Changes in microstructure of a reduced cobalt catalyst during performing FT synthesis from syngas determined by in situ high-pressure syngas adsorption

Article abstract of DOI:10.1016/S1381-1169(01)00340-5

Syngas adsorptions at 1.2 MPa and room temperature on a precipitated cobalt catalyst reduced by H₂ and on that used for high-pressure FT synthesis were studied by diffuse reflectance FTIR spectroscopy. The syngas adsorption on the freshly reduced sample gives rise to two weak bands at 2013 and 2033 cm^-1, which are assigned to linear adsorption of CO on metallic cobalt particles. The bands at 2013 and 2033 cm^-1 sharply increase by heating the sample after performing syngas adsorption for over 1 h, up to 100°C in the high-pressure syngas flow. However, the intensities of the bands begin to reduce once the temperature remained constant. The increase of the bands during the heating may be attributed to recombination of the dissociated CO on the catalyst. High ability of the sample for CO dissociation indicates that fine metallic cobalt particles have been formed on the surface. Syngas adsorption on the sample after use for FT synthesis gives rise to two strong bands at 1999 and 2021 cm^-1, and several weak bands at 2150, 2025, 1897 and 1822cm^-1. These bands do not increase while heating the sample to 100°C in the high-pressure syngas flow, indicating low ability of the sample for CO dissociation; so large particles have been formed on the surface. The bands at 1999, 2021 and 1897 cm^-1 are assigned to CO adsorption on cobalt carbides; the bands at 2025 and 1822 cm^-1 may be associated with sintered metallic cobalt species and the band at 2150cm^-1 due to Co²⁺ species. The features of the high-pressure syngas adsorption indicate that, during FT synthesis, fine metallic cobalt particles on the surface are gradually transformed to a mixture of cobalt carbides, sintered metallic cobalt and small amount of cobalt oxides.

Full text of DOI:10.1016/S1381-1169(01)00340-5

Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
Changes in microstructure of a reduced cobalt catalyst during
performing FT synthesis from syngas determined by in situ
high-pressure syngas adsorption
1
2
∗
Guozhu Bian , Tetsuya Nanba , Naoto Koizumi, Muneyoshi Yamada
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 07, Aramaki,
Aoba-ku, Sendai 980-8579, Japan
Received 27 April 2001; received in revised form 4 July 2001; accepted 22 August 2001
Abstract
Syngas adsorptions at 1.2 MPa and room temperature on a precipitated cobalt catalyst reduced by H2 and on that used
for high-pressure FT synthesis were studied by diffuse reflectance FTIR spectroscopy. The syngas adsorption on the freshly
−
1
reduced sample gives rise to two weak bands at 2013 and 2033 cm , which are assigned to linear adsorption of CO on
−1
metallic cobalt particles. The bands at 2013 and 2033 cm sharply increase by heating the sample after performing syngas
adsorption for over 1 h, up to 100 C in the high-pressure syngas flow. However, the intensities of the bands begin to reduce
◦
once the temperature remained constant. The increase of the bands during the heating may be attributed to recombination of
the dissociated CO on the catalyst. High ability of the sample for CO dissociation indicates that fine metallic cobalt particles
have been formed on the surface. Syngas adsorption on the sample after use for FT synthesis gives rise to two strong bands
−
1
−1
at 1999 and 2021 cm , and several weak bands at 2150, 2025, 1897 and 1822 cm . These bands do not increase while
◦
heating the sample to 100 C in the high-pressure syngas flow, indicating low ability of the sample for CO dissociation; so
large particles have been formed on the surface. The bands at 1999, 2021 and 1897 cm are assigned to CO adsorption on
−1
−1
cobalt carbides; the bands at 2025 and 1822 cm may be associated with sintered metallic cobalt species and the band at
− 2+
1
2
150 cm due to Co species. The features of the high-pressure syngas adsorption indicate that, during FT synthesis, fine
metallic cobalt particles on the surface are gradually transformed to a mixture of cobalt carbides, sintered metallic cobalt and
small amount of cobalt oxides. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: FTIR; Syngas adsorption; Cobalt catalyst; FT synthesis; Microstructure
1. Introduction
∗
Corresponding author. Tel.: +81-22-217-7214;
Iron and cobalt-based catalysts are FT synthesis
fax: +81-22-217-7293.
catalysts most commonly used. The morphologi-
cal changes of the catalysts have been well stud-
ied for the purpose of understanding the catalytic
property-structure relationship. It is now widely ac-
cepted that the composition of iron-based catalysts
undergoes significant changes during the reactions,
and that the formation of iron carbides always occurs
E-mail address: yamada@erec.che.tohoku.ac.jp (M. Yamada).
