Synthesis of LaMnO3+δby Firing Gels Using Citric Acid

Article abstract of DOI:10.1006/jssc.1996.7229

A gel was prepared by adding citric acid to an aqueous solution of lanthanum nitrate and manganese nitrate. From thermal analysis and IR measurement, the gel prepared with 0.0023 mol of citric acid is a mixture of LaMn(C₆H₅O₇)(NO₃)₃, La(NO₃)₃·xH₂O, and Mn(NO₃)₂·xH₂O; the gel with 0.007 mol of citric acid LaMn(C₆H₅O₇)(NO₃)₃; and the gel with 0.015 mol of citric acid is LaMn(C₆H₅O₇)₂. Hexagonal perovskite-type LaMnO_3+δwas obtained by firing the gels. The specific surface area, the La/Mn ratio, and the catalytic activity for CO oxidation on LaMnO_3+δwere measured. The catalytic activity for CO oxidation on LaMnO_3+δsynthesized using citric acid is better than that on LaMnO_3+δsynthesized using poly(acrylic acid). For these results, citric acid is inferred to improve the crystallinity (regularity of the ions) of the LaMnO_3+δsurface.

Full text of DOI:10.1006/jssc.1996.7229

JOURNAL OF SOLID STATE CHEMISTRY 129, 60—65 (1997)
ARTICLE NO. SC967229
Synthesis of LaMnO3؉d by Firing Gels Using Citric Acid
Hideki Taguchiꢀ, Shin-ichiro Matsu-ura, and Mahiko Nagao
Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700, Japan
and
Testuo Choso and Kenji Tabata
Research Institute of Innovation Technology for the Earth (RITE), Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan
Received August 9, 1996; in revised form November 11, 1996; accepted November 13, 1996
LaMnO
are strongly influenced by the PAA concentra-
ꢁ>B
A gel was prepared by adding citric acid to an aqueous solution
of lanthanum nitrate and manganese nitrate. From thermal ana-
lysis and IR measurement, the gel prepared with 0.0023 mol
tion. Therefore, we synthesized LaMnO
using gels with
ꢁ>B
various PAA concentrations at low temperature and exam-
ined the relationship among the PAA concentration, the
crystal structure, and the oxygen content (9). PAA plays an
important role in making a gel from the aqueous solution of
lanthanum nitrate and manganese nitrate and provides
of citric acid is
a mixture of LaMn(C6H5O7)(NO3)3,
La(NO3)3 xH O, and Mn(NO ) xH O; the gel with 0.007 mol
·
·
2
3 2
2
of citric acid LaMn(C6H5O7)(NO3)3; and the gel with 0.015 mol
of citric acid is LaMn(C6H5O7)2. Hexagonal perovskite-type
LaMnO3؉d was obtained by firing the gels. The specific surface
area, the La/Mn ratio, and the catalytic activity for CO oxida-
tion on LaMnO3؉d were measured. The catalytic activity for CO
the heat of combustion for the synthesis of LaMnO
The rapid combustion of PAA caused many cracks in
.
ꢁ>B
LaMnO , resulting in a large specific surface area. From
ꢁ>B
oxidation on LaMnO3؉d synthesized using citric acid is better the results of catalytic activity and the amount of absorbed
than that on LaMnO3؉d synthesized using poly(acrylic acid). For
these results, citric acid is inferred to improve the crystallinity
(regularity of the ions) of the LaMnO3؉d surface. ( 1997 Academic
Press
oxygen on (La Sr )MnO synthesized using PAA and the
ꢀ\V
V
ꢁ
solid-state reaction, the surface of the samples synthesized
using PAA had insufficient crystallization and fewer adsorp-
tion sites in comparison with the samples synthesized using
the solid-state reaction (10).
