Effect of Sodium on the Structure-Performance Relationship of Co/SiO2 for Fischer-Tropsch Synthesis

Article abstract of DOI:10.1002/cjoc.201600748

A series of Co/SiO₂ catalysts with different sodium (Na) loadings (0, 0.1, 0.2, 0.5 and 1 wt%) were prepared and evaluated for Fischer-Tropsch reaction to study the effect of Na on the catalyst structure and catalytic performance. The addition of Na was found to decrease the catalytic activity and hydrocarbon selectivity, but increase CO₂ selectivity due to the enhanced WGS activity. The addition of Na also resulted in higher selectivity to oxygenates (alcohols and aldehydes) and O/P ratio as well as the shift of hydrocarbons to lower carbon numbers. Structure characterization revealed a decrease in the surface area and particles size for the calcined samples with the addition of Na. Co₂C was formed during the reaction process for the Na-promoted catalysts. As a result, a new Co/Co₂C bifunctional active sites were generated for oxygenates formation leading to increasing oxygenates selectivity. In addition, the Co₂C nanoparticles alone may also act as dual active sites for oxygenate formation at high reaction pressure over the promoted catalysts with high Na loading.

Full text of DOI:10.1002/cjoc.201600748

FULL PAPER
DOI: 10.1002/cjoc.201600748
Effect of Sodium on the Structure-Performance Relationship of
Co/SiO2 for Fischer-Tropsch Synthesis
Yuanyuan Dai,a,b Fei Yu,a,b Zhengjia Li,a,c Yunlei An,a,d Tiejun Lin,a Yanzhang Yang,a,b
Liangshu Zhong,*,a Hui Wang,a and Yuhan Sun*,a,e
a CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences, Shanghai 201203, China
b University of the Chinese Academy of Sciences, Beijing 100049, China
c School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China
d College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China
e School of Physical Science and Technology, Shanghai Tech University, Shanghai 201203, China
A series of Co/SiO2 catalysts with different sodium (Na) loadings (0, 0.1, 0.2, 0.5 and 1 wt%) were prepared and
evaluated for Fischer-Tropsch reaction to study the effect of Na on the catalyst structure and catalytic performance.
The addition of Na was found to decrease the catalytic activity and hydrocarbon selectivity, but increase CO2 selec-
tivity due to the enhanced WGS activity. The addition of Na also resulted in higher selectivity to oxygenates (alco-
hols and aldehydes) and O/P ratio as well as the shift of hydrocarbons to lower carbon numbers. Structure charac-
terization revealed a decrease in the surface area and particles size for the calcined samples with the addition of Na.
Co2C was formed during the reaction process for the Na-promoted catalysts. As a result, a new Co/Co2C bifunc-
tional active sites were generated for oxygenates formation leading to increasing oxygenates selectivity. In addition,
the Co2C nanoparticles alone may also act as dual active sites for oxygenate formation at high reaction pressure
over the promoted catalysts with high Na loading.
Keywords Fischer-Tropsch synthesis, structure-performance relationship, sodium, cobalt carbide, oxygenates
Introduction
on the catalytic performance. Previous studies show that
the addition of alkali metals promotes catalyst carburi-
zation by facilitating the dissociation of CO on the cat-
alyst surface.[18] Moreover, alkali metals accelerate the
CO dissociative adsorption rate and result in an increase
of the surface coverage of adsorbed CO.[19] The addition
of alkali metals will also inhibit olefin readsorption, and
thus increase olefin selectivity.[19]
Generally speaking, alkali metals are the common
promoters of Fe-based catalysts for the FT synthe-
sis.[17,20-22] However, the typical promoters for Co-based
catalysts are noble metals, transition metals and some
rare earth metal oxides. Alkali metals are always con-
sidered to possess a negative impact on Co-based FTS
catalysts as the addition of alkali metals always leads to
the decrease in catalytic activity. Lillebø et al.[23] have
reported that there was not significant change of micro-
calorimetric measurements by H2 and CO chemisorption
for Co/Al2O3 with 0.1 wt% Na concentration. However,
the activity was 33%－43% lower than that of the cata-
lyst without Na. Na could decrease the reducibility of
cobalt oxide,[24] block active site[25] and promote catalyst
Fischer-Tropsch (FT) synthesis is a catalyzed surface
polymerization process that produces major chemical
building blocks such as paraffins, olefins and oxygen-
ates from syngas.[1,2] FT synthesis has attracted much
attention both in academia and industry due to the de-
pletion of oil-based resource and the increase of global
demand for clean fuel and various chemicals.[3,4] The
group VIII metal elements such as Fe, Co and Ni, owing
to their electronic properties, have been investigated for
the FT reaction.[5] Co-based catalysts are considered
optimal for the production of middle distillates and
waxs from H2-rich syngas because of the high stability,
and low activity for the water-gas-shift (WGS) reac-
tion.[6,7] Compared with Fe-based FT catalysts, the
products obtained from Co-based FT catalyst are mainly
linear paraffins with few olefins and alcohols.
Electronic promoters are always used for FT cata-
lysts to further improve the catalytic activity and prod-
uct selectivity. Many promoters, such as calcium,[8] zir-
conia,[9] copper,[10,11] manganese[12-15] and alkali met-
als,[8,16-18] have been shown to exhibit different effects
*
E-mail: zhongls@sari.ac.cn, sunyh@sari.ac.cn
Received October 31, 2016; accepted January 14, 2017; published online XXXX, 2017.
