Zirconia supported La, Co oxides and LaCoO3 perovskite: Structural characterization and catalytic CO oxidation

Article abstract of DOI:10.1016/S1381-1169(01)00422-8

ZrO₂-supported La, Co oxide catalysts with different La, Co loading (2, 6, 8, 12 and 16 wt.% as Lack₃) were prepared by impregnation of tetragonal ZrO₂ with equimolar amounts of La and Co citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. Catalytic CO oxidation was performed at 298-800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. Lack₃ perovskite was also detected for loading higher than 6%. XAS experiments suggested that at high loading Lack₃ and Co₃O₄ were formed, while at low loading, La, Co oxide species interacting with support, and hard to be structurally defined, prevailed. The catalysis study evidenced that the catalytic activity was due to segregated and highly dispersed cobalt oxide species.

Full text of DOI:10.1016/S1381-1169(01)00422-8

Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
Zirconia supported La, Co oxides and LaCoO perovskite:
3
structural characterization and catalytic CO oxidation
a
b
b
b
b,∗
S. Colonna , S. De Rossi , M. Faticanti , I. Pettiti , P. Porta
a
Istituto di Struttura della Materia, CNR, Via Fosso del Cavaliere 100, 00133 Rome, Italy
Centro di Studio, Struttura e Attività Catalitica di Sistemi di Ossidi’, CNR, c/o Dipartimento di Chimica,
Università degli Studi di Roma, La Sapienza’, Piazzale Aldo Moro 5, 00185 Rome, Italy
b
Received 19 July 2001; received in revised form 12 October 2001; accepted 12 October 2001
Abstract
ZrO2-supported La, Co oxide catalysts with different La, Co loading (2, 6, 8, 12 and 16 wt.% as LaCoO3) were prepared
by impregnation of tetragonal ZrO2 with equimolar amounts of La and Co citrate precursors and calcination at 1073 K. The
catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface
area determination. Catalytic CO oxidation was performed at 298–800 K. XRD revealed the presence of tetragonal zirconia
with traces of the monoclinic phase. LaCoO3 perovskite was also detected for loading higher than 6%. XAS experiments
suggested that at high loading LaCoO3 and Co3O4 were formed, while at low loading, La, Co oxide species interacting with
support, and hard to be structurally defined, prevailed. The catalysis study evidenced that the catalytic activity was due to
segregated and highly dispersed cobalt oxide species. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Zirconia-supported La; Co perovskites; CO oxidation
1
. Introduction
One of the main drawbacks of the perovskites in
catalytic investigations is their very low surface area
2
−1
Oxides with perovskite structure (ABO3) and con-
(some m g ) when they are prepared by solid state
reaction of the oxide precursors at high temperature
[4,5]. Although, a method based on citrates precur-
sors [6,7] allows the production of perovskites even
taining transition metal ions have attracted consider-
able interest for many years in the field of material
science and heterogeneous catalysis [1–3]. A great
variety of metal ions can be introduced in the per-
ovskite structure provided that their ionic radii fit the
sizes of the 12 coordinated A and octahedral B sites,
e.g. rA > 0.90 Å and rB > 0.51 Å. Therefore, the
ABO3 perovskite lattice can be considered an ideal
host matrix for many transition or nontransition metal
ions.
2
−1
with a few tens (m g ) of specific surface area after
calcination at 1073 K [8–11], rapid sintering at higher
temperatures cannot be avoided. We tried to extend
the interface between the active material and gas
phase by dispersing the perovskite components on a
suitable support. In this respect zirconia seems to be
a good choice because the interaction with the active
phase is generally not strong enough as to favor bonds
formation of the various oxides with the support
therefore preventing the formation of the active com-
pound and not so weak to cause the aggregation of
∗
Corresponding author. Tel.: +39-6-4991-3378;
fax: +39-6-490-324.
E-mail address: piero.porta@uniroma1.it (P. Porta).
1
381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00422-8

