Physico-chemical investigation of catalytic oxidation sites in 4%Rh/CeO2 catalysts prepared by impregnation and deposition–precipitation methods

Article abstract of DOI:10.1016/j.chemphys.2019.110472

4 wt% Rh/CeO₂ catalysts were prepared by two different methods: deposition–precipitation (DP) and impregnation (Imp). X-ray diffraction (XRD), differential scanning calorimetry and thermogravimetry (DSC-TG), temperature-programmed reduction (TPR), and electron paramagnetic resonance (EPR) were used for physicochemical characterization. The solids were tested in propylene and carbon black oxidation reactions. The 4%Rh/CeO₂ (DP) showed better catalytic performance in both reactions compared to the catalyst prepared by the impregnation method. The XRD technique evidenced the formation of Rh₂O₃ phase in the DP-solid after calcination of this latter at 400 °C for 4 h. EPR evidenced, only in the DP-solid, the presence of O₂ ^? species in interaction with the CeO₂ surface whereas Rh⁴⁺ ions in the form of clusters were identified in both solids. The TPR technique showed that the DP-solid was reduced by hydrogen at lower temperature compared to the impregnated one. The higher catalytic performance of the DP-solid was attributed to the presence of O₂ ^? species along with the presence of Rh₂O₃ phase in ceria and to the better reducibility and lower particle size of the rhodium species.

Full text of DOI:10.1016/j.chemphys.2019.110472

Chemical Physics 527 (2019) 110472
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Physico-chemical investigation of catalytic oxidation sites in 4%Rh/CeO2
catalysts prepared by impregnation and deposition–precipitation methods
a,b
c
a
a,b
a
Madona Labaki , Samer Aouad , Sara Hany , Cynthia Abou Serhal , Edmond Abi-Aad ,
a,⁎
Antoine Aboukaïs
a
Unité de Chimie Environnementale et Interactions sur le Vivant EA 4492 (UCEIV), MREI, Université du Littoral – Côte d’Opale (ULCO), 145 Avenue Maurice Schumann,
5
9140 Dunkerque, France
b
Laboratory of Physical Chemistry of Materials (LPCM/PR2N), Faculty of Sciences, Lebanese University, Fanar, 90656 Jdeidet El Metn, Lebanon
Department of Chemistry, Faculty of Arts and Sciences, University of Balamand, P.O. Box. 100, Tripoli, Lebanon
c
A R T I C L E I N F O
A B S T R A C T
4 wt% Rh/CeO catalysts were prepared by two different methods: deposition–precipitation (DP) and impreg-
Keywords:
2
CeO
2
nation (Imp). X-ray diffraction (XRD), differential scanning calorimetry and thermogravimetry (DSC-TG), tem-
perature-programmed reduction (TPR), and electron paramagnetic resonance (EPR) were used for physico-
chemical characterization. The solids were tested in propylene and carbon black oxidation reactions. The 4%Rh/
Deposition–precipitation
Impregnation
−
O
2
CeO (DP) showed better catalytic performance in both reactions compared to the catalyst prepared by the
2
Oxidation
Rh
impregnation method. The XRD technique evidenced the formation of Rh
2
O
3
phase in the DP-solid after calci-
2 3
O
−
nation of this latter at 400 °C for 4 h. EPR evidenced, only in the DP-solid, the presence of O
2
species in
4
+
interaction with the CeO surface whereas Rh ions in the form of clusters were identified in both solids. The
2
TPR technique showed that the DP-solid was reduced by hydrogen at lower temperature compared to the im-
−
pregnated one. The higher catalytic performance of the DP-solid was attributed to the presence of O
2
species
along with the presence of Rh
2
O phase in ceria and to the better reducibility and lower particle size of the
3
rhodium species.
1
. Introduction
Ceria (CeO
used Rh/γ-Al
air.
