Partial oxidation of methane over bimetallic copper-cerium oxide catalysts

Article abstract of DOI:10.1016/j.molcata.2009.12.014

The performance of bimetallic copper-cerium oxides was investigated for the catalytic partial oxidation of methane. They were synthesized by two routes using either the intermetallic compound CeCu₂ as catalytic precursor or the sol-gel method via cerium and copper nitrates (1:2) in the presence of urea. The catalysts prepared by the sol-gel method were always less active and selective than the catalyst prepared by the intermetallic route, which was comparable in catalytic behavior to noble metal catalysts on alumina (e.g. 5 wt% Rh/Al₂O₃, Conv. CH₄ > 90%, Sel. H₂ > 99%, H₂/CO ≈ 2.0) at T = 750 °C. To our knowledge, this is a novelty for copper based catalysts. The copper-cerium oxide catalysts were also quite stable in the temperature range studied, 350-800 °C, and for a long period of time on stream. Such catalytic behavior seems to be determined by the surface morphology and composition and by an unusual interaction between the copper and cerium oxide that hinders the deactivation of the catalyst at high temperatures, which is a direct consequence of the synthetic method used in this work. Therefore, the bimetallic copper-cerium oxide catalyst obtained by the intermetallic route seem to be a good option to produce syngas with the advantage of being less expensive than the catalysts based on noble metals.

Full text of DOI:10.1016/j.molcata.2009.12.014

Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Partial oxidation of methane over bimetallic copper–cerium oxide catalysts
a
b
b
a
Ana C. Ferreira , A.M. Ferraria , A.M. Botelho do Rego , António P. Gon c¸ alves ,
c
d
a
a,∗
A. Violeta Girão , Rosário Correia , T. Almeida Gasche , Joaquim B. Branco
Instituto Tecnológico e Nuclear, Unidade de Ciências Químicas e Radiofarmacêuticas, Estrada Nacional 10, 2686-953 Sacavém, Portugal
Universidade Técnica de Lisboa, IST, Centro de Química - Física Molecular and IN, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Universidade de Aveiro, CICECO, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
a
b
c
d
Universidade de Aveiro, I3N- Departamento de Física, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2009
Received in revised form
The performance of bimetallic copper–cerium oxides was investigated for the catalytic partial oxidation
of methane. They were synthesized by two routes using either the intermetallic compound CeCu2 as
catalytic precursor or the sol–gel method via cerium and copper nitrates (1:2) in the presence of urea.
The catalysts prepared by the sol–gel method were always less active and selective than the catalyst
prepared by the intermetallic route, which was comparable in catalytic behavior to noble metal cata-
1
4 December 2009
Accepted 24 December 2009
Available online 11 January 2010
◦
lysts on alumina (e.g. 5 wt% Rh/Al2O3, Conv. CH4 > 90%, Sel. H2 > 99%, H2/CO ≈ 2.0) at T = 750 C. To our
knowledge, this is a novelty for copper based catalysts. The copper–cerium oxide catalysts were also
Keywords:
Bimetallic
◦
quite stable in the temperature range studied, 350–800 C, and for a long period of time on stream. Such
catalytic behavior seems to be determined by the surface morphology and composition and by an unusual
interaction between the copper and cerium oxide that hinders the deactivation of the catalyst at high
temperatures, which is a direct consequence of the synthetic method used in this work. Therefore, the
bimetallic copper–cerium oxide catalyst obtained by the intermetallic route seem to be a good option to
produce syngas with the advantage of being less expensive than the catalysts based on noble metals.
Copper–cerium oxide
Methane
Catalytic partial oxidation
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
major drawbacks of this reaction are the deactivation caused by
sintering, changes in the oxidation state of the metal active phase
and carbon deposition [1]. The most obvious way to minimize
these phenomena is to work at lower temperatures; however the
methane partial oxidation reaction has to proceed at high tempera-
The development of efficient catalysts for the oxidation of
methane and other volatile organic compounds (VOC’s) has
attracted renewed attention during the last years. For the indus-
try, methane is a source of hydrogen (fuel cell technology) and
an important precursor of syngas that can be used for the pro-
duction of hydrocarbons (via Fischer–Tropsch synthesis). For the
environment, methane is a pollutant.
The catalytic partial oxidation of methane (POM) constitutes a
proper way to produce syngas [1]. This reaction is mildly exother-
mic and provides a suitable H /CO ratio for the methanol and
Fischer–Tropsch synthesis. These characteristics make POM an
attractive and realistic alternative to the steam reforming reaction.
The catalysts reported to be active for the POM to syngas are
either noble metals (like Ir, Pt, Pd, Rh and particularly Ru) or Ni-
based compounds [2–6]. Despite the high activity of noble metal
based catalysts, the high cost of such systems limits their extensive
industrial application. Ni catalysts, usually supported on alumina
or silica, have been also extensively studied [7–10]. However, the
◦
tures (usually higher than 700 C) to achieve significant conversion
of methane [11].
Many efforts have focused on the development of metal cata-
lysts with long-term operation stability. Noble metal catalysts are
less sensitive to carbon deposition than other catalysts [12–14].
