Structure sensitivity of oxidative dehydrogenation of cyclohexane over FeOx and Au/Fe3O4 nanocrystals

Article abstract of DOI:10.1021/cs3007582

Shape-controlled nanoscale FeO_x and Au/Fe₃O ₄ catalysts with an inverse spinel structure were prepared and tested for the oxidative dehydrogenation of cyclohexane. The reaction was studied in situ in SAXS/TPRx mode with isothermal steady-state holds. {111}-Bound Fe ₃O₄ nanooctahedra are highly stable under reaction conditions at 300 C, but {100}-bound nanocubes begin to agglomerate above 250 C. The selectivity to cyclohexene and benzene over CO₂ depends strongly on the iron oxide shape and its interaction with the gold. When gold is added onto the iron oxide, the formation of benzene over cyclohexene is favored over both shapes. The highest benzene yield was measured on the Au/Fe ₃O₄ octahedra. A parallel study of a commercial polycrystalline Au/Fe₂O₃ powder was conducted, and the activity and selectivity of this catalyst were compared with the nanoshapes. After leaching of the gold with a sodium cyanide solution and heat treatment, the atomically dispersed gold on the iron oxide surface selectively catalyzed the ODH of cyclohexane to benzene. Stabilization of cationic gold was found by in situ XANES conducted during the ODH reaction.

Full text of DOI:10.1021/cs3007582

Research Article
pubs.acs.org/acscatalysis
Structure Sensitivity of Oxidative Dehydrogenation of Cyclohexane
over FeOx and Au/Fe3O4 Nanocrystals
S. Goergen,†,∥ C. Yin,‡,∥ M. Yang,† B. Lee,§ S. Lee,§ C. Wang,† P. Wu,† M. B. Boucher,† G. Kwon,‡
S. Seifert,§ R. E. Winans,§ S. Vajda,*,‡,⊥,¶ and M. Flytzani-Stephanopoulos*,†
†Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States
§
⊥
‡Materials Science Division, X-ray Science Division, Nanoscience and Technology Division, Argonne National Laboratory, 9700
South Cass Avenue, Argonne, Illinois 60439, United States
¶Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520,
United States
ABSTRACT: Shape-controlled nanoscale FeOx and Au/Fe3O4 catalysts with
an inverse spinel structure were prepared and tested for the oxidative
dehydrogenation of cyclohexane. The reaction was studied in situ in SAXS/
TPRx mode with isothermal steady-state holds. {111}-Bound Fe3O4
nanooctahedra are highly stable under reaction conditions at 300 °C, but
{100}-bound nanocubes begin to agglomerate above 250 °C. The selectivity
to cyclohexene and benzene over CO2 depends strongly on the iron oxide
shape and its interaction with the gold. When gold is added onto the iron
oxide, the formation of benzene over cyclohexene is favored over both
shapes. The highest benzene yield was measured on the Au/Fe3O4 octahedra.
A parallel study of a commercial polycrystalline Au/Fe2O3 powder was
conducted, and the activity and selectivity of this catalyst were compared with
the nanoshapes. After leaching of the gold with a sodium cyanide solution and heat treatment, the atomically dispersed gold on
the iron oxide surface selectively catalyzed the ODH of cyclohexane to benzene. Stabilization of cationic gold was found by in situ
XANES conducted during the ODH reaction.
KEYWORDS: gold, iron oxide, oxidative dehydrogenation, cyclohexane, shape effect, benzene, structure selectivity
Platinum- and vanadia-based catalysts are well studied and
among the most efficient for cyclohexane dehydrogenation.5−8
1. INTRODUCTION
A
The dehydrogenation of cyclohexane to cyclohexene and
benzene is an important process for the production of chemicals
and polymers. The heterogeneously catalyzed reaction is
typically carried out at high temperatures in anaerobic con-
ditions. A stepwise removal of hydrogen leads to the formation of
cyclohexene and eventually to benzene. The breakage of C−H
bonds is the rate-limiting step of the dehydrogenation reaction
and competes with the scission of C−C and CC bonds. The
secondary production of hydrocarbon fragments and coke
causes deactivation of the catalyst. By adding hydrogen to the
reactant stream, the hydrocarbon cracking can be largely
suppressed.1,2
In recent years, oxidative dehydrogenation has been
considered as a promising alternative to current industrial pro-
cesses. Especially in the case of ethane and propane, considerable
progress has been made.3 The oxidative pathway is thermody-
namically favored and proceeds at much lower temperatures. In
the presence of oxygen, hydrogen is converted to water, and
hydrocarbon fragments are recovered in the form of carbon
oxides. Combustion largely prevents coke formation and, thus,
catalyst deactivation.4 Despite a growing research effort,
however, catalyst design remains an important challenge to
improve dehydrogenation activity and selectivity.