In the present study, we tried to use citric acid to make
gels of lanthanum nitrate and manganese nitrate and deter-
mined the structural formula of the gels. Then we syn-
INTRODUCTION
thesized LaMnO
structure, oxygen content, crystallite size, specific surface
area, La/Mn ratio, and catalytic activity for CO oxidation
by firing the gels in air. The crystal
ꢁ>B
Perovskite-type LaMO
(M"Cr, Mn, Fe, Co, and Ni)
ꢁ>B
shows high catalytic activity for the oxidation of carbon
monoxide (CO) or hydrocarbons (1—5). According to
Seiyama, the catalytic activity for the oxidation of CO on
on LaMnO
were measured. These results will provide
ꢁ>B
some information about the synthesis of LaMnO
the crystallinity of the LaMnO
and
ꢁ>B
LaNiO is higher than it is on NiO (6). LaMO
is gener-
ꢁ
ꢁ>B
surface.
ꢁ>B
ally synthesized at high temperature using a solid-state
reaction, so the specific surface area of LaMO
than 5 mꢂ/g (2, 3). To improve the catalytic activity, it is
necessary to synthesize LaMO
face area.
Taguchi et al. reported that LaMnO
with +20 mꢂ/g were easily synthesized using poly(acrylic
acid) (PAA) at low temperature (7, 8). Recently, we noticed
that the crystal structure and the oxygen content of
is less
ꢁ>B
EXPERIMENTAL
with a large specific sur-
ꢁ>B
Powders of La(NO ) 6H O and Mn(NO ) 6H O
and LaCoO
·
·
ꢁ ꢁ
ꢂ
ꢁ ꢂ
ꢂ
ꢁ>B
ꢁ>B
were weighed in equimolar amounts (0.007 mol) and dis-
solved in a small amount of distilled water. Citric acid
(0.0023, 0.006, 0.007, 0.008, 0.009, and 0.015 mol) was used
to make a gel. Gelation was achieved by keeping the solu-
tion at 100°C for 12 hr. The gel was fired at 300—900°C
for 6 hr in air. The heating rate was 10°C/min. For
ꢀ To whom correspondence should be addressed.
60
0022-4596/97 $25.00
Copyright ( 1997 by Academic Press
All rights of reproduction in any form reserved.

LaMnO
SYNTHESIS FROM GELS
61
ꢁ>B
comparison, LaMnO
and the solid-state reaction (10).
Thermal analysis (DTA and TG) of the gels was per- (Molecular Sieve 5A) kept at 50°C during the measure-
formed at a heating rate of 10°C/min to determine the ments.
was also synthesized using PAA a flow reactor at a flow rate of 150 cmꢁ · min\ꢀ. The prod-
ꢁ>B
ucts were analyzed by gas chromatography using a column
reaction temperature of the gel. Infrared absorption (IR)
spectra of the gels were measured using the KBr method.
The crystal phases of the samples were identified by X-ray
powder diffraction using monochromatic CuKa radiation.
The oxygen content of the samples was determined by an
RESULTS AND DISCUSSION
Gel formation was possible in the molar range from
oxidation—reduction (redox) method (11). The crystallite size 0.0023 to 0.015 mol of citric acid. A brown smoke that
(D ) of the samples was calculated from the half-width of evolved from the nitrate ion was given out from the gels at
ꢃꢂꢄ
a diffraction peak (024) using Scherrer’s equation (12). The 100°C, and the amount of the smoke increased with increas-
XPS measurements for the La3d, Mn2p, O1s, and C1s levels ing amount of the citric acid. Figures 1a, 1b, and 1c show the
of the samples were carried out using MgKa radiation at results of thermal analysis (DTA and TG) of the gels with
room temperature. The binding energy was calibrated using 0.0023, 0.007, and 0.015 mol of citric acid, respectively. In
the C1s level from the usual contamination. The specific Fig. 1a, we observed small endothermic peaks (90, 130, 270,
surface area of the samples was determined by the BET and 340 °C) and a large exothermic peak (160°C) on the
method for nitrogen adsorption. The catalytic activities for DTA curve. The total weight loss below 500°C is +58%. In
the oxidation of CO were measured at 195—300°C using Fig. 1b, we observed both a small endothermic peak (80°C)
a conventional flow system. The samples (0.10 g) were pre- and a large exthothermic peak (190°C) on the DTA curve.
heated at 300°C in a pure oxygen stream for 3 hr. A mixed Abrupt weight loss was observed at 195°C, and the total
gas of CO (1.0%), O (4.0%), and He (balance) was fed in weight loss below 500°C is +58%. In Fig. 1c, we observed
ꢂ
FIG. 1. Thermal analysis (DTA and TG) of the gels with 0.0023, 0.007, and 0.015 mol of citric acid.

