Comparative studies on the properties of new and conventional perovskite-type LaCoO3

Article abstract of DOI:10.1246/bcsj.78.1450

The physical properties and redox chemistry of perovskite-type LaCoO ₃ prepared by reactive grinding and by the citrate method, have been comparatively studied by temperature-programmed reduction (TPR), X-ray diffraction (XRD), N₂ adsorption-desorption, and infrared spectroscopy (IR). XRD and N₂ adsorption-desorption measurements show that LaCoO₃ prepared by reactive grinding possesses a smaller particle size and thus a greater specific surface area. An IR study suggests that there would exist a large amount of oxygen defects in LaCoO₃ prepared by reactive grinding. TPR results illustrate that the reduction of LaCoO₃ to Co⁰/La₂O₃ requires a higher temperature for a sample prepared by reactive grinding than for the one prepared by the citrate method. Due to a more difficult reduction, LaCoO ₃ prepared by reactive grinding displays a better stability of the perovskite structure than LaCoO₃ prepared by the citrate method under a reducing atmosphere.

Full text of DOI:10.1246/bcsj.78.1450

1450 Bull. Chem. Soc. Jpn., 78, 1450–1456 (2005)
Ó 2005 The Chemical Society of Japan
Comparative Studies on the Properties of New
and Conventional Perovskite-Type LaCoO3
ꢀ
Lin Huang, Mahbod Bassir, Echchahed Bousselham, and Serge Kaliaguine
Department of Chemical Engineering, Laval University, Ste-Foy, Quebec, Canada G1k 7P4
Received July 7, 2004; E-mail: huanglin1@yahoo.com
The physical properties and redox chemistry of perovskite-type LaCoO3 prepared by reactive grinding and by the
citrate method, have been comparatively studied by temperature-programmed reduction (TPR), X-ray diffraction (XRD),
N2 adsorption–desorption, and infrared spectroscopy (IR). XRD and N2 adsorption–desorption measurements show that
LaCoO3 prepared by reactive grinding possesses a smaller particle size and thus a greater specific surface area. An IR
study suggests that there would exist a large amount of oxygen defects in LaCoO3 prepared by reactive grinding. TPR
results illustrate that the reduction of LaCoO3 to Co0/La2O3 requires a higher temperature for a sample prepared by
reactive grinding than for the one prepared by the citrate method. Due to a more difficult reduction, LaCoO3 prepared
by reactive grinding displays a better stability of the perovskite structure than LaCoO3 prepared by the citrate method
under a reducing atmosphere.
Perovskite-type oxides are of interest in applied fields of
physics and chemistry because of important physical charac-
teristics such as ferroelectricity, magnetism, high-temperature
superconductivity, and electro-optic effects. Transition metal-
containing perovskite-type oxides have found potential catalyt-
ic applications in both the oxidation and reduction of signifi-
cance over more than two decades.1–12 LaCoO3 is one of the
typical mixed oxides of this family with a rhombohedral dis-
tortion of the cubic perovskite lattice. The deformation in
structure may result in large changes in the electronic and
chemical properties. Therefore, LaCoO3 has been extensively
studied and exploited as catalyst precursors for the combustion
of hydrocarbons and volatile organic compounds,2,6,8,10–14
DeNOx,3–5,7 the electrochemical reduction of oxygen,9 and
the hydrogenation and hydrogenolysis of hydrocarbons.2,15–21
Nevertheless, like other perovskite-type materials, the uses
of LaCoO3 as catalyst precursors are limited by its low specific
surface area. Conventional methods for preparing perovskite-
type materials involve a high-temperature calcination step,
which leads to a severe sintering of crystals producing a large
particle size and thus a low specific surface area. The so-called
‘‘citrate method’’, by which the bottom calcination temperature
at 500–600 ꢁC is adopted,22,23 yields crystalline LaCoO3 with a
specific surface area as low as around 8 m2/g only.10
correlated to both a high specific surface area and a high defect
density created by reactive grinding.10,11
In the light of the preliminary results on the physical and
catalytic properties of new perovskites, we took stronger inter-
est in exploring the surface chemistry and structural properties
of new perovskites by comparing with those of conventional
perovskites. We attempted to understand the discrepances in
the microstructural feature between new and conventional per-
ovskites in association with their apparent differences in parti-
cle size and specific surface area. We paid special attention to
studying the redox behavior and structural stability under re-
ducing atmospheres of new and conventional perovskites, in
order that we could predict the performances and advantages
of new perovskites when applied to catalytic hydrogenation.
This paper reports on comparative characterization results
of new and conventional LaCoO3 perovskites by TPR, XRD,
and IR. Different spectroscopic characteristics and reduction
behaviors are demonstrated. These results reveal the difference
in the structural stability between the two LaCoO3 perovskites
under reducing conditions due to different preparation meth-
ods.
