Pore size effects in Fischer Tropsch synthesis over cobalt-supported mesoporous silicas

Article abstract of DOI:10.1006/jcat.2001.3496

Pore size effects on Fischer Tropsch reaction rates and selectivities over cobalt catalysts were studied at atmospheric pressure using periodic (SBA-15 and MCM-41) and commercial mesoporous silicas as catalytic supports. The catalysts were characterized by nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and oxygen titration. Fischer Tropsch reaction rates were found much higher on cobalt catalysts with the pore diameter exceeding 30 A than on the narrow pore catalysts. A larger diameter of catalyst pores also led to significantly higher C₅₊ selectivities. The catalytic effects were interpreted in terms of different cobalt particle size and reducibility in the wide pore and narrow pore silicas. XRD and XPS showed that the size of supported cobalt species strongly depended on the pore size, increasing with increases in catalyst pore diameter. TGA and oxygen titration indicated higher extent of overall reduction of cobalt species in wide pore supports. Lower reducibility of small cobalt particles is likely to be one of the reasons responsible for the lower Fischer Tropsch reaction rates and higher methane selectivities on narrow pore cobalt catalysts.

Full text of DOI:10.1006/jcat.2001.3496

Journal of Catalysis 206, 230–241 (2002)
doi:10.1006/jcat.2001.3496, available online at http://www.idealibrary.com on
Pore Size Effects in Fischer Tropsch Synthesis over Cobalt-Supported
Mesoporous Silicas
Andrei Y. Khodakov, ,1 Anne Griboval-Constant, Rafeh Bechara, and Vladimir L. Zholobenko†
∗
∗
∗
∗
Laboratoire de Catalyse de Lille, Universit e´ des Sciences et Technologies de Lille, B aˆ t. C3, Cit e´ Scientifique, 59655 Villeneuve d’Ascq, France;
and †School of Chemistry and Physics, Keele University, Staffordshire ST5 5BG, United Kingdom
Received July 13, 2001; revised November 21, 2001; accepted December 12, 2001
new catalysts with higher productivities, higher C5+ yields,
Pore size effects on Fischer Tropsch reaction rates and selectivi- and lower methane selectivities. A large number of papers
ties over cobalt catalysts were studied at atmospheric pressure using published so far focus on optimization of metal function in
periodic (SBA-15 and MCM-41) and commercial mesoporous sili-
cas as catalytic supports. The catalysts were characterized by nitro-
gen adsorption, X-ray diffraction (XRD), X-ray photoelectron spec-
troscopy (XPS), thermogravimetric analysis (TGA), and oxygen
titration. Fischer Tropsch reaction rates were found much higher on
cobalt catalysts with the pore diameter exceeding 30 A˚ than on the
narrow pore catalysts. A larger diameter of catalyst pores also led
to significantly higher C5+ selectivities. The catalytic effects were
interpreted in terms of different cobalt particle size and reducibility
monometallic (Co, Fe, Ru, Mo) and bimetallic FT catalysts
(
3–12).
The effect of support and its porosity on FT reaction rate
and hydrocarbon selectivities remains, however, still un-
clear. Mass transport limitations for carbon monoxide and
hydrocarbons in catalyst pores, pore filling, and condensa-
tion of heavier hydrocarbons, different adsorption proper-
ties of metal particles located in narrow and wide pores are
in the wide pore and narrow pore silicas. XRD and XPS showed often employed to explain the effect of pore sizes on FT re-
that the size of supported cobalt species strongly depended on the action rates and selectivities. Anderson et al. (13) for exam-
pore size, increasing with increases in catalyst pore diameter. TGA ple, attributed the observed increase in methane selectivity
and oxygen titration indicated higher extent of overall reduction
of cobalt species in wide pore supports. Lower reducibility of small
cobalt particles is likely to be one of the reasons responsible for the
lower Fischer Tropsch reaction rates and higher methane selectivi-
with decreasing average pore diameter to mass transport
phenomenon. It was suggested that greater rate of diffu-
sion of hydrogen inside pores filled with liquid products
compared to that of carbon monoxide caused an increase
in H2/CO ratio in catalyst pores, and thus, a shift toward
formation of lighter hydrocarbons. Lapszewicz et al. (14)
showed that variation of product distribution as a function
of catalyst pore diameter could be the result of changing ad-
sorption patterns of hydrogen and carbon monoxide rather
than mass transfer phenomena. Vanhove et al. (15) found
ties on narrow pore cobalt catalysts.
ꢀc 2002 Elsevier Science (USA)
Key Words: Fischer Tropsch synthesis; cobalt catalysts; meso-
porous materials; porosity; nanoparticles.
INTRODUCTION
Fischer Tropsch (FT) synthesis is hydrocarbon produc- that textural properties of Co catalysts supported by alu-
tion from carbon monoxide and hydrogen. FT synthesis mina could modify the chain length of hydrocarbons. The
allows making valuable long chain hydrocarbons from rel- effect was assigned to different residence times of hydrocar-
atively cheap natural and associated gas, which are first bons in alumina pores due to pore condensation and filling.
converted to synthesis gas by vapor reforming and partial Bartholomew and Reuel (16) reported strong support ef-
oxidation. Rather than burning off unused gas or leaving fects in carbon monoxide hydrogenation on cobalt cata-
it untapped, FT synthesis yields products that are useful, lysts. The order of decreasing CO hydrogenation activity
cleaner (lower in sulphur and heavier metals than crude was TiO > SiO > Al O > C > MgO. The specific activity
2
2
2
3
oil), easier and cheaper to transport (1, 2). New require- of cobalt on all studied catalysts also decreased with in-
ments for the residual sulphur (30–50 ppm) in diesel fuel creasing metal dispersion. Iglesia et al. (17, 18) showed that
are a prime reason for renewed interest in FT synthesis in FT specific activity was proportional to metal dispersion
Europe.
