XAS characterization and CO oxidation on zirconia-supported LaFeO3 perovskite

Article abstract of DOI:10.1016/S1381-1169(02)00231-5

ZrO₂-supported La, Fe containing catalysts were prepared by impregnation of tetragonal ZrO₂ with citrate-type precursors and calcination at 1073 K. Catalytic activity towards CO oxidation was tested at 400-900 K. XRD analysis for the La-Fe zirconia supported samples showed only the tetragonal zirconia phase with traces of monoclinic zirconia. At higher loading XRS also revealed the presence of perovskite LaFeO₃ phase. XAS analysis, carried out for the ZLaFe-2, -6, and -12 materials, showed the formation of LaFeO₃ dispersed on zirconia also at a loading < 6%. In addition to perovskite, a small quantity of very dispersed α-Fe₂O₃ should be formed. With regard to the catalytic performance, pure zirconia and ZLa-6 displayed very low activity in CO oxidation. ZLaFe-2 was substantially more active so pointing to Fe as essential for high catalytic activity. α-Fe₂O₃ was the more active catalyst even in comparison with LaFeO₃. ZLaFe catalysts displayed catalytic activity increasing with the iron content. Activity data suggested higher surface dispersion at lower loading.

Full text of DOI:10.1016/S1381-1169(02)00231-5

Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
XAS characterization and CO oxidation on zirconia-supported
LaFeO3 perovskite
S. Colonnaa, S. De Rossib, M. Faticantib, I. Pettitib,∗, P. Portab
a
Istituto di Struttura della Materia, CNR, Via Fosso del Cavaliere 100, 00133 Rome, Italy
b
Dipartimento di Chimica, Istituto IMIP (ex centro di Studio su “ruttura ed Attività Catalitica di Sistemi di Ossidi”SACSO), Università degli
Studi di Roma, CNR, Sapienza Piazzale Aldo Moro 5, 00185 Rome, Italy
Received 6 February 2002; accepted 23 April 2002
Abstract
ZrO2-supported La, Fe containing catalysts were prepared by impregnation of tetragonal ZrO2 with citrate-type precur-
sors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy
(XAS) and BET specific surface area (SA) determination. The catalytic activity towards CO oxidation was tested in the range
400–900 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaFeO3 perovskite was
detected for La, Fe loading higher than 6%. XAS suggested that LaFeO3 was formed at lower loading too. The catalytic
investigation evidenced that the presence of Fe is necessary for making highly active catalysts, whose performance improves
with increasing the iron content.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Zirconia-supported La–Fe perovskite; XAS; Catalytic CO oxidation
1. Introduction
riety of properties that these compounds exhibit
is due to the fact that around 90% of the metal-
lic elements in the periodic table can give stable
perovskite-type structures and also multicomponent
perovskites by partial substitution of cations in A
and/or B position. The only rule to be needed in
order to form a perovskite-type structure is a ionic
radii requirement introduced by Goldschmidt [3], the
Most of the catalysts used in modern industrial
applications is based on mixed metal oxides. Among
these, perovskite-type oxides represent a prominent
and very studied class of materials [1,2]. The per-
ovskite oxides have general formula ABO3 in which
A is the larger cation and B the smaller one. The
ideal perovskite-type structure is cubic with the A
cation at the center of the cube 12-fold coordinated
with oxygen anions and the B cations occupying
the corners of the cube in a six-fold coordination.
Deviations from the ideality with orthorhombic
(e.g. LaFeO3) or rhombohedral (e.g. LaMnO3 and
LaCoO3) symmetry are also known. The great va-
so called tolerance factor (t) defined by the equation
√
t = (rA + rO)/ 2(rB + rO), where 0.75 < t < 1.0
and rA, rB and rO are the ionic radii of the cations
A and B and of the oxygen anions, respectively.
The possibility of including several metal cations
into the structure accounts for the large variety of
reactions in which perovskites are used as cata-
lysts, for instance, total oxidation of hydrocarbons,
CO oxidation, NOx decomposition, hydrogenation
and hydrogenolysis reactions and so on. Moreover,
∗
Corresponding author. Fax: +39-06-490324.
E-mail address: ida.pettiti@uniroma1.it (I. Pettiti).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(02)00231-5

