Effect of Re and Al2O3 Promotion on the Working Stability of Cobalt Catalysts for the Fischer–Tropsch Synthesis

Article abstract of DOI:10.1134/S0023158420020111

Abstract: The results of the working stability studies of cobalt catalysts based on SiO₂ and Al₂O₃ promoted with Re and Al₂O₃ in the synthesis of hydrocarbons from CO and H₂ in continuous tests for 200–300 h are presented. The prepared catalysts were characterized by transmission electron spectroscopy, temperature-programmed reduction with hydrogen, temperature-programmed desorption of CO, and X-ray fluorescence spectroscopy and tested at a temperature 200°C, a pressure of 0.1 MPa, and a GHSV of 100 h^–1. It was determined that a cobalt–silica catalyst promoted with Al₂O₃ had the highest activity. It was established that the addition of Al₂O₃ to a cobalt–silica catalyst increased the conversion of CO and selectivity for C₅₊ hydrocarbons and inhibited the agglomeration of Co particles under the action of a reaction atmosphere in the Fischer–Tropsch synthesis. It was found that the initial conversion of CO increased by a factor of 2 upon the introduction of 0.1 wt % rhenium into the Co/γ-Al₂O₃ catalyst; however, the rate of its deactivation increased in this case due to an almost twofold increase in the size of cobalt particles in the course of synthesis after operation for 300 h.

Full text of DOI:10.1134/S0023158420020111

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 2, pp. 310–317. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Kinetika i Kataliz, 2020, Vol. 61, No. 2, pp. 278–286.
Effect of Re and Al2O3 Promotion on the Working Stability of Cobalt
Catalysts for the Fischer–Tropsch Synthesis
R. E. Yakovenkoa, *, I. N. Zubkova, G. B. Narochniya, O. P. Papetaa,
O. D. Denisova, and A. P. Savost’yanova
aPlatov South-Russian State Polytechnic University (NPI), Novocherkassk, 346428 Russia
*е-mail: jakovenko@lenta.ru
Received July 9, 2019; revised October 18, 2019; accepted October 25, 2019
Abstract—The results of the working stability studies of cobalt catalysts based on SiO2 and Al2O3 promoted
with Re and Al2O3 in the synthesis of hydrocarbons from CO and H2 in continuous tests for 200–300 h are
presented. The prepared catalysts were characterized by transmission electron spectroscopy, temperature-
programmed reduction with hydrogen, temperature-programmed desorption of CO, and X-ray fluorescence
spectroscopy and tested at a temperature 200°C, a pressure of 0.1 MPa, and a GHSV of 100 h–1. It was deter-
mined that a cobalt–silica catalyst promoted with Al2O3 had the highest activity. It was established that the
addition of Al2O3 to a cobalt–silica catalyst increased the conversion of CO and selectivity for C5+ hydrocar-
bons and inhibited the agglomeration of Co particles under the action of a reaction atmosphere in the
Fischer–Tropsch synthesis. It was found that the initial conversion of CO increased by a factor of 2 upon the
introduction of 0.1 wt % rhenium into the Co/γ-Al2O3 catalyst; however, the rate of its deactivation increased
in this case due to an almost twofold increase in the size of cobalt particles in the course of synthesis after
operation for 300 h.
Keywords: Fischer–Tropsch synthesis, cobalt catalyst, working stability, C5+ hydrocarbons
DOI: 10.1134/S0023158420020111
INTRODUCTION
The stable operation of catalysts in a continuous
mode is important for the industrial use of catalysts in
order to produce C5+ hydrocarbons. Recently, a tech-
nology was developed and a pilot batch of cobalt–sil-
ica gel catalysts promoted with alumina was manufac-
tured at ZAO Samara Catalyst Plant [19]. It was of
interest to determine the stability of this catalyst and
compare it with that of well-known samples.
Therefore, the aim of this work was to study the
working stability of cobalt catalysts on various sup-
ports in the synthesis of hydrocarbons from CO and
H2 in continuous long-term tests.