2 2 3 2
and almost independent of the support (SiO , Al O , TiO ,
FT synthesis proceeds on metal supported catalysts. The ZrO modifiedbySiO andTiO ). Thesupportwasfoundto
2
2
2
efficiency of FT synthesis can be improved by design of slightly influence C5 selectivity due to transport enhanced
+
secondary reactions such as α-olefin readsorption.
Microporous and mesoporous oxides are common sup-
ports for metal catalysts. Traditional mesoporous materials
1
To whom correspondence should be addressed. E-mail: andrei.
khodakov@ec-lille.fr.
230
0
ꢀ
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.

PORE SIZE EFFECTS IN FISCHER TROPSCH SYNTHESIS
231
are irregularly spaced and their pore sizes are broadly dis- polypropylene flask and heated in an oven at 373 K for 24 h.
tributed. Thus, it is rather difficult to figure out in which The resulting gel was washed with distilled water, then dried
pores Fischer Tropsch synthesis takes places. Moreover, at room temperature for 24 h and calcined in air at 773 K
◦
at FT reaction conditions the products, wax and carbon for 6 h. The rate of temperature ramping was 1 /min.
deposits could easily block a part of catalyst pores mak-
Si3 sample (MCM-41 type) was synthesized adapting the
ing them unavailable for reacting molecules. Recently dis- procedure described in (23). This material was prepared by
covered MCM-41 and SBA-15 periodic mesoporous silicas stirring 38.5 g of 25 wt% cetyltrimethylammonium bromide
(
19–26) represent a new class of inorganic oxides. Their (CTMAB, Aldrich) solution with 1.28 g of 35% ammonia
2
surface areas are approaching 1000 m /g, pore size distri- solution (BDH), 8.64 g of tetramethylammonium hydro-
butions in periodic mesoporous silicas are very narrow. The xide pentahydrate (TMAOH, Aldrich), 8.95 g of fumed
pore sizes can be adjusted from 20 to 300 A at the stage of silica (SiO2, Aldrich), and 43.77 g of deionized water. The
˚
synthesis of these materials using various surfactants and final molar ratio was 37.3SiO2 : 6.6CTMAB : 6.6NH4OH :
different reaction conditions. Thus, periodic mesoporous 11.7TMAOH : 678H2O. The synthesis mixture was kept
silicas can be considered as model supports, which allow at 351 K for 66 h, then filtered and washed with deion-
the effect of pore size on catalytic behavior of supported ized water and ethanol, to remove excess of surfactant be-
catalysts to be studied.
fore calcination. The final product was dried overnight at
This work addresses the effects of pore size on the struc- 373 K and then carefully calcined in nitrogen and oxygen to
ture of Co species and catalytic behavior in FT synthesis of 773 K.
Co catalystssupported bymesoporous silicas. Bothperiodic
and commercial mesoporous silicas are used as catalytic 1.5 to 10 , CuKα radiation) of the siliceous MCM-41 ma-
Low angle X-ray diffraction patterns (2-theta range from
◦
◦
supports. Nitrogen adsorption, X-ray diffraction (XRD), terials showed an intense peak at ∼2.5 and low intensity
◦
thermogravimetric analysis (TGA), oxygen titration, and peaks at 3–6 characteristic of the hexagonal structure of
X-ray photoelectron spectroscopy (XPS) are used to char- MCM-41.
acterizethecatalysts. Characterizationandcatalyticactivity
data are compared at relatively low cobalt contents (about copolymer (P123) poly (ethylene glycol)-block-poly (pro-
wt%). Previously, Reuel and Bartholomew showed (16) pylene glycol)-block-poly (ethylene glycol), average Mn
that low cobalt loadings provided the best comparison of ca. 5800 (Aldrich, 43,546-5) as a template. To prepare Si4
SBA-15 silicas (Si4 and Si5) were obtained using block
5
catalytic supports for Fischer Tropsch catalysts.
sample, 6 g of P123 copolymer was dispersed in 45 g of
water and 180 g of 2M HCl under stirring. After complete
dissolving P123, 12.75 g of tetraethyl orthosilicate (TEOS)
were added to the solution.
EXPERIMENTALS AND METHODS
To synthesize Si5 silica, 90 g of dimethylformamide, 180 g
of water, and 180 g of 4M HCl were mixed in a glass flask
1
. Catalyst Preparation
Si1 sample (MCM-41 type) was prepared using a surfac- under stirring. Then 12 g of P123 was dispersed in the re-
tant with a shorter chain length (C12 compared to usual sulting solution under stirring. After complete dissolving of
C16) following procedure reported by Matthae et al. (21). P123, 24 g of TEOS were added.
The material was prepared by mixing 33.06 g of a 25 wt% so-
The mixtures used in the synthesis of Si4 and Si5 silicas
lution of the surfactant, dodecyl-trimethylammonium bro- were kept at 313 K under stirring for 24 h. Then they were
mide (C12TMAB, Aldrich), with 8.42 g of TMAOH, and transferred to a hermetically closed polypropylene flask
1
.4 g of 35% ammonia solution. The silica source, 8.81 g of and heated in an oven at 373 K for 48 h. The gels were
fumed silica (Aldrich) was added with 20.65 g of deion- washed with distilled water, dried at room temperature for
ized water so that the final molar ratio was 73.4SiO2 : 24 h, and calcined in air for 10 h using the same procedure
2
2.8TMAOH : 13C12TMAB : 7NH4OH : 1420H2O. The mi- as in the synthesis of Si2 silica. The details of synthesis pro-
xture was stirred for 30 min, then kept at 363 K for 72 h, then cedures are also available from Refs. (24–26).
filtered and washed with water and ethanol. The final prod- Samples Si6 and Si7 were commercial fumed silicas:
uct was dried at 363 K for 24 h and then carefully calcined Aerosil 380 (Degussa) and Cab-o-sil M5 (Cabot), respec-
in nitrogen and oxygen to 773 K. tively. These materials were agglomerated by wetting and
Si2 silica (MCM-41 type) was synthesized as described in dried in an oven at 393 K overnight.