number of other materials include metal-doped zeolites and
metal phosphates.9−11 In recent years, gold nanoparticles
supported on metal oxides have been shown to be attractive
catalysts in a number of selective oxidation and hydrogenation
reactions.12,13 Their potential for oxidative dehydrogenation was
pointed out by Bond as early as 1972.14 More recently, the
Hutchings group has reported that metal oxide-supported Au−
Pd alloys are highly selective catalysts for the formation of
benzene from cyclohexane and cyclohexene,15 and Vajda and co-
workers reported nanometer and subnanometer cobalt oxide
catalysts for the same reaction.16−18 Gold supported on various
oxide supports, including iron oxide, has also been studied for the
oxidative dehydrogenation of ethane, propane, 1-butene and
α,ω-diols.19−21
Today, only little is understood regarding the relationship
between the gold−metal oxide structures and their dehydrogen-
ation activity. The addition of gold is generally observed to
increase the activity and selectivity of metal oxides. The nature of
Received: November 23, 2012
Revised: January 31, 2013
© XXXX American Chemical Society
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the underlying support, however, plays a crucial role in the
catalyst performance. Metal oxide supports with high oxygen
mobility in conjunction with closely contacted gold nano-
particles were found to favor the dehydrogenation reaction.21 Sa
et al. recently correlated the high dehydrogenation activity of
gold−lanthanum oxide with the ability of lanthanum oxide to
stabilize cationic gold at high temperatures.20 In contrast to the
anaerobic reaction, in which metallic clusters are believed to be
the active sites,10,11,22 cationic gold has been related to high
alkene selectivity with low carbon oxide formation during the
oxidative dehydrogenation of C2+ aliphatic alkanes.20 Similarly,
Vajda et al. reported an extraordinary activity of size-selected
Pt8−10 clusters deposited on aluminum oxide surfaces for the
oxidative dehydrogenation of propane. In comparison with
conventional Pt and vanadia catalysts, the under-coordinated
atoms in subnanometer Pt clusters were shown to favor C−H
scission over the breakage of C−C and CC bonds.23
To remove the organic surfactants from the iron oxide surface,
the final samples were suspended in a 10% aqueous
tetramethylammonium hydroxide solution. The aqueous iron
oxide suspension was purified by sequential precipitation and
suspension in distilled water. A small amount of sodium
hydroxide was added to facilitate recovering the iron oxide
nanoparticles by centrifugation.
Gold (2 atom % per total Fe atom) was deposited on the iron
oxide nanoparticles by deposition−precipitation in the presence
of sodium hydroxide (DP-NaOH). To 100 mL of an aqueous
suspension of iron oxide (4 mg mL−1), 45 mg of HAuCl4·3H2O
in 5 mL of distilled water was added dropwise to 100 mL of an
aqueous suspension of iron oxide (4 mg mL−1). The pH of the
suspension was adjusted to 8−9 by addition of sodium
hydroxide. The final sample was sonicated for 1 h, washed with
hot water (70 °C) and dried in vacuum at 80 °C overnight.
2.2. Catalyst Characterization and Activity Tests.
Transmission electron microscopy (TEM) was performed on a
JEOL 200cx at 200 kV. For high-resolution imaging, a JEOL
2010 was used. Powder samples were suspended in ethanol via
sonication, and a single drop of the corresponding suspension
was applied to a carbon-film-coated copper grid. The average
nanoparticle size was estimated from ∼100 nanoparticles. X-ray
Atomically dispersed gold species, strongly interacting with
the metal oxide support, have been the focus for a number of
other reactions.24 Comparing parent and NaCN-leached gold-
based catalysts, Fu et al. have discovered that atomically
dispersed cationic gold species stabilized by ceria are the active
sites for the water-gas shift reaction.25 Similar observations have
been made in the case of gold/iron oxide catalysts, and the
presence of a preponderance of isolated gold atoms has been
verified by atomic resolution STEM.26,27 In a first-of-a-kind
catalytic study carried out with gold deposited on ceria single
crystal nanoshapes, Si and Flytzani-Stephanopoulos have shown
that the {110} surfaces of ceria provide more anchoring sites for
atomically dispersing the gold and, hence, a better overall activity
for the water-gas shift reaction.28 The shape effect of ceria was
demonstrated to be indirect, as the apparent activation energies
of the reaction are the same for all shapes. It has further been
shown that atomically dispersed gold with strong Au−O-M
bonds forms only on CeO2{110}/{111}, ZnO{1000}, and
Fe3O4{111}surfaces, thus resulting in enhanced water-gas shift
and methanol steam reforming reaction rates on these
surfaces.28,29
In the present study, we apply a similar strategy to identify
potential shape effects of gold/iron oxide catalysts for the
oxidative cyclohexane dehydrogenation reaction. Nanoscale
single crystal iron oxide supports with cubic and octahedral
shapes, bound by {100} and {111} surfaces, respectively, were
prepared and used as supports. Scanning tunneling microscopy
and theoretical studies showing that positively charged gold
atoms can be stabilized on Fe3O4 (111),30 and Fe3O4 (100)
surfaces31 have already been reported. Shape-controlled catalysts
are compared with a commercial, bulk Au/Fe2O3 catalyst
obtained from the World Gold Council (WGC). Shape, size,
and oxidation state of the catalysts were monitored in situ by
SAXS and XANES.
2. EXPERIMENTAL SECTION
2.1. Catalyst Preparation. Shape-controlled FeOx and Au/
Fe3O4 catalysts were prepared as reported earlier.29 Octahedral
and cubic FeOx nanocrystals were synthesized by thermal
decomposition of iron oleate. Nanooctahedra were formed in the
presence of tetraoctylammonium bromide, and sodium oleate-
mediated preparations yielded nanocubes. The synthesis was
carried out in a 9:1 mixture of 1-octadecene and diphenylether at
a reflux temperature of 300 °C and applying a heating rate of 3 °C
min−1. Products were recovered and washed by sequential
precipitation and resuspension in a 1:1 hexane/ethanol mixture.
