Fe2O3 hollow microspheres as highly selective catalysts for the production of α-olefins

Article abstract of DOI:10.1039/c8nj04115f

We rationally designed and facilely synthesized an excellent Fischer-Tropsch synthesis (FTS) catalyst based on mesoporous Fe₂O₃ microspheres derived from Fe-glycerate via an annealing treatment in air. The synthesis of a Fe-containing precursor is the key step in the fabrication process of Fe₂O₃ microspheres with different interior structures (solid spheres, yolk-shell spheres and hollow spheres). The as-prepared catalysts were characterized systematically to elucidate their structure-dependent catalytic properties. The fine control of Fe₂O₃ with different interior structures and tunable pore sizes distinctly optimized the FTS activity and shifted the product distribution towards heavy hydrocarbons (especially, α-olefins) compared with the traditional iron catalysts. In light of the characterization results, the excellent FTS performance of the Fe₂O₃ hollow spheres can be attributed to the hollow and mesoporous structures.

Full text of DOI:10.1039/c8nj04115f

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Fe2O3 Hollow Microspheres as Highly Selective Catalysts to α-
olefins
Received 00th January 20xx,
Accepted 00th January 20xx
Jiaqiang Sun,* Pengfei Wang and Jiangang Chen*
DOI: 10.1039/x0xx00000x
We rationally designed and facile synthesized an excellent Fischer-Tropsch synthesis (FTS) catalyst based on mesoporous
Fe2O3 microspheres derived from Fe–glycerate via an annealing treatment in air. The synthesis of Fe-containing precursor
is the key step in the fabrication process of Fe2O3 microspheres with different interior structure (solid spheres, yolk-shell
spheres and hollow spheres). The as-prepared catalysts have been characterized systematically to elucidate their
structure-dependent catalytic property. The fine control of Fe2O3 with different interior structure and tunable pore size
optimized distinctly the FTS activity and shifted the product distribution towards heavy hydrocarbons (especially, α-olefin)
compared with the traditional iron catalysts. In light of the characterization results, the excellent FTS performance of Fe2O3
www.rsc.org/
hollow
spheres
can
be
attributed
to
the
hollow
and
mesoporous
structures.
surface because H2 diffuses more quickly than CO, thus leading
to a higher selectivity to CH4. The small pore size usually shows
lower heavy hydrocarbon selectivity. The large pore is able to
diminish the diffusion resistance and lead to high heavy
hydrocarbon content in the products, however, the large pore
diameter mostly contains small surface area, resulting in low
CO conversion levels. It suggests that the propagation of the
carbon chain have close relationship with the pore size of the
Introduction
Effectively controlling the architecture of the catalysts is
important, because the activity and selectivity of catalysts
depend greatly upon the surface/interface structure of the
1
catalysts. Particularly, hollow structures with a permeable
outer shell bring many outstanding benefits to catalysis
properties, in virtue of the large surface area, porosity,
2
permeability and internal void space. The high surface area
8
catalyst support. However, these observations are generally
the result of complex interplay among many factors induced
by changing the pore size of the support. These factors include
not only the confinement effect, which leads to changes in the
re-adsorption probability of α-olefins in the confined spaces
and the diffusion situation, but also the particle size effect, the
reducibility and the dispersion of active species. These factors
are all crucial in determining FT catalytic behavior. Bao and co-
workers prepared Fe2O3 nanoclusters inside and outside
carbon nanotubes (CNTs) with similar sizes and they found
that the Fe2O3 nanoclusters confined inside CNTs were
and open porous channels of the hollow structures provide
more accessible active sites for catalytic reaction. Hollow
structures have recently emerged as ideal and powerful
3
platforms for nanoreactors and confined catalysis 4. The
design and preparation of heterogeneous catalysts with
simultaneously enhanced activity and selectivity have
5
attracted intense interest. Low selectivity significantly limits
the application of the Fischer-Tropsch process for synthesis of
6
specific narrow hydrocarbon fractions. The development of
new strategies and novel catalysts that can tune the selectivity
to desired products is an important goal for Fischer-Tropsch
Synthesis (FTS). Considerable development has been achieved
in recent years, but the effective control of product selectivity
is still challenging. The previous studies have already
