S.W. Kang et al. / Journal of Catalysis 349 (2017) 66–74
67
pans (Tzero, ꢄ40
l
L) under a flow of N2 (50 mL minꢁ1). The sample
2. Experimental section
mass was 3.7 mg. Melt infiltration was followed in situ for the Fe
(NO3)3ꢂ9H2O/CMK-3 physical mixture by a heat-cool-heat cycle
to 75, ꢁ90, and 75 °C, respectively, at 2.5 °C minꢁ1 with 5 min
isothermal steps at each temperature extreme.
2.1. Chemicals
Pluronic P123 (Mw = 5800), tetraethyl orthosilicate (TEOS, 98%),
and iron nitrate nonahydrate (Fe(NO3)3ꢂ9H2O, ACS reagent, ꢃ98%)
were purchased from Aldrich. Single-layer graphene was pur-
chased from ACS Material, LLC. Sucrose, hydrochloric acid (HCl,
35 wt%), and sulfuric acid (H2SO4, 95%) were purchased from Sam-
chun Pure Chemical Co. Ltd. The chemicals were used as received
without further purification.
2.4. High-temperature Fischer-Tropsch synthesis
The Fischer-Tropsch reaction tests were conducted using a
fixed-bed reactor of stainless steel. The catalysts (80 mg) were
diluted with glass beads (4.2 g, 425–600 lm) and then charged
into the fixed-bed reactor. The catalysts were slightly oxidized dur-
ing the catalyst loading process, but were reactivated under a CO
flow of 40 mL minꢁ1 at 350 °C for 4 h. After the activation treat-
ment, reaction was carried out at 32ꢁ01°C and 15 bar using a synthe-
sis gas (H2/CO = 1.0, GHSV = 30 NL gcat hꢁ1). The composition of the
outlet gases was analyzed using an online gas chromatograph (Agi-
lent, 3000A Micro-GC) equipped with a molecular sieve and plot Q
columns. The gas flow rates were measured using a wet-gas flow
meter (Shinagawa Corp.). The composition of the wax and liquid
products was analyzed by means of an offline GC (Agilent, 6890
N) with a simulated distillation method (ASTM D2887).
2.2. Synthesis of Fe5C2@CMK-3 and Fe5C2/graphene nanocatalysts
Ordered mesoporous carbon CMK-3 was prepared using the
hard template SBA-15. First, Pluronic P123 (8.0 g), deionized water
(251.4 g), and HCl (48.6 g) were added to a 0.5 L polypropylene
bottle and the mixture was stirred at 35 °C. After complete dissolu-
tion of Pluronic P123, TEOS (17.0 g) was added to the solution and
aged for 24 h at 35 °C. The reaction mixture was then transferred to
a Teflon-lined autoclave and heated at 150 °C for 24 h. The white
precipitate was filtered, washed twice with deionized water, and
then dried in an oven at 60 °C for 1 d. Finally, the mesoporous silica
SBA-15 sample was obtained by calcination under air at 550 °C for
5 h. For the synthesis of the CMK-3 mesoporous carbon support, a
carbon precursor solution was prepared by dissolving sucrose
(4.13 g) and H2SO4 (0.47 g) in H2O (15 g). SBA-15 mesoporous silica
(3 g) was added to this solution, which was then mixed to homo-
geneity. The mixture was placed in a drying oven for 6 h at
100 °C. The oven temperature was subsequently increased to
160 °C and maintained at this temperature for 2 h. After the addi-
tion of sucrose (2.48 g), H2SO4 (0.28 g), and H2O (15 g), the silica–
polymerized-sucrose composite was again treated at 100 and
160 °C. The sample was carbonized by heating to 900 °C and main-
tained at that temperature for 2 h under flowing Ar. Finally, the
carbon–silica composite was washed with hydrofluoric acid (4 M)
at room temperature to remove the silica template. The obtained
template-free carbon CMK-3 was filtered, washed with ethanol,
and dried at 100 °C. To prepare a Fe5C2@CMK-3 nanocatalyst with
20 wt% Fe, 1.85 g of Fe(NO3)3ꢂ9H2O was physically ground with
1.0 g of CMK-3 in a mortar for several minutes until the powder
became homogeneously black. Then, the powder mixture was aged
in a polypropylene bottle at 50 °C in an oven. After aging for 24 h,
the sample was cooled under ambient atmosphere and then trans-
ferred to an alumina boat in a tube-type furnace. Finally, the iron
salt-incorporated CMK-3 sample was slowly heated at the ramping
2.5. Characterization
The characterization of high-resolution transmission electron
microscopy (TEM) with energy electron loss spectroscopy (EELS)
of the obtained catalysts was performed using an FEI Titan 80–
300 operated at 300 kV. For scanning TEM analysis, samples were
prepared by putting a few drops of the corresponding colloidal
solutions on lacey carbon coated copper grids (Ted Pellar, Inc).
