A novel nitrogen-doped porous carbons derived from graphene for

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A novel nitrogen-doped porous carbons derived from graphene for effective CO2 capture Liying An, Shenfang Liu, Linlin Wang, Jiayi Wu, Zhenzhen Wu, Changdan Ma, Qiankun Yu, and Xin Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06122 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

A novel nitrogen-doped porous carbons derived from graphene for effective CO2 capture Liying Ana, Shenfang Liua, Linlin Wangb, Jiayi Wua, Zhenzhen Wua, Changdan Maa, Qiankun Yua, Xin Hu*,a aKey

Laboratory of the Ministry of Education for Advanced Catalysis Materials,

Zhejiang Normal University, Jinhua 321004, China bCollege

of Engineering, Zhejiang Normal University, Jinhua 321004, PR China

*Corresponding

author’s e-mail: [email protected]; phone: 86-151-0579-0257; fax:

86-579-8228-8269

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Table of Contents

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Abstract In this work, graphene-derived N-enriched porous carbons were synthesized by urea modification and KOH activation of thermally shocked graphene oxide. The prepared sorbents were characterized by various techniques and investigated as potential CO2 capture materials. The as-prepared sorbents possess high CO2 adsorption capacity of 2.40 mmol/g (25°C) and 3.24 mmol/g (0°C) at 1 bar, which is higher than most graphene-based carbons reported previously. The nitrogen incorporation and narrow microporosity are the two major factors that determine CO2 uptake for these graphene-derived carbonaceous adsorbents under ambient conditions. The adsorption kinetic data of the optimized sample were well-described by the classical Fick’s diffusion model with a high CO2 diffusion rate. The fast CO2 adsorption kinetics can be attributed to the short diffusion paths of this sample, which is composed by thin layer of graphene sheets. Moreover, these graphene-derived sorbents also demonstrated excellent stability and recyclability, high selectivity of CO2 over N2, suitable heat of adsorption and excellent dynamic CO2 capture capacity. As a result, these graphene-derived porous carbons deserve the consideration in removal of CO2 from exhausted flue gas.

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1. Introduction With the continuous increase of atmospheric CO2 concentrations, there is an imperative need for inexpensive and effective removal of CO2 from flue gas1, 2. Among various CO2 capture technologies, CO2 adsorption by solid adsorbents is a promising approach because of its merits of low capital investment, low energy consumption, ease of operation and free of equipment corrosion3-7. The vital of this technique is to obtain solid adsorbents with excellent performances including 1) high CO2 adsorption capacity, 2) high selectivity of CO2 over N2, 3) rapid CO2 adsorption/desorption kinetics, 4) moderate heat of enthalpy, and 5) excellent thermal, chemical and mechanical stability. Of different sorbents explored for CO2 adsorption such as zeolites8, metal oxides9, metal-organic Frameworks10 (MOFs), porous polymers11,

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and ionic liquids13-15, carbonaceous materials are receiving increased

interest because of their multi-merits such as minimal costs, high surface area, tunable pore structure, hydrophobic property, inertness to chemicals, thermal and mechanical stability, as well as their environmentally benign nature16-24. To date, various porous carbons synthesized from different methods i.e. physical or chemical activation using different precursors such as coal25, petroleum coke26-28 and biomass materials29-36 have been widely researched for CO2 capture. It has been revealed that the narrow micropore (< 1nm) of the carbonaceous materials determines their CO2 adsorption capacities at the ambient conditions16, 37, 38. Furthermore, integration of N element into the carbon skeleton can obviously change the electronic distribution of the carbon surface, which in turn increase the surface polarity of the carbon materials. Since CO2 4