demonstrated that the catalytic behaviors of catalysts are also
strongly dependent on the diffusion of reactants and products
9
transformed into iron carbide species more efficiently. But,
the catalytic performance of catalysts is also dramatically
affected by the metal–support interactions. 10 Thus, discussion
on the sole effect of the pore size is quite difficult. The use of
mesoporous materials with suitable pore size distributions
directly as catalyst without support has provided some new
insights into the pore size effect. The mesoporous catalysts
with controllable pore diameters are suitable for the product
of α-olefin, due to offering high specific surface area, short
diffusion length of reaction gases, and efficient channels for
mass transport. The mesopores may function as nanoreactor
to control the chain length, either by shape selectivity or by
enhancing the readsorption of intermediates. 11 In other words,
the nanospaces of mesoporous materials can be expected to
regulate the product selectivity. Therefore, the rational design
7
in the pore of the catalyst support. The diffusion limitation
usually causes higher local H2/CO ratios on the active metal
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and controllable synthesis of Fe2O3 hollow spheres with The Fe–glycerate precursors with different interior structure
mesoporous structures are highly desirable for the (solid spheres, yolk-shell spheres and hollow spheres) were
enhancement of its catalytic performance but still remain as prepared by a one-pot solvothermal method. 13 To gain further
significant challenges. Recently, thermal decomposition of insights into the fabrication process of the Fe-glycerate
metal-containing precursor has been proved as a feasible precursor spheres, a time-dependent morphology evolution
route for preparing porous or hollow metal oxides study was conducted at 180 °C (Figure 1). The morphology of
micro/nanostructures owing to the facile modulation of the the Fe–glycerate precursor spheres obtained from different
12
pore structure.
Therefore, the preparation of hollow stages of solvothermal reaction has been investigated by SEM
structures using metal-containing precursor with large specific and TEM. At the early stage of the solvothermal reaction, solid
surface area, tunable framework and high porosity as spheres with thin nanoflakes (Figure 1a, b) are formed by the
precursors/templates may provide a unique opportunity to heterogeneous nucleation process. The as-prepared precursor
develop a new class of highly tailorable catalysts.
spheres are highly uniform with a diameter of about 650 nm.
Herein, we report a simple, tunable and scalable Fe- Further extending the reaction time, it results in the partial
containing precursor-mediated synthesis strategy for the hollowing of the pristine solid spheres (Figure 1c, d).
preparation of highly uniform mesoporous Fe2O3 microspheres Eventually after reaction for 8 h, the original solid spheres
with different interior structure (solid spheres, yolk-shell evolve into well-defined hollow spheres constructed by a large
spheres and hollow spheres). We then explore the interior amount of radially standing nanosheets (Figure 1e, f).
structure and pore size effect of Fe2O3 for forming α-olefins in Representative TEM images (Figure 1f) further confirm the
FTS. The feature of hollow and porous structures provides a well-defined hollow structure with a shell thickness of around
larger surface area and more active sites to adsorb and 150 nm. Along with morphology observations, the phase purity
activate reaction gases. As compared with conventional Fe2O3 of the products was examined by XRD (Figure S1). All the peaks
nanoparticles, Fe2O3 hollow spheres with mesoporous of the as-synthesized Fe–glycerate solid spheres, yolk-shell
structure exhibit excellent catalytic activity, higher α-olefin spheres and hollow spheres precursors can be well-indexed to
selectivity and higher stability. The excellent catalytic the high crystal Fe–glycerate phase, which is a Fe-based
performance suggests that Fe2O3 hollow spheres are promising alkoxide typically produced in glycerol. 13, 14
candidates for FTS catalysts. More importantly, the present
study indicates that the hollow porous architectures can
effectively enhance the performance of catalysts.
Results and discussion
Structural Characterization of metal-containing precursor and
Fe2O3 microspheres
The fabrication process of Fe2O3 microspheres with different
interior structure (solid spheres, yolk-shell spheres and hollow
spheres) involves two steps. The one-pot self-templated
formation of Fe–glycerate spheres is the key step to obtain
high-quality Fe2O3 microspheres.
