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Effect of Porosity Parameters and Surface Chemistry on Carbon Dioxide Adsorption in Sulfur-Doped Porous Carbons En-Jie Wang, Zhu-Yin Sui, Ya-Nan Sun, Zhuang Ma, and Bao-Hang Han Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04370 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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Effect of Porosity Parameters and Surface Chemistry on Carbon Dioxide Adsorption in Sulfur-Doped Porous Carbons En-Jie Wang,† Zhu-Yin Sui,*,‡ Ya-Nan Sun,‡ Zhuang Ma,*,† Bao-Hang Han*,‡,§

† School of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, China ‡ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100049, China

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ABSTRACT: In this work, a series of highly porous sulfur-doped carbons are prepared through physical activation methods by using polythiophene as a precursor. The morphology, structure, and physichochemical properties are revealed by a variety of characterization methods, such as scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and nitrogen sorption measurement. Their porosity parameters and chemical compositions can be well-tuned by changing the activating agents (steam and carbon dioxide) and reaction temperature. These sulfur-doped porous carbons possess specific surface area of 670–2210 m2 g–1, total pore volume of 0.31–1.26 cm3 g–1, and sulfur content of 0.6–4.9 atom %. The effect of porosity parameters and surface chemistry on carbon dioxide adsorption in sulfur-doped porous carbons is studied in detail. After careful analysis for carbon dioxide uptake at different temperatures (273 and 293 K), pore volumes from small pore size play an important role in carbon dioxide adsorption at 273 K, while surface chemistry is the key factor at a higher adsorption temperature or lower relative pressure. Furthermore, sulfur-doped porous carbons also possess good gas adsorption selectivity and excellent recyclability for regeneration.

KEYWORDS: Polythiophene; Porous Carbon; Sulfur-Doping; Physical Activation; Gas Adsorption

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INTRODUCTION Currently, carbon dioxide that is mainly produced from burning of fossil fuels is thought to be a major contributor resulting in global warming.1 There has been a huge increase in the level of carbon dioxide emission since the industrial revolution. 2 Nowadays it is very urgent to find efficient ways to cut down the atmospheric concentration of carbon dioxide. Chemical absorption in liquid-phase systems has been applied to capture carbon dioxide in the atmosphere.3 In this process, the use of amine-containing molecules is unavoidable, thus making this technology suffer from severe drawbacks, such as high toxicity, corrosion of equipment, and high energy consumption to regenerate the adsorbents. Porous solid materials (e.g., metal–organic frameworks, zeolites, carbon-based porous materials) are good candidates, which have been widely investigated for carbon dioxide uptake.4,5,6,7,8 Among them, carbon-based porous materials might be the most promising carbon dioxide adsorbent on the basis of their easy availability, low cost, low energy consumption for regeneration, and large specific surface area.9,10,11 Heteroatoms have been introduced into the framework of carbon-based porous materials to further improve their adsorption capacity for carbon dioxide. Especially for nitrogen-doped porous carbons, a great amount of work has been reported. The introduction of nitrogen can provide some basicity, thus enhancing the acid–base interaction based on the weak acidic nature of carbon dioxide.12 Two kinds of strategies can usually be used to prepare nitrogen-doped porous carbons: i) the high-temperature


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thermal treatment of porous carbons with ammonia,13,14 and ii) the carbonization and/or activation of nitrogen-containing precursors such as polypyrrole,12 polyaniline,15 and melamine.16 Although many reports have indicated that nitrogen-doping can enhance carbon dioxide adsorption performance, several groups believe that nitrogen-doping has no influence on carbon dioxide adsorption. For example, Sevilla et al. indicated that nitrogen-free and nitrogen-doped (∼3 wt %) microporous carbons exhibit analogous carbon dioxide capture capacity. They came to the conclusion that nitrogen functionalities had no appreciable effect on carbon dioxide adsorption.17 Adeniran et al. prepared highly microporous carbon materials with identical porosity through activation of various carbon precursors. They claimed that nitrogen-doping appears to show a negative effect on carbon dioxide uptake.18 Sánchez-Sánchez et al. conducted a detailed study on the effect of porous texture and surface chemistry on carbon dioxide uptake capacity. They indicated that these two factors played vital roles in carbon dioxide adsorption.19 Compared to nitrogen-doped porous carbons, sulfur-doped porous carbons (SPCs) have also been extensively studied owing to their excellent performance in the fields of catalysis,20,21 energy storage,22,23 and photoactivity.24 In addition, it has been reported that SPCs can be used as carbon dioxide adsorption media. For example, Hong et al. prepared

