Sewage-free preparation of 2D metal oxides by a rapid freezing soft template method for extraordinarily activating solar-driven humidity VOC combustion

Article abstract of DOI:10.1039/d1cy00029b

Humidity volatile organic compounds (VOCs) cause significant harm to human beings. Two-dimensional (2D) metal oxides exhibit excellent activity towards humidity VOC catalytic combustion. Organic compounds as soft templates have been widely used in the preparation of 2D metal oxides. However, due to the structural fragility of soft templates, the mineralization process is necessary for acquiring 2D metal oxides, which results in the discharge of sewage. Herein, in this study, a novel soft template method based on rapid freezing to generally prepare 2D metal oxides without producing sewage is reported. It is worth emphasizing that the as-synthesized 2D Co₃O₄has an 8 nm level of thickness and high specific surface area, which enable it to exhibit high activity towards the combustion VOCs, including formaldehyde, CH₄, and acetone, and fully oxidize VOCs to CO₂due to the excellent CO oxidation activity of 2D Co₃O₄. Further, 2D Co₃O₄is added into the light selective absorbing device to obtain a high temperature of 295 °C under 1 sun irradiation. Consequently, 2D Co₃O₄shows a nearly 100% combustion efficiency towards humidity formaldehyde, CH₄, acetone, and CO under ambient sunlight irradiation, comparable to the corresponding thermal catalytic reactions and far beyond the photocatalytic reactions, showing the potential for practical applications.

Full text of DOI:10.1039/d1cy00029b

Catalysis
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Technology
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Sewage-free preparation of 2D metal oxides by a
rapid freezing soft template method for
extraordinarily activating solar-driven humidity
VOC combustion†
Cite this: Catal. Sci. Technol., 2021,
11, 2456
a
a
a
b
a
Dachao Yuan,‡ Luping Ma, Jianguo Zhao, Xianhua Bai, Jianchang Li,
a
b
Jianjun Hao* and Yaguang Li
*
Humidity volatile organic compounds (VOCs) cause significant harm to human beings. Two-dimensional
2D) metal oxides exhibit excellent activity towards humidity VOC catalytic combustion. Organic
(
compounds as soft templates have been widely used in the preparation of 2D metal oxides. However, due
to the structural fragility of soft templates, the mineralization process is necessary for acquiring 2D metal
oxides, which results in the discharge of sewage. Herein, in this study, a novel soft template method based
on rapid freezing to generally prepare 2D metal oxides without producing sewage is reported. It is worth
emphasizing that the as-synthesized 2D Co
area, which enable it to exhibit high activity towards the combustion VOCs, including formaldehyde, CH
3 4
and acetone, and fully oxidize VOCs to CO due to the excellent CO oxidation activity of 2D Co O
3 4
O has an 8 nm level of thickness and high specific surface
4
,
2
.
Received 4th January 2021,
Accepted 25th January 2021
3
Further, 2D Co O4 is added into the light selective absorbing device to obtain a high temperature of 295
3 4
°C under 1 sun irradiation. Consequently, 2D Co O shows a nearly 100% combustion efficiency towards
humidity formaldehyde, CH , acetone, and CO under ambient sunlight irradiation, comparable to the
4
DOI: 10.1039/d1cy00029b
corresponding thermal catalytic reactions and far beyond the photocatalytic reactions, showing the
potential for practical applications.
rsc.li/catalysis
ensuring the complete oxidation of VOCs into CO (ref. 12) to
avoid the secondary pollution of CO. Extensive efforts are
2
Introduction
Humidity volatile organic compounds (VOCs) are the main
pollutant gases, which have adverse effects on human
used to exploit metal oxides for numerous types of catalytic
1
3–15
reactions.
In particular, because of the bizarre electronic properties
1
,2
health.
Due to the strong adsorption between water
3
16,17
molecules and catalytic sites, VOCs are hardly adsorbed by
the catalytic sites. It makes VOCs difficult to react and usually
requires temperatures above 100 °C to avoid water
condensation, causing energy consumption. Thermal catalytic
combustion is the mainstream technology for the removal of
and large specific surface surfaces,
the metal oxides
synthesized in a two-dimensional (2D) nanosheet morphology
1
8,19
are excellent candidates for numerous applications.
