Hollow Carbon Spheres with Abundant Micropores for Enhanced CO2

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Hollow carbon spheres with abundant micropores for enhanced CO2 adsorption Xuena Li, Shiyang Bai, Zhengjian Zhu, Ji-Hong Sun, Xiaoqi Jin, Xia Wu, and Jian Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04131 • Publication Date (Web): 14 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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Hollow carbon spheres with abundant micropores for enhanced CO2 adsorption

Xuena Li,a Shiyang Bai,a* Zhengjian Zhu,a Jihong Sun,a Xiaoqi Jin,a Xia Wu,a and Jian Liub a

Beijing Key Laboratory for Green Catalysis and Separation,

Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, P. R. China b

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

* To whom correspondence should be addressed. E-mail: [email protected]; Tel: +86-10-67396118. Fax: +86-10-67391983. Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, Beijing University of Technology, 100 PingLeYuan, Chaoyang District, Beijing, 100124, P. R. China

ACS Paragon Plus Environment

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Hollow carbon spheres with abundant micropores for enhanced CO2 adsorption Xuena Li,a Shiyang Bai,a* Zhengjian Zhu,a Jihong Sun,a Xiaoqi Jin,a Xia Wu,a and Jian Liub a

Beijing Key Laboratory for Green Catalysis and Separation, Department of

Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, P. R. China b

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

Abstract The interest in the design and controllable fabrication of hollow carbon spheres (HCSs) emanates from their tremendous potential applications in adsorption, energy conversion and storage, and catalysis. However, the effective synthesis of uniform HCSs with high surface area and abundant micropores remains a challenge. In this work, HCSs with tuneable microporous shells were rationally synthesized via hard-template method using resorcinol (R) and formaldehyde (F) as a carbon precursor. The HCSs with very high surface area (1369 m2/g) and abundant micropores (0.53cm3/g) can be obtained in the assistance of addition inorganic silane (TEOS) simultaneously with the carbon source (RF). Interestingly, the extra abundant micropore showed favourable adsorption for CO2, resulted in one and half times increase in the CO2 adsorption capacity than normal HCSs under the same condition. Meanwhile, these HCSs hold the potentials in the gas separation such as CO2 and N2. 1. Introduction


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Monodisperse hollow carbon spheres (HCSs) with the high specific surface area, large pore volume, good conductivity and excellent mechanical stability have attracted rapid research interest due to their widely applications in adsorption,1-9 catalysis,10-13 Li-S batteries,14-18 fuel cells and supercapacitor electrodes, etc.19-21 Previously, various methods have been demonstrated for the synthesis of HCSs and the hard template method has been recognised as the most common one. For example, Su and co-workers reported the synthesis of N-doped HCSs with single, double and N-doped shells by assembling carbon patches throughout a chemical vapor deposition (CVD) method.22 Zhang and co-workers synthesized bowl-like or balloon-like HCSs by using sodium dodecyl benzene sulfonate or P123 as templates by adapting a hydrothermal carbonization synthesis. However, neither of these methods can be applied to precisely control the porosity of the carbon shell.23 Alternatively, by nanocasting of solid core/mesoporous shell silica spheres, Chai and Yoon successfully prepared carbon capsules with a hollow core and a mesoporous shell.24 In addition, Arnal and co-workers successfully obtained porous hollow carbon nanostructures using binary core-shell as a template (SiO2@ZrO2) and furfural alcohol as carbon precursor, the resultant HCSs have BET surface area up to 2343 m2/g.25 Although these methods realized adjustable porosity and enhanced the BET surface area of HCSs effectively, the drawback could be the high cost procedure or the multi-step complicated synthetic routes. Liu et al. first successfully prepared microporous carbon spheres via polymerizing resorcinol (R) and formaldehyde (F) in the guidance of Stöber method which depended on that the condensation of R/F were


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similar to that of silane precursor such as tetraethyl orthosilicate (TEOS) in ethanol/water mixtures under ammonia solution at room temperature.26,27 According to the similarity principle between silane and RF, many different carbon nanospheres with core-shell, hollow, and yolk-shell structures have been fabricated.28 However, in most cases, the high specific surface area of HCSs was not so easy to be obtained, which limited its further application in adsorption and supercapacitors.

