Easy Synthesis of Ordered Mesoporous CarbonâCarbon Nanotube

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Easy synthesis of ordered mesoporous carbon-carbon nanotube nanocomposite as a promising support for CO2 photoreduction Yangang Wang, Qing Cai, Mingcui Yao, Shifei Kang, Zhigang Ge, and Xi Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03974 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Sustainable Chemistry & Engineering

Easy synthesis of ordered mesoporous carbon-carbon nanotube nanocomposite as a promising support for CO2 photoreduction Yangang Wang*ab, Qing Caib, Mingcui Yaoa, Shifei Kangb, Zhigang Gea, Xi Li*a a

College of Biological Chemical Science and Engineering, Jiaxing University, No. 56

South Yuexiu Road, Nanhu District, Jiaxing 314001, China b

Department of Environmental Science and Engineering, University of Shanghai for

Science and Technology, No. 516 Jungong Road, Yangpu District, Shanghai 200093, China

*

Corresponding

authors.

Tel/Fax:

+86

573

83640037.

E-mail

[email protected] (Y. Wang), [email protected] (X. Li).

ACS Paragon Plus Environment

address:

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: An easy solid-liquid grinding hard-templating route was demonstrated for the preparation

of

ordered

mesoporous

carbon-carbon

nanotube

(OMC-CNT)

nanocomposite by using natural soybean oil as a carbon source, which is a kind of reproducible liquid seed fat derived from soybean. The as-prepared OMC-CNT nanocomposite has bimodal mesostructures with relative large pore volume and high surface area which can potentially be used as support in real catalysis/photocatalysis. Indeed, we have demonstrated that such OMC-CNT nanocomposite used as a support for g-C3N4 catalyst exhibited an excellent photocatalytic performance in the reduction of CO2 with H2O. The excellent photocatalytic activity of the g-C3N4/OMC-CNT catalyst can be ascribed to its unique structure of carbon support with numerous favorable properties such as interconnected and hierarchical mesostructure, large surface area and high electronic conductivity, which contributes to the diffusion and mass transfer of reactants or products, meanwhile, the high electronic conductivity of carbon nanotube will promote the separation and migration efficiency of photogenerated electron-hole pairs during photocatalytic process. KEYWORDS: carbon nanotube, mesoporous carbon, nanocomposite, CO2 photoreduction


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INTRODUCTION The recently emerged nanostructured carbons such as carbon nanotubes (CNTs), ordered mesoporous carbon (OMC), and graphene have greatly promoted the science and engineering of carbon materials. Since the first report of CNTs with a cylindrical nanostructure in 1991 by Iijima,1 CNTs have gained a great deal of scientific and technological interest due to their high electronic conductivity, admirable mechanical and physical properties, amazing thermal conductivity, and excellent chemical stability which make them promising for important applications as catalytic supports and electrode materials.2-10 However, the high production cost, relatively low specific surface areas and entanglement problem restrict their wide-spread applications.11 To overcome these problems, recently some groups have reported to prepare novel OMC-CNT nanocomposites by combining the advantages of both carbon materials,12-16 these OMC-CNT nanocomposites with three-dimensionally (3D) interconnected pore structure, high specific surface area, and improved electronic conductivity exhibited excellent electrochemical performances in areas of energy storage systems.15,16 To date, chemical vapour deposition (CVD) technique is a commonly used approach for synthesizing OMC-CNT nanocomposites, and the employed carbon sources including benzene, acetylene, metal phthalocyanine, methane.11-13,15,17 Nonetheless, most of these carbon precursors produced from fossil fuels may not be sufficiently available in near future. What is more, because of the disadvantages of the fossil fuels such as flammable, explosive, toxic, and poor controllability, it is not very



