Nitrogen doped porous carbons from lotus leaf for CO2 capture and

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Environmental and Carbon Dioxide Issues

Nitrogen doped porous carbons from lotus leaf for CO2 capture and supercapacitor electrodes Shenfang Liu, Pupu Yang, Linlin Wang, Yuliang Li, Zhenzhen Wu, Rui Ma, Jiayi Wu, and Xin Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00886 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019

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Nitrogen doped porous carbons from lotus leaf for CO2 capture and supercapacitor electrodes Shenfang Liu a, Pupu Yang a, Linlin Wangb, Yuliang Lic, Zhenzhen Wua, Rui Ma *, a, Jiayi Wua, 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, 688 Yingbin Ave. Jinhua

321004, PR China CCanwell

Medical Co. Ltd., Jinhua, Zhejiang, 321016, China

*Corresponding

author’s e-mail: [email protected] (R.M.); [email protected] (X.H.); phone:

86-151-0579-0257; fax: 86-579-8228-8269.

1

ACS Paragon Plus Environment

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Abstract In this work, N-doped porous carbons were synthesized by an one-step sodium amide activation of carbonized lotus leaf at 450-500°C. The CO2 adsorption properties of the as-synthesized carbonaceous materials were carefully investigated. In addition, the supercapacitor performance of the optimized sample was also preliminarily explored to examine its potential as the electrode material. These lotus leaf-derived carbons possess good CO2 adsorption capacity up to 3.50 and 5.18 mmol/g at 25 and 0°C under atmospheric pressure, respectively. It was found that the synthetic effects of narrow microporosity, N content, pore size and pore size distribution of the sorbents decide their CO2 adsorption abilities under the ambient conditions. These lotus leafbased carbons also demonstrate many excellent CO2 adsorption properties such as good selectivity of CO2 over N2, quick adsorption kinetics, moderate heat of adsorption, excellent recyclability and high dynamic adsorption capacity. In addition, preliminary electrochemical studies show that the optimized sample has high capacitance (266 F/g) and excellent stability in cycling tests. These results indicate these lotus leaf-derived N-doped porous carbons have good potential in the application of CO2 capture and supercapacitor.

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1. Introduction Excessive CO2 emission has caused serious climate changes, which is an urgent problem to be solved.1, 2 To mitigate CO2 emission, various CO2 capture techniques have been widely explored such as amine scrubbing,3 membrane separation,4 ionic liquid absorption5 and adsorption by solid sorbents6-9 etc. Of these methods, adsorption by solid sorbents is considered as the most promising and sustainable technology for CO2 capture. Solid sorbents with good CO2 capture properties are required for the successful implementation of this technique. Currently, widespread studies have been conducted on the CO2 adsorption of various porous materials such as zeolites,10 porous carbons,11-17 metal-organic frameworks (MOFs),18 and porous polymers19,

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etc.

Among these adsorbents, porous carbons stand out due to their well-known multiple merits like low cost, easy processability, stable chemical properties, adjustable porosity and hydrophobicity.21-24 It has been reported that narrow micropores (< 1.0 nm) is critical for CO2 capture capacities of the porous carbons under ambient condition.25-29 In addition, nitrogen doping into the carbon framework can further reinforce the interplay between adsorbents and CO2 molecules.30-32 A large amount of research work has been devoted to the development of nitrogen-enriched porous carbons with superior CO2 capture properties. In addition to the environmental problems caused by CO2 emissions, the depletion of fossil fuels is another major global challenge. In response to the upcoming energy crisis, the development of new energy storage systems, such as supercapacitors, is one of the effective solutions. The performance of supercapacitors greatly depends on the 3


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properties of the electrode materials. Porous carbons have been widely investigated as the electrode material and exhibited promising results.33 In order to advance the supercapacitor performance of carbon materials, in addition to constructing developed porous structure, doping nitrogen into the carbon skeleton is also an effective strategy. The nitrogen-doping can induce pseudocapacitive effects as well as boost the wettability of carbons to electrolyte solution.34 Therefore, N-doped porous carbons are both prospective electrode materials of supercapacitors and promising CO2 adsorbents. Here, biomass material (lotus leaf)-based N-doped porous carbons were prepared by one-step sodium amide activation at relatively low temperature ranges. One advantage of NaNH2 lies in its strong basicity and nucleophilicity,35, 36 which make NaNH2 effectively perform the activation and thus create developed porosity under temperature ranges of 450 ° C-550 ° C. Such low activation temperature can greatly reduce the corrosion of reaction equipment, which is superior to the conventional KOH activator. Another virtue of NaNH2 is its double roles as both activator and nitrogen source, which can integrate N into the carbon framework during the activation process simultaneously. Therefore, N-doped porous carbons can be successfully achieved by an one-step and low-temperature sodium amide activation process. This strategy will save the complicated and tedious multi-step post-nitrogen-doping processes and highly cut the preparation cost of adsorbents. In addition, the lower activation temperature is good to both the generation of narrow microporosity and preservation of N-containing groups in the resulting sorbents, both factors are important in deciding CO2 adsorption abilities of carbons under 25°C and atmospheric pressure. 4