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S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
these oxides can be excellent materials not only for
catalytic investigations but also for establishing cor-
relations between reactivity and surface and/or bulk
properties.
2. Experimental
ZLaFe supported catalysts with different La–Fe
loading (2, 4, 6, 12 and 16 wt.%) were prepared by
impregnation of tetragonal ZrO2 with the appropriate
amount of citrate precursors.
The preparation of perovskite-type oxides is a very
important task, particularly if the goal is to obtain
high surface area (SA) materials good for catalytic
purposes. A method based on citrate precursors was
used [4] which allows to obtain perovskites with a
few tens m2 g−1 of specific SA after calcination at
1073 K. In our laboratory many perovskite-type ox-
ides have been already synthesized, characterized for
their solid state properties and catalytically tested for
methane and CO oxidation [5–11] and for methane
coupling [12]. Moreover, the dispersion of an ac-
tive phase on a suitable support is the most com-
mon and convenient way to increase the possibility
of contacting the active material and the reactant
species. Therefore, a project started in our laboratory
in order to investigate the structural and catalytic
properties of materials prepared by impregnating a
support with proper precursors and submitting this
system to the same thermal treatments used to pre-
pare the bulk perovskite. Some supported perovskite
phases have already been reported in the literature
[13–18]. In our investigation the precursors were
always citrate-type and tetragonal zirconia was the
first support to be prepared and used. The zirco-
nia support should represent a good choice thanks
to its high SA and moderate interactions with per-
ovskite. Previous investigations on ZrO2-supported
La–Mn [19] and La–Co [20] materials revealed that
in both systems perovskite was formed at a loading
higher than 6%. In addition, the La–Mn/ZrO2 sam-
ples showed the formation of disordered La and Mn
oxide species at low loading, while the La–Co/ZrO2
samples showed the formation of disordered La–Co
mixed oxide species at low loading, and of Co3O4 at
high loading.
Tetragonal ZrO2 with a large SA was prepared by
the method of Chuah et al. [21]. An aqueous solution
of ZrOCl2 0.075 M was added, dropwise (1 ml min−1
)
and under magnetic stirring, to a 5 M NH4OH solu-
tion. The hydrous zirconia, after digestion in its mother
solution at 373 K for 48 h, was filtered, washed with
water until the AgNO3 test gave no opalescence in the
washing 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−1).
The dispersion of LaFeO3 on the ZrO2 support was
performed by the citrate method [4,14]. Two solutions
were added to a weighted amount of ZrO2, an aque-
ous solution of citric acid was added first and subse-
quently a solution containing La(NO3)3 and Fe(NO3)3
in equimolar proportions. The molar ratio between the
citric acid and the overall metal nitrates was fixed at
1. The resulting solution was kept at 383 K until dry-
ness and the samples, after grinding, were calcined at
1073 K for 5 h. For a comparison between catalytic
activities, two samples containing respectively La and
Fe with a 6 wt.% loading (labeled ZLa−6 and ZFe−6)
were also prepared. For a phase check, unsupported
LaFeO3 was prepared by the citrate method [4,22].
Phase analysis was performed by XRD using a
Philips PW 1029 diffractometer with Ni-filtered Cu
K␣ radiation and equipped with a computer for data
acquisition and analysis (software APD-Philips).
BET SA of the samples (SA, m2 g−1) was evalu-
ated by nitrogen adsorption at 77 K in a vacuum glass
apparatus.
XAS measurements at the Fe K-edge were per-
formed for the ZLaFe−2, −6 and −12 supported
samples, and for LaFeO3 and ␣-Fe2O3 as reference
materials. These measurements were collected at the
beamline GILDA, ESRF, Grenoble (France) in fluo-
rescence detection mode for the catalyst samples and
in transmission mode for the reference compounds.
The beamline monochromator was equipped with two
Si (3 1 1) crystals. Powder samples were deposited
on Millipore membranes or mixed to an appropriate
amount of boron nitride (BN) and pressed into pellets.
In order to carry on and broaden the research in
the field of supported perovskites, the present study
reports the main results concerning the preparation,
XRD and XAS characterization and catalytic inves-
tigation towards CO oxidation of zirconia supported
La–Fe samples with different metals loading (here-
after, labeled ZLaFe−x where x stands for the wt.% of
loading of the supported species assumed as LaFeO3
perovskite in the calculation).

