Mn-Fe nanoparticles on a reduced graphene oxide catalyst for enhanced olefin production from syngas in a slurry reactor

Article abstract of DOI:10.1039/c8ra02193g

Fe nanoparticles (NPs) supported on reduced graphene oxide (rGO) nano-sheets were promoted with Mn and used for the production of light olefins in Fischer-Tropsch reactions carried out in a slurry bed reactor (SBR). The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, N₂ physisorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopic (XPS) methods. Mn was shown to preferentially migrate to the Fe NP surface, forming a Mn-rich shell encapsulating a core rich in Fe. The Mn shell regulated the diffusion of molecules to and from the catalyst core, and preserved the metallic Fe phase by lowering magnetite formation and carburization, so decreasing water gas shift reaction (WGSR) activity and CO conversion, respectively. Furthermore, the Mn shell reduced H₂ adsorption and increased CO dissociative adsorption which enhanced olefin selectivity by limiting hydrogenation reactions. Modification of the Mn shell thickness regulated the catalytic activity and olefin selectivity. Simultaneously the weak metal-support interaction further increased the migration ability owing to the utilization of a graphene-based support. Space velocities, pressures and operating temperatures were also tested in the reactor to further enhance light olefin production. A balanced Mn shell thickness produced with a Mn concentration of 16 mol Mn/100 mol Fe was found to give a good olefin yield of 19% with an olefin/paraffin (O/P) ratio of 0.77. Higher Mn concentrations shielded the active sites and reduced the conversion dramatically, causing a fall in olefin production. The optimum operating conditions were found to be 300 °C, 2 MPa and 4.2 L g^-1 h^-1 of 1:1 H₂:CO syngas flow; these gave the olefin yield of 19%.

Full text of DOI:10.1039/c8ra02193g

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Mn–Fe nanoparticles on a reduced graphene oxide
catalyst for enhanced olefin production from
syngas in a slurry reactor†
Cite this: RSC Adv., 2018, 8, 14854
abc
c
a
c
AL-Hassan Nasser,
Lisheng Guo, Hamada ELnaggar, Yang Wang,
c
a
c
Xiaoyu Guo, Ahmed AbdelMoneim* and Noritatsu Tsubaki
*
Fe nanoparticles (NPs) supported on reduced graphene oxide (rGO) nano-sheets were promoted with Mn
and used for the production of light olefins in Fischer–Tropsch reactions carried out in a slurry bed reactor
(SBR). The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD),
transmission electron microscope (TEM), Raman spectroscopy, physisorption, temperature
N
2
programmed reduction (TPR) and X-ray photoelectron spectroscopic (XPS) methods. Mn was shown to
preferentially migrate to the Fe NP surface, forming a Mn-rich shell encapsulating a core rich in Fe. The
Mn shell regulated the diffusion of molecules to and from the catalyst core, and preserved the metallic
Fe phase by lowering magnetite formation and carburization, so decreasing water gas shift reaction
2
(WGSR) activity and CO conversion, respectively. Furthermore, the Mn shell reduced H adsorption and
increased CO dissociative adsorption which enhanced olefin selectivity by limiting hydrogenation
reactions. Modification of the Mn shell thickness regulated the catalytic activity and olefin selectivity.
Simultaneously the weak metal–support interaction further increased the migration ability owing to the
utilization of a graphene-based support. Space velocities, pressures and operating temperatures were
also tested in the reactor to further enhance light olefin production. A balanced Mn shell thickness
produced with a Mn concentration of 16 mol Mn/100 mol Fe was found to give a good olefin yield of
Received 13th March 2018
Accepted 3rd April 2018
19% with an olefin/paraffin (O/P) ratio of 0.77. Higher Mn concentrations shielded the active sites and
reduced the conversion dramatically, causing a fall in olefin production. The optimum operating
DOI: 10.1039/c8ra02193g
ꢀ
ꢁ1 ꢁ1
conditions were found to be 300 C, 2 MPa and 4.2 L g
h
2
of 1 : 1 H : CO syngas flow; these gave
rsc.li/rsc-advances
the olefin yield of 19%.
required if more competitive systems are to be designed. The
common strategy for achieving this goal is via promotion of the
1
Introduction
The use of Fischer–Tropsch synthesis (FTS) technology for the Fe catalyst systems.
conversion of syngas into useful end products is an important
The most common promoters for the enhancement of light
process. Light olens are essential components in the petro- olen production are the alkali metals, especially Na and K,
chemical industry, acting as a starting point for many produc- which increase the selectivity of Fe systems towards light olens
1–4
12–14
tion lines. Fe catalysts commonly used in FTS industrial dramatically.
Mn is comparably effective in achieving this
plants are characterized by their high selectivities for the light target. Extensive research has been done over the years for
5–7
olen fractions. They also give high water gas shi reaction various mixtures and combinations of Mn with alkali metals on
5
,8,15,16
(
WGSR) activities, rendering them more exible in processing various supporting materials for this purpose.
