Surface structure of cobalt, palladium, and mixed oxide-based catalysts and their activity in methane combustion studied by means of micro-Raman spectroscopy

Article abstract of DOI:10.1002/jrs.5261

This paper reports the results of the study of surface properties and catalytic activity of cobalt, palladium, and mixed oxide catalysts supported on alumina. Catalytic materials were prepared by the wetness impregnation method directly from solutions of metal salts and using the ultrasound irradiation. The material surface was investigated by micro-Raman spectroscopy, and the activity in methane combustion was monitored. The results indicated the presence of different metal oxides on the catalysts' surfaces. Cobalt spinel Co₃O₄ was the main phase of the cobalt catalysts as well as species resulted from interactions with alumina support. In the case of palladium catalysts, the PdO was most prevalent. The article also discusses the observed variation in band position in the Raman spectra and compares the band assignment with literature data. The palladium and mixed oxide catalysts showed average conversion higher than 50% for methane combustion at 673.15?K (400?°C) especially those after sonication.

Full text of DOI:10.1002/jrs.5261

Received: 22 July 2017
DOI: 10.1002/jrs.5261
Revised: 28 August 2017
Accepted: 30 August 2017
R E S E A R C H A R T I C L E
Surface structure of cobalt, palladium, and mixed
oxide‐based catalysts and their activity in methane
combustion studied by means of micro‐Raman spectroscopy
1
2
3
1
Damian K. Chlebda
| Roman J. Jędrzejczyk
| Przemysław J. Jodłowski
| Joanna Łojewska
1
Faculty of Chemistry, Jagiellonian
University, Kraków, Poland
Abstract
2
This paper reports the results of the study of surface properties and catalytic
activity of cobalt, palladium, and mixed oxide catalysts supported on alumina.
Catalytic materials were prepared by the wetness impregnation method directly
from solutions of metal salts and using the ultrasound irradiation. The material
surface was investigated by micro‐Raman spectroscopy, and the activity in
methane combustion was monitored. The results indicated the presence of
different metal oxides on the catalysts' surfaces. Cobalt spinel Co O was the
Malopolska Centre of Biotechnology,
Jagiellonian University, Kraków, Poland
3
Faculty of Chemical Engineering and
Technology, Cracow University of
Technology, Kraków, Poland
Correspondence
Damian K. Chlebda, Faculty of Chemistry,
Jagiellonian University, Gronostajowa 2,
3
4
3
0‐387 Kraków, Poland.
main phase of the cobalt catalysts as well as species resulted from interactions
with alumina support. In the case of palladium catalysts, the PdO was most
prevalent. The article also discusses the observed variation in band position in
the Raman spectra and compares the band assignment with literature data.
The palladium and mixed oxide catalysts showed average conversion higher
than 50% for methane combustion at 673.15 K (400 °C) especially those after
sonication.
Email: damian.chlebda@uj.edu.pl
Funding information
National Science Centre, Poland, Grant/
Award Number: 2015/19/N/ST8/00181
and 2015/17/D/ST8/01252
KEYWORDS
cobalt oxide, in situ micro‐Raman, methane combustion, palladium oxide, structure
[
13,14]
1
| INTRODUCTION
catalysts.
It is well known that the reduced form
of palladium does not yield as high efficiency as the
oxidised form.
[
13]
In recent years, palladium‐based catalysts have become
This leads to carbon deposition on
of great importance, particularly in industrial processes
the catalyst surface, causing catalyst deactivation. A
[1–5]
such as the combustion of methane,
oxidation of
similar situation can be observed in the presence of a
[6,7]
[8]
[15,16]
carbon monoxide,
and removal of nitrogen oxides.
toxin such as sulfur oxide.
Thus, the development
One of the reason is palladium's ability to exhibit a
of suitable catalysts that overcome some of the draw-
[9]
[17]
great activity even in lean conditions. Therefore, it
can be applied successfully to catalytic processes in
excess oxygen for the production of energy from natural
gas. However, some drawbacks of using palladium cata-
backs of palladium is still crucial.
One promising
direction of development is to use palladium metal
only as a dope, compared to the other metals in the
[1,18]
proposed catalyst, as shown in our previous studies.
