One-pot synthesis of carbon-coated Fe3O4 nanoparticles with tunable size for production of gasoline fuels

Article abstract of DOI:10.1039/c8nj01280f

Carbon-coated Fe₃O₄ nanoparticles with tunable size were fabricated through a facile one-pot solvothermal method and employed as catalysts for Fischer-Tropsch (FT) synthesis to optimize the product distribution of hydrocarbons. Each spherical nanoparticle (Fe₃O₄@C) was observed to consist of a core-shell structure with an Fe₃O₄ core and a carbon shell. Furthermore, the Fe₃O₄ particle size can be effectively tailored by altering the initial dosage of glucose during catalyst preparation. Fe₃O₄@C with a suitable structure exhibited notable catalytic activity (CO conv. over 98%), high C₅-C₁₁ selectivity (46.6%) and superior stability in the FT synthesis. It was proposed that Fe₃O₄@C with a small particle size facilitated the formation of more active sites, which are responsible for the enhanced catalytic activity. However, the increase in particle and pore size promoted the selectivity towards C₅₊ hydrocarbons and lower olefins. This may provide a novel route to directly synthesize gasoline fuels during FTS.

Full text of DOI:10.1039/c8nj01280f

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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One-pot synthesis of Carbon-coated Fe3O4 nanoparticles with
tunable size for production of gasoline fuels
Junling Tu,a Jiaojiao Yuan,*a Shimin Kang,a Yongjun Xu a and Tiejun Wang*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Carbon-coated Fe3O4 nanocparticles with tunable size were fabricated through a facile one-pot solvothermal method, and
employed as catalysts for Fischer-Tropsch (FT) synthesis to optimize the product distribution of hydrocarbons. Each
spherical nanoparticle (Fe3O4@C) was observed to consist of a core-shell structure with a Fe3O4 core and a carbon shell.
Furthermore, the Fe3O4 particle size can be effectively tailored by the initial dosage of glucose during catalyst preparation.
Fe3O4@C with suitable structure exhibited notable catalytic activity (CO conv. over 98 %.), high C5-C11 selectivity (46.6%)
and superior stability in the FT synthesis. It was proposed that Fe3O4@C with small particle size facilitated the formation of
more active sites, which is responsible for the enhanced catalytic activity. However, the increase in particle and pore size
promoted the selectivity to C5+ hydrocarbons and lower olefins. This may provide a novel route to directly synthesize
www.rsc.org/
gasoline
fuels
during
FTS.
flexible product distribution and favourable engineering
characteristics.8-10 Moreover, FTS is a structural sensitive
reaction, its catalytic activity and selectivity are demonstrated
to depend on the shape and particle size of the catalysts.11-14
For example, Galvis et al.,12 reported that the activity
increased 6-8 fold when the average iron carbide size
decreased from 7 to 2 nm for carbon nanofiber supported iron
catalysts. On the other hand, Barkhuizen et al.,15 found that
the decreasing particle size of K-promoted iron catalysts led to
the decrease of surface-specific activity. Park et al.,16 indicated
that a large particle size led to a higher of C5+ selectivity and
lower of CH4 selectivity for nano-sized iron catalysts. Co-
precipitation method, generally used to prepare Fe-based
catalysts, is the preferred choice owing to the simple operation
process and mild condition. However, it is difficult to control
the structure of the catalysts. In addition, the nanoparticles
aggregation and deactivation of Fe-based catalysts are still
major challenges to the practical application.17-19
1. Introduction
The Fischer-Tropsch synthesis (FTS), one of the most promising
processes for converting alternative energy sources, such as
natural gas, coal and biomass, into liquid fuels and other high-
value products, has attracted much renewed attention among
industrial applications and academic studies because of the
growing demand for fuels and chemicals and the predicted
decline in the long-term availability of crude oil.1-4 One of the
main challenges for FTS is the difficulty to adjust the
hydrocarbons distribution within the designed range (gasoline,
diesel or jet fuel) due to the Anderson-Schulz-Flory law. Thus,
there have been continuous attempts to design highly active
and selective catalysts for FTS. Recently, many types of
catalysts (including Co, Fe and bifunctional catalysts) and
related processes have been developed towards the solution
of this problem.5-7 For example, Subramanian et al.,6 reported
that the encapsulation of Co nanoparticles in nano-sized
porous silica spheres resulted in higher activity per catalyst
weight and stability with a shift of the chain length distribution
of hydrocarbons to lower values. Jiao et al.,7 developed a
bifunctional catalyst affording two types of active site to
produce light olefins from syngas.