hollow

sulfur-doped

carbon

poly(3,4-ethylenedioxythiophene)-derived

spheres

through

copolymers.

The

carbonization as-prepared

of

porous

materials display carbon dioxide adsorption capacities of 4.0–4.4 mmol g–1 at 196 K and


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1.0 bar.25 Seredych et al. synthesized nanoporous sulfur-doped carbons, which show adsorption capacity of ~ 4 mmol g–1 at ambient pressure. They indicated that surface interactions including polar interaction, acid–base interaction, and hydrogen bonding play a vital role in carbon dioxide uptake.26 Seema et al. obtained a sulfur-doped microporous carbon from chemical activation of a graphene/polythiophene composite. The material showed a carbon dioxide adsorption capacity of 4.5 mmol g–1 at ambient conditions.

27

Although some papers have reported carbon dioxide adsorption

performance of SPCs, it is necessary to explore the influencing factors that determine carbon dioxide adsorption. Polythiophene (PTh) has a high sulfur content in the polymer backbone and is easily synthesized in large quantities. To date, SPCs prepared from physical activation of sulfur-containing polymers are rarely reported. Herein, we prepare a series of SPCs with high porosity through physical activation of PTh by using steam and carbon dioxide as activating agents. Activating agents and reaction temperature have remarkable effect on specific surface area, pore volume, and chemical composition. Through analyzing porosity parameters and surface chemistry, it can be found that pore volume from small pore size (less than 1.0 nm) is vital to high-performance adsorbents for carbon dioxide uptake in addition to specific surface area and sulfur-doping.

EXPERIMENTAL SECTION Preparation of SPCs


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PTh was synthesized through a common oxidation polymerization method by using FeCl3 as an oxidant. 28 The typical experimental procedure is as follows. Thiophene (4.0 g) was added into 50 mL of chloroform solvent at room temperature. Sonication was applied to make thiophene monomer well-disperse in chloroform. Then, the oxidant, FeCl3 (8.0 g), dissolved in 100 mL of chloroform was added to the resulting thiophene solution under continuous stirring. The reaction temperature was kept at ~ 0 °C and the polymerization time was 10 h. A precipitate can be formed in the solution after polymerization. The above mixture was filtered and washed with a great amount of methanol. The obtained precipitate was suspended in 100 mL of hydrochloric acid (1.0 M) and sonicated for 2 h. The mixture was filtered and washed with a great amount of water to remove any impurity. PTh was obtained after drying at 80 °C for 24 h in an oven. SPCs were prepared through physical activation of PTh by using carbon dioxide and steam as activating agents, respectively. For carbon dioxide activation, PTh was transferred into a tube furnace, then heated at 10 °C min‒1 under nitrogen flow. When the tube furnace was heated up to the set temperature (900, 950, and 1000 °C), the sample was carbonized for 1 h, then the gas was switched from nitrogen to carbon dioxide. After carbon dioxide activation for 1 h, porous carbons were prepared. The SPCs obtained from carbon dioxide activation procedure were defined as SPC-CO2-T, where T stands for carbon dioxide activation temperature (T = 900, 950, and 1000). For steam activation, PTh was transferred into a tube furnace, then heated at 10