1
Permanent address: Department of Chemical Physics, University
of Science and Technology of China, Hefei, Anhui 230026, PR
China.
2
Present address: Catalytic and Electrochemical Purification
Group, Institute for Environmental Management Technology,
National Institute of Advanced Industrial Science and Technology,
Japan.
1
381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00340-5

2
20
G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
to constitute the active sites for FT synthesis [1–4].
On the other hand, cobalt has a lesser tendency to
produce carbides than Fe. The active components on
cobalt catalysts are usually considered to remain in
metallic states during FT synthesis [5–8].
Recently, however, Ducreux et al. [9] observed the
formation of cobalt carbides on a supported cobalt cat-
alyst during FT synthesis by in situ XRD, which was
also related with the deactivation process of the sam-
ple. These observations have been supported by Tihay
et al. [10], who reported that when an Fe–Co-based
catalyst is used for FT synthesis, metallic Co particles
on the surface are partly transformed to carbides, as
revealed by high-resolution transmission electron mi-
croscopy (HRTEM) observations.
Cobalt species are usually observed to exist as the
metallic phase during FT synthesis; those observations
may suggest small sizes and low concentration of the
cobalt carbide particles formed during the reaction. It
was also found [9,10] that those cobalt carbides are
unstable, which were decomposed to metallic cobalt
and polymeric carbon easily, so that they have rarely
been observed by ex situ techniques. Therefore, it may
be favorable to identify cobalt carbides by using some
surface-sensitive experimental techniques and by in
situ investigations of the catalysts after performing FT
synthesis.
We have recently investigated the high-pressure
syngas adsorption on reduced and used iron catalysts
by in situ diffuse reflectance FTIR [11]. The adsorp-
tion of CO on metallic iron particles and that on iron
carbides can be clearly distinguished. Syngas adsorp-
tion on the cobalt species may be comparable to that
on the iron species. Although the adsorption prop-
erties of oxidized and freshly reduced cobalt-based
catalysts have been widely investigated [12–19], re-
garding to the cobalt catalyst used for high-pressure
FT synthesis; however, nearly no infrared data are
available. In order to elucidate the morphological
changes during high-pressure FT synthesis, includ-
ing whether or not cobalt carbides are formed, we
characterized in the present work the processes of
high-pressure syngas adsorption on freshly reduced
cobalt catalyst and on that used for FT synthesis
from syngas at different temperatures. In situ diffuse
reflectance FTIR spectroscopy is applied to investiga-
tions of the syngas adsorption. The characterizations
after catalytic test give information about the transfor-
mation of fine metallic cobalt phase into a mixture of
cobalt carbides, sintered metallic particles and small
amount of cobalt oxides.
2. Experimental
The precipitated cobalt oxide sample used in this
work was prepared in the usual batch-wise man-
ner by adding ammonia solution into a solution of
Co(NO3)2·6H2O. The obtained precipitates were
washed with deionized water, followed by drying at
◦
◦
120 C for 12 h and calcination in air at 350 C for 4 h.
The diffuse reflectance FTIR spectra were recorded
−
1
with a resolution of 4 cm and accumulation of 20 on
a Bio-Rad FTS 6000 FTIR spectrometer supplied with
a MCT detector and a diffuse reflectance attachment.
The infrared cell with ZnSe windows (Spectra-Tech
Inc.) can be performed at high pressures and high
temperatures. One set of stainless steel gas lines was
connected to the cell, which allows in situ reduction
and FT synthesis followed by high-pressure syngas
adsorption.
For performing FTIR measurements, the oxi-
◦
dized samples were in situ reduced at 300 C and
atmospheric pressure in a flow of ultrahigh purity
(>99.999%) H2 used as supplied. In situ high-pressure
FT synthesis on the reduced sample was performed
in a syngas flow (H2/CO = 2.0) at 1.2 MPa, 200 and
◦
240 C, respectively. High-pressure syngas adsorption
on the reduced and used samples was performed in the
syngas flow at 1.2 MPa and room temperature. The
syngas was also an ultrahigh purity one (>99.999%),
which was used as supplied. He (>99.999%) used
for flushing and thermal desorption of the adsorbed
species was further purified by passing it through
a deoxy trap. The gas flows were maintained at
−
1
60 ml min by mass flow control meters. Approxi-
mately 25 mg of powder sample was used for each
experiment.
Powder XRD patterns of the catalyst samples were
obtained on a Miniflex diffractometer (Rigaku) us-
ing Cu K␣ radiation with a Ni filter. Cobalt phases
were detected by comparing diffraction patterns with
those in the standard powder XRD file compiled by
the Joint Committee on Powder Diffraction Standards
published by the International Center for Diffraction
Data. The reduced and used samples were passivated

G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
221
in a stream of 1% O2/He at room temperature before
being transferred from the reactor for XRD measure-
ments.