Figure 1. TEM images of as-synthesized catalysts: (a, c) FeOx
nanooctahedra, (b, d) FeOx nanocubes, (e) Au/Fe3O4 nanooctahedra,
and (f) Au/FeOx nanocubes.
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diffraction (XRD) analysis was performed on a Rigaku 300
instrument with a rotating anode generator and a mono-
chromatic detector. Cu Kα1 radiation was used with a power
setting of 50 kV and 250 mA. Typically, a scan rate of 2° min−1
with a 0.02° step size was used. Bulk elemental analysis of the
samples was conducted by inductively coupled plasma optical
emission spectrometry (ICP-OES, Leeman Laboratories Inc.).
The BET surface area of the samples was determined by single-
point N2 adsorption/desorption cycles in a Micromeritics
AutoChem 2920. The samples were pretreated in helium at
200 °C for 30 min.
In situ small-angle X-ray scattering (SAXS) experiments were
conducted at the Advanced Photon Source at Argonne National
Laboratory at beamline 12-ID-B and 12-ID-C. One-side-polished
standard P-doped N-type silicon chips with a thickness of
300 μm were used as supports for the powder samples (MTI
Corporation). The silicon chips were first cleaned with aqua regia
(3:1 HCl/HNO3), rinsed with ethanol, and calcined at 400 °C
for 3 h. The iron oxide powders were suspended in ethanol
(3 mg/mL) and drop-coated on the pretreated silicon chips.
A loading of ∼10 mg/chip was achieved. After deposition,
the coated chip was dried at 80 °C in vacuum. The chip was
loaded into a specially designed reaction cell, sealed with Kapton
windows, and mounted on a computer-controlled goniome-
ter.32−37
Temperature-programmed oxidative cyclohexane dehydro-
genation was conducted with a 0.4% cyclohexane/4% O2 gas
mixture in helium. The reaction cell was operated in a continuous
flow mode at 800 Torr and at a flow rate of 30 mL min−1. The
temperature was increased from room temperature up to 300 °C
with a heating rate of 10 °C min−1. At 50 °C intervals, the
temperature was held for 20 min to allow for steady state to be
reached and SAXS data collection.
The gas composition at the cell outlet was analyzed using a
differentially pumped mass spectrometer (Pfeiffer Vacuum
Prisma Plus QMS 220 M2). An 12 keV X-ray beam with two-
dimensional Pilatus 2 M (at 12-ID-B) or Platinum mosaic CCD
(at 12-ID-C) detector was used for recording the SAXS images.
A grazing-incidence geometry with an incidence angle of 0.1° was
applied; however, information arising from the grazing-incidence
geometry was compromised by the thickness of the sample layer
on the silicon chip (>1 μm). Therefore, the collected data were
handled similarly to transmission SAXS experiments. The
2-dimensional X-ray images were analyzed by taking cuts in
the in-plane direction. To correlate the experimental line-cut data
with theory, the X-ray scattering intensity, I, of randomly
oriented particles was calculated as follows:
Table 1. Physicochemical Properties of FeOx and Au−FeOx
Samples
BET surface area
iron oxide
gold content
(m2 g−1, Δ 5 nanoparticle size (nm, (at. %, ICP,
The scattering intensity of polyhedral particles in a powder
sample is expressed by the following equation with use of the
local monodisperse approximation:38,39
sample
m2 g−1
)
TEM, Δ 2 nm)
Δ
0.2%)
FeOx octahedra
Au/FeOx octahedra
FeOx cubes
62
78
27
22
2.0
2.0
I(q) = I(0)(P(q) S(q) + nAu PAu(q))
(1)
103
83
12
Au/FeOx cubes
13
where q is the modulus of the scattering vector, q;39 I(0) is the
scaling factor, I(q → 0) = I(0); P(q) is the orientation averaged
form factor; and S(q) is the structure factor. When particles are
aggregated,
Au/Fe2O3 (WGC,
parent)
44
15−60
2.0 (3.1 by XPS)
Au/Fe2O3 (WGC,
NaCN-leached)
44
15−60
0.7 (0 by XPS)
Figure 2. XRD pattern of as-synthesized and calcined catalysts: (a, b) FeOx nanooctahedra, (c, d) Au/Fe3O4 nanooctahedra, (e, f) FeOx nanocubes,
(g, h) Au/FeOx nanocubes; (a, c, e, g) as-synthesized, (b, d, f, h) calcined at 200 °C, − − Fe3O4 (PDF #01-077-1545),  γ-Fe2O3 (PDF #01-089-5894),
* Au (PDF #04-004-5106).
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S(q) = cq−D + Sh(q; Rh, v )
f
(2)
f
The first term serves to model the aggregates with parameters c
and Df; where c is a scaling constant, and Df is the Porod power-
law exponent for the aggregate.40 The second term is the
structure factor of homogenously distributed particles. In this
work, it models the local structure in the aggregates. We used the
structure factor derived from the PY hard sphere potential, of
which parameters are the radius Rh and the volume fraction vf of
the hard sphere.38
P(q) of polydispersed polyhedra is calculated as follows:39
P(q) =
n(r)⟨|F(q, r)|2⟩ dr
∫
(3)
where n(r) is the size distribution function, which in this work is
the Schultz distribution function.38 |F(q, r)|2 is the form factor of
the polyhedral particle whose size parameter is r, and |F(0, r)|2 =
V(r)2, where V(r) is the volume of the particle. r is the distance
from the center of mass to a vertex of a polyhedron. The angled
bracket indicates the orientation average, details of which can be
found elsewhere.39 When there are Au particles on the
octahedron, the scattering function for them is added, as
shown in eq 1, where nAu is the number ratio of Au nanoparticles
per octahedron, and the Au nanoparticle is modeled as a
sphere with a polydispersity determined by the Schultz
distribution function, too.
