Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-Based Spinel for Benzene Oxidation: In Situ DRIFTS Study Coupled with Raman and XAFS Spectroscopy

Article abstract of DOI:10.1021/acscatal.6b03547

Co₃O₄ spinel has been widely investigated as a promising catalyst for the oxidation of volatile organic compounds (VOCs). However, the roles of tetrahedrally coordinated Co²⁺ sites (Co²⁺_Td) and octahedrally coordinated Co³⁺ sites (Co³⁺O_h) still remain elusive, because their oxidation states are strongly influenced by the local geometric and electronic structures of the cobalt ion. In this work, we separately studied the geometrical-site-dependent catalytic activity of Co²⁺ and Co³⁺ in VOC oxidation on the basis of a metal ion substitution strategy, by substituting Co²⁺ and Co³⁺ with inactive or low-active Zn²⁺(d⁰), Al³⁺(d⁰), and Fe³⁺(d⁵), respectively. Raman spectroscopy, X-ray absorption fine structure (XAFS), and in situ DRIFTS spectra were thoroughly applied to elucidate the active sites of a Co-based spinel catalyst. The results demonstrate that octahedrally coordinated Co²⁺ sites (Co²⁺_Oh) are more easily oxidized to Co³⁺ species in comparison to Co²⁺_Td, and Co³⁺ are responsible for the oxidative breakage of the benzene rings to generate the carboxylate intermediate species. CoO with Co²⁺_Oh and ZnCo₂O₄ with Co³⁺_Oh species have demonstrated good catalytic activity and high TOF_Co values at low temperature. Benzene conversions for CoO and ZnCo₂O₄ are greater than 50% at 196 and 212 °C, respectively. However, CoAl₂O₄ with Co²⁺_Td sites shows poor catalytic activity and a low TOF_Co value. In addition, ZnCo₂O₄ exhibits good durability at 500 °C and strong H₂O resistance ability.

Full text of DOI:10.1021/acscatal.6b03547

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Research Article
pubs.acs.org/acscatalysis
Geometrical-Site-Dependent Catalytic Activity of Ordered
Mesoporous Co-Based Spinel for Benzene Oxidation: In Situ DRIFTS
Study Coupled with Raman and XAFS Spectroscopy
Xiuyun Wang,† Yi Liu,† Tianhua Zhang,† Yongjin Luo,*,‡ Zhixin Lan,† Kai Zhang,† Jiachang Zuo,‡
Lilong Jiang,*,† and Ruihu Wang*,§
†National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, People’s Republic
of China
‡Fujian Key Laboratory of Pollution Control & Resource Reuse, Fujian Normal University, Fuzhou 350007, People’s Republic of
China
§State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, People’s Republic of China
S
* Supporting Information
ABSTRACT: Co3O4 spinel has been widely investigated as a promising catalyst for the oxidation of
volatile organic compounds (VOCs). However, the roles of tetrahedrally coordinated Co2+ sites
(Co2+T ) and octahedrally coordinated Co3+ sites (Co3+O ) still remain elusive, because their oxidation
d
h
states are strongly influenced by the local geometric and electronic structures of the cobalt ion. In this
work, we separately studied the geometrical-site-dependent catalytic activity of Co2+ and Co3+ in VOC
oxidation on the basis of a metal ion substitution strategy, by substituting Co2+ and Co3+ with inactive
or low-active Zn2+(d0), Al3+(d0), and Fe3+(d5), respectively. Raman spectroscopy, X-ray absorption fine
structure (XAFS), and in situ DRIFTS spectra were thoroughly applied to elucidate the active sites of a
Co-based spinel catalyst. The results demonstrate that octahedrally coordinated Co2+ sites (Co2+O ) are more easily oxidized to
h
Co3+ species in comparison to Co2+T , and Co3+ are responsible for the oxidative breakage of the benzene rings to generate the
d
carboxylate intermediate species. CoO with Co2+ and ZnCo2O4 with Co3+ species have demonstrated good catalytic activity
Oh
Oh
and high TOFCo values at low temperature. Benzene conversions for CoO and ZnCo2O4 are greater than 50% at 196 and 212 °C,
respectively. However, CoAl2O4 with Co2+ sites shows poor catalytic activity and a low TOFCo value. In addition, ZnCo2O4
Td
exhibits good durability at 500 °C and strong H2O resistance ability.
