Effect of cobalt weight content on the structure and catalytic properties of Co/CNT catalysts in the fischer–tropsch synthesis

Article abstract of DOI:10.1134/S0023158416050062

Cobalt-based Fischer–Tropsch synthesis (FTS) catalysts containing 1 to 40 wt % cobalt supported on multi-walled carbon nanotubes (CNTs) have been investigated. The CNTs have been characterized by low-temperature nitrogen adsorption, scanning electron microscopy, and X-ray photoelectron spectroscopy. All catalysts have been prepared by impregnating, with an ethanolic solution of cobalt nitrate, the CNTs preoxidized with concentrated nitric acid and have been tested in the FTS at 220°C and atmospheric pressure. Correlations have been established between the cobalt weight content of the catalyst and the Co particle size determined by transmission electron microscopy and X-ray diffraction. The Co content and particle size have an effect on the activity and selectivity of the catalyst and on the target fraction (C₅₊) yield in the FTS. The highest CO conversion is observed for the catalyst containing 20 wt % Co; the highest selectivity and activity, for the catalyst containing 5 wt % Co; the highest C₅₊ yield, for the catalyst containing 10 wt % Co.

Full text of DOI:10.1134/S0023158416050062

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 640–646. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © S.A. Chernyak, G.E. Selyaev, E.V. Suslova, A.V. Egorov, K.I. Maslakov, A.N. Kharlanov, S.V. Savilov, V.V. Lunin, 2016, published in Kinetika i
Kataliz, 2016, Vol. 57, No. 5, pp. 645–651.
Effect of Cobalt Weight Content on the Structure and Catalytic
Properties of Co/CNT Catalysts in the Fischer–Tropsch Synthesis
a, b,
a
a
a
a
a
S. A. Chernyak *, G. E. Selyaev , E. V. Suslova , A. V. Egorov , K. I. Maslakov , A. N. Kharlanov ,
a, b
a, b
S. V. Savilov , and V. V. Lunin
a
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
b
*
e-mail: сhernyak.msu@gmail.com
Received November 25, 2015
Abstract—Cobalt-based Fischer–Tropsch synthesis (FTS) catalysts containing 1 to 40 wt % cobalt supported
on multi-walled carbon nanotubes (CNTs) have been investigated. The CNTs have been characterized by
low-temperature nitrogen adsorption, scanning electron microscopy, and X-ray photoelectron spectroscopy.
All catalysts have been prepared by impregnating, with an ethanolic solution of cobalt nitrate, the CNTs pre-
oxidized with concentrated nitric acid and have been tested in the FTS at 220°C and atmospheric pressure.
Correlations have been established between the cobalt weight content of the catalyst and the Co particle size
determined by transmission electron microscopy and X-ray diffraction. The Co content and particle size have
an effect on the activity and selectivity of the catalyst and on the target fraction (С ) yield in the FTS. The
5+
highest CO conversion is observed for the catalyst containing 20 wt % Co; the highest selectivity and activity,
for the catalyst containing 5 wt % Co; the highest C yield, for the catalyst containing 10 wt % Co.
5+
Keywords: heterogeneous catalysis, Fischer–Tropsch synthesis, carbon nanotubes, particle size, cobalt
DOI: 10.1134/S0023158416050062
INTRODUCTION
materials are inert towards the catalytic metal and pro-
vide favorable conditions for obtaining a highly dis-
persed catalyst [6]. Among the carbon materials
The Fischer–Tropsch synthesis (FTS) is the cata-
lytic hydrogenation of carbon monoxide for the pro-
duction of synthetic fuel and lubricants [1]. This pro-
cess has been carried out on an industrial scale by big
and small companies (Shell, Sasol, Chevron, Synfuels
China, Velocys, etc.) all over the world. The main
advantage of the synthetic hydrocarbon feedstock
resulting from the FTS is its purity, since all possible
impurities are removed from synthesis gas before it is
processed [2]. All Group VIII metals are catalytically
active in the FTS, but only iron and cobalt are used in
industry for the reason that they are much less expen-
sive than noble metals [3]. In the presence of nickel,
the heavy hydrocarbon selectivity is low and the major
reaction product is methane [4]. An advantage of the
(fibers, activated carbons, carbon black), carbon
nanotubes (CNTs) attract particular attention, for
they possess the following unique combination of
properties: high mechanical strength; high thermal
stability; specific surface area variable in a wide range;
possibility of functionalizing their surface with various
groups and heteroatoms in order to tune the pH of the
surface, their hydrophilicity, and their donor–accep-
tor properties [7]. All these factors, as well as the
increase in CNT production and the steadily decreas-
ing CNT price, make CNTs very promising supports
for catalysts for a wide variety of processes [8].
