Highly active rod-like Co3O4 catalyst for the formaldehyde oxidation reaction

Article abstract of DOI:10.1016/j.catcom.2017.09.003

In this study, rod-like Co₃O₄ (Co₃O₄-rod) nanocatalyst was prepared by a modified precipitation method, which showed an excellent performance for formaldehyde oxidation to carbon dioxide and water. The light-off temperature (10% conversion) and the temperature for complete conversion of formaldehyde was at 60 °C and 120 °C, respectively. XPS and CO₂-TPD analyses indicated that Co₃O₄-rod had abundant Co^{3 +} and [sbnd]OH (hydroxyl) species on the surface compared with other morphologies of Co₃O₄ investigated (e.g. Co₃O₄-sheet and Co₃O₄-cube). It was proposed that Co^{3 +} ions should be considered as active sites of good oxidation ability for formaldehyde catalytic oxidation.

Full text of DOI:10.1016/j.catcom.2017.09.003

Accepted Manuscript
Highly active rod-like Co3O4 catalyst for the formaldehyde
oxidation reaction
Zeyun Fan, Wenjian Fang, Zhixiang Zhang, Mingxia Chen,
Wenfeng Shangguan
PII:
S1566-7367(17)30376-X
doi: 10.1016/j.catcom.2017.09.003
CATCOM 5185
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
16 May 2017
6 September 2017
7 September 2017
Please cite this article as: Zeyun Fan, Wenjian Fang, Zhixiang Zhang, Mingxia Chen,
Wenfeng Shangguan , Highly active rod-like Co3O4 catalyst for the formaldehyde
oxidation reaction. The address for the corresponding author was captured as affiliation for
all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.09.003
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ACCEPTED MANUSCRIPT
Highly active rod-like Co3O4 catalyst for the formaldehyde oxidation reaction
*
Zeyun Fan, Wenjian Fang, Zhixiang Zhang, Mingxia Chen, Wenfeng Shangguan
shangguan@sjtu.edu.cn
Research Center for Combustion and Environmental Technology,
Shanghai Jiao Tong University,
Shanghai, P. R. China.
*
Corresponding author at: Full postal address: Research Center for Combustion and
Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
Abstract: In this study, rod-like Co3O4 (Co3O4-rod) nanocatalyst was prepared by a
modified precipitation method, which showed an excellent performance for
formaldehyde oxidation to carbon dioxide and water. The light-off temperature (10%
conversion) and the temperature for complete conversion of formaldehyde was at 60

C and 120 C, respectively. XPS and CO2-TPD analyses indicated that Co3O4-rod
3
+
had abundant Co and -OH (hydroxyl) species on the surface compared with other
morphologies of Co3O4 investigated (e.g. Co3O4-sheet and Co3O4-cube). It was
3
+
proposed that Co ions should be considered as active sites of good oxidation ability
for formaldehyde catalytic oxidation.
Key words: Co3O4; morphology; rod-like; formaldehyde removal; hydroxyl species.
1
.Introduction
1

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Formaldehyde (HCHO) emission from decorative and building materials into indoor
air is harmful to human health [1]. Catalytic oxidation is one of the chemical methods
for HCHO removal. The most effective catalysts are those of noble-metal-loaded
materials, such as Pt/TiO2, Au/Co3O4-CeO2, and Pd/CeO2[2-4]. However, the
widespread use of these materials is limited due to their high cost. Owing to the
excellent catalytic performance in many aspects, Co3O4 has attracted wide attention
[5-8]. Recently, Co3O4 catalysts using various preparing methods have been used for
formaldehyde and CO oxidation reactions [9-14]. The results showed that the catalytic
3
+
ability related to reactive oxygen species and Co cations on the surface. The surface
3
+
Co cations could be controlled by selectively exposing specific crystal planes. Xie et
3
+
al.[12] reported that Co3O4 nanorods exposed active Co on the exposed {110}
planes and showed low temperature catalytic oxidation of CO even at -77 C. Hu et al.
[
13] indicated that Co O nanobelts were more active than Co O nanocubes for CO
3 4 3 4
oxidation. Zeng et al. [14] investigated the effects of Co3O4 catalyst morphology on
CO oxidation and found that Co3O4 nanosheets showed high catalytic activity in
comparison with Co3O4nanocubes and nanorods. However, the promotion of catalytic
activity induced by the different morphology of Co3O4 is still ambiguous. Moreover,
the light-off temperature (10% conversion) and the temperature for complete
conversion of formaldehyde on Co3O4 is still higher than room temperature.
In the present work, rod-like Co3O4 (Co3O4-rod) nanocatalyst was prepared which
selectively exposed specific crystal planes, inspired by previous works [12]. This
3
+
rod-like Co3O4 showed abundant Co cationic species on its surface along with the
2