The development of technologies for the produc-
tion of synthetic hydrocarbons based on the Fischer–
Tropsch (FT) synthesis is of considerable current
interest primarily due to the need of converting non-
traditional carbon-containing resources (associated
petroleum gas, coal, biomass, etc.) and more stringent
requirements imposed on the products obtained [1–
3]. The FT synthesis is a heterogeneous catalytic pro-
cess, which allows one to obtain a wide range of hydro-
carbon products from CO and H2 depending on the
catalyst used and synthesis conditions [4, 5]. Sup-
ported cobalt catalysts based on Al2O3, SiO2, and TiO2
are most widely used in industry [6–8]. With the use
of supports based on Al2O3 and TiO2, a portion of
cobalt forms difficult-to-reduce compounds with the
support, which are inactive in the FT synthesis [9, 10].
It is possible to decrease the degree of metal–support
interactions and increase selectivity for the formation
of target products by the introduction of promoters—
the noble metals Pt, Re, and Ru [16–18].
EXPERIMENTAL
Preparation of Hydrocarbon Synthesis Catalysts
The following four FT synthesis catalysts were pre-
pared: Co/SiO2, Co–Al2O3/SiO, Co/γ-Al2O3, and
Co–Re/γ-Al2O3.
The catalysts were prepared by the impregnation of
porous supports. KSKG silica gel from OOO Salavat
Catalyst Plant (GOST [State Standard] 3956-76) and
AN γ-Al2O3 from OOO Novomichurinsk Catalyst
Plant (TU [Technical Specifications] 2163-142-
60201897-2010) were used as the supports.
Abbreviations: FT, Fischer–Tropsch synthesis; TEM, transmis-
sion electron microscopy; TPR-H , temperature-programmed
2
reduction with hydrogen, TPD, temperature-programmed
desorption; XRF, X-ray fluorescence analysis; and GHSV, gas
hourly space velocity.
310

EFFECT OF Re AND Al2O3 PROMOTION ON THE WORKING STABILITY
311
An impregnation solution of cobalt nitrate CO was carried out by increasing the temperature from
(Co(NO3)2) with a concentration of 55% was used to 100 to 800°C at a rate of 20 K/min.
prepare the catalysts. The support granules were
immersed in the impregnation solution of cobalt
nitrate and kept for 0.5 h at a temperature of 70–80°С.
Rhenium was introduced into the catalysts by coim-
pregnation with cobalt nitrate and ammonium per-
rhenate (NH4ReO4). The concentration of ammo-
nium perrhenate in the impregnation solution was
maintained in such a way that the Re content of the
prepared catalyst was 0.1 wt %. The procedure used for
preparing the Co–Al2O3/SiO2 catalyst included the
joint impregnation of the support with cobalt and alu-
minum nitrates [19].
After impregnation, an excess of the impregnation
solution was removed, and the wet catalyst granules
were subjected to heat treatment under the tempera-
ture-programmed conditions: 80°C, 4 h; 100°С, 1 h;
120°С, 1 h; 140°С, 1 h; and 400°С, 4 h; the heating
rate was 10 K/min.
The study of the catalysts by transmission electron
microscopy (TEM) was carried out using a Tecnai G2
Spirit BioTWIN microscope (FEI, the Netherlands)
with an accelerating voltage of 120 kV. A sample of the
initial catalyst was preliminarily reduced with a nitro-
gen–hydrogen mixture (10% H2 + 90% N2) by linear
heating from room temperature to 500°C for 1 h. The
samples were ground in a flow of CO2 and applied to
copper gauze in the form of suspensions (isopropyl
alcohol–catalyst).
The particle size distribution of cobalt crystallites
in prereduced catalysts was determined using trans-
mission electron microscopy. The average crystallite
size was calculated from the following formula [20]:
ndi3
i
0

(1)
dav(Co ) =
,
2
nd
i
i
where ni is the number of particles with a diameter di.
The dispersity of cobalt (D, %) was calculated
Catalyst Characterization Procedures
based on these data [21]:
The cobalt content of the catalyst samples was
determined by X-ray fluorescence analysis (XRF) on
an ARLQUANT’X spectrometer (Thermo Scientific,
Switzerland) under the following conditions: an atmo-
sphere of air, a Teflon substrate, and an effective irra-
diation area of 48.9 mm2.