Experimental
Conventional LaCoO3 was prepared according to the citrate
method.23 10 g of La(NO3)3 6H2O (99.99%), 6.72 g of Co(NO3)2
To overcome the disadvantage of conventional preparative
methods, we recently developed a reactive grinding technique
allowing for the preparation of perovskite-type materials hav-
ing high specific surface areas and nanosized particles.24,25
ꢂ
ꢂ
6H2O (98+%), and 10 g of citric acid were each dissolved in 20
mL of distilled water. The solutions were then mixed together and
the water was evaporated at 80 ꢁC until a viscous liquid was
obtained. This liquid was treated under a vacuum at 70 ꢁC for
ꢁ
Owing to the low grinding temperature (below 40 C), a high
specific surface area of 100 m2/g can be reached with the aid
of grinding additives. The new method has the advantages of
providing a high density of active sites for catalytic reactions
and lowering the preparative cost of perovskite-type materials.
Cobalt-containing perovskites prepared by reactive grinding
have been found to possess high catalytic activity for the oxi-
dation of hydrocarbons.10,11 The good catalytic performance is
ꢁ
12 h. The resulting solid was calcined at 600 C for 12 h, which
gave rise to a specific surface area of 7.8 m2/g. New LaCoO3 was
prepared from La2O3 and Co3O4 via reactive grinding as describ-
ed previously by us.25 La2O3 (99.99%) was first calcined at 600
ꢁC for 24 h to transform any lanthanum hydroxide to lanthanum
oxide. Then a mixture of La2O3 and Co3O4 (97.49%) at an atomic
ratio of La/Co = 1 was introduced into a vial with three balls.
Published on the web August 4, 2005; DOI 10.1246/bcsj.78.1450

L. Huang et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1451
800
700
600
500
400
300
200
100
0
The vial and balls were made of tungsten carbide. The high-ener-
gy ball milling was processed at actual temperatures kept below
40 ꢁC. ZnO and NaCl were used as post-treatment additives to
enhance the specific surface area of LaCoO3. After completion
of ball milling, ZnO was leached out of the sample with a 2 M
solution of NH4Cl and NaCl was eliminated with distilled water.
Residual Zn and Na were less than 0.3% and 0.001%, respective-
ly. The resultant sample was dried in an oven at 120 ꢁC overnight.
In the final conventional and new LaCoO3 perovskites, the atomic
ratios of La to Co were 1.0.
(b)
(a)
TPR was carried out on a multifunctional catalyst testing and
characterization apparatus RXM-100 designed by Advanced Sci-
entific Designs, Inc. The apparatus was equipped with a quadru-
pole mass spectrometer UTI100 and a thermal conductivity detec-
tor (TCD). Prior to a TPR run, a sample of 100 mg was subjected
to 2 h of calcination in flowing 20% O2/He (30 mL/min) at the
desired temperature, followed by 1 h of evacuation under a vacu-
um at the calcination temperature. A TPR profile was obtained by
passing 5% H2/Ar at the desired flow rate through the sample
which was heated from ambient temperature using a desired
temperature program. The effluent gas was passed through a dry
20
30
40
50
60
70
80
2
/ degrees
θ
Fig. 2. XRD patterns of conventional LaCoO3 calcined at
ꢁ
(a) 600 C (7.8 m2/g); (b) 950 C.
ꢁ
Table 1. Particle and Surface Characteristics of LaCoO3
Calcination
Particle
BET surface
LaCoO3
temperature/ꢁC diameteraÞ/nm area/m2 gꢄ1
New
New
120
500
800
600
950
10.5
17.0
42.0
17.0
38.1
77.9
56.1
32.7
7.8
ꢁ
ice/ethanol trap (ꢃꢄ75 C) placed before TCD to remove water
New
Conventional
Conventional
from the exit stream. H2, Ar, 5% H2/Ar, and He had a purity of
99.999%. The amount of H2 consumed during a TPR could be
obtained by integrating the TPR profile area. The reduction degree
of the sample could be calculated accordingly.
—
a) Estimated in terms of Sherrer formula.
XRD was performed on a PHILIPS or SIEMENS D5000 dif-
fractometer to determine the crystal structures of powdered sam-
appearing at 31.8, 35.8, and 48.5ꢁ in Fig. 1 are assigned to
trace amounts of tungsten carbide, ball milling material, which
is slightly mixed to new LaCoO3 after reactive grinding. It can
be seen from Figs. 1 and 2 that the peak width of new LaCoO3
ꢀ
ples. Cu Kꢀ radiation was used (ꢁ ¼ 1:5406 A). XRD spectra
were recorded with step scans from 20 to 80ꢁ in 2ꢂ angle.
IR spectra were measured on a Nicolet MAGNA-760 spectrom-
eter in the 700–50 cmꢄ1 region at a resolution of 4 cmꢄ1. Pow-
dered samples were supported on a polyethylene film card, a
ST-IR Card purchased from Thermo Electron Corp.