3
. Results and discussion
3
.1. High-pressure syngas adsorption on freshly
reduced cobalt catalyst samples
Freshly in situ H2-reduced cobalt sample was sub-
jected to the adsorption experiment in a syngas flow
−
1
(
H2/CO = 2, 60 ml min ) at 1.2 MPa and room tem-
perature. Fig. 1A shows the spectra recorded with
time after raising pressure to 1.2 MPa, from which
only very weak bands can be observed until syngas
adsorption for 15 h. The spectrum attached in the
Fig. 1A is the amplified one of Fig. 1A(d), referenc-
ing to the spectrum recorded at the initial 15 s of the
syngas adsorption. From which two bands at 2013
and 2033 cm can be clearly observed. According
to the literature [12,16,18,19], the bands in the region
of 2050–1980 cm are usually associated with the
stretching modes of linear carbonyl on zero-valent
−
1
−
1
−
1
Co. Therefore, the bands at 2013 and 2033 cm can
be assigned to linearly adsorbed CO on tops of metal-
lic cobalt particles. Since the adsorption is performed
at the high pressure of 1.2 MPa, any bands due to CO
δ+
adsorption on Co (0 < δ ≤ 2), which should ap-
−
1
pear in the region of 2050–2180 cm [12,15,17,20],
would be hidden in the very strong bands of gaseous
CO and therefore difficult to detect.
The adsorption experiment was reproduced and
after syngas adsorption was performed for 1 h, the
◦
sample was quickly heated up to 100 C in the
Fig. 1. Infrared spectra recorded from syngas adsorption on
H2-reduced Co catalyst at 1.2 MPa and room temperature for
1 min (a), 20 min (b), 1 h (c), 3 h (d) and 15 h (e) (A); and those
recorded after heating the reduced Co sample, which previously
adsorbed syngas at 1.2 MPa and room temperature for 1 h, in the
high-pressure syngas flow. The spectra recorded after
the temperature reached 100 C are shown in Fig. 1B.
By increasing the temperature up to 100 C, the bands
◦
◦
−
1
at 2013 and 2033 cm become very stronger. The
spectrum attached on Fig. 1B is the amplified one of
Fig. 1B(a), referencing to the same spectrum as used
for the amplified spectrum attached in Fig. 1A. It is
seen that the bands become about 10 times as strong
as those on the amplified spectrum shown in Fig. 1A.
The spectra in Fig. 1B also indicate that, with remain-
ing temperature at 100 C, however, the intensities
of the bands at 2013 and 2033 cm gradually de-
crease in the syngas flow. By a careful inspection, it
◦
high-pressure syngas flow up to 100 C (a), and then remaining
◦
at 100 C for 1 min (b), 5 min (c), 1 h (d) and 3 h (e) (B).
is found that raising temperature in the range of about
◦
50–130 C will lead to obvious increase in intensities
−
1
of the bands at 2013 and 2033 cm ; whereas the
intensities of the bands become reduced once the tem-
perature is remained constant. These results suggest
that the tremendous increases of the bands during
◦
−
1

2
22
G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
Fig. 3. Infrared spectra recorded after heating the H2-reduced Co
sample up to 100 C in the high-pressure syngas flow; the sample
previously adsorbed syngas at 1.2 MPa and room temperature for
Fig. 2. Infrared spectra recorded from syngas adsorption on
H2-reduced Co catalyst 1.2 MPa and 100 C for 1 min (a), 20 min
◦
◦
(b), 1 h (c) and 3 h (d).
20 min (a) or 15 h (b).
the raising temperature should not be due to further
adsorption of gaseous CO molecules on the catalyst
surface.
The above results may be explained as that, dur-
ing the syngas adsorption at room temperature, large
amounts of CO molecules are dissociatively adsorbed
on the reduced cobalt sample, which can not be de-
tected by FTIR; with raising temperature, the surface
carbon and oxygen atoms from dissociated CO re-
combine to form CO molecules, which are thereafter
molecularly adsorbed on the surface and can be de-
tected by FTIR. When the syngas adsorption is per-
formed at room temperature for longer times, more
CO molecules become adsorbed on the catalyst sur-
face; therefore, larger increases in intensities of the
To further elucidate the cause for the significant
increase in intensities of the bands at 2013 and
−
1
2
033 cm by raising temperature in the high-pressure
syngas flow, more investigations have been per-
formed. It is found that, as displayed in Fig. 2, while
the syngas adsorption is performed at 1.2 MPa and
◦
1
00 C, only two weak bands are evolved at 2013
−
1
and 2033 cm after syngas adsorption for 3 h. The
intensities of the bands are even lower than those of
the bands in Fig. 1A.
