Nature of active sites in the cobalt-zirconium oxide catalyst

Article abstract of DOI:10.1007/s11172-011-0277-6

The temperature-programmed reduction, powder X-ray diffraction, and oxygen adsorption methods were applied to study the phase composition and the nature of the active surface of the catalyst Co(10%)/ZrO₂. The results of the physicochemical studies were compared with the data on the activity and selectivity of the catalysts in the synthesis of hydrocarbons from CO and H ₂. The mechanism for the reduction of the cobalt phases in the Co(10%)/ZrO₂ system was proposed. The main features governing the formation of active sites of the synthesis of high-molecular-weight hydrocarbons were considered.

Full text of DOI:10.1007/s11172-011-0277-6

Russian Chemical Bulletin, International Edition, Vol. 60, No. 9, pp. 1835—1841, September, 2011
1835
Nature of active sites in the cobalt—zirconium oxide catalyst
D. A. Grigoryeva and M. N. Mikhailova,b
aUnited Research and Development Center,
Building 2, 55/1 Leninsky prosp., 119333 Moscow, Russian Federation
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (495) 730 6102. Eꢀmail: mik@ioc.ac.ru
The temperatureꢀprogrammed reduction, powder Xꢀray diffraction, and oxygen adsorption
methods were applied to study the phase composition and the nature of the active surface of the
catalyst Со(10%)/ZrO2. The results of the physicochemical studies were compared with the
data on the activity and selectivity of the catalysts in the synthesis of hydrocarbons from CO and
H2. The mechanism for the reduction of the cobalt phases in the Co(10%)/ZrO2 system was
proposed. The main features governing the formation of active sites of the synthesis of highꢀ
molecularꢀweight hydrocarbons were considered.
Key words: cobaltꢀbased catalyst, Fischer—Tropsch synthesis, zirconia, catalyst activation,
active sites.
The Fischer—Tropsch synthesis is an important stage
in casingꢀhead gas utilization and transportation of natuꢀ
ral gas. The key factor of this process is the selection of an
optimal catalyst, which determines the yield and compoꢀ
sition of hydrocarbon products.1 The most part of cataꢀ
lysts of the Fischer—Tropsch synthesis contain cobalt
compounds immobilized on the support. One of the main
steps in formation of the active phase of the cobalt cataꢀ
lysts is reductive activation necessary for the reduced coꢀ
balt catalysts to be formed on the catalyst surface.2 The
supports used for the catalysts of the Fischer—Tropsch
synthesis are not inert materials and during preparation
and activation they can form poorly reducible compounds
of cobalt. These compounds decrease the activity of the
catalyst.3 It is known that the active sites of the classical
catalysts of the Fischer—Tropsch synthesis contain cobalt
atoms in two different states: as metallic particles and
system the active component is reduced so easily that meꢀ
tallic cobalt is already formed at the synthesis temperature
(250 °C). ZrO2 is not characterized by a strong interaction
with cobalt oxide and, hence, it is of interest to reveal the
nature of active sites of the Co/ZrO2 catalysts and their
relationship to the formation of solid solutions Со—О—Zr
(see Ref. 7) for the isomorphic substitution of the Zr atoms
by the Co atoms.8 For this purpose, we prepared a series
of catalysts Со(10%)/ZrO2 and studied their properties
under various conditions of preliminary activation by the
temperatureꢀprogrammed reduction (TPR), powder Xꢀray
diffraction analysis, and oxygen adsorption method.
Experimental
The tetragonal modification of zirconia was used as a supꢀ
port. The procedure for preparation of support granules and their
physicochemical characteristics have earlier been described.6
Cobalt was introduced into the catalyst by the impregnation
of the support with an aqueous solution of cobalt nitrate
Co(NO3)2•6H2O (10 wt.%). After impregnation, the catalysts
was calcined for 1 h at 270 °С in air.
Powder Xꢀray diffraction analysis was carried out on a Dronꢀ3
diffractometer. The samples were scanned at a rate of 0.5 °С min–1
using CuꢀKα radiation and cerium oxide as an internal standard,
which was introduced in an amount of 10 wt.% with respect to
the sample weight.
