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Applied Catalysis A: General 321 (2007) 79–85

Effects of impregnation solvent on Co/SiO2 catalyst for

Fischer-Tropsch synthesis: A highly active and stable

catalyst with bimodal sized cobalt particles

Yi Zhang a, Yong Liu a, Guohui Yang a, Shouli Sun b, Noritatsu Tsubaki a,*a Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan

b Gas Reaction Technologies, Inc., 861 Ward Drive, Santa Barbara, CA 93111, USA

Received 8 May 2006; received in revised form 9 January 2007; accepted 15 January 2007

Available online 18 January 2007

Abstract

Silica-supported cobalt (20 wt%) catalysts were prepared by incipient-wetness impregnation of silica with different cobalt nitrate solutions. The

catalyst prepared from dehydration ethanol solution exhibited stable and the highest activity, as well as significantly low methane selectivity.

Cobalt crystalline size of the catalyst prepared from dehydrated ethanol was smaller than that of the catalyst prepared from aqueous solution, and

existed two different size where the large particles showed low bulk density with cluster-like structure. But only larger clusters existed in the

catalyst prepared from aqueous solution. The increased amount of active sites and more reactive adsorbed CO on the surface determined the highest

activity of the catalyst prepared from dehydrated ethanol solution in liquid-phase Fischer-Tropsch synthesis (FTS) reaction.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Fischer-Tropsch synthesis; Co/SiO2 catalyst; Ethanol solvent; Preparation method; Syngas

1. Introduction

Fischer-Tropsch synthesis (FTS), directly converting coal

gasifier gas or natural gas into higher hydrocarbons, has

attracted great attentions because of the increase of demands for

environmentally friendly liquid fuels. A key step in improving

the activity of FTS is the development of highly active catalysts.

Supported cobalt catalysts have been demonstrated as the

preferred catalysts due to its high selectivity for production of

long-chain paraffins and low water-gas shift (WGS) reaction

reactivity [1,2]. Recent studies indicated that the activity of

cobalt catalysts depended on the numbers of active sites after

the reduction, while the active site numbers should be

determined by the metallic Co particle size, loading amount,

dispersion and reduction degree [3,4]. Synthesis of highly

dispersed Co catalysts requires strong interaction between the

support and the Co precursor, which forms fine CoO or Co3O4

* Corresponding author. Tel.: +81 76 445 6846; fax: +81 76 445 6846.

E-mail addresses: tsubaki@eng.u-toyama.ac.jp,

tsubaki@eng.toyama-u.ac.jp (N. Tsubaki).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.01.030

clusters [5]. It has been reported that dispersion of supported

cobalt is usually influenced by Co3O4 dispersion, which formed

from the catalyst precursor [6,7].

Silica, as a common support used in the catalysts of FTS, has

the characteristics of a higher surface area, porosity, stability

and weaker metal–support interaction than aluminum as

support. However, the weak interaction between cobalt and

silica in silica-supported catalysts favors the reduction of cobalt

precursor and promotes agglomeration of supported cobalt

particles, reducing the dispersion of supported cobalt and the

numbers of active sites [8]. It is considered that solvents used to

dissolve Co precursors have a remarkable effect on the

interaction between cobalt and silica. Ho et al. [9] observed

ethanol as a solvent of impregnation instead of water for the

catalysts started from cobalt nitrate improved the dispersion of

supported cobalt and retained a high extent of reduction of the

cobalt phase. They found that ethoxyl groups on the silica gel or

Co3O4 surface during impregnation of ethanolic cobalt nitrate

solution might hinder the aggregation of Co3O4 by physically

interfering during the thermal decomposition of cobalt nitrate

and affect the sintering process of the cobalt metal during

reduction as well [10]. Ming and Barker [11] investigated the

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–8580

influence of pH value of impregnation solution on the formation

of cobalt silicates. They observed that cobalt ions reacted with

the surface of silica gel to form various cobalt silicates or

hydrosilicates at pH � 5. Although these species can only been

reduced at elevated temperatures (exceeding 1000 K), it has

been indicated that a certain amount of these cobalt silicates is

necessary to obtain highly dispersed cobalt catalysts [12].

However, none of these studies have been investigated with

respect to the effect of ethanol as a solvent of impregnation on

the catalytic performance for FTS reaction.

