Partial oxidation of methane to synthesis gas using LnCoO3 perovskites as catalyst precursors

Article abstract of DOI:10.1006/jcat.1997.1580

In this work a series of cobalt-containing perovskites LnCoO₃ (Ln = La, Pr, Nd, Sm, and Gd) has been studied as catalyst precursors for the partial oxidation of methane to synthesis gas. All the perovskite precursors were prereduced in situ, producing cobalt metal finely dispersed over the rare earth sesquioxide support described here as Ln-Co-O. Of the catalyst tested the system Gd-Co-O showed exceptionally better performance for CO and H₂ production (with methane conversion of 73% and selectivities of 79 and 81% for CO and H₂, respectively, at 1009 K). The production of synthesis gas over the other catalysts decreased in the following order: Sm-Co-O ? Nd-Co-O > Pr-Co-O. The catalyst La-Co-O was active for methane combustion and only traces of CO and H₂ were observed under the reaction conditions. XRD and XPS analyses of the catalyst La-Co-O showed that under the reaction conditions the cobalt metal is completely reoxidized, regenerating the original LnCoO₃ perovskite structure. For the reaction over Nd-Co-O the cobalt is only partially reoxidized to NdCoO₃. For Gd-Co-O and Sm-Co-O, the most stable and active catalysts for the partial oxidation of methane no reoxidation to LnCoO₃ was observed. TPR and XRD studies showed that the perovskite NdCoO₃ is reduced in two steps, first to NdCoO_2.5 and further to Co°/Nd₂O₃ and in both stages it was demonstrated that the reoxidation with O₂ is capable of recovering the perovskite structure. TPO experiments with reduced La-Co-O, Nd-Co-O, Sm-Co-O, and Gd-Co-O catalysts indicated that reoxidation of cobalt also takes place in two steps: first by oxidation of the supported Co° to the spinel Co₃O₄ (Co²⁺Co³⁺₂O₄) followed by a further oxidation of the Co²⁺ to Co³⁺ with a simultaneous solid state reaction with Ln₂O₃, regenerating the perovskite structure. It was observed that the temperature for the second oxidation step is strongly dependent on the nature of the lanthanide. Based on these results it is proposed that the deactivation of the catalysts Ln-Co-O by reoxidation of cobalt metal is related to the thermodynamic stability of the parent perovskite structure. We also present evidence that hydroxyl groups on the rare earth oxide, specially in the La-Co-O system, might make some contribution to the reoxidation of cobalt metal during the reaction via a reverse spillover process.

Full text of DOI:10.1006/jcat.1997.1580

JOURNAL OF CATALYSIS 167, 198–209 (1997)
ARTICLE NO. CA971580
Partial Oxidation of Methane to Synthesis Gas Using LnCoO3
Perovskites as Catalyst Precursors
,1
ꢀ
ꢀ
ꢀ
R. Lago, G. Bini,† M. A. Pen˜a, and J. L. G. Fierro
ꢀ
Instituto de Cata´lisis y Petroleoqu´ımica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain; and †Dipartimento de Chimica Industriale e
Materiali, University of Bologna, Vle. Risorgimento 4, 40136 Bologna, Italy
Received August 12, 1996; revised December 12, 1996; accepted December 17, 1996
1. INTRODUCTION
In this work a series of cobalt-containing perovskites LnCoO3
(Ln = La, Pr, Nd, Sm, and Gd) has been studied as catalyst precur-
sors for the partial oxidation of methane to synthesis gas. All the per-
ovskite precursors were prereduced in situ, producing cobalt metal
finely dispersed over the rare earth sesquioxide support described
The research on methane conversion has recently
focused on the direct catalytic oxidation to synthesis gas:
CH4 + 1/2 O2 → CO + 2H2 (1Hꢂ298 K = ꢄ22.6 kJ/mol).
[1]
–
–
–
–
here as Ln Co O. Of the catalyst tested the system Gd Co O
showed exceptionally better performance for CO and H2 produc-
tion (with methane conversion of 73% and selectivities of 79 and
81% for CO and H2, respectively, at 1009 K). The production of syn-
thesis gas over the other catalysts decreased in the following order:
The major advantages of this route over the steam reform-
ing are the H2/CO ratio of ca. 2, suitable for downstream
processes and the exothermicity of the reaction which elim-
inates the need for a fuel gas (1). Many catalysts for the par-
tial oxidation of methane to synthesis gas, consisting of sup-
ported metals such as Ni, Co, and Fe and noble metals Pd, Ir,
Rh, Ru, Pt, etc., have been described in the literature (2–9).
This work deals with the use of perovskites of the type
LnCoO3 as precursors of the catalyst for the partial oxi-
dation of methane to synthesis gas. These perovskites may
offer interesting features as precursors for supported metal
catalysts. For example, careful reduction can be carried out
in order to produce a finely dispersed transition metal over
the sesquioxides Ln2O3. Cobalt and nickel supported on
rare earth oxides have been demonstrated to be effective
catalysts for methane partial oxidation. Choudhary et al.
(10) obtained high activity and selectivity for the conver-
sion of methane to H2 and CO in the presence of CoO–rare
earth oxide catalysts. They claimed that the reduced catalyst
–
–
–
–
–
–
–
–
Sm Co O ꢁ Nd Co O > Pr Co O. The catalyst La Co O was ac-
tive for methane combustion and only traces of CO and H2 were
observed under the reaction conditions. XRD and XPS analyses of
–
–
the catalyst La Co O showed that under the reaction conditions
the cobalt metal is completely reoxidized, regenerating the original
–
–
LnCoO3 perovskite structure. For the reaction over Nd Co O the
–
–
cobalt is only partially reoxidized to NdCoO3. For Gd Co O and
–
–
Sm Co O, the most stable and active catalysts for the partial ox-
idation of methane no reoxidation to LnCoO3 was observed. TPR
and XRD studies showed that the perovskite NdCoO3 is reduced in
two steps, first to NdCoO2.5 and further to Coꢂ/Nd2O3 and in both
stages it was demonstrated that the reoxidation with O2 is capable of
recovering the perovskite structure. TPO experiments with reduced
–
–
–
–
–
–
–
–
La Co O, Nd Co O, Sm Co O, and Gd Co O catalysts indicated
that reoxidation of cobalt also takes place in two steps: first by ox-
idation of the supported Coꢂ to the spinel Co3O4 (Co2+Co3+2O4)
followed by a further oxidation of the Co2+ to Co3+ with a simulta-
neous solid state reaction with Ln2O3, regenerating the perovskite
structure. It was observed that the temperature for the second oxi-
dation step is strongly dependent on the nature of the lanthanide.
