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Nitrogen and Sulfur Co-Doped Hierarchically Porous Carbon for Adsorptive and Oxidative Removal of Pharmaceutical Contaminants Wenjie Tian , Huayang Zhang, Xiaoguang Duan, Hongqi Sun, Moses O. Tade, Ha Ming Ang, and Shaobin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01748 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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ACS Applied Materials & Interfaces

Nitrogen and Sulfur Co-Doped Hierarchically Porous Carbon for Adsorptive and Oxidative Removal of Pharmaceutical Contaminants

Wenjie Tian, Huayang Zhang, Xiaoguang Duan, Hongqi Sun*, Moses O. Tade, H.M. Ang, and Shaobin Wang* Department of Chemical Engineering and CRC for Contamination Assessment and Remediation of the Environment (CRC CARE), Curtin University, GPO Box U1987, Western Australia, Australia

ABSTRACT Heteroatoms (nitrogen and sulfur) co-doped porous carbons with high surface areas and hierarchically porous structures were successfully synthesized via a direct pyrolysis of a mixture of glucose, sodium bicarbonate and thiourea. The resulting N-S co-doped porous carbons (N-S-PCs) exhibit excellent adsorption abilities and are highly efficient for potassium persulfate (PS) activation when employed as catalysts for oxidative degradation of sulfachloropyridazine (SCP) solutions. The adsorption capacities of N-S-PC-2 (which contains 4.51 at.% of N and 0.22 at.% of S, and exhibits SBET of 1608 m2 g−1) are 73, 7 and 3 times higher than graphene oxide (GO), reduced-graphene oxide (rGO) and commercial single-walled carbon nanotube (SWCNT), respectively. For oxidation, the reaction rate constant of N-S-PC-2 is 0.28 min−1. This approach not only contributes to the large-scale production and application of high-quality catalysts in water remediation, but also provides an innovative strategy for production of heteroatom-doped porous carbons for energy applications.

KEYWORDS: Sulfur and nitrogen co-doping, persulfate, antibiotics, porous carbon, adsorption

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1. INTRODUCTION The heavy pressure caused by the fresh water scarcity and increasing water consumption have urged rigorous pollution control and effective remediation technologies.1 Particular attention should be paid to emerging contaminants. More recently, worldwide discharge of pharmaceuticals in municipal wastewater has been recognized as one of the emerging environmental issues.2-6 Among these pharmaceuticals, sulfonamide antibiotics have become one of the major contributors. Sulfonamides have been among the most extensively used antibiotics in aquaculture, animal husbandry and also in human medicine since they were discovered in 1930s.7 Sulfonamides are polar amphoteric compounds that are water-soluble and are easy to migrate in the environment.1-2 Sulfonamides would not undergo biodegradation neither under aerobic nor anaerobic conditions.8 In addition, due to their anionic character and antibacterial nature, they may bypass the depuration activity in municipal sewage treatment plants.1, 9 As a result, sulphonamides are regularly detected in the environment such as hospital waste dumps, fish farming wastewaters, animal manure effluents, and manure waste lagoons from swine farms.10 Continuous exposure to the sulphonamides contained in water supplies, even in trace amounts, could induce high levels of microbial resistance in wildlife and humans.

2-6

Moreover, highly toxic effects of

sulfadiazine on daphnia magna, green algae and lemna minor have already been observed.1112

Also, more evidences have suggested the antibiotic resistance transfer between aquatic

bacteria and human pathogens.13

It is therefore critical to develop and apply efficient water treatment method to remove sulfonamides from various effluents. So far, several trails have been reported in removal of sulfonamides, such as adsorption,7,

14-15

membrane filtration,16 chemical remediation and

photocatalytic degradation.8 Among them, adsorption is feasible and economical to conduct, yet it has been limited by the capability for ultralow pollutant concentration because of the adsorption-desorption equilibrium. It also requires a proper post-treatment. In pursuit of complete decomposition of the target organic pollutants into harmless substances, advanced oxidation processes (AOPs) have demonstrated competing capabilities.9,

17

Based on the

electrochemical generation of hydroxyl radicals (•OH), several studies have been done on sulfonamides degradation using electrochemical advanced oxidation processes (EAOPs).9, 12


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Alternatively, AOPs achieved by activating some superoxides such as hydrogen peroxide (H2O2), peroxymonosulfate (HSO5−, PMS) and persulfate (S2O82−, PS), tend to be simpler, which have not been addressed for degradation of sulfonamides yet.

