Highly dispersed Co nanoparticles embedded in a carbon matrix as a robust and efficient Fischer-Tropsch synthesis catalyst under harsh conditions

Article abstract of DOI:10.1039/d0cy01424a

Preventing the deactivation behavior of Co-based catalysts is a significant challenge during the Fischer-Tropsch synthesis reaction. In this study, a series of catalysts with Co nanoparticles embedded in a matrix of porous carbon are directly synthesizedviaa unique melting approach. It is demonstrated in this work that the loading of Co is highly controllable, and ranges from 20.6-44.0 wt% in the as-prepared samples. The catalyst shows a higher selectivity towards heavy hydrocarbons and a lower selectivity towards methane when compared to the MOF-derived Co@C catalyst tested at a similar CO level. Notably, no obvious deactivation of the catalysts is observed at a high operating temperature of 260 °C, with high CO conversion levels recorded. The special carbon rich environment of the catalyst could inhibit the oxidization and agglomeration of the active phase to prevent deactivation.

Full text of DOI:10.1039/d0cy01424a

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ISSN 2044-4761
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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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Highly Dispersed Co Nanoparticles Embedded in Carbon Matrix as
Robust and Efficient Fischer-Tropsch Synthesis Catalyst Under
Harsh Condition
Received 00th January 20xx,
Accepted 00th January 20xx
Zhe Caiab‡, Shuai Lyuc‡, Yao Chend, Chengchao Liuc, Yuhua Zhangc, Faquan Yua*, Jinlin Lic*
DOI: 10.1039/x0xx00000x
Preventing the deactivation behavior of Co-based catalyst is a significant challenge during the Fischer-Tropsch synthesis
reaction. In this study, a series of catalysts with Co nanoparticles embedded in matrix of porous carbon is directly synthesized
via a unique melting approach. It is demonstrated in this work that the loading of Co is highly controllable, which ranges
from 20.6 wt% ~ 44.0 wt% in the as-prepared samples. The catalyst shows a higher selectivity towards heavy hydrocarbons
and a lower selectivity towards methane when compared to the MOFs-derived Co@C catalyst tested at similar CO level.
Notably, no obvious deactivation of the catalysts is observed at high operating temperature of 260 ℃ , with high CO
conversion levels recorded. The special carbon rich environment of catalyst could inhibit the oxidized and agglomeration of
the active phase to prevent deactivation.
www.rsc.org/
organic frameworks)-derived carbon-based catalysts have been
used in the catalytic conversion of syngas to hydrocarbons with
unparalleled performances reported.9-11 For instance, Gascon et
1. Introduction
Fischer-Tropsch synthesis (FTS) is a promising strategy for
converting coal, natural gas, and biomass into clean fuels and
chemicals via syngas (H2+CO).1-3 Being the earliest industrial FTS
catalyst, Co-based catalyst possesses numerous advantages
such as high activity and high selectivity for heavy
hydrocarbons, low CO2 selectivity, and reasonable cost.4-5
Generally, the catalytic performance of Co catalyst is highly
dependent on the dispersion and reduction of Co species, which
is determined by the specific nanostructure of the catalyst. As
such, this would mean that manipulating the nanostructure of
the material may lead to enhanced catalytic performance.
In the past decades, carbon-based nanomaterials have been
widely used as catalysts or catalyst scaffolds in various
application as such energy-related or environmental-related
fields. This is largely due to their fascinating properties such as
well-developed pore structures, high surface areas, and suitable
interaction with the active metals.6-8 Recently, MOFs(metal-
al. synthesized a series of Fe@C catalysts with excellent
catalytic activity and high selectivity via direct pyrolysis of
Basolite Fe300 precursor. The resultant Fe@C was comprised of
well-dispersed θ-Fe3C which were confined within a porous
carbon matrix.12 Qiu et al. obtained a Co@CN catalyst through
the pyrolysis of ZIF-67. The catalyst possessed large pore size
and good dispersity, and it was able to demonstrate excellent
catalytic activity.13 By regulating the ratio of CTAB
(cetyltrimethylammonium bromide) and Co precursor in the
MOFs, Chen et al. developed a series of Co@C catalysts with
high Co loading (ca. 50 wt%) and high degree of dispersion.14
MOFs is a type of coordination polymer that is formed by the
self-assembly of organic ligands and transition metal ions.