Scheme 1. (A) Illustration of the hydrolysis procedure of R/F and TEOS; (B) The possible formation mechanism of HCSs. In this paper, uniform monodisperse HCSs were successfully synthesized by R/F coating on the as-synthesized SiO2 spheres (sSiO2), and their controllable morphology was evaluated through changing the dosage of the ammonia solution, R/F and the diameter of sSiO2 during the sol-gel process. Furthermore, considering the previous reports,26 silane can be interacted with R/F, because of their matched


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hydrolysis-condensation rate and analogous geometry as shown in Scheme 1A.29 Therefore, TEOS has been employed as the pore constructor participated in the shell coating process. Through this facile procedure, well-developed porosity can be generated not only by carbonization but also by silica etching, which resulted in very high specific surface area and larger pore volume. In recent years, carbon dioxide (CO2) has drawn particular attention for being the main anthropogenic contributor to global warming. Various types of adsorbents including

Amine-modified

mesoporous

silica,30-37

metal-organic

frameworks

(MOFs),38, 39 metal oxide40, zeolites41, 42 and carbonaceous materials.43-46 have been examined for CO2 capture. Table S1 in Supporting Information showed us some CO2 adsorption capacities of different adsorbents. Among them, MOFs have gained much attention because of the comparatively high CO2 adsorption capacities (up to 8.5 mmol/g even at ambient conditions).39 Despite the excellent adsorption capacities of MOFs, these materials are not stable enough under the flue gas47 and much more expensive than most of the other absorbents, especially the majority of carbonaceous adsorbents. Because of the low price, high specific surface area and adjustable surface properties, the carbonaceous materials demonstrated a pronounced potential in CO2 capture and stability. Therefore, the as-synthesized HCSs as a potential adsorbent for CO2 capture has been preliminary investigated, and the extra pores generated by etching of silicas are proved to be beneficial for the adsorption of CO2. 2. Experimental Section

2.1. Materials


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TEOS, resorcinol (R), formaldehyde (F), cetyltrimethyl ammonium bromide (CTAB) and hydrofluoric acid (HF) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Ethanol (EtOH) and ammonia solution (25%, NH4OH) was brought from Beijing Chemical Works. All chemicals were of analytically pure grade and the deionized water was used in all experiments.

2.2. Synthesis of HCS Monodisperse silica colloidal particles with various diameters were synthesized following the Stöber Method

27

: 7 mL aqueous ammonia was added into a solution

containing 28 mL of deionized water and 65 mL ethanol. After stirring for 5 minutes, 4 mL of TEOS was added into the above-prepared mixture solution and the reaction was kept stirring for 1 h to yield uniform silica spheres. The SiO2 spheres was retrieved by centrifugation, and further dehydrated at 343 K for 12 h. The as-prepared SiO2 powder 50 mg were homogeneously dispersed in the mixture of deionized water and ethanol by ultrasonic, followed by the addition of CTAB (75 mg), and then stirring at room temperature for 15 min, then resorcinol, TEOS, formaldehyde and ammonia solution were added to the dispersion with continuous stirring, the molar ratio of SiO2/CTAB/NH4OH/R/F/TEOS/H2O/EtOH = 1: 0.25: x: y: 6.33y: z: 83.33: 7.96 [where x involves 2, 4, 8, 12 (designated as HCS-Nx), y involves 0.15, 0.30, 0.45, 0.75, 1.05, 1.50 (designated as HCS-Ry), z involves 0, 0.09, 0.18, 0.54, 0.72 (designated as HCS-Sz)]. After few minutes, the solution became cloudy and the stirring was continued for 24 h at room temperature (25 °C). The resulting product, SiO2@RF/(SiO2) core-shell spheres, were centrifuged and purified by ethanol three


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times, then dried at 70 °C for 12 h. The core-shell carbon nanostructures SiO2@C/(SiO2) were obtained by the carbonization of SiO2@RF/(SiO2), the program was performed under nitrogen atmosphere in a tube furnace, first heating with 1 °C/min from room temperature up to 350 °C and kept for 2 h, resumed using the heating rate of 1 °C/min up to 600 °C and kept for 4 h. After silica etching with 10 wt% HF aqueous solution for 24 h and washing the samples with deionized water until the pH≈7, the as-synthesized sSiO2@C/(SiO2) were converted into HCSs. The detailed synthesis parameters were listed in Table S1 corresponding to different samples.

