Ruthenium promoted cobalt catalysts prepared by an autocombustion method directly used for Fischer-Tropsch synthesis without further reduction

Article abstract of DOI:10.1039/c4cy00402g

Ru promoted Co/SiO₂ Fischer-Tropsch synthesis (FTS) catalysts with high reduction levels were synthesized through an autocombustion method using citric acid (CA) as a reductant and nitrate ions as oxidants. The as-synthesized catalysts were used directly in FTS reaction without further reduction. The effects of the ruthenium promoter, citric acid contents and reductant types on the catalyst structures and FTS performance were systematically studied. Results indicated that the introduction of a small amount of Ru (1 wt%) improved the reduction and dispersion of cobalt during the autocombustion process, and significantly enhanced the FTS activity. The CO conversion of the catalyst increased rapidly from 0.8 to 41.4% after Ru promotion. The citric acid contents (molar ratio of citric acid to nitrates: CA/N) in the precursor also played an important role in controlling the structures and FTS performance of the catalysts. With the increase of CA/N, the metal reduction level increased and the Co crystalline size decreased but the activity of the catalyst first increased and then decreased with gradually increasing CA/N. An excessive amount of the reductant could result in more residual carbon species and decrease the activity of the catalyst. For different types of reductants (at the same molar ratio of reductants to nitrates), the catalyst prepared by citric acid exhibited the highest activity whereas the catalyst synthesized by oxalic acid showed the lowest methane selectivity. The Ru promoted cobalt catalysts prepared by the autocombustion method, which omits the complex and high energy consumption reduction process, can be used directly for highly efficient FTS and thus will be more promising in the future. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00402g

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Ruthenium promoted cobalt catalysts prepared
by an autocombustion method directly used
for Fischer–Tropsch synthesis without
further reduction
Cite this: Catal. Sci. Technol., 2014,
4, 3099
ab
bc
b
b
b
b
Rungravee Phienluphon, Lei Shi, Jian Sun, Wenqi Niu, Peng Lu, Pengfei Zhu,
a
b
b
b
Tharapong Vitidsant,* Yoshiharu Yoneyama, Qingjun Chen* and Noritatsu Tsubaki*
Ru promoted Co/SiO
2
Fischer–Tropsch synthesis (FTS) catalysts with high reduction levels were
synthesized through an autocombustion method using citric acid (CA) as a reductant and nitrate ions as
oxidants. The as-synthesized catalysts were used directly in FTS reaction without further reduction. The
effects of the ruthenium promoter, citric acid contents and reductant types on the catalyst structures
and FTS performance were systematically studied. Results indicated that the introduction of a small
amount of Ru (1 wt%) improved the reduction and dispersion of cobalt during the autocombustion
process, and significantly enhanced the FTS activity. The CO conversion of the catalyst increased
rapidly from 0.8 to 41.4% after Ru promotion. The citric acid contents (molar ratio of citric acid to
nitrates: CA/N) in the precursor also played an important role in controlling the structures and FTS
performance of the catalysts. With the increase of CA/N, the metal reduction level increased and the
Co crystalline size decreased but the activity of the catalyst first increased and then decreased with
gradually increasing CA/N. An excessive amount of the reductant could result in more residual carbon
species and decrease the activity of the catalyst. For different types of reductants (at the same molar ratio
of reductants to nitrates), the catalyst prepared by citric acid exhibited the highest activity whereas
the catalyst synthesized by oxalic acid showed the lowest methane selectivity. The Ru promoted cobalt
catalysts prepared by the autocombustion method, which omits the complex and high energy consump-
tion reduction process, can be used directly for highly efficient FTS and thus will be more promising in
the future.
Received 31st March 2014,
Accepted 17th May 2014
DOI: 10.1039/c4cy00402g
www.rsc.org/catalysis
Due to high activities and selectivities to large molecular
weight hydrocarbons, cobalt-based catalysts are widely used
1
. Introduction
1
–5
Growing demand coupled with the possibility of running out
of oil has significantly increased interest in the production of
renewable and clean fuels. Fischer–Tropsch synthesis (FTS)
has received more attention than ever since it is considered
as an effective process to produce wide-range liquid hydro-
carbon fuels and high-value added chemicals from relatively
abundant resources, such as natural gas, coal and biomass,
for FTS.