In Memory of Professor Enze Min
Chin. J. Chem. 2017, XX, 1—9
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Dai et al.
FULL PAPER
carburization. Anton et al.[16] have studied the effect of
Na on Co/Cu/ZnO/Al2O3 in FTS. They found that cata-
lysts with low Na loading (≤0.6 wt%) presented strong
initial deactivation due to the phase change of Co0.
However, the catalytic activity remained almost con-
stant with further increase of the Na loadings (≥0.8
wt%). Close contact of metallic Co0 and Co2C can occur,
which was assumed to generate additional active sites
for alcohols formation. This conclusion was also veri-
fied by other researchers.[26-29] Recently, a breakthrough
in the selective production of lower olefins from syngas
for CoMn catalysts has been reported by our group.[30]
Using sodium carbonate as the precipitant, the obtained
CoMn catalysts presented Co2C nanoprisms with ex-
posed facets of (101) and (020) during the reaction pro-
cess. Very high selectivity to lower olefins (60.8 C%)
with low methane selectivity was detected. However,
the catalysts using ammonium carbonate as the precipi-
termined by a PE Optima 2100DV inductive coupled
plasma emission spectrometer (ICP). The surface area
and pore structure of all the samples were determined
by N2 physical adsorption-desorption at 196 ℃, using
the multipoint BET analysis method, with a TriStar II
3020 instrument. Prior to the measurements, the sam-
ples were degassed at 200 ℃ for 6 h. The phase and
average cobalt oxide crystallite size for the catalysts was
determined by powder X-ray diffraction (XRD) per-
formed on a Rigatku Ultima 4 X-ray diffractometer with
Cu Kα radiation (40 kV, 40 mA) in the range of 5°－90°.
The identification of different phase was performed by
comparing the patterns with JCPDS standard cards.
The reduction characteristics of the catalysts were
studied by H2 temperature programmed reduction
(H2-TPR) on a Micromeritics ChemiSorb 2920 with a
thermal conductivity detector (TCD). Typically, the cat-
alyst sample (50 mg) was placed in a U-shaped quartz
reactor and pretreated in flowing He of 40 mL•min–1 at
200 ℃ for 1 h, followed by cooling down to 60 ℃.
After pretreatment, the temperature was raised to
800 ℃ at a rate of 10 ℃•min–1 with 50 mL•min–1 of
5% H2/Ar mixture gas.
tant presented metallic cobalt and CoxMn1－ O and ex-
x
hibited typical FT performance. It was suggested that
the residual Na enhanced the formation of Co2C. Alt-
hough there are a lot of works concerning Na-promoted
Co-based FT catalyst, detailed informations about the
effect of Na on catalyst structure and catalytic perfor-
mance (activity and selectivity) are still lacking. In this
paper, Na-promoted Co/SiO2 catalysts were chosen as
model catalysts and the effect of Na on the struc-
ture-performance relationship was investigated in de-
tails.
For semiquantitative analysis, the samples were
scanned from 30° to 50° using a step scanning mode, of
which the step size was 0.02°. The content of different
phases was calculated by means of reference intensity
ratios (RIR). Generally, the integrated intensities of the
most intense peak of each phase were obtained from
fitting and deconvolution results of XRD patterns. Then
the mass fraction of x phase (wx) was calculated from
the Eq. 1 as follows:
Experimental
Catalysts preparation
I x
(1)
wx＝
Commercially available SiO2 with specific surface
area of 200 m2•g1 was used as support. Co/SiO2 was
firstly prepared by the incipient wetness impregnation
technique. Co weight loading was held constant at 20
wt% with respect to the sum of Co3O4 and SiO2 support.
10 g of SiO2 was impregnated with an aqueous solution
(14 mL) of 13.57 g Co(NO3)2•6H2O. After impregnation,
the sample was dried at room temperature for 12 h, then
dried at 100 ℃ for 5 h, finally the catalyst was cal-
cined at 330 ℃ for 3 h with a heating rate of 1 ℃/min.
The obtained catalyst was marked as Co/SiO2-0Na, and
used for all further modification by Na impregnation.
The Na-promoted catalysts were obtained by further
impregnation with aqueous solution of Na2CO3. Na
weight loadings were held 0.1 wt%, 0.2 wt%, 0.5 wt%
and 1 wt% with respect to the sum of Co3O4 and SiO2
support. The drying and calcination steps were per-
formed as described for the unpromoted supported cat-
alysts. According to the loading amount of Na, the cata-
lysts were marked as Co/SiO2-0.1Na, Co/SiO2-0.2Na,
Co/SiO2-0.5Na and Co/SiO2-1Na accordingly.
Ii
R IR x  iN1
R IR i
where I is the integrated intensity of the intensest peak
and the RIR values were available from the PDF cards.
The spent catalysts were analyzed with transmission
electron microscopy (TEM) to determine the size dis-
tribution and lattice fringe. The images were obtained
using an accelerating voltage of 200 kV and a Tecnai
G220 high-resolution microscope equipment. Samples
for TEM were prepared by dispersing the sample in
ethanol followed by ultrasonication.
Catalytic evaluation
The Fischer-Tropsch synthesis was performed in a
fixed-bed reactor at 20 bar, 220－250 ℃, H2/CO＝2
V/V and GHSV＝2000 mL g1•h1 using 1.5 g catalyst
(particle size of 40－60 mesh) diluted with 3 g quartz
sand (particle size of 40－60 mesh). Prior to reaction,
the catalyst was in-situ reduced by pure H2 at 400 ℃
with gas flow rate of 200 mL/min for 5 h. After reduc-
tion, the reactor was cooled to 220 ℃ under He at-
mosphere with gas flow rate of 200 mL/min. Then the
pressure was increased gradually up to 20 bar and syn-
Catalyst characterization
The metal composition and the Na content were de-
2
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Chin. J. Chem. 2017, XX, 1—9