The temperatureꢀprogrammed reduction of the freshly calꢀ
cined catalysts and the samples subjected to preliminary activaꢀ
tion in hydrogen (weighed sample 250 mg) was carried out on
an AutoChem II 2920 instrument (Micromeritics) in a flow
(10 mL min–1) of the Н2 (5%) + Ar (95%) gas mixture. The
cations of the oxide formed due to the strong interaction
+
of cobalt oxide with the oxide support, viz., Соδ
—
СоО•MxOy.4,5 For example, in the presence of the classiꢀ
cal catalysts of the Fischer—Tropsch synthesis Со/Al2O3,
spinel structures are formed and the cobalt atoms (a part
of them) become inactive in catalysis. In addition, the
reduction of the cobalt catalysts on the classical supports
(aluminum, silicon, and titanium oxides) require high temꢀ
peratures (400—500 °C). Therefore, one of the main diꢀ
rections of improvement of the Fischer—Tropsch syntheꢀ
sis is the development of catalytic systems characterized
by a weak metal—support interaction. No poorly reducꢀ
ible cobalt zirconates are formed in the Co/ZrO2 cataꢀ
lysts. It has recently been shown6 that in the Co/ZrO2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1804—1810, September, 2011.
1066ꢀ5285/11/6009ꢀ1835 © 2011 Springer Science+Business Media, Inc.

1836
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
Grigoryev and Mikhailov
temperature was linearly increased from 100 to 900 °С with a rate
of 10 °С min–1
Results and Discussion
.
Oxygen adsorption on the samples preꢀactivated in hydroꢀ
gen (weighed sample 300 mg) was carried out on an AutoChem II
2920 instrument (Micromeritics). After reduction, the sample
surface was purified from weakly sorbed compounds with a heliꢀ
um flow (30 mL min–1) for 60 min and the sample was cooled
down to ~20 °С. Then the lowꢀtemperature (at –110 °С) and
highꢀtemperature (at 450 °С) titration with oxygen was perꢀ
formed. For this purpose, oxygen from the loop (volume 1 mL)
swept with the О2 (15%) + Не (85%) mixture was introduced by
impulses into a helium flow (60 mL min–1) that was passed
through the weighed sample. The degree of reduction of oxygen
was calculated on the basis of the assumption that the oxidation
proceeds according to the stoichiometric equations to Co3O4
The powder Xꢀray diffraction patterns of the calcined
catalyst Co(10%)/ZrO2 and the samples activated at
250 °C for 1 and 24 h and at 350 °C for 1 h are presented in
Fig. 1. The Xꢀray diffraction pattern of the unreduced
sample exhibits lines due to the oxide phase Со3О4 (see
Fig. 1, curve а). The activation of the catalysts at both 250
and 350 °C leads to the complete disappearance of the
lines of the crystalline phase Со3О4 (see Fig. 1, curves b—d),
which is caused by the complete reduction of this phase to
form the cobalt phases amorphous to Xꢀrays. It was shown9
by the powder Xꢀray diffraction study of the Со/ZrO2
systems that the Со3О4 phase disappeared and the СоО
phase appeared upon hydrogen reduction at temperatures
higher than 200 °C. All diffraction peaks disappear from
the Xꢀray diffraction patterns after activation at 300 °C.
This can be explained by the fact that in the temperature
range from 200 to 300 °C bulk CoO is transformed into
the amorphous oxide phase or reduced to the amorphous
phase of metallic cobalt. The lines of the bulk phase of
metallic cobalt appeared in the Xꢀray diffraction patꢀ
terns of the reduced samples only upon the activation at
T > 400 °C.10
3 Со0 + 2 О2
Со3О4,
Со3О4.
3 СоО + 0.5 О2
Using the lowꢀtemperature adsorption of oxygen, the disꢀ
persity and surface area of metallic cobalt were calculated acꢀ
cepting that each cobalt atom adsorbs one oxygen molecule. The
size of crystallites of metallic cobalt was determined assuming
that the cobalt crystallites were spherical and did not interact
with the support
An analysis of the TPR curves gives more complete
data on the state of cobalt in the catalysts. The TPR curves
of the calcined and preꢀactivated in hydrogen samples are
given in Fig. 2. Three peaks can be distinguished in the
TPR spectrum of the calcined Co(10%)/ZrO2 sample (see
Fig. 2, curve а). The first peak at 195—250 °C with
a maximum at 225 °C corresponds to the reduction of
oxide Со3О4 to СоО.11 The second peak with a maximum
at 350 °C can be assigned to the reduction of CoO to
metallic cobalt.12 The third broad peak at 420—670 °C
where ρCo is the density of metallic Со equal to 8.9 g cm–3; λ is
the Co content in the catalyst, wt.%; RCo is the degree of reducꢀ
tion of Со, %; m is the catalyst weight, g; 0.01 is the coefficient
of recalculation of the values expressed in percents to the value
in weight fraction; NA = 6.02•1023 is Avogadro´s number;
S0 = 0.66225•10–19 is the average surface area per one surface
cobalt atom, m2; and AdО2 is the lowꢀtemperature amount adꢀ
sorbed of oxygen in μmol g–1
.