Figure 2. (a) XRD patterns of the Fe2O3 solid spheres, yolk-shell spheres and
Figure 1. SEM and TEM images of the Fe–glycerate precursors after different
hollow spheres. TEM and HRTEM images of the Fe2O3 solid spheres (b, c), yolk-
reaction time: (a, b) 2 h; (c, d) 4 h; (e, f) 8 h.
shell spheres (d, e) and hollow spheres (f, g)
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Figure 2a shows the XRD pattern of the as-prepared Fe2O3 respectively, coupled with the satellite peak at
15
sample after calcination at 350 °C for 3 h. All the peaks could clearly verify the formation of α-Fe2O3. While the Fe 2p3/2
∼
719.0 eV,
be assigned to the α-Fe2O3 (JCPDS 33-0664) and γ-Fe2O3 (JCPDS peak at 710.4 eV and the Fe 2p1/2 peak at 724.0 eV are
16
39-1346). For all catalysts, the crystallite size of α-Fe2O3 and γ- assigned to the γ-Fe2O3.
The signal of K could not be
Fe2O3 was calculated from the reflection peak (104) at 2θ= discerned in the spectrum of all the catalysts owing to the low
33.2 and the reflection peak (311) at 2θ= 35.6, respectively, K content. The amorphous C dominates the C 1s spectra of the
according to the Scherrer equation. The particle sizes of α- catalysts (Figure S2), 17 which is beneficial to the homogeneous
Fe2O3 and γ-Fe2O3 are 19.5 nm and 10.9 nm for solid spheres, distribution of the iron oxide, and thus favoring the formation
respectively. The particle sizes of α-Fe2O3 and γ-Fe2O3 are 21.0 of the active surface iron carbide species upon exposure to
nm and 11.1 nm for yolk-shell spheres, respectively. The syngas. 18 Because γ-Fe2O3 and Fe3O4 have very similar XRD
particle sizes of α-Fe2O3 and γ-Fe2O3 are 20.5 nm and 10.9 nm patterns, we performed further characterization by the
for hollow spheres, respectively. As can be seen, the Fe2O3 Mössbauer spectroscopy to distinguish the phases of iron
crystalline size of the prepared catalysts is similar, indicating oxide. Figure S3 show Mössbauer spectroscopy of mesoporous
that the iron oxide particle size effect can be negligible. The spheres after calcination. These original Mössbauer
Fe–glycerate precursors convert into Fe2O3 mesoporous spectroscopy data of calcinated mesoporous spheres were
spheres with well-retained structure after annealing in air fitted into a sextet and a doublet at 300 K. The analyzed results
(Figure 2b-g). As shown in Figure 2c, e, g, the Fe2O3 spheres are are summarized in Table S1. The phase of a sextet was
composed of interconnected nanoparticles. Highly crystalline considered as α-Fe2O3 from the isomer shift values relative to
Fe2O3 with different interior structure possess a highly porous the α-Fe. The phase of a doublet was thought to be γ-Fe2O3
texture, and their pores are clearly visible in the TEM images. with superparamagnetic property at 300 K, which is a deficient
19
HRTEM images further confirm the polycrystalline feature.
inverse spinel.
Therefore, the mesoporous spheres were
XPS was employed to identify the surface nature of the considered as a mixture of α-Fe2O3 and γ-Fe2O3 phase. Owing
catalysts. The survey XPS spectra of the catalysts reveal that C, to the outstanding effect of potassium promotion on iron-
O, and Fe are the primary surface species (Figure 3a). Figure 3b based catalyst for FTS, namely improving activity and olefins to
20
shows the Fe 2p spectra of the catalysts. The peaks with BEs of paraffins ratio,
all the catalysts contain a certain
711.1 and 724.8 eV for the Fe 2p3/2 and 2p1/2 levels of Fe3+,
concentration of potassium. The K loading of Fe2O3 solid, yolk-
shell and hollow spheres samples are 0.61, 0.63 and 0.67 wt%,
respectively, according to the ICP-AES results.
