Fischer-Tropsch to olefins over Co2C-based catalysts: Effect of thermal pretreatment of SiO2 support

Article abstract of DOI:10.1016/j.apcata.2021.118283

SiO₂ supported Co₂C-based catalysts were used for Fischer-Tropsch to olefins (FTO), and the effect of thermal pretreatment of SiO₂ support under different temperatures on the Co₂C morphology and catalytic performance was investigated. It was found that the interaction between cobalt and support was weakened when SiO₂ was pretreated at high temperature (990 °C) due to the decreased content of surface Si[sbnd]OH groups. The relative weak interaction between cobalt and support benefited the formation of cobalt manganese composite oxide after calcination and reduction, and thus promoted the generation of Co₂C nanoprisms with promising FTO performance. In contrast, for the SiO₂ support pretreated at 350 °C or 650 °C, the strong interaction between cobalt and support led to phase separation of cobalt and manganese. As a result, only Co₂C nanospheres were generated which displayed low activity and high methane selectivity.

Full text of DOI:10.1016/j.apcata.2021.118283

Applied Catalysis A, General 623 (2021) 118283
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Fischer-Tropsch to olefins over Co C-based catalysts: Effect of thermal
2
pretreatment of SiO support
2
a,
b
a
a
,
b
a
a
a,
c,
a,c,
Liusha Li , Fei Yu , Xiao Li , Tiejun Lin , Yunlei An , Liangshu Zhong *, Yuhan Sun
*
a
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201203, PR
China
b
University of the Chinese Academy of Sciences, Beijing, 100049, PR China
c
School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
SiO2 supported Co C-based catalysts were used for Fischer-Tropsch to olefins (FTO), and the effect of thermal
2
Syngas conversion
Fischer-Tropsch to olefins
2 2
pretreatment of SiO support under different temperatures on the Co C morphology and catalytic performance
was investigated. It was found that the interaction between cobalt and support was weakened when SiO
2
was
Co
2
C
◦
–
pretreated at high temperature (990 C) due to the decreased content of surface Si OH groups. The relative
Metal-support interaction
Thermal treatment
weak interaction between cobalt and support benefited the formation of cobalt manganese composite oxide after
calcination and reduction, and thus promoted the generation of Co C nanoprisms with promising FTO perfor-
2
◦ ◦
mance. In contrast, for the SiO support pretreated at 350 C or 650 C, the strong interaction between cobalt and
2
2
support led to phase separation of cobalt and manganese. As a result, only Co C nanospheres were generated
which displayed low activity and high methane selectivity.
1
. Introduction
Na and Mn were investigated in details. It was found that the Na pro-
moter acting as an electronic donor can promote the formation and
Olefins are well known as fundamental building blocks, and widely
stable existence of Co
and structural additive exhibited a significant effect on the final
morphology of Co
C nanostructure during reaction process [19–22].
Before FTO reaction, catalyst activation was conducted for the forma-
tion and stabilization of Co
2
C phase, while the Mn promoter as an electronic
applied for the production of various polymers, cosmetics, drugs and
detergents [1]. In general, olefins are produced by thermal or catalytic
cracking of naphtha and dehydrogenation of alkanes. However, with the
concern of exhausted petroleum resource, direct synthesis of olefins
from syngas, which is derived from coal, natural gas, biomass and even
2
2
C nanoprisms using syngas [23–25]. Co
2
C
nanoprisms were mainly generated from bulk CoMn oxides prepared via
coprecipitation method. In general, the application of catalyst supports
is expected to decrease active metal content and improve mechanical
CO
2
, is becoming more attractive due to the abundant reserves [1–6].
Fischer-Tropsch to olefins (FTO) [1,5–11] and oxide-zeolite bifunctional
catalytic process [2,3,12] are the reported routes for direct synthesis of
olefins from syngas, and have been extensively studied in recent years.
