Nitrogen doped porous hollow carbon spheres for enhanced benzene removal

Nitrogen doped porous hollow carbon spheres for enhanced benzene removal

Accepted Manuscript Nitrogen doped porous hollow carbon spheres for enhanced benzene removal Junwen Qi, Yang Li, Guoping Wei, Jiansheng Li, Xiuyun Sun...

4MB Sizes 0 Downloads 21 Views

Accepted Manuscript Nitrogen doped porous hollow carbon spheres for enhanced benzene removal Junwen Qi, Yang Li, Guoping Wei, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Weiqing Han, Lianjun Wang PII: DOI: Reference:

S1383-5866(17)31089-4 http://dx.doi.org/10.1016/j.seppur.2017.07.021 SEPPUR 13878

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

6 April 2017 25 June 2017 11 July 2017

Please cite this article as: J. Qi, Y. Li, G. Wei, J. Li, X. Sun, J. Shen, W. Han, L. Wang, Nitrogen doped porous hollow carbon spheres for enhanced benzene removal, Separation and Purification Technology (2017), doi: http:// dx.doi.org/10.1016/j.seppur.2017.07.021

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nitrogen doped porous hollow carbon spheres for enhanced benzene removal

Junwen Qi, Yang Li, Guoping Wei, Jiansheng Li*, Xiuyun Sun, Jinyou Shen, Weiqing Han, Lianjun Wang*

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Corresponding author. Tel: +86 25 84303216; Fax: +86 25 84315941; E-mail address: [email protected]; [email protected]

Abstract: In this work, nitrogen doped hollow carbon spheres (NHCS) were synthesized via modified “silica-assistant” route and further examined as adsorbents for the removal of benzene, which is a typical volatile organic compounds (VOCs). To understand the structure and composition on benzene removal, solid carbon spheres (SCS), nitrogen doped solid carbon spheres (NSCS) and hollow carbon spheres (HCS), were also prepared for comparison. The structure and morphology characterization revealed that the resultant carbon spheres possess uniform spherical shape, abundant micropores and mesopores with solid or hollow structure and high pore volume. Combined with Raman and XPS spectra, the composition of carbon spheres was successfully tailored via introduction of nitrogen atom into carbon matrix. To present the removal performance of carbon spheres for VOCs, the dynamic adsorption behavior was evaluated. Attributed to the combined effect of hollow structure and nitrogen doping, NHCS presents remarkable dynamic adsorption performance for benzene vapor with adsorption capacity of 766 mg/g. The enhancement on desorption behaviors resulting from hollow structure of HCS is confirmed when compared with SCS. Carbon spheres exhibit good regenerability based on desorption experiment. The mechanism of enhanced adsorption performance caused by nitrogen doping is clarified as the combined action of adjustment on porous structure and composition. These results indicate that NHCS would become a promising candidate for VOCs removal. Key words: carbon sphere; hollow structure; nitrogen doping; VOCs; adsorption

1

1. Introduction Considerable concerns have been raised due to the substantial emissions of volatile organic compounds (VOCs) into air, which are not just the risks for human beings and ecosystem, but also leading serious air pollution such as photochemical smog and haze [1-3]. Numerous technologies are investigated for elimination of VOCs, including adsorption [4, 5], catalytic combustion [6], photocatalytic [7], biological methods [8] and others [9-11]. Among these VOCs control techniques, adsorption has always been a prior choice for its efficiency, convenience, low energy and cheap operating costs [12]. In the adsorption process, proper adsorbent is acknowledged as the most important parameter for the efficient removal of VOCs. Therefore, rationally design and regulation of the structure and composition of adsorbent is of great significance. Porous carbon materials (PCMs), especially activated carbons (ACs) were widely used as adsorbent for VOCs removal due to its abundant porous structure and low cost [13, 14]. In order to improve adsorption properties for VOCs, many efforts have been conducted to prepare ACs with high porosity by developing new precursors [15], using different activation methods [16]. While the mainly microporous structure (<2 nm) of ACs can slow the diffusion velocity of adsorbate to reduce mass transfer efficiency [17]. Besides that, the irregular pore structure could also restrict the access of VOCs molecules to the internal active site, which isn’t conducive to regeneration in thermal desorption [18]. Apart from the porous structure, the composition of porous carbon is another crucial factor to determine the adsorption performance of VOCs [19, 20]. The relationship between adsorption capacity and composition was investigated through adopted modified ACs by impregnating with acids or bases as adsorbent for VOCs [21]. The oxygen-doped ACs prepared with an oxidation treatment were conducted for ethanol and n-octane adsorption [22]. Nitrogen doped ACs which were obtained through activation-oxidation-amination procedure presented higher

