Synthesis of urchin-like FeF2 nanoarchitectures and their conversion into three-dimensional urchin-like mesoporous α-Fe2O3 nanoarchitectures for methane activation

Article abstract of DOI:10.1002/ejic.201402152

Novel urchin-like FeF₂ nanoarchitectures have been fabricated by an unconventional nonhomogeneous ionic liquid/diphenyl ether solvothermal method. Subsequent solid-state thermal annealing was utilized to convert the FeF₂ nanoarchitectures into 3D urchin-like mesoporous α-Fe₂O₃ nanoarchitectures. The reaction system and temperature played important roles in the morphology of FeF₂ and the growth of the 3D urchin-like α-Fe₂O₃. A study of methane activation over the 3D urchin-like mesoporous α-Fe₂O₃ nanoarchitectures revealed that methane was activated and converted into carbon dioxide at a low temperature (230 °C). Compounds containing C-C bonds were produced at 600 °C. This 3D urchin-like mesoporous α-Fe₂O₃ shows excellent potential as a catalyst for methane conversion in the chemical industry. Novel urchin-like FeF₂ nanoarchitectures have been fabricated by an unconventional nonhomogeneous ionic liquid/diphenyl ether solvothermal method and converted into 3D urchin-like mesoporous α-Fe₂O₃ nanoarchitectures by solid-state thermal annealing. The α-Fe₂O₃ nanoarchitectures exhibit good catalytic properties in methane activation reactions.

Full text of DOI:10.1002/ejic.201402152

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DOI:10.1002/ejic.201402152
Synthesis of Urchin-Like FeF2 Nanoarchitectures and
Their Conversion into Three-Dimensional Urchin-Like
Mesoporous α-Fe2O3 Nanoarchitectures for Methane
Activation
Bing Dong,[a] Hengqiang Zhang,[a] Aiguo Kong,*[a,b]
Yingying Kong,[a] Fan Yang,[a] and Yongkui Shan*[a]
Keywords: Mesoporous materials / Nanostructures / C–H activation / Iron / Fluorine / Ionic liquids
Novel urchin-like FeF2 nanoarchitectures have been fabri-
cated by an unconventional nonhomogeneous ionic liquid/
diphenyl ether solvothermal method. Subsequent solid-state
thermal annealing was utilized to convert the FeF2 nanoar-
chitectures into 3D urchin-like mesoporous α-Fe2O3 nano-
architectures. The reaction system and temperature played
important roles in the morphology of FeF2 and the growth of
the 3D urchin-like α-Fe2O3. A study of methane activation
over the 3D urchin-like mesoporous α-Fe2O3 nanoarchitec-
tures revealed that methane was activated and converted
into carbon dioxide at a low temperature (230 °C). Com-
pounds containing C–C bonds were produced at 600 °C. This
3D urchin-like mesoporous α-Fe2O3 shows excellent poten-
tial as a catalyst for methane conversion in the chemical
industry.
Of the 3D nanoarchitectures reported, considerable at-
tention has been focused on α-Fe2O3 nanoarchitectures be-
cause of their environmentally benign nature and extensive
applications in, for example, catalysis, pigments, adsorp-
tion, and sensors.[9–11] Many α-Fe2O3 3D nanoarchitectures,
such as hierarchical mesoporous structures,[12] hollow
microspheres,[13] porous plates,[2] nanorod arrays,[14] ellip-
soids,[15] cage-like hollow spheres,[16] and flower-like
nanoarchitectures,[17,18] have been synthesized from nano-
particles as the building units.