1
62
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
the active components in large particles with loss of
exposed surface area. Note that the specific activity of
supported LaCoO3 and La0.8Sr0.2CoO3 perovskites
in the oxidation of propane was found to be signifi-
cantly enhanced only through the dispersion of a few
layers on ZrO2 [12,13].
In a previous paper devoted to La, Mn oxide sys-
tems supported on ZrO2 [14] we observed that at low
loading metal oxides were dispersed on the surface
with low catalytic activity. Samples with high loading
evidenced the presence of perovskite-type species and
showed higher activity for both methane combustion
and CO oxidation.
It was then interesting to study the analogous La,
Co oxide systems supported on ZrO2 in order to in-
vestigate the nature of the species formed at various
loadings and study the catalytic behavior towards CO
oxidation. In this paper we report the results of this
investigation.
383 K until dryness and the samples, after grinding,
were calcined at 1073 K for 5 h. For comparison, un-
supported LaCoO3 was also prepared by the citrate
method as reported in [9].
2
−1
BET surface area of samples (SA, m g ) was
evaluated by nitrogen adsorption at 77 K in a vac-
uum glass apparatus. Phase analysis was performed
by X-ray powder diffraction using a Philips PW
1029 diffractometer with Ni-filtered Cu K␣ radiation.
◦
◦
Spectra were recorded in a 2θ range from 20 to 60 .
X-ray absorption spectroscopy (XAS) measure-
ments at the La and Co K-edges were done for
ZLaCo-2, ZLaCo-6 and ZLaCo-12 materials, and for
LaCoO3, Co3O4 and La2O3 as reference samples.
These measurements were done at the beam line
GILDA, ESRF, Grenoble (France) in fluorescence
detection for the catalysts samples and in transmis-
sion mode for the reference compounds. During the
data collection the samples were held at the liquid
nitrogen temperature. The beam line monochroma-
tor was equipped with two Si(3 1 1) crystals for Co
K-edge measurements and two Si(5 1 1) crystals for
La K-edge measurements. Powder samples were
deposited on millipore membranes or mixed to an
appropriate amount of boron nitride (BN) and pressed
into pellets. The XANES part of the experimental
signal was obtained by subtracting a linear pre-edge
and normalizing to one in correspondence of the first
EXAFS oscillation. The EXAFS analysis was per-
formed by using the complete FEFF8 package [16].
2
. Experimental
La, Co oxides/ZrO2 catalysts with different La–Co
loading (hereafter designated as ZLaCo with the num-
ber after the label indicating the nominal perovskite
content: 2, 6, 8, 12 and 16 wt.%) were prepared
by impregnation of tetragonal ZrO2 with equimolar
amounts of citrate precursors.
Tetragonal ZrO2 with a large surface area was pre-
pared by the method of Chuah et al. [15]. An aqueous
solution of 0.075 M ZrOCl2 was added, drop wise
2
Fourier transforms (FTs) for the signal k ␹(k) were
−
1
calculated in the range 2.5 < k < 11 Å by using a
Kaiser window. Structural information was obtained
by fitting the FT(R) function in the 1.0–4.0 Å range
(1.0–4.3 Å for the ZLaCo-12 sample, on La K-edge).
−1
(
1 ml min ) and under magnetic stirring, to a 5 M
NH4OH solution. The hydrous zirconia, after diges-
tion in its mother solution at 373 K for 48 h, was
filtered, washed with water until the AgNO3 test for
The CO oxidation with O was studied in a flow
2
–
Cl detection gave no opalescence in the washing
system in the range from room temperature to 800 K
solution, then dried in a furnace overnight at 383 K
and finally calcined at 1073 K for 5 h (raising slowly
the temperature by 1 K min ).
using 0.5 g of catalyst supported on a silica fritted
disk internal to a silica reactor vertically positioned
in a tubular electrical heater.
−
1
The dispersion of La, Co oxides on the ZrO2 sup-
port was performed by the citrate method [6–11].
Two solutions were added to a weighted amount of
ZrO2: an aqueous solution of citric acid was added
first and subsequently a solution containing La(NO3)3
and Co(NO3)2 in 1:1 proportions. The molar ratio
between the citric acid and the overall metal nitrates
was fixed at 1. The resulting solution was kept at
A Ni–NiCr thermocouple was positioned in the
middle of the catalyst bed. An ASCON XS propor-
tional programmer powered the heater and was set to
produce a linear temperature ramp of 1 K min from
298 to 800 K.
−1
The composition and the total flow rate of the
reactants were adjusted to 1% CO, 20% O2, balance
3
−1
He by volume and 100 cm STP min , employing