safety to synthesize OMC-CNT nanocomposites by such traditional CVD method. As a result, it is crucial to find low-cost and reproducible carbon sources for preparing OMC-CNT nanocomposites with a controllable and easy method. In this report, we present an easy strategy to synthesize OMC-CNT nanocomposite by using natural soybean oil as a carbon source, which is a kind of reproducible liquid seed fat derived from soybean. The synthesis was made using a simple solid-liquid grinding hard-templating route as reported previously by our research group.18 In a typical synthesis, soybean oil and iron oxide nanoparticles modified mesoporous silica SBA-15 with a weight ratio of 2:1 were mixed and grounded for 30 min to obtain a uniform mixture. Pyrolysis experiment was performed at 900 oC under N2 ambient with a gradual rise of temperature, during which Fe nanoparticles were reduced in situ and would auto-catalyze for the formation of carbon nanotubes. The resultant nanocomposite was treated with sodium hydroxide solution to remove the SBA-15 template, the detailed description of preparation procedure is included in the experimental section. Meanwhile, the as-prepared OMC-CNT nanocomposite used as novel support for g-C3N4 photocatalyst displayed an excellent catalytic performance in the photoreduction of CO2 with H2O to produce value-added fuels. EXPERIMENTAL SECTION Synthesis of Iron Oxide Nanoparticles Modified Mesoporous SBA-15 Template Ordered mesoporous silica SBA-15 was prepared based on previous procedure except for the use of 10-fold amount.19 Iron oxide nanoparticles modified SBA-15 template


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was achieved by a traditional wetness impregnation path as follows. Typically, 10 mmol Fe(NO3)3•9H2O were dissolved into 25 ml of ethanol, after that 2 g of SBA-15 template was added in order to obtain a homogeneous mixture under stirring for 4h. The obtained mixture was dried under room temperature followed by calcination at 500 oC for 4 h with a heating ramp of 2 oC min-1. Synthesis of OMC-CNT Nanocomposite In a typical synthesis process, 1.5 g of above obtained iron oxide nanoparticles modified SBA-15 template and 3 g of soybean oil were mixed and grounded for 30 min to obtain a uniform mixture. Then the mixture was moved to a tube furnace and heated to 900 oC under N2 atmosphere for 4h with a linear heating ramp of 2 oC min-1, during which Fe nanoparticles were reduced in situ and would auto-catalyze for the formation of carbon nanotubes on the mesoporous carbon. The resultant nanocomposite was etched by using a 2M sodium hydroxide aqueous solution to remove the SBA-15 template at room temperature for three times. The ordered mesoporous carbon-carbon nanotube (denoted as OMC-CNT) was obtained by filtration, washed with deionized water and dried overnight at 70 oC. Synthesis of g-C3N4/OMC-CNT Nanocomposite Catalyst The g-C3N4 used in this work was synthesized by heating melamine to 550 oC for 4h in an alumina crucible with a cover under air atmosphere according to a previous report.20 The g-C3N4/OMC-CNT nanocomposite catalyst was synthesized with a simple ultrasound-assisted method at room temperature.21 Typically, 2.5 g of g-C3N4 was added into 25 mL methanol and sonicated for 30 min. Then, 0.5 g of the



OMC-CNT nanocomposite was added into the above solution and stirred for 24 h under dark at room temperature. The nanocomposite catalyst was obtained after the methanol was evaporated by using a rotary evaporator at 60 oC. Structural Characterization The powder X-ray diffraction patterns were operated on a Bruker D8 Advanc X-ray diffractometer using CuKα source. Scanning electron microscopy (SEM) was performed on a TESCAN VEGA-3-SBH scanning electron microscope operating at 25 kV. Transmission electron microscope (TEM) images were obtained using a JEOL JEM-2010 electron microscope with an acceleration voltage of 200 kV. Nitrogen sorption isotherm at 77 K was carried out on a BeiShiDe 3H-2000PS4 apparatus to measure the specific surface area, pore volume, and pore size distribution. Electrochemical impedance spectroscopy (EIS) and photocurrent measurements were recorded using a conventional three-electrode system on a CHI 660E electrochemical analyzer, where the working electrode was the prepared sample film, the counter electrode was a Pt flake, and the reference electrode was Ag/AgCl. Photocatalytic Measurements Photocatalyic CO2 reduction experiments were performed in a home-made photoreaction apparatus (2700 mL) with a quartz window in front of light irradiation. In each test, catalyst powder (0.1g) was uniformly dispersed on the stainless omentum which was installed in the middle of reactor. Before the test, the apparatus was evacuated several times and purged with the CO2 and water vapor mixture (water vapor was bubbled by CO2, and their volume concentration were 4.5 % and 95.5 %,