For instance, Torres Galvis et al. developed a supported Fe-based FTO
strength, and it remains great interest to fabricate supported Co
nanoprisms by simple impregnation method. Previously, the effect of
different supports including SiO and carbon nanotubes (CNTs)
, γ-Al
on the FTO reaction was studied, and the results indicated that Co
nanoprisms were formed for the inert CNT with high olefins selectivity
and low methane selectivity [19]. However, Co C nanospheres were
observed for both SiO and Al supported CoMn catalyst due to the
strong interaction between cobalt and supports. The formed Co
2
C
2
2 3
O
catalyst with sodium and sulfur as promoters, and found that the
2
C
=
selectivity to lower olefins (C2ꢀ 4
) reached 61 C% [1]. As for
oxide-zeolite catalysts, Jiao et al. reported that the catalyst obtained
2
from coupling ZnCrO
x
and mesoporous SAPO zeolite realized 80 C%
2
2 3
O
=
selectivity for C2ꢀ 4 with CO conversion of 17 % [2].
2
C
In our previous study, Co
2
C nanoprisms were found to exhibit
nanospheres led to a relatively high methane selectivity. With the ad-
vantages of low price, oxygen resistance and strong mechanical
strength, oxide supports are still the best potential choices for practical
=
excellent FTO performance with a C2ꢀ 4 selectivity of 60.8 C% and low
methane selectivity of 5.0 C% at mild conditions [4,13–20]. The roles of
*
Corresponding authors at: CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of
Sciences, Shanghai, 201203, PR China.
E-mail addresses: zhongls@sari.ac.cn (L. Zhong), sunyh@sari.ac.cn (Y. Sun).
https://doi.org/10.1016/j.apcata.2021.118283
Received 6 May 2021; Received in revised form 2 July 2021; Accepted 4 July 2021
Available online 8 July 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
Table 1
a
2
Catalytic performance for various CoMnNa/SiO catalysts .
Product Selectivity (C%, CO
2
-free)
Olefin/Paraffin Ratio
◦
Catalyst
Temperature ( C)
CO Conv.(C%)
CO
2
Sele. (C%)
Ole.
Oxy.
Para.
CH
4
C
2
C
3
C
4
O/P
2
60
3.1
9.7
57.4
51.4
49.3
60.4
57.1
45.7
75.9
78.5
79.9
5.1
4.7
5.3
7.0
4.6
4.2
10.0
10.7
9.7
37.6
43.9
45.4
32.6
38.2
50.1
14.1
10.8
10.4
18.7
25.7
35.9
19.7
26.1
28.5
3.9
2.9
1.9
1.4
2.6
1.9
1.2
10.4
8.5
6.8
11.2
10.8
10.7
11.2
11.2
10.5
17.2
17.5
18.1
8.8
1.5
1.2
1.1
1.9
1.5
0.9
5.4
7.2
7.6
CoMnNa/SiO
2
2
2
-350
-650
-990
265
4.4
10.5
15.0
9.1
8.0
2
2
70
60
5.2
7.3
4.0
8.8
CoMnNa/SiO
265
4.3
12.3
12.9
37.7
42.8
46.0
8.6
2
2
70
60
5.3
7.7
11.9
19.0
23.9
12.8
12.1
12.5
CoMnNa/SiO
265
70
3.8
2
4.3
a
/CO = 0.5 and WHSV = 2000 mL gꢀ 1
ꢀ 1
Reaction condition: 5 bar, H
2
h
. Ole.: olefins; Oxy.: oxygenates; Para.: paraffins.
2.2. Characterization
adsorption experiments were conducted on a Micromeritics 2420
industrial application of supported catalysts. And we wonder whether it
is possible to weaken the interaction between oxide supports and cobalt
for better FTO performance over supported CoMn catalysts.