The textural properties of the precursor and the catalysts were
evaluated by N2 adsorption-desorption isotherm measurement
and were summarized in Table S2. The first pore was almost
identical (about 3.5 nm) for the precursor, however, the
second pore was different obviously (Figure S4). The bimodal
precursor all displayed large specific surface area. All reported
pores at 3.5 nm are the result of N2 cavitation within the
pores. BET surface area and total pore volume of the precursor
with a distinct bimodal mesoporous structure increased from
the solid spheres to hollow spheres, with the evolution of the
microsphere interior structure. The pyrolysis of bimodal
mesoporous Fe-glycerate precursor produces mesoporous
Fe2O3 with a BET surface area of 78 for solid spheres, 87 for
yolk-shell spheres, 92 m2g-1 for hollow spheres, respectively.
Compared with the precursor, the respective catalysts showed
bigger average pore size and pore volume, which might be due
to the gasification of organic ligands or collapse of the pore
structure during thermal treatment in air. It could be also
clearly observed the obviously different mesoporous
distribution. It can be clearly seen that only one peak appears
on the pore size distribution curves of the Fe2O3 spheres
derived from the desorption branches using the Barett-Joyner-
Halenda model (Figure S5). All the Fe2O3 catalysts clearly
showed wider pore distribution and the average pore size of
spheres increased with the increasing of hollow interiors.
Figure 3. XPS survey spectra and Fe 2p XPS spectra of the Fe2O3 solid spheres,
yolk-shell spheres and hollow spheres
.
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Figure 4. XRD patterns of the Fe2O3 solid spheres (a), yolk-shell spheres (b) and hollow
spheres (c) catalysts after reduction.
The pores centered at around 15.0 nm were attributed to the
mesoporous of the Fe2O3 solid spheres. While the average
pore size of yolk-shell spheres and hollow spheres were 20.1
nm and 20.5 nm, respectively. The total pore volumes of the
Fe2O3 solid spheres, yolk-shell spheres and hollow spheres
were calculated to be 0.33 cm3 g−1, 0.39 cm3 g−1and 0.44 cm3
g−1, respectively. The Fe2O3 hollow spheres with the largest
pore size showed the highest BET surface area and the largest
pore volumes than that of others. The data suggested that the
pore structure of catalyst was not completely destroyed and
still present mesoporous structure. Therefore, Fe-glycerate
with large specific surface area, tunable framework and high
porosity have been considered as more promising templates
or precursors to fabricate mesoporous iron oxides catalysts,
primarily via thermolysis.
Figure 5. (a) Plots of CO conversion of the Fe2O3 solid spheres, yolk-shell spheres
and hollow spheres against time on stream. (b) Hydrocarbon product selectivity
of the catalysts after 400 h on stream at 2.0 MPa, 280°C, H2/CO = 2 and GHSV =
10 Lg−1h−1
.
mesoporous microspheres after 24
h of reactions are
summarized in Table S3. Furthermore, relevant data from
literature for other iron catalysts are also listed in the Table 2.
22
The reduction behavior of the Fe2O3 mesoporous
microspheres was investigated by H2-TPR and the profiles are
shown in Figure S6. All catalysts showed similar reduction
behavior that consisted of two distinctive steps. The low-
temperature peak was due to the reduction of γ-Fe2O3 and α-
Fe2O3 to Fe3O4; the reduction at above 400 °C consisted of
heavily overlapped peaks corresponding to the consequential
The catalysts prepared by the Fe-containing precursor-
mediated synthesis route display much higher FTY and
excellent C5+ selectivity in comparison with others catalysts.
FTY of the yolk-shell and hollow spheres after 24 h of reactions
is about 8.7 and 14.0 μmol COgFe-1s-1, respectively, which
indicates that the full hollow structure of spheres is more
favorable to the catalytic activity than the partial hollow
structure of spheres. This remarkable difference is related to
the different internal structure that affects the carbidization
degree upon exposure to syngas. The time on stream evolution
of CO conversion for the catalysts is presented in Figure 5a,
thereby providing insights in the stability of these catalysts.