During the pyrolysis process at high temperature, the organic
ligands are destroyed and the metal species are reduced by the
surrounding free carbon, simultaneously. The porous M@C
construction would benefit from the reconstruction of the
carbon which is catalyzed during the reduction of the ions
species. The synthesized materials possess rich porous
structure, with highly dispersed nanoparticles, and high
reduction degree.15
a. Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key
Laboratory for Novel Reactor and Green Chemistry Technology, Hubei
Engineering Research Center for Advanced Fine Chemicals, and School of
Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan
430205, P. R. China. E-mail: fyu@wit.edu.cn
b. The College of Post and Telecommunication of Wuhan Institute of Technology,
Wuhan 430205, P. R. China.
c. Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-Central University for Nationalities,
Wuhan 430074, Hubei Province, P. R. China. E-mail: jinlinli@aliyun.com
d. Key Lab for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering and Technology, Collaborative Innovation Center of
Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072,
China.
Thus, motivated by the specific generative process of M@C
derived from MOFs, we report the design of a series of Co@C
catalysts via an innovate melting technique. The as-synthesized
catalysts exhibit excellent physical and chemical properties,
while at the same time demonstrating remarkable catalytic
performance during FTS. Even when the catalysts are exposed
to harsh conditions, they are still able to show good stability,
which can be attributed to their special encapsulated structure.
‡ These authors are contributed equally to this work.
† Footnotes relating to the title and/or authors should appear here.
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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analyzer is used to investigate the surface area, pore size, and
2. Experimental
2.1. Materials
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total pore volume of the catalyst by nitroDgeOnI: 1p0h.1y0s3i9so/Dr0pCtiYo0n14a2t4A−
196 ℃. Prior to the measurement, the sample is degassed at
200 ℃ for 6 h. The total pore volume is obtained at a relative
pressure of 0.99. The Brunauer-Emmett-Teller (BET) method is
used to acquire the surface area of the samples. The pore size
distributions are assessed from the desorption curves of the
isotherms by utilizing the Barrett-Joyner-Halenda (BJH)
method. The laser Raman spectroscopy (LRS) spectra of the
The Co@C catalysts with different Co loadings are prepared
using a melt method. 3 g glucose and 5 g urea are mixed and
stirred at 140 ℃ to form a clear molten solution. Then 2.18 g
Co(NO3)3·6H2O (0.0075 mol) is added into the solution and
stirred for 5 min to achieve a transparent solution. The solution
is then dried at 180 ℃ for 20 h. After which, the black dried
sample is ground into powder and calcined at 750 ℃ for 2 h
under N2 atmosphere to obtain the catalyst, denoted as 35-
Co@C. By adjusting the amount of Co(NO3)3·6H2O to 0.0043 mol,
0.01mol and 0.14 mol we have prepared a series of catalysts,
and they are denoted as 20-Co@C, 50-Co@C, and 65-Co@C,
respectively.
0.36 g tetrabutyl titanate and 0.40 g Al(NO3)3·9H2O was co-
added with 2.18 g Co(NO3)3·6H2O for Ti and Al promoted
catalyst respectively, other steps remain the same. Denoted as
35-Co@C-Ti and 35-Co@C-Al, respectively.
The Co@C-M catalyst was prepared using ZIF-67 as a sacrifcial
template according to the following procedure: ZIF-67 was
heated to 600 ℃ in a quartz tubular reactor by flowing Ar at 80
mL/min with heating rate 3 ℃/min and kept at 600 ℃ K for 2 h.