It is estimated that solvent used to prepare Co precursors

plays an important role in the precipitation and calcination of

the supported cobalt. In this study, effects of various organic

solvents, which were used to prepare dehydrated cobalt nitrate

precursors, were investigated, and the various catalysts were

characterized by XRD, H2 and CO chemisorption, oxygen

titration, TEM, TPSR, TPD and FT-IR.

2. Experimental

2.1. Preparation of catalysts

Co/SiO2 catalyst containing 20 wt% Co was prepared by

incipient-wetness impregnation of the supports with different

cobalt nitrate solutions. The support was commercially

available silica gel (ID gel, Fuji Davison, specific surface

area: 270 m2 g�1, pore volume: 1.22 cm3 g�1 and average pore

diameter: 8.7 nm). The dehydrated cobalt nitrate, which was

used to form different cobalt nitrate solutions, was prepared by

heating commercially available Co(NO3)2�6H2O at 373 K for

3 h. The impregnated samples were evacuated in a desiccator

for 1 h and dried by slow evaporation at room temperature. The

two catalysts using deionized water and dehydrated ethanol as

solvent were designated by CoH and CoE, respectively.

All samples were sieved to desired pellet sizes (80–100 mesh).

All catalysts were reduced at 373 K for 1 h and 673 K for 10 h in

series and then cooled to room temperature to be passivated by

1% oxygen diluted by nitrogen at room temperature.

2.2. Characterization of catalysts

2.2.1. XRD

X-ray diffraction of the reduced catalyst was carried out with

a RINT 2400 X-ray diffractometer, and all samples were

scanned using Cu Ka radiation in the range of 208 < 2u < 808in the step mode (0.028, 1.5 s). The mean crystallite size (D) of

cobalt metal was determined from the line half broadening of

the diffraction lines using the Scherrer equation.

2.2.2. Hydrogen and carbon monoxide chemisorption

measurement

Hydrogen and carbon monoxide chemisorption experiments

for CoH and CoE were conducted in a static volumetric glass

high-vacuum system. Research grade gases were used for

catalyst pretreatment and adsorption without further purifica-

tion. Before H2 and CO adsorption, the passivated catalysts

were in situ reduced in H2 at 673 K for 1 h, and then evacuated.

H2 adsorption was measured at 673 K for 1 h; CO chemisorp-

tion was implemented at room temperature [13,14]. Dispersion

percentage was based on the chemisorbed H2, assuming H:Co

of 1:1 surface stoichiometry [14].

2.2.3. Oxygen titration measurement

The extent of cobalt reduction was obtained from the weight

gain after reoxidation of reduced catalyst using oxygen titration

method. Experiments were carried out in lab-built equipment

constructed by stainless-steel tubing, fitting, valve and mass

flow controllers in which a quartz microreactor was used. The

passivated catalysts of about 0.1 g were re-reduced by hydrogen

at 673 K for 1 h, and then the whole system was purged by

helium stream of 25 ml/min for 1 h in order to remove the

hydrogen absorbed on the catalyst surface. The oxygen pulses

were introduced into the reactor by a six-port valve to reoxidize

the catalysts. The reduction degree was calculated by assuming

stoichiometric conversion of metallic Co to Co3O4 [15,16],

from the consumed oxygen of the pulses.

2.2.4. TEM measurement

For transmission electron microscope (TEM) measurement,

a catalyst pellet was embedded into resin and cut by microtome

method (Leica Ultracut UCT). The observation was imple-

mented on JEOLTEM-2010. Average particle size is estimated,

according toP

nid3i =P

nid2i .

2.2.5. TPSR and TPD

Temperature-programmed surface reaction (TPSR) experi-

ments were carried out using similar equipment with oxygen

titration. The passivated catalysts were in situ reduced by

hydrogen at 673 K for 1 h, and then the system was purged by

helium stream of 25 ml/min for 1 h. Subsequently, the

absorption of CO was performed at flow rate of 15 ml/min

for 5 min. After removing the physically absorbed CO using

helium stream, hydrogen stream of 30 ml/min was introduced

into the microreactor heated at the rate of 5 K min�1. Products

such as CH4, CO and CO2 were analyzed by a gas

chromatography equipped with a methanator and GC-FID.

Experiments on temperature-programmed desorption (TPD)

were conducted similarly as those of TPSR, only the carrier gas

was helium instead of H2.