As a promising

•−

alternative to •OH, strongly oxidizing sulfate radicals (SO4 ) will also be generated in the activation of PMS or PS,18-20 making it highly efficient. It is known that PS could be activated by various approaches such as heating,21 UV-light irradiation,22 catalysis by transition metal ions, metallic oxides23 or carbon based materials.24 Carbon-based materials, such as graphene, carbon nanotubes, porous carbon and activated carbons, have received much attention recently, because of their high efficiencies in water remediation and prevention of potential secondary contamination from toxic metal leaching.25 In our previous studies, chemically reduced graphene oxide, graphene, and carbon nanotubes all showed high efficiency to activate PMS or PS for phenol degradation.18-20 However, the preparation of graphene oxides and carbon nanotubes commonly involves multi-steps, high cost and harsh treatment (such as using sulfuric acid). For widespread applications, scalable, economical and simple synthesis would make such carbon-based catalysts more appealing.

Hierarchically porous carbon materials with a high surface area, well-defined porosity, tuneable surface chemistry, good electrical conductivity and excellent chemical stability are attracting great attention on account of their potentials in addressing energy and environmental issues.26-28 Typical approaches include hard or soft templating processes, and the porosity can be generated by removing those hard or soft sacrificial constituents after carbonization.26-28 However, there exists some common drawbacks with the hard-templating method.26 Although soft-templating methods are more facile, most of the already explored softtemplates are based on rather expensive and non-renewable surfactants or blockcopolymers.26, 28 Therefore, it is highly desirable to develop scalable, economical, efficient and feasible methods to fabricate hierarchically porous carbon structures. Since pristine carbons are very poor in their catalytic performance such as oxygen reduction or the activation of superoxides, heteroatom doping has been widely adopted to improve their catalytic abilities and nitrogen is recognized as a preeminent dopant.25, 29 Very recently, sulfur is receiving intensive attention as a complementary element to N.

29, 30

As it is known, N is

able to tune electronic properties of the carbon materials, whereas S is able to induce high 3 ACS Paragon Plus Environment


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chemical reactivity.29 More importantly, both experimental studies and quantum calculations have proven that catalysts dually doped with N and S exhibited better performances than that with solely N or S doping due to a synergistic effect.29-32 Therefore, introduction of N, S atoms into the structure of porous carbons is promising for more efficient catalysis.

Herein we first describe a method for synthesis of N-S co-doped porous carbons with welldefined pore structures by a pyrolysis process of glucose, sodium bicarbonate and thiourea at 700 oC under nitrogen flow. Glucose was used as the carbon source, which is of great significance because of its low value, huge amount, easy access, rapid regeneration and environmental friendliness. With similar attributes, sodium bicarbonate, one of the most widely used osteoporosis agent in food industry, was applied as the pore-forming agents. Thiourea was also used to allow for dual doping of nitrogen and sulfur. Compared to the hard-templating method, the craft we adopted is more feasible, economical and involves no harsh treatment. On the other hand, sodium bicarbonate is also cost-effective compared with the expensive soft templates.26, 28 The N, S co-doped porous carbons (N-S-PCs) were applied for adsorption and degradation of sulfachloropyridazine (SCP), a broad spectrum of sulfonamide. It was observed that N-S-PC-2 (which contains 4.51 at.% of N and 0.22 at.% of S, and exhibits SBET of 1608 m2 g−1) demonstrated not only excellent adsorption ability but also a remarkable catalytic oxidation capability for SCP removal, making it an attractive alternative for water remediation. The mechanism of adsorption and degradation was also discussed.

2. EXPERIMENTAL SECTION 2.1. Chemical reagents. D-(+)-glucose (≥ 99.5%), sodium bicarbonate (≥ 99.7%), thiourea (≥ 99.0%), potassium persulfate (≥ 99.0%), sodium nitrite (≥ 99.0%), and 5, 5-dimethyl-1pyrroline N-oxide (≥ 97.0%) were purchased from Sigma-Aldrich. Commercial single-walled carbon nanotube (SWCNT, ≥ 95.0%) was purchased from Chengdu Organic Chemicals, China. All chemicals were used without further purification.