2.3. Characterization Scanning electron microscopy (SEM) images were recorded on Hitachi S-4300 electron microscope at an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images were obtained by JEM-1200EX microscope (JEOL, Japan) operating at 120 kV. Samples for TEM measurements were made by casting one drop of the sample’s n-hexane solution on carbon-coated copper grids. Nitrogen adsorption isotherms were measured at -196 °C on ASAP 2020 volumetric adsorption analyzer manufactured by Micromeritics (Norcross, GA, USA). All the samples were outgassed at 200 °C for 2 h before adsorption measurement. Here, the specific surface area and micropore size distribution are calculated by BET (Brunauer-Emmett-Teller) equation and Horvath-Kawazoe method, respectively. Fourier transform infrared spectroscopy (FT-IR) spectrum was measured using KBr discs in the region of 4000-400 cm-1 by a Nicolet Nexus 470 spectrometer to analyze the composition of the resultant copolymer. The X-ray photoelectron spectroscopy (XPS) was performed


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using a ULVAC-PHI PHI Quantera II spectrometer equipped with a monochromated Al Ka X-ray source. The element analysis of C, H, and N was performed on Elementar Vario MACRO cube Elemental analyzer.

2.4. Adsorption measurement The adsorption performance of the HCSs were measured using Gemini VII 2390 surface area analyzer Micromeritics manufactured by Micromeritics (Norcross, GA, USA). All the samples were pre-treated at 120 °C for 2 h before adsorption measurement. The test pressures were range from 0 to 1 atm. 3. Results and Discussion In the synthesis process, R and F were used as the carbon precursor, and CTAB was involved as the connector, which not only induced spontaneous deposition of RF resin onto the surface of sSiO2 but also promoted the polymerization of RF at low concentration.28 In the NH4OH-catalyzed sol-gel procedure, the occurance for polycondensation of RF adsorbed on the sSiO2 surfaces through CTAB with RF in the emulsion droplets of RF/CTAB/NH4OH took place simultaneously.26 Therefore, the RF can be easily coated onto the surface of sSiO2 and grow into tuneable shell through co-condensation reaction. As can be seen in Figure 1, after the RF coating the shell of the SiO2@RF spheres presented a rough surface and each SiO2@RF core-shell sphere contained only one SiO2 core at centre. The SiO2@C was obtained by subsequent calcination of the SiO2@RF in N2 atmosphere and the surface of the particles became smooth again with a decreased diameter, which because of the continuous pyrolysis of phenolic resin in the carbonization process and the thermal


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contraction effect of SiO2 sphere.22 After etching silica with 10 % HF solution, the uniform hollow carbon spheres with a diameter of about 240 nm and a shell thickness about 30 nm were fabricated. Moreover, the shell thickness of the HCSs can be conveniently tuned from 7 to 80 nm just by changing the synthesis parameters, including the NH4OH solution dosages or R/F carbon precursor (Figure 1e-h and S2).

Figure 1. SEM images (insert are the corresponding TEM images) of a) sSiO2, b) sSiO2@RF, c) sSiO2@C, d) HCS, and (e-h) the HCSs with different shell thickness. According to the previous report, the RF system had similar hydrolysis polymerization reaction kinetics to the silica precursors.26 The effect of silane doped in the carbon precursor in the constructing of HCSs was investigated for the first time. In order to illuminate the role of silane clearly, the molar ratio of TEOS to SiO2 in the synthesis process were altered from 0 to 0.72. The spherical morphology and particle size would not be destroyed by the addition of silane (Figure 2). However, when the molar ratio was up to 0.72, the shells and the spherical morphology presented a certain degree of broken (Figure 2e).


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Figure 2. SEM images (insert are the corresponding TEM images) of (a) HCS-S0 (HCS-N4/R0.45), (b) HCS-S0.09, (c) HCS-S0.18, (d) HCS-S0.54, and (e) HCS-S0.72. Figure 3 presented the N2 adsorption-desorption isotherms and corresponding pore size distribution of the silane-assisted HCSs. It was evident from the isotherms (Figure 3A) that all of samples possessed a type I isotherms, which were characteristic of monolayer adsorption on microporous solids, and type H4 hysteresis loop as a result of the existence of several different types of pores in the structure.49, 50 An initial rise below the relative pressure (p/p0) of about 0.1 could be accounted for the adsorption in micropores. A very slight adsorption at p/p0 around 0.4-0.9 was related to capillary condensation of nitrogen in the mesopores, partly due to the windows between macropores that have been formed by the thermal melt and the carbon burn-off during the calcinations.51 A steep adsorption occurred at p/p0 around 0.9-1.0 could be assigned to the macropores generated by the sSiO2 template. Pore size