Effect of Sodium on the Structure-Performance Relationship of Co/SiO2
gas (H2/CO＝2, V/V) was introduced with gas flow rate
of 50 mL/min. The tail gas after passing through a hot
trap (120 ℃) and a cold trap (0 ℃), was analyzed
on-line with an Agilent 7890B gas chromatograph by
using a carbon molecular sieves column (TDX-1) with a
thermal conductivity detector (TCD), using helium as
the carrier gas. Hydrocarbons with carbon number from
1 to 7 (C1 to C7) were analyzed using a KCl-modified
alumina capillary column (19095P-K25) with an argon
carrier and a hydrogen flame ionization detector (FID).
The aqueous products, oil products and wax products,
collected from cold trap and hot trap, were analyzed
off-line by GC. The aqueous products were analyzed
with two Porapak Q columns equipped with a TCD (for
H2O and MeOH detection) and FID (for C1－C5 oxy-
genates detection), respectively. The oil products were
analyzed through an HP-1 column with N2 carrier by
FID. The wax products were dissolved in CS2 and ana-
lyzed through an MXT-1 column with N2 carrier by FID.
The CO conversion and product selectivities were de-
termined after at least 24 h on-stream when the steady
state was reached.
catalyst to 88.9 m2•g1 for the catalyst with 1.0 wt% of
Na, in agreement with the literature.[16,22]
Table 1 The composition and structure of Co/SiO2 catalysts
with different Na loadings
Coa/ Naa/ BET surface
wt% wt% area/(m2•g1)
Particle size of
Co3O4 /nm
Sample
b
Co/SiO2-0Na 21.1
－
198.2
193.1
161.4
132.2
88.9
18.3
17.2
17.3
17.5
17.6
Co/SiO2-0.1Na 21.3 0.13
Co/SiO2-0.2Na 21.6 0.35
Co/SiO2-0.5Na 21.9 0.58
Co/SiO2-1Na 21.4 1.11
a Measured by ICP; b calculated by XRD.
XRD patterns of Co/SiO2 catalysts with different Na
loadings at different stages were shown in Figure 1(a).
After calcination, only crystalline Co3O4 and SiO2
phases were observed by XRD, and no obvious diffrac-
tion peak belonging to the Na component was detected
due to the low Na loading. The characteristic reflection
peaks at 2θ of 19.0°, 31.4°, 36.9°, 37.9°, 45.0°, 55.9°
and 59.4° corresponded to the Co3O4 phase (PDF# No.
42-1467), and the peak at 2θ of 21.9° was attributed to
SiO2 (PDF# No. 39-1425). The crystallite sizes of cobalt
oxides were calculated from the Co3O4 (311) peak ac-
cording to the Scherrer equation and listed in Table 1.
The average particle size of cobalt oxides for the un-
promoted sample is 18.3 nm, while there is a slight de-
crease for the Na-promoted samples. Figure 1(b)
showed the XRD patterns of reduced catalysts. The
peaks at 2θ of 44.2°, 51.5° and 75.9° were ascribed to
Results and Discussion
Catalysts characterization
The content of cobalt and sodium, textural properties
and particle size were measured by ICP, N2-adsorption
and XRD as listed in Table 1. The cobalt and sodium
concentrations determined with ICP were basically con-
sistent with the nominal concentrations. With the addi-
tion of Na, the specific surface area (SBET) of the cata-
lysts decreased from 198.2 m2•g1 for the unpromoted


