Study on Fe-Co alloy role over RANEY Fe-Co bimetallic catalysts in Fischer-Tropsch synthesis

Article abstract of DOI:10.1039/c6ra22785f

To further understand the role of Fe-Co alloys in Fischer-Tropsch synthesis, a series of RANEY Fe-Co mono/bimetallic catalysts were prepared through alkali leaching solidified Fe-Co-Al alloy. There are three advantages about these catalysts: (i) RANEY structure can provide certain surface area for catalytic reaction; (ii) these catalysts avoid the influence of the metal-support interaction due to the absence of support; (iii) the composition of active phase can be determined easily. The FT results showed that not all the Fe-Co alloy could enhance the C₅₊ selectivity, only when the ratio of Fe/Co is 3:7 can C₅₊ selectivity be promoted effectively. Besides, we also found that Fe-Co alloy did not show intended synergy effects on the FT activity, instead, Co-dominant catalysts exhibited a higher CO conversion.

Full text of DOI:10.1039/c6ra22785f

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Page 1 of 18
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Study on Fe-Co Alloy Role over Raney Fe-Co
Bimetallic Catalysts in Fischer-Tropsch Synthesis
Yongbiao Zhaiab, Yingying Xueab, Zheng Chenab, Min Chenac, Buhuan Wangab,
Jiangang Chena*
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, P. R. China
b University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
c College of Environmental Engineering, Shanghai University, Shanghai, 200444, P.
R. China
Abstract
To further understand the role of FeꢀCo alloy in FischerꢀTropsch synthesis, a series of
Raney FeꢀCo mono/bimetallic catalysts were prepared through alkali leaching
solidified FeꢀCoꢀAl alloy. There are three advantages about these catalysts: (i) Raney
structure can provide certain surface area for catalytic reaction; (ii) these catalysts
avoid the influence of the metalꢀsupport interaction due to the absence of support; (iii)
the composition of active phase can be determined easily. The FT results showed that
not all the FeꢀCo alloy could enhance the C5+ selectivity, only when the ratio of Fe/Co
is 3:7 can C5+ selectivity be promoted effectively. Besides, we also found that FeꢀCo
alloy did not show intended synergy effects on the FT activity, instead, Coꢀdominant
catalysts exhibited a higher CO conversion.
Key words: Raney FeꢀCo alloy, bimetallic catalysts, FischerꢀTropsch synthesis.
*
Corresponding author. Eꢀmail: Chenjg@sxicc.ac.cn

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1. Introduction
FischerꢀTropsch (FT) synthesis is a heterogeneous catalytic process that converts
syngas (CO and H2) into a wide variety of hydrocarbons.1 2 Syngas can be produced
from fossil and renewable feedstocks, such as natural gas, coal and biomass.
Generally, the hydrocarbon products of FT synthesis contain valueꢀadded chemicals
=
=
(C2 ꢀC4 ) and clean liquid fuels (gasoline and diesel).3ꢀ5 They could be considered as a
real alternative to traditional fuels from crude oil. Therefore, FT synthesis has
received much renewed interest in recent years. In previous studies on the catalysts
for FT synthesis, bimetallic catalysts have been investigated in detail but the obtained
results were rather confusing.6 Ma et al. reported an enhanced activity for bimetallic
catalysts compared to monometallic catalysts,7 while Duvenhage reported an opposite
trend.8 Besides, the contradictory results on the selectivity were also reported in some
literature reports.9 10
Iron and cobalt are the typical active metals used in FT synthesis. Coꢀbased
catalysts show high catalytic activity and are suitable for the production of longꢀchain
paraffins.11 The comparatively less expensive Feꢀbased catalysts exhibit high
selectivity toward lower olefins and catalyze the water gas shift (WGS) reaction,
which can be used to adjust H2/CO ratio in syngas and make the FT synthesis more
flexible to different types of feedstocks.12 We therefore focus on bimetallic FeꢀCo
catalysts in this work. In most studies on bimetallic systems, FeꢀCo active phase was
dispersed on the surface of supports through a variety of methods.13ꢀ16 Supported
catalysts displayed high specific surface area and enhanced dispersion of active phase.

