The promoting effect of cerium on the characteristics and catalytic performance of palladium supported on alumina pillared clays for the combustion of propene

Article abstract of DOI:10.1016/j.apcata.2013.08.055

The aim of this work was to study the effect of the presence of various amounts of cerium on palladium supported on an alumina pillared clay for the catalytic oxidation of propene. For this purpose, a montmorillonite clay was intercalated and pillared with alumina, then used as a support for palladium. The palladium loading was selected on the basis of previous findings from our group. Cerium loadings of between 0 and 1 wt.% were added after the incorporation of palladium onto the support. The resulting catalysts were characterized by nitrogen physisorption at -196 °C, X-ray powder diffraction (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The results obtained suggest that the added Ce partially blocks the microporous volume of the metal catalyst and interacts with the Pd to form new metallic species that enhance the behavior of the initial catalyst in the oxidation of propene.

Full text of DOI:10.1016/j.apcata.2013.08.055

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The promoting effect of cerium on the characteristics and catalytic
performance of palladium supported on alumina pillared
clays for the combustion of propene
∗
A. Aznárez, S.A. Korili, A. Gil
Department of Applied Chemistry, Building Los Acebos, Public University of Navarra, Campus of Arrosadia, E-31006 Pamplona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The aim of this work was to study the effect of the presence of various amounts of cerium on palla-
dium supported on an alumina pillared clay for the catalytic oxidation of propene. For this purpose,
a montmorillonite clay was intercalated and pillared with alumina, then used as a support for palla-
dium. The palladium loading was selected on the basis of previous findings from our group. Cerium
loadings of between 0 and 1 wt.% were added after the incorporation of palladium onto the support.
Received 22 February 2013
Received in revised form 28 August 2013
Accepted 29 August 2013
Available online xxx
◦
The resulting catalysts were characterized by nitrogen physisorption at −196 C, X-ray powder diffrac-
Keywords:
Pillared clays
Palladium supported catalysts
Propene/propylene combustion
Cerium promoting effect
tion (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The
results obtained suggest that the added Ce partially blocks the microporous volume of the metal catalyst
and interacts with the Pd to form new metallic species that enhance the behavior of the initial catalyst
in the oxidation of propene.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
high temperatures. As such, the thermal stability of the pillars must
be increased to avoid the sintering of pillared clays. One way to
achieve this goal is to introduce mixed-oxide pillars, such as Al–Ga,
Al–Si, Al–Zr, Al–Fe, and Al–Cu, amongst others [5]. Another appro-
priate way to overcome this problem is to add a rare earth metal.
Thus, Sterte [6] found that the incorporation of lanthanide elements
during preparation of the intercalating agent resulted in materials
whose basal spacing was greater than that of conventional materi-
als. Likewise, an improvement in the thermal stability of Al-pillared
The increasing demand for green energy and the coming into
force of more stringent regulations regarding exhaust emissions
have been the driving force behind much of the recent research into
catalytic combustion technologies. The benefits of catalytic com-
bustion with respect to its traditional thermal counterpart are well
established and consist mainly in the formation of lower amounts
of by-products due to the lower operating temperatures and higher
efficiency [1]. In this respect, although noble metal-based cata-
lysts are well-known for their high performance in the catalytic
combustion of hydrocarbons, several studies have investigated the
possibility of enhancing the performance of supported noble metal
catalysts by adding suitable promoters [2,3].
Pillared IntercaLated Clays (PILCs) are an interesting class of
two-dimensional microporous materials that can be prepared by
the intercalation of inorganic or organic compounds between the
silicate layers of clay minerals, which results in an increased basal
spacing, pore volume and specific surface area. Due to their textu-
ral properties and acidity characteristics, these solid materials are
very attractive for adsorption and catalytic applications [4]. Unfor-
tunately, PILCs are not sufficiently stable and tend to collapse, with
a resulting loss of textural properties and catalytic performance, at
clays was reported by Tokarz and Shabtai [7], who prepared pil-
lared clay catalysts by first exchanging the clay with Ce3+ or La
3+
,
then exchanging these clays with refluxed, partly hydrolysed Al(III)
solutions. The addition of a rare earth metal improves both the
thermal stability and the adsorptive and catalytic properties of the
pillared materials. CeO2 has been reported to be able to store and
3+
4+
liberate oxygen as a result of the redox properties of the Ce /Ce
pair, which can result in good performance for the combustion of
Volatile Organic Compounds (VOCs). Supported palladium catalyst
is known to be effective for the combustion of VOCs and many
research works investigate the possibility of enhancing the catalytic
activity by using CeO2 as suitable support or adding this oxide as
promoter to the supports. The properties of the supports are modi-
fied by the presence of CeO . Farrauto et al. [8] reported a beneficial
2
effect of the presence of CeO as support on the stabilization of PdO.