2

adsorption capacities for CO2 and benzene [23]. But the post treatments such as impregnating and oxidation would lead to pore clogging or consumption of ACs [24, 25]. In addition to that, the processes were complicated and time consuming. While in situ introduction of heteroatom to tailor compositions of porous carbon would avoid these adverse effects [26, 27]. Hence, it is urgent to develop PCMs with microporous-mesoporous structure and suitable composition for adsorption of VOCs. Carbon spheres, a novel kind of PCMs, have received considerable attentions due to unique physical and chemical characteristics, such as large surface area, low specific density, large controllable inner pore volume and open-framework structures [28-32]. Consequently, carbon spheres have been used in a variety of applications, including drug delivery, energy storage and environmental treatment [33-37]. It’s noticed that carbon spheres with developed mesoporous and hollow structure would facilitate mass transfer efficiency when used as adsorbent for pollutant from environmental media [38]. Moreover, the changeable composition provides the opportunity to apply this novel material for VOCs removal. However, to the best of our knowledge, there are currently only relatively limited studies performed on carbon spheres as adsorbent for VOCs removal. In this present work, we prepare nitrogen doped hollow carbon spheres (NHCS) via modified “silica-assistant” route. Solid carbon spheres (SCS), nitrogen doped solid carbon spheres (NSCS) and hollow carbon spheres (HCS) were synthesized for comparison. The regulation of porous structure and composition on carbon spheres was achieved via altering reactant condition. Dynamic adsorption performances of VOCs on obtained carbon spheres were investigated. The influence of hollow structure was evaluated by desorption experiment, and the promotion of composition was also demonstrated. 2. Experimental 2.1.Chemicals and materials

3

Resorcinol, hexadecyl trimethylammonium chloride (CTAC), tetraethoxysilane (TEOS) and melamine were purchased from Sinopharm Chemical Reagent Co., Ltd. Formaldehyde solution (HCHO, 37-40%), anhydrous ethanol, ammonia aqueous solution (NH4OH, 25-28%) and hydrofluoric acid solution (HF, 40%) were obtained from Nanjing Chemical Reagent Co., Ltd. Benzene, toluene and xylene were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. All chemicals were analytical pure grade and used as received without further purification. Millipore water (18.2 MΩ) was used in all experiments. 2.2.Preparation of carbon spheres Carbon spheres were synthesized based on previous reported “silica-assistant” route [32, 39] with some modification. Typically, 2g CTAC was dissolved in the mixture solution of 40 mL ethanol and 100 mL deionized water. Then 0.5 mL NH4OH solution was added and stirred vigorously for 10 min at 70 oC. Subsequently, 0.55 g resorcinol was added and continually stirred for another 30 min. Next, 3 mL TEOS and 0.74 mL HCHO solution were added to the reaction system. After stirring of another 30 min, 0.55 mL HCHO solution and 0.5 g melamine were added. The reaction solution was stirred for 24 hours continually. The collected solid product was dried in the oven at 70 °C overnight and then calcined in a tubular furnace at 700 °C for 3 h with heating rate of 3 °C/min under N2 flow. Nitrogen doped hollow carbon spheres, denoted as NHCS, were obtained after etching of silica with 10% HF. Hollow carbon spheres (HCS) were prepared in the same way with the exception of adding melamine. For comparison, nitrogen doped solid carbon spheres (NSCS), solid carbon spheres (SCS) were prepared as same as NHCS and HCS, except the amount of TEOS decreases to 1 mL. 2.3.Characterization Morphological structures were examined by scanning electron microscopy (SEM) on FEI Quanta 250F, as well as transmission electron microscopy (TEM) on FEI Tecnai 20. Nitrogen adsorption-desorption isotherms were measured using Micromeritics ASAP-2020 at

4

liquid nitrogen temperature (77 K). Before measurements, all the samples were degassed in vacuum at 250 oC for 6 h. The specific surface area of the sample was calculated using the Brunauer-Emmett-Teller (BET) method within the relative pressure range of 0.05-0.20. The micropore surface area was calculated using the V-t plot method. The total pore volume was the adsorbed amount at a relative pressure of 0.975. The pore size distribution (PSD) curves were calculated using Barrett-Joyner-Halenda model (BJH). Raman spectra were obtained by a LabRAM Aramis spectrometer at 532 nm. The X-ray photoelectron spectroscopy (XPS) measurement was performed by using a PHI Quantera II ESCA System with Al Kα radiation at 1486.8 V. 2.4.Adsorption experiments The dynamic adsorption was carried out by a flow method reported by Hu et al [40]. Carbon spheres diluted with quartz sands, were loaded in a fixed-bed reactor with length of 10.0 mm and internal diameter of 4.0 mm. The adsorption temperature was 25 oC. Before measurements, each sample was degassed at 100 oC overnight in the oven. Nitrogen, as the carrier gas, was used to keep a total flow rate of 50 mL/min. The concentration of benzene was controlled at 500 ppm. The concentration changes before and after adsorption measurements were tested by using a gas chromatography equipped with a flame ionization detector (Agilent 7890A, United States). Breakthrough was defined as the ratio of benzene concentration of the outlet (CA) to the inlet (C0) in the stream reached 0.10, and the adsorption time was called the breakthrough time. The equilibrium dynamic adsorption capacity of the adsorbent was calculated from the breakthrough curves. In order to investigate the effect on desorption performance of hollow structure, the dynamic desorption performance of benzene from a benzene/nitrogen mixture was evaluated by using a thermogravimetric analyzer (TGA Q600, United States). Prior to the adsorption experiment, all the samples were pretreated at 150 oC in nitrogen flow (50 mL/min) for 60

