Activation of the Surface of Carbon and Nitrogen-Doped Carbon Nanotubes by Calcium Nitrate: Catalytic Properties of Cobalt Supported Catalysts of the Fischer–Tropsch Process Based on Them

Article abstract of DOI:10.1134/S0023158419010129

Abstract: Using the methods of scanning and transmission electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, the influence of concentration, time, and the procedure for the oxidative treatment of the surface of carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs) in the mixture with calcium nitrate was studied. All prepared materials were tested as supports of cobalt-supported catalysts in the Fischer–Tropsch process. It is shown that an increase in the Ca(NO₃)₂ concentration from 15 to 25 wt % leads to the destruction of CNTs, and the time of treatment and oxygen concentration in the oxidizing mixture have almost no effect on the morphology and physicochemical characteristics of the materials. It is found that heterosubstitution in nanotubes and a change in the surface relief of the carbon support lead to an increase in CO conversion, and the catalyst activity increases in the series: Co/CNTs → Co/N-CNTs → Co/CNTs(Ca) → Co/N-CNTs(Ca).

Full text of DOI:10.1134/S0023158419010129

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 1, pp. 87–95. © Pleiades Publishing, Ltd., 2019.
Russian Text © E.V. Suslova, S.V. Savilov, A.V. Egorov, V.V. Lunin, 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 1, pp. 108–117.
Activation of the Surface of Carbon and Nitrogen-Doped Carbon
Nanotubes by Calcium Nitrate: Catalytic Properties of Cobalt
Supported Catalysts of the Fischer–Tropsch Process Based on Them
a,
a
a
a
E. V. Suslova *, S. V. Savilov , A. V. Egorov , and V. V. Lunin
a
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
*e-mail: suslova@kge.msu.ru
Received July 5, 2018; revised September 20, 2018; accepted September 28, 2018
Abstract—Using the methods of scanning and transmission electron microscopy (SEM and TEM), X-ray
photoelectron spectroscopy (XPS) and Raman spectroscopy, the influence of concentration, time, and the
procedure for the oxidative treatment of the surface of carbon nanotubes (CNTs) and nitrogen-doped carbon
nanotubes (N-CNTs) in the mixture with calcium nitrate was studied. All prepared materials were tested as
supports of cobalt-supported catalysts in the Fischer–Tropsch process. It is shown that an increase in the
Ca(NO ) concentration from 15 to 25 wt % leads to the destruction of CNTs, and the time of treatment and
3
2
oxygen concentration in the oxidizing mixture have almost no effect on the morphology and physicochemical
characteristics of the materials. It is found that heterosubstitution in nanotubes and a change in the surface
relief of the carbon support lead to an increase in CO conversion, and the catalyst activity increases in the
series: Co/CNTs → Co/N-CNTs → Co/CNTs(Ca) → Co/N-CNTs(Ca).
Keywords: Fischer–Tropsch process, carbon nanotubes, nitrogen-doped carbon nanotubes, chemical activa-
tion, cobalt
DOI: 10.1134/S0023158419010129
INTRODUCTION
functional groups and/or defects on a CNT surface
and to uniform metal deposition. The resulting cata-
lysts for the Fischer–Tropsch process are character-
ized by high selectivity for to the target C fraction
Currently, carbon nanomaterials (CNMs) are one
of the most promising catalyst supports for a wide
range of processes [1, 2]. For instance, nonstructured
carbon materials (activated carbons [3], carbon fibers
5
+
and stable life cycle [19]. The described approaches
cannot be considered the only possible ones. Physical
[
4, 5], carbon spheres [6], their nitrogen-doped ana-
(
treatment with CO , water vapor or their mixtures at
2
logs [7]) and structured carbon materials (tubes [8],
graphene nanoflakes [9, 10], graphene [11], and oth-
ers) are described as cobalt supports in catalysts for the
Fischer–Tropsch synthesis: production of liquid
hydrocarbons from syngas. For such catalyst supports,
structure defects, condition of a surface, purity, and
the predominance of mesopores are very important
characteristics, since they ensure the uniform distribu-
tion of nanoparticles of a metal catalyst in the course
of their deposition and, as a result, high selectivity [1].
