 ± y
3 ± y
La Ag MnO
13.2
14.9
12.7
12.2
0.8
0.2
La Ag MnO
6.3
97.7
0.7
0.3
La Ag MnO
5.6
100
100
0.8
0.2
7
La Ag MnO
15.5
0.8
0.2
*
350°C.
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SYNTHESIS, MORPHOLOGY, AND ACTIVITY
а)
717
(
(b)
100 nm
100 nm
(c)
(d)
200 nm
100 nm
Fig. 2. Electron micrographs of La0 Ag MnO3 ± y (sample 3): (a) ×76000, (b) ×31000, and (c) × 33000; (d) backscattered
.8
0.2
electron image of the same sample (×33000).
activity in methane oxidation (Table 4). With the same methane oxidation relative to that of the sample pre-
sample, 100% soot conversion is attained at the lowest pared at a stoichiometric ratio of the components
temperature (500°C), although the ignition tempera- (Fig. 6a, Table 4). At the same time, catalytic activity
ture for samples 3 and 4 is lower.
in soot oxidation depends on the polymer content in
the opposite way (Fig. 6b, Table 4): the maximum
activity is exhibited by sample 7 (Table 4), which was
prepared using an increased proportion of organic
matter and contains the largest amount of silver with
the largest particle size. This sample is characterized
by the lowest ignition temperature (265.5°C) and
The catalytic activity of the oxides depends on the
conditions under which they were prepared.
La Ag MnO samples prepared using different
3 ± y
0.8
0.2
organic components (PVA and glycerol) or different
proportions of a given organic component (Table 4,
samples 3, 5–7) show different activities in the meth-
ane and soot oxidation reactions (Figs. 6a, 6b,
Table 4). Samples 3 and 5 (Table 4), synthesized using
1
00% conversion temperature (400°C).
The data obtained for methane oxidation can be
a stoichiometric mixture but different organic compo- explained in terms of the compositions and morphol-
nents and characterized by similar perovskite and sil- ogy of the oxides. The highest activity is shown by the
ver particle sizes, differ insignificantly in their catalytic samples that are characterized by a developed texture,
properties. Increasing the proportion of the polymer the largest amount of silver in the perovskite phase, the
component in the initial PSC (Table 1, samples 3, 6, smallest size of silver and perovskite particles, and the
7
), which leads to an increase in the silver particle size, presence of the metallic silver phase (samples 2 and 5).
diminishes the catalytic activity of the material in Apparently, these factors maximize the amount of
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

7
18
RUSSKIKH et al.
(а)
(b)
100 nm
300 nm
(c)
(d)
1
μm
200 nm
Fig. 3. Backscattered electron images of La0.8Ag0.2MnO3 ± y: (a) sample 3, ×33000; (b) sample 6, ×17000; (c) sample 7, ×9000;
d) sample 5, ×20000.
(
weakly bound oxygen on the surface (owing the oxide composition can be adapted to a particular cat-
microheterogeneity of the sample and the defective- alytic reaction, namely, the oxidation of a gas or solid.
ness of the perovskite phase), the species that deter- For the efficient simultaneous catalytic oxidation of
mines the oxidizing activity of the catalysts [23, 24]. At solids and gases, combined catalysts containing silver
the same time, the highest activity in the oxidation of particles of different sized can be used in practice.
soot, for which the inner surface of perovskite is inac-
cessible, is shown by the sample with the highest silver
phase content and the largest silver particle size. It is
not impossible that, in this case, catalytic activity is
mainly due to the presence of silver particles. Appar-
ently, the size effect associated with the formation of
an oxide film on the surface of large silver particles
CONCLUSIONS
Perovskite-type mixed oxide with the general for-
mula La Ag MnO (х = 0–0.3) were synthesized
3 ± y
1
– x
x
by the pyrolysis of nitrate-containing compositions
under various conditions (using glycerol or PVA as the
organic component and different salt : polymer ratios
in the initial composition). The dependence of the
composition of the resulting mixed oxide on the above
conditions and pyrolysis temperature was investigated.
(
but not on the surface of small particles, which are
more resistant to oxidation) is manifested here. This
issue needs to be investigated in greater detail using
specialized methods [25].
Thus, in the optimization of catalyst composition It was found that increasing the amount of silver intro-
with due regard for economic parameters) and syn- duced into the PSC increases the maximum tempera-
(
thesis conditions, it is necessary take into consider- ture reached in pyrolysis. Increasing the polymer con-
ation the temperature regime under which the catalyst tent of the initial PSC over the stoichiometric value
is to be operated. This study also demonstrates that, by changes the way pyrolysis proceeds and, in addition,
varying the synthesis conditions, the resulting mixed- raises the temperature maximum in the reaction
KINETICS AND CATALYSIS
Vol. 57
No. 5
2016