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
271
During the data collection samples were held at the
liquid nitrogen temperature. The XANES part of the
experimental signal was obtained by subtracting a
linear pre-edge and normalizing to one in correspon-
dence of the first EXAFS oscillation. The EXAFS
analysis was performed by using the complete FEFF8
package [23]. Fourier transforms (FTs) for the sig-
nal k3␹(k) were calculated in the range 2.7 < k <
11.5 Å−1 by using a Kaiser window. Structural infor-
mation was obtained by fitting the FT(R) function in
the 1.0–4.0 Å range.
phase) was revealed. Fig. 1 shows the XRD patterns
for all the ZLaFe supported samples and LaFeO3 un-
supported perovskite; asterisks indicate the peaks that
are related to the perovskite phase. Patterns for the
ZLa−6 and ZFe−6 samples (not reported in the fig-
ure) show only peaks related to the zirconia support.
In Table 1 the phases detected by XRD are reported.
For all the ZLaFe samples the SA values are in the
range 47–85 m2 g−1. The SA values for ZLa−6 and
ZFe−6 samples are 45 and 32 m2 g−1, respectively.
The ZrO2 support and the LaFeO3 and ␣-Fe2O3 ref-
erence compounds show SA values equal to 83, 6 and
2 m2 g−1, respectively. All the SA values are reported
in Table 1.
The structural investigation of the ZLaFe−2, −6
and −12 samples was undertaken by XAS spec-
troscopy at the Fe K-edge. The high local sensitivity
of this technique allows to better evidence structural
characteristics of the supported phases and to make
a comparison between samples with low and high
metals loading.
The CO oxidation with O2 was studied in a flow
system in the range 400–900 K using 0.5 g of catalyst
supported on a silica fritted disk internal to a silica re-
actor vertically positioned in a tubular electrical heater.
A Ni–NiCr thermocouple was positioned in the middle
of the catalyst bed. An ASCON XS proportional pro-
grammer powered the heater and was set to produce
a linear temperature ramp of 1 K min−1 in the experi-
mental range. The composition and the total flow rate
of the reactants was adjusted to 1% CO, 20% O2, bal-
ance He by volume and 100 cm3 STP min−1, employ-
ing independent electronic mass flow controllers MKS
model 1259 driven by a MKS unit model 147. The
space velocity resulted 12 000 cm3 STP h−1 g−1. At
time intervals of 20 min, that is every 20 K, a sample of
1 cm3 of effluent from the reactor was sampled and an-
alyzed by gas-chromatography, using an Alltech CTR
1 column made of two coaxial columns held at RT. The
First derivatives of the XANES spectra are reported
in Fig. 2. The supported ZLaFe samples show a maxi-
mum at 7126 eV resembling that of pure LaFeO3 per-
ovskite. A broadening of this maximum towards lower
energy values is also visible and could be attributed to
a small and dispersed amount of ferric oxide present
on the zirconia surface together with a perovskite-like
phase.
®
inner column was packed with Porapak , whereas
This hypothesis was confirmed by the EXAFS
analysis. Fig. 3 shows the Fourier transforms (solid
lines) of the EXAFS signals for the ZLaFe samples
compared to pure LaFeO3 and ␣-Fe2O3. In the same
figure the dashed lines represent the fits obtained by
using the standard EXAFS formula. For the ZLaFe
samples, FTs reveal intense and well defined first shell
features in the 0.8–2.0 Å range, due to oxygen nearest
neighbors, while peaks related to following coordina-
tion shells are not so evident. The first shell features
of the samples appear in the same position as those
shown in the FT of pure LaFeO3. In fact, for both the
LaFeO3 and ␣-Fe2O3 reference compounds the first
shell peaks are visible in the same 0.8–2.0 Å range,
but for ␣-Fe2O3 these peaks are shifted to lower R
values as compared with LaFeO3 (and with ZLaFe
samples too). In a previous work by Okamoto et al.
[24], ZrO2-supported Fe oxide catalysts, with an iron
loading ranging from 1 to 10 wt.%, were investigated
the outer one contained a molecular sieve packing. A
thermal conductivity detector was used to reveal CO2,
O2 and residual CO. The mass balance with respect
to carbon was 100 2%. Before each run a standard
pretreatment at 773 K with oxygen in flow at 100 cm3
STP min−1 and 0.5 h was carried out. For some cata-
lysts the run was repeated after the oxygen treatment
and the conversion values were reproduced to within
3% in the whole range of reaction temperatures.
3. Results and discussion
3.1. Structural properties
Phase analysis, performed by X-ray diffraction,
showed that up to 6% loading no other phase but
tetragonal zirconia (with traces of the monoclinic