2
O
3-CeO as catalyst to oxidize gasoline oxygenates by wet
2
2
) has a great reputation in catalysis and was well in-
The EPR technique is highly sensitive compared to other spectro-
scopic techniques (XPS, IR, UV–visible, etc.) making it suitable for the
investigation of adsorbed paramagnetic species at solid surfaces. These
paramagnetic species can be intermediate compounds implied in cata-
lytic oxidation and reduction reactions. Therefore, their identification
could help in accessing reactions mechanisms which depend on the
redox properties of the active sites located at the support surface and
the support itself.
vestigated due to its high oxygen storage capacity and its use in cata-
lytic oxidation reactions [1–3]. It is also well known that adding tran-
sition metals over ceria enhances the activity of the latter [4,5].
Rhodium supported on CeO is extensively used as catalyst in many
2
reforming reactions and it is also involved in a number of oxidation
reactions [6–12]. Haneda et al. [6] have shown that the Ir-Rh/CeO
ZrO improves three-way catalytic activity by oxidizing CO and C
molecules and reducing NO one. Shimizu et al. [7] have prepared Rh/
2
3 6
H
-
2
The aim of the present study is to compare the catalytic perfor-
CeO
2
catalysts by a supercritical CO
2
impregnation technique and they
mances of two series of 4%Rh/CeO catalysts (prepared by depos-
2
used them for automotive exhaust. The catalysts have been character-
ition–precipitation and conventional impregnation) towards the total
oxidation of propylene and carbon black particles and to use physico-
chemical techniques such as EPR and TPR to explain the performances
obtained. Propylene is an example of the pollutants volatile organic
compounds and carbon black is used as soot model.
ized by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction
(
XRD), and transmission electron microscopy (TEM) to analyze the Rh
loading, Rh structure, and the morphology of the particles, respectively.
Li et al. [8] have investigated Rh/CeO -SiC catalyst in the partial oxi-
2
dation of ethanol for hydrogen production. Cuauhtémoc et al. [9] have
⁎
Corresponding author.
E-mail addresses: mlabaki@ul.edu.lb (M. Labaki), samer.aouad@balamand.edu.lb (S. Aouad), sara.hany@univ-littoral.fr (S. Hany),
cynthia.abou-serhal@univ-littoral.fr (C. Abou Serhal), edmond.abiaad@univ-littoral.fr (E. Abi-Aad), antoine.aboukais@univ-littoral.fr (A. Aboukaïs).
https://doi.org/10.1016/j.chemphys.2019.110472
Received 30 May 2019; Received in revised form 29 July 2019; Accepted 8 August 2019
Available online 10 August 2019
0301-0104/ © 2019 Elsevier B.V. All rights reserved.

M. Labaki, et al.
Chemical Physics 527 (2019) 110472
2
. Experimental part
2.1. Catalysts preparation
Cerium hydroxide was prepared by precipitation of cerium (III)
nitrate hexahydrate, Ce(NO
3
)
3
·6H O (ACROS) with sodium hydroxide
2
NaOH (ACROS). The suspension was filtered, washed several times with
hot deionized water, and dried at 100 °C for 24 h. CeO
calcination under air at 400 °C for 4 h.
2
was obtained by
The rhodium chloride RhCl
3
·xH O (STREM CHEMICALS) was used
2
as rhodium precursor and introduced into ceria according to two
methods: wet impregnation and deposition–precipitation.
Wet impregnation: The supported rhodium/CeO
2
catalysts were
was
prepared by the conventional wet impregnation method. CeO
2
impregnated with rhodium solution. The slurry was stirred 2 h at am-
bient temperature and then water was slowly evaporated at 75 °C in a
rotary evaporator. The as-dried sample was put in an oven set at 100 °C
Fig. 1. Determination of T
i
(beginning of CB combustion under air), Tmax
(
maximum combustion rate), and T (end of CB combustion) on the DSC and TG
f
−1
curves.
for 20 to 24 h. The obtained powder was calcined at 400 °C (1 °C.min
,
−
1
4
h, 2 L dried air.h ). Two rhodium loadings were prepared: 0.5 wt%
and 4 wt%. The samples will be respectively designated by 0.5% Rh/
CeO (Imp) and 4% Rh/CeO (Imp).