Introduction
The fabrication of three-dimensional (3D) nanoarchitec-
tures with controlled morphology, orientation, and dimen-
sionality has attracted increasing interest due to their
unique physical and chemical properties.[1] For example,
complex 3D superstructures assembled from nanometer-
sized building blocks usually exhibit a greater resistance to
agglomeration, higher specific surface area, better mechani-
cal strength, and a lower pressure drop, which allow higher
reactant gas velocities than fine powder catalysts.[2] In ad-
In particular, 3D urchin-like α-Fe2O3 nanoarchitectures
dition, complex nanoarchitectures used as catalysts are with different shapes and morphologies have recently been
often more easily separated and recycled than nano-sized fabricated and show potential applications in various
units.[3] Substantial effort has been directed towards the de- fields.[1–6,19] Urchin-like α-Fe2O3 nanoarchitectures pro-
velopment of new methods for the assembly of low-dimen- duced by a simple solution-based reaction and sequential
sional nano-building blocks into complex 3D nanoarchitec- calcination exhibit enhanced sensing ability towards eth-
tures. A large number of materials with 3D nanoarchitec- anol.[1,20] Urchin-like α-Fe2O3 hollow microstructures syn-
tures, such as flower-like CeO2,[4] porous Cr2O3 spheres,[5] thesized by a simple ligand-assisted hydrothermal route
MnO2 hollow nanoarchitectures,[6] Bi2Te3 nanotube ar- show a Morin transition from the low-temperature antifer-
rays,[7] and 3D Co-Pt nanoarchitectures,[8] have been fabri- romagnetic phase to a weakly ferromagnetic phase at
cated by various methods.
181 K, which is much lower than the transition temperature
of bulk α-Fe2O3.[2,21] 3D urchin-like α-Fe2O3 microstruc-
tures comprising crystalline α-Fe2O3 nanoflakes grown per-
pendicularly on the surface of spheres prepared by a one-
step thermal oxidation of Fe spheres show good field-
emission performance and may be good candidates for use
in field-emission devices.[3,22] A composite mixture con-
sisting of an urchin-like α-Fe2O3 structure and an epoxy
resin exhibits excellent electromagnetic wave absorption
ability and is a promising radar-absorbing material that is
[a] Department of Chemistry, East China Normal University,
Shanghai 200241, China
E-mail: ykshan@chem.ecnu.edu.cn
http://chem.ecnu.edu.cn
[b] Shanghai Key Laboratory of Molecular Catalysts and
Innovative Materials, Fu Dan University,
Shanghai 200433, China
E-mail: agkong@chem.ecnu.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402152.
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capable of dual-frequency electromagnetic wave absorp- quid ([Omim]BF4) alone produced FeF2 nanorods with a
tion.[4,23] Urchin-like α-Fe2O3 and Fe3O4 aggregates con- diameter of about 50 nm and a length of up to 300 nm (see
sisting of sea shuttle fibrous bunches grown radially from a Figure S2b). Therefore the success of fabricating urchin-like
center of the aggregate obtained by a facile glucose-guided FeF2 nanoarchitectures was due to the presence of both the
hydrolyzing/reducing process exhibit excellent microwave ionic liquid ([Omim]BF4) and diphenyl ether. However, the
absorption properties.[5,24] The α-Fe2O3/Pt urchin-like morphology, purity, and crystallinity of the urchin-like
microsphere composite fabricated by a two-step process ex- FeF2 nanoarchitectures were also influenced by other reac-
hibits catalytic behavior in methanol oxidation and reduces tion parameters, including temperature, time, and stirring
the consumption of Pt in CO oxidation and has potential rate, as is often the case.
applications in electrocatalysis, such as in fuel cells, and as
The powder X-ray diffraction patterns (Figure 1) of the
biosensors.[6,25] The above results indicate, therefore, that products obtained in the ionic liquid ([Omim]BF4)/diphenyl
the investigation of 3D urchin-like α-Fe2O3 nanoarchitec- ether system at different reaction temperatures shows that
tures with ordered and functional properties is of great sig- the samples obtained at temperatures of 250 °C and higher
nificance for both fundamental research and practical are tetragonal FeF2 (space group P42/mnm, a = 4.703 Å,
applications.
c = 3.306 Å, PDF-45-1062) with no impurity phase being
In this study, a unique nonhomogeneous ionic liquid/di- detected (Figure 1, b,c). On the other hand, at a lower reac-
phenyl ether solvothermal method has been used for the tion temperature (200 °C), FeF2 was obtained together with
first time to control the assembly of urchin-like FeF2 a considerable amount of iron hydroxide as an impurity
nanoarchitectures comprising 1D FeF2 nanorods. Solid- phase (Figure 1, a).
state thermal annealing of the FeF2 nanoarchitectures read-
ily gave mesoporous α-Fe2O3 nanorod-assembled urchin-
like nanoarchitectures, the α-Fe2O3 porous nanorods grow-
ing radially from the center of the nanoarchitectures. In
methane conversion reactions, these mesoporous urchin-
like α-Fe2O3 nanoarchitectures showed excellent catalytic
activity in the conversion of methane into carbon dioxide
at a low temperature (230 °C).