The high activity of cobalt catalysts requires both
appropriate sizes of cobalt crystallines and high dispersion of
cobalt active sites. The high dispersion of cobalt needs a
large surface area of the support and a strong interaction
6
between cobalt and the support but a strong metal support
interaction may decrease the reducibility and activity of
the catalyst. Noble metals promotion (such as ruthenium),
which can improve both the reduction and dispersion of
cobalt, is an effective method to enhance the activity of the
1
–3
via synthetic gas (H + CO).
2
7
–9
cobalt catalysts. Besides, synergistic bimetallic interactions
between cobalt and ruthenium can also increase the FTS
a
7–10
Department of Chemical Technology, Faculty of Science, Chulalongkorn University,
Bangkok 10330, Thailand. E-mail: tharapong.v@chula.ac.th; Fax: +66 2 2185813;
Tel: +66 2 2187517
activity and C5+ selectivity.
Even so, the conventional prep-
aration of cobalt-based FTS catalysts involves impregnation,
drying, calcination and reduction steps. Among these steps,
the catalyst reduction is particularly important, but it
increases the cost and complexity of FTS. Therefore, a pro-
moted Co based catalyst with a high metal reduction level
b
Department of Applied Chemistry, School of Engineering, University of Toyama,
Gofuku 3190, Toyama 930-8555, Japan. E-mail: tsubaki@eng.u-toyama.ac.jp,
chenqingjunsh@gmail.com; Fax: +81 76 4456848; Tel: +81 76 4456846, +81 76 4456848
c
Shenyang University of Chemical Technology, Shenyang, 110142, China
This journal is © The Royal Society of Chemistry 2014
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and no need for a reduction process will be more promising
for FTS.
ammonia solution to obtain a value of 7. Then, the solution
was stirred and refluxed at 80 °C for 2 h. Subsequently, the
neutralized solution was evaporated and condensed at 70 °C
on a hot plate with continuous stirring to obtain 20 ml of
solution. Next, the obtained solution was impregnated over
Autocombustion is a novel method to produce catalysts
1
1,12
with high reduction levels.
thesis, a low temperature is required to start the combustion
exothermic decomposition of a redox mixture of metal salts
In the autocombustion syn-
2
−1
(
5 g of silica (Q-50, Fuji Silysia Co.; surface area: 79 m g ,
3
−1
and reductants), and then the combustion can continue with-
out external energy supply. So this method has the advan-
pore volume: 1.1 cm g ). After the impregnation, the sam-
ple was dried in air at 120 °C for 12 h and then calcined in
argon at 400 °C for 3 h to obtain the Ru promoted Co/SiO
2
1
3
tages of low cost and high energy efficiency. Furthermore,
during the combustion process, the metal can be reduced by
the released gases (such as H , CH , CO, and so on) from the
FTS catalyst. The catalyst was subsequently passivated by 1%
oxygen in argon at room temperature for 4 h. The prepara-
tion procedure of ruthenium promoted catalysts by the
autocombustion method was shown in Fig. 1. The obtained
catalysts with different CA/N values are denoted as 0.15RuCo,
0.2RuCo, 0.25RuCo, 0.3RuCo, 0.35RuCo and 0.4RuCo. Differ-
ent types of reductants (formic acid and oxalic acid) with the
molar ratio of reductant to oxidants of 0.3 were also used
to prepare the catalysts under the same preparation condi-
tions as citric acid. Monometal (cobalt/ruthenium) catalysts
with the CA/N of 0.3 were also prepared by the same
autocombustion method and were denoted as 0.3Co and
0.3Ru, respectively.
2
4
pyrolysis of the reductant. Therefore, the as-prepared catalyst
might be used directly for the FTS reaction without further
reduction. However, the autocombustion process is very com-
plex and usually occurs violently. It is more difficult to con-
trol the reduction level than the conventional H reduction.