Co3O4
Co-FCC
Co-HCP



(a)
(b)

｡

CoSi-1Na









CoSi-1Na



CoSi-0.5Na
CoSi-0.2Na
CoSi-0.2Na
CoSi-0.1Na
CoSi-0Na
CoSi-0.1Na
CoSi-0Na
20
40
2/(degree)
60
80
20
40
60
80
2/(degree)

Co-FCC
Co-HCP
Co2C
*
(c)

*
(d)
*
*
CoSi-1Na
*
*
*
*
*
*
*
CoSi-1Na
CoSi-0.5Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-0.2Na
CoSi-0.1Na
*
*
CoSi-0.1Na
CoSi-0Na
*


*





CoSi-0Na

20
40
2/(degree)
60
80
200
400
600
800
Temperature/oC
Figure 1 XRD patterns of calcined (a), reduced (b) and spent (c) Co/SiO2 catalysts; (d) TPR profiles of calcined Co/SiO2 catalysts.
Chin. J. Chem. 2017, XX, 1—9
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Dai et al.
FULL PAPER
FCC Co (PDF# No. 15-0806), and the peaks at 2θ of
44.2°, 44.8°, 47.6°, 62.7°, 75.9° and 84.2° were as-
signed to HCP Co (PDF# No. 05-0727). After the re-
duction under 400 ℃ by H2, the Co3O4 phase appeared
to be converted into Co0 (including FCC and HCP) and
FCC Co was the dominant cobalt phase.[31]
The XRD patterns of spent catalysts were shown in
Figure 1(c). For the spent Co/SiO2-0Na sample, only
metallic phase was detected, while Co2C (PDF# NO.
50-1371) with the characteristic peaks at 2θ of 37.6°,
41.5°, 43.0°, 45.9°, 57.1° and 64.8° were clearly ob-
served for Co/SiO2-0.1Na. By semiquantitative analysis,
the proportion of Co2C among all Co-containing phases
was 20%. The proportion of Co2C increased with the
increase in Na loading. When the Na loading was higher
than 0.2 wt%, the Co2C phase was the dominant Co
species and the metallic Co phase was hardly observed
from the XRD patterns. Obviously, the presence of Na
promoted the formation of cobalt carbide. This phe-
nomenon was also found by Anton et al.[16] and it was
reported that Na enhanced the rate of CO dissociation
resulting in carbonization. Na can acted as an electronic
donator to cobalt resulting in stronger CO adsorption
and enhanced CO dissociation, benefiting the formation
and stabilization of Co2C. As a result, there was only the
Co2C phase present in the sample with higher Na load-
ing.
H2-TPR was carried out from room temperature to
800 ℃ under the flow of 5% H2/Ar as shown in Figure
1(d). All catalysts exhibited two main peaks and the first
peak was assigned to the reduction of Co3O4 to CoO
while the second peak was ascribed to the reduction of
CoO to Co0. For Co/SiO2-0Na, the first reduction stage
from Co3O4 to CoO ranged from 200 to 270 ℃. The
second reduction stage from CoO to Co0 occurred from
270 to 360 ℃. The addition of Na to the Co/SiO2 cata-
lyst slightly suppressed the reduction of Co3O4 and CoO
due to the interaction of cobalt oxide with Na. The se-
cond reduction stage of CoO to Co0 also shifted to
higher temperature with the increase of Na loading.
When the Na loading was higher than 0.5 wt%, all re-
duction stages shifted to higher temperature resulting in
the first reduction peak overlaped with the second re-
duction peaks. Khobragade[25] claimed that alkali metals
inhibited the dissociative adsorption of H2 resulting in
the suppression of the reduction, which was consistent
with our H2-TPR results.
The TEM images of spent Co/SiO2 catalysts with
different Na loadings were shown in Figure 2. All sam-
ples presented a certain degree of aggregation as the
magnetic properties of the samples. The sample without
Na showed metallic phases (FCC Co and HCP Co) and
no Co2C phase was found. However, the Co2C phase
was present in all Na-promoted samples, which was
constant with the XRD results. For the Co/SiO2-0.1Na
sample, both metallic phase and Co2C were found and
metallic phase was the dominant cobalt phase. For sam-
ples with higher Na loading, only the Co2C phase
Figure
2
TEM images of various spent catalysts. a:
Co/SiO2-0Na; b: Co/SiO2-0.1Na; c: Co/SiO2-0.2Na; d: Co/SiO2-
0.5Na; e: Co/SiO2-1Na.
could be found. In addition, all Co2C in the different
Na-promoted samples showed sphere-like morphology.
Recently, we have reported that Co2C nanoprisms with
the specific exposed facets [(101) and (020)] were the
effective active phase for Fischer-Tropsch to olefins
(FTO) reaction.[30] As there was no obvious Co2C nano-