The freshly prepared catalysts were studied by tem-
perature-programmed reduction with H2 (TPR-H2)
using a ChemiSorb 2750 analyzer (Micromeritics, the
United States) with a thermal conductivity detector
(TCD). A sample of 0.1–0.15 g was placed in a quartz
reactor arranged in a temperature-programmed fur-
nace. Before the onset of TPR-H2, the catalyst sample
was kept in a flow of He (20 mL/min) for 2 h at a tem-
perature of 200°С to remove moisture and other
adsorbed gases. Then, the sample was cooled to room
temperature and the flow was switched to a mixture of
10% H2 and 90% N2 (20 mL/min). The reduction pro-
cess was studied in a temperature range of 20–800°C
with a heating rate of 5 K/min.
The catalysts were studied by the temperature-pro-
grammed desorption (TPD) of CO using a ChemiSorb
2750 analyzer (Micromeritics, the United States) with
a thermal conductivity detector (TCD). A sample of
about 0.1 g was placed in a U-shaped quartz reactor
and kept in a flow of helium (20 mL/min) for 2 h at a
temperature of 200°С to remove moisture and other
adsorbed gases. Reduction with a hydrogen–nitrogen
mixture (10% H2 and 90% N2) was carried out for 1 h
at a temperature of 400°C. The pulse adsorption of CO
was carried out at 20°С in a flow of helium
(20 mL/min) until the surface of the sample was satu-
rated. After saturation, the sample was purged with
helium (20 mL/min) for 1 h at a temperature of 100°C
to remove physically adsorbed gas. The desorption of
96
(2)
D =
,
dav(Co0)
where dav(Co0) is the average crystallite size of metallic
cobalt, nm.
The standard deviation was determined from the
formula [30]
Σin (di − d(Co0))2
i
(3)
σ =
.
 n
i
Hydrocarbons were removed from the pores of cat-
alysts after the synthesis (washed out) in a Soxhlet
extractor. For this purpose, a Schott filter with a cata-
lyst sample was placed in a reservoir located at the cen-
ter of the extractor. 1,2-Dimethylbenzene (o-xylene)
and n-heptane were used as solvents. The catalyst
washing procedure included the extraction of hydro-
carbons with o-xylene (2 h at a temperature of 144°C)
and then extraction with n-heptane for 1 h at a tem-
perature of 98°C.
The Fischer–Tropsch synthesis of hydrocarbons
was carried out in a steel flow reactor (din = 16 mm)
with a fixed catalyst bed (10 cm3) at atmospheric pres-
sure (0.1 MPa), a GHSV of 100 h–1, and a constant
temperature (T) of 200°C. The catalyst samples were pre-
liminarily reduced in a flow of H2 at GHSV= 3000 h–1 and
T = 400°С for 1 h. The catalysts were activated with
synthesis gas by a stepwise increase in the temperature
(2.5 K/h) from 150 to 200°С. After the catalyst activa-
tion, the synthesis was carried out in a continuous
mode for 300 h.
The rate of catalysts deactivation was judged from
the parameter RCD (%/h), which shows a decrease in
KINETICS AND CATALYSIS Vol. 61 No. 2 2020

312
YAKOVENKO et al.
Table 1. Physicochemical properties of the starting and
Figure 1 shows the TEM images and particle-size
distribution diagrams of cobalt crystallites for reduced
catalysts.
spent catalysts
Starting
Spent
С(Со),
Cobalt particles 5–20 nm in size occurred on the
surface of the Co/SiO2 catalyst (Fig. 1a); the average
particle size was 14.8 nm (Table 1).
Sample
*D,
*D,
%
*dav, nm
*dav, nm
%
%
Co/SiO2
21.9
22.8
19.9
19.7
6.5
10.9
4.4
–
7.1 2.0
–
14.8 2.8
8.8 1.6
21.8 5.9
13.9 3.6
The addition of Al2O3 to the silica gel catalyst (Fig. 1b)
significantly changed the dispersity of the active com-
ponent: its value increased from 6.5 to 10.9%. The
introduction of alumina facilitated a decrease in the
cobalt particle size and narrowed the particle-size dis-
tribution. It is likely that an additive of 1.0 wt % Al2O3
inhibited the aggregation of cobalt metal nanoparti-
cles, which manifested itself in the observed decrease
in the average particle size. The distribution became
almost unimodal; the average cobalt particle size was
8.8 nm, which is considered optimal for FT synthesis
catalysts [22].