Microcatalytic experiments were carried out in a steel tubuꢀ
lar reactor (inner diameter 12 mm) on a flowꢀtype catalytic setꢀ
up. The conditions of the process were as follows: Р = 2 MPa,
Н2 : СО : N2 = 63 : 31.5 : 5.5 (vol.%). The catalysts reduced with
hydrogen and the catalyst that was not preliminarily subjected to
reductive treatment were activated in a synthesis gas flow (voluꢀ
metic flow rate 1000 h–1). The catalyst was heated to 170 °С with
a rate of 2 °C min–1. Then the temperature of the process was
increased stepwise until the conversion of carbon monoxide
reached a value >70%.
I
Co3O4
d
c
The initial mixture and gaseous synthesis products were anaꢀ
lyzed by gas adsorption chromatography using a thermal conꢀ
ductivity detector and helium as a carrier gas. Molecular sieves
CaA (3 m × 3 mm) served as the first column, and the second
column was HayeSep. The first column was used for the analysis
of CO and N2 (isothermal regime, 80 °С), whereas СО2, СН4,
and hydrocarbons С2—С4 were analyzed on the second column
(temperatureꢀprogrammed regime, 80—200 °С, 8 °C min–1).
The composition of the liquid synthesis products was determined
by gas liquid chromatography using a flameꢀionization detector
and helium as a carrier gas. A capillary column (50 m) with the
DBꢀPetro 0.5 stationary phase was used for analysis (temperaꢀ
tureꢀprogrammed regime, 50—250 °С, 3 °C min–1).
b
a
36
38
40
42
44
46
48
2θ/deg
Fig. 1. Powder Xꢀray diffraction patterns of the samples of the
Co(10%)/ZrO2 catalyst calcined at 270 °С (a) and reduced with
hydrogen at 250 °С for 1 h (b), at 250 °С for 24 h (c), and at
350 °С for 1 h (d).

Active sites in Co—Zr oxide catalyst
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
1837
with a maximum at 565 °C reflects the reduction of surꢀ
face cobalt oxides13 weakly interacting with zirconia14 to
form the metal. A shoulder of the peak at 665—750 °C is
observed in the TPR curve of the calcined sample. It was
suggested15 that this peak corresponds to the recrystalꢀ
lization of the support (transition from the metastable tetꢀ
ragonal to mоnoclinic phase16) and reduction of solid
solutions Со—О—Zr (see Ref. 17). Compared to the
Co(10%)/ZrO2 sample calcined at 400 °C,6 the peaks in
the TPR spectrum of the catalyst heated at 270 °C are
shifted to low temperatures. A decrease in the temperature
of preliminary treatment of the catalyst increases the disperꢀ
sity of oxide Со3О4.18 For the Со/ZrO2 system due to the
weak metal—support interaction, an increase in the disperꢀ
sity leads to an increase in the reduction ability of the
catalyst,14 whereas for the Со/Al2O3 catalyst an increase in
the dispersion of cobalt oxide favors the formation of spinel
structures and deteriorates the reduction properties.19
The degree of cobalt reduction in the activated catalyst
was estimated from the value of the surface area under the
TPR curve for the calcined and reduced Со(10%)/ZrO2
samples using the formula
surface area under the curve of the TPR spectrum of the
calcined catalyst; and S(TPRR(Т, τ)) is the surface area
under the curve of the TPR spectrum for the catalyst actiꢀ
vated with hydrogen at temperature T and time τ.
The results of calculations of the degree of reduction of
the active component under various activation conditions
are given in Table 1.