N
2
Much efforts have been made to tune the interactions between oxide
instrument for the textural properties of CoMnNa/SiO
2
catalysts. Before
◦
supports (such as Al
2
O
3
, SiO
2
2
and TiO ) and cobalt, which have a sig-
measurements, the samples were degassed at 200 C for 5 h. The specific
surface area (SBET) was calculated through BET method, and the pore
volume (VBJH) and pore size distribution were determined by BJH
method.
nificant influence for the reducibility and catalytic performance
[
26–28]. Generally, the modification methods includes precoating with
carbon interlayer, introducing reactive inorganic oxide, surface hydro-
phobic treatment and modifying with surface silanol groups [27,29–39].
X-ray diffraction (XRD) patterns were obtained by a Rigaku Ultima
IV X-ray powder diffractometer at a range of 20–80 ◦ with a speed of 4 /
min. JCPDS standard cards were employed for the identification of
characteristic peaks.
◦
2 3
For instance, Vissers and co-workers reported that carbon-covered Al O
via pyrolysis organic molecules (cyclohexene or ethylene) on the surface
of Al reduced the strong interaction between cobalt and Al
which displayed higher catalytic activity [29]. Feller et al. discovered
that the interaction between cobalt and SiO decreased while the
reduction degree of catalysts increased via introducing zirconium into
the Co/SiO catalysts, resulting from the formation of a weaker
cobalt-zirconium interaction [27]. Besides, Rytter et al. modified Al
2
O
3
2 3
O ,
Transmission electron microscopy (TEM) and high-resolution trans-
mission electron microscopy (HRTEM) were performed on a JEOL JEM
2011 electron microscope operated at 200 kV.
2
2
2
Hydrogen temperature-programmed reduction (H -TPR) measure-
2
O
3
ments were carried out on Micromeritics Autochem-II 2920 instruments
equipped with a thermal conductivity detector (TCD). Typically, 50 mg
fresh sample was loaded into a quartz tube. Before reduction, the sample
with chloro or methoxy active ligands, and found that the interaction
was prevented and the reduction of catalysts was easier due to hydro-
◦
phobicity of Al
2
O
3
surface [38]. Recently, Okoye Chine et al. pretreated
was pretreated with a He flow at 200 C for 2 h, and then the temper-
◦
SiO with ethylene glycol and thermal method, and discovered that the
2
ature cooled down to 60 C in He flow until the baseline was stable. At
◦
increased proportion of isolated silanol groups with ethylene glycol
treatment enhanced the strong interaction, resulting in low activity for
FTS [39]. In contrast, the proportion of isolated silanol groups did not
increase for thermal method pretreatment.
last, the samples were heated to 800 C in a flow of 10 % H
2
/Ar, and the
H
2
consumption was collected by TCD.
Fourier transform infrared spectrum (FTIR) experiments were car-
ried out, and recorded by the Nicolet iN10 (Thermo Scientific) infrared
ꢀ 1
Herein, we prepared SiO
found that the thermal treatment of SiO
support interaction, which greatly affect the formation of CoMn com-
posite oxides and the final morphology of Co C nanostructures. Various
2
supported catalysts for FTO reaction, and
spectrophotometer at a range of 400ꢀ 4000 cm . After the background
was collected, the spectrum of sample was collected with transmission
mode.
2
can effectively tune the metal-
2
characterization techniques were used to trace the phase and structure
evolution of catalysts, and the structure-performance relationship was
also investigated.
