Surface oxygen vacancies on Co3O4 mediated catalytic formaldehyde oxidation at room temperature

Article abstract of DOI:10.1039/c5cy01709b

Three kinds of Co₃O₄ catalysts with different concentrations of surface oxygen vacancies were successfully synthesized through a solvothermal and subsequent thermolysis method. Room temperature catalytic removal of formaldehyde, which is one of the major indoor air pollutants and is seriously harmful to human health, is achieved by these Co₃O₄ catalysts. The existence of surface oxygen vacancies was confirmed by XPS and Raman spectroscopy. The concentration of oxygen vacancies was obtained via TGA data, which could reach 1.93% (atom ratio) in this study. H₂-TPR and O₂-TPD results reveal the essential role played by surface oxygen vacancies in the catalytic oxidation of formaldehyde, which is reflected in the improved mobility of oxygen and the facile formation of reactive oxygen species (ROS). As a consequence, the catalytic activity shows a direct relationship with the concentration of surface oxygen vacancies. The Co₃O₄ nanobelts exhibited the best performance for formaldehyde oxidation due to their larger specific surface area, higher low-temperature reducibility and abundant active surface oxygen species. The high catalytic activity and stability of this Co₃O₄ catalyst without the introduction of noble metals as a co-catalyst shows its potential practical application for formaldehyde removal at room temperature. This study reveals the key importance of surface oxygen vacancies in the catalytic oxidation activity and complements the common viewpoint that Co³⁺ is the major active species in Co₃O₄-based systems, as well as provides a possibility for developing high performance catalysts through surface modification.

Full text of DOI:10.1039/c5cy01709b

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Surface oxygen vacancies over Co3O4 mediated catalytic
formaldehyde oxidation at room temperature
Zhong Wanga,b, Wenzhong Wang*,a, Ling Zhanga, and Dong Jianga,b
Received 00th Janu ary 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Three kinds of Co3O catalysts with different concentration of surface oxygen vacancies were successfully synthesized
4
through a solvothermal and subsequent thermolysis method. Room temperature catalytic removal of formaldehyde,
www.rsc.org/
which is one of the major indoorair pollutants and seriously harmful to human health, is achieved by these Co3O catalysts.
4
The existence of surface oxygen vacancies were confirmed by XPS, Raman spectra. The concentration of oxygen vacancies
were obtained via TGA data, which could reach 1.83% (atom ratio) in this study. H2-TPR and O2-TPD results reveal the
essential role of surface oxygen vacancies played in catalytic oxidation of formaldehyde, which are reflected by the
improved mobility of oxygenand facile formation of reactive oxygen species (ROS). As a consequence, the catalytic activity
shows a direct relationship with the concentration of surface oxygen vacancies. The Co3O4 nanobelts exhibited the best
performance of formaldehyde oxidation due to larger specific surface area, higher low-temperature reducibility and
abundant active surface oxygen species. The high catalytic activity and stability of this Co3O catalyst without the
4
introduction of noble metals as co-catalyst shows a potential practical application for formaldehyde removal at room
temperature. This study reveals the key importance of surface oxygen vacancies in the catalytic oxidation activity and
complements the common viewpoint of Co3+ is the major activity species in the Co3O4-based systems, and provides a
possibility for developing high performance catalysts throughsurface-modification.
practical use of these catalysts, and consequently, provides a
strong motivation to seek more economical and efficient way
for catalytic HCHO removal.
As one of the commonly available and general spinel
structured oxides, Co3O4 have received extensive attention of
1. Introduction
Formaldehyde (HCHO) is a typical and major indoor air
pollutant and is well recognized to be seriously harmful to
human health.1 The effective removal of HCHO from indoor air
has become an important subject in environmental science.
Great efforts have been made to combat HCHO pollution,
among which, room temperature catalytic oxidative
decomposition is considered to be the most promising one
because this process is environmentally friendly and energy
saving.2 Oxide-supported noble metal (Pt, Pd, Au, and Ag)
various fields, such as Li-O2 batteries14, magnetic materials15
gas sensing16, particularly, in the field of heterogeneous
catalysis.17-20 It is well established that Co3O4 nanocrystals are
an ideal substitute for noble metals for CO oxidation at low
temperature.21, 22 Xie et al.17 reported that Co3O4 nanorods, in
which {110} facets are exposed primarily, could catalyze CO
oxidation completely at a temperature as low as -77 C and
even exhibit high activity in the presence of water. Recently,
Co3O4 nanocrystals have also been widely reported in catalytic
,
catalysts, such as Pt/MnOx-CeO23, Pt/TiO24, Pd/TiO25, Au/CeO2
,
6
Au/Co3O47, Pt/AlOOH8, Pt/Al2O32, Au/FeOx , Ag/HMO10 etc.