N2 adsorption–desorption measurements were undertaken on
an OMNISORB100 analyzer to determine the BET specific sur-
face areas of samples.
ꢁ
calcined at 120 C is indeed larger than those of LaCoO3 cal-
cined at higher temperatures, indicating that the former pos-
sesses a smaller particle size than the latter. In the cases with
smaller particle sizes, two broad peaks at around 32.9 and
33.3ꢁ result in one superimposing peak at around 33.1ꢁ
(Figs. 1(a), (b), and Fig. 2(a)). In Table 1 are listed the particle
and surface parameters of some studied LaCoO3 materials.
The particle size does increase with increasing calcination
temperature of LaCoO3. It was estimated by the Sherrer meth-
od that the mean particle diameters of new LaCoO3 after cal-
cination at 120, 500, and 800 ꢁC were 10.5, 17.0, and 42.0 nm,
respectively and those of conventional LaCoO3 after calcina-
Elementary analyses of La, Co, Na, and Zn were performed by
atomic absorption spectroscopy on a Perkin-Elmer 1100B spec-
trometer.
Results and Discussion
Figures 1, 2, 3, 4, and 5 show comparative XRD patterns, IR
spectra, and TPR profiles of new and conventional LaCoO3.
The XRD spectra of new LaCoO3 (Fig. 1) are consistent with
those of conventional LaCoO3 (Fig. 2). Note that small peaks
ꢁ
tion at 600 and 950 C were 17.0 and 38.1 nm, respectively.
Correspondingly, N2 adsorption–desorption measurements
indicated that the BET specific surface areas of new LaCoO3
after calcination at 120, 500, and 800 ꢁC were 77.9, 56.1,
and 32.7 m2/g, respectively. These data confirm the relation-
ship that the greater is the particle size, the smaller is the spe-
cific surface area due to the sintering of perovskite crystals as
the calcination temperature of LaCoO3 increases. It can also be
noticed from Figs. 1 and 2 that the perovskite structure of new
LaCoO3 remains plausibly unchanged after calcination at 800
ꢁC, because only a small extra peak at 36.9ꢁ for trace amounts
of Co3O4 is observed, and the perovskite structure of conven-
tional LaCoO3 is retained perfectly stable after calcination at
1400
1200
1000
(c)
800
600
400
200
0
(b)
(a)
ꢁ
950 C.
20
30
40
50
60
70
80
The IR spectrum of conventional LaCoO3 exhibited a set of
notable bands at 667 w, 594 s, 559 sh, 419 m, 337 m, and 178
m cmꢄ1 as shown in Fig. 3, which are in agreement with those
reported in the literature.26 In contrast, an IR spectrum of new
2
θ
/ degrees
Fig. 1. XRD patterns of new LaCoO3ꢁcalcined at (a) 120
ꢁC (77.9 m2/g); (b) 500 C; (c) 800 C.
ꢁ

1452 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Studies on LaCoO3
30
25
20
15
10
5
0.2
0.15
0.1
(d)
(c)
0.05
0
(b)
(a)
(b)
700
600
500
400
300
200
100
Wavenumbers (cm-1
)
Fig. 3. IR spectra of (a) conventional LaCoO3; (b) new
LaCoO3 (3 m2/g); (c) new LaCoO3 (45 m2/g); (d) new
LaCoO3 (35 m2/g).
(a)
900
0
0
100
200
300
400
500
600
700
800
1000
Temperature (°C)
Fig. 5. TPR profiles of (a) new LaCoO3 calcined at 500 ꢁC
(56ꢁ.1 m2/g); (b) conventional LaCoO3 under 5% H2/Ar at
10 C/min.
0.2
0.15
(c)
0.1
0.25
0.2
(b)
0.05
(a)
0
600
0.15
550
500
450
400
350
300
250
200
150
100
-1
)
Wavenumbers (cm
(b)
0.1
Fig. 4. IR spectra of new LaCoO3ꢁcalcined at (a) 120 ꢁC
0.05
0
(a)
ꢁ
(77.9 m2/g); (b) 500 C; (c) 800 C.
