Journal of Natural Gas Chemistry 18(2009) –
La2−xCexCu1−yZnyO4 perovskites for high temperature
water-gas shift reaction
S. S. Maluf, E. M. Assaf∗
Instituto de Quı́mica de São Carlos, Universidade de São Paulo, Av. Trabalhador Sãocarlense, 400 São Carlos, SP 13560-970, Brazil
[ Manuscript received November 10, 2008; revised February 13, 2009 ]
Abstract
The performance of La2−x Cex Cu1−y Zny O4 perovskites as catalysts for the high temperature water-gas shift reaction (HT-WGSR) was investigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites
exhibited the La2 CuO4 orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the
La1.90 Ce0.10 CuO4 perovskite alone, when calcined at 350/700 ◦ C, also showed a (La0.935 Ce0.065 )2 CuO4 perovskite with tetragonal structure, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those
calcined at 350/700 ◦ C and, among these, La1.90 Ce0.10 CuO4 was outstanding, probably because of the high surface area associated with the
presence of the (La0.935 Ce0.065 )2 CuO4 perovskite with tetragonal structure and orthorhombic La2 CuO4 phase.
Key words
perovskite; shift reaction; lanthanum; copper; cerium
1. Introduction
The water-gas shift reaction (WGSR) is mostly used in the
production of hydrogen via the steam reforming of hydrocarbons and is of importance for future energy technologies such
as fuel cells [1]. Polymer electrolyte fuel cells (PEFC) have
been extensively studied [2−4] due to their attractive properties, such as high power density, low emissions of NOx , dust,
noise, etc., low temperature operation and compactness [5,6].
In this system, hydrogen is used as a fuel; it is supplied from
steam reforming of hydrocarbons such as methane, propane
and kerosene. Therefore, the H2 obtained via methane steam
reforming undergoes a purification process before application
in fuel cells. This process consists of the high temperature
water-gas shift reaction (reducing the CO gas of 10% to 3%),
the low temperature water-gas shift reaction (reducing the CO
gas of 3% to 0.5%) and the preferencial oxidation reaction
(reducing to 5−10 ppm of CO) as the last step.
The problem is that the reformed gas contains CO at the
level of 1%−10% which adsorbs irreversibly on the Pt electrode of the PEFC at the operating temperature (ca. 80 ◦ C)
and hinders the electrochemical reaction [7,8]. Therefore,
CO must be removed from the reformed gases to less than
∗
10−20 ppm before feeding the gas mixture to the Pt electrode.
The water-gas shift reaction is desirable for removal of a large
amount of CO since it is a moderately exothermic reaction
(∆H 298 = −41.1 kJ/mol) and the reaction temperature is easy
to control. The equilibrium conversion of CO is dependent
largely on the reaction temperature: since the shift reaction
is an exothermic reaction, lower temperature is favored for
higher CO removal.
Catalysts for the high-temperature (>300 ◦ C) WGSR
are typically based on iron-chromium oxides, while those
used in the low-temperature (<300 ◦ C) WGSR are copperzinc oxide based [4,9]. However, the copper-based catalysts, although active, are handicapped (especially for fuel
cell applications) by their sensitivity to exposure to air and
water condensation [1].
It has been shown that oxides with high oxygen storage
capacity such as CeO2 can exhibit high WGSR activity in conjunction with various metal promoters [1,3,10−13]. The role
of ceria in such systems is proposed to be via a ceria-mediated
redox process, where the oxygen storage capacity of ceria and
the metal are the active elements [11,13].
In the presence of Cu, ceria reduction is proposed to start
at low temperatures (<200 ◦ C) [10]. Ceria is also responsible
Corresponding author. Tel: +551633739951; Fax: +551633739952; E-mail: eassaf@iqsc.usp.br
The work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo.
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
doi:10.1016/S1003-9953(08)60091-2
2
S. S. Maluf et al./ Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
to enhance reducibility and to stabilize the Cu particles towards sintering [14]. However, the effect of ceria on the Cu
particles is not clear.
Additionally, an interesting oxide structure, focused on
this paper, is the perovskite. The general structural formula
is ABO3 , where A and B are usually rare earth and transition
metal cations, respectively, and the corresponding perovskites
show high activity and thermal stability. These have frequently been exploited in the development of catalysts since
the substitution of foreign cations allows their catalytic properties to be modified systematically.
