Flame-synthesized LaCoO3-supported Pd. 1. Structure, thermal stability and reducibility

Article abstract of DOI:10.1016/j.jcat.2007.10.004

Nanosized LaCoO₃ (LCO) and 0.5 wt% Pd/LaCoO₃ (PdLCO) were synthesized in a single step by flame-spray pyrolysis (FP) and characterized by N₂ adsorption-desorption at 77 K (BET), electron microscopy (HRTEM, STEM-EDXS), in situ XRD, in situ fluorescence XANES and EXAFS (around the Pd K-edge), EPR, and H₂ TGA-TPR. The stability of the perovskite structure under different treatments and the location of Pd were addressed by calcination at 600 and 800 °C and successive reduction in 10% H₂/He to 300 and 600 °C. The as-prepared PdLCO exhibited a high surface area (ca. 100 m²/g). Palladium appeared to be finely dispersed on the FP material and was partially incorporated in the perovskite lattice. Calcination at 800 °C caused sintering and substantial incorporation of Pd at the B-site of the ABO₃ framework. EXAFS revealed that the Pd{single bond}O distance was shorter than in PdO and further decreased with increasing calcination temperature, simultaneously with the appearance of a Pd-La contribution. The reduction process involved both Pd and Co. In the 100-300 °C range, the reduction of Co³⁺ to Co²⁺ (from LaCoO₃ to La₂Co₂O₅) and the segregation of Pd in the form of metal particles occurred. The reduction of Co was already reversible at 120 °C, and the perovskite structure was restored after exposure to oxygen. In contrast, Pd remained in the metallic state. Therefore, the final structure of PdLCO after mild reoxidation consisted of Pd and Co particles supported on LaCoO₃. In contrast, reduction at 600 °C led to the formation of a Pd-Co alloy. The composition of PdLCO reduced at different temperatures is likely to strongly influence the catalytic processes involved in combustion exhaust after treatment.

Full text of DOI:10.1016/j.jcat.2007.10.004

Journal of Catalysis 252 (2007) 127–136
www.elsevier.com/locate/jcat
Flame-synthesized LaCoO3-supported Pd
1. Structure, thermal stability and reducibility
Gian Luca Chiarello a, Jan-Dierk Grunwaldt b, Davide Ferri b, Frank Krumeich b, Cesare Oliva a,
Lucio Forni a,∗, Alfons Baiker b
a
Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, v. C. Golgi 19, I-20133 Milano, Italy
b
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, 8093 Zurich, Switzerland
Received 18 July 2007; revised 25 September 2007; accepted 2 October 2007
Abstract
Nanosized LaCoO (LCO) and 0.5 wt% Pd/LaCoO (PdLCO) were synthesized in a single step by flame-spray pyrolysis (FP) and characterized
3
3
by N adsorption–desorption at 77 K (BET), electron microscopy (HRTEM, STEM-EDXS), in situ XRD, in situ fluorescence XANES and EXAFS
2
(around the Pd K-edge), EPR, and H TGA-TPR. The stability of the perovskite structure under different treatments and the location of Pd were
2
◦
◦
addressed by calcination at 600 and 800 C and successive reduction in 10% H /He to 300 and 600 C. The as-prepared PdLCO exhibited a
high surface area (ca. 100 m /g). Palladium appeared to be finely dispersed on the FP material and was partially incorporated in the perovskite
lattice. Calcination at 800 C caused sintering and substantial incorporation of Pd at the B-site of the ABO framework. EXAFS revealed that the
2
2
◦
3
Pd–O distance was shorter than in PdO and further decreased with increasing calcination temperature, simultaneously with the appearance of a
◦
3+
2+
Pd–La contribution. The reduction process involved both Pd and Co. In the 100–300 C range, the reduction of Co to Co (from LaCoO
3
◦
to La Co O ) and the segregation of Pd in the form of metal particles occurred. The reduction of Co was already reversible at 120 C, and the
2
2 5
perovskite structure was restored after exposure to oxygen. In contrast, Pd remained in the metallic state. Therefore, the final structure of PdLCO
◦
after mild reoxidation consisted of Pd and Co particles supported on LaCoO . In contrast, reduction at 600 C led to the formation of a Pd–Co
3
alloy. The composition of PdLCO reduced at different temperatures is likely to strongly influence the catalytic processes involved in combustion
exhaust after treatment.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Flame-spray pyrolysis; Pd/LaCoO perovskite; In situ fluorescence XANES; EXAFS; In situ XRD-QMS–TGA; Exhaust gas catalysis
3
1. Introduction
for example, exhibit higher selectivity to N2 compared with the
traditional Pt or Pd supported on Al2O3 or SiO2, when used as
catalysts for the selective reduction of NO by H2 (H2-SCR) un-
der lean-burn conditions [9–12]. Indeed, along with the reaction
occurring on the noble metal surface, the formation of different
active nitrogen-containing species adsorbed on the perovskite
surface has been observed. These species are reduced to N2
by hydrogen spillover from the noble metal [13]. An important
and successful example of the precious metal–perovskite com-
bination is LaFe0.95Pd0.05O3 [17,18]. The advantage of such
perovskite systems is that Pd can be reversibly incorporated into
the lattice on oxidation and segregated out as small particles.
This behavior represents a sort of self-regeneration mechanism,
which confers a superior resistance to the catalyst by prevent-
ing sintering of Pd particles. This material satisfies the stringent
Pd-based materials are well known highly active catalysts
for a wide range of heterogeneous reactions, from fine chem-
ical synthesis [1] to partial or full oxidation reactions [2–4].