Based on these results it is proposed that the deactivation of the
–
CoO Yb2O3 produced methane conversion of 59.4% with
80% selectivity for H2 and CO at temperatures as low as
673 K, far enough from the product distributions predicted
byequilibrium. Asalreadyexplained (5), the apparent devi-
ation of product distributions from thermodynamic equilib-
rium at high space velocities may have resulted from a small
hot zone in the reactor and consequentlyincorrect measure-
ment of the true reaction temperature. Several papers in the
recent literature report the effective role played by La2O3
as a support and promoter for nickel catalysts (11–14). Also,
the reduction conditions of the perovskite can be varied in
order to control the metal particle size which has an impor-
tant effect on coke formation. It has been demonstrated
that the rate of coke formation over nickel catalysts during
–
–
catalysts Ln Co O by reoxidation of cobalt metal is related to the
thermodynamic stability of the parent perovskite structure. We also
present evidence that hydroxyl groups on the rare earth oxide, spe-
–
–
cially in the La Co O system, might make some contribution to
the reoxidation of cobalt metal during the reaction via a reverse
c
spillover process.
ꢃ 1997 Academic Press
1
To whom correspondence should be addressed. Fax: +34 1 585 4760.
E-mail: jlgfierro@icp.csic.es.
198
0021-9517/97 $25.00
c
Copyright ꢃ 1997 by Academic Press
All rights of reproduction in any form reserved.

POM REACTION ON PREREDUCED LnCoO3 PRECURSORS
199
the steam reforming of hydrocarbon depends on the metal
Temperature-programmed reduction (TPR) and
crystallite size. Borowiecky (15) showed that rate of coke temperature-programmed oxidation (TPO) experiments
formation increases linearly with the size of the nickel crys- were carried out in a semiautomatic Micromeritics TPD/
tallites in the range from 6 to 35 nm. Rostrup-Nielsen (16) TPR 2900 apparatus interfaced to a microcomputer. TPR
proposed that coke formation during steam reforming of profiles were obtained by passing a 10% H2/Ar flow
methane requires a minimum of 16 neighboring sites on a (50 cm3/min) through the sample. The temperature was
nickel surface. Another way to improve the resistance to increased from 323 to 1046 K at a rate of 10 K/min, and the
coking is to add an earth alkaline oxide such as MgO and amount of H2 consumed was determined with a TCD; the
CaO to the catalyst (17). It has been proposed that the for- effluent gas was passed through a cold trap placed before
mation of solid solution such as NiO/MgO in the catalyst the TCD in order to remove water from the exit stream.
precursor is responsible for the high activity and stability TPO profiles were recorded in a similar way. For this, the
observed (13, 18, 19). In this respect, perovskites of the reduced catalysts were cooled to ambient temperature in a
type Ln1ꢄyAyMO3 (where A is an alkaline earth and M He flow and then exposed to a 3% O2/He flow (50 cm3/min)
a transition metal) is an interesting precursor to produce a while increasing the temperature at a rate of 10 K/min up
finelydispersed metalcontainingalkaline earth oxide which to a final temperature of 1200 K.
might show improved coke resistance.
The cobalt metal surface area was determined by hydro-
Hayakawa et al. (20) studied the perovskite Ca1ꢄxSrxTiO3 gen chemisorption measurements were obtained in a con-
mixed with nickel oxide for methane oxidation at 1028 K, ventional pyrex glass volumetric adsorption apparatus. A
both before and after pretreatment with methane. Before 200-mg sample of the perovskite was reduced at 1250 K
methane pretreatment the catalyst produced mainly C2 hy- for 3.5 h under a stream of 33% H2/N2 and then out-
drocarbonsand CO2 but no syngas. After methane pretreat- gassed at 773 K overnight at pressures lower than 10ꢄ4 Torr
ment at 1048 K for 1 h, the mixture became very selective (1 Torr = 133.3 Pa). The adsorption isotherms were mea-
to synthesis gas, giving 70.9% conversion of methane with sured at 313 K after 2 h equilibration at this temperature.
94% selectivity to CO and H2. Based on XRD analyses This temperature was used since the chemisorption pro-
it was proposed that the methane pretreatment produced cess is known to be highly activated on cobalt (22). The
metallic nickel supported on the perovskite which was re- total amount of chemisorbed H atoms was used to deter-
sponsible for the synthesis gas formation. Similar results mine the number of vacant Coꢂ atoms at the surface using
were obtained for cobalt and iron (8). Slagten and Olsbye the stoichiometry H/Cos = 1 (22). The reproducibility was
–
–
(9) studied the systems La M O (M = Co, Ni, Rh, and Cr) within 10% .
for the partial oxidation of methane to syngas. They ob- X-ray photoelectron spectra were acquired with an
served very high activity for the system La Rh O, whereas ESCALAB 200R spectrometer equipped with a MgKꢀ
–
–
–
–
the catalyst La Co O (which was a mixture of LaCoO3, 120-W X-raysource (hv = 1253.6eV). Both H2-reduced and
La2O3, and Co3O4) produced mainly CO2. If the catalyst used catalysts were analyzed by XPS. The prereduced cata-
–
–
La Co O was kept at 1073 K after 30 h reaction the activ- lysts in H2 flow at 1023 K for 3.5 h and then used in the reac-
ity changed to give mainly CO, which they assigned to the tion were all quenched to room temperature under argon
in situ reduction of cobalt.
and immediatelydrenched in isooctane. Despite thiscareful
This work presents some results on the use of cobalt con- procedure to isolate the catalyst it was not possible to com-
taining perovskite LnCoO3 as catalyst precursors for the pletely avoid oxidation of the surface metallic cobalt during
partial oxidation of methane to synthesis gas. Different lan- transferral of the sample from the reactor to the XPS spec-
thanides (Ln = La, Pr, Nd, Sm, Gd, and Dy) have been used trometer. The samples were pressed into small aluminum
in order to study the effect of the rare earth support on the cylinders and then mounted on a sample rod placed in a
cobalt catalyst.
pretreatment chamber and outgassed at room temperature
for 1 h prior to being transferred to the analysis chamber.