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has higher quadruple moment than N2, the stronger interaction between CO2 and the polar sites associated with nitrogen function groups is advantageous for the selective removal of CO2 from the flue gas39, 40. Therefore, at present, the researchers have devoted their great efforts to developing new strategies of synthesizing nitrogen-doped porous carbons with excellent CO2 adsorption properties. As a “rising star” in material science, graphene is drawing more and more attention due to its unique molecular structure and many superior properties such as light weight, super-flexible, tough mechanical strength, excellent chemical stability, large surface area, and tunable porosity41. Graphene-based hybrid materials have shown great performance in various research fields such as supercapacitors42, energy storage43, and catalysis44 to name a few. Considering the unparalleled features in structure, chemical, mechanical and thermal properties, graphene is undoubtedly a promising candidate for CO2 capture application. First, graphene is a strictly 2D material with high flexibility and specific surface area. A porous network can be easily constructed by the randomly cross-linking of graphene layers. Second, graphene's planar geometry makes it easy to modify and functionalize to further adjust its porosity and surface chemistry properties. In addition, graphene has excellent mechanical strength and hardness, which is especially important for industrial large-scale CO2 capture. The high thermal conductivity of graphene is beneficial to its application in the temperature swing adsorption (TSA) process. More importantly, graphene is chemically inert so that it can stand the harsh condition of real flue gas, which contains considerable quantities of water and other impurities i.e. 5



O2, SO2, and NOX. Therefore, nitrogen-doped porous carbons derived from thin layers of graphene sheets could have attractive CO2 adsorption properties compared to the conventional carbonaceous CO2 adsorbents. It is also very stimulating to examine the CO2 adsorption properties of graphene based porous carbonaceous adsorbents. Herein, N-rich porous carbons were synthesized using graphene oxide (GO) as the precursor, urea as the nitridation agent and KOH as the activator. In order to avoid the loss of specific surface area of final adsorbents because of the stacking of graphene layers, the GO precursor was subjected to thermal shocking at 250°C and atmospheric pressure before the nitridation reaction with urea. Then, the N-enriched porous carbons were obtained by KOH activation of urea-modified graphenes under different conditions. In this work, the impact of two synthesis parameters i.e. activation temperature and KOH amount on the porosity and surface chemistry properties of the adsorbents was investigated. In addition, the relation among CO2 adsorption capacity under ambient conditions and textural, surface chemistry properties of the obtained adsorbents were studied to provide further insight on CO2 adsorption mechanism for these nitrogen-enriched carbonaceous materials. 2. Experimental Methods 2.1 Materials Graphite powder (8000 mesh) was purchased from Aladdin and used as received. Sulfuric acid (98 wt.%), potassium permanganate (analytical purity), hydrogen peroxide (30 wt.%), urea (analytical purity), and potassium hydroxide (analytical purity) were obtained from Sinopharm Chemical Reagent Co., Ltd without further 6


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purification. 2.2 Synthesis of GO GO was prepared from graphite powder with a modified Hummer’s method45. The detailed preparation procedure can be found in the supporting information. 2.3 Thermal shocking of GO The as-synthesized GO powder was heated at 250°C under ambient air. The thermally exfoliated GO was denoted as T-G. 2.4 Urea modification of T-GO Same amount of T-GO and urea are mixed together, which is then heated at 350°C in air for 5 hours. After careful washing to get rid of the unreacted urea, the urea modified sample was obtained and assigned as T-GU. For comparison purpose, GO was also modified by urea following the same procedure as shown above. The resulting sample was named as GU. 2.5 KOH activation of T-GU The detailed KOH activation procedure can be found in the supporting information. The as-prepared N-doped porous carbons was named as T-GU-X-Y, where X is the activation temperature and Y stands for the ratio of KOH to T-GU. For comparison, the carbonaceous materials prepared by KOH activation of T-G were denoted by T-G-X-Y and the adsorbents synthesized by KOH activation of GU were assigned as GU-X-Y. 2.6 Characterization The morphology and structure of porous carbon materials was investigated by 7



scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, wide angle X-ray diffraction (XRD), and N2 sorption at -196°C. The elemental composition of samples was examined by an Elemental Analyzer. The surface chemistry of the materials was analyzed by X-ray photoelectron spectroscopy (XPS). The CO2 adsorption capacity of carbon samples was determined at 0 and at 25°C using a Beishide 3H-2000PS2 sorption analyzer. The dynamic CO2 adsorption capacity of the sorbents was studied on a fixed-bed reactor. The adsorption kinetics of CO2 was determined using a thermogravimetric analysis instrument. Detailed information about the characterization techniques can be found in the supporting information. 3. Results and Discussion 3.1 Porous texture The as-synthesized GO precursor was characterized by different techniques, with results shown in the Figure S1 (Supporting information). SEM image of GO shows lamellar morphology (Figure S1a), with thickness of roughly 1.40 nm measured by the atomic force microscopy (AFM) (Figure S1b). Powder X-ray diffraction pattern of GO exhibits a single diffraction (002) peak at a 2θ of ca. 9.2° corresponding to the c-axis interlayer distance (d-spacing) of 0.96 nm between GO layers (Figure S1c). Raman spectrum shows broad D and G bands for GO at 1356 and 1590 cm-1, respectively (Figure S1d). Moreover, the intensity ratio of the D band to G band (ID/IG =0.72) was similar to that reported in the literature46. The above characterization suggests the successful synthesis of GO nanosheets. The BET surface area (SBET) of 8


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GO was about 6 m2/g (Table 1), which is much less than the theoretical value of an individual graphene sheet (2630 m2/g). This is likely due to self-aggregation or serious stacking of graphene oxide sheets during the drying process. To prevent these stacking and to increase the specific surface area of GO precursor, thermal shocking of GO at 250°C and ambient air was explored. After this process, the exfoliated graphene nanoplates was expanded from the GO, as evidenced by the great increase in sample volume during the process of thermal shocking. As a result, the obtained T-G has a SBET of 190 m2/g, which is over 30 times higher than that of GO. As described later, this enhancement in the specific surface area of precursor is proved to be beneficial for increasing the porous texture properties of the graphene-based nitrogen-doped porous carbons. Incorporation of nitrogen into the T-G from heating a mixture of T-G/urea at 350 °C under ambient air causes an almost 70% decrease in its surface area, along with a decrease in the total pore volume (V0). The deterioration of the textural parameters of T-GU can be a result of pore-blocking by the numerous N species introduced upon urea treatment. In contrast, the surface area of GU shows a higher value than that of GO, which could be resulted from reaction of urea with the external surface of GO aggregation to create some porosity. The N-enriched graphene was further activated by KOH to improve the porous texture properties. As illustrated in Table 1, each textural parameter of the activated samples exhibits a great increase compared to the precursor, with SBET ranging from 731 to 1264 m2/g, V0 ranging from 0.46 to 0.98 cm3/g and micropore volume (Vt) ranging from 0.29 to 0.71 cm3/g. It is found that increasing activation temperature or KOH amount increase each textural 9



parameter of the adsorbents. The N2 sorption isotherms of these N-enriched porous carbons are demonstrated in Figure 1a. All the sorbents display type I + type IV isotherms, with an obvious N2 uptake at low relatively pressure (P/P0 < 0.01), a slightly steep adsorption at medium relatively pressure (0.1N-6 for 12