2
.3. Catalytic evaluation
The CoMnNa/SiO catalysts were evaluated in fixed-bed reactor
2
constructed with 8 mm inner diameter. Typically, 1.0 g of 40–60 mesh
2
. Experimental section
fresh catalysts mixed with 3.0 g silica of same mesh. Prior to reaction,
◦
the catalyst was in-situ reduced at 300 C for 5 h under a 10 % H
2
/N
2
2
.1. Catalyst preparation
ꢀ 1 ꢀ 1
flow with WHSV of 8000 mL g
h . After reduction, the temperature
◦
was cooled to 250 C. Before feeding, the reactor was swept in a He flow
CoMnNa/SiO
2
supported catalysts were prepared by incipient
support (480
m /g, Aladdin) was firstly pretreated under different temperatures (350
for 30 min. Then the feed gas (syngas, H
2
/CO = 0.5, v/v) with a WHSV
wetness impregnation method. Before preparation, SiO
2
ꢀ 1 ꢀ 1
of 2000 mL g
h
was introduced into the system, and the pressure
2
was increased to 5 bar (gauge pressure). After activation for 24 h, the
temperature was raised to desired reaction temperature for CO hydro-
genation tests. The gas phase products were analyzed online by gas
chromatograph (Agilent 7890B) equipped with thermal conductivity
◦
◦
◦
C, 650 C and 990 C) for 5 h in maffle furnace, and named as SiO
SiO -650, SiO -990, respectively. Typically, 6.61 g SiO support was
impregnated with mixture liquid of 7.41 g Co(NO O (Sinopharm
⋅6H
Chemical Reagent Co., Ltd), 5.21 g aqueous of 50 wt% Mn(NO
Sinopharm Chemical Reagent Co., Ltd.), 0.22 g NaNO (Sinopharm
2
-350,
2
2
2
3
)
2
2
3 2
)
(
TCD) detector and flame ionization (FID) detectors after transiting the
(
3
◦
◦
hot trap (120 C) and cold trap (0 C). The liquid products were collected
in the hot trap and cold trap, and analyzed off-line by Shimadzu GC2010
Chemical Reagent Co., Ltd.) and suitable deionized water. The content
of Co, Mn, Na was fixed at 20 wt%, 10 wt%, and 0.6 wt%, respectively,
with respect to the sum of samples. The samples were then dried under
plus. In addition, there may be some deviations resulting in the lower C
and C products, which was also reported by other researchers in FT
8
9
◦
room temperature for 24 h, and dried at 120 C for 12 h. At last, the
study [40,41]. The data were taken when the performance kept in
relatively stable states after reaction for 24 h. And each FTO testing held
for 48 h under the desired temperature. The mass balance, oxygen
◦
CoMnNa/SiO
2
samples were calcined at 350 C in maffle furnace for 4 h.
2

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
Fig. 1. ASF distributions and chain growth probability calculated by fitting curve from C3 to C7 using ASF model for CoMnNa/SiO
2
-350 (a), CoMnNa/SiO
2
-650 (b),
(1)
/CO = 0.5 and WHSV = 2000 mL gꢀ
1
h .
ꢀ 1
CoMnNa/SiO
2
-990 (c) at the reaction condition of 5 bar, H
2
balance, and carbon balance were calculated, and all higher than 95 %.
The catalytic activity for CoMnNa/SiO catalysts was determined by
n
CO,inlet ꢀ nCO,outlet
CO conversion =
× 100%
2
n
CO,inlet
Eq. (1) as follows, where the nCO,inlet means the moles of CO at the inlet
while the nCO,outlet means the moles of CO at the outlet.
Meanwhile, the selectivity of various products was calculated by Eqs.
2) and (3). Among them, the N and n means the moles and carbon
(
i
i
Fig. 2. Detailed hydrocarbon distributions for CoMnNa/SiO
2
-350 (a), CoMnNa/SiO
2
-650 (b), CoMnNa/SiO
2
-990 (c) at the reaction condition of 5 bar, H
2
/CO = 0.5
and WHSV = 2000 mL gꢀ
1
h .
ꢀ 1
3

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
◦
2
Fig. 3. Stability test for CoMnNa/SiO -990 catalyst at 265 C.
number of product i, respectively.
2
for CoMnNa/SiO -990, indicating that the produced hydrocarbons were
more concentrated in low-carbon numbers. The detailed hydrocarbon
distribution was also summarized as shown in Fig. 2. It was clear that the
hydrocarbon distribution was concentrated in the range of C1ꢀ 12 for all
of the catalysts, and olefins were the main products in hydrocarbons.