2
Many studies have been conducted to improve the metal
reduction level during the autocombustion, which include
1
3,14
varying the type of organic reductants,
contents,
the reductant
1
5–19
20–22
and the pH value of the precursors.
But
these methods cannot change the reduction temperature and
the rate of cobalt essentially. A promoted autocombustion
method by introducing a second metal Ru, which can
decrease the reduction temperature of Co and increase the
reduction rate by H2 spillover, may obviously improve the
reduction of Co and enhance the FTS activity. In addition, if
combined with the optimization of types of organic reduc-
tants and the reductant contents, Ru promoted Co catalysts
with high reduction levels and activities might be prepared
by using this method.
In this work, Ru promoted Co/SiO2 FTS catalysts with
a high reduction level were developed by the autocombustion
method. Catalyst characterization results indicated that
the introduction of ruthenium significantly improved the
reduction of cobalt during autocombustion and can help to
achieve a high reduction level of cobalt at a very small
amount of the reductant. The as-prepared catalyst without
further reduction exhibited a high FTS activity comparable to
As a reference catalyst, the air combustion–reduction
catalyst Mair-reduction refers to the catalyst prepared by the
similar autocombustion method with M metal salt and citric
acid (M is Co, Ru or RuCo). The differences are that the cata-
lysts were obtained by calcination in air at 400 °C for 3 h and
then reduction at 400 °C for 10 h in H . For comparison, con-
2
N
ventional impregnated catalysts (denoted as M , where M
was the loaded metal Co, Ru or RuCo) with 10 wt% cobalt
and/or 1 wt% ruthenium were also prepared. The catalysts
were calcined in air at 400 °C for 3 h and then reduced in H
2
at 400 °C for 10 h. All of the catalysts were passivated by 1%
oxygen in argon at room temperature for 4 h.
2
the conventional H reduced catalyst. The effects of the
ruthenium promoter, citric acid contents and reductant types
on the structures and FTS performance of the catalysts have
been studied.
2
. Experiment
2
.1 Preparation of the catalysts
Ru promoted Co/SiO FTS catalysts were synthesized through
2
the autocombustion method using citric acid (CA) as a reduc-
tant and nitrate ions as oxidants. The contents of cobalt and
ruthenium in the catalysts are fixed at 10 wt% and 1 wt%,
respectively. The nitrate compounds (Co(NO ) and Ru(NO ) )
3 2 3 3
are denoted as N and citric acid is denoted as CA. First, the
cobalt nitrate, ruthenium nitrate and CA were dissolved in
1
00 ml of distilled water based on the different molar ratios
of citric acid to nitrates (CA/N) of 0.15, 0.2, 0.25, 0.3, 0.35
Fig. 1 Schematic flow chart of the catalysts preparation by the
and 0.4. The solution's pH was adjusted using 28 wt% of
autocombustion method.
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2
.2 Characterization of the catalysts
ruthenium nitrates as the oxidants and citric acid as the
reductant. After the impregnation of the precursors over SiO2
The behavior of the autocombustion process was character-
ized by thermogravimetic and differential thermal analyses
(
Q-50, shown in Fig. 1), the dried samples were characterized
−
1
by thermal analysis to elucidate the evolution of the Ru pro-
(
DTA/TGA-60, Shimadzu) at a heating rate of 10 °C min
moted Co/SiO catalyst during the autocombustion process.
2
from room temperature to 800 °C in a nitrogen atmosphere.
The TG/DTA results of the dried samples prepared with
different reductant contents (molar ratio of citric acid to
nitrates: CA/N) are shown in Fig. 2.
The DTA curves of the samples exhibited one endothermic
peak and two exothermic peaks. The endothermic peak below
X-ray diffraction (XRD) patterns of the prepared catalysts
were measured using a Rigaku RINT 2200 X-ray powder dif-
fractometer with a monochromatic Cu Kα radiation source at
4
0 kV and 40 mA in the 2θ range of 10–80 degrees. The aver-
age crystalline sizes of the powders were calculated by the
Scherrer equation. The pore structures of the catalysts were
1
00 °C could be assigned to the removal of adsorbed water.