4
50°C) [23] or chemical (treatment with inorganic
salts or alkalies [24]) activation is widely used in the
synthesis of mesoporous carbon nanostructures with
various applications (materials for electrodes of energy
storage devices [20], composite materials [21], adsor-
bents for chromatography and gas storage [22], etc.).
The modification of a CNT surface with calcium
nitrate with further nickel deposition proved to be rather
efficient in the catalytic hydrogenation of 4-chloroace-
tophenone [25]. Landau et al. noted that this has led to
Multi-walled carbon nanotubes (CNTs) meet all the formation of nano-onions on the CNT surface.
the requirements for carbon-based catalyst support: Nano-onions are the sites for additional stabilization
they are chemically inert, thermally stable, and can be of nickel and provide greater catalyst activity [25].
easily compacted. To ensure the effective stabilization There is information on the use of CNTs obtained by
of cobalt nanoparticles on a CNT surface, CNTs are the pyrolytic method on (Fe,Co)/CaCO as iron sup-
3
usually preliminarily treated with nitric acid [12, 13], ported Fischer–Tropsch catalysts [26], and in Ref.
hydrogen peroxide [14, 15], or doped with nitrogen [27], the authors showed that, due to this, iron
[
16, 17] or phosphorus [18] atoms. Alternatively, the nanoparticles did not migrate along the CNT surface
surface already doped CNTs can be oxidized [17]. All and did not sinter in the Fischer–Tropsch process.
these methods of treatment lead to the formation of The cases of postactivation of the CNT surface (e.g.,
87

8
8
SUSLOVA et al.
by KOH) followed by the deposition of palladium par-
ticles for the catalytic oxidation of ethanol were also
described [28]. However, data on the use of CNTs and
their nitrogen-doped analogs with a relief surface as a
support for Fischer–Tropsch cobalt catalysts are not
available in the literature.
In this work, cobalt supported catalysts based on
CNTs and N-CNTs with a relief surface are prepared
for the first time, and the formation of this relief is
studied under various conditions. The resulting mate-
rials are tested in the Fischer–Tropsch process. The
influence of the support microstructure on the main
catalytic characteristics is studied.
CNTs and N-CNTs Synthesis
CNTs and N-CNTs were synthesized by pyrolytic
decomposition of hexane and acetonitrile, respec-
tively, according to the method described in [9].
Before the beginning of synthesis, MgO impregnated
with an aqueous solution of a mixture of cobalt nitrate
and ammonium molybdate was placed in a tubular
quartz reactor (diameter, 50 mm; length, 1.5 m),
purged with nitrogen at a flow rate of 200 mL/min
with a simultaneous temperature increase to 750°C,
after which the nitrogen flow rate was increased to
5
00 mL/min, and the flow was bubbled through hex-
ane or acetonitrile for 5 h. The target products were
cleaned of (Co,Mo)/MgO impurities by boiling the
synthesis products in a solution of hydrochloric acid
for 5–6 h, filtered on a Büchner funnel, and dried at
130°C for 10 h. To remove amorphous impurities, CNTs
and N-CNTs were annealed in air at 400°C for 3 h.