2
The effect of CeO addition to alumina-support is studied by various
2
authors [9–13] on supported catalysts for the methane combus-
tion. Similarly, various authors have reported that the presence of
Ce in Pd/Al-pillared clays produces a substantial decrease in the
∗ Corresponding author. Tel.: +34 948 169602; fax: +34 948 169602.
E-mail address: andoni@unavarra.es (A. Gil).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.08.055
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temperature required for the complete oxidation of benzene
14,15]. In all these studies the properties of the supports are
X-ray photoelectron spectroscopy (XPS) analyses were per-
formed using an SSI X-probe (SSX-100/206) spectrometer from
Surface Science Instruments (USA). The analysis chamber was oper-
[
modified and after wet impregnation with a palladium solution,
new catalysts are obtained. In the present work, various amounts
of cerium are added to a supported palladium catalyst and the
characteristics and catalytic performance on the combustion of
propene studied. The catalyst was selected from a previous work
−
7
ated under ultrahigh vacuum at a pressure close to 5 × 10 Pa, and
the sample was irradiated with monochromatic Al K␣ (1486.6 eV)
radiation (10 kV; 22 mA). Charge compensation was achieved using
an electron flood gun adjusted to 8 eV and placing a nickel grid
3.0 mm above the sample. The pass energy for the analyzer was set
at 150 eV for both wide and narrow scans, and an area of approx-
imately 1.4 mm2 was analyzed. Under these conditions, the full
width at half-maximum (fwhm) of the Ag 3d5/2 peak from a silver
standard sample was about 1.6 eV. For these measurements, the
binding energy (BE) values were referred to the C-(C,H) contribu-
tion of the C 1s peak at 284.8 eV. Data treatment was performed
using the CasaXPS program (Casa Software Ltd., UK), and some
spectra were deconvoluted using the least squares fitting routine
incorporated in this software with a Gaussian/Lorentzian (85/15)
product function and after subtraction of a non-linear baseline.
Molar fractions were calculated using peak areas normalized on the
basis of acquisition parameters and sensitivity factors provided by
the manufacturer. The C 1s, O 1s, Mg 2s, Si 2p, Al 2p, Ce 3d and Pd 3d
peaks were used for quantitative analysis. Based on the XPS analy-
sis, the XPS surface ratio of a given element is defined as the atomic
concentration of the element (%) with respect to the concentration
of the major element of the support (Si) (%).
[
16] considering the possibility of modification of the catalytic
behavior by the presence of a promoter.
2
. Experimental
2.1. Catalyst preparation
The starting material was a montmorillonite from Tsukinuno,
supplied by the Clay Science Society of Japan. The raw clay was
pillared with alumina according to a conventional pillaring proce-
dure [17], then used as catalytic support. The supported palladium
catalyst was prepared by wet impregnation of the support with
a solution of palladium (Pd(NO ) , palladium(II) nitrate solution,
3
2
1
0 wt.% in 10 wt.% HNO , Sigma–Aldrich) to obtain a material with
3
a metal loading of 0.1 wt.%. The metal salt/clay slurry was evapo-
rated under reduced pressure in a rotavapor and the resulting solid
◦
◦
dried at 120 C for 16 h before being calcined in air at 500 C for 4 h
to form the final supported catalyst. The modified catalysts were
prepared by treating 5 g of the Pd catalyst with 50 cm3 of aqueous
Ce(NO ) ·6H O solutions (99.99%, Sigma–Aldrich) to obtain load-
2.3. Catalytic performance
3
3
2
ings of between 0 and 1 wt.%. The metal salt/catalyst slurries were
evaporated under reduced pressure in a rotavapor and the resulting
solids dried at 120 C for 16 h before being calcined in air at 500 C
for 4 h to form the final modified catalysts. The catalytic series are
referred to as wt. CePd, where wt. indicates the cerium content.