700
600
500
400
300
200
100
Wavenumbers (cm-1
)
LaCoO3 presented rather broad and ill-resolved bands. With-
out adding any grinding additives, LaCoO3 achieved by me-
chanical grinding, which displayed a specific surface area of
3 m2/g, resulted in a quite similar spectrum to that of conven-
tional LaCoO3 with regular broadening of all the bands
(Fig. 3(b)). After continued milling treatments in the presence
of NaCl and ZnO as additives, the specific surface areas at-
tained to 45 and 35 m2/g and the spectral bands further broad-
ened in the meantime (Figs. 3(c) and 3(d)). The 594 and 559
cmꢄ1 bands were superimposed into a large one and the 419
and 337 cmꢄ1 bands into another large one. By virtue of the
perfect XRD spectrum of LaCoO3 prepared by reactive grind-
ing with ZnO, the observed broad IR bands can still reasonably
be assigned to the structural vibrations of the LaCoO3 perov-
skite in spite of the bad spectral resolution. Furthermore, cal-
cination treatments of such a sample at higher temperatures
rarely altered the IR spectral shape, as shown in Fig. 4. After
calcination at 800 ꢁC, the appearance of a weak additional band
at 578 cmꢄ1 and a shift of the 357 cmꢄ1 band to 380 cmꢄ1 are
attributed to the formation of trace amounts of Co3O4 and
La2O3, referring to the IR spectra of Co3O4 and La2O3 pre-
sented in Fig. 6. This is consistent with the XRD result. The
IR spectral evolution in Fig. 4 implies that the IR band broad-
ening of new LaCoO3 is independent of the particle size of
LaCoO3. Such an IR spectral difference between new and con-
ventional LaCoO3 is not understood. The unexpected IR spec-
tral shape of new LaCoO3 may tentatively be attributed to a
considerable amount of oxygen defects in the bulk of LaCoO3.
The high oxygen defect density of new LaCo1ꢄxFexO3 perov-
skites has been assumed by us according to their high catalytic
activity for the oxidation of volatile organic compounds.10
New LaCoO3 showed a different TPR profile than that of
conventional LaCoO3 under identical experimental conditions,
as shown in Fig. 5. The TPR behavior of the latter is consistent
with that reported in the literature,8,27,28 two principal peaks
Fig. 6. IR spectra of (a) La2O3; (b) Co3O4.
ꢁ
appearing at 414 and 595 C and the second one being nearly
triple the first one in area. The TPR profile of the former cal-
cined at 500 C presented four visible peaks at 411, 530, 709,
and 860 ꢁC, suggesting that there would exist at least four
types of Co3þ sites in the crystallite structure of new LaCoO3.
The first peak was similar to that in the case of conventional
LaCoO3. The subsequent H2 consumptions became irregular
ꢁ
ꢁ
and the last principal peak was observed at 709 C in equiva-
lent area to the first one, unlike the case of conventional
LaCoO3. Evidently, the reduction chemistry of new LaCoO3
ꢁ
is complex. The temperature (800 C) at which new LaCoO3
is roughly fully reduced is 113 ꢁC higher than the correspond-
ing one (687 ꢁC) for conventional LaCoO3. At 640 ꢁC, the re-
duction of conventional LaCoO3 was nearly full while the re-
duction percent of new LaCoO3 reached only 67. This clearly
accounts for the fact that new LaCoO3 is more difficult to re-
duce than conventional LaCoO3. The emergence of multiple
TPR peaks for LaCoO3 may be due to a distortion of the cubic
perovskite lattice and the oxygen defect, which would possibly
create distinct Co3þ environments and reducibilities in the
crystallite structure of LaCoO3. Quantitative analysis indicated
that the reduction process
2LaCoO3 þ 3H2 ! La2O3 þ 2Co0 þ 3H2O;
ð1Þ
was completed on both new and conventional LaCoO3 after
TPR. The different reducibilities of Co3þ in new and conven-
tional LaCoO3 are related to the distinct natures of Co3þ sites.
In order to shed light on the effects of the physical proper-
ties of LaCoO3 on its chemical properties and structural stabil-
ity, we investigated the redox behavior of new and convention-
al LaCoO3 and explored their structural stability in 5% H2/Ar.
For selecting appropriate pretreatment conditions and experi-

L. Huang et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1453
aÞ
18
16
14
12
10
8
Table 2. Principal TPR Peak Maxima of New LaCoO3 as
a Function of Calcination Temperature
Calcination temperature/ꢁC
Principal peak
(b)
120
250
500
800
6
4
1
2
361
384
368
700
389
716
475
756
2
(a)
0
0
50
100
150
200
a) With a specific surface area of 77.9 m2/g after calcination
at 120 C.
Time (min)
ꢁ
Fig. 8. TPR profiles under 5% H2/Ar of new LaCoO3 cal-
ꢁ
cined at 500 C (56.1 m2/g) at (a) 10 C/min from 23 to
ꢁ
16
14
12
10
8
ꢁ
ꢁ
550 C followed by 2 h of isothermic heating at 550 C;
ꢁ
ꢁ
(b) 10 C/min from 23ꢁto 600 C followed by 2 h of iso-
thermic heating at 600 C.