EXPERIMENTAL
Initial Substances and Materials
The materials were used: cobalt (II) nitrate hexa-
hydrate (extra purity grade), calcium nitrate tetrahy-
drate (chemical purity grade), magnesium nitrate
CNTs and N-CNTs Treatment by Calcium Nitrate
(
(
(
chemical purity grade), ammonium molybdate
chemical purity grade), and hydrochloric acid
chemical purity grade), nitric acid (chemical purity
Treatment with calcium nitrate was carried out as
follows. Calcium nitrate solution in tetrahydrofuran
was added to a sample of CNTs or N-CNTs (the con-
centration of Ca was 15 or 25 wt % relative to CNTs or
N-CNTs) and then subjected to ultrasonic treatment
for 1 h, the solvent was evaporated on a rotor. The
mixtures were dried in air at 130°C for 10 h, placed in
a tubular quartz reactor, and heated in a flow of nitro-
gen (200 mL/min) to 500°C. On the CNT surface of
the following reactions occurred [25]:
grade, density 1.4 kg/L), hexane (chemical purity
grade), ethanol (chemical purity grade), hydrogen
(
99.99%), nitrogen (99.999%), He (99.995%), CO
99.95%), and Ar (99.999). Tetrahydrofuran (chemi-
(
cal purity grade) was kept over granulated KOH
(
chemical purity grade) for 7 days, then filtered
through a layer of Al O (for chromatography) and
2
3
distilled over NaA molecular sieves, selecting a frac-
tion a boiling point of 66°C.
Ca
(
NO3 )2 + 2C → CaCO + 2NO + CO ,
3
x
y
CaCo → CaO + CO ↑ .
3
2
Methods of Analysis and Physicochemical Studies
The temperature was changed to 450 or 720°C and
The morphology and composition of CNTs were
determined by scanning electron microscopy (SEM)
and energy dispersive X-ray spectroscopy (EDX)
using a JEOL JSM-6390LA instrument (JEOL,
Japan). Microscopic images were taken at an acceler-
ating voltage of 20–25 kV. Transmission electron
microscopy (TEM) was used to study the structure of
CNTs, N-CNTs, and exhausted Co/CNMs catalysts
using a JEOL 2100F instrument (JEOL, Japan) with
an accelerating voltage of 200 kV.
the N : O mixture with a ratio of 95 : 5 or 70 : 30,
2
2
respectively, flowed through a reactor for 25 min. The
products were cooled to room temperature in a flow of N ,
2
then boiled in hydrochloric acid solutions to remove
calcium, washed in a Büchner funnel to neutral pH of
the wash water and dried at 130°C for 10 h. The EDX
method was used to check that none of the samples
contained even trace amounts of calcium. Table 1
shows the notation for the samples, the conditions for
their treatment by Ca(NO ) ⋅ 4H O, and their physi-
3
2
2
To determine the surface composition of N-CNTs,
X-ray photoelectron spectroscopy (XPS) was used.
The spectra were recorded on an Axis Ultra DLD instru-
ment (Kratos Analytical, UK) using monochromatic
cochemical characteristics.
Catalyst Preparation and Fischer–Tropsch Process
AlK radiation (1486.6 eV). Survey XPS spectra were
α
Catalysts with a cobalt content of 15 wt % were pre-
pared by support impregnation [9, 29]. An alcohol
solution of cobalt nitrate was added to the samples of
CNTs, N-CNTs or CNTs, N-CNTs after treatment
recorded with an analyzer pass energy of 160 eV and a step
of 1 eV. High-resolution spectra were recorded with an
analyzer pass energy of 20 eV and a step of 0.05 eV.
Raman spectra were recorded on a LabRam with calcium nitrate, the samples were treated on the
HR800 UV spectrometer (Horiba–Jobin Yvon, Japan) ultrasound bath at 60°C for 2 h, the solvent was evap-
using a 5-mW excitation by an argon laser with a wave- orated, and then the samples were dried at 130°C for
length of 514 nm. For each experiment, Raman spectra 13 h. Then, 1.5–2.5 g of the catalyst was placed in a
were obtained at least three points and averaged.
quartz tubular reactor (diameter, 10 mm), reduced in
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ACTIVATION OF THE SURFACE
Table 1. Sample notation and their preparation conditions
89
Weight fraction
of Ca, %
#
Notation
Ttreatment, °С
N : O
2
2
1
2
3
4
5
6
7
CNTs_initial
–*
15
15
25
25
–*
15
–**
450
720
450
720
–**
720
–**
CNTs(15Ca_450)
CNTs(15Ca_720)
CNTs(25Ca_450)
CNTs(25Ca_720)
N-CNTs_initial
70 : 30
95 : 5
70 : 30
95 : 5
–**
N-CNTs(15Ca_720)
95 : 5
*
Calcium is not available in the sample composition.