5

min to remove the physisorbed moisture and other organic compounds. Then, the temperature was cooled to the adsorption temperature (25 oC). After the temperature stabilized, the simulated gas flow containing 5000 ppm benzene was introduced into the chamber. The gas flow was switched to nitrogen when the adsorption equilibrium was achieved. Finally, benzene vapor was desorbed by increasing temperature to 250 oC at 1 oC/min in nitrogen flow, and meanwhile the sample weight was recorded as a function of time. 3. Results and discussions 3.1. Characterization of carbon spheres The morphology and structure of carbon spheres were investigated by FESEM. As can be seen in Fig. 1a and 1d, both NCS and HCS are well-defined and monodisperse spheres with diameter of about 350 nm. There are no obviously changes between SCS and NSCS as well as HCS and NHCS, which could be seen in Fig. 1b and 1e. It can be conducted that the nitrogen doping have negligible effect on morphology of carbon spheres [41, 42]. At higher magnification images of NSCS and NHCS, the rough surface is observed. It’s noticed that, the cavity with diameter of 200 nm and shell thickness of 75 nm of NHCS could be seen clearly in Fig. 1f. From the TEM image (Fig. 2a and 2b), NSCS exhibits the regularly spherical shape with solid core and rough surface, as well as SCS. While for HCS and NHCS (Fig. 2c and 2d), the central cavity with dark edge and size of about 200 nm is displayed, which agreed with the results of SEM. This hollow structure on HCS and NHCS, could facilitate the mass transfer of adsorbate and restrict pore clogging during adsorption and desorption process [43, 44]. Furthermore, the abundant amorphous worm-like pores are presented on carbon spheres, which is favorable for organic molecule adsorption. The porosity of carbon spheres was investigated by nitrogen adsorption-desorption isotherms along with the corresponding PSD curves as shown in Fig. 3. All the carbon

6

spheres exhibit typical type IV isotherm with a H2-type hysteresis loop, a characteristic of mesopores. The adsorption isotherm presents a capillary condensation step in the relative pressure (P/P0) of 0.25-0.45, corresponding the small mesopores on the frameworks of carbon spheres. Meanwhile, the other steep capillary condensation step is shown in P/P0 higher than 0.9, which can be ascribed to the inter-particle void formed by the spheres’ packing [32]. From the corresponding PSD curves (Fig. 3b), carbon spheres exhibit similar distributions with a sharp peak centered at 2.7 nm, indicating the uniform mesoporous structure. It’s noteworthy that the hysteresis loop of HCS and NHCS is much bigger than SCS and NSCS, suggesting more mesopores on HCS and NHCS. The specific surface areas and pore volume of carbon spheres are listed in Table 1. The surface area and pore volume of NSCS and NHCS decreased slightly compared with carbon spheres without nitrogen doping. The tiny decrease may due to the introduction of heteroatom which lead to slight destruction of pores [45, 46]. Owing to the hollow structure, the mesopore surface area of HCS is 2.7 times that of SCS, resulted in a higher pore volume of 1.29 cm3/g. The abundant pore structure, i.e. high surface area and pore volume of carbon spheres provide prerequisites for application of adsorption for VOCs. Raman spectra were used to further investigate the structure of carbon spheres. As shown in Fig. 4, all the samples exhibit two significant peaks around 1350 cm-1 (D-band) and 1560 cm-1 (G-band), corresponding the defects/disordered structure and conjugated structure [47]. Generally, the integrated intensity ratio of D-band to G-band (ID/IG) is used as indicator of the degree of graphitization [48]. The SCS and HCS present analogous Raman spectra with ID/IG of 2.56 and 2.62, indicating similar chemical property. While the value of ID/IG increase to 2.69 and 2.72 after nitrogen doping. The increase of ID/IG means that more defects and distortion of carbon matrix are formed by heteroatom doping, which is consistent with the results of nitrogen adsorption-desorption isotherms.

7

In order to inquire into the influence of nitrogen doping on carbon spheres, XPS analysis was conducted to identify the element contents and state of nitrogen species. The XPS survey spectra and deconvoluted spectra are plotted in Fig. 5, and the element contents are also listed in Table 1. From Fig. 5a, SCS and HCS exhibit similar composition of carbon and oxygen. After nitrogen doping, N1s peak at about 401 eV are founded with nitrogen content of 3.4% and 3.7% on NSCS and NHCS, respectively. The bonding configurations of nitrogen atoms in NSCS were investigated through high-resolution N1s XPS spectrum. The high-resolution XPS spectra of N1s can be deconvoluted into three peaks corresponding different electronic states of nitrogen species: pyridinic-N (398.6 eV), pyrrolic-N (400.9 eV), and N-oxide (403 eV), respectively (Fig.5b). The electron distribution and polarity of carbon matrix can be tailored via introduction of nitrogen species, which can affect the affinity between adsorbate and carbon spheres [46, 49]. Furthermore, nearly same distribution of nitrogen species is observed on NHCS, due to the same synthesis procedure (Fig. 5c). Based on above characterizes, it can be confirmed that carbon spheres with different structure (solid and hollow) and composition (nitrogen doping or not) are successfully prepared through modified “silica-assistant” route. 3.2. Adsorption studies 3.2.1. Dynamic adsorption of benzene The fixed bed packed with adsorbents is adopted when it comes to practical adsorption processes for VOCs. The direct method, i.e. breakthrough measurement is mostly used to evaluate the dynamic performance of VOCs adsorption [50]. To investigate dynamic adsorption performance, the breakthrough curves for benzene of carbon spheres are shown in Fig. 6 and the corresponding breakthrough time, equilibrium time and adsorption capacity are listed in Table 2.