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distribution (PSD) and cumulative pore volume curves obtained from N2 adsorption by density functional theory (DFT) method are presented in Figures 3B, C and Table 1, which show that all samples exhibit two PSD peaks, mainly located at 0.5-0.6 nm and 1.3-1.5 nm. As can be seen from Figure 3B and C, all the samples showed almost equal pore volumes in the range below 1 nm (∼0.1 cm3/g); however, HCS-S series exhibited increasing cumulative volume (between the pore width of 1-2 nm) with increasing of silane dosage, mainly due to the presence of larger micropores (1.3-1.5 nm) which seemed to be mainly contributed by the etching of silica. In other words, although there was also two kind of micropore for the sample of HCS-S0, the quantity

400

A

300 200 100 0 0.0

0.2

0.4

0.6

0.8

Relative Pressure (p/p0)

1.0

-1

1.5

1.0

a b c d e

B

0.5

0.0 1

0.6

3

a b c d e

Cumulative Pore Volume (cm g )

3 -1

500

3 -1

600

dV/dlog(W) Pore Volume (cm g

-1

)

)

of larger micropore increased remarkable with the increasing of silane dosage.

Quantity Adsorbed (cm g STP

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

Pore Width (nm)

0.5 0.4

a b c d e

C

0.3 0.2 0.1 0.0 1

2

3

4

Pore Width (nm)

Figure 3. N2-sorption isotherms (A), corresponding pore size distributions (B) and Cumulative pore volume below the pore width of 5 nm (C) of (a) HCS-S0 (HCS-N4/R0.45), (b) HCS-S0.09, (c) HCS-S0.18, (d) HCS-S0.54, and (e) HCS-S0.72. As the textural parameters summarized in Table 1, the BET surface area of 390 m2/g including both micropore and mesopore were obtained for HCS, which increased to 574-1376 m2/g with increasing dosage of added TEOS. Meanwhile, these HCSs also exhibited high micropore surface areas of 462-1246 m2/g and micropore volumes


5

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of 0.19-0.53 cm3/g. The sample HCS-S0.54 present the highest proportion of micropore surface area (91.3%) and micropore volume (62.7%). The data of sample HCS-S0.72 are similar to that of HCS-S0.54, while the spherical particles were seriously broken, which may be caused by exceed micropores. Above all, the HCSs with controllable abundant micropore could be easily fabricated through the addition of TEOS, and altered in a certain range. Table 1. Physical and chemical characteristic of all related samples Physical Properties

Chemical Properties

SBETa (Smib)

Vt (Vmib)

Wmec

CO2

N2

C

N

(m2/g)

(cm3/g)a

(nm)

capacityd

capacityd

(wt%)

(wt%)

(mmol/g)

(mmol/g)

Sample

HCS-S0 390 (348)

0.35 (0.14)

0.53, 1.26

1.82

0.25

1.329

73.67

HCS-S0.09

574 (462)

0.38 (0.19)

0.53, 1.26

1.96

0.25

2.334

84.81

HCS-S0.18

706 (640)

0.49 (0.26)

0.64, 1.27

2.31

0.26

1.408

80.61

HCS-S0.54

1369 (1250)

0.83 (0.52)

0.64, 1.46

2.63

0.29

1.528

85.65

HCS-S0.72

1376 (1246)

0.83 (0.53)

0.64, 1.46

2.26

0.24

1.363

80.79

(HCS-N4/R0.45)

Notation: a The BET surface area. b The micropore surface area Smi and micropore volume Vmi were estimated from the t-plot method. c The pore size distribution was calculated by DFT method. d

CO2 uptake at 273 K and 1 atm.

To prove the universality of this method, a series of HCSs with different hollow interior size were precisely prepared by varying the initial sizes of template sSiO2


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during the synthetic procedure. As shown in Figure S3, the HCSs with different cavity were obtained simply by using the sSiO2 with particle sizes of 172 nm, 211 nm, 269 nm and 285 nm as the templates, while the other condition parameters followed the same ratio as HCS-S0.54. On the basis of the above results, the synthesis mainly followed the procedures as blow: ammonia as catalyst added firstly caused the formation of RF emulsion droplets through the hydrogen bondings,26 and then the RF resin polymers were deposited on spherical silica surfaces through the combination of electrostatic attraction where CTAB as a bridge.52 Therefore, the hydrolysis rate of R and F would accelerate with the increase dosage of the ammonia catalyst or carbon precusor, causing the gradually enlargment of resultant droplets, just as Figures 2.27, 28 More important,

after

joining

the

silane

TEOS,

ammonia

promoted

the

hydrolysis-condensation polymerization of both TEOS and RF, forming RF-silica shell.49,