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°C min‒1 under nitrogen flow. When the tube furnace was heated up to the set temperature (800, 850, and 900 °C), the sample was carbonized for 1 h, then the gas was switched from nitrogen to steam. The steam was generated from the gasification of water at the set temperature and the flow rate of water was 1 mL min–1. After steam activation for 1 h, porous carbons were prepared. The SPCs obtained from steam activation procedure were defined as SPC-H2O-T, where T stands for steam activation temperature (T = 800, 850, and 900). Gas Sorption Measurements Carbon dioxide, methane, and nitrogen sorption isotherms were carried out on a Micromeritics TriStar II 3020 porosity analyzer. Before adsorption analysis, the as-prepared SPCs were placed in a degassing station and treated for 12 h at 120 °C under vacuum. Furthermore, SPC-H2O-850 and SPC-CO2-950 were degassed under vacuum condition at 60 °C for 6 h to explore their regeneration ability.

RESULTS AND DISCUSSION In this study, a series of SPCs were obtained from physical activation of PTh by using carbon dioxide and steam as activating agents. Carbon dioxide and steam can react with carbon under high-temperature conditions (CO2 + C = 2 CO; H2O + C = CO + H2).29,30 With the consumption of carbon atoms, a great amount of pores would be produced. PTh is used as a precursor because it possesses abundant sulfur atoms in its polymer skeletons, which is beneficial to the eventual formation of sulfur-doped


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carbons with high porosity during pyrolysis and activation processes. The morphology of SPCs was revealed by scanning electron microscopy (SEM). As displayed in Figure S1 (Supporting Information), SEM images show that all of the SPCs exhibit similar morphology and particle size distribution. The particle sizes of the as-prepared SPCs are in the range of several to several tens micrometers. According to the energy dispersive analysis (Figure S2, Supporting Information), there are three elements (carbon, oxygen, and sulfur) in SPC-H2O-850 and SPC-CO2-950. In addition, the signal of Fe element is not observed, indicating the complete removal of Fe during the sample preparation.

b (002) (100) SPC-H O-900 2 SPC-H2O-850 SPC-H2O-800

10

20

30

40

50

Intensity / a.u.

Intensity / a.u.

a

60

(002) (100) SPC-CO -1000 2

SPC-CO2-950 SPC-CO2-900

10

20

c

D

30

40

50

G

SPC-H2O-900

SPC-H2O-850

d

D G SPC-CO2-1000

SPC-CO2-950 SPC-CO2-900

SPC-H2O-800 600

900

1200

1500

60

2 theta / degree

Intensity / a.u.

2 theta / degree

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

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1800

Raman Shift / cm-1

2100

600

900

1200

1500

1800

2100

Raman Shift / cm-1

Figure 1. XRD patterns of SPC-H2O-T (a) and SPC-CO2-T series of samples (b); Raman spectra of SPC-H2O-T (c) and SPC-CO2-T series of samples (d).


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X-ray diffraction (XRD) patterns of SPCs were given to reveal their structural information. As shown in Figures 1a and 1b, all of SPCs display similar XRD patterns, and no evident sharp peaks can be observed, suggesting their amorphous structure. It can be seen that two wide and weak diffraction peaks located at around 24° and 42° appear from the SPCs, which are characteristics of amorphous carbonaceous materials.31 Raman spectroscopy was used to characterize the as-prepared SPCs and reveal the effect of different activation conditions on their microstructure. As shown in Figures 1c and 1d, SPCs possess two peaks centered at around 1350 and 1590 cm‒1, which can be ascribed to D-band and G-band, respectively. ID/IG ratio is associated with the graphitization degree, and a high ID/IG value indicates poor degree of graphitization.32 According to Raman spectra of SPCs, the ID/IG values of SPC-H2O-800, SPC-H2O-850, and SPC-H2O-900 are in range of 0.98–1.02. For SPC-CO2-900, SPC-CO2-950, and SPC-CO2-1000, their ID/IG values are calculated to be 1.21–1.34. It can be observed that SPCs prepared through a carbon dioxide activation method possess higher ID/IG values than those prepared through the steam activation method, indicating that more defects might be introduced during carbon dioxide activation.