35
30
25
20
(d)
6
4
2
0
(c)
(c)
15
10
(b)
(a)
(b)
(a)
1000
5
0
200
400
600
800
1000
1200
0
Temperature (°C)
0
200
400
600
800
1200
Temperature (°C)
Fig. 7. TPR profiles under 5% H2/Ar of new LaCoO3 cal-
ꢁ
cined at 500 ꢁC (56.1 m2/g) at (a) 1 ꢁC/min; (b) 3 C/
ꢁ
Fig. 9. TPR profiles under 5% H2/Ar at 10 C/min of (a)
the sample in Fig. 8(a) following 1 h of re-oxidation at
500 ꢁC under 20% O2/He; (b) the sample in Fig. 8(b) fol-
ꢁ
ꢁ
min; (c) 5 C/min; (d) 10 C/min.
mental parameters, the TPR condition tests were conducted on
new LaCoO3 samples as functions of the calcination tempera-
ture and the heating rate under 5% H2/Ar. As can be seen in
Table 2, the principal TPR peaks shifted obviously to higher
ꢁ
lowing 1 h of re-oxidation at 500 C under 20% O2/He;
(c) Co3O4/La2O3 (Co:La = 1:1 atomic Ratio).
that the sample prior to TPR had the nature of the LaCoO3 per-
ovskite. It can be inferred that new LaCoO3 is reversibly re-
duced at below 550 C under 5% H2/Ar and that the perov-
skite lattice structure of new LaCoO3 is still retained after re-
ꢁ
reduction temperatures at a heating rate of 10 C/min as the
ꢁ
calcination temperature increased. The first peak maximum
went from 361 to 475 ꢁC and the second one from 684 to
756 ꢁC, while the calcination temperature was varied from
ducꢁtion under such conditions. When programmed reduction at
ꢁ
ꢁ
120 to 800 C. This demonstrates that increasing the calcina-
10 C/min from 23 to 600 C followed by 2 h of isothermic
reduction at 600 C was processed, another H2 consumption
ꢁ
tion temperature of LaCoO3 leads to an increase in the reduc-
tion temperature of LaCoO3, which can be explained by the
particle size effect on the reduction of LaCoO3. According
to Fig. 7, the TPR peaks shifted to lower reduction tempera-
tures as the heaꢁting rate decreased on new LaCoO3 samples
calcined at 500 C. This trend is expected. Tꢁhe peak maxima
curve was obtained (Fig. 8(b)), whose integral area gave a re-
duction degree of 85% on LaCoO3. Following 1 h of re-oxida-
tion at 500 ꢁC under 20% O2/He, the TPR profile of the result-
ing sample (Fig. 9(b)) deviated from that of new LaCoO3. This
illustrates that the perovskite latticeꢁstructure of new LaCoO3
is destroyed after reduction at 600 C ꢁand that the reduction
of new LaCoO3 at roughly above 600 C under 5% H2/Ar is
irreversible.
ꢁ
at 410 and 709 C shifted to 342 and 631 C, respectively,
while the heating rate was decreased from 10 to 1 ꢁC/min.
We reckon that the calcination temperature of 500 ꢁC is proper
for the pretreatment of new LaCoO3 and comparable to that
(600 ꢁC) used for conventional LaCoO3. Also, the heating rate
In a series of experiments, we discovered that increasing the
calcination temperature of LaCoO3 can enhance the stability of
the LaCoO3 perovskite under reducing conditions. For new
LaCoO3 calcined at 800 ꢁC, the perovskite lattice structure
is maintained stable after reduction at 600 ꢁC under 5% H2/Ar.
In the case of conventional LaCoO3, programmed reduction
at 10 ꢁC/min from 23 to 450 ꢁC followed by 2 h of isothermic
reduction at 450 ꢁC resulted in a H2 consumption curve as seen
in Fig. 10(a), whose integral area corresponded to a reduction
degree of 48% on LaCoO3. After 1 h of re-oxidation of the re-
sulting sample at 500 ꢁC, an identical TPR profile with that of
conventional LaCoO3 appeared (Fig. 1ꢁ1(a)). Programmed re-
ꢁ
of 10 C/min is the most practical for TPR experiments, con-
sidering both the reduction temperature and the reduction time
factors.