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best catalytic performance towards formaldehyde oxidation than previous reports over
Co3O4-based catalysts [9-11]. Additionally, different morphologies of Co3O4 were
prepared to compare the activity promotion, and XPS investigations were carried out
to relate the presence of cobalt oxidized and hydroxyl species with activity
3
+
performance. It is suggested that hydroxyl and Co cationic species on the surface
should be considered to play vital role for the formaldehyde oxidation reaction.
2
2
.Experimental
.1 Catalysts preparation
Three types of morphologies, rod-like, sheet-like, and cube-like Co3O4 catalysts
coded as Co3O4-rod, Co3O4-sheet, and Co3O4-cube, respectively, were prepared
according to previous works [12], [15] and [16]. The typical preparation procedures
are described in what follows.
2
.1.1 Co3O4-rod catalyst
5
.0 g of Co(CH3COO)2·4H2O was dissolved in 60 ml of (CH2OH)2 in a three-neck
flask, and heated to 160 C gradually in an oil bath. Then, 200 ml of 0.2 M sodium
carbonate was dropped into the vigorously stirring solution with a continuous flow of
nitrogen. The suspension was still stirred for 1 h. After filtrated and washed with
water, the residue was dried at 50 C under vacuum for 8 h and calcined at 450 C for
4
2
2
h in air.
.1.2 Co3O4-sheet catalyst
.5g of Co(NO3)2·6H2O and 3 g of polyvinylpyrrolidone (PVP, Mw = 15,000) were
3

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added to a three-neck flask containing 20 mL mixture of ethanol and distilled water
(v/v 1:1, bubbled by nitrogen for 30 min before using). Then, 50 mL of a 0.4 M
-
1
NaOH was slowly dropped into the mixture at the rate of 30 mL·h . The suspension
was transferred into a Teflon-lined autoclave and kept at 120 C for 4 h. After
centrifuging and washing with water and ethanol, the black precipitate was dried at 60

C for 8 h. The dried precipitate was calcined at 450 C for 4 h to obtain the final
product.
2
.1.3 Co3O4-cube catalyst
0
.24 g of CoCl2·6H2O and 0.3 g of PVP were added in 30 ml of deionized water.
Another 20 ml water solution containing0.1 M KOH and 0.03 M KIO3 was prepared.
Then, the two solutions were mixed and stirred for half an hour. The suspension was
sealed in a Teflon-lined autoclave at 180 C for 18 h. The following steps were the
same as with the Co O -sheet (Section 2.1.2).
3
4
2
.2 Characterization of catalysts
X-ray diffraction (XRD) patterns of the catalysts were recorded using a D8
ADVANCE (Bruker) with Cu Kα radiation. N2 adsorption-desorption isotherms were
obtained using a TriStar II 3020 apparatus (Micromeritics), and the specific surface
2
area (m /g) was estimated by the BET method. The morphology of the samples was
investigated using Transmission Electron Microscopy (JEM2100 operating at 200 kV).
Fourier transform-infrared spectroscopy (FTIR) data were recorded using a Nicolet
6
700 FTIR spectrometer equipped with a MCT detector in transmission mode (KBr
4

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pellet technique). X-ray photoelectron spectroscopy (XPS) was used for surface
elemental analysis and oxidation states of cobalt (AXIS Ultra DLD instrument
(
Kratos) at 300 W, Al Kα excitation). H -temperature programmed reduction (H -TPR)
2
2
and CO2-temperature programmed desorption (CO2-TPD) traces were obtained using
a Chemisorb 2720 TPx apparatus (Micromeritics). The samples (100 mg) were
−1
preheated in helium from room temperature to 450 °C at the rate of 30 °C·min and
then cooled to room temperature. Then, adsorption was conducted using a 3% CO2/He
−1
at the flow rate of 35 mL·min for 20 min. Finally, the CO2-TPD test was initiated
−1
under He flow (35 mL·min ) from room temperature to 550 °C (heating rate
−1
1
0 °C·min ). The H2-TPR were performed from room temperature to 720 °C at a
−1
−1
heating rate of 10 °C·min under a 5% H2/N2 flow (35 mL·min ) after the
preheating process mentioned above.
Further experimental details for all the characterization techniques applied are
provided in Supporting Information (SI).
2
.3 Catalytic oxidation of formaldehyde
HCHO oxidation was carried out in a U-shape fixed bed quartz tubular reactor with a
-mm-inner-diameter under atmospheric pressure. The catalyst (100 mg, 40–60 mesh)
4
was placed in the tube between two quartz wool layers. The influence of external and
internal diffusion was evaluated as depicted in Fig. S1. HCHO vapor was obtained by
passing a flow of N2 through a saturator containing formaldehyde (99%, SCRC), the
former placed in a water bath (T = 50 C). By mixing the N2 stream carrying HCHO
−1
vapor with the flow of O2 in N2, a typical feed gas composition (100 mL·min ,
5