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respectively.) at 20 ml min-1 for 2 h to reach sorption equilibrium. Then the apparatus was closed tightly and the Xe arc lamp (500W) was switched on to start the reaction. The distance between the substrate and Xe lamp was about 15 cm, the reaction pressure and temperature were controlled at 110 KPa and 30 oC, respectively. During the irradiation, the gas phase products were collected at different time periods and measured by gas chromatography. RESULTS AND DISCUSSION The morphology and structural features of the obtained OMC-CNT nanocomposite were elucidated by SEM and TEM. SEM images in Figure 1(a,b) showed that the rod-like SBA-15 template (Figure S1, Supporting Information) was replicated in the resulting carbon nanocomposite and a great number of CNTs were formed on the surface of rod-like primary particles, indicating the successful preparation of the OMC-CNT nanocomposite. A closer observation from TEM images in Figure 1(c-f) revealed that the presence of many curved CNTs with 30-60 nm in diameter protruded from the external surface of templated OMC particles (Figure 1d and e), and these OMC primary particles were well interconnected by the CNTs to form a dandelion-shaped nanocomposite. HRTEM images in Figure1f and Figure S2 (Supporting Information) confirm that the formed CNTs are typical multi-walled carbon nanotubes with paralled graphene layers (~8 nm of wall thickness). The low-angle XRD pattern for the OMC-CNT nanocomposite in Figure 2a shows a well-defined diffraction peak indexed as (100) reflection of two-dimensional hexagonal (P6mm) structure, which indicates that the OMC-CNT nanocomposite has



retained some regular mesostructures, as proved by above TEM image in Figure 1d. From the wide-angle XRD pattern (Figure 2a inset), it is observed that besides a small peak near ～45o which is ascribed to the (110) reflection of α-Fe, other two distinct peaks appeared at around 25 and 44 o can match well with graphitic carbon. Figure 2b reveals the nitrogen adsorption-desorption isotherm of the OMC-CNT nanocomposite. The isotherm shows a typical IUPAC type-IV curve with an H1 and H2 hysteresis loops at P/P0 of 0.45-0.95, which suggests the existence of hierarchical pore distribution. The pore size and distribution curve derived from the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method is given in Figure 2b as inset. It can be found that the OMC-CNT nanocomposite had hierarchical pore structures centered at around 4.0 and 38 nm which can be attributed to the dissolution of mesoporous silica walls and the space between the particles of OMC-CNT nanocomposite. Such bimodal mesostructures are especially useful for heterogeneous catalytic and electrochemical systems because of the easy diffusion/transfer of reactive molecules.22-28 Additionally, this OMC-CNT nanocomposite has a relative high Brunauer-Emmett-Teller (BET) surface area (275.9 m2/g) and large pore volume (1.079 cm3/g), which could endow it with a wide range of potential applications as catalyst/photocatalyst supports, absorbents or electrode materials. The problem of global warming caused by the rapidly increasing atmospheric carbon dioxide (CO2) concentration from the combustion of fossil fuels has raised serious security concerns. Recent progess suggests that the photocatalytic reduction of CO2 with H2O into value-added fuels by semiconductors is a newly-found sustainable


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path to mitigate the growth of CO2 emissions and relieve the future energy supply demand.29-31 In the present work, we intend to explore the potential use of our prepared OMC-CNT nanocomposite as a support for g-C3N4 catalyst in the photoreduction of CO2 with H2O. OMC-CNT supported g-C3N4 catalyst was prepared by a simple self-assembly coating approach according to a previous study,21 and the weight ratio of g-C3N4 to OMC-CNT was 5. The XRD pattern and photograph of the obtained g-C3N4/OMC-CNT was given in Figure S3 (Supporting Information), it can be observed that the bright yellow of g-C3N4 became brownish-black color after supporting on the OMC-CNT nanocomposite. It was suggested that the darkened color of photocatalysts would enhance light absorption from ultraviolet to visible area and thus improve their photocatalytic performance.32-34 TEM images of the g-C3N4/OMC-CNT are given in Figure S4 (Supporting Information). It can be clearly seen that the surface of OMC-CNT was homogeneously decorated with sheet-like g-C3N4, and no significant separation between g-C3N4 and OMC-CNT because of the strong self-assembly property of g-C3N4 after sonication.21,35 The photocatalytic reactions were performed in a home-made stainless steel reactor under simulated solar irradiation and the main gaseous products were measured by gas chromatography. For comparison, two control experiments were conducted under the same operating conditions: (1) g-C3N4/OMC (OMC was prepared using soybean oil as precursor according to our previous method,36 and the weight ratio of g-C3N4 to OMC was also controlled at 5); (2) bare g-C3N4. Figure 3 illustrates the evolution curves of two main gaseous products as functions of the irradiation time over all samples, and the yields