Other
8–11
2
syngas streams with various H : CO ratios.
However, further promoters used for olen enhancement include Zn and
6,7,12,17
enhancement of their orientation towards light olens is Mg.
Mn is known to increase the olen selectivity by increasing
dissociative CO adsorption on the catalyst surface whilst
a
Materials Science and Engineering Department, Egypt-Japan University of Science and
Technology, New Borg El-Arab, Alexandria 21934, Egypt. E-mail: ahmed. reducing H
16,18
2
adsorption.
This chokes off the hydrogenation
abdelmoneim@ejust.edu.eg
reactions which are responsible for converting the olens to
paraffins. However, Mn at very high levels was reported to have
a detrimental effect on the CO conversion in some cases,
b
Chemical Engineering Department, Faculty of Engineering, Alexandria University,
Alexandria 11432, Egypt
c
Department of Applied Chemistry, School of Engineering, University of Toyama,
5,16,19
especially when carbon supports were used.
In reality, the
Gofuku 3190, Toyama city, 930-8555, Japan. E-mail: tsubaki@eng.u-toyama.ac.jp
rate of carburization of metallic Fe was retarded by an excess of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra02193g
5
Mn oxide which led to a signicant decrease in FTS activity.
1
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Thus it was found that an optimum level exists at which there is rGO via ultrasonic assisted impregnation. The details are
a balance between the reduced CO conversion and the described in the ESI.†
enhanced olen selectivity. Support materials including
20
21
22
23
ceramic-silica, alumina, zeolites, and titania were inten-
2.3 Characterization
sively used to support Fe NPs. Considering the strong metal–
support interactions (SMSI), other supports were developed
ꢀ
The XRD patterns of the as prepared, calcined (300 C, 3 h, He
1
8
ꢁ1
00%, 30 ml min ), reduced, and spent catalysts were obtained
using carbon-based materials like activated carbon, carbon
using a Rigaku Ultima IV X-ray diffractometer. The samples
were exposed to Cu-Ka radiation (l ¼ 0.154 nm) at 40 kV and 20
5,16
24,25
nanotubes (CNTs)
and graphene.
Carbon supports are
linked to the Fe particles by weaker bonds which allow for
higher catalyst activity and stability at the right set of operating
conditions.
Graphene is a relatively new material which has emerged as
a promising candidate for many applications in the past
decade. It is characterized by a high specic surface area, 2D
structural features, and high thermal and electrical conductiv-
ities caused by the delocalized p electrons migrating freely on
the sheet surface; this all renders this material an interesting
choice for electronic, electrochemical and catalytic applica-
ꢀ
ꢀ
ꢁ1
mA within the 5–80 2q range at a rate of 0.02 min . The
strongest intensity peak of the phase being studied was chosen
for crystallite size calculation by the Scherrer equation. TEM
images of the samples were obtained by a Hitachi H-7650 TEM
instrument operating at 100 kV. The images were used to
calculate the average particle size of each catalyst, by analyzing
at least 100 particles. The Raman spectra of the as prepared
catalysts were collected using a Renishaw Invia Raman micro-
scope equipped with a 532 nm green laser source. N2 phys-
isorption tests were carried out in a NOVA-2200e Quantachrome
6
,26–29
tions.
Common pathways for the production of graphene
Instruments analyzer in which the samples (50–60 mg) were
30
are chemical exfoliation methods like the Hummers' method,
modied Hummers' method
ꢀ

rst degassed at 200 C for 2 h. Aerwards the N
2
adsorption
24,26,31,32
and the improved
isotherms were recorded at 77 K to get the BET (Brunauer–
Emmett–Teller) area, total pore volume and the average pore
size by the BJH (Barrett–Joyner–Halenda) method. TPR tests
were conducted on the as prepared catalyst samples (30 mg)
using a BELCAT-II, MicrotracBEL instrument. The samples were
loaded into the quartz cell, heated up to 300 C (10 min ),
kept at this temperature for 2 h in an inert He atmosphere (30
ml min ), and then the temperature was cooled down to 50 C.
33
Hummers' method. In these methods, strong oxidizing
mediums are used to introduce oxygenated groups between the
graphite layers to form graphite oxide (GtO). The GtO can then
be easily exfoliated mechanically or ultrasonically to produce
the low layer number graphene oxide sheets, which can then be
reduced into graphene, usually referred to as reduced graphene
oxide (rGO) or chemically reduced graphene oxide.
ꢀ
ꢀ
ꢁ1
ꢁ
1
ꢀ
In catalytic applications, it is important to provide stable
positions for the NPs planted on the graphene sheets. A very
smooth graphene sheet with no defects will allow the NPs to
migrate freely on heating during reduction or reaction stages
which can cause sintering and segregation of the catalyst
particles. So rough sheets with high defect densities are
preferred for such applications where the defects act as anchor
points for the catalyst particles, hindering their motion and
ꢀ
ꢀ
ꢁ1
The temperature was then ramped up to 800 C (5 min ) and
nally kept at 800 C for 30 min while the sample was exposed
to a ow of 5% vol H /Ar at 30 ml min . H in the effluent gas
2 2
ꢀ

ꢁ1
stream was analyzed by an online TCD detector to observe the
reduction behavior of the samples. XRF measurements were
carried out on a Philips PW 2404 wave dispersive X-ray Fluo-
rescence instrument to analyze the composition of as prepared
catalysts. The XPS spectra were observed using a Thermo
Fischer Scientic ESCALAB 250Xi instrument equipped with an
Al-Ka irradiation source. All binding energies were corrected by
using the C1s line at 284.5 eV as an internal standard.