2.2.6. FT-IR

FT-IR spectra were recorded using a Nicolet Magna 550

spectrometer. For each sample, catalyst of 30–40 mg was

pressed into a disk with diameter of 10 mm. The disk was

placed in an infrared cell equipped with CaF2 windows. CO was

adsorbed on the catalysts at room temperature (PCO = 200 Torr)

for 10 min. Then, the cell was evacuated for 5 min. The detail

procedure of the measurement was described elsewhere [17].

2.3. Activity tests for FTS

All tests were performed in a typical flow-type semi-batch

reactor at 513 K with 1 MPa overall pressure by feeding a gas

mixture of CO:H2 = 1:2. The passivated catalyst (1 g) and

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–85 81

solvent of 20 ml (n-hexadecane) were poured into the reactor.

Effluent gas released from the reactor was cooled with a dry-ice

trap, and then was analyzed by on-line gas chromatography. CO

and CO2 were analyzed by using an active charcoal GC column

equipped with a thermal conductivity detector (TCD). The light

hydrocarbons were analyzed using Porapak-Q column with FID.

The liquid products collected in the dry-ice trap and those

remaining in the reactor were combined and analyzed using a

Silicone SE-30 column with FID. Argon was employed as an

internal standard with a concentration of 3% in the feed gas.

3. Results and discussion

3.1. Catalytic activity of Co-based catalysts prepared from

different solvents

The activity and selectivity of the catalysts prepared from

different solvent were shown in Table 1. Maximum CO

conversions were used in Table 1 for the deactivated catalysts. It

is clearly shown that the catalyst activity greatly depended on

the type of solvent. The catalyst prepared from dehydrated

ethanol (CoE) exhibited the highest activity and lowest

methane selectivity; but the catalyst prepared from cyclohex-

anol displayed the lowest activity and highest methane

selectivity. Using 95% ethanol as solvent alternatively, the

catalyst activity was much lower than that of catalyst prepared

from dehydrated ethanol. This phenomenon indicated that

solvent other than water was important in the catalyst

preparation. Van Steen et al. used temperature program

reduction (TPR) method to study the reduction behavior of

Co/SiO2 catalysts prepared from different solutions, pointing

out that the TPR spectra of the catalysts prepared from water, n-

alcohols, acetone, or THF were similar [18]. But the reduction

spectrum of the catalyst prepared using 90% dehydrated cobalt

nitrate was remarkably different from that of the catalyst made

using the original Co(NO3)2�6H2O, indicating that the effect of

water ligand in the cobalt complex could not be omitted.

Generally, decreasing the polarity of the solvent caused an

increase in the interaction between the cobalt complex in the

solution and the silica gel surface. It is known that, with silanol

Table 1

Catalytic behaviors of various 20 wt% Co/SiO2 FTS catalysts prepared from

different solvents

Solvent CO conversion

(%)

CH4 sel.

(%)

CO2 sel.

(%)

a

Water 64.28 7.49 4.80 0.85

Methanol 62.40 7.64 3.36 0.85

Ethanola 89.96 5.60 3.45 0.91

Ethanolb 67.81 5.26 3.80 0.91

Acetone 64.05 6.79 3.28 0.85

n-Propanol 64.65 7.22 4.08 0.85

DMF 51.63 5.33 1.62 0.87

THF 57.92 6.25 2.93 0.85

Cyclohexanol 10.31 12.13 0.98 0.82

Reaction conditions: 513 K, 1.0 MPa, W/F = 5 g-cat h mol�1, CO/H2 = 1/2.a Dehydrated ethanol.b Ninety-five percent ethanol.

groups, water, as a solvent, formed a glassy layer of

immobilized water due to the formation of hydrogen bond

on the silica surface [19,20]. If solvents that are less able to

form these hydrogen bonds, such as ethanol, are used, the

silanol group can form these H-bonds directly with the original

water ligands of the cobalt complex, as it is impossible to

remove all water ligands from Co(NO3)2�6H2O. This interac-

tion might promote the fixation of the precursor onto the silica

surface and increase the cobalt dispersion. Different solvent

determined formation type between cobalt nitrate itself and

silica surface, resulting in variation of dispersion, surface

morphology and reduction degree of cobalt particles. On the

other hand, the CoE catalyst exhibited the highest a value

(chain growth probability) as shown in Table 1, and higher C5+

selectivity than CoH catalyst as shown in Table 2, indicating

that CoE catalyst was advantageous for formation of long-chain

hydrocarbons, besides the highest CO conversion.