Results and Discussion
Urchin-Like FeF2 Nanoarchitectures
It is well known that the morphologies and fascinating
properties of nanoarchitectures are actually an outward em-
bodiment of their internal solid-state structures and local
interactions or bonding between the constitutive compo-
Figure 1. Power XRD pattern of FeF2 prepared at a) 200, b) 250,
and c) 280 °C.
nents themselves, which are very sensitive to reaction pa-
The SEM images of FeF2 prepared in the ionic liquid
rameters such as the polarity of the solution, concentrations ([Omim]BF4)/diphenyl ether system at different tempera-
of the reactants and foreign ions, temperature, pressure, sur- tures reveal different morphologies (Figure 2). FeF2 ob-
factant, and precursor. Inspired by their high sensitivity to tained at 200 °C exhibits flower-like nanostructures as-
the synthetic environment, the morphology and crystallinity sembled from shorter nanorods accompanied by an impu-
of these nanostructured materials together with the special rity phase of irregular nanoparticles of iron hydroxide (Fig-
coupling or synergistic properties resulting from the com- ure 2, a), which is in good agreement with the XRD results.
bined effect of the ordered building blocks can be con- The as-prepared FeF2 at 250 °C shows 3D urchin-like
trolled by tailoring these parameters or designing alterna- nanoarchitectures consisting of nanorods that grow radially
tive synthetic routes. In our investigation, the ionic liquid from the center of the aggregate (Figure 2, b), with the
([Omim]BF4)/diphenyl ether mixture was utilized to synthe- nanorods of FeF2 showing diameters of about 50 nm and
size novel urchin-like FeF2 nanoarchitectures. In this syn- lengths of up to 570 nm. In addition, it is observed that the
thesis, iron(III) acetylacetonate as an iron source was dis- sizes of the urchin-like nanoarchitectures range from 2 to
solved in the solvent mixture comprising the ionic liquid 10 μm (Figure 2). The corresponding SAED image of the
([Omim]BF4) as a fluorine source and diphenyl ether. The nanorods reveals a very good crystalline nature, with the
experimental results indicate that both the ionic liquid and diffraction stripes or points corresponding to the (110),
diphenyl ether were necessary for the formation of urchin- (101), (111), and (211) planes agreeing well with calcula-
like FeF2 nanoarchitectures. The sole use of diphenyl ether tions of the tetragonal phase of FeF2 (Figure 2, e). This is
during the thermal decomposition of iron(III) acetylacet- the first time that such 3D urchin-like FeF2 nanoarchitec-
onate resulted in the product Fe3O4 with an irregular par- tures assembled from nanorods have been reported. At a
ticle morphology (see Figure S1a and Figure S2a in the reaction temperature of 280 °C, the morphology of the
Supporting Information), whereas the use of the ionic li- urchin-like FeF2 nanoarchitectures remained unchanged, as
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shown in Figure 2c. However, at a higher reaction tempera- the thermal stability and phase behavior of the ionic liquid/
ture, the length of the nanorods, as the building blocks of diphenyl ether mixture system were studied systematically
the urchin-like FeF2 nanoarchitectures, decreased, which at different temperatures. The results are shown in Figure 3.
led to a decrease in size of the urchin-like FeF2 nanoarchi- They indicate a strong temperature dependence of the sta-
tectures. As the reaction temperature was raised further, the bility and phase behavior of the ionic liquid/diphenyl ether
nanorods grew along the flank, which resulted in the forma- system. Below 200 °C, the ionic liquid and diphenyl ether
tion of more complicated aggregates of the urchin-like FeF2 are stable and almost insoluble in each other (Figure 3, a).
nanoarchitectures (Figure 2, d). Thus, the reaction tempera- At more elevated temperatures (Ն250 °C), the solubility of
ture not only affects the composition and purity of the diphenyl ether in the ionic liquid increased and the ionic
product, but also plays an important role in the mor- liquid phase turned brown, which implies that some ionic
phology of FeF2.