P (q) =
n(r)|F(q, r)|2 dr
∫
Au
s
(4)
, where |FS(q, r)|2 is the form factor of a sphere with a radius r.
In situ Au and Fe XANES were employed to examine the
oxidation states of the catalysts under reaction conditions. The
spectra were collected using the beamline 12-BM of the
Advanced Photon Source at Argonne National Laboratory.
A 1 mg portion of the sample was loaded into a quartz capillary
(1 mm in o.d.). The sample was kept in place by adding quartz
wool at both ends. A gas mixture of 0.08% cyclohexane and 0.8%
oxygen in helium was used, and a total flow rate of 5 mL min−1
was applied. The capillary was heated by two electrical resistances
above and below the capillary, and the temperature was
controlled by a thermocouple inside the capillary.41 The samples
were treated at various temperatures for 1 h and characterized at
room temperature after cooling in the cyclohexane/oxygen gas
mixture. Au XANES spectra were taken in the fluorescence mode
with a 13-channel Ge detector. The detector was placed at 90°
with respect to the incident beam, and the capillary was placed at
45° to both the incident beam and the Ge detector. The X-ray
absorption edge energy at the Au LIII edge was calibrated by
assigning the 11919 eV energy to the first inflection point in the
XANES spectra of the metallic gold foil. A gold foil was used as
the reference for Au0 and Au(OH)3 for Au3+, respectively. Fe
XANES spectra were taken in the transmission mode. The Fe K-
edge was calibrated by assigning 7122 eV to the first inflection
point in the XANES spectra. Fe3O4 and γ-Fe2O3 samples were
used as reference. The reported XANES data are the averages of
three scans (lasting ∼11 min per scan). No changes were
detected between the first and the last scan. The data were
processed by the Athena program.42
Figure 3. Fe K-edge fluorescence XANES spectra collected during
oxidative dehydrogenation of cyclohexane with 0.08% cyclohexane/
0.8% O2 in helium. Fe2O3 and Fe3O4 are reference samples. The
percentages indicate the content of Fe3O4 in the samples, which is
obtained by linear combination using the Athena software.
of catalyst was mixed with 0.2 g of quartz sand. The catalyst was
pretreated for 1 h at 200 °C in air. A total flow rate of 70 mL min−1
was applied. The temperature was increased from room
temperature to 400 °C with a heating rate of 5 °C min−1. The
temperature was held for 1.5 h to reach steady-state conditions.
The gas composition at the reactor outlet was analyzed by a mass
spectrometer (MKS) and with a NDIR CO/CO2 gas analyzer.
3. RESULTS AND DISCUSSION
3.1. Characterization of Iron Oxide and Gold−iron
Oxide Nanoshapes. Single crystal metal oxide nanoparticles
with controlled shapes are excellent to investigate the relation-
ship between catalyst structure and activity at atmospheric
pressure. Shape-controlled and monodispersed iron oxide
nanoparticles with sizes smaller than 50 nm are currently best
synthesized by thermal decomposition of metal−organic
precursors.43 Thermolysis of iron oleate in the presence of
tetraoctylammonium bromide is known to stimulate the
preferential growth of nanooctahedra, and sodium oleate
mediates the crystallization of nanocubes.44 To use the iron
oxide shapes as a catalyst support, we applied a treatment in
aqueous tetramethylammonium hydroxide to remove the
organic surfactant from the nanoparticle surface.45 Final iron
Steady-state oxidative cyclohexane dehydrogenation experi-
ments were performed in a fixed-bed quartz microreactor at
atmospheric pressure. The gas composition of 0.4% cyclo-
hexane/4% oxygen gas mixture in helium was the same as for the
in situ reaction and SAXS tests described above. A 0.05 g portion
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Figure 4. Temperature-programmed oxidative cyclohexane dehydrogenation (0.4% cyclohexane/4% O2 in helium) with isothermal holds of 20 min
each: (a) FeOx octahedra, (b) Au/Fe3O4 octahedra, (c) FeOx cubes, (d) Au/FeOx cubes (data collected in the reaction cell with in situ SAXS analysis).
oxide samples are shown in Figure 1, and their physicochemical
properties are summarized in Table 1. The nanocrystals have a
specific surface area of 70−100 m2 g−1. Both the cubic and
octahedral nanoparticles were monodisperse, with average
particle sizes of 12 (cubes) and 25 (octahedra) nm (Table 1).
For nanooctahedra, the average particle size may vary from batch
to batch between 15 and 30 nm. In contrast to nanocubes, the
size-control of nanooctahedra was found to be sensitive to small
variations in the experimental conditions. A different batch was
used for gold deposition; hence, the difference in size for FeOx
and Au/FeOx nanooctahedra. A size variation between 15 and
30 nm was not found to impact the oxidative dehydrogenation
activity.