9
have been widely studied due to their high catalytic activity at
room temperature. If noble metals were not introduced,
however, these catalysts will exhibit lower catalytic activities
and high temperature is generally required to activate the
oxidation reactions.11-13 The high cost of noble metals and the
potential need for additional thermal energy limits the
oxidation of VOCs, such as CH419, C2H423, C3H824, toluene25 and
26, 27
formaldehyde7,
. However, these studies focused on
structure-, dimension- and crystal facet-dependent activities,
while the surface defect as one other important influence
factor of catalyst activity, has rarely been studied.
It is well known that the catalytic oxidation of
hydrocarbon over Co3O4 involves the participation of reactive
oxygen species (ROS).28-30 Therefore, it is reasonable to expect
that the catalyst with high oxygen mobility and the facile
formation of highly active electrophilic oxygen species (O - or
a. State Key La borato ry of High Perfo rmance Ceramics and Su perfine
Microstructure, Shangha i Institute of Ceramics, Ch inese Academy of Science s,
1295 Dingxi Road, Shanghai 200050 , People’s Republic of Ch ina. Fax: +86-21-
5241-3122, Email: wzwang@mail.sic.ac.cn
-
O2 ) will lead to superior catalytic oxidation activity. It has been
b. University of Ch inese Academy of Science, Beijing 100049, People’s Republic of
China.
-
2-
widely reported that oxygen can be facilely charged (.O2 , O2
,
or O42-) by the localized electrons at the surface oxygen
vacancies (Vo) through molecular oxygen activation channel.31,
Electronic Supp lementary Information (ESI) available: [HRTEM images, N2
adsorption-deso rption iso therms, concen tratio n changes of f ormaldehyde and
CO2, and FT IR spectra]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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32
This charged state of oxygen is the critical step to generate Teflon-lined stainless steel tank and heated at 120 C for 10 h.
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DOI: 10.1039/C5CY01709B
ROS, which will play an important role in following catalytic The precipitate was collected by centrifugation and rinsed with
oxidation reaction. On the other hand, as reported by Metiu deionized water and absolute ethanol for three times. The
and co-workers, oxygen vacancies exposed at the surface are products were then freeze-dried and calcined in a program-
strong Lewis basic sites.33 Taking the Lewis acidity nature of controlled furnace at 250 C for 4h.
2.1.3 Synthesis of Co3O4 nanosheets
Co3O4 nanosheets were obtained via converting the
(NH4)Co8(CO3)6(OH)6·4H2O precursor synthesized by
formaldehyde (initial reactant) and formic acid (intermediate
product) into consideration34, a catalytic surface with higher
Lewis basicity (surface oxygen vacancies) corresponds to a
higher adsorption and catalytic activity for HCHO oxidation
elimination.
a
hydrothermal method.14 First, 4 mmol of Co(NO3)2·6H2O, 8
mmol of urea and 0.4 g of CTAB were dissolved in 40 mL
deionized water under vigorously stirring until a transparent
red solution formed. The resulting homogeneous solution was
transferred into a Teflon-lined stainless steel tank with a
volume of 50 mL which was then heated in an oven at 140 C
for 12h. The precipitate was collected by centrifuged and
rinsed with de-ionized water and absolute ethanol for three
times. The products were then freeze-dried and calcined in a
program-controlled furnace at 250 C for 4h.
Herein, three kinds of Co3O4 catalysts with different
concentration of surface oxygen vacancies were synthesized
through a solvothermal and subsequent thermolysis route. The
origin of the surface oxygen vacancies come from the reducing
ambient and thermal treatment during the synthesis
procedures.35 The formation of surface oxygen vacancies and
their relative concentrations were characterized by XPS,
Raman spectra and TGA data. These catalysts were employed
in the catalytic oxidation of HCHO at room temperature, and
their catalytic activity shows a direct correlation with the
corresponding variation of surface oxygen vacancies
concentrations. H2-TPR and O2-TPD results reveal the
mechanism of surface oxygen vacancies mediated catalytic
oxidation activity. This work will deepen our understanding of
the effect of surface oxygen vacancies on the catalytic
oxidation reaction and paves a way for developing high
performance catalysts through surface-modification.