1000

SPC-H2O-900

600 SPC-H2O-850 400 SPC-H2O-800 200

0 0.0

0.2

0.4

0.6

0.8

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b 0.69

1.30 1.69 2.31

SPC-H2O-800

1

10

100

Pore Width (nm)

Pore Volume, dV/dD

c SPC-CO2-1000

600 SPC-CO2-950 SPC-CO2-900

200

SPC-H2O-850

0.80 1.09

1.0

800

400

SPC-H2O-900

0.59 0.80 1.09 1.63 0.53

Relative Pressure (P/P0)

d 1.14 1.69 0.77

SPC-CO2-1000

0.53 0.80 1.09

SPC-CO2-950

0.53

0.69 1.05 0 0.0

0.2

0.4

0.6

0.8

1.0

1

-1

1.5

f

3

e

SPC-H2O-900

1.2

0.9

SPC-H2O-850

0.6

SPC-H2O-800 0.3

0.0 1

10

100

Pore Width (nm)

100

Pore Width (nm)

Cumulative Pore Volume (cm g )

1.5

SPC-CO2-900 10

Relative Pressure (P/P0)

3

-1

a

800

Pore Volume, dV/dD

3 -1 Adsorbed Volume, STP / cm g

1000

Cumulative Pore Volume (cm 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

3 -1 Adsorbed Volume, STP / cm g

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1.2

SPC-CO2-1000

0.9

SPC-CO2-950

0.6

SPC-CO2-900

0.3

0.0 1

10

100

Pore Width (nm)

Figure 2. (a) Nitrogen sorption isotherms and (b) pore size distribution profiles of SPC-H2O-800, SPC-H2O-850, and SPC-H2O-900; (c) nitrogen sorption isotherms and (d) pore size distribution profiles of SPC-CO2-900, SPC-CO2-950, and SPC-CO2-1000; cumulative pore volume data of SPC-H2O-T (e) and SPC-CO2-T series of samples (f).


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The porous properties of SPCs were analyzed in detail on the basis of nitrogen adsorption‒desorption isotherms and pore size distribution profiles (Figure 2), which were obtained from nitrogen sorption measurements at 77 K. Figure 2a shows nitrogen adsorption‒desorption isotherms of SPCs obtained from steam activation of PTh. There is a sharp increase in the amount of nitrogen adsorbed at the low relative pressure (P/P0 ˂ 0.01) for the SPC-H2O-800 and SPC-H2O-850, while the isotherms tend to be flat at the high relative pressure (P/P0 ˃ 0.1). In the case of SPC-H2O-900, the uptake of nitrogen rises gradually with the increase in the relative pressure in the whole range, suggesting the presence of the larger pores besides micropores. The pore size distribution profiles of SPC-H2O-800, SPC-H2O-850, and SPC-H2O-900 are displayed in Figure 2b. For SPC-H2O-800 and SPC-H2O-850, their pore sizes are in the range of micropores (less than 2.0 nm). However, for the SPC-H2O-900 activated at 900 °C, a great amount of mesopores are detected, indicating that the reaction activity between steam and carbon increases with increasing the activation temperature. According to the last IUPAC classification, these characteristics indicate that SPC-H2O-800 and SPC-H2O-850 exhibit a Type I(a) isotherm and SPC-H2O-900 shows a Type I(b) isotherm.33 As depicted in Figures 2c and 2d, SPCs obtained from carbon dioxide activation of PTh exhibit a similar tendency. The porosity parameters of SPCs are shown in Table 1. The Brunauer–Emmett–Teller (BET) specific surface area and total pore volume results of SPC-H2O-800, SPC-H2O-850, and SPC-H2O-900 are in the range of 710–2210 m2 g–1 and 0.37–1.26 cm3 g–1, respectively. The BET specific surface


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areas and total pore volumes of SPC-CO2-900, SPC-CO2-950, and SPC-CO2-1000 are in the range of 670–1900 m2 g–1 and 0.31–0.98 cm3 g–1, respectively. Figures 2e and 2f show cumulative pore volume data of SPC-H2O-T and SPC-CO2-T series of samples. It is obvious that the ultramicropore volumes are not correlated with the total pore volumes. Narrow micropore volumes play a vital role in the carbon dioxide uptake. Therefore, the ultramicropore volumes were further calculated from the carbon dioxide isotherms at 273 K by applying Dubinin–Radushkevich equation (Figure S3, Supporting Information). Table 1 gives the ultramicropore volumes of SPC-H2O-T and SPC-CO2-T series of samples.