However, the use of nickel perovskite-type oxides seems also a
good alternative to reduce the formation and deposition of car-
bon on the catalysts. Lago et al. [15] observed that the activity and
selectivity of a series of LnCoO3 (Ln = La, Pr, Nd, Sm, or Gd) per-
ovskites was high for the POM reaction to syngas. Choudhary et al.
[16] reported that complex oxides with a perovskite structure, like
2
LaNiO , La0.8Ca (or Sr)0.2NiO and LaNi
resistant to coking. Tsipouriari and Verykios [17] studied the par-
CoXO (X = 0.2–1.0), were
3
3
1−X
3
tial oxidation of methane to synthesis gas over a Ni/La O3 catalyst
2
and have found that the CH conversion and the H selectivity were
4
2
close to thermodynamic predictions.
On the other hand, the ability of ceria to store, release and
transport oxygen makes it very attractive as catalyst in oxidation
reactions. Cerium is capable of adjusting its electronic configura-
∗ Corresponding author. Tel.: +351 219946116.
E-mail address: jbranco@itn.pt (J.B. Branco).
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.12.014

4
8
A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
tion [18] and ceria itself has catalytic activity for the oxidation of
methane [19,20]. Otsuka et al. [21–23] showed that ceria is able
as described elsewhere [40] and (ii) the sol–gel route (samples
CeCu O ) using the reaction of aqueous solutions containing sto-
2
4
to directly convert methane to syngas at temperatures higher than
ichiometric amounts of Ce(III) and Cu(II) nitrates (Aldrich, purity
99.99% cerium nitrate; purity 98.0% copper nitrate) in the presence
of urea (Panreac, purity 99.0%). All the reagents were used with-
out further purification. The initial concentrations were 0.1 M in
Ce(III), 0.2 M in Cu(II), and 3 M in urea. The volume of the initial
solution (blue color) was 100 mL. The water of this solution was
◦
6
00 C.
Ceria was also used as a promoter of both the activity and selec-
tivity of supported Ni or Pt catalysts for partial oxidation of methane
24,25] or CO2 reforming of methane [26]. Tang et al. [27] reported
[
that nickel supported on ceria (high Ni-loading of 13 wt%) was
◦
◦
active for POM at T = 750 C. However, this catalyst rapidly deac-
evaporated on a hot-plate at 80 C with continuous stirring and a
tivates due to carbon deposition.
In recent years, Cu–Ce–O composite catalysts received consid-
erable attention [28–31]. Their high catalytic activity seems to be
gel formed upon cooling. This gel was decomposed in an oven at
◦
250 C to yield the precursor, sample CeCu O 250, and calcined at
2
4
◦
two different temperatures, 600 and 900 C in order to obtain sam-
connected to both CuO and CeO [28,29]. In 1995, Liu reported that
ples with different mean particle size (samples CeCu O4 600 and
2
2
CuO–CeO2 catalysts were very active for the oxidation of methane
in the presence of CO, conversion ≈95% [32]. Other copper–ceria-
based materials have been regularly examined as active catalysts
for total oxidation reactions, such as CO oxidation [29,32–34] and
CH4 combustion [29,32,35,36].
CeCu O4 900, respectively). Calcination time was 7 h for all three
samples [52].
2
2.2. Catalyst characterization
The influence of the preparation method was also reported
by various researchers. Several methods for the preparation of
copper–cerium oxide catalysts were used such as combustion,
thermal decomposition, co-precipitation and sol–gel [28,31,37].
Compared with other methods, sol–gel has advantages such as (i)
easy control of the chemical composition and (ii) low-temperature
synthesis and has been employed to prepare different types of
materials [38].
In this context, binary intermetallic compounds of lanthanide
or actinide metals combined with d metals (namely Ni, Co, Mn, or
Fe) have drawn the attention of many authors due to their catalytic
properties [39–47] and were described as a new type of supported
catalysts precursors, which can be more active and selective than
those obtained by conventional routes [40–42,45–51].
XRD patterns were obtained in reflection geometry with a
PANalitycal X’Pert Pro diffractometer using Cu K␣ monochromatic
radiation (ꢀ = 1.5406 Å). The operational settings for all scans were
◦
voltage = 45 kV; current = 40 mA; 2ꢁ scan range 5–80 using a step
◦
◦
size of 0.03 at a scan speed of 0.02 /min. For identification pur-
poses, the relative intensities (I/I ) and the d-spacing (Å) were
0
compared with standard JCPDS powder diffraction files [53]. The
mean CuO and CeO2 particle diameter were determined by means
of the Scherrer’s equation using the XRD (0 0 2) and (1 1 1) peaks,
respectively [54].
The surface morphology and the particle size of the samples
were obtained using a HR-FESEM Hitachi SU-70, operating at
15 keV. The chemical composition was determined by EDS, using
a B-U Bruker Quantax 400 EDS system (EDS data available as
supplementary material).