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−1
GHSV of 69,000 h ) containing 100 ppm HCHO and 21 vol% O2 was obtained.
The concentration of CO2 in the outlet gas stream from the reactor was measured by
gas chromatography (GC 9560, HUAAI, flame ionization detector with a CH4
conversion oven, TDX-01 packed columns). Experimental data were recorded
beginning at the point at which the reaction reached a steady-state at each reaction
temperature, where data acquisition continued over the span of 1 h. The HCHO
conversion was estimated using the following equation (1):
[
CO2 ]out
HCHO conversion (%) =
100%,
(1)
[
HCHO]in
where [CO2]out is the CO2 concentration (ppm) in the outlet stream and [HCHO]in is
the inlet HCHO concentration (ppm) to the reactor.
3
3
.Results and discussion
.1 TEM studies
Fig. 1 shows a low-magnification TEM image of the Co3O4-rod sample. In the image,
the nanorods stretched from the cluster core, showing a coral-like morphology. This
2
-1
morphology leads to a large specific surface area (83.5 m ·g ), which is much larger
than that of Co3O4-sheet and Co3O4-cube (Table 1). The TEM images of sheet-like
and cube-like Co3O4 are presented in Fig. S2. In Fig. S2a, Co3O4-sheet exhibits a
platelet morphology. The platelets with some cracks stacked up each other are also
seen in the image. The cubic shape of the sample is distinguished easily (Fig. S2b).
The edges and corners of the sample were not very sharp, and the particles were
sintered under the calcined conditions (temperature at 450 C for 4 h). The XRD
6

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patterns (Fig.S3) show that all diffraction peaks are in good agreement with those of
spinel Co3O4 (JCPDS 65-3103)[17], indicating that no impurity phase exists in the
materials prepared in this work. The crystallite size of Co O samples evaluated by
3
4
using Scherrer's equation and the particle size of the samples estimated from TEM
images are presented in table S2. The width and thickness of the Co3O4-sheet are
about 100 nm and 19 nm, respectively. The edge length of Co3O4-cube particle (79
nm) estimated from TEM images is much larger than the crystallite size of
Co3O4-cube (9.7 nm). Those large cube-like particles might be formed by primary
particles through a self-multiplication process [18], and the specific surface area
decreased sharply. Accordingly, the specific surface area of Co3O4-cube is the lowest
among the three samples.
3
.2 FTIR studies
Fig.2 shows the IR spectra of Co3O4 samples. All of the samples have strong and
-
1
sharp peaks at 665 and 570 cm due to Co–O vibrations in octahedral sites of the
spinel Co3O4, consistent with the finger print IR absorptions of Co3O4 phase [19, 20].
-
1
The broad IR band at 3430 cm is attributed to the stretching vibrations of the O–H
bond [21]. Compared to the Co3O4-sheet and Co3O4-cube samples, the content of OH
on Co3O4-rod is much higher, suggesting that hydroxyl groups can be formed easily
-
1
on the surface of Co3O4-rod. The peak at 1635 cm is assigned to the adsorption of
-
1
water. In addition, the weak IR bands around 1060 cm are attributed to the C–O
stretching region due to the adsorption of organic species during the process of
7

