Synthesis of LaMnO3+δ by firing gels with poly(acrylic acid) in a pure argon stream

Article abstract of DOI:10.1006/jssc.1996.0069

Perovskite-type LaMnO_3+δ was synthesized by firing the gels with 0.007-0.033 M of poly(acrylic acid) (PAA) in a pure argon stream. The oxygen content, crystallite size (D₀₂₄), specific surface area, and catalytic activity for CO oxidation on LaMnO_3+δ were measured to characterize the LaMnO_3+δ surface. In spite of a small specific surface area, the catalytic activity for CO oxidation on LaMnO_3+δ fired in the pure argon stream is twice that on LaMnO_3+δ fired in air. These results indicate that firing the gels in the pure argon stream improved the crystallinity (regularity of the ions) of the LaMnO_3+δ surface.

Full text of DOI:10.1006/jssc.1996.0069

JOURNAL OF SOLID STATE CHEMISTRY 121, 495–497 (1996)
ARTICLE NO. 0069
BRIEF COMMUNICATION
Synthesis of LaMnO by Firing Gels with Poly(Acrylic Acid) in a
3؉␦
Pure Argon Stream
1
Hideki Taguchi, Akio Sugita, and Mahiko Nagao
Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700, Japan
Received September 26, 1995; accepted October 19, 1995
resulting in a large specific surface area. Recently, we mea-
Perovskite-type LaMnO3؉␦ was synthesized by firing the gels sured both the catalytic activity for CO oxidation and the
with 0.007–0.033 M of poly(acrylic acid) (PAA) in a pure argon amount of adsorbed oxygen on (La1Ϫx Sr )MnO synthe-
x
3
stream. The oxygen content, crystallite size (D024), specific sur-
face area, and catalytic activity for CO oxidation on LaMnO3؉␦
were measured to characterize the LaMnO3؉␦ surface. In spite
of a small specific surface area, the catalytic activity for CO
oxidation on LaMnO3؉␦ fired in the pure argon stream is twice
that on LaMnO3؉␦ fired in air. These results indicate that firing
the gels in the pure argon stream improved the crystallinity
sized using PAA and the solid-state reaction (7). Then, it
is apparent that the surface of the samples firing the gels
using PAA has an insufficient crystallization and is lacking
in adsorption sites in comparison with the samples synthe-
sized using the solid-state reaction.
In order to improve the LaMnO3ϩͳ surface, it is necessary
to control the combustion of PAA. In the present study,
we tried to synthesize LaMnO3ϩͳ by firing the gels in a
pure argon stream. Thereafter, the crystal structure, oxy-
gen content, crystallite size, specific surface area, and
catalytic CO oxidation on LaMnO3ϩͳ were measured.
These results will provide some information about the
stability of LaMnO3ϩͳ synthesized in the pure argon
stream and the crystallinity (regularity of the ions) on
the LaMnO3ϩͳ surface.
(regularity of the ions) of the LaMnO3؉␦ surface.
 1996 Aca-
demic Press, Inc.
INTRODUCTION
Perovskite-type LaMnO3ϩͳ is available to be used as a
catalyst that oxidizes hydrocarbons or carbon monoxide
(
CO) oxidation (1–3). LaMnO3ϩͳ is generally synthesized
at high temperature using a standard ceramic technique.
Therefore, the specific surface area of LaMnO ϩͳ is less
than 5 m /g (2–3). By using poly(acrylic acid) (PAA),
3
EXPERIMENTAL
2
Taguchi et al. synthesized LaMnO3ϩͳ with a large specific
surface area and defined the conditions to synthesize
LaMnO3ϩͳ in air (4). They measured the La/Mn ratio,
specific surface area, and catalytic activity for CO oxidation
on LaMnO3ϩͳ in order to characterize the LaMnO3ϩͳ sur-
face (5).
The preparation of the gels using PAA has been de-
scribed in detail elsewhere (4). Average molecular weight
of PAA was 2000. In the present study, the PAA concentra-
tion was from 0.007 to 0.033 M. The gels were fired at
400–900ЊC in the pure argon stream for 6 h. The heating
rate was 1ЊC/min up to 300ЊC, and then 10Њ/min.
PAA is an important chemical to make a gel from the
solution, and to provide the heat of combustion for the
synthesis of LaMnO3ϩͳ . From the results of differential
thermal analysis (DTA) of the gel, the gel gave the extho-
thermic peaks in the temperature range from 160 to 300ЊC,
and these peaks correspond to the combustion of PAA
The crystal phases of the samples were identified by X-
ray powder diffraction (XRD) using monochromatic CuKͰ
radiation. The oxygen content in each sample was deter-
mined by an oxygen-reduction (redox) method (8). The
crystallite size (D024) of the samples was calculated from
the half-width of a diffraction peak (024) using Scherrer’s
equation (9). The specific surface area of the samples was
determined by the BET method for nitrogen adsorption.
The catalytic activities for CO oxidation were measured
at 195 to 300ЊC using a conventional flow system. The
samples (0.1 g) were preheated at 300ЊC in a pure oxygen
(
6). Because of the rapid combustion of PAA, the crystal
structure of LaMnO3ϩͳ becomes orthorhombic. The rapid
changes in temperature cause many cracks in LaMnO3ϩͳ
,
1
To whom correspondence should be addressed.
4
95
0022-4596/96 $18.00
Copyright  1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