In our laboratories, we have been using binary intermetal-
lic compounds LnCu2 (Ln = La, Ce, Pr, Nd) [40], ThCu2 and AnNi2
The X-ray photoelectron spectroscopy (XPS) measurements
were performed in a spectrometer XSAM800 (KRATOS) operated
in the fixed analyzer transmission (FAT) mode, with pass energy of
20 eV. The non-monochromatized Al K␣ X-radiation of 1486.6 eV
was produced using a current of 10 mA and a voltage of 12 kV.
(
An = Th, U) as catalytic precursors of bimetallic oxides, e.g.
3
CuO·Ln CuO or 2CuO·ThO2 [42]. Such compounds exhibited
2
4
selectivity for the 4-methylpentan-2-ol decomposition and the
lanthanide-containing phase, CeO or Ln CuO , seems to play a role
2
2
4
◦
in the formation of the CuO active sites [40]. After reduction, these
bimetallic oxides were described as supported copper catalysts on
lanthanide oxides, 2Cu·CeO and 4Cu·Ln O , which were active and
Samples were analyzed at 45 takeoff angle (TOA) in an ultrahigh
−
7
vacuum (UHV) chamber (<10 Pa) at room temperature. Details on
spectra acquisition and data treatment were published elsewhere
[55]. For quantification purposes, sensitivity factors were 0.66 for
O 1s, 0.25 for C 1s, 6.3 for Cu 2p and 10.0 for Ce 3d.
2
2
3
selective for the mesityl oxide hydrogenation to 4-methylpentan-
2
-one (methylisobutylketone, MIBK) [51]. Their catalytic activity
and selectivity was associated with the lanthanide-containing
phase that seems to play an important role in the formation of the
copper and/or nickel active sites.
Raman spectra (Stokes) were taken in the backscattering con-
figuration at room temperature and analyzed using a LabRaman
HR800/UV (Jobin-Yvon) spectrometer. The samples were excited
with a He–Cd laser (325 nm) and the microscope attachment is
based on an NUV system that uses a 40× long distance objective.
Here we report and compare the activity, selectivity and sta-
bility of bimetallic copper–cerium oxide catalysts for catalytic
gas-phase partial oxidation of methane in the temperature range
H -TPR experiments were performed under pure hydrogen
2
◦
of 350–850 C and at atmospheric pressure. They were prepared
in an apparatus previously described [42,51]. The samples were
placed in a quartz U tube (6 mm diameter), then submitted to a
by two routes using either the intermetallic CeCu2 as catalytic pre-
cursor or the sol–gel method. All the samples were characterized
by powder X-ray diffraction (XRD), scanning electron microscopy
3
flow of hydrogen (50 cm /min) under atmospheric pressure, and
◦
◦
heated up to 950 C at a rate of 10 C/min. The degree of reduction
was determined by the quantitative analysis of water, the reduc-
tion product, as described elsewhere [56]. This procedure allows
enhancing the sensitivity of the method as well as performing TPR
(
SEM), energy dispersive X-ray spectrometry (EDS), UV-Raman
spectrometry (RAMAN), X-ray photoelectron spectroscopy (XPS)
and temperature-programmed reduction (H -TPR).
2
experiments under pure hydrogen. Prior to H -TPR, all samples
2
◦
3
were pretreated at 150 C under He (F = 50 cm /min) during 10 min
to eliminate any problem due to physisorbed water. Optimized
resolution was obtained by careful choice of the sample weight
2
. Experimental
3
2.1. Catalysts preparation
(≤30 mg) and hydrogen flow (F = 50 cm /min) taking into account
the criteria established by Ballivet-Tkatchenko and Delahay [57]
The bimetallic copper–cerium oxides were prepared by two
for the temperature-programmed reduction of V O5 that we have
2
routes: (i) the intermetallic route (sample 2CuO·CeO ) using the
also confirmed for the reduction of CuO. The TPR curves became dis-
torted, flat at their maximum and there is a broadening of the peaks
for K > 20 s (P > 3 C) and the optimal conditions comprise between
2
controlled oxidation of CeCu2 under air (Air Liquide, O :N = 20:80
2
2
◦
◦
◦
(
vol.%), purity 99.995%) at 10 C/min heating rate up to 950 C,

A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
49
◦
0–20 s and 1.5–3 C for K and P, respectively. Under our experi-
1
mental conditions, the calculated K and P values were ≈15 s and
◦
≈
3 C. Water was analyzed on a Shimadzu 9A instrument equipped
with thermal conductivity detector (TCD), helium was used as car-
3
◦
rier gas (F = 50 cm /min, TDetector = 105 C, 150 mA), connected to a
CR3 Shimadzu integrator.
2.3. Catalytic activity
The catalytic partial oxidation of methane was carried out at
atmospheric pressure in a fixed-bed U-shaped quartz reactor, plug-
3
flow type reactor, with a quartz frit and an inside volume of 15 cm .
Mass flow controllers were used to control CH4 (Air Liquide, purity
9
9.9995%), air (Air Liquide, purity 99.9995%) and He (Air Liquide,
purity 99.9995%) flows. A thermocouple was placed near the cat-
alytic bed for continuous monitoring of the sample temperature.