−
1
bands at 2013 and 2033 cm are observed by raising
temperature up to 100 C.
◦
As shown in Fig. 3, it is also found that heating
◦
the sample up to 100 C in the high-pressure syngas
The above speculation is confirmed by flash-
ing measurement in a helium flow as well. After
the reduced cobalt sample was subjected to the
high-pressure syngas adsorption at room temperature
for 1 h, pressure was reduced to atmospheric pressure
and then syngas was replaced by helium. The bands
of gaseous CO disappear by simple helium flushing
for about 0.5 min. The flashing was carried out after
helium flushing for 3 min; at this moment no gaseous
CO exists in the system. The spectra presented in
Fig. 4 are those recorded before and after flashing
the sample in the helium flow. The spectra shown are
flow, after performing syngas adsorption at room tem-
perature for 20 min, produces a small increase of the
−
1
bands at 2013 and 2033 cm . On the other hand,
while the sample is heated after syngas adsorption
at room temperature for 15 h, a tremendous increase
−
1
in intensities of the bands at 2013 and 2033 cm
is observed. These results clearly indicate that the
−
1
increases of the bands at 2013 and 2033 cm dur-
ing raising temperature have close correlation with
the previous high-pressure syngas adsorption at room
temperature.

G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
223
It has been reported from X-ray photoelectron spec-
troscopy analyses [22–24] that, on crystalline iron sur-
face, CO adsorbs molecularly at low temperatures such
◦
as −150 C. The adsorbed CO molecules desorb se-
quentially at elevated temperatures and dissociate on
◦
the sample at above 100 C. The dissociated carbon
and oxygen atoms will recombine to CO molecules
◦
and desorb from the surface at above 500 C. On this
work, the temperatures for CO dissociation and for the
dissociated carbon and oxygen atoms to recombine to
CO molecules are both greatly reduced in comparison
with the results for crystalline iron. Therefore, the cat-
alyst samples have relatively higher capability for CO
dissociation; which indicate that those metallic cobalt
particles formed on the surfaces are fine ones.
On the spectra shown in Fig. 4, which were
recorded after syngas was adsorbed for 1 h and then
high pressure syngas was replaced by a helium flow at
atmospheric pressure, no bands due to CO adsorption
Fig. 4. Infrared spectra recorded from syngas adsorption on reduced
Co catalyst at 1.2 MPa and room temperature for 1 h, followed
by helium flushing at atmospheric pressure for 3 min (a), flashing
on Co2 and Co species are observed, therefore,
most of the cobalt species on the surface have been
converted to metallic phase by the hydrogen reduc-
tion. This result is in agreement with the data from
X-ray photograph spectroscopy [25]. Jiang et al. [18]
reported that while the cobalt sample is reduced in
+
1+
◦
the sample in the helium flow up to 100 C (b) and remaining at
◦
00 C for 2 min (c).
1
referencing to the spectrum recorded at the end of hy-
drogen reduction before sample is exposed to syngas.
◦
◦
From Fig. 4, it is observed that flashing to 100 C
hydrogen at 300 C and 1.2 MPa, syngas adsorp-
in the helium flow results in an obvious increase in
tion on the reduced sample gives bands at 2080 and
−
1
−1
intensities of the bands at 2013 and 2033 cm . It
is also observed that these bands thereafter quickly
diminish and almost disappear in about 2 min.
Combining the results from quickly raising tem-
perature in the high-pressure syngas flow and the
results from He flashing, it can be concluded that,
during desorption from the surface, parts of the dis-
sociative CO molecules may go through the molecu-
larly adsorption states, in this step increasing in the
C–O bond strength and, therefore, being detected by
FTIR with the same frequencies as the molecularly
adsorbed CO molecules [21]. The recombined CO
molecules are slowly desorbed in the high-pressure
syngas flow. Upon helium flashing at atmospheric
pressure the desorption becomes very fast. The recom-
bination of dissociated CO has rarely been observed
in literature, which may be due to the fact that CO
adsorption on cobalt catalysts are usually performed
at low pressures, such as <100 Torr, and there-
fore, desorption of the recombined CO occurs very
faster.
2050 cm , which are assigned to CO adsorption on
δ+
0
Co (0 < δ < 2) and oxygen-rich Co sites, respec-
tively. Therefore, hydrogen reduction at low pressure,
as used in this work, favors to complete reduction of
the oxidized cobalt species.