The pressure in the ion-pumped analysis chamber was be-
low 3 ꢅ 10ꢄ9 Torr during data acquisition. The spectra were
2. EXPERIMENTAL
All the perovskites were prepared by the method of collected for 20to 90min, dependingon the peak intensities,
the amorphous citrate percursor (21) and were present at a pass energy of 10 eV (1 eV = 1.602 ꢅ 10ꢄ19 J). The in-
as pure phases with no contaminants of Ln2O3 or cobalt tensities were estimated by calculating the integral of each
oxides according to the powder X-ray diffraction (XRD) peak after smoothing and subtraction of the S-shaped back-
analyses. XRD patterns were recorded with a Philips PW ground and fitting the experimental curve to a combination
1716/30 diffractometer using nickel filtered CuKꢀ radiation of Gaussian and Lorentzian lines of variable proportion.
(ꢁ = 0.1538 nm), under constant instrumental parameters. All binding energies (BE) were referenced to the adventi-
For each sample, Bragg’s angles between 5 and 75ꢂ were tious C 1s line at 284.9 eV. This reference gave BE values
scanned at a rate of 2ꢂ/min.
within an acuracy of ꢆ0.1 eV.

200
LAGO ET AL.
Temperature-programmed desorption (TPD) experi-
ments were performed on aliquots of catalysts (50 mg)
placed in a quartz reactor attached to a vacuum line and gas
handling system. The samples were reduced as described
above and outgassed at 773 K for typically 3 h until no
desorption from the catalyst was observed by the MS de-
tector. Subsequently, they were cooled down to 313 K and
exposed to a 100-Torr CO pulse for 15 min. Once the gas
phase was removed, the samples were heated to 1073 K at a
constant heating rate of 5 K/min and the desorbing species
were monitored with a MS detector. A Balzers QMA 125
quadrupole mass spectrometer, capable of monitoring 16
masses simultaneously, connected in line with the reactor,
was used for the analysis of the desorption products.
The catalytic tests were carried out in a quartz fix bed
flow microreactor (4 mm inner diameter). A space velocity
(HGSV) of 2 ꢅ 105 hꢄ1 and a mixture CH4/O2/Ar = 2 :1 :4
(molar ratio) at a flow rate of 166.6 cm3/min were used.
The catalyst weight (50 mg) was held in the middle of the
reactor using quartz wool. Flow rates were controlled by
Unit mass flow controllers. All the catalyst were reduced
in situ with a 33% H2/Ar mixture (50 cm3/min) at 1023 K
for 3.5 h prior the reaction. After the reduction, the H2
flow was stopped and the flows of methane and argon ad-
justed for the reaction. The oxygen was then slowly intro-
duced, producing an increase in the bed temperature due to
exothermic oxidation reactions. Two thermocouples were
used, one outside the reactor to control the furnace temper-
ature and the other one inside in contact with the catalyst
through a quartz sheath to measure the bed temperature.
The effluents of the reactor were sampled by means of a
six-port valve and then analyzed on-line by GC (Hewlett
Packard HP 5890), using Ar as the carrier, which allowed
direct analysis of hydrogen and other products.
FIG. 1. Temperature-programmed reduction profiles of LnCoO3 per-
ovskite oxide precursors.
the lower temperature of reduction and split in two compo-
nents, indicating that the reduction process is activated and
probably controlled by mass transport of H2 or/and H2O
across the lattice.
To identify the reduced species formed in the TPR steps
in Fig. 1 XRD studies of NdCoO3 subjected to different
reduction treatments were carried out (Fig. 2). The XRD
pattern displayed in Fig. 2a corresponds to NdCoO3 cubic
perovskite structure. Upon reduction at 623 K under H2
flow for 15 min the diffraction peaks are essentially at the
same position but with a very strong broadening and in
some peaks a splitting is observed (Fig. 2b). This indicates
3. RESULTS
3.1. Catalyst Characterization
The temperature programmed reduction profiles of the that the perovskite structure is basically the same, however
cobalt containing perovskites are displayed in Fig. 1. All the distorted due to the creation of anion vacancies. These re-
perovskites showed similar reduction profiles consisting of sults suggest that the stoichiometry of the phase formed
two sets of peaks at approximately 633 and 833 K. It can be in the first step 1 electron reduction can be represented as
observed that especially for LaCoO3 and PrCoO3 the sec- NdCoO2.5. The reoxidation of this sample at 973 K under
ond reduction peak shiftsto higher temperatures. For exam- oxygen flow can easily regenerate the original perovskite
ple, the reduction peak for SmCoO3 at ca. 785 K is observed structure (XRD not shown here), demonstrating the re-
at 844 K for LaCoO3. In all cases the hydrogen consumption versibility of this process. On the other hand, the phase
for the first reduction step (peak at ca. 633 K) was always NdCoO2.5 is not stable and at high temperatures (1123 K
approximately half of the hydrogen consumption obtained for 1 h) under He phases of CoO and Nd2O3 hexagonal
for the second reduction step (peak at 833 K). Careful ther- are formed (Fig. 2c). When the perovskite was treated with
mogravimetric reduction experiments (not shown here) hydrogen at 778 K for 15 min, a three-electron reduction
demonstrated that the first step is a one-electron reduc- was observed and XRD analysis showed the presence of
tion process (CO3+ + e → Co2+) whereas the second step is Nd2O3 hexagonal and very weak and broad peaks which
a two-electron reduction process (Co2+ + 2 e → Coꢂ). TPR could be due to metallic cobalt (Fig. 2d). Upon heating at
profiles of the first step of reduction display a long tail in 1123 K in helium the highly dispersed cobalt metal sinters

POM REACTION ON PREREDUCED LnCoO3 PRECURSORS
201
takes place regenerating the perovskite NdCoO3 structure.
