Promotion effect of the addition of Eu to Co/silica catalyst for Fischer-Tropsch synthesis

Article abstract of DOI:10.1016/j.catcom.2013.03.014

Co/silica (Eu-Co/SiO₂) catalysts with 1, 5, 10, and 20% Eu were synthesized and their catalytic activities towards Fischer-Tropsch synthesis were investigated in a slurry-phase reactor. The addition of 1% Eu improved the activity of the Co/SiO₂ catalyst, while more than 5% Eu reduced it. XRD and XANES analyses revealed that Eu has a negligible effect on the Co metal particle sizes of the reduced catalysts and that 1% Eu enhanced the reducibility of the Co oxides while excess Eu was ineffective. This is the first report demonstrating the promotion effect of Eu on FTS reaction of Co/SiO₂ catalyst.

Full text of DOI:10.1016/j.catcom.2013.03.014

Catalysis Communications 36 (2013) 75–78
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Promotion effect of the addition of Eu to Co/silica catalyst for
Fischer–Tropsch synthesis
a
a
a
Yoshinori Suzuki a, , Masatoshi Kuchida , Yuichiro Sakama , Hiroshi Saiki ,
⁎
Isao Karube a, Noritatsu Tsubaki b
a
Graduate School of Bionics, Tokyo University of Technology, 1401-1 Katakura-cho, Hachioji, Tokyo 192-0982, Japan
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Co/silica (Eu–Co/SiO2) catalysts with 1, 5, 10, and 20% Eu were synthesized and their catalytic activities
towards Fischer–Tropsch synthesis were investigated in a slurry-phase reactor. The addition of 1% Eu improved
the activity of the Co/SiO2 catalyst, while more than 5% Eu reduced it. XRD and XANES analyses revealed that Eu
has a negligible effect on the Co metal particle sizes of the reduced catalysts and that 1% Eu enhanced the reduc-
ibility of the Co oxides while excess Eu was ineffective. This is the first report demonstrating the promotion effect
of Eu on FTS reaction of Co/SiO2 catalyst.
Received 17 January 2013
Received in revised form 1 March 2013
Accepted 4 March 2013
Available online 21 March 2013
Keywords:
Rare earth
© 2013 Elsevier B.V. All rights reserved.
Cobalt-based catalyst
Slurry-phase reactor
XANES
1. Introduction
highly exothermic character of the FTS reaction, the reaction heat is easily
to be removed in a slurry-phase reactor.
Fischer–Tropsch synthesis (FTS) is a technique to produce clean liquid
hydrocarbon fuels from syngas, which comprises CO and H2, in the pres-
ence of metal catalysts. FTS also enables the conversion of biogas to fuel;
therefore, the technique is considered to be a potential solution to the
problem of depleting petroleum resources.
In the present study, Eu-doped Co/SiO2 (Eu–Co/SiO2) catalysts were
synthesized and evaluated in a slurry-phase reactor. Eu was loaded onto
the Co/SiO2 catalyst at various ratios. The resultant catalysts were char-
acterized by X-ray diffraction (XRD) and X-ray absorbance near-edge
structure (XANES) analyses; XANES is a useful method for determining
the reducibility of Co on catalysts [10,15]. Finally, we elucidated the
relationship among loading amount of Eu, reducibility of Co oxide on
the catalyst, and the FTS activity.
It is well-known that Co-based catalysts show high catalytic activity
for FTS [1,2]. Cobalt is generally supported on SiO2, γ-Al2O3, etc. in a
highly dispersed form, which increases its catalytic activity [3–7].
It has been reported that the addition of a secondary metal to
Co-based catalysts further improves the catalytic activity [8–11].
Noble metals such as Ru, Pt, and Pd have been investigated as the
secondary metal for supported Co catalysis [8,9,11]. Rare earth (RE)
elements have also been recently studied as promoters [12–14],
and some of them successfully improved the catalytic activity. Zeng
et al. showed that the addition of La, Ce, Pr, and Sm decreased the
rate of production of methane, carbon dioxide, and C2–C4 products,
and increased C5+ selectivity; however, Nd had an opposite effect
on γ-Al2O3-supported catalysts [12]. The effects of added REs on
FTS catalysts are complicated and remain poorly understood. Additional-
ly, most reported studies on RE-promoted catalysts were performed in
fixed-bed reactors; therefore, little information on RE-promoted catalysts
in slurry-phase reactors is available. While it is difficult to control the
reaction temperature during reaction in a fixed-bed reactor due to the
2. Experimental
2.1. Catalyst preparation
SiO2 supports (specific surface area: 261 m2 g−1, average pore
size: 10 nm, pore volume: 1.28 mL g−1, particle size: 75–150 nm)
were kindly donated by Fuji Silysia Chemical, Ltd. SiO2-supported
catalysts containing 10 wt.% (with respect to SiO2) Co and 10 wt.%
Eu (10Co/SiO2 and 10Eu/SiO2, respectively) were prepared via incipient
wetness impregnation of the supports with aqueous solutions of Co
nitrate and Eu nitrate, respectively. Catalysts containing 10 wt.% Co and
1, 5, 10, or 20 wt.% Eu (10Co1Eu/SiO2, 10Co5Eu/SiO2, 10Co10Eu/SiO2,
and 10Co20Eu/SiO2, respectively) were prepared via co-impregnation
with aqueous solutions containing the appropriate amounts of Co nitrate
and Eu nitrate. The precursors were dried in air at 393 K for 12 h and
calcined at 673 K at a ramping rate of 2 K min−1 for 2 h. The catalysts
were then activated in a hydrogen atmosphere at a flow rate of
⁎
Corresponding author. Tel.: +81 42 637 5046; fax: +81 42 637 2112.
E-mail address: yosuzuki@stf.teu.ac.jp (Y. Suzuki).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.014