Unless otherwise stated, a gaseous mixture with a CH /O2 molar
4
ratio of 2 [CH4 (28%), air (O2 14%, N2 58%)] was used and the reac-
tion studied with an adequate Gas Hourly Space Velocity (GHSV,
mL of CH /g of catalyst h) of 8500, m = 100 mg. The outlet gas
Fig. 1. XRD patterns for the bimetallic copper–cerium oxides obtained by the inter-
4
◦
◦
metallic route (A) and by sol–gel (urea) after calcination at 250 C (B); 600 C (C) and
was cooled in an ice-water trap and their composition analyzed
on-line by gas chromatography using a Restek ShinCarbon ST col-
umn (L = 2.0 m, ꢂ = 1/8 in., ID = 1 mm, 100/200 mesh) and an Agilent
◦
9
00 C (D).
4
890D GC equipped with a thermal conductivity detector (TCD) and
To further characterize the sol–gel and the intermetallic sam-
a 6-port gas sampling valve with a 0.250 ␮L loop.
ples, temperature-programmed reduction (H -TPR) and Raman
2
Catalyst activity (r ) was defined as the number of mL of
studies were also undertaken. Fig. 3 shows the Raman spectra
obtained for all the copper–cerium oxides that are dominated by
i
methane converted per g of catalyst and per hour (mLCH4 /g h).
Unless otherwise stated, the values reported in this paper
represent the activities of the catalysts after 1 h on stream.
The conversion of CH4, the selectivities of CO, CO2, C2
−
1
two main peaks at 466 and 1175 cm , which were assigned to
cubic CeO2 [59,60]. Two minor bands were also observed at about
−
1
298 and 623 cm
.
−
1
(
(
[
C H + C H ) and H2 were calculated as follows: Conv.CH4
The frequency at 466 cm is related to symmetrical stretching
of the Ce–O8 vibrational unit, corresponding to the triply degen-
erate F2g mode of fluorite CeO2 and the only one allowed in first
2
4
2
6
%) = {([CH ] − [CH ]o)/[CH ] } × 100; Sel. (%) = {[CO]o/([CH ] −
4
i
4
4
i
CO
4
i
CH ]o)} × 100; Sel.
(%) = {[CO ]o/([CH ] − [CH ]o)} × 100;
4
CO
2
4
i
4
and
2
−
1
Sel.
(%) = {2 × [C2]o/([CH ] − [CH ]o)} × 100
Sel.H
2
order Raman scattering. The bands at about 623 and 1175 cm
C2
4
i
4
(
%) = {[H ]o/2 × ([CH ] − [CH ]o)} × 100, where [CH4]i is the
were linked to activation of density of states of phonon (DOS)
in CeO2 lattice due to the presence of CeO2 defects that induce
the relaxation of Raman selection rules. These bands are related,
respectively, to Raman inactive transverse and longitudinal opti-
cal phonon modes at the Brillouin zone center and were linked to
oxygen vacancies in the CeO2 lattice [60–62]. Since oxygen vacan-
cies are active sites for combustion reactions, this finding confirms
that copper–lanthanide oxides are promising for applications in
catalytic oxidation processes. The almost undetectable band at
2
4
i
4
inlet flow rate of methane and [CH ]o, [CO]o, [CO ]o, [C2]o and
4
2
[
H ]o are outlet flow rates for methane and products. The amount
2
of reagents and products was confirmed by an external standard
method using reference mixtures of CH , CO, CO , H and C2 (Air
4
2
2
Liquide). The confidence level was better than 95%. The activity
of the catalysts was compared to that of a commercial catalyst
(
[
5.0 wt% Rh/Al O , Aldrich), one standard in this research field
2 3
58].
−
1
2
98 cm was assigned to CuO, which could indicate that copper
species are well dispersed and in close contact with CeO2 [63].
However, the nearly absence of differences in the respective lat-
3
. Results and discussion
tice parameters of the CeO fluorite type cell suggests that no large
2
3.1. Catalysts characterization
amount of copper has been incorporated in the lattice [60,64,65].
Fig. 4 shows typical H -TPR profiles obtained for the reduction
2
Fig. 1 shows the X-ray diffraction patterns obtained before cat-
of 2CuO·CeO (intermetallic route) and CeCu O catalysts obtained
2
2
4
◦
alytic tests. For both series of copper–cerium oxides, sol–gel and
intermetallic samples, the patterns were consistent with those
of standard CuO and CeO2 monoclinic and cubic phases, respec-
by the sol–gel/urea route after calcination at 250, 600 and 900 C.
The H -TPR profiles show one main reduction peak in the tem-
2
◦
perature range of 174–265 C and a second reduction peak between
687 and 701 C (see inset). For the first reduction peak, the tempera-
◦
tively, reported before for the oxidation of CeCu [40], with a nearly
2
absence of differences in the respective lattice parameters.