The catalytic removal of pollutants from the exhaust of internal
combustion engines (e.g., as part of a three-way catalyst) is per-
haps one of their most important applications [5]. Among these,
Pd supported on perovskite-like mixed oxides are promising
materials in which the chemical properties of the noble metal
are coupled with those of the support [6–16]. These materials,
*
Corresponding author. Fax: +39 02 50314300.
E-mail address: lucio.forni@unimi.it (L. Forni).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.10.004

128
G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
vehicle ultra-low-emission standards and has been commer-
cialized for automotive exhaust aftertreatment in Japan [19].
LaCoO3 is particularly attractive, because it is one of the most
reducible ABO3-type perovskites [20] and displays a rich phase
behavior under reducing conditions [21,22], which can be sig-
nificantly influenced on addition of a noble metal such as Pd,
Pt, Rh, or Cu [11,22,23].
The usual synthetic route to perovskite-supported noble met-
als is based on conventional wet chemistry procedures, involv-
ing the preparation of the oxidic support, followed by loading
of the noble metal(s) by impregnation from aqueous solution.
These routes require several batch steps and are experimentally
tedious. Moreover, the resultant crystal phase before catalysis
is often not composed solely of a perovskite-supported noble
metal but rather is a mixture of metal oxides, possibly includ-
ing a perovskite.
Flame-spray pyrolysis (FP) [24,25] is an efficient and at-
tractive single-step method suitable for continuous large-scale
production of catalytic materials in the form of nanoparti-
cle powders having high BET specific surface areas (SSAs)
[26–32]. Several studies have investigated the preparation of
supported metals [28,32–37] and alloys [36,38–40]. Only re-
cently it has been shown that perovskites also can be prepared
by this method [41–43]. Therefore, the aim of the present
work was to prepare a 0.5 wt% Pd/LaCoO3 (PdLCO) cata-
lyst by FP and to determine the structure of Pd and of the
perovskite support through the detailed characterization of the
flame-made PdLCO material after preparation, during calci-
nation to 800 ◦C, and during successive reduction to 600 ◦C
and compare it with the Pd-free LaCoO3 (LCO). This mate-
rial is a potential candidate for the low-temperature H2-SCR
of NO in the presence of excess oxygen [11,44]. In partic-
ular, changes in the crystal phase were monitored using in
situ X-ray diffraction (XRD) coupled with quadrupolar mass
spectrometric–thermogravimetric analysis (QMS–TGA). The
changes in the oxidation state of palladium were monitored
in the fluorescence mode by in situ X-ray absorption near-
edge structure (XANES) and extended X-ray absorption fine-
structure (EXAFS) at the Pd K-edge. Finally, additional impor-
tant information was gained from the comparison of the high-
resolution electron microscopy (HRTEM) and STEM-EDXS
patterns of PdLCO and of a 0.5 wt% Pd/LaCoO3 sample (imp-
PdLCO) obtained by impregnation of LCO with a Pd salt.
All of the information collected here represents a solid basis
for the catalytic application of this material, particularly for the
interpretation of the catalytic behavior of this system in H2-
SCR under lean-burn conditions, which will be the target of a
future study [44].
at 60 ◦C, to obtain a 1:1 metal–ion ratio and a 0.15 M over-
all metal concentration (sample LCO). The desired amount of
Pd(CH3COO)2 (Fluka, purum) was added when preparing the
Pd-containing sample (PdLCO).
The liquid solution was fed at a rate of 4 mL/min by a micro-
annular gear pump (HNP Mikrosysteme model mzr 2905) at
the center of a vertical nozzle, together with 5 L/min of O2
(1 bar pressure drop across the nozzle). The obtained spray
was ignited, and the flame was supported by a premixed
methane/oxygen (1.0 L/min O2 and 0.5 L/min CH4) annular
flame (12 mm diameter, 0.15 mm thick) surrounding the noz-
zle. The powder thus produced was collected on a glass fiber
filter (Whatman GF/D, 25.7 cm diameter) connected to a vac-
uum pump (Busch Seco SV 1040C).
A 0.5 wt% Pd/LaCoO3 (impPdLCO) was also prepared by
incipient wetness impregnation of a portion of the FP-made per-
ovskite (LCO) with a solution of (NH4)2PdCl4 (Fluka, puriss.).
In detail, the LCO was precalcined at 800 ◦C and then dispersed
in an aqueous solution containing the desired amount of Pd, so
as to have a final 0.5 wt% of noble metal on the support. The
suspension was slowly stirred for 6 h and then evaporated in a
rotavapor. The powder thus obtained was then dried overnight
at 90 ◦C and finally calcined in flowing 10% O2/He at 500 ◦C
for 2 h before use.
2.2. Characterization methods
The BET SSA was measured by N2 adsorption/desorption
at 77 K on a Micromeritics ASAP 2010 apparatus, after out-
gassing in vacuo at 300 ◦C for at least 6 h. For HRTEM and
STEM, the material was dispersed in ethanol and deposited
onto a perforated carbon foil, supported on a copper grid. The
analysis was done on a Tecnai F30 microscope (FEI, Eind-
hoven; field emission cathode) operated at 300 kV. STEM im-
ages, obtained with a high-angle annular dark field (HAADF)
detector, reveal metal particles with bright contrast (Z contrast).
Energy dispersive X-ray spectroscopy (EDXS) was applied us-
ing an EDAX system attached to the Tecnai F30 microscope.