76
Y. Suzuki et al. / Catalysis Communications 36 (2013) 75–78
80 mL min−1 at 673 K for 15 h. Finally, the catalysts were passivated
by 1% oxygen in nitrogen at a flow rate of 10 mL min−1 at room
temperature.
40
35
30
25
20
15
10
5
2.2. FTS reaction
10Co/SiO2
FTS reactions were performed in a flow-type semi-batch slurry-
phase reactor with an inner volume of 100 mL. The passivated catalyst
(1.00 g) and n-dodecane (20 mL) were loaded into the reactor. The
conditions for the FT reaction were a temperature of 523 K, a total
pressure of 2.0 MPa, and an agitation speed of 500 rpm. Syngas
comprising H2/CO at a ratio of 1:1 (with 3% Ar) was used to mimic
CO-rich biomass-derived gas; the syngas was supplied at a flow
rate of 20 mL min−1. During the reaction, the effluent gas from the
reactor was first cooled in a dry-ice trap and then input into
on-line gas chromatographs (GC-3200s, GL Science, Inc.) equipped
with thermal conductivity detectors (TCDs). The liquid products collect-
ed in the dry-ice trap and products remaining in the solvent were ana-
lyzed via gas chromatography using a silicone SE-30 column and TCD.
The chain growth probability, α, was calculated using the Anderson–
Schulz–Flory (ASF) procedure [16].
10Co1Eu/SiO2
10Co5Eu/SiO2
10Co10Eu/SiO2
10Co20Eu/SiO2
10Eu/SiO2
0
0
4
8
12
16
20
24
Time (h)
Fig. 1. CO conversions over time in the FTS experiments.
summary, a small amount of Eu improves the catalytic activity of
Co/SiO2; however, excess Eu decreases the activity. This tendency
differs from that of Ru-promoted Co catalysts reported by Sun et al.
[8]; they reported that increasing Ru loading from 0 to 2% molar
ratio increased the CO conversion. This occurs because Ru has a
high catalytic activity towards FTS. The promotion effects of Eu are
unique because Eu itself shows little catalytic activity for FTS.
Although the addition of 1% Eu increased the CO conversion, CH4 and
CO2, which are undesirable byproducts, were produced at the similar
amount as with 10Co/SiO2. The chain growth probabilities of 10Co/SiO2
and 10Co1Eu/SiO2 were both 0.79. However, more Eu resulted in lower
α values. From these results, 1% Eu is the most preferable amount for
the promotion of Co/SiO2 catalysts under these conditions.
2.3. XRD analysis
XRD patterns of the catalyst samples before and after reduction were
obtained using a TXJ-E165 MX-Labo (MAC Science Co/Ltd.) instrument
employing Cu Kα radiation. The reduced catalysts were passivated
using the same procedure described in Section 2.1. The samples were
ground using an agate mortar before the measurements.
2.4. XAFS analysis
Co K-edge XANES spectra of the catalysts were measured at BL-12C
at the Photon Factory, KEK (Tsukuba, Japan). All spectra were collected
in transmission mode. XANES spectra of Co foil, CoO, and Co3O4 were
measured as reference compounds. The powders of CoO, Co3O4, and
catalysts were diluted to 1 wt.% Co with boron nitride for the XANES
measurements. The XANES spectra were analyzed using REX2000 ver.
2.5.9 (Rigaku Co.).
3.2. XRD analysis
Fig. 2a shows XRD patterns of 10Co/SiO2, 10Co1Eu/SiO2, 10Co5Eu/
SiO2, 10Co10Eu/SiO2, and 10Eu/SiO2 before reduction. Diffraction peaks