However, the surface morphology of the copper–cerium oxides
particles was significantly different depending on the synthetic
method. The analysis by SEM/EDS shows that the copper–cerium
oxide obtained by the intermetallic route is composed by aggre-
gates of CeO2 with variable size and platelets of CuO, whereas
the sol–gel samples appear to be better described as a homoge-
neous solution of both oxides (EDS data available as supplementary
material) (Fig. 2). The average particle size is ≈50 nm in the case of
the sol–gel samples, which agrees well with the values calculated
from the Scherrer’s equation (40–60 nm).
ture at maximum mass loss rate (Tm) increases with the calcination
temperature, except for the sample calcined at 250 C. For the sec-
◦
ond mass losses the Tm is practically independent of the calcination
◦
temperature, within the experimental error (± 5 C).
The standard Gibbs free energy for copper oxide reduction is
considerably more favorable than that of CeO2 into Ce O3 (e.g.
2
−98.9 kJ/mol for the reduction of CuO and +24.4 kJ/mol for the
reduction of CeO ) [66]. Therefore, the reduction of CuO is expected
2
at first and the Tm of the first reduction peaks was attributed to the
reduction of CuO [40,42,51]. The Tm of second reduction peaks fits
the published data for the surface reduction of CeO2 [67]. The XRD

5
0
A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
Fig. 2. SEM images obtained before (upper) and after reaction (lower) for the bimetallic copper–cerium oxides: (A) intermetallic route, (B) sol–gel reaction, calcination at
◦
9
00 C.
◦
patterns obtained after the H -TPR study were identical to those
to that of pure CuO, within the Tm experimental error (± 5 C). One
2
already published for the reduction of 2CuO·CeO2 that show pure
crystalline phase of metallic copper along with CeO2 [68], which
confirms that the second reduction peaks correspond to the sur-
face reduction of CeO2 instead of a bulk reduction with formation
possible answer to such difference can be found on the amorphous
character of the sample calcined at 250 C, compared to the XRD
behavior of the other samples (Fig. 1), which could indicate that
the sample calcined at 250 C behave as a mixture of highly dis-
◦
◦
of Ce O3 [67,68].
persed nanoparticles of pure oxides (CuO plus CeO ) without any
2
2
The H -TPR quantitative analysis of the water produced on each
contact between them.
2
mass loss gives a H O:Ce molar ratio of 2.2, 1.9, 2.2 and 2.1 ± 0.2 for
Liu and Flytzani-Stephanopoulos [33] reported Tm values for the
reduction of copper oxide clusters strongly interacting with ceria
2
2
CuO·CeO (intermetallic route), CeCu O 900, CeCu O 600 and
2
2
4
2
4
◦
CeCu O 250 (sol–gel route), respectively. The Tm obtained for the
in the temperature range of 125–175 C, while larger CuO parti-
2
4
◦
first reduction peak of the sol–gel samples is lower than that of pure
CuO, whereas in the case of 2CuO·CeO2 it shifts to higher temper-
ature. Moreover, the Tm is lower for the samples calcined at 600
cles, non-associated with CeO , were reduced at ≈200 C. Wrobel
2
◦
et al. [69] reported copper reduction in the range of 120–150 C
for CuO/CeO2 catalysts. The same results were also reported by
Ratnasamy et al. [70], where two overlapping reductions were
attributed to disperse copper oxide clusters, strongly interacting
with CeO2.
◦
◦
and 900 C than for that calcined at 250 C (222 vs. 174 or 192) that
present a temperature at maximum reduction rate corresponding
Fig. 3. UV-RAMAN spectra for the bimetallic copper–cerium oxide obtained by the
sol–gel and the intermetallic routes.
Fig. 4. H2-TPR profiles for the bimetallic copper–cerium oxides and pure CuO.

A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
51
Fig. 6. Temperature effect on the selectivity to hydrogen (bold symbols) and carbon
dioxide (open symbols) over the bimetallic copper–cerium oxides.
POM over CuO and CeO was also undertaken. Pure CeO was inac-
2
2
◦
◦
tive at T ≤ 650 C, whereas the activity of pure CuO starts at 450 C.
Higher temperatures increase dramatically the activity of CeO that
2
◦
at T ≥ 750 C becomes equal to that of pure CuO ≈ 20% and to that
of the copper–cerium oxides obtained by the sol–gel route.
Through the analysis of the catalytic performance for all the
samples, it can be said that the synthetic method used for prepara-
tion of the catalysts influence the activity and thus the calcination
temperature of the sol–gel samples, however in terms of a small
effect. As shown in Fig. 6, the selectivity to hydrogen is always lower
over the CeCu O4 catalysts (sol–gel samples) than over 2CuO·CeO2
2
(
intermetallic sample). In contrast, carbon dioxide was the main
◦
product over the sol–gel catalysts (T < 650 C), whereas the selec-
tivity to CO2 decreased significantly with the temperature over
◦
2
CuO·CeO2 and became residual (<2%) at T ≥ 650 C.
Moreover, the activity of 2CuO·CeO is only comparable to that
2
Fig. 5. Temperature effect on the conversion of methane over the bimetallic
copper–cerium oxides: (A) intermetallic and (B) sol–gel route.