62
TAGUCHI ET AL.
small exthothermic peaks (150 and 170°C) and large ex- spectra, the gel is considered to be LaMn(C H O )(NO ) .
ꢅ ꢆ ꢇ ꢁ ꢁ
thothermic peaks (340 and 350°C). There are two steps in The molecular weights of LaMn(C H O )(NO )
ꢅ ꢆ ꢇ
the TG curve, and total weight loss below 500°C is +57%. and LaMnO are 568.85 and 241.85, respectively. The
ꢁ
ꢁ ꢁ
The exthothermic peak shifts to higher temperature with theoretical weight loss is +57% in the reaction
increasing amount of citric acid.
LaMn(C H O )(NO ) PLaMnO
and is nearly equal
ꢅ ꢆ ꢇ
ꢁ ꢁ
ꢁ>B
Figure 2 shows the IR spectra of the gels with 0.0023, to the observed weight loss. As for the gel with 0.015 mol of
0.007, and 0.015 mol of citric acid. The absorption peaks or citric acid, excess citric acid was added to make the gel, and
shoulders in Fig. 2 could be assigned to the methylene group the brown smoke was given out from the gel. The disappear-
(+720 cm\ꢀ for a rocking vibration and +1465 cm\ꢀ for ance of the peaks at +1440 and +820 cm\ꢀ indicates that
a
scissoring band), the carboxylate anion (+1550— the gel is LaMn(C H O ) . The molecular weight of
ꢅ ꢆ ꢇ ꢂ
1650 cm\ꢀ for strong asymmetric stretching and LaMn(C H O ) is 571.85. The theoretical weight loss is
ꢅ ꢆ ꢇ ꢂ
+1400 cm\ꢀ for weak symmetric stretching), and the +58% in the reaction LaMn(C H O ) PLaMnO
,
ꢅ ꢆ ꢇ ꢂ
ꢁ>B
nitrate ion (+1440 and +820 cm\ꢀ for NO stretching) and is nearly equal to the observed weight loss.
(13, 14). The gels with 0.0023 and 0.007 mol of citric acid had
Figure 3 shows the relationship between the crystal struc-
the absorption peaks due to the nitrate ion. The gel with ture of the samples and synthesis condition (the amount of
0.015 mol of citric acid had sharp peaks due to the car- citric acid and the firing temperature). Hexagonal
boxylate anion, and the NO stretching was not observed.
From the results of thermal analysis and IR measure- with 0.006 mol of citric acid. Above 0.007 mol of citric acid,
ment, we determined the chemical composition of the gels. however, hexagonal LaMnO
As for the gel with 0.0023 mol of citric acid, it is obvious firing the gels at low temperature. The oxygen content
LaMnO
was not obtained below 700°C by firing the gel
ꢁ>B
was easily obtained by
ꢁ>B
that the gel lacks citric acid. The gel is considered to be (3#d) of LaMnO
is independent of the synthsis condi-
ꢁ>B
a mixture of LaMn(C H O )(NO ) , lanthanum nitrate, tion and is shown in Table 1. The crystallite size (D ) of
ꢅ ꢆ ꢇ
ꢁ ꢁ
ꢃꢂꢄ
and manganese nitrate. The exothermic peak at 170°C and LaMnO
was calculated from the half-width of the dif-
ꢁ>B
endothermic peaks correspond to the combustion of fraction peak (024) using Scherrer’s equation (12). D
is
ꢃꢂꢄ
LaMn(C H O )(NO ) and the decomposition of nitrates. shown in Table 1, and has a minimum value at 0.007 mol of
ꢅ ꢆ ꢇ
ꢁ ꢁ
Since the amount of water of crystallization in lanthanum citric acid. The specific surface area of LaMnO
is also
ꢁ>B