However, a narrower hydrocarbon distribution was observed for
n
CO2
S
S
CO2
=
× 100%
(2)
(3)
n
CO,inlet ꢀ nCO,outlet
N
i
× n
i
= ∑ (
× 100%
i
N
i
× n
i
)
CoMnNa/SiO
for CoMnNa/SiO
that of CoMnNa/SiO
product over CoMnNa/SiO
products with carbon number in the range of 1~7 dominated the
2
-990. Moreover, the methane proportion in hydrocarbons
-350 and CoMnNa/SiO -650 was much higher than
-990. In addition, it is hard to collect the liquid
2
2
3
. Results and discussion
2
◦
2
-350 catalyst under 270 C as the light
3
.1. Catalytic performance
products. The stability for CoMnNa/SiO
2
-990 catalyst was also tested at
The CoMnNa/SiO catalysts were evaluated under 5 bar syngas at
2
◦
◦
◦
◦
265 C as shown in Fig. 3. Both the activity and products selectivity
almost remained unchanged during 100 h test, indicating promising
stability.
several temperatures (260 C, 265 C and 270 C) after in-situ reduction,
and the results were summarized in Tables 1 and S1. CO conversions of
CoMnNa/SiO
2
-350 and CoMnNa/SiO
2
-650 catalysts were quite low with
◦
less than 5 C% at 265 C. However, CO conversion was up to 19.0 C% for
CoMnNa/SiO
and the ratio of olefin to paraffin (O/P) for the catalyst with the support
2
-990. As for the product selectivity, the olefins selectivity
3.2. Structure characterization
◦
of SiO
catalysts. However, methane selectivity for CoMnNa/SiO
CoMnNa/SiO -650 was almost up to 26 C%, which was much higher
than that obtained by CoMnNa/SiO -990 (3.8 C%). In addition, methane
selectivity increased with the reaction temperature for all catalysts,
especially for CoMnNa/SiO -350 and CoMnNa/SiO -650. Paraffins
selectivity for CoMnNa/SiO -990 (10.8 C%) was much lower than that
obtained by CoMnNa/SiO -350 (43.9 C%) and CoMnNa/SiO -650 (38.2
C%). Moreover, oxygenates selectivity for all the catalysts was lower
than 11.0 C%. Obviously, the CoMnNa/SiO -990 catalyst exhibited high
activity, high olefins selectivity and very low methane selectivity, which
was much better than that obtained by CoMnNa/SiO -350 and
selectivity for
-990 suggested syngas derived from coal or biomass with
C-based FTO reaction.
As shown in Fig. 1, the hydrocarbon distribution for CoMnNa/SiO
50 and CoMnNa/SiO -650 followed the classical ASF distribution
under different reaction temperatures. However, the hydrocarbon dis-
tribution significantly deviated from the ASF model for CoMnNa/SiO
90, consistent with the considerably low methane selectivity. As for the
chain growth probabilities, the values for CoMnNa/SiO -350 and
CoMnNa/SiO -650 were both around 0.58, and basically not affected by
reaction temperature. On the other hand, the value was relatively low
2
pretreatment at 990 C were the highest among all of the studied
The distinguished difference for catalytic performance suggested
there should exist great difference for the structure of various CoMnNa/
2
-350 and
2
SiO catalysts. Therefore, the structure of various catalysts was inves-
2
tigated in detail by a series of techniques. The composition of calcined
catalysts was firstly tested by XRF and listed in Table S2. For all the
calcined catalysts, the content of Co, Mn and Na was approximately 18.7
wt%, 10.0 wt% and 0.6 wt%, respectively, basically consistent with the
theoretical loading amount. Compared with the unsupported CoMn
2
2
2
2
2
2
2 2
catalysts without SiO , the supported CoMnNa/SiO catalysts exhibited
a similar catalyst activity with reduced cobalt content [4]. In addition,
the specific surface area and pore structure properties of various samples
2
2
were showed in Fig. S1 and Table S2. The SBET for CoMnNa/SiO -350,
2
2
2
CoMnNa/SiO
CoMnNa/SiO
2
2
-650 catalysts. In addition, the high CO
2
CoMnNa/SiO
2
-650 and CoMnNa/SiO
2
-990 was 260.6 m /g, 246.9 m /g
and 45.8 m /g, respectively. Meanwhile, type Ⅳ isothermal curve and
type H hysteresis loop were observed for all the samples, revealing
2
low H
2
/CO ratio was more suitable for Co
2
2
2
-
similar mesoporous structure [42].