Ref. (22). One gram of 28% NH4OH solution was added to Cobalt was introduced to silicas by aqueous incipient
1 g of 25% solution of cetyltrimethyl ammonium chloride wetness impregnation using solutions of cobalt nitrate pre-
2
under stirring. This solution was combined with 5.3 g of pared to obtain 5 wt% Co content in the final catalysts
tetramethylammonium hydroxide pentahydrate (Aldrich, (Table 2). The samples were dried in an oven at 373 K over-
9
7%), followed by the addition of 5.6 g of fumed silica (Cab- night and then calcined in the flow of dry air at 773 K for 5 h.
o-sil M-5, Cabot) and 11.4 ml of water under stirring. The The catalysts were pressed and sieved to obtain 200–500 µm
reacting mixture was transferred into a hermetically closed size fractions. Co content in the samples was measured by

2
32
KHODAKOV ET AL.
atomic absorption at the “Service Central d’Analyses du Eq. [1] is suggested to calculate the relative concentration
C.N.R.S” (Vernaison, France).
of Co metal phases (M, %—percentage of cobalt in metal
phases)
2
. Catalyst Characterization
R, %
4
− 1
1
00
M, % =
[1]
Surface area and pore size distribution. The BET sur-
3
face area and pore size distribution in mesoporous silicas
were measured using nitrogen sorption at 77 K. Prior to
the experiments, the samples were outgassed at 473 K for
where R, % is the extent of overall reduction and R ≥
25%.
The relative concentrations of cobalt metal phases are
used for calculating FT turnover rates (Table 3).
5
h. The isotherms were measured using a Micrometrics
ASAP 2010 system. The total pore volume (TPV) was cal-
culated from the amount of vapor adsorbed at a relative
pressure close to unity assuming that the pores are filled
with the condensate in liquid state. The pore size distribu-
tion curves were calculated from the desorption branches
of the isotherms using Barrett–Joyner–Halenda (BJH) for-
mula (27).
Oxygen titration. The catalysts were first reduced in hy-
drogen at 753 K for 5 h followed by purging in flowing
helium for 25 min at 723 K to remove any chemisorbed hy-
drogen; 0.1 ml pulses of oxygen were then introduced to the
flow of He at 723 K. The consumption of O2 by the cata-
lysts was measured by a thermal conductivity detector. The
experiments were completed when the detector showed no
consumption of O2.
X-raydiffraction. XRDpatternswererecordedatroom
temperature by a Siemens D5000 diffractometer using
CuKα radiation. Average Co3O4 particle size was calcu-
lated from the Sherrer equation (28) using a (440) Co3O4
XRD peak at 2θ = 65.344. Instrument line broadening was
corrected by calibration using a mechanical mixture of bulk
crystalline Co3O4 and silica.
XPS. XPS spectra were recorded using a Leybold
Heraeux LHS 10 spectrometer equipped with a 300 W Al
Kα source (hν = 1486.6 eV) operated at 13 kV and 25 mA.
The Si2p line (BE = 103.4 eV) from silica support was used
as a reference. The measurements were performed at room
−
8
temperature in high vacuum (10 Torr). Assuming both
uniform distribution of Co particles in catalyst grains and
TGA. The extent of Co reduction in hydrogen was mea-
sured using a Sartorius 4102 electronic ultramicrobalance
equipped with a controlled atmosphere cell. Co oxidized
catalysts (20–25 mg) were placed in a quartz crucible and
were dehydrated in dry air at 773 K for 3 h. Then the sam-
ples were cooled down to room temperature. After that
treatment, the catalysts were heated in a flow of hydrogen
from room temperature to 773 K. The rate of temperature
2
high specific surface area (>100 m /g) of the catalysts, the
sizes of Co3O4 particles can be calculated using a simplified
Kerkhof–Moulijn formula [33]:
ꢂ
ꢃ
ꢀ
ꢁ
ꢀ
ꢁ
d
λpp
1
− exp −
ICo
ISi
ICo
ISi
=
,
[2]
d
exp
monolayer
λpp
◦
ramping was 1 /min. The extent of overall cobalt reduc-
tion (R, %) was calculated from the weight loss in the at-
mosphere of hydrogen at 753 K assuming stoichiometric
reduction of Co3O4 to metallic cobalt. The data were cor-
rected by subtracting weight losses of silica supports treated
under the same conditions.
where (ICo/ISi)exp is the experimental electron intensity
ratio for Co2p and Si2p bands, d is the Co3O4 particle size,
λpp is the inelastic mean free path (IMFP) of the Co2p
photoelectron passing through Co3O4 supported phase,
(
ICo/ISi)monolayer is the predicted electron intensity ratio for
TGA technique used in the present work to measure ex-
tents of cobalt reduction was successfully tested for sup-
ported Co/Al2O3 catalysts in our previous report (29). An
excellent agreement was observed between the extents of
Co reduction measured by the TGA (temperature ramping
Co2p and Si2p bands assuming monolayer coverage of sil-
˚
ica by Co3O4 phase. The value of λpp = 18.5 A for Co2p
electrons in Co3O4 was calculated using Seah and Dench
formula (34). (ICo/ISi)monolayer was obtained according to
Eq. [3] using FAT mode (E0 = 50) and photoelectron cross
section values (σSi, σCo) from (35)
◦
of 1 /min) and by magnetic measurements (29).