The perovskites oxides have several advantages relative
to other oxides simply, because the ability to accommodate
a variety of ions of different valence, leading to a high electronic conductivity and high mobility of oxygen. Then, these
oxides are classified as non-stoichiometric or oxides with oxygen non-stoichiometric. In addition, they show better thermal
stability than oxides of transition metals.
The perovskites have been studied for several reactions,
including oxidation of CO and hydrocarbons [15−18], CO
oxidation by NO, CO hydrogenation, NOx decomposition and
eletrocatalysis [19−21]. However, there is little work on the
use of perovskites as catalysts for the water-gas shift reaction
[10,22,23]. For these reasons, in the present study, La2−x Cex Cu1−y Zny O4 perovskites were prepared, characterized and
tested for high temperature water-gas shift reaction to estimate
the activity and production of hydrogen.
2. Experimental
2.1. Catalysts
Samples were prepared by Pechini [24], which it is
a polymerization method, using La(NO3 )3 ·6H2 O (VETEC),
Cu(NO3 )2 ·3H2 O (Riedel-de Haën), Ce(NO3 )3 ·6H2 O (AlfaAesar) and Zn(NO3 )2 ·6H2 O (Synth). The nitrates were dissolved in water (in the desired ratios) and stirred at 60 ◦ C.
This solution was added to a solution of citric acid in ethylene
glycol (60 : 40 w/w) and the resulting mixture was evaporated
(110−120 ◦ C) for 24 h with vigorous stirring in order to carry
out the polymerization. Products were dried at 80 ◦ C for 24 h.
For calcination step, it was realized by two different heating schedules: the first consisted of pre-calcination at 350 ◦ C
for 2 h and calcination at 700 ◦ C for 4 h; the second of precalcination at 550 ◦ C for 1.5 h and calcination at 900 ◦ C for
10 h. All thermal treatments were performed in air, flowing at
50 ml/min, with a 5 ◦ C/min heating rate.
The nominal compositions of the catalysts were:
La2 CuO4 , La1.95 Ce0.05 CuO4 , La1.90 Ce0.10 CuO4 and La1.95 Ce0.05 Cu0.8 Zn0.2 O4 .
2.2. Characterization techniques
Samples were characterized by the following methods:
(1) Energy dispersive X-ray spectroscopy (EDS) to determine chemical compositions; the equipment used was an EDX
Link Analytical QX 2000 coupled to a LEO 440 electron microscope linked to an Oxford detector.
(2) X-ray diffraction (XRD) to determine crystal structures, in a Rigaku Multiflex diffractometer, with a Cu Kα
(1.5406 Å) radiation source; the X-ray patterns were recorded
for 2θ values ranging from 20o to 70o .
(3) N2 physisorption to determine surface area by the BET
method, at 77 K, using a Quantachrome NOVA 2000.
(4) Temperature programmed reduction (TPR) measurements were performed in a quartz U-shaped tube reactor with
a mixture of H2 (1.96%)/Ar flowing at 30 ml/min. The catalyst sample (50 mg) was heated from room temperature to
1000 ◦ C at a rate of 10 ◦ C/min. The water produced during
the reduction was removed by driving the effluent gas through
a tube containing silica gel. The outlet gas was analyzed by a
thermal conductivity detector (TCD) and the H2 consumption
was measured by comparing the corresponding peak area with
that produced by a standard CuO sample.
(5) X-ray absorption near-edge structure (XANES) spectra at the Cu K-edge for samples were measured at the
Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil. A Si (111) monochromator was used to select
the X-ray beam from the synchrotron light produced by the
1.37 GeV electron storage ring with a maximum current of
200 mA. The Cu K-edge absorption spectra were recorded
in the transmission mode, in a range of photon energy from
8900 to 9400 eV, using a CCD camera. In an atmosphere of
H2 /CO2 (1 : 1 v/v) flowing at 30 ml/min, the oxidized samples
were heated in situ from 22 to 400 ◦ C, at 10 ◦ C/min, and held
at 400 ◦ C for 30 min. The XANES spectra at the Cu K-edge
were collected in situ at various temperatures. The purpose of
the XANES analysis is to provide information on the different
oxidation states of copper (Cu2+ , Cu+ , and/or Cu0 ) under the
reaction conditions (temperature, atmosphere of the reaction
products).
2.3. Catalytic tests
Catalytic activity measurements were carried out at
360 ◦ C in a fixed-bed tubular glass micro-reactor, so as to analyze the activity and hydrogen production. This temperature
was chosen because there was no catalytic activity for temperatures lower than 360 ◦ C.