SYNTHESIS, MORPHOLOGY, AND ACTIVITY
а) (b)
719
(
Conversion, %
Conversion, %
100
1
00
1
2
3
4
8
0
0
0
0
0
9
0
0
6
4
2
8
10
300
5
4
50
3
00
400
500
0
0.1
0.2
0.3
x
Temperature, °С
Fig. 4. Methane conversion as a function of (a) temperature and (b) the amount of Ag introduced into La
Ag MnO3 ± y: х =
x
1
– x
(
1) 0, (2) 0.1, (3) 0.2, and (4) 0.3.
(а)
(b)
Conversion, %
Conversion, %
100
1
9
00
0
0
0
0
0
0
0
0
9
0
8
6
6
4
4
2
2
80
7
8
0
x = 0
1
x = 0.1 2
x = 0.2 3
x = 0.3 4
4
0
3
50
00
5
1
0
0
2
00 300 400 500 600 700 800
0.1
0.2
0.3
Temperature, °С
x
Fig. 5. Soot conversion as a function of (a) temperature and (b) the amount of Ag introduced into La
(2) 0.1, (3) 0.2, and (4) 0.3.
Ag MnO3 ± y: х = (1) 0,
x
1
– x
medium. The resulting La1 – xAg MnO
(х = 0–0.3) formly distributed on the surface of the perovskite
x
3 ± y
samples are not single phases: they contain metallic phase. The introduction of silver into LaMnO3 ±
y
silver as a minor phase. They show a high catalytic enhances the catalytic activity of the oxide in meth-
activity in methane and soot oxidation. Crystallo- ane and soot oxidation with oxygen; however, the
graphic parameters of these phases were calculated, dependence of the catalytic activity on the amount of
and the average particle size of mixed oxide particles silver introduced is nonlinear. The catalytic activity
was estimated by XRD. The effect of synthesis condi- of La Ag MnO depends on the method of its
3 ± y
1
–x
x
tions on the morphology of the phases was studied. synthesis. As the polymer content of the initial PSC
The samples synthesized from PSCs containing excess is increased, the catalytic activity of the oxide in
polymer, contain larger particles of both perovskite methane oxidation decreases and its catalytic activity
and metallic silver phases. The specific surface area in soot oxidation increases. Therefore, the catalyst
changes insignificantly on passing to polymer-rich preparation procedure should be chosen with regard
compositions owing to the fact that the resulting for the conditions under which the catalyst is to be
aggregated, micron-sized perovskite particles retain operated, namely, temperature and specific features
high porosity. The metallic silver particles are uni- of the catalytic process.
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

7
20
RUSSKIKH et al.
(а)
(b)
Conversion, %
Conversion, %
100
1
00
1
2
3
1
2
3
8
0
0
0
0
0
8
0
0
0
0
0
6
4
2
6
4
2
3
00
400
500
0
200
400
600
800
Temperature, °С
Temperature, °С
Fig. 6. Temperature dependences of (a) methane and (b) soot conversion over La
different salt : polymer ratios: (1) sample 3, (2) sample 6, and (3) sample 7.
Ag MnO
synthesized from PSCs at
3 ± y
1
– x
x
ACKNOWLEDGMENTS
9. Antsiferov, V.N., Makarov, A.M., and Ostroushko, A.A.,
Problemy poroshkovogo materialovedeniya (Topics in
Powder Materials Science), part VII: Vysokoporistye
pronitsaemye yacheistye materialy – perspektivnye nos-
iteli katalizatorov (High-Porosity Permeable Cellular
Materials as Promising Catalyst Supports), Yekaterin-
burg: Ural. Otd. Ross. Akad. Nauk, 2006.
In this study, we used equipment of the Modern
Nanotechnologies shared facilities at Ural Federal
University. The study was supported by the Russian
Foundation for Basic Research grant no. 14-33-
5
0886), the Ministry of Education and Science of the
Russian Federation (project no. 2343, UID RFME-
FI59414X0011), and the Government of the Russian
Federation through program 211 (agreement
no. 02.A03.21.0006).
1
0. Antsiferov, V.N., Ostroushko, A.A., and Makarov, A.M.,
Sintez, svoistva i primenenie katalizatorov okisleniya
sazhi na osnove modifitsirovannykh vysokoporistykh
yacheistykh materialov (Soot Oxidation Catalysts Based
on Modified High-Porosity Cellular Materials), Yekat-
erinburg: Ural. Otd. Ross. Akad. Nauk, 2007.
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