2.2. Characterization
The Powder X-ray diffraction (XRD) spectrum is collected by
using a Bruker D8 powder diffractometer (Cu-Kα radiation, 40
kV and 40 mA, with a Vantec-1 detector). The morphologies of
the samples are investigated under a FEI Tecnai G2 F20
transmission electron microscope (TEM), with an accelerating
voltage of 200 kV. The precise mass loading is characterized
with thermogravimetric analysis (TGA) by using a NETZSCH TG
209F3 instrument with a ceramic crucible as the sample holder.
The TGA is conducted according to the following program:
temperature increases from 30 to 900 ℃ with a ramping rate of
10 ℃•min-1 in the presence of continuous flow of air (flow rate
30 mL•min-1). A Zeton Altamira AMI-300 instrument which is
equipped with a thermal conductivity detector (TCD) is used to
carry out the H2 temperature-programmed reduction (TPR).
Before the testing, the catalyst (0.05 g) is firstly purged with
high purity argon (>99.99%) at 120 ℃ for 1 h, and the catalyst is
then cooled to 70 ℃. 5% H2/Ar (flow rate of 30 ml•min−1) is used
during the temperature increment from 70 to 750 ℃ (with a
ramping rate of 10 ℃•min−1), and the temperature is held at
750 ℃ for 30 min, in this process, Tilon LC-D200M mass
spectroscopy instrument is used to test the tail gas (MS). The
amount of H2 dispersion and dispersity of cobalt were measured
by hydrogen temperature programmed desorption (H2-TPD)
using the Zeton Altamira AMI-200 unit. The catalysts were
reduced at 450 ℃ for 10 h and cooled to 100 ℃ in flowing
hydrogen. Prior to increasing the temperature from 100 ℃ to
450 ℃ at a rate of 10℃/min, the samples were held at 100 ℃
for 1 h under an argon stream. Then the catalysts were held at
450 ℃ for 2 h. Meanwhile, the TCD detector began to record
the signal until it returned to the baseline. The amount of
desorbed hydrogen was calculated by comparing the integrated
TPD spectrum with the mean areas of calibrated hydrogen
pulses. Micromeritics Tristar 3000 surface area and porosity
samples are collected by
a confocal Renishaw Raman
microprobe RM-1000. A continuous wave argon ion laser (Ar+,
514.5 nm) is projected through the samples in the exposure to
air at room temperature. A 30 s focus duration with a 50 ×
objective lens, and a scanning range of 80 to 4000 cm−1 with a
resolution power of 2 cm − 1 are used. X-ray Photoelectron
Spectroscopy (XPS) is conducted using a VG Multilab 2000
system with Al-K α radiation as the X-ray source. All binding
energies are calibrated using the C 1s peak (284.6 eV).
2.3. Catalytic test
Fischer–Tropsch synthesis reaction is carried out in a fixed-bed
reactor (stainless steel, id = 9.5 mm). In a typical setup, the
catalyst (0.1 g) is firstly mixed with inert quartz sand particles (1
g). Before the measurement, the catalyst is reduced by a pure
flowing H2 stream (GHSV=6 g SL-1 h-1) at 450 ℃ for 10 h. After
which, the catalyst is cooled to 100 ℃. Then, the syngas (H2/CO
= 2, GHSV=4 g SL-1 h-1) is led into the tube and the pressure was
increased to 1.0 MPa, the temperature is gradually increased to
the target temperature. During the reaction, feed gas and outlet
gases such as H2, CH4, CO, CO2, C2~C4 products, etc. are tested
online using an Agilent Micro GC3000 kitted out with molecular
sieves, Plot-Q and Al2O3 capillary columns. CO steady state
conversion and hydrocarbon selectivity are collected at 15–40
h. The wax and water products are collected in a hot trap
(100 ℃) and the oil and water products are gathered in a cold
trap (0 ℃) after more than 80 h of operation to achieve a good
mass balance at close to steady state, typically.
3. Results and discussion
3.1. Synthesis and structure
The synthesis process of Co@C is based on a melting method
which is illustrated in Scheme 1. The amount of Co loading is
tuned by controlling the amount of cobalt nitrate added in the
reaction system. The pyrolysis process is crucial for the
formation of the encapsulated structure.
Scheme 1: Melting strategy used in the synthesis of Co@C FTS catalyst.