The apparatus for catalytic tests consisted of flow controllers, the reactor unit, and the analytical system. The flow
system consisted of a set of mass-flow controllers (AAlborg,
four channels) which accurately controlled the flow of feed
gases (55 ml/min, 5%CO/N2 mixture). The water for the reaction was supplied by a saturator, previously calibrated, at a
constant temperature. The feed gas was passed through the
saturator and it was wet enough for the reaction to occur. The
molar ratio utilized was H2 O : CO = 2.3 : 1.
The mass of catalyst used was 100 mg (60−80 mesh)
and it was introduced into the reactor on a porous plate.
The reactor was placed in a furnace with a thermocouple
placed close to the catalytic bed, to ensure precise temperature
measurements.
3
Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
All the reaction products were analyzed in-line by gas
chromatography (Varian, Model 3800) with two TCDs, using an automatic injection valve. The reaction product stream
was divided into two outlet streams, which were analyzed
differently in order to obtain accurate and complete quantification of the reaction products. One of these aliquots was
used to analyze hydrogen, which was separated in a 13X
molecular-sieve packed column, using N2 as carrier gas. The
other aliquot was used to analyze CO2 and CO separated in
Porapak N-packed columns, using He as carrier gas. The water present in the stream was condensed and removed before
injection into the chromatograph.
3. Results
3.1. Energy dispersive X-ray spectroscopy (EDS)
Table 1 shows the average mass percentage of the components, obtained by EDS, and it is seen that the real values
are very close to the nominal composition (except in La2 CuO4
sample) showing the method is efficient. The nominal compositions were calculated assuming La2−x Cex Cu1−y Zny O4 as
the formula of the prepared perovskites. The amount of La
was generally higher than the theoretical content. These results are in agreement with Merino et al [25].
Table 1. EDS results for samples
Sample
a La CuO
2
4
a La
1.95 Ce0.05 CuO4
a La
1.90 Ce0.10 CuO4
a La
1.95 Ce0.05 Cu0.8 Zn0.2 O4
b La CuO
2
4
b La
1.95 Ce0.05 CuO4
b
La1.90 Ce0.10 CuO4
b La
1.95 Ce0.05 Cu0.8 Zn0.2 O4
a
nominal
81.3
79.3
77.2
79.3
81.3
79.3
77.2
79.3
La
experimental
74.6
79.9
77.6
80.4
71.7
79.5
77.8
80.1
nominal
18.7
18.7
18.8
14.9
18.7
18.7
18.8
14.9
Content (%)
Cu
experimental
nominal
25.4
−
19.1
2.0
19.3
4.0
14.5
2.0
28.9
−
18.6
2.0
18.1
4.0
13.6
2.0
Ce
experimental
−
1.0
3.1
1.2
−
1.9
4.1
1.9
nominal
−
−
−
3.8
−
−
−
3.8
Zn
experimental
−
−
−
3.9
−
−
−
4.4
Calcination at 350/700 ◦ C; b calcination at 550/900 ◦ C
3.2. X-ray dif fraction (XRD)
X-ray patterns of pervoskites, calcined at 350/700 ◦ C and
550/900 ◦ C, respectively, are presented in Figures 1 and 2.
In Figure 1, the diffraction lines corresponding to orthorhombic La2 CuO4 (JCPDS # 82-2142) show that it is
present as the main phase in all solids, with high crystallinity
and well-defined symmetry. The X-ray patterns of La2 CuO4
also show, besides the dominant La2 CuO4 phase lines, three
small peaks at 2θ = 35.9o, 38.9o and 49.1o of a segregated
CuO phase (JCPDS # 80-1917). With 0.05%Ce, the signals
of CuO disappear and the La2 CuO4 peaks remain. However,
when the Ce loading was increased to 0.10%, the intensity
of signals decreased and significant changes occurred: three
new peaks at 2θ: 23.3o, 45.2o and 51.6o appeared; the signal at 43.7o was divided into two (43.2o and 43.5o) and the
intensity of the 55.9o signal was weak, compared to other
diffractograms.
Analyzing these results, signals are found that refer to
the (La0.935Ce0.065 )2 CuO4 perovskite with tetragonal structure (JCPDS # 48-0796), whose main peaks are at 2θ = 31o ,
55.8o, 45.2o, 23.3o, 43.2o and 43.6o. This is a consequence
of the replacement of La by Ce, which introduces structural
disorder, since Ce has a greater ionic radius than La, and can
cause a change in the crystal lattice, as the chemical structure
is very complex [25].