KEYWORDS: spinel, ordered mesopore, benzene oxidation, sustainable chemistry, DFT calculation
oxidation and catalytic oxidation of VOCs.2,12−15 It was
1. INTRODUCTION
reported that the oxidation of VOCs over TMO catalysts
Volatile organic compounds (VOCs) are carbon-based
occurs according to a Mars−van Krevelen type redox
chemicals that can cause environmental and health prob-
cycle.15−18 Co3O4 consists of one tetrahedrally coordinated
lems.1−5 To meet more and more stringent regulations of VOC
Co2+ site and two octahedrally coordinated Co3+ sites;19,20
emissions, several promising techniques, including conventional
adsorption over Co3+ or Co2+ sites and activation of C−H are
control processes (biological degradation, adsorption, etc.) and
two crucial steps in the VOC oxidation over Co3O4.15
emerging technologies (plasma catalysis, catalytic oxidation,
Additionally, the C−H bond breaking on metal oxides occurs
via direct interaction of σ and σ* C−H orbitals with d-type
orbitals of cobalt cations.16,21 However, the roles of Co3+ or
Co2+ sites in the activation of C−H bonds and oxidative
breakage of the C−H bonds have remained elusive because
their oxidation states are strongly influenced by the local
geometric and electronic structures of the cobalt ion. For
etc.) have been proposed for VOC removal.6−8 Among these
techniques, catalytic oxidation is considered to be one of the
most effective pathways for the removal of VOCs, owing to its
low operating temperature and high efficiency.9−11 Catalysts
based on precious metals can exhibit outstanding catalytic
performance at relatively low temperatures, but high noble-
metal loading is generally required, which greatly limits their
practical application because of high cost and easy sintering at
high temperature.
instance, Co3O4 nanorods with predominately exposed Co3+
Oh
sites show superior catalytic activities toward ethylene
A promising alternative to the precious-metal-based catalysts
is the use of transition-metal oxides (TMOs) including MnO2,
Co3O4, NiO, etc., among which Co3O4 is one of the most
efficient catalysts in many catalytic reactions, such as CH4
Received: December 14, 2016
Revised: January 13, 2017
Published: January 19, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acscatal.6b03547
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oxidation.17 Similar studies have shown that Co3O4 nanocryst-
als display high low-temperature activity and selectivity in the
catalytic oxidation of C3H8, which is contributed by
Co3+species.18 Conversely, some other researchers have found
a good correlation between the specific catalytic activity and the
relative proportion of Co2+ ions during VOC oxidation.14,21
Thus, it is very essential to elucidate the intrinsic roles of the
two types of cobalt sites.
2.3. Characterizations. Powder X-ray diffraction (XRD) was
performed on a Panalytical X’Pert Pro diffractometer using Co Kα
radiation. N2 physisorption measurement was performed on an ASAP
2020 apparatus; the sample was degassed in vacuo at 180 °C at least 6
h before each measurement. H2 temperature-programmed reduction
(H2-TPR) was performed on an AutoChem II 2920 instrument
equipped with a TCD detector, in which the sample was pretreated
under an Ar flow (30 mL/min) at 400 °C for 0.5 h. After the sample
was cooled to room temperature, the temperature was increased to
800 °C at 5 °C/min in a gas flow of 10 vol % H2/Ar (30 mL/min).