3.2. High-pressure syngas adsorption on the
reduced cobalt catalyst used for FT synthesis
◦
at 200 C
To investigate the morphological changes of the re-
duced cobalt catalyst during FT synthesis, the sample
used for in situ FT synthesis from syngas was also
subjected to the experiment of high-pressure syngas
adsorption. The FT synthesis was performed for 6 h at
◦
1.2 MPa, 200 and 240 C, respectively. After the reac-
tions, the samples were cooled to room temperature
in the high-pressure syngas flow.
While the FT synthesis was performed at 1.2 MPa
◦
and 200 C, the sample was less active and, according
to FTIR observations, water and hydrocarbons were

2
24
G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
Fig. 6. Infrared spectra from the ratio of Fig. 5B(a) to Fig. 5A(c)
a), and the ratio of Fig. 5B(b) to Fig. 5B(a) (b).
(
sities of the bands slowly enhance with adsorption
time.
The above experiment was reproduced and, after
syngas adsorption for 1 h, the sample was quickly
◦
heated up to 100 C in the high-pressure syngas flow.
The spectra recorded after the temperature reached
◦
1
2
00 C are shown in Fig. 5B. Four bands at 1999, 2021
−
1
013, and 2033 cm are clearly observed. In compar-
ison with the spectra recorded before raising tempera-
−1
ture (Fig. 5A), two more bands at 2013 and 2033 cm
appear. The ratio of Fig. 5B(a) to Fig. 5A(c) is pre-
sented in Fig. 6a, from which two minus bands at
−
1
2
013 and 2033 cm can be clearly observed. How-
Fig. 5. Infrared spectra recorded from syngas adsorption on the
ever, the intensities of the bands become obviously
reduced compared to those for the freshly reduced
sample (attached of Fig. 1B). The spectrum shown in
Fig. 6b is the ratio of Fig. 5B(b) to Fig. 5B(a), on
which apparently four plus bands at 1999, 2021 2013,
reduced Co catalyst, after use for FT synthesis at 1.2 MPa and
◦
2
00 C for 6 h, at 1.2 MPa and room temperature for 1 min (a),
0 min (b), 1 h (c) and 3 h (d) (A); and those recorded after heating
2
the used Co sample, which previously adsorbed syngas at 1.2 MPa
and room temperature for 1 h, in the high-pressure syngas flow up
−
1
◦
◦
to 100 C (a), and remaining at 100 C for 5 min (b) and 1 h (c)
B).
and 2033 cm are observed. Therefore, adsorbed CO
species are gradually desorbed from the surface at the
elevated temperature. For correctly assigning the weak
(
−
1
bands at 1897 and 1822 cm , it is useful to note that
the adsorbed species responsible for the broadband at
formed. The spectra recorded after cooling the sam-
◦
−1
ple to room temperature (<30 C) are presented in
1897 cm are also desorbed during heating and re-
◦
Fig. 5A. It is observed that the syngas adsorption gives
maining the sample at 100 C, whereas the adsorbed
species responsible for the broadband at 1822 cm
−
1
−1
rise to two strong bands at 1999 and 2021 cm and
two weak bands at 1897 and 1822 cm 1. The inten-
−
do not desorb from the surface.

G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
225
partially transformed during the FT synthesis; and
the new-formed species exhibit significantly low abil-
ity for CO dissociation. According to the literature
−
1
[
12–14], the weak band at 2150 cm should be as-
2
+
centers. These
signed to CO adsorption on Co
observations indicate that small amount of metallic
cobalt species was oxidized, which may be due to the
formation of water and CO2 on the surface during FT
synthesis [26].
−
1
The bands in the region of 1900–1800 cm have
0
been assigned to bridged CO on Co species [12,16].
As shown above, during the thermal desorption ex-
periment in the helium flow, the broad bands at 2025
−
1
and 1822 cm do not disappear with raising temper-
◦
ature up to 300 C, while the bands at 2021, 1999
−
1
and 1822 cm
vanish with raising temperature up
◦
to 150 C. Therefore, these bands may be assigned
to two kinds of Co species, respectively. The sur-
Fig. 7. Infrared spectra recorded after helium flushing of the Co
catalyst, which was previously used for FT synthesis at 1.2 MPa
and 200 C for 6 h and syngas adsorption at 1.2 MPa and room
0
◦
face species responsible for the bands at 2025 and
−
1
1822 cm exhibits high thermal stability and strong
temperature for 1 h, at atmospheric pressure for 3 min (a); and then
◦
◦
heating the sample in the helium flow up to 100 C (b), 150 C
binding strength with the adsorbed CO. Therefore, it
may be a new kind of sintered metallic cobalt phase
formed during the reaction, which can connect with
adsorbed CO to form stable sub carbonyl species. On
the other hand, the strong doublet bands at 1999 and
◦
◦
(c); 200 C (d) and 300 C (e).