Bulk Co3O4 was synthesized by precipitation method
according to previous literature.38
2.2 Characterization
Powder X-ray diffraction (XRD) was measured to
characterize the as-prepared catalyst with a Rigaku D/MAX
2250V diffractometer using monochromatized Cu Kα
(λ=0.15418 nm) ray radiation source, operated at 40 kV tube
voltage and 100 mA tube current. The scanning rate was
2°/min in the range of 2θ=20-80°. Surface area measurements
were performed at 77K using a Micromeritics Tristar 3000
Brunauer-Emmett-Teller (BET) analyzer. The field emission
scanning electron microscope (FE-SEM) images were acquired
2. Experimental
2.1 Synthetic procedures
2.1.1 Synthesis of Co3O4 nanobelts
on
a JEOL JSM-6700F filed emission scanning electron
microscope. Transmission electron microscopy (TEM, JEOL
JEM-2100F, accelerating voltage 200 kV) characterization was
performed to evaluate the morphologies and microstructures
of the catalysts. High-resolution transmission electron
microscopy (HRTEM) images were obtained and analyzed using
the Digital Micrograph software (Gatan Inc.). Reduction
behavior of catalysts was examined by temperature-
programmed reduction (H2-TPR) under a pure H2 flow (50
mL/min) at a heating rate of 10 C/min, the amount of H2
uptake during the reduction was measured by thermal
conductivity detector of a gas chromatography instrument.
The oxygen temperature programmed desorption (O 2-TPD)
analysis was performed on a Micromeritics Chemosorb 2750
instrument. The procedures were as followed: (1) each sample
was pretreated under a He flow (50 mL/min) a t 300 C for 40
min; (2) the sample was purged with 4% O 2/He gas flow for 1h
at room temperature for O2 adsorption; (3) the sample was
heated to 850 C at a heating rate of 10 C /min under a pure
He gas flow. The signal of O2 desorption was measured by
Co3O4 nanobelts was synthesized through a topochemical
transformation route from Co(CO3)0.5(OH)·0.11H2O nanobelts
due to their close structural matching according to previous
literature.36 In a typical synthesis, 10 mL of aqueous ammonia
(30%) was first mixed with 25 mL of EG under stirring for 2 min
to form a homogeneous solution. Then, 1.5 mL of aqueous
sodium carbonate solution (1 mol/L) was added to the mixture
and stirred for another 2 min. Afterwards, 5 mL of aqueous
cobalt nitrate solution (1 mol/L) was added to the mixture,
continue stirring for 20 min. The resulting homogeneous
solution was transferred into a Teflon-lined stainless steel tank
with a volume of 50 mL which was then heated at 170 C for
17h. The autoclave was then cooled down to room
temperature, precipitate was collected by centrifugation and
rinsed with deionized water and absolute ethanol for three
times. The products were then freeze-dried and calcined in a
program-controlled furnace at 250 C for 4 h.
2.1.2 Synthesis of Co3O4 nanoplates
thermal
conductivity
detector.
X-ray
photoelectron
Co3O4 nanoplates was obtained from Co(OH)2 as
precursor.37 Briefly, 1.2 g of Co(NO3)2·6H2O was dissolved in a
mixture of 5 mL ethanol and 5 mL deionized water. Then, 1 g
of PVP was added and the solution was stirred for 30 min.
Afterwards, 25 mL of NaOH (0.4 mol/L) aqueous solution was
slowly added. The resulting suspension was transferred into a
spectroscopy (XPS) analysis was performed on an ESCALAB 250
instrument (Thermo Scientific Ltd.) to study the chemical
states of the samples. The C 1s signal was used to correct the
charge effects. IR spectra were recorded on a Burker Tensor 27
spectrometer. Room temperature Raman spectra were
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recorded on a confocal microprobe Jobin Yvon Lab Ram Infinity (see in SI). The FFT spot patterns (Fig. 2 a3) can be indexed as
_ _
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DOI: 10.1039/C5CY01709B
Raman system with a laser excitation of 532 nm. Fourier (111), (113) and (220) crystal planes along the [110] zone axis.
transform infrared (FTIR) spectra were recorded with a Nicolet Fig. 2 b1 shows the TEM images of Co3O4 nanoplates. The
iS10 FTIR spectrometer ex-situ in air. Thermogravimetric (TG) morphology of the nanoplates is hexagonal platelet with
analysis was recorded on an NETZSCH STA 449C. Specimens thickness of about 20 nm. The HRTEM image (Fig. 2 b2) shows
were placed in Pt crucibles, and TG-DTA data were recorded
during heating from room temperature to 700 C under
flowing air (100 mL/min). A heating rate of 10 C/min was used
for all experiments.