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T-GU-700-6. Among all the nitrogen functional groups, it has been proposed that N-5 exhibited the highest influence on CO2 adsorption via the strongest interaction with CO2 molecules39. 3.4. CO2 Adsorption properties Figure 5 shows the CO2 adsorption isotherms at 25°C and 0 °C for the T-GU-X-Y series adsorbents. CO2 adsorption capacity of each sample increases with the increase of pressure without the indication of adsorption saturation at both temperatures, indicating the sorbents can own higher CO2 adsorption capacity at higher pressures. At 25°C and 0°C, the maximum CO2 adsorption capacity for these sorbents is 2.40 and 3.24 mmol/g under atmospheric pressure, respectively, which exists a big gap compared with previously reported porous carbons synthesized from conventional precursors51, 52. However, this value is higher than most graphene-based carbons ever reported. For instance, Balasubramanian et al. synthesized holy graphene frameworks by HNO3 etching and self-assembly of GO sheet, which displayed the maximum CO2 adsorption capacity of 2.12 mmol/g at 0°C and 1 bar53. Han et al. synthesized three-dimensional graphene porous materials by hydrothermal treatment of aqueous GO dispersion54. The optimum adsorbent exhibited the maximum CO2 uptake of 2.40 mmol/g at 1 bar and 0°C. Under the same test conditions, in another work of their group, the porous carbonaceous materials prepared through steam activation of graphene aerogel possessed CO2 uptake of 2.45 mmol/g55. Furthermore, they fabricated a series of GO based composite porous materials such as graphene-polyethylenimine (PEI)56, graphene-terpyrine57 and graphene-manganese 13



oxide58 hybrid porous materials for the CO2 capture application, which showed CO2 adsorption ability less than 2.7 mmol/g at 1 bar and 0°C. In addition to the above examples, the CO2 adsorption capacity in this study is superior to many previously presented carbonaceous materials59-62, and other representative porous materials such as MOPs (microporous organic polymers)63, COFs (covalent organic frameworks)64 and PAFs (porous aromatic framework)65. Table S1 (Supporting information) summaries the comparison of CO2 adsorption results for different samples at 1 bar and 0°C. From Table 1 and Figure 5, T-GU-700-4 has a relatively low CO2 adsorption ability of 1.04 and 1.80 mmol/g at 25°C and 0°C, respectively. When the KOH/precursor ratio increased to 6, a great enhancement in CO2 uptake i.e. 2.40 and 3.24 mmol/g was found in T-GU-700-6 at both temperatures. However, further increasing in KOH amount and activation temperature leads to the deterioration of CO2 adsorption ability for T-GU-700-8 and T-GU-750-6. After careful observing the porous textural data shown in Table 1, it can be found that the CO2 uptake is not determined by the value of SBET, V0, or Vt of these sorbents. For example, although T-GU-700-8 possesses the highest value in SBET, V0, or Vt among all the nitrogen-doped porous carbons, its CO2 uptake is much lower than that of T-GU-700-6 with less developed porous structure. The highest CO2 uptake of T-GU-700-6 can be due to its highest value of Vn among all the samples. This finding agrees well with the conclusion drawn by previous literature that the CO2 uptake under low pressure is decided by the narrow microporosity of the adsorbents17, 32, 66. 14


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On the other hand, when comparing T-GU-700-6 with the control sample T-G-700-6, it is found the former has a higher CO2 adsorption ability, which can be due to the nitrogen doping on T-GU-700-6 by urea modification. From the above discussion, it can be concluded that both developing narrow microporosity and incorporation of nitrogen into the framework are beneficial for the CO2 adsorption ability of these adsorbents under ambient conditions. When comparing the CO2 uptake of T-G-700-6 with that of T-GU-750-6, it is found that even though the latter has more than double amount of nitrogen content than the former, it still has a lower CO2 adsorption capacity due to its lower value of Vn. This finding seems to indicate that narrow microporosity plays a more important role than nitrogen content in deciding CO2 adsorption capacity for these graphene-based carbonaceous materials. The above results indicate that T-GU-700-6 is a potential CO2 adsorbent. However, to fully explore its potential for practical applications, the recyclability and stability, selectivity of CO2 over N2, heat of adsorption, CO2 uptake kinetics and dynamic CO2 uptake must also be considered. To examine the stability and recyclability of T-GU-700-6 in the CO2 capture, five successive runs of adsorption-desorption isotherms were recorded at 25°C. After every run, the sample was heated at 150°C for 2h in a vacuum and then re-used in the next CO2 sorption test. As shown in Figure 6a, only 5% loss corresponding to the original

CO2

adsorption

ability

was

achieved

after

five

consecutive

adsorption/desorption runs indicating the excellent stability and recyclability for T-GU-700-6. 15



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In this study, to obtain the selectivity of T-GU-700-6 for CO2 over N2, isotherms of CO2 and N2 were recorded at 25° C separately (Figure 6b). The achieved CO2 and N2 adsorption isotherms were then fitted by the linear equation. The ideal adsorption solution theory (IAST) developed by Myers and Prausnitz67 has accurately predicted binary gas mixture adsorption in many carbonaceous materials30,

40, 68.

