Effects of Sm on Fe-Mn catalysts for Fischer-Tropsch synthesis

Article abstract of DOI:10.1039/c9ra05337a

Sm-promoted FeMn catalysts were prepared by the co-precipitation method and characterized by N₂ adsorption, XRD, CO-TPD, H₂-TPD, CO₂-TPD, H₂-TPR, XPS and MES. It was found that compared with the un-promoted catalyst, when Sm was added at a proper content, the catalyst showed a larger BET surface area and promoted the formation of iron particles with a smaller size. The presence of Sm could increase the surface charge density of iron, which enhanced the Fe-C bond and promoted the stability and amount of CO dissociated adsorption, as confirmed by XPS and CO-TPD. Furthermore, according to MES, Sm could promote the formation of Fe₅C₂, which was the active phase of FTS. In addition, Sm could also enhance the basicity of the catalysts and suppress the H₂ adsorption capacity, which inhibited the hydrogenation reaction and the conversion of olefins to paraffins, as verified by the results of CO₂-TPD and H₂-TPD. According to the FTS performance results, compared with the observations for the un-promoted catalysts, when the molar ratio of Sm to Fe was 1%, the CO conversion increased from 63.4% to 70.4%, the sum of light olefins in the product distribution increased from 26.6% to 32.6, and the ratio of olefins to paraffins increased to 4.18 from 4.09.

Full text of DOI:10.1039/c9ra05337a

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Effects of Sm on Fe–Mn catalysts for Fischer–
Tropsch synthesis†
Cite this: RSC Adv., 2019, 9, 32240
Zhonghao Han,a Weixin Qian,a Hongfang Ma,a Haitao Zhang,a Qiwen Sunb
a
*
and Weiyong Ying
Sm-promoted FeMn catalysts were prepared by the co-precipitation method and characterized by N2
adsorption, XRD, CO-TPD, H2-TPD, CO2-TPD, H2-TPR, XPS and MES. It was found that compared with
the un-promoted catalyst, when Sm was added at a proper content, the catalyst showed a larger BET
surface area and promoted the formation of iron particles with a smaller size. The presence of Sm could
increase the surface charge density of iron, which enhanced the Fe–C bond and promoted the stability
and amount of CO dissociated adsorption, as confirmed by XPS and CO-TPD. Furthermore, according to
MES, Sm could promote the formation of Fe5C2, which was the active phase of FTS. In addition, Sm
could also enhance the basicity of the catalysts and suppress the H2 adsorption capacity, which inhibited
the hydrogenation reaction and the conversion of olefins to paraffins, as verified by the results of CO2-
TPD and H2-TPD. According to the FTS performance results, compared with the observations for the
un-promoted catalysts, when the molar ratio of Sm to Fe was 1%, the CO conversion increased from
63.4% to 70.4%, the sum of light olefins in the product distribution increased from 26.6% to 32.6, and the
ratio of olefins to paraffins increased to 4.18 from 4.09.