nitrate and manganese nitrate varies with temperature, they shown in Table 1, and has the maximum value at 0.008 mol
are defined as La(NO ) · xH O and Mn(NO ) · xH O. of citric acid. Above 0.007 mol of citric acid, it is obvious
ꢁ ꢁ
ꢂ
ꢁ ꢂ
ꢂ
Therefore, it is very difficult to calculate the theoretical that the heat of combustion of LaMn(C H O )(NO ) in-
ꢅ ꢆ ꢇ
ꢁ ꢁ
ꢃꢂꢄ
weight loss of the gel with 0.0023 mol. As for the gel with creased with the amount of citric acid, so that D
in-
0.007 mol of citric acid, citric acid is equivalent with lan- creased and the specific surface area decreased as shown in
thanum nitrate and manganese nitrate, and brown smoke Table 1. Below 0.007 mol of citric acid, the heat of combus-
was given out from the gel. As the methylene group, car- tion of LaMn(C H O )(NO ) decreased due to the co-
ꢅ ꢆ ꢇ
ꢁ ꢁ
boxylate anion, and nitrate ion are detected in the IR existence of La(NO ) · xH O and Mn(NO ) · xH O, and
ꢁ ꢁ ꢂ ꢁ ꢂ
ꢂ
FIG. 3. Relationship between crystal structure of the samples and
synthesis condition (amount of citric acid and firing temperature). (᭹)
FIG. 2. IR spectra of the gels with 0.0023, 0.007, and 0.015 mol of citric hexagonal LaMnO , (᭡) mixture of hexagonal LaMnO
and La O ,
ꢁ>B
ꢁ>B
ꢂ ꢁ
acid.
(;) amorphous

LaMnO
SYNTHESIS FROM GELS
63
ꢁ>B
TABLE 1
Relationship between Oxygen Content (3؉d), Crystallite Size
(D024), Specific Surface Area (S) of LaMnO3؉d and Synthesis
Condition (Amount of Citric Acid and Firing Temperature)
Synthesis condition
D
S
ꢃꢂꢄ
(nm)
Amount (mol)
Temp. (°C)
3#d
(mꢂ/g)
0.006
0.007
0.008
0.009
500
500
500
500
—
—
—
3.23
3.20
3.17
12.2
14.8
18.4
16.3
18.5
15.2
0.006
0.007
0.008
0.009
700
700
700
700
3.19
3.20
3.18
3.19
21.5
16.4
20.2
22.4
12.9
14.2
15.7
13.1
FIG. 5. Relationship between specific surface area (S) and firing tem-
perature of the gel with 0.007 mol of citric acid. Specific surface area (S) of
LaMnO
synthesized using PAA and solid-state reaction is also plotted.
ꢁ>B
we could not obtain LaMnO
Since we could not observe the change in DTA and TG
by firing the gel at 500°C.
ꢁ>B
acid and PAA. Figure 5 shows the relationship between the
specific surface area and the firing temperature of the gel
with 0.007 mol of citric acid. The specific surface area of
above 500°C, the increase of D
at 700°C is considered to
ꢃꢂꢄ
be caused by sintering of LaMnO
. From the results of
ꢁ>B
LaMnO
synthesized using citric acid decreases from
Table 1, we considered that LaMnO
0.007—0.008 mol of citric acid had a very fine powder. Here-
after, we used the gel with 0.007 mol of citric acid to charac-
firing the gel with
ꢁ>B
ꢁ>B
+16.7 (400°C) to +3.0 mꢂ/g (900°C). At low temperature,
the specific surface area of LaMnO
PAA is larger than the specific surface area of LaMnO
synthesized using citric acid.
The XPS spectra of the La3d , Mn2p , and O1s levels
synthesized using
ꢁ>B
terize the surface of LaMnO
Figure 4 shows the relationship between D
firing temperature of the gel with 0.007 mol of citric acid. In
.