◦
of catalysts based on noble metals (T ≥ 550 C) reported to be very
active for the POM to syngas reaction [2–6]. As an example, it was
◦
found that the behavior of 2CuO·CeO2 at 800 C is similar to that
Therefore, the H -TPR results of the copper–lanthanide oxides
2
of a commercial catalyst of rhodium (5 wt%) on alumina (Table 1).
obtained by the sol–gel route may be attributed to reduction and
formation of highly dispersed CuO that strongly interacts with
CeO2 thus lowering the temperature at the maximum mass loss
Therefore, it seems to be a good alternative to produce syngas
◦
since it provides at T ≥ 550 C a suitable H /CO ratio of ≈2 for the
2
methanol and Fischer–Tropsch synthesis, with the advantage of
being less expensive than the noble metal catalysts.
◦
rate (except for the sample calcined at 250 C) [71]. To explain the
result obtained with the intermetallic sample, the possibility of an
unusual synergistic effect between CuO and CeO2 that lead to a
higher reduction temperature must be taken into account, which
is certainly connected to this unusual synthetic route that provide
bimetallic copper–cerium oxides more stable to reduction.
The bimetallic copper–cerium oxides activity and selectivity
◦
was also quite stable in the temperature range studied (350–800 C)
and for a long period of time on stream. Fig. 7 shows the perfor-
◦
mance of the sol–gel, intermetallic and rhodium samples at 800 C.
For all the samples and at each temperature studied (350, 450, 550,
◦
6
50, 750 and 800 C), no deactivation was observed for a period of
3.2. Activity and stability of catalysts
≈18 h of time on stream (total time on stream superior to 100 h).
To our knowledge, the results obtained over 2CuO·CeO are a
2
The copper–cerium oxides were active and selective for the cat-
novelty on copper based catalysts, which corroborate the advan-
tage of the intermetallic compound CeCu2 as precursor for the
preparation of bimetallic copper–cerium oxide catalysts.
alytic partial oxidation of methane. The conversion of methane
◦
becomes measurable in the temperature range of 350–800 C and
increases with the reaction temperature (Fig. 5).
The catalyst obtained by the intermetallic route presents
always the highest activity and converts about 90% of methane
at 750 C (Fig. 5A), whereas over the sol–gel samples the con-
Table 1
◦
Ce/Cu, O/(Ce + Cu) and C/(Ce + Cu) atomic ratios computed from the XPS Ce 3d, Cu
2p, O 1s and C 1s core level peaks, before and after reaction.
version of methane is considerably lower ≈20% (Fig. 5B). Among
the copper–cerium oxides obtained by the sol–gel method, the
Catalysts
Ce/Cu
O/(Ce + Cu)
C/(Ce + Cu)
◦
catalysts calcined at 250 and 600 C presented the same perfor-
0.6 (0.3)a
0.6 (0.9)
0.7 (0.8)
0.7 (0.5)
2
CuO·CeO2
2.6 (2.1)
1.9 (2.2)
1.7 (1.9)
2.3 (1.9)
1.9 (1.0)
1.2 (2.7)
1.1 (1.9)
1.0 (3.8)
◦
mance: become active at 350 C and reach an activity of about
CeCu2O4 250
CeCu O 600
◦
◦
2
0% at T ≥ 550 C. The catalyst calcined at 900 C was inactive until
2
4
◦
◦
CeCu2O4 900
T = 650 C but at 800 C, the conversion of methane was ≈20%, as for
a
the other sol–gel samples. For comparison proposes, the study of
Within parentheses the values obtained after reaction.

5
2
A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
In order to explain our results and to approach the true nature
of the active sites, an analysis of the catalysts after reaction was
undertaken. Figs. 2 and 8 show some of the SEM pictures obtained
after reaction (AR). Important modifications occur on the bimetallic
copper–cerium oxides surface morphology that in the case of the
intermetallic catalyst become more porous, whereas in the case
of the sol–gel samples the surface is now composed of aggregates
of cerium oxide grains and platelets of copper oxide (EDS analysis
shows evidence of copper agglomerations at the surface) (Fig. 2).
Sample CeCu O 250 shows also evidence of few tubes that mor-
2
4
phologically appear to be carbon nanotubes (Fig. 8A), which was
confirmed by the analysis of the sample after a long period of time
on stream (Fig. 8B). The surface of the sample is covered by tubu-
lar structures that appear to be carbon nanotubes with variable
diameter (≈30–70 and 100 nm) and variable length.
However, the element mapping shows that cerium is clearly
detected in the place of tubes (EDS, supplementary material). Nev-
ertheless, quantification of carbon by SEM-EDX is not possible in
these conditions because the powders were previously covered
with a thin film of carbon of which the thickness cannot be mea-
sured without adequate equipment in order to obtained proper
secondary images from SEM. The average particle size was com-
parable to those obtained before reaction.
After reaction and especially after a long period of time on
stream (stability studies), the formation of other oxide phases that
could correspond to the formation of new solid solutions between
CuO and CeO was by no means detected. However, the appearance
2
of Cu O (Fig. 9) and Cu (Fig. 10) due to copper oxide partial and total
2
reduction, respectively, was identified by XRD.