liquid had decomposed (Figure 3, b,c). At the same time,
under vigorous stirring the ionic liquid and diphenyl ether
mixture became a macro-emulsion-like system in the tem-
perature range 200–280 °C (Figure 3, a–c). Secondly, IR
(Figure 4) and TG (Figure 5) analyses of the as-prepared
FeF2 were carried out in air. The TG analysis revealed a
weight loss of about 6 wt.-% between 250 and 400 °C, which
suggests that organic compounds or carbonaceous materi-
als may exist in the as-prepared FeF2. The bands at 2920,
2840, 1380, 3100, and 1160 cm–1 in the IR spectra of the
as-prepared FeF2 clearly show that the [Omim]BF4 ionic
liquid was adsorbed onto the surface of the as-prepared
FeF2. In addition, the solubility of iron(III) acetylacetonate
in the ionic liquid and diphenyl ether and their miscibility
were investigated. The results illustrate that iron(III) acetyl-
acetonate is less soluble in the ionic liquid than in diphenyl
ether and that the solubility of diphenyl ether in the ionic
liquid is greater than that of the ionic liquid in diphenyl
ether, all of which increase with increasing temperature.
Figure 2. SEM images of FeF2 prepared at a) 200, b) 250, c) 280,
and d) 300 °C, and e) the corresponding SAED pattern of the FeF2
prepared at 250 °C.
The volume ratio of the ionic liquid ([Omim]BF4) to di-
phenyl ether and stirring rate also affected the morphology
and composition of the products. If the volume ratio of the
ionic liquid ([Omim]BF4) to diphenyl ether was less than
0.1, the product was a mixture of FeF2 nanorods and iron
oxide. The pure urchin-like FeF2 could only be obtained
when the volume ratio of ([Omim]BF4) to diphenyl ether
was between 0.1 and 10 (see Figure S1c in the Supporting
Figure 3. Incompletely mutually soluble ([Omim]BF4)/diphenyl
ether system at different temperatures.
In the light of the experimental results and above dis-
Information). Our research results attest to the fact that the cussion, we believe that the novel urchin-like FeF2 nano-
stirring rate had to be high enough to form macro-emul- architectures form in the ionic liquid domains in the ionic
sions from the ionic liquid ([Omim]BF4) and diphenyl ether liquid/diphenyl ether macro-emulsions containing iron(III)
system,[26] otherwise the pure and perfect urchin-like FeF2 acetylacetonate, which acts as the fluorine and iron source,
nanoarchitectures could not be obtained under any experi- respectively.[27] The growth of the urchin-like FeF2 nano-
mental conditions (see Figure S3).
architectures occurs in three stages. In the initial stage, the
To explore why the ionic liquid/diphenyl ether mixture decomposition of the ionic liquid releases the fluoride ions
system can lead to a novel morphology of FeF2 and how as a result of a small amount of water in the ionic liquid
the conditions of synthesis may influence the morphology, and the presence of the transition-metal ion;[28] the fluoride
we performed a series of comparative experiments. First, ions then react with iron(III) acetylacetonate to form iron
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experimental results have shown that a reaction tempera-
ture of about 250 °C is beneficial for the preparation of the
novel urchin-like FeF2 nanoarchitectures (Figure 2, b). At
this temperature, the rates of the chemical reactions and
reactant diffusion between the different domains provide
sufficient iron and fluorine sources for FeF2 nucleation and
surface growth of the FeF2 nanoparticle aggregates to give
novel urchin-like FeF2 nanoarchitectures with a perfect
surface morphology.
In addition to the roles mentioned above, the ionic liquid
([Omim]BF4) plays a significant role in the shape-controlled
synthesis of the novel urchin-like FeF2 nanoarchitectures.
In view of the mechanism proposed by other authors,[32] we
can regard the ionic liquid ([Omim]BF4) as a shape-regulat-
ing surfactant or capping agent during the growth of the
FeF2 nanorods due to their preferential adsorption onto
particular crystal facets while also retaining their ability to
coordinate with metal ions, as identified by the IR and TG
analyses (see Figures 4 and 5), thereby leading to the pre-
ferred growth orientation of the ionic liquid/capped nuclei.
As such, in this case, the ionic liquid ([Omim]BF4) was used
not only as a solvent and fluorine source (precursor), but
also, more importantly, as an all-in-one solvent/precursor/
capping agent (or shape-regulating surfactant).