Co–Al2O3/SiO2
Co/γ-Al2O3
Co–Re/γ-Al2O3
13.5
3.5
6.9
27.6 5.2
* According to TEM data. C(Co), % is the concentration of cobalt
in the catalyst; d , nm is the Co crystallite size; and D, % is the
av
dispersity of cobalt. Dashes indicate that the corresponding values
were not determined.
the degree of conversion of CO after catalyst operation
for 1 h. RCD was determined using the formula
The particles of cobalt in the Co/γ-Al2O3 catalyst
(Fig. 1c) had an average size of 21.8 nm (Table 1). The
introduction of rhenium into the composition of
Co/γ-Al2O3 (Fig. 1d) led to a significant decrease in
the average size of cobalt nanoparticles from 21.8 to
13.9 nm (Table 1) and, as a consequence, to an
increase in dispersity from 4.4 to 6.9%. The data on
the effect of the addition of rhenium to the γ-Al2O3-
based catalyst are consistent with published results
[23–27].
XCi O − XCf O
(4)
RCD
=
,
τ
i
where
is the initial degree of conversion of CO,
XCO
f
%;
is the final degree of conversion of CO, %; and
τ is time, h.
The synthesis gas and gaseous synthesis products
were was analyzed by gas-adsorption chromatography
on a Kristall 5000 chromatograph (Khromatek, Rus-
sia) with a thermal conductivity detector, a Hayesep R
column (Khromatek, Russia), and a column with NaX
molecular sieves. The former column was used for the
analysis of С1–С5 hydrocarbons and СО2 (carrier gas,
helium; flow rate, 15 mL/min), and the latter was used
for the analysis of CO, Н2, and N2 (carrier gas, argon;
XCO
Figure 2 and Table 2 show the TPR-H2 curves of
catalyst samples and their characteristics, respectively.
The TPR-Н2 curves of all of the samples contained
two intense reduction peaks (Fig. 2) due to the Co3+ →
Co2+ and Co2+ → Co0 transitions in ranges of 262–324
and 337–583°С, respectively. A shift of peaks 1 and 2
flow rate, 15 mL/min) under temperature-program- to a range of higher temperatures was observed for the
ming conditions (80–240°C; heating rate, 8 K/min).
Co–Al2O3/SiO2 catalyst. This can be a consequence of
interactions between the supported particles of cobalt
oxide and Al2O3 [28].
RESULTS AND DISCUSSION
The ratio S2/S1 between the areas of the two main
peaks (Table 2) of SiO2-based catalysts, which indi-
cates the amount of hydrogen consumed in the
Table 1 summarizes the characteristics of the pre-
pared and spent catalysts. These data indicate that the
cobalt content of the test catalysts varied in a range of Co3+ → Co0 transition, is 3, which corresponds to a
19.7–22.8%.
theoretically expected value based on the reaction
Table 2. Characterization of the TPR-H2 profiles of the catalysts
Peak 1
Peak 2
S2/S1
Catalyst
area S1, %
area S2, %
Т, °С
Т, °С
Co/SiO2
262
295
319
324
25.0
25.0
51.1
50.4
337
371
583
524
75.0
75.0
48.9
49.6
3.00
3.00
0.96
0.98
Co–Al2O3/SiO2
Co/γ-Al2O3
Co–Re/γ-Al2O3
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EFFECT OF Re AND Al2O3 PROMOTION ON THE WORKING STABILITY
(а) (b)
313
100 nm
100 nm
Concentration, %
Concentration, %
20
18
16
14
12
10
8
35
30
25
20
15
10
5
6
4
2
0
0
0
2
4
6
8
10 12 14 16 18 20
Particle size, nm
0
2
4
6
8
10 12 14 16 18 20
Particle size, nm
(c)
(d)
200 nm
200 nm
Concentration, %
Concentration, %
20
18
16
14
12
10
8
20
18
16
14
12
10
8
6
6
4
4
2
2
0
0
0
4
8
12 16 20 24 28 32
Particle size, nm
0
2
4
6
8 10 12 14 16 18 20 22 24
Particle size, nm
Fig. 1. TEM micrographs of reduced catalysts and bar diagrams of cobalt particle-size distributions: (a) Co/SiO , (b) Co–
2
Al O /SiO , (c) Co/γ-Al O , and (d) Co–Re/γ-Al O .