In order to improve the catalytic activity and efficiently
break the distribution of the produced hydrocarbons for the
FTS process, one interesting progress is the design of an iron-
based core-shell catalyst with incorporated metal
nanoparticles, due to its properties of space confinement and
synergistic function for restraining the agglomeration of
nanoparticles.20-24 Recently, Han et al.,22 prepared a thin-sheet
Al-fiber@meso-Al2O3@Fe-Mn-K catalyst for the FTS to produce
lower olefins (FTO), delivering a high iron time yield at 90% CO
conversion with 40% selectivity to C2-C4 olefins. Chen et al.,24
encapsulated Fe2O3 nanoparticles incorporated into Carbon
Nanotubes (CNT) and found that the catalytic activity and C5+
hydrocarbons selectivity improved greatly for Fe2O3@CNT. Qin
et al.,25 encapsulated iron nanoparticles into graphitic carbon
and found that Fe@C showed desirable selectivity of 52.4% to
Iron-based materials have been demonstrated as attractive
catalysts for FTS with lower H2/CO ratio syngas as raw
feedstock. Their high water-gas shift activity, lower cost,
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C2-C4 olefins and high stability in FTS. Besides, some evidences using
a single monochromator Renishaw system 1000
have proved that the core-shell structure may modify the equipped with a thermoelectrically cooled CCD detector and
physical and chemical properties of the catalyst. The intrinsic holographic super-notch filter. Brunauer–Emmett–Teller (BET)
structure-performance relationship, such as the effects of core surface area data were obtained following N2 physisorption at
size, shell thickness and pore structure in FTS on the designed 77 K using a Micromeritics ASAP 2010 instrument. Before
core-shell strucures still need further investigations.
measurement the samples were treated at 393 K for 6 h under
In this work, Fe3O4@C catalysts were prepared with a range vacuum. Both the average pore volume and pore size
of tailored particle size via an improved one-step solvothermal distribution were calculated by applying the Barrett–Joyner–
method. The performance of the catalysts in FTS was studied Halenda (BJH) method. H2-temperature programmed
in detail to provide new information on the effect of core-shell reduction (H2-TPR) experiments were carried out in an
structure on the catalytic behavior.It was demonstrated that Automatic Chemisorption Analyzer (CPB-1, Quantachrome)
the Fe3O4@C with suitable size showed high catalytic activity with a thermal conductive detector (TCD).
and selectivity for production of gasoline range products. This
2.3 Catalytic evaluation
result indicates that the synthesized core-shell material seems
a potential catalyst for further production of gasoline fuels The catalytic activity and product selectivity of the as-prepared
from syngas.
catalysts were evaluated in a high-pressure fixed-bed reactor
with a 10 mm inner diameter quartz tube inside. For each test,
1.0 g catalyst sieved into 40-60 mesh was diluted with quartz
powder. Prior to catalytic test, the catalyst was in situ
activated with syngas (H2/CO=1) at 573 K for 12 h (ramping
rate: 2 K/min). The reaction was conducted in 2.0 MPa syngas
(47.5%CO, 47.5%H2, 5% N2) at a flow rate of 3000mL h-1 at 553
K. 0.5%O2/N2 was used for passivation after reaction. The
permanent gases were analyzed using an online gas
chromatography (GC, Agilent, 7890A) by 5A molecular sieve
column and Plot Q column with a thermal conductivity
detector (TCD) and low hydrocarbons were analyzed by Al2O3
capillary column with a hydrogen flame ionization detector
(FID). Liquid products were collected by a cold trap and
analyzed using an offline GC (Shimadzu, GC-2010) by SE-30
column equipped with FID. The product selectivity was
calculated based on total hydrocarbons in weight.