2.2. Preparation of Carbon Materials. For the synthesis of N-S-PCs, glucose, sodium bicarbonate and thiourea were firstly dissolved in pure water, followed by evaporation of the aqueous solution at 105 oC in air. After that, the dried mixture was put into a tube furnace and calcined at 700 oC for 2 h with a heating rate of 5 °C/min under N2 flow. The carbonized 4 ACS Paragon Plus Environment

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materials were grinded to powder, and then washed by water and ethanol for several times. The final carbon samples were obtained after drying. In this process, the mass ratio between glucose and sodium bicarbonate was kept constant to be 1:1. The mass proportion of thiourea in the mixture varied from 5, 15 to 25% to manipulate the doping level so as to tune the functionality of the porous carbons. Accordingly the samples were referred as N-S-PC-1, NS-PC-2 and N-S-PC-3. For reference samples, blank porous carbon (PC) with no heteroatom doping was prepared by calcination of glucose and sodium bicarbonate, while non-porous carbon (NONPC) was obtained by calcining pure glucose only. GO was produced from a natural graphite powder by the modified Hummers’ method,18 while rGO was prepared according to our previous synthesis method.19

2.3. Characterization. Morphologies of the carbon samples were investigated on a transmission electron microscope (JEOL 2100, TEM) and scanning electron microscope (FEI Verios XHR 460, SEM). X-ray diffraction (XRD) patterns were conducted on a Bruker D8-Advanced X-ray instrument. Surface elemental contents and chemical states were determined by X-ray photoelectron spectroscopy (XPS) with a Kratos AXIS Ultra DLD system under ultra high vacuum (UHV) condition by monochromated Al-Kα X-rays. XPS spectra were fitted using Kratos Vision and Casa XPS software. All spectra were calibrated to yield a primary C 1s component at 284.6 eV. A Shirley background was first subtracted, followed by component fitting using Voigt functions with a 30% Lorentzian component. Elemental analysis of the samples were measured on an elemental analyzer (PerkinElmer 2004 II). A thermogravimetric analysis instrument (TGA/DSC1 STARe system, METTLERTOLEDO) was also employed. The Brunauer-Emmett-Teller (BET) specific surface area and the pore size distribution of the samples were determined by N2 adsorption/desorption at – 196 °C using a Micromertics Tristar 3000. The samples were degassed in vacuum at 110 °C overnight before the test. Electron paramagnetic resonance (EPR) was performed on a Bruker EMS-plus instrument to detect the free radicals, with 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent and the quantitative information were analyzed by Xeon software (Bruker).

2.4. Adsorption and Catalytic Oxidation Procedures. Typical adsorption experiments were carried out at 25 oC by dispersing the carbon samples (0.05 g/L) in SCP solutions (20 mg L–1, pH 7). At certain time intervals, 1 mL of the solution was withdrawn using a syringe and 5 ACS Paragon Plus Environment


filtered by filters with 0.45 µm millipore films. The concentrations of these samples were determined by an ultra-high performance liquid chromatography (UHPLC). SCP oxidation tests were conducted in a 500 mL glass reactor at 25 oC. The carbon samples (0.05 g L-1) and PS (6.5 mM) were added to the SCP (20 mg L–1, pH 7) solutions together to initiate the reaction. At each time interval, 1.0 mL solution of water samples were taken, filtered and quenched immediately by mixing with 0.5 mL of sodium nitrite solution (0.1 M). Each experiment was repeated and the results were reproducible. For the reusability tests, the catalyst was collected by vacuum filtration after each of 3 h reaction, then washed with deionized water for several times and dried in an oven at 60 °C.

3. RESULTS AND DISCUSSION 100

0.0

80

-0.2 Glucose Glucose/Sodium bicarbonate Glucose/Sodium bicarbonate/Thiourea

60

-0.4

-0.6

40

DTG (% °C−1)

Weight loss (%)

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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-0.8 20

-1.0 0

100

200

300

400

500

600

700

800

900

T(°C)

Figure 1. TGA and DTG curves of the precursors heated under argon atmosphere.

TGA and DTG were conducted by heating the precursors to synthesize NONPC, PC and NS-PC-2 under argon atmosphere to gain insights into the formation mechanism of porous carbon during pyrolysis. As indicated by the curves of pure glucose (Figure 1), there was a mild weight loss between 180 and 250 °C, which could be ascribed to the dehydration of glucose. Following that, a further larger decomposition process was observed with an apparent weight loss occurred between 250 and 400 °C. After that, the curve was flat to 900 °C. However, in terms of glucose/sodium bicarbonate as well as glucose/sodium bicarbonate/thiourea systems, their TGA and DTG curves were distinct from those of pure glucose, with weight loss happening only at one temperature interval. Apart from that, the beginning temperature was decreased to 115 °C while ended earlier at 280 °C. This indicated 6 ACS Paragon Plus Environment

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that the addition of sodium bicarbonate might promote the self-assembly among the precursors before pyrolysis, which could facilitate the noncovalent interactions among them, such as hydrogen bonding, van der Waals forces and electrostatic interaction, thereby beneficial for the synthesis of high-quality carbons. The further mass loss above 700 °C could be attributed to thermal decomposition of Na2CO3, removal of some heteroatoms or reduction of Na2CO3 at higher temperatures.33 Therefore, to avoid such chemical loss, the fabrication temperature was set at 700 °C.