The IAST

adsorption selectivity of T-GU-700-6 for CO2 over N2 was calculated in the simulated flue gas stream (10% CO2 and 90% N2) from the experimental single-component isotherms. The estimated CO2/N2 selectivity under this condition for T-GU-700-6 is 12, which is comparable to or larger than those of carbonaceous adsorbents reported previously28, 38, 69-73. Isosteric heat of adsorption (Qst) was a suitable way of estimating the adsorbent-adsorbate interplay, which is important for both CO2 uptake and adsorbent regeneration. A too low Qst of a sorbent is harmful for CO2 adsorption, but a too high Qst would result in difficulty in the regeneration of sorbents. A moderately high Qst is preferred, which can ensure both efficient CO2 capture and succeeding easy desorption to regenerate the adsorbents. For calculation of Qst, CO2 adsorption of T-GU-700-6 was measured at both 25°C and 0°C, respectively. Then, Qst was calculated by employing the Clausius-Clapeyron equation to the adsorption isotherms74, 75. Plot of Qst as a function of CO2 uptake are illustrated in Figure 6c. The value of Qst found for T-GU-700-6 is in the range of 12-33 kJ mol-1, typical for a physisorption process and comparable to the values presented for other carbonaceous adsorbents76-78. Another finding for this plot is that Qst decreases with the increase of 16


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CO2 uptake reflecting the surface heterogeneity for T-GU-700-6. The higher Qst in the low CO2 loading is especially advantageous for the flue gas separation, which needs strong adsorbent-CO2 interaction at relatively low CO2 partial pressure. From the Qst value of T-GU-700-6, it is judged that this sorbent will have lower CO2 uptake at higher temperature. To confirm this, the CO2 adsorption/desorption isotherms of T-GU-700-6 was tested at 70 °C and 1 bar, with result shown in Figure S4 (supporting information). It can be clearly seen that only 0.83 mmol/g of CO2 uptake is achieved for T-GU-700-6, which is greatly decreased compared with the value obtained at 25 or 0°C. The dynamic CO2 uptake of T-GU-700-6 under continuous gas flowing conditions was determined by a fixed-bed reactor, with measurement details presented in Supporting information. Based on the breakthrough curve shown in Figure 6d, the dynamic CO2 uptake of T-GU-700-6 is calculated to be 0.30 mmol/g from a CO2/N2 (10:90 v/v) gas mixture at 25 °C. This value is consistent with the static pure CO2 adsorption capacity of T-GU-700-6 at partial pressure of 0.1 bar suggesting its effective CO2 capture from the continuous flowing gas mixture. For practical applications, carbonaceous sorbents also need to exhibit fast adsorption kinetics. Adsorption kinetics of T-GU-700-6 was measured using a thermogravimetric analysis instrument, with detailed procedure demonstrated in Supporting information. As shown in Figure 7a, CO2 adsorption of T-GU-700-6 was very fast at the beginning and reached 95% saturation adsorption capacity within about 4.0 min and finally reached a plateau after 6.0 min, suggesting it was 17



approaching equilibrium. The short diffusion paths of this sample, which is composed by thin layer of graphene sheets, can account for its fast CO2 adsorption kinetics. In addition, the adsorption kinetics could be properly described using a classical Fick’s diffusion model exhibited in the following equation: 1