2.3 Evaluation of catalytic activity
The catalytic oxidation activity of the as-prepared samples
(50 mg) for formaldehyde degradation (260 ppm) was tested in
a gas-closed vitreous reactor (capacity 650 mL) with a double-
walled jacket with water for temperature control. During the
experiment, the reaction temperature was kept at 25 C by
circulating water. The relative humidity was controlled by
evaporating certain amount of water injected in the reactor
through heating after being purged by pure air. It could be
controlled in the range of 20-98% at 25 C. The activity of the
catalysts on HCHO oxidation was estimated by the
concentration change of HCHO before and after the oxidation
process. HCHO concentration in the reactor was analyzed by
phenol spectrophotometric method. 10 mL gas sample
containing trace HCHO was absorbed by 15 mL phenol reagent
solution (1×10-4 wt %) for 5 min. Then, 1.2 mL (1 wt %)
ammonium ferric sulfate solution was added as the coloring
reagent. After being shaken and waiting for 15 min in the dark,
HCHO concentration was determined by measuring light
absorbance at 630 nm with a spectrophotometer (Hitachi U-
3010 UV-vis spectrophotometer). CO and CO2 were monitored
by GC analysis (GC 7900, Teccomp) equipped with a TDX-01,
Figure 1. FESEM images of Co3O4 nanobelts (a1-a2),
nanoplates (b1-b2) and nanosheets (c1-c2).
_
_
the (220) and (202) crystal planes with 0.276 nm d-spacing and
interfacial angle of 60°, which indicated that the surface of the
nanoplates are {111} exposed. Its corresponding FFT image (Fig.
2 b3) shows an hexagon spot array, which can be indexed as
[111] zone axis. TEM image of the Co3O4 nanosheets is showed
in Fig. 2 c1, which present a triangle shape composed by
nanoparticles with average size of about 30 nm. From the
lattice resolved HRTEM image (Fig. 2 c2) and corresponding
80-100 mesh packed column followed by
a
methane
conversion furnace and a flame ionization detector (FID).
-
-
FFT spot pattern (Fig. 2 c3), it can be indexed as (111) and (220)
crystal planes along the [112] zone axis.
3. Results and discussion
3.1 Morphological structure
The field-emission scanning electron microscope (FE-SEM)
images of Co3O4 catalysts are exhibited in Fig. 1. As shown in
Fig. 1 a1, the nanobelts have a regular morphology with the
length of 200 nm and width of 40 nm. The enlarged image (Fig.
1 a2) demonstrated that the nanobelts were composed by
nanoparticles with a size range of 14-17 nm. Fig. 1 b shows the
typical feature of nanoplates, which present a hexagonal
shape with side length of 180 nm and thickness of 17-19 nm.
SEM image of the Co3O4 nanosheets is presented in Fig. 1 c1,
the triangle nanosheets were composed by nanoparticles with
a size varying from 24 to 28 nm (Fig. 1 c2).
Fig. 2 a1 shows the TEM images of Co3O4 nanobelts. The
thermal conversion process results in porous structure and a
uniform size distribution with average size of ~ 15 nm is
demonstrated. The corresponding high-resolution TEM image
(Fig. 2 a2) shows the d-spacing about 0.282 nm and 0.241 nm,
which_a_re in good agreement with the standard data of (220)
and (113) interplanar spacings for spinel structured Co3O4.
More HRTEM images of Co3O4 nanobelts are given in Fig. S1
Figure 2. TEM, HRTEM, and fast-Fourier-transform (FFT)
images of Co3O4 nanobelts (a1-a3), nanoplates (b1-b3) and
nanosheets (c1-c3).
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3.2 Textural characterization of catalysts
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DOI: 10.1039/C5CY01709B
N2 adsorption-desorption isotherms of different Co3O4
catalysts and their corresponding pore size distribution are
shown in Fig. S2. All the samples show a typical H3 type loop,
which indicate the aggregation of Co3O4 nanoparticles,
revealing the slit-shaped pores of these catalysts. Their
corresponding pore size distributions curves show that the
pore sizes of Co3O4 catalysts are dominantly distributed
between 2.0 nm and 6.0 nm. BET surface areas of the Co3O4
catalysts are listed in Table 1.
To further study the phase structure of Co3O4 catalysts,
XRD measurements were carried out. Fig. 3a shows the XRD
patterns of as-prepared Co3O4 samples. The peaks at 2θ of
31.3°, 36.8°, 44.8°, 59.5°, 65.2° correspond to the (220), (311),
(400), (511), and (440) planes. All the diffraction peaks match
well with the characteristic structure of the spinel Co3O4
(JCPDS No. 65-3103) without any impurity phase. The average
particle sizes of all the samples calculated from the XRD data
(FMWH of (311) peak) using the Scherrer equation are shown
in Table 1. All these samples exhibit an average particle size
between 15 to 25 nm, in good accordance with the
observation from the SEM and TEM images. Compared with
Co3O4 nanosheets, the diffraction peaks of Co3O4 nanoplates
and nanobelts show an obvious shift to higher degrees (Fig.
3b), which can be ascribed to the presence of a structural
disorder in the nanoplates and nanobelts.39 The structural
defects are supposed to larger the Co-O bond length and
favorable for the formation of oxygen vacancies, which will
make the extraction of O easier from the surface of Co3O4. 21
As indicated above, these catalysts with high oxygen mobility
will significantly promote the catalytic oxidation activity.