With the assistance from TGA results (Figure 1), the overall procedures for the synthesis of N-S-PCs can be illustrated in Scheme 1. In the evaporation process at 105 °C, the selfassembly process of glucose, sodium bicarbonate and thiourea occurred. Then during the heat-treatment of the mixture at 700 °C under a nitrogen flow, well-defined hierarchically porous structures could be generated.

Glucose

Thiourea

Sodium bicarbonate

Glucose/sodium 105 °C bicarbonate/thiourea solution Evaporation

700 °C, N2 Pyrolysis

Scheme 1. Synthesis of N, S co-doped porous carbons.

3.1 Characterization of Materials. XRD patterns of the prepared carbons depicted two weak broad peaks located at about 25 and 44°, corresponding to (002) and (101) diffractions of carbon, respectively (Figure S1). SEM images shown in Figure 2a confirmed that NONPC from direct carbonization of glucose was nonporous, while porous carbon was successfully produced when sodium bicarbonate was applied (Figure 2b, PC) and N-S-PC-2 also possessed a porous structure. TEM images (Figures 2d and S3) demonstrated that N-S-PC-2 composed of a three-dimensional interconnected pore system. There were evident macrospores on the surfaces of the porous carbons and N-S-PC-3 possessed an obvious wider spacing between holes than other porous carbons (Figures S2 and S4). To further examine their distinctions, the BET specific surface areas and pore structure details of these samples



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were measured by N2 adsorption-desorption and corresponding results are summarized in Figure 3 and Table 1.

a

b

c

d

Figure 2. SEM images of (a) NONPC, (b) PC and (c) N-S-PC-2. (d) Representative TEM image of N-S-PC-2.

According to Figure 3, the porous carbons showed type-І isotherms with desorption hysteresis and a steep increase at relative low pressures, followed by a moderate increase at intermediate relative pressures (Figure 3a). This highlights the formation of a hierarchical pore architecture consisting of micro- (< 2 nm) and mesopores (2-50 nm).27 NONPC possessed an appreciable low BET surface area (2 m2 g−1) with virtually no pores. In contrast, a high BET surface area (SBET, 459 m2 g−1) and an improved total pore volume (Vt, 0.46 cm3 g−1) were obtained in PC. The BET surface areas of porous carbons were further enhanced after N, S co-doping, to 1044 m2 g−1 in N-S-PC-1, reaching 1608 m2 g−1 in N-S-PC-2, and then decreasing in N-S-PC-3 (1016 m2 g−1). The total pore volumes of N-S-PCs were also improved greatly compared to PC, with N-S-PC-2 having the highest total pore volume (1.0 cm3 g−1). The measured average pore sizes of all porous carbons were in the range of 0.28~49.47 nm, so the mesopore volume (Vmeso) can be obtained by subtracting micropore volume (Vmic) from Vt. As such, the measured Sexternal can be attributed to the contribution 8 ACS Paragon Plus Environment

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from mesopores. As shown in Table 1, all porous carbons consisted of a hierarchical mesopore/micropore structures. It is worth noting that the Vmic increased steadily with increasing N, S doping amount, while Vmes increased first, and then decreased in N-S-PC-3. It is proposed that, on one hand, the decomposition of thiourea is conducive for pore generation and enhancement of the BET surface area. On the other hand, heteroatom doping can potentially block the pore channels of porous carbons, leading to higher Vmic and lower Vmes. It is also noted that the mesopores in these porous carbons were dominated by ponysize mesopores, peaked near 2 nm (Figure 3b). However, their plots did not go to zero in a wider mesopore-size range, indicating that large-size mesopores also existed.

0.10

700

a

b

N-S-PC-2 N-S-PC-3

dV/dD Pore Volume (cm3/g⋅nm)

NONPC PC N-S-PC-1

600 500

3

Volume Sorption (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


400 300 200 100

NONPC PC N-S-PC-1 N-S-PC-2 N-S-PC-3

0.08

0.06

0.04

0.02

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Relative Pressure (P/P0)

1.0

0.00 0

2

4

6

8

10

12

14

16

18

Pore diameter (nm)

20

22

24

26

28

Figure 3. (a) Nitrogen sorption isotherms at –196 °C and (b) BJH pore size distributions of the carbon samples.