The shape of the TPR spectra after the activation of
the catalysts with hydrogen depends on the activation conꢀ
ditions (see Fig. 2, curves b—d). The shape of the former
two peaks of hydrogen absorption and the intensities of
the third and fourth peaks change as the reduction condiꢀ
tions (temperature or duration) become more severe (see
Fig. 2). The intensity of the first peak in the TPR spectra
of the preꢀactivated samples increases with an increase in
the duration and temperature of the hydrogen treatment.
Evidently, this peak cannot be attributed to the reduction
of cobalt oxides Со3О4 and СоО and, most likely, this
peak indicates the formation of new surface cobaltꢀconꢀ
taining phases during the preꢀactivation of the Co(10%)/
ZrO2 system with hydrogen. These phases can be surface
cobalt oxohydroxides,20 which can be identified in the
TPR spectra of the preꢀactivated samples by the first peak
of hydrogen absorption
RCoT,τ (%) =
•100%,
2 СоО + 0.5 Н2
СоООН + Со0.
T,
where RCo τ is the degree of reduction of cobalt activated
with hydrogen at temperature T and time τ; S(TPR) is the
The peaks at 340—360 °С correspond to cobalt hydrꢀ
oxide Со(ОН)2 (see Ref. 17), which is an intermediate
compounds formed by the reduction of the СoO(OH)
phase to the Со0 particles
I
350
565
СоООН + 0.5 Н2
Со(ОН)2.
225
720
The intensity of the third reduction peak increases afꢀ
ter the preꢀactivation of the Co(10%)/ZrO2 catalyst with
hydrogen at 250 °С (see Fig. 2, curves b and с). This can
occur due to the partial transition of unreduced bulk coꢀ
balt oxides to surface oxides. The probability of this redisꢀ
tribution is rather high, since the activation is carried out
near the Hüttig temperature (temperature of mobility) for
metallic cobalt (253 °С),21 which leads to the agglomeraꢀ
tion of particles of the reduced metal.
a
b
550
345
255
775
505
350
240
Table 1. Degree of reduction (RCo), specific surface (SCo), disꢀ
persity (DCo), and particle size (dCo) of metallic cobalt deterꢀ
mined from the oxygen titration data under various activation
conditions
770
c
205
505
345
715
T,
d
τ
τ
/h
ТB
/°С
RCo
SCo
DCo
(%)
dCo
/nm
(%)
/m2 (g of catalyst)–1
200 300 400 500 600 700 800 T/°C
1
24
1
250 09.3 (8.8*)
250 15.5 (15.2*)
350 34.2 (31.7*)
1.3
1.7
2.6
24.6
19.1
13.2
4.1
5.2
7.5
Fig. 2. Temperatureꢀprogrammed reduction curves of the
Co(10%)/ZrO2 catalyst calcined at 270 °С (a) and reduced with
hydrogen at 250 °С for 1 h (b), at 250 °С for 24 h (c), and at
350 °С for 1 h (d).
* According to the TPR data.

1838
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
Grigoryev and Mikhailov
Under more severe conditions of preꢀactivation of the
Co(10%)/ZrO2 catalyst, the intensity of the peak correꢀ
sponding to surface oxides decreases noticeably (see Fig. 2,
curve d). According to the TPR data for the preꢀcalcined
sample, the reduction of these oxides starts at temperaꢀ
tures higher than 420 °С (see Fig. 2, curve а); however,
due to hydrogen spillover from the surface of metallic coꢀ
balt22 formed during activation, the reduction of surface
cobalt oxides on the Co(10%)/ZrO2 catalyst becomes posꢀ
sible at a lower temperature (350 °С).
The data on the activity and selectivity at 250 °С for
the Co(10%)/ZrO2 catalysts preꢀreduced with hydrogen
and for the sample with no preliminary activation, as well
as the compositions of liquid hydrocarbons obtained on
these systems, are given in Tables 2 and 3, respectively.
The activity of the reduced samples at a synthesis temꢀ
perature of 250 °С increases when the preꢀactivation conꢀ
ditions become more severe. The selectivity to methane
and highꢀmolecularꢀweight hydrocarbons is almost the
same for all samples.
The presence of the Со0 phase and an increase in its
content in the catalyst also favor the reduction of cobalt in
the composition of the Со—О—Zr solid solutions.10
A nearly complete absence of this phase after reduction
with hydrogen at 350 °С is due to the transition of cobalt
to surface oxides СоО. This transition becomes possible at
temperatures higher than the Tamman temperature23 (the
temperature at which ionic pairs of solids begin to diffuse)
for cobalt oxide, viz., 347 °С.24 Thus, at 350 °С cobalt
oxide СоО can readily diffuse from the support lattice
to the ZrO2 surface and can more rapidly be reduced to
the metal.