In addition, Raman spectra of the sol–gel samples after reac-
−
1
tion exhibit one extra peak at ≈1586 cm (Fig. 11). After a long
time on stream (stability studies) another extra band is observed at
−
1
≈
1415 cm . As an example, Fig. 11 shows also the Raman spectra
◦
obtained for the sol–gel sample calcined at 900 C after the sta-
bility test at 800 C. These bands can be assigned to the existence
◦
Fig. 7. Partial oxidation of methane over the bimetallic copper–cerium oxide as a
function of time on stream at 800 C: (A) samples obtained by the sol–gel reaction
and by (B) sample obtained by the intermetallic route and commercial catalyst 5 wt%
Rh/Al2O3.
◦
of carbon (graphite) on the surface that increases drastically after
a long time on stream [75,76], confirming the surface modifica-
−1
tions revealed by SEM. In particular, the band at ≈1586 cm has
been attributed to the existence of considerable fraction of carbon
at grain boundaries [77]. For the intermetallic sample such bands
were practically inexistent, even after a long time on stream at high
temperature.
This raises the question about the nature of the active sites for
the catalytic POM over copper–lanthanide oxides and the roles
played by copper and ceria on their formation. Ceria is catalytically
active for the oxidation of methane [19,20]. Otsuka et al. [21–23]
showed that ceria is able to directly convert methane to syngas
Finally, the surface of the bimetallic oxides was also studied by
XPS before and after reaction. The Ce 3d region presents a com-
plex structure of doublets typical of Ce4+ in CeO2 [78,79] (Fig. 12A).
The Cu 2p region is composed by two doublets with spin–orbit
separations of 19.9 ± 0.1 eV and a multiplet structure character-
istic of Cu2+ species, like CuO or Cu(OH)2. Cu 2p3/2 components,
centered at 933.8 ± 0.1 and 936.8 ± 0.5 eV, are assigned to Cu2+
◦
at temperatures higher than 600 C. Moreover, recent studies have
shown that the oxygen vacancies in ceria are very efficient for the
activation of gold and copper nanoparticles for the water gas shift
reaction [72–74].
Fig. 8. SEM pictures obtained after reaction (A) and after a long time on stream (B) for the bimetallic copper–cerium oxide obtained by the sol–gel route after calcination at
◦
2
50 C.

A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
53
Fig. 9. XRD patterns obtained after reaction for the bimetallic copper–cerium oxides
prepared by the intermetallic route (A) and by sol–gel (urea) after calcinations at
◦ ◦
50 C (B) and 600 C (C).
2
Fig. 12. XPS Ce 3d (A) and Cu 2p (B) regions for the intermetallic and sol–gel samples
before reaction (BR).
bound to O and OH groups, respectively (Fig. 12B). However, the
main peak centered at 933.8 eV is wide enough (3.3 ± 0.2 eV) to
include also copper in other oxidation states, as shown by the
+
dashed lines drawn in Fig. 12B, corresponding to Cu (Cu O) and
2
0
0
Fig. 10. XRD patterns obtained after the stability study for the bimetallic
Cu . However, Cu is unlikely to be detected on the extreme sur-
face since it is expected to oxidize immediately after the sample is
handled under the atmosphere. After reaction, no change in the
binding energies (BE) of the Ce 3d and Cu 2p regions could be
found.
copper–cerium oxides prepared by the intermetallic route (A) and by sol–gel (urea)
◦
◦
after calcinations at 250 C (B) and 900 C (C).
C 1s is dominated by a peak centered at 285 eV corresponding to
aliphatic carbon contamination, but in sol–gel samples and espe-
cially after reaction, it also includes an important contribution from
graphite (∼284.2 eV) that corroborates the results of Raman spec-
troscopy (Fig. 13). The main component of the O 1s region, centered
at 529.8 ± 0.1 eV, is attributed to oxygen of metal or lanthanide
oxides. At higher BE one can find two extra components centered
at 531.8 ± 0.1 and 533.2 ± 0.2 eV, both presenting the contribution
of oxygen in hydroxide and organic oxygen [80].
The quantitative XPS results show that the atomic Ce/Cu ratio
is very similar in both synthetic methods. However, after reaction,
there is a considerable enrichment in copper for the intermetallic
samples, whereas for sol–gel samples the surface is depleted of cop-
per, since the stoichiometric ratio should be Ce/Cu = 0.5 (Table 1).
Table 1 also presents the atomic oxygen/metal ratio. If the stoi-
chiometric oxygen existed, this ratio should be:
Fig. 11. UV-RAMAN spectra obtained after reaction and after the stability (SS)
studies over the bimetallic copper–cerium oxide prepared by the sol–gel and inter-
metallic routes.
0
2Ce/Cu + 1
Ce/Cu + 1
=
Ce + Cu

5
4
A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
seem to be the result of a smaller amount of copper species at
the catalysts surface and deactivation due to carbon deposition
that poisons their active sites [81]. Some copper particles may
remain embedded in the ceria lattice and/or partially covered by
carbon and could not be accessible to the reagents, which could also
◦
explain the slowdown of the increase of the activity for T ≥ 550 C
(Fig. 5).