8

The analogous breakthrough curves of HCS and SCS are shown in Fig. 6. While HCS represents the longer breakthrough time of benzene (86 min) compared with SCS (55 min), which can be seen obviously. Another 155 min is needed to reach adsorption equilibrium for both SCS and HCS. Meanwhile, adsorption capacity obtained on HCS (474 mg benzene/g adsorbent) is 1.41 times of SCS (337 mg benzene/g adsorbent). This phenomenon could be ascribed to more accessible porous area formed by hollow structure and larger pore volume. In other words, hollow structure provide one more interface with adsorption active sites, which could promote the adsorption capacity [51, 52]. After nitrogen doping, the breakthrough time extended about 65 min, indicating the better dynamic adsorption performance. The time to get equilibrium is a bit longer than nitrogen-free carbon spheres. The enhanced adsorption capacities for benzene on NSCS and NHCS are also gained with about 70% increase. This may be due to the introduction of nitrogen atom, which could improve the affinity between benzene and carbon spheres through changing electron distribution and polarity of carbon matrix [53]. It’s noticed that the adsorption capacity of benzene on NHCS is the highest (766 mg/g) among these carbon spheres, which is comparable with literature reports [54, 55]. This remarkable dynamic adsorption performance could be attributed to the combined effect of hollow structure and nitrogen doping on carbon spheres. 3.2.2. Dynamic desorption of benzene The desorption performance of adsorbents is very important for their regeneration during applied in practical processes. In order to investigate the enhancement on desorption performance of hollow structure, the dynamic desorption behavior was carried out on saturated HCS and SCS. The corresponding results are shown in Fig. 7. The desorption ratio, defined as the ratio of weight decrease and weight increase after equilibrium adsorption. After pure nitrogen replaced benzene/nitrogen mixture, the adsorbed benzene began to release with

9

the increase of temperature. The desorption curve of HCS is more steep than SCS (Fig. 7), indicating less mass transfer resistance [17]. This could be ascribed to the hollow structure and abundant mesopores on HCS as described above. Practically, 80% of desorption ratio takes 68 min for HCS, while nearly two times (120 min) for SCS. When the temperature increase to about 225 oC, the adsorbed benzene were totally released from SCS and HCS, implying good regenerability of SCS and HCS. Based on desorption performance of carbon spheres, hollow structure could improve desorption efficiency under mild heat treatment. 3.2.3. Effect of aromaticity on adsorption performance The above investigated dynamic adsorption behaviors of HCS and NHCS indicate that nitrogen doping enhance benzene adsorption capacities. This may be achieved through the regulation of electron distribution via nitrogen doping on the carbon spheres. To clarify the mechanism of enhanced adsorption performance, benzene, toluene and xylene with different aromaticity were selected as probe adsorbates on HCS and NHCS. As we all know, benzene as typical aromatic compound, owns the uniform distribution of electron cloud. The increase of methyl substituent on benzene ring could lead to the decrease of uniformity and aromaticity. When benzene was used as adsorbate, the breakthrough time and adsorption capacity of NHCS are 1.83 and 1.62 times to that of HCS (Fig. 8a and Table 2). Nevertheless, the breakthrough curves almost overlapped when it comes to xylene (Fig. 8c). The electron distribution on carbon spheres was changed by nitrogen doping, which was confirmed via Raman and XPS spectra. The formation of defect and disorder zones supplied more accessible pores and functional groups on the carbon spheres, which is beneficial for adsorption of aromatic compounds. Besides that, the p-type lone pair electrons provided by nitrogen atom could form p-π coordinate bond with benzene ring, which could also enhance the adsorption performance [56]. While for xylene, limited to small amount of nitrogen and low aromaticity, weak coordinate effect could not promote adsorption behavior any more.

10

Combining the above analysis, it can conduct that the nitrogen doping into carbon spheres could change the electron distribution and polarity to enhance adsorption performance for VOCs. 4. Conclusions In summary, carbon spheres with different structure and composition successfully prepared through modified “silica-assistant” route. The obtained well-defined carbon spheres showed abundant micropores and mesopores with solid or hollow structure. Combined with nitrogen adsorption-desorption isotherms, Raman and XPS spectra, it was confirmed that the composition was tailored via introduction of nitrogen atom into carbon matrix. Breakthrough curves revealed that NHCS presented remarkable dynamic adsorption performance for benzene vapor. The highest adsorption capacity (766 mg/g) was attributed to the combined effect of hollow structure and nitrogen doping on NHCS. When compared with SCS, HCS with hollow structure showed enhancement desorption behaviors. Carbon spheres exhibited good regenerability based on desorption experiment. The mechanism of enhanced adsorption performance caused by nitrogen doping was clarified as the combined action of adjustment on porous structure and composition. Abundant porous and hollow structure, high pore volume, nitrogen doping, large adsorption capacity and excellent desorption performance indicate that NHCS would become a promising VOCs adsorbent. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51478224).