The degree of reduction of cobalt (RCo) and its strucꢀ
tural characteristics, namely, the surface area of metallic
cobalt (SCo), dispersion (DCo), and diameter (dCo) of Со0
particles were determined in the experiments on the lowꢀ
temperature and highꢀtemperature adsorption of oxygen
on the preꢀactivated Co(10%)/ZrO2 samples. The results
are presented in Table 1 along with the value of the degree
of reduction of active metal determined by the TPR data
of the preꢀreduced catalysts. The values of RCo obtained
by the TPR and oxygen adsorption methods are consistent
(see Table 1).
The degree of reduction of the active component inꢀ
creases by a factor of 1.7 as the time of the activation at
250 °С was increased from 1 to 24 h. An increase in the
temperature of the preliminary hydrogen treatment to
350 °С makes it possible to increase the degree of reducꢀ
tion of cobalt in the Co(10%)/ZrO2 catalyst to 32%. When
the time and temperature of activation increase, the surꢀ
face area of Со0 and the crystallite size of the reduced
metal increase and the value of dispersity decreases (see
Table 1).
The group composition of the hydrocarbon product
obtained at 250 °С is similar: normal alkanes (80—83 wt.%)
and isoalkanes (15—16 wt.%) are predominantly formed
with an insignificant amount of olefins (2—4 wt.%). The
degree of polymerization decreases with an increase in the
severity of the preꢀactivation conditions.
The study of the influence of the conditions for the
synthesis of hydrocarbons in the presence of the Co(10%)/
ZrO2 samples on their catalytic characteristics showed
that the maximum yield of hydrocarbons for all systems
was 75—77 g m–3. The synthesis conditions and cataꢀ
lytic characteristics for the systems studied are listed in
Tables 4 and 5.
When the performance of the samples is compared at
similar conversions of the raw materials (ССО = 74—78%)
and selectivity to highꢀmolecularꢀweight hydrocarbons
(SC5+ = 55—58%), it can be seen that the temperatures of
synthesis at which these parameters achieved are different
for all catalysts.
The fraction of the gasoline component in the compoꢀ
sition of hydrocarbons obtained in the presence of the
Co(10%)/ZrO2 samples increases with softening of the
activation conditions. The content of isoalkanes and oleꢀ
fins increases (see Table 5) due to an increase in the probꢀ
ability of cracking and isomerization with an increase in
the temperature of the process (see Table 4). Thus, by
changing the activation conditions of the Co(10%)/ZrO2
catalytic system, one can obtain hydrocarbon products in
high yields in a wide range of both fractional and group
compositions.
Table 2. Conversion of CO (CСО), selectivity to methane (SCH
)
4
and hydrocarbons С5+ (SC ), and yield of the liquid hydroꢀ
5+
carbon product (YC ) for the Co(10%)/ZrO2 catalysts at 250 °C,
Thus, the conditions of preliminary reduction of the
Co(10%)/ZrO2 samples affect the content of Со0 and its
structural characteristics (see Table 1). The surface area,
dispersion, and crystallite diameter of the active metal
exert a considerable effect on the activity and selectivity
of the catalyst in the formation of highꢀmolecularꢀ
weight hydrocarbons for the classical catalysts of the Fisꢀ
cher—Tropsch synthesis.25,26 No similar dependence were
found for the Со/ZrO2 system. Therefore, it was of interꢀ
est to study the influence of the activation conditions on
the catalytic properties of the Co(10%)/ZrO2 samples in
the synthesis of hydrocarbons from CO and Н2.
5+
a pressure of 2 MPa, and a flow rate of 1000 h–1 under various
activation conditions
τ/h
ТB/°С
CСО (%)
YC /g m–3
SC
SCH
4
5+
5+
%
—*
1
24
1
—
30
54
63
77
30
53
65
77
55
32
30
26
27
250
250
350
56
60
57
* Without activation.