The X-ray diffraction (XRD) patterns of the as-prepared Co@C
catalysts with different Co loading are presented in Figure 1. The
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XRD spectra of all Co@C catalysts reveal three prominent
diffraction peaks located at 44.5°, 51.9°, and 76.0°, which can
be assigned to the (111), (200), and (220) crystalline planes of
the FCC-Co (face-centered cubic Co0), respectively. Self-
reduction behavior during pyrolysis process (calcination) is well
documented for Co-based catalyst which uses carbon material
as the supporter.16-17 Thus, based on the XRD result, the Co
species presented in Co@C catalysts after pyrolysis at 750 ℃ is
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DOI: 10.1039/D0CY01424A
Figure 2. (a) Low magnification (inset showing the Co size distribution
Co0. This indicates that the reduction degree of Co species in the graph), and (b) High resolution (inset showing the graphite layers) TEM
images of 35-Co@C catalyst.
as-synthesized catalysts is high after the pyrolysis process. The
diffraction peak at 26.5° corresponds to the (002) planes of Surface elemental composition and chemical environment of
graphitic carbon, which could be a result of the conversion of the as-synthesized Co@C are characterized with X-ray
free carbon under the catalysis of the Co0 particles. Hence, photoelectron spectroscopy (XPS), and the results are shown in
based on the XRD results, it can be confirmed that Co@C is Table S2 and Figure 3. There is no clear distinction in the surface
composed of Co0 and graphitic carbon.
elemental composition for Co@C with different Co contents,
while it is worth noting that the survey XPS data shows a low Co
content of less than 2.7 at% and a high C content over 88 at%
for all samples. This result suggests that the abundant Co
species is surrounded by C matrix, which is consistent with the
TEM results (Figure 2b). The chemical state of the Co is
determined based on the Co 2p XPS spectrum. As shown in
Figure 3, the presence of the peak at ∼781.0 eV indicates the
existing oxidation state of Co on the surface of particles. On the
other hand, the peak at 778.5 eV is ascribed to Co0,19 suggesting
that there is a considerable number of accessible active sites on
the external surface. Thus, the XPS result is consistent with the
XRD results (Figure 1). It is generally known that the Co0
particles in nanoscale can be oxidized easily when exposed to
air. The unique reducing chemical environment in Co@C
catalysts would thus be responsible for its high reducibility.
Figure 1. XRD patterns of Co@C catalysts with different Co loadings.
Figure 2 presents the typical transmission electron microscope
(TEM) images of 35-Co@C. Based on Figure 2a, it can be
observed that the Co particles located on the carbon matrix are
well dispersed. Particle-size histogram obtained from the TEM
analysis show an average nanoparticle diameter of 23.5 nm,
with a wide size distribution ranging from 7.5 to 30 nm. The high
resolution TEM (HRTEM) image, presented in Figure 2b, reveals
a few layers graphite coating on the particles. The TEM images
of the 20-Co@C, 50-Co@C, and 65-Co@C are shown in Figure
S1. As shown in Figure S1, Co particles in 20-Co@C exhibit
uniform size distribution in the framework of carbon, which are
smaller in size as compared to the Co particles in 35-Co@C. Even
though Co particles are dispersed in the 50-Co@C and 65-Co@C,
the size distributions of Co particles in these samples are
relatively broad, which makes it hard to analysis the statistical
data accurately. The precise Co content presented in the various
Co@C samples is determined using thermogravimetric (TG)
analysis, and the results are shown in Table S1. It can be
observed that the Co loading in Co@C increases with the
amount of cobalt nitrate added. The Co loading in 65-Co@C is
Figure 3. Co 2p XPS spectra of Co@C catalysts with different Co mass
the highest at 44.0 wt%. This Co loading is higher as compared loading.
to some of the intricately designed MOFs-derived Co@C.18
The reducibility of Co@C catalysts is studied by temperature-
programmed reduction-mass spectrum (TPR-MS) test. It can be
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seen that the Co@C catalysts exhibit three distinct peaks at 210-
250 ℃ , 330-360 ℃ , and 510-550 ℃ respectively, which
coincide well with the shape of the CH4 (m/z=16) signal
measured by MS. Besides, weak H2O (m/z=18) signal are also
observed for all catalysts at 210-250 ℃, based on the MS result.