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
163
independent electronic flow controllers MKS model
259 driven by an MKS unit model 147. The space
except tetragonal zirconia (with only traces of the
monoclinic phase), was revealed. Fig. 1 shows that
in the ZLaCo-8, ZLaCo-12 and ZLaCo-16 samples,
in addition to the ZrO2 X-ray diffraction (XRD) pat-
tern, small peaks are visible. Their intensity increases
at the increase of La–Co loading, their position cor-
responding to that of the strongest peaks of LaCoO3
perovskite.
1
3
−1 −1
velocity resulted 12 000 cm STP h
g .
At time intervals of 20 min, that is every 20 K, a
3
sample of 1 cm of effluent from the reactor was sam-
pled and analyzed by gas-chromatography, using an
Alltech CTR 1 column made of two coaxial columns
held at RT. The inner column was packed with
®
Preparation method leads to the stabilization of
ZrO2 in its tetragonal phase characterized by a BET
Porapak , whereas the outer one contained a molecu-
lar sieve packing. A thermal conductivity detector was
used to reveal CO2, O2 and residual CO. The mass bal-
ance with respect to carbon was 100± 2%. Before each
run a standard pretreatment at 773 K with oxygen in
2
−1
surface area of 82 m g even after 5 h calcination at
073 K, making it suitable as catalyst support. Disper-
1
sion of active components and following calcination
step yielded final surface areas of ZLaCo samples in
3
−1
flow at 100 cm STP min for 0.5 h was carried out.
2
−1
the range 60–80 m g
.
First derivatives of the XANES spectra are shown
in Fig. 2. More evidently at high cobalt loading, the
first derivative profiles show some structures at lower
energy with respect to the maximum that are typical
for Co3O4. It seems that, with increasing Co content,
3
. Results and discussion
Phase analysis, performed by X-ray powder diffrac-
tion, showed that up to 6% loading no other phase,
Fig. 1. XRD patterns of ZrO2, LaCoO3 and ZLaCo catalysts. Asterisks indicate the most intense X-ray lines of LaCoO3.

1
64
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
Fig. 2. First derivatives of the Co K-edge XANES spectra for
Co3O4, LaCoO3 and ZLaCo supported systems.
cobalt oxide can be formed on the surface of the
zirconia support, thus inhibiting the formation of the
perovskite phase.
This hypothesis was confirmed by the EXAFS anal-
ysis. The fine structure of the samples was investi-
gated at the Co K-edge (for ZLaCo-2, ZLaCo-6 and
ZLaCo-12) and at the La K-edge (for ZLaCo-2 and
ZLaCo-12).
2
Fig. 3. Co K-edge: FTs of the k χ(k) signals for the ZLaCo-2,
ZLaCo-6 and ZLaCo-12 samples compared to pure LaCoO . The
continuous line represents the experimental data and the dots are
3
the fit profiles.
Fig. 3 shows the FTs (solid lines) of the EXAFS
signals collected at the Co K-edge for the samples
compared to pure LaCoO3 (upper panel). In the same
figure the dotted lines represent the fits obtained by
using the standard EXAFS formula. For LaCoO3, the
FT presents a first shell peak at about 1.8 Å, second
shell peaks in the 2.0–4.0 Å range due to both Co–Co
and Co–La contributions and a broad, not very intense,
peak at about 5.0 Å. As to the supported samples, the
FTs show a broadening of the first shell peak, partic-
ularly evident for the ZLaCo-6 sample, revealing het-
erogeneity of the Co–O contributions due to surface
disorder probably related to the presence of different
cobalt-containing oxide phases. Moreover, for the FTs,
the greatest differences appear in the 2.0–5.0 Å range.
The ZLaCo-2 material shows the most intense second
shell peak at about 3.2 Å, while for the ZLaCo-6 and
ZLaCo-12 samples the most relevant second shell
peak is centered at about 2.5 Å. In addition, the Co
richer samples present a broadened third shell peak at
about 4.8 Å, a distance that is slightly shorter than that
of the broad peak appearing in the FT of pure LaCoO3.
The FTs of the LaCoO3 and Co3O4 reference com-
pounds were fitted by using the crystallographic bond
distances data [17,18], and leaving free the E0 shift,
2
the S term and the Debye–Waller factors. In the fitting
0
procedure of the ZLaCo samples two Co–O contribu-
tions were used to reproduce the near neighbors coor-
dination. The second shell structures were fitted using
both Co–Co and Co–La contributions. Debye–Waller,
2
0
E0 and S parameters were fixed to the value obtained
from the fit of the reference compounds. The coor-
dination numbers (N) and bond distances (R) were
left to vary in the fit. The Debye–Waller factors were