Therefore, the two main reduction steps of NdCoO3 can be
written as:
1 e/mol: 2 NdCoO3 + H2 ꢀ 2 NdCoO2.5 + H2O
[2]
3 e/mol: 2 NdCoO3 + 3H2 ꢀ Nd2O3 + 2Coꢂ + 3H2O. [3]
These reactions are reversible and as long as the metal-
lic cobalt remains well dispersed the perovskite struc-
ture can be easily regenerated by oxidation. These re-
sults are in agreement with the work of Crespin and Hall
(23), who observed basically the same reduction steps for
LaCoO3.
The cobalt metal area of the reduced perovskites was
determined by hydrogen chemisorption experiments. The
results are shown in Table 1. The chemisorption mea-
surements revealed that the cobalt metallic surface area
was similar for all the perovskites. This is supported by
the Co/Ln surface ratio (Table 1) obtained by XPS which
also suggests similar metallic dispersion. The XPS analy-
ses of the reduced perovskites showed the presence of Coꢂ
(778.6 eV) but also a doublet at approximately 780.5 and
796.2 eV which correspond to Co 2p3/2 and Co 2p1/2, re-
spectively, for the Co3+ ion (Fig. 9A). It can be also noted
in Fig. 9A that the intensity of the component at approxi-
mately 778.6 eV, associated to the metallic cobalt, does not
change substantially from catalyst to catalyst. As can be
seen below, this is not the case for the catalyst used in which
the proportion of reduced cobalt (Coꢂ) decreased dramat-
ically or even disappeared (see Fig. 9B). Shake-up satellite
lines with 4.7 eV over the Co3+ lines were also detected, in-
dicating the presence of Co2+ (24). These oxidized species
of cobalt are probably formed by air oxidation during the
transferral of the reduced sample from the reactor to the
XPS spectrometer. Also, Marcos et al. (25) have shown that
the reduction of the perovskite LaCoO3 produced a La2O3
oxide covered by hydroxyl groups which, upon heating and
evacuation in the XPS chamber, partly reoxidizes the cobalt
crystallites.
FIG. 2. Powder X-ray diffraction patterns for NdCoO3: (a) as pre-
pared, (b) reduced in hydrogen at 623 K for 15 min, (c) after reduction
(623 K, 15 min) and heated under He at 1123 K for 1 h (sintering), (d) re-
duced in hydrogen at 778 K for 15 min, (e) after reduction (778 K, 15 min)
and sintering, (f) after reduction (778 K, 15 min), sintering at 1123 K and
reoxidation under O2 flow at 973 K. A, NdCoO3;B, Nd2O3 (cub); C, Nd2O3
(hex); D, CoO; E, Co3O4; and F, Coꢂ.
TABLE 1
Characterization of the Reduced Perovskites
and shows much stronger lines in the XRD pattern (Fig. 2e).
The presence of small amounts of CoO, probably formed
by the reoxidation of cobalt by water or oxygen still present
in the sample during sintering, was also observed. The reox-
idation of the reduced sample in Fig. 2d (before sintering)
also reproduced the perovskite original structure with the
same XRD pattern (not shown here). However, after sin-
tering (Fig. 2f) the reoxidation produces a mixture of the
phases NdCoO3, Co3O4, and Nd2O3 (Fig. 2f), showing that
the process is not completely reversible. It was observed
that if the sample in Fig. 2f was kept under oxygen flow at
Coꢂ
Co/Ln
ratio
BET area BET area area (Coꢂ
(m2/g)
(m2/g)
atoms/g)
by XPSa
XRD
Precursor perovskite (red. cat.) (red. cat.) (red. cat.) (red. cat.)
LaCoO3
NdCoO3
SmCoO3
GdCoO3
8.6
5.6
5.1
4.0
5.4
4.5
—
3.77 ꢅ 1019
3.98 ꢅ 1019
3.69 ꢅ 1019
3.21 ꢅ 1019
0.71
0.50
0.58
0.75
La2O3 (hex)
Nd2O3 (hex)
Sm2O3 (cub)
Gd2O3 (cub)
5.0
Note. red. cat., catalysts reduced at 1023 K under 33% H2/Ar flow for
3.5 h.
a For the XPS Co/Ln ratio determination all cobalt signals (metal, oxi-
temperatures of 1123 K for 1 h a slow solid state reaction dized forms, and shake-up satellite lines) were included.

202
LAGO ET AL.
Powder XRD analysis of the reduced LnCoO3 per-
ovskites showed only the presence of the sesquioxides
La2O3 (hex), Nd2O3 (hex), Sm2O3 (cub), and Gd2O3 (cub).
The apparent absence of reflections for the metallic cobalt
indicates a high metallic dispersion with Coꢂ particles
smaller than 2 nm. Considering the approximate value of
3 ꢅ 1019 atoms/m2 for a close packing arrangement (26), the
number of surface cobalt metallic atoms obtained for the
reduced catalysts is similar to the number of surface cobalt
ions on the original perovskite surface (between 2 ꢅ 1019
to 5 ꢅ 1019 atoms/g for GdCoO3 with BET surface area of
4.0 m2/g and LaCoO3 8.6 m2/g, respectively). It therefore
seems that after reduction of the perovskites a large pro-
portion of the cobalt metal is located in the bulk of the
material.
Temperature-programmed oxidation analyses were per-
formed for the reduced catalysts in order to investigate the
stability of these systems toward gas phase oxygen (Fig. 3).
