Synthesis and CO2 capture properties of mesoporous carbon nitride materials

Chemical Engineering Journal 203 (2012) 63–70

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Synthesis and CO2 capture properties of mesoporous carbon nitride materials Qing-Fang Deng a, Lei Liu a, Xiu-Zhen Lin a, Gaohui Du b, Yuping Liu a, Zhong-Yong Yuan a,⇑ a b

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

h i g h l i g h t s " Mesoporous carbon nitrides and carbon nitride–carbon composites were synthesized for CO2 capture. " High CO2 adsorption capacity with good reusability and CO2/N2 selectivity is obtained. " The introduction of carbon coating layer improved the microporosity, favoring the CO2 capture. " Both the high surface area and the presence of N contributed to the high CO2 capture capacity.

a r t i c l e

i n f o

Article history: Received 26 April 2012 Received in revised form 27 June 2012 Accepted 27 June 2012 Available online 5 July 2012 Keywords: Mesoporous Carbon nitride Carbon CO2 adsorption

a b s t r a c t Mesoporous carbon nitride (MCN) and carbon nitride–carbon (MCN/C) composites with a partly graphitized structure are synthesized by using mesoporous silica SBA-15 as a hard template and ethylenediamine and carbon tetrachloride as precursors, possessing uniform mesopore size of 6.3 nm, high surface area of 278–338 m2/g, and high nitrogen content of 20.5–24.9% with abundant basic sites. The synthesized MCN and MCN/C composites are used as adsorbents for CO2 capture, showing high adsorption capacity and good reusability. The presence of carbon coating layer in MCN/C composites introduced more micropores, which favored the capture of CO2, exhibiting high CO2 capture capacity of 3.05 mmol/g at 0 °C and 2.35 mmol/g at 25 °C and 760 mm Hg, with good CO2/N2 selectivity and excellent regeneration stability. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Global warming, which mainly results from the emission of green house gas such as CO2 and CH4, has attracted a great deal of public concerns since it would probably bring about severe environmental problems [1]. Much effort has then been devoted to reduce the concentration of CO2 by a capture process. Traditionally, large-scale capture of CO2 produced from fossil-fuel combustion is based on the sorption by liquid amines such as monoethanolamine, diethanolamine and methyldiethanolamine. However, these systems have some disadvantages such as poor chemical stability and mass transport efficiency, corrosion of equipment, toxicity, and flow problems caused by viscosity [2–4]. Therefore, CO2 adsorption on solid adsorbents, including zeolites [5], metal–organic framework compounds [6], porous silica [7] and carbon materials [8] have received considerable research interest recently. Porous carbon-base materials, which are of low-cost, wide availability, large surface area, easy-to-design pore structure, surface functionalization and low energy requirements, are considered to ⇑ Corresponding author. Tel.: +86 22 23509610; fax: +86 22 23502604. E-mail address: [email protected] (Z.-Y. Yuan). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.124

be one of the most promising adsorbents for capturing CO2 [9– 11]. Moreover, the incorporation of basic nitrogen groups into carbon can improve CO2 adsorption capacity [12–15]. Up to date, nitrogen-containing carbon materials used for CO2 capture were mostly prepared by post-treatment in ammonia atmosphere at high temperature [15] and chemical reactions of amino groups on the surface of pre-functionalized carbons [16], though such a surface functionalization does not change the properties of carbons significantly. In contrast, in situ doping of carbons by using nitrogen-containing chemical compounds as precursor can realize a homogeneous incorporation of nitrogen into the carbon material with a controlled chemistry [17]. However, N-doped carbons obtained in this carbonization-activation process generally display a poor porosity, which may limit their application in CO2 adsorption. Thus, it is still a challenge to develop porous carbonbase materials that combine large surface area and high density of nitrogen in framework to enhance the CO2 adsorption uptake. Carbon nitride (CN) is a well-known and fascinating material with application potential in many fields, such as in catalysis, photocatalysis and electron emission device [18–20], because the incorporation of nitrogen atoms in the carbon can enhance the mechanical, conducting, field-emission, and energy-storage