XRD patterns reveal that both iron oxide cubes and octahedra
are formed of a cubic inverse spinel phase (Figure 2). Variations
in diffraction intensities are primarily related to the amount of
Figure 5. Steady-state reaction rates of oxidative dehydrogenation of
cyclohexane: , FeOx octahedra; , FeOx cubes; , Au/Fe3O4
sample used for analysis. As can be seen in the high-resolution
images of Figure 1, nanooctahedra are bound by {111} surface
planes, and nanocubes are bound by {100} surface planes. The
black color and the magnetic properties of the octahedra samples
is in agreement with the Fe3O4 (magnetite) phase. A slight shift
of the diffraction peaks toward lower 2θ values and a brownish
hue of the cubic sample indicate the presence of a trace amount
of γ-Fe2O3. After calcination in air at 200 °C, both iron oxide
nanooctahedra and nanocubes have a red-brown color, and the
position of XRD peaks is in agreement with the presence of
γ-Fe2O3.
◊
⧫
□
■
●
○
octahedra; , Au/FeOx cubes; , Au/Fe2O3 WGC (parent); , Au/
Fe2O3 WGC (leached); ⊗, Au/Fe2O3 WGC (leached, reduced); +,
2.5% Au, 2.5% Pd−TiO2 (ref 15); ×, NiFe2O4 (ref 41). Data collected
from both the microreactor and the in situ SAXS reactor systems have
been considered.
The resolution of these conventional characterization
techniques makes it difficult to discriminate between the Fe3O4
and γ-Fe2O3 phases, which have very similar atomic plane
spacings. For this purpose, qualitative Fe K-edge XANES analysis
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Table 2. Activity of Metal Oxide Catalysts for the Oxidative Dehydrogenation of Cyclohexane
conversion (%)
O2/C6H12 molar
activation energy
(kJ mol−1
sample
ratio
250 °C 300 °C 350 °C 400 °C
)
ref
FeOx octahedra
10
2
1
2
3
2
7
6
18
22
16
38
45
35
17
45
56
45
75
87
89
61
66
73
64
61
60
67
80
3
7
4
3
6
3
3
this work
this work
this work
this work
this work
this work
this work
Au/Fe3O4 octahedra
FeOx cubes
10
10
10
10
10
10
6
Au/FeOx cubes
11
12
11
5
Au/Fe2O3 (WGC, parent)
Au/Fe2O3 (WGC, NaCN-leached)
Au/Fe2O3 (WGC, NaCN-leached, reduced in 5% H2 at
200 °C)
Co3O4, 12 nm cubes
NiFe2O4
10
54
84
59
98
16
45
45
15
47
11
11
11
11
11
11
11
2
NiCo2O4
2
2.5% Au, 2.5% Pd−TiO2
VOx/Al2O3
2.5
2
63−107
83−88
71−92
50
V2O5/SiO2
1−2
0.5−2
0.6
V2O5−Nb2O5/SiO2
vanadium phosphate
iron phosphate
cerium phosphate
H-ZSM-5
0.6
77
0.6
88
0.5−10
0.5−1
59−71
90−136
Co-ZSM-5
Table 3. Properties of FeOx and Au−FeOx Nanoparticles Obtained from SAXS Analyses
a
SAXS fitting parameters (Å2)
surface area
b
c
sample
temp (°C)
model
r1
Δ1
r2
Δ2
(Å2/particle)
(m2 g−1
)
FeOx octahedra
25
300
25
octahedron
octahedron
two-particle
two-particle
octahedron
cube
143
142
111
113
113
55
15
14
11
9
3.2·105
3.2·105
3.5·105
3.5·105
56
56
AuFe3O4 octahedra
FeOx cubes
5.0
5.0
0.2
0.2
124
124
300
300
25
9
6
1.0·105
8.8·104
147
114
300
400
25
cube
57
7
cube
60
7
Au/FeOx cubes
cube
64
8
1.2·105
1.1·105
107
110
300
400
cube
62
8
sphere
67
31
a
r = half edge length in the case of cubes and octahedra and the radius in the case of spheres. Δ = width of the size distribution. r1, Δ1: iron oxide. r2,
b
c
Δ2: gold. Calculated from the Porod law (eq 8) Calculated, assuming a particle density of 5·10−24 Å3 g−1.
was carried out at room temperature and at 225 °C in a
cyclohexane/oxygen atmosphere (Figure 3). The Fe K-edge
features originate from the transition of 1s electrons to empty 3d
and 4p orbitals. The small pre-edge peak is characteristic for 1s →
3d transitions for iron atoms in a tetrahedral environment. Since
1s → 3d transitions are symmetry-forbidden in the case of
octahedral coordinated iron, this feature is characteristic of the
spinel structure. The absorption maximum around 7133 eV has a
higher intensity in the case of Fe3O4 than for γ-Fe2O3, reflecting
the change in oxidation state.46,47 The absorption maxima of all
iron oxide samples are situated between the two reference
samples, indicating the presence of both Fe3O4 and γ-Fe2O3. The
ratio of Fe3O4 phase in each iron oxide and the gold/iron oxide
nanoshapes is estimated by the linear combination results of the
references through Athena software, and the Fe3O4 percentages
are shown in Figure 3 following each sample name.
not show any gold nanoparticles on the iron oxide nano-
octahedra or nanocube surfaces (Figure 1), but the presence of
small subnanometer clusters cannot be excluded, considering the
TEM resolution. In recent work, the gold atoms and sub-
nanometer clusters on iron oxide were imaged by aberration-
corrected High Angle Annular Dark Field (HAADF)/
STEM.30,31 Evidence for the presence of strongly interacting
subnanometer clusters and cations of gold with the iron oxide
surface was obtained from H2-TPR. Indeed, low-temperature
hydrogen reduction features indicate the presence of Au−O−Fe
species on both the nanooctahedra and nanocubes, but in a much
higher amount in the octahedral samples.29 The presence of
strongly bound gold species was confirmed by NaCN leaching.