The perovskite with Zn produced an X-ray pattern similar to that of La1.95 Ce0.05 CuO4 perovskite, showing total dominance of the La2 CuO4 orthorhombic phase, but
with less intense diffraction lines. Thus, as the degree of
ion substitution increases, the diffraction line intensity falls,
and also there is a slight diffraction-line shift to higher values
of the Bragg angle, θ.
The peaks of CeO2 were not observed because the Ce is
a highly dispersed phase or incorporated to the structure of
the La2 CuO4 because some peaks of samples with Ce have
2θ values shifted to lower values in relation to the La2 CuO4
sample (Figure 1).
Figure 1.
X-ray diffractograms of perovskites calcined at (a)
350/700 ◦ C. (1) La2 CuO4 , (2) La1.95 Ce0.05 CuO4 , (3) La1.90 Ce0.10 CuO4 ,
(4) La1.95 Ce0.05 Cu0.8 Zn0.2 O4
4
S. S. Maluf et al./ Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
The X-ray patterns of Figure 2 show high crystallinity
and dominance of the La2 CuO4 orthorhombic phase, in the
perovskites calcined at 550/900 ◦ C. The La2 CuO4 sample behaved similarly to perovskites calcined at 350/700 ◦ C. With
sample La1.95 Ce0.05 CuO4 , the copper oxide peak disappeared
and a new signal appeared at 2θ = 28.2o, while for sample
La1.90 Ce0.10 CuO4 , this signal increased and two new peaks
appeared: 46.7o and 55.2o . These could be due to CeO2 ,
but with 2θ values shifted (#JCPDS 81-0792). When Zn was
added, peaks at 2θ = 28.2o and 46.7o decreased significantly
and the peak at 55.2o overlapped with a peak at 56.0o to form
a single peak. This shows that there was a decrease of segregated CeO2 on substitution of Cu by Zn, while the La2 CuO4
orthorhombic phase remained. Summarizing, for this series
of perovskites, high temperatures associated with the substitution of La by Ce in the samples favored segregation of CeO2 .
this type of material [25−27], as the structure is well defined
and crystalline (according to the XRD results).
Results show that for samples calcined at 350/700 ◦ C the
surface area increased as cerium was added. This increase was
very significant for La1.90Ce0.10 CuO4 (350/700 ◦ C), probably because the partial substitution of La by Ce can change
the oxidation state of the copper and/or produce structural
defects (as the presence of (La0.935Ce0.065 )2 CuO4 perovskite
with tetragonal structure), which can be associated with the
physicochemical properties of the material [15−18] producing an increase in the observed surface area. These results are
in agreement with the literature [25,26,28].
Perovskites calcined at 550/900 ◦ C showed very low surface areas, as a result of the sintering that occurred at the
higher temperature to which they were submitted.
Table 2. Surface area results
Temperature of thermal
Sample
treatment (◦ C)
La2 CuO4
350/700
La1.95 Ce0.05 CuO4
350/700
La1.90 Ce0.10 CuO4
350/700
La1.95 Ce0.05 Cu0.8 Zn0.2 O4
350/700
La2 CuO4
550/900
La1.95 Ce0.05 CuO4
550/900
La1.90 Ce0.10 CuO4
550/900
La1.95 Ce0.05 Cu0.8 Zn0.2 O4
550/900
Surface area
(m2 /g)
2.5
2.4
8.0
3.8
−*
0.2
0.3
0.8
* Measurement not possible.
3.4. Temperature programmed reduction
The TPR patterns are presented in Figures 3 and 4. For
perovskites calcined at 350/700 ◦ C (Figure 3), all TPR patterns were similar, i.e, two reduction peaks, a more intense
one at 400 ◦ C and a weaker one at 520 ◦ C.
Figure 2. X-ray diffractograms of perovskites calcined at 550/900 ◦ C.
(1) La2 CuO4 , (2) La1.95 Ce0.05 CuO4 , (3) La1.90 Ce0.10 CuO4 , (4)
La1.95 Ce0.05 Cu0.8 Zn0.2 O4
Comparing the XRD results obtained for the two series
of perovskites, it is concluded that both series show the prevalence of La2 CuO4 orthorhombic phase; with milder heat treatment (350/700 ◦ C), the presence of 0.10% Ce favored the
formation of (La0.935Ce0.065 )2 CuO4 perovskite with tetragonal structure; the replacement of Cu by Zn, in both series of
perovskites, caused no structural changes in samples and the
diffraction line intensity of perovskites calcined at 550/900 ◦ C
was higher, probably because the high temperatures of calcination enhanced the crystallinity of the samples. And, it can
also be concluded that the Pechini method, with the conditions here utilized (high solubility of reactants, slow rate of
drying and calcination), is suitable for the preparation of pure
perovskites.