Figure 4. IR spectra of a) the [Omim]BF4 IL and b) the as-prepared
FeF2.
The XPS spectra (Figure 6) and PXRD patterns of the
products obtained in all the above reactions involving
iron(III) acetylacetonate also show the formation of FeF2
in the ionic liquid/diphenyl ether macro-emulsions, based
on a comparison with PDF-45-1062 from the ICDD
Figure 5. TG analysis of the as-prepared FeF2 at 250 °C.
fluoride. In the second stage, fast nucleation[29] and aggre- (tetragonal FeF2). The XPS and XRD patterns clearly show
gation dominate, with the higher concentration of the reac- that the Fe3+ was reduced to Fe2+ during the formation of
tive species (fluorine and iron ions) providing the opportu- the FeF2 nanoarchitectures. This phenomenon is similar to
nity to induce fast nucleation and create nanoparticle aggre- the reduction of Fe(NO3)3 in the ionic liquid solvent
gates as the cores of the urchin-like FeF2 nanoarchitectures. BMIBF4 under microwave irradiation to produce crys-
In the final stage, slow nucleation, Ostwald ripening, and talline, aggregated bar-shaped FeF2.[28] This reaction occurs
fast autocatalytic surface growth[30,31] dominate. During because the ionic liquid effectively acts as a reducing
this stage, iron(III) acetylacetonate in the diphenyl ether do- agent.[28a] On the basis of the discussions above, the novel
mains can continuously enter into the ionic liquid domains urchin-like FeF2 nanoarchitectures grow as illustrated in
by concentration diffusion resulting from the fast nucle- Scheme 1.
ation thereby satisfying the demand for iron for surface
growth of the FeF2 nanoparticle aggregates. At lower reac-
tion temperatures (Յ200 °C), the low rate of ionic liquid
decomposition and the lower concentration of iron(III)
acetylacetonate led to no obvious difference in the rates of
nucleation and surface growth. The features of the second
and third stages cannot be sufficiently realized, which re-
sults in the formation of FeF2 flower-like nanoarchitectures
and the appearance of Fe3O4 nanoparticles arising from de-
composition of iron(III) acetylacetonate in the diphenyl
ether domains. At higher reaction temperatures (Ն280 °C),
the chemical reactions {[Omim]BF4 IL decomposition and
the reaction of fluoride ion with iron(III) acetylacetonate}
occur more rapidly, leading to faster nucleation and con-
sumption of the majority of iron(III) acetylacetonate, which
gives rise to the formation of smaller urchin-like FeF2
nanoarchitectures due to an insufficient supply of iron(III)
acetylacetonate as the iron source for the surface growth
of the FeF2 nanoparticle aggregates in the last stage. Our cined at 500 °C for 4 h.
Figure 6. XPS spectra of a) the as-prepared FeF2 and b) FeF2 cal-
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Scheme 1. Schematic illustration for the growth process of urchin-like FeF2 nanoarchitectures.
3D Urchin-Like α-Fe2O3 Nanoarchitectures
at 800 °C are in good agreement with those of bulk α-Fe2O3
(see Figure S4d). Typical SEM and TEM images (Figure 7,
d,e) reveal that the urchin-like nanoarchitectures of FeF2
have collapsed and converted into larger nanoparticles of
α-Fe2O3 with no porosity as a result of further crystalli-
zation after annealing at 800 °C. The SAED image of this
sample shows a regular spot pattern (Figure 7, f), which
confirms the single-crystalline nature of the α-Fe2O3 par-
ticles. In comparison with the SAED image of the urchin-
like α-Fe2O3 nanoarchitectures (Figure 7, c) obtained from
FeF2 by annealing in air at 500 °C for 4 h, the results ob-
tained at 800 °C strongly indicate that annealing at high
temperature improves the crystallinity and morphology of
the haematite α-Fe2O3 nanoarchitectures.
The method used here for the preparation of the urchin-
like α-Fe2O3 is different to the methods described in the
literature.[33,34] In this study, the urchin-like α-Fe2O3
nanoarchitectures were prepared in two steps. First, the ur-
chin-like FeF2 nanoarchitectures were obtained by iono-
thermal treatment and then transformed into the α-Fe2O3
phase with mesoporous morphologies by thermal annealing
without significantly altering the skeleton construction.