2
3
2
2
3
2 3
KINETICS AND CATALYSIS Vol. 61 No. 2 2020

314
Intensity
YAKOVENKO et al.
Intensity
324
319
689
672
524
4
3
4
3
583
337
371
295
772
317
2
1
744
220
207
2
337
262
1
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
800
Temperature, °С
Temperature, °С
Fig. 2. TPR-H curves of the catalysts: (1) Co/SiO ,
Fig. 3. TPD of CO profiles of the catalysts: (1) Co/SiO ,
2
2
2
(2) Co–Al O /SiO , (3) Co/γ-Al O , and (4) Co–Re/γ-
(2) Co–Al O /SiO , (3) Co/γ-Al O , and (4) Co–Re/γ-
2 3 2 2 3
2
3
2
2
3
Al O .
Al O .
2 3
2
3
stoichiometry of the stepwise reduction of cobalt oxide formed from chemisorbed CO by the Boudoir reaction
Co3O4 to Co0 [29].
[30]. The introduction of Al2O3 and Re promoters into
the catalysts led to a significant increase in the amount
of desorbed CO in the temperature range of the
appearance of the first peak. Lapidus et al. [31] found
that the first peak in the TPD of CO profile corre-
sponds to the weakly bound adsorption of CO in a
molecular form on the oxide catalyst surface. It char-
acterizes bifunctional active surface centers involved
in the process of polymerization.
The addition of rhenium to the Co/γ-Al2O3 catalyst
led to a shift in the temperature maximum of peak 2
toward a lower temperature without a significant effect
on the reduction temperature of cobalt oxide Co3O4 to
CoО (peak 1). The experimental data on the effect of
the rhenium promotion of γ-Al2O3-based catalysts on
the reduction of CoO to Co0 are consistent with the
conclusions of Tavasoli et al. [23] on the role of rhe-
nium as a reduction promoter. Smaller S2/S1 area
ratios for these catalysts may indicate that a portion of
cobalt occurred in a difficult-to-reduce form.
Table 3 shows the average values of the main
parameters of the catalytic activity of samples in the
synthesis of hydrocarbons for 300 h of continuous
operation.
The adsorption of CO on reduced catalysts was
investigated using the TPD of CO. Figure 3 shows the
TPD curves for CO from the reduced samples.
All of the test catalysts were active in the synthesis
of hydrocarbons from CO and H2. Under the experi-
mental conditions, Co–Al2O3/SiO2 was most active:
the conversion of CO was 78.6%, and selectivity and
productivity for C5+ hydrocarbons were 64.2% and
All of the catalyst samples exhibited pronounced
profiles of the temperature-programmed desorption
of CO. Two major desorption peaks were observed.
The first low-temperature peak with a maximum at
207–220°C for catalysts based on SiO2 or 317–337°C
for samples based on γ-Al2O3 can be attributed to the
desorption of chemisorbed CO. The second high-
temperature peak with a maximum at 672–772°C
probably appeared as a result of the desorption of CO2
11.3 kg m–3 h–1, respectively. Promotion with alumina
led to a 10.8% increase in the catalyst productivity due
to an increase in the conversion of CO and selectivity
for C5+. In addition, the rate of catalyst deactivation
decreased by 9.6% upon the introduction of alumina.
Table 3. Activity of the catalysts in the Fischer–Tropsch synthesis*
Selectivity, %
Productivity
of С5+, kg m–3 h–1
СО conversion,
%
RCD, %/h
Sample
СН4
С2–С4
С5+
СО2
Co/SiO2
75.3
78.6
41.9
18.0
16.5
14.4
17.9
15.4
15.4
12.1
10.7
62.8
64.2
71.1
65.4
3.8
3.9
2.4
6.0
0.052
0.047
0.035
0.102
10.2
11.3
6.4
Co–Al2O3/SiO2
Co/γ-Al2O3**
Co–Re/γ-Al2O3
68.5
9.5
–1
* Conditions: 0.1 MPa, 100 h , and 200°C.