CoE was also very stable, which kept the highest activity and

low methane as well as CO2 selectivity for 100 h continuously.

But CoH was not stable. The activity with stream on time of

both kinds of catalysts was shown in Fig. 1.

3.2. The structure properties of catalysts CoE and CoH

The characterization results of CO or H2 chemisorption, O2

titration, XRD and TEM were summarized in Table 2. CoE had

the smaller cobalt crystallite size than CoH, determined by

H2ad, TEM or XRD. However, its reducibility was slightly

poorer than that of CoH. The lower reduction degree of CoE

Fig. 1. The activity and selectivity stream-on-time for CO hydrogenation over

catalyst CoH and CoE. (A) CoH and (B) CoE. Reaction conditions: 503 K,

1.0 MPa, W/F = 5 g-cat h mol�1, and H2/CO = 2.

Table 2

Characterization of catalyst CoE and CoH

Catalyst Sel. (%) CO uptake (mmol/g) Reduction degree (%) Particle size (nm) TOF (�100 s�1)

C2–C4 C5+ H2ad TEM XRD

CoH 9.8 82.7 95.7 81.9 12.1 10.7 9.2 5.4

CoE 5.2 89.2 125.4 68.8 10.7 7.5 8.7 7.5

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–8582

could be ascribed to the decreased polarity of the solvent, which

increased the interaction between the cobalt complex and the

silica gel surface, resulting in the formation of more cobalt

species that could only be reduced at higher temperature. The

CO uptake of CoE catalyst was larger than that of CoH,

indicating higher activity of this catalyst. The TOFs of CoE and

CoH catalysts were calculated. As shown in Table 2, the TOF of

Fig. 2. TEM photos of CoE and CoH clusters.

CoE catalyst was higher than that of CoH catalyst, even though

its reduction degree was lower than that of CoH catalysts.

From TEM of CoH in Fig. 2, it is clear that the size of the

dispersed cobalt cluster is uniform. We counted the diameter

sizes of all the observed clusters and showed the statistical

graph in Fig. 3 for CoH. Uniformly distributed cobalt cluster

size of CoH gave a sharply concentrated distribution graph in

Fig. 3, with most clusters having size of 150 nm.

On the other hand, TEM photo of CoE in Fig. 2 distinctly

indicated that large clusters coexisted with small clusters

independently. By counting all the observed clusters, statistical

relationship between cluster size and clusters number exhibited

a dual-peak distribution for CoE in Fig. 3. Most small clusters

had a size of 40 nm while most large clusters were of 150 nm.

The TEM images in Fig. 2 showed that cobalt clusters are

smaller in CoE than in CoH. A significant difference was

observed. For CoH, the size of cobalt clusters was uniform. For

CoE, the size of cobalt clusters was bimodal. Large and small

cobalt clusters coexisted, as illustrated in Fig. 3. TEM images of

Co/SiO2 before reduction, showed the presence of droplets of

cobalt nitrate. The size of these droplets was similar to that of

the cobalt cluster in the reduced sample [21]. Fig. 4 represents a

single cluster structure of both catalysts. For CoE, the TEM

photo of large cobalt particle is exhibited. Interestingly, the

large clusters in CoE were not in the condensed bulk state and

Fig. 3. The cobalt clusters diameter distribution of catalyst CoE and CoH.

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–85 83

had a loose cluster-like structure. CoH cluster had high bulk

density while CoE large cluster was in a loosely constructed

form, providing large metallic surface area and high Co

dispersion. It is clear that the cobalt ion dissolved in dehydrated

ethanol favored the distribution of Co over the whole silica

particle, resulting in higher dispersion of supported cobalt.

3.3. CoH and CoE characterized by TPD and TPSR

Desorption profiles of CH4, CO2 and CO on the two catalysts

measured by TPD and TPSR were described in Fig. 5. For CO-

TPD profiles, the catalyst CoE had two peaks of desorbed CO2.