Table 1. Porous properties of SPCs prepared at different conditions. Sample

SBET (m2 g–1) a

SLangmuir (m2 g–1) b

Smicro (m2 g–1) c

Vtotal (cm3 g–1) d

Vmicro (cm3 g–1) e

Vultramicro (cm3 g–1) f

SPC-H2O-800

710

940

690

0.37

0.34

0.29

SPC-H2O-850

1110

1650

1030

0.65

0.55

0.35

SPC-H2O-900

2210

3510

2120

1.26

1.12

0.30

SPC-CO2-900

670

860

660

0.31

0.30

0.33

SPC-CO2-950

1230

1570

1210

0.58

0.55

0.35

SPC-CO2-1000

1900

2550

1830

0.98

0.84

0.28

a

b

Specific surface area based on the BET method; Specific surface area obtained from the Langmuir equation; c Micropore surface area determined by the t-plot method; d Total pore volume at P/P0=0.97; e Micropore volume determined by the t-plot method; f Ultramicropore volume obtained from carbon dioxide adsorption isotherms by applying the Dubinin–Radushkevich equation.


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b

SPC-H2O-900

Intensity / a.u.

C 1s

O 1s S 2p

SPC-H2O-850 SPC-H2O-800

C 1s

SPC-CO2-1000 O 1s S 2p SPC-CO2-950 SPC-CO2-900

800 700 600 500 400 300 200 100

800 700 600 500 400 300 200 100

Binding Energy / eV

Binding Energy / eV

c

S 2p SPC-H2O-900

SPC-H2O-850

SPC-H2O-800 160

162

164

166

168

170

172

S 2p

d Intensity / a.u.

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

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SPC-CO2-1000

SPC-CO2-950

SPC-CO2-900 160

162

Binding Energy / eV

164

166

168

170

172

Binding Energy / eV

Figure 3. XPS survey spectra of SPC-H2O-T (a) and SPC-CO2-T series of samples (b); high-resolution S 2p spectra of SPC-H2O-T (c) and SPC-CO2-T series of samples (d).

Chemical compositions of the as-prepared SPC samples were confirmed by X-ray photoelectron spectroscopy (XPS) analysis. As shown in Figures 3a and 3b, all of these samples display the presence of carbon, sulfur, and oxygen elements. Table S1 (Supporting Information) gives carbon, oxygen, and sulfur contents of SPCs in this work. It can be seen that sulfur species decrease obviously with increasing steam or carbon dioxide activation temperatures. The decreased sulfur contents are mainly ascribed to decomposition/reaction of thermally unstable sulfur species during the physical activation processes. According to the high-resolution C 1s spectra (Figure S4,