[
10]
lysts
have to be taken into account in respect of
Palladium–cobalt oxides dispersed on alumina may be
the alternative that is sought. The use of methodology
mentioned above to produce precursors of heteroge-
neous catalysts is very interesting because, compared
to other systems, this catalyst shows some advantages
planning reactors and controlling the catalytic process.
The harsh reaction conditions usually lead to the
[11,12]
sintering of the active metal component
and the
reduction of the palladium on the surface of
J Raman Spectrosc. 2017;1–10.
wileyonlinelibrary.com/journal/jrs
Copyright © 2017 John Wiley & Sons, Ltd.
1

2
CHLEBDA ET AL.
[19]
mainly related to structure.
However, there is a need
2 | MATERIALS AND METHODS
for further investigation into the surface structure of
this catalyst. This in turn will provide comprehensive
knowledge about the structure evolution and the
stabilisation effects of mixed oxide catalysts and their
2.1 | Catalyst preparation
The catalysts used in this study were prepared by means
of incipient wetness impregnation. Due to its simplicity,
this is one of the methods most frequently used in labora-
tories to deposit the active metal on the support surface.
[20,21]
application in many industrial processes.
The
structure of cobalt and palladium catalysts has been
[22–25]
investigated by many researchers,
but studies do
not provide a unified comparison of structural proper-
ties or clarify characteristics of metal structures created
during the calcination process.
The aluminium(III) oxide derived from Al(OH) (supplied
3
by Aldrich Chemical Co.) by calcination at 873.15 K
(600 °C) for 6 hr was used as a support. The selected metal
salts (Co(NO ) 6H O; Pd(NO ) 6H O, supplied by
Modification of the preparation steps for obtaining
highly active material and development in the pro-
posed oxides systems, utilising their size, shape, and
3
2
2
3 2
2
Aldrich Chemical Co.) were first dissolved in an appropri-
ate volume of deionised water to obtain desired concen-
tration of solutions (cf. Table 1). The prepared solutions
were added to the alumina support. The impregnation
time was equal to 1 hr. The catalyst precursor was dried
in an air at 333.15 K (60 °C) and calcined at 873.15 K
(600 °C) for 6 hr at the heating rate of 2 K/min to obtain
the active metal catalyst. The details concerning the prep-
aration steps for six samples of catalyst and two reference
materials used in this study are presented in Table 1.
For the mixed oxide catalyst, alumina was first
impregnated with a lower molarity solution—Pd(NO3)2
—dried and calcined as described above. Then, the sam-
ple was used as a support material for cobalt catalysts.
The standard reference materials Co O and PdO were
[26]
function, are key to success.
Commonly used steps
in preparation include the exposition of the maximum
possible amount of oxide nanoparticles on high‐surface
[27]
area carriers to obtain high metal dispersion.
This
step has a direct impact on final activity in selected
catalytic process. Another method to reach high cata-
lytic activity is the application of an external agent that
causes incensement in dispersion of the active metal
oxide over the surface. A relatively new approach in
this context is sonochemistry, which further reduces
[28]
the particle size.
Sonication is a process in which
no direct coupling of the ultrasound with chemical
[28]
species occurs on the molecular level.
Instead, the
3
4
sonication effect is based on cavitation (the formation,
growth, and collapse of bubbles in a liquid media)
and leads to the formation of particles with narrow
prepared by heating cobalt nitrate and palladium nitrate
at 873.15 K (600 °C) for 6 hr in air. They were further used
as pure standards for Raman band comparison.
[29]
size distribution.
Additional sonicated reference materials were pre-
pared to obtain smaller particles. The sonication process
was performed with solutions of cobalt and palladium
nitrates in water solutions, by probe sonication with a
Misonix Sonicator S‐4000 (QSonica LLC, Newtown, CT,
USA) equipped with a (½" diameter horn sonication
parameters: P = 90 W, f = 20 kHz). Sonication was carried
out in an ice bath to reduce defects and the evaporation
of the solution. The temperature of the solution was
In this study, we investigated spectroscopic properties
and the activity in methane combustion of different
cobalt–palladium and mixed oxide‐based catalysts sup-
ported on aluminium(III) oxide. The catalysts prepared
with the incipient wetness impregnation method were
compared to those prepared with the application of ultra-
sound irradiation in the preparation method that results
in particle size reduction.