After treatment in a 0.05% NaCN solution, the gold was entirely
removed from the nanocubes, but a small amount of gold was still
present on the nanooctahedra, as confirmed by ICP analysis and
by the absence or presence of gold by Au-XANES.
Gold (2 at. %) was deposited by DP-NaOH on both the iron
oxide nanocubes and nanooctahedra. After gold deposition, the
specific surface areas remained unchanged (Table 1). TEM did
XRD analyses revealed the presence of some 10 nm, large, gold
nanoparticles in both samples (Figure 2). Semiquantitative
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Figure 6. In situ Au LIII fluorescence XANES spectra collected during oxidative dehydrogenation of cyclohexane with 0.08% cyclohexane/0.8% O2 in
helium: (a) reference gold samples, (b) parent 5 wt % Au/Fe2O3 (WGC), (c) leached Au/Fe2O3 (WGC), and (d) Au/Fe2O3 (WGC) after NaCN
leaching and reduction in 5% H2 at 200 °C for 1 h.
estimates, based on reference intensity ratios, suggest that ∼1 wt
% of gold is present as metallic nanoparticles. Having the same
dimension as the iron oxide support itself, these large
nanoparticles are unlikely to interact strongly with the iron
oxide surface. They can, of course, catalyze a different reaction
pathway than the Au−O−Fe sites, and this is considered below.
Before and after calcination in air at 200 °C, the iron oxide is
enriched in γ-Fe2O3. In oxidative reaction conditions over FeOx
and Au−FeOx nanoshapes, γ-Fe2O3 is expected to remain stable.
The Fe K-edge XANES spectra in Figure 3 show that the bulk
composition of both Au−FeOx nanoshapes is a mixture of Fe3O4
and Fe2O3, with the former phase dominating in the octahedra
(Figure 3, Fe3O4 percentages labeled in blue). During reaction in
a cyclohexane/oxygen atmosphere at 225 °C, the magnetite
phase of the nanooctahedra is stable, while the nanocubes are
partly oxidized into γ-Fe2O3. We may attribute these differences
to the presence of gold, interacting stronger with the {111}
surfaces, thus stabilizing the Fe3O4 phase.
collection of the corresponding steady-state data. Cyclohexane is
converted over the iron oxide and gold/iron oxide nanoshapes
into cyclohexene, benzene, carbon dioxide, and water. For all
samples studied, the dehydrogenation products started to appear
above 200 °C. This temperature is typical for activation under
oxidative conditions,15,48 but it is low compared with the
corresponding anaerobic processes. Both cyclohexene and
benzene are formed over the iron oxide nanoshapes, but benzene
is the only dehydrogenation product in the presence of gold. In
the generally accepted hypothesis of a sequential reaction
pathway, benzene is formed when the abstraction of additional
hydrogen atoms from the olefinic intermediate is favored over
the desorption of cyclohexene.7,49 The addition of gold thus
provides the necessary energy to further activate C−H bonds.
From the steady-state reaction data, an Arrhenius-type plot
was constructed, as shown in Figure 5. It can be seen that the
overall reaction rates per gram of catalyst are comparable for all
samples studied. The activity is also very close to that of the
reference 5 wt % Au−Fe2O3 catalyst obtained from the World
Gold Council. However, if the rates are normalized with the
surface area (Tables 1 and 3), they are higher (by ∼50%) for the
octahedral catalysts than the cubes. The rates for the FeOx and
Au/FeOx samples studied in this work are higher than those for
the NiFe2O4 catalysts reported by Turek et al.,48 but are lower
than the AuPd−TiO2 catalyst studied by the Hutchings group,15
3.2. Oxidative Dehydrogenation Activity and Selectiv-
ity. The catalytic activity for the oxidative cyclohexane
dehydrogenation reaction was tested in a gas mixture of 4% O2
and 0.4% cyclohexane in helium. Figure 4 shows the product
formation as a function of temperature and time from the TPRx
data collected in the reaction cell with in situ SAXS. An
isothermal hold of 20 min at each temperature allowed the
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as shown in Figure 5. The rates are also comparable to those
reported for 12 nm Co3O4 nanocubes in the same gas
composition as what is used here.16 The apparent activation
energies for the cyclohexane conversion over iron oxide and
gold/iron oxide samples are in the range of 60−70 kJ mol−1
(Table 2). Slightly higher apparent activation energy is shown for
the leached and prereduced commercial reference catalyst.
The evolution of the oxidation state of gold in reaction
conditions was monitored by in situ XANES experiments using
the commercial reference catalyst and leached samples thereof.
We selected the reference sample over the nanoshapes because it
contained more gold in its leached state and was thus conducive
to the intended analysis. From the white-line intensity of the
XANES spectra in Figure 6, it can be seen that the parent catalyst
contains mostly metallic gold, whereas the leached catalysts
before and after reduction contain primarily cationic gold. Mild
reduction in H2 helps to mobilize gold atoms from the iron oxide
core to the surface without reducing it.26 For the Au/Fe2O3
catalysts, Allard et al.27,50 have shown that atomic gold species
were strongly bound, even after redox heat treatments up to
673 K. In work with Au/Fe3O4 (111) single crystals, gold atoms
over the uncapped O atoms were shown to be stable to 773
K.30,51 The insets in Figure 6 show that the oxidation states of the
catalysts do not change significantly during the oxidative
dehydrogenation reaction. For all three samples, a small decrease
of the intensity at the Au-L III absorption edge, due to a partial
reduction of the cationic gold species, can be noticed.