50

Moreover, the similar hydrolysis-condensation rate of TEOS-RF and

resulting structure led to the homodispersion of two different species just like one,29 as illustrated in Scheme 1a. Then, the extra micropores were generated along with the disappearance of silica through the follow-up HF corrosion. The micropores in HCSs were fabricated by the decomposition of RF and release of the small molecules, and the corrosion of silica, which was much bigger than the small molecules decomposed from RF. So, along with the increase amount of TEOS added, the PSD exhibited only little increase, but the contribution of the larger micropore around 1.3-1.5 nm to the cumulative pore volume became remarkable. Therefore, Besides almost the same


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particle size and shell thinkness of the HCSs shown in Figure 2, the gradually increased pore size distrubtions and cumulative pore volume between the pore width of 1-2 nm obtained from N2 sorption isotherm were also a strong evidence to demonstrate the homogeneous condensation of silane with RF rather than the disturbing to the hydrolysis-condensation of RF. The possible synthesis mechanism were diagrammatized in Scheme 1B.

HCS-S0 HCS-S0.09 HCS-S0.18 HCS-S0.54 HCS-S0.72

2.0

A

1.5 1.0

B

R-1 R-2 R-3 R-4 R-5

-1

)

2.5

CO2 adsorption

CO2 uptake (mmol g

-1

)

2.5

Gas uptake (mmol g

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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2.0 1.5 1.0 0.5

0.5 N2 adsorption

0.0

0.0 0

100

200

300

400

500

600

Absolute Pressure (mmHg)

700

0

100

200

300

400

500

600

700

Absolute Pressure (mmHg)

Figure 4. CO2 and N2 adsorption at 0 °C for HCSs prepared with different dosage of TEOS (A); and the recycle adsorption of sample HCS-S0.54 (B). As reported in the literature, the presence of microporosity, especially small micropores with sizes of below 1 nm, are very important for CO2 adsorption.54-58 In this paper, we studied the CO2 adsorption capacity of resultant HCSs. As shown in Table 1, all prepared HCSs possess abundant microporosity, which could be useful for CO2 adsorption. Figure 4 showed the CO2 adsorption isotherms and Table 1 listed the CO2 adsorption capacity measured at 273 K and 1 atm. As profiled in Figure 4A and Table1, the CO2 capacity range from 1.82 to 2.63 mmol/g with the increase of microporosity which was caused by the increase dosage of TEOS, indicating that the extra micropore fabricated by silane additive quite benifited the adsorption of CO2,


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resulting in over one-fold increase in the CO2 uptake between sample HCS-S0.54 and HCS-S0. The most important reason was that sample HCS-S0.54 presented the highest proportion of micropore surface area (91.3%) and micropore volume (62.7%). Besides, although the microporous volume of sample HCS-S0.72 was a little higher than sample HCS-S0.54, the spherical morphology of the former was destroyed, which resulted in the decreasing of adsorption capacity of cavity for gas. At identical temperature and pressure, the adsorption ability of HCSs for N2 was also measured; the capacity of CO2 was nine times more than that of N2, suggesting HCSs a potential selective adsorbent for CO2 and N2 seperation. The separation coefficient of CO2/N2 was from 7.3 to 9.4, suggesting that HCSs could also be a potential selective adsorbent for CO2 and N2 seperation. The high selectivity may be caused by the dispersion interaction between CO2 and HCS framwork, which cannot be ignored in the adsorption of CO2 in porous material especially in microporous material.59, According to some reports,

59-61

60

the dispersion interaction was considered as a

site-specific one which was much larger when CO2 molecules cut or approached the nanopore entrance and wall. Compare with CO2, the dispersion interactions between N2 and HCS framework were much lower, because N2 molecules were much more difficult to be polarized than CO2. Therefore the high adsorption selectivity of CO2 was achieved. Moreover, HCS-S0.54 was chosen as model to investigate the stability of these samples, and the results exhibited unexpectedly adsorption performance, and kept the higher CO2 uptake (2.60 mmol/g) even after 5 recycle.