Received 12th July 2019
Accepted 26th September 2019
DOI: 10.1039/c9ra05337a
rsc.li/rsc-advances
higher alpha values than that of an unpromoted catalyst in
FTS.8,9 It is generally believed that manganese promoters in
1. Introduction
iron-based catalysts are benecial for the formation of low-
carbon olens for FTS.10 The reason for this nding may be
that manganese can enhance the adsorption capacity of CO and
promote the formation of iron carbide.11 The study showed that
the Mn promoter was benecial for the formation of light
carbon olens.12,13 Some studies have shown that the intro-
duction of Mn as a structure promoter into iron-based catalysts
for the Fischer–Tropsch synthesis can enhance the stability of
catalysts and promote the WGS reaction.14,15 In contrast, other
reported Mn promoters enhanced the selectivity of olens and
C5+ but restrained the selectivity of methane.16,17 In addition,
some studies have shown that structural promoters such as
Due to the shortage of fossil energy and the increasingly severe
environmental problems, Fischer–Tropsch synthesis (FTS) has
aroused much attention1 as it can convert syngas into raw oil
and chemicals without sulfur, nitrogen and aromatic hydro-
carbons.2 Many studies have shown that only iron and cobalt
catalysts have the value of being industrialized.3 Cobalt-based
catalysts have higher FTS activity as well as higher selectivity
of high-carbon hydrocarbons and paraffins.4 Compared to
cobalt-based catalysts, iron-based catalysts are attractive for
their high water gas shi (WGS) and high CO2 selectivity due to
the Boudouard mechanism.5,6
In the Fischer–Tropsch synthesis, many metals have been
introduced as promoters. The impact of the Group I alkali
metals upon the activity of the iron catalysts was studied and it
was found that potassium promotion provided an outstanding
catalyst at the activity viewpoint for both the FTS and WGS
reactions.7 Other research studies suggested that all Group II
metal promoters yielded similar carbon utilization but with
20
SiO2,18,19 Al2O3 and TiO2 can improve the selectivity, activity
and stability of iron-based catalysts.21
In recent years, rare earth metals have been reported as
promoters. The catalytic performance of the precipitated Fe–Cu
catalyst promoted with La, Ce, Nd, Eu and Th oxides was
investigated and the results showed that rare earth metal oxides
and Th promoters could increase the dispersion and stabiliza-
tion of the iron-based catalysts, inhibiting their growth and
further reduction.22 Some studies suggest that the introduction
of a small amount of rare earth metal will greatly increase the
performance of iron-based catalysts, but excessive rare earth
metal promoters will signicantly inhibit FTS.23,24 The effect of
La2O3 on a precipitated iron catalyst showed that the addition of
aEngineering Research Center of Large Scale Reactor Engineering and Technology,
Ministry of Education, State Key Laboratory of Chemical Engineering, East China
University of Science and Technology, Shanghai 200237, China. E-mail: wying@
ecust.edu.cn; Fax: +86 21 64252192; Tel: +86 21 64252151
bState Key Laboratory of Coal Liquefaction and Coal Chemical Technology, Shanghai
201203, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05337a
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La2O3 increased the formation of methane while suppressing 2920 instrument. About 50 mg catalysts were purged in a ow of
+
the selectivity for C5 hydrocarbons.25
Although many rare earth metal promoters have been was treated in 10% H2/90% Ar (v/v) at a ow rate of 50 ml minꢂ1
studied, few studies have been performed with samarium as and the reduction temperature was increased from room
argon at 350 C for 2 h and then cooled to 50 C. Each sample
ꢀ
ꢀ
,
ꢂ1
ꢀ
ꢀ
promoters. The aim of this work was to investigate the inuence temperature to 800 C at a rate of 10 C min
.
of samarium upon the properties of iron-based catalysts in the X-ray photoelectron spectroscopy (XPS) was performed on
FTS reaction. Iron-based catalysts modied by samarium were a VG ESCALAB 250Xi electron spectrometer equipped with
prepared by the co-precipitation method and tested under FTS a hemispherical analyzer operating in a constant pass energy
conditions in a xed tube reactor to analyse the promotion mode and an Al Ka X-ray source operating at 10 mA and 12 kV.
effect of samarium on the activity and product distribution. The The samples were reduced by H2 at 350 ꢀC and 0.1 Mpa for 10 h
inuences of Sm2O3 on the structure, adsorption properties, before the measurement. The reduced catalysts were placed in
reduction properties and FTS catalytic performance of the iron- nitrogen to avoid oxidation.
¨
Mossbauer spectroscopy (MES) of the catalyst samples was
based catalysts were researched via N2 adsorption, XRD, CO-
¨
¨
TPD, H2-TPD, CO2-TPD, H2-TPR, XPS and Mossbauer performed on an MR-351 constant-acceleration Mossbauer
spectroscopy.
spectrometer (FAST, Germany) at room temperature with 57Co in
a Pd matrix. The spectra were collected over 512 channels in the
¨
mirror image format. The Mossbauer parameters including an
2. Experimental
2.1. Catalyst preparation
isomer shi (IS), quadruple splitting (QS), and hyperne eld
(Hhf) were used to identify the components of the iron phase.
The reacted catalysts were placed in nitrogen to avoid oxidation.