The characteristics of CNT-supported cobalt cata-
cobalt catalysts over the nickel catalysts is their higher lysts have been the subject of many studies. It was
time-on-stream stability and the possibility of con- demonstrated that use of CNTs as the catalyst support
ducting the process at atmospheric pressure and lower in the FTS provides means to decrease the cobalt
temperatures [3, 5].
oxide-to-cobalt metal reduction temperature and to
increase the degree of reduction of cobalt [9]. It was
confirmed many times that the activity and selectivity
of CNT-based composites are higher than those of
catalysts supported on oxides [9, 10] and other carbon
materials [11, 12].
Although the FTS has been widely used in industry
for a long time, the problem of enhancing the effi-
ciency and stability of catalysts for this process is still
being addressed all over the world. In any catalytic
process, it is essential to find an appropriate catalyst
support. The most common supports are Al O , SiO ,
and TiO [1]. As distinct from these oxides, carbon depend on the cobalt particle size, but the relevant
2
3
2
The catalytic activity and selectivity in the FTS
2
640

EFFECT OF COBALT WEIGHT CONTENT
641
data available from the literature are contradictory. above parameters were calculated using the BET and
The optimum Co particle size was reported to be 3.5 to BJH models.
1
5 nm [13–18]. The particle size can be varied by mod-
The composition of the CNT surface was studied
by X-ray photoelectron spectroscopy (XPS). Photo-
electron spectra were recorded on an Axis Ultra DLD
spectrometer (Kratos Analytical, United Kingdom)
ifying the surface with functional groups [19], by using
the microemulsion method [20], and by varying the
weight fraction of the metal [13, 21]. Cobalt content
has a marked effect on the cobalt particle size, sub-
strate conversion, and the selectivity and activity of the
catalyst. As a rule, a decrease in the metal concentra-
tion leads to a decrease in the average diameter of
metal crystallites and, accordingly, changes the cata-
lytic properties of the catalyst [13]. The literature con-
cerning this issue is also contradictory. For example, a
study of the effects of the weight fraction of CNT-sup-
ported cobalt demonstrated that the rate of the FTS
and the number of active sites increase as the metal
concentration is increased from 15 to 40% [22]; at the
same time, it was discovered [23] that CO conversion
increases as the cobalt content of the CNT-supported
using monochromatic AlK radiation (1486.6 eV).
α
Survey spectra were obtained at an analyzer pass
energy of 160 eV and a scanning step size of 1 eV.
Temperature-programmed reduction with hydro-
gen (Н -TPR) was carried out using a laboratory setup
2
including a gas preparation system, a reactor placed in
a tubular furnace, and a thermal-conductivity detec-
tor. Samples were reduced with a 5% H /Ar mixture
2
(
23 mL/min) at a constant heating rate of 12°C/min.
Co/CNT composites were pre-heat-treated at 300°С
in an N atmosphere for 2 h.
2
X-ray diffraction (XRD) patterns were obtained on
catalyst is raised from 5 to 25 wt % and becomes lower an STADI-P automated powder diffractometer
at a cobalt content of 35 wt %.
(STOE & Cie GmbH, Germany) using СoK radia-
α
tion (λ = 1.78896 Å) and the Bragg−Brentano geome-
try. Particle sizes were calculated via the Scherrer
equation. Before being examined, the catalysts were
calcined in flowing nitrogen (30 mL/min) at 300°С.