The initial CO conversion with the solid spheres catalysts was
16.6% and decreased to 6.6% after FTS for 216 h, then
remaining stable. The CO conversion of the yolk-shell and
hollow spheres catalysts after activation exhibited an initial
decrease slightly in the induction period and they displayed
21
reduction of Fe3O4 (Fe3O4 to FeO and FeO to metallic Fe).
This facile reduction of γ- Fe2O3 is of utmost importance in
chemical reactions. 21The reduction temperature of γ-Fe2O3
shifted from 323 °C for the solid spheres to 315 °C for the
hollow spheres in the low-temperature region.
Catalytic Properties
Prior to the FTS reaction, the Fe2O3 spheres were activated in
CO at 300 °C for 16 h. After reduction, all the peaks in the XRD
pattern shown in Figure 4 can be indexed as the monoclinic
phase Hägg carbide (Fe5C2, JCPDS 20-0508), with no other
phase being detected, indicating the Fe2O3 catalyst was fully
carburized. The Fe2O3 mesoporous microspheres catalysts
were tested under the industrially relevant conditions of
280 °C, 2.0 MPa, H2/CO ratio of 2 and GHSV of 10 Lg-1h-1. The
catalytic activities and hydrocarbon products distribution
(expressed as C-mol% on a carbon basis) for the Fe2O3
steady-state operation after 24
h on stream. The CO
conversion of the yolk-shell spheres catalysts was 17.3% in the
steady state. The hollow spheres catalysts realized the CO
conversion of 33.8% during the 400 hours of reaction. We
attribute these results to their different internal space that can
act as nanoreactor. The difference found as a consequence of
different structure is the activity and stability of the catalyst.
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One of the most important requirements for an FTS catalyst is
to obtain the maximum production of the heavy hydrocarbon
fraction (especially, α-olefins) while suppressing methane
selectivity to the lowest level possible. The hydrocarbon
selectivity of the Fe2O3 spheres after 400 h on stream is shown
in Figure 5b. The hollow spheres catalysts showed high
selectivity toward heavy hydrocarbon products (C5+, 70.6%)
while yielding a methane product fraction lower than 6%. The
selectivity of C5-11 (gasoline fraction) and C12–18 hydrocarbons
(diesel fraction) for the hollow spheres catalysts are 43.2 %
and 17.1 %, respectively, which is desirable for their
application in the FTS process.
The yolk-shell spheres catalysts also exhibited high selectivity
to C5+ (~73.7%) while comparatively little methane (3.5%). The
selectivity of C5-11 (gasoline fraction) for the yolk-shell spheres
is 39.9%, while the selectivity of C12-18 is 21.9 %. In particular,
the α-olefin (in the C5+ hydrocarbons) fraction amounted to
51.6% and 45.9% for the yolk-shell and hollow spheres in all
the hydrocarbon products, respectively. This is higher than the
values reported to date for iron catalysts, including the
FexOy@C spheres [22d], Fe-rGO, [22e], Fe5C2 nanoparticles, 23a and
other catalysts derived from the pyrolysis of metal–organic
frameworks (Fe3O4@Fe5C2 core-shell nanoparticles and Fe@C).
Figure 6. ASF distribution plots for the catalysts after 400 h on stream at 2.0 MPa,
280°C, H2/CO = 2 and GHSV = 10 Lg−1h−1. n is the number of carbon atoms in a
product, and Wn is the weight fraction of the product with carbon number equal
to n.