*
* Heat treatment was not carried out.
a flow of hydrogen at 400°C for 4 h, cooled in a stream preparations were detected depending on the Ca con-
of hydrogen to 205°C, and the mixture with the ratio centration and the temperature of the subsequent oxi-
–1
CO : H : N = 1 : 2 : 0.7 (overall flow rate, 1200 h ; dative annealing. With an increase in calcium content
2
2
from 15 to 25 wt % in the treated mixtures, the mor-
phology of carbon nanomaterial samples changed sig-
nificantly: ragged defects appeared due to the destruc-
tion of several upper layers of CNTs, which was con-
firmed by TEM (Figs. 2a–2d), against the background
of significant degradation of the original CNT struc-
nitrogen served as an internal standard) passed
through the reactor at an atmospheric pressure. The
duration of the experiments was at least 3 days. The
content of CO, CO , and hydrocarbons in the prod-
2
ucts was determined on an Agilent 6890N gas chro-
matograph (Agilent, USA) equipped with a DB5 col-
umn and a flame ionization detector for separation
and quantitative analysis and a HayeSep column with
ture. Note that the temperature and the N : O ratio in
2
2
the composition of the oxidizing mixture had a smaller
effect on the nature of the CNT surface.
a Porapak R sorbent for separation and analysis of N ,
2
CO, CO , and CH . The calculation of the product
In the case of N-CNTs, in oxidative conversion,
2
4
yields was carried out in accordance with the method morphology changes are less pronounced (Figs. 2d, 3e).
described in [29]. In the C fraction, C –C hydro- At the same time, in the treatment of N-CNTs, the
5+
5
15
carbons were detected. Exhausted catalysts were pas- composition of surface graphene layers is significantly
sivated with a mixture of 1% O /Ar at room tempera- modified. According to XPS, the nitrogen content in
2
the N-CNT (15Ca_720) after treatment decreased from
ture for their further analysis by TEM.
1.58 to 1.02 at % (Fig. 3a). It cannot be excluded that this
result is partly due to the thermal effect. It was previously
found that as a result of N-CNT treatment at 1000°C,
the amount of nitrogen in the N-CNT composition
significantly decreases, which the authors of [31] asso-
ciated with the elimination of molecular nitrogen
encapsulated between N-CNT layers and nitrogen in
RESULTS AND DISCUSSION
Physicochemical Characteristics of CNTs and N-CNTs
before and after Oxidative Treatment
with Calcium Nitrate
The morphology of the obtained CNTs and the pyridine states. In the N1s XPS spectra obtained for
N-CNTs was studied by SEM and TEM. According to our preparations, we can distinguish five basic nitrogen
states: pyridine state with energies of 390.1–399.3 eV,
SEM, the CNT thickness was 14–17 nm, while the
CNTs itself were tangled into agglomerates of ~5 μm pyrrole state (399.8–401.5 eV), substitution state
401.1–402.7 eV), nitrogen oxides (>402.6 eV) [32],
and nitrogen encapsulated between layers (404.3 eV)
(
(
Fig. 1a). According to the TEM, the thickness of the
internal channel was 7–12 nm, and the number of car-
bon layers parallel to the internal channel was 6–10 [33]. Nitrogen in these states is present in N-CNTs both
before and after treatment, and their relative fractions
remain almost unchanged (Fig. 3b).
(
Fig. 1b). According to the SEM, the N-CNT thick-
ness was 15–20 nm (Fig. 1c). N-CNTs had a bamboo
structure typical of nitrogen-doped CNTs, character-
ized by multiple bridges in the internal channels
For bulk characterization of all the studied carbon
nanostructures, one can use Raman spectroscopy. In
the Raman spectrum of CNMs, usually three main
(
Fig. 1d). This is due to the stress arising from the
incorporation of nitrogen atoms into graphene planes:
to minimize surface energy, graphene layers are
deformed and bent [30].