TABLE 1 Details about the prepared six catalysts and two reference materials
Number of active
metals
Average metal particle size
(nm) in solution
Name
Solutions used for impregnation
Samples
Co/γ‐Al
2
O
3
1
1
2
1 M Co(NO
0.001 M Pd(NO
0.001 M Pd(NO
3
)
2
6H
2
2
2
2
2
2
2
2
O
13 ± 10
84 ± 33
—
Pd/γ‐Al
2
O
3
3
3
)
)
6H
6H
O
2
2
O
O
Co/Pd/γ‐Al
2 3
O
1
M Co(NO
3
3
)
)
2
2
6H
6H
Co‐son/γ‐Al
Pd‐son/γ‐Al
Co‐son/Pd‐son/γ‐Al
2
O
3
1
1
2
1 M Co(NO
O
7 ± 4
35 ± 24
—
2
O
3
0.001 M Pd(NO
0.001 M Pd(NO
3
3
)
)
6H
6H
O
2
2
O
O
2 3
O
1
3 2
M Co(NO ) 6H
References
Co
PdO
3
O
4
(spinel)
1
1
—
—
—
—

CHLEBDA ET AL.
3
maintained below 313.15 K (40 °C). Cyclic ultrasonic
pulses were generated for about 20 min to the solution
to achieve maximum possible deagglomeration of parti-
cles. Prepared earlier alumina support was proceeded
with the methodology described above with the sonically
treated metal salts.
temperature‐programmed experiments were performed
with a Hiden CatLab reactor, and the gases were analysed
by a Hiden QIC20 mass spectrometer. Amount of catalyst
used during reaction equals 0.025 g. The programme
included several steps: The catalysts were conditioned at
773.15 K (500 °C) in 21% O (Air Products, 5.1) balanced
2
The particle sizes were measured after deposition and
evaporation of the metal salts solution on the high‐grade
mica substrate using Atomic Force Microscope XE100
by He (Air Products, 5.1) for 20 min; after cooling to
298.15 K (25 °C), 0.400 mol% of CH in He (Air Products,
4
calibration mixture, ±0.5% rel.), and 21% O balanced by
2
(
<
Park System, South Korea) with a silicon tip (tip diameter
10 nm) in noncontact mode. Three 5 × 5 μm area scans
He were introduced, and the temperature was increased
from 323.15 K (50 °C) to 773.15 K (500 °C) at a heating
rate of 10 K/min in steps. At every 50 K step, the temper-
ature remained for 30 min. The total flow rate was
were conducted over the substrates surface to ensure a sta-
tistical sampling of nanoparticles. For 100 particles in the
analysed cases, the particle diameter has been calculated
from the projected area using ImageJ software. Common
standard deviation values were estimated for the mean
diameters (number average diameters) for each sample.
The results are summarised in Table 1.
3
50 cm /min corresponding to weight hourly space velocity
−
1
equal to 120 L(hg) at every step of procedure. The
signals from CO (m/z = 44), CH (m/z = 16), and H O
2
4
2
(m/z = 18) were continuously monitored by the mass
spectrometer. For comparison of the catalysts' activity,
the conversion was calculated.
2
.2 | In situ micro‐Raman spectroscopy
analysis
3
| RESULTS AND DISCUSSION
The in situ micro‐Raman analysis was performed with a
confocal UV/visible Raman microscope system—LabRAM
HR800 spectrometer (Horiba Jobinn Yvonne, France). The
spectra were collected under backscattering geometry by
using a 50× objective lens with a spot diameter of about
PdO and spinel Co O are catalytically active materials
that are used extensively in heterogeneous and homoge-
neous catalysis.
strictly upon their structure and their interactions with
the support. Knowledge about the crystal structures of
these metal oxides is essential to provide detailed Raman
characteristics of different variants of catalysts prepared
by the standard impregnation method and by sonication.