ꢁ>B
ꢁ>B
and the
ꢃꢂꢄ
ꢆꢈꢂ
ꢁꢈꢂ
of LaMnO
synthesized using citric acid are shown in
Fig. 4, we also plotted D
synthesized using poly(acrylic) acid (PAA) and the solid-
calculated from LaMnO
ꢁ>B
ꢃꢂꢄ
ꢁ>B
Figs. 6a, 6b, and 6c, respectively. A satellite peak was ob-
served on the high binding energy side of the main La3d
peaks by +4 eV. This satellite was interpreted as the excita-
tion of an electron from the anion valence band into the
state reaction. D
increases with temperature. At 900°C,
synthesized using the solid-state reac-
ꢆꢈꢂ
ꢃꢂꢄ
ꢁ>B
D
of LaMnO
ꢃꢂꢄ
tion is less than D
of LaMnO
synthesized using citric
ꢃꢂꢄ
ꢁ>B
lanthanum f band (15). The Mn2p
asymmetric toward the high binding energy site. The bind-
level is broad, and
ꢁꢈꢂ
ing energies of the La3d and Mn2p levels are shown in
ꢆꢈꢂ
ꢁꢈꢂ
Table 2 and are independent of the synthesis condition:
834.3$0.2 eV for the La3d level and 642.0$0.2 eV for
ꢆꢈꢂ
the Mn2p level, respectively. We could not observe the
ꢁꢈꢂ
large variation in both the binding energies and FWHM of
the La3d
and Mn2p
levels. The O1s level has two
ꢆꢈꢂ
ꢁꢈꢂ
peaks, and these peaks are independent of the synthesis
condition. The lower binding peak is +529.3$0.2 eV and
assignable to a lattice oxygen. The higher binding peak is
+531.0$0.2 eV and assignable to an adsorbed oxygen or
hydroxyl group (16, 17). The La/Mn ratio of the LaMnO
ꢁ>B
surface is also shown in Table 2. Except for LaMnO
ꢁ>B
firing the gel with 0.007 mol of citric acid at 500°C, the
La/Mn ratio is 1.2—1.3 and is independent of the synthesis
condition.
The conversion from CO to CO was measured for
FIG. 4. Relationship between crystallite size (D ) and firing temper-
ꢃꢂꢄ
of LaMnO
ꢂ
LaMnO
synthesized using citric acid and is shown in
ature of the gel with 0.007 mol of citric acid. D
ꢃꢂꢄ
thesized using PAA and solid-state reaction is also plotted.
syn-
ꢁ>B
ꢁ>B
Fig. 7. The gel with 0.007 mol of citric acid was fired at

64
TAGUCHI ET AL.
FIG. 6. XPS spectra of the La3d , Mn2p , and O1s levels of LaMnO
firing the gels with 0.007 mol of citric acid.
ꢆꢈꢂ
ꢁꢈꢂ
ꢁ>B
400—900°C. The conversion increased in the firing temper- acid increased in the firing temperature range from 400
ature range from 400 to 600°C and had a maximum value, to 600°C, and had a constant value in the firing temperature
then decreased in the firing temperature range from 700 to range from 600 to 900°C. It is obvious that LaMnO
900°C. The rate of reaction (R) at a given temperature was synthesized using citric acid has a large rate of reaction at
ꢁ>B
calculated using the equation
low temperature. On firing at 900°C, the rate of reaction of
LaMnO
synthesized using solid-state reaction is larger
ꢁ>B
F;C;C
by +0.1 cmꢁ · min\ꢀ · m\ꢂ than those of LaMnO
synthesized using citric acid and PAA. Since we cannot find
the difference in the specific surface areas of LaMnO
ꢀ
ꢁ>B
R"
,
m;S
ꢁ>B
where F is the gas flow per minute, C the initial concentra- synthesized using citric acid, PAA, and solid-state reaction,
tion of CO, C the conversion per gram from CO to CO , it is considered that the La/Mn ratio of LaMnO
m the mass of the sample and S specific surface area of the thesized using solid-state reaction is different from the
sample (18). Figure 8 shows the relationship between the La/Mn ratio of LaMnO
rate of reaction at 270°C and the firing temperatures of the PAA.
Voorhoeve et al. reported that the oxidation of CO occurs
PAA and the solid-state reaction is also plotted in Fig. 8. at the metal ions of the surface (19). Both the metal ion
syn-
ꢀ
ꢂ
ꢁ>B
synthesized using citric acid and
ꢁ>B
gels. The rate of reaction for LaMnO
synthesized using
ꢁ>B
The rate of reaction for LaMnO
synthesized using citric content and the surface crystallinity (regularity of the ions)
ꢁ>B
TABLE 2
Relationship between Binding Energies of La3d5/2 and
Mn2p3/2 Levels of LaMnO3؉d and Synthesis Condition (Amount
of Citric Acid and Firing Temperature)
Synthesis condition
Amount
(mol)
Temp.