Helium flushing of the adsorbed species at atmo-
−
1
−1
spheric pressure followed by thermal desorption up
2021 cm and the weak broad band at 1897 cm
◦
to 300 C was also performed after syngas adsorp-
may be assigned to linear and bridging adsorption of
CO on cobalt carbides, respectively.
tion for 1 h. A few spectra are shown in Fig. 7. Only
a small decrease in intensities of the bands at 1999
The main reasons for assigning the bands at 1999,
−
1
−1
and 2021 cm was observed after helium flushing for
min. With elimination of strong bands due to gaseous
2021 and 1897 cm to CO adsorption on cobalt car-
3
bides may be summarized below. (1) The formation of
cobalt carbides during FT synthesis has been demon-
strated by in situ XRD and HRTEM analyses [9,10]; it
is probable that syngas adsorption on the used samples
arise bands due to CO adsorption on cobalt carbides.
−
1
CO, a broad band at about 2150 cm is seen. The re-
sults from thermal desorption experiment indicate that
nearly no increase is observed for the bands at 2013
and 2033 cm by raising temperature up to 100 C;
the bands at 1999, 2021 and 1897 cm begin to di-
minish at about 100 C and disappear at about 150 C.
With elimination of the bands at 1999 and 2021 cm
a broad band at about 2025 cm can be observed.
The bands at 2025 and 1822 cm do not disappear
−
1
◦
−
1
−1
(2) The bands in the region of 2040–1800 cm have
◦
◦
−
0
been due to CO adsorptio on Co species [12,16–19],
1
cobalt carbides are Co0 species [10,27,28], so the
bands of CO adsorption on cobalt carbides should ap-
pear in this region as well. (3) According to the liter-
ature [12,15–20,29], the features of CO adsorption on
cobalt-based catalysts and those on iron-based cata-
,
−
1
−
1
◦
with raising temperature up to 300 C.
Syngas adsorption on the used cobalt sample gives
rise to a new set of bands, and the bands at 2013
δ+
lysts are similar. For example, CO adsorption on Fe
−
1
δ+
and 2033 cm become very weaker compared with
those for the freshly reduced sample. These results
indicate that significant changes in microstructure
have occurred during the reaction. Fine metallic
cobalt particles formed by hydrogen reduction are
or Co (0 < δ ≤ 2) gives bands in the region of
−
1
0
0
2200–2050 cm ; on Fe or Co species linear CO ap-
−
1
pears in 2050–1980 cm and bridged CO appears in
1900–1700 cm . It was revealed in our previous work
[11] that on freshly H2-reduced iron catalyst syngas
−
1

2
26
G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
adsorption arises weak bands at 2013 and 2033 cm 1
−
;
and that on the same sample used for high-pressure
FT synthesis syngas adsorption gives two very strong
−
1
bands at 1999 and 2021 cm , which are assigned to
CO adsorption on iron carbides. So, in this work, the
−
1
bands at 1999 and 2021 cm might be assigned to
CO adsorption on cobalt carbides as well. (4) The an-
other possible assignment of the surface species corre-
−
1
lated with the bands at 1999, 2021 and 1897 cm may
be some aggregated metallic cobalt phase transformed
from the fine metallic cobalt particles. However, as
described later in this paper, the surface species asso-
−
1
ciated with the bands at 1999, 2021 and 1897 cm is
unstable, which can be converted back to fine metallic
◦
cobalt species by helium treatment at 100 C. There-
fore, the assignment is denied because any aggregated
metallic cobalt phase formed during the FT synthesis
is more stable than those fine metallic particles on the
freshly reduced sample.
Fig. 8. Infrared spectra recorded from syngas adsorption on used
Co catalyst at 1.2 MPa and room temperature for 1 min (a), 20 min
(
b), 1 h (c) and 3 h (d). The FT synthesis was performed at 1.2 MPa
◦
and 240 C for 6 h.
3
.3. High-pressure syngas adsorption
on the reduced catalyst after use for high-pressure
FT synthesis at 240 C
◦
−1
2
00 C; the band at 1897 cm disappears simulta-
◦
neously. Therefore, the adsorbed CO molecules be-
come more closely bound on the cobalt carbide parti-
cles in comparison with those on the sample after use
While the freshly H2-reduced sample was used for
◦
FT synthesis at 1.2 MPa and 240 C, the sample was
rather active. After reaction for 6 h, the sample was
cooled to room temperature in the syngas flow to
perform high-pressure syngas adsorption. The spectra
recorded are displayed in Fig. 8.