The two exothermal peaks at about 200 and 270 °C were
ascribed to the redox reaction of nitrates with citric acid,
meanwhile the TG curves decreased deeply. During this pro-
cess, nitrates and citric acid were decomposed, forming
cobalt and ruthenium oxides. Large amounts of gases such
as H , CO, H O, CH , NO, CO , NH and NO were released.
determined by N physisorption using a NOVA 2200e appara-
2
tus. The samples were degassed at 200 °C for 2 h before the
analysis. The specific surface area (S) was obtained by the
BET method. The total pore volume (V) was calculated by the
single point method and the average pore size was achieved
by 4V/S. The determination of the metal content in the cata-
lyst was carried out using a scanning electron microscope
equipped with an energy-diffusive X-ray (EDX) spectroscopy
attachment (SEM-EDS, JEOL JSM-6360LV).
2
2
4
2
3
2
2 4
During this process, the formed H , CO and CH acted as the
reductants to reduce the metal oxides to metallic Co and Ru.
Ru might be firstly reduced due to the low reduction temper-
ature. The reduced Ru could act as a promoter to improve
the Co reduction. When the temperature was higher than
The reduction behaviors were studied by hydrogen
temperature-programmed reduction (H -TPR) (BELCAT-B-TT).
2
Before reduction, the catalysts were heated at 150 °C for 2 h
300 °C, the weight losses of the catalysts were very small.
For the catalysts with different reductant contents, it is
in argon flow. After that, the temperature was cooled down to
3
clear that the decomposition and redox reaction of nitrates
and citric acid took place in a low rate at low CA/N (0.15–0.3).
With the increase of CA/N, the intensity of the exothermic
peak increased, indicating that the combustion process
occurred more severely. At the same time, the weight loss in
the temperature region of 180–250 °C increased with increas-
ing CA/N. It needs to be noted that the weight losses in
5
2
0 °C. Then, 5% H –Ar mixture gas with a flow rate of 30 cm
−
1
min was passed through the catalysts. The temperature was
linearly raised from 50 °C to 800 °C at a heating rate of
−
1
5
.0 °C min . The effluent gas was analyzed using a thermal
conductivity detector (TCD).
2
.3 Fischer–Tropsch synthesis tests
0
.35RuCo and 0.4RuCo are much smaller than that in
The FTS performance of the catalysts was tested in a semibatch
slurry-phase reactor with an inner volume of 80 ml. 1 g of pas-
sivated catalyst was ground to fine particles in 20 ml of
n-hexadecane, and then the mixture was transferred into the
0.3RuCo. This is because an excessive amount of citric acid
might result in obvious carbon deposition over the catalysts.
The deposition of carbon species could decrease the weight
loss of the catalysts.
reactor. Syngas with a H /CO molar ratio of 2 was used as a reac-
2
tant and 3% Ar was used as an inner standard. During the reac-
tion, the effluent gas from the reactor was analyzed by online
gas chromatography. A thermal conductivity detector (TCD)
was used to analyze the gaseous products (CO, CO , and CH ).
3
.2 Physical and chemical properties of Ru promoted
catalysts synthesized by the autocombustion method
3.2.1 Crystal phase analysis of the as-synthesized catalysts.
2
4
1 5
Light hydrocarbons (C –C ) were online analyzed using a flame
2
The XRD patterns of Ru promoted Co/SiO catalysts prepared
ionization detector (FID) with a Porapak-Q column. The liquid
products collected in a dry-ice trap and hydrocarbons dissolved
in the solvent were combined and all of them were analyzed
using a FID with a silicone SE-30 column. The FTS reaction
by different methods are presented in Fig. 3a. The XRD pat-
terns of Co and Ru monometal catalysts were also measured
and the results were shown in Fig. 3b and c, respectively. It
can be seen that the catalyst prepared by the Ru promoted
autocombustion method (0.3RuCo) exhibited weak peaks for
2
conditions were: H /CO = 2, T = 240 °C, P (total) = 1.0 MPa,
−
1
and W/F = 10 gh mol , where W is the weight of the catalyst
2
metallic Co and SiO . No diffraction peaks for cobalt oxides
and F is total syngas flow rate (including H , CO and 3% Ar).