The perovskites were completely reduced in a TPR exper-
iment with a stream of 10% H2 in Ar heating from room
temperature to 923 K at a rate of 10 K/min. The temperature
was then rapidly decreased to avoid sintering of the cobalt
metal and TPO analysis performed. TPO profiles showed
that the oxidation takes place in two steps. The first one at
near 473 K (peak I) has an oxygen consumption which sug-
gests that the cobalt is oxidized to two Co3+ and one Co2+
Therefore, this oxidation probably corresponds to the for-
mation of the cobalt oxide spinel according to the reaction.
.
–
–
FIG. 4. (A) TPO profiles of the Sm Co O system: (a) sample pre-
reduced at 900 K and then reoxidized at 1123 K, (b) after cycle (a) re-
duced at 900 K and subsequently oxidized at 623 K. (B) TPR profiles of
–
–
3Coꢂ + 2O2 ꢀ Co3O4.
[4]
the Sm Co O system: (a) original sample reduced at 900 K, (b) sample
(a) reoxidized at 1123 K followed by reduction at 900 K, (c) sample (b)
reoxidized at 623 K and then heated at 1123 K under He, followed by
reduction at 900 K.
The second step (peak II) takes place at a much higher
temperature and the oxygen consumption suggests the
following process:
Co3O4 + 1/4 O2 + 3 Ln2O3 ꢀ LnCoO3
[5]
In this step apparently the Co2+ is oxidized to Co3+ simul-
taneously with a solid state reaction to regenerate the per-
ovskite structure. The occurrence of these reactions are also
supported by simple experiments involving sequences of
TPO and TPR measurements, shown in Figs. 4A and 4B.
It can be observed that the TPR profile, obtained with the
sample reduced in the first TPR (Fig. 4B, profile a), and re-
oxidised in the first TPO up to 1123 K (Fig. 4A, profile a),
is very similar to the TPR of the fresh sample (Fig. 4B,
profile a), suggesting the presence of a SmCoO3 perovskite
structure. On the other hand, if the reduced sample, after
a complete reduction–oxidation cycle, is then reoxidized in
a TPO experiment, which is interrupted at 623 K (Fig. 4A,
profile b), and the sample heated up to 1123 K under He,
the TPR analysis shows only a broad peak at ca. 623 K
(Fig. 4A, profile c), which is similar to the reduction of the
spinel Co3O4 reported in the literature (27).
FIG. 3. Temperature-programmed oxidation profiles for the reduced
–
–
–
–
–
–
–
–
La Co O, Nd Co O, Sm Co O, and Gd Co O catalyst systems.

POM REACTION ON PREREDUCED LnCoO3 PRECURSORS
203
It is interesting to note in Fig. 3 that the temperature for
the first oxidation step is similar for all the catalysts whereas
the temperature of the second step is strongly dependent
on the nature of the lanthanide. For lanthanum, oxidation
occurs at 939 K, whereas for gadolinium a much higher tem-
perature is necessary (1109 K). Also the area and the shape
of the second peak varied significantly for the different lan-
thanides. Very sharp peaks were obtained for Gd and Sm,
whereas a broad and weak peak was observed for La. The
Apeak I/Apeak II area ratio of the first and the second peak
from the TPO profiles shown in Fig. 3 are 12.5, 10.0, 9.0,
–
–
–
–
–
–
–
–
and 7.5for La Co O, Nd Co O, Sm Co O, and Gd Co O,
–
–
–
–
respectively. The systems La Co O and Nd Co O can be
observed to deviate significantly from the expected area
ratio which is 8.0 considering the stoichiometry of Eqs. [4]
and [5]. This suggests that part of the cobalt which in the
–
–
–
–
Gd Co O and Sm Co O systems is oxidized only at tem-
peratures higher than 973 K, is being oxidized at lower tem-
–
–
–
–
peratures for La Co O and Nd Co O. In fact, Crespin et
al. (23) showed that if the reduced LaCoO3 was kept at
a temperature as low as 673 K under oxygen the metal-
lic cobalt was completely reoxidized, regenerating the per-
ovskite structure.
3.2. Carbon Monoxide TPD Experiments
Figures 5A–5C display TPD profiles of CO adsorbed on
–
–
–
–
–
–
the reduced systems La Co O, Nd Co O, and Gd Co O,
respectively. In Fig. 5A, the presence of basically two peaks
of CO can be observed, peak I at ca. 453 K and peak II
at ca. 693 K. Cortes and Droguett (28) studied cobalt sup-
ported on kieselguhr and also obtained a CO TPD with two
peaks at very similar temperatures which they assigned to
surface Co-linear carbonyl species (peak I) and Co-bridged
carbonyl (peak II). Further TPD and IR studies were then
reported in the literature supporting the linear and bridged
assignments of these CO TPD peaks (29–31). Figure 5A
shows that the intensity of CO peak I is similar for all the
FIG. 5. Temperature-programmed desorption profiles of CO from
–
–
–
–
–
–
the reduced La Co O, Nd Co O, and Gd Co O catalysts: (A) CO (m/e =
28), (B) CO2 (m /e = 44), and (C) H2 (m /e = 2).
cludes the possibility of the water gas shift reaction as a
source of H2 and CO2. Two differences are noted in the
CO2 signals in Fig. 5B. The first one is that relatively large
–
–
–
–
–
–
systems studied, La Co O, Nd Co O, and Gd Co O. On
–
–
amounts of CO2 are produced for the La Co O system,
the other hand, it can be clearly seen that the CO peak II for
–
–
decreasing for Nd Co O, and very little is produced for
–
–
La Co O almost disappears while it increases in intensity
–
–
Gd Co O. The second feature is the desorption tempera-
ture of CO2, which increases in the order La > Nd > Gd.