   2 Dc t  mt 6   2 exp 2 mmax   rc 

where mt/mmax is the fractional adsorption uptake and the ratio Dc/rc 2 (min-1) is known as the diffusion time constant. The Fick’s diffusion model can correlate the adsorption kinetics data well with the squared correlation coefficient (R2) for regressions of around 0.91 (Fig. 7b). Furthermore, the diffusion time constant for CO2 of T-GU-700-6 is calculated to be 0.5088 min-1, which is higher than the biomass-derived carbon reported previously32. This high value of Dc/rc 2 suggests high CO2 diffusion rate during the adsorption process, which can greatly reduce the adsorption cycle time in potential practical applications. 4. Conclusion In this study, graphene-derived N-enriched carbonaceous adsorbents were prepared using GO as precursor, urea as the nitridation agent and KOH as the activator. To avoid the self-aggregation of graphene layers and thus improve the porous textural properties of final N-doped porous adsorbents, GO was subjected to thermal shocking before urea modification and KOH activation. The results show that this thermal expansion of GO precursor is beneficial for both porosity improvement and nitrogen enrichment of the ultimately synthesized porous carbons. The obtained

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graphene-derived carbonaceous adsorbents exhibit excellent CO2 uptake at atmospheric pressure i.e. 2.40 and 3.24 mmol/g at 25°C and 0°C, respectively, which is larger than most graphene-based carbons ever reported. Detailed studies found that both narrow microposity and N-incorporation are favorable for CO2 adsorption under ambient conditions. Furthermore, these graphene-derived carbonaceous materials possess many other advantages such as excellent stability and recyclability, high selectivity of CO2 over N2, suitable heat of adsorption, high dynamic uptake as well as fast CO2 adsorption kinetics. As a relatively new type of adsorbents, these graphene-derived porous carbons deserve the consideration in CO2 capture from exhausted flue gas. Acknowledgments This work is supported by Zhejiang Provincial Natural Science Foundation (LY17E010002), and National Undergraduate Training Program for Innovation and Entrepreneurship of China (201810345024). Supporting Information: Detailed synthesis procedure and characterization of sorbents, schematic diagram of the CO2 capture system (Scheme S1), detailed characterization of GO precursor (Figure S1), XRD spectra (Figure S2), Raman spectra (Figure S3) and CO2 sorption isotherms at 70 °C (Figure S4) of T-GU-700-6.

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Nitrogen-Doped

Porous

Coconut

Shell-Based

CO2

Sorbent

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30


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Table 1. Porous properties, elemental compositions, and CO2 uptakes of precursors and sorbents derived from different conditions. SBETa (m2/g)

V0b (cm3/g)

Vtc (cm3/g)

Vnd (cm3/g)

N (wt%)

GO

6

0.07

-

0.08

0.67

43.72

T-G

190

1.21

-

0.10

0.71

GU

12

0.08

-

0.04

T-GU

63

0.46

-

T-GU-700-4

731

0.46

T-GU-700-6

1032

T-GU-700-8

Sample

C (wt%)

H (wt%)

CO2 uptake (mmol/g) 25°C

0°C

3.48

0.39

0.51

60.56

2.02

0.48

0.65

6.52

75.88

1.02

0.13

0.14

0.09

12.94

78.33

1.08

0.46

0.54

0.29

0.35

4.42

88.30

1.77

1.04

1.80

0.82

0.61

0.59

2.59

82.42

1.38

2.40

3.24

1264

0.98

0.67

0.36

2.03

86.03

0.91

1.60

1.93

T-GU-750-6

1089

0.98

0.71

0.37

1.94

85.50

0.49

1.64

2.15

T-G-700-6

1296

1.10

0.75

0.43

0.86

77.12

0.90

1.97

2.52

GU-700-6

360

0.42

0.15

0.18

1.32

85.05

1.31

0.95

1.43

a

Surface area was calculated using the BET method at P/P0=0.04-0.32. pore volume at P/P0= 0.99. c Evaluated by the t-plot method. d Pore volume of narrow micropores (

A novel nitrogen-doped porous carbons derived from graphene for

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