2.3. Catalytic test
2
2
Deposition–precipitation: Rhodium (4 wt%) was loaded on CeO
2
The catalysts were tested in the total oxidation of propylene. The
experiments were carried out in a conventional fixed bed micro
preliminary suspended in water. The precipitation was carried under
control of the following parameters: pH was maintained constant at 8
−1
reactor between 25 °C and 500 °C (1 °C.min ). The reactive flow
-
1
−1
by adding drops of 0.1 mol.L NaOH solution, the temperature was
fixed at 80 °C, and the stirring speed was constant. The slurry was
stirred during four hours, filtered, washed with hot water, dried over-
(100 mL.min ) consisted of a mixture of air and 6000 ppm of propy-
lene. Before the catalytic test, the solid (100 mg) was activated under a
−1
−1
flow of air (2 L.h ) at 400 °C (1 °C.min ). The reactants and products
were identified and quantified by a Varian CP-4900 micro gas chro-
matography equipped with thermal conductivity detectors (TCD).
The carbon black (CB)–catalyst mixture was prepared in tight con-
tact conditions. 2 wt% of CB and 98 wt% of catalyst were manually
grinded for 15 min. About 30 mg of CB–catalyst mixture were loaded in
an alumina crucible and heated from room temperature to 1000 °C
−1
night in an oven 100 °C, and then calcined at 400 °C (1 °C.min , 4 h,
−
1
2
L dried air.h ). The sample will be designated by 4% Rh/CeO (DP).
2
2
.2. Catalysts characterization
−1
−1
XRD measurements were performed with an automatic powder
(heating rate: 5 °C min ) in air flow (75 mL·min ). Three character-
istic temperature values are determined from DSC–TG curves (Fig. 1): T
diffractometer BRÜKER AXS D8 Advance, using CuKα radiation and a
scintillation counter LynxEye. The diffraction patterns were recorded in
a step-scan mode with a step of 0.02° (2 θ), counting time 2 s, in the
angular interval 20–80° at room temperature. The Eva program was
used for diffraction data processing. Identification of the crystalline
phases was made with the Joint Committee on Powder Diffraction
Standards (JCPDS) files from the International Center for Diffraction
Data (ICDD). Differential scanning calorimetry and gravimetric analysis
i
where the CB oxidation begins, T
max
where the rate of oxidation is at its
maximum, and T where the combustion ends. T
f
max
values were used to
compare the catalytic performances and CB oxidation rates.
3
. Results and discussion
−
1
3.1. Physico-chemical characterization
(
DSC-TG) was conducted in air flow (75 mL.min ) at a heating rate of
−
1
5
°C.min
from room temperature to 400 °C followed by an isotherm
3
.1.1. X-ray diffraction
at the same temperature for four hours with about 30 mg of sample with
a Netzsch STA 409 apparatus.
Fig. 2 shows the XRD patterns obtained for both 4% Rh/CeO (DP)
2
and (Imp). The lines attributed to CeO
2
cerianite phase (JCPDS-ICDD
EPR measurements were performed at two different temperatures:
3
7
4-0394) located at 2θ: 28.55°; 33.07°; 47.48°; 56.34°; 59.44°; 69.86°;
−
196 °C and room temperature, on an EMX BRUKER spectrometer with
7.08°; and 79.83°, were observed for both solids. In addition, a line
a cavity operating at a frequency of 9.5 GHz (X band). The measure-
ments were performed on 50–55 mg of the calcined solids previously
with a weak intensity, located at 35.2°, was observed and assigned to
−
5
Rh
the XRD pattern of 4% Rh/CeO
able to detect Rh by XRD in rhodium incorporated into MCM-41.
2
O
3
hexagonal phase (JCPDS-ICDD 41-0541). This line was absent in
treated under vacuum (1.2 × 10 bar). The magnetic field was
modulated at 100 kHz and the power supply was sufficiently low to
avoid saturation effects. The g values were determined from precise
frequency and magnetic field values.
2
(Imp). Mulukutla et al. [13] were also
O
2 3
The temperature-programmed reduction (TPR) measurements were
performed on a device AMI-200 from ZETON ALTAMIRA. A trap for
removing water during reduction was mounted in the gas line prior to
3.1.2. Thermal analysis
Fig. 3a and b show the DSC and TG curves obtained respectively for
4% Rh/CeO
2
(DP) and 4% Rh/CeO (Imp), before calcination, heated
−1
2
the thermal conductivity detector. A hydrogen-argon mixture (5% H
2
/
under dry air at a rate of 5 °C· min
from room temperature up to
−
1
Ar) was used to reduce the samples at a flow rate of 30 mL.min . The
400 °C followed by a 4 h dwell at the final temperature.