2. Experimental
2.1 Preparation of catalysts
Carbon-coated Fe3O4 nanocparticles (Fe3O4@C) were
fabricated by an improved solvothermal reaction of FeCl3,
Glucose and urea in ethylene glycol (EG) solvent. In a typical
synthesis, 1.35 g of FeCl3·6H2O (5 mM) was dissolved in 40 mL
of EG to form a clear solution. Under vigorous stirring, 3 g of
CO(NH2)2 (50 mM) and 0.2 g of C6H12O6·H2O (0.2 mM) were
added to the solution. The mixture was vigorously stirred for
30 min, and transferred into a Teflon-lined stainless-steel
autoclave, which was then sealed and heated at 473 K for 12 h.
The product was separated by a magnet and washed several
times with distilled water and ethanol. The fresh-made
nanocatalyst was dried overnight at 313 K and directly used for
FTS reaction without further thermal treatment to maintain
their nanostructure and morphology. Other Fe3O4@C catalysts
with tunable size were prepared following the similar
procedures, except the mole ratio of glucose/Fe is 1:10, 2:10,
3. Results and discussion
3.1 Synthesis and characterization of Fe3O4@C
3:10 were varied. The obtained nano-Fe3O4@C catalysts were The Fe3O4@C nanocparticles with tunable size were prepared
denoted as NFCx, where (x) is the molar ratio of glucose/Fe. For at 473 K for 12 h by using a one-pot solvothermal method, as
comparison, the naked Fe3O4 catalyst was obtained by glucose described in the Experimental section. Typically, the Fe3O4
removal.
nanoparticles were formed via partially reduction of Fe3+ to
Fe2+ by EG under alkaline condition by the decomposition of
urea. When these small magnetic particles formed in the
2.2 Measurements and Characterizations
X-ray diffraction (XRD) patterns were collected on a D/MAX-PC solution, they exposed a reactive surface outside, catalyzed
2500 powder X-ray diffractometer (Rigaku, Japan), using Cu Kα following the carbonization of glucose at 473 K. This leaded to
radiation (40 kV and 100 mA) with scanning angle (2θ
) from10o in situ deposition of carbonaceous products around the Fe3O4
to 80o. Scanning electron microscopy (SEM) measurements particle surface and formed a carbon shell26, 27. In the synthesis
were carried out to characterize the morphologies of the as- system, glucose is the key factor for the formation of core-shell
prepared samples using a HITACHI S-4800 scanning electron structure nanoparticles. Glucose is not only the inevitable
microscope with 2 kV accelerating voltage. High-resolution stabilizer, leading to monodisperse Fe3O4 nanoparticles by
transmission electron microscopy (HRTEM) measurements presenting the further aggregation, but also the carbon source
were performed on a JEOL-2010 Model (Japan) electron for the Fe3O4@C uniform core-shell structure.
microscope operated at an accelerating voltage of 200 kV. The
The crystal structure of the as-prepared catalyst was
X-ray photoelectron spectroscopy (XPS) measurements were measured by XRD. As shown in Figure 1, the diffraction peaks
conducted on an ESCALab250 X-ray photoelectron at 2θ of 30.08o, 35.46o, 43.01o, 56.92o, 62.52o can be indexed
spectrometer, using Mg Kα radiation as the excitation source. to the cubic phase of Fe3O4 (JCPDS card No. 75-1069), and the
Laser Raman spectra (LRS) of the samples were measured broadened peak with a relatively low peak intensity at 2θ of
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20o-30o is related to the existence of amorphous carbon. No
impurity peak was observed, indicating that high purity
crystalline Fe3O4 was successfully synthesized through this
method. Furthermore, Fig. 1 shows a decrease of the peak
intensity of Fe3O4, indicating that an increase of glucose
dosage during catalyst preparation reduces the crystallite size
of NFCx. The crystallite size was calculated with Debye-
Scherrer formula by taking the diffraction peak at 2θ=35.46o
(shown in Table 1). It indicates that the increasing of the mole
ratio of glucose/Fe from 1:10 to 3:10 results in a decrease of
the average particle size of the Fe3O4 core from 32.2 nm to
14.4 nm. For comparison, the diffraction peaks of the naked
Fe3O4 catalyst (Fig. S1, ESI†) were observed to be strong and
sharp signal with a crystallite size of 60.6 nm. In this system,
Fe3+ is transformed to FeOOH under alkaline conditions
obtained by the decomposition of urea and further reduced to
Fe3O4 by boiling EG.28 Meanwhile, carbonaceous intermediates
In order to obtain a validation for the presence of a carbon
shell, Laser-Raman spectrum was further conducted to identify
the vibrations bands of the carbon species on the surface
layers. As indicated in Fig. 2, the observed diffraction peak for
all of the fresh NFCx catalysts at about 700 cm-1 can be indexed
to Fe3O4. Meanwhile, the two broadened peaks centered at
1328 cm−1 (D-band, distorted) and 1600 cm−1 (G-band,
graphite) indicate the existence of amorphous and graphite-
like carbons on the surface of NFCx.31-33 Evidently, an increase
of the peak intensity of surface carbon occured as the glucose
dosage increases during synthesis, which is related to more
amorphous carbon formed on the surface layers. For the
naked Fe3O4 sample, the disappearance of G and D bands
(shown in ESI†, Fig. S2) in the Raman spectra indicates that
there are no carbon species on the surface.