To
meet the mass production and low cost, organic compounds,
21
such as P123 (ref. 20) and PVP, as soft templates have been
widely used to synthesize 2D materials. Due to the structural
fragility of soft templates, it is necessary to add a mineralizer
4
,5
VOCs with the merit of high catalytic efficiency.
oxides, such as Co
Metal
6
7,8
9,10
11
3
O
4
,
CeO
2
,
x
MnO ,
3
and LaMnO ,
2
0
have exhibited excellent VOC removal efficiency. Among
them, Co O has high activity towards CO oxidation to CO ,
to form 2D metal oxides. Therefore, it requires a complex
washing process to remove the mineralizer and obtain
products, resulting in sewage discharge, which is harmful to
the environment. In view of the green chemistry, overcoming
the structural fragility of soft templates to omit sewage for
synthesizing metal oxides as 2D nanosheets is highly
desirable.
3
4
2
a
College of Mechanical and Electrical Engineering, Hebei Agricultural University,
Baoding 071001, P. R. China. E-mail: hjjpaper@163.com
b
Hebei Key Lab of Optic-electronic Information and Materials, The College of
Physics Science and Technology, Institute of Life Science and Green Development,
Hebei University, Baoding, 071002, China. E-mail: liyaguang@hbu.edu.cn
Herein, in order to solve the sewage discharge problem of
the soft template method, we developed a novel soft template
method to synthesize 2D metal oxides. In this method, a
rapid freezing method was used to conquer the structural
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00029b
‡
These authors contributed equally.
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fragility of the soft templates to omit the mineralization
process and washing process, thus resulting in zero sewage
discharge during the synthesis of 2D metal oxides. Among
the as-synthesized 2D materials, 2D Co O has the merits of
solution containing Co-PVP into ice (Fig. 1d). Then, the water
was removed by freeze-drying to obtain Co-PVP. Fig. S1†
shows that the morphology of Co-PVP is a uniform two-
dimensional morphology, and as depicted in Fig. 1e, we
could use the rapid freezing method to retain the 2D
morphology of Co-PVP without adding any mineralizer.
Therefore, the washing process and wastewater were absent
from the whole synthesis process. After the subsequent
3
4
a porous structure and large specific surface area, which
showed excellent activity towards the combustion of
numerous humidity VOCs. Further, we developed a light
selective absorbing device to nearly fully absorb sunlight and
confine the sunlight converted heat energy to convert 1 sun
annealing treatment, we could obtain Co
morphology, named as 2D Co O (Fig. 1f).
3
O
4
in 2D
2
2
irradiation to
a
300 °C level high temperature.
3
4
Consequently, the system of 2D Co O supported by the light
From the XRD pattern in Fig. S2a,† we can see that cubic
phase Co (JCPDS no. 43-1003) existed in 2D Co . The
peaks at 19°, 31.3°, 36.9°, 38.2°, 44.5°, 55.6°, 59.4°, and 65.3°
3
4
selective absorbing device exhibited high activity in ambient
sunlight-driven humidity VOC combustion, which are
comparable to corresponding thermal catalytic reactions and
superior to the photocatalytic reactions.
3
O
4
3 4
O
are assigned to the (111), (220), (311), (222), (400), (422),
2
5
(511), and (440) planes, respectively.
SEM and TEM
techniques were employed to visualize the morphology and
structure of 2D Co O . It was clear that 2D Co O exhibited a
3
4
3 4
Experimental section
2
D structure (Fig. 2a). The 2D morphology of 2D Co
further confirmed by enlarged SEM and TEM images
Fig. 2b and c), and they revealed that 2D Co has a porous
3 4
O was
Experimental details are given in the ESI.†
(
3 4
O
morphology, which might be caused by the decomposition of
PVP. The high-resolution TEM (HRTEM) image in Fig. 2d
Results and discussion
Taking Co
3
O
4
as an example, we showed the difference in the
showed that 2D Co
(220) plane of cubic Co
The thickness of 2D Co
3 4
EDS mapping of 2D Co O , we could see that oxygen and
3 4
O exhibited clear lattice fringes from the
preparation principle between the traditional soft template
method and rapid freezing soft template method. In this
work, PVP was used as a soft template. Co(NO
3
O
4
with a lattice spacing of 0.286 nm.