corresponding to Co3O4 appear in all the spectra. No diffraction peaks
for Eu compounds such as Eu oxide were evident even in the pattern
of 10Eu/SiO2, which contains 10% Eu; this suggests that Eu is highly
dispersed and exists mainly in an amorphous phase. Although the spec-
tra of 10Co/SiO2 and 10Co1Eu/SiO2 were almost identical, the higher
Eu-loading catalysts featured lower peak intensities because of the
relatively low content of Co. XRD patterns of the catalysts after reduc-
tion are shown in Fig. 2b. Peaks attributable to Co metal and CoO were
evident in the XRD patterns of the reduced Co containing catalysts.
The intensities of the peaks decreased with increase in Eu ratio. These re-
sults are consistent with the XRD results of the catalysts before reduction.
Since, it is difficult to determine the average particle sizes of Co metal on
the reduced catalysts directly from the peaks in Fig. 2b using Scherrer's
equation, they were estimated from the Co3O4 particle sizes on the cata-
lysts before reduction. The average particle sizes of Co3O4 on the cata-
lyst before reduction were calculated from the most intense Co3O4
line (i.e., 2θ = 36.8°) using Scherrer's equation. Then, the average
3. Results and discussion
3.1. FTS reaction
To reveal the effect of gas composition, FTS reactions with syngas of
H2/CO = 1, and 2 were conducted using 10Co/SiO2 catalyst. The CO
conversion of 10Co/SiO2 with syngas of H2/CO = 1 was 31.6%, while
the CO conversion of 65.4% was obtained when we used syngas with
H2/CO = 2. Generally, CO-rich biomass-derived gas gives lower CO
conversion than syngas of H2/CO = 2, which is the stoichiometrically
optimal gas ratio for FTS on cobalt-based catalyst. Here after, all exper-
iments were carried out with syngas of H2/CO = 1 simulating syngas
derived from biomass.
To determine the effect of Eu on the FTS, the catalytic activities of
10Co/SiO2, 10Co1Eu/SiO2, 10Co5Eu/SiO2, 10Co10Eu/SiO2, 10Co20Eu/
SiO2, and 10Eu/SiO2 were examined using the slurry-phase reactor.
Plots of CO conversion over time are shown in Fig. 1. The CO conversions
of all the catalysts except 10Eu/SiO2 increased in the first 3 h, indicating
that the passivated phase of the catalysts was reduced resulting in in-
creased catalytic activities. After 3 h, the CO conversions were stable.
The catalytic activities of Co, Eu, and Co–Eu/SiO2 after reaching the
steady state are summarized in Table 1. The addition of 1% Eu to
10Co/SiO2 increased the CO conversion to 35.8%. However, the addition
of more Eu (i.e., 5, 10, and 20%) decreased the CO conversion from that
of 10Co1Eu/SiO2. The CO conversion of 10Eu/SiO2 was only 2.34%,
which indicates that Eu itself is almost catalytically inactive for FTS. In
Table 1
Catalytic activities of Co/SiO2, Eu–Co/SiO2 and Eu/SiO2 catalysts for FTS.
Catalyst
CO conversion (%)a
CH4 (%)a
CO2 (%)a
α
10Co/SiO2
31.6
35.8
33.8
31.8
26.2
2.34
1.81
1.42
2.65
1.29
2.13
–
1.68
1.64
1.29
1.47
1.81
–
0.79
0.79
0.78
0.77
0.75
–
10Co1Eu/SiO2
10Co5Eu/SiO2
10Co10Eu/SiO2
10Co20Eu/SiO2
10Eu/SiO2
a
The values are averages of the values at T = 18, 21, and 24 h in Fig. 1.