Figure 3. (a) XRD patterns of Co3O4 nanosheet, nanoplates and
nanobelts. (b) Magnified peaks in 2θ range from 35 to 40º.
3.3 XPS spectra
X-ray photoelectron spectroscopy (XPS) was employed to
gain further insight into the compositions and chemical states
on the surface of the synthesized Co3O4 catalysts. The typical
patterns of Co 2p and O 1s are presented in Fig. 4. According
to the literature26, the curve of Co 2p spectra were fitted into
four peaks at binding energies of ~779.5, ~781.2, ~794.6 and
~797.1 eV, which are corresponding to Co3+ 2p3/2, Co2+ 2p3/2
,
Co3+ 2p and Co2+ 2p1/2 respectively.(Fig. 4a) The energy
1/2
difference between the Co 2p3/2 peak and the Co 2p1/2 peak is
40
~15 eV, consistent with previous reports. 21,
The ration of
Co3+/Co2+ can be calculated from the XPS results as shown in
Table 1. The small difference of Co3+/Co2+ ration among these
three samples are conform to the H2-TPR test results as
discussed below.
Figure 4. XPS spectra of the Co 2p (a) and O 1s (b) for various Co3O4 catalysts.
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Table 1. Textural characteristic and surface composition of different Co3O4 samples.
Catalyst
BET
surface
area
(m2/g)
61
Particle Co3+/Co2+ k/SBET
Oads/Olatt
(XPS)
C/Co
Concentration
size
(XPS)
(min-
1*m-
2*g)
1
(XPS)
of Oxygen
Vacancies
(atom %)
1.93
(XRD)
(nm)
nanobelts
nanoplates
nanosheets
17
21
25
40.3/59.7
33.7/66.3
36.1/63.9
36.2/63.8
27.3/72.7
24.6/75.4
0.205
0.137
0.149
21
0.145
-
1.02
39
0.14
3.4 Catalyst activity
confirms the occurrence of catalytic oxidation, rather than
adsorption, of formaldehyde on our Co3O4 catalysts. However,
the selectivity of converting HCHO to CO2 was hard to be
determined in this study due to the strong interaction
between CO2 and the catalyst surface.
The catalytic activities of different Co3O4 catalysts
towards HCHO oxidation at room temperature are presented
in Fig. 5. Each sample was measured for five cycles. Error bars
in Fig. 5 (a) shown less than 10% difference with each cycle,
which reflects the statistical reproducibility and stability for all
the samples. As can be seen from the diagram, among the
nanosheets, nanoplates and nanobelts, the nanobelts showed
the highest catalytic activity for HCHO oxidation, which can
completely eliminate 260 ppm formaldehyde in 30 minutes.
This eliminate efficiency was comparable to, or even higher
than, some oxide-supported noble metal catalysts. 41-43 The
nanoplates showed less active while stable catalytic
performance, which can oxidize 100 ppm formaldehyde in 60
minutes. The initial decrease in the nanosheets can be
ascribed to adsorption because its content showed no change
in the later detections. To verify the catalytic oxidation rather
than adsorption had happened on Co3O4 nanobelts, the sealed
reactor was heated to 80 C for 15 min immediately after
HCHO removal experiment, and then the concentration of
HCHO in gas phase was monitored, which showed no
detectable HCHO. FTIR experiments were performed to
investigate the surface adsorbed species before and after the
oxidation reaction (Fig. S3). For the original catalyst, the bands
at 1385 cm-1 and 1520 cm-1 are attributed to surface
adsorbed nitrate species and carbonate species, respectively,
which are originated from the reactants of cobalt nitrate and
sodium carbonate.44 After reaction, no band located at 2845
cm-1, which ascribed to the CH2 asymmetric stretch of
formaldehyde, was detected.7 It has been reported that
formate and dioxymethylene species on the surface of catalyst
are critical by-products during HCHO oxidation.26, 45 However,
neither the bands at 1570 cm-1 and 1359 cm-1 attributed to
vas(COO) and vs(COO) for the adsorbed formate species 46, nor
the peaks at 2710 cm-1 and 2810 cm-1 belonging to the
In order to investigate the reusability of Co3O4 nanobelts
catalyst in HCHO catalytic oxidation, the recycle performance
was checked under the same conditions as mentioned above.
After 20 cycles, the catalyst maintained 67% of its initial
activity (eliminate 174 ppm HCHO in 30 min). The gradual
deactivation can be ascribed to the accumulation of
bicarbonate species on catalyst surface during the reaction as
confirmed by the FTIR study, which inhibit the adsorption and
activation of HCHO and O2. The used catalyst was then
regenerated at 120 C under vacuum for 4h to remove the
surface absorbed bicarbonate species (Fig. S3). The
regenerated catalyst can recover to 86% of its initial activity
(eliminate 223 ppm HCHO in 30 min), indicated the stable
activity and high reproducibility of this Co3O4 catalyst.