prisms for the spent Na-promoted Co/SiO2, the catalytic
performance of the Co2C on Na-promoted Co/SiO2
samples might be different from Co2C nanoprisms. The
particle sizes of Co2C for Co/SiO2-0.2Na, Co/SiO2-
0.5Na and Co/SiO2-1Na were 17.7, 14.5 and 17.3 nm,
respectively, suggesting the Na loading amount had lit-
tle effect on the Co2C size.
STEM/EDX was also used to study the distributions
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Effect of Sodium on the Structure-Performance Relationship of Co/SiO2
of different elements in spent Co/SiO2-1Na catalyst as
shown in Figure 3. Clearly, the cobalt phase was dis-
persed on SiO2. However, the signal for Na was rather
weak and seemed as signal noise due to the very low Na
loading.
the promoter effect of Na became more remarkable.
Co/SiO2-0.2Na sample exhibited lower CO conversion
(28%) but higher oxygenates selectivity than Co/SiO2-
0.1Na. Among all Na-promoted catalysts, Co/SiO2-
0.5Na sample showed the highest oxygenates selectivity
at about 14.5 C%. The aldehydes selectivity for the un-
promoted sample was less than 1% but higher than 1.5%
for Na-promoted samples. In addition, the alcohol selec-
tivity of Na-promoted samples was much higher than
that of the unpromoted sample, suggesting that the addi-
tion of Na improved both alcohols and aldehydes selec-
tivity.
As far as the product distributions were concerned as
shown in Table 3, the oxygenates distribution for the
Co/SiO2-0Na sample was in the range of lower carbon
(C1－C11) slate and no C12 oxygenates were obtained.
＋
However, the produced hydrocarbons exhibited higher
carbon number and the C12 hydrocarbon selectivity
＋
was 31.7 wt%. With the addition of Na, C12 oxygen-
＋
ates could be obtained while the selectivity to C12 hy-
＋
drocarbons decreased. The O/P ratios were also investi-
gated as depicted in Figure 4. Obviously, the O/P ratios
for the Na-promoted sample were much higher than that
of the Na-free sample. For the Co/SiO2-0wt%Na sample,
the O/P ratios with carbon number above 5 were almost
zero. However, that values increased greatly with the
addition of Na, indicating that Na could inhibit olefin
re-adsorption and decrease the olefin hydrogenation
rates.[18]
Figure 3 The elemental mapping of the spent Co/SiO2-1Na
catalyst. Na (green), Co (red), Si (purple).
From the characterization results, it can be conclud-
ed that Na could suppress the reduction by inhibiting the
dissociative adsorption of hydrogen, and promote the
formation of Co2C by enhancing CO adsorption and
dissociation.
CoSi-0Na
4
3
2
1
0
CoSi-0.1Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-1Na
Fischer-Tropsch synthesis
Table 2 shows the catalytic performance over the
studied Co/SiO2 catalysts. The catalyst without Na dis-
played high activity for the FT reaction with low selec-
tivity to both CO2 and oxygenates but very high hydro-
carbon selectivity. The activity and product distributions
on this catalyst were in good agreement with the re-
ported catalytic performance of typical Co-based FT
catalyst with Co0 as the active phase.[7] The addition of
Na resulted in a decrease in the catalytic activity with a
great change of product selectivity. For catalysts with
low Na loading (Co/SiO2-0.1Na) at 250 ℃, the oxy-
genates selectivity increased to 9.5 C% at a CO conver-
sion of about 40%. With further increase of Na loading,
0
5
10
15
20
Carbon number
Figure 4 Ratios of olefin to paraffin for Co/SiO2 catalysts with
different Na loadings.
Table 2 The catalytic performance of various Co/SiO2 catalysts (20 bar, H2/CO＝2 and GHSV＝2000 mL•g1•h1)
Selectivity/C%
Sample
Temperature/℃
CO conversion/C%
Oxygenates
2.6
CHn CO2 Alcohols
Aldehyde
0.9
210
250
250
250
250
250
26.7
94.4
39.6
27.6
29.2
32.6
96.7 0.6
91.6 6.1
81.5 9.0
44.3 44.2
38.4 47.1
38.0 48.1
1.7
1.6
Co/SiO2-0Na
2.0
0.4
Co/SiO2-0.1Na
Co/SiO2-0.2Na
Co/SiO2-0.5Na
Co/SiO2-1Na
9.5
6.7
2.8
11.5
10.0
12.7
11.3
1.5
14.5
1.8
14.0
2.7
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Dai et al.
FULL PAPER
Table 3 The product distribution of various Co/SiO2 catalysts (250 ℃, 20 bar, H2/CO＝2 and GHSV＝2000 mL•g1•h1)