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Supporting Information), SPCs possess three types of carbon species, including C=C (284.8 eV), C–S or C–O (286.8 eV), and π–π* transitions (291.2 eV), respectively, which is consistent with other carbon-based materials.34 Figures 3c and 3d present their high-resolution S 2p spectra. The surface sulfur species of SPCs can be deconvoluted into three different peaks. The signals at 164.1 and 165.3 eV are assigned to thiophene-like sulfur groups, while the higher binding energy peak at 168.8 eV can be resulted from oxidized sulfur groups.35 Figure 4 displays carbon dioxide sorption isotherms of SPC-H2O-T and SPC-CO2-T series of samples at 273 K and 1.0 bar. All of these SPCs present enhanced uptake capabilities with increasing carbon dioxide pressure, and their sorption isotherms are almost reversible. Table 2 summarizes the carbon dioxide adsorption capacities of these SPCs at 273 K and 1.0 bar. It can be seen that carbon dioxide adsorption capacities of SPC-H2O-T and SPC-CO2-T series of samples are in the range of 3.44–3.89 mmol g–1 and 3.52–4.16 mmol g–1, respectively. Remarkably, SPC-H2O-850 (3.89 mmol g–1) and SPC-CO2-950 (4.16 mmol g–1) possess the highest carbon dioxide adsorption capacities in SPC-H2O-T and SPC-CO2-T series of samples, respectively. Although SPC-H2O-900 (2210 m2 g–1) and SPC-CO2-1000 (1900 m2 g–1) show the highest specific surface areas, their carbon dioxide adsorption capacities are only 3.44 and 3.52 mmol g–1, respectively. Therefore, carbon dioxide uptake capacities of SPCs are not positively correlated with their specific surface areas. Furthermore, sulfur-doping is also responsible for the carbon dioxide adsorption because of their


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polar interaction, acid–base interaction, and/or hydrogen bonding interaction between the carbon surface and carbon dioxide molecules. 36,37 According to XPS analysis, SPC-H2O-800 (3.4 atom %) and SPC-CO2-900 (4.9 atom %) exhibit the highest sulfur contents in SPC-H2O-T and SPC-CO2-T series of samples, respectively. However, their carbon dioxide adsorption capacities are 3.61 and 3.92 mmol g–1, respectively, which are lower than those of SPC-H2O-850 (3.89 mmol g–1) and SPC-CO2-950 (4.16 mmol g–1). Therefore, carbon dioxide uptake capacities of SPCs are either not positively

5

-1

CO2 Adsorbed (mmol g )

correlated with their sulfur contents.

a

273 K

4

SPC-H2O-850

3 SPC-H2O-800

SPC-H2O-900

2 1 0

0

200

400

600

800

Absolute Pressure (mmHg)

5

-1

CO2 Adsorbed (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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4

273 K

b

SPC-CO2-950

3 SPC-CO2-900

SPC-CO2-1000

2 1 0

0

200

400

600

800


Figure 4. (a) Carbon dioxide sorption isotherms of SPC-H2O-800, SPC-H2O-850, and SPC-H2O-900 at 273 K; (b) carbon dioxide sorption isotherms of SPC-CO2-900, SPC-CO2-950, and SPC-CO2-1000 at 273 K.


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Carbon dioxide uptake capacity usually increases with increasing the specific surface area or sulfur content. However, this rule is not suitable for the obtained SPCs in our work. Therefore, there are some other factors that could affect the amount of carbon dioxide adsorbed. It has been reported that pore volume plays a very important role in carbon dioxide adsorption under atmospheric pressure.38,39,40 According to Table 1, it can be seen that SPC-H2O-900 possesses the largest total pore volume (1.26 cm3 g–1) and micropore volume (1.12 cm3 g–1) among all the as-prepared SPCs. However, its carbon dioxide adsorption capacity is very low as compared to those of other SPCs. Therefore, total pore volume and micropore volume are not decisive factors that affect carbon dioxide uptake. Recently, some literatures have indicated that small pore sizes (˂ 1.0 nm) are beneficial to the enhancement of carbon dioxide adsorption capability.18,41 It can be seen from Table 2 that although SPC-H2O-900 possesses the largest micropore volume among SPCs, its cumulative pore volume is only 0.15 cm3 g–1 when pore size is less than 1.0 nm. SPC-CO2-950, which shows the highest carbon dioxide capacity (4.16 mmol g–1), possesses the largest cumulative pore volume of 0.29 cm3 g–1 among all the as-prepared SPCs when pore size is less than 1.0 nm. The above results indicate that carbon dioxide adsorption capacity at 273 K is related to pore volumes from small pore size (less than 1.0 nm).