Table 1. Textural characteristics of the porous carbon samples. Samples

SBET a /

Smic b

Sexternal b

Vt c /

Vmic b / cm3

Vmesod /

Average pore

m2 g−1

/ m2 g−1

/ m2 g−1

cm3 g−1

g−1

cm3 g−1

size range / nm

PC

459

79

380

0.46

0.04

0.42

0.28-43.78

N-S-PC-1

1044

388

656

0.71

0.23

0.48

0.28-49.47

N-S-PC-2

1608

550

1058

1.00

0.33

0.67

0.29-38.12

N-S-PC-3

1016

644

372

0.59

0.37

0.22

0.29-35.30

a

Surface area calculated using the BET method. Evaluated by the t-plot method. c Total pore volume calculated at P/P0 = 0.99. d Mesopore volume obtained through the difference between Vt and Vmic. b



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Based on the above, the formation mechanism of the hierarchical pore structures are proposed as follows: The macropores observed in SEM images might be formed by the release of CO2 from the surfaces when NaHCO3 decomposed. Then Na2CO3 was left and encapsulated in the inner pore channels created by CO2 bubbling, which should be responsible for creating mesopores and micropores. Even after water washing, trace levels of encapsulated Nacompounds might be left in the inner pores since they were hard to be removed completely and might be difficult to be detected by XPS surface analysis.34 Thiourea addition also created mesopores as well as micropores. It is deduced that macropores only exist on the surface of the porous carbons because the inner macrospores created by CO2 were filled by micropores and mesopores. The number of macropores is low so macropores are hard to be detected. It was known that the functionalities of carbons are closely linked to their surface chemical states, especially to N, S-containing functional groups.30, 35 Elemental analysis results were shown in Table S1. XPS analysis of N-S-PCs was then performed to investigate the compositional information of the materials. The surface chemical compositions and the resulting spectra are presented in Figure 4. Specifically, the atomic ratios of N in N-S-PCs were determined to be from 3.42, 4.51 to 9.61% for N-S-PC-1, -2 and -3, respectively. In the high-resolution N 1s spectra, the spectra were resolved to four peaks centered at around 398, 400, 401 and 405 eV, corresponding to pyridinic-N (N-1), pyrrolic-N (N-2), graphitic-N (N3), and oxidized-N (N-4), respectively.36-37 With the increase of thiourea proportion, the contents of N-1 and N-3 increased notably (Figure 4d). This might be meaningful considering that pyridinic N and graphitic N are conducive for enhanced catalytic performance in oxygen reduction reaction (ORR).38 One the other hand, S contents did not change as much as N, merely in the scope of 0.20, 0.22 and 0.38 at.% for N-S-PC-1, -2 and -3, respectively. All the high resolution S 2p peaks of N-S-PCs were fitted to three components centered at around 164, 165, and 168 eV, respectively. The former two peaks correspond to S 2p3/2 (S-1) and S 2p1/2 (S-2) positions of thiophene-S, derived from spin-orbit splitting of thiophenic sulfur atoms incorporated into carbon framework.39 The last peak possibly arose from some oxidized S (S-3).29,

38, 40-41

Compared with N-S-PC-1, the amount of S-3 was slightly higher in N-S-PC-2, while it was dominant in N-S-PC-3 and the amount of thiophene-S decreased accordingly (Figure 4f). This may be detrimental, as S-C bonds are presumed to play a more vital role in ORR 10 ACS Paragon Plus Environment

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catalytic activity compared with S-O bonds.38 As shown in Figures 4c and d, several N binding configurations can exist. In contrast to N-doping modality, S atoms are prone to be doped at the edges or defects of the carbon network.38 As a result, the overall sulfur contents were much lower than nitrogen in N-S-PCs.

C1s

a

b

O1s N1s

O

N-S-PC-1 (C:90.51% O:5.87% N:3.42% S:0.20%)

Intensity (a.u.)

S2p

N-S-PC-2 (C:90.24% O:5.03% N:4.51% S:0.22%)

N-S-PC-3 (C: 84.08% O:5.94% N:9.61% S:0.38%)

H 1200

1000

800

600

400

200

0

Binding Energy (eV)

10

N-2 N-3

N-4

Intensity (a.u.)