Q.-F. Deng et al. / Chemical Engineering Journal 203 (2012) 63–70

2. Experimental 2.1. Synthesis Mesoporous silica SBA-15 was synthesized according to a previous report [28]. 4 g of triblock copolymer Pluronic P123 ((EO)20(PO)70(EO)20) was dissolved in a mixture of 30 g of distilled water and 120 ml of 2 M HCl aqueous solution by stirring at 40 °C for 5 h. Then 9 g of tetraethoxysilane (TEOS) was added into the above solution under stirring and aged at 40 °C for 24 h. The resulting gel solution was transferred into a Teflon-lined autoclave and heated at 100 °C for 48 h. The product was filtered, dried at 80 °C for 24 h, and then calcined at 550 °C in air for 6 h. Mesoporous carbon nitride materials were prepared using SBA15 as the hard template. In a typical synthesis, the calcined SBA-15 (0.5 g) was added to a mixture of ethylenediamine (1.35 g) and carbon tetrachloride (3 g), and the mixture was refluxed and stirred at 90 °C for 6 h. The resulting dark-brown-colored solid was placed in a drying oven for 12 h, and then heat-treated at 600 °C for 5 h in a nitrogen flow. The MCN product was obtained by using 5 wt.% HF aqueous solutions to remove the silica template. Also, the obtained template-carbon nitride composites were dispersed in 5 g of aqueous solution containing 1.25 g of sucrose and 0.14 g H2SO4, which was then dried in an oven at 100 °C and 160 °C for 5 h, respectively. After carbonization at 600 °C for 2 h in N2 flow, the MCN/C composites were obtained by using 5 wt.% HF solutions to remove the silica template. 2.2. Characterization X-ray diffraction (XRD) patterns were obtained on a Bruker D8 FOCUS diffractometer, using Cu Ka radiation at 40 kV and 150 mA. N2 adsorption–desorption isotherms were collected at liquid nitrogen temperature using a Quantachrome NOVA 2000e sorption analyzer. The specific surface areas (SBET) of the samples were calculated following the multi-point BET (Brunauer–Emmett–Teller) procedure. The pore-size distributions were determined from the adsorption branch of the isotherms using the BJH (Barett–Joyner– Halenda) method and NLDFT (non-local density functional theory) models. Before carrying out the measurement, each sample was degassed at 200 °C for more than 6 h. Transmission electron microscopy (TEM) analysis was performed on a Jeol JEM 2010 microscope, operating at 200 kV. The sample was dispersed in ethanol and treated with ultrasound for 5 min, and then deposited on a copper grid coated with preformed holey carbon film. Temperature-programmed desorption (TPD) experiments were carried out in a flow apparatus of Quantachrome CHEMBET 3000, using a U-shaped quartz micro-reactor. About 0.2 g of sample was heated to 600 °C under the flow of pure He gas with a rate of 10 °C/min, and kept at this temperature for 1 h. After cooled to

80 °C, kept at the temperature to adsorption CO2 with high-purity CO2, the gas was switched to pure He gas until the baseline was stable. Then the CO2-TPD curve was measured at a rate of 10 °C/ min. The CO2 adsorption isotherms of the samples were measured using a Quantachrome AUTOSORB-1MP analyzer at 0 °C and 25 °C. Each sample was outgassed for 12 h at 200 °C to remove the guest molecules from the pores before starting the adsorption measurements and then cooled down to room temperature, followed by the introduction of CO2 into the system. CO2 adsorption was also characterized by a simultaneous DSC– TGA analysis using a TA SDT Q600 instrument under ambient pressure (1.0 atm) [29]. The samples were first activated by heating to 350 °C under an N2 flow to remove adsorbed small molecules, and then cooled to 25 °C in an N2 flow. During the sorption experiments, the samples were purged with CO2. Upon introduction of the gas, a weight gain was observed due to CO2 physical absorption on the sample surface. CO2 uptake of the samples was also tested at 35 and 75 °C. Desorption of CO2 was performed by purging with N2 (100 ml/min). The cycles of adsorption and desorption were repeated in order to test the ability of the sorbents to retain their CO2 sorption capacity. In the case of adsorption–desorption cycles, after the sample was saturated, the samples were heated at 150 °C in a nitrogen atmosphere and the CO2 was desorbed, then cooled down to 25 °C, and allowed to adsorb CO2 again for 180 min. This process was repeated for four runs to evaluate any changes in CO2 uptake. 3. Results and discussion 3.1. Material characteristic Fig. 1a shows the small-angle XRD patterns of the silica template (SBA-15), MCN and MCN/C composites. The SBA-15 sample exhibited three well-ordered peaks which can be indexed as the (1 0 0), (1 1 0), and (2 0 0) reflections, indicating a two-dimensional hexagonal pore structure with p6mm symmetry [28]. After nanocasting with carbon tetrachloride and ethylenediamine precursors and subsequent removing of the silica template by HF, a similar pattern to the SBA-15 template was observed in the obtained MCN, but with low intensity and broad diffractions (Fig. 1a), suggesting the decrease in the ordering of mesostructure. Furthermore, the reflections of MCN shifted to slightly higher 2h values, suggesting a minor shrinkage of the pore structure after template removal, as a result of the high-temperature carbonization [30].