The X-ray diffraction (XRD) patterns of the samples were analyzed
by high power powder X-ray diffractometer (Rigaku D/MAX-2500,
18 kW). X-ray photoelectron spectroscopy (XPS) studies were car-
ried out using a Sigma Probe (Thermo VG Scientific, Inc.) with a
micro-focused monochromator X-ray source. The samples for XPS
were prepared by placing a few drops of the colloidal solutions
on small pieces (5 mm ꢀ 5 mm) of gold wafer, which were then
allowed to dry under N2 flow. Mössbauer spectra were obtained
with a fixed absorber and a moving source. A Mössbauer spectrom-
eter of the electromechanical type, with a 50 mCi 57Co source in a
rhodium matrix, was used in constant-acceleration mode. The iron
loading amounts were measured by inductively coupled plasma
optical emission spectrometry (ICP-OES, Thermo Scientific iCAP
6300). N2 sorption isotherms were measured at ꢁ196 °C with a
TriStar II 3020 surface area analyser. Before measurement, the
samples were degassed in a vacuum at 300 °C for 4 h. The X-ray
absorption experiment with synchrotron radiation was performed
at the Pohang Accelerator Laboratory (PAL).
rate of 2.7 °C minꢁ1 to 350 °C under CO flowing at 200 mL minꢁ1
.
The sample was held at 350 °C for 4 h under a continuous CO flow.
After the thermal treatment, the resulting black powder was
cooled to room temperature and submerged in anhydrous ethanol
(20 mL) under N2 flowing at 500 mL minꢁ1 to prevent rapid surface
oxidation of the active particles. The Fe5C2@CMK-3 powder
immersed in ethanol was separated simply using a magnet and
completely dried in a vacuum oven at 50 °C. For the preparation
of the Fe5C2/graphene, all the procedures were identical to the syn-
thesis of the Fe5C2@CMK-3 nanocatalyst, except for the use of
single-layer graphene powder (1.0 g) as the support material.
2.6. Computational approaches
The amorphous carbon surface was optimized via the large-
scale atomic molecular massively parallel simulator (LAMMPS).
The Erhart/Albe-tersoff potential was applied to calculate the
interatomic forces of C–C [23]. The constant number of atoms,
volume and temperature (NVT) ensemble condition was used to
represent the experimental environment of the optimal Fe5C2
particle generation condition (350 °C), with sufficient time steps
(500 ps). After that, DFT calculations were performed for the
Fe5C2 particle on both amorphous carbon and crystalline graphene
substrates using the Vienna Ab initio Simulation Package (VASP).
Exchange-correlation energies were treated using the Perdew-
Burke-Ernzerhof (PBE) functional based on generalized gradient
approximation (GGA) [24]. More details are described in the
supporting information.
2.3. Differential scanning calorimetry
The melting behavior of Fe(NO3)3ꢂ9H2O in the presence of
mesoporous carbon CMK-3 was studied using differential scanning
calorimetry (DSC) (Q20, TA Instruments). The temperature and
heat flow were calibrated with a certified indium sample and mea-
surements were performed with hermetically sealed aluminum