Oxygen temperature programmed surface reaction (O2-TPSR)
experiments were performed on the same apparatus as that for H2-
TPR. Before the experiment, the sample was pretreated in He at 300
°C for 0.5 h. After the sample was cooled to 50 °C, the He flow was
switched to a pulse injection of 10 vol % benzene/Ar until complete
adsorption of benzene, followed by purging in He for 10 min. Finally,
the TPSR run was started under a flow of 40 mL/min of 3 vol % O2/
He ramping at 5 °C/min to 500 °C. A mass spectrometer (Cirrus) was
used for online monitoring of effluent gases. The signals at mass to
charge (m/z) ratios of 18 (H2O), 28 (CO), 44 (CO2), and 78 (C6H6)
were monitored, and the profile of CO was deducted from the
contribution of the m/z 28 fragment of CO2. X-ray photoelectron
spectroscopy (XPS) analysis was performed on a Physical Electronics
Quantum 2000 instrument, equipped with a monochromatic Al Kα
source (Kα = 1486.6 eV) at 300 W under UHV. Catalyst charging
during the measurement was compensated by an electron flood gun.
Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HR-TEM) measurements were
carried out on a JEM-2010 microscope operating at 200 kV in the
mode of bright field. Inductively coupled plasma atomic emission
spectroscopy (ICP-AES) analysis was carried out using an Ultima2
spectrometer. The Si component was determined using a PANalytical
Axios X-ray fluorescence (XRF) spectrometer with a rhodium tube as
the source of radiation. Raman spectra of samples were collected under
ambient conditions on a Renishaw spectrometer. A laser beam (λ 532
nm) was used for excitation. X-ray absorption fine structure (XAFS)
measurements were performed on the 1W2B beamline of Beijing
Synchrotron Radiation Facility. The spectra of the Co K-edge of the
samples and reference compounds were recorded at room temperature
in transmittance and fluorescence modes, respectively. A Si (111)
double-crystal monochromator was used to reduce the harmonic
content of the monochromatic beam.
2.4. In Situ DRIFTS. In situ diffuse reflection infrared Fourier
transform spectroscopy (DRIFTS) was recorded on a Nicolet Nexus
FT-IR spectrometer in the range of 650−4000 cm−1 with 32 scans at a
resolution of 4 cm−1. Prior to each experiment, the sample was
pretreated at 300 °C for 0.5 h in a gas flow of N2 to remove any
adsorbed impurities and then cooled to 150 °C. The background
spectrum was collected under N2 and automatically subtracted from
the sample spectra. Afterward, 1000 ppm of benzene balanced with N2
was introduced to the cell at a flow rate of 30 mL/min at 150 °C, and
then DRIFTS spectra were recorded. After physisorbed benzene was
removed by flushing wafer with N2 for 3 h, subsequently 20% O2/N2
was introduced to investigate the reactivity of preadsorbed benzene
with N2 + O2 at 150 or 250 °C.
2.5. DFT Calculations. All spin-polarized DFT calculations were
carried out using the Vienna ab Initio simulation program (VASP)
with the gradient-corrected PW91 exchange-correction function. For
valence electrons, a tight convergence of the plane-wave expansion was
obtained with a kinetic energy cutoff of 500 eV, and the ionic cores
were described with the projector augmented wave (PAW) method.
The Brillouin zone of the Monkhorst−Pack grid was set at 2 × 2 × 1.
For energy calculations, the electronic energy was converged to 10−5
eV, and the positions of the atoms were allowed to relax until all forces
were smaller than 0.02 eV/Å.
Herein, we separately studied the catalytic activities of Co2+
and Co3+ for VOC oxidation on the basis of a metal ion
substitution strategy. Three-dimensionally (3D) ordered
mesoporous Co3O4 was prepared by a nanocasting method,
and then Co2+ or Co3+ sites were replaced with catalytically
inactive or low-activity Zn2+ (d0), Al3+ (d0), and Fe3+ (d5),
respectively. Benzene, one of the carcinogenic VOCs, was used
as a target toxic gas for testing the catalytic activity of catalysts.
Our results indicate that Co2+ sites are more easily oxidized to
Oh
Co3+ species in comparison to tetrahedrally coordinated sites,
and Co3+ is responsible for the oxidative breakage of the
benzene rings to generate the intermediate species (i.e.,
carboxylates), giving rise to good catalytic activity and high
TOFCo values at low temperature.