−
1
temperature was linearly raised at a rate of 5 °C.min
temperature till 900 °C and then the sample was kept one hour at
00 °C. The current of the thermal conductivity detector was 75 mA.
from room
For the DP solid, the percentage of weight loss from room tem-
perature up to 400 °C and before the 4 h-dwell at this latter temperature
is of 3.1%. This loss occurs in two steps: the first one is due to the loss of
physisorbed water on the solid (2.1%) and the second one to the des-
9
Before the reducing step, the sample was flushed by an argon flow of
−
1
−1
−1
3
0 mL.min
during one hour at 150 °C.min
(5 °C.min ). The
orption of water present in RhCl
3
·xH O (1%). Indeed, when this latter
2
sample mass used was about 50 mg.
precursor was brought from room temperature up to 400 °C and kept at
this temperature for 4 h (Fig. 3c), the total weight loss recorded is about
2

M. Labaki, et al.
Chemical Physics 527 (2019) 110472
Fig. 4. EPR spectra recorded at −196 °C of: a) 4% Rh/CeO
room temperature for 1 h, b) 4% Rh/CeO (DP) evacuated at 400 °C for 1 h, c)
% HCl/CeO evacuated at 400 °C for 1 h.
2
(DP) evacuated at
2
1
2
XRD- and to the formation of CeOCl or Ce-Cl species on ceria surface as
reported by Force et al. [10]. These formations are illustrated by the
following reaction:
3
2
2
RhCl3/CeO2 + 4CeO2 + O2
Rh2O3/6ClCeO2
For the Imp solid, the weight loss also takes place in two steps, with
a total percentage of 4.7%. The first step, 3.4%, corresponds to the loss
of physisorbed water and the second step, 1.3%, to dehydration of the
precursor RhCl
3
·xH O. No trace of gain was detected during the 4 h-
2
dwell at 400 °C. This result can explain the absence of Rh
2
O formation
3
in the impregnated solid.
Fig. 2. XRD patterns of 4% Rh/CeO
2
(DP) and 4% Rh/CeO (Imp).
2
3
.1.3. Electron paramagnetic resonance
2
4%. Since, in the prepared catalyst, the rhodium weight corresponds
3.1.3.1. 4% Rh/CeO (DP). The EPR spectrum, recorded at −196 °C, of
2
to 4% of the catalyst total weight, the water loss resulting from the
4% Rh/CeO (DP) subjected to vacuum at room temperature during 1 h,
2
4
00
precursor dehydration must be equal to about 24 ×
= 0.96%, value
is given in Fig. 4a. It is a complex spectrum. It is apparently formed by
1
3
+
close to 1%, the value obtained for the second loss step. In addition, the
the superimposition of three signals. The first one, Ce , has an axial
symmetry with g = 1.965 and g = 1.945. This signal was extensively
very slight gain of weight (0.1–0.2%) after 150 min of calcination at
⊥
//
3
+
4
00 °C can be due to the oxidation of RhCl
3
into Rh
2
O
3
-detected by
studied by numerous authors [1,3,14–16] and was attributed to Ce
−
1
−1
Fig. 3. DSC-TG analysis under air flow (75 mL.min ) at a heating rate of 5 °C.min from room temperature to 400 °C followed by an isotherm at 400 °C for four
hours: a) 4% Rh/CeO (DP), b) 4% Rh/CeO (Imp), and c) RhCl ·xH O.
2
2
3
2
3

M. Labaki, et al.
Chemical Physics 527 (2019) 110472
Fig. 5. EPR spectra recorded at room temperature of: a) 4% Rh/CeO
2
(DP)
Fig. 6. EPR spectra recorded at −196 °C of 4% Rh/CeO
2
(Imp): a) evacuated at
-
evacuated at room temperature for 1 h, b) 4% Rh/CeO (DP) evacuated at
2
room temperature for 1 h, b) evacuated at 400 °C for 1 h.
4
00 °C for 1 h.