Water vapor, the existence of which is unavoidable in
ambient environment, has been known to play great influence
on low-temperature catalytic oxidation reactions.47 In order to
investigate this factor, HCHO removal over Co3O4 nanoplates
and nanobelts at room temperature with five relative humidity
(RH) levels were carried out, as shown in Fig. 5d-4e. It is
obvious that the HCHO removal efficiency was improved with
the increase of RH up to 80% for nanobelts and up to 60% for
nanoplates, respectively. Further increase of RH will somewhat
inhibit the catalytic activity. These results suggest that water
plays dual-character in determining the performance of Co3O4
catalysts. At low RH, the improved oxidation efficiency can be
ascribed to the formation of hydroxyl groups dissociated from
water on the surface oxygen vacancies sites , which favors the
adsorption and transfer of oxygen48 and benefits both the
formation and the consumption of reaction intermediates44
.
asymmetric and symmetric CH stretch of dioxymethylene26
,
Previous study even found that surface –OH could oxidize
HCHO into formate and CO2 in the absence of atmospheric
oxygen.49 When the RH beyond some critical concentration,
water vapor will leads to severe catalytic deactivation due to
its strong adsorption on the active sites, hindering O 2 and
HCHO adsorption.44
were detected. The big difference is the appearance of a band
located at ~1630 cm-1, which can be ascribed to bicarbonate
species due to the adsorption of CO2 and H2O on the catalyst
surface.21 The change of CO2 concentration during the reaction
process were measured, which presented a fluctuant increase
due to the CO2 generating-adsorption process (Fig. S3),
showed a good accordance with the FTIR results. This result
The reaction rate constant (k) of nanoplates (Fig. 5b) and
nanobelts (Fig. 5c) were calculated using pseudo-first-order
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kinetic model and the degree of fitting (R2) reached more than oxidation over Co3O4 catalysts could be surface oxygen
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0.98, which indicated that the pseudo-first-order kinetic model vacancies mediated. Then, the existencDeOs I:o1f0.s10u3rf9a/Cce5CYo0x1y7g0e9nB
could predict the reaction kinetics accurately. The reaction vacancies were characterized by Raman spectra and O1s XPS
rate constant (k) of nanobelts is 20 times that of nanoplates, and their concentrations were calculated from the TGA data.
which are 0.14 min-1 and 0.0071 min-1 respectively. To avoid The relationship between the concentrations of surface
the inconformity effect caused by the difference of specific oxygen vacancies and the catalytic formaldehyde oxidation
surface area, reaction rate constant of different Co3O4 samples activity were fully discussed. And the results of H2-TPR and O2-
were normalized by their BET surface area respectively, and TPD revealed that the mobility of oxygen and the ability of
the results are shown in Table 1. The normalized reaction rate low-temperature oxygen activation were improved greatly by
constant of nanobelts is still 7 times that of nanoplates
the surface oxygen vacancies, which are crucial factors for
improving the catalytic activity.
3.5 Surface oxygen vacancies characterization
Fig. 4b shows the O 1s spectrum and its two fitted
Gaussian peaks. The peak at ~530.2 eV corresponds to lattice
oxygen species (Olatt ), while the one located at ~531.8 eV can
be ascribed to surface adsorption oxygen (Oa ds). 55 The rations
of Oads/Ol att of different catalysts were shown in Table 1. The
ration can be a yardstick to measure the amount of surface
vacant oxygen, thus higher ration indicates larger surface
oxygen vacancies. Therefore, the order of surface oxygen
vacancies concentration is nanobelts
>
nanoplates
>
nanoshees. The relative concentration of surface oxygen
vacancies shows a good consistence with the catalytic activity
mentioned above.
The existence of surface oxygen vacancies are further
confirmed by the Raman spectra. As reported, Raman bands at
~225, 480, 520, 683 cm-1 correspond respectively to the F1 2g,
Eg, F2 2g, and A1g modes of the pure Co3O4.39 Among them, the
band at ~225 cm-1 and 683 cm-1 can be ascribed to Raman
vibration of Co2+-O2- and Co3+-O2- respectively.21 As shown in
Fig. 6, the Raman bands of the nanosheets fit very well with
the standard pure Co3O4, indicated that the Co3O4 nanosheets
Figure 5. Catalytic activities. (a) Formaldehyde concentration
changes as a function of reaction time for the different Co3O4
catalysts (RH= 20%). The error bars represent the maximum
and minimum values and the dot indicate the average value
from five experimental cycles. (b)-(c) Linear estimation of
apparent rate constants (k) of Co3O4 nanoplates and nanobelts have negligible structure disorders. Compared wit h the
in panel (a) using the pseudo-first-order reaction kinetics. (d)-
(e) The effect of relative humidity on formaldehyde removal
efficiency over Co3O4 nanoplates and nanobelts.