O
was 8.8 nm (Fig. S2b†). From the
3
4
3
)
2
·6H
2
O and
PVP were added to water to make a transparent aqueous
solution. It is known that PVP is a typical soft template that
cobalt were uniformly dispersed (Fig. S2c and d†). Based on
this strategy, we successfully obtained other 2D metal oxides,
including CuO (Fig. S3a and d†), ZnO (Fig. S3b and e†), and
2
1
can bond to Co ions as Co-PVP
and form a two-
2
3
dimensional micelle, as shown in Fig. 1a. However, Co-PVP
as a soft template is fragile, and the movement of the
solution and solute molecules could lead to the collapse of
the soft templates. Therefore, in the traditional soft template
Fe
method for synthesizing 2D metal oxides.
The nitrogen isotherms of 2D Co show typical IV type
2 3
O (Fig. S3c and f†), representing the universality of this
3 4
O
with hysteresis loops, indicating that the mesoporous
2
0,24
method,
added to form 2D hydroxides to obtain enough rigidity
Fig. 1b), and the impurities are formed as well. The
mineralizers (such as urea and Na CO ) were
2 3
(
subsequent washing process was used to remove impurities,
thus inevitably producing wastewater, as shown in Fig. 1c. In
our method, we used liquid nitrogen to quickly freeze the
Fig.
3 4
1 Preparation diagram of 2D Co O via the traditional soft
template method (a–c) and rapid freezing soft template method (a and
d–f) at different stages.
Fig. 2 (a and b) SEM images, (c) TEM image, (d) HRTEM image of 2D
Co O .
3 4
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Fig. 3 (a) N
2
adsorption–desorption isotherm of 2D Co
3 4
O . (b and c)
XPS spectra of Co and O elements from 2D Co
absorption spectra of 2D Co
3 4
O
. (d) UV-vis-IR
3
4
O .
structure is present in 2D Co O (Fig. 3a). This can be further
3
4
Fig.
4
Temperature-dependent formaldehyde oxidation (a), CH4
proved by the pore size distribution (Fig. S4†). The specific
3
oxidation (b), acetone oxidation (c), and CO oxidation (d) of 2D Co O
4
2
−1
surface area of 2D Co
3
O
4
was calculated to be 115.9 m g .
3 4
and nano Co O .
The high surface area and porous structure of 2D Co O
3
4
would be beneficial for the catalytic reactions in terms of the
mass transfer and accessibility of active catalysts to the
reactant. As displayed in Fig. 3b, the spectrum of Co 2p from
VOC oxidation performance of 2D Co
3
O
4
should be ascribed
to the porous structure and large surface area. Fig. S6–S8†
shows that nearly 100% selectivity towards CO formation
was achieved in the oxidation of formaldehyde, CH , and
acetone at 120 °C, 300 °C, 300 °C through 2D Co O . Fig. 4d
2
D Co
3
O
4
showed spin–orbit splitting into 2p1/2 and 2p3/2
2
components. Both components contain the same qualitative
information, and Co 2p3/2 had two components at the band
energy (BE) values of 777.95 and 781.75 eV, and Co 2p1/2 also
had two components with BE of 795.35 and 796.35 eV, which
4
3
4
shows that 2D Co
oxidize 97% and 61% of CO to CO
The excellent CO oxidation activity should be the main
reason for the full oxidation of VOCs of 2D Co . Fig. S9†
3
O
4
and nano Co
3 4
O could catalytically
at 150 °C, respectively.
2
3
+
2+
26
were assigned to Co and Co ions, respectively. Fig. 3c
shows that O1s had signals displayed at the BE values of
3 4
O
5
29.9 and 531.3 eV. The former was surface lattice oxygen
shows the catalytic stability characterization of formaldehyde,
2
7,28
(Olatt),
whereas the latter was surface adsorption oxygen
The presence of surface-active oxygen species and
CH , CO, and acetone at the temperature of 120 °C, 300 °C,
4
2
6,29
(O
ads).