3.3. BET surface area
The surface areas of the prepared catalysts are summarized in Table 2. These values are lower than the areas of conventional supported catalysts, but such behavior is common in
Figure 3. TPR profiles of perovskites calcined at 350/700 ◦ C
The first peak (400 ◦ C) is assigned to reduction of copper ions in the perovskite to metal: Cu2+ → Cu+ (Equation
1) and the peak at 520 ◦ C is related to reduction of cuprous
oxide: Cu+ → Cu0 (Equation 2) [18,29−31].
5
Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
2La2 CuO4 + H2 → Cu2 O + 2La2O3 + H2 O
(1)
(2)
Cu2 O + H2 → 2Cu0 + H2O
For La2 CuO4 sample, there is a weaker shoulder at
340 ◦ C, attributed to the reduction of the segregated CuO
species that appears in the XRD results [18,29−31].
Experimental and nominal amounts of H2 consumed are summarized in Table 3. For La2 CuO4 and
La1.95 Ce0.05 Cu0.8 Zn0.2 O4 samples, the experimental and
nominal amounts of H2 consumed are practically equals,
while for other perovskites, the experimental value is lower
than the nominal. This occurs probably because all copper of
the samples is not reduced as considered in the calculation of
nominal values. As the degree of substitution of these samples rises, it occurs as a slight shift of the peak maximum to
high temperature, showing higher stability in these samples
probably because the Cu species interact strongly with Ce.
Additionally, Table 3 shows the amount of H2 consumed
in the 520 ◦ C peak increase as the degree of substitution of the
samples and this can be attributed to increase of reduction of
Cu+ species since the replacement of La3+ by Ce4+ produces
an excess of positive charge in the structure of the perovskite,
which is balanced by reduction of Cu2+ to Cu+ species [32].
Alternatively, this peak may be the consequence of reduction
of Ce4+ species, which are reduced in this temperature range
[4,29,33−35].
Table 3. TPR data for perovskites calcined at 350/700 o C
Sample
La2 CuO4
La1.95 Ce0.05 CuO4
La1.90 Ce0.10 CuO4
La1.95 Ce0.05 Cu0.8 Zn0.2 O4
Total amount of H2 (mol)
nominal
experimental
2.0×10−4
2.1×10−4
2.1×10−4
1.5×10−4
1.9×10−4
1.1×10−4
−4
1.4×10
1.3×10−4
In the TPR profile of La1.95 Ce0.05 Cu0.8 Zn0.2 O4 , the peak
at 520 ◦ C was shifted to 600 ◦ C and a shoulder appeared in
the range 650 ◦ C to 800 ◦ C, that can probably be ascribed
to the reduction of Ce4+ present in the bulk of the sample
[4,29,33−35].
Figure 4 shows the TPR patterns of perovskites calcined at 550/900 ◦ C. All these spectra have three reduction
peaks: 380 ◦ C, 420 ◦ C and 520 ◦ C. The previous comments
regarding perovskites calcined at 350/700 ◦ C also apply to
Figure 4. TPR profiles of perovskites calcined at 550/900 ◦ C
Total experimental
amount of H2 (mol/g)
40.5×10−4
19.8×10−4
15.7×10−4
18.9×10−4
H2 amount
at 520 ◦ C peak (mol)
1.7×10−5
2.2×10−5
3.5×10−5
3.5×10−5
perovskites calcined at 550/900 ◦ C. Thus the shoulder at
380 ◦ C in La2 CuO4 is from the reduction of segregated CuO,
seen in XRD results. The signal at 420 ◦ C is from reduction
of Cu2+ → Cu+ (Equation 1) and the peak at 520 ◦ C shows
reduction of Cu+ → Cu0 (Equation 2) [29−31]. With the
addition of the Ce, the area of the peak at 380 ◦ C decreases
(since the content of segregated CuO falls) and the signals at
420 ◦ C and 520 ◦ C begin to separate, to turn the peaks more
definable. For La1.95 Ce0.05 Cu0.8 Zn0.2 O4 sample, the peak at
520 ◦ C shifted to 600 ◦ C, similar behavior was obtained for
the sample calcined at 350/700 ◦ C.