Propene combustion was carried out using an automated bench-
scale catalytic unit (Microactivity Reference, PID Eng & Tech). The
reactor was a tubular, fixed-bed, downflow type with an internal
diameter of 0.9 cm. Catalyst samples were mixed with an inert
material, at a weight ratio of 1:4, in order to dilute the catalyst
bed and avoid hot spot formation. The propene concentration in
the feed was 0.5% and the oxygen-to-hydrocarbon molar ratio
was 20, with helium as the balance gas, up to a total feed flow
◦
◦
2.2. Catalyst characterization
Nitrogen adsorption experiments (Air Liquide, 99.999%) at
3
of 150 cm /min. The catalyst was stabilized for 120 min at each
◦
−
196 C were performed using a static volumetric apparatus
temperature to ensure steady-state conversion. Space velocities
(
Micromeritics ASAP 2010 adsorption analyser). All samples (0.2 g)
(
GHSV), calculated at standard temperature and pressure and based
◦
were degassed for 24 h at 200 C at a pressure lower than 0.133 Pa.
The Langmuir surface area (SLang) was calculated from nitrogen
adsorption data over the relative pressure range 0.01–0.05, consid-
−1
on the volume of the catalytic bed, were about 20,000 h . Prior to
the catalytic tests, one of the following pre-treatments were applied
to the catalysts: (1) in flowing air (100 cm /min): heating to 150 C
at 2 C/min, 2 h at 150 C and cooling to room temperature, or (2) in
flowing H (Air Liquide, 99.999%) (100 cm /min): heating to 300 C
at 2 C/min, 2 h at 300 C and cooling to room temperature. The
reactant and reaction product streams were analyzed online using
an Agilent 6890 gas chromatograph system.
3
◦
2
ering a nitrogen molecule cross-sectional area [18] of 0.162 nm .
◦
◦
The total pore volume (Vp) was estimated from the amount of
nitrogen adsorbed at a relative pressure of 0.99, assuming that the
density of the nitrogen condensed in the pores is equal to that of liq-
3
◦
2
◦
◦
◦
3
uid nitrogen at −196 C (0.81 g/cm ) [18]. The micropore volumes
(
V␮p) were calculated using the Dubinin–Radushkevich equation
19] over the relative pressure range 0.01–0.034.
[
After extraction by acid digestion, the metal contents were
3. Results and discussion
determined by inductively coupled plasma optical emission
spectroscopy (ICP-OES) using a Varian Vista-MPX instrument.
X-ray diffraction (XRD) patterns of nonoriented powder sam-
ples were obtained using a Siemens D-5000 diffractometer fitted
with a Ni-filtered Cu K␣ radiation source, at 40 kV and 30 mA.
Temperature-programmed reduction (TPR) studies were carried
out using a Micromeritics TPR/TPD 2900 instrument. An initial
The most important differences between the nitrogen adsorp-
tion isotherms of the samples are found at relative pressures lower
than 0.10 (see Fig. 1), in other words in the micropore region.
The textural properties of the materials obtained are presented in
Table 1. The Al-PILC used as catalytic support in this work shows
2
a specific surface area of 158 m /g. A comparison of the textu-
pre-treatment with N (Air Liquide, 99.999%) was carried out at
ral properties of the support, the Pd catalyst and the Ce-modified
catalysts showed that the surface and volume accessibilities are
affected by the presence of the metals, especially in the case of Ce.
The loss of specific surface area caused by the presence of Pd is
around 26%, with this value increasing to 53–63% in the presence
of Ce. For 0CePd as well as for Ce-modified catalysts, the decrease
in the microporous surface is much more significant than that in
the external surface. These results are attributed to the presence
of the smaller particles of palladium on the microporous region
2
◦
◦
1
3
50 C for 90 min, at a heating rate of 10 C/min, under a flow of
3
0 cm /min, to condition the samples. TPR tests were then car-
◦
ried out from room temperature up to 800 C, at a heating rate of
1
◦
0 C/min, under a total flow of 30 cm /min (5% H in Ar, Air Liq-
2
3
uide). Water and other compounds that might be formed during
metal reduction and precursor decomposition were retained by a
molecular sieve trap. Hydrogen consumption was measured using
a thermal conductivity detector (TCD).
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3
Table 1
◦
Textural properties derived from N2 adsorption at – 196 C, chemical analyses and temperatures at which the conversion of propene reaches 10% and 90%.