of CO and CH4 are increased with the prolonging of reaction time. The g-C3N4/OMC-CNT catalyst exhibits a significantly higher activity than that of the two control experiments. After 7 h of simulated solar irradiation, the CO and CH4 yield of 25.1 and 14.7 umol g-1-cat., respectively, are achieved on the g-C3N4/OMC-CNT catalyst, noticeably both values are about 1.5 and 2 times better than the products on g-C3N4/OMC and bare g-C3N4, respectively. In addition, we also detected a small amount of oxygen (O2) in the produced gaseous products by using a TCD-based gas chromatograph. Under simulated solar irradiation, the dominating reactions involved in the photocatalytic reduction of CO2 with H2O to produce CO and CH4 are listed below:37-39 hv − + g-C3 N 4 ⎯⎯ → ecb + h vb

(1)

2H 2 O + 4h + → 4H + + O2

(2)

CO 2 + 2H + + 2e − → CO + H 2O

(3)

CO2 + 8H + + 8e− → CH 4 + 2H 2O

(4)

According to above Eqs. (1)-(4), the electrons ( e − ) and holes ( h + ) are produced with the help of the light and semiconductor photocatalyst. The excited valence band electrons on the conduction band will react with the surface-absorbed CO2 molecules to produce value-added fuels (CO and CH4), whereas the holes staying on the valence band could oxidize H2O to generate oxygen (O2). CO2 photoreduction process involves a multi-electron step, it respectively requires 2 and 8 electrons to form 1CO and 1CH4 molecule, 4 holes and 2H2O molecules are needed to form 1O2 molecule. Thereby, not only CO and CH4 are formed, but also O2 is produced in this reaction.


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Therefore, the excellent photocatalytic activity of the g-C3N4/OMC-CNT catalyst can be ascribed to its unique structure of carbon support, which accounts for various beneficial properties. First, the interconnected and hierarchical mesostructure with a large surface area facilitates the diffusion of the reactants into the bulk of catalysts and accelerates the intraparticulate molecular transfer. Meanwhile, the bright yellow of g-C3N4 supported on the back OMC-CNT support enhances light absorption because of the darkened color of nanocomposite catalyst. Finally, the high electronic conductivity of carbon nanotube on OMC-CNT support will promote the separation and migration efficiency of photogenerated electron-hole pairs during CO2 photoreduction process, resulting in an increased CO2 photocatalytic reduction activity under solar irradiation (Figure 4). In order to provide reasonable evidence and to verify the above conclusion, the EIS analysis was carried out to investigate the separation process of photogenerated electron-hole pairs. Figure 5a shows the EIS Nyquist plots of the g-C3N4/OMC-CNT, g-C3N4/OMC and bare g-C3N4. Generally, a larger arc radius of the EIS Nyquist plot indicates a lower charge transfer efficiency.40,41 Evidently, the arc radius of the g-C3N4/OMC-CNT is much smaller than that of the g-C3N4/OMC and bare g-C3N4, suggesting that the interfacial electron transfer and effective separation of photoinduced electron-hole pairs are significantly enhanced in the g-C3N4/OMC-CNT catalyst. The higher electron transfer efficiency and more efficient separation capability of photoinduced electron-hole pairs for the g-C3N4/OMC-CNT catalyst were also confirmed by its photocurrent response in Figure 5b. The photocurrent



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density of g-C3N4/OMC-CNT is about 4.6 µA cm-2, which is much larger than that of g-C3N4/OMC (3.2 µA cm-2) and bare g-C3N4 (0.5 µA cm-2). The stability is also very important with respect to the practical application of a photocatalyst.

Herein,

the

photocatalytic

reduction

of

CO2

reaction

on

g-C3N4/OMC-CNT catalyst was repeated up to six times under the same experiment conditions, where the illumination time was set to 7 h for every cycle. As shown in Fig. 6, no apparent deactivation was observed even if after six consecutive runs (the CO2 photoreduction efficiency declined by only 5 %), indicating that the g-C3N4/OMC-CNT catalyst possesses very good stability during the CO2 photoreduction reaction. CONCLUSIONS Ordered mesoporous carbon-carbon nanotube nanocomposite was successfully prepared by a simple solid-liquid grinding hard-templating route using natural soybean oil and iron oxide nanoparticles modified mesoporous silica SBA-15 template. The as-prepared OMC-CNT nanocomposite has bimodal mesostructures with relative large pore volume and high surface area which can potentially be used as support in real catalysis/photocatalysis. Indeed, we have demonstrated that this OMC-CNT nanocomposite used as a support for g-C3N4 catalyst displayed an excellent catalytic performance for CO2 photoreduction with H2O. This simple strategy is effective for the generation of novel nanostructured carbon supports for a range of catalytic reactions.