In this work, we studied the effect of cobalt content
on the particle size, structure, and catalytic properties
of Co/CNT composites using up-to-date physico-
chemical methods.
Synthesis of Supports and Catalysts
EXPERIMENTAL
The supports for cobalt catalysts were CNTs syn-
thesized by the pyrolytic decomposition of hexane at
Starting Compounds and Materials
7
50°С for 6 h in the presence of a Co–Mo/MgO cata-
The starting compounds were cobalt(II) nitrate
hexahydrate (special-purity grade), magnesium
nitrate (reagent grade), ammonium molybdate
lyst in a tubular quartz reactor 50 mm in diameter [24].
The resulting CNTs were treated with hydrochloric
acid to remove magnesium, molybdenum, and cobalt
impurities and were then dried in a drying oven
(
reagent grade), hydrochloric acid (reagent grade),
nitric acid (reagent grade, density of 1.4 kg/L), hexane
(
Binder, Germany) at 130°С.
(
(
reagent grade), ethanol (reagent grade), hydrogen
99.99%), nitrogen (99.999%), helium (99.995%),
The CNT surface was functionalized by treating
CNTs with a concentrated boiling solution of nitric
carbon monoxide (99.95%), and argon (99.993%).
acid (68% HNO : CNTs = 200 mL : 10 g) under reflux
3
for 5 h. Next, the CNTs were filtered, washed with dis-
Analysis and Physicochemical
Characterization Methods
tilled water until neutral pH, and dried at 130°С.
Cobalt catalysts were prepared by impregnating
nitric acid–oxidized CNTs with an ethanolic solution
of cobalt nitrate. Thereafter, the solvent was evapo-
rated under ultrasonication in an ultrasonic bath (Fer-
roplast-Medikal, Russia) at 60°С. The resulting mate-
rials were dried in a drying oven at 130°С for 16 h. The
cobalt content of the catalysts was 1, 5, 10, 15, 20, or
The morphology and composition of CNTs were
studied by scanning electron microscopy (SEM) on a
JEOL JSM-6390LA microscope (JEOL, Japan).
SEM images were obtained at an accelerating voltage
of 20−25 kV. The structure of spent Co/CNT catalysts
was studied by transmission electron microscopy
4
0 wt %. (Hereafter, the catalysts are designated as
xCoCNT, where x is the cobalt content, wt %.)
(
TEM) on a JEOL 2100F microscope (JEOL, Japan)
at an accelerating voltage of 200 kV. Prior to being
examined by TEM, the spent catalysts were passivated
with a 1% О + Ar gas mixture at room temperature.
2
Catalytic Tests
The specific surface area (SBET), average pore
Catalytic tests were carried out in a vertical flow
radius (R ), and pore volume (V ) of materials quartz reactor (10 mm i.d.) placed in a tubular fur-
pore
pore
were determined by low temperature nitrogen adsorp- nace. The testing procedure was described in an earlier
tion on an AUTOSORB-1C/MS/TPR sorption ana- paper [24]. Supported cobalt nitrate was decomposed
lyzer (Quantachrome, United States). The samples to by heating the catalyst to 400°С in flowing nitrogen
be examined were degassed at 250°С for 3 h. The (30 mL/min). The catalyst was pre-reduced in flowing
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

6
42
CHERNYAK et al.
2
μm
0.5 μm
(а)
(b)
2
μm
0.5 μm
(c)
(d)
Fig. 1. SEM images of (a, b) CNTs washed with HCl and (c, d) CNTs oxidized with HNO3.
hydrogen (40 mL/min) at 400°С for 4 h. CO was
Table 1 lists the specific surface areas and pore sizes
hydrogenated at 220°С and atmospheric pressure for and volumes of CNTs derived from low-temperature
4
0 h at a gas ratio of H : CO : N = 2 : 1 : 0.7, which adsorption data. The oxidation of the CNTs increases
2 2
their specific surface area and pore volume and con-
siderably decreases their pore size. This effect is typi-
cal of modification of CNTs with an acid and can be
explained by the opening of tube channels, dissolution
of intrachannel metal particles, and generation of sur-
face defects through the breaking of carbon–carbon
bonds in outer layers of the CNTs [25]. These defects
was maintained using a gas flow controller (Meta-
Chrom, Russia). The internal standard was nitrogen.