The Sm-promoted catalysts were prepared by the co-
precipitation method. A solution containing iron(III) nitrate
nonahydrate (>99.99 wt% Shanghai Macklin Biochemical Co.,
Ltd), manganese nitrate (50 wt% Shanghai Macklin Biochem-
ical Co., Ltd) and samarium(III) nitrate hexahydrate (>99.99 wt%
Shanghai 3A Chemical Co., Ltd) with the desired ratio was
precipitated by anhydrous sodium carbonate (>99.99 wt%
Shanghai Macklin Biochemical Co., Ltd) at 65 ꢀC. The pH value
of the solution was kept at 8.0 ꢁ 0.3. Then, the solution was
stirred for 0.5 h and aged for 1 h. Aer washing, ltration and
drying, the precipitate was treated at 550 ꢀC for 3 h. The
FeMnSm catalysts were prepared by the above methods. The
molar compositions of the catalysts were 100Fe/10Mn/xSm (x ¼
0, 0.5, 1, 1.5 and 2), which were labeled as Sm0, Sm0.5, Sm1,
Sm1.5 and Sm2, respectively.
2.3. FTS performance
The FTS performance of the catalysts was conducted in a xed
bed reactor (ID ¼ 10 mm). The detailed description of the
reactor and the product analysis system has been provided
elsewhere.26 The particle size of the catalysts was 60–80 mesh,
and 0.3 g of the catalyst sample was mixed with 0.6 g of the same
particle sized quartz grains. The catalysts were reduced with H2
at 350 ꢀC, 0.10 MPa and GHSV 4000 ml gꢂ1 hꢂ1 for 10 h. The FTS
catalyst activity tests were maintained at 300 ꢀC, 1.0 MPa, H2/CO
¼ 2 and 12 000 ml gꢂ1 hꢂ1. Aer the FTS reaction, the outlet
gases CO, H2, CH4, etc. were analyzed by online GC Agilent
7890A with a thermal conductivity detector (TCD). The waxes
were dissolved in CS2 and detected off-line by GC Agilent 7890A
with a ame ionization detector (FID).
2.2. Catalyst characterization
N2 physical adsorption was performed at 77 K with a Micro-
meritics ASAP 2020 instrument. The samples were evacuated at
120 ꢀC for 4 h before the adsorption measurement. Specic
surface areas were measured using the Brunauer Emmett Teller
3. Result and discussion
3.1. Catalyst characterization
3.1.1. Textural properties of catalysts. The textural and
(BET) method. Pore volumes and pore sizes were measured by structural properties of the corresponding catalysts were char-
the Barrett–Joyner–Halenda (BJH) procedure. acterized by N2 physical adsorption and XRD. The N2 physical
The XRD patterns of the samples were obtained on a diffrac- adsorption–desorption isotherms of the FeMnSm catalysts are
tometer operating with Cu Ka radiation at 40 kV with a scanning shown in Fig. 1. No signicant increase in the catalysts was
rate of 6ꢀ minꢂ1 and 2q angles ranging from 10 to 80ꢀ.
detected aer N2 uptake until the relative pressure (P/P0)
CO-TPD, H2-TPD and CO2-TPD were carried out using increased to 0.9. A rapid increase was seen when P/P0 was
a Micromeritics Autochem 2920 apparatus with a thermal beyond 0.9. Therefore, the existence of a larger mesoporous and
conductivity detector (TCD). About 100 mg samples were intergranular pore space was proven.27 During FTS, these
ꢀ
reduced under an H2 ow at 350 C for 2 h and were cooled to characteristics were helpful to reduce the mass transfer resis-
60 ꢀC under an He gas ow. Aerwards, CO/H2/CO2 were tance and shorten the residence time of the feed gas, thereby
introduced into the catalyst bed for 30 min. Then, the catalyst promoting the formation of low-carbon hydrocarbons and
bed was purged by the ow for 60 min. Subsequently, the inhibiting the formation of high-carbon hydrocarbons.28
samples were heated to 800 ꢀC at a rate of 10 ꢀC minꢂ1. The
desorbed products were detected with the TCD detector.
The BET surface areas, pore volumes and pore sizes of the
fresh catalysts with various samarium contents are listed in
H2-TPR of the fresh catalysts was carried out in a conven- Table 1. It can be seen that the BET surface areas of the catalysts
tional atmospheric quartz ow reactor by a Micromeritics ASAP rst increase with the increase in samarium from 0 to 1%. This
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Fig. 2 XRD patterns of the FeMnSm catalysts with different Sm
loadings.