This conversion is thermodynamically favorable because the
α-Fe2O3 phase is very stable in air at high temperature, and
the conversion reaction generates volatile products further
driving the reactive equilibrium to the right-hand side.
Furthermore, the nanoscale dimensions of FeF2 nanorod
aggregated structures reduce the kinetic energy barrier for
the diffusion of atoms and ions. Such intrinsic properties
facilitated the complete transformation of FeF2 into α-
Fe2O3. However, the morphology and crystallinity of the α-
Fe2O3 nanostructures depend on the annealing tempera-
ture.
The PXRD patterns (see Figure S4 in the Supporting In-
formation) of the samples obtained by annealing FeF2 show
that the FeF2 in urchin-like FeF2 nanoarchitectures were
converted into rhombohedral α-Fe2O3 (haematite, JCPDS
¯
no. 33-0664, space group R3c, a = 5.036 Å, c = 13.75 Å) at
500 °C for 4 h in air with no other crystalline impurity
phases observed. This result is in good agreement with the
remarkable weight loss (16 wt.-%) observed in the TG
analysis of FeF2 between about 410 and 800 °C (Figure 5).
The SEM and TEM images (Figure 7, a,b) and XPS spectra
of the calcined FeF2 reveal that the 3D urchin-like mor-
phology of the FeF2 nanoarchitectures is well preserved in
the thermally annealed products composing hematite α-
Fe2O3, without fluoride, at 500 °C. At the same time, abun-
dant mesopores can be observed in the nanorods of the
obtained urchin-like α-Fe2O3 nanoarchitectures (Figure 7,
b), which possess a higher BET surface area (48.6 m2 g–1)
than the urchin-like FeF2 nanoarchitectures (29 m2 g–1) and
a sharp pore-size distribution centered at 3.4 and 7.5 nm
determined from nitrogen adsorption/desorption measure-
ments (see Figure S5). On increasing the annealing tem-
perature, the crystallinity of the obtained haematite α-
Fe2O3 increases (see Figure S4), thereby leading to lower
BET surface areas (Table 1), which implies that a change of
Figure 7. SEM images of FeF2 calcined at a) 500 °C for 4 h and
d) 800 °C for 2 h. TEM images of FeF2 calcined at b) 500 °C for
4 h and e) 800 °C for 2 h. SAED images of α-Fe2O3 prepared at
c) 500 and f) 800 °C, respectively.
It is worth noting that the thermal annealing introduces
morphology occurs during thermal annealing. The sharp porosity into the α-Fe2O3 nanorods in the urchin-like
diffraction peaks in the PXRD pattern of samples annealed nanoarchitectures (Figure 7, b), which is probably due to
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Table 1. Physical properties of the prepared α-Fe2O3.
nanoarchitectures was studied under the same conditions.
The experimental results are shown in Figure 8 and show a
surprising phenomenon, that is, methane was converted
into products containing C–O bonds (CO or CO2) at
230 °C, which is 190 °C lower than over bulk α-Fe2O3. Such
a low temperature for the activation of methane is very rare.
The lowest temperature at which products containing
C–C bonds were formed over the porous urchin-like α-
Fe2O3 nanoarchitectures was 600 °C, 150 °C lower than
over bulk α-Fe2O3.