ACCEPTED MANUSCRIPT
sample preparation[6].
3
.3 XPS studies
Fig. 3 shows the Co 2p and O 1s XP spectra of the catalysts. In Fig.3a, two main
peaks are observed in the spectra of Co 2p, due to spin-orbit splitting into 2p1/2
component at the high BE and 2p3/2 at the low BE side. The Co 2p3/2 spectra could be
3
+
fitted into two components at BE = 779.4 and 780.6 eV, corresponding to Co and
2
+
3+
2+
Co cations, respectively [22]. The Co /Co ratio of Co3O4-rod (3.3) is larger than
that of Co3O4-sheet (1.49) and Co3O4-cube (0.73), as shown in Table 1, indicating that,
3
+
among the three samples, there is the largest concentration of Co ions on the surface
of Co3O4-rod, which could be linked to the increase of formaldehyde oxidation rate.
In the O 1s core level spectra, especially in the spectrum of Co3O4-rod, three peaks
marked as O , O , and O can be identified (Fig. 3b). The peak at 529.9 eV (O )
I
II
III
I
corresponds to surface lattice oxygen, that at 531.3 eV (OII) to an oxygen of low
coordination, and that at 533.1 eV (OIII) to hydroxyl species associated with adsorbed
water [23]. As listed in Table 1, the OIII/OI ratio of Co3O4-rod (0.25) is higher than
that of Co3O4-sheet (0.08) and Co3O4-cube (0.09). These results could indicate that
Co3O4-rod has abundant hydroxyl (OH) groups on its surface, which is consistent
with the FTIR results (Section 3.2).
3
.4 H2-TPR and CO2-TPD studies
The effects of different morphologies on the redox behavior and surface basicity were
8

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investigated by the H2-TPRand CO2-TPD technique, respectively. There are two
regions of reduction at around 300 C and 350–600 C (Fig. S4). The former region is
3
+
2+
characterized by the reduction of Co to Co , whereas the latter to the reduction of
2
+
Co to Co [24]. The hydrogen consumption at a lower temperature is obvious for the
Co3O4-rod compared to Co3O4-sheet, indicating that the former catalyst possesses a
3
+
greater concentration of Co . Meanwhile, Co3O4-cube does not show a clear
3
+
hydrogen consumption in this temperature range, suggesting that reduction of Co to
2
+
Co might occur at higher temperatures. The large surface area and the exposed {110}
3
+
planes may lead to a larger concentration of Co on Co3O4-rod surface, which
appears to be beneficial to the catalytic oxidation of HCHO, as reported also in the
case of CO oxidation[12]. The lower reduction temperature (300 C) of Co3O4-rod
also indicates good oxidation ability than other similar Co3O4-based systems [9].
CO desorption curves are presented in Fig. 4 for the three samples. The amount of
2
CO2 desorbed from catalysts (Table S2) follows the order Co3O4-rod > Co3O4-sheet >
-
1
Co3O4-cube, and the Co3O4-rod has the largest total basicity, i.e. 4.19 mmol·g . The
desorption range below 100 C can be attributed to the decomposition of bicarbonates
formed on Brönsted sites (-OH groups)[25]. It could be inferred from the distinct
desorption peak at 79 C, that the concentration of surface hydroxyl groups is larger
on the surface of Co3O4-rod than on the surface of the other samples, result which is
consistent with the XPS and FTIR studies.
3
.5 Catalytic activity studies
9

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The catalytic activity of the three Co3O4samples towards the oxidation of
formaldehyde is shown in Fig. 5. The Co3O4-rod catalyst has the best catalytic activity
with the light-off temperature (10% conversion rate) at 60 C, which is lower than
that of Co3O4-sheet (90 C) and Co3O4-cube (110 C). The conversion of HCHO
completed at 120 C, where Co3O4-sheet and Co3O4-cube catalysts entirely convert
HCHO at 120 and 140 C, respectively. A stability test for formaldehyde oxidation
was performed over the period of 24 h at T = 120 C for the Co3O4-rod catalyst (Fig.
S5). The results indicate that the Co3O4-rod catalyst has an excellent catalytic
performance for formaldehyde oxidation comparing with other Co3O4-based materials
reported (Table S3), especially for the Co3O4 catalyst without any morphology
controlling in our previous work [28]. According to the characterization results,
Co3O4-rod shows distinct physical and chemical properties. Firstly, the relatively
larger specific surface area provides an increasing concentration of active sites.
3
+
Secondly, a greater concentration of Co species on the surface not only can be
considered as an important kinetic parameter of the oxidation reaction [12], but can
probably increase the concentration of oxygen anionic defects (low coordinated
oxygen species) and favor the adsorption of H2O molecules that lead to OH species by
dissociation [26]. The latter species facilitate oxidation of formate species (reaction
intermediate) to CO2 and H2O [27]. Therefore, the catalytic activity for formaldehyde
oxidation on the Co3O4-rod catalyst surface should be attributed to its large specific
3
+
surface area and surface concentrations of Co and OH groups.
1
0