7,24,26
increasing their dispersion.
In this study a series of Fe-based catalysts loaded on rGO
sheets were prepared with various Mn loading levels. These
catalysts were used in an FTS slurry bed reactor (SBR) to test
their performance against that of a Mn-free Fe/rGO catalyst.
Then the promotional effect of Mn was investigated in detail
2.4 FTS SBR performance evaluation
based on the catalytic performance as well as characterization The catalyst FTS performance tests were carried out in a SBR
results. At the same time, the optimum conditions were inves- reaction system; full details are given in the ESI as shown in
tigated to gain a high yield of light olens.
Fig. S1.†
3
Results and discussion
2
Materials and methods
3.1 XRD and TEM
2.1 Graphite oxide preparation
The XRD patterns for GtO and rGO are shown in Fig. S2.† The
results for GtO show a sharp peak at about 10 indicating an
Graphite oxide (GtO) was prepared by the modied Hummers'
ꢀ
24,32
method, the details of which are given elsewhere
and
˚
interlayer spacing of 8.5 A. The rGO shows a broad low intensity
described in the ESI.†
ꢀ
peak at 24 which is common when graphene nano-sheets are
amorphously stacked with a small number of graphene layers
2.2 Fe/rGO preparation by solvothermal coprecipitation
24,34
per nano-sheet, as reported by others.
The XRD spectra for
The Fe/rGO catalyst was prepared by the coprecipitation the as prepared catalysts aer hydrazine hydrate (HH) reduc-
6,7
method, and the Mn promoter was introduced into the Fe/ tion, and where necessary, incipient wetness impregnation
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Fig. 1 XRD spectra of (A) as prepared catalysts, and (B) catalysts after calcination for 3 h.
3
5
(
IWI), are shown in Fig. 1(A). A very good match is observed Al
when comparing them with the hematite (a-Fe ) JCPDS card, the FeMn catalyst is benecial for improving catalytic
which shows that hematite is the major Fe phase in the as performance.
2 3
O catalysts. Therefore, an appropriate nanoparticle size for
2 3
O
6,7
prepared catalysts. Some distortions are observed in the
While hematite is still the dominant component in the
spectra of FeMn16 (concentration of 16 mol Mn/100 mol Fe) calcined catalysts, the spectra in Fig. 1(B) have some minor
and FeMn29 (concentration of 29 mol Mn/100 mol Fe) but these peaks which match well to peaks in the bixbyite Fe(
are not clear enough to indicate the explicit presence of Mn-rich standard card as reported by Maiti et al. for FeMn catalysts.
Mn O
x
2ꢁx)
3
3
6
A
separate phases. The Mn is either in an amorphous form or is similar FeMn solid solution was also found according to
highly dispersed with very small crystallite sizes, thus Mn M ¨o ssbauer results; the solid solution contained a mixture of
9
,13,15,37,38
cannot be clearly detected by XRD. All this is in agreement with Fe
2
O
3
and Mn
2
O
3
as in other reports.
It is worth
24
our previous investigation of a similar system, as well as with noticing that the same behavior is also expected with systems
8
–10,13,15,17
other work on FeMn FTS catalysts.
that form spinel magnetite Fe O in which mixed spinels of
3 4
5
The hematite crystallite sizes for the as prepared catalysts F(
Mn O single phase oxides formed. According to the XRD
3ꢁx)
x 4
calculated from the Scherrer equation are of similar values data, the grain sizes did not show any signicant changes aer
about 18.5–19 nm), as shown in Table 1. The same particle size calcination, with basically the same average value of 19 nm for
is expected for the different catalysts since, without calcination, the hematite crystallites (Table 1).
(
the IWI loaded promoters will not have any signicant effect on
Aer TPR of the catalysts, the reduced samples were trans-
the catalyst particle size. Generally, the agglomeration and ferred to the XRD equipment to record their diffraction spectra.
0
sintering of Fe-based catalysts lead to low catalytic activity. It The samples gave clear peaks resembling those of a-Fe as
20,36
has been reported that CO conversion in the FTS is dependent shown in Fig. 2(A).