Reduction of supported Co3O4 species in hydrogen
has been a subject of a large number of investigations
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ E ꢁ−0.23
Co
ICo
ISi
nCo
nSi
σCo
σSi
(
6, 29–32). It has been shown that all Co3O4 crystallites
can be reduced to CoO at the temperature range of 473–
73 K; the concentration of Co metal phases being con-
k
=
,
[3]
Si
E
monolayer
bulk
k
5
trolled by the ease of CoO → Co reduction step. Thus, the where (nCo/nSi)bulk is the ratio of bulk atomic concentra-
Co
Si
relative concentrations of Co metal phases in the reduced tions of Co and Si atoms, Ek and Ek are the kinetic en-
catalysts can be calculated assuming complete reduction of ergies of Co2p3/2 and Si2p electrons, respectively.
Co3O4 to CoO and partial reduction of CoO to Co metal. Equation[2]showsthatforagiven (nCo/nSi)bulk, (ICo/ISi)exp
On the basis of the stoichiometry of these two reactions, ratio increases with decreasing particle sizes, (ICo/ISi)exp

PORE SIZE EFFECTS IN FISCHER TROPSCH SYNTHESIS
233
ratio close to (ICo/ISi)monolayer indicates monolayer cover-
age of the support by cobalt atoms.
Catalytic experiments. Prior to catalytic experiments,
the samples were reduced in a flow of hydrogen at 753 K for
5
h. Catalytic behavior was studied in a fixed bed flow mi-
croreactor operating at 463 K and at atmospheric pressure
with H2/CO ratio of 2. The catalyst loadings were typically
between 0.6 and 0.7 g. Carbon monoxide contained 5% of
nitrogen, which was used as an internal standard. To avoid
possible condensation of the reaction products, the reactor
and gas transfer lines were placed in an oven and contin-
uously heated at 433 K. The reaction products were ana-
lyzed online by gas chromatography. Analysis of CO, CO2,
CH4, and H2 were performed using a 13X molecular sieve
column and a thermoconductivity detector. Hydrocarbons
(
C1–C18) were analyzed by a flame-ionization detector us-
ing a capillary HP-PONA column.
RESULTS AND DISCUSSION
1
. Catalyst Porosity
The isotherms of nitrogen adsorption and desorption on
mesoporous silicas and the corresponding pore size distri-
bution curves calculated using BJH method (27) are shown
in Figs. 1 and 2. The BET surface area, total pore volume
and average pore diameter are presented in Table 1. The
BET surface area in all periodic mesoporous silicas (Si1,
2
Si2, Si3, S4, S5) was higher than 650 m /g, whereas the BET
surface area of the commercial silicas (Si6, Si7) was much
2
lower (∼200–300 m /g). The shape of the isotherms of the
Si1 and Si2 sample (Fig. 1a) is typical for MCM-41 silicas
(
19–22). It corresponds to the adsorption of N2 on the walls
of narrow mesopores. The average pore diameter calcu-
˚
lated using BJH method was lower than 20 A. The surface
area, pore volume, and average pore diameter of Si1 and
Si2 silicas are in good agreement with previous reports (19–
2
2). The lower closure point of the nitrogen hysteresis loop
of the isotherm of Si3 silica (Fig. 1a) is situated at a rela-
tive pressure close to 0.42. The pore distribution curve of
˚
Si3 presented in Fig. 2a exhibits two peaks at 38 and 25 A.
FIG. 1. Nitrogen adsorption–desorption isotherms (a, b, c) of meso-
According to Gregg and Sing (36), the peak in the pore porous silicas. Isotherms of Si2, Si3, Si5, and Si7 silicas are offset for clarity.
˚
distribution curve at 38 A is likely to be an artifact reflect-
ing the tensile strength effect of the adsorbate. Thus, the
˚
average pore diameter of the Si3 silica is about 25 A.
BJH pore sizes distribution curves (Fig. 2) were narrower
Nitrogen adsorption isotherms of Si4, Si5, Si6, and Si7 for the Si4 and Si5 periodic mesoporous silicas, than for
silicas (Figs. 1b and 1c) belong to type IV according to the commercial Si6 and Si7. The average pore diameters ranged
˚
˚
classification of Brunauer et al. (37). Type IV isotherms oc- from 42 A in Si4 silica to 330 A in Si7.
cur on porous adsorbents possessing pores in the diameter
range from 20 to 500 A. All the isotherms show a reversible crease both in specific surface area and in total pore volume
Impregnation of silicas with cobalt nitrate produces a de-
˚
part and a type A hysteresis loop at higher pressures. Ac- (Table 1). At low cobalt loadings (5 wt%) the observed de-
cording to De Boer (38), the type A hysteresis loop is asso- crease in surface area after catalyst impregnation is primar-
ciated with the presence of “cylinder shape” pore of rather ily attributed to clogging support pores by cobalt species
constant cross section. The P/Po position of the inflection that makes them inaccessible for nitrogen adsorption. This
points is related to a diameter in the mesopore range. The effect was especially pronounced for narrow pore supports

2
34
KHODAKOV ET AL.
TABLE 1
Adsorption Properties of Mesoporous Silicas and Cobalt Catalysts
Adsorption properties of
mesoporous silicas
Adsorption properties of
cobalt catalysts
SBET,
m /g
TPV,
cm /g
Pore diameter,
Synthesis
Ref.
SBET,
m /g
TPV,
cm /g
Pore diameter,
2
3
˚
2
3
˚
Silica
A
Co Catalysts
A
Si1
Si2
Si3
Si4
Si5
Si6
Si7
1481
742
1285
679
887
311
213
0.75
0.59
1.04
0.78
1.91
1.38
0.84
≈20
≈20
25
42
91
(21)
(22)
(23)
(25)
(26)
CoSi1
CoSi2
CoSi3
CoSi4
CoSi5
CoSi6
CoSi7
738
462
397
586
674
275
206
0.41
0.46
0.26
0.62
1.11
1.22
0.76
20
≈20
20
43
75
280
330
Aerosil 380
Cab-o-sil M5
260
230
(
Si1, Si2, and Si3, Tables 1 and 2). This observation is consis- were detected for all oxidized catalysts. The XRD peaks
tent with previous reports (39–41). Impregnation also leads were much broader for Co3O4 crystallites in narrow pore
to a slight decrease in support concentration in modified silicas (CoSi1, CoSi2, and CoSi3). The Co3O4 crystallite
catalysts, a “dilution effect,” which could also contribute to sizes (Table 2) were calculated from the widths of XRD
decrease in catalyst surface areas.