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However, numerous researches on supported catalysts met limited success because of
the presence of strong metalꢀsupport interaction that prevented the reduction of oxide
precursor.17 18 Ma et al. had observed that the performance of FeꢀCo catalysts
depended on the type of support materials used.7 Support materials such as SiO2, TiO2
and MgO have been shown to react with Fe or Co to give Co2SiO4, Co2TiO4 and
FeOꢀMgO during catalysts preparation, thermal treatment or catalytic reaction.
Unfortunately, these mixed compounds are not active in FT synthesis.19 In this case,
carbonaceous materials are widely used as a promising support, but there are still
some uncertainties. It is difficult to determine the active phase because Fe, Co or
FeꢀCo alloy may be coexistent in the catalysts.20ꢀ22
To avoid these shortcomings, Raney FeꢀCo catalysts were prepared in this work
through alkali leaching of a rapid quenched (Fe1ꢀxCox)50Al50 alloy. Firstly, Raney
structure can provide a large surface area for catalytic reaction;23 24 secondly, these
catalysts effectively prevent the effect of metalꢀsupport interaction due to the absence
of support; thirdly, the FeꢀCo alloy can be formed easily during the highꢀtemperature
melting stage. Besides, the rapid quenching could produce refined and uniform
microstructures, which lead to improvements in catalytic performance. Sun et al. had
shown that FeꢀMn catalysts leached from rapid quenched FeꢀMnꢀAl alloy displayed
superior activity, selectivity to alkenes and longer hydrocarbons.25
2. Experimental section
2.1 Catalyst preparation
The ternary (Fe1ꢀxCox)50Al50 alloys were prepared by vacuum induction

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technique. The prescribed amounts of metallic Fe, Co and Al powders were melted
and held at 1400 oC in a vacuum induction furnace for 1 h to ensure the homogeneity
of melt. The molten material was then quenched in water. Subsequently, the brittle
alloy were ground and sieved, and the 60ꢀ80 mesh fractions were used throughout the
study.
The bimetallic FeꢀCo catalysts were prepared by adding FeꢀCoꢀAl alloy into the
o
NaOH solution (6.0 mol/L) at 70 C for alkali leaching. The black powders were
washed thoroughly with distilled water and absolute ethanol. The asꢀprepared
catalysts were stored in ethanol before characterization and catalytic testing.
2.2 Characterization
Xꢀray diffractometer was employed to analyze the XRD profiles of asꢀprepared
mono/bimetallic Raney Fe1ꢀxCox (RꢀFe1ꢀxCox) catalysts. The XRD patterns were
obtained on a DXꢀ2700 diffractometer using Cu Kα radiation (λ=0.1538 nm) and the
scanning range was 30 ꢀ 90°. The compositions of all the catalysts were determined
by inductively coupled plasma optical emission spectrometer (ICPꢀOES). The BET
surface area (SBET), pore volume (Vpore), and average pore size (Dpore) were tested via
N2 physical adsorption at ꢀ196℃ using a Micromeritics ASAP 2420 instrument. All
the samples were degassed under vacuum before measurement. The morphology of
catalysts was investigated by scanning electron microscope (SEM, JSMꢀ6701F,
Japan).
2.3 Catalytic testing
The performance of FTS was evaluated on a fixedꢀbed equipped with a stainless

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steel tubular reactor. Prior to the FTS reaction, the asꢀprepared catalysts were heated
at 100 oC for 2 h under flowing H2 (GHSV=2000, P=2 bar) to remove ethanol, which
isolate the catalysts and air during catalyst loading. After that, the catalysts were
cooled down to room temperature, a flow of syngas was fed into the catalysts and the
temperature was increased to 240 oC for asꢀprepared catalysts. During the testing, the
pressure of syngas was maintained at 20 bar.
o
o
The reaction products passed through a 130 C hot trap and a 5 C cold trap at
working pressure, and then the gaseous products were analyzed onꢀline by gas
chromatography (GC 920). A carbon molecular sieve column connected with a
thermal conductivity detector was used to separate and quantify H2, N2, CH4, CO and
CO2, whereas C1ꢀC8 hydrocarbons were separated in a capillary porapackꢀQ column
and detected in a flame ionization detector.
3. Results and discussion
The XRD patterns of mono/bimetallic RꢀFe1ꢀxCox catalysts are shown in Figure 1.
For the monometallic RꢀFe catalyst, the peaks of 44.7, 65.1 and 82.3° may correspond
to metallic iron [PDF no.06ꢀ0696]. In the case of monometallic RꢀCo sample, the
diffraction peaks at 2θ value of 44.2, 51.5 and 75.9° can be corresponding to metallic
cobalt [PDF no.15ꢀ0806]. For the bimetallic Raney catalysts (x range from 0.1 to 0.7),
metallic Co diffraction peaks disappeared completely and Fe diffraction peaks shifted
to the higher angle zone (2θ=45.2, 65.7 and 83.2°), which implied that Co was
homogeneously dispersed and FeꢀCo alloys were formed [PDF no.50ꢀ0795]. However,
in the patterns of RꢀFe0.2Co0.8 and RꢀFe0.1Co0.9 catalysts not only the diffraction peaks