Electron paramagnetic resonance (EPR) and ferromagnetic res-
onance (FMR) spectra were recorded at 298 and 77 K using
a Bruker Elexsys spectrometer at the working frequency of
9.4 GHz. In situ XRD analysis was performed at the bending
magnet beamline B2 at HASYLAB, Hamburg, Germany. The
storage ring typically operates at 4.45 GeV with a ring cur-
rent of 80–120 mA. The powder diffractograms were obtained
above the Pd K-edge at λ = 0.4720 Å using a Si(111) double
crystal as a monochromator. The 2θ axis of the XRD patterns
was converted to the more common laboratory wavelength of
the CuKα radiation (λ = 1.5406 Å) for a better comparison.
The obtained patterns were compared with the Inorganic Crys-
tal Structure Database (ICSD) data for phase identification. The
average crystal diameter was calculated by fitting with a Gauss
function the profile of the reflection at 2θ = 47.7◦ (selected
as the most intense single peak) to calculate the integral peak
width β (rad) (i.e., the peak area/peak intensity ratio) and by
applying Scherrer’s equation:
2. Experimental
2.1. Catalyst preparation
La(CH3COO)3·xH2O (Aldrich, purity >99.9%, H2O 7.5
wt%, determined by TGA) and Co(CH3COO)2·4H2O (Merck,
purum) were dissolved in a mixture of propionic acid,
n-propanol, and water (5:4:1 vol%) under vigorous stirring
DXRD (nm) = λ (nm)/(β cosθ).

G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
129
XANES and EXAFS data were collected using a Si(111)
double-crystal monochromator in the step scanning mode at the
Norwegian–Swiss beamline (SNBL, BM01B) of the European
Synchrotron Radiation Facility (ESRF, Grenoble, France). The
spectra were obtained in transmission and fluorescence modes
at room temperature using a specially designed in situ cell [45].
The spectra were collected around the Pd K-edge (24.350 keV).
Because of the low Pd content and of the strong absorbance of
the perovskite matrix, the transmitted X-ray beam was rather
weak, leading to a low signal-to-noise ratio, so that only the
fluorescence mode was successfully used to record spectra. The
fluorescence signals from the 13 detector elements (13 element
Ge detector, Canberra) were directly preamplified and then fed
into the XIA digital pulse processing electronics (DXP 2X).
The PdKα fluorescence regions were selected and calibrated on
the basis of a full spectrum for each channel, collected before
the measured scans. The resulting 13 SCA data were added up
manually after each measurement, with the spectra background-
corrected and normalized using WINXAS 3.1 software [46].
Data fitting was performed in the R space on the Fourier-
transformed k3-weighted EXAFS functions (Fourier transform
in the range of 3.5–14 Å; S02 = 0.7). Theoretical scattering am-
plitudes and phase shifts of the Pd–Pd and Pd–O shells (first
shell only) were calculated using the FEFF code [47]. In both in
situ XRD and EXAFS experiments, the powder was pelletized,
crushed, and sieved to 50–100 µm. The reaction mixtures for in
situ oxidation and reduction were 21% O2/He and 5% H2/He.
Fig. 1. In situ XRD of PdLCO during calcination in 21% O /He at the indicated
2
temperatures: (∗) LaCoO ; (+) Co O ; (!) La CoO phases.
3
3
4
2
4
2.3. H2-TGA-TPR experiments
Thermogravimetric analysis during reduction with H2
(TGA-TPR) of LCO and PdLCO was performed in a Netzsch
STA 409 thermal analyzer [48] under the following operat-
ing conditions: 50 mL/min of 10% H2/He and a temperature
ramp of 5 ◦C/min up to 700 ◦C. The outlet of the thermal ana-
lyzer was connected to an Omnistar Pfeiffer Vacuum GSD 301
O2 quadrupolar mass spectrometer (QMS) through a heated
(150 ◦C) stainless steel capillary tube. Before the TPR exper-
iments, the as-prepared powder was treated in the same TGA
apparatus in flowing 20% O2/He, while heating at 5 ◦C/min
from room temperature up to 800 ◦C.
Fig. 2. HRTEM image and ED ring pattern (inset) of as-prepared FP-made
PdLCO.
The as-prepared material was stable up to 400 ◦C under ox-
idizing conditions; sintering occurred at higher temperatures.
The in situ XRD measurements during calcination up to 800 ◦C
(Fig. 1) showed an increase of peak intensity only at 600 ◦C.
At this temperature, the SSA decreased to 20 m2/g. After cal-
cination at 800 ◦C, the reflections of the perovskite became
narrower and more intense, and the SSA decreased even fur-
ther, down to 8 m2/g. The crystal sizes calculated by applying
Scherrer’s formula were 5.3 nm for the sample calcined up to
400 ◦C, 8.1 nm for the sample calcined at 600 ◦C, and 35.6 nm
for the sample calcined at 800 ◦C. In the latter case, a reflection
appeared at 2θ = 36.9◦, attributed to a segregated Co3O4 phase.
Such a segregation is strictly correlated with the presence of
Pd. Indeed, no Co3O4 formation was observed on FP-LCO af-
ter calcination at 800 ◦C for 3 h (see Fig. 7 of [44]). On the other
hand, a similar phenomenon was recently observed with flame-
made La0.9Ce0.1CoO3 [49], where the formation of the Co3O4
phase occurred during the calcination above 600 ◦C, after seg-
3. Results and discussion
3.1. Calcination of PdLCO at 800 ◦C
The materials prepared by FP demonstrated an SSA of
ca. 106 m2/g. According to the XRD pattern (Fig. 1), the as-
prepared powder had a perovskite-like crystal phase. No fea-
tures could be attributed to specific Pd-containing particles in
PdLCO.