Note that the shape of pore size distribution curves re- In the CoSi3 sample, XRD patterns were very broad and
mains however the same, though the average pore diameter had low intensity. This made it impossible to measure the
peaks (28) using the Sherrer equation (2θ = 65.344 (440)).
of the catalysts was smaller after cobalt introduction.
widths of XRD peaks and Co3O4 particle sizes for that sam-
ple. Table 2 shows that the sizes of Co3O4 crystallites de-
pend on the average pore diameters in mesoporous silicas;
larger Co3O4 crystallites are found in silicas with wider
pores.
The results of XPS experiments are presented in Figs. 4
and 5 and Table 2. The O1s binding energy near 533.1 eV is
characteristic of oxygen in SiO2 (42, 43). Co2p XPS spectra
2
. Cobalt Species and Particle Sizes
The XRD patterns of Co supported catalysts are pre-
sented in Fig. 3. XRD patterns characteristic of Co3O4
(
Fig. 4) show that Co3O4 is the dominant cobalt phase in
oxidized catalysts. This finding is consistent with the obser-
vation of Co3O4 XRD patterns in all catalysts and our pre-
vious X-ray absorption data (44). For wider pore catalysts
(
CoSi6 and CoSi7) the shapes and binding energies of the
Co2p line (Fig. 4) were very similar to those of bulk Co3O4
45). Higher intensities of the satellite peak and a shift of
(
the XPS lines toward the higher energy region observed
for narrow pore catalysts (CoSi1, CoSi2, CoSi3, CoSi4, and
2
+
CoSi5), indicate the presence of Co ions in interaction
with the surface (46).
The sizes of Co3O4 particles reported in Table 2 were
obtained from the intensity of XPS signal using Kerkhof–
Moulijn method (33). The XPS intensity ratio (ICo/ISi)exp
measured for the different relative atomic concentrations
of cobalt (nCo/nSi)bulk is presented in Fig. 5. The straight line
calculated using formula [3] represents the monolayer limit
corresponding to maximum of dispersion of the Co3O4 sup-
ported phase. Lower relative intensity of Co2p XPS exper-
imental signal (ICo/ISi)exp indicates large Co3O4 particles.
The reason for the lower intensity of (ICo/ISi)exp signal for
larger particles is that a considerable number of Co atoms
are located in the bulk of cobalt particles where they are not
˚
detectable by XPS (depth of analysis ≈60 A). Figure 5 and
FIG. 2. Pore size distribution curves calculated from nitrogen desorp-
tion isotherms for (a) narrow pore and (b) wide pore silicas.
Table 2 show that Co O particles are considerably larger
3
4

PORE SIZE EFFECTS IN FISCHER TROPSCH SYNTHESIS
235
TABLE 2
Characterization of Co Catalysts
Co3O4 crystallite
diameter from, A
˚
Estimated cobalt
dispersion,
%
Extent of overall reduction
at 753 K measured by
TGA, %
Amount of O2 consumed
in oxygen titration, mole
O2/mole Co
Co content,
wt%
Co catalyst
XRD
XPS
CoSi1
CoSi2
CoSi3
CoSi4
CoSi5
CoSi6
CoSi7
5.47
5.67
5.05
6.95
5.39
5.75
4.75
57
43
n.d.
92
121
147
230
8
16
18
70
67
96
76
22.5
29.8
n.d.
13.9
10.6
8.7
43.2
62.9
22.8
72.8
94.8
97.2
94.4
0.212
0.202
0.091
0.373
0.466
0.417
0.673
5.6
in wider pore (Si4, Si5, Si6, Si7) than in narrow pores silicas silicas followed by calcinations could lead to enrichment
Si1, Si2, Si3). of Co3O4 particles on the exterior of the SiO2 grains (47).
Table 2 also shows that Co3O4 particle sizes measured Higher concentration of Co3O4 near the outer surface of
(
from XRD using the Scherrer equation were much larger catalyst grains could lead to higher intensity of the Co2p
than those measured by XPS. Similar differences between
the sizes of Co3O4 particles in silica supported catalysts
measured by XPS and XRD were observed in previous
reports. Castner et al. (47) found for example, that XRD
determined Co3O4 particle sizes were about 2 times larger
than those measured by XPS. These differences seem to be
related to the limitations of XPS and XRD methods.
The validity of the Kerkhof and Moulijn model rests on
the assumption of uniform distribution of the supported
phase between the bulk and outer surface of catalyst grains.
Previous works however, showed that impregnation of
FIG. 3. XRD patterns of Co catalysts: (1) CoSi1; (2) CoSi2; (3) CoSi3;
FIG. 4. XPS spectra of the Co2p level for Co catalysts supported by
(4) CoSi4; (5) CoSi5; (6) CoSi6; and (7) CoSi7.
mesoporous silicas.

2
36
KHODAKOV ET AL.
that the positions of the first slope in the TGA curves and
temperatures of Co3O4 to CoO reduction, respectively, are
not influenced by Co3O4 particle sizes. The second slope at
600–700 K can be assigned to the reduction of CoO phase
to metallic cobalt. The position of the second slope in TGA
curves depends on the particle size; catalysts containing
smaller particles are reduced to metallic cobalt at temper-
atures higher than those with larger particles. The extents
of cobalt reduction at 753 K calculated from TGA data are
shown in Table 2. The extent of reduction was significantly
lower for narrow pore CoSi1, CoSi2, CoSi3, and CoSi4 than
for wider pore CoSi5, CoSi6, and CoSi7 samples.