–1
lines are observed: the G line at 1550–1600 cm ,
characterizing tangential C–C vibrations, the D line at
–1
–1
1
250–1350 cm and G' line at 2700 cm correspond-
Based on the example of CNTs treatment with cal- ing to asymmetric vibrations of C–C bonds in the
cium nitrate, changes in the characteristics of the region of defects in the structure of carbon nanomate-
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9
0
SUSLOVA et al.
(а)
1 µm
(b)
10 nm
(c)
1 µm
(d)
20 nm
Fig. 1. SEM and TEM images of fresh (a, b) CNTs and (c, d) N-CNTs.
rials [34]. The Raman spectra of CNTs and N-CNTs
The CO conversion increased almost 2.5 times in
before and after their treatment with 15 wt % the case of CNT surface modification with calcium
Ca(NO ) remained almost unchanged. The ratio of (15Co/CNTs and 15Co/CNTs(15Ca_720) catalysts) and
3
2
slightly decreased with calcium treatment of N-CNTs
15Co/N-CNTs (15Ca_720) catalysts). Note that
intensities (I) of D and G peaks slightly decreased, and
the ratio of intensities of G' and G peaks increased for
both CNTs and N-CNTs, which indicates the
improvement of the structure (Fig. 4). Earlier, similar
results were obtained in [31]: the I /I ratio of
(
both approaches (the introduction of nitrogen hetero-
atoms into the carbon layers and the CNT surface
modification with calcium nitrate) led to a significant
increase in the CO conversion. It is known [35, 36]
that with an increase in the defectiveness of a carbon
support, which is favored by the tubular structure of
CNTs, the number of valence-unsaturated carbon atoms
that participate in the chemisorption of reagent mole-
cules increases. The approach proposed in this work is
aimed at increasing the defectiveness of the surface.
D
G
N-CNTs before and after heat treatment at 1000°C
remained almost unchanged.
Characteristics of Cobalt Supported
Fischer–Tropsch Catalysts
All synthesized catalysts were tested in the
The activity of the catalysts grew both with the
Fischer–Tropsch process at an atmospheric pressure introduction of nitrogen and with the treatment of
and 205°С. The CO conversion, activity, and selectiv- support by calcium nitrate. The most active was
ity to various products were calculated (Table 2).
15Co/N-CNTs(15Ca_720) catalyst. The values of CO
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ACTIVATION OF THE SURFACE
91
(а)
10 nm
(b)
10 nm
(c)
10 nm
(d)
10 nm
(e)
10 nm
Fig. 2. TEM images of the samples: (a) CNTs(15Ca_450), (b) CNTs(15Ca_720), (c) CNTs(25Ca_450), (d) CNTs(25Ca_720),
e) N-CNTs(15Ca_720).
(
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SUSLOVA et al.
Pyridine N
(а)
N
(
b)
N-CNTs(15Ca_720)
0
0
0
0
0
.6
.5
.4
.3
.2
N-CNTs
N-CNTs(15Ca_720)
Pyrrole N
N
H
N-CNTs
0
.1
Substitution N
0
4
12 408 404 400 396 392
N+
Binding energy, eV
Nitrogen content
Fig. 3. (a) N1s XPS spectra of N-CNTs and N-CNTs(15Ca_720) samples; (b) the ratio in the concentration of nitrogen atoms in
N-CNTs before and after treatment.
D
G
G′
N-CNTs(15Ca_720)
N-CNTs_initial
CNTs(15Ca_720)
CNTs(15Ca_450)
CNTs_initial
1000
1500
2000 2500
3000
Raman shift, cm–1
Fig. 4. Ramam spectra of the CNTs, CNTs(15Ca_450), CNTs(15Ca_720), N-CNTs, and N-CNTs(15Ca_720) samples.
conversion in the presence of all the three catalysts
The selectivity of catalysts strongly depends on the
1
5Co/CNTs(15Ca_720), 15Co/N-CNTs, and 15Co/N- size and narrowness of the distribution of metal parti-
CNTs(15Ca_720) were close. The 15Co/N- cles [15, 37]. Figure 5 shows TEM images of all
CNTs(15Ca_720) catalyst showed the lowest selectiv- Fischer–Tropsch catalysts. Note that, in none of the
ity to the target C fraction.