** The catalyst was tested in a continuous mode for 200 h.
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EFFECT OF Re AND Al2O3 PROMOTION ON THE WORKING STABILITY
315
decrease in selectivity for C5+ hydrocarbons due to
increased formation of methane and CO2.
(а)
Conversion of CO, %
An increase in selectivity for methane with time
can be due to the accumulation of liquid C5+ hydro-
carbons in the pores of the catalyst, which may lead to
a decrease in the number of available active chain
growth centers [32]. On the other hand, as noted by
Iglesia et al. [36], the accumulation of synthesis prod-
ucts in the pores of the catalyst facilitated an increase
in the molar ratio H2/CO inside a catalyst granule due
to the different diffusion rates of the reactants. Indeed,
Khasin [37] found that the diffusion coefficients of
hydrogen and CO through a film of n-tetradecane at
200°С are 1.6 × 10–4 and 2.2 × 10–4 m2/s, respectively.
This leads to an increase in the ratio Н2/СО on the
catalyst surface and an increase in selectivity for meth-
ane formation [38].
The first 50 h of synthesis were characterized by the
highest rate of decrease in the activity of all of the sam-
ples. It is likely that the active component of the cata-
lyst was stabilized during this period under the influ-
ence of a reaction atmosphere.
The Co/SiO2 and Co–Al2O3/SiO2 catalysts exhib-
ited similar initial conversions of CO and selectivity
for the formation of C5+ hydrocarbons and CH4. How-
ever, after ~250 h of synthesis, a sharp decrease in
activity was observed in the Co/SiO2 sample, which
was accompanied by an increase in the fraction of
methane formed and a decrease in the productivity of
С5+ hydrocarbons.
100
90
80
70
60
50
40
3
20
10
1
2
3
4
300
0
50
100
150
(b)
200
250
Selectivity for CH4, %
30
25
20
15
10
5
1
2
3
4
0
50
100
150
200
250
300
Time, h
A sharp increase in the degree of CO conversion
from 48 to 90% was observed in the initial period upon
the addition of rhenium to the γ-Al2O3-based catalysts
(Fig. 4a). Vada et al. [39] found a similar effect and
observed a rapid decrease in the activity of the Co–
Re/γ-Al2O3 catalyst in comparison with that of a sam-
ple without rhenium.
The loss of catalyst activity is usually caused by a
combination of factors that have different effects on
the behavior of the catalytic system in the course of
synthesis. The formation of a large amount of water at
a high degree of CO conversion is considered a main
reason for the deactivation of cobalt catalysts based on
oxide supports. The effect of active metal oxidation
under the action of Н2О leads to a decrease in the
metal surface area of a cobalt catalyst [11]. In this case,
the rate of oxidation of the active phase is directly pro-
portional to the partial pressure of water [40].
Fig. 4. Dependence of (a) CO conversion and (b) selectiv-
ity for CH on the synthesis time: (1) Co/SiO , (2) Co–
4
2
Al O /SiO , (3) Co/γ-Al O , and (4) Co–Re/γ-Al O .
2
3
2
2
3
2 3
The introduction of rhenium into the composition
of the Co/γ-Al2O3 catalyst increased the degree of
conversion from 41.9 to 68.5%. At the same time, the
selectivity for C5+ hydrocarbons decreased from 71.1 to
65.4% due to the more intense formation of gaseous
reaction products C1–C4 and CO2. A significant
increase in selectivity for CO2 (by a factor of 2.5) in the
promoted sample may be associated with an increase
in the rate of water gas shift reaction due to the facili-
tation of the dissociative adsorption of CO [32].
The rhenium promotion of the Co/γ-Al2O3 catalyst
led to an increase in the rate of its deactivation by a
factor of 2.9. Jacobs et al. [33] and Dalai and Davis
[34] also noted that the introduction of noble metals
into the composition of cobalt catalysts facilitated
their accelerated deactivation.
For the TEM study of the catalysts after the synthe-
sis, they were unloaded in a flow of nitrogen and sub-
jected to two-stage washing with o-xylene and n-hep-
tane in order to remove hydrocarbons from the pores.