were identified in this catalyst. This finding indicated that
most of the cobalt oxides were reduced to metallic Co and
the formed Co was well dispersed. For Ru promoted catalysts
2
3
. Results and discussion
prepared by air combustion–reduction (0.3RuCoair-reduction
)
3
2
.1 Evolution of Ru promoted Co/SiO catalysts during the
and conventional (RuCo ) methods, both SiO and metallic
N
2
autocombustion process
Co crystals were identified, but the intensity of Co crystalline
peaks was much higher than that of the autocombustion cat-
alyst. It can be concluded that the catalysts prepared by the
In this work, Ru promoted Co/SiO FTS catalysts were pre-
2
pared by the autocombustion method using cobalt and
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Fig. 2 TG/DTA curves of Ru-promoted Co catalysts with different CA/N values: (a) 0.15RuCo, (b) 0.2RuCo, (c) 0.25RuCo, (d) 0.3RuCo,
e) 0.35RuCo and (f) 0.4RuCo.
(
autocombustion method exhibited a higher dispersion of
cobalt than those prepared by the air combustion–reduction
and conventional methods.
that the major phase of this catalyst is CoO. It meant that the
amount of citric acid was not enough to reduce Co
3
O
4
completely and some CoO was left. With the increase of CA/N
in the precursors, the diffraction peaks of CoO gradually
decreased while the diffraction peaks of metallic Co became
stronger. When CA/N increased to 0.3, pure metallic Co was
obtained. However, further increasing CA/N to 0.4, small and
board peaks of metallic Co were detected. This result indi-
cated that the metallic Co sites were relative small and well-
dispersed at a high reductant content.
In the case of the cobalt catalyst (0.3Co, Fig. 3b) prepared
by the autocombustion method without Ru promotion, weak
diffraction peaks for metallic cobalt crystalline were identi-
fied but the peak intensity of Co crystalline in 0.3Co is
weaker than that in 0.3RuCo. Furthermore, CoO crystalline
was also observed in the 0.3Co catalyst. These results indi-
cated that only part of cobalt was reduced at a low reduction
content (CA/N = 0.3) without ruthenium promotion. There-
fore, ruthenium played an important role in the reduction of
cobalt during the autocombustion process. No obvious dif-
During the autocombustion process, citric acid was
decomposed into a large amount of gases such as H , H O,
2 2
CO, CH and CO , which could reduce the cobalt oxides to
4
2
2
3–25
fraction peaks except for SiO
2
were found in the 0.3Ru cata-
metallic Co.
With the increase of the citric acid content,
lyst. This result was probably due to the low content and well
dispersion of Ru in the catalyst.
The effect of CA/N on the reduction and dispersion of
cobalt was also studied by XRD, and the results are presented
in Fig. 4 and Table 1. The narrow and intense diffraction
peaks of 0.15RuCo are indexed to the CoO phase, showing
more H
oxides were reduced to metallic Co. In our previous work,
the Co/SiO catalyst was synthesized by an autocombustion
method using citric acid as a reductant and burnt in argon,
but the highest activity was achieved at a very high reductant
content (CA/N = 1). In this work, a small amount of reductant
2 4
, CO and CH were formed, and thus more cobalt
1
5
2
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Fig. 4 The X-ray diffraction patterns of the as-synthesized catalysts
with different contents of the reductant.
pores of the support. With the increase of reductant content,
the BET surface area of the as-synthesized catalyst first
increased, reached a maximum at 0.3RuCo, and then
decreased. This is because a higher amount of the reductant
led to smaller Co particle sizes, which could improve the
BET surface area. However, an excessive amount of the reduc-
tant could result in the oversupply of the carbonic residues,
and cause the decrease of the surface areas. For the 0.4RuCo
catalyst, although it had the smallest Co crystalline size, its
2
−1
surface area was limited to 74 m g due to the accumula-
tion of excessive carbonic residues.
The metal contents of the catalysts prepared by auto-
combustion and conventional impregnation methods were
measured by scanning electron microscopy with EDS, and
the results are collected in Table 1. The contents of Ru and
Co prepared by the autocombustion method are close to the
desired values but the content of Co in the conventionally
prepared catalyst is much higher, which might be attributed
to the fact that more Co was impregnated on the external
surface of the catalysts.