A
closer interaction between copper and ceria in the
copper–ceria oxides and/or its higher dispersion certainly explains
the higher copper reducibility on the sol–gel samples. It is clear that
the CuO phase is reduced to metallic copper or at least partially to
Cu O at the end of reaction. The XRD signals attributed to the Cu
2
and Cu O phases were low, but very intense after a long time on
2
stream (stability study). Furthermore, the catalysts were only active
◦
at temperatures higher than 350 C, which could be correlated with
the formation of Cu O and metallic copper, as expected from the
2
H -TPR results, pointing out a major role of such species; it is known
2
that a partially reduced metal clearly enhances the catalytic activity
[
82].
At last, the unusual H -TPR behavior of the intermetallic sam-
2
ples raises the possibility of an unusual interaction between copper
and ceria species that could also contribute to their high activity
and stability, particularly to a slowdown of the sintering of the cop-
per particles at high reaction temperatures due to physical barriers
established by the CeO particles. As previously reported for nickel
2
based catalysts, bulk diffusion of nickel atoms in lanthanum oxide
perovskite-type structure is inhibited as a consequence of the phys-
ical barriers established by the La O3 particles [83,84]. Moreover,
2
the metallic Ni crystallites maintain a high dispersion on the La O3
2
surface. On the basis of this argument, Fierro and coworkers [85]
concluded that the good stability of the catalyst is mainly due to the
ability to stabilize the Ni crystallites in a high dispersion degree on
a La O3 matrix, thus limiting the extent of the formation of aggre-
2
gates of nickel particles. As a consequence, the formation of coke
residues that would encapsulate or cover metallic Ni is also limited
and hence the deactivation of the catalyst.
Fig. 13. XPS C 1s regions for regions for the intermetallic (A) and sol–gel samples
B) before (BR) and after reaction (AR).
(
The same argument can be applied to the 2CuO·CeO2 cata-
lyst. Nevertheless, only the possibility of an unusual interaction
between copper and ceria species on the intermetallic sample can
explain the low reducibility of copper on that sample, which rein-
forces the importance of the close contact between CuO and CeO2
that is a direct consequence of the synthetic method used in this
work. However, this point deserves a deeper analysis in the near
future.
Given the Ce/Cu ratio found for samples before reaction, the value
should be around 1.4. This means that, in the all the samples, the
oxygen exceeds the stoichiometric amount since the oxygen bound
to carbon is almost negligible when compared to the total amount
of oxygen. After reaction, the amount of oxygen in all the samples
is similar within the experimental error. In fact, in the oxygen rich
samples the relative amount of oxygen decreases a lot after reac-
tion, whereas the sol–gel 250 and 600 samples slightly increase
that amount.
4. Conclusions
Another interesting finding is that after reaction, the relative
amount of carbon increases for the sol–gel samples while for
the intermetallic ones it decreases (Fig. 13 and Table 1). This
means that the reaction evolution seems to clean the surface of
the intermetallic sample but poisons the surface of the sol–gel
samples which correlates well with the Raman results. A possi-
ble explanation for this behavior is that the carbon seen by XPS
in the intermetallic sample is completely external and then, eas-
ily removable by temperature increase and gas flow. Contrarily,
in the other samples it is rather mixed with the other compo-
nents and instead of being removed they can act as nucleation
sites for further carbon compounds deposition poisoning the cata-
lyst.
Therefore, the high activity and selectivity of the 2CuO·CeO2
sample (intermetallic route) seem to be related to their stability
to deactivation and to a major enrichment of the surface com-
position on copper and oxygen, as shown by the XPS results,
which correlates with the surface morphologic differences found
between the sol–gel and intermetallic route sample as shown by
SEM/EDS. The lower activity and stability of the sol–gel samples
The results obtained in the present work show that the
copper–cerium oxide catalyst (2CuO·CeO ) obtained by the inter-
2
metallic route was very active and selective for the partial oxidation
of methane and production of syngas suitable for the methanol
and Fischer–Tropsch synthesis (Conv. CH > 90%, Sel. H > 99%,
4
2
◦
H /CO ≈ 2.0) at T = 750 C, which to our knowledge is a novelty on
2
copper based catalysts. The catalytic performance of 2CuO·CeO
2
is only comparable to that of noble metals commercial catalysts,
e.g. 5 wt% Rh/Al O . The influence of synthetic method on the cat-
2
3
alytic performance was confirmed by comparison of such results
with those of CeCu O4 catalysts obtained by the sol–gel method
2
in the presence of urea that were less active and selective than
2
CuO·CeO . The copper–cerium oxide catalysts were quite stable
2
◦
in the temperature range studied, 350–800 C, for a long period of
time on stream and less expensive than the catalysts based on noble
metals, which is an advantage of the intermetallic route. This work
will be extended to other intermetallic compounds in order to find
correlations along the lanthanide series and to study the influence
of the d metal on their catalytic behavior.

A.C. Ferreira et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 47–55
55
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[
2
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Appendix A. Supplementary data
[
[
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