The peak at 210-250 ℃ could be ascribed to the overlay of the
reduction of cobaltous oxide and the methanation of unstable
carbon species. Peaks at 330-360 ℃ and 510-550 ℃ are the
methanation of more stable carbon species. Interestingly, the
methanation temperature of the third peak decreases with Co
loadings, which implies that the methanation of carbon species
may be closely related to the catalysis of Co for Co@C catalyst.
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DOI: 10.1039/D0CY01424A
Figure 5. N2 adsorption-desorption isotherms of the as-synthesized Co@C
catalysts.
Table 1. Surface area, pore volume, and pore diameter of catalysts
Sample
Surface area,
m2/g
Pore Volume, Pore diameter,
cm3/g
0.13
0.06
0.03
0.03
nm
3.1
3.5
3.7
3.9
20-Co@C
35-Co@C
50-Co@C
65-Co@C
381.3
284.5
241.6
218.8
3.2 Fischer-Tropsch synthesis
The FTS catalytic performance of the Co@C catalysts is
investigated in a fixed bed reactor at a temperature of 240 ℃
and at a pressure of 1.0 MPa. The overall results are listed in
+
Table 2. CH4 and C5 selectivity of 20-Co@C are 14.1 % and
73.4%, respectively, with a CO conversion of 19.1%. With the
increase in the Co loading, the CO conversion increases without
any changes to the selectivity. Another Co@C catalyst derived
from MOFs precursor is synthesized as a comparison (Co@C-M).
The morphological features of Co@C-M, is characterized using
the TEM. As shown in Figure S2, a similar image of a highly
dispersed Co nanoparticles are supported onto a carbon matrix.
The overall catalytic performances of Co@C-M and 65-Co@C
are then compared at similar CO conversion level. The cobalt-
time-yield (CTY) of Co@C-M is higher than that of 65-Co@C,
while the C5+ selectivity of Co@C-M is lower at a conversion of
Figure 4. H2-TPR and MS profiles of different Co@C catalyst. a: 20-Co@C; b:
35-Co@C; c: 50-Co@C; d: 65-Co@C
The pore structure of the as-synthesized Co@C materials are ca. 30%. Furthermore, the undesirable CH4 selectivity of Co@C-
evaluated using the N2 sorption experiments. The N2 M reaches 24.3%.
adsorption-desorption isotherms of the as-synthesized Co@C The catalytic performance of the catalyst is the result of the
catalysts are shown in Figure 5, and the detailed pore specific nanostructure of which. For the Co@C material derived
parameters are listed in Table 1. It is observed that the 65-Co@C from a carbon-based precursor, its detail texture was intricate
exhibits a BET surface area of 218.8 m2g-1 while 20-Co@C due to the unordered reconstruction of carbon and metal
possesses a BET surface area of 381.3 m2g-1. With the decrease species during pyrolysis process. Therefore, the correlation of
in the Co loading, the surface area increases significantly. This structure and performance for Co@C catalysts must be
clearly indicates that the overall structure of the material discussed carefully. During the pyrolysis process of MOFs, the
presented in this work is determined by reaction procedures of organic metal skeleton was destroyed, and metal ions were
the carbon under the catalysis of Co at high-temperature reduced to form particles synchronously. The unpredictable
pyrolysis process. All the samples display type IV isotherms and pyrolysis process may result in a wide distribution of particle
they exhibit a steep N2 hysteresis, which are the characteristics size, many nanoclusters or even single atomic particles which
of non-uniform banded pores or close-packed pores. This pore are difficult to be found by conventional TEM measurement
structure is composed of interlaced graphitic and non-graphitic may exist in catalysts.21 Fu et al. suggested that smaller Co
carbon that are derived from reorganization of free carbon particles were more active in the promotion of direct carbon
catalyzed by adjacent Co0 particles during high temperature nanotubes methanation during a H2 temperature-programmed
pyrolysis.