The formation of O
D
and S signals is due to the presence of Cl ions,
D
ions. The second one, nominated O
// = 2.030, = 2.010, and
attributed to O
D
g
, possessing an axial symmetry with
coming from the RhCl precursor, in ceria [24,31]. Indeed, 1 wt% Cl
3
g
g
⊥
iso = 2.017 was unambiguously
was impregnated on CeO already calcined at 400 °C using a solution of
2
−
4+
2
species located on Ce
ions of ceria. Indeed, a
HCl. The obtained product was calcined at 400 °C for 4 h. The EPR
similar signal with a slight difference in the g// value was obtained after
spectrum of this sample, after being subjected to O adsorption, ex-
2
adsorption of molecular O
2
on pure ceria [17–20]. The O
D
signal
hibited two signals similar to O and S (Fig. 4c). The small difference
D
D
disappeared when the spectrum is recorded at room temperature
in the g values is due to the absence of rhodium on the latter solid.
−
(
Fig. 5a). This means that the O
2
species is weakly adsorbed on
When the EPR spectrum of 4% Rh/CeO (DP) subjected to vacuum
2
ceria. The third signal, R
D
, is symmetric and centred at g = 2.17 with
at 400 °C during 1 h is recorded at room temperature (Fig. 5b), the R ,
D
4
+
3+
peak-to-peak width ΔH = 96 G. This latter can be attributed to Rh
R , and S disappeared whereas, the Ce , O , and S remained ob-
P
P
P
D
5
(
Z = 45; 4d with S = 5/2; 3/2; 1/2) of RhO
2
small particles [21–23].
servable but their intensities decreased, compared to those obtained at
–
This latter oxide can be formed beside of the Rh
2
O
3
one but in an
−196 °C, following the Curie law. The stability of O species re-
2
amount not detectable by XRD. In addition, since the fine structure is
not observed, it could be said that S = 1/2.
sponsible for the O
P
signal is an indication of the solid oxidizing
property.
The EPR spectrum, recorded at −196 °C, of 4% Rh/CeO
2
(DP)
subjected to vacuum at 400 °C during 1 h, is illustrated in Fig. 4b. This
3
.1.3.2. 4% Rh/CeO
2
(Imp). Fig. 6a shows the EPR spectrum, recorded
kind of treatment can be considered as a smooth reduction of the solid.
3
+
at −196 °C, of 4% Rh/CeO
2
(Imp) subjected to vacuum at room
Apparently, beside the Ce and R signals, the spectrum is formed of
D
temperature during 1 h. The spectrum is composed of three signals:
four signals: i) the first one O
P
, apparently similar to O
D
, with a little
3
+
–
the first one was already attributed to Ce
nominated R , is a broad symmetric signal centered at g = 2.16 with
ΔH = 225 G, and the last one (D ) is not well-resolved. When the
spectrum is recorded at room temperature, the intensities of the above
signals decreased following the Curie law. The R signal can be
ions, the second one,
difference in the g// value (2.032), but, on the contrary, the O
2
species
I
corresponding to O
room temperature (Fig. 5b); ii) an isotropic signal, called S
g = 2.0015 with a peak-to-peak width ΔH = 10 G, it was attributed to a
trapped electron in the ceria matrix [24,25]; iii) a signal, nominated R
with an axial symmetry is characterized by g// = 2.16, A// = 115 G,
P
remained stable when the spectrum is recorded at
I
D
, centred at
I
P
,
4
+
attributed to Rh
in the form of clusters. The oxidation of RhCl ,
3
which is in the form of large particles loaded on ceria surface due to the
g
⊥
= 1.984, A = 19 G, and giso = 2.043 [26–29]. Since the signal
⊥
preparation method (impregnation), is responsible for the formation of
possesses a hyperfine structure of two lines, it was unambiguously at-
4
+
Rh
clusters.
tributed to Rh species (I = 1/2). It is well known that the metallic
0
1
8
When the solid 4% Rh/CeO (Imp) was treated under vacuum at
+
2
rhodium (Rh ; Z = 45; 5 s 4d ) cannot be detected by EPR except at
low temperatures (less than −196 °C) [23,27,30] due to the presence of
3
4
00 °C for 1 h (Fig. 6b), the Ce signal and R
the D signal became well resolved with an axial symmetry and a hy-
perfine structure of two lines (g// = 2.176, A// = 53 G, g = 2.034,
= 25 G, and giso = 2.081) but its intensity is very weak (Fig. 6b).