Fig. 3 X-ray diffraction patterns of the catalysts prepared by different
methods (a: Ru promoted Co catalysts, b: Co catalysts, c: Ru catalysts).
3.2.3 Reduction properties of the as-synthesized catalysts.
The reduction properties of Ru promoted Co/SiO
prepared by the autocombustion method were characterized
by H temperature programmed reduction (TPR). For com-
2
catalysts
2
(
CA/N = 0.3) could reduce the cobalt oxides completely. This
parison, TPR curves of the monometal catalysts and the cata-
lysts prepared by the conventional impregnated method were
also measured. The TPR results of different catalysts are
result further proved that the adding of Ru promoted the
reduction of cobalt. In addition, Ru also acted as a structural
promoter to increase the dispersion of cobalt particles and
decrease the average cobalt cluster sizes.
.2.2 Pore structure and elemental composition analysis.
The pore structure parameters of the as-synthesized catalysts
are also shown in Table 1. The BET surface area of the sup-
port silica (Q-50) was 79 m g , whereas the surface area of
shown in Fig. 5. The reduction of the calcined catalyst Co
can be assigned to a two-step reduction of Co O to CoO and
N
8
,25
3
4
3
then to metallic Co. The presence of Ru resulted in a
decrease in the reduction temperature of cobalt in the cal-
cined RuCoN catalyst. The reduction of RuO to metallic Ru
2
2
−1
usually takes place at a lower temperature than the reduction
2
−1
26–28
the as-synthesized catalyst 0.3RuCo was 90 m
g
. The
of Co
3
O
4
.
It did not exhibit a separate peak in the cal-
increase of BET surface area after autocombustion was
mainly derived from some Co nanoparticles loaded in the
cined RuCoN catalyst and was probably overlapped with the
first reduction peak (180 °C) of Co . The peak at 250 °C is
3 4
O
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Table 1 Physical and chemical parameters of the catalysts
EDS (wt%)
a
2
−1
a
3
−1
a
b
Catalysts
Surface areas (m g
)
Pore volume (cm g
)
Pore diameter (nm)
Crystallite size (nm)
Co
Ru
c
0
0
0
0
0
0
.15RuCo
.2RuCo
.25RuCo
.3RuCo
.35RuCo
.4RuCo
80
79
77
90
81
74
74
83
0.68
0.91
1.32
1.19
0.95
1.05
1.08
0.70
21
20
21
37
32
21
21
21
11.3
10.0
9.1
6.7
6.0
5.0
15.5
13.7
11.0
9.9
9.3
10.6
10.3
10.2
12.1
12.8
1.2
0.7
0.7
1.3
1.0
1.3
0.9
—
RuCo
Co
N
N
a
b
c
Determined by the N
2
adsorption method. Calculated by the Scherrer formula, using the peak at 2θ = 44.05°. CoO crystalline size.
Fig. 6 TPR profiles of the catalysts with different reductant contents.
Fig. 5 TPR profiles of the catalysts prepared by different methods.
due to the reduction of Co
3
O
4
, which is promoted by Ru via
about 350–450 °C in the 0.4RuCo catalyst was assigned to the
hydrogenation of residual amorphous carbon.
2
9
hydrogen spillover. For the catalysts prepared by the
autocombustion method, there were nearly no reduction
peaks in 0.3Ru. This result was probably attributed to the fact
that the amount of Ru was very low (1 wt%) and most of the
Ru was reduced during the autocombustion process. How-
ever, for the monometal catalyst 0.3Co, a relative high reduc-
3.3 FTS performance of Ru promoted catalysts prepared by
the autocombustion method
3.3.1 Effect of Ru promotion on the FTS performance of
the catalysts. The FTS catalytic activities of the catalysts
prepared by different methods were measured in a slurry bed
reactor under the reaction conditions of 240 °C, 1.0 MPa and
2
tion temperature and a large amount of H consumption are
observed. This phenomenon suggested that the reduction of
cobalt was more difficult and most of the cobalt was not
reduced in 0.3Co. In the case of 0.3RuCo, the reduction tem-
2
H /CO = 2. The activity curves and product selectivities are
perature of cobalt was much lower and the H consumption
was much smaller than those of RuCo and 0.3Co, demon-
N
shown in Fig. 7 and Table 2. In this work, the reaction time
is five hours and the measured activities were the initial
activities. Generally, the initial activity may not correlate well
with the steady-state activity. Fortunately, the initial activities
of the catalysts are relative stable as shown in Fig. 7, so we
can make the correlation between the structures and perfor-
mance of the catalysts.