−
Cat
1
–1
The total flow rate was 1.2 L g h , and the catalyst
weight was 2 g. The reaction products were analyzed
on an Agilent Technologies 6890N gas chromatograph
(
Agilent Technologies, United States). Hydrocarbons
were separated on a DB-Petro capillary column using accommodate oxygen-containing functional groups,
a flame-ionization detector. The CO , CO, and CH
mainly carboxyls [24, 26]. It was indeed demonstrated
2
4
concentrations were determined using a thermal-con- by XPS that the oxidation of CNTs with nitric acid
ductivity detector and a HayeSep column packed with causes an oxygen buildup on their surface (Table 1).
The observed changes in the porosity and composition
of the CNT surface are favorable for a uniform distri-
bution of cobalt nitrate over the surface and for an
increase in the dispersion of metal particles resulting
from catalyst reduction [19–21, 24].
Porapak R. Spent catalysts were deactivated with a
1
% O /Ar mixture at room temperature.
2
RESULTS AND DISCUSSION
The nitric acid–oxidized CNTs were loaded with
The morphology of the original and nitric acid– cobalt by impregnating them with an ethanolic solu-
oxidized CNTs was studied by SEM (Fig. 1). The oxi- tion of cobalt nitrate. The catalysts containing 5, 10,
2
0, or 40 wt % cobalt were characterized by XRD.
Their diffraction patterns are shown in Fig. 2.
The XRD peaks at 19°, 31.3°, 36.9°, 44.8°, 55.7°,
59.4°, and 65.2° are characteristic of the cobalt oxide
dized CNTs formed denser agglomerates than the
original CNTs and, accordingly, had a higher bulk
density. The CNT diameter in both samples was 10–
30 nm.
KINETICS AND CATALYSIS
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EFFECT OF COBALT WEIGHT CONTENT
643
Table 1. Textural properties of the original and oxidized mum at 150–200°С arises from the decomposition of
CNTs and their oxygen content according to XPS data
residual cobalt nitrate. The peak at 200–300°С corre-
2
sponds to the reduction of Co O to СоО; the peak at
3
4
Sample SBET, m /g Rpore, nm Vpore, mL/g О*, at %
0
3
00–450°С, to the reduction of СоО to Со [11, 12,
CNT-HCl
CNT-HNO3
192
235
18.8
11.0
0.2
0.7
0.3
6.8
24]. The hydrogen uptake above 450°С is due to the
CNTs–hydrogen interaction yielding methane [10].
Variation of the amount and particle size of cobalt
only causes changes in the intensities of the peaks cor-
responding to the cobalt oxide reduction stages but
does not shift these peaks along the temperature axis.
Note that the reduction temperature of all cobalt frac-
tions is independent of their particle size.
*
Atomic concentration of carbon-bonded surface oxygen deter-
mined by XPS.
Table 2. Со О and Со crystallite sizes determined by XRD
3
4
0
Catalyst
d(Co O ), nm
d(Co )*, nm
3
4
All catalysts were tested in the FTS at atmospheric
pressure and 220°С. The catalytic properties of the
samples are presented in Table 3. The CO conversion,
catalytic activity, selectivities toward the formation of
different products, and the yield of the target hydrocar-
bon (C ) fraction were calculated. The C yield was
5
CoCNT
3.3
5.2
10.5
13.9
2.5
3.9
7.8
1
0CoCNT
0CoCNT
0CoCNT
2
4
10.5
5+
5+
determined as the product of the CO conversion (α)
*
0
Cobalt metal particle size calculated via the formula d(Co) =
.75 × d(Co O ) [15].
and C selectivity (S, %): Yield = α × S. This quantity
3
4
5+
indicates the proportion of СО (%) converted into
heavy hydrocarbons.
phase Co O , and the reflections at 26.4° and 44.4° are
3
4
All of the above characteristics depend nonlinearly
on the weight percentage of cobalt in the catalyst. The
CO conversion increases as the cobalt content is
increased to 20 wt % and then falls dramatically. The
due to the CNTs. The width of the cobalt oxide peaks
decreases with an increasing cobalt content, indicating
an increase in the degree of crystallinity of Co O .