The amount of oxygen required to oxidize reduced Co
species was measured by oxygen titration at 723 K (Table 2).
This technique, often based on the assumption of stoichio-
FIG. 5. Relation between measured XPS intensity ratio (ICo/ISi)exp
and the ratio of bulk atomic concentrations of Co and Si atoms (nCo/
nSi)bulk: (1) CoSi1; (2) CoSi2; (3) CoSi3; (4) CoSi4; (5) CoSi5; (6) CoSi6; metric oxidation of metallic cobalt to Co O is commonly
3
4
and (7) CoSi7.
used to evaluate the extent of Co reduction (7, 18, 50). Re-
cently, we showed (31) that in inert atmosphere at tem-
peratures higher than 623 K supported CoO phase could
be more stable than Co3O4. Decomposition of supported
Co3O4 particles to CoO was observed for example, on heat-
ing Co supported silica samples in inert atmosphere (31).
Therefore, oxygen titration conducted in helium could re-
sult in oxidation of Co metal phases to CoO instead of
Co3O4 or to the mixture of CoO and Co3O4. Thus, we
believe that O2 titration assuming selective formation of
Co3O4 upon oxidation provides underestimated extents of
the overall reduction relative to TGA method. O2 titra-
tion data are used in this work qualitatively in conjunction
with more reliable values of extents of reduction measured
by TGA. Table 2 shows higher consumption of oxygen by
wide pore cobalt catalysts in agreement with higher extents
of reduction measured by TGA.
XPS signal and therefore to underestimating Co3O4 parti-
cle sizes using the Kerkhof and Moulijn model. The Co3O4
particle sizes evaluated from XRD patterns were found
slightlylargerthantheporediametersofmesoporoussilicas
(
Table 2) probably because of the following reasons. First,
the particles of Co3O4 could adopt a slightly elongated
shape in mesopores. Second, it is known (48) that measur-
ing sizes of crystallites from the half-width of the diffraction
profile could slightly overestimate crystallite diameters.
Third, some very small cobalt particles could be missed by
XRD because of significant XRD line broadening.
Holmen and co-workers showed (7) that Co3O4 particle
sizes could be converted to the corresponding cobalt metal
particle sizes according to the relative molar volumes of
metallic cobalt and Co3O4. The resulting conversion fac-
˚
tor for the diameter (d, A) of a given Co3O4 particle be-
0
ing reduced to metallic cobalt is: d(Co ) = 0.75 d(Co3O4).
Cobalt dispersion (D, %) can be calculated from average
˚
metal particle sizes (A) assuming spherical uniform parti-
cles with site density 14.6 atoms/nm , by the use of formula:
2
D = 960/d (7, 49).
Inthepresentwork, thediameterofCo3O4 particlesmea-
suredbyXRDisusedtoestimateCometalparticlesizesand
metal dispersions. The estimated values of Co dispersion
are shown in Table 2. A reasonable agreement between Co
metal dispersions and particle sizes calculated from XRD
and from H2 chemisorption on cobalt supported silica cata-
lysts was previously reported in the literature (7, 18).
3
. Cobalt Reducibility
Weight loss of the catalysts in hydrogen as a function
of temperatures is presented in Fig. 6. The curves exhibit
two steps: at 500–600 K and at 600–700 K. In agreement
with previous reports (6, 29–32), the weight loss at 500–
FIG. 6. Weight loss of catalysts in hydrogen: (1) CoSi1; (2) CoSi2;
3) CoSi3; (4) CoSi4; (5) CoSi5; (6) CoSi6; and (7) CoSi7. The curves
were corrected by subtracting contributions of supports and normalized
(
6
00 K is attributed to the Co3O4 → CoO transition. Note to 5 wt% loadings.

PORE SIZE EFFECTS IN FISCHER TROPSCH SYNTHESIS
237
4
. Catalytic Behavior
Catalytic measurements were made in a differential cata-
lytic reactor, carbon monoxide conversion was lower than
%. C1–C18 hydrocarbons and water were detected as re-
5
action products. Carbon dioxide was not observed at the
reaction conditions.
Figure 7 shows FT reaction rate, methane, and C5+ se-
lectivites as functions of time on stream on the CoSi4 sam-
ple. FT reaction rates decrease slowly with time-on-stream,
whereas hydrocarbon selectivities remain unchanged. Af-
ter 7 h on stream the reaction rates decreased by about 10–
1
5% relative to the initial values. The catalytic behavior of
FT catalysts is usually evaluated in the literature (7, 15–18,
6, 51) using quasi steady state reaction rates and selectiv-
ities. It was shown previously (46) that a quasi steady state
FIG. 8. Typical product distribution on CoSi1 and CoSi7 catalysts
T = 463 K, H2/CO = 2, time-on-stream = 7 h).
(
4
for Co silica supported catalysts at similar FT reaction con- rates increase several times as the pore size increases from
˚
ditions could be normally reached after 4 h on stream. Thus, 20 to 330 A. The catalysts with pore diameter smaller than
˚
the reaction rates were calculated from carbon monoxide 30 A exhibit low activity, a significant increase in FT reac-
˚
conversions after 7 h on stream and gas hourly space veloc- tion rates is observed as the pore size increases to 40 A.
ities; then they were normalized by the number of cobalt Considerably higher methane (30%) and lower C5+ selec-
atoms loaded in the reactor. The FT turnover rates (reac- tivities were observed on narrow pore silicas, whereas the
tion rates normalized by the total number of Co atoms on chaingrowthprobabilities(α)wereintherangeof0.66–0.78
the surface of metal particles) were obtained from FT re- for both narrow and wide pore catalysts.
action rates, relative concentrations of cobalt metal phases
calculated using Eq. [1], and cobalt dispersion (Table 3).