Figures 8, 9, 10, and 11 show the redox behaviors of new
LaCoO3 calcined at 500 ꢁC and conventional LaCoO3. In
the case of new LaCoO3, programmed reduction at 10 ꢁC/
min from 23 to 550 ꢁC followed by 2 h of isothermic reduction
at 550 ꢁC produced a H2 consumption curve as seen in
Fig. 8(a), whose integral area corresponded to a reduction de-
ꢁ
gree of 81% on LaCoO3. After 1 h of re-oxidation at 500 C
under 20% O2/He, the resulting sample was subjected to
ꢁ
duction at 10 C/min from 23 to 500 C followed by 2 h of
ꢁ
ꢁ
TPR from 23 to 900 C at 10 C/min. A similar TPR profile
to that of new LaCoO3 was observed (Fig. 9(a)), suggesting
isothermic reduction at 500 ꢁC led to another H2 consumption
curve (Fig. 10(b)), whose integral area gave a reduction degree

1454 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Studies on LaCoO3
18
16
14
12
10
8
(f)
1400
1200
1000
800
600
400
200
0
(e)
(b)
(a)
(d)
6
(c)
(b)
4
2
(a)
0
20
30
40
50
60
θ /degrees
70
80
90
0
20
40
60
80
100
120
140
160
180
2
Time (min)
ꢁ
Fig. 12. XRD patterns for new LaCoO3 calcined atꢁ500 C
(56.1 m2/g) (a) after TPR under 5% H2/Ar at 10 C/min
from 23 to 400 ꢁC followed by 2 h of isothermic heating at
400 ꢁC; (b) after TPR under 5% H2/Ar at 10 ꢁC/min from
23 to 500 ꢁC followed by 2 h of isothermic heating at 500
Fig. 10. TPR profiles of conventional LaCoO3 under 5%
ꢁ
ꢁ
H2/Ar at (a) 10 C/min from 23 to 450 Cꢁ followed by
ꢁ
2 h of isothermic heating at 450 C; (b) 10 C/min from
23 to 500 ꢁC followed by 2 h of isothermic heating at
ꢁ
500 C.
ꢁC; (c) after TPR under 5% H2/Ar at 10 C/min from 23
to 550 ꢁC followed by 2 h of isothermic heating at 550 ꢁC;
ꢁ
25
20
ꢁ
(d) after 1 h of re-oxidation at 500 C under 20% O2/He
following (c); (e) after TPR under 5% H2/Ar at 10 ꢁC/min
from 23 to 600 ꢁC followed by 2 h of isothermic heating at
(b)
15
ꢁ
ꢁ
600 C; (f) after 1 h of re-oxidation at 500 C under 20%
O2/He following (e).
10
5
(a)
800
0
0
100
200
300
400
500
600
700
900
(e)
700
Temperature (°C)
600
(d)
Fig. 11. TPR profiles under 5% H2/Ar at 10 ꢁC/min of (a)
the sample in Fig. 10(a) following 1 h of re-oxidation at
500 ꢁC under 20% O2/He; (b) the sꢁample in Fig. 10(b) fol-
lowing 1 h of re-oxidation at 500 C under 20% O2/He.
500
(c)
400
300
(b)
200
100
(a)
0
of 70% on LaCoO3. However, the sample could not have the
TPR profile of conventional LaCoO3 as shown in Fig. 11(b),
20
30
40
50
60
θ /degrees
70
80
90
2
ꢁ
following 1 h of re-oxidation at 500 C. The results demon-
ꢁ
Fig. 13. XRD patterns for conventional LaCoO3 (a) after
TPR under 5% H2/Ar at 10 ꢁC/min from 23 to 400 ꢁC fol-
lowed by 2 h of isothermic heating at 400 ꢁC; (b) after 1 h
of re-oxidation at 500 ꢁC under 20% Oꢁ2/He following (a);
(c) aꢁfter TPR under 5% H2/Ar at 10 C/min from 23 to
strate that only at roughly below 450 C can the perovskite
lattice structure of conventional LaCoO3 maintain its stability
under 5% H2/Ar.
These TPR results are evidenced by XRD and IR data.
Figures 12 and 13 show the XRD evolution during the redox
treatment process of new and conventional LaCoO3. After
ꢁ
450 C followed by 2 h of isothermic heating at 450 C;
(d) after 1 h of re-oxidation at 500 ꢁC under 20% O2/
He following (c); (e) after TPR under 5% H2/Ar at 10
ꢁ
the reduction of new LaCoO3 at 400 and 500 C under 5%
ꢁ
H2/Ar, following the same procedure as mentioned above,
the resulting samples gave the same XRD pattern as that of
LaCoO3 with lower diffraction intensities. When the reduction
ꢁC/min from 2ꢁ3 to 500 C followed by 2 h of isothermic
heating at 500 C.
ꢁ
temperature was raised to 550 C, a slight downward shift of
In contrast, the XRD spectrum of conventional LaCoO3 was
ꢁ
peaks of LaCoO3 occurred together with the appearance of a
weak peak at 44.5ꢁ. The latter peak isꢁassigned to Co0. Re-ox-
idation of the reduced sample at 500 C restored the peak po-
sitions of LaCoO3 and slightly increased the peak intensity.
The simultaneous appearance of a weak peak at 37.1ꢁ is attrib-
uted to the formation of a trace amount of Co3O4 by the oxi-
dation of a trace amount of Co0. The XRD observations turn
out to show that the perovskite lattice structure of new LaCoO3
essentially remains unchanged, having experienced reduction
at below 550 ꢁC under 5% H2/Ar. Once the reduction temper-
ature reached 600 ꢁC, all peaks of LaCoO3 almost disappeared
along with the concomitant appearance of a peak of Co0 at
observed only after reduction at below 450 C under 5% H2/
Ar, as shown in Fig. 13. Re-oxidation at 500 ꢁC did not modify
the XRD pattern of LaCoO3 except that it enhanced the dif-
ꢁ
fraction intensity for reduction at 450 C. Reduction at above
500 ꢁC resulted in the disappearance of all the peaks of
LaCoO3 and the appearance of a peak of Co0 at 44.3ꢁ. It is
evident that the perovskite lattice structure of conventional
LaCoO3 is less stable than that of new LaCoO3 under identical
reducing conditions.