reduction process.22 Then the Co@C-M sample was
characterized with H2-TPR. As shown in Figure S3, temperature
of carbon methanation for Co@C-M was quite lower than Co@C
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catalyst prepared by melting method (488 ℃ vs. 534 ℃). It specific shape. After magnification the image it could be found
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DOI: 10.1039/D0CY01424A
indicated that the plentiful small Co particles were well that lots of cobalt particles distributed on the carbon
distributed in the Co@C-M. Many researchers suggest that the framework. Furthermore, carbon nanotubes can be seen clearly
catalytic performance of CO hydrogenation was dependent of on the parts of Co particles. To provide an insight into the
cobalt particle size for catalysts with sizes less than 6 nm, CH4 formation mechanism of the as-prepared materials, we have
selectivity increased with the decreased of Co size.23 We attained the XRD, N2-adsorption, and Raman data of the
believed that the small Co particles (＜ 6 nm) undetected by catalysts prepared at different pyrolysis temperatures to track
TEM could be responsible for the higher CH4 selectivity of the evolution process of 35-Co@C. As shown in Figure 7, it can
Co@C-M. The TOF value of the catalyst is calculated based on be clearly seen there are dramatic changes in the component
the H2 chemisorption data (Table S4). It can be seen that the and specific surface area of the catalyst with the increase in
TOF value increases with the Co loading. Based on the above- pyrolysis temperature. Co0 only appears as the pyrolysis
mentioned discussion, the size distribution of Co particle in temperature rises to 550 ℃. When the pyrolysis temperature
Co@C is broad due to the uncontrolled structural evolution increases from 550 ℃ to 750 ℃ , the diffraction peak that
during the pyrolysis process. Thus, it is expected that there is a belongs to Co0 become sharper, and the diffraction peak at 26.5°
larger number of small Co0 particles (＜6 nm) present in the that belongs to graphite also appears. Besides, the specific
catalyst with lower Co loading. This, would lower the TOF of the surface area increases sharply from 12 m2/g to 284.5 m2/g, and
catalyst as a whole.
the intensity ratio of D band to G band increases from 0.95 to
1.35, simultaneously. This result implies that after the
appearance of Co0, there is a dramatic change in the
construction of the catalyst. The self-reduction behavior during
the pyrolysis process, i.e., calcination, is well documented for
Co-based catalyst that uses carbon material as the supporter. It
is expected that Co0 particles are achieved via the reduction of
precursor by the surrounding carbon species at high
temperature. On the other hand, Co0 is known to be an effective
catalyst for the growth of carbon nanotubes over a wide range
of temperatures,24 i.e., the texture properties of catalyst are
highly dependent on the catalytic effect of Co0. The graphitic
carbon that is formed near the Co0 particles extrudes the free
carbon that is relatively far away from Co0, which results in the
formation of disordered pore structure. However, this process
is difficult to regulate precisely, and therefore, this would result
in Co particles exhibiting a heterogeneous surface structure and
a wide distribution of Co particles. Thus, based on these data,
we have illustrated the entire formation process of the material
with a schematic diagram(Figure 7 d).
Table 2. Catalytic performance of various Co@C catalysts
Hydrocarbon
selectivity (%)
C2-
CO
CTYc
CO2 Sel.
(%)
Catalysts
Conv.
(%)
+
CH4
C5
C4
20-Co@C
35-Co@C
50-Co@C
19.1
26.6
25.3
32.5
64.5
25.4
50.8
＜0.5
＜1
1.38
1.38
0.90
1.10
--
--
--
--
--
2.0
1.6
1.6
1.4
5.8
1.4
3.3
--
14.1
15.9
16.2
16.0
36.0
20.8
26.2
--
12.5
14.6
14.3
18.3
18.2
16.0
17.9
--
73.4
69.5
69.5
65.7
45.8
62.2
55.9
--
65-Co@C
35-Co@C-Al
35-Co@C-Ala
35-Co@C-Ti
35-Co@C-550
35-Co@C-650
Co@C-Mb
--
1.8
--
--
--
30.7
1.76
24.3
10.6
65.1
Reduction conditions: in pure hydrogen at 450 ℃, 1 bar and 3 SL·g−1·h−1 for 10
h. Reaction condition: H2/CO = 2, 240 ℃, 1.0 MPa and 4 SL·g−1·h−1. CO steady
state conversion and hydrocarbon selectivity are collected at 15–40 h.