There were no other peaks observed
on particle size, and hydrocarbon selectivity is strongly affected besides those of metallic iron in the Mn-free sample (Fe).
by the catalyst particle size in the range of 2.0–12.0 nm for Fe/ However, aer Mn addition, some additional peaks were clearly
ꢀ
ꢀ
ꢀ
visible as small bumps at 35 , 40 and 43 which resembled
those of MnO (manganosite) and FeO (wustite) when compared
with their respective JCPDS cards. This proves that Mn has
a restricting effect on the Fe reduction by preventing some FeO
Table 1 Particle size data from the XRD data and TEM images
Size (nm)
0
ꢀ
from being reduced to Fe . This is in agreement with a lot of
Catalyst
Phase
Method 2q/
Fe
FeMn16 FeMn29
previous investigations which state clearly that separate MnO
and FeO phases as well as mixed solid solutions between the
As prepared Fe
2
2
O
O
3
3
XRD
XRD
TEM
XRD
TEM
33
33
—
35
—
57
18.9 19.0
18.9 18.1
19.9 18.2
14.2 14.1
14.7 9.0
12.8 10.6
18.4
20.2
14.7
8.0
8.6
8.8
Calcined
Fe
Fe
two
techniques.
were
detected
by
various
characterization
15,36,38,39
Carburized
3
O
4
The spectra for the carburized catalysts aer syngas treat-
ꢀ
Carbides XRD
ment are shown in Fig. 2(B). The largest peak (35 ) is from the
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Fig. 2 XRD spectra of (A) reduced, and (B) spent catalysts.
6
5
oxygen-poor Fe
3
O
4
magnetite produced by the oxidation of carburization of Fe and the subsequent carbide crystallization,
metallic Fe on exposure to the H
activity. The formed magnetite is shielded from further The TEM pictures along with the particle size distribution
2
O produced from the FTS and it also decreased magnetite formation.
15
reduction into carbides by the layers of carbides being formed (PSD) histograms of the catalysts before and aer the FTS
at the catalyst's surface. These layers impede the seeping out of reaction are compared in Fig. S3 and S4.† The PSD results are in
9
,15
oxygen from the magnetite core.
good agreement with the XRD results except in two cases: the
0
ꢀ
The presence of a-Fe (the peak at 45 ) aer carburization calcined FeMn29 and the carburized FeMn16 (Table 1). In both
increased as the Mn loading increased. In the Fe catalyst (0% cases the XRD overestimates the grain size slightly by 5 nm.
Mn) the metallic peak nearly disappeared completely, showing
Accordingly, the TEM PSD values differed from the XRD in
that the Fe had no resistance to carburization. However, the cases of the hematite phase aer calcination as well as the
a signicant shoulder peak could be detected in the FeMn16 magnetite phase aer carburization, both of which decreased in
catalyst and a clear distinct peak of metallic iron was even more average particle size as the Mn level increased. It was shown
obvious with FeMn29. All these ndings imply that Mn earlier from the XRD results that Mn forms solid solutions in
5,15,38
impeded the carburization of Fe.
It can be deduced that both the hematite and spinel phases. It was also demonstrated
high Mn levels in FeMn16 and especially FeMn29 hindered the that a high Mn content rendered higher stability of the metallic
0
carburization reactions and preserved some Fe in the NP core. Fe phase, shielding the Fe-rich core from the carburizing
The peaks of Fe-carbides, probably Fe
aer the reaction. Fe (also known as the Hagg carbide) is the crystallization.
main active phase responsible for the FTS activity of the cata- doping since the outer Mn-rich layer restricted H
5 2 2
C and Fe C, appeared species which hindered carburization and retarded the carbide
5,15,16,41
5
C
2
The magnetite content decreased with Mn
O formation
2
5,8,10,15
ꢀ ꢀ
0
lyst.
The carbide peaks (at 43 and 56 ) increased in during early FTS reaction stages which preserved the inner Fe
15
intensity as the molar ratio Mn/100Fe increased from 0 to 16, metallic core formed aer reduction. Therefore, the falling
but the increase in Mn molar content to 29/100Fe caused these particle sizes with FeMn29 are not at all unexpected aer taking
peaks to fade again signicantly showing the reduced crystal- these observations into consideration.
linity and grain sizes in this sample. In addition, different iron
carbides are reported to exhibit distinct inuences on the TEM and XRD results at the same Mn level can also explained by
product selectivity. Fe C produced lower CH and higher C5+ the high carbide mass density compared to that of the other
The fall in the carbide phase particle size as stated by the
2
4
40
9,42,43
selectivity than c-Fe
5
C
2
,
thus the appearance of Fe
2
C owing to precursory phases,
which causes the decrease in particle
the addition of Mn is benecial for excellent performance. This size regardless of other effects.
observation will be important in explaining the FTS activity
trends later on. The same observation can be drawn from the
crystallites' sizes calculated for the carburized catalysts for both 3.2 Raman
the magnetite and the carbide components (Table 1). The size of
The Raman spectra for the samples are given in Fig. S5.† The
the magnetite crystallites decreased steadily (14.2 to 8 nm) with
the rise in Mn content, and the grain size of the carbide phase
decreased from 12.8 for the Mn-free sample down to 8 nm when
the Mn content reached 29 mol/100 mol Fe. This again implies
that a very high Mn content exhibited negative effects on the
characteristic D and G bands appear at about 1350 and
ꢁ1
1
600 cm respectively. The D band corresponds to the vibra-
tions associated with defects or sheet edges. It is thus expected
to increase in intensity as the degree of disorder and number of
defects in the sheets increase, while the G band corresponds to
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physical properties signicantly according to Table 2. This is in
24,25
agreement with previous work done on similar systems.