It is clear from Figure 6 that the hydrocarbons selectivity for
all the catalysts follow the Anderson–Schulz–Flory (ASF)
distribution, with the α value of 0.78 for the hollow spheres
catalysts, similar to that obtained with the yolk-shell sphere
catalysts, while the solid sphere catalyst presents the lower α
value of 0.69. This can be rationalized from the simplified
“surface carbide” or “carbene insertion-paraffin-metal ring
coordination intermediates” mechanism, which is widely
accepted for FTS. 26 In this model, methylene generated by CO
dissociation hydrogenation and metal form the first paraffin-
metal ring coordination. The carbon chain grows by the
insertion of methylene monomer units (CH2) to the adsorbed
paraffin-metal ring coordination. The methylene insertion
process will be affected by the space steric hindrance of cyclic
coordination, making methylene easier to attack α site. The
chain growth is terminated by forming α-olefins or by
hydrogenation to produce paraffins. The hollow spheres
catalysts provides probably more CO chemisorptions and
larger space, which promoting markedly the product shifting
towards heavy hydrocarbons (especially, α-olefin) by
improving the carbon-chain growth. The lower methane
selectivity can be expected when using the K modified yolk-
shell and hollow sphere catalysts with larger pore diameter
24
The C2–4 and C5–11 (gasoline fraction) hydrocarbons become
dominant for the solid spheres catalysts. The solid spheres
catalysts with a small amount of the diesel fraction displayed a
high selectivity to methane (~9.6%). The CO2 selectivity evolves
in a trend from 7.6% for the solid spheres catalyst to 25.1% for
the hollow spheres catalyst. Moreover, the selectivity did not
undergo significant changes during the whole reaction period
for the Fe2O3 mesoporous microspheres. As the BET results
(Table S2, Figure S7) shown, the average pore size of the
spheres used was increased after FTS reaction, ensuring better
accessibility of the embedded iron species to the reactants.
But the solid spheres may caused more blocking pore, which
hindered reactants to approach the active sites, leading to
lower CO conversion rates and higher methane. The catalyst
with appropriate larger pore size showed higher middle
distillate selectivity, especially the yolk-shell spheres catalyst
showed higher C5–18 selectivity. Furthermore, the formation of
CH4 was remarkably suppressed with increasing pore size. It
could also be seen that the yolk-shell sphere and hollow
sphere catalyst with larger mesoporous size showed higher
olefin selectivity, which was better than that of the solid
sphere. The high α-olefin selectivity of the yolk-shell and
hollow spheres catalyst was likely due to its larger mesoporous
27
that favour the chain growth and the termination step. The
hollow spheres catalysts showed high catalytic activities
combined with high selectivities to the desired products. It
should be point out that the reaction performance of this
catalyst should be the synergetic effects of pore size and
structure.
size, which could favors the desorption of olefin and diffusion
25
of reactants and production.
The mesoporous catalysts
typically possess high surface areas and controllable pore
diameters, which are suitable for the preparation of heavy
hydrocarbons. The ratio of olefin to paraffin (denoted as O/P)
for the solid spheres, the yolk-shell spheres and the hollow
spheres in the C5-C11 range hydrocarbons is 2.2, 3.3 and 2.5,
respectively. As illustrated by Figure 5b, the O/P ratio in
hydrocarbon decreases with increasing chain length (n>5) for
all the catalysts.
Active Phase in FTS
The structural and chemical phase characterization of the
catalysts was carried out after 400 h on stream in FTS. The
cubic phase Fe3O4 (JCPDS 65-3107) and the monoclinic phase
Hägg carbide (Fe5C2, JCPDS 20-0508) appear in the XRD
patterns for the catalysts used (Figure S8). While the
diffraction peak at 24.3° marked with triangle is readily
assignable to C, which was mainly caused by the carbon
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temperature. 28 In the present work, the catalysts were mainly
converted to χ-Fe5C2 and ϵ′-Fe2.2C. Fe3O4 was also found after
400 h of FTS, which was mainly caused by the gradual
oxidation of the iron carbide phase with the water produced
upon exposure to syngas. 29 However, after 400 h of synthesis
gas treatment, the solid spheres exhibit lower ϵ′-Fe2.2
C
contents (25%) compared to the hollow spheres (29%). The
lower degree of total carbides and the higher content of Fe3O4
for the solid spheres could directly result in relatively lower
30
catalytic activities and worse stability. The hollow spheres
are more favorable to the exposure of iron species on the
catalyst compared with the solid spheres. In addition, the
hollow spheres facilitates the reduction and carburization of
Fe2O3 compared to the solid spheres, promoting the formation
of carbonaceous species in surface layers.
Figure 7. Fe 2p XPS spectra of the Fe2O3 solid spheres, yolk-shell spheres and hollow
spheres after 400 h on stream.