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ASSOCIATED CONTENT Supporting Information SEM image of ordered mesoporous SBA-15 template; HRTEM images of the formed carbon nanotubes; XRD pattern (a) and photograph (b) of the g-C3N4/OMC-CNT catalyst; TEM images of the g-C3N4/OMC-CNT catalyst which were taken from different regions. ACKNOWLEDGEMENTS This work was partially supported by the National Natural Science Foundation of China (Grant No. 21103024 and No. 61171008), Yancheng Huanbo Energy Technonogy Limited Company, Longyuan Youth Innovative Talents Program, and Technology Development Project of Jiaxing University and University of Shanghai for Science and Technology. REFERENCES (1) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature. 1991, 354, 56-58. DOI: 10.1038/354056a0 (2) Fu, Y.; Zhang, L.; Chen, G. Preparation of A Carbon Nanotube-copper Nanoparticle Hybrid by Chemical Reduction for Use in the Electrochemical Sensing of Carbohydrates. Carbon. 2012, 50, 2563-2570. DOI: 10.1016/j.carbon.2012.02.014 (3) Baughman, R. H.; Zakhidov, A. A.; De Heer, W. A. Carbon Nanotubes-the Route toward Applications. Science. 2002, 297, 787-792. DOI: 10.1126/science.1060928



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Figure 1. SEM images (a, b) and TEM images (c-f) of OMC-CNT nanocomposite.



(b) 750 600

(101)

3 -1

Vads(cm g )

(002)

Fe (110)

(100)

(a)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

1

2

20

30

40

2-Theta

3

4

2-Theta

50

60

450 300 10

70

Pore size (nm)

150

5

6

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

Figure 2. (a) XRD patterns and (b) N2 adsorption-desorption isotherm and corresponding pore size distribution curve (inset) of the OMC-CNT nanocomposite.


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(a)

(b) Yields of CH4 (µmol/g-cat.)

25

Yields of CO (µmol/g-cat.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g-C3N4/OMC-CNT 20

g-C3N4/OMC bare g-C3N4

15 10 5 0

15

100

200

300

400

g-C3N4/OMC bare g-C3N4

9 6 3 0

0

g-C3N4/OMC-CNT

12

0

Irradiation time (min)

100

200

300

400

Irradiation time (min)

Figure 3. Yields of CO (a) and CH4 (b) as functions of irradiation time over the g-C3N4/OMC-CNT, g-C3N4/OMC and bare g-C3N4.



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CO, CH4 Reduction

CO2

CB

OMC-CNT

g-C3N4

VB

H2O Oxidation

H+, HO•

Figure 4. Plausible mechanism for the photoreduction of CO2 with H2O over the g-C3N4/OMC-CNT.


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(b)

3500

g-C3N4/OMC-CNT g-C3N4/OMC

2500

bare g-C3N4

2000 1500 1000 500 0

0

1000

2000

7.5

g-C3N4/OMC-CNT

-2

3000

Current Density(µA.cm )

(a)

-Z''/ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3000

4000

5000

6.0

g-C3N4/OMC

off

on

bare g-C3N4

4.5 3.0 1.5 0.0

0

Z'/ohm

30

60

Time(s)

90

120

Figure 5. EIS spectra (a) and photocurrent responses (b) of the g-C3N4/OMC-CNT, g-C3N4/OMC and bare g-C3N4.



Yields of Products (µmol/g-cat.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25 20 15 10 5 0

CO CH4

0

5 10 15 20 25 30 35 40 45 Time (h)

Figure 6. Cyclic photocatalytic performance of g-C3N4/OMC-CNT catalyst under simulated solar irradiation.


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For Table of Contents Use Only

CO, CH4 Reduction

CO2

CB g-C3N4

VB

OMC-CNT

H2O Oxidation

H+, HO•

Reproducible soybean oil was employed as carbon source to synthesize ordered mesoporous carbon-carbon nanotube, which can be used as a promising support for CO2 photoreduction.



Table of Contents Graphic

CO, CH4 Reduction

CO2

CB g-C3N4

VB

OMC-CNT

H2O Oxidation

H+, HO•

Reproducible soybean oil was employed as carbon source to synthesize ordered mesoporous carbon-carbon nanotube, which can be used as a promising support for CO2 photoreduction.


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