Fig. 1 N2 physical adsorption–desorption isotherms of the FeMnSm
catalysts.
dissociated adsorption of CO.30 No signicant difference was
observed for the lower desorption peaks, showing that Sm had
little effect on the CO molecular adsorption. However, obvious
differences could be observed for the high temperature
desorption peaks. When the Sm content increased from 0 to
1%, the peaks shied to a higher temperature zone and the area
increased, which indicated that both the CO desorption
temperature and desorption amount increased. This was
because Sm can be considered as a promoter, which can donate
its electrons to iron atoms; this can lead to an increase in the
surface charge density of the iron atoms, contributing to the
adsorption of CO and the formation of the Fe–C bonds.31,32
However, both the desorption temperature and area decreased
when the Sm content continued to increase from 1% to 2%,
indicating that excessive Sm was not conducive to CO adsorp-
tion. According to the BET result, this probably resulted from
the fact that Sm covered the active sites of the catalyst, blocking
the channels of the catalyst and occupying the active sites of the
catalyst, which reduced the adsorption of CO on the iron
catalyst.
result can be due to the fact that the introduction of a small
amount of rare earth metal promoters can increase the disper-
sion of iron atoms. With the continuous increase in the Sm
promoter content from 1% to 2%, the specic surface area of
the catalyst decreased, which indicated that rare earth oxides
partly blocked the pore path of the catalysts. This may reduce
the activity of the catalysts.
The X-ray powder diffraction patterns of fresh catalysts with
different Sm loadings are shown in Fig. 2. All the fresh catalysts
with different Sm contents showed the characteristic peaks of a-
Fe2O3, which had a characteristic structure with 2q values of
24.2, 33.2, 35.7, 40.9, 49.5, 54.1, 57.7, 62.5, and 64.1ꢀ; the XRD
pattern of hematite Fe2O3 according to JCPDS #99-0060 is
shown at the bottom of Fig. 2. On increasing the Sm content
from 0 to 1%, the intensity of these peaks showed a decreasing
trend, which indicated that the crystallinity of the iron phases
became poor with the addition of samarium. The crystallite
sizes of Fe2O3 were calculated by the Scherrer's equation
according to the full spectrum data and are listed in Table 1.
Therefore, Sm could maintain high dispersion of iron particles.
3.1.2. Desorption and reduction properties of catalysts.
The results of CO-TPD are shown in Fig. 3. The peak tempera-
ture and desorption amount are shown in Table S1.† Two
desorption peaks could be observed for all samples. According
to previous studies,29 the low temperature peak at about 85 ꢀC is
related to the molecular adsorption of CO and the high
temperature peak beyond 550 ꢀC can be assigned to the
The results of H2-TPD are shown in Fig. 4. The peak
temperature and desorption amount are shown in Table S2.† It
Table 1 Result of the N2-physisorption for the fresh catalysts
SBET (m2 gꢂ1
)
Dpa (nm)
VPb (cm3 gꢂ1
)
DFe (nm)
c
Catalyst
Sm0
Sm0.5
Sm1
Sm1.5
Sm2
22.8
25.2
26.7
23.5
23.0
23.5
22.0
20.9
21.5
22.8
0.18
0.19
0.18
0.16
0.18
18.0
17.9
16.1
16.4
16.9
a
b
Fig. 3 CO-TPD patterns of the FeMnSm catalysts with different Sm
loadings.
BJH desorption average pore size. BJH desorption pore volume.
Calculated by the Scherrer equation according to the results of XRD.
c
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can be seen that the H2-TPD patterns of all the catalysts contain
two desorption peaks. The rst one lower than 100 ꢀC corre-
sponds to the weakly adsorbed H species on the iron-based
catalyst surface.33 The second peak in the range of 240–290 C
ꢀ
was ascribed to the strongly chemically adsorbed hydrogen on
the deep sites of the iron surface.34 When the Sm content
increased from 0 to 2%, the desorption peak in the low
temperature zone hardly changed, indicating that Sm had no
obvious effect on the low temperature adsorption of H2 on the
catalysts. On the other hand, the desorption amount of sample
Sm1 in high temperature zone is the smallest. This was because
the addition of an appropriate amount of Sm increased the
charge density of Fe, which inhibited the formation of the Fe–H
bond and weakened the adsorption of H2.35
Fig. 5 CO2-TPD patterns of the FeMnSm catalysts with different Sm
loadings.