The FTS activities of monometal catalysts prepared by the
autocombustion method are very low. The CO conversions of
0.3Co and 0.3Ru were only 0.8 and 1.7%, respectively. The
low activity of 0.3Co is mainly due to most of the cobalt not
being reduced during the autocombustion (Fig. 3 and 5).
Although most of Ru was reduced in the 0.3Ru catalyst, the
small loading amount and the high dispersion of Ru might
2
strating that most of the cobalt in 0.3RuCo was reduced
during the autocombustion process. These results confirmed
that Ru promoted the cobalt reduction during the auto-
combustion process.
TPR profiles of Ru promoted Co catalysts with different
reductant contents are compared in Fig. 6. With the increase
of reductant content, the peak for cobalt reduction (250 °C)
decreased gradually and even disappeared at the CA/N ratio
of 0.35. This result indicated that the reduction level of
cobalt increased with the reductant content. However, an
excessive amount of the reductant could result in severe
residual carbon species in the catalyst. The broad peak at
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and achieved the similar activity and selectivity to the catalyst
prepared by the conventional method.
It needs to be noted that the chain growth factors (alpha
value) of the catalysts prepared by the autocombustion method
are in the region of 0.81–0.83, which is much smaller than the
3
0,31
traditional cobalt catalysts (about 0.88).
There are two pos-
sible reasons for these results. One is that the cobalt crystalline
sizes prepared by the autocombustion method were much
smaller and the finely dispersed metallic crystallines were not
suitable for carbon chain growth reaction. The other is that the
relative high reaction temperature (240 °C) resulted in more
methane and less long chain hydrocarbon formation.
3.3.2 Effect of citric acid contents on the FTS performance.
FTS catalytic activities and selectivities of Ru-promoted cata-
lysts with different CA/N values are presented in Fig. 8 and
Table 3. All of the catalysts prepared by the autocombustion
method exhibited highly stable initial activities for the FTS as
shown in Fig. 8. It is clear that the 0.15RuCo catalyst exhibited
a limited CO conversion as low as 21.4%, which is mainly
restricted by its unreduced CoO (as shown in Fig. 4 and
Table 1). With the increase of CA/N, the content of reduced
cobalt increased and the CO conversion generally increased
but the oversupplied of CA (0.4RuCo) resulted in the decrease
of CO conversion. The small sizes of cobalt crystallines and a
lot of carbonic residues accumulated on their surfaces resulted
in the low CO conversion. For the 0.4RuCo catalyst, the meth-
ane selectivity was high, which may be also due to the small
sizes of cobalt crystallines. The finely dispersed metallic crystal-
lines are not suitable for carbon chain growth but enhance the
Fig. 7 Activity curves of the catalysts prepared by different methods
(
3
2 2
FTS reaction conditions: H /CO = 2, 240 °C, 1 MPa, W/F (CO, H and
% Ar) = 10 gh mol ).
−
1
Table 2 The FTSa performance of the catalysts prepared by different
methods
Selectivity (%)
Catalysts
Conversion of CO (%) CO
2
CH
4
C
5+
α
0
0
0
0
0
.3RuCo
.3Co
.3Ru
.3RuCoair-reduction 60.4
.3Coair-reduction
41.4
0.8
1.7
3.5
—
—
8.5
4.1
3.4
3.0
—
21.1 59.8 0.83
—
—
—
—
—
—
21.1 64.1 0.82
14.4 68.3 0.81
19.7 60.6 0.77
25.4 50.4 0.76
26.5
45.1
27.7
8.0
1
5
RuCo
Co
Ru
N
formation of methane.