The HRTEM image showed microaggregations of single-
crystal nanospheres, 4 to 10 nm in diameter (Fig. 2). The high
crystallinity and size homogeneity were also seen on electron
diffraction (ED) analysis (inset of Fig. 2). Indeed, the ED pat-
tern consisted of well-defined rings, confirming the high crys-
tallinity of the sample detected by HRTEM.

130
G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
Fig. 3. EPR spectra at 298 K (a, c and d) and at 77 K (b) of PdLCO: (a) as
◦
◦
◦
prepared, (b) calcined at 600 C, (c) calcined at 800 C, (d) calcined at 800 C
◦
and reduced at 600 C. The bump k is due to the instrument cavity and not to
the sample. The intensity of traces (a, b and c) was multiplied by a factor 10.
regation of a CeO2 phase. In the present case, the presence of
Pd likely induces an excess of B-site cations, thus provoking
the segregation of Co as Co3O4. The peaks of this phase are
not visible in the pattern of PdLCO calcined at 600 ◦C, proba-
bly because of the very small size of the crystals. The sintering
occurring after calcination at 800 ◦C produced larger crystals,
detectable by XRD.
Information on the oxidation state of Pd can be obtained by
EPR, because the paramagnetic 4d7 and 4d9 electron configu-
ration of Pd3+ and Pd+ can be distinguished from the diamag-
netic 4d8 and 4d6 configuration of Pd2+ and Pd4+. The forma-
tion of Pd3+ ions has been observed on oxidation of palladium-
containing catalysts at sufficiently high temperature [50–53].
In particular, well-characterized EPR spectra of Pd3+ species
in the g ≈ 2 spectral region have been reported with Pd-doped
yttria oxidized at 600 ◦C [53]. In contrast, such a spectrum was
not observed either in as-prepared PdLCO or after calcination
at 600 and 800 ◦C. This indicates that only Pd2+ and/or Pd4+
were/was present in PdLCO (Fig. 3). The absence of Pd3+ is a
major difference with respect to LaFePdOx [18], in which XPS
showed that Pd was predominately in this oxidation state in the
calcined and reoxidized material.
In the as-prepared material, a weak ferromagnetic resonance
spectrum was recorded (Fig. 3a) that disappeared after sample
calcination at 800 ◦C; therefore, this signal could be attributed
to the nanosized structure of these particles [54]. The residual
low-intensity broad EPR signal observed with the sample cal-
cined at 800 ◦C (Fig. 3c) is attributable to the segregated Co3O4
phase [51].
XANES and EXAFS at the Pd K-edge measurements of
PdLCO as-prepared, precalcined at 600 ◦C, and precalcined at
800 ◦C are reported in Fig. 4. In all of these samples, XANES
spectra (Fig. 4a) exhibits a typical white line at 24.37 keV, in-
dicating that the Pd is oxidized. This result is supported by the
Fourier-transformed EXAFS spectra (Fig. 4b) and data fitting
(Table 1), which show a clear contribution at 1.7 Å (scale not
corrected for the phase shift) due to the oxygen first-neighbor
shell, thus demonstrating that Pd is surrounded by oxygen. Note
3
Fig. 4. XANES fluorescence spectra (a) and Fourier transformed k -weighted
EXAFS spectra (b) taken in the fluorescence mode at the Pd K-edge of the
◦
as-prepared PdLCO and after calcination at 600 and 800 C.
that no evidence of the presence of Pd in oxidation state +4
was found in the EXAFS spectra at the Pd K-edge; however, the
Fourier-transformed EXAFS spectra of the as-prepared samples
and the samples calcined at 600 ◦C differ greatly from those of
pure PdO or of PdO supported on ZrO2 [45,55]. This is due to
the absence of a second shell at ca. 2.6 Å, typical of the Pd–
Pd distance in PdO (see the reference spectrum of pure PdO in
Fig. 4b) and the slightly shorter Pd–O distance compared with
that in PdO. This demonstrates that Pd was very finely dispersed
in the as-prepared FP-made material. Moreover, the FP-made
samples exhibited a contribution at 3.2 Å whose intensity in-
creased with increasing calcination temperature, accompanied
by a slight contraction of the Pd–O distance (Table 1, entries
1 to 3). Calculations of theoretical EXAFS spectra using the
FEFF code predicted a similar contribution due to the Pd–La
bond distance. This important result demonstrates that Pd in-
corporates into the ABO3 perovskite lattice at the B site and
that the degree of incorporation increases with increasing tem-
perature [56]. In contrast, FT-EXAFS of the sample calcined
at 800 ◦C showed a change in the Pd–Pd and Pd–La environ-
ments; in particular, besides the intense Pd–La contribution at
3.2 Å, a shoulder appeared more markedly at 2.7 Å, due to the
Pd–Pd distance in palladium-based species (very likely PdO,
although we cannot completely exclude the presence of some
metallic Pd). This change might indicate a partial segregation of

G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
131
Table 1
a
Fit of the EXAFS data at the Pd K-edge
2
Entry
Sample
Shell
CN
R
σ
ꢀE
Residual
(%)
0
2
(Å)
(Å )
(eV)
1
2
3
4
5
6a
6b
As-prepared PdLCO
PdLCO, calcined 600 C
PdLCO, calcined 800 C
impPdLCO, reduced 300 C
PdLCO, calcined 800 C, reduced 300 C
PdLCO, calcined 800 C, reduced 600 C
PdLCO, calcined 800 C, reduced 600 C
Pd–O
Pd–O
Pd–O
Pd–Pd
Pd–Pd
Pd–Pd
Pd–Pd
Pd–Co
5.3
5.6
6.3
8.1
6.9
6.4
7.5
4.3
2.013
2.008
2.008
2.73
2.73
2.68
0.0026
0.0025
0.0031
0.0069
0.0061
0.0069
0.0074
0.0074
11.9
12.0
11.2
1.8
1.8
12.0
2.3
2.5
4.0
8.9
8.1
2.8
13
◦
◦
◦
◦
◦
◦
◦
◦
◦
2.70
2.70
3.7
6.3
a
2
CN: coordination number; R: interatomic distance; σ : Debye–Waller factor; ꢀE : energy shift.