In all samples, the total experimental value of H2 consumed (Table 4) was lower than the nominal amount, and this
may be attributed to the same previous comments regarding
preovskites calcined at 350/700 ◦ C.
Similar to perovskites calcined at lower temperatures, the
presence of cerium provokes an increase in consumption of
H2 in the 520 ◦ C peak (Table 4), indicating that surface Ce4+
species are reduced together with copper [4,29,33−35], or that
the Cu+ content increases as a consequence of replacement of
La by Ce [32].
The TPR patterns show that perovskites calcined at high
temperatures produced more defined peaks, as a consequence
of the severe thermal treatment during calcinations. Nevertheless, these perovskites showed a lower real consumption of
H2 than the perovskites calcined at low temperatures. This is
probably due to the sintering process that would have occurred
at 900 ◦ C.
Table 4. TPR data for perovskites calcined at 550/900 o C
Sample
La2 CuO4
La1.95 Ce0.05 CuO4
La1.90 Ce0.10 CuO4
La1.95 Ce0.05 Cu0.8 Zn0.2 O4
Total amount of H2 (mol)
nominal
experimental
2.6×10−4
1.6×10−4
1.9×10−4
1.6×10−4
−4
2.5×10
1.5×10−4
2.3×10−4
2.1×10−4
Total experimental
amount of H2 (mol/g)
26.5×10−4
22.1×10−4
15.4×10−4
16.6×10−4
H2 amount
at 520 ◦ C peak (mol)
2.0×10−5
4.2×10−5
3.7×10−5
7.4×10−5
6
S. S. Maluf et al./ Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
3.5. Catalytic tests
Figure 5 shows the CO conversion for catalysts calcined
at 350/700 ◦ C. The LaCuOx and La1.95Ce0.05 CuO4 catalysts produced the same conversion (60%). When the content of Ce was increased (0.10%), the conversion rose to
75%. Meanwhile, the La1.95 Ce0.05 Cu0.8 Zn0.2 O4 perovskite
performed worse than all other catalysts, as this sample favored the reverse shift reaction (CO2 +H2 → CO+H2 O).
La1.90Ce0.10 CuO4 perovskite presented the best performance
(75%). This can be explained by its high surface area (Table 2), which may be connected with the formation of the
(La0.935Ce0.065 )2 CuO4 perovskite with tetragonal structure.
Another possible explanation would be that catalytic performance is associated with the filling of the valence orbitals of
the metals, the higher energy level of the filled valence orbital,
the higher the availability of electrons to the reaction. Since
the electron configurations of La and Ce are [Xe] 5d1 6s2 and
[Xe] 6s2 4f 2 , respectively, Ce has a higher filled energy level
than La, increasing the energy of the valence band of the compound and providing more electrons for reaction. Hence, the
presence of Ce improves the catalytic performance, relative
to La2 CuO4 . Also, the replacement of La by Ce induces the
reduction Cu2+ → Cu+ (probably more catalytically active),
as mentioned in the TPR section, to compensate the excess
positive charge produced by replacement of La by Ce.
The presence of Zn did not have any beneficial effect,
probably because of its electron configuration. Since Zn and
Cu have electron distributions [Ar] 4s2 3d10 and [Ar] 4s1 3d10 ,
respectively, thus showing that Zn, with its complete octet is
more stable and does not need to donate or receive electrons.
3.6. X-ray absorption near edge structure (XANES)
Figure 5. CO conversion by perovskites calcined at 350/700 ◦ C
Figure 6 shows the CO conversion for samples calcined at
500/900 ◦ C. All catalysts (except La1.95 Ce0.05 Cu0.8 Zn0.2 O4 )
performed similarly, i.e., after 2 h of reaction the catalysts
presented the same catalytic behavior until the CO conversion
of 32%. The La1.95Ce0.05 Cu0.8 Zn0.2O4 catalyst showed the
lowest CO conversion and was deactivated more rapidly than
other finalizing the reaction after 4 h of test.
Figure 6. CO conversion by perovskites calcined at 550/900 ◦ C
Comparing the two series of perovskites, a better performance was seen in catalysts submitted to mild heat treatments,
because these catalysts had higher surface areas, according to
the BET results (Table 2). In terms of CO conversion, the
Since the catalytic tests showed that the perovskites calcined at 350/700 ◦ C performed better, it was decided to apply
the XANES analysis to these perovskite samples.