References [1] Ken Sexton, John L. Adgate, Gurumurthy Ramachandran, Gregory C. Pratt, Steven J.

11

Mongin, Thomas H. Stock, M.T. Morandi, Comparison of Personal, Indoor, and Outdoor Exposures to Hazardous Air Pollutants in Three Urban Communities, Environ. Sci. Technol., 38 (2004) 423-430. [2] R. Volkamer, L.T. Molina, M.J. Molina, T. Shirley, W.H. Brune, DOAS measurement of glyoxal as an indicator for fast VOC chemistry in urban air, Geophys. Res. Lett., 32 (2005) 93-114. [3] R. Volkamer, J.L. Jimenez, F.S. Martini, K. Dzepina, Q. Zhang, D. Salcedo, L.T. Molina, D.R. Worsnop, M.J. Molina, Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected, Geophys. Res. Lett., 33 (2006) 254-269. [4] D.T. Tefera, L.M. Jahandar, M. Fayaz, Z. Hashisho, J.H. Philips, J.E. Anderson, M. Nichols, Two-dimensional modeling of volatile organic compounds adsorption onto beaded activated carbon, Environ. Sci. Technol., 47 (2013) 11700-11710. [5] C. Hung, H. Bai, M. Karthik, Ordered mesoporous silica particles and Si-MCM-41 for the adsorption of acetone: A comparative study, Sep. Purif. Technol., 64 (2009) 265-272. [6] L.F. Liotta, Catalytic oxidation of volatile organic compounds on supported noble metals, Appl. Catal., B, 100 (2010) 403-412. [7] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Photocatalytic purification of volatile organic compounds in indoor air: A literature review, Atmos. Environ., 43 (2009) 2229-2246. [8] L. Yang, L. Jing, B. Lu, A. Jiang, C. Wan, Study on the removal of indoor VOCs using biotechnology, J. Hazard. Mater., 182 (2010) 204-209. [9] Y. Li, Z. Fan, J. Shi, Z. Liu, W. Shangguan, Post plasma-catalysis for VOCs degradation over different phase structure MnO2 catalysts, Chem. Eng. J., 241 (2014) 251-258. [10] Z. Wang, G. Xiu, T. Qiao, K. Zhao, D. Zhang, Coupling ozone and hollow fibers membrane bioreactor for enhanced treatment of gaseous xylene mixture, Bioresour. Technol., 130 (2013) 52-58. [11] Z. Cheng, P. Sun, L. Lu, J. Chen, L. Jiang, J. Yu, The interaction mechanism and characteristic evaluation of ethylbenzene/chlorobenzene binary mixtures treated by ozone-assisted UV 254nm photodegradation, Sep. Purif. Technol., 132 (2014) 62–69. [12] H. Wang, M. Tang, L. Han, J. Cao, Z. Zhang, W. Huang, R. Chen, C. Yu, Synthesis of hollow organosiliceous spheres for volatile organic compound removal, J. Mater. Chem. A, 2 12

(2014) 19298-19307. [13] N. Mohan, G.K. Kannan, S. Upendra, R. Subha, N.S. Kumar, Breakthrough of toluene vapours in granular activated carbon filled packed bed reactor, J. Hazard. Mater., 168 (2009) 777-781. [14] H. Wang, M.J. Lashaki, M. Fayaz, Z. Hashisho, J.H. Philips, J.E. Anderson, M. Nichols, Adsorption and Desorption of Mixtures of Organic Vapors on Beaded Activated Carbon, Environ. Sci. Technol., 46 (2012) 8341-8350. [15] R.R. Bansodea, J.N. Lossoa, W.E. Marshallb, R.M. Raoa, R.J. Portierc, Adsorption of volatile organic compounds by pecan shell- and almond shell-based granular activated carbons, Bioresour. Technol., 90 (2003) 175-184. [16] A. Silvestre-Albero, J.M. Ramos-Fernández, M. Martínez-Escandell, A. Sepúlveda-Escribano, J. Silvestre-Albero, F. Rodríguez-Reinoso, High saturation capacity of activated carbons prepared from mesophase pitch in the removal of volatile organic compounds, Carbon, 48 (2010) 548-556. [17] H. Wang, M. Tang, K. Zhang, D. Cai, W. Huang, R. Chen, C. Yu, Functionalized hollow siliceous spheres for VOCs removal with high efficiency and stability, J. Hazard. Mater., 268 (2014) 115-123. [18] Y. Liu, Z. Li, X. Yang, Y. Xing, C. Tsai, Q. Yang, Z. Wang, R.T. Yang, Performance of mesoporous silicas (MCM-41 and SBA-15) and carbon (CMK-3) in the removal of gas-phase naphthalene: adsorption capacity, rate and regenerability, RSC Adv., 6 (2016) 21193-21203. [19] B. Dou, J. Li, Y. Wang, H. Wang, C. Ma, Z. Hao, Adsorption and desorption performance of benzene over hierarchically structured carbon-silica aerogel composites, J. Hazard. Mater., 196 (2011) 194-200. [20] M. Houari, B. Hamdi, O. Bouras, J.C. Bollinger, M. Baudu, Static sorption of phenol and 4-nitrophenol onto composite geomaterials based on montmorillonite, activated carbon and cement, Chem. Eng. J., 255 (2014) 506-512. [21] K.J. Kim, C.S. Kang, Y.J. You, M.C. Chung, M.W. Woo, W.J. Jeong, N.C. Park, H.G. Ahn, Adsorption-desorption characteristics of VOCs over impregnated activated carbons, Catal. Today, 111 (2006) 223-228. [22] J.F. Vivo-Vilches, E. Bailón-García, A.F. Pérez-Cadenas, F. Carrasco-Marín, F.J. 13