0
◦
Fig. 5. Room temperature XRD of LCO and PdLCO after calcination at 800 C.
metallic Pd particles, which reoxidize on cooling in an oxygen-
rich atmosphere [57]. Similarly, in a previous investigation on
FP-made Pd/Al2O3, Strobel et al. reported that during calcina-
tion up to 1000 ◦C, PdO-particles sintered and decomposed to
metallic Pd at T > 800◦C, but reoxidized back to PdO during
cooling at T < 600◦C [38].
Regardless, after calcination of PdLCO at 800 ◦C, Pd was
still predominately oxidized and within the perovskite lattice,
as confirmed by the comparison of the XRD patterns of PdLCO
and LCO after calcination at 800 ◦C (Fig. 5). A slight shift of
the reflections to higher θ values can be observed for PdLCO,
due to distortion of the elementary crystal cell caused by the
insertion of Pd.
3.2. Reduction of LCO
Fig. 6. Thermogravimetric analysis data (TGA, black line) and relative first
derivative (DTG, grey line) for reduction in 10% H /He of LCO (a) and
2
◦
PdLCO (b). Heating rate 5 C/min. Inset: horizontal translation and overlap-
The TGA profile (Fig. 6a) shows that the reduction of LCO
occurred in three steps, as reported in Table 2. Reduction of
LaCoO3 is well documented in the literature. In situ XRD
during reduction (see Fig. S1 in the supplementary material),
similar to those obtained by us, were recently reported for a
sol–gel (SG)-made LaCoO3 [11]. The SG sample reduced in
three steps, through “undetectable cobalt species” or, alterna-
tively, through the intermediate formation of La2CoO4, which,
however, was not clearly assigned in the diffractograms. In-
deed, the reduction of LaCoO3 through the intermediate phases
of the general formula Lam+1ComO3m+1 anticipates an irre-
versible decomposition of the perovskite into La2CoO4 (m = 1)
ping of the two TGA curves of (a) and (b).
and CoO. Such a pathway has been reported only for reduc-
tion processes of LaCoO3 at high temperature (T > 600◦C)
[58]. In contrast, the reduction to metallic Co and La2O3 at low
temperature (T < 600◦C) typically occurs through intermedi-
ates with nominal composition LaCoO3−δ [21]. In particular,
two different intermediate phases of the homologous series
LanConO3n−1 were identified: La3Co3O8 (n = 3, δ = 0.33)
and La2Co2O5 (brownmillerite type, n = 2, δ = 0.5), in which
the crystal structure retains the perovskite-type arrangement of

132
G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
Table 2
◦
Assignment of the processes occurring during the TGA-reduction of LCO preannealed at 800 C (Fig. 6a)
a
Step
T
Assignment
XRD phase
La Co O
t.w.l.
e.w.l.
◦
1A
2A
3A
350 C
400 C
550 C
LaCoO + (1/3)H → LaCoO
+ (1/3)H O
−2.2%
−1.1%
−6.5%
−2.2%
−1.1%
−6.5%
3
2
2.67
2
3
3
8
5
◦
LaCoO
+ (1/6)H → LaCoO + (1/6)H O
La Co O
2.67
2
2.5
2
2 2
◦
LaCoO + H → (1/2)La O + Co + H O
La O + Co
2 3
2.5
2
2
3
2
Overall step
a
LaCoO + (3/2)H → (1/2)La O + Co + (3/2)H O
The assignment was made on the basis of the crystal phases detected by in situ XRD at the same temperature and by comparing theoretical (t.w.l.) and
−9.8%
−9.8%
3
2
2
3
2
experimental (e.w.l.) wt% losses.
Table 3
◦
Assignment of the processes observed during the TGA-reduction of PdLCO preannealed at 800 C (Fig. 6b)
a
Step
1B
T
T
T
e.w.l.