According to the literature,
crystallises in the normal spinel structure, where the dis-
tribution of cations is shown to be Co (Co ) O
2 4
(space group O ‐Fd3m). The Co cations are located
3
4
[
30–33]
Their catalytic abilities depend
1
μm and power of 1 mW at λ = 532 nm. All spectra were
calibrated using a crystalline Si wafer and with respect to
−
1
the Raman peak of Si at 520.7 cm . The optimum
experimental conditions used for the Raman spectrum
measurement were found to correspond to 1,800
grooves/mm grating and collection time of 30 s for visible
[
34]
the cobalt oxide (Co O )
3 4
2+
3+
2−
(532 nm) excitation.
7
2+
The catalyst and reference samples were loaded in a
h
3
+
catalytic reactor cell Linkam‐CCR1000 (Scientific Instru-
ments Ltd, Waterfield, Surrey, England) fitted with quartz
windows. The sample was mounted on an unreactive
disposable quartz wool placed inside the ceramic holder.
inside parentheses at tetrahedral sites, and the Co
cations are outside parentheses at octahedral sites. Based
on group theory (as calculated: Γ = A1g (R) + Eg
(R) + F1g (in) + 3F (R) + 2A2u (in) + 2E (in) + 4F
(IR) + 2F2u (in)),
Raman active (R). These are one A1g mode, one E mode,
and three F2g modes. The remaining modes are active in
IR spectroscopy (IR) or inactive (in). The Raman
spectrum registered at 383.15 K (110 °C) after dehydration
in our study is shown in Figure 1a. It consists of five
Raman‐active modes, as predicted above by group theory.
The five prominent observed Raman bands correspond to
2g
35]
u
1u
[
The samples were calcined in the oxidative environment
there are only five modes that are
3
(
[
(
constant total flow rate of 30 cm /min 21% O /79% He
2
g
Air Products, calibration mixture, Poland]) at 773.15 K
500 °C) for 30 min to remove impurities and the layers
of adsorbed water. The Raman spectra were acquired at
83.15 K (110 °C) in an inert helium gas flow (Air
3
3
Products, 5.1, Poland) with a flow rate of 30 cm /min in
total. The flow rate of the reactant gases was controlled
by a set of mass flow controllers.
−
1
−1
−1
the A1g (674 cm ), F (607 cm ), F (511 cm ), E
g
2g
2g
−
1
−1
(
470 cm ), and F (190 cm ) modes of the crystalline
2g
[
1,36]
Co O phase and are consistent with previous reports.
3
4
2.3 | Methane combustion activity tests
The structure of palladium oxide has already been
well described in the literature. Based on this information,
PdO crystallises in the tetragonal cooperite structure
The combustion of methane was carried out in a micro
fixed‐bed reactor under atmospheric pressure. The

4
CHLEBDA ET AL.
−
1
−1
at 646 cm ; a broad band between 1,000 and 1,200 cm ;
−
1
and a weak band at 1282 cm . The presence of many
bands can be explained by the strong resonance Raman
scattering that led to the appearance of many extra lines
[
39,40]
in the spectrum.
The resonance signal enhancement
observed at acquisition temperature can be correlated to
an exciton model of the electronic structure, as discussed
[
39]
in the literature.
The Raman test of palladium oxide
(
6
Figure 1b) reveals a characteristic band centred at
46 cm that corresponds to the B1g mode. The band
−
1
position wavenumber is in good agreement with others
presented in the literature, for instance, those registered
by Otto et al. and by Bell et al. with the 514.5 nm
+
[41,42]
wavelength of the Ar laser.
They also registered a
dominant sharp band just as in our study. The band at
−
1
6
46 cm is the first‐order scattering phonon mode and
involves only the motion of oxygen atoms along the c‐axis.
The second band predicted by group theory appears at
−
1
4
39 cm but with lower intensity. It represents another
first‐order Raman scattering mode—E —and reflects the
g
motion of oxygen atoms perpendicular to the c‐axis. Due
to the aforementioned signal enhancement, the PdO
spectrum consists of multiple smaller peaks that come
from second‐order scattering processes, from overtones
and their combinations or from forbidden LO‐phonon
scattering studied and described in detail by McBride
[
39]
et al.
Details about the bands present on a reference
Raman spectra of cobalt and palladium oxides were
compared to the literature data and are shown in
Table 2. In most cases, the bands appear in the same
wavenumber range and are in good agreement with those
reported in the literature.