As shown in Fig. 8, syngas adsorption on the
used sample gives rise to two strong bands at 1999
−
1
and 2021 cm , and two weak bands at 1897 and
1
−
1
822 cm . The spectra are similar to those from
syngas adsorption on the sample after use for FT
◦
synthesis at 200 C, but the intensities of the bands
become significantly stronger.
The experiment was reproduced and after syngas
adsorption for 1 h, the sample was quickly heated up
◦
to 100 C in the high-pressure syngas flow. By this
way, however, no bands are observed at 2013 and
−
1
2
033 cm (the data are not shown). The data from
thermal desorption experiment are shown in Fig. 9.
−
1
Fig. 9. Infrared spectra recorded from a helium flushing of the
used Co catalyst, which was previously used for FT synthesis
at 1.2 MPa and 300 C for 6 h and syngas adsorption at 1.2 MPa
The bands at 2013 and 2033 cm are not observed
−
1
as well; and the bands at 1999 and 2021 cm re-
◦
◦
main unchanged by raising temperature to 100 C.
and room temperature for 1 h, at atmospheric pressure for 3 min
With further increasing temperature, the bands at 1999
◦
(
a); and then heating the sample in the helium flow up to 100 C
−
1
◦ ◦ ◦
(b), 150 C (c), 200 C (d) and 300 C (e).
and 2021 cm begin to diminish and vanish at about

G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
227
◦
for FT synthesis at 200 C. It is observed once again
−
1
that the broad bands at 2150, 2025 and 1822 cm
do not disappear until raising the temperature up to
◦
3
00 C.
Elimination of the bands at 2013 and 2033 cm 1
−
reveals complete disappearance of the fine metallic
cobalt particles from the surface. In the meantime,
it is also observed that more sintered metallic cobalt
species and cobalt carbides are formed. Therefore, al-
most all of the fine metallic cobalt particles on the sur-
face have been transformed to sintered metallic cobalt
species, cobalt carbides and small amount of cobalt
oxide by performing FT synthesis at elevated temper-
ature. The temperatures for adsorbed CO molecules
to desorb from the surface of cobalt carbide particles
become significantly higher, which may indicate large
sizes of the particles.
Fig. 10. XRD patterns of the cobalt catalyst samples: (1) oxidized,
Syngas adsorption on the cobalt samples used for
FT synthesis reveals the formation of cobalt carbides
on the surfaces. However, in comparison with the data
of syngas adsorption on used iron samples [11], the
bands due to CO adsorption on cobalt carbides are
obviously weaker than those due to CO adsorption on
iron carbides. In addition, during syngas adsorption on
◦
(
2) after H2 reduction at 300 C for 6 h, (3) after use for FT
◦
synthesis at 250 C, 1.0 MPa for 6 h.
all of which are characteristics of Co3O4 phase. The
pattern of the catalyst sample reduced with hydrogen
−
1
◦
used iron samples, no bands at 2025 and 1822 cm
at 300 C for 6 h has dominant peaks at 41.7, 44.6,
◦
0
were observed. These results demonstrate that cobalt
species have a lesser tendency to form carbides than
the iron species.
47.5 and 75.9 , which correspond to Co phase. The
pattern of the reduced cobalt catalyst after use for
FT synthesis at 1.0 MPa, 250 C for 6 h shows same
◦
It was also observed that, after the cobalt sample
peaks as that of the freshly reduced sample.
◦
utilized for the FT synthesis at 1.2 MPa, 240 C for 6 h
XRD pattern of the freshly reduced sample reveals
the formation of metallic cobalt species, which is in
agreement with the results from high-pressure syngas
adsorption. Syngas adsorption on the cobalt samples
◦
is treated in a helium flow at 100 C, syngas adsorp-
tion on the treated sample gives rise to weaker bands
−
1
at 1999 and 2021 cm , and the bands at 2013 and
−1
◦
2
033 cm appear again (the data are not shown). This
after use for FT synthesis at 1.2 MPa and 240 C for
result indicates that the surface species correlated to
6 h indicates that noticeable amounts of cobalt car-
bides are formed; however, XRD pattern of the sample
after use for FT synthesis at 1.2 MPa and 250 C for
−
1
the bands at 1999 and 2021 cm are unstable, which
might be decomposed to metallic cobalt and polymeric
carbon easily. The observations that cobalt carbides
are unstable are in agreement with the results from the
literature [9,10].
◦
6 h shows that only metallic cobalt is present. These
results indicate that only those cobalt species on the
surface are transformed during the reaction. The ob-
servations that only metallic cobalt species are de-
tected by XRD analysis are consistent with those in the
literature [6–8]. Although bulk cobalt phase remains
as metallic cobalt during the FT synthesis, the parti-
cles on the surface are gradually changed; the forma-
tion of cobalt carbides may have great effects on FT
synthesis, which should be investigated in the future
work.