272
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
Fig. 1. XRD patterns of ZrO2, LaFeO3 and ZLaFe samples. Asterisks indicate the most intense X-ray lines of LaFeO3.
by EXAFS spectroscopy revealing FTs very similar in
shape and peaks position to the FT of ␣-Fe2O3. This
evidence can be a further confirmation that some-
thing different from ␣-Fe2O3 has been formed on the
surface of the zirconia support in our ZLaFe samples.
The FTs of the LaFeO3 and ␣-Fe2O3 reference
compounds were fitted by using the main crystallo-
graphic bond distances data [25,26], and leaving free
the E0 shift, the S2 term and the Debye–Waller fac-
tors. In the fitting p0rocedure of the ZLaFe samples two
Table 1
Samples, XRD phases, SA and kinetic parameters for CO oxidation
Catalyst
XRD phases
molFe
SA
Ea
Kinetic constant (first-order with respect to CO) at 573 K
g−1 × 103 (m2g−1
)
(kJ mol−1
)
kW (molCO
h
kS (molCO
h
kFe(molCO
2
h
2
2
−1 g−1 × 102)
−1 m−2 × 104)
−1 molFe
−1
)
ZrO2
ZrO2T
ZrO2T
ZrO2T
ZrO2T
–
83
53
85
63
63
79
47
32
45
6
80
29
39
52
68
46
51
79
42
80
90
0.07
0.19
0.34
0.36
0.41
0.74
0.47
0.08
0.04
0.15
2.7
0.08
0.37
0.40
0.57
0.65
0.93
0.99
0.26
0.08
2.5
–
ZLaFe−2
ZLaFe−4
ZLaFe−6
ZLaFe−8
0.082
0.165
0.247
0.330
0.494
0.659
1.08
–
24
21
15
12
15
7.1
0.8
–
ZrO2T + LaFeO3
ZLaFe−12 ZrO2T + LaFeO3
ZLaFe−16 ZrO2T + LaFeO3
ZFe−6
ZLa−6
LaFeO3
␣-Fe2O3
ZrO2T
ZrO2T
LaFeO3
␣-Fe2O3
4.12
12.5
0.4
2.1
2
134
ZrO2T: tetragonal zirconia.

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
273
Fig. 2. First derivatives of XANES spectra for the ZLaFe samples compared to LaFeO3 and ␣-Fe2O3 reference compounds.
Fe–O contributions were used to reproduce the first
near neighbors coordination. The second shell struc-
tures were fitted by using both Fe–La and Fe–Fe con-
tributions. Debye–Waller, E0 and S02 parameters were
fixed to the value obtained from the fit of the refer-
ence compounds. The coordination numbers (N) and
bond distances (R) were left to vary in the fit. The
Debye–Waller factors were fixed to avoid the strong
correlation with the coordination numbers. The fit re-
sults, reported in Table 2, reveal that in the three sam-
ples the distances related to the Fe–O and Fe–La bond
length are very similar to those of LaFeO3 perovskite.
Fig. 3. Fe K-edge: Fourier transforms of the k3χ(k) signals for the ZLaFe−2, −6 and −12 samples compared to pure LaMnO3 and
␣-Fe2O3. The continuous line represents the experimental data and the dots are the fit profiles.