2. EXPERIMENTAL SECTION
2.1. Catalyst Preparations. Three-dimensionally ordered meso-
porous Co3O4 was synthesized according to the modified literature
method.22 A 0.5 g portion of KIT-6 (purchased from nanoscience and
technology companies of Jicang in Nanjing, People’s Republic of
China) was added to a stirred solution of 1.0 g of Co(NO3)2·6H2O in
ethanol (40 mL); the suspension was stirred at room temperature for
12 h. After the removal of solvent, the remaining solid was dried at 120
°C overnight followed by calcination at 400 °C for 2 h. The KIT-6
hard template was removed using 2 M NaOH solution, and the
resultant solid was dried at 120 °C for 24 h and finally calcined at 400
°C for 2 h. The obtained sample is denoted as Co3O4. When the as-
obtained Co3O4 was treated with 3.5 vol % H2 at 250 °C for 2 h, the
resultant reduced sample is denoted as CoO.
The synthetic procedures of ZnCo2O4, CoAl2O4, and CoFe2O4
were similar to that of Co3O4 except that Co(NO3)2·6H2O was
partially replaced by salt nitrates with 1:2 molar ratios of Zn to Co, Co
to Al, and Co to Fe, respectively.
2.2. Catalytic Activity Tests. The catalytic activities of samples
were evaluated in a continuous flow fixed-bed quartz reactor with 0.1 g
of catalyst. A thermocouple was inserted inside the catalytic bed to
measure the reaction temperature. A gas mixture composed of 498
ppm of benzene, 20% O2, and balance N2 was fed into the reactor, and
the total flow rate was kept at 150 mL/min by a mass flow controller,
equivalent to a weight hourly space velocity (WHSV) of 90000 mL/(g
h). After steady operation for 30 min, the activity of the catalyst was
tested. Benzene conversions were analyzed by a gas chromatograph
equipped with a flame ionization detector. In the case of water vapor
addition, 9.5 vol % of H2O was introduced at 350 °C via a mass flow
controller using a water saturator. The benzene conversion (Xbenzene
)
and turnover frequency of TOFCo were calculated according to the
equations
Cin − Cout
Xbenzene
=
× 100%
cout
(1)
CbenzeneXbenzene
V
gas
TOFCo
=
(s−1
)
nCo
(2)
where Cin and Cout are the inlet and outlet benzene concentrations,
respectively, Vgas is the total molar flow rate, Cbenzene is the benzene
concentration in the inlet gas, and nCo is the molar amount of Co in
total catalyst calculated by inductively coupled plasma OES
spectrometer results.
3. RESULTS AND DISCUSSION
3.1. Structural and Textural Properties and Reduc-
ibility. Low-angle XRD patterns (Figure S1 in the Supporting
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Figure 1. (A) XRD patterns of Co3O4, ZnCo2O4, CoAl2O4, CoFe2O4, and CoO. (B) H2-TPR profile of Co3O4.
Information) of Co3O4, CoO, CoAl2O4, ZnCo2O4, and
CoFe2O4 show a peak at 1.0−1.2°, indicating the existence of
a well-ordered mesoporous structure. The wide-angle XRD
pattern (Figure 1A) of Co3O4 shows well-resolved reflections
that can be assigned to the Co3O4 phase with a cubic spinel-
type structure (PDF: 00-042-1467). Interestingly, ZnCo2O4,
CoAl2O4, and CoFe2O4 show similar peaks, while the peak
intensities and positions are unavoidably affected because of
different ionic radii and electronic state of foreign atoms. These
results suggest the maintenance of the cubic-spinel structure
after partial cobalt is substituted by zinc, aluminum, or iron.