5+
cases was it possible to achieve a narrow size distribu-
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Table 2. CO conversion and selectivity in the presence of supported cobalt catalysts
Activity,
93
Selectivity
Catalyst
CO conversion, %
molC5+ s –1
−1
× 10–6
g
Co
CO2
CH4
C –C
C5+
2
4
1
5Co/CNTs*
5Co/CNTs(15Ca_720)
5Co/N-CNTs
5Co/N-CNTs(15Ca_720)
4.6
11.1
13.3
12.9
0.70
1.51
2.01
2.11
6.3
4.3
7.5
5.2
18.5
20.0
16.3
28.1
13.6
8.0
61.6
69.0
67.5
53.5
1
1
8.7
1
13.9
Reaction conditions: Т = 205°С and atmospheric pressure. * Data published earlier in [29].
tion of Co particles. In all samples, both particles with sites for Co (Fig. 3a). Based on this assumption, it
a diameter of 1–2 nm and larger agglomerates (30– becomes obvious why a decrease in the total nitrogen
5
0 nm) were present. The 15Co/CNTs catalyst was a content in the support from 1.58 at % for N-CNTs to
mixture of Co particles, 6–15 nm in size, that were 1.02 at % for N-CNTs(Ca) led to a lower CO conver-
located in the pores and voids between the CNTs sion (Table 2). In this study, the Co particle size in the
(
Fig. 5a). When using CNTs without any modifica- composition of the 15Co/N-CNTs catalyst was ~5 nm.
tions, the catalyst metal particles weakly interact with Some of these particles were encapsulated inside the
the hydrophobic carbon surface of CNTs. The metal channels (Fig. 5c). Similar results were previously
particles migrate and agglomerate in the Fischer– described in catalytically active Co and Fe particles
Tropsch process, which significantly deteriorates the encapsulated inside the CNT and N-CNT channels [16,
catalyst performances [38]. However, with excessively 36, 46, 47]. Note that the 15Co/N-CNTs(15Ca_720)
strong interaction, which is achieved, for example, by catalyst practically did not contain encapsulated
a high Co dispersion on the surface of CNTs with a cobalt, the Co dispersion over the surface was higher
significant content of oxygen-containing groups, it than that of the other catalysts studied (Fig. 5d). This
turns out that Co does not crystallize and does not cat- is probably due to the higher concentration of oxygen
alyze the reductive conversion of CO and CO [9, 39]. on the surface of the N-CNTs(15Ca_720) support due
2
The CNT surface is usually modified to stabilize Co or to oxidation of the N-CNTs surface with nitrogen
other metal nanoparticles. Namely, oxygen-contain- oxides in the course of calcium nitrate treatment.
ing groups are introduced [5, 40]. However, during the According to XPS, the N-CNTs sample contained
operation of the catalyst, some of these groups are 1.54 at % oxygen on the surface, and the N-CNTs(Ca)
eliminated from the CNT surface, which leads to cat- sample contained 1.65 at % oxygen.
alyst degradation [40].
After treatment with calcium nitrate, Co particles
with sizes of 1–2 nm are stabilized on the CNT and
N-CNT surfaces, including the sites of ragged defects
One of the main tasks in the study and optimization
of supported catalysts can be considered a search for a
balance between the metal–support interaction and
the number of active sites. With the introduction of
nitrogen atoms, it becomes possible to significantly
increase the surface hydrophilicity [41] and achieve a
significantly higher dispersion of Co or other metal
particles on the surface [16, 18, 42–45]. For example,
in Ref. [16], the size of Co particles dispersed on the
CNT surface oxidized with nitric acid was 16.2 nm.