Maldonado-Hódar, Tailoring activated carbons for the development of specific adsorbents of gasoline vapors, J. Hazard. Mater., 263 (2013) 533-540. [23] S.A. Opatokun, A. Prabhu, A.A. Shoaibi, C. Srinivasakannan, V. Strezov, Food wastes derived adsorbents for carbon dioxide and benzene gas sorption, Chemosphere, 168 (2016) 326-332. [24] M.K. Aroua, C.Y. Yin, F.N. Lim, W.L. Kan, W.M. Daud, Effect of impregnation of activated carbon with chelating polymer on adsorption kinetics of Pb 2+, J. Hazard. Mater., 166 (2009) 1526-1529. [25] Y. Han, G. Hwang, H. Kim, B.Z. Haznedaroglu, B. Lee, Amine-impregnated millimeter-sized spherical silica foams with hierarchical mesoporous-macroporous structure for CO2 capture, Chem. Eng. J., 259 (2015) 653-662. [26] C. Bai, J. Li, S. Liu, X. Yang, X. Yang, Y. Tian, K. Cao, Y. Huang, L. Ma, S. Li, In situ preparation of nitrogen-rich and functional ultramicroporous carbonaceous COFs by “segregated” microwave irradiation, Microporous Mesoporous Mater., 197 (2014) 148-155. [27] T.C. Mendes, C. Xiao, F. Zhou, H. Li, G.P. Knowles, M. Hilder, A. Somers, P.C. Howlett, D.R. Macfarlane, In situ-activated N-doped mesoporous carbon from a protic salt and its performance in supercapacitors, ACS Appl. Mater. Interfaces, 8 (2016) 35243-35252. [28] R. Liu, S.M. Mahurin, C. Li, R.R. Unocic, J.C. Idrobo, H. Gao, S.J. Pennycook, S. Dai, Dopamine as a carbon source: the controlled synthesis of hollow carbon spheres and yolk-structured carbon nanocomposites, Angew. Chem. Int. Ed., 50 (2011) 6799-6802. [29] A.H. Lu, W.C. Li, G.P. Hao, B. Spliethoff, H.J. Bongard, B.B. Schaack, F. Schüth, Easy synthesis of hollow polymer, carbon, and graphitized microspheres, Angew. Chem. Int. Ed., 49 (2010) 1615-1618. [30] F. Böttger-Hiller, P. Kempe, G. Cox, A. Panchenko, N. Janssen, A. Petzold, T. Thurn-Albrecht, L. Borchardt, M. Rose, S. Kaskel, Twin Polymerization at Spherical Hard Templates: An Approach to Size-Adjustable Carbon Hollow Spheres with Micro- or Mesoporous Shells, Angew. Chem. Int. Ed., 52 (2013) 6088-6091. [31] R.J. White, K. Tauer, M. Antonietti, M.-M. Titirici, Functional hollow carbon nanospheres by latex templating, J. Am. Chem. Soc., 132 (2010) 17360-17363. [32] Z.-A. Qiao, B. Guo, A.J. Binder, J. Chen, G.M. Veith, S. Dai, Controlled Synthesis of 14