Oxygenates distribution/wt%
C2 C5
Hydrocarbon distribution (O&P, wt%)
C2 C5
Sample
C1
C12
0
C1
C12
－
－
＋
－
－
＋
4
11
4
11
Co/SiO2-0Na
Co/SiO2-0.1Na
Co/SiO2-0.2Na
Co/SiO2-0.5Na
Co/SiO2-1Na
10.7
6.3
31.9
55.2
42.7
47.6
43.5
57.4
35.7
46.4
32.7
31.9
19.5
13.2
34.8
36.6
36.9
7.2
41.6
37.7
18.4
17..0
15.7
31.7
23.0
12.7
7.5
2.8
2.7
27.8
34.1
39.0
40.3
8.3
12.7
12.4
7.0
12.3
7.0
The chain growth probabilities (α) of the produced
hydrocarbons and oxygenates were drawn in Figure 5.
All products deviated from the ASF distribution, featur-
ing two alpha-breaks distributions. For hydrocarbon
distributions of Co/SiO2-0Na and Co/SiO2-0.1Na, the
alpha breaks were located at C5 and C10 making the hy-
drocarbon selectivities into three segments, and the al-
pha breaks of other samples were located at C9 and C13.
The chain growth factor of hydrocarbons (α＝0.87 cal-
culated from C13－C23) for Co/SiO2-0wt%Na was much
higher than that of oxygenates (α＝0.55 calculated from
C4－C8), suggesting that Co-based FT catalysts pos-
sessed higher selectivity to heavy paraffins than oxy-
genates. However, the addition of Na resulted in the
shift of hydrocarbons to lower carbon numbers and the
shift of oxygenates to higher carbon numbers. For ex-
ample, when Na loadings were higher than 0.2 wt%, the
temperature for all the studied catalysts as expected.
CO2 selectivity also increased with increasing tempera-
ture for all samples due to the severe WGS reaction.
However, CO2 selectivities for Co/SiO2-0Na and
Co/SiO2-0.1Na were lower than those of other samples
and still below 10 C% even at 250 ℃. At low tempera-
ture (220－230 ℃), Co/SiO2-0Na catalyst showed the
lowest CH4 selectivity of about 7.3% at 220 ℃ and
8.4% at 230 ℃. At the same temperature, the catalysts
with high Na loadings showed rather high CH4 selectiv-
ity of about 15%－19%, and the CH4 selectivity slightly
increased when the temperature further increased from
230 to 250 ℃. However, Co/SiO2-0Na exhibited the
highest CH4 selectivity (about 20%) at 250 ℃. The
changing trend of C ＋ROH selectivity of all catalysts,
＋
5
contrary to that of CH4 selectivity, decreased with in-
creasing temperature.[8] It seemed that higher tempera-
ture would favor surface species to desorb and resulted
in high CH4 selectivity and low selectivity to heavy
products.[11]
proportion of C12 hydrocarbons was less than 23 wt%.
＋
For the oxygenates distributions, the addition of Na in-
creased the fraction of the C2－C4 at the expense of
C5－C11. The hydrocarbons chain growth factors of
Na-promoted sample were about 0.8 calculated from the
C13－C23 slates and were lower than that of unprompted
sample. However, the oxygenates chain growth factors
slightly increased once Na was added. In addition, the
chain growth factors of all products almost kept con-
stant for the studied Na promoted samples with different
Na loadings.
As shown in Figure 7, the olefin to paraffin (O/P) ra-
tio of C2 decreased remarkably with increase of reac-
－
4
tion temperature due to enhanced secondary hydrogena-
tion of olefins for the sample without Na. When the Na
loading was 0.1 wt%, the change in C2 ＝/C2
with
o
－
－
4
4
temperature was similar to that for Co/SiO2-0Na. How-
＝
ever, for the sample with higher Na loadings, C2-4
/
o
C2
ratios almost kept unchanged with the increase of
－
4
＝
The effect of reaction temperature for different
Co/SiO2 catalysts was also investigated. As shown in
Figure 6, CO conversion increased with the increase of
reaction temperature. The changing trend of C2
se-
－
lectivity with temperature was similar to C2 ＝/C2-4
4
o
－
4
ratios.
0
CoSi-0Na
CoSi-0.1Na
CoSi-0Na
(a)
(b)
-2
-4
CoSi-0.1Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-1Na
CoSi-0.2Na
-2
-4
-6
-8
CoSi-0.5Na
CoSi-1Na
n
n
n
n
-6
-8
-10
-10
0
5
10
15
20
25
0
5
10
15
20
25
Carbon number (n)
Carbon number (n)
Figure 5 Product distribution of hydrocarbons (a) and oxygenates (b) of various Co/SiO2 catalysts with different Na loadings.
6
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Chin. J. Chem. 2017, XX, 1—9