2
S␮pb (m /g)
2
Sextc (m /g)
2
Vpd (cm /g)
3
V␮pe (cm /g)
3
T10f ( C)
◦
T90f ( C)
◦
T10g ( C)
◦
T90g ( C)
◦
Catalyst
Al-PILC
SLanga (m /g)
Pd (wt.%)
Ce (wt.%)
159
117
58
75
66
140
100
44
59
53
19
18
14
17
13
0.111
0.092
0.063
0.081
0.066
0.060
0.044
0.022
0.029
0.025
–
–
–
252
254
–
–
326
322
–
–
229
234
249
254
–
315
303
306
320
0
0
0
1
CePd
0.13
0.12
0.12
0.11
0.01
0.03
0.17
1.64
.01CePd
.1CePd
CePd
–
–
a
b
c
d
e
f
Specific surface area calculated using the Langmuir equation.
Specific micropore surface area.
Specific external surface area obtained from the t-plot method.
Specific total pore volume.
Specific micropore volume derived from the Dubinin-Radushkevich (DR) equation.
Temperatures after pre-treatment in air.
Temperatures after pre-treatment in hydrogen.
g
and of the bigger ones on the external surface. The cerium phase
is mainly situated on the external surface, blocking the entrance
to the micropores, and in interaction with the palladium bigger
particles, as confirmed by the TPR analysis. These results could be
explaining if the impregnation aqueous solution of cerium can pro-
duce the dissolution of the palladium with a new reorganization of
the metal on the surface of the Al-PILC.
support. As far as the crystalline phases of Ce are concerned, diffrac-
◦
◦
◦
tion peaks at 28.6 ((1 1 1), I = 100%)), 33.2 ((2 0 0), I = 30%)), 47.5
◦
((2 2 0), I = 52%) and 56.4 ((3 1 1), I = 42%)) (JCPDS file no. 34-0394)
were only detected for the 1CePd catalyst and correspond to CeO2
oxide. No peaks resulting from other Ce species, or for the 0.01CePd
and 0.1CePd catalysts, were detected. This could also be related to
the low Ce content.
The XRD patterns for the supported palladium and Ce-modified
catalysts are presented in Fig. 2. The fact that patterns for these
samples did not reveal the presence of PdO or Pd0 could be related
to the low loading of this metal and its high dispersion on the
The TPR profiles for the catalysts are presented in Fig. 3. In the
case of the 0CePd catalyst, the first region of H consumption, which
2
◦
is observed between 100 and 200 C, is attributed to the reduction
of small PdO species (oxide clusters). The subsequent reduction
◦
process in the temperature range of 200–460 C is attributed to
the reduction of bigger PdO particles that interact strongly with
the surface of the support [20]. The significant reduction process
observed at 640 C was assigned to the reduction of structural Fe
6
4
2
0
0
0
0
AL-PILC
◦
3+
0
0
0
1
CePd
.01CePd
.1CePd
CePd
cations of the support. It should also be noted that the reduction
◦
processes at temperatures higher than 550 C can be ascribed to
reduction and thermal decomposition of the sample (thermogravi-
metric analysis does not form part of this work). In agreement with
the above, a similar reduction process was observed for the CePd
◦
catalysts at between 100 and 200 C. As in the case of the Pd cat-
alyst, for the catalysts 0.01CePd and 0.1CePd this process is due
to the reduction of small PdO particles. In the case of 1CePd, how-
◦
ever, the reduction process that commences at 100 C is much more
pronounced than those observed for the other catalysts, probably
due to the reduction of small PdO particles interacting with the
cerium oxide. The reduction maximum in the temperature range
◦
2
00–460 C shifted to higher temperatures for all the Ce-modified
0.0
0.2
0.4
0.6
0.8
1.0
catalysts. Various authors have related the presence of a maximum
p/pº
◦
at 360 C to the reduction of non-stoichiometric Ce oxides [12,13].
◦
Thus, the addition of varying amounts of Ce to the catalyst results in
an interaction between the Ce and both large PdO particles and the
Fig. 1. Nitrogen adsorption isotherms for the CePd catalysts at 196 C.
0
CePd
0.01CePd
.1CePd
1CePd
1
CePd
.1CePd
.01CePd
CePd
0
0
0
0
Al-PILC
25
30
35
40
45
2
50
55
60
65
70
0
100
200
300
400
500
600
700
800
Temperature (ºC)
Theta (º)
◦
Fig. 2. XRD patterns for the CePd catalysts calcined in air at 500 C for 4 h.
Fig. 3. TPR patterns for the CePd catalysts.
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Table 2
Binding Energies (BEs) and atomic ratios obtained from XPS experiments.