The Cu K-edge absorption spectra of perovskites calcined
at 350/700 ◦ C are shown in Figure 7. The spectra were obtained between room temperature and 400 ◦ C, in the presence
of CO2 and H2 , (products of reaction). In the XANES experiments, water and CO were not present.
Figures 8 and 9 show the first and last spectra obtained
for each sample at 24 ◦ C and 400 ◦ C, respectively. All spectra presented in Figure 8 have the same profile: a narrow
and intense peak at 8997 eV and another broad and low intensity peak at 9012 eV. These signals are typical of Cu2+ ,
so at the beginning of the reaction the copper is oxidized
[4,36−38]. As the temperature increased, the above characteristics were maintained up to 400 ◦ C, when the spectra
began to change until reaching the shape seen in Figure 9.
In this figure, the spectra have a low intensity broad doublet
(8993 eV and 9002 eV). This profile is characteristic of metallic Cu [4,36−38]. Additionally, a shoulder at 8980 eV of low
intensity appears in the spectra at 400 ◦ C. According to the literature, a signal in this region indicates Cu+ species [39−41].
This agrees with the comments made on the TPR results, suggesting an increased presence of Cu+ species to compensate
the replacement of La3+ by Ce4+ .
Therefore, it is concluded that the copper, in the presence
of the reaction products H2 and CO2 (and in the absence of
H2 O and CO), begins the reaction in the oxide form and, when
the temperature has reached 400 ◦ C, it is in the metallic form
and remains so until the reaction ends. This reduction occurs
because of the presence of hydrogen in the reaction mixture,
under the conditions used.
Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
7
Figure 7. Cu K-edge XANES spectra of samples in the range room temperature to 400 ◦ C in H2 /CO2
samples, because the replacement of La3+ by Ce4+ causes an
excess positive charge, which is compensated by transformation of Cu2+ to Cu+ . This hypothesis is confirmed by XANES
results for perovskites calcined at 350/700 ◦ C, which showed
a signal at 8980 eV, assigned to Cu+ . Moreover, the XANES
results revealed that the perovskites, in the presence of the
shift reaction products H2 and CO2 , changed from the oxide
(up to 400 ◦ C) to the metal state (at 400 ◦ C), under experimental conditions.
Perovskites that exhibited the best catalytic performance
were those calcined at mild temperatures and, among these,
the outstanding pervskite was La1.90Ce0.10 CuO4 , probably because of the high surface area associated with the presence of
(La0.935Ce0.065 )2 CuO4 perovskite with tetragonal structure.
Figure 8. Cu K-edge XANES spectra of samples at 24 ◦ C
Acknowledgements
The authors thank FAPESP for financial assistance, Brazilian
Synchrotron Laboratory (LNLS) for the XANES experiments and
DEQ/UFSCar for the BET and XRD analyses.
References
Figure 9. Cu K-edge XANES spectra of samples at 400 ◦ C
4. Conclusions
From the results obtained in this study, it is concluded
the La2 CuO4 phase with orthorhombic structure was present
in all samples. The La1.90 Ce0.10 CuO4 perovskite, calcined
at 350/700 ◦ C, also had the (La0.935Ce0.065 )2 CuO4 perovskite
with tetragonal structure, producing a surface area higher than
the other perovskites.
TPR results suggest a raised content of Cu+ species in
[1] Deng W, De Jesus J, Saltsburg H, Flytzani-Stephanopoulos M.
Appl Catal A, 2005, 291: 126
[2] Atake I, Nishida K, Li D, Shishido T, Oumi Y, Sano T, Takehira
K. J Mol Catal A: Chem, 2007, 275: 130
[3] Hilaire S, Wang X, Luo T, Gorte R J, Wagner J. Appl Catal A,
2004, 258: 271
[4] Rφnning M, Huber F, Meland H, Venvik H, Chen D, Holmen A.
Catal Today, 2005, 100: 249
[5] Scherer G G. Solid State Ionics, 1997, 94: 249
[6] Ahmed S, Krumpelt M. Int J Hydrogen Energ, 2001, 26: 291
[7] Schmidt V M, Bröcherhoff P, Höhlein B, Menzer R, Stimming
U. J Power Sources, 1994, 49: 299
[8] Vidal F, Busson B, Six C, Pluchery O, Tadjeddine A. Surf Sci,
2002, 502/503: 485
[9] Newsome D S. Catal Rev Sci Eng, 1980, 21: 275
[10] Li Y, Fu Q, Flytzani-Stephanopoulos M. Appl Catal B, 2000, 27:
179
[11] Fy Q, Weber A, Flytzani-Stephanopoulos M. Catal Lett, 2001,
77: 87
[12] Jacobs G, Chenu E, Patterson P M, Williams L, Sparks D,
Thomas G, Davis B H. Appl Catal A, 2004, 258: 203
[13] Ricote S, Jacobs G, Milling M, Ji Y, Patterson P M, Davis B H.