N-1

N-S-PC-1

Nitrogen Content (at. %)

c

N-S-PC-2

d N-1: Pyridinic N N-2: Pyrrolic N N-3: Graphitic N N-4: Oxidized N

8

6

4

2

N-S-PC-3 398

400

403

405

Binding Energy (eV)

408

410

0.5

N-S-PC-1

N-S-PC-3

N-S-PC-3 166

168

Binding Energy (eV)

170

S-1: C-S-C 2p3/2 S-2: C-S-C 2p1/2

N-S-PC-2

164

N-S-PC-2

f S-3

162

N-S-PC-1

0.6

S-1 S-2

e

160

0

Sulfur Content (at. %)

395

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


172

0.4

S-3: C-SOX-C

0.3

0.2

0.1

0.0

N-S-PC-1

N-S-PC-2

N-S-PC-3

Figure 4. (a) Wide survey of XPS spectra of N-S-PCs, (b) schematic illustration of doped N, S atoms, (c) high resolution N 1s spectra of N-S-PCs, (d) the content of four nitrogen species, (e) S 2p spectra of N-S-PCs, and (f) the content of sulfur species.

High-resolution C1s spectra of all the carbon samples were also analyzed, as presented in Figure S5. Although the XRD results of all carbons did not show virtual difference, it is 11 ACS Paragon Plus Environment


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evident in their high-resolution C 1s peak that NONPC had a lower sp2 C content than other porous carbons, evidenced by its wider main peak and enhanced C-H, C-O ratios. Figures S5d-e further proved the successful incorporation of N and S species into carbon frameworks.

TGA and DTG of NONPC and PC (Figure S6, under air) depict that the creation of pore structures lowered the thermal stability by approximately 100 °C. However, heteroatom doping successfully made the porous carbons more thermally stable. The onset burning temperatures of N-S-PCs under air were comparable to that of NONPC.

3.2 SCP Removal. Comparative studies were performed on N-S-PC-2 and some reference carbons of GO, rGO and commercial SWCNT first, followed by a comparison among the carbons prepared under different conditions in this study. Figure 5 exhibits the adsorption and degradation of SCP on different carbon materials. It is shown in Figure 5a that GO could hardly adsorb SCP, whereas SWCNT and rGO provided about 21% and 7% of SCP adsorption, respectively. Remarkably, when N-S-PC-2 was introduced, the adsorptive removal of SCP reached 60%, far better than the reference carbons. Figure 5b indicates that N-S-PC-2 possessed the best adsorption performance among all the porous carbons prepared with different chemical contents. The detailed adsorption information of these carbon samples is summarized in Table S2. The calculated adsorption capacity of N-S-PC-2 can reach 220 mg g−1, which was 73, 7 and 3 times higher than GO, rGO and commercial SWCNT, respectively. NONPC had little effect, yet PC could adsorb 16% of SCP in 30 min. The adsorption capacities (qm) of SCP over N-S-PC-1 and N-S-PC-3 were calculated to be 190 mg g−1 and 126 mg g−1, respectively. However, there were no obvious differences in their surface-area-normalized adsorption capacities (qm/SBET, Table S1), indicating that the adsorption performances of as-prepared porous carbons were basically consistent with their BET surface area. The qm/SBET values of the porous carbons were also similar to GO and rGO, yet SWCNT was higher, indicating that the SWCNT had decent adsorption ability. The adsorption capacity of N-S-PC-2 was also compared with some frequently-used activated carbon adsorbents on pharmaceutical compounds under the similar conditions, as given in Table S3. It is shown that N-S-PC-2 was comparable or better than these activated carbons from the viewpoint of both adsorption capacity and surface-area-normalized adsorption capacities with a much lower dosage.


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Figure 5. (a) and (b) SCP removal by adsorption (Adsorbent 0.05 g L-1, SCP 20 mg L-1, and T 25 oC). (c) and (d) SCP oxidative degradation (Catalyst 0.05 g L-1, PS 6.5 mM, SCP 20 mg L-1, and T 25 oC).

The catalytic SCP oxidation with PS is depicted in Figures 5c and d. It is noted that PS alone could hardly degrade SCP (less than 15% removal in 180 min), and neither can GO and rGO for PS activation. Commercial SWCNT was better, which could achieve 100% SCP removal in 150 min, but still far lower if compared to N-S-PC-2. A rapid decomposition of SCP was observed on N-S-PC-2, with complete SCP removal in 20 min at only 0.05 g L-1 catalyst loading. The rate constant (k) calculated from the first order kinetic model in N-S-PC-2/PS system was 0.28 min−1 (R2 = 0.994). As to the other carbon samples (Figure 5d), there was almost no effect on NONPC. PC showed a moderate ability in PS activation. N-S-PC-1 achieved 100% SCP removal in 45 min (k = 0.22 min−1, R2 = 0.996), and N-S-PC-2 exhibited the best performance, whereas N-S-PC-3 only decomposed 97% SCP in 180 min (k = 0.015 min−1, R2 = 0.962).