496
BRIEF COMMUNICATION
TABLE 1
Oxygen Content (3 ؉ ␦), Crystallite Size (D024), Specific Sur-
face Area (S), and Rate of Reaction (R) of LaMnO3؎␦ for the
Oxidation of CO at 270؇C
Firing condition
PAA
Temp.
Concentration
%)
D
(nm)
024
S
R
Ϫ
(ЊC)
Atmosphere
2
(cm3 и min и m 2
)
1
Ϫ
(
3 ϩ ͳ
(m /g)
0
0
0
.007
.020
.033
700
700
700
Ar
Ar
Ar
3.20
3.21
3.18
18.8
25.5
25.5
21.1
11.4
16.7
0.54
0.44
0.43
0
0
.007
.020
900
900
Ar
Ar
3.20
3.18
25.5
33.3
9.2
4.8
0.22
0.48
0
0
.020
.020
700
900
Air
Air
3.18
3.18
18.0
31.5
23.5
5.4
0.21
0.22
FIG. 1. The relationship between the crystal structure of the samples
and the synthesis condition (PAA concentration and firing temperature).
The samples were synthesized by firing the gels for 6 h in a pure argon
LaMnO3ϩͳ for the gel fired with 0.020 M of PAA at 700ЊC
is very low, other values of the specific surface area de-
crease with increasing the PAA concentration and firing
temperature. The variation in D024 and the specific surface
area suggests the incrrease in the particle size of
stream. (᭺; hexagonal LaMnO
and La ; ϫ, amorphous).
3 3
ϩͳ ; ᭝, a mixture of hexagonal LaMnO
ϩͳ
2
O
3
stream for 3 h. A mixed gas of CO (1.0%), O (2.0%), and
2
LaMnO3ϩͳ
.
He (balance) was fed in a flow reactor at a flow rate of
In order to investigate the stability of LaMnO3ϩͳ
,
3
Ϫ
1
1
50 cm и min . The products were analyzed by gas chro-
matography using a column (Molecular Sieve 5A) kept at
0ЊC during the measurements.
LaMnO3ϩͳ was fired at 700ЊC for 6–36 h in the pure argon
stream. Figure 2 shows the XRD patterns of LaMnO3ϩͳ
for the gel fired with 0.020 M at 700ЊC for 6, 12, 24, and
5
3
6 h. The XRD patterns of LaMnO3ϩͳ fired for 12, 24,
RESULTS AND DISCUSSION
Figure 1 shows the relationship between the crystal struc-
and 36 h were indexed as the hexagonal perovskite-type
structure (10). The diffraction peaks (300), (214), (220),
and (208) were observed in hexagonal LaMnO3ϩͳ fired for
ture of the samples and the synthesis conditions (PAA
concentration and firing temperature) in the pure argon
stream. From the XRD patterns, it is obvious that the
sample fired at low temperature is amorphous or is a mix-
ture of hexagonal LaMnO3ϩͳ and La O . In the concentra-
6
(
h. With increased firing time, the intensity of the peak
018) decreased the peaks (400) and (224) became one
broad peak. The slight decrease in 2␪ of each peak indicates
2
3
tion range from 0.007 to 0.033 M of PAA, the sample fired
at high temperature is indexed as a hexagonal perovskite-
type structure. The region for the formation of LaMnO3ϩͳ
in the pure argon stream is wide in comparison with the
region for the formation of LaMnO3ϩͳ in air (4).
Table 1 shows the oxygen content (3 ϩ ͳ) of LaMnO3ϩͳ
fired in the pure argon stream. From the results of Fig. 1,
it is obvious that firing temperature in the pure argon
stream is higher than the firing temperature in air. When
the gels are fired in air, the oxygen content of LaMnO3ϩͳ
increases with increasing firing temperature (4). However,
the oxygen content of LaMnO3ϩͳ fired in the pure argon
stream is ca. 3.18–3.21 and is independent of the synthesis
condition. Table 1 also shows the crystallite size (D024) and
the specific surface area (S) of LaMnO3ϩͳ for gels fired in
the pure argon stream. D024 increases with increasing PAA
concentration and firing temperature. Although we can
3
FIG. 2. XRD patterns of a LaMnO ϩͳ -fired gel with 0.020 M at 700ЊC
not understand the reason why the specific surface area of for 6, 12, 24, and 36 h in a pure argon stream.