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4
.Conclusions
The Co3O4-rod catalyst was prepared by a modified precipitation method for
formaldehyde oxidation, whereas Co O -sheet and Co O -cube were prepared by a
3
4
3
4
hydrothermal method for comparison. All three catalysts were of Co3O4 spinel
structure. Among the catalysts, the Co3O4-rod showed the best performance for the
formaldehyde oxidation reaction. The light-off temperature (10% conversion) of
formaldehyde oxidation on Co3O4-rod catalyst was at 60 C, much lower than that of
Co3O4-sheet and Co3O4-cube catalysts. The Co3O4-rod catalyst completely converted
HCHO at 120 C with excellent stability (24 h on reaction stream). It was further
analyzed that the Co3O4-rod not only had the highest specific surface area but also the
3
+
largest surface concentration of Co cations and -OH groups on the surface, which
were responsible for high HCHO oxidation activity.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21577088) and the National Key Research
&
Development Plan
（
2017YFC0211804）.
1
1

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2

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Figure Captions:
Fig.1. TEM image of Co3O4-rod catalyst.
Fig.2. FTIR spectra obtained over the Co3O4-rod, Co3O4-sheet and Co3O4-cube
catalysts.
Fig.3. Co 2p (a) and O 1s (b) X-ray photoelectron spectra obtained over the
Co3O4-rod, Co3O4-sheetand Co3O4-cubecatalysts.
Fig.4. CO2-TPD profiles recorded over the Co3O4-rod, Co3O4-sheet and Co3O4-cube
catalysts.
Fig.5. Catalytic performance in terms of HCHO conversion (%) of Co O -rod,
3
4
Co3O4-sheet and Co3O4-cube solids for the oxidation of formaldehyde. Reaction
−1
conditions: Total flow rate = 100 mLmin , [HCHO]in = 100 ppmv, [O2]in = 21 vol. %,
−1
N2 balance; GHSV = 69,000 h ; W = 0.1g.
1
7

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Fig. 1.
1
8

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Fig. 2.
Co O -cube
3
4
Co O -sheet
3
4
Co O -rod
3
4
1635.20
H O
2
3
430.97
OH
6
65.25
5
70.42
Co O4
3
4
000 3500 3000 2500 2000 1500 1000 500
-
1
Wave namber (cm )
1
9

ACCEPTED MANUSCRIPT
Fig. 3.
(a)
3
+
2+
Co
Co
Co O -cube
3
4
Co O -sheet
3
4
Co O -rod
3
4
7
70 775 780 785 790 795 800 805 810
B. E. (eV)
(b)
Co O -cube
3
4
Co O -sheet
3
4
OI
OII
Co O -rod
3
4
OIII
5
26 528 530 532 534 536 538 540
B. E. (eV)
2
0

ACCEPTED MANUSCRIPT
Fig. 4.
1
72
1
23
Co O -rod
3
60
3
4
7
0
9
1
77
3
53
1
10
Co O -sheet
3
4
1
38
7
Co O -cube
3
4
0
100
200
300
400
500
600
o
Temperature ( C)
2
1

ACCEPTED MANUSCRIPT
Fig. 5.
1
00
Co O -cube
3
4
Co
3
O
4
-sheet
8
6
4
2
0
0
0
0
0
Co O -rod
3
4
2
0
40
60
80
100 120 140
o
Temperature ( C)
2
2

ACCEPTED MANUSCRIPT
Table:
Table 1
3
+
2+
BET and surface Co /Co and OIII/OI atom ratios (XPS) for the three types of Co3O4
morphologies.
*
BET
Surface element molar ratios
sample
2
-1
3+
2+
(
m ·g )
Co /Co
OIII/OI
0.09
Co3O4-cube
Co3O4-sheet
Co3O4-rod
3.7
0.73
1.49
3.03
8.5
0.08
83.5
0.25
*
Calculated by fitting of the XPS peak.
2
3

ACCEPTED MANUSCRIPT
Graph abstract
Highlights:


Rod-like Co
3
O
4
was synthesized by a modified precipitation method.
shows good performance on low-concentration formaldehyde
Rod-like Co
removal.
3
O
4
3
+


Abundance of Co ions on rod-like Co
3
O surface exhibit high oxidation ability.
4
Hydroxyl species on the surface promote the activity of formaldehyde oxidation.
2
4