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SCP

PS CO2 SO4

2−

H2O

Scheme 2. Catalytic process of SCP on N-S-PC-2. (The grey, blue, red, yellow and green atoms are C, N, O, S and H atoms, respectively.)

The excellent catalytic effect of N-S-PC-2 can be explained in several aspects. To begin with, its high BET surface area and well-defined pore structure (Figure 3 and Table 1) enable more active sites to be exposed. Also, better adsorption capability is able to promote the surface reactions where PS activation occurs.25 To be specific, during PS activation process, once the SCPs and S2O82− (PS) were adsorbed, SCPs will be in-situ decomposed, followed by an adsorption-decomposition cycle until fully SCP removal, as illustrated in Scheme 2. Therefore, the high adsorption capacity of N-S-PC-2 was favorable for accelerating the degradation reaction rate. The other factors are associated with N, S atom doping species and amounts, which can be investigated from the point of density functional theory.30 Compared with carbon (2.55) atoms, N atoms are more electronegative (3.04). As a result, N atoms, especially pyridinic and graphitic N, can create a net positive charge on the adjacent carbon atoms. On the other hand, the electronegativity of sulfur atoms (2.58) is close to carbon, which can reduce the energy difference among unoccupied carbon molecular orbitals.29 It was thus suggested that appropriate amount of nitrogen and sulfur doping can break the chemical inertness of carbon and exert a synergistic effect on the improvement of catalytic activity owing to the redistribution of spin and charge densities and creation of more active sites.29, 42 Therefore, N-S-PC-2 with a moderate doping amount exhibited the best promoting effect. However, as indicated in calculation results,30 over-doping of N or S might break the charge balance of the covalent carbon electron system, disrupt the charge redistribution, and then weaken the synergistic effect. Figure S7 presents the activity of recycled N-S-PC-2 in SCP degradation. As seen, 80% removal was obtained in the second run and 44% removal in the third run, still much better than the performance of the first run testing of GO and rGO.


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The influence of solution temperature on PS activation over N-S-PC-2 was conducted at 25, 35, and 45 oC (Figure 6). It is suggested that higher reaction rate was achieved by elevating the reaction temperature, as illustrated in Figure 6a. The concentration of catalyst was reduced to 0.0025 g L-1, considering its excellent degradation performance. Specifically, complete SCP removal was obtained in 20 min at 35 oC (k = 0.30 min−1, R2 = 0.98) and 15 min at 45 oC (0.44 min−1, R2 = 0.99). It is worth noting that, although the catalyst concentration was very low, the SCP removal could still be achieved in 30 min by N-S-PC-2 (k = 0.16 min−1, R2 = 0.99), which exhibited a powerful degradation capability. The SCP degradation curves of N-S-PC-2 can be fitted well by the first-order kinetics with high values of regressions coefficients (R2 = 0.99). Based on the Arrhenius equation, the activation energy (Ea) of N-S-PC-2 for catalytic SCP oxidative degradation was obtained to be 39.1 kJ mol−1.

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Figure 6. (a) Effect of solution temperature on catalytic activity (Catalyst 0.0025 g L-1, PS 6.5 mM, SCP 20 mg L-1), (b) Estimation of the activation energy.

3.3 Catalytic Mechanism of PS Activation on N-S-PCs. It is reported that, in most AOPs, the reactive radicals produced from the PS activation play a dominant role in attacking and degrading organics. Thus, in-situ electron paramagnetic resonance (EPR) was employed to obtain more insights on the radical generation and evolution. DMPO was adopted as a spin trapping agent to capture the free radicals. Figure 7a shows that N-S-PC-2 was able to effectively activate PS to generate a mass of reactive radicals. Radical evolution graph (Figure 7b and Figure S8) reveals that both SO4•− and •OH were quickly produced in the first five minutes. Owing to the consumption by SCP oxidation, the intensities of •OH slightly dropped, but did not change much (Figure S8). This demonstrates the strong ability of N-SPC-2 for continuous activation of PS to generate •OH radicals. Similarly, the intensities of 15 ACS Paragon Plus Environment


SO4•− did not show evident changes in this process. It is also noted that the signal of DMPO– SO4 was much weaker than DMPO–OH. This can be ascribed to the superior high adsorption capability of N-S-PC-2, which can adsorb not only SCP but also anionic S2O82− and SO4•−. Hence the generated SO4•− radicals might also be adsorbed on the surface of N-S-PC-2, which greatly accelerate the SCP oxidation process.