The main peak located at 530 K and a shoulder peak located at

448 K. For CoH catalyst, one main peak located at lower

temperature, 435 K, and another broad band existed from 495

to 720 K. The total desorbed amount of CO2 of CoE was 1.6

times to that of CoH. In addition to CO2, the desorption of CO

Fig. 4. TEM of CoE and CoH (a single large cluster).

was also detected. CoE catalyst has two CO desorption peaks

which were located at 380 and 520 K, respectively, and those of

CoH appeared at 355 and 630 K. Based on above, the ratio of

CO desorbed as CO2 was 71% on CoE catalyst and 60% on

CoH catalyst in CO-TPD, indicating that more CO desorbed as

CO2 on CoE catalyst than on CoH catalyst. Because the part of

CO desorbed as CO2 in CO-TPD processing was active-state

CO in FT synthesis [17], it is considered that the CoE catalyst

was more active than CoH catalyst in FTS.

The peak temperature of CH4 formation in CO-TPSR was

reported to closely relate to the catalytic activity of CO

hydrogenation [17]. The reactivity of carbon species after CO

adsorption at room temperature was studied by TPSR. The rate

of methane formation as a function of temperature was shown

in Fig. 5. Because of the smaller cobalt particle size of CoE

catalyst as shown in Table 2, the C–O bond of adsorbed CO

should be difficult to be cleaved due to the geometric effect.

However, the temperature of the maximum methanation rate

was 393 K for CoE catalyst, lower than 404 K for CoH catalyst.

These irregular findings suggested that CO was in a more

reactive state on CoE catalyst than on CoH catalyst, according

to the results of CO-TPD and CO-TPSR, resulting in higher

TOF of this catalyst as shown in Table 2.

3.4. FT-IR measurement

FT-IR spectra of the adsorbed CO on CoH and CoE catalysts

were compared in Fig. 6. For CoH, an intense peak at

2006.9 cm�1 coupled with one shoulder peak at 1935.6 cm�1

were observed. The peak located at 2006.9 cm�1 can be readily

assigned to CO adsorbed on metallic cobalt in linear geometry

Fig. 5. TPSR and TPD profiles of catalyst CoH and CoE.

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–8584

[22–25]. The 1935.6 cm�1 shoulder peak should be assigned to

the bridge-type absorbed CO. For CoE catalyst, linear-type

absorbed CO band shifted from 2006.9 cm�1 on CoH to

2000.1 cm�1, and the band of the bridge-type absorbed CO at

1933.6 cm�1 increased significantly, referring that bridge-type

CO formed in large amount on CoE catalyst. Two additional

bands at 2061.1 and 2037.0 cm�1 appeared on CoE. These

shoulder peaks could be assigned to the surface carbonyl

species [25].

Generally large cobalt particle provide large flat metallic

surface, favoring bridge-type adsorbed CO, resulting in lower

selectivity of CH4 and higher chain growth probability (a), as

well as lower CH4 peak temperature in CO-TPSR. However,

reverse trend appears here, as the smaller Co particles on CoE

had more bridge-type CO, contributing to lower selectivity of

CH4 and higher chain growth probability (a) of this catalyst.

Generally, the linear adsorbed CO peaks on smaller Co particles

would shift to higher wavenumber. However, the linear

adsorbed CO peak of CoE catalyst located at lower

wavenumber (2000.1 cm�1) than that of CoH (2006.9 cm�1),

indicating the structure properties of metallic cobalt of CoE

catalyst, loosely constructed large clusters, influenced CO

adsorption state on the surface of metallic cobalt, resulting in

formation of more reactive adsorbed CO on CoE. It is clear that

loosely constructed large clusters in CoE favored formation of

bridge-type CO. CO might be adsorbed onto two adjacent Co

domains or two neighboring elementary Co particles inside a

large Co cluster simultaneously, to accomplish a bridge-type

adsorption mode. It has been pointed out that the bridge-type

CO is much more active than linear-type CO for FTS [17].

Therefore, the highest catalytic activity and the highest a value

of CoE catalyst can be attributed to more reactive linear

adsorbed CO and the increased bridge-type adsorbed CO,

which is easily dissociated to carbon and oxygen, providing

another evidences from CO-TPD and CO-TPSR.