Mesoporous Carbon Nanostructures via a “Silica-Assisted” Strategy, Nano Lett., 13 (2013) 207-212. [33] Y. Fang, D. Gu, Y. Zou, Z. Wu, F. Li, R. Che, Y. Deng, B. Tu, D. Zhao, A Low-Concentration Hydrothermal Synthesis of Biocompatible Ordered Mesoporous Carbon Nanospheres with Tunable and Uniform Size, Angew. Chem. Int. Ed., 49 (2010) 7987-7991. [34] J. Liu, T. Yang, D.-W. Wang, G.Q. Lu, D. Zhao, S.Z. Qiao, A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres, Nat. Commun., 4 (2013) 2798. [35] S. Feng, W. Li, Q. Shi, Y. Li, J. Chen, Y. Ling, A.M. Asiri, D. Zhao, Synthesis of nitrogen-doped hollow carbon nanospheres for CO2 capture, Chem. Commun., 50 (2014) 329-331. [36] Y. Liu, T. Chen, T. Lu, Z. Sun, D.H.C. Chua, L. Pan, Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization, Electrochim. Acta, 158 (2015) 403-409. [37] C. Liu, J. Wang, J. Li, R. Luo, J. Shen, X. Sun, W. Han, L. Wang, Controllable synthesis of functional hollow carbon nanostructures with dopamine as precursor for supercapacitors, ACS Appl. Mater. Interfaces, 7 (2015) 18609-18617. [38] L. Guo, L. Zhang, J. Zhang, J. Zhou, Q. He, S. Zeng, X. Cui, J. Shi, Hollow mesoporous carbon spheres-an excellent bilirubin adsorbent, Chem. Commun., 40 (2009) 6071-6073. [39] L. Chao, W. Jing, J. Li, M. Zeng, L. Rui, J. Shen, X. Sun, W. Han, L. Wang, Synthesis of N-Doped Hollow-Structured Mesoporous Carbon Nanospheres for High-Performance Supercapacitors, ACS Appl. Mater. Interfaces, 8 (2016) 7194-7204. [40] Q. Hu, J.J. Li, Z.P. Hao, L.D. Li, S.Z. Qiao, Dynamic adsorption of volatile organic compounds on organofunctionalized SBA-15 materials, Chem. Eng. J., 149 (2009) 281-288. [41] S. Maldonado, S. Morin, K.J. Stevenson, Structure, composition, and chemical reactivity of carbon nanotubes by selective nitrogen doping, Carbon, 44 (2006) 1429-1437. [42] H. Chen, F. Sun, J. Wang, W. Li, W. Qiao, L. Ling, D. Long, Nitrogen Doping Effects on the Physical and Chemical Properties of Mesoporous Carbons, J. Phys. Chem. C, 117 (2013) 8318-8328. [43] B. Liu, X. Li, Q. Zhao, J. Liu, S. Liu, S. Wang, M.O. Tade, Insight into the Mechanism of Photocatalytic Degradation of Gaseous o-dichlorobenzene over Flower-Type V2O5 Hollow 15

Spheres, J. Mater. Chem. A, 3 (2015) 15163-15170. [44] X. He, H. Sun, M. Zhu, M. Yaseen, D. Liao, X. Cui, H. Guan, Z. Tong, Z. Zhao, N-Doped porous graphitic carbon with multi-flaky shell hollow structure prepared using a green and 'useful' template of CaCO3 for VOC fast adsorption and small peptide enrichment, Chem. Commun., (2017) 3442-3445. [45] S.-M. Li, S.-Y. Yang, Y.-S. Wang, H.-P. Tsai, H.-W. Tien, S.-T. Hsiao, W.-H. Liao, C.-L. Chang, C.-C.M. Ma, C.-C. Hu, N-doped structures and surface functional groups of reduced graphene oxide and their effect on the electrochemical performance of supercapacitor with organic electrolyte, J. Power Sources, 278 (2015) 218-229. [46] F. Sun, J. Gao, X. Liu, Y. Yang, S. Wu, Controllable nitrogen introduction into porous carbon with porosity retaining for investigating nitrogen doping effect on SO2 adsorption, Chem. Eng. J., 290 (2016) 116-124. [47] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework, Nat. Commun., 5 (2014) 5261. [48] Z.S. Wu, W. Ren, L. Xu, F. Li, H.M. Cheng, Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries, ACS Nano, 5 (2011) 5463-5471. [49] M.P. Zhu, Z.F. Tong, Z.X. Zhao, Y.Z. Jiang, Z.X. Zhao, A Microporous Graphitized Biocarbon with High Adsorption Capacity toward Benzene Volatile Organic Compounds (VOCs) from Humid Air at Ultralow Pressures, Ind. Eng. Chem. Res., 55 (2016) 3765-3774. [50] M. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Behaviour of activated carbons with different pore size distributions and surface oxygen groups for benzene and toluene adsorption at low concentrations, Carbon, 43 (2005) 1758-1767. [51] C. Han, D. Yang, Y. Yang, B. Jiang, Y. He, M. Wang, A.Y. Song, Y. He, B. Li, Z. Lin, Hollow Titanium Dioxide Spheres as Anode Material for Lithium Ion Battery with Largely Improved Rate Stability and Cycle Performance by Suppressing the Formation of Solid Electrolyte Interface Layer, J. Mater. Chem. A, 3 (2015) 13340-13349. [52] S. Pan, J. Li, G. Wan, C. Liu, W. Fan, L. Wang, Nanosized yolk-shell [email protected] Zr(OH)x spheres for efficient removal of Pb (II) from aqueous solution, J. Hazard. Mater., 309 (2016) 1-9. 16

[53] A. Nieto-Márquez, D. Toledano, P. Sánchez, A. Romero, J.L. Valverde, Impact of nitrogen doping of carbon nanospheres on the nickel-catalyzed hydrogenation of butyronitrile, J. Catal., 269 (2010) 242-251. [54] D. Fairén-Jiménez, F. Carrasco-Marín, C. Moreno-Castilla, Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows, Langmuir, 23 (2007) 10095-10101. [55] M.A. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Benzene and toluene adsorption at low concentration on activated carbon fibres, Adsorption, 17 (2011) 473-481. [56] Y. Zhao, X. Wu, J. Yang, X.C. Zeng, Ab initio theoretical study of non-covalent adsorption of aromatic molecules on boron nitride nanotubes, PCCP, 13 (2011) 11766-11772.