Effect of Sodium on the Structure-Performance Relationship of Co/SiO2
100
40
(a)
(b)
80
60
40
20
30
20
10
0
0
220
230
240
250
220
230
240
250
Temperature/oC
Temperature/oC
20
90
80
70
60
50
(c)
(d)
15
10
220
230
240
250
220
230
240
250
Temperature/oC
CoSi-0Na
Temperature/oC
CoSi-0.1Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-1Na
Figure 6 CO conversion (a), product selectivity to CO2 (b), CH4 (c) and C5＋＋ROH (d) over various Co/SiO2 catalysts at different
reaction temperatures.
5
CoSi-0Na
CoSi-0Na
30
20
10
(a)
(b)
CoSi-0.1Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-1Na
CoSi-0.1Na
CoSi-0.2Na
CoSi-0.5Na
CoSi-1Na
4
3
2
1
0
220
230
240
250
220
230
240
250
Temperature/oC
Temperature/oC
＝
Figure 7 C2 ＝/C2 o ratio (a) and C2
4
selectivity (b) of various Co/SiO2 catalysts at different reaction temperatures.
－
－
－
4
4
For the cobalt catalyst, the Co0 active phase will ex-
hibit high activity for producing linear heavy hydrocar-
bons with low activity for the WGS reaction.[32] In all
Na-promoted samples, Co2C was found after the reac-
tion based on the XRD and TEM results, demonstrating
that Na promoted the formation of cobalt carbide. Co-
balt carbide formation is a possible cause for FT catalyst
deactivation as it is less active than metallic Co.[33,34]
According to the previous study, Co/Co2C presented as
dual active sites for higher alcohols formation,[26] on
which Co0 functions for CO dissociation and chain
propagation while Co2C for CO non-dissociative activa-
tion and insertion. Thus, for Na promoted sample con-
taining metallic Co and Co2C phases, the performance
showed high oxygenates selectivity. For those samples
with high Na loading, only the Co2C phase was ob-
served. However, oxygenates were still produced, sug-
gesting that the Co2C nanoparticles alone may also act
as the dual active sites for oxygenate formation at high
pressure.
From the TEM images of spent catalysts, Co2C was
found in all Na-promoted samples with sphere-like
morphology. In our previous study, there existed a
strong facet effect for the Co2C nanostructure during
syngas conversion.[30] Co2C nanoprisms had high selec-
tivity to lower olefins at about 55 C% and the ratios of
＝
＝
＝
o
o
C2 /C2 , C3 /C3 and C4 /C4o were 22, 45 and 30 re-
spectively at atmospheric pressure. In order to compare
the catalytic performance between Co2C nanoprisms
and sphere-like Co2C nanoparticles, the Co/SiO2-0.2Na
Chin. J. Chem. 2017, XX, 1—9
© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
7