nanosheets, the Raman bands of the nanoplates and the
nanobelts show an obvious shift to higher frequencies. The
blue-shift of the Raman bands can be ascribed to the surface
oxygen vacancies due to its photon-confinement effects.56, 57 It
It should be noted that, the Co3O4 catalysts here show no should noted here that the overall enhancement of peak
direct relationship with neither the Co3+ content nor the BET intensity in nanoplates sample probably due to the influence
surface area (see Table 1), in contrast with previous reports17, of crystallinity and the orientation effect during Raman test.
27, 50
where Co3+ was generally treated as the mainly active
species. Recent studies suggested that the surface oxygen
vacancies could dramatically enhance the electrocatalytic
performance of Co3O4 through delocalize the electrons around
the Co-O bands.51 Theoretical simulation also presented that
the surface oxygen vacancies may provide a unique way for
enhancing the catalytic activity of rutile MnO2.52 Moreover,
oxygen vacancies (OVs) on the surface of BiOCl were found
-
that prefer to activate oxygen into O2 or O22-, depending on
the location of OVs, which can promote the catalytic oxidation
reactions significantly.53 Additionally, Song et al. reported that
the TiO2 with surface oxygen vacancies could catalytic oxidize
formaldehyde at room temperature without light.54 Therefore,
based on the above results and owing to the reducing ambient
during the catalysts synthesis procedures, it is reasonable to
expect that the catalytic performance of formaldehyde
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Figure 6. Raman spectra of Co3O4 nanosheets, nanoplates and 3.6 Thermal redox behavior of catalysts
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DOI: 10.1039/C5CY01709B
nanobelts.
The reducibility of different Co3O4 catalysts were studied
In order to further investigate the exact concentration of
oxygen vacancies in nanobelts and nanoplates, the TGA curves
in flowing air gas were measured. As shown in Fig. 7(a), bulk
Co3O4 and Co3O4 nanosheets showed two weight loss stages in
the temperature range of 40-700 C. According to the
literatures58, the first weight loss can be ascribed to the loss of
physical adsorption water molecules on the Co3O4 surface,
while the second one can be assigned to the loss of
chemisorption waters and the release of oxygen atoms on the
surface of Co3O4 under high temperature. Compared with the
bulk Co3O4, TGA curves of the nanoplates and nanobelts
presented a similar trend of weight loss in the temperature
range of 40-200 C, but a more gentle weight loss in 200-400
C and another similar trend of weight loss after 400 C. As
mentioned above, the initial weight loss can be attributed to
the release of physical adsorbed water molecules on the
catalyst surface. After that, the oxygen vacancies can be re-
filled with oxygen under high temperature, leading to a gentle
weight loss of nanoplates and nanobelts. The weight loss after
reoxidation was due to the desorption of chemisorption
waters. TGA differential spectra of the nanosized samples with
bulk Co3O4 are shown in Fig. 7(b). An increasing D-value
(differential of residual mass between nanosized samples and
bulk sample) has been presented after 200 C, corresponding
to the gentle weight loss stage in TGA curves. Then, D-value
tends to a constant after 400 C, indicating the final similar
weight loss in TGA curves. From the D-value data, the amount
of refilled oxygen can be obtained, which are directly related
to the oxygen vacancies concentration. The D-value of Co3O4
nanobelts, nanoplates and nanosheets are 0.51%, 0.27% and
0.036%, respectively, indicating the oxygen vacancies
concentration (the atom fraction of oxygen vacancies in total
oxygen atoms) are 1.93%, 1.02% and 0.14%, respectively.
by H2-TPR technique (Fig. 8 a). The TPR profile presented two
well-resolved reduction peaks, corresponding to the reduction
of Co3+ to Co2+ (low-temperature peak) and Co2+ to Co0 (high-
temperature peak), respectively. Compared with the Co3O4
nanosheets, the reduction peaks of the nanoplates and the
nanobelts showed remarkable shift to lower temperature,
which could be attributed to the surface oxygen vacancies.59
Surface oxygen vacancies promoted the reducibility of catalyst,
facilitated the Co3+/Co2+ redox and oxygen mobility. The
synergistic effects between Co3+/Co2+ and O/Vo redox speed
up the oxygen transfer during HCHO oxidation process. As
illustrated in Fig. 9, the Co2+ may initially interact with oxygen
and then is oxidized into Co3+. With the participation of oxygen
va cancies, the activated oxygen is continuously transferred to
HCHO, making HCHO oxidation happens . As known, high
oxygen mobility will promote the catalytic oxidation activity.21
Consequently, the reducibility shows a good accordance with
their catalytic activity. The ratio of Co3+/Co2+ can be obtained
from the proportion of their reduction peak area. T he small
difference among three samples agreed with the XPS results
which were shown in Table 1.