FIGURE 1 Raman spectra for the reference materials—Co
spinel structure and the PdO—prepared by high temperature
calcination; the main peaks upon 532‐nm laser excitation of the
3 4
O
The structural properties of prepared catalysts based
on γ‐Al O3 support were also determined by micro‐
2
Raman spectroscopy. Registered Raman spectra for cata-
lysts prepared by the standard incipient wetness impreg-
nation method for catalysts Co/γ‐Al O , Pd/γ‐Al O and
oxides have been marked
2
3
2 3
9
(
space group D4h ‐P4 /mmc) where each oxygen atom is
mixed Co/Pd/γ‐Al O3 are shown in Figure 2a–c. As
2
2
surrounded tetrahedrally by metal atoms and each metal
γ‐Al O does not give Raman transitions, signals regis-
2
3
[
37]
atom is at the centre of a rectangle of oxygen atoms.
This creates a structure built from edge‐sharing chains
tered in the Raman spectrum come from the structures
of the oxides, from oxide‐support interaction, or (although
less likely) from a small amount of α‐Al O (as impuri-
[38]
of PdO planes.
Despite that, all of the Pd–O bonds
4
2 3
are equidistant, the singular PdO units are rectangular
ties). For the Co/γ‐Al O catalyst, bands with maximum
4
2 3
rather than perfect squares. This results in distance
compression leading to shorter nearest‐neighbour O–O
positions along two axes in space. Based on the group the-
ory calculation Γ = A2u (IR) + 2E (IR) + B (R) + Eg
wavenumber similar to that observed in the reference
spinel sample can be observed. These five Raman active
peaks indicate the Co O4 spinel structure. The most
3
−
1
intensive Raman mode, at 668 cm (A1g mode), describes
the characteristics of the octahedral sites, and the band is
symmetrically sharp. The following bands also describe
the spinel cobalt oxide structure: bands at 606, 515, 475,
u
1g
(R) + B2u (in), there are only two modes that are Raman
active—the B1g and E modes. The Raman spectrum of
g
pure palladium oxide used as the reference in this paper
is shown in Figure 1b. As can be seen, the spectra consist
of several bands with different intensities. These are a
−
1
and 190 cm correspond to the E and F modes, and
g
2g
it is likely that they are related to the combined vibrations
of the tetrahedral site and octahedral oxygen motions.
−
1
weak band at 275, 337, 439, and 571 cm ; a strong band

CHLEBDA ET AL.
5
TABLE 2 Details about the bands present on Raman spectra of cobalt and palladium oxides and about the literature data
Laser line
(nm)
−
1
Compounds
Co (spinel)
Co (Co
Bands wave number (cm
)
References
3
O
4
190 m, 470 m, 511 m, 607 w, 674 s
488, 528, 694
94 w, 482 m, 522 m, 618 w, 691 s
82, 519, 621, 690
This work
Jongsomjit et al.
532
514
514
780
2
+
3+
2−
[43]
)
2
O
4
[44]
1
4
Hadjiev et al.
[36]
Tang et al.
PdO
275 w, 337 w, 439 w, 571 w, 646 s, 1000–1200 b, 1282 w
This work
532
532
514
514
[
39]
[45]
2
1
2
73 w, 330 w, 431 s, 625 s, 1,000–1,300 b
00–200, 320, 651
78, 341, 423, 445, 650
McBride et al.
and Demoulin et al.
[
41]
Otto et al.
[
5]
Janbey et al.
Note. s = strong; m = medium, w = weak, b = broad.
2 3 2 3 2 3
FIGURE 2 Raman spectra for catalysts Co/γ‐Al O , Pd/γ‐Al O , and Co/Pd/γ‐Al O prepared by the standard incipient wetness
impregnation method
−
1
Obtained bands are consistent with previous investiga-
sample. The region between 240 and 550 cm seems to
be anomalous, with high intensity in comparison to the
reference material (Figure 1b) and some bands arising
[1,46]
tions
with γ‐Al O as a catalyst support. It must be
2
3
noted that, in the spectrum, there is one additional weak
−
1
−1
band around 606 cm , which may indicate the creation
from background signals. Although the band at 427 cm
of a small amount of cobalt–alumina structures. Addition-
can be attributed to the first order scattering E mode,
g
−
1
ally, all bands except those at 190 cm are slightly blue
shifted in comparison to characteristics of the reference
material. The observed shift may be connected with the
interaction of the cobalt oxide structure with the alumina
support. This kind of disorder is not observed for the F2g
Raman mode, resulting in a constant band position of
the rest of the bands observed, at 273, 340, 442, 477, 570,
−
1
716 cm , and others, result from phonon interaction
and second order scattering processes. This indicates
that the crystallite size creates the collective lattice
[
48,49]
vibrations that enhance the Raman signal.