Dai et al.
FULL PAPER
Table 4 The catalytic performance of various Co/SiO2 catalysts on atmospheric pressure (250 ℃, 1 bar, H2/CO＝2 and GHSV＝2000
mL•g1•h1)
Product selectivity (C%, CO2-free)
Olefin/Paraffin ratio
Sample
CO Conversion/C%
＝
C2-4
o
CH4
17.2
7.6
C2-4
9.8
4.7
2.0
C5＋＋ROH
65.9
C2
C3
C4
Co/SiO2-0Na
Co/SiO2-0.2Na
Co/SiO2-1Na
17.7
3.1
7.1
0.1
8.7
10.8
1.5
0.6
19.6
27.6
68.1
20.2
19.8
13.1
13.6
2.8
10.2
60.2
and Co/SiO2-1Na catalysts were also evaluated at at-
mospheric pressure as showed in Table 4. All the sam-
ples with the Co2C phase in our study showed higher
O/P ratios than unpromoted Co/SiO2. However, the O/P
ratios were still lower than those of Co2C nanoprisms
with exposed (101) and (020) facets.[30] In addition,
higher CH4 selectivity was obtained for sphere-like
Co2C. It further illustrated that the morphology and ex-
posed facet had strong effect on the catalytic perfor-
mance of Co2C for syngas conversion.
and the Chinese Academy of Sciences (No. QYZDB-
SSW-SLH035, Youth Innovation Promotion Associa-
tion).
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Conclusions
The effect of sodium on the structure and catalytic
performance of Co/SiO2 for the Fischer-Tropsch syn-
thesis was investigated in details. The samples with dif-
ferent Na loadings were characterized by various meth-
ods. The results showed that Na could change the struc-
ture of the catalyst, resulting in decreasing surface area
and particles size and increasing the reduction tempera-
ture. From the XRD and TEM results of spent samples,
the addition of Na promoted the formation of Co2C with
sphere-like morphology.
CO conversion and products selectivity were
strongly influenced by the Na promoter. The addition of
Na greatly decreased the catalytic activity and increased
CO2 selectivity. However, oxygenates selectivity, espe-
cially alcohol selectivity, increased when Na was added.
It seemed that the addition of Na led to the transfor-
mation of the catalytic performance from typical FT
reaction to oxygenate formation. The hydrocarbon
products shifted to lower carbon numbers for
Na-promoted samples. However, the oxygenate chain
growth factors slightly increased once Na was added. In
addition, the chain growth factors of all products almost
kept as constant for the promoted samples with different
Na loadings. The O/P ratios of samples with Na were
much higher than those without Na. The metallic cobalt
shifted to cobalt carbide during the reaction process due
to the presence of Na, indicating that Na could promote
the formation of Co2C serving as the active site for ox-
ygenate and olefin formation.
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8
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(Zhao, X.)
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9