CO2-TPD was performed to get information about the surface
basicity of the FeMnSm catalysts, as shown in Fig. 5. The peak
temperature and desorption amount are shown in Table S3.†
The CO2-TPD proles of the FeMnSm catalysts could be divided
Fe. The FeO phase was unstable below 590 ꢀC. Therefore, Fe3O4
was partially reduced to a-Fe, and the remaining species were
reduced to FeO.36 It is generally believed that the Fe species in
the catalyst during reduction and reaction undergo the
following transformation: Fe2O3 / Fe3O4 / Fe5C2.37 By
comparing the rst reduction peak of the sample, it can be
found that the peak temperature decreased when Sm increased
from 0 to 1%, indicating that the introduction of samarium into
the iron-based catalyst enhanced the reduction process of a-
Fe2O3 to Fe3O4.38 Nevertheless, when the Sm content continued
to increase to 2%, the reduction peak temperature increased,
indicating that the addition of excessive Sm was disadvanta-
geous to catalyst reduction.
3.1.3. Electronic effect of Sm promoter. The Fe 2p spectra
of the catalysts named Sm0, Sm0.5 and Sm1 aer reduction are
displayed in Fig. 7 and the molar ratios of the surface elements
are listed in Table S4.† Two main peaks located at about
710.9 eV and 724.7 eV were attributed to Fe 2p3/2 and Fe 2p1/2,
respectively.39 The electronegativity data showed that Fe (1.83) >
Sm (1.17). The greater the electronegativity, the stronger the
ability of the atoms to attract electrons. According to the result
of XPS, both the peaks shied to lower binding energy for the Fe
ꢀ
into one desorption peak at a lower temperature (100 C) and
one peak at a higher temperature (450–550 ꢀC). The rst peak at
ꢀ
about 100 C could be contributed by weakly absorbed CO2 on
the surface of the catalysts. The second one corresponded to the
desorption of the strongly chemisorbed CO2. When the Sm
content increased from 0 to 1%, the chemical adsorption peak
area and desorption temperature increased signicantly,
showing that the basicity of the catalyst increased. However,
when the Sm content continued to increase to 2%, the low
temperature adsorption of CO2 increased and the strong
adsorption decreased, proving that when Sm exceeded the
appropriate value, the basicity of the catalyst decreased and the
adsorption of CO2 changed from chemical adsorption to phys-
ical adsorption. A possible reason could be that the catalyst
surfaces were covered and the active sites binding to CO2 were
reduced.
The H2-TPR proles of the catalysts are shown in Fig. 6.
Three obvious reduction peaks can be observed. The rst peak
between 250 and 300 ꢀC represents a-Fe2O3 / Fe3O4. The
second peak between 400 and 500 ꢀC corresponds to the Fe3O4
/ FeO process. The last peak over 600 ꢀC represents FeO / a-
Fig. 4 H2-TPD patterns of the FeMnSm catalysts with different Sm Fig. 6 H2-TPR patterns of the FeMnSm catalysts with different Sm
loadings.
loadings.
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phase. The sextets with a hyperne eld (Hhf) of 214–218, 170–
180 and 92–108 kOe could be attributed to c-Fe5C2.42 It is
generally believed that Fe5C2 is the reactive phase of the FTS
reaction.43,44 Compared with the observation for the Sm-free
catalyst, the content of Fe5C2 increased from 64.4% to 76.9% in
the Sm1 catalyst based on the BET, CO-TPD and XPS studies. A
possible explanation for this might be that a small amount of Sm
could increase the dispersion of the iron-based catalyst, resulting
in the increase in the Fe and CO binding sites.45 Moreover, the
strong electron interaction between Sm and Fe promoted CO
adsorption and enhanced the Fe–C bond, leading to more Fe5C2
formation. However, in the Sm2 sample, the content of c-Fe5C2
decreased to 60.4%, which may be related to the blockage of the
catalyst pores, resulting in a decrease in the number of active
sites of the catalysts. Meanwhile, similar conclusions were
reached from BET and CO-TPD. Furthermore, the remaining iron
phases could be considered as un-reduced Fe3+ with a small grain
superparamagnetic (spm) state. The content of Fe3+ (spm) in the
Sm1 sample was the lowest, which meant that the reduction
degree of Sm1 was the highest and was consistent with the
conclusion from H2-TPR.