Although the overall activity of FeOx and Au/Fe3O4 samples is
not significantly different, the catalyst structure has a strong
impact on the benzene and carbon dioxide selectivity (Figure 7).
This figure was constructed on the basis of selectivity at low
conversions using the steady-state data of Table 2. As expected
from thermal activation, the carbon dioxide selectivity increases
while the benzene selectivity decreases with increasing temper-
ature. The highest benzene selectivity is observed for the Au/
Fe3O4 nanooctahedra. On Au/FeOx nanocubes, the benzene
selectivity is initially high, but decreases dramatically with
increasing temperature. The carbon dioxide formation is lower
on the octahedra than on the cubes, irrespective of the presence
of gold. The ability of an oxide to work as a total oxidation
catalyst is known to be related to the reducibility of the metal
oxide. Highly reducible Mn2O3, for instance, converts cyclo-
hexane into mainly carbon oxides.11 Jin et al. also observed a
similar relationship between the reducibility of mixed Mg−O−V
catalysts and their selectivity for cyclohexane dehydrogenation
and total oxidation.52 The promotional effect of gold on the
benzene selectivity of the iron oxide nanooctahedra observed
here may be attributed to the interaction of gold with the iron
oxide and the stabilization of the Fe3O4 phase by Au cations, as
the XANES data in Figure 3 indicate. Under oxidative conditions,
the γ-Fe2O3 phase will otherwise dominate. The addition of gold
on the iron oxide nanocubes did not prevent their agglomeration
upon heating to >250 °C, as found by the in situ SAXS
experiments, discussed below, and this may have also contributed
to their further loss of active Au−O−Fe sites and ensuing loss of
selectivity to benzene at 400 °C (Figure 7d).
Figure 7. Product selectivity in the oxidative dehydrogenation of
cyclohexane at the conditions of Figure 4 and from the fixed-bed experi-
ments conducted in the microreactor (0.4% cyclohexane/4% O2 in
helium, steady-state data): (a) FeOx octahedra, (b) FeOx cubes, (c) Au/
Fe3O4 octahedra, (d) Au/FeOx cubes, (e) Au/Fe2O3 WGC (parent), (f)
Au/Fe2O3 WGC (leached), and (g) Au/Fe2O3 WGC (leached, reduced).
be seen in Figure 7e. The metallic gold nanoparticles alone can be
further ruled out as selective ODH sites by examining catalysts
devoid of metallic gold after mild leaching with sodium cyanide
solutions. The selectivities of the leached samples derived from
the commercial 5 wt % Au−Fe2O3 (WGC) catalyst are different
from the parent catalyst. Although the latter favors CO2 (at >95%)
at all temperatures, the leached samples show considerable
selectivity to benzene. These samples comprise the residual gold
as cations (Figure 6c and d) interacting with iron oxide as
Au−O−Fe species.26,27 Hence, we can conclude that Au−O−Fe
species are responsible for the enhanced benzene selectivity and
the metallic gold nanoparticles on the parent iron oxide sample
catalyze the combustion reaction. In view of these findings,
different preparation techniques should be examined in an effort
to produce many more Au−O−Fe sites on the gold nanoshapes
to achieve higher activity and, thus, higher benzene yields for
potential practical application of these catalysts. The data
collected here are useful in that they have identified an important
trend: namely, it is possible to stabilize the active and selective
Fe3O4 phase of iron oxide, even in excess oxygen atmospheres, by
the addition of small amounts of gold through the formation of
Au−O−Fe sites.
An alternative explanation for the increased selectivity of the
gold-promoted iron oxide nanoshapes needs to be considered in
view of the presence of weakly bound gold nanoparticles and
clusters on their surfaces. This was addressed by considering the
commercial reference 5 wt % Au−Fe2O3 (WGC) sample, which
has a preponderance of gold nanoparticles on its surface.23,24
This material catalyzes the complete combustion to CO2, as can
3.3. In Situ SAXS Characterization. The stability in size
and shape of the iron oxide and gold/iron oxide nanoshapes in
reaction conditions was probed by in situ SAXS experiments.
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Figure 8. In situ SAXS characterization performed during oxidative cyclohexane dehydrogenation with 0.4% cyclohexane/4% O2 in helium: (a) FeOx
octahedra, (b) Au/Fe3O4 octahedra, (c) FeOx cubes, and (d) Au/FeOx cubes. Fitting parameters are summarized in Table 3. The catalytic test at 400 °C
was conducted ex situ over 90 min.
Figure 8 shows the I(q) curves derived from scattering images
collected at various temperatures. I is the scattering intensity, and
q is the magnitude of the scattering vector. The X-ray scattering
intensity originates primarily from the iron oxide support.
Average particle sizes derived from the fitting of experimental
I(q) curves are summarized in Table 3. They are in good
agreement with sizes obtained from TEM images (Table 1).
Experimental data of iron oxide and gold/iron oxide are fitted
using an octahedral model. For the gold-containing sample, the
simple octahedral model does not match the experimental data in
the high q region (Figure 9a). A two-particle model assuming
subnanometer spherical particles on the octahedral surface
describes reasonably well the experimental data (Figure 9b). The
iron oxide and gold/iron oxide nanooctahedra are stable in
reaction conditions up to 400 °C. No significant change in the
position and slope of the oscillations in the scattering curves was
observed (Figure 8a, b). The stability in shape and size is also
confirmed by TEM (Figure 10a, b).