Catalyst
Binding energies (eV)
Atomic Surface Ratio
O 1s
Si 2p
Mg 2s
Al 2p
Pd 3d5/2
Ce/Si
O/Si
Pd/Si
Al/Si
Al-PILC
532.0
532.1
532.1
532.1
532.1
102.9
103.0
103.0
103.0
103.0
89.5
89.6
89.6
89.6
89.6
74.9
75
75
75
75
–
–
–
3.11
3.51
3.50
3.94
3.90
–
0.60
0.60
0.61
0.60
0.62
0
0
0
1
CePd
338.1
338.1
338.1
338.1
336.5
336.5
336.5
336.5
–
335.1
–
0.0011
0.0011
0.0011
0.0012
.01CePd
.1CePd
CePd
0.0023
0.0052
0.0176
–
support, resulting in the formation of non-stoichiometric Ce oxides,
which are reduced at 360–380 C. The reducibility of the bigger PdO
particles is diminished by the presence of these oxides.
crystallite size, which leads to lower Pd/Ce/support interaction.
Monteiro et al. [22], who studied the role of the Pd precursors in
the oxidation of CO over Pd/Al O3 and Pd/CeO /Al O catalysts,
◦
2
2
2
3
The binding energies (BEs) and atomic ratios of the catalysts,
as determined by XPS, are presented in Table 2. Typical BEs for
montmorillonite are 102.75 (Si 2p3/2), 74.8 (Al 2p3/2) and 532 eV
concluded that Pd-Ce interaction was promoted by ceria species
in its reduced state (Ce3+). Thus, in the lower-loaded Ce catalysts
(0.01CePd and 0.1CePd) the interaction Pd/Ce/support and the per-
centage of reduced ceria (Ce3+) are higher than in the higher-loaded
ones (1CePd).
(
O 1s) [21]. The addition of Pd, followed by Ce, did not result in
significant changes to the BEs of the principal elements in the sup-
port. The XPS results also show that all the catalysts, with the
same Pd loading but different cerium loading, have the same sur-
face atomic Pd/Si ratio. Pd-Ce interaction contributes, as it has
been previously reported [22], to either palladium redispersion
or maintenance of a highly dispersed state. A comparison of the
ratio between surface and total Ce (from chemical analysis, see
Table 1) gives the following order with respect to Ce amount: 0.074
The conversion of propene over the 0CePd catalyst after various
pre-treatments in flowing air or hydrogen is shown in Fig. 4A. The
catalytic conversion was calculated in terms of propene consump-
tion and CO2 formation. The values obtained were compared and
found to be generally in good agreement. Furthermore, our results
show that the complete oxidation of propene depends on the pre-
treatment of the 0CePd catalyst only slightly, being the catalytic
performance of the catalyst slightly improved by the pre-treatment
in flowing hydrogen. Various authors [9–11] also described that
the presence of CeO2 does not significantly affect catalytic per-
formances in the combustion of methane, but markedly affects
high temperature catalytic behavior in line with the stabilization
of active PdO species. To ensure a better comparison, the T10 and
T90 values (temperatures at which propene conversion reaches 10%
and 90%, respectively) are included in Table 1. After treatment of the
(
0.01CePd) > 0.030 (0.1CePd) > 0.011 (1CePd). As a result, it can be
concluded that, in accordance with the nitrogen adsorption results,
the amount of Ce that penetrates the structure of the support dur-
ing synthesis increases with Ce content. The O/Si ratio suggests
that cerium addition increases the presence of oxygen on the sur-
face. Deconvolution of the XPS spectra of the catalysts into their
individual components showed that there were three forms of pal-
ladium present on the surface. The BEs measured for the Pd 3d5/2
line, which are in accordance with those reported in the litera-
ture, were 335.1, 336.5 and 338.1 eV. The BE at 335.1 eV can be
◦
catalyst at 300 C, some of the noble metal oxide, even that which
interacts strongly with the support, is reduced (see Fig. 3). Thus,
the catalytic effect is due to the fact that reduction of the oxides,
0
attributed to Pd , and that at 336.5 eV, which is comparable to an
0
oxidized palladium foil standard (336.7 eV), was attributed to bulk
PdO. The peak at 338.1 eV was attributed to highly dispersed and
deficiently coordinated Pd2+ in contact with the support to form
palladium–aluminate structures. Thus, the XPS results show that at
low surface concentrations oxidized palladium exists in two chem-
ical states, namely bulk PdO and palladium-support structures.