When nitrogen atoms were introduced in addition to
surface oxidation, it became 5.4 nm. Nitrogen atoms
in different states are involved in the stabilization of
metal particles in a different manner. A lone pair of
electrons in pyrrole nitrogen in the N-CNT structure
is involved in the conjugation of the π-system and,
therefore, its component has little or no effect (if we
take into account the electron redundancy of the
cycle) on the stabilization of Co particles in the cata-
(
Figs. 5b, 5d). It can be assumed that these particles
contribute to a strong increase in catalyst activity,
which was previously found in Ref. [27]. In this work,
in the presence of 15Co/CNTs(15Ca_720) and
1
5Co/N-CNTs(15Ca_720) catalysts, the highest
selectivity to methane was observed. Note that, in Ref.
16], an increased yield of methane was found in the
[
presence of a catalyst supported on nitrogen-doped
CNTs compared with a catalyst based on CNTs oxi-
dized by nitric acid. A decrease in CO conversion in
the presence of the 15Co/N-CNTs(15Ca_720) cata-
lyst compared to 15Co/N-CNTs is probably due to the
formation of cobalt agglomerates (Fig. 5d) and, as a
result, a decrease in the number of active sites. Of all
the samples studied in this work, the 15Co/N-
CNTs(15Ca_720) catalyst proved to be the most selec-
tive toward the target C fraction. The selectivity to
5+
lyst. The lone electron pair in pyridine nitrogen and CO is 31–32% higher for both Ca(NO ) -treated cat-
2 3 2
the uncompensated charge in the substitution nitro- alyst supports 15Co/CNTs(15Ca_720) and 15Co/N-
gen create additional coordination and stabilization CNTs(15Ca_720) compared to their nontreated ana-
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SUSLOVA et al.
(а)
10 nm
(b)
50 nm
(c)
20 nm
(d)
20 nm
Fig. 5. TEM images of exhausted and passivated supported cobalt catalysts: (a) 15Co/CNTs, (b) 15Co/CNTs(15Ca_720),
c) 15Co/N-CNTs, (d) 15Co/N-CNTs(15Ca_720).
(
logs. For the 15Co/CNTs(15Ca_720), 15Co/N- nitrogen atoms, as well as the chemical activation of
CNTs, and 15Co/N-CNTs(15Ca_720) catalysts, the the CNTs and N-CNTs surfaces by calcium nitrate
selectivity to CO increased with increasing CO con-
2
followed by oxidative heat treatment, affects the cata-
version. The same results were previously obtained in lytic parameters of supported cobalt catalysts. In both
Ref. [37, 48] and can be explained by an increase in the
yield of water, one of the products of CO hydrogena-
tion, which then enters the water-gas shift reaction.
approaches, an increase in the catalyst activity was
achieved.
ACKNOWLEDGMENTS
CONCLUSIONS
We are grateful to Drs. S.A. Chernyak and
K.I. Maslakov for obtaining SEM images and XPS
spectra, as well as Dr. A.S. Ivanov for valuable com-
In conclusion, it should be noted that the Fischer–
Tropsch process is structure-sensitive [49] and
depends on almost all physicochemical parameters of
catalysts and their supports. In this work, it was shown ments and discussion. The work was supported by the
how heterosubstitution in the CNT structure with Russian Science Foundation (project no. 18-13-
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ACTIVATION OF THE SURFACE
95
0
0217) and used equipment purchased due to the 25. Landau, M., Savilov, S., Ivanov, A., Lunin, V., Titel-
man, L., Koltypin, Y., and Gedanken, A., J. Mater.
Sci., 2011, vol. 46, p. 2162.
Moscow State University Development Program.
2
6. Motchelaho, M., Xiong, H., Moyo, M., Jewell, L., and
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Translated by Andrey Zeigarnik
KINETICS AND CATALYSIS Vol. 60 No. 1 2019