Figures 14 and 15 show the IR spectral evolution during the
redox treatment processes of new and convꢁentional LaCoO3.
After the reduction of new LaCoO3 at 500 C under 5% H2/
Ar according to the same manner as mentioned above, the re-
sulting sample displayed three broad bands at 546, 370, and
166 cmꢄ1 (Fig. 14(a)). This spectrum resembles that for new
ꢁ
44.4ꢁ. Subsequent re-oxidation at 500 C could result in no
peak except that of Coꢁ3O4 at 37.1ꢁ. It is thus clear that the
reduction at above 600 C under 5% H2/Ar caused a collapse
of the perovskite lattice structure of new LaCoO3 via the
sintering of cobalt atoms.
ꢁ
LaCoO3 calcined at 500 C (Fig. 4(b)) with a significant shift
of the 357 cmꢄ1 band to 370 cmꢄ1, indicating that the reduced

L. Huang et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1455
destroyed with the formation of Co0/La2O3 after reduction at
0.35
0.3
ꢁ
600 C under 5% H2/Ar.
In the case of conventional LaCoO3, reduction at 400 C
ꢁ
(d)
(c)
0.25
0.2
gave rise to six broad bands at 654 sh, 602, 540, 405 sh,
339, and 175 cmꢄ1, as can be seen in Fig. 15(a). The band po-
sitions are compatible to those of conventional LaCoO3. How-
ever, the intensity of the bands at 602, 540, and 405 cmꢄ1 is
relatively low with respect to that of the bands at 594, 559,
and 419 cmꢄ1 in the spectrum of conventional LaCoO3
(Fig. 3(a)). This suggests that the reduced sample consists of
the remaining LaCoO3 and a new component. The latter does
not belong to Co0/La2O3, since the bands for La2O3 do not ap-
pear in Fiꢁg. 15(a). Extended exposure of the reduced sample to
air at 23 C brought about a slight increase in the spectral in-
tensity. This accounts for the fact that the new component is
hardly converted back to LaCoO3 by oxidation in air at ambi-
0.15
0.1
(b)
0.05
0
(a)
600
550
500
450
400
350
300
250
200
150
100
Wavenumbers (cm−1
)
Fig. 14. IR spectra for new LaCoO3 calcined at 500 ꢁC
(56.1 m2/g) (a) after TPR under 5% H2/Ar at 10 ꢁC/
min from 23 to 500 ꢁC followed by 2 h of isothermic heat-
ing at 500 ꢁC; (b) after 1 h of re-oxidation at 500 ꢁC under
20% O2/He following (a); (c) after TPR under 5% H2/Ar
at 10 C/min from 23 to 600 C followed by 2 h of iso-
thermic heating at 600 ꢁC; (d) after 1 h of re-oxidation
at 500 C under 20% O2/He following (c).
ꢁ
ent temperature. Upon re-oxidation at 500 C, the relative in-
ꢁ
ꢁ
tensity of the bands at 594, 553, and 411 cmꢄ1 to the bands at
337 and 177 cmꢄ1 increased largely as shown in Fig. 15(c).
The obtained spectrum coincides with that of conventional
LaCoO3. The results demonstrate that the perovskite lattice
structure of the new component is still retained and that
LaCoO3 can be effectively regenerated from this component
by oxidation at 500 ꢁC. The reduced sample is believed to have
a formula like LaCoO3ꢄx. When the reduction temperature was
raised to 500 ꢁC, the observed spectrum resembled that of
La2O3, as presented in Fig. 15(d). This implied that LaCoO3
is already transformed to Co0/La2O3 under such reducing con-
ditions.
ꢁ
0.25
0.2
(d)
0.15
0.1
0.05
0
(c)
(b)
(a)
700
600
500
400
Wavenumbers (cm-1
300
200
100
)
It is obvious that the results in studies of the redox chemis-
try of LaCoO3 by TPR, XRD, and IR are fairly consistent.
It appears apparent from the present work that the stability
of the LaCoO3 perovskite structure under a reducing atmo-
sphere is dependent on the redox behavior of LaCoO3 itself.