The bands
positions are similar to other reported studies concerning
−
1
[46,50]
1
90 cm .
The spectrum of Pd/γ‐Al O (Figure 2b) exhibits one
the Pd/γ‐Al O system.
2
3
The mixed oxide system prepared by consecutive
2
3
−
1
sharp and symmetric B1g mode band at 647 cm that
complies with the formation of larger PdO particles on
and within the support structure. No band shift in com-
parison to the reference material has been observed. In
fact, as noted in the literature, PdO is a very Raman‐active
oxide, but this is easily detectable only if the thickness of
impregnation steps exhibits similar or higher activity in
[1]
methane abatement than in our previous studies ; thus,
the structure of these catalysts is also very interesting.
The Raman spectrum derived from in situ measurement
after dehydration from the Co/Pd/γ‐Al O3 sample is
2
shown in Figure 2c. The cobalt spinel pattern is again
[40,47]
an oxide is greater than 1 nm.
6
The dominant band at
47 cm (B1g mode) is symmetric and sharp, suggesting
the formation of larger, bulk‐like PdO particles in the
easily distinguished in the collected spectra. The band at
−
1
−1
680 cm has a clearly visible shoulder, with an additional
−
1
band at 630 cm . Going through the literature, it is hard

6
CHLEBDA ET AL.
to find the proper Raman spectrum of this catalytic sys-
tem that will have two visible metal structures. Due to
the laser interaction with the surface and the higher
amount of cobalt, the palladium structure is usually hid-
den and the Raman spectrum corresponds more to the
monometal catalyst structure. The Co–Pd system has been
studied by various groups, with focus mostly on catalytic
properties rather than just the structure. It is known that
the preparation of an alumina‐based catalyst by sequen-
tial deposition of Co and Pd results in the creation of
which will interact well with the support. Such interac-
tion will leave a fingerprint in the spectra.
The only spectra that looks identical to the unsonicated
was registered for Pd‐son/γ‐Al O (Figure 3b). The band
2
3
wavenumber indicates the PdO structure—a sharp band
−
1
at 647 cm (B ) and two weak bands with maximums
1g
−
1
−1
at 442 cm (E ) and 277 cm (second order scattering
g
effect). Notwithstanding, the registered spectra exhibit a
high level of background that masks the low‐intensity
bands visible for reference material. The shift observed
for the sonicated cobalt catalyst (Figure 3a) is interesting:
[19]
core‐shell structure.
When palladium is deposited on
−
1
a surface as the first metal, the less mobile cobalt atoms
four bands (665, 600, 510, 466, and 187 cm ) are signifi-
cantly downshifted in comparison to the reference mate-
rial, and the direction of that shift is opposite to those
observed for the unsonicated catalyst. The described
(
which exhibit stronger metal–substrate interaction)
[51]
partly cover the palladium structure.
Afterwards,
mobile palladium atoms change position, so the final
structure is of a core‐shell type. The presence of a second
metal on the surface influences the electronic and
structural properties of the catalyst. The PdO bands also
appear on the spectra of the bimetallic cobalt–palladium
−
1
behaviour also affects the band at 187 cm (F ), the
2
g
position of which for the unsonicated sample remains con-
stant. Similar changes have been reported in the literature
[52]
for the nanoparticles dispersed in other media.
The
−
1
catalyst at 273 cm , and others are tangled both with
agglomeration state of nanoparticles and the reduction in
their size can significantly influence their Raman proper-
ties. As a result, the red shift and broadening of the Raman
modes are observed. Furthermore, in a Co‐son/γ‐Al O
−
1
an intensive band at the 300–600 cm region and the
−
1
strong band at 680 cm of cobalt spinel. Furthermore,
the band positions are slightly blue shifted in comparison
with the pure cobalt oxide sample. The observed shift is
2
1
3
−
spectrum (Figure 3a), a shoulder around 500 cm is
formed that is not observed for the reference material.