To further investigate the nature of adsorbed and
activated oxygen species that will be involved in the
formaldehyde oxidation reactions 29, O2-TPD experiments were
carried out as shown in Fig. 8b. According to the literatures 27, 39
the desorption peak at temperature about 200C can be
ascribed to molecular adsorption oxygen O- 2, and the
desorption peak at temperature around 340C belongs to the
surface chemical adsorption oxygen O-. These surface reactive
oxygen species (ROS) are known to promote the catalytic
oxidation reactivity.6, 30 The desorption peak at temperature
higher than 700C belongs to the release of bulk phase lattice
oxygen. Compared with the nanosheets, the nanoplates and
the nanobelts presented obvious shifts of O 2 desorption peaks
to lower temperature and higher intensities of desorption
peaks. This phenomenon can be ascribed to the presence of
surface oxygen vacancies. Their relative concentrations can be
obtained by the desorption peak area.21 Because O2 activation
played an important role in catalytic oxidation reactions, the
facile supply and replenish of reactive oxygen species (ROS)
probably lead to an outstanding catalytic performance for
HCHO oxidation.
,
Figure 7. (a) TGA curve of different Co3O4 samples (b) The TGA
differential spectra of nanoplates, nanobelts and nanosheets
with bulk Co3O4.
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Figure 8. H2-TPR and O2-TPD profiles of Co3O4 nanobelts, nanoplates and nanosheets.
3.7 Mechanism discussion
Based on the above results, it is concluded that the
catalytic performances of formaldehyde oxidation over Co3O4
catalysts are surface oxygen vacancies mediated. Catalytic
activity shows a direct relationship with the concentration of
surface oxygen vacancies. To further confirm the important
role of surface oxygen vacancies played in this process, all the
three samples were heat treated at 400 C for 3 hours in air,
according to the TGA results, to refill the oxygen vacancies,
and then their catalytic formaldehyde oxidation activities at
room temperature were checked. It is interesting to find out
that all the catalysts lost their catalytic activities after
annealing, which demonstrated the essential role of surface
oxygen vacancies played in the catalytic formaldehyde
oxidation reaction forcefully.
Accordingly, formaldehyde catalytic oxidation over Co3O4
catalyst with surface oxygen vacancies is schematically
exhibited in Fig. 9. The synergistic effect between Co3+/Co2+
and O/Vo redox greatly accelerates the active oxygen transfer .
Moreover, the surface oxygen vacancies are favorable to
reduce the chemisorption energy of oxygen molecules, and the
surface chemisorbed O2 can accept delocalized electrons from
Figure 9. Schematic illustration of HCHO oxidation over Co3O4
with surface oxygen vacancies.
Conclusions
In summary, three kinds of Co3O4 catalysts with different
concentration of surface oxygen vacancies were synthesized
and their catalytic performances of HCHO oxidation at room
temperature were evaluated. XPS, Raman spectra and TGA
data confirmed the existence of surface oxygen vacancies. H2-
TPR and O2-TPD results reveal the essential role of surface
oxygen vacancies played in catalytic oxidation of formaldehyde,
which are reflected by the improved mobility of oxygen and
facile formation of reactive oxygen species (ROS).
Consequently, catalytic activity shows a direct relationship
with the concentration of surface oxygen vacancies. This study
surface oxygen vacancies, be transformed into reactive oxygen
54
species (ROS).39,
Surface oxygen vacancies could also
effectively bind reactants and assist in their reactions.33, 45, 60
Thus, when Co3O4 with surface oxygen vacancies are exposed
to formaldehyde atmosphere, the HCHO molecules are
adsorbed on the surface of catalyst. Surface oxygen vacancies
facilitate the activation of oxygen molecules and the easy
replenishment of ROS. Then HCHO molecules can be oxidized
into formate species rapidly, and finally mineralized into CO 2
and H2O by reacting with ROS.
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27. B. Bai, H. Arandiyan and J. Li, Appl. Catal. B: Environ., 2013,
not only provides insights into the effect of surface oxygen
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provides
a
possibility for developing high performance
catalysts through surface-modification.
30. L. F. Liotta, H. Wu, G. Pantaleo and A. M. Venezia, Catal. Sci.
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Basic Research Program of China (2013CB933200) and the
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Table of Contents:
This study reveals the essential role of surface oxygen vacancies played in catalytic oxidation
reactions, and complements the common viewpoint of Co3+ is the major activity species in the
Co3O4-based systems.