I
one disappeared whereas
+
8
3+
I
the unpaired electron in the s orbital. In addition, Rh (4d ) and Rh
6
⊥
(
4d with high spin) possess respectively a total electron spin S = 1 and
A
⊥
2
. Since the fine structure is absent in the spectrum, therefore R signal
P
2
+
3
+
6
Such a signal, attributed to Rh
doped lithium dithionate (Li
species can be obtained by a soft reduction of some RhCl
species, was obtained in rhodium
·2H O) [28]. In our sample, these
species which
could not be due to such species. Rh (4d with low spin) has a S = 0
2
+
7
2
S
2
O
6
2
and is silent in EPR. Finally, the Rh
ions (4d ) has S = 3/2 in high
3
spin and S = 1/2 in low spin. Since the fine structure is absent in the
2
+
could resist to the oxidation during the calcination because of their
spectrum, hence R
P
P
signal could be ascribed to Rh
, with ΔH = 15 G and centred at g = 2.043. This
signal, consequently it can be at-
species in the form of clusters. Since the R signal,
ions, remained stable after this smooth reduction,
low spin; iv) a
localisation in the bulk of large particles. The disappearance of the R
I
symmetric signal, S
4
+
3+
signal can be due to the reduction of Rh
into Rh
since this latter
latter value is equal to giso of the R
P
2
+
4
species is in general silent toward the EPR technique. The symmetric
tributed to Rh
D
+
signal with a weak intensity, centred at g = 2.0015 (S
D
), can un-
attributed to Rh
2
+
ambiguously be attributed to a trapped electron in ceria matrix. Finally,
this means that the Rh
species responsible for the R
P
signal can be
3
+
the isotropic signal S centred at g = 2.29 with ΔH = 65 G, can be due
I
obtained by reduction the Rh
ions of Rh
O
2 3
oxide and not by the
2
+
reduction of Rh4+ ions.
to traces of Rh
clusters [29,32,33] resulting from the reduction of
some Rh3+ species obtained after reduction of Rh
4+
clusters.
4

M. Labaki, et al.
Chemical Physics 527 (2019) 110472
Fig. 8. Conversion of propylene during its total oxidation versus reaction
temperature over CeO
2
, 4% Rh/CeO
2
(DP), 4% Rh/CeO (Imp), and 0.5% Rh/
2
CeO (Imp).
2
of Rh
2
O
3
in different environments and/or with different particle size
0
[
13,38]. Rh
2
O
3
is reduced into Rh [36,39,40]. Conversely, Boutros
et al. [38] stated that the first reduction peak (at lower temperature) is
due to the reduction of Rh into Rh O and RhO and the second one to
the reduction of Rh O and RhO.
The TPR profile of 4% Rh/CeO (Imp) shows an intense peak at
2
O
3
2
2
2
8
2 °C and a small peak at 204 °C. The first peak could be ascribed to the
4+ 3+
reduction of the Rh
clusters –evidenced by EPR– into Rh
and
2
+
4+
Rh . Indeed, EPR showed the reduction of Rh
clusters after one
hour under vacuum at 400 °C. The peak at 204 °C could be due to the
reduction of some Rh3+ ions, located in the bulk, into Rh
2+
.
Fig. 7. TPR profiles: a) CeO
2
, b) 4% Rh/CeO
2
(Imp), c) 4% Rh/CeO
2
(DP), d)
4
% Rh
2
O
3
/CeO (DP).