(a)

(b) MCN

SBA-15

MCN

Intendity (a.u.)

properties and present basic sites [20–26]. Compared to bulk CN, mesoporous CN materials with high surface area and large porosities possess large number of active chemical sites exposed on the surface, resulting in enhanced performance in such applications [27]. Porous CN spheres were synthesized by using spherical mesoporous cellular silica foams (MCFs) as a hard template and ethylenediamine and carbon tetrachloride as precursors, which have high nitrogen content of 17.8% with CO2 capture capacity of 2.90 mmol/g at 25 °C [27]. In this contribution, mesoporous carbon nitrides (MCN) and carbon nitride–carbon (MCN/C) composites were synthesized by using mesoporous silica SBA-15 as the hard template, and their CO2 capture property, CO2/N2 selectivity and the cycling stability were investigated.

Intendity (a.u.)

64

MCN/C MCN/C

1

2

3

2θ (degree)

4

20

40

60

80

2θ (degree)

Fig. 1. Small-angle (a) and wide-angle (b) powder XRD patterns of the template SBA-15, MCN and MCN/C.

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SBA-15 MCN/C MCN

SBA-15 MCN/C MCN

3

400

0.6 0.5 0.4

300

0.3

3

dV/dD (cm /g/nm)

Volume adsorbed (cm /g)

(b)

(a)

500

200

0.2

100

0.1 0.0

0 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

1.0 0

5

10

15

20

Pore diameter (nm)

Fig. 2. (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding NLDFT pore size distribution curves of the template SBA-15, MCN and MCN/C. The volume was shifted by 60 for the curve of MCN/C data and dV/dD value was shifted by 0.12 and 0.2 for the curve of MCN/C, SBA-15, respectively.

As for MCN/C composites, a similar pattern was observed, proving a good replication of the mesoporous structure, even after coating with carbon layer. Wide-angle XRD patterns of both MCN and MCN/C composites showed a strong reflection around 25.9° (2h), i.e. an interlayer d-spacing of 0.343 nm, indicating the presence of turbostratic ordering of carbon and nitrogen atoms in the CN graphene layers with a uniform distribution of nitrogen atoms throughout the samples, which is similar to that in the nonporous carbon nitride sphere (Fig. 1b) [30]. The N2 adsorption–desorption isotherms and corresponding pore size distribution curves of the silica template (SBA-15), MCN and MCN/C composites are shown in Fig. 2, and their textual properties are listed in Table 1. The isotherms of SBA-15 are of typical type IV with a H1-type hysteresis loop in the relative pressure (P/P0) range of ca. 0.4–0.85, characteristic of mesoporous materials with narrow pore size distribution centered at 7.0 nm (Fig. 2b), having BET surface area of 677 m2/g and pore volume of 0.835 cm3/g. However, for the replica MCN, the specific surface area and total pore volume were significantly reduced to 278 m2/g and 0.38 cm3/g, respectively. Furthermore, the steep increase of the nitrogen sorption in MCN occurred at lower relative pressures, indicating smaller pore sizes; indeed, a slightly broadened poresize distribution centered at 6.2 nm is observed. It is important to note that the pore size of MCN (6.2 nm) is larger than the wall thickness of the mesoporous silica template (5.0 nm). This could be explained by shrinkage of the carbon nitride rods in the mesochannels of the silica template. When the pores of the SBA-15 template were completely filled with the MCN precursor, during thermal treatment and condensation, volume shrinkage had to be considered, and then the removal of silica yielded a larger interstitial pore size than that would be expected from a replication of the silica pore wall [31], though the incomplete filling of