.3.3 Effect of reductant types on the FTS performance. In
N
3
N
—
—
—
this work, Ru promoted catalysts with different types of
organic reductants were also prepared and tested for the FTS
performance. For all of the catalysts, the molar ratio of the
reductant to nitrates is fixed at 0.3. The FTS activities and
selectivities of these catalysts are listed in Table 4. It can be
found that the organic reductant types showed a remarkable
a
The reactions were carried out under the conditions of H
2
/CO = 2,
−
1
2
40 °C, 1 MPa, W/F (CO, H
2
and 3% Ar) = 10 gh mol , and reaction
time of 5 h.
result in the low activity (Fig. 3 and 5). The Ru promoted Co
catalyst prepared by the autocombustion method (0.3RuCo)
exhibited a high FTS activity with the CO conversion of 41.1%.
Furthermore, there was no obvious activity loss in the five
hours of reaction time on stream, indicating a high stability of
this catalyst (Fig. 7). It is believed that the promotion of Ru on
the reduction and dispersion of Co (Table 2 and Fig. 5) played
important roles in the high activity of 0.3RuCo. Ru itself in the
0
.3RuCo catalyst may also contribute significantly to the high
FTS activity by improving the nature of Co as an effective pro-
7–10
moter, as demonstrated by other authors.
The improvements in the FTS activity by Ru were also
found for the catalysts prepared by air combustion–reduction
(
N
0.3RuCoair-reduction) and conventional impregnated (RuCo )
methods. It is worth noting that the CO conversion and
hydrocarbon selectivity of 0.3RuCo are similar to those of
N
RuCo . It can be concluded that the Ru promoted catalyst
Fig. 8 Activity curves of the catalysts with different reductant
prepared by the autocombustion method may not undergo
the complex and high energy consumption reduction process,
2
contents (FTS reaction conditions: H /CO = 2, 240 °C, 1 MPa, W/F
(CO, H and 3% Ar) = 10 gh mol ).
2
−1
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Table 3 The FTSa performance of the catalysts with different contents
of reductant
catalysts. Reductant types also had a remarkable influence on
the FTS performance. The maximum catalytic activity was
achieved when citric acid was used as the reductant. The Ru
promoted cobalt catalysts prepared by the autocombustion
method, which omits the complex and high energy consump-
tion reduction process, can be used directly for high efficiency
FTS and thus will be more promising in the future.
Selectivity (%)
Catalysts
Conversion of CO (%)
CO
2
CH
4
C
5+
α
0
0
0
0
0
0
.15RuCo
.2RuCo
.25RuCo
.3RuCo
.35RuCo
.4RuCo
21.4
28.1
34.1
41.4
32.7
28.7
2.8
2.8
2.9
3.5
3.3
3.2
14.1
17.1
17.3
20.5
20.1
23.0
72.4
66.6
65.7
59.8
58
0.89
0.87
0.87
0.83
0.77
0.77
Acknowledgements
57.4
a
The reactions were carried out under the conditions of H
40 °C, 1 MPa, W/F (CO, H and 3% Ar) = 10 gh mol , and reaction
2
2
/CO = 2,
This work was supported by co-funding of the Thailand
Research Fund (through the Royal Golden Jubilee Ph.D. Pro-
gram) and Chulalongkorn University (grant no. PHD/0010/2553),
the Higher Education Research Promotion and the National
Research University Project of Thailand, the Office of the Higher
Education Commission (EN 272 A), the Ratchadaphiseksomphot
Endowment Fund (CU-CLUSTER-FUND), the Research Program
on Materials for Future Energy and the Center of Excellence on
Petrochemical and Materials Technology.
−
1
2
time of 5 h.
Table 4 The FTSa performance of the catalysts prepared using different
organic reductants
Selectivity (%)
Reductant
Conversion of CO (%)
CO
2
CH
4
C
5+
α
Formic acid
Oxalic acid
Citric acid
38.7
32.8
41.4
2.2
1.7
3.5
19.4
65.0
0.81
15.7 67.9 00 .. 87 93 References
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