After Co3O4 was treated by H2 at 250 °C, the XRD pattern of
CoO shows three diffraction peaks with 2θ values of 42.6, 49.6,
and 72.8°, which can be assigned to the cubic CoO phase
(PDF: 01-075-0393), indicating a complete phase trans-
formation from Co3O4 to CoO. The H2-TPR profile (Figure
1B) of Co3O4 shows three reduction signals. The weak peak
observed at 164 °C can be assigned to the reduction of surface
oxygen species.13,20 The second peak, at approximately 249 °C,
is associated with the reduction of Co3+ to Co2+ accompanying
the corresponding structural change to CoO.13,23 The high-
temperature peak centered at 340 °C is associated with the
reduction of Co2+ to Co0.24 On the basis of these results, the
reduction of mesoporous Co3O4 to CoO can be realized at 250
°C under a 3.5 vol % H2 atmosphere.
structure with cubic Ia3d symmetry, which are consistent
with the low-angle XRD results. It is worth noting that the 3D
morphology of CoO is similar to that of Co3O4, indicating that
the phase transformation occurs in a pseudomorphic manner,
without reconstruction of the whole mesoporous framework.
Moreover, the lattice fringes can be observed in HR-TEM
images of these catalysts, revealing the highly crystalline nature
of mesoporous frameworks (for more details see the
Supporting Information).22,25 These results indicate good
replication of the KIT-6 template (Figure 2A). XRF analysis
(Table S1 in the Supporting Information) confirms that 4.8−
5.1 wt % of residual silica is present in the prepared catalysts.
Additionally, the texture properties such as surface area are also
important parameters for catalysis. It is shown that the surface
areas of the catalysts (Table 1) are close to each other, in the
Table 1. BET Surface Areas, Reaction Temperatures, TOFCo
Values, and Activation Energies (Ea) of Catalysts for
Benzene Oxidation
a
BET surface T10% T50% T90% TOFCo
sample
area (m2/g) (°C) (°C) (°C) (10−3 s) Ea (kJ/mol)
Co3O4
80
76
79
77
83
178
145
201
164
175
215
196
245
263
0.30
0.67
0.14
0.87
0.45
70
64
CoO
CoAl2O4
ZnCo2O4
CoFe2O4
112
212
223
236
261
TEM images (Figure 2) of Co3O4, CoO, CoAl2O4, ZnCo2O4,
and CoFe2O4 exhibit arrays of high-ordered mesoporous
a
The TOFCo value at low conversions under a kinetically controlled
regime at 167 °C for benzene oxidation.
range of 76−83 m2/g. These results suggest that that only
minor textural properties change over CoO, ZnCo2O4,
CoAl2O4, and CoFe2O4 with respect to Co3O4.
3.2. Cobalt Oxidation States and Structure. 3.2.1. Co-
balt Oxidation States. The valence states of cobalt in Co3O4,
CoO, CoAl2O4, ZnCo2O4, and CoFe2O4 have been studied by
X-ray absorption near-edge structure (XANES), where the
position of the absorption edge could be used as an indicator.
Figure 3A suggests that the valence state of cobalt ions in
catalysts follows the order ZnCo2O4 > Co3O4 > CoFe2O4. It
should be mentioned that Co2+ is present in CoO and
CoAl2O4, and the average oxidation state of cobalt in Co3O4 is
+2.67. In comparison to pure Co3O4, changes in the main
absorption edge position are observed in CoO, CoFe2O4,
CoAl2O4, and ZnCo2O4, as shown in Figure 3B. The average
oxidation state of cobalt in CoFe2O4 is +2.36, suggesting that
Co2+ and Co3+ coexist and that the dominant state of Co ions is
+2. The pre-edges of the Co K-edge of CoO, CoAl2O4,
CoFe2O4, ZnCo2O4, and Co3O4 are illustrated in Figure 3C.
Figure 2. TEM and HR-TEM images of (A) KIT-6 template, (B)
Co3O4, (C) CoO, (D−F) CoFe2O4, (G−I) ZnCo2O4, and (J−L)
CoAl2O4.