The research groups of Fischer et al. and Kock et al. corre-
late this band position with the creation of a CoAl O
2 4
−
1
around 4 cm and is noticeable for all visible bands
−
1
except those at 190 cm .
The sonicated series of catalysts Co‐son/γ‐Al O ,
2
3
[
53,54]
Pd‐son/γ‐Al O , and mixed Co‐son/Pd‐son/γ‐Al O were
structure on the surface.
It can therefore be con-
2
3
2 3
also investigated by Raman spectroscopy, and the spectra
registered are shown in Figure 3a–c, respectively. In the
case of sonication, the ultrasound generation and cavita-
tion process lead to the creation of well‐dispersed and
low‐size particles—down to nanometres in diameter. In
this case, the Raman spectra can reflect the creation of
nanoparticles. It is possible to obtain small nanoparticles,
cluded that the cobalt atoms could also be adsorbed on
the alumina surface and be incorporated into its lattice
upon thermal treatment. As an effect of sonication, the cre-
ated small particles of cobalt may have a greater tendency
to interact with the γ‐Al O support. Additionally, thermal
2
3
treatments induce the migration of a fraction of the cobalt
[55]
into the alumina to form an aluminate‐like phase. As a
2 3 2 3 2 3
FIGURE 3 Raman spectra for catalysts Co‐son/γ‐Al O , Pd‐son/γ‐Al O , and Co‐son/Pd‐son/γ‐Al O prepared by the impregnation
method preceded by sonication with salt solutions

CHLEBDA ET AL.
7
dominant phase in the analysed catalyst, the spinel struc-
ture of Co O oxide can be detected, with a small amount
3
4
of CoAl O spinel. Raman spectra for the mixed catalyst
2
4
prepared by sonication (Figure 3c) show the modes charac-
teristic for Co O , consisting of bands at 670, 604, 511, 468,
3
4
−
1
and 190 cm . It must be noticed that, in the spectrum, two
−
1
weak bands of palladium oxide at 373 and 313 cm are
also visible. The presence of PdO is debatable, as it is not
−
1
evidenced by the intense band around 654 cm as in the
cases described above. The dominant signal from a spinel
structure of cobalt oxide can, in this case, also be explained
by the interaction of laser light with the material, as the
laser penetrates deeper layers. Therefore, islands of small
incorporated amounts of palladium oxide on a cobalt oxide
structure are registered faintly. Taking into account the
structural information, it is more possible that the Raman
bands of palladium oxide may be covered by overlapping
−
1
the 670 cm characteristics of bands of cobalt oxide. The
registered shift of a band position is similar to those
observed for unsonicated samples, and the band at
−
1
1
90 cm is resistant to this red shifting in comparison to
the reference material. The discussed variation in band
position in the Raman spectra of analysed samples and
band assignment are summarised in Table 3.
3.1 | Catalytic activity
The catalysts were tested in methane catalytic combus-
tion, with the maximum concentration of methane in
the reactant flow of 0.400 mol%. The temperature depen-
dence of methane conversion (as light‐off curves) over
tested catalysts (selectivity to CO2 was 100% for all
samples) and the normalised reaction rate are plotted in
Figure 4a,b, respectively. Additionally, three parameters,
T , T , and T , describing the methane conversion
10
50
90
levels at 10%, 50%, and 90%, were calculated and
presented in Table S1. For the sonicated Pd–Co bimetallic
and Pd monometallic catalysts, similar high activity was
observed, although for both Co monometallic catalysts,
the activity does not surpass 50% conversion even at high
temperature. Low activity at tested temperatures is
consistent with other literature reports that a cobalt‐based
catalyst reaches T above 873.15 K (600 °C) on alumina
10
[56]
support.
Interestingly, catalyst conversion is slightly
different for comparable catalysts prepared by the stan-
dard and sonicated methods for palladium. The obtained
results on γ‐Al O supported catalysts in comparison to
2
3
[57]
our previous study on structured catalysts
(γ‐Al O3
2
washcoated on kanthal steel) show similar activity of
sonicated catalysts to the reported Co1Pd0.001 and
Pd0.001 catalysts deposited on kanthal steel. However,
the supported catalysts prepared by standard wetness
method reveal the shift of light‐off temperatures to the

8
CHLEBDA ET AL.