(°C)
La3d
(eV)
FWHM Mn2p
(eV)
FWHM La/Ma
(eV)
ꢆꢈꢂ
ꢁꢈꢂ
(eV)
0.007
0.007
0.007
500
700
900
834.1
834.3
834.4
6.6
6.5
6.4
641.8
641.8
642.2
3.4
3.7
3.7
1.5
1.2
1.3
0.006
0.007
0.008
700
700
700
834.3
834.3
834.2
6.5
6.5
6.4
642.0
641.8
642.0
3.6
3.7
3.9
1.3
1.2
1.2
FIG. 7. Conversion from CO to CO on LaMnO
with citric acid.
firing the gels
ꢂ
ꢁ>B

LaMnO
SYNTHESIS FROM GELS
65
ꢁ>B
citric acid, hexagonal perovskite-type LaMnO
tained at 300—900°C. The catalytic activity for CO oxidation
indicates that the rate of reaction for LaMnO
thesized using citric acid is better than that for LaMnO
synthesized using PAA. It is considered that citric acid
improves the crystallinity (regularity of the ions) of the
was ob-
ꢁ>B
syn-
ꢁ>B
ꢁ>B
LaMnO
surface.
ꢁ>B
ACKNOWLEDGMENT
The authors express their thanks to Dr. Yasuo Takeda of Mie University
for measuring the thermal analysis of the gels.
REFERENCES
FIG. 8. Relationship between the rate of reaction (R) at 270°C and
firing temperature of the gels with 0.007 mol of citric acid. The rate of
LaMnO
synthesized using PAA and solid-state reaction is also plotted.
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ꢁ>B
2. T. Nitadori, S. Kurihara, and M. Misono, J. Catal. 98, 221 (1986).
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play an important role to the catalytic activity. As seen in
Fig. 6c, it is obvious that there are two kinds of oxygen at
the surface; one is lattice oxygen, and the other is oxygen
adsorbed on the metal ions. CO is produced by the reac-
6. T. Seiyama, ‘‘Metal Oxides and Catalysis,’’ p. 211. Koudan-Sha Ltd.,
Tokyo, 1979.
ꢂ
tion of CO with oxygen adsorbed on the metal ions of the
outermost surface. After the desorption of CO from the
surface, oxygen is again adsorbed on the metal ions. In the
present study, we can understand that the low rate of reac-
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(1994).
18. Y. Ogino, T. Onoda, S. Shikagawa, M. Karube, Y. Saito, K. Tabe,
T. Tamura, H. Matsumoto, M. Misono, and K. Yoshida, ‘‘Catalysis,’’
p. 880. Maruzen, Tokyo, 1986.
ꢂ
tion for LaMnO
600°C was caused by the deviation of the La/Mn ratio.
Because of the rapid combustion of PAA, LaMnO
thesized using PAA has many cracks, resulting in a larger
specific surface area and insufficient crystallization (10). The
synthesized using citric acid below
ꢁ>B
syn-
ꢁ>B
large rate of reaction for LaMnO
synthesized using citric
ꢁ>B
acid indicated that LaMnO
has sufficient crystallization
ꢁ>B
and has many adsorption sites. From these results, citric
acid is better than PAA for improving the crystallinity of the
LaMnO
surface.
ꢁ>B
CONCLUSION
Thermal analysis and IR spectra indicate that the gel
with 0.0023 mol of citric acid is mixture of
LaMn(C H O )(NO ) , La(NO ) · xH O, and Mn(NO ) ·
a
ꢅ ꢆ ꢇ
ꢁ ꢁ
ꢁ ꢁ
ꢂ
ꢁ ꢂ
xH O, the gel with 0.007 mol of citric acid is
ꢂ
LaMn(C H O )(NO ) , and the gel with 0.015 mol of citric
19. R. H. Voorhoeve, D. W. Johnson, J. P. Remeika, and P. K. Gallagher,
Science 195, 827 (1977).
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acid is LaMn(C H O ) . By firing the gel with 0.007 mol of