Figure 9. Comparison of two models for the fitting of experimental
SAXS data for Au/Fe3O4 octahedra. Fitting parameters are summarized
in Table 3.
Figure 10. TEM images collected after oxidative dehydrogenation of
cyclohexane at 400 °C for 90 min: (a) FeOx octahedra, (b) Au/Fe3O4
octahedra, (c) FeOx cubes, and (d) Au/FeOx cubes.
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A simple cubic model accounts for the experimental data of
both iron oxide and gold/iron oxide cubes. No clear difference
was found among the data from those cubes. These results are in
accordance with a lower Au−O−Fe interaction observed by
hydrogen TPR.29 For both cubic samples, the iron oxide support
starts to change above 250 °C. The particle size of iron oxide
nanocubes is stable up to 400 °C, but the I(q) curves undergo a
subtle change in the slope in the Porod region, where the I(q)
decrease as q4, probably related to a change in the surface area
(Figure 8c). According to the Porod law, the slope is
proportional to the nanoparticle surface for large q, with
where a is the edge length of the cube, and therefore, its surface
area will decrease to 80.6%. The surface area of the bare iron
oxide cube decreased to 88% with a minor size change. The
nanoparticles displayed in Figure 8c have, indeed, a more
rounded morphology. In contrast to the bare iron oxide support,
the gold/iron oxide cubes start to sinter at 400 °C (Figure 8d).
The particle distribution became polydispersed, as indicated by
strongly damped oscillations. As a result, the particle size
increased and the surface area per particle decreased, as shown in
Table 3, indicating not only a shape change but also a volume
increase via sintering. The TEM image in Figure 10d shows that
the gold/iron oxide nanocubes had started to sinter with the
formation of larger randomly shaped particles. The sintering of
gold/iron oxide nanocubes was accompanied by a sharp drop in
benzene selectivity (Figure 7d).
2πN(Δρ)2Sp
I(q) →
q4
(5)
where N, Δρ and Sp are the number of particles in the X-ray
beam, the difference in electron density between a particle and its
surrounding, and the surface area of the particle,40 respectively.
The Porod intensity (Ip) at large q can be defined as
4. CONCLUSIONS
Iron oxide and gold/iron oxide nanooctahedra and nanocubes
with a cubic inverse spinel structure are active catalysts for the
oxidative dehydrogenation of cyclohexane with measurable
activity at 200 °C. In situ SAXS experiments were useful in
discriminating stable from unstable shapes due to heating in the
reaction mixture. The nanocubes were unstable to heating above
250 °C, but Au addition suppressed this instability. Both
cyclohexene and benzene dehydrogenation products are formed
on the bare iron oxide nanoshapes, but the overall selectivity to
them is low, the major product being CO2. Addition of gold
promotes the selectivity to benzene over cyclohexene, and
suppresses the full combustion to CO2. Selectivity to benzene
exceeds 82% at 350 °C for the Au-promoted Fe3O4 nano-
octahedra. The addition of gold is found to stabilize the Fe3O4
phase of iron oxide, presumably through Au−O−Fe sites that
keep the iron oxide in the mixed oxidation state of Fe3O4, even in
the presence of excess oxygen in the gas. The enhanced
selectivity to benzene is not due to gold nanoparticles, as proved
by the study of samples after removal of the gold nanoparticles by
cyanide leaching.
I(q) q4 ≡ Ip
(6)
A graphical representation of the Porod intensity as a function of
q is shown in Figure 11.
Overall, the findings of this work establish for the first time the
critical structural parameters that may be employed to tune the
Fe3O4 catalyst activity and selectivity for the ODH of
cyclohexane. Preparing and stabilizing atomically dispersed
gold on the surface stabilizes the Fe3O4 phase and selectively
produces benzene. This may be a general finding for the gold
promotion of other oxide surfaces and for the dehydrogenation
of other hydrocarbon molecules, as well, and warrants further
investigation.
Figure 11. Porod plots corresponding to the I(q) curves in Figure 8: (a)
FeOx octahedral, (b) FeOx cubes, (c) Au/FeOx cubes. Black, at 25 °C;
red, at 300 °C.
From the Porod law and the relation below,
I(0) = N(Δρ)2Vp2
(7)
AUTHOR INFORMATION
Corresponding Author
*(M.F.-S.) Phone: (+1) 617-627-3048. Fax: (+1) 617-627-3991.
E-mail: maria.flytzani-stephanopoulos@tufts.edu. (S.V.) E-mail:
vajda@anl.gov.
■
where Vp is the volume of the particle,
Finally, we get the relation
Ip
2πSp
=
Vp2
I(0)
Author Contributions
(8)
∥Equally contributing authors.
.The surface areas, calculated according to eq 8, are summarized
in Table 3. They are close to those measured by nitrogen
physisorption and use of the BET equation (Table 1). Our
calculations show that the surface area of the iron oxide cubes
slightly decreases with increasing reaction temperature. The
subtle change can probably be attributed to a transition from the
cubic to a more stable, spherical shape. Note that when an ideal
cube is transformed to an ideal sphere without changing the
particle volume, the diameter of the sphere will be (3/4π)3/22a,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support of this
work by the Air Force Office of Scientific Research MURI
program, Contract No. FA9559-08-1-0309. The work performed
at Argonne was supported by the U.S. Department of Energy,
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