FIGURE 4 (a) Light‐off curves of methane catalytic combustion over tested catalysts; the red lines represent the 10%, 50%, and 90% of
conversion rate; and (b) reaction rate; reaction conditions: flow of 0.400 mol% CH in He and 21% O balanced by He to total flow rate of
0 cm /min (weight hourly space velocity equal to 120 L(hg) ); temperature profile from 323.15 K to 773.15 K (50 to 500 °C) at a heating rate
of 10 K/min in steps [Colour figure can be viewed at wileyonlinelibrary.com]
4
2
3
−1
5
0
higher values. The parameters from Table S1 let compare
the prepared catalyst by the activity during selected
conversion levels (at 10%, 50%, and 90% of maximum
conversion). The most active catalysts at low temperature
are Pd‐son/γ‐Al O and Co‐son/Pd‐son/γ‐Al O , which
crucial role of activation of Pd /PdO species on the cobalt
spinel surface.
2
3
2
3
4 | CONCLUSION
reached the 10% conversion at 586.15 K (313 °C) and
6
19.15 K (346 °C) respectively.
The structural properties of the cobalt and palladium‐
based catalysts with γ‐Al O as support were determined
It can be assumed that, during CH4 combustion
2
3
processes, the catalyst structure may undergo modifica-
tion. However, it is difficult to follow the oxidation
and reduction processes on a catalyst surface by Raman
by spectroscopic investigation using the micro‐Raman
method. Two different methods of preparation via stan-
dard wetness impregnation and their modification with
sonochemistry were presented, and the resulting structure
of the adsorbed metal oxide particles was characterised.
The Raman spectra disclosed that the active cobalt form
of the catalysts in catalytic combustion is Co O spinel
[23]
spectroscopy,
due to overlapping spectral information
from different phases often in small concentration. For
the most used palladium catalyst, some researchers report
that up to 873.15 K (600 °C), there is no reduction of large
3
4
[
23]
PdO particles and thus CH does not modify bulk PdO
structure, and for the palladium‐based catalysts, the PdO
oxide is dominant phase. For the sonicated cobalt catalyst,
the cobalt–alumina spinel structure was observed as an
4
0
and the activity depends strictly on the Pd /PdO ratio
x
and their dispersion in the support structure. For bimetal-
lic catalysts, the predicted activity should be better than
monometallic; thus, the cobalt loaded on the catalysts
−
1
additional shoulder on the bands around 500–510 cm .
The band positions for different series of catalysts and
for the prepared reference materials were not stable. The
particle size and the interaction of metal particles with
the support caused a shift in maximum band wavenum-
ber. Raman bands red shift and broadening for agglomer-
ated nanoparticles were noticed for the nanoparticles
obtained by sonication. The activity test for methane
abatement under oxidative conditions proved the higher
activity of palladium and mixed palladium–cobalt catalyst
prepared by sonication than the standard wetness impreg-
nation method.
0
[55]
may lead to the activation of Pd /PdO species.
The
high activity of palladium doped cobalt catalysts is
consisted with other works based on the same active
[57,58]
metals reported in literature.
Synergism on the addi-
tion of small amount of palladium to cobalt was observed
[59]
also for the CO hydrogenation.
Yet still the origin of
the very high activity of the cobalt spinel catalyst doped
with palladium is unclear, and many explanations are
postulated by researchers. Those describe the electronic
[51]
and ensemble (structural) effects ; also, it was noticed
that in Co–Pd system the mobility of the lattice oxygen
is increased by to weakening of the Co−O bond by the
presence of palladium. The availability of oxygen prevents
the metallic Pd formation leading to the higher activity in
ACKNOWLEDGEMENT
This study was supported by National Science Centre,
Poland research grant No. 2015/19/N/ST8/00181. The
[58]
hydrocarbons combustion. Thus, it can be assumed the

CHLEBDA ET AL.
9
authors are also grateful to the National Science Centre,
Poland—Project 2015/17/D/ST8/01252.
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How to cite this article: Chlebda DK,
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structure of cobalt, palladium, and mixed oxide‐
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