In Fig. 5C it is observed that also hydrogen (m/e = 2) is
–
–
–
–
for Nd Co O and especially for Gd Co O. As observed in
Fig. 5B, desorption of the CO peak II is accompanied by the
formation of significant amounts of CO2. This CO2 is prob-
ably produced during CO TPD by the disproportionation
of CO,
–
–
produced in large amounts for La Co O compared to that
–
–
–
–
seen with Nd Co O and Gd Co O. These results seem to
suggest that the reaction with surface hydroxyl groups de-
scribed in Eq. [7] is present during the CO TPD exper-
iments. In this respect, the basicity of the rare earth ox-
ides, which decreases in the order La2O3 > Nd2O3 > Gd2O3
(35, 36), might play an important role in the reaction. For
example, due to its strong basicity, La2O3 is known to form
very high concentrations of stable hydroxyls and oxycar-
bonate species in a wide temperature range (24, 25, 37–40).
2 COsurf ꢀ CO2 + Csurf
,
[6]
and/or by the reaction of CO with superficial hydroxyl
groups (32–34),
COsurf + OHsurf ꢀ CO2 + 1/2 H2.
[7]
No H2O was observed in TPD blank experiments with the LaO(OH) decomposes to form La2O3 only at temperatures
–
–
–
–
–
–
catalysts La Co O, Nd Co O, and Gd Co O, which pre- higher than 800 K (38). Therefore, these hydroxyl groups

204
LAGO ET AL.
on the surface of La2O3 react with CO desorbing during
the TPD in peak II (Fig. 5A) producing CO2 and H2. The
higher desorption temperature of CO2 observed in the TPD
–
–
for La Co O (Fig. 5B) might be related to the basicity of
La2O3 which results in a stronger interaction with the acid
–
–
molecule CO2. In the case of the Gd Co O system where
the Gd2O3 oxide is less basic, a relatively low concentration
of hydroxyl groups is probably produced and therefore the
presence of the reaction described in Eq. [7] is consider-
ably diminished. The CO disproportionation (Eq. [6]) dur-
ing the TPD experiments has not been investigated in this
work and a more detailed study is necessary to comment on
it. TPD studies on Rh supported on silica promoted with
La2O3 (32, 41) suggested that although the disproportion-
ation of CO was present, the contribution of Eq. [7] to the
formation of CO2 was much more important.
3.3. Catalytic Testing
Among the cobalt containing perovskites, GdCoO3,
SmCoO3, NdCoO3, PrCoO3, and LaCoO3, tested as cata-
lyst precursors for the partial oxidation of methane, the
–
–
Gd Co O system showed exceptionally better perfor-
mance for synthesis gas formation (Figs. 6A–6C). At 1009 K
a steady-state methane conversion of 73% with selectiv-
ities of 79 and 81% for CO and H2, respectively, is ob-
–
–
–
–
served for the catalyst Gd Co O. The catalysts Sm Co O
–
–
and Nd Co O, of lower activity, show similar steady-state
methane conversions in the temperature range studied. On
the other hand, the H2 and CO selectivities are much higher
–
–
over Sm Co O.
FIG. 6. (A) Methane conversion and (B) H2 and (C) CO selectivities
–
–
The catalyst La Co O is active for the methane combus-
tion and only traces of H2 and CO were observed under
the reaction conditions used (Figs. 6A–6C). It is interesting
to observe that, at a constant temperature, the steady-state
yields of H2 and CO followed inversely the order of the
ionic radii of the rare earth (Fig. 7), considering the oxida-
tion state +3, coordination number 8, and the sesquioxides
of structural type A for La2O3, Pr2O3, and Nd2O3 and type
B for Sm2O3 and Gd2O3 (42). Therefore, the gadolinium
which has the shortest ionic radius shows the highest CO
and H2 yields. On the other hand, for the catalyst contain-
ing lanthanum (the largest ion) no syngas formation was
observed (Fig. 7).
–
–
in the presence of Ln Co O systems prereduced under a 30% H2/Ar flow
at 1023 K for 3.5 h.
3.4. Characterization of Used Catalysts
XPS studies were further carried out to know the oxi-
dation state of cobalt in the catalysts after on-stream pro-
–
–
cess. The La Co O catalyst after reaction at 1023 K for
19 h showed the Co 2p spin-orbit splitting at 780.5 and
796.2 eV, which correspond to Co 2p3/2 and Co 2p1/2 levels,
All the catalytically active systems for syngas production
showed hysteresis behavior. Figures 8A–8C display the hys-
tereses observed in the methane conversion and H2 and CO
–
–
–
–
selectivities over the catalysts Gd Co O, Sm Co O, and
–
–
Nd Co O. When the reaction temperature over the prere-
duced catalyst was increased from ca. 823 K up to ca. 1073 K
the conversion of methane and selectivities for CO and H2
also increased. However, if the reaction temperature was
then decreased the observed conversion and selectivities
were always higher than the values obtained when the tem-
perature had been increased.
FIG. 7. Steady-state H2 ( ) and CO (ꢃ) yields as a function of the
lanthanide ionic radii.

POM REACTION ON PREREDUCED LnCoO3 PRECURSORS
205
respectively, for the Co3+ ion. Moreover, the observation
TABLE 2
of shake-up satelite lines with approximately 4.7 eV over
the Co3+ indicates the presence of Co2+ (23). No line at
778.6 eV was observed, suggesting the absence of metallic
cobalt on the surface (Fig. 9B, spectrum a). The Co 2p3/2
XRD and XPS Analyses of the Catalyst
after On-Stream Operationa
Precursor
LaCoO3
Co/Ln surface ratio by XPSb
0.7
Crystalline phases (XRD)
–
–
–
–
core-level spectra of catalysts Gd Co O, Sm Co O, and
LaCoO3
–
–
Nd Co O after reaction showed the lines at 778.6 and 780.3
eV, indicating the presence of both metallic cobalt and ox-
idized forms of cobalt, respectively, on the surface of the
used catalysts (Fig. 9B). The XPS Co/Ln ratios determined
(including the satellite lines for both Co 2p3/2 and the lan-
thanide) shown in Table 2 suggest that the cobalt dispersion
over the catalyst surface after reaction follows the order
La2O3 (tr)
Nd2O3 (hex)
NdCoO3
NdCoO3
0.2
SmCoO3
GdCoO3
0.3
0.5
Sm2O3 (cub)
Gd2O3 (cub)
Note. tr, traces; hex, cub, the hexagonal and cubic crystalline phases,
respectively.
a Reaction at 1023 K for 19 h.