3
2
2
Fig. 4 showed the XRD patterns of CoMnNa/SiO catalysts at
different stages. After calcination, Co
(PDF #42-1169) were observed for CoMnNa/SiO
SiO -650 catalysts. However, CoMnNa/SiO -990 displayed diffraction
peaks of MnCo 4.5 (PDF #32-0297) and Co (PDF #74-2120).
Herein, we noticed that CoMn composite oxide was formed only for the
calcined CoMnNa/SiO -990. In addition, according to the elemental
mapping of the calcined catalysts in Fig. 5, obvious elemental separation
3
O
4
(PDF #74-2120) and MnO
2
2
-
2
-350 and CoMnNa/
9
2
2
α
2
2
O
3 4
O
2
α
2
4

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
2
Fig. 4. XRD patterns of the calcined (a), reduced (b) and spent (c) CoMnNa/SiO catalysts with rough scan and fine scan.
of cobalt and manganese appeared for CoMnNa/SiO
and manganese were highly overlapped for CoMnNa/SiO
Furthermore, Co Mn1-xO was observed for the reduced CoMnNa/SiO
90 (Fig. 4b). However, the diffraction peaks of CoO (PDF #71-1178)
and MnO (PDF #75-0257) were identified for the reduced CoMnNa/
SiO -350 and CoMnNa/SiO -650 samples, which further revealed the
2
-350 while cobalt
#65-1457) was the main phase of Co species for all spent CoMnNa/SiO
catalysts. Moreover, CoO, as another form of Co species, as well as MnO
were all observed for CoMnNa/SiO -350 and CoMnNa/SiO -650 cata-
lysts. In addition, a small amount of Co Mn1-xO and MnO was detected
for CoMnNa/SiO -990 catalyst. Moreover, no peaks associated with Co
2
2
-990.
x
2
-
2
2
9
x
0
2
2
2
were observed in XPS study (Fig. S2), indicating that Co species existed
in an oxidized state at the surface. Besides, previous studies also
revealed that no metallic Co was observed after activation process using
phase separation of Co and Mn.
2
The XRD patterns of spent catalysts (Fig. 4c) showed that Co C (PDF
5

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
2 2
Fig. 5. X-ray energy dispersive spectroscopy (EDS) mapping of the fresh calcined CoMnNa/SiO -350 (a) and CoMnNa/SiO -990 (b).
2 2 2
Fig. 6. (HR)TEM images of various spent catalysts for CoMnNa/SiO -350 (a), CoMnNa/SiO -650 (b) and CoMnNa/SiO -990 (c).
6

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
Based on our previous study, the precursor of cobalt manganese
composite oxide was beneficial to the formation of Co
with exposed (020) and (101) facets [4,13,20]. For both CoMnNa/-
SiO -350 and CoMnNa/SiO -650 samples, phase separation of Co and
Mn was observed without the formation of CoMn composite oxide.