with
a low degree of polymerization are formed by
dehydration of glucose, which leads to in situ deposition of
carbonaceous products around the Fe3O4 particle surface to
form
a carbonaceous shell via further intermolecular
dehydration.29, 30 The observed inverse relationship between
the average size and the amount of glucose was probably
attributed to the tendency of carbonaceous intermediates to
increase the viscosity of the solution and hence inhibit the
migration and aggregation rate of nanoparticles.
NFC1
NFC2
NFC3
311
Fig. 3 SEM and TEM images of the NFCx catalysts (a-c, SEM
images of NFC1, NFC2, NFC3, respectively; d, TEM images of
NFC2).
Fe
O
4
440
3
511
JCPDS 19-0629
400
220
30
422
111
10
20
40
50
60
70
80
2 Theta (degree)
The morphology of the as-obtained samples was
characterized by SEM and TEM. As shown in Figure 3, all the
samples consist of monodispersed nanoparticles, presenting a
spherical shape with an average size varying from 18-33 nm.
The structure of a single NFC2 was characterized by HRTEM
(Figure 3d). Clearly, the spherical nanoparticle has a shell
structure with a thickness of 2 nm and a core with a size of 18
nm. The nanoparticle with lattice space value of 0.48 nm fits
well with Fe3O4. Moreover, the diameter of the synthesized
Fe3O4@C can be tailored by changing the concentration of
glucose (Fig. 3a-c). When the mole ratio of glucose/Fe was set
at 1:10, the nanoparticles exhibited a mean size of 33 nm
(NFC1, Fig. 3a). When the mole ratio of glucose/Fe was
increased to 2:10, the average size of the nanoparticles
decreased to 21 nm (NFC2, Fig. 3b). Nanoparticles with a mean
size of 18 nm were observed as the mole ratio of glucose/Fe
increased to 3:10 (NFC3, Fig. 3c). The naked Fe3O4 sample was
Fig. 1 X-ray diffraction patterns of the as-prepared samples
♦
♥
♥
∇
∇
∇
NFC3
NFC2
NFC1
♦
♥
♦
∇ Fe3O4
♥ Peak D
♦ Peak G
200
400
600
800
1000
1200
1400
1600
1800
Raman Shift (cm-1)
Fig. 2 Laser-Raman spectrums of the synthesized samples
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composed of hollow iron-based spheres with an average size
of 300 nm in the absence of glucose (Fig. S3, ESI†). It indicates
that the glucose dosage adjusts efficiently the particle size of
Fe3O4@C during preparation, which aligned with the XRD
results. Thus, it can be suggested that glucose plays an
essential role in tuning the particle size of NFCx during catalyst
preparation.
NFC3
NFC2
NFC1
Textural properties of the catalysts were further determined
by measuring N2 absorption–desorption isotherms and
Thermogravimetric analysis (TGA). As listed in Table 1, the BET
surface areas of the NFC1, NFC2, and NFC3 were calculated to
be 60, 99, and 117 m2/g, with corresponding average pore
diameters of 19.0, 11.6, and 9.8 nm, respectively. It is noted
that the relative large pore could be ‘‘ink-bottle’’ type
mesopores resulting from the void between Fe3O4@C
nanoparticles in the samples. This result can be attributed to
that the increase of mole ratio of glucose/Fe during catalyst
preparation leads to a decrease of particle size, resulting in
high surface area of the NFCx. However, high carbon content
for small nanoparticles (26.3 % for NFC3) may bring about
blocking of some pores and the reduction of the pore size. In
addition, the shell thickness of each samples was also
approximately determined based on a “core-shell sphere
model” by density (5.18 g/cm3 for Fe3O4 and 1.95 g/cm3 for
that of amorphous carbon) and the carbon shell content (Table
1). The shell thickness of NFCx were calculated to be all about 2
nm, which is near the value of the particle size observed from
SEM. It is observed that the surface area of the naked Fe3O4
sample is only 9 m2/g (Table S1, ESI†) because of aggregation
of nanoparticles (Fig. S3, ESI†). The relatively high surface area
and large porosity of Fe3O4@C can provide a large contact area
for reactants and promote diffusion of products. These results
demonstrated that the as-prepared Fe3O4@C nanoparticles
with tunable structure (various particle size, surface area and
pore size) are promising catalysts to improve the catalytic
activity and selectivity in FT synthesis.