For the Co/Ln signal ratio determination all cobalt signals, metal,
oxidized forms, and shake-up satellite lines were included.
–
–
–
–
–
–
–
–
La Co O > Gd Co O > Sm Co O > Nd Co O.
b
–
–
XRD analyses of the used catalysts Gd Co O and
–
–
Sm Co O showed similar patterns to the reduced catalysts.
Very strong and sharp peaks for the sesquioxides Gd2O3
(Fig. 10A) and Sm2O3 can be observed. On the other hand,
–
–
ovskite LaCoO3 (Fig. 10B). The XRD for Nd Co O after
reaction revealed the presence ofthe phasesNd2O3 and also
the perovskite NdCoO3 (Fig. 10C). For all used catalysts no
clear evidence for the presence of simple cobalt oxides such
as CoO, Co2O3, and Co3O4 could be found by XRD.
For the reduced perovskites NdCoO3 and GdCoO3 the
Nd2O3 (hex) and Gd2O3 (cub) phases were observed. Coin-
cidentally, the strong diffraction lines of the metallic cobalt
planes (101), (110), (103), and (112) with interplanar dis-
tances of 0.1910, 0.1252, 0.1149, and 0.1066 nm, respectively,
are also observed for the oxides of gadolinium, samarium,
and neodymiun. Therefore, it was not possible to obtain in-
formation about the metallic particles from the XRD anal-
yses.
–
–
XRD analysis of the La Co O catalyst after reaction at
1023 K for 19 h clearly showed the formation of the per-
4. DISCUSSION
According to the powder XRD analyses reduction of the
perovskite precursors produced metallic cobalt dispersed
on the rare earth sesquioxides, Coꢂ/Ln2O3. The results re-
ported in the literature (8–10) suggest that metallic cobalt
is probably the active phase for the oxidation of methane to
synthesis gas. It is interesting to observe that, although the
reduced perovskites possess similar cobalt metallic surface
area as revealed by H2 chemisorption and XPS data, they
showed strikingly different catalytic properties. For exam-
–
–
ple, the catalyst Gd Co O showed very high activity for
–
–
synthesis gas formation, whereas La Co O produced only
CO2 and H2O.
–
–
In the case of the La Co O system, the XRD and XPS
results showed that under the reaction conditions studied
the metallic cobalt is reoxidized back, producing the orig-
inal perovskite structure LaCoO3. Therefore, it is not sur-
prising that the only reaction products observed were water
and carbon dioxide. This agrees with previous works on this
perovskite and other forms of cobalt oxide which have been
shown to be active catalysts for methane combustion and
FIG. 8. Effect of the reaction temperature on the (A) methane con-
version and (B) H2 and (C) CO selectivities over prereduced GdCoO3,
SmCoO3, and NdCoO3 perovskite precursors.

206
LAGO ET AL.
FIG. 9. Co 2p3/2 core level spectra of (A) reduced LnCoO3 (under a 30% H2/Ar flow at 1023 K for 3.5 h) and (B) used (reaction at 1023 K for 19 h)
–
–
–
–
–
–
–
–
catalysts. (a) La Co O, (b) Nd Co O, (c) Sm Co O, and (d) Gd Co O.
also for CO and H2 oxidation (43). The high Co/Ln surface ity changed to give mainly CO which they assigned to the
ratio determined by XPS for the used catalyst is expected in situ reduction of cobalt.
– –
for a perovskite-like surface. Slagten and Olsbye (9) stud-
The used catalyst Nd Co O shows fairly high amounts
ied the perovskite LaCoO3 (containing some impurities of of the perovskite NdCoO3 formed during the reaction.
La2O3 and Co3O4) for the partial oxidation of methane to This perovskite promotes the nonselective oxidation of
syngas and observed the production of mainly CO2. If the methane and also CO and H2. On the other hand, the
–
–
–
–
catalyst was kept at 1073 K after 30 h on-stream the activ- Gd Co O and Sm Co O systems, which showed the best

POM REACTION ON PREREDUCED LnCoO3 PRECURSORS
207
–
–
The hysteresis behavior observed for the Co Ln O cata-
–
–
lysts (except La Co O) (Figs. 8A–8C) has been discussed
for alumina supported nickel catalysts during the partial
oxidation of methane (6, 7). Lunsford et al. (6), using XRD
and XPS, demonstrated that only for reaction temperatures
higher than 973 K is the precalcined NiAl2O4 reduced to
produce metallicnickelwhich isactive for syngasformation.
Decreasing the temperature to 673 K, the oxygen consump-
tion is not complete and all the surface nickel is oxidized
back to the spinel NiAl2O4. Apparently, a similar process
occurs in our catalytic systems. At low reaction tempera-
tures, where the oxygen conversions are lower, oxidation
of the surface metallic cobalt takes place. These oxidized
forms of cobalt are active for the total oxidation of methane
and also for the oxidation of CO and H2 formed during the
reaction. At reaction temperatures as high as 1023 K the
–
–
oxygen conversion is complete (for Gd Co O) or almost
complete, so a reducing atmosphere is produced. In this
situation the Coꢂ is favored and an increase in the selec-
tivities for syngas is observed. If the temperature is then
decreased the metallic cobalt is slowly reoxidized and the
H2 and CO selectivities also decrease. Also, with an increase
in the reaction temperature, the Coꢂ present in the bulk of
the catalyst may migrate, causing an increase in catalytic
activity.