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Table 2. Porosity parameters and CO2 uptake capacities of SPCs at different conditions. CO2 uptake (mmol g–1)

Pore volume (cm3 g–1) a

273 K (1.0 bar)

293 K (1.0 bar) b

293 K (0.15 bar)

SPC-H2O-800

0.24

3.61

2.71 (2.30)

0.94

SPC-H2O-850

0.24

3.89

2.73 (2.45)

0.77

SPC-H2O-900

0.15

3.44

2.07 (1.84)

0.48

SPC-CO2-900

0.25

3.92

2.85 (2.49)

0.94

SPC-CO2-950

0.29

4.16

2.87 (2.42)

0.81

SPC-CO2-1000

0.19

3.52

2.42 (2.08)

0.42

Sample

a

b

Pore volume from small pore size (less than 1.0 nm); These values in parentheses are carbon dioxide uptake data of SPCs under wet conditions at 293 K. The carbon dioxide uptake behaviors of these SPCs at 293 K were also investigated. Table 2 summarizes their adsorption capacities for carbon dioxide at 1.0 bar. Obviously, there is a similar trend for carbon dioxide adsorption of SPCs at 293 K as compared with 273 K. However, we can find out an interesting phenomenon after careful observation. The difference in the carbon dioxide uptake capacities between SPC-H2O-800 and SPC-H2O-850 becomes smaller with increasing the adsorption temperature from 273 to 293 K. For SPC-CO2-900 and SPC-CO2-950, they have a similar tendency. According to previous report, this phenomenon might be ascribed to the surface chemistry that plays an important role at higher temperatures.19 The isosteric heat of adsorption (Qst) is calculated by fitting carbon dioxide adsorption isotherms at different temperatures (273 and 293 K) and applying the Clausius–Clapeyron equation (Figure S5, Supporting Information).42 Figure S6 (Supporting Information) presents Qst curves of the as-prepared SPCs. Their Qst values are in the range of 19–26 kJ mol–1,


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which depend on the amount of carbon dioxide adsorbed. These Qst values are comparable to those of many other porous adsorbents reported previously.11,43,44 The low Qst value indicates that the uptake of carbon dioxide is via physical adsorption, which facilitates the regeneration of the adsorbents.44 It is well-known that regeneration ability is very crucial in practical application in addition to adsorption capacity. SPC-H2O-850 and SPC-CO2-950 were investigated to evaluate their regeneration ability. (Figure S7, Supporting Information) It can be seen that their carbon dioxide uptake capacities are almost constant and no obvious change can be observed in ten cycles, suggesting that SPC-H2O-850 and SPC-CO2-950 possess excellent recyclability for regeneration. Recently, there has been some research focus on the postcombustion carbon dioxide capture. Solid adsorbents for postcombustion capture of carbon dioxide would inevitably be exposed to humidity because the cost of dehydrating the incoming gas stream would be high. To test the effect of humidity on the carbon dioxide uptake, the as-prepared SPCs were exposed to the humid condition (75 % relative humidity obtained from a saturated sodium chloride) for 24 h. These wet materials were measured for carbon dioxide adsorption without additional treatment. Their adsorption isotherms and corresponding data at 293 K were shown in and Figure S8 (Supporting Information) and Table 2. These SPCs loses 11–16 % of their original capacity measured under the dry condition. These capacity loss values are superior to those of metal–organic framework (~20 % drop),45 microporous carbon molecular sieve (39 % drop),46 and


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conjugated microporous polymer (29–60 % drop).47

b

293 K

3

CO2 2

CH4

1

N2 0

0

200

400

600

800

CO2 2

CH4 1

N2 0

0


d

293 K

3

600

CO2 2

CH4 1

N2 0

0

200

400

600

800

CO2 2

CH4 1

N2 0

0

f

293 K

SPC-CO2-950

2

CH4 1

N2 0

200

400

600

400

600

800

3

293 K

SPC-CO2-1000

-1

-1

CO2

0

200


Gas Adsorbed (mmol g )

3

800

293 K

SPC-CO2-900


e

400

-1

SPC-H2O-900


3

200


-1


c

293 K

SPC-H2O-850

-1

SPC-H2O-800


3

-1


a


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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800


CO2

2

CH4 1

N2 0

0

200

400

600

800


Figure 5. Carbon dioxide, methane, and nitrogen adsorption isotherms of (a) SPC-H2O-800,

(b)

SPC-H2O-850,

(c)

SPC-H2O-900,

(d)

SPC-CO2-900,

(e)

SPC-CO2-950, and (f) SPC-CO2-1000 at 293 K.