3
4
Table 2 lists Co crystallite sizes calculated from XRD
data via the Scherrer equation. Clearly, the cobalt par-
ticle size increases with an increasing metal concen-
tration.
С
selectivity depends on the metal content of the
+
5
catalyst in a complicated way. In order to explain these
results, all of the spent catalysts were examined by
TEM (Fig. 4). The TEM images were used to deter-
mine the average particle diameter and the standard
deviation characterizing the distribution width.
According to TEM data, the average particle size in
the spent 20СоCNT catalyst is smaller than the Co
particle diameter in the reduced sample calculated
using the Scherrer equation. Particle size determina-
tion for this sample was complicated by the simultane-
ous presence of very small (~1–2 nm) and large
The catalysts containing 5, 20, and 40% Со were
characterized by Н -TPR (Fig. 3). All of the TPR pro-
2
files show four hydrogen uptake maxima. The maxi-
Intensity, arb. units
Co O
3
4
graphite
4
0CoCNT
0CoCNT
Hydrogen uptake, arb. units
2
4
0CoCNT
0CoCNT
10CoCNT
2
5CoCNT
5CoCNT
10 15 20 25 30 35 40 45 50 55 60 65 70
0
100 200 300 400 500 600 700 800 900
2θ, deg
Temperature, °С
Fig. 2. X-ray diffraction patterns of the catalysts containing
, 10, 20, and 40% cobalt, all calcined at 300°С.
Fig. 3. TPR profiles for the catalysts containing 5, 20, and
40% cobalt.
5
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

6
44
CHERNYAK et al.
(
>15 nm) crystallites because of the intensive glow of Table 3. Performance characteristics of the catalysts in the
Fischer–Tropsch process
the latter in the electron beam. It can be assumed that
the 20СоCNT catalyst contained large Со particles
that were outside the area examined by TEM.
CO
conversion,
%
Selectivity, %
Catalyst
СоCNT
CH4 C –С
C5+
CO2
2
4
The cobalt particles in the 1СоCNT catalyst
(
4.9 nm, see Fig. 4a) are larger than the same particles
1
2
13
25
30
39
20
55
34
36
46
53
55
29
14
14
18
16
18
13
46
44
27
19
18
3
6
in the 5СоCNT, 10СоCNT, and 15СоCNT samples
(
3.6, 3.9, and 4.2 nm, respectively, Figs. 4b, 4c, 4d), 5СоCNT
and the particle size distribution in the 1СоCNT cat-
alyst is wider: the standard deviation for 1СоCNT,
10СоCNT
6
5
СоCNT, 10СоCNT, and 15СоCNT is 2.8, 1.6, 1.5, 15СоCNT
9
and 1.8 nm, respectively. The 20СоCNT sample con-
tains larger particles (5.3 nm, Fig. 4e) with a wide size
20СоCNT
12
9
distribution (standard deviation of 3.1 nm). The large 40СоCNT
average size of Co particles in the spent 1СоCNT sam-
ple may be due to the tendency of the small metal par-
ticles to agglomeration [27]. Earlier, we found that, in the XPS data for the catalyst supports used in this
carbon materials containing 0.56–5 wt % supported study and the same data for the supports examined
cobalt, the size of the metal particles is ~2 nm and the earlier [21] demonstrated that amorphous cobalt
particles are hydrogenation-inactive [21]. At the same forms on CNTs with a high surface concentration of
time, the 1СоCNT and 5СоCNT catalysts examined oxygen (9.2 at %) and crystalline cobalt forms on less
in this work contain cobalt particles with d > 4 nm and strongly oxidized supports (6.8 at %, according to this
display a noticeable activity. A comparison between study). Thus, it can be presumed that the state of
(а)
(b)
(c)
d(Co) = 4.9 ± 2.8
d(Co) = 3.6 ± 1.6
d(Co) = 3.9 ± 1.5
20 nm
0 2 4 6 8 101214161820 20 nm
0 2
4
6
8 101214161820 20 nm
0
2
4
6
8 10 12 14 16 1820
d(Co), nm
d(Co), nн mм
d(Co), nm
(d)
(e)
(f)
d(Co) = 4.2 ± 1.8
d(Co) = 5.3 ± 3.0
20 nm
0
2
4
6
8 101214161820 20 nm
50 nm
0
2 4 6 8 10 12 14 16 1820
d(Co), nm
d(Co), nm
Fig. 4. TEM images and Co particle size distribution for spent catalysts: (a) 1CoCNT, (b) 5CoCNT, (c) 10CoCNT, (d)
5CoCNT, (e) 20СоCNT, and (f) 40СоCNT.