As observed by TPR, the nature of the Ln affects
the reducibility of Co in the perovskites LnCoO3. The
√
Goldschmidt tolerance factor t = (rLn +rO )/[ 2(rCo +rO )]
obtained for the structures of LaCoO3, PrCoO3, NdCoO3,
SmCoO3, and GdCoO3 were 0.899, 0.885, 0.878, 0.867, and
0.857, respectively. These tolerance factors indicate that
considering solely geometric factors lanthanum, the largest
ion in the series, forms the most stable perovskite struc-
ture. This trend is reflected in the TPR results, where the
perovskite LaCoO3, the most stable structure, is reduced at
the higher temperatures, 844 K (Fig. 11).
FIG. 10. X-ray diffraction patterns for (a) calcined perovskite precur-
sors LnCoO3, (b) after reduction (under a 30% H2/Ar flow at 1023 K for
3.5 h), and (c) after reaction (1023 K for 19 h). A, LnCoO3; C, Ln2O3.
performances for syngas production, under reaction did
not reoxidize back to the original perovskites. According
–
–
to the XPS Co/Ln surface ratio (Table 1), the Gd Co O
catalyst shows a cobalt dispersion (Co/Ln = 0.75) higher
–
–
than that of Sm Co O (0.58) which may be responsible for
the different activities of these catalysts. These phase trans-
formations involving the perovskite and the simple oxides
under oxidative and reductive conditions have been care-
fully studied by XRD for NdCoO3. NdCoO3 was observed
to be consecutively reduced first to NdCoO2.5 and then to
metallic cobalt dispersed over Nd2O3. These two reduced
forms in the presence of oxygen at high temperature were
demonstrated to be easily reoxidized back to the perovskite
structure. However, if the metallic cobalt is sintered the re-
oxidation is not completely reversible, producing a mixture
of NdCoO3, Nd2O3, and CoO. Similar results for LaNiO3
and LaCoO3 have been reported in the literature (44–47).
FIG. 11. Goldschmidt’s tolerance factor t and reduction temperature
obtained from temperature-programmed reduction experiments for the
LnCoO3 perovskites.

208
LAGO ET AL.
earth oxide according to the reaction
Ln2O3 + H2O ꢀ 2 LnO(OH).
[9]
This reaction is more favorable for the La2O3 (as suggested
by the CO TPD experiments; Figs. 5A–5C), which is prob-
ably due to its stronger basic character. It is possible that in
the reaction conditions these hydroxyl groups, by reverse
spillover, can reach the cobalt metal crystallites in the sur-
face and in the bulk of the catalyst promoting its reoxida-
tion. However, more detailed study is necessary to investi-
gate this point. Another effect which could be present has
been reported for the system M/Ln2O3/support (M = Rh
and Pd; and Ln = La, Ce, Pr, Nd, and Sm) (31–33), where
metal particles covered with partially reduced rare earth
FIG. 12. Goldschmidt’s tolerance factor t and oxidation temperature
obtained from temperature-programmed oxidation experiments for the oxide “islands” show drastic changes in their surface and
LnCoO3 perovskites.
catalytic properties.
5. CONCLUSION
Likewise, the TPO experiments showed that reoxidation
of cobalt to form the perovskite structure is more favorable
for larger lanthanides. Figure 12 shows a good correlation
This work suggests that the high activity and selectivity
–
–
–
–
of the catalysts Gd Co O and Sm Co O for the partial
oxidation of methane to synthesis gas is due to the stabil-
ity of the cobalt in its reduced state over the sesquioxides
between the oxidation temperature obtained from the TPO
profiles and the Goldschmidt tolerance factor. Katsura et al.
(48) studied the thermodynamics of the oxidation of iron
to the rare earth perovskites between 1473 and 1673 K ac-
cording to the reaction
–
–
–
–
Gd2O3 and Sm2O3. In the case of La Co O and Nd Co O
reoxidation of cobalt to the original perovskite structure
causes loss of activity and selectivity. Reoxidation of the
catalysts seems to be related to the thermodynamic stabil-
ity of the parent perovskite structure. It is also suggested
that the presence of hydroxyl groups on the rare earth ox-
Fe (s) + 1/2 Ln2O3 + 3/4 O2 (g) ꢀ LnFeO3 (s). [8]
They showed that reoxidation to form the perovskite
structure is favored in the order La > Nd > Sm > Gd with
standard Gibbs energies of ꢄ288.0, ꢄ274.6, ꢄ267.9, and
ꢄ263.7 kJ/mol, respectively.
–
–
ide, especially in the La Co O system, might make some
contribution to the reoxidation of cobalt metal via a reverse
spillover phenomenon.
The question of whether, under reaction conditions,
other factors affect the stability and activity of the cata-
lysts is under investigation. For example, the basicity
of the rare earth oxides which increases in the order
Gd < Sm < Nd < Pr < La is known to have a strong effect
on the oxidative coupling of methane (35, 36). Marcos
et al. (25) studied the reduced perovskites La1ꢄyAyCoO3
(A = Sr and Th) and showed that at increasing oxide ba-
sicities, Sr > La > Th, the surface becomes covered by hy-
droxyls. They demonstrated that upon heating in a vacuum
these hydroxyl groups can oxidize the cobalt metal crystal-
lites. Also, veryrecentlyWengetal. (49) proposed that in the
catalyst Rh/Al2O3 during the partial oxidation of methane,
adsorbed water or OH groups on the alumina support can
act as an oxygen source for oxidation reactions taking place
on the Rh surface. It is envisaged here that the presence of
hydroxyl groups on the Ln2O3 support could favor the reox-
idation of cobalt during the reaction which deactivates, the
catalyst and leads finally to regeneration of the perovskite
structure. During the reduction and the reaction the H2O
present creates hydroxyl groups on the surface of the rare
ACKNOWLEDGMENTS
This work was supported by Comision Interministerial de Ciencia y
Tecnologia, Spain (Contract MAT95-0894). One of the authors (RML) is
grateful to the Ministerio de Educacion y Ciencia, Spain, for the award of
a postdoctoral fellowship. The authors acknowledge Mr. E. Pardo for his
assistance in recording the photoelectron spectra.
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