The higher the temperature for completing the reductive reac-
tion of LaCoO3 in Eq. 1, the more stable the LaCoO3 perov-
skite structure under a reducing atmosphere. Since the temper-
ature at which new LaCoO3 is completely reduced is markedly
higher than that for conventional LaCoO3, the former displays
a significantly higher destruction temperature of the perovskite
structure under a reducing atmosphere than the latter. As the
reduction temperature of LaCoO3 increases with increasing
calcination temperature of LaCoO3, the stability of the
LaCoO3 perovskite structure under a reducing atmosphere
tends to be reinforced by increasing the calcination tempera-
ture. The good stability of the perovskite structure under re-
ducing conditions is of significant importance for perovskite-
type oxide catalyzed hydrogenation reactions. As long as the
perovskite structure remains, the cobalt may be present in
the form of highly dispersed Co0 in the lattice. It is expected
that such perovskite catalysts are quite effective for hydroge-
nation. Furthermore, La3þ is known as a promoter of Co/SiO2
for CO hydrogenation.29 It may be imagined that LaCoO3
would be a competitive hydrogenation catalyst precursor.
Fig. 15. IR spectra for conventional LaCoO3 (a) after TPR
under 5% H2/Ar at 10 ꢁC/min from 23 to 400 ꢁC followed
by 2 h of isothermic heating at 400 ꢁC; (b) after 2 weeks of
re-oxidation at 23 ꢁC under 20% O2/He following (a); (c)
ꢁ
after 1 h of re-oxidation at 500 C under 20% O2/He fol-
ꢁ
lowing (b); (d) after TPR under 5% H2/Ar at 10 C/min
ꢁ
from 23 to 500 C followed by 2 h of isothermic heating
ꢁ
at 500 C.
sample contained the remaining LaCoO3 together with a new
component. Apparently, the perovskite lattice structure in the
reduced sample remained, since no additional bands such as
those for La2O3 were observed. After the re-oxidation of the
reduced sample at 500 ꢁC, there was little modification in
spectral pattern as shown in Fig. 14(b) relative to Fig. 14(a),
except for a retro-shift of the 370 cmꢄ1 band to 360 cmꢄ1. This
seems to show that the new component was re-oxidized to
LaCoO3. Thus, the reduced sample is assumedꢁto be in the
form of LaCoO3ꢄx. However, reduction at 600 C led to the
appearance of another set of bands at 501, 359, and 168
cmꢄ1 (Fig. 14(c)) instead of the bands at 546, 370, and 166
cmꢄ1 for new LaCoO3. These three new bands are similar to
those characteristic of La2O3 presented in Fig. 6(a). Since
the vibrational spectral features of chemical bonds in the per-
ovskite lattice no longer emerge, it is concluded that the
LaCoO3 perovskite was already disintegrated into Co0/La2O3.
Subsequent re-oxidation at 500 ꢁC produced a notable band at
579 cmꢄ1 (Fig. 14(d)), which corresponded to one of the vibra-
tional modes of the Co–O bond in Co3O4 (Fig. 6(b)). This con-
firms that the perovskite lattice structure of new LaCoO3 was
Conclusion
We have compared the XRD spectra, IR spectra, and TPR
profiles of new and conventional LaCoO3. These characteriza-
tion and test results reveal the differences in the physical prop-

1456 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Studies on LaCoO3
(2001).
10 V. Szabo, M. Bassir, A. van Neste, and S. Kaliaguine,
Appl. Catal., B, 37, 175 (2002).
11 S. V. Nguyen, V. Szabo, D. Trong On, and S. Kaliaguine,
Micropor. Mesopor. Mater., 54, 51 (2002).
12 N. K. Labhsetwar, A. Watanabe, and T. Mitsuhashi, Appl.
Catal., B, 40, 21 (2003).
13 R. J. H. Voorhoeve, J. P. Remeika, P. E. Freeland, and B.
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14 H. Arai, T. Yamada, K. Eguchi, and T. Seiyama, Appl.
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erty and surface chemistry between them. Although they both
have the same perovskite X-ray structure, new LaCoO3 pos-
sesses a smaller particle size and a greater specific surface
area. Moreover, new LaCoO3 is suggested to have many oxy-
gen defects. Compared to conventional LaCoO3, new LaCoO3
is obviously difficult to reduce to Co0/La2O3.
We have also studied the stability of the LaCoO3 perovskite
structure under reducing conditions by observing the redox be-
havior of LaCoO3. The reducibility of Co3þ in LaCoO3 deter-
mines the stability of the perovskite lattice structure. Due to
the higher temperature required for completely reducing new
LaCoO3, the perovskite lattice structure of new LaCoO3 is
more stable than that of conventional LaCoO3.
15 J. O. Petunchi, J. L. Nicastro, and E. A. Lombardo,
J. Chem. Soc., Chem. Commun., 1980, 467.
16 K. Ichimura, Y. Inoue, and I. Yasumori, Bull. Chem. Soc.
Jpn., 53, 3044 (1980).
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) with a
strategic grant.
17 K. Ichimura, Y. Inoue, and I. Yasumori, Bull. Chem. Soc.
Jpn., 54, 1787 (1981).
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9