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Figure 7. (a) EPR spectra of PS activation with N-S-PC-2 (Catalyst 0.05 g L-1, PS 6.5 mM, SCP 20 mg L-1, T 25 oC, and DMPO 0.16 M. ♦ DMPO-OH, ● DMPO-SO4), (b) Radical evolution during PS activation on N-S-PC-2.

Despite from radical reaction, our group recently discovered that N-doping could induce nonradical approaches for catalytic phenol oxidation with PMS.20 To fully disclose the catalytic mechanism in this system, radical quenching experiments were conducted. Ethanol was applied as a radical scavenger, which can react quickly with both hydroxyl and sulfate radicals produced in PS activation process.43 If the radicals were necessary for organic degradation reactions, the reaction would then be deterred by addition of quenching reagents. Figure 8 shows the quenching effect of ethanol on PC and N-S-PC-2/PS systems. It is noted that ethanol greatly lowered the SCP degradation efficiency in PC/PS system. When this ratio was elevated to 1000:1, the reaction rate was nearly reduced to zero. The slight SCP removal can be ascribed to adsorption, as indicated in Figure 5b. However, differences were observed in N-S-PC-2/PS. Figures 8b and c indicate that, although SCP degradation efficiency decreased with increasing ethanol/PS molar ratios, 100% SCP removal could still be achieved in 180 min at 1000:1 molar ratio. This result demonstrated that N and S functional groups might not only enhance the radical pathway but also bring in the non-radical oxidation. According to our previous experimental study,18-19 the sp2 carbon plays a key role for 16 ACS Paragon Plus Environment

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PMS/PS activation and the sp3 carbon is inactive. There are plentiful free-flowing π electrons in the effective sites of the sp2 carbon, which can be activated for a better catalytic activity by conjugating with the lone-pair electrons of N.44 It is hence deduced that N and S could also invoke the covalent π electrons in the sp2 carbon of N-S-PC-2. The activated electrons would combine with PS and activate the O−O bond in PS to react with the adsorbed SCP molecules. Therefore, both radical and non-radical approaches existed in N-S-PC-2/PS system, which worked together for complete SCP decomposition in a short time.

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Figure 8. Effects of radical scavenger on SCP degradation in (a) PS/PC, and (b) PS/N-S-PC2, inset: (c) Reaction rates of SCP oxidation with PS on N-S-PC-2 under different ratios of ethanol (Catalyst 0.05 g L‒1, PS 6.5 mM, SCP 20 mg L–1 and T 25 oC).

4. CONCLUSIONS In summary, we firstly propose an easily-handled NaHCO3-based soft-templating pyrolysis method for preparation of N-S co-doped porous carbon materials. N and S doping plays a crucial role in determining the overall functionalities. The prepared N-S-PC-2 demonstrated a well-developed hierarchical porosity and high surface area. This work unambiguously shows great potential of N-S co-doped porous carbon materials for SCP removal. This simple, green, and economical protocol is highly promising for the large-scale production of high-quality catalysts for water remediation. This synthetic approach can also be extended to fabrication of functional carbon materials with applications in other fields such as separation, energy and medicine.

■ ASSOCIATED CONTENT Supporting Information 17 ACS Paragon Plus Environment


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Adsorption details of the synthesized carbons; adsorption capacity comparison with reported activated carbons; XRD patterns, representative SEM images and magnified SEM images; high resolution C 1s spectra, TGA and DTG curves of all the prepared carbon samples; stability tests of PS/N-S-PC-2 and specific EPR spectra at different time intervals.

■ AUTHOR INFORMATION Corresponding Authors. *Email: [email protected] (S. Wang), [email protected] (H. Sun).

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors acknowledge the partial support from Australian Research Council (DP130101319), technical assistance from The University of Western Australia Centre for Microscopy, Characterisation and Analysis (CMCA) and Curtin University Electron Microscope Facility, as well as the WA X-Ray Surface Analysis Facility, funded by the Australian Research Council LIEF grant (LE120100026). H.S. thanks the support from Curtin Research Fellowship and Opening Project (KL-13 02) funded by State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University. ■ REFERENCES (1) Haidar, M.; Dirany, A.; Sires, I.; Oturan, N.; Oturan, M. A. Electrochemical Degradation of the Antibiotic Sulfachloropyridazine by Hydroxyl Radicals Generated at a BDD Anode.

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TOC

SCP

N2 700 °C

PS CO2

Glucose/Thiourea/NaHCO3

SO42−

H2O


and Sulfur-Codoped Hierarchically Porous Carbon ... - ACS Publications

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