Correspondingly, Co C nanospheres were formed under reaction con-
ditions conducted in the current work. Therefore, we speculated that
stronger interaction between cobalt and SiO hindered the formation of
cobalt manganese composite oxide for the CoMnNa/SiO -350 and
CoMnNa/SiO -650 catalysts. The average crystallite sizes of
2
C nanoprisms
2
2
2
2
2
2
cobalt-containing phases at different stages were calculated by Scher-
rer’s equation, and summarized in Table S3. The crystallite sizes for
CoMnNa/SiO -990 were larger than the others at all stages especially
2
after reaction, which implying the weaker interaction between cobalt
and SiO support. Moreover, H -TPR profiles of fresh CoMnNa/SiO
catalysts were investigated as shown in Fig. 7. The curve of CoMnNa/-
2
2
2
SiO
2
-350 was similar to that of CoMnNa/SiO
2
-650, and both showed
◦
◦
◦
three main peaks at 225.7 C, 287.0 C and 566.8 C, respectively. The
first two peaks were ascribed to reduction of Co
3
O
4
to CoO and CoO to
0
◦
Co , respectively [47,48]. Notably, the intense peak around 566.8
C
was attributed to the signal of strong interaction between highly
dispersed Co species and SiO support [37,49]. On the other hand,
-990 exhibited three obviously different peaks around
2 2
Fig. 7. H -TPR profiles of various fresh CoMnNa/SiO catalysts after
2
calcination.
CoMnNa/SiO
2
◦
◦
◦
◦
2
46.8 C, 343.4 C and 426.8 C. The first peak at 246.8 C was ascribed
in situ XAFS characterization [43,44]. Although Co
phase for all of the spent catalysts, the morphology of Co
samples exhibited significant difference as shown in Fig. 6. A number of
Co C nanospheres were discovered from randomly selected HRTEM
images for both CoMnNa/SiO -350 and CoMnNa/SiO -650 catalysts. In
contrast, a large amount of Co C nanoprisms with exposed (020) and
101) facets were observed for CoMnNa/SiO -990 catalyst. According to
the previous study [45,46], the spherical Co C nanoparticles usually
displayed low activity and high methane selectivity, consistent with the
relatively poor FTO performance for CoMnNa/SiO -350 and CoMn-
Na/SiO -650 catalysts. In contrast, the Co nanoprisms always
2
C was the main
2 4.5 3 4
to the reduction of MnCo O and Co O , while the second peak was
2
C for various
2 4.5 x
attributed to reduction of the rest MnCo O to Co Mn1-xO [18]. Be-
◦
sides, the broad peak around 426.8 C was assigned to the further
◦
2
reduction of Co Mn1-xO [18,19]. Specially, the peak around 566.8 C
x
2
2
was absent for CoMnNa/SiO -990 catalyst, indicating that interaction
2
2
between Co species and SiO2 support was relative weak for
(
2
CoMnNa/SiO -990.
2
2
It was well known that the strong interaction was usually derived
from the interaction between Si
the content of Si OH groups should possess a significant impact on the
interaction. FTIR spectra of SiO2 with different pretreatment tempera-
–
OH groups and cobalt [37]. Therefore,
2
–
2
2
C
ꢀ 1
exhibited high activity, low methane selectivity and high olefins selec-
tivity, and was in good agreement with the catalytic results for CoMn-
ture was tested as shown in Fig. 8a. The absorption bands at 480 cm ,
ꢀ 1
ꢀ 1
820 cm and 1050 cm were all attributed to vibration of Si
Si Si [39,50]. The intensity of these absorption bands was similar
for different SiO . The absorption bands at 1631 cm and 3444 cm
–
O from
Na/SiO
it was clear that the hydrocarbon distribution for the catalysts with Co
nanospheres followed the typical ASF model while dramatically devi-
ated ASF distribution for the catalysts with Co C nanoprisms. Based on
the above results, we can draw the conclusion that the diverse
morphology of Co C is the main underlying reason for the striking dif-
2
-990 catalysts [4]. Combined with the ASF distribution (Fig. 1),
O
– –
ꢀ 1
ꢀ 1
2
C
2
were both ascribed to the
absorption bands at 965 cm was assigned to the
–
OH vibration from H O [39]. Specially, the
2
1
ꢀ
2
–
OH vibration of
OH groups [39]. It is noteworthy that the absorption bands of
–
OH groups for SiO -990 was significantly weakened compared with
Si
Si
–
2
2
ꢀ
1
ꢀ 1
ference of FTO performance.