1
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EFFECT OF COBALT WEIGHT CONTENT
645
activity and selectivity of the catalyst in the FTS. For
example, it was found that the Со/CNT catalyst with
a Co particle size of 6–12 nm shows a lower activity
and selectivity than the samples in which the particle
−
5
−1 −1
A × 10 , mol CO s g
С5+ yield, %
Co
12
1
.4
.2
.0
1
8
6
4
2
0
0
size is 1–2, 2–4, or 4 nm. The CO selectivity in the
2
1
water gas shift (WGS) reaction depends directly on the
CO conversion. The same correlation was also
observed in another study [9]. As the CO conversion
was increasing, the amount of water, a by-product of
hydrocarbon formation, in the reaction zone
increased. In turn, water displaced the equilibrium in
1
0
0
0
0
.8
.6
.4
.2
0
the WGS reaction toward СО formation.
2
The highest activity is shown by the 5СоCNT cat-
alyst (Fig. 5) because, owing to its high dispersion, the
largest amount of cobalt is involved in the process,
even though the metal content of the catalyst is fairly
0
5
10 15 20 25 30 35 40
Co content, wt %
low. At the same time, the highest C yield was
Fig. 5. Catalytic activity and С5+ yield as a function of the
cobalt content of the catalyst.
5+
observed with the 10CoCNT. This is due to the fact
that, compared 5СоCNT, this catalyst affords a higher
CO conversion at comparable liquid-product selectiv-
ities.
cobalt on the CNT surface correlates with the number
of oxygen-containing functional groups on the surface.
Thus, the optimum cobalt content of the Со/CNT
catalyst under the conditions examined is 10 wt %,
since this metal content maximizes the heavy hydro-
carbon yield. Further increasing the cobalt content
leads to Co agglomeration, catalyst destabilization,
and loss of activity. The dispersion of cobalt on oxi-
dized CNTs at low metal concentrations is determined
by the degree of oxidation of the support. For a given
support, the cobalt oxide-to cobalt metal reduction
temperature is independent of the cobalt content and
particle size.
The decrease in CO conversion as a result of
increasing the cobalt content of the catalyst from 20 to
4
0% is apparently due to worse stabilization of the
metal and the formation of large Co agglomerates
Fig. 4f), which causes a marked decrease in the num-
(
ber of active sites on the surface. A decrease in CO
conversion at high Co concentrations was also
observed by other authors [23]. These authors attri-
bute this effect to the existence of a “capacity limit”
for the uniform distribution of metal particles on the
surface. As the Со concentration is further increased,
metal particles deposit not on the CNT surface but on
one another to form agglomerates, thus decreasing the
effective surface area of the metal.
ACKNOWLEDGMENTS
This study was part of a program of the Presidium
The literature concerning the dependence of the of the Russian Academy of Sciences and was sup-
heavy hydrocarbon selectivity on the cobalt content of ported by the Russian Foundation for Basic Research
the catalyst is contradictory. It was observed [28] that (project no. 15-03-09029). The authors acknowledge
the С yield decreases with an increasing weight con- partial support from Lomonosov Moscow State Uni-
5+
centration of cobalt. The authors of that study versity Program of Development.
explained this effect by the fact that smaller particles
are more active in the formation of the С fraction.
5+
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Translated by D. Zvukov
KINETICS AND CATALYSIS
Vol. 57
No. 5
2016