Sample
BET Surface Average pore Pore volume
area [m2 g–1]
size [nm]
[cm3 g–1]
α-Fe2O3-400 °C
α-Fe2O3-500 °C
α-Fe2O3-800 °C
55.1
48.6
3.2
3.4
3.4, 7.5
–
0.29
0.26
–
the decomposition of FeF2 and crystallization of α-Fe2O3
as well as the release of a large quantity of gases (fluorine)
during the annealing process.[20,35] Simultaneously, an in-
crease in the crystal density caused by the conversion of
FeF2 into α-Fe2O3 in the solid-state reaction [ρ(FeF2) =
4.1 gcm–3, ρ(α-Fe2O3) = 5.2 gcm–1] can also promote the
production of porosity.[36,37]
It is interesting that the prepared urchin-like α-Fe2O3
nanoarchitectures exhibit more mesopores that the porous
α-Fe2O3 nanowires obtained from α-FeF3·3H2O nanowires
by thermal treatment in air,[36] that is, an improved pore
morphology can be seen in these urchin-like α-Fe2O3
nanoarchitectures. A lot of the pores are open to the outer
surface, isolated from each other, and randomly distributed
in the nanorods, with the pore walls formed from the sinter
of α-Fe2O3 nanoparticles and not their agglomeration. This
type of pore structure is similar to that of a sponge and
shows clear differences to that in multiple polycrystalline
aggregates with void spaces between the grains.[36]
Methane Activation Over Urchin-Like α-Fe2O3
Nanoarchitectures
C–H bond activation in methane under environmentally
benign and economically feasible conditions has been an
important and interesting challenge.[38] Haematite is an ex-
tremely attractive candidate for catalysis applications be-
cause it is the most thermodynamically stable phase of iron
Figure 8. a) Methane conversion and b) selectivity of the products
formed over porous urchin-like α-Fe2O3 nanoarchitectures at 3:1
methane to oxygen molar ratio at different reaction temperatures.
To better understand the mechanisms responsible for the
oxide and is environmentally friendly, nontoxic, and exhib- high catalytic activity of the porous urchin-like α-Fe2O3
its tunable catalytic activity depending on the morphology nanoarchitectures for the conversion of methane at low
of the material.[39,40] To search for a novel, highly efficient, temperature, XPS was used to investigate the chemical
and benign catalyst, and to explore the catalytic behavior of environment and electronic structures of the elements at the
the prepared urchin-like α-Fe2O3 nanoarchitectures in the surface of the catalysts. Figure 9 (a,b) shows the Fe2p spec-
activation of the C–H bond in methane, we have designed tra of the porous urchin-like α-Fe2O3 nanoarchitectures and
and achieved the catalytic conversion of methane over α- bulk α-Fe2O3. The Fe2p spectrum of the former shows the
Fe2O3 catalysts with different morphologies. In this investi- structure typical of iron oxides with broad main peaks
gation, for the sake of simplicity and convenience, the tem- (Fe2p3/2 and Fe2p1/2) and very weak shake-up satellites. The
perature of formation of products containing C–O or C–C electron-binding energy of Fe2p3/2 is 710.4 eV, which corre-
bonds was used as the criterion of catalytic activity because sponds to trivalent Fe and is less than the corresponding
it is believed that the destruction of C–H bonds and con- peak of bulk α-Fe2O3 (710.9 eV). The shift in the binding
struction of C–O or C–C bonds may occur at the same energy of Fe2p3/2 confirms that there must be a strong elec-
temperature in the conversion of methane. The lower the tronic interaction in the materials studied. The XPS spectra
temperature is, the higher the catalytic activity of the cata- of O1s of the porous urchin-like α-Fe2O3 nanoarchitectures
lyst. With bulk α-Fe2O3 as the catalyst, products containing and bulk α-Fe2O3 (Figure 9, c,d) show an intense main peak
C–O bonds (CO or CO2) were observed when the reaction with a shoulder on the high binding energy side that can be
temperature reached 420 °C and the lowest temperature for fitted by three components with binding energies of 529.4–
the formation of products containing C–C bonds (ethylene 529.6, 531.1–531.2, and 533.3–533.4 eV, which have been
or ethane) was 750 °C in a system containing CH4 and O2 assigned to lattice oxygen (O2–latt) species, adsorbed oxygen
–
in a ratio of 3:1. For comparison, the catalytic conversion (O–, O2 , or O22–) species associated with oxygen vacan-
of methane over the prepared urchin-like porous α-Fe2O3 cies,[41,42] and physisorbed H2O at the surface, respectively.