3.4
2
H TPR
H TPR proles are compared in Fig. 4, and resemble the typical
2
1
5,36,38,41,46
scheme
formation process in which hematite a-Fe
magnetite Fe , then to FeO, and nally to the metallic Fe.
Although Mn can stabilize the phases preceding the metallic
for Fe reduction with a three step trans-
2 3
O
is reduced to
3 4
O
47
Fe phase by forming various solid solutions with them, this
stabilization hinders the reduction of a-Fe O to Fe as indi-
0
2
3
cated by the TPR results. The introduction of Mn results in
a delay in the reduction of hematite to magnetite, and so the

rst peak merges in with the magnetite reduction peak in
ꢀ
Fig. 3 The I
D
/I
G
band intensity ratios for GtO, rGO and the catalysts a much smaller peak that is on average 40–60 C higher than
before the FTS reaction.
that of Fe. The shoulder FeO reduction peak which starts at
about 550 C is smoother and more diminished aer Mn
ꢀ
addition. The retarding effect results from the formation of the
the degree of graphitization and increases with the number of bixbyite solid solution along with hematite aer calcination,
4
4,45
graphene layers per nano-sheet.
The band intensity ratio I /I
the degree of defects and exfoliation of the graphene sheets.
which was observed in the XRD patterns and is also conrmed
36,48,49
D
G
can be taken as a measure of from the Fe O –Mn O phase diagrams.
2
3
2 3
It is followed by
the formation of spinel solid solution comprising two phases,
7,44
36
It can be seen from Fig. 3 that the ratio increases from 0.73 in one Mn-rich and the other Fe-rich. On further reduction these
GtO to reach 1.01 aer ultrasonic exfoliation and reduction with are transformed to the MnO and FeO phases to form a solid
15,36
rGO, which shows that the ultrasonic exfoliation is successful in solution manganowustite with 20% FeO.
These solid solu-
reducing the number of layers per sheet signicantly. Fe and tions are formed when Mn migrates preferentially at high
Mn decoration had a negligible effect on the ratio with it temperatures to the particle surface forming a Mn-rich
1
5,16,38,47
showing only a slight fall to 0.93 with FeMn16. The results of the crust
2
that isolates the Fe species from the reducing
D band in the vicinity of 2600–2800 cm are compared in agents and eventually causes the segregation of Fe-rich phases
ꢁ1
15,16,38,47
Fig. S6.† It has been mentioned that the 2D band will shi to in the core of the particles.
ꢁ
1
values higher than 2700 cm if the number of layers per sheet
2
6,44,45
is high;
2
the results here show that the peaks are below
700 cm demonstrating that the prepared graphene has low
graphitization.
3
.5 XPS
ꢁ
1
The surface compositions of the as prepared, calcined and
carburized catalysts as atomic ratios obtained from the XPS data
3.3 Nitrogen physisorption
The BET areas along with the average pore sizes and volumes
are listed in Table 2. The coprecipitation of Fe on the rGO sheets
2
ꢁ1
caused a severe decrease in area from 395 to 218 m g and
a very slight increase in pore volume, while the average pore size
˚
decreased by 2 A. The area and pore volume decreased steadily
2
ꢁ1
with the rise in Mn content to reach minimums of 169 m g
3
ꢁ1
and 0.26 cm g , respectively, with 29 mol Mn/100 mol Fe. The
˚
pore size decreased from 16.5 A for rGO to an average uniform
˚
value of about 14.9 A for the three catalysts. In reality, the
introduction of suitable Mn additives does not change surface
Table 2 Nitrogen physisorption results for rGO, Fe, FeMn16 and
FeMn29
BET area
Pore volume
Average pore
size ( A˚ )
2
ꢁ1
3
ꢁ1
Catalyst
(m g
)
(cm g
)
rGO
Fe
FeMn16
FeM29
395.3
218.1
194.6
169.0
0.34
0.35
0.33
0.26
16.5
14.81
15.01
14.86
Fig. 4 H TPR results for the catalysts Fe, FeMn16 and FeMn29.