Fig.
loadings.
7 XPS patterns of the FeMnSm catalysts with different Sm
atom with the increase in Sm. Previous studies have shown that
higher surface electron densities will exhibit lower bond ener-
gies in XPS.40 The electronegativity of Fe is higher than that of
Sm. Therefore, the Sm atom would donate electrons to the Fe
atom, resulting in Fe being the central atom with higher charge
density. In this way, Sm as a promoter could be considered as
a weak electron donor, which could accelerate the dissociation
of adsorbed CO and decrease H2 absorption,34,41 resulting in
a higher concentration of carbon species on the catalyst surface.
3.1.4. Phase composition of catalysts aer reaction. The
bulk iron phases in the Sm-promoted catalysts aer the reaction
3.2. FTS performance
The activities and product distribution of the catalysts with
different Sm loadings for FTS measured under the conditions of
300 ꢀC, 1.0 MPa, 12 000 ml gꢂ1 hꢂ1 and H2/CO ¼ 2 in a xed bed
are shown in Table 3. According to the results, the CO conversion
for the catalysts increased from 63.4% to 70.4% with the increase
in the Sm loading from 0 to 1%. It is possible that these results
were due to a combination of various factors. First, the BET
surface area increased with the introduction of a small amount of
Sm, and the large BET surface area provided more active sites,
leading to higher CO conversion. Second, according to H2-TPR,
the lowest rst peak temperature for the sample Sm1 meant that
it was the easiest to reduce to Fe3O4 and then convert to Fe5C2,
which was also reected in MES. Lastly, the results of XPS and
CO-TPD showed that the addition of an appropriate amount of
Sm can increase the electron density of the Fe surface, promoting
CO adsorption as well as enhancing the formation of the Fe–C
bonds. This beneted the generation of Fe5C2, as conrmed by
MES, resulting in higher activity.46,47 When the Sm content
increased from 1% to 2%, CO conversion decreased, which may
be due to the decrease in the BET surface area. In this way, the
number of active sites decreased. Meanwhile, the introduction of
excessive Sm was not conducive to the reduction of the catalysts.
Both factors hindered CO adsorption and inhibited iron carbide
formation, nally leading to low FTS activity.48
¨
were measured by Mossbauer spectroscopy (MES). The results
are shown in Fig. 8. The MES parameters are listed in Table 2.
As shown in Table 2, the sextets with a hyperne eld (Hhf) of
487–490 and 453–455 kOe can be attributed to Fe3O4. The former
represents the tetrahedron site (A site) in the Fe3O4 phase, and
the latter represents the eight position sites (B site) in the Fe3O4
The FTS product selectivity of the catalysts with different Sm
loadings and their detailed data are also listed in Table 3. The
]
catalysts presented selectivity of C2–4 in the HC distribution,
which increased from 26.6% to 32.6% aer the addition of
samarium from 0 to 1%, but it decreased to 27.7% as the Sm
amount continued to increase to 2%. Research studies have
shown that the formation of low-carbon olens is related to the
basicity of the catalyst surface.49,50 In short, increasing the
surface basicity in the FTS catalyst can suppress H2 adsorption.
Fig. 8 Mossbauer spectra of the Sm-promoted catalysts after the
reaction.