The percentage of the chemical states of palladium and cerium
and thus formation of the active phase in the reaction (Pd ), occurs
during the reductive pre-treatment.
The effect of the presence of Ce on the catalytic behavior is
presented in Fig. 4B–D. In Fig 4B it is shown a decrease in the
catalytic activity of the 1CePd catalyst, in respect of the 0CePd
◦
sample, when a reductive pre-treatment at 300 C is applied. It is
explained due to its bigger CeO2 crystallite size, which leads to
lower Pd/Ce/support interaction, and thus to a smaller catalytic
activity. An improvement of the activity for 0.01CePd and 0.1CePd
catalysts, in comparison with the 0CePd catalyst, is observed for
3+
4+
(
0
Ce /Ce ) in the surface of the catalysts is presented in Table 3. For
CePd the percentage of bulk PdO (70%) is considerably higher than
that of palladium-support structures (30%). In the case of the Ce-
modified catalysts, the percentage of palladium-support structures
increases (50–85%) while the percentage of bulk PdO decreases
◦
high temperatures, approximately from 300 C. The high activity of
the pre-reduced samples is discussed in terms of reduction of CeO2
in the vicinity of the metal particles [23,24], and a decreasing CeO2
crystallite size that leads to higher metal/Ce interaction and hence
greater activity [25]. Monteiro et al. [22] pointed out a cause in
not observing hydrocarbon oxidation at low temperatures. They
concluded that during CO oxidation at low temperatures, oxy-
gen uptake went directly to replenish the ceria oxygen-deficient
lattice, instead of oxidizing CO. There would then be a fierce
competition between the CeOx species and Pd sites by the molecule
(
10–50%), with respect to 0CePd catalyst, which is related to the
Pd/Ce/support interaction. On increasing the metal loading from
3
+
0
.1%wt. to 1%wt. it is shown that the percentage of Ce decreases
4
+
while that of Ce increases. The higher cerium loading in 1CePd
leads to a bigger CeO2 crystallite size, as shown by XRD analysis.
In line with these results, the lower percentage of palladium-
support structures in 1CePd (50%) with regard to 0.01CePd (85%)
and 0.1CePd (90%) is explained also in terms of a bigger CeO2
Table 3
Palladium and cerium surface composition (%) obtained from XPS experiments.
Catalyst
BEs (EV)–Pd 3d5/2
Oxidized palladium
Reduced Palladium
Ce3+
Ce4+
n≥+2
335.1 (Pd0)
338.1 (Pd
)
336.5 (Bulk PdO)
0
0
0
1
CePd
30
85
90
50
70
10
10
50
–
5
–
–
100
95
100
100
–
5
–
–
–
–
51
41
–
–
49
59
.01CePd
.1CePd
CePd
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Fig. 4. Conversion of propene over palladium and palladium-cerium modified catalysts after air and hydrogen pre-treatments.
of oxygen. Thus, oxygen would be available to CO oxidation only
after complete re-oxidation of ceria reduced species. Ceria reduced
like to thank P. Eloy, Université catholique de Louvain (Belgium),
for XPS measurements. A. Aznárez acknowledges financial support
from the Public University of Navarra through a PhD fellowship.
3
+
species (Ce ) were not able to help CO to oxidize due to lack of
oxygen in its lattice. Hence, the highest rates of CO oxidation were
only observed at higher temperatures as a result of strong com-
petition for oxygen molecules. Unfortunately, this enhancement
disappeared with an oxidative pre-treatment and in lean condi-
tions, and the reactivity was similar to that of 0CePd, see Fig. 4C. It
has been reported that in these conditions, the active sites of the
interface Pd/Ce/support and Pt/Ce/support are converted into an
oxidized and less active state, disappearing the promoter effect of
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5
[
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◦
It is concluded that a reduction pre-treatment at 300 C resulted
[
◦
in a higher activity than a reduction pre-treatment at 500 C, or an
◦
oxidative pre-treatment at 150 C. The Ce loading plays a key role,
[
as when big CeO2 crystallites are formed, a lower Pd/Ce/support
interaction and thus a smaller catalytic activity are observed.
[
[
[
[
Acknowledgements
This work was funded by the Spanish Ministry of Science and
Innovation (MICINN) and the European Regional Development
Fund (FEDER) (project MAT2007-66439-C02). The authors would
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(1995) 367–376.
Please cite this article in press as: A. Aznárez, et al., Appl. Catal. A: Gen. (2013), http://dx.doi.org/10.1016/j.apcata.2013.08.055