2
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are compared in Table 3. The Mn/Fe molar ratio is higher on the Table
4
The effect of Mn loading level on the FTS reaction
performance
surface than the bulk values measured by XRF methods (Table
S1†), and there is a decrease in the Fe/C and a rise in the Mn/C
molar ratios as the overall Mn content is increased. This gives
a
a
a
Fe
FeMn16
FeMn29
further evidence that Mn preferentially migrates to the catalyst CO conversion %
particle surface to form the Mn-rich mixed oxide phase which Fraction
83
84
74
Selectivity C mol%
CO
CH
2–4 olen
Total olen
2
38
22
8
14
22
70
16
86
0.169
7
34
24
12
21
5
72
7
79
0.271
12
0.55
35
23
10
22
2
73
5
78
0.278
10
0.51
was described in the XRD discussion. The migration of Mn
leads to Mn-rich shell formation which will play a critical role
during the FTS activation and reaction stages, as will be claried
4
C
later on. A similar promoter promotional effect derived from C9+ HC
preferential migration was demonstrated by different Total paraffin
5
0,51
Total iso
Total par + iso
O/P
Olen yield
a
researchers.
surface migration ability of the K promoter being applied to
improve CO hydrogenation performance via inuencing
More recently, Guo et al. reported the enhanced
2
52
surface hydrogenation ability and carbide content. The sharp
increase of carbide content with the K promoter addition led to
weaker hydrogenation ability and stronger C–C coupling ability.
Besides, it was reported that the selectivity of olens is generally
positively correlated with the strength of the surface basicity of
0.57
a
ꢀ
ꢁ1
ꢁ1, Time On Stream (TOS) ¼ 8 h.
H
2
/CO ¼ 1, 2 MPa, 340 C, 4.2 L g
h
and heavier hydrocarbons. Besides that, Mn helps to stabilize
the active iron carbide phase and acts as an electron donor,
thereby changing the properties of Fe in a similar manner to
53
the catalyst. Therefore, the increase of surface basicity owing
to Mn migration is benecial to olen formation. Furthermore,
the addition of Mn was able to increase the dissociative
54
alkali promoters. Thus, the modication of Fe-based catalysts
via Mn-promoter doping could offer an excellent performance.
Indeed, an increase in olen selectivity is noticed from Table 4,
as indicated by the rising olen/paraffin (O/P) ratio. However,
the selectivity for heavy hydrocarbons (C9+) was sharply reduced
by the rising Mn content. This discloses that in our Fe–Mn/rGO
catalyst, the selectivities for lighter fractions were preferentially
enhanced with an increasing tendency towards olenic species.
16,18
adsorption of CO and decrease the H
2
adsorption.
Conse-
quently the appropriate surface content of Mn resulting from
the migration phenomenon indeed changed the surface physi-
cochemical properties of the catalyst, such that it realized
a favorable performance for olen formation.
3
.6 FTS SBR performance evaluation
3
.6.1 Promotional effect of Mn dopant. The results for the Similar enhanced catalytic performance via Mn-promoter
55
Mn loading effect on the Fe FTS activity are displayed in Table 4. migration was also observed in CoMn FTS catalysts. Mn was
The CO conversion was not affected when the Mn molar ratio closely associated with Co via the migration phenomenon,
was 16/100 mol Fe, if compared with the Mn-free sample. exhibiting a stabilizing effect on the adsorption of CO, C, H, O
X
However, a further increase in Mn loading to 29/100 mol Fe and CH . The stabilizing effect, leading to the increased selec-
reduced the conversion by 10% to 74%. This behavior with Mn- tivity towards olens and C5+ species, was observed experi-
promoted Fe catalysts has been observed mainly on carbon mentally. These literature reports indicate that the regulation of
5
,8,16
nanotubes in other investigations,
and the same observa- promoter migration is probably a proper way of controlling the
46
tions were also found with unsupported catalysts in which the activity and selectivity of catalysts.
FTS activity was negatively affected by high Mn loading levels
3.6.2 Tunable activity and selectivity via Mn shell thick-
aer passing through a maximum at an optimum Mn loading ness. By analyzing the data we have obtained so far from the
level. The reasons are the fact that Mn masks the active sites at characterization and reactor experiments, we can draw the
excessively high concentrations and can also hinder the following conclusions about the inuence of Mn on the Fe FTS
carburization of Fe and reduce its crystal size, which appears to catalytic system. Mn, when added to the Fe catalysts and
agree well with our XRD, XPS and TEM characterization results. exposed to thermal stresses, such as calcination, tends to
Generally, the doping of Mn increases surface basicity and migrate to the surface of the catalyst particles to form a Mn-rich
increases the dissociative adsorption of CO while reducing the shell comprised of Mn and Fe mixed oxides as indicated by the
16,18
H
2
adsorption,
thus it enhances the selectivity for olens XRD, TEM, TPR and XPS results. This shell encapsulates a core
rich in Fe which is also composed of mixed Fe and Mn oxides.
Table 3 XPS elemental surface analysis
This core–shell construction is preserved during further catalyst
transformations, namely reduction and carburization. The Mn
shell varies in concentration as the Mn loading level is
increased; this was demonstrated by the XPS results which
showed a rise in Mn/Fe and Mn/C ratios as the Mn wt% was
raised.
As prepared
Calcined
FeMn16
Atomic ratio (%)
Fe
Fe
FeMn29
Mn/Fe
Fe/C
Mn/C
0.00
1.54
0.00
0.00
1.50
0.00
69.28
1.17
0.81
72.99
1.24
0.90
A thicker Mn-rich/Fe-poor shell obstructs diffusion, regu-
lating the chemical interaction of the catalyst core with the
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surrounding chemicals. During the reduction reaction, it
hinders the full reduction of FeO by forming a manganowustite
0
mixed oxide phase that is not easily reduced to metallic Fe .