a: Reaction temperature was 220 ℃.
b: Reaction temperature was 230 ℃.
c: Cobalt time yield (CTY) = mol of CO converted to hydrocarbons (excluding
CO2) per time (s) per weight of Co, expressed as 10-5 molCOgCo-1s-1.
Figure 6. SEM image of 35-Co@C catalyst.
The overall activity of Co@C catalyst was also highly dependent
on the unique microstructure of which. Although the Co loading
mass of as-synthesized Co@C was high, its catalytic activity was
pale by comparison. To clarify the structure of our Co@C
catalyst further, the surface morphology of the catalyst was
Figure 7. a: XRD patterns, b: Raman spectrum, c: N2 adsorption-desorption
isotherms of 35-Co@C catalyst with different pyrolysis temperature; d：
schematic diagram of the evolution of material during the heating process.
detected by scanning electron microscope (SEM). Figure S4 and Subsequently, we tested the catalytic performance of 35-Co@C
Figure 6 were the typical SEM of 35-Co@C sample. It can be prepared at different pyrolysis temperatures, and the results
seen that morphology of the catalyst was bulk like without a are listed in Table 2. It can be seen that although the
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characteristic diffraction peak that belongs to Co0 is obvious for Such diverse structures of particles are derived from the
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35-Co@C-650 and 35-Co@C-550 samples, the CO conversions uncontrollable pyrolysis process. It has bDeOenI: 1r0e.1p0o3r9t/eDd0CthY0a1t4t2h4Ae
for both samples are negligible. This result indicates that the confined space between graphite and metal could promote CO
syngas is unable to approach the active sites of the catalyst. The reaction,25 while thicker graphite layer on surface would
N2-adsorption data reveals that the as-prepared 35-Co@C-550 masking the active site completely. Large number of covered-
possesses a relatively low specific surface area. On the other active sites could responsible to the lower activity of our Co@C.
hand, the specific surface area of 35-Co@C-650 is relatively high, To improve the catalytic performance of the Co@C catalyst, we
with a low pore volume. The Co0 particles in 35-Co@C-650 and have added Al and Ti precursor during the melting process to
35-Co@C-550 samples are likely to be blocked by the compact enhance this material. The detailed catalytic performance data
carbon matrix. When the pyrolysis temperature is set to 750 ℃, are listed in the Table 2. It can be observed that when the
the specific surface area of the catalyst increases from 12 m2/g catalyst is enhanced with Al and Ti, the catalytic activity is
to 284.5 m2/g. This result suggests a looser carbon structure, significantly improved, even though the CH4 selectivity
which facilitates the diffusion of syngas to the surface of the increases concurrently. We have also tested the catalytic
active site.
performance of 35-Co@C-Al at 220 ℃ , whereby a CO
conversion of 25.4% is recorded. This result is almost similar to
the CO conversion recorded for 35-Co@C at 240 ℃ , which
suggests that Al could improve the catalytic performance of 35-
Co@C significantly. However, further improvements in the
structure of the catalyst and the reduction in methane
selectivity are needed. We have also tested the catalytic
property of 35-Co@C without reduction. It can be seen that the
CO conversion is 20.5% at 240 ℃, and the CH4 selectivity is up
to 38.5%. The overall performance of the catalyst without
reduction is inferior to that of the reduced catalyst. The surface
compositions of the reduced 35-Co@C are analyzed and
compared to those of the fresh catalyst. After the reduction
process, the Co/O ratio increases from 0.27 to 0.31, while the C
content decreases from 92.4% to 77.5%(Table S2). This result
implies that more Co0 is accessible to the syngas. Besides, the
TPR-MS result reveals that a proper reduction process can
remove some of the free carbon from the Co@C, which would
facilitate the exposure of more active phases. Thus, based on
the results, we propose that an appropriate reduction process
is necessary to enhance the performance of Co@C catalyst.