the (CH2NH2)2 and CCl4 precursors in the mesochannels of SBA15 could not be totally excluded. While the MCN/C composites displayed a relative higher BET surface area of 338 m2/g with a lower total pore volume of 0.331 cm3/g, in comparison with the MCN sample. There is little change on mesopore size for MCN/C composites compared to MCN sample, suggesting that the mesostructure is well-preserved after coating with carbon on MCN. As revealed by the t-plot calculation, the micropore volume of MCN/C is larger than that of MCN (Table 1), indicating the introduction of carbon improved the microporosity and thus contributed to the total surface area. Fig. 3 shows the TEM images of MCN and MCN/C composites. MCN shows a good replication of 2D pore structure of the SBA15 template (Fig. 3a), which is in agreement with the results of the small-angle XRD and nitrogen adsorption analysis. High-resolution TEM images revealed a partly graphitized structure of MCN, showing local graphitic order with bent layer planes and weak edge termination (Fig. 3b), which is consistent with the XRD result. In the TEM images of MCN/C composites, the MCN particles are surrounded by an irregular layer of amorphous microporous carbon coating (Fig. 3c), and the graphitic ordering is easily observed in the involved MCN particles (Fig. 3d). The nature and coordination of the carbon and nitrogen atoms in MCN and MCN/C composites were investigated by FT-IR and XPS techniques. Fig. 4 shows the FT-IR spectra of MCN and MCN/ C composites. Both of the samples showed three major broad bands centered around 1072, 1544, 3413 cm1, which can be ascribed to aromatic CAN stretching bonds, aromatic ring modes and the stretching vibrations of the NAH groups in the aromatic ring, respectively [32]. In addition, the absence of the band around 2200 cm1 confirms that there are no C„N bonds in the samples, indicating the presence of 1,3,5-triazine ring [26,33,34]. In order to know the details of the chemical bondings of carbon and nitrogen atoms in MCN and MCN/C composites, the C1s and N1s XPS peaks were deconvoluted into Gaussian–Lorenzian shapes (Fig. 5). The C1s spectrum was deconvoluted into three peaks with binding energies of 289.3, 285.7, and 284.1 eV, in which the peak centered at 284.1 eV is assigned to pure graphitic sites in the amorphous CN matrix, whereas the peak at 285.7 eV is attributed to the sp2 C atoms bonded to N in an aromatic ring, and the highest energy contribution at 289.3 eV can be assigned to the sp2-hybridized carbon in the aromatic ring attached to the NH2 group [30]. In the N1s spectra, two peaks at about 398.7 and 400.6 eV can be observed. The lower binding energy peak (398.7 eV) is attributed to nitrogen atoms bonded with the graphitic carbon atoms, while the higher peak at 400.6 eV is assigned to N atoms trigonally bonded to sp2 carbons [26] or bonded with the sp2 carbon atoms and hydrogen atoms [35]. The XPS results are in good agreement with the values reported for non-porous carbon nitride samples [35–37]. It is clear that both of the MCN and MCN/C composites contain a large number of pyrrolic-type and pyridinic-type of nitrogen, which are believed to be promising for CO2 capture. Moreover, the amount of nitrogen calculated from XPS analysis is

Table 1 The textural properties and the nitrogen content of the SBA-15 template and the synthesized mesoporous carbon nitrides.

a b c d e f

Samples

SBETa (m2/g)

Vmicrob (cm3/g)

Vtotalc (cm3/g)

Dpored (nm)

d001e (nm)

twallf (nm)

N content (wt.%)

SBA-15 MCN MCN/C

677 278 338

– 0.009 0.057

0.84 0.38 0.33

7.0 6.2 6.2

10.4 9.5 9.4

5.0 – –

– 20.5 24.9

Multi-point BET surface area. Micropore volume determined by the t-plot method. Total pore volume determined at relative pressure P/P0 = 0.98. Pore size determined from nonlocal density functional theory (NLDFT) modeling. Determined from small-angle powder XRD measurements show in Fig. 1a. pffiffiffi Wall thickness determined from twall ¼ d001  ð2= 3Þ  Dpore .

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Tansmittance (%)

Fig. 3. TEM images of (a and b) MCN and (c and d) MCN/C. (b) and (d) are the high resolution images of MCN and MCN/C samples, respectively, showing partly graphitized structure of carbon nitride.

4000

-1

1544 cm

MCN -1 1072 cm

-1

3413 cm

MCN/C

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm ) Fig. 4. FT-IR spectra of MCN and MCN/C samples.