400
500
600
700
800
900
1000
1100
1200
Temperature (K)
Fig. 4 H2-TPR profiles of the NFCx catalysts
H2-TPR experiments were conducted to investigate the
influence of the structure on the reduction behaviour of the
synthesized samples. As shown in Fig. 4, the TPR patterns
exhibit two main peaks at about 640-680 K and 700-1030 K,
corresponding to the transformations of Fe2O3→Fe3O4 and
Fe3O4 → FeO + Fe, respecꢀvely.9 It should be noted that the
first low peak can be considered as the surface partially
oxidization of Fe3O4 → Fe2O3 in air. The intensity and area of
reduction peak at 640-680 K increases evidently for the NFCx
catalysts with the Fe3O4 size decreases, because small particles
are more active. As presented in Fig. 4, the two reduction
peaks of NFC1, NFC2, and NFC3 shift toward lower temperature
with the decrease in particle size, which suggests the Fe3O4@C
catalysts with small nanoparticles favour for reduction. In
addition, the reduction peaks of each Fe3O4@C shift to lower
temperature compared with the naked Fe3O4 sample (Fig. S4,
ESI†). This phenomenon could be related to the size effect and
the change in the chemical environment caused by electron
transport between Fe3O4 and the carbon shell. As reported by
Chen et al.,24 a similar result was attributed to a confined
effect of the CNTs on the encapsulated iron oxides. Phase
transformation over the catalysts was also investigated by XRD.
As shown in the ESI† (Fig. S5), it can be observed that the
diffraction patterns of each Fe3O4@C present peaks at 39.31o,
41.15o, 43.4o, 44.07o and 44.65o, attributing to iron carbide (χ-
Fe5C2). No Fe3O4, FeO and Fe was detected, suggesting that
iron oxides were almost completely converted to χ-Fe5C2,
which was viewed as the active phase in FT synthesis.34, 35
Therefore, it reveals that the synthesized core-shell materials
can be used as promising catalysts for FT synthesis for
production of hydrocarbon with high catalytic activity.
Table 1 Textural properties of the NFCx catalysts
Particlesize (nm)
Carboncontent
Surfacearea poresize
Sample
（
wt.%
）
(m2/g)
(nm)
XRD
SEM
NFC1
NFC2
NFC3
17.2
21.5
26.3
32.2
17.7
14.4
37
22
18
60
99
19.0
11.6
9.8
117
3.2 Fischer–Tropsch synthesis performance
The catalytic performances of the Fe3O4@C catalysts for FT
synthesis were tested in a fixed bed reactor operating at 553K
and 2.0 Mpa using syngas (H2/CO = 1). CO conversion is
expressed as the molar of CO moles converted to
hydrocarbons and CO2. The selectivity to hydrocarbon is
expressed as the weight percentage in all the hydrocarbons.
The CO conversion and the hydrocarbon distribution of the
samples are presented in Fig. 5 and Table 2.