Figure 4 shows the dependence of CO conversion The TEM data (Fig. 5a, Table 1) showed that the dis-
and selectivity for CH4 on the duration of the synthe- persity and particle-size distribution of Co in the Co–
Al2O3/SiO2 sample that worked for ~300 h were com-
sis. In general, all of the test catalysts demonstrated a
regular decrease in the conversion of CO with time parable with those of the initial reduced catalyst
[35]. A decrease in activity was accompanied by a (Fig. 1b). It is likely that, in addition to an increase in
KINETICS AND CATALYSIS Vol. 61 No. 2 2020

316
YAKOVENKO et al.
(а)
(b)
50 nm
200 nm
Concentration, %
35
Concentration, %
20
18
16
14
12
10
8
30
25
20
15
10
5
6
4
2
0
0
0
2
4
6
8
10 12 14 16 18 20
Particle size, nm
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Particle size, nm
Fig. 5. TEM micrographs of the catalysts after synthesis and bar diagrams of cobalt particle-size distributions: (a) Co–
Al O /SiO and (b) Co–Re/γ-Al O .
2
3
2
2 3
the conversion of CO and the process selectivity for
C5+ hydrocarbons, the introduction of Al2O3 into a
The addition of rhenium to the Co/γ-Al2O3 catalyst
led to a significant increase (by a factor of ~2) in the
cobalt–silica gel catalyst inhibited the aggregation of conversion of CO in the initial period of catalyst oper-
Co nanoparticles under the action of a reaction atmo- ation (the first 50 h). However, the rate of deactivation
sphere in the FT synthesis conditions to facilitate sta- on the promoted sample was almost three times
ble catalyst operation.
higher, probably, due to a twofold increase in cobalt
particle sizes under the action of the reaction atmo-
sphere. Nevertheless, the conversion of CO even after
200 h of synthesis in the presence of Co–Re/γ-Al2O3
was higher by a factor of 1.5.
The addition of rhenium to the Co/γ-Al2O3 catalyst
led to a significant increase (by a factor of ~2) in the
conversion of CO in the initial period of catalyst oper-
ation (the first 50 h). However, the rate of deactivation
on the promoted sample was almost three times
higher, probably, due to a twofold increase in cobalt
particle sizes under the action of the reaction atmo-
sphere. Nevertheless, the conversion of CO even after
200 h of synthesis in the presence of Co–Re/γ-Al2O3
was higher by a factor of 1.5.
The Co–Re/γ-Al2O3 catalyst after 300 h of opera-
tion exhibited a significant increase in the average par-
ticle size of Co (from 13.9 to 27.6 nm), which was a
consequence of the agglomeration of cobalt particles
under the action of a reaction atmosphere.
CONCLUSIONS
We studied the working stability of the supported
cobalt catalysts in the synthesis of hydrocarbons from
CO and H2 in continuous tests. It was determined that
the cobalt–silica gel catalyst promoted by Al2O3 had
the highest activity. The introduction of Al2O3 into
Co/γ-Al2O3 increased the degree of CO conversion
and selectivity for C5+ hydrocarbons and inhibited the
An increase in selectivity for the formation of
aggregation of Co particles in the FT synthesis condi- methane in the course of synthesis was observed on all
tions under the action of a reaction atmosphere.
of the catalysts. This can be a consequence of the
KINETICS AND CATALYSIS
Vol. 61
No. 2
2020

EFFECT OF Re AND Al2O3 PROMOTION ON THE WORKING STABILITY
317
18. Mikhailova, Ya.V., Sviderskii, S.A., Krylova, A.Yu.,
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accumulation of liquid C5+ hydrocarbons in the cata-
lyst pores, which leads to an increase in the molar ratio
H2/CO inside a catalyst granule due to different rates
of diffusion of the reactants.
As a result of this work, we determined that the rate of
catalyst deactivation increased in the order Co/γ-Al2O3 <
Co–Al2O3/SiO2 < Co/SiO2 < Co–Re/γ-Al2O3.
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This work was supported by the Ministry of Education
and Science of the Russian Federation (state contract no.
10.2980.2017/4.6) and the President of the Russian Federa-
tion (grant no. MK-364.2019.3 for young scientists) and
performed with the use of the equipment of the Nanotech-
nology Center of Collective Use at the Platov South-Rus-
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS Vol. 61 No. 2 2020