SiO -350 and SiO -650. Peak fitting from 1400 cm to 600 cm and
2
2
ꢀ 1
ꢀ 1
Fig. 8. (a) FTIR spectra of SiO
2
2
with different pretreatment temperature; (b) Peak fitting of FTIR spectra from 1400 cm to 600 cm for SiO supports.
7

L. Li et al.
Applied Catalysis A, General 623 (2021) 118283
the corresponding semi-quantitative results were also shown in Fig. 8b
catalysts and optimization of pretreatment procedure is necessary to
greatly improve catalytic performance.
ꢀ 1
and Table S4. With a benchmark of the peak area at 1050 cm
ꢀ
1
(
Si
groups) was in the order of SiO
SiO -990. Especially, the relative peak area drastically decreased when
the pretreatment temperature increased up to 990 C, indicating the
decrease of the content of Si OH groups. Moreover, the IR mapping
images at 965 cm with a randomly selected area for SiO supports
were shown in Fig. S2, which also indicated a decrease trend for the
–
O
–
Si), the relative peak area (A1/A2) at 965 cm
(Si
–
OH
2
-fresh > SiO
2
-350 ≈ SiO
2
-650 >
CRediT authorship contribution statement
2
◦
Liusha Li: Conceptualization, Validation, Formal analysis, Investi-
gation, Data curation, Writing - original draft. Fei Yu: Resources, Formal
analysis, Writing - review & editing. Xiao Li: Formal analysis, Writing -
review & editing. Tiejun Lin: Validation, Formal analysis, Writing -
review & editing. Yunlei An: Writing - review & editing. Liangshu
Zhong: Conceptualization, Validation, Supervision, Project adminis-
tration, Funding acquisition, Writing - review & editing. Yuhan Sun:
Supervision, Writing - review & editing.
–
ꢀ
1
2
content of Si
temperature.
–
OH groups with the increase of pretreatment
3
.3. Mechanism discussion
Generally, the surface silanol of SiO
when contacting with impregnation liquid, as the point of zero charge
PZC) for SiO (around 2) was always smaller than the pH of impreg-
nation liquid [51]. As a result, the surface of SiO support was negatively
charged and could anchor the cations in the impregnation solution via
2
support would be deprotonated
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
(
2
2
◦
◦
electrostatic adsorption. For SiO
the rich surface silanol of SiO
drated cations including [Co (H
impregnation process. As the ion radius for [Co (H
2
via pretreatment at 350 C and 650 C,
2
support made it easy to anchor the hy-
This work was financially supported by the National Key R&D Pro-
2
+
2+
6
O) ]
2
O)
6
]
and [Mn (H
2
during
was smaller
2+
would possess stronger
gram of China (2017YFB0602202), Natural Science Foundation of China
2
+
2
O) ]
6
(
91945301), Program of Shanghai Academic/Technology Research
2
+
than that of [Mn (H
electrostatic interaction with SiO
result, cobalt was concentrated on the surface of SiO
ration occurred between cobalt and manganese. In contrast, the sharply
decreased surface silanol for SiO -990 support made it difficult to an-
chor the hydrated cations including [Co (H
during impregnation process. On the other hand, as the hydrated cations
2
O)
6
]
, [Co (H
support than [Mn (H
and phase sepa-
2 6
O) ]
Leader (20XD1404000), Key Research Program of Frontier Sciences,
CAS (Grant No. QYZDB-SSW-SLH035), the “Transformational Technol-
ogies for Clean Energy and Demonstration”, Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant No.
XDA21020600), Youth Innovation Promotion Association of CAS.
2
+
2
2
O)
6
]
. As a
2
2
2
+
2+
6
O) ]
2
O)
6
]
and [Mn (H
2
Appendix A. Supplementary data
2
+
2+
of [Co (H
2
O)
6
]
and [Mn (H
2
O) ] possess the same charge number
6
and coordination structure (octahedral coordination), they are easily to
mix together during the drying process. Finally, CoMn composite oxide
was formed after calcination.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118283.
Prior work has demonstrated that the CoMn composite oxides pre-
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