Assignment
onset
◦
max
◦
offset
◦
100 C
144 C
165 C
−2.2%
LaCo(Pd)O → La Co(Pd) O
3
0
3
3 8
PdO → Pd
Co O → CoO
3
4
◦
◦
◦
0
2B
3B
4B
a
165 C
220 C
390 C
186 C
273 C
484 C
220 C
330 C
580 C
−0.9%
−1.1%
−5.6%
CoO → Co
◦
◦
◦
0
La Co(Pd) O → La Co O + Pd
3
3
8
2
2 5
0
◦
◦
◦
La Co O → La O + Co
2
2
5
2 3
The assignment was made on the basis of the in situ XANES and XRD results reported in Figs. 7 and 8, respectively.
the cations. Therefore, LaCoO3−δ is generally composed of
LaCoO3 and La3Co3O8 for 0.00 < δ < 0.33 and of La2Co2O5
and La3Co3O8 for 0.33 < δ < 0.50. These partially reduced
structures are thermodynamically unstable with respect to
LaCoO3 and are easily reoxidized after exposure to an oxygen-
containing atmosphere at relatively low temperature, but above
room temperature [58]. The reoxidation is also facilitated by the
relatively open crystal structure, in which the oxygen transport
within the matrix is facilitated. In contrast, the full reduction to
metallic cobalt and La2O3 is irreversible, at least at T < 500◦C,
with Co being reoxidized to CoO and not back to the perovskite
already at room temperature after prolonged exposure to air, as
confirmed by ex situ XRD measurements (not reported).
Crystal-phase transformations occurring during the reduc-
tion process of LCO, monitored by in situ XRD (see supple-
mentary material), showed that the LaCoO3 phase was stable up
to 280 ◦C, whereas its complete reduction occurred at 550 ◦C,
in full agreement with the TGA curve shown in Fig. 6a. Note
that the experimental weight loss given in Table 2 is in good
agreement with the theoretical weight loss referred to the sug-
gested transformations. This indicates that calcination at 800 ◦C
is likely sufficient to saturate all of the oxygen vacancies of
LCO, because the weight loss is due solely to loss of oxygen.
dramatically affected the stability of PdLCO under hydrogen
and lowered the temperature of reduction. This process oc-
curred through an additional step—the reduction of Pd2+ to
metallic Pd—besides the reduction of Co3+ to metallic Co.
Furthermore, two distinct reduction phenomena occurred, one
involving the bulk (i.e., the Pd-doped perovskite) and the other
involving the surface (i.e., the surface PdO and Co3O4 species).
The lattice reduction began at 100 ◦C, in parallel to the reduc-
tion of the surface PdO particles to metallic Pd. The formation
of metallic clusters likely triggered the dissociative adsorption
of H2, leading to the more active H adatoms. This would ex-
plain why this material was reduced at lower temperature, as
has been observed for other Pd-containing materials [59]. Fur-
thermore, the formation of the La3Co3O8 phase was probably
thermodynamically favored by the insertion at the B sites of the
framework Pd2+. Indeed, Pd2+ ions are better accommodated
in this structure, where two out of three B sites are occupied by
bivalent cations (B2+/B3+ ratio = 2).
The in situ XRD patterns collected during reduction to
700 ◦C, presented in Fig. 7, closely match the TGA-TPR pro-
file of Fig. 6b. The LaCoO3 phase was stable up to 100 ◦C.
The diffractogram recorded at 150 ◦C showed the formation
of La3Co3O8 (CoII/CoIII ratio = 2) [21]. The reflections rel-
ative to this phase became better defined at 200 ◦C, whereas the
Co3O4 peak at 2θ = 36.9◦ disappeared, replaced by a reflection
at 2θ = 42.5◦, attributed to CoO. The formation of La2Co2O5
[21] occurred at 300 ◦C, along with the reduction of CoO to
metallic Co particles on the surface (2θ = 44.2◦), which also
were detected by STEM and whose size can be estimated be-
tween 25 and 50 nm (see supplementary material). Finally, the
reduction of the whole bulk Co2+ to metallic Co occurred at
550 ◦C. In particular, the pattern recorded at 550 ◦C showed
two La2O3 phases: trigonal, in a hexagonal coordinate sys-
tem, and cubic. The latter appeared already in the diffractogram
recorded at 500 ◦C and disappeared at 700 ◦C. Moreover, a very
intense and typical [54] ferromagnetic signal spectrum of the
3.3. Reduction of PdLCO
Two species of Pd were observed at this point in PdLCO
calcined at 800 ◦C: segregated PdO particles and Pd2+ cations
inserted at the B site of the perovskite. The H2-TGA-TPR
curve of the FP-made PdLCO preannealed at 800 ◦C (Fig. 6b)
showed that in the 50–650 ◦C temperature range, the reduc-
tion proceeded in four steps, which are better resolved in the
first-derivative curve. The assignment of the phase transforma-
tions responsible for the weight loss was made on the basis of
the in situ XANES and XRD data (Table 3). The TGA curves
of LCO and PdLCO clearly indicate that the presence of Pd

G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
133
◦
Fig. 7. Normalized in situ XRD of PdLCO, precalcined at 800 C, during reduc-
◦
tion in 5% H /He at different temperatures up to 700 C. Patterns attribution:
2
(a) LaCoO ; (b) La Co O ; (c) La Co O ; (d) trigonal, in hexagonal coordi-
3
3
3
8
2
2 5
nate system La O and Co phases. The main peaks of the following phases:
2
3
(ꢀ) Co O ; (×) CoO; (+) cubic La O ; (∗) metallic Co; are indicated.
3
4
2 3
Co nanoparticles was detected by EPR analysis on PdLCO cal-
cined at 800 ◦C and reduced at 600 ◦C (Fig. 3d). The size of
these nanoparticles can be estimated by HRTEM as on the or-
der of 1 nm (see supplementary material).