BRIEF COMMUNICATION
497
the increase in the cell constant with increasing firing time. LaMnO3ϩͳ surface was improved by firing the gels in the
From these results, it is obvious that hexagonal LaMnO3ϩͳ pure argon stream. Therefore, the atmosphere plays an
fired at 700ЊC for 6–36 h in the pure argon stream is stable. important role in the control of the surface structure of
The conversion from CO to CO was measured for LaMnO3ϩͳ
.
2
LaMnO3ϩͳ fired in the pure argon stream. The rate of
reaction (R) at a given temperature is calculated using the
following equation:
CONCLUSION
Perovskite-type LaMnO3ϩͳ was synthesized by firing the
gels with 0.007–0.033 M of PAA in the pure argon stream.
The variation in the crystallite size (D024) and the specific
surface area suggests that the particle size of LaMnO3ϩͳ
fired in the pure argon stream is larger than that of
LaMnO3ϩͳ fired in air. However, the rate of reaction for
CO oxidation on LaMnO3ϩͳ fired in the pure argon stream
is twice that on LaMnO3ϩͳ fired in air. These results indicate
that the crystallinity of the LaMnO3ϩͳ surface is easily
improved by changing the firing condition.
F ϫ C ϫ CV
m ϫ S
R ϭ
,
where F is the gas flow per minute, C the initial concentra-
tion of CO, C the conversion per gram from CO to CO ,
V
2
m is the mass of the sample, and S is the specific surface
area of the sample (11). The rate of reaction at 270ЊC is
shown in Table 1. Except the rate of reaction for LaMnO3ϩͳ
for the gel fired with 0.007 M of PAA at 900ЊC, the rate
3
Ϫ
1
Ϫ
2
of reaction is ca. 0.43–0.54 cm и min и m . The oxygen
content, the crystallite size (D024), the specific surface area,
the rate of reaction of LaMnO3ϩͳ for the gels fired in air
are also shown in Table 1. The variation in D024 and the
specific surface area suggests that the particle size of
LaMnO3ϩͳ increases with change in the atmosphere. The
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