Active sites in Co—Zr oxide catalyst
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
1839
Table 3. Composition of hydrocarbons С5+ obtained on the Co(10%)/ZrO2 catalysts in the Fischer—Tropsch synthesis at
250 °С, a pressure of 2 MPa, and a flow rate of 1000 h–1
τ/h
ТB/°С
Composition (wt.%)
Composition of paraffins (wt.%)
P*
Olefins
nꢀParaffins
Isoparaffins
С5—С10 С11—С18
С19+
—**
1
24
1
—
4
2
3
2
80
83
82
82
16
15
15
16
39
44
49
52
45
43
40
39
16
13
11
9
0.85
0.84
0.81
0.80
250
250
350
* Р is the probability of chain growth α.
** Without activation.
The high activity observed for all Co(10%)/ZrO2 samꢀ
ples and exhibited at various temperatures of the synthesis
is explained by specific features of formation of the sites
active in the Fischer—Tropsch synthesis.
On the samples reduced at 350 °С, the major part of
cobalt exists in the metallic state and is formed during
direct reduction with hydrogen.
Under the action of the reaction conditions, the cobalt
oxide compounds forming solid solutions in the Co—O—Zr
system are transformed into the cobalt phase to become
a fraction of the active sites. This favorably distinguishes
zirconia from alumina and other classical supports of the
cobalt catalysts of the Fischer—Tropsch synthesis characꢀ
terized by the irreversible formation of compounds upon
insertion of cobalt into the support lattice.
The same levels of conversion on the Co(10%)/ZrO2
samples reduced with hydrogen at 250 °С for 1 and 24 h
were attained at higher temperatures (265 and 260 °С,
respectively). Probably, in these systems the necessary
amount of Со0 and oxide components of the active sites
appear only during the synthesis. The transformation of
the oxide phases of cobalt into metallic cobalt at the initial
steps of the synthesis occurs due to the reaction with carꢀ
bon monoxide. This is confirmed by high (up to 100% in
the first hours) values of selectivity to the formation of
carbon dioxide on the unactivated catalysts. Cobalt oxides
can be reduced under a high partial hydrogen pressure18 in
the reactor (~1.2 MPa) at temperatures higher than 200 °С
corresponding to the onset of the first reduction peak in
the TPR spectrum of the preꢀcalcined sample Co(10%)/
ZrO2 (see Fig. 2, curve а). The oxohydroxide phases of Co
are transformed into Со0 under the same conditions.
The action of CO on the cobalt catalysts results in the
adsorptionꢀactivated increase in the dispersion of the coꢀ
balt phases,27 which prevents crystallite sintering.28 Since
the total number of active sites of the catalyst is a function
of both the dispersity of cobalt and its reducibility,25 the
Co(10%)/ZrO2 system, which underwent no preꢀactivaꢀ
tion and is characterized, hence, by the lowest degree of
cobalt reduction, exhibits the minimum activity at 250 °С.
On further increase in the synthesis temperature the diffuꢀ
sion of metallic cobalt on the catalyst surface is intensiꢀ
fied,29 resulting in the deep transformation of the active
surface.26 Therefore, the sample, which was not subjected
to preꢀactivation, attains the maximum productivity only
at 275 °С. Similar transformations of the solid catalyst
surface accompanied by a change in the character of the
active sites were observed by other researchers30 under the
Fischer—Tropsch conditions.
The results of physicochemical studies and catalytic
tests of the Co(10%)/ZrO2 system activated under condiꢀ
tions of varying severity make it possible to consider the
mechanism of transformations of the active phase of the
catalysts in the processes of reductive activation and direct
synthesis of hydrocarbons from CO and Н2 (Scheme 1).
Three cobaltꢀcontaining phases can exist on the cataꢀ
lyst surface after the calcination of zirconia impregnated
with an aqueous solution of cobalt nitrate at 270 °С for 1 h:
bulk cobalt oxide Со3О4 distributed over the Со3О4 surꢀ
face and a solid solution Со—О—Zr formed upon the
isomorphic substitution of zirconium atoms by cobalt atoms
during preparation. When the Со(10%)/ZrO2 catalysts is
activated with hydrogen at 250 °С, the bulk oxide Со3О4
is reduced to СоО and forms the surface oxide (see Scheme 1,
steps 1 and 5). The formation of metallic cobalt bound to
the support surface is possible at this temperature due to
the stepwise reduction of surface CoO through the steps of
Table 4. Conversion of CO (CСО) and selectivity to methane
(SCH ) and hydrocarbons С5+ (SC ) for the Co(10%)/ZrO2 cataꢀ
4
lysts in the Fischer—Tropsch syn5t+hesis at the maximum yield of
highꢀmolecularꢀweight hydrocarbons (YC
)
5+
τ/h
ТB
Т
CСО (%)
YC /g m–3
SC
SCH
4
5+
5+
°С
%
—*
1
24
1
—
275
265
260
250
78
74
75
77
77
76
75
77
55
26
26
24
27
250
250
350
58
57
57
* Without activation.