Co dispersion (Table 2) was calculated from XRD Co3O4
particle size using the formulas from Ref. (7).
The hydrocarbon selectivities were calculated on carbon
basis. The chain growth probabilities, α, were calculated
from the slope of the curve ln(Sn/n) versus n, where n is the
carbon number of the hydrocarbon and Sn the selectivity to
corresponding hydrocarbon (52). The catalytic data for the
Co catalysts supported by mesoporous silicas are presented
in Table 3. Typical hydrocarbon distributions are presented
in Fig. 8.
Figure 9 and Table 3 show that the catalytic behavior of
Co catalysts supported by mesoporous silicas is strongly
affected by porous structure of the support. The reaction
FIG. 7. FT reaction rate, methane, and C5+ selectivites as functions
of time on stream on CoSi4 sample (T = 463 K, H2/CO = 2).
FIG. 9. (a) FT reaction rates and (b) hydrocarbon selectivities as func-
tions of support pore sizes.

2
38
KHODAKOV ET AL.
TABLE 3
Catalytic Behavior of Co Catalysts Supported by Mesoporous
larger particles and this interaction was likely to stabilize
small oxidized particles and clusters in silica. We found that
the sizes of supported Co3O4 crystallites are controlled by
support pore diameters in mesoporous silicas. Larger pore
diameters lead to larger Co3O4 particles. The reducibility
of Co species is higher in larger pore silicas, because larger
pore silicas contain larger, and thus easier to reduce, Co3O4
crystallites.
Thus, silica porosity affects the structure of Co supported
species in two ways: first, the support pore diameter con-
trols the size of encapsulated Co species; second, cobalt
reducibility is influenced by support porosity; reducibility
Silicas (H
sion <5%)
2
/CO = 2, P = 1 bar, Tr = 463 K, 7 h On-Stream, Conver-
Reaction
rates,
Turnover
CH4
selectivity,
C-%
C5+
selectivity,
C-%
Co
catalyst
frequency,
−
4
−1
−4 −1
10
s
10
s
α
CoSi1
CoSi2
CoSi3
CoSi4
CoSi5
CoSi6
CoSi7
0.5
0.1
9.16
0.66
—
17.50
13.99
17.55
51.72
24.9
22.7
34.5
15.2
15.3
17.2
16.9
50.1
57.0
44.4
65.0
68.4
59.5
60.7
0.66
0.78
0.71
0.74
0.77
0.05
1.55
1.38
1.47
2.68
0.73 of cobalt species being much lower in narrow pore supports.
0.70
Let us now discuss the effect of pore size on the cata-
lytic behavior ofCo supported mesoporous silicas. Catalytic
data show that FT reaction rates were much lower on Co
catalysts supported by narrow pore materials. Catalyst char-
acterization revealed that narrow pore catalysts contained
smaller cobalt particles. Thus, the activity of the catalysts
5
. Pore Size Effects on the Structure of Cobalt Particles
and Their Catalytic Performance in FT Synthesis
Characterization results show that the structure, re- seems to be affected by the size of cobalt particles. The FT
ducibility of supported cobalt species, and their catalytic be- turnover rates estimated from FT reaction rates, relative
havior are strongly influenced by catalyst porosity (Table 2). concentrations of cobalt metal phases and dispersion, were
The size of supported Co3O4 particles measured by both also found to be lower for smaller particles than for larger
XRD and XPS is found to depend on pore sizes, smaller particles (Table 3).
Co3O4 particles being observed in narrow pore silicas.
These results are in agreement with previous findings.
The dependence of Co3O4 particle sizes on catalyst pore Reuel and Bartholomew (16) found that turnover frequen-
diameters (Table 2) suggests preferential localization of cies of Co/Al2O3, Co/SiO2, Co/TiO2, and Co/C catalysts
Co3O4 particle in silica pores. Previous studies have shown prepared by impregnation method increased with decreas-
that narrow pore size distribution in periodic mesoporous ing metal dispersion. The trend of decreasing activity with
silicas could stabilize supported nanoparticles. Zhang et al. increasing dispersion was assigned either to changes in sur-
(
53) reported the procedure to prepare nanosized ZnS par- face structure with decreasing particle size or to electronic
ticles in ordered mesoporous silicas. Iwamoto et al. (54) modifications as a results of more intimate interactions of
showed that MCM-41 materials could stabilize nanoparti- the small crystallites with the support. It was also suggested
cles of iron oxides. MCM-41 and SBA-15 mesoporous ma- (51) that FT synthesis was structure sensitive and favored
terials were also used (55, 56) to fabricate nanowires of sites to which CO was strongly coordinated. Since only par-
noble metals encapsulated in silica pores. In line with these tially reduced Co samples were studied in that work, it
reports we found that the sizes of Co3O4 particles in meso- was rather difficult to draw unambiguous conclusions about
porous silica are controlled by pore sizes.
structural sensitivity of FT synthesis. Iglesia et al. (17, 18)
Differences in Co3O4 particle sizes lead to differences in showed that FT synthesis rates were proportional to metal
cobalt reducibility. Much lower reducibility of small Co3O4 dispersion and were independent of the support. They
particles in narrow pore supports was observed by TGA. attributed lower activity of small cobalt particles observed
The extents of reduction vary from 20–60% for narrow pore in previous reports to the differences in the extent of re-
silicas (Si1, Si2, Si3) to 90–100% for wider pore supports duction as a function of metal dispersion, chemical identity,
(
Table 2). A lower consumption of oxygen by CoSi1, CoSi2, and surface properties of the support. It was suggested that
and CoSi3 catalysts measured by oxygen titration also in- partially reduced cobalt species did not catalyze FT syn-
dicates a lower extent of reduction of Co species in nar- thesis and that small particles could be easily reoxidized by
row pore silicas. It can be suggested therefore, that small water and other reaction products at the conditions of FT
cobalt particles in narrow pores are more difficult to re- synthesis. Note that it is rather difficult to separate catalytic
duce than larger particles in wider pores. This suggestion is effects which could arise from possible reoxidation of small
consistent with earlier data (6, 29–32). It is known that re- Co particles at FT reaction conditions or from structural
ducibility of Co3O4 crystallites depends on their sizes; the sensitivity of FT synthesis over Co particles of different
ease of reduction increasing from smaller to larger parti- sizes.