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
165
Table 1
Coordination numbers (N) and bond distances (R) obtained from EXAFS data for ZLaCo and LaCoO3 samples
Co K-edge
Co–O
Co–O
Co–Co
Co–Co
Co–La
Co–La
Co–Co
ZLaCo-2
N ± 0.5
2.1
1.96
2.4
2.09
–
–
–
–
0.5
3.62
4.2
3.80
3.8
3.93
R (Å) ± 0.02
ZLaCo-6
N ± 0.5
2.0
1.92
2.1
2.07
0.8
2.91
0.1
3.34
0.9
3.64
–
–
–
–
R (Å) ± 0.02
ZLaCo-12
N ± 0.5
3.4
1.92
1.4
2.08
1.4
2.93
0.3
3.25
1.0
3.65
–
–
–
–
R (Å) ± 0.02
LaCoO3
N
R (Å)
6.0
1.93
–
–
–
–
–
–
2.0
3.25
6.0
3.31
6.0
3.81
La K-edge
La–O
La–O
La–O
La–Co
La–Co
La–La
La–La
ZLaCo-2
N ± 0.5
5.7
2.49
4.5
2.60
3.2
3.27
2.1
3.51
1.8
3.77
1.2
4.01
–
–
R (Å) ± 0.02
ZLaCo-12
N ± 0.5
3.0
2.34
6.2
2.54
0.5
3.63
5.3
3.34
1.8
3.51
1.7
3.84
2.5
4.29
R (Å) ± 0.02
LaCoO3
N
3.0
2.43
6.0
2.69
3.0
3.00
2.0
3.25
6.0
3.31
6.0
3.81
–
–
R (Å)
fixed to avoid the strong correlation with the coordi-
nation numbers. The fit results, reported in Table 1,
reveal that in the ZLaCo-2 sample a La–Co mixed ox-
ide phase dispersed on the zirconia surface probably
occurred, whose structure is not equal to perovskite.
In fact, the fitted Co–Co (3.93 Å) and Co–La (3.62
and 3.80 Å) bond distances are longer than those ex-
pected for the LaCoO3 perovskite (Co–Co: 3.81 Å
and Co–La: 3.31 Å, from crystallographic data [18]).
In the samples with higher Co loading (ZLaCo-6
and ZLaCo-12), the prevailing formation of dispersed
Co3O4-like species together with some well dis-
persed La–Co mixed oxide was evidenced. In fact,
the fitted Co–Co bond distances (in the 2.91–2.93 and
of the support, and/or the presence of small crysta-
llites.
In order to verify these evidences, the EXAFS
analysis was also performed on La K-edge for the
ZLaCo-2 and ZLaCo-12 catalysts. Fig. 4 shows the
FTs for these samples compared to that of LaCoO3
(upper panel). As in Fig. 3 the solid lines are the ex-
perimental data and the dotted ones represent the fits.
In the 1.0–4.3 Å range, the FTs of the ZLaCo samples
show three most intense structures, the first is a dou-
ble peak in the range 1.0–2.2 Å, then a single peak
at about 2.6 Å (much more intense for the ZLaCo-12
compound) and a double peak in the 2.8–4.3 Å range.
These structures are shifted to higher distances in the
most concentrated ZLaCo-12 sample. Fits were per-
formed using three La–O, two La–Co and one La–La
contributions. For ZLaCo-12 a further La–La shell
was added as really improving the quality of the fit.
The Debye–Waller factors were fixed to the values
obtained from the fits of the reference compounds.
The results, reported in Table 1, confirm that, in the
3
.25–3.34 Å ranges, Table 1) resemble those of Co3O4
(2.85 and 3.35 Å, from crystallographic data [19]) and
a single Co–La contribution was found with a bond
distance at about 3.65 Å and a very low coordination
number (N ∼ 1) for both samples. In all the fits,
the reduced coordination numbers, found for all the
coordination shells, indicate a disorder on the surface