Comparison of the two TGA curves of LCO and PdLCO
(inset of Fig. 6a) reveals an interesting feature. The third step
of LCO reduction (Co2+ to Co0) occurred through an initial
small weight loss in the 390–490 ◦C range (labelled as 3ꢀA in
Fig. 6a), followed by a rapid drop. This suggests that the re-
duction process involves first the more accessible Co cations
of the outer atomic layers, and that it extends to the whole
particle only at sufficiently high temperatures. In the case of
PdLCO, the former weight loss was absent, because the Co
cations involved were already reduced at T < 300◦C (step 2B).
Therefore, this region appeared flat (the circled zone in the in-
set of Fig. 6a). In fact, the sum of the weight losses of steps 2B
(−0.9%) and 4B (−5.6%) is −6.5%, equal to the weight loss
of step 3A. However, the major loss (step 4B) was identical to
that of LCO, but shifted to lower temperature. Therefore, the
weight loss of step 2B allows for quantification of the amount
of Co in the Co3O4 phase and thus the amount of surface metal-
lic Co forming after reduction at 300 ◦C. Indeed, representing
the weight loss due to the reaction CoO + H2 → Co + H2O, the
oxygen atoms leaving the solid as H2O were in 1:1 molar ratio
with respect to the Co2+ ions. The amount thus calculated cor-
responds to 14% of the overall Co or, in other words, 3.5 wt%
of the material. Such a high value suggests that the Co atoms
involved in formation of the Co3O4 phase were not only those
segregated from the matrix after substitution by Pd2+, but also
those present in the external layer and at the edge positions.
Their mobility is favored by the high calcination temperature
(800 ◦C), for example, through surface decomposition involv-
ing the segregated metallic Pd particles:
Fig. 8. In situ fluorescence XANES spectra at the Pd K-edge (a) and selected
◦
Fourier-transformed EXAFS spectra (b) of PdLCO preannealed at 800 C dur-
◦
ing reduction in flowing 5% H /He at different T up to 600 C. The arrows
2
indicate the shift with increasing temperature.
Indeed, a shoulder can be observed in the diffractogram of
PdLCO calcined at 800 ◦C at ca. 2θ = 31◦, where the main
reflection of the La2CoO4 phase dropped (Fig. 1). The peak
appears broad, because few lattice planes contributed to the
diffraction. However, it is noteworthy that XRD detected the
presence of diffuse scattering due to structural defects, typical
of FP-made materials. These defects may make these very ac-
tive catalysts in oxidation reactions (e.g., the flameless combus-
tion of hydrocarbons), because they facilitate oxygen mobility
within the matrix.
The transformations observed during reduction of PdLCO
were also followed by in situ XANES and EXAFS. The
XANES and the Fourier-transformed EXAFS data at selected
temperatures are reported in Fig. 8. The XANES spectra con-
firm that Pd reduction occurred in two steps: at 100 ◦C for a first
fraction of Pd (e.g., PdO particles), and at above 140 ◦C for the
Pd ions incorporated into the perovskite lattice. Complete Pd re-
duction was achieved at ca. 550 ◦C, that is, after the loss of the
perovskitic structure (compare with Fig. 6a). Fig. 8a shows a
clear change in the near-edge structure with increasing temper-
ature, similar to that reported previously for Pd–Co/SiO2 [60]
(but different from that reported for Pd/ZrO2 [45,55]), indicat-
ing a change in the electronic structure due to the progressive
2LaCoO3 + Pd → La2CoO4 + CoO + PdO.

134
G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
formation of a Pd–Co alloy. The Fourier-transformed EXAFS
data show that, compared with the theoretical Pd–Pd distance
in Pd foil, the Pd–Pd distance shifted to lower R values with in-
creasing reduction temperature. This shift was not observed in
the impPdLCO sample reduced at 300 ◦C (Fig. 8; Table 1). The
Pd–Pd distance in PdLCO contracted from 2.75 Å to a lower
value than expected in small noble metal nanoparticles. In all
cases, the coordination number was relatively small (<12, with
12 the value for bulk Pd metal), demonstrating that the par-
ticle size was rather small (ca. 30 Å). Moreover, the average
coordination number (CN) of Pd was smaller in PdLCO than in
impPdLCO (entry 4). Thus, more surface atoms were present
in PdLCO, and hence the particle size was smaller, resulting in
a greater Pd dispersion. PdLCO reduced at 600 ◦C exhibited a
significantly shorter Pd–Pd distance (entry 6) that can be rea-
sonably fitted only by assuming that Pd and Co are neighbors
in the first shell of a Pd–Co alloy. In fact, with this approach,
the residual error was only 3.7%, compared with 13% when
formation of the alloy is neglected. The CN of Pd in the alloy
(entry 6b), assuming dodecahedral coordination, revealed that
the Pd central atom has an average of 7 Pd atoms and 5 Co
atoms around it. Moreover, the total CN of 11.8 1 indicates a
considerably larger particle size with respect to the Pd particles
in PdLCO prereduced at 300 ◦C. The surface segregation of Pd,
principally as small particles (<10 nm), in FP-made PdLCO
after the reduction process has been confirmed by STEM-EDX
investigations as well (see supplementary material).
It is worth mentioning that a comparison of the TPR pro-
file of the Pd/LaCoO3 sample obtained by impregnation of a
SG-made LaCoO3 [11] with that of the FP-made PdLCO iden-
tified two major differences: (i) The reduction process of the
SG sample occurred in three steps instead of four, and (ii) the
first two steps were rather broad and consecutive in the 100–
400 ◦C range, whereas the overall reduction was completed at
ca. 600 ◦C instead of 500 ◦C. The missing reduction step in the
SG sample was likely due to the absence of the Co3O4 phase,
thus confirming that its formation is strictly related to the pres-
ence of Pd in the perovskite lattice. The lattice Pd generated
an excess of B-type cations, with the consequent segregation of
Co. However, the presence of Pd in the lattice made the per-
ovskite more accessible to reducing agents, so that it reduced at
lower temperatures.