1840
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
Grigoryev and Mikhailov
Table 5. Composition of hydrocarbons С5+ obtained on the Co(10%)/ZrO2 catalysts in the Fischer—Tropsch synthesis at the
maximum yield of highꢀmolecularꢀweight hydrocarbons
τ/h
ТB/°С
Composition (wt.%)
Composition of paraffins (wt.%)
P
Olefins
nꢀParaffins
Isoparaffins
С5—С10 С11—С18
С19+
—*
1
24
1
—
8
4
5
2
66
76
76
82
26
20
19
16
70
56
58
52
27
36
35
39
3
8
7
9
0.74
0.77
0.76
0.80
250
250
350
* Without activation.
Scheme 1
i. Impregnation with an aqueous solution of Co(NO3)2•6H2O, then calcination (270 °С), 1 h.
Oxm is bulk oxide, Oxs is surface oxide, Solv is solid solution, and A is active site.
formation of cobalt oxohydroxide and hydroxide (steps 6,
7, and 8) and it is favored by a prolonged activation. Bulk
cobalt(II) oxide is reduced to an insignificant extent at
250 °С (step 2).
carbon monoxide at slow heating to the initial temperaꢀ
ture of the synthesis (170 °С) (step 10). Then the oxide is
reduced to oxide CoO due to the reaction with carbon
monoxide and hydrogen under elevated pressure (2 MPa)
(step 3). A further temperature increase and the presence
of the reductive medium in the reactor (synthesis gas)
favor the transition of CoO (step 4) of the oxohydroxide
and hydroxide phases of cobalt (steps 7 and 8) to surface
particles of metallic cobalt well dispersed on the support.
This process involves hydrogen spillover with primarily
The activation of the Co(10%)/ZrO2 system at 350 °С
leads to the formation of metallic cobalt due to the direct
reduction of bulk (steps 1 and 2) and surface oxides (steps 1,
5, and 4) and due to the involvement of cobalt from the
+
δ
solid solutions (steps 9 and 4). The formation of Соn
due to transformations through the steps of hydroxide forꢀ
mation occurs to an insignificant extent.
formed Соn + and zirconia. At high conversions of the raw
δ
The initial formation of active sites of synthesis of highꢀ
molecularꢀweight hydrocarbons from metallic cobalt occurs
under the conditions of preliminary activation (step 11).
Based on results of the catalytic experiments (see Table 4)
it can be stated that the number of active sites increases
with an increase in the severity of conditions of preꢀactiꢀ
vation with hydrogen.
When the synthesis of hydrocarbons from CO and Н2
is conducted under a high pressure of the reactants and at
the synthesis temperatures, the bulk oxide Со3О4 is redisꢀ
tributed into the surface phase Со3О4 under the action of
materials, the catalyst surface is deeply rearranged to form
additional active sites of synthesis of highꢀmolecularꢀ
weight hydrocarbons involving the Соn phase and the
oxide phases of cobalt10 (step 11).
In this work, we used the TPR, powder Xꢀray diffraction
analysis, and oxygen adsorption methods to study specific
features of the preliminary activation of the Co/ZrO2 catꢀ
alytic system with hydrogen at the process temperature
(250 °С) and varying times of activation (1 and 24 h). The
results obtained are compared with those acquired for the
sample activated at the elevated temperature (350 °С, 1 h).
+
δ

Active sites in Co—Zr oxide catalyst
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 9, September, 2011
1841
15. D. I. Enache, M. RoyꢀAuberger, R. Revel, Appl. Catal. A:
General, 2004, 268, 51.
16. T. Yamaguchi, Catal. Today, 1994, 20, 199.
17. L. A. Bruce, G. J. Hope, J. F. Mathews, Appl. Catal., 1983,
8, 349.