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100
conversion (shown in Table 2). This result can be ascribed to
more active sites on the surface of small nanoparticles with
higher surface area and metal dispersion. This affects the heat
and mass transfer as well as the transport pathways for the
reactant molecules and catalytic products.38, 39 Therefore, the
NFC3 catalyst with the smaller size exhibits higher iron-time-
(a)
90
80
70
NFC1
NFC2
NFC3
yield (FTY, expressed as moles of CO conversion per gram of
-1
iron per second) value of 3.3
×
10−5 molCOgFe s-1. As the
reaction time is prolonged, NFC2 and NFC3 keep distinguished
high catalytic activity (CO conv. > 97 %) without undergoing
visible deactivation, but the NFC1 catalyst suffers little
deactivation with 5 % decrease of the CO conversion during 48
h on stream. In contrast, the initial CO conversion of the naked
Fe3O4 catalyst is only approximate 55%, and its catalytic
conversion gradually decrease by 50 % (Fig. S6, ESI†) casued by
sintering (Fig. S7a, ESI†) with reacꢀon ꢀme on stream. This
indicates that the carbon-coated Fe3O4 catalyst has higher
catalytic activity and stability than the naked catalyst. It is
related to that the shell of carbon layer suppresses
aggregation of nanoparticles inside the core, thus improving
the stability of the nanoparticle structures (Fig. S7b-d, ESI†)
and giving rise to their catalytic durability.
60
50
0
5
10
15
20
25
30
35
40
45
50
Time on stream (h)
CH4
(b)
=
C2-4
C2-4
0
40
30
20
10
0
C5-11
C12+
In addition to the activity and the stability of the catalysts,
the product selectivity is also an area of interest. As shown in
Fig. 5b and Table 2, the Fe3O4@C core–shell catalyst with
suitable particle size (NFC1) displays excellent C5-11 (gasoline
range products) selectivity (46.6%). The Anderson–Schulz–
Flory (ASF) model (eq. S1, ESI†), which is used to predict the
NFC1
NFC2
NFC3
Fig. 5 (a) CO conversions of synthesized catalysts as a function of
time on stream, and (b) production distribution for synthesized
catalysts after reaction for 24 h.
Table 2 The catalytic performance of various catalysts in the product distribution in FTS, indicates that the maximum
FTS reaction. Reaction conditions: catalyst = 1.0 g, H2/CO = 1, selectivity is achievable about 44 % for the C5-11 fraction, with a
573 K, 2.0 MPa, 24 h.
Performance
chain growth probability (
α
On the basis of this model, the
) of 0.78, as shown in Fig. S8 (ESI†).
values of the NFC1, NFC2, and
α
NFC1
NFC2
NFC3
NFC3 were calculated to be 0.78, 0.74 and 0.66, respectively
(Table 2). It is well known that the particle size, crystal form
and textural properties (including surface area, pore structure)
of the iron carbides show some inherent correlations with
product distribution in FTS.12, 39 It is possible that the suitable
iron carbides size with appropriate pore and the confinement
effect of the core-shell structure of Fe3O4@C enhances the
formation of C5–11 hydrocarbons. The results indicate that
Fe3O4@C can be tuned to effectively produce gasoline fuels
during FTS.
CO conversion (%)
H2 conversion (%)
CO2 selectivity (%)
FTY (10−5 molCOgFe-1 s-1)
88.9
73.8
35.6
2.6
97.2
75.5
37.1
3.0
98.3
77.2
37.1
3.3
Hydrocarbon selectivity (wt.%, CO2-free))
CH4
17.7
26.4
55.9
46.6
19.2
29.1
51.7
43.0
20.3
31.2
48.5
40.4
C2-4
C5+
At this point, even NFCx exhibited a similar core-shell
structure, but these three catalysts displayed different
catalytic performances (Fig.5b and Table 1). As illustrated in
Fig.5b, the C5+ selectivity and olefin/paraffin ratio of C2-4
manifest an increasing from 48.5 % to 55.9 % and 0.5 to 0.8,
respectively, following the order: NFC3<NFC2<NFC1. However,
the selectivity to CH4 and C2–4 exhibit an opposite trend (20.3
% to 17.7 % and 31.2 % to 26.4 %, respectively). Generally, the
macroscopic catalyst performance relies on its microstructure
and chemical composition. In the ASF model, a CH3 group
formed through the CO dissociation and hydrogenation is
employed as a chain initiator and adsorbed on the catalyst
surface. The chain growth depends on the insertion of
methylene monomer units (CH2) to the adsorbed alkyl species,
C5-11
Olefin/paraffin (wt./ wt.)