This was conrmed by the TPR results which showed a dimin-
ished shoulder peak corresponding to the FeO reduction.
Another feature of this shell was observed during the FTS
reaction itself, in which the Mn-rich shell restricted the diffu-
sion of water from the reaction medium to the Fe-rich core and
15
caused the slow formation of the magnetite phase, as observed
from the TEM and XRD data which showed a fall in magnetite
particle size and peak intensity. The XRD data also showed an
0
ꢀ
increase in the metallic Fe shoulder peak at 45 giving strong
evidence that the increasing thickness of the Mn-rich outer
0
5
crust preserved more Fe and decreased Fe
is important since magnetite is the main phase responsible for
CO production via the WGSR reaction and consequently Mn
should indirectly decrease the WGSR activity; this was observed
to a mild extent in our FTS reaction data with rising Mn levels. cause a decrease in CO conversion and CO
On the other hand, the Mn shell has the very benecial effect is evident from Fig. 5. The methane selectivity stayed constant
of increasing the selectivity towards olen production; this at about 24% at ow rates of up to 8.4 L g
effect is caused by the enhancement of dissociative CO increased slightly to 27% at 12.6 L g
adsorption and decreased H
reaction medium more favorable for olen production by at 20% with 8.4 L g
3 4
O formation. This
Fig. 5 The effect of space velocity on the FTS reaction performance.
2
2
selectivity and this
ꢁ
1
ꢁ1
h
and then
. The overall olen
This renders the selectivity showed a similar trend to that of methane, stabilizing
ꢁ1
ꢁ1
h
16,18
2
adsorption.
ꢁ1
ꢁ1
ꢁ1
h
, and then rising to 24% at 12.6 L g
ꢁ1
decreasing the hydrogenation activity. Therefore, the modi-
h . C2–4 olens showed similar behavior, but the olens in the
cation of the Mn shell thickness plays a crucial role in catalytic range 4–8 were basically unaffected by changes in GHSV as
activity and olen selectivity. Simultaneously the weak interac- demonstrated in Fig. 5. The overall olen yield was at
ꢁ
1
ꢁ1
tion between the metal and the graphene-based support further a minimum of 9% at 8.4 L g
h . Heavy hydrocarbons and the
increases the migration ability. This regulatory Mn–Fe core– closely related isoparaffins showed the opposite trend with local
ꢁ
1
ꢁ1
shell structure has not previously been reported in graphene- maximums of 15 and 13% at 8.4 L g
h
respectively, while
based supported catalysts.
paraffins decreased from 72% to about 67% as the space
To date, reports detailing the use of graphene and graphene velocity increased. Finally the Anderson–Schulz–Flory (ASF)
derivatives in the FTS industry are limited in parameter (a) showed a trend reecting the heavy hydrocarbon
6
,7,11,19,21,24,25,56–60
ꢁ1 ꢁ1
number
studies are required to give a clearer role of these materials in
the FTS reaction. Some of the published works used graphene an increase in pressure, thus it is expected that the olen
and more systematic and detailed pattern, with a local maximum of 0.6 at 8.4 L g
h .
Generally, olen double bond hydrogenation is enhanced by
6,7,11,19,24,56–58
61,62
as a support for Fe NPs,
catalyst systems.
while others utilized Co selectivity will be adversely affected by rising pressures.
Cheng et al. aimed at improving light effect of pressure on the FTS catalytic performance of the
The
21,25,59,60
6,7
olen selectivities by using Mg and K promoters. They also FeMn16 catalyst was investigated and is shown in Fig. 6 and
provided a comparison between the alkaline earth metal Table S3.†
1
9
promoters. Moussa et al. provided some insight into the role
of K and Mn promoters in the general performance of Fe/rGO FTS activity was observed and the CO conversion fell from 84 to
catalysts but did not discuss in detail the effects on olen 62%, accompanied by a surge in CH selectivity from 24% up to
selectivities. The rest of the publications concentrated on 38%. There was a slight decline in WGSR activity with a fall in
comparing graphene prepared with different methods or other CO selectivity down from 34 to 25%. The WGSR activity is
When the pressure was lowered from 2 MPa to 1 MPa, a lower
4
2
support materials. Lastly, all of the literature dealing with gra- intrinsically related to the FTS activity. The WGSR reaction
phene supports carried out the FTS reaction in xed bed reac- depends on the presence of water which is a side product of
tors. Therefore, this is the rst work to our knowledge that tries FTS, therefore a low FTS activity means a low water content and
to provide a description of the Fe–Mn NPs on the rGO support, hence reduced WGSR activity and low CO
and reports the use of rGO supported catalysts in SBRs.