Y. Suzuki et al. / Catalysis Communications 36 (2013) 75–78
77
a
b
10Co
10Co
10Co1Eu
10Co1Eu
10Co5Eu
10Co5Eu
10Co10Eu
10Eu
10Co10Eu
10Eu
10
30
50
70
90
10
30
50
70
90
2 Theta (deg.)
2 Theta (deg.)
Fig. 2. XRD patterns of Co/SiO2, Eu/SiO2, and Eu–Co/SiO2 catalysts: (a) Before reduction and (b) after reduction and passivation. (♦) Co3O4, (▲) CoO, (●) Co0.
particle sizes of Co metal on the reduced catalysts were calculated
using the following equation [11]:
increased the content of oxidized species as compared to the
un-doped Co catalyst (Fig. 3d). These results indicate that the addition
of a small amount of Eu promotes the reduction of Co oxides, while a
significant amount of Eu inhibits reduction. The effect of the Co/Eu
ratio on the reducibility of Co oxides suggests an interaction between
Co and Eu. Reducibility in noble-metal-promoted Co/γ-Al2O3 was
previously investigated by XANES analysis [15]. Accordingly, it was de-
termined that the addition of small quantities of Au, Ag, or Pt enhances
the reducibility of Co oxide on γ-Al2O3. However, there are few studies
on the effects of REs on Co reducibility on SiO2-supported catalysts. Our
results show that the addition of only a small amount of Eu increases
the reducibility of Co oxides on SiO2 supports.
The CO conversion results in the FTS experiments are related to the
amount of Co metal in the catalysts, as determined by the XANES anal-
ysis: Catalysts with relatively large amounts of Co metal gave high CO
conversion. This corresponds to the generally accepted mechanism
that suggests that the active sites of Co-based catalysts for FTS are the
reduced Co metal surface sites.
ꢀ
ꢁ
d Co0 ¼ 0:75dðCo3O4Þ;
where d(Co0) and d(Co3O4) represent the average particle size of Co
metal on the reduced catalysts and Co3O4 particles on the catalysts
before reduction, respectively. The results are shown in Table 2.
The Co metal particle sizes of all catalysts were almost identical.
The fact that the addition of a secondary metal to Co-supported cat-
alysts changes the particle size of the Co metal is well known [11,12].
Interestingly, the effect of the secondary metal on the particle size de-
pends on the support type. For instance, the addition of a small amount
of Pt to Co/SiO2 catalysts decreased the particle size of the Co metal
while an opposite effect was observed for Al2O3-supported catalysts
[11]. Zeng et al. investigated the effect of REs on γ-Al2O3-supported Co
catalysts and reported that the addition of La, Ce, and Nd increases the
particle sizes of the Co metal on SiO2 [12]. However, our XRD results
imply that the addition of Eu to Co/SiO2 has a negligible effect on the
crystallite size of the Co metal.
RE-promoted Co/SiO2 and Co/γ-Al2O3 catalysts have been studied in
fixed-bed reactors: Some REs enhance the catalytic activity and selec-
tivity [12–14]. To date, there is little information on RE-promoted
Co-based catalysts in slurry-phase reactors. In the present study, it
was found that Co/SiO2 promoted with a small amount of Eu contains
a relatively large amount of Co metal and shows high CO conversion.
3.3. XANES analysis
Co K-edge XANES spectra of the reference compounds, 10Co/SiO2,
10Co1Eu/SiO2, and 10Co10Eu/SiO2 are shown in Fig. 3a–d. The spec-
tra of the Co–Eu catalysts were successfully fitted by the linear com-
bination method using XANES spectra of Co foil, CoO, and Co3O4 as
end members with low residuals (Table 2). The fitting results for
10Co/SiO2 showed that the catalyst contains 60.1% Co metal, 30.8%
CoO, and 9.2% Co3O4 (Fig. 3b). The addition of 1% Eu increased the
ratio of Co metal species in the catalyst to 65.0% (Fig. 3c). However,
the addition of 10% Eu reduced the content of Co metal species and
4. Conclusions
Eu-promoted Co/SiO2 catalysts were prepared and their catalytic
activities towards FTS were tested in a slurry-phase reactor. The addi-
tion of a small amount of Eu improved the FTS activity. The reason for
this improvement was revealed by XRD and XANES analyses of the
catalysts: The addition of a small amount of Eu increased the amount
Table 2
Characterization of catalysts by XRD and XANES analyses.
Catalyst
Cobalt content
(g/gSiO2)
Europium content
(g/gSiO2)
XRDa
XANESb
Size of Co0 crystallites (nm)
% Co0
% CoO
% Co3O4
Residual
(%)
10Co
0.1
0.1
0.1
0.1
0
13.7
12.8
12.1
12.7
60.1
65.0
–
30.8
28.2
–
9.2
6.8
–
1.1
0.9
–
10Co1Eu
10Co5Eu
10Co10Eu
0.01
0.05
0.1
53.3
32.7
14.0
1.8
a
From analysis of 2θ peak at 36.8° for Co3O4 (311) and assuming a contraction of 0.75 in converting to the metal [11].
Obtained by linear combination fittings.
b

78
Y. Suzuki et al. / Catalysis Communications 36 (2013) 75–78
Fig. 3. Normalized XANES spectra of (a) reference compounds including Co foil, CoO, and Co3O4; (b) 10Co/SiO2; (c) 10Co1Eu/SiO2; and (d) 10Co10Eu/SiO2. The spectra of 10Co/SiO2,
10Co1Eu/SiO2, and 10Co10Eu/SiO2 were fitted with the spectra of the reference compounds using the linear combination method.
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