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C1s

1200

B

Intensity (a.u.)

A

Intensity (a.u.)

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

O1s F1s

1000

800

N1s

600

400

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200

0

410

408

Binding Energy (eV)

406

404

402

400

Binding Energy (eV)

398

396

Figure 5. (A) XPS spectra and (B) N 1s spectrum of HCS-S0.54. It was essential to find out that CO2 was mainly physically or chemically absorbed on the carbon studied. The possible chemical adsorption of CO2 on these samples could take place on nitrogen species. To check this issue, the composition of HCSs were investigated by XPS and Elemental analysis. There was no evidence of Si signal after etching with HF, which indicated the complete removal of the silica (Figure 5A). Combining with the elemental analysis, the compositions of HCSs were dominated by carbon and oxygen as well as nitrogen. The nitrogen originated from ammonia solution which was used as a catalyst for polymerization of R, F and TEOS. Although it seemed to provide us a simple method to introduce nitrogen into carbon based materials, the content of nitrogen presented no definite rule along with the synthesis condition. The N 1s spectrum (Figure 5B) was fitted into three peaks with binding energies of 398.7 ± 0.2, 400.7 ± 0.2, and 403.5 ± 0.2 eV that correspond to pyridinic-N, quaternary-N and pyridine-N-oxide.62,

63

The peak area ratio of S398:

S400.7: S403.5 was 1: 1.78: 0.35, suggested that after the HF corrosion the main state of nitrogen on the HCS was quaternary-N which was not active in the CO2 capture at anhydrous condition.31, 34 Therefore, the mainly physically absorbed principle was


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proved. 4. Conclusions In summary, we proposed a method for fabricating monodispersed hollow carbon spheres with controllable microporosity and high surface area by using the SiO2 spheres as template. The morphology and structure of the HCSs can be modified easily by adjusting the synthesis parameters. Partcularly, NH4OH and RF have been found effecting on the morphology and the shell thickness of spheres. For the first time, silane was used as a porous structure formation agent, which can construct extra-micropores in the shells of HCSs and generate the HCSs with extremely high surface area and abundant micropore. Our results demonstrated that the silica doping can increase the surface area and microporosity significantly from 390 to 1376 m2/g and 0.14 to 0.53 cm3/g, respectively. Furthermore, the HCSs with extra-micropores present much higher capacity than that of normal HCSs for CO2 uptake. Moreover, the HCSs showed a very good ability in the separation of CO2/N2 with the separation coefficient of up to 9.4. Owing to these advantages, the reported HCSs will be very promising for more far-reaching applications. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: CO2 adsorption capacities of different types of adsorbents, synthesis detail of HCSs, FT-IR spectra of sSiO2, sSiO2@RF, sSiO2@C and HCS, SEM and TEM images of HCSs with different shell thickness, TEM images of HCSs with different cavity sizes.


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Acknowledgements This project was supported by the National Natural Science Foundation of China (21403011, 21276005 and 21576005), the Beijing Municipal Natural Science Foundation (2152005), and the Scientific Research Project of Beijing Educational Committee (KM201610005015). References (1) Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow Micro-/Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987-4019. (2) Liu, T. T.; Qu, L. L.; Qian, K.; Liu, J.; Zhang, Q.; Liu, L. H.; Liu, S. M. Raspberry-like hollow carbon nanospheres with enhanced matrix-free peptide detection profiles. Chem. Commun. 2016, 52, 1709-1712. (3) Hao, T.; Saunders, M.; Dodd, A.; O'Donnell, K.; Jaroniec, M.; Liu, S. M.; Liu, J. Triconstituent co-assembly synthesis of N,S-doped carbon–silica nanospheres with smooth and rough surfaces. J. Mater. Chem. A. 2016, 4, 3721-3727. (4) Liu, J.; Wickramaratne, N. P.; Qiao, S. Z. Jaroniec, M. Molecular-based design and emerging applications of nanoporous carbon spheres. Nat. Mater. 2015, 14, 763-774. (5) Yang, T. Y.; Zhou R. F.; Wang, D. W.; Jiang, S. P.; Yamauchi, Y.; Qiao S. Z.; Monteiro, M. J.; Liu, J. Hierarchical mesoporous yolk–shell structured carbonaceous nanospheres for high performance electrochemical capacitive energy storage. Chem.

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Abstract Graphic


Hollow Carbon Spheres with Abundant Micropores for Enhanced CO2

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