The heavy fractions (C9+) remained almost constant at 6%,
.6.3 Effect of operating conditions. The effects of different although the reduced pressure levels did shi the ASF param-
2
selectivity.
3
operation conditions were investigated in order to gain high eter to lower values, falling from 0.55 to 0.49, which is a typical
olen selectivity. The effect of ow rate (expressed as the gas response to the fall in pressure. This decrease is attributed to
ꢁ
1
ꢁ1
hourly space velocity (GHSV) in L g
h
) was studied at three the increased methane formation rather than decreased C9+
) and the results are shown in selectivity obviously.
Fig. 5. The effect of ow rate on the performance of Fe–Mn FTS
The FTS performance of FeMn16 was evaluated at various
systems was investigated previously in both xed bed and temperatures to nd the best temperature for light olen
ꢁ
1
ꢁ1
levels (4.2, 8.4 and 12.6 L g
h
13
61
slurry bed systems. The increase in ow rate was expected to production; the results are shown in Fig. 7 and Table S4.† The
14860 | RSC Adv., 2018, 8, 14854–14863
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ꢀ
fell to 49%; this fall gives further evidence that 300 C is the best
condition to suppress all saturated products in favor of olens.
ꢀ
The fall may be attributed to hydrogenation activity at 300 C
which is insufficient to convert the hydrocarbon chains into
paraffins before desorption from the active site, especially given
61
the increased desorption due to heating. Once this bottle neck
temperature level is surpassed, the hydrogenation rate is
accelerated and higher paraffin selectivities are reached.
The heavy hydrocarbon (C9+) fraction selectivity was very
ꢀ
high at 23% at 280 C, as would be expected typically in an FTS
system, and then it decreased sharply as the temperature
increased to stabilize at about 6%. Isoparaffin selectivity is more
intimately related to the C9+ selectivity since the susceptibility to
branching and isomerization increases with chain length due to
the larger number of possible isomers. That is the reason why
Fig. 6 The effect of pressure on the FTS reaction performance.
conversion decreased sharply from 84 to 67% as the tempera- the isoparaffin selectivity mirrored that of C with a max of
9
+
ꢀ
ꢀ
ꢀ
ture fell from 340 to 320 C, but only decreased to 64% at 300 C 10% at 280 C and a slight fall to 7% at higher temperatures.
ꢀ
aer which it collapsed down to 35% at 280 C. An increase in
The chain growth probability parameter (a) behaved as ex-
conversion with increased temperature will provide more water pected, showing a slight decrease with the rise in temperature,
as a side product from the FTS reaction which will in turn and indicating the obvious effect of heat in reducing the average
increase the WGSR activity and the CO selectivity as shown in product chain length due to faster chain desorption from the
2
Fig. 7. Methane selectivity rises with temperature due to the active sites of the catalyst.
reduced stability of long chain hydrocarbons which can
18
undergo chain ssion (cracking) reactions.
The main reactions inuencing the selectivity of the paraf-
ns and olens are chain growth and olen hydrogenation Manganese acted as a promoter of an Fe NP catalyst by forming
reactions. While chain growth, or the main FTS reaction, is the a Mn-rich layer around a core rich in Fe. The outer layer of Mn is
rst step in the process, the hydrogenation reactions occur benecial for the formation of olens via tuning the gas
4
Conclusion


aerwards to increase the paraffin selectivities. Higher adsorption ability and surface properties, as veried by XPS and
temperatures provide enough activation energy for the hydro- TPR, etc. However, excessively high Mn levels shielded the Fe
genation of double bonds and thus a fall in olen selectivity is active sites and hindered diffusion of the reactants to the
ꢀ
found as the temperature rises from 280 to 340 C. In our work, catalyst particle core which caused a decrease in catalyst
the olen yield depends on both CO conversion and olen activity. Modication of the surface Mn shell thickness could
selectivity and so the slight stabilization in conversion level regulate catalytic activity as well as product selectivity. The
ꢀ
between 320 and 300 C and the continuous rise in olen tunable properties are of great signicance to the fabrication of
selectivity in this temperature range led to an optimum olen high efficiency carbon material supported Fe NP catalysts, and
ꢀ
yield of 19% at 300 C. A very similar result was found for the promote the catalytic performance. Besides that, suitable
FTS performance of the Fe–Mn–K FTS SBR system in the 260– operational conditions further enhanced the production of
ꢀ
ꢀ
61
3
00 C range; this had the highest O/P ratios at 300 C.
A
olens. The best process conditions for high olen production
ꢀ
ꢁ1
coprecipitated Fe–Mn system tested in a xed bed reactor also using the FeMn16 catalyst were at 2 MPa, 300 C and 4.2 L g
ꢀ
13
ꢁ1
gave an optimum O/P ratio at 280 C.
h
using syngas with a 1 : 1 H : CO ratio. At these conditions,
2
In general paraffin selectivity experienced an increase from the O/P ratio was 0.7 and the olen yield was 19%.
ꢀ
5
9% to 72% with rising temperatures except at 300 C where it
Conflicts of interest
There are no conicts to declare.
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