274
S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
Table 2
Coordination numbers (N) and bond distances (R) obtained from EXAFS data for ZLaFe, LaFeO3 and ␣-Fe2O3 samples
FeK-edge shell
Fe–O
Fe–O
Fe–La
Fe–La
Fe–Fe
Fe–Fe
ZLaFe−2
N
0.3
2.3
0.9
1.9
2.9
2.8
0.5
R (Å)
0.02
1.96
2.11
3.32
3.76
3.83
3.02
ZLaFe−6
N
0.3
2.5
1.96
1.3
2.09
2.6
3.28
3.39
3.71
3.2
3.78
0.5
3.03
R (Å)
0.02
ZLaFe−12
0.3
N
2.4
1.3
3.9
4.7
4.2
0.4
R (Å)
0.02
1.97
2.09
3.32
3.80
3.83
3.06
LaFeO3
N
R (Å)
4.0
1.99
2.0
2.10
4.0
3.35
4.0
3.76
6.0
3.93
–
–
␣-Fe2O3 shells
N
Fe–O
3.0
Fe–O
3.0
Fe–Fe
1.0
Fe–Fe
3.0
Fe–Fe
3.0
Fe–O
3.0
Fe–O
3.0
Fe–Fe
6.0
Fe–O
3.0
Fe–Fe
1.0
R (Å)
1.95
2.11
2.89
2.97
3.37
3.40
3.60
3.70
3.78
3.99
3.2. Catalytic properties
The coordination numbers are smaller than those ex-
pected, probably owing to a very disordered distribu-
tion of perovskite-like species on the zirconia surface
and/or to the presence of small crystallites. However,
the coordination numbers increase with increasing the
La and Fe content, indicating a greater degree of par-
ticles aggregation at higher loading.
The activity of zirconia support was measurable
starting from 570 K and the CO conversion was 70% at
800 K (Fig. 4). Similar conversion values were shown
by the catalyst ZLa−6, pointing to a low activity of
both zirconium and lanthanum oxide species.
As to the Fe–Fe bond lengths, the ZLaFe samples
show a distance at about 3.80 Å that is slightly shorter
than that of pure LaFeO3 (3.93 Å). Moreover, the fits
improved remarkably by adding a further Fe–Fe con-
tribution which resulted to be a short bond distance
(about 3.03 Å) with very small coordination numbers
(N = 0.4–0.5). Such a contribution could suggest the
presence of a small amount of very dispersed ␣-Fe2O3
which exhibits short Fe–Fe bond distances at 2.97 and
3.37 Å, respectively, and a longer one at 3.70 Å. The
simultaneous presence of different phases, mainly per-
ovskite and some ferric oxide, very disordered and
dispersed on the zirconia surface, should be confirmed
also considering that the 3.70 Å Fe–Fe distance, be-
longing to ␣-Fe2O3, could affect the values of the
longer distances found in the ZLaFe samples. These
bond lengths resulted to be shorter than 3.93 Å, that is
the value of Fe–Fe bond distance in LaFeO3. There-
fore, the fitted distances, found in the ZLaFe supported
samples, resulted to be intermediate values between
the Fe–Fe bond distances in the two oxidic phases.
Concerning the iron containing catalysts, they were
substantially more active in comparison with ZrO2
and ZLa−6. The transition metal is therefore essential
for developing high activity. Most probably this is so
because of the ease of surface local change of the
iron oxidation state with ensuing improvement of the
redox steps of the reaction. Except the more dilute
Fe containing catalyst, ZLaFe−2, that reaches 83%
conversion at 800 K, the iron based catalysts reached
100% conversion below 800 K, the ␣-Fe2O3 sample
showing 100% conversion at 580 K already.
The conversion level trends with the reaction tem-
perature, shown in Fig. 4, clearly indicates a quite large
range of apparent activation energy, Ea, values for the
different samples. Quantification of Ea was obtained
from Arrhenius plots (not shown) in which the kinetic
constants (first-order with respect to CO and pseudo
zeroth-order with respect to oxygen, present in large
excess) were reported versus the reciprocal of temper-
ature (K). The kinetic constants were evaluated in the
range from some percentage to about 80% conversion.

S. Colonna et al. / Journal of Molecular Catalysis A: Chemical 187 (2002) 269–276
275
Fig. 4. CO conversion as a function of the reaction temperature.
Table 1 reports the Ea data and the kinetic constants
per gram of catalyst (kW), per unit surface area (kS)
and per mol of iron (kFe), evaluated from the Arrhe-
nius plots at the reaction temperature of 573 K (in few
cases extrapolation was necessary).
traces of monoclinic zirconia. At higher loading XRD
also shows the presence of the perovskite LaFeO3
phase.
XAS analysis, performed for the ZLaFe−2, −6 and
−12 materials, reveals the formation of LaFeO3 dis-
persed on zirconia also at a loading lower than 6%.
In addition to perovskite, a small amount of very dis-
persed ␣-Fe2O3 should be formed.
As to the catalytic performance, pure zirconia and
ZLa−6 display very low activity in CO oxidation.
ZLaFe−2 is substantially more active so pointing to
Fe as essential for high catalytic activity. ␣-Fe2O3 is
by far the more active catalyst even in comparison
with LaFeO3. ZLaFe catalysts display catalytic activ-
ity increasing with the iron content and, as expected,
activity data suggest higher surface dispersion at lower
loading.
Based on kS (Table 1), among the pure compounds,
the order of activity is ␣-Fe2O3 ꢀ LaFeO3 ꢀ ZrO2.
This is true in all the temperature range, being the ap-
parent activation energies of the samples rather close
(90, 80, 80 KJ mol−1, respectively). Concerning the
ZLaFe supported samples, they show Ea values lower
than those of the pure compounds suggesting that the
structure of the active site and/or the enthalpy of the
CO adsorption change with composition in a complex
way. Table 1 shows that the kS values of the ZLaFe
samples are similar, however showing a definite in-
crease with iron content from 2 to 16%. A parallel
decrease in the kFe values at the increase of the iron
content for ZLaFe catalysts is consistent with a higher
dispersion of the active material at lower loading.
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