3 4
150 °C, 300 °C, respectively, and it showed that 2D Co O
exhibited excellent stability towards all the VOC tests.
3
+
Co cationic species were beneficial to promote the catalytic
process. The UV-vis-IR spectrum (Fig. 3d) shows a typical
light absorption spectrum of 2D Co
absorption throughout the whole solar spectrum.
D Co O showed high activity towards numerous types of
humidity VOC catalytic reactions. Fig. 4 shows the catalytic
performance of 2D Co , compared with that of nano Co
synthesized by the sol–gel method (Fig. S5†), which had a
With efficient catalysts, we plan to use sunlight to replace
traditional energy on driving the catalytic reactions. At
present, the main catalysis driven by sunlight is
photocatalysis, i.e., sunlight-photogenerated electron and
hole-chemicals. In addition to photocatalysis, the sunlight-
3 4
O with full light
2
3
4
4
,24,30,31
3
O
4
3
O
4
driven thermal catalysis is another strategy,
that is,
sunlight thermal energy chemicals. As shown in Fig. 5a,
different from the direct irradiation of sunlight in
photothermal catalysis, we constructed a reaction tube as a
light selective absorbing device by decorating it with a
2
−1
porous morphology and specific surface area of 33.2 m g
Fig. S5c†). The feed gas was a mixture of O /N with a
volume ratio of 20%/80% passed through water, and the
concentration of humidity VOCs (formaldehyde, CH , and
acetone) was 1000 ppm. As shown in Fig. 4a, nano Co
(
2
2
4
3 4
spectrally selective coating and vacuum layer. 2D Co O was
3
O
4
loaded in the interior of the reaction tube. In this system, the
spectrally selective coating was used to absorb sunlight and
generate thermal energy. The light absorbance of the
spectrally selective coating was completely different from that
of 2D Co O . The spectrum absorption curve of spectrally
shows poor catalytic activity towards the complete conversion
of 1000 ppm of formaldehyde at a high temperature of 180
°
C, while 2D Co
formaldehyde at a low temperature of 120 °C. Not only
formaldehyde, but 2D Co could also catalytically oxidize
7% of CH and 98% of acetone at 300 °C, in comparison
with 18% of CH and 55% of acetone using nano Co O at
3 4
O can entirely oxidize 1000 ppm of
3
4
3
O
4
3 4
selective coating and 2D Co O indicated a high light
absorption ranging from 0.4–2.0 μm (Fig. 5b), revealing the
nearly full sunlight absorption property. In addition,
9
4
4
3 4
corresponding temperatures (Fig. 4b and c). The superior
3 4
spectrally selective coating and 2D Co O showed nearly zero
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Fig. 5 (a) Sketch map of the reaction tube with the spectrally selective
coating and vacuum layer. (b) Normalized light absorption spectra of
the spectrally selective coating and 2D Co
μm. (c) The IR mapping of the spectrally selective coating under 1 sun
irradiation. (d) The temperature of 2D Co loaded in the spectrally
3 4
O ranging from 0.4 to 20
3
O
4
selective coating-based reaction tube under sunlight irradiation with
different intensities.
Fig. 6 The efficiency of formaldehyde oxidation (a), CH
acetone oxidation (c), and CO oxidation (d) for 2D Co
3 4
D Co O under sunlight irradiation with different intensities.
4
oxidation (b),
O + device and
3
4
2
light absorption and violent light absorption, respectively,
and also at the infrared (IR) region of 3–20 μm (Fig. 5b).
Kirchhoff's law has evidenced that the materials having high
showed a relatively higher efficiency. Table S1† lists the state-
of-the-art photocatalytic formaldehyde removal performances.
To the best of our knowledge, the best reported turnover
3
2
IR absorption could emit more heat energy by IR radiation.