20.5% for MCN and 24.9% for MCN/C, consistent with the value obtained from the chemical elemental analysis, indicating that a large number of nitrogen atoms were bond to C atoms uniformly. 3.2. CO2 capture performance The nitrogen-containing groups are believed to be Lewis base active sites for binding acidic CO2 [38]. Since the prepared MCN

and MCN/C samples possess high content of nitrogen, their CO2 adsorption performance was investigated. Fig. 6 shows the CO2 adsorption isotherms at 0 °C and 25 °C of the MCN and MCN/C samples. The MCN sample presents high CO2 capture capacity of 2.16 mmol g1 at 0 °C and 1.76 mmol g1 at 25 °C and 760 mm Hg, which is comparable with that of mesoporous carbons with high surface area and large microporosity synthesized by low-temperature autoclaving method [15]. While the CO2 uptake of MCN/C composite (3.05 mmol g1 at 0 °C and 2.35 mmol g1 at 25 °C and 760 mm Hg) is superior to that of MCN both at 0 °C and 25 °C. The larger CO2 capacity of MCN/C composites is attributed to the higher surface area, higher micropore volume and considerably higher nitrogen content, indicating both the porous structure and nitrogen content played an important role in the adsorption of CO2. The existence of a large number of micropores and small mesopores in the MCN/C materials contributed to its excellent CO2 capture performance, owing to the relatively weak van der Waals forces and the capillary condensation effect [39]. In contrast, the high N content give rise to a superior CO2 capture capacity for MCN/C composites, which attributed to the abundant Lewis base sites that promote chemical bonding between specific functionalities in the solid absorbent and CO2 [27]. This can be confirmed by the obvious desorption peak in the corresponding CO2-TPD results (Fig. 7). The onset desorption temperatures is ca. 125 °C, suggesting a relatively weak basicity from the nitrogen groups. The MCN/C samples show the larger desorption peak than MCN, indicating the more basic sites in MCN/C due to the higher content

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(a)

(b)

N1s

Intensity (a.u.)

Intensity (a.u.)

C1s

280

282

284

286 288 290 Binding Energy (eV)

292

(c)

392

294

394

396

398 400 402 404 Binding Energy (eV)

406

(d)

N1s

Intensity (a.u.)

Intensity (a.u.)

C1s

408

280

282

284

286

288

290

292

294

392

394

396

Binding Energy (eV)

398

400

402

404

406

408

Binding Energy (eV)

Fig. 5. (a) C1s and (b) N1s XPS spectra of MCN, and (c) C1s and (d) N1s XPS spectra of MCN/C.

2.5

3.5

(b)

o

CO2

2.0

0 C o

25 C 1.5

1.0

3.0

CO2 uptake (mmol/g)

CO2 uptake (mmol/g)

(a)

o

CO2

0 C o

25 C

2.5 2.0 1.5 1.0

0.5 o

N2 25 C 0.0

0

100

200

300

400

500

600

700

Pressure (mmHg)

0.5 0.0

N2 25 oC 0

100

200

300

400

500

600

700

Pressure (mmHg)

Fig. 6. CO2 and N2 adsorption isotherms of (a) MCN and (b) MCN/C at 0 and 25 °C, obtained on a Quantachrome Autosorb-1MP analyzer.

of nitrogen groups than that of MCN, which contributes to the CO2 higher uptake. The basic sites were responsible for the high extent of capture of CO2 molecules due to the formation of weak covalent bonds between CO2 molecules and the basic sites. Moreover, the isosteric heats of adsorption on MCN and MCN/C were calculated using the CO2 isotherms at 0 and 25 °C based on the Clausius–Clapeyron equation [40]. The isosteric heat of adsorption is in the range of 15.8–19.7 and 20.5–26.2 kJ mol1 for MCN and MCN/C composites, respectively, indicating the strong interaction between CO2 molecules and the absorbent [41]. The heats of

adsorption for MCN/C is higher than MCN and increased with the N content, implying N functional groups played an important role in the initial interaction between CO2 and the carbon surface. This result further confirms that the larger amount of N groups is beneficial to the CO2 adsorption capacity. N2 adsorption isotherms at 25 °C on MCN and MCN/C were also measured and compared to the CO2 adsorption isotherms (Fig. 6). The MCN/C exhibits the N2 adsorption uptake of only 0.15 mmol g1 at ambient pressure, which is much lower than its CO2 adsorption capacity. At identical pressure for both CO2 and N2, the amount of

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6

MCN/C MCN

TCD signal (a.u.)