3
.4. XRD patterns of the oxidized, reduced
and used cobalt samples
Bulk cobalt phases of the samples were determined
by XRD measurements. The results are shown in
Fig. 10. The pattern of the oxidized cobalt sample
◦
has peaks at 31.3, 36.9, 44.7, 55.9, 59.3 and 65.4 ,

2
28
G. Bian et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 219–228
[3] S.A. Eliason, C.H. Bartholomew, Appl. Catal. A 186 (1999)
4
. Summary
229.
[
4] M.D. Shroff, D.S. Kalakkad, A.G. Sault, A.K. Datye, J. Catal.
High-pressure syngas adsorption on freshly reduced
156 (1995) 185.
cobalt sample gives rise to two weak bands at 2013 and
[5] R.B. Anderson, in: P.H. Emmett (Ed.), Catalysis, Vol. IV, Van
Nostrand, New York, 1956, pp. 29–55.
−
1
2
033 cm , which are assigned to linear adsorption of
CO on metallic cobalt particles. The bands at 2013 and
[6] B. Ernst, A. Bensaddik, L. Hilaire, P. Chaumette, A.
Kiennemann, Catal. Today 39 (1998) 329.
[7] B. Ernst, S. Libs, P. Chaumette, A. Kiennemann, Appl. Catal.
A 186 (1999) 145.
−
1
2
033 cm sharply increase by heating the sample up
◦
to 100 C in the high-pressure syngas flow after syn-
gas adsorption for above 1 h. The increase of the bands
during the heating may be attributed to recombina-
tion of the dissociated CO on the catalyst. High ability
of the sample for CO dissociation indicates that fine
metallic cobalt particles have been formed on the sur-
face. The results of high-pressure syngas adsorption
on the reduced sample after use for high-pressure FT
synthesis from syngas indicate that fine metallic cobalt
particles on the surface are gradually transformed to
a mixture of cobalt carbides, sintered metallic cobalt
and small amount of cobalt oxides. All the bands due
to CO adsorption on the used sample do not increase
[8] G.P. Huffman, N. Shah, J. Zhao, F.E. Huggins, T.E. Hoost,
S. Halvorsen, J.G. Goodwin Jr., J. Catal. 151 (1995) 17.
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Stud. Surf. Sci. Catal. 119 (1998) 125.
10] F. Tihay, G. Pourroy, M. R-chard-Plouet, A.C. Roger, A.
Kiennemann, Appl. Catal. A 206 (2001) 29.
11] G. Bian, A. Oonuki, N. Kobayashi, N. Koizumi, M. Yamada,
Appl. Catal. A 219 (2001) 13.
[12] G. Busca, R. Guidetti, V. Lorenzelli, J. Chem. Soc., Faraday
Trans. I 86 (1990) 989.
13] G. Busca, F. Trifiro, A. Vaccari, Langmiur 6 (1990) 1440.
14] G. Busca, V. Lorenzelli, V. Sanchez-Escribano, R. Guidetti,
J. Catal. 131 (1991) 167.
[15] K. Sato, Y. Inoue, I. Kojima, E. Miyazaki, I. Yasumori, J.
Chem. Soc., Faraday Trans. I 80 (1984) 841.
16] J. Ansorge, H. Foerster, J. Catal. 68 (1981) 182.
17] J.E. Bailie, C.H. Rochester, G.J. Hutchings, J. Chem. Soc.,
Faraday Trans. 93 (13) (1997) 2331.
[
[
[
[
◦
by heating the sample to 100 C in the high-pressure
[
[
syngas flow, indicating low ability of the sample for
CO dissociation. Therefore, the particles transformed
from fine metallic particles during the FT synthesis
are large ones.
[
18] M. Jiang, N. Koizumi, T. Ozaki, M. Yamada, Appl. Catal. A
209 (2001) 59.
[
[
19] E.C. Decanio, D.A. Storm, J. Catal. 132 (1991) 375.
20] A. Lapidus, A. Krylova, V. Karzansku, V. Borovkov, A.
Zaitsev, Appl. Catal. 73 (1991) 65.
Acknowledgements
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This work was supported by Research for the Future
Program of Japan Society for the Promotion of Science
under the project ‘Synthesis of Ecological High Qual-
ity Transportation Fuels’ (JSPS-RFTF98P01001). The
authors are grateful to the Center of Interdisciplinary
Research of Tohoku University for use of the infrared
spectrometer.
2
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[
23] J. Benziger, R.J. Madix, Surf. Sci. 94 (1980) 119.
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