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Note that the content of adsorbed oxygen on the porous on these results and those of other studies, we believe that
urchin-like α-Fe2O3 nanoarchitectures was 6.4% higher the high catalytic activity of the porous urchin-like α-Fe2O3
than that on bulk α-Fe2O3 (Figure 9). This implies that the nanoarchitectures for the conversion of methane at low
prepared porous urchin-like α-Fe2O3 nanoarchitectures temperature can be ascribed to their larger surface area and
have a higher density of surface oxygen vacancies.[42] Based higher oxygen vacancy density, which can accelerate the
dissociation of oxygen molecules at the surface and increase
the mobility of lattice oxygen.[43]
Conclusions
Three-dimensional mesoporous urchin-like α-Fe2O3
nanoarchitectures have been successfully fabricated by a
novel FeF2 to α-Fe2O3 conversion. The morphology of the
3D urchin-like FeF2 nanoarchitectures is preserved in the
α-Fe2O3 materials after undergoing solid-state thermal an-
nealing. At the same time, special slit-shaped meso-struc-
tured and hierarchical porosity are also introduced into the
α-Fe2O3 nanorod building blocks of urchin-like α-Fe2O3
nanoarchitectures by controlling the pyrolysis temperature
of the FeF2 nanoarchitectures. The synthesis of 3D urchin-
like FeF2 nanoarchitectures was the key to obtaining 3D
urchin-like α-Fe2O3 nanoarchitectures. An unconventional,
nonhomogeneous ionic liquid/diphenyl ether system played
an important role in the formation of the 3D urchin-like
FeF2 nanoarchitectures.
The urchin-like α-Fe2O3 nanoarchitectures composed of
1D porous nanorods exhibited unique catalytic properties
in methane activation; methane was activated over α-Fe2O3
and converted into carbon dioxide at 230 °C and C–C com-
pounds at 600 °C. The results of our investigation indicate
that the high catalytic activity of the porous urchin-like α-
Fe2O3 nanoarchitectures for the conversion of methane at
low temperature may result from their larger surface area
and higher oxygen vacancy density. Thus, the mesoporous
urchin-like α-Fe2O3 nanoarchitectures prepared by the con-
version of FeF2 nanoarchitectures show excellent potential
as catalysts or catalyst supports for methane conversion.
Experimental Section
Preparation of Mesoporous Urchin-Like α-Fe2O3 Nanoarchitectures:
All reagents were of analytical grade and used without any further
purification. The ionic liquid ([Omim]BF4) was synthesized accord-
ing to the reported method,[44] and the freshly prepared ionic liquid
contained about 5 wt.-% water, as determined by TG analysis. 3D
urchin-like α-Fe2O3 nanoarchitectures were prepared by solid-state
annealing of FeF2 nanoarchitectures. The typical synthesis of
urchin-like FeF2 nanoarchitectures is as follows: Iron(III) acetyl-
acetonate (1.0 g) was added to mixtures of the [Omim]BF4 ionic
liquid (10 mL) and diphenyl ether (40 mL) in a 200 mL high-pres-
sure reactor equipped with mechanical stirring. Nitrogen purging
for 20–40 min at a flow rate of 25 mLs–1 was utilized to eliminate
oxygen from the reactor, and then the pressure of nitrogen within
the sealed reactor was increased to 0.5 MPa. After that, the reac-
tion mixture was heated at 250 °C under mechanical stirring
(1000 rpm) for 10 h. To separate FeF2, ethanol (50 mL) was added
to the cooled reaction mixture. The FeF2 product was collected by
centrifuging and washing with ethanol to remove the solvent. For
the preparation of porous urchin-like α-Fe2O3 nanoarchitectures,
Figure 9. Fe2p XPS spectra of a) porous urchin-like α-Fe2O3
nanoarchitectures and b) bulk α-Fe2O3. O1s XPS spectra of c) po-
rous urchin-like α-Fe2O3 nanoarchitectures and d) bulk α-Fe2O3.
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Received: March 7, 2014
Published Online: ᭿
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3D Nanoarchitectures
B. Dong, H. Zhang, A. Kong,* Y. Kong,
F. Yang, Y. Shan* ........................... 1–10
Synthesis of Urchin-Like FeF2 Nanoarchi-
tectures and Their Conversion into Three-
Dimensional Urchin-Like Mesoporous α-
Fe2O3 Nanoarchitectures for Methane Ac-
tivation
Novel urchin-like FeF2 nanoarchitectures
have been fabricated by an unconventional
nonhomogeneous ionic liquid/diphenyl
ether solvothermal method and converted
into 3D urchin-like mesoporous α-Fe2O3
nanoarchitectures by solid-state thermal
annealing. The α-Fe2O3 nanoarchitectures
exhibit good catalytic properties in meth-
ane activation reactions.
Keywords: Mesoporous materials / Nanos-
tructures / C–H activation / Iron / Fluor-
ine / Ionic liquids
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