For CoE catalyst, as supported by the TEM observation of a

single large cobalt cluster with loosely assembled smaller

Fig. 6. FT-IR spectra of catalyst CoE and CoH after adsorbed CO. Cobalt

loading: 20 wt% and PCO = 200 Torr.

clusters inside, CO adsorption mode at the surface of these

smaller clusters can be altered by the strong interaction from

surrounding cobalt clusters. As from Fig. 6, the appearance of

peaks of 2037.0 and 2061.1 cm�1, accounted for cobalt

carbonyl formation, indicated that at the corner of small cobalt

cluster where enough space is available, multiple CO can

simultaneously attack the same corner cobalt atom to form

carbonyl structure. Different from a large cobalt crystalline

with normal atom density where much flat surfaces exist to

favorite bridge-type adsorbed CO, smaller cobalt crystalline

has more corner atom to form carbonyl structure. This

conclusion can be also applied to the smaller clusters assembled

inside a large cobalt particle of CoE catalyst. With the same

reason, carbonyl IR peaks were not observed at CoH catalyst

where corner atoms were few, as proved in Fig. 6. But different

from an isolated smaller cobalt particle, CO adsorbed at one

small particle can interact with the adjacent cobalt surface of

another cobalt small particle inside the confined nano-scale

space between two neighboring small particles in a large cluster

of CoE catalyst, as illustrated in Fig. 7. Linearly adsorbed CO

molecule pulled simultaneously by two cobalt surfaces showed

a very weakened CO bondage compared with its analogy on one

cobalt surface, with the decreased wave number from 2006.9 to

2000.1 cm�1. In more obvious cases, the bondage of linearly

adsorbed CO interacted with two cobalt surfaces was so weak

that red shift moved it to give a peak similar to bridge-type

adsorbed CO (1933.6 cm�1) apparently, explaining the reason

why CoE catalyst had a stronger bridge-type adsorbed CO IR

peak than CoH catalyst in Fig. 6. It seems that there exist

transient states between linear adsorption and bridge adsorp-

tion, for adsorbed CO pulled by multiple cobalt surfaces,

supported by the shoulder peaks between linearly adsorbed CO

peak (2000.1 cm�1) and bridge-type adsorbed CO

(1933.6 cm�1) peak for CoE catalyst in Fig. 6. Furthermore,

small crystalline are difficult to be reduced after oxidation,

compared with large crystalline. The same phenomenon

happened to the smaller clusters loosely assembled into a

large cluster for CoE catalyst here, supported by the lower

reduction degree of this catalyst as in Table 2. All mentioned

above can elucidate the high activity of CoE catalyst as CO

bond cleavage is the rate-determining step in FTS reaction.

Concerning high carbon chain growth probability and low

methane selectivity of CoE catalyst, confined nano-space

between neighboring small particles in a large cluster of CoE

catalyst is favorable to promote carbon chain growth by

Fig. 7. Scheme of CO adsorptions and their interactions at the inner surface of

large cobalt cluster of CoE catalyst.

Y. Zhang et al. / Applied Catalysis A: General 321 (2007) 79–85 85

stopping the release of methane, formed by hydrogenation of

CH2 monomer, from inner cobalt surfaces.

From above-mentioned, the Co/SiO2 catalyst prepared from

dehydrated ethanol (CoE) formed bimodal size cobalt clusters,

unlike uniformly distributed cobalt clusters of CoH catalyst,

contributing to higher dispersion of supported cobalt, as

determined by H2ad, TEM and XRD. The large amount of CO

uptake on CoE catalyst was determined by CO chemisorption,

as shown in Table 2, indicating higher catalytic activity of this

kind of catalyst. It is considered that different structure

properties of metallic cobalt in CoE catalyst contributed to

formation of more reactive adsorbed CO, as proved by CO-

TPD, CO-TPSR and FT-IR, resulting in higher TOF as

compared in Table 2, leading to the highest catalytic activity

and the highest a value in FTS reaction.

4. Conclusions

The catalysts prepared from different cobalt nitrate solution

were investigated. The catalyst prepared from dehydrated

ethanol exhibited stable and the highest activity than the

catalysts prepared from any other cobalt nitrate solution. For

CoH, the size of cobalt particles was uniform. But for CoE, the

size of cobalt particles was bimodal, namely coexisting large

and small cobalt particles. The cobalt ion dissolved in

dehydrated ethanol favored the distribution of Co over the

whole silica surface, resulting in higher dispersion of supported

cobalt. It is considered that different structure properties of

metallic cobalt, loosely constructed form clusters, in CoE

catalyst, contributed to formation of more reactive adsorbed

CO, as proved by CO-TPD, CO-TPSR and FT-IR, resulting in

higher TOF, and determining the highest catalytic activity and

the highest a value in FTS reaction.

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