Appl Catal A, 2006, 303: 35
8
S. S. Maluf et al./ Journal of Natural Gas Chemistry Vol. 18 No. 2 2009
[14] Fernndez-Garcı́a M, Gómez Rebollo E, Guerrero Ruiz A,
Conesa J C, Soria J. J Catal, 1997, 172: 146
[15] Tejuca L G, Fierro J L G. Properties and Applications of
Perovskite-Type Oxides. Nova York: Marcel Dekker, 1983
[16] Ponce, S, Peña M A, Fierro J L G. Appl Catal B, 2000, 24: 193
[17] Ciambelli P, Cimino S, De Rossi S, Faticanti M, Lisi L, Minelli
G, Pettiti I, Porta P, Russo G, Turco M. Appl Catal B, 2000, 24:
243
[18] Peter S D, Garbowski E, Perrichon V, Primet M. Comptes Rendus Chimie, 2004, 7: 57
[19] Koponen M J, Suvanto M, Pakkanen T A, Kallinen K, Kinnunen
T J J, Härkönen M. Solid State Sci, 2005, 7: 7
[20] Koponen M J, Suvanto M, Kallinen K, Kinnunen T J J,
Härkönen M, Pakkanen T A. Solid State Sci, 2006, 8: 450
[21] Peña M A, Fierro J L G. Chem Rev, 2001, 101: 1981
[22] Tsagaroyannis J, Haralambous K J, Liosos Z, Petroutsos G,
Spyrellis N. Mater Lett, 1996, 28: 393
[23] Bradow R, Jovanovic D, Petrovic S, Jovanovic Z, TerleckiBaricevic A. Ind Eng Chem Res, 1995, 34: 1929
[24] Pechni M P. US Patent 3 330 697. 1967
[25] Merino N A, Barbero B P, Grange P, Cadús L E. J Catal, 2005,
231: 232
[26] Pillai U R, Deevi S. Appl Catal B, 2006, 65: 110
[27] Civera A, Pavese M, Saracco G, Specchia V. Catal Today, 2003,
83: 199
[28] Patel S, Pant K K. Chem Eng Sci, 2007, 62: 5436
[29] Tabakova T, Boccuzzi F, Manzoli M, Sobczak J W, Idakiev V,
Andreeva D. Appl Catal A, 2006, 298: 127
[30] Fierro G, Lojacono M, Inversi M, Porta P, Lavecchia R, Cioci F.
J Catal, 1994, 148: 709
[31] Luo M F, Zhong Y J, Yuan X X, Zheng X M. Appl Catal A,
1997, 162: 121
[32] Mizuno N, Yamato M, Tanaka M, Mizuno M M. Chem Mater,
1989, 1: 232
[33] Nishida K, Atake I, Li D, Shishido T, Oumi Y, Sano T, Takehira
K. Appl Catal A, 2008, 337: 48
[34] Ricote S, Jacobs G, Milling M, Ji Y Y, Patterson P M, Davis B
H. Appl Catal A, 2006, 303: 35
[35] Nagai M, Zahidul A Md, Matsuda K. Appl Catal A, 2006, 313:
137
[36] Chen D, Wu Z. Radiat Phys Chem, 2006, 75: 1921
[37] Szizybalski A, Girgsdies F, Rabis A, Wang Y, Niederberger M,
Ressle T. J Catal, 2005, 233: 297
[38] Liu S H, Wang H P, Wang H C, Yang Y W. J Electron Spectrosc
Relat Phenom, 2005, 144-147: 373
[39] Oguchi H, Nishiguchi T, Matsumoto T, Kanai H, Utani K, Matsumura Y, Imamura S. Appl Catal A, 2005, 281: 69
[40] CaballeroA, Morales J J, Cordon A M, Holgado J P, Espinos J
P, Elipe A R G. J Catal, 2005, 235: 295
[41] Amano F, Suzuki S, Yamamoto T, Tanaka T. Appl Catal B, 2006,
64: 282