¨
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Table 2 MES parameters of the Sm-promoted catalysts after the reactiona
Catalyst
Sm0
Assignment
Hhf (kOe)
IS (mm sꢂ1
)
QS (mm sꢂ1
)
G/2 (mm sꢂ1
)
Area (%)
Fe3+(spm)
Fe3O4(A)
Fe3O4(B)
c-Fe5C2
0.30
0.32
0.63
0.30
0.23
0.61
0.21
0.30
0.73
0.25
0.26
0.13
0.35
0.32
0.65
0.27
0.23
0.34
1.09
ꢂ0.02
0.00
ꢂ0.11
ꢂ0.09
0.45
0.29
0.18
0.32
0.20
0.22
0.17
0.13
0.10
0.22
0.15
0.27
0.17
0.23
0.23
0.35
0.22
0.23
0.21
9.3
9.1
17.2
13.6
44.0
6.8
489.17
454.49
215.84
171.34
100.00
Sm1
Sm2
Fe3+(spm)
Fe3O4(A)
Fe3O4(B)
c-Fe5C2
0.98
3.8
4.8
488.10
454.56
217.10
175.96
107.5
ꢂ0.24
0.16
13.2
23.8
36.7
16.4
9.7
11.8
18.2
10.6
43.4
6.4
ꢂ0.14
0.10
0.10
Fe3+(spm)
Fe3O4(A)
Fe3O4(B)
c-Fe5C2
0.98
487.65
453.21
214.18
170.85
92.45
ꢂ0.01
0.02
ꢂ0.13
ꢂ0.08
0.11
a
Reaction condition: 300 ꢀC, H2/CO ¼ 2, 1.0 MPa and 12 000 ml gꢂ1 hꢂ1
.
Table 3 Catalyst performance of the FeMnSm catalysts with different Sm loadingsa
HC distribution (%)
]
0
+
Catalyst
CO conversion (%)
CO2 selectivity (%)
C1
C2–4
C2–4
C5
O/P
Sm0
Sm0.5
Sm1
Sm1.5
Sm2
63.4
68.4
70.4
64.2
48.8
43.5
45.6
49.3
45.4
45.0
13.9
15.1
15.4
16.7
16.0
26.6
28.9
32.6
28.7
27.2
6.5
7.0
7.8
9.0
8.9
52.9
48.8
44.0
45.4
47.7
4.09
4.13
4.18
3.19
3.06
a
Reduction conditions: T ¼ 350 ꢀC, P ¼ 0.1 MPa, 4000 ml gꢂ1 hꢂ1, H2, TOS ¼ 10 h. Reaction conditions: T ¼ 300 ꢀC, P ¼ 1.0 MPa, 12 000 ml gꢂ1
hꢂ1, H2/CO ¼ 2.
The H2-TPD and CO2-TPD results indicated that the catalyst the BET surface areas, pore volumes and the formation of Fe
surface basicity increased, while the H2 concentration on the particles with a smaller size. The presence of Sm could enhance
catalyst surface decreased from sample Sm0 to Sm1; however, the dissociated adsorption of CO and inhibit the adsorption of H2.
this trend changed in the opposite direction from sample Sm1 Furthermore, the electron transfer between samarium and iron
to Sm2. Strong alkalinity and lower hydrogen adsorption helped increase the surface charge density of the iron atoms, as
properties inhibited the hydrogenation reaction, which sup- conrmed by XPS. Sm could also promote the formation of Fe5C2,
pressed the conversion of olens to paraffins. Therefore, sample leading to high FTS activity. In addition, the introduction of an
Sm1 with the strongest basicity and the weakest hydrogen appropriate amount of Sm enhanced the basicity of the catalysts
adsorption showed the highest selectivity of C2–4] and the ratio and inhibited the hydrogenation reaction, leading to the formation
of olens to paraffins (O/P). In addition, the selectivity of C5+ of more light olens. As a result, the Sm1 sample exhibited 70.3%
]
was negatively correlated with conversion, which was consistent CO conversion, 32.6% C2–4 yield and O/P ratio of 4.18 at 300 ꢀC,
with the results reported in the literature.51,52
1.0 MPa and WHSV of 12 000 ml gꢂ1 hꢂ1
.
Conflicts of interest
4. Conclusions
There are no conicts to declare.
The Sm-promoted FeMn catalysts were prepared by the co-
precipitation method and their performance was investigated in
a xed bed reactor. It was found that the highly selective produc-
tion of light olens can be achieved at higher activities over the
Acknowledgements
FeMn catalysts promoted by a proper amount of Sm. In regard to We gratefully acknowledge the nancial support from the
the catalyst texture, Sm introduced into the samples could increase National High Technology Research and Development Plan of
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