5 4 3 2 1 0 50

100

150

200

250

300

350

Temperature (OC) Fig. 7. CO2-TPD curves of MCN and MCN/C.

nitrogen adsorbed is less than one tenth of the amount of CO2, suggesting that CO2 and N2 could be selectively adsorbed, and the MCN and MCN/C could be potential selective adsorbents for CO2 and N2 separation. Previous reports had shown that at elevated temperature, activated carbon had a negligible CO2-adsorption capacity due to weak

chemical interactions with the carbon surface [42]. The CO2 adsorption capacity of the synthesized MCNs was also examined at higher temperatures (35 and 75 °C) by thermogravimetric analysis, and the results are shown in Fig. 8. It is seen that the CO2 adsorption capacities of the synthesized MCN/C and MCN are 1.28 and 1.08 mmol/g at 35 °C, which decrease to 0.83 and 0.79 mmol/g at 75 °C, respectively. The CO2 adsorption performance of MCN and MCN/C is better than that of commercial activated carbon (0.89 mmol g1 at 25 °C and 0.23 mmol g1 at 75 °C) [43], and comparable with that of hierarchical mesostructured titanium phosphonate hybrid materials with high surface area [44–46]. For CO2 adsorption, gas uptake is related to morphological properties such as specific surface area, the micro-architecture of the material and the interaction strength between the adsorbent and adsorbate [47,48]. Thus, it is reasonable that MCN/C composite, with a higher specific surface area and higher nitrogen content, has a larger CO2 adsorption capacity than MCN. At room temperature, the high uptake is attributed to both physical and chemical adsorption. When the temperature increased to 75 °C, some of physically captured CO2 molecules escaped from the surface of the pore walls and thus the CO2 adsorption capacity decreased. For practical applications, in addition to a high CO2 adsorption capacity, easy regeneration is another very important feature of a good adsorbent. The reversibility of CO2 adsorption on MCN and

1.2

1.4

(a)

o

35 C

1.0

CO2 uptake (mmol/g)

0.8 o

75 C

0.6 0.4 0.2

0

50

100

150

0.8 o

75 C

0.6 0.4 0.2 0.0

200

0

50

100

150

200

Time (min)

Time (min)

Fig. 8. TGA records of CO2 adsorption for (a) MCN and (b) MCN/C at 35 and 75 °C.

a

1.0

b

1.4 1.2

0.8

CO2 adsorption (mmol/g)

0.0

(b)

1.2

o

35 C

CO2 adsorption (mmol/g)

CO2 uptake (mmol/g)

1.0

0.6 0.4 0.2 0.0

1.0 0.8 0.6 0.4 0.2 0.0

0

200

400

600

800

time (min)

1000

1200

1400

0

200

400

600

800

1000

time (min)

Fig. 9. The cycling stability of (a) MCN and (b) MCN/C at 25 °C, measured on a TA SDT Q600 instrument.

1200

1400

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MCN/C composites at 25 °C has been tested over four cycles. As seen in Fig. 9, the CO2 adsorption capacity of MCN and MCN/C for the four cycles are nearly identical, retaining stable at around 1.50 and 1.03 mmol g1, respectively. Thus, the MCN and MCN/C materials reported here can be easily and totally regenerated over multiple cycles without any loss of adsorption capacity, suggesting a good cycling stability of the material as a CO2 adsorbent.

[15]

[16] [17] [18]

4. Conclusions Mesoporous carbon nitrides have been synthesized by using mesoporous silica SBA-15 as a hard template. The obtained MCN and MCN/C materials had high BET surface area, large pore volume and high nitrogen content (20.5 wt.% and 24.9 wt.%) with abundant basic sites. The introduction of carbon in MCN/C composite improved its microporosity, which favored the capture of CO2. Both the high surface area and the presence of N contributed to the high CO2 capture capacity. The MCN/C composites exhibit high CO2 capture capacity of 3.05 mmol/g at 0 °C and 2.35 mmol/g at 25 °C and 760 mm Hg, as well as good CO2/N2 selectivity and excellent regeneration stability, being one of the promising materials for CO2 capture.

[19] [20]

[21]

[22] [23] [24] [25]

[26]

Acknowledgements

[27]

This work was supported by the National Natural Science Foundation of China (20973096 and 21073099), the National Basic Research Program of China (2009CB623502), the Specialized Research Fund for the Doctoral Program of Higher Education (20110031110016), and the Program for Innovative Research Team in University (IRT0927).

[28]

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