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of Co appeared at 2θ value of 44.2, 51.5 and 75.9°, but also the characteristic
diffractions of FeꢀCo alloy phase appeared at 45.2°. According to the FeꢀCo binary
phase diagram,26 the solid solubility of Co in Fe is about 70 atom%, thus metallic Co
can be separate out from the melt with decreasing temperature when the Co content is
greater than 70 atom%. The similar results can be seen in other literatures.27
Figure 1c shows amplified XRD patterns of the RꢀFe1ꢀxCox catalysts (x range
from 0.1 to .07) near 2θ=45°, in which an obvious shift toward higher angle zone can
be found with the increase of Co content. This can be attributed to the difference in
atomic radius between Fe and Co, since the atomic radius of Co is smaller than that of
Fe.
The bulk compositional and textural properties of asꢀprepared mono/bimetallic
RꢀFe1ꢀxCox catalysts are summarized in Table 1. After alkali leaching, the Al content
decreased drastically from 50% to no more than 4.5%, which agrees to the
disappearance of FeꢀCoꢀAl phase in Figure 1. Although these catalysts were prepared
by the same experimental method, the content of Co significantly influenced the
surface area and crystallite size. As inferred from Table 1, the surface area rises from
29.6 m2gꢀ1 to 57.5 m2gꢀ1 when the Co content increases from 0 to 90%, while the
mean crystallite size decreases from 16.9 nm to 12.6 nm. This trend has also been
reported elsewhere.6 For example, the studies on FeCo catalysts reported a decrease in
particle size for bimetallic catalysts compare to the monometallic catalysts, and
particle size decreased with the increase of Co content.
The catalysts were also characterized by SEM, and the typically SEM

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micrographs were presented in Figure 2. Compared to the spongy morphology of
Raney Fe and Co (Figure 2a and 2b) characteristic, we note that the average particle
size of bimetallic RꢀFe0.2Co0.8 and Fe0.1Co0.9 (Figure. 2c and 2d) decreases obviously
with the increase of Co content. This trend is consistent with the results of average
crystallite size in Table 1.
The catalytic performance of mono/bimetallic RꢀFe1ꢀxCox catalysts were
evaluated by using a fixedꢀbed reactor at 240 oC and 20 bar (GHSV=2000, H2/CO=2).
The CO conversion of the catalysts as a function of time on stream is presented in
Figure 3. It was observed that all the catalysts investigated in this study were stable
within 100 h. For the monometallic catalysts, RꢀCo has a higher CO conversion than
that of RꢀFe under the same reaction conditions. This result was consistent with
available literature data. For the bimetallic catalysts, the addition of Co (x ≤0.7) to
Feꢀbased catalysts did not yield any drastic effect on CO conversion compared to the
monometallic RꢀFe catalyst. This result suggests that FeꢀCo alloy does not possess
any additive properties compared to the pure Fe catalysts. This observation is
agreement with conclusions drawn from other literature studies.16,20 Notably, Coꢀrich
samples (x≥0.8) significantly improved the CO conversion, and the RꢀFe0.1Co0.9
sample had the highest conversion (72.5%) among all the catalysts. According to the
foregoing discussions, Co could be separated out from supersaturated Fe0.1Co0.9 melt
(Fe0.1Co0.9→2/3 Co + 1/3 Fe0.3Co0.7), thus we can assume that the structure of
supersaturated Fe0.1Co0.9 may similar to the Fe0.3Co0.7ꢀsupported Co catalyst. Besides,
the average crystallite size of precipitatedꢀCo is smaller than that of monometallic

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RꢀCo and the surface area is larger than monometallic RꢀCo (Table 1). These
combined factors result in the highest CO conversion of RꢀFe0.1Co0.9.28 The similar
results has also been demonstrated elsewhere.9
The steadyꢀstate CO conversions and product selectivity of mono/bimetallic
RꢀFe1ꢀxCox catalysts are summarized in Table 2. C5+ selectivity and CO conversion are
shown in Figure 4. As expected, monometallic RꢀFe exhibited comparatively lower
selectivity to C5+ hydrocarbons than that of RꢀCo catalyst. Monometallic Co catalysts
o
o
can be typically evaluated in the temperature range of 220 C to 240 C to achieve a
high selectivity to longꢀchain paraffins. But the Fe catalysts need alkali metal
promoters to enhance the longꢀchain hydrocarbons selectivity. For the bimetallic
RꢀFe1ꢀxCox catalysts, the selectivity of C5+ hydrocarbons increased slightly with the
increase of Co content when the value of x is smaller than 0.7. Surprisingly, the
selectivity of C5+ hydrocarbons rises sharply to 62.7% from 33.3% when the value of
x is 0.7, and the relatively high selectivity (55ꢀ62%) is maintained when the value of x
is greater than 0.7. These results indicated that not all the FeꢀCo alloy can enhance the
C5+ selectivity effectively. In fact, the ratio of Fe/Co is very critical in FeꢀCo alloy. On
the basis of our results, Fe0.3Co0.7 is the most crucial phase to enhance the selectivity
of C5+ hydrocarbons. Coville and coꢀworker also reported that Co2Fe alloy may be the
main component to improve the selectivity of longꢀchain paraffins.29 Combining CO
conversion and C5+ selectivity, RꢀFe0.2Co0.8 and RꢀFe0.1Co0.9 displayed the best
catalytic performance, because their C5+ selectivity was about 60% and CO
conversion remained at a high level (58.4% and 72.5%).