C2-4
0.8
0.6
0.5
Chain growth probability (
α
)
0.78
0.74
0.66
As shown in Fig. 5a, the NFC3 catalyst exhibits a higher initial
activity (CO conv. = 98 %) as compared with NFC2 (CO conv. =
97 %) and NFC1 (CO conv. = 92 %). The high catalytic activity of
the Fe3O4@C core–shell catalysts could be primarily attributed
to abundant of iron carbides (especially, χ-Fe5C2) on the
surface layer (Fig. S5, ESI†).36, 37 Moreover, the rate of CO
conversion followed the order of the particle size:
NFC3>NFC2>NFC1, smaller particle size leads to higher CO
which can be terminated by the formation of α-olefins or the
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production of paraffins via hydrogenation.38, 40 Thus, numerous size, shell content and pore size can be successfully adjusted
factors in relation to chain initiation, propagation and by changing the concentration of glucose. During FT synthesis,
termination (such as CO dissociation, hydrogenation, the Fe3O4@C catalysts exhibited excellent catalytic
adsorption, desorption and heat transfer) may contribute to performance. The catalyst with large particle and pore size
the different catalytic performances.
(NFC1) promoted a shift in the product distribution towards
In this study, the varied production distribution may be heavy hydrocarbons combined with high selectivity to gasoline
attributed to the following main aspects: particle size of iron fuels (46.6 %). The catalyst with small particle size and high
carbides, pore size and confinement effect of the carbon shell. It surface area (NFC3) favored the formation of more iron
has been demonstrated that particle size of iron carbide plays carbide, enhancing the catalytic activity to an extremely high
an important role in FTS.12, 41, 42 As proposed by Galvis et al.,12 level (CO conv. >98 %), but resulting in more CH4. The results
CH4 formation, followed by strong hydrogenation ability, takes demonstrated that the core-shell structure played an essential
place at highly active on small iron carbide particles, which role in the catalytic activity and selectivity.
have more abundant low coordination sites residing at corners
and edges. In addition, it is found that CHx coverages increase
Conflicts of interest
with decreasing particle size because of smaller particles
having a higher number of highly active low coordination sites.
It is also established that the turnover frequency increased
There are no conflicts to declare.
with CHx coverage.41 Consequently, the NFC3 catalyst with the
-1
smallest size shows the highest FTY (3.3
x
10−5 molCOgFe s-1)
Acknowledgements
and selectivity to CH4 (20.3 %) (Table 2). As for bigger particles,
lower H coverages on the surface leading to lower methane
selectivity and higher lower olefin selectivity are observed. The
H coverage was suppressed on NFC1 in line with lower
selectivity to CH4 (17.7 %) and higher selectivity to lower olefin
(11.7 %). In fact, pore size also has strong influence on the
production distribution in FTS.18, 43, 44 Thus, porous materials
(especially, mesoporous materials) have been employed as
nanoreactor to improve the chain growth via enhancing the
adsorption of alkyl species as well as suppressing the
hydrogenation reaction. Smaller pore sizes with limited heat
and mass transfer rate inhibit the chain growth reaction, giving
rise to more lower hydrocarbons (CH4 and C2–4). In contrast,
higher olefin and C5+ selectivity were observed over larger pore
iron catalysts.18 Therefore, higher selectivity to C5+
hydrocarbons (55.9 %) and lower olefins (11.7 %) were
observed for the NFC1 catalyst with the largest pore size about
19.0 nm. In addition, the confinement effect of the carbon
shell may also account for the catalytic performances in FTS via
improving reducibility for more active carbide species,
trapping of the reaction intermediates, and prohibiting the re-
absorption of light olefins for the secondary reaction.24, 45, 46 It
appears that these combined effects expedite the different
catalytic activity and product distribution under the studied
reaction conditions. Furthermore, the influences such as
carbon deposition, Fe−C bond strength, and surface coverages
on the FT performance of Fe3O4@C could be the potential
factors to affect catalytic performance of FTS and should be
encourage in the future. In summary, these results suggest
that carbon-coated Fe3O4 nanocparticles with suitable
structure is promising for gasoline fuels production in FT
synthesis.
The authors gratefully acknowledge the financial support from
the Natural Science Foundation of China (No. 51606042,
51561145010), National Key Technologies R&D Program of
China (No. 2016YFD04003032), and China Postdoctoral Science
Foundation (No. 2017M622700).
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New Journal of Chemistry
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DOI: 10.1039/C8NJ01280F
Fe3O4@C nanoparticles with tunable size exhibit excellently catalytic performance on gasoline fuels direct synthesis.