1
66
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
the two fitted La–Co bond distances, 3.34 Å, results
to be similar to the distance in LaCoO3 (3.31 Å) but
the other one, 3.51 Å, is still too long for attributing
properly to a perovskite structure. Also for the two
fitted La–La bond distances, the first, 3.84 Å, is simi-
lar to LaCoO3 (3.81 Å), the second one, 4.29 Å, is too
long for perovskite. So, the most reliable hypothesis
for the ZLaCo-12 catalyst is the formation of different
phases on the surface of the zirconia support: a mixed
La–Co oxide whose structure is evolving towards the
perovskite and some well dispersed single lanthanum
and cobalt oxides. Finally, in the ZLaCo-12 sample
the total coordination numbers of the La–Co and
La–La contributions are enhanced with respect to the
most diluted ZLaCo-2 sample, revealing a greater
aggregation of the La–Co mixed oxide on the surface
of the support at higher loading.
ZLaCo catalysts show activity in CO oxidation
already at room temperature, whereas ZrO2 is active
only above 550 K. In particular, the initial CO conver-
sion was 100% for ZLaCo-8, ZLaCo-12, ZLaCo-16
and for a sample of Co3O4 examined for refer-
ence, while the conversion was 90, 78, and 10%,
for ZLaCo-6, ZLaCo-2 and LaCoO3, respectively
(Fig. 5). Hence, a study of the dependence of the
activity on the temperature is possible only for the
support ZrO2 and the perovskite LaCoO3. The results
2
Fig. 4. La K-edge: FTs of the k χ(k) signals for the ZLaCo-2 and
ZLaCo-12 samples compared to pure LaCoO3. The continuous line
represents the experimental data and the dots are the fit profiles.
most diluted sample, a dispersed La–Co mixed oxide
phase was probably formed on the surface of the zir-
conia support. The two shorter La–O bond distances
are close to those of LaCoO3, but all the other dis-
tances are really longer, thus testifying against the
formation of a perovskite-type structure. Low values
of the coordination numbers for the La–Co and La–La
shells should also confirm a disorder on the surface of
the support, and/or the presence of small crystallites.
For the ZLaCo-12 sample, the situation seems to be
different and more complicated. In fact, the fit reveals
the presence of a very long La–O bond distance at
3
.63 Å, with low coordination numbers (N = 0.5),
which could be attributed to some well dispersed
lanthanum oxide species (in La2O3, La–O = 3.67 Å,
from crystallographic data [19]). Moreover, one of
Fig. 5. Percentage of CO conversion at 298 K on: (᭺) ZLaCo-2;
(
ᮀ) ZLaCo-6; (ꢀ) ZLaCo-8; (᭛) ZLaCo-12; (᭝) ZLaCo-16; (᭜)
LaCoO3; (᭹) Co3O4.

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
167
Fig. 6. Percentage of CO conversion vs. temperature for LaCoO3
and ZrO2.
Fig. 8. Deactivation with time on stream in isothermal runs at
298 K for Co3O4, LaCoO3, and ZLaCo-2, ZLaCo-16 supported
systems.
for these latter samples are shown in Fig. 6, where,
it can be seen that 100% conversion is achieved for
LaCoO3 at 500 K, at which temperature ZrO2 is still
inactive. From the Arrhenius plot (Fig. 7), apparent
activation energy values of 64 and 84 kJ mol can
be derived for LaCoO3 and ZrO2, respectively.
All the catalysts, except ZrO2, showed a continuous
deactivation with time on stream in isothermal runs at
ates, which do not decompose at room temperature
and progressively block the active sites. Other rea-
sons for the deactivation as, for instance, formation of
carbon residues are less probable due to the presence
of excess oxygen in the feed.
−
1
By following the deactivation curves it is possible
to establish a rank for the activity of the various cat-
∼
alysts, that is: ZLaCo-16 = ZLaCo-12 > ZLaCo-8 >
2
98 K even after many hours (Fig. 8). The deactiva-
∼
ZLaCo-6 = ZLaCo-2 (Fig. 5). Thus, the higher the
tion is most probably due to a strong chemisorption
of the product, possibly under form of surface carbon-
cobalt content the higher the activity in CO oxida-
tion. This finding together with the high activity of
Co3O4, compared with the low activity of LaCoO3
perovskite and the lack of activity at room tempera-
ture of the zirconia support, strongly suggests that on
all the supported systems the activity for CO oxida-
tion is due to the presence of segregated and highly
dispersed Co3O4, which escape detection by XRD.
The LaCoO3 perovskite, when present, only poorly
contribute to the catalytic activity.
4. Conclusions
XRD analysis for the zirconia-supported samples
shows only the tetragonal zirconia phase with traces of
monoclinic zirconia up to the loading of 6%. At higher
loading XRD also shows the presence of the perovskite
Fig. 7. Arrhenius plots for LaCoO3 and ZrO2.

1
68
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 180 (2002) 161–168
LaCoO3 phase. XAS analysis confirms the formation
of LaCoO3 together with cobalt oxide species dis-
persed on zirconia at high La–Co loading, whereas at
low loading the presence of disordered oxide species
strongly interacting with support is suggested.
The ZrO2 support does not contribute to the activ-
ity of supported sample due to its very low activity.
All the ZLaCo and LaCoO3 are active at room tem-
perature. However, they undergo deactivation with
time on stream due to the strong chemisorption of
the product most probably under form of carbonates,
which do not decompose at room temperature. The
activity of the supported samples is related to the
presence of dispersed Co3O4, which escape detec-
tion by XRD. The contribution to the activity of the
LaCoO3 perovskite when present is probably low.
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