cles. It was suggested (31) that interaction between metal
Our data are consistent with the suggestion that lower
and support in smaller particles was much stronger than in activity of small cobalt particles could be attributed to their

PORE SIZE EFFECTS IN FISCHER TROPSCH SYNTHESIS
239
lower reducibility. Lower FT catalytic activities were always
observed on partially reduced cobalt species. Tables 2 and 3
show that FT reaction rates and turnover frequencies in-
crease dramatically as the extents of reduction reach 90%.
It is also seen that FT turnover frequencies, which represent
FT reaction rates normalized by the number of available Co
surface atoms, increase with increasing Co particle sizes.
The dependence of FT turnover frequencies on Co particle
sizes suggests either possible reoxidation of smaller cobalt
particles at FT reactions conditions or structural sensitivity
of FT synthesis.
Table 3 displays higher methane selectivity on smaller
cobalt particles located in narrow pore silicas. Methane
selectivity drops from 30 to 15% as silica pore sizes in-
FIG. 10. Relation between methane selectivity and extent of the over-
all catalyst reduction.
˚
creasesfrom20to100 A. Previousstudiesalsoreporthigher
methane selectivity on small Co particles. At least three
suppositions have been used to explain higher methane se-
lectivity in FT synthesis over small Co particles. First, high methane selectivity and the extent of overall reduction of
methane selectivity was attributed to the sites of weak car- cobalt supported mesoporous silicas, higher methane se-
bon monoxide adsorption (51). It is generally assumed that lectivies being observed over less reduced samples. Note
heavier hydrocarbons are favored when carbon monox- that such catalysts are always found to contain smaller
ide and intermediates are strongly adsorbed by metal Co O particles (Table 2). Thus, higher methane selectiv-
3
4
sites.
ities observed on narrow pore silicas are likely to be at-
Second, Reuel and Bartholomew (16) assigned higher tributed to the presence of either unreduced cobalt species
methane selectivity observed on well-dispersed low loading or the small cobalt particles, which produce higher relative
cobalt catalysts to the presence of stable unreduced oxide amounts of methane than large cobalt particles, rather than
phases capable of catalyzing water-gas shift reaction: CO+ to either higher H /CO ratio in catalyst pores due to the
2
H2O → CO2 +H2, and thereby increasing the H2/CO ratio water-gas shift reaction or diffusion limitations for carbon
at the catalyst surface. The water-gas shift reaction suggests monoxide.
the emission of measurable amounts of carbon dioxide. Our
In the present work, catalytic behavior of Co supported
data do not agree with this interpretation of higher methane catalysts was studied at atmospheric pressure and at low
selectivities on narrow pore catalysts, since carbon dioxide conversion levels. Higher pressures and higher carbon
was not detected among reaction products.
monoxide conversions would probably lead to the satu-
Third, diffusion limitations for carbon monoxide in cata- ration of catalyst pores by liquid reaction products. The
lyst pores could also increase H2/CO ratio in catalyst pore concentrations of hydrocarbons adsorbed in catalyst pores,
and thus, methane selectivities (13). It is known however, especially the heaviest ones, depend on their partial pres-
that an increase in effective H2/CO ratio usually leads to sure in the catalyst bed. The partial pressure of the products
both an increase in methane selectivity and to higher chain at the same conversion and selectivity levels depends on the
termination probability and to lower coefficient of chain total pressure in the reactor. An increase in total pressure
propagation α. Table 3 shows that the decrease in pore even at the same conversions and selectivities could result
sizes results only in higher methane selectivity, whereas the in condensation of hydrocarbons, which are normally in the
coefficient of chain propagation seems not to be affected gaseous state at atmospheric pressure.
by variation in catalyst pore diameters.
In addition, the partial pressure of reaction products de-
Our catalytic results (Table 3) also show that lower C5+ pends on their yields. It is known (2, 17) that the reac-
selectivity observed on narrow pore samples is likely to tant pressure in Fischer Tropsch synthesis affects usually
be related not to lower probability of chain propagation chain growth and chain termination probabilities as well
of narrow pore silicas, but to higher methane selectivity. as the rates of secondary reactions. Iglesia, for example,
Therefore, the contribution of mass transport limitations showed (17) that chain termination probability to paraf-
to higher methane selectivities observed on narrow pore fin on Co/TiO catalysts dramatically decreased as the re-
2
catalysts is unlikely to be significant.
actant pressure increased from 540 to 2000 kPA. Higher
Analysis of literature data (16, 51) shows that higher chain growth probability leads to higher heavier hydrocar-
methane selectivies are observed in FT synthesis when Co bon selectivities and therefore to higher yields of heavier
catalysts are not completely reduced or contain smaller hydrocarbons at higher reactant pressures even at the same
cobalt particles. Figure 10 shows a relationship between carbon monoxide conversion. A simple increase in carbon

2
40
KHODAKOV ET AL.
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1
1
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Characterization techniques uncover that both the size of
supported Co3O4 crystallites and their reducibility strongly
depend on the pore diameter of mesoporous silicas. Small
pores present in MCM-41 materials lead to a smaller size
2
(
1992).
2
2
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(
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