Conclusions
In summary, we develop a facile solvothermal approach
combined with subsequent calcination to synthesize highly
uniform Fe2O3 mesoporous microspheres. Dedicate control
over the synthesis of Fe–glycerate precursors will result in
Fe2O3 spheres with many structure-induced merits. Compared
to other iron catalysts, the mesoporous materials typically
possess high surface areas and controllable pore diameters,
which are suitable for the preparation of heavy hydrocarbons
(especially, α-olefin). With the high structural integrity,
complex interior architectures, and enlarged surface area, the
hollow Fe2O3 microspheres exhibit superior catalytic
performances for FTS. It is implied that exploiting the unique
structural transformation of Fe–glycerate precursors, thus
controlling pore size and structure of Fe2O3, is an efficient
method to prepare highly active, selective and stable FTS
catalysts.
Figure 8. Mössbauer spectra of the Fe2O3 solid spheres, yolk-shell spheres and hollow
spheres after 400 h on stream.
Experimental
deposition. The peaks at 707.4 and 720.2 eV in the Fe 2p XPS
spectrum (Figure 7) also demonstrate the existence of Fe5C2. 23
The presence of two weak peaks at 710.8 and 724.3 eV in all
samples indicated the existence Fe3O4, suggesting that the
surface of the iron carbide was slightly oxidized. 23a Mössbauer
spectroscopy is powerful to identify and quantify the iron
phases formed in FTS. Hence, to further identify the active
phases of the FTS catalysts, the phases of iron specie were also
Catalyst Preparation.
0.404 g of Fe(NO3)3∙6H2O, 0.025 g of K(NO3)3 and 15 mL of
glycerol were dissolved in 105 mL of isopropanol under
magnetic stirring. After stirring for 10 min, 1 mL of deionized
water was added into the above solution. The homogeneous
solution prepared above was transferred directly into a 200 mL
Teflon-lined stainless steel autoclave and maintained at 190 °C
for 2h, 4h or 8 h. After cooling to room temperature naturally,
the precipitate was separated by centrifugation, washed with
several times with ethanol and dried at 80 °C for 4 h. The as-
obtained Fe-glycerate precursor was then calcinated at 350°C
for 4h in air with a heating rate of 1.0 °Cmin−1.
then determined by Mössbauer spectroscopy. Figure
8
presents the 57Fe Mössbauer spectrum of all the catalysts after
400h of FTS and the deconvoluted subspectra; the
corresponding fitting parameters are listed in Table S4.
The asymmetric line shapes of the Mössbauer spectroscopy of
mesoporous spheres after FTS is mainly due to defective
crystal structure. Mössbauer spectrum results revealed that all
the catalysts used attained similar iron carbide species. Iron
carbides are generally acknowledged as the main active phase
in FTS, among which, Hägg carbide (χ-Fe5C2) and cementite (θ-
Fe3C) are mostly reported as heterogeneous catalysts for FTS.
Iron carbides (ε’-Fe2.2C, ε-Fe2C) are identified active at low
Characterization
Powder X-ray diffraction (XRD) was performed on a Shimadzu
XRD-6000 diffractometer with Cu Kα radiation (λ= 1.5418 Å) in
the 2θ range from 10° to 70°. The morphology of as-
synthesized samples were monitored by using a scanning
electron microscope (SEM, Zeiss Supra 55). The structure and
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composition of the products were characterized by means of a
high-resolution transmission electron microscope (HRTEM,
JEM 2100F) and X-ray photoelectron spectroscopy (XPS,
ESCALAB 250). The content of K was also analyzed by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES, Thermo iCAP6300). The surface area was determined by a
Notes and references
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2
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No.21503256 and 21373254) and
the autonomous research project of SKLCC (No. 2013BWZ004).
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Fe2O3 Hollow Microspheres as Highly Selective Catalysts to αꢀolefins
Jiaqiang Sun,* Pengfei Wang and Jiangang Chen*
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, China
TOC
Fe2O3 derived from Fe–glycerate with different interior structure and tunable pore
size optimized distinctly the product selectivity.