−
1
We used an IR camera to detect the temperature of the
spectrally selective coating and 2D Co O under 1 sun
frequency (TOF) value was 0.138 min
irradiation. We increased the inlet air rate to 500 sccm to
test the maximum formaldehyde oxidation rate of the 2D
under 1 sun
3
6
3
4
−
2
irradiation (1 kW m ). The infrared (IR) images show that
the temperature of the spectrally selective coating and 2D
Co
3
O
4
+
device under
1
sun irradiation, and the
Co O4 was 303 °C (Fig. 5c) and 75.4 °C (Fig. S10†),
formaldehyde oxidation performance is depicted in Fig. S11.†
Fig. S11† shows that the maximum formaldehyde oxidation
concentration was about 47 293 ppm when the light intensity
increased to 1 sun. We assumed that all Co atoms in 2D
3
respectively, under 1 sun irradiation. It confirmed that
the spectrally selective coating could create ultrahigh
temperature just under ambient sunlight irradiation due to
its property of high sunlight absorption and low IR radiation.
3 4
Co O were used in this reaction, and TOF of formaldehyde
−
1
As 2D Co
3
O
4
was loaded in the light selective absorbing
oxidation through this system was 5.1 min under 1 sun
irradiation without additional energy import. It confirmed
that the conversion rate of formaldehyde through the light
device, the high temperature could be used to heat 2D Co O ,
3
4
which reached 140 and 295 °C under 0.25 and 1 sun
irradiation, respectively (Fig. 5d), and capable of operating
under numerous types of catalytic reactions.
3 4
selective absorbing device-assisted 2D Co O was 37 times of
the best reported photocatalytic formaldehyde decomposition
value. The formaldehyde catalytic oxidation stability of the
We have carried out the photocatalysis of 2D Co
named as 2D Co ) and sunlight-driven thermal catalysis of
D Co O loaded in a light selective absorbing device, as
3 4
O
(
2
3
O
4
2D Co
the formaldehyde oxidation concentration over the light
selective absorbing device-supported 2D Co remained the
3 4
O + device was also recorded. As shown in Fig. S12,†
3
4
shown in Fig. 5a (named as 2D Co
3
O
4
+ device). Fig. 6a–d
3 4
O
show that 2D Co + device could eliminate almost 100%
3
O
4
same after a 10 h test, indicating the highly stable sunlight-
driven formaldehyde oxidation under 1 sun irradiation.
formaldehyde, CH , acetone, and CO when the irradiated
4
sunlight was higher than 0.25 sun, 1 sun, 1 sun, and 0.3 sun,
respectively, comparable to the thermal catalytic humidity
VOC combustion, as shown in Fig. 4. However, 2D Co O only
Conclusion
3
4
showed 10%, 0%, 0%, and 0% of the elimination rate for
formaldehyde, CH , acetone, and CO, respectively, under 1
sun irradiation. Compared with other sunlight-driven VOC
elimination systems, under sun irradiation, the
photocatalytic removal efficiency for CH , acetone, and CO
was very low, almost only 1/100 of the efficiency of 2D Co O
In this study, we have developed a rapid freezing soft
template method to prepare 2D metal oxides, including
Co O , CuO, ZnO, and Fe O , without producing wastewater.
4
3
4
2 3
1
The obtained 2D Co O catalyst possessed a porous structure
3 4
2
−1
4
and a large specific surface area of 115.9 m g . As a result,
the 2D Co O catalyst showed the complete oxidation of
3
4
3 4
3
3–35
+
device,
and photocatalytic removal of formaldehyde
humidity formaldehyde, CH , acetone, and CO at 120 °C, 300
4
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C, 300 °C, and 150 °C, respectively. Furthermore, a novel
Catalysis Science & Technology
°
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can heat 2D Co O to 140 and 295 °C under 0.25 and 1 sun
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3
4
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3
4
selective absorbing device could eliminate nearly 100%
humidity formaldehyde, CH , acetone, and CO when the
4
irradiated sunlight was higher than 0.25 sun, 1 sun, 1 sun,
and 0.4 sun, respectively. The TOF of formaldehyde oxidation
−
1
through this system was 5.1 min under 1 sun irradiation,
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Conflicts of interest
There are no conflicts to declare.
1
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9 H. Liu, L. Li, C. Guo, J. Ning, Y. Zhong and Y. Hu, Chem.
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