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Conclusion
A series of mono/bimetallic RꢀFe1ꢀxCox catalysts have been prepared successfully
by alkali leaching solidified FeꢀCoꢀAl alloy. Powder XRD patterns showed that Co
and Fe0.3Co0.7 alloy were coexistent in the bimetallic RꢀFe1ꢀxCox (x≥0.8), because Co
could be separated out from supersaturated melt, which was encouraged by the solid
solubility of Co in Fe (70 atom%). Furthermore, the FT results show that FeꢀCo alloy
did not exhibit any synergy effect on CO conversion. At the same time, we find that
not all the FeꢀCo alloy can enhance the C5+ selectivity. Only when the ratio of Fe/Co
is 3:7 can C5+ selectivity be promoted. As a result, supersaturated RꢀFe0.2Co0.8 and
RꢀFe0.1Co0.9 displayed excellent FT activity as well as C5+ selectivity.
Acknowledgements
We gratefully acknowledge the financial support from the National Natural
Science Foundation of China (Grant No. 21503256 and 21373254) and the
autonomous research project of SKLCC (Grant No. 2013BWZ004).
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Fig. 1. (a and b) XRD patterns of the mono/bimetallic Raney Fe1ꢀxCox catalysts. (c)
amplified XRD patterns of the Raney Fe1ꢀxCox catalysts near 2θ=45°.

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Fig. 2. Representative SEM micrographs of Raney Fe (a), Raney Co (b), Raney
Fe0.2Co0.8 (c) and Raney Fe0.1Co0.9 (d).

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Fig. 3. CO conversion as a function of time in FT synthesis for the mono/bimetallic
Raney Fe1ꢀxCox catalysts (T=240 oC, P=20 bar, GHSV=2000 hꢀ1, H2/CO=2).

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Fig. 4. The steadyꢀstate CO conversion (line) and C5+ selectivity (column) on the
mono/bimetallic Raney Fe1ꢀxCox catalysts (T=240 oC, P=20 bar, GHSV=2000 hꢀ1,
H2/CO=2).

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Table 1. Physicochemical characteristics of the asꢀleached Raney Fe1ꢀxCox catalysts
SBET
Vpore
dpore
(nm)
dFeꢀCo
catalyst
Al wt %(a)
(m2gꢀ1)
(cm3gꢀ1)
(nm)(b)
RꢀFe
3.9
3.7
4.4
4.0
3.7
3.3
4.1
29.6
37.1
46.7
51.8
55.3
57.5
31.4
0.121
0.149
0.196
0.209
0.214
0.253
0.155
19.9
16.4
12.7
12.1
11.8
11.5
19.8
16.9
16.1
14.8
13.5
13.1
12.6
15.6
RꢀFe0.8Co0.2
RꢀFe0.5Co0.5
RꢀFe0.3Co0.7
RꢀFe0. 2Co0.8
RꢀFe0.1Co0.9
RꢀCo
(a)Bulk composition determined by ICPꢀOES analysis.
(b)Mean crystallite size calculated from the XRD of Fe(110), Co(111) and FeꢀCo(110)
using the Scherrer equation

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Table 2. Catalytic results for the mono/bimetallic Raney Fe1ꢀxCox catalysts in FT
synthesis
Hydrocarbon selectivity (%)
CO conversion
(%)
CO2 selectivity
(%)
Catalyst
CH4
25.7
21.3
20.2
12.8
14.7
17.1
13.9
C2ꢀC4
44.8
48.1
43.5
24.5
24.0
24.1
36.8
C5+
29.2
30.6
33.3
62.7
61.3
58.8
49.3
RꢀFe
33.6
36.1
33.7
38.8
58.4
72.5
63.7
16.7
14.3
12.9
12.3
8.5
RꢀFe0.8Co0.2
RꢀFe0.5Co0.5
RꢀFe0.3Co0.7
RꢀFe0. 2Co0.8
RꢀFe0.1Co0.9
RꢀCo
5.4
3.8

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Table of content
Supersaturated R-Fe0.2Co0.8 and R-Fe0.1Co0.9 catalysts exhibited the best CO
conversion and higher C5+ selectivity.