This allowed us to study the processes of reduction of
various phases of cobalt and suggest the structure and comꢀ
position of the active sites for the Co/ZrO2 catalysts. Moreꢀ
over, the main features of their formation during hydrogen
activation under various conditions and in the process of
the subsequent synthesis of hydrocarbons from CO and
Н2 were outlined and the differences in the behavior of the
studied catalytic systems were revealed.
18. P. A. Chernavskii, A. Y. Khodakov, G. V. Pankina, J.ꢀS.
Girardon, E. Quinet, Appl. Catal., 2006, 306, 108.
19. A. Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier,
B. Moisson, P. Chaumette, J. Catal., 1997, 168, 16.
20. J. van de Loosdrecht, S. Barradas, E. A. Caricato, N. G.
Ngwenya, P. S. Nkwanyana, M. A. S. Rawat, B. H. Sigweꢀ
bela, P. J. van Berge, J. L. Visagie, Top. Catal., 2003, 26, 121.
21. A. Moulijn, A. E. van Diepen, F. Kapteijn, Appl. Catal. A,
2001, 212, 3.
22. G. Jacobs, Y. Ji, B. H. Davis, D. Cronauer, A. J. Kropf,
C. L. Marshall, Appl. Catal. A, 2007, 333, 177.
23. J. Thewlis, Encyclopedic Dictionary of Physics, 1962, 171.
24. L. Zhang, L. Dong, W. Yu, L. Liu, Y. Deng, B. Liu, H. Wan,
F. Gao, K. Sun, L. Dong, J. Colloid Interface Sci., 2011,
355, 464.
25. J.ꢀS. Girardon, E. Quinet, A. GribovalꢀConstant, P. A. Cherꢀ
navskii, L. Gengembre, A. Y. Khodakov, J. Catal., 2007,
248, 143.
26. A. Y. Khodakov, R. Bechara, A. GribovalꢀConstant, Appl.
Catal. A, 2003, 254, 273.
27. V. A. de la Penꢁa O´Shea, J. M. CamposꢀMartýn, J. L. G.
Fierro, Catal. Commun., 2004, 5, 635.
28. B. Jongsomjit, J. G. Goodwin, Jr., Catal. Today, 2002,
77, 191.
References
1. A. Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev., 2007,
107, 1692.
2. W. H. Lee, C. H. Bartolomew, J. Catal., 1989, 120, 256.
3. C. R. Reuel, C. H. Bartholomew, J. Catal., 1984, 85, 78.
4. A. L. Lapidus, Izv. Akad. Nauk SSSR, Ser. Khim., 1991, 2681
[Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1991,
40, 2335].
5. W. H. Lee, C. H. Bartholomew, J. Catal., 1989, 120, 256.
6. A. L. Lapidus, D. A. Grigor´ev, M. N. Mikhailov, A. N.
Loginova, Izv. Akad. Nauk, Ser. Khim., 2010, 1629 [Russ.
Chem. Bull., Int. Ed., 2010, 59, 1657].
7. L. Shi, J. Chen, Y. Sun, Stud. Surf. Sci. Catal., 2007, 167, 97.
8. I. D. Lick, A. Carrascull, M. Ponzi, E. N. Ponzi, I. L. Botto,
Mater. Chem. Phys., 2005, 92, 327.
9. J. G. Chen, Y.ꢀH. Sun, Stud. Surf. Sci. Catal., 2004, 147, 277.
10. D. I. Enache, B. Rebours, M. RoyꢀAuberger, R. Revel,
J. Catal., 2002, 205, 346.
29. N. E. Tsakoumis, M. Ronning, O. Borg, E. Rytter, A. Holꢀ
men, Catal. Today, 2010, 154, 162
30. H. Schulz, Z. Nie, F. Ousmanov, Catal. Today, 2002, 71, 351.
11. H. Lin, C. Wang, H. Chiu, S. Chien, Catal. Lett., 2003,
86, 63.
12. G. R. Moradi, M. M. Basir, A. Taeb, A. Kiennemann, Catal.
Commun., 2003, 4, 27.
13. J. Lojewska, W. Makowski, T. Tyszewski, R. Dziembaj, Catal.
Today, 2001, 69, 409.
14. S. Kittiruangrayab, T. Burakorn, B. Jongsomjit, P. Praserthꢀ
dam, Catal. Lett., 2008, 124, 376.
Received July 21, 2011