2
3
.2. Catalytic tests
3
.1.4. Temperature-programmed reduction
Fig. 7 shows the TPR profiles of CeO
2
, 4% Rh/CeO (Imp), 4% Rh/
2
3.2.1. Oxidation of propylene
CeO
2
(DP), and 4% Rh
2
O
3
/CeO
2
(DP). The latter is a sample prepared
Fig. 8 illustrates the conversion of propylene versus the reaction
temperature in the presence of the pure ceria support and the 4% Rh/
CeO catalysts. The catalyst 0.5% Rh/CeO (Imp) was also evaluated. It
by using Rh
2
O
3
as precursor instead of RhCl . In the case of ceria
3
(
Fig. 7a), two broad reduction peaks were observed at 472 °C and
2
2
7
76 °C and were ascribed, respectively, to the reduction of ceria surface
is observed that addition of rhodium shifts the temperature of propy-
lene conversion to lower values, whatever is the rhodium content or the
preparation method. Furthermore, it is worthy to note that in all cases,
3
+
with the formation of Ce
species and an oxygen lacuna, and to the
removal of bulk oxygen from the ceria structure along with the re-
4
+
3+
duction of Ce
into Ce
ions [17,34–36].
propylene was exclusively converted into CO . No other by-product was
2
Adding rhodium to ceria, following both DP and Imp methods, leads
in one side to the shift of the two reduction peaks obtained for the pure
ceria to lower temperatures and in the other side to the appearance of
new reduction peaks at a lower range of temperatures (< 220 °C). The
shift of ceria reduction peaks to lower temperatures suggests that rho-
formed.
Furthermore, 4% Rh/CeO (DP) is more reactive than 4% Rh/CeO
2
2
(Imp), with a (T
5
0(Imp)
− T
50(DP)
) of about 35 °C, T being the tem-
50
perature of 50% propylene conversion. This result is in agreement with
those reported in the literature [3,17,34,35] where the depos-
ition–precipitation method gave better activities in oxidation reactions
than the impregnation one. Indeed, the deposition–precipitation leads
to the formation of nanosized particles dispersed on the surface of the
solid, which are highly efficient in catalysis, whereas the impregnation
method leads to the formation of large particles. The size of these
particles depends in general on the active sites concentration added on
the support. It decreases with this latter parameter and can become the
same dimensions as those of nanoparticles. Indeed, it was demonstrated
that when catalysts are prepared by the impregnation method with low
4
+
dium makes ceria reduction easier. Furthermore, the amount of Ce
3
+
reduced into Ce
either on the surface or in the bulk of ceria, de-
creases after rhodium addition. The latter fact suggests a modification
in ceria reduction behavior due to rhodium.
The peaks obtained at low temperatures (60 and 141 °C) for the 4%
–
Rh/CeO
2
(DP) (Fig. 7c) can be due to Rh
2
O
3
and O
2
ions reduction.
Indeed, the TPR profile of Rh
spectively at 53 and 148 °C. These two temperatures are close to those
obtained during 4% Rh/CeO (DP) reduction. The small differences
2
O
3
/CeO (Fig. 7d) shows two peaks, re-
2
2
which exist could be due to the fact that in the latter, Rh
2
O
3
is formed
rhodium contents, 0.5% Rh/CeO (Imp), its catalytic activity is similar
2
upon calcination of RhCl
3
3
. The reduction peak at 60 °C was attributed to
to that obtained for the 4% Rh/CeO (DP).
2
−
the reduction of Rh
2
O
and O
2
ions present on the CeO
2
surface
and
Other factors could be also in the origin of the higher performance
−
whereas the peak at 141 °C could be due to the reduction of Rh
2
O
3
of the 4% Rh/CeO (DP). This catalyst exhibits also the presence of O
2
2
−
O
2
ions rather located in the bulk (not on the surface) [37]. It was
species along with the presence of Rh O phase and also lower reduc-
2
3
stated that the existence of two reduction peaks is due to the reduction
tion temperatures in TPR than 4% Rh/CeO2 (Imp). All these factors
5

M. Labaki, et al.
Chemical Physics 527 (2019) 110472
for the better catalytic performance of 4%Rh/CeO (DP) solid in the
2
total oxidation of propylene and carbon black reactions. The easier
reducibility of rhodium oxides species and the lower size of rhodium
particles are also in the origin of the higher catalytic activities of the
solids prepared by DP method.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
The University of Littoral – Côte D’Opale (ULCO), the Lebanese
University, and the Agence Universitaire de la Francophonie – Région
du Moyen-Orient (AUF) are gratefully acknowledged for financial
supports. The authors would also like to thank the ARCUS E2D2 project,
the French Ministry of Foreign Affairs, and the «Région Hauts de
France» for financial support.
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