17

Figure Captions

Fig. 1. SEM images of carbon spheres: SCS (a), NSCS (b and c), HCS (d) and NHCS (e and f). The third column (c and f) are the corresponding SEM images under high magnification. Fig. 2. TEM images of carbon spheres: SCS (a), NSCS (b), HCS (c) and NHCS (d). Fig. 3. Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of carbon spheres. Fig. 4. Raman spectra of carbon spheres. Fig. 5. XPS survey spectra of carbon spheres (a) and high-resolution N1s XPS spectra of NSCS (b) and NHCS (c). Fig. 6. The breakthrough curves for benzene of carbon spheres. Fig. 7. The desorption curves for benzene of SCS and HCS. Fig. 8. Effect of aromaticity: breakthrough curves for benzene (a), toluene (b) and xylene (c) of HCS and NHCS.

18

Fig. 1. SEM images of carbon spheres: SCS (a), NSCS (b and c), HCS (d) and NHCS (e and f). The third column (c and f) are the corresponding SEM images under high magnification.

19

Fig. 2. TEM images of carbon spheres: SCS (a), NSCS (b), HCS (c) and NHCS (d).

20

0.8

(a)

SCS HCS NSCS NHCS

(b)

0.7

dV/dD (cm /(g*nm))

800

3

600

3

Volume Adsorbed (cm /g STP)

1000

400

SCS HCS NSCS NHCS

0.6 0.5 0.4 0.3 0.2 0.1

200 0.0

0.0 0.2

0.4

0.6

0.8

1.0

1

2

3

4

5

6

7

8

Pore Diameter (nm)

Relative Pressure (P/P0)

Fig. 3. Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of carbon spheres.

21

Intensity (a.u.)

D

G

ID/IG=2.72

NHCS

ID/IG=2.69

NSCS

ID/IG=2.62

HCS

ID/IG=2.56

SCS

500

1000

1500

2000

2500

-1

Raman Shift (cm ) Fig. 4. Raman spectra of carbon spheres.

22

Fig. 5. XPS survey spectra of carbon spheres (a) and high-resolution N1s XPS spectra of NSCS (b) and NHCS (c).

23

100

SCS HCS NSCS NHCS

80

CA/C0 (%)

60

40

20

0 0

50

100

150

200

250

300

350

Time (min) Fig. 6. The breakthrough curves for benzene of carbon spheres.

24

400

Desorption Ratio (%)

0

SCS HCS

20 Change gas flow

40

60

80

100 0

50

100

150

200

Time (min)

Fig. 7. The desorption curves for benzene of SCS and HCS.

25

250

HCS NHCS

(a)

100

CA/C0 (%)

80

CA /C0 (%)

60

40

20

100

HCS NHCS

80

80

60

60

40

50

100

150

200

250

Time (min)

300

350

400

40

0

0

0

(c)

20

20

0

HCS NHCS

(b)

CA /C0 (%)

100

0

50

100

150

Time (min)

200

250

0

50

100

150

200

250

300

350

400

Time (min)

Fig. 8. Effect of aromaticity: breakthrough curves for benzene (a), toluene (b) and xylene (c) of HCS and NHCS.

26

Table Captions

Table 1 Structural parameters of carbon spheres Table 2 Dynamic adsorption parameters of benzene on carbon spheres

27

Table 1 Structural parameters of carbon spheres S Samples

BET

S

micro

S

meso

(m2/g)

Elemental Pore Pore volume diameter composition (cm3/g) (nm) C (%) O (%) N (%)

(m2/g)

(m2/g)

SCS

1138

870

268

0.73

2.7

92.4

7.6

-

HCS

1210

653

557

1.29

2.7

92.6

7.4

-

NSCS

1068

804

264

0.58

2.7

90.2

6.4

3.4

NHCS

1082

603

479

1.11

2.7

89.8

6.5

3.7

28

Table 2 Dynamic adsorption parameters of benzene on carbon spheres Samples

Breakthrough time (min)

Equilibrium time (min)

Adsorption capacity (mg/g)

SCS

55

210

337

HCS

86

240

474

NSCS

115

285

600

NHCS

157

345

766

29

Graphical Abstract

30

Highlights 

Carbon spheres with different structure and composition were used for VOCs removal.



Nitrogen doped hollow carbon spheres presented remarkable adsorption performance.



The regenerability and enhancement of hollow structure were confirmed.



The mechanism of enhanced adsorption performance caused by nitrogen doping was clarified.

31