Figure 8. a, c and e: three typical HRTEM images of 35-Co@C catalyst; b, d
and f: the corresponding surface diagram.
HRTEM was used to observe the subtle surface structure of the
particles. The results show that the surface of Co@C material is
not exactly the same. Three typical particle surface states were
observed and their HRTEM images and corresponding
schematic diagrams were lied in Figure 8. The morphology first
typical particles were presented in Figure 8a, it can be seen that
the particle was covered with more than ten layers of graphitic
layer (Figure 8 a and b). It was expected that these particles
Figure 9. CO conversion vs. time for the as-synthesized Co@C catalysts in
the steam.
were inactive. In the second case, several junctions between Preventing and limiting catalyst deactivation is one of the most
graphite flakes and Co were visible on the surface of particles important topics in the development of the next generation of
(Figure 8 c and d). In the third case, it can be seen that the industrial catalysts. Thus, in this work, the stability of the as-
surface structure of the particles is more complex and varied. synthesized Co@C is also investigated. As shown in Figure 9, all
Parts of Co surface were covered by graphitic carbon with Co@C samples present excellent stability at 240 ℃ for 120 h.
different layer while parts were fully exposed (Figure 8 e and f). To examine the catalytic stability of Co@C under a harsh
6 | J. Name., 2012, 00, 1-3
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condition, we raise the reaction temperature further to 260 ℃. layers on surface of Co would cover the active siteVsi,ewthAirsticslehOonulinlde
DOI: 10.1039/D0CY01424A
Detail performance data listed in Table S3. The CO conversion be taken into account when designing a superior Co@C
of 65-Co@C is stable at 58% at 260 ℃ for 50 h, and the rest of materials. Thus, based on the collective results, this approach
the samples also shown good stability at similar reaction opens another pathway towards the development of metal
condition.
nanoparticles with both high stability and reactivity as highly
For a typical Co-based catalyst used in Fischer-Tropsch synthesis, efficient catalyst in the hydrogenation application.
deactivation of the catalyst typically originates from the
decrease in the amount of accessible active surface area due to
oxidation and sintering of Co particles.26-27 Thus, to investigate
the stability of the as-synthesized catalysts, the phase
compositions of all Co@C after 200 h in the stream are
characterized using XRD. As shown in Figure 10, no phase
transformation can be observed for all Co@C after 200 h in the
stream. Water is one of the primary byproducts in FTS reaction,
and the partial pressure of water increases with CO conversion.
Small Co and Fe particles would be oxidized by the water, which
easily leads to a decrease in the active sites. The stability of the
active phase in the reaction process is also thanks to the special
carbon environment around it. It should be noted that the
diffraction peak assigned to graphite carbon were broadened
obviously compared to fresh catalysts, indicating that the
crystallinity of graphite carbon decreased after reaction. It
suggested that the graphite structure was not stable in Fischer-
Tropsch synthesis reaction. Raman spectroscopy was used to
determine the carbon type, structural ordering degree, and the
presence of defects in Co@C materials. It can be seen that all
the catalysts presented typical D-band and G-band of carbon,
indicating large number of defects distributed on graphite
carbon (Figure S5). These defects could be an important reason
for the instability of graphite layer during reaction process.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21571147, 21972170 and 21902187).
Notes and references
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4. Conclusions
In this work, we have successfully developed a series of Co@C
via a unique melting approach. The catalysts possess abundant
porous structure that can facilitate the mass transfer process.
Highly dispersed Co nanoparticles can improve the catalytic
efficiency significant. Furthermore, the graphitic layers that are
coated on the particles can render a unique chemical
environment for the actives phase. This could prevent the
particles from being oxidized by the water, which can help to
improve the stability of the catalyst. While the multiple graphite
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DOI: 10.1039/D0CY01424A
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