Nitrogen and methane adsorption experiments were conducted to assess the


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separation abilities of SPCs at 293 K (Figure 5). We employed initial slope ratio to determine the selectivity for carbon dioxide (Figure S9 and Table S2, Supporting Information). Among the as-prepared SPCs, SPC-H2O-800 shows the highest CO2/N2 selectivity of 29.3, and the CO2/N2 adsorption selectivities of the rest SPCs range from 14.7–25.9. These selectivities are comparable to those exhibited by heteroatom-doped porous carbons (9.6–21.7)48 and porous organic polymer CPOP-1 (25).49 Furthermore, the carbon dioxide selectivity of SPCs over methane was also obtained by employing initial slope methods. The CO2/CH4 selectivities of these SPCs are in the range of 3.0 to 5.1. Considering the fact that the partial pressure of carbon dioxide is 0.15 bar under postcombustion conditions, carbon dioxide uptake capacities of SPCs at 0.15 bar were also given (Table 2). Both SPC-H2O-800 and SPC-CO2-900 show the highest amounts of adsorbed carbon dioxide (0.94 mmol g–1) in SPC-H2O-T and SPC-CO2-T series of samples at 293 K and 0.15 bar. SPC-H2O-800 and SPC-CO2-900 display lower specific surface areas and pore volumes than SPC-H2O-850 and SPC-CO2-950, respectively. Therefore, sulfur-containing functional groups should have a positive effect on the carbon dioxide adsorption at a low relative pressure. Table S3 (Supporting Information) provides carbon dioxide adsorption performance comparison of SPCs with other porous materials. Although the adsorption capacities achieved in our work are lower than that of metal–organic framework, the values are comparable to those of many other porous materials, including porous carbons, graphene-based porous materials, and porous


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organic polymers. Therefore, the as-prepared SPCs could have potential for carbon dioxide adsorption.

CONCLUSIONS In this work, carbon dioxide and steam activation methods are applied to prepare highly porous sulfur-doped carbons for carbon dioxide uptake by using PTh as a precursor. The porous and chemical properties of the as-prepared SPCs have been studied in detail. Their porosity parameters and chemical compositions can be well-tailored by changing activating agents (steam and carbon dioxide) and reaction temperature. These SPCs with specific surface areas of 670–2210 m2 g–1 show high carbon dioxide adsorption capacities of 3.44–4.16 mmol g–1 at 273 K and 1.0 bar. After careful analysis for the carbon dioxide uptake performance, both pore volumes from small pore size (less than 1.0 nm) and surface chemistry are responsible for the carbon dioxide adsorption. The as-prepared SPCs with tunable porosity might be applied in other fields, such as supercapacitors, batteries, water treatment, and catalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/. Instrumental characterization; SEM images of SPCs; high-resolution C 1s spectra


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of SPC-H2O-T and SPC-CO2-T series of samples; Virial analysis of carbon dioxide adsorption data (273 and 293 K) for SPC-H2O-T and SPC-CO2-T series of samples; Qst for SPCs as a function of carbon dioxide adsorption amounts; the cycling performance of SPC-H2O-850 and SPC-CO2-950 for carbon dioxide uptake; the carbon dioxide uptake of SPCs under wet condition at 293 K.

AUTHOR INFORMATION Corresponding Authors *Phone: +86 10 8254 5576. E-mail: [email protected]. *Phone: +86 10 8254 5708. E-mail: [email protected]. *Phone: +86 244 5861 003. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The financial support of the National Natural Science Foundation of China (Grants no. 51602070 and 21574032) is acknowledged.

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