◦
Fig. 9. Normalized XRD of PdLCO: (a) calcined at 800 C and reduced at
◦
◦
◦
◦
300 C; (b) calcined at 800 C, reduced at 300 C and reoxidized at 160 C
in flowing 21% O /He; (c) calcined at 800 C, reduced at 300 C and ex-
posed overnight to air at r.t.; (d) calcined at 800 C, reduced at 600 C and
◦
◦
2
◦
◦
exposed overnight to air at r.t. Phase attribution: (a) La Co O ; (b) LaCoO ;
2
2
5
3
(c) La Co O ; (d) La O (∗); and CoO (◦).
3
3
8
2 3
surface oxidation of these particles might have occurred already
at lower temperatures [12]. The temperature of 160 ◦C was cho-
sen because it is the temperature at which the maximum NO
conversion is reached in the NO/H2/O2 reaction for perovskite-
supported noble metals [9–13]. Importantly, XANES spectra
indicated that Pd remains in the metallic state after reoxida-
tion of the perovskite at 160 ◦C. Thus, the final composition of
the material obtained after calcination at 800 ◦C, reduction at
300 ◦C, and reoxidation at 160 ◦C can be written as 0.5 wt%
Pd/3.5 wt% Co/LaCo1−xO3−δ. This formulation indicates that,
unlike the traditional material, the FP-prepared material exhib-
ited surface metallic Co along with Pd particles. Because Pd
strongly affects Co reducibility (vide supra), this may anticipate
a possible synergy between Pd and Co nanoparticles and the
perovskite support in catalytic processes involved in the control
of exhaust emissions. Furthermore, the selective catalytic re-
duction of NO (SCR) can proceed efficiently with Co/SiO2 and
Co/Al2O3 if Co is well dispersed on the support surface [61,62].
On the other hand, PdLCO reduced at 600 ◦C produced a
Pd–Co/Co/La2O3 mixed phase. After overnight exposure to air
at room temperature or at 160 ◦C, this material oxidized to Pd–
Co/CoO–La2O3 (Fig. 9d), and no significant change occurred
in the XANES and EXAFS spectra. These findings indicate that
the Pd–Co alloy was more stable to reoxidation than the dis-
persed Co particles alone.
3.4. Stability of reduced PdLCO in O2-rich atmosphere
In the previous sections, we showed that reduction of
PdLCO at 300 ◦C produced Pd/Co/La2Co2O5. This phase was
thermodynamically metastable, and it reoxidized to Pd/Co/
La3Co3O8 after overnight exposure to air at room tempera-
ture (Fig. 9c). Furthermore, in situ XRD experiments (Fig. 9b)
showed that Pd/Co/La2Co2O5 oxidized to Pd/Co/LaCoO3 in
flowing 21% O2/He at 160 ◦C. In particular, no reflections
of Co3O4 or CoO were detected at this temperature. A weak
peak of the Co3O4 crystal phase at 2θ = 36.9◦ could be seen
only on the XRD pattern recorded at 250 ◦C, indicating that
the bulk reoxidation of the segregated Co particles occurred
at T > 160◦C. However, we cannot exclude the possibility that
It is noteworthy that the different oxidizability of the Co par-
ticles forming from reduction of the perovskite support with
size on the order of 1 nm compared with the greater than one
order of magnitude larger segregated Co particles is closely
connected to the size difference. Indeed, it is well known that
the smaller their size, the more readily these particles oxidize.
Finally, it should be mentioned that the formation of a Pd–
Co alloy is reputedly detrimental, because the lower melting
point of the alloy compared with that of metallic Pd promotes

G.L. Chiarello et al. / Journal of Catalysis 252 (2007) 127–136
135
particle growth at high temperature, with a consequent loss in
catalytic activity [63]. However, we found that the Pd–Co al-
loy was more active than metallic Pd in the low-temperature
(ca. 160 ◦C) NO/H2/O2 reaction [44].
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4. Conclusion
The main result of the present investigation is that some
properly selected activation thermal treatments allow one to
tune-up the structure of the FP-prepared Pd–LaCoO3-system.
The one-step FP synthetic route produced a pure perovskite-like
material doped with the precious metal, entering into the B site
of the perovskite lattice as Pd2+. This material exhibited signif-
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it interesting for catalytic applications. The material was stable
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The authors thank the Hamburger Synchrotronstrahlungsla-
bor (HASYLAB) at the Deutsches Elektronen-Synchrotron
for providing beamtime for the in situ XRD studies, and the
Swiss–Norwegian beamline at ESRF for providing in situ
EXAFS measurements in the fluorescence mode. They also
thank C. Baehtz at HASYLAB and H. Emmerick and W. van
Beek at ESRF are acknowledged for their kind support dur-
ing the experiments, and Dr. M. Maciejewski for the TGA
measurements and fruitful discussions. G.L.C. is grateful for
a scholarship from the University of Milano.
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Supplementary material
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Catal. Lett. 104 (2005) 9.
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Surf. Sci. 252 (2006) 7862.
The online version of this article contains additional supple-
mentary material.
Please visit DOI: 10.1016/j.jcat.2007.10.004.
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