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Figure 3. (A) Normalized Co K-edge XANES spectra over CoO powder (reference sample), Co foil, and Co3O4, CoO, CoAl2O4, CoFe2O4 ,and
ZnCo2O4. (B) Energy position of the main absorption of Co K-edge of Co3O4, CoO, CoAl2O4, CoFe2O4, and ZnCo2O4. (C) Pre-edge of XANES
spectra at the Co K-edge of Co3O4, CoO, CoAl2O4, CoFe2O4, and ZnCo2O4. (D) XPS spectra of Co 2p over Co(NH3)6Cl6, CoO reference, and Co-
based spinel catalysts. Co foil, Co(NH3)6Cl6, and CoO reference were purchased from Sigma-Aldrich. (E) XPS spectra of O 1s over Co-based
catalysts. (F) Relation of interatomic distance between Oh and Td atoms in the Co-based spinel structure.
For a 3d transition metal, the pre-edge peaks are assigned to the
forbidden 1s → 3d transition,26−28 and the change in the pre-
edge peak intensity is indicative of the changes in the site
occupation in tetrahedral and octahedral symmetry: namely,
narrower and more intense for the former and broader and less
intense for the latter.28 This is mainly because tetrahedral
symmetry is highly noncentrosymmetric and this enables p → d
transitions that contribute to the pre-edge peak.26 When both
tetrahedral and octahedral sites are occupied, the pre-edge peak
will be the sum of these contributions, and the increase in
intensity will be directly proportional to the tetrahedral site
occupation.28 Thus, the pre-edge intensity (Figure 3C) of
cobalt ions in CoAl2O4 is higher than that of CoO, CoFe2O4,
and ZnCo2O4, suggesting that more Co ions occupy tetrahedral
sites in the former catalyst.
The surface cobalt oxidation states were investigated by X-ray
photoelectron spectroscopy (XPS). CoO powder and Co-
(NH3)6Cl3 were chosen as reference materials for pure Co2+
and Co3+ (Figure 3D), respectively. Pure Co2+ exhibits two
shakeup peaks at ca. 785 and 802 eV,14,16,29 while pure Co3+
displays only a very weak shakeup peak at 791 eV. In
comparison to the CoO reference, the Co 2p XPS spectra of
mesoporous CoO, CoAl2O4, and CoFe2O4 exhibit two shakeup
peaks at 786 and 804 eV, which are typical for Co2+. Co 2p XPS
spectra of ZnCo2O4 indicate that the primary surface cobalt
species are Co3+, which is similar to the case for the
Co(NH3)6Cl3 reference. The Fe 2p XPS spectrum (Figure S2
in the Supporting Information) of CoFe2O4 demonstrates that
the dominant state of Fe ions is +3. However, a relative weak
band at 711.2 eV may also indicate the presence of small
amounts of Fe2+.30 O 1s XPS spectra of the catalysts are shown
in Figure 3E; the peak at 530.4−532.5 eV can be assigned to
lattice oxygen (Olatt),14 while that at 532.1−534.9 eV can be
assigned to surface-adsorbed oxygen species (Oads), resulting
from the adsorption of gaseous O2 into oxygen vacancies.31 The
peak positions of O 1s in CoAl2O4, CoFe2O4, and ZnCo2O4 are
different from those of Co3O4 and CoO. This is because the
structural defects of catalysts have changed after Co2+ or Co3+
sites are replaced by Zn2+, Al3+, or Fe3+. The Oads/(Olatt + Oads)
ratios (Table S1 in the Supporting Information) in the catalysts
follow the trend CoAl2O4 (56%) > CoFe2O4 (54%) >
ZnCo2O4 (44%) > CoO (42%) > Co3O4 (37%), indicating
the formation of more surface oxygen defects of CoAl2O4,
ZnCo2O4, and CoFe2O4 in comparison to Co3O4. Additionally,
structural transformation from the Co3O4 to CoO procedure is
also beneficial in enhancing the formation of surface vacancies
by oxygen removal from the surface oxygen ion binding with
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