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The choice of biomass materials as the carbon precursor is due to their low cost, wide availability and sustainability37. As one of the typical biomass materials, lotus leaf is famous for its ultrahydrophobicity. Many researches concerning self-cleaning have been motivated by this famous “lotus effect”. In this manuscript, the possibility of lotus leaf as raw materials for the preparation of nitrogen-doped porous carbons was investigated. A series of N-doped porous carbons with different porous textural and surface chemical properties were synthesized by adjusting the amount of NaNH2 and activation temperature. The CO2 adsorption properties of the as-obtained carbons were carefully investigated. In addition, the supercapacitor performance of the optimized sample was also preliminarily explored. The results reveal that these lotus leaf-derived N-doped porous carbonaceous materials are promising in the application of CO2 capture and supercapacitor. 2. Experimental 2.1 Synthesis and characterization of lotus leaf-derived N-doping porous carbons The oven-dried lotus leaves were milled and sifted. The lotus leaves’ powders with particle size between 74-150 µm were selected to perform carbonization. Carbonization was conducted at 500 °C for 1 h under the protection of nitrogen flow. The resulting product is labeled as LC. The yield of LC is about 39%. Sodium amide activation processes is reported in the supporting information. In the activation process, two parameters were adjusted i.e. activation temperature (450, 500, or 550ºC) and mass ratio of NaNH2 to LC (1, 2 or 3). The obtained nitrogenenriched porous carbons were labeled as LC-X-Y. X represents the activation 5


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temperature and Y means the ratio of NaNH2 to LC. The yield of these N-doped porous carbons ranges from 31% to 72%. The detailed material characterization, adsorption and electrochemical measurement are recorded in the supporting information. 3. Results and Discussion 3.1 Textural information of lotus leaf-based porous carbons Figure 1 shows the N2 adsorption-desorption isotherms for these lotus leaf-derived N-doped porous carbons. Generally, all the samples exhibit type I isotherms, which illustrate that they belong to microporous materials. A rapid increase in the adsorbed N2 volume is found in the low relative pressure (P/P00.1, the isotherms demonstrate an almost flat adsorption feature. When the P/P0 is close to 1.0, an obvious increase in adsorption was found for the samples synthesized at higher activation temperature (500° C and 550 ° C) and NaNH2/LC ratio (2 and 3) indicating the presence of some macropores in these adsorbents. Furthermore, small hysteresis between adsorption and desorption branches at P/P0 of 0.4-0.9 was also found for some samples illustrating the presence of mesopores. The co-existence of micropore and mesopore is affirmed by the pore size distribution (PSD) curves of these porous carbons (Figure 2). It can also be found that the N2 adsorption-desorption isotherms of adsorbents prepared at NaNH2/LC ratio of 1 show much narrower knee than those synthesized at NaNH2/LC ratio of 2 and 3, indicating the narrow pore size distribution for the adsorbents synthesized at small NaNH2 amount. These hypotheses can also be verified by the PSD results displayed in Figure 2. The summary of each porous textural characteristic for the N-doped porous 6


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carbons can be found in Table 1. These lotus leaf-derived carbons possess highly developed porous structure with maximum BET surface area (SBET), total pore volume (V0), and micropore pore volume (Vt) of 1924 m2/g, 1.24 and 0.90 cm3/g, respectively. These findings indicate that lotus leaf is a viable carbon precursor and NaNH2 is effective in developing porosity at the temperature range of 450°C to 550°C. It should be noted that at each activation temperature, the adsorbents prepared at NaNH2/LC ratio of 2 show the highest values in each porous textural characteristic. The reason of this phenomenon is currently not clear, which could be related to the complicated solidstate and gas erosion reactions in the NaNH2 activation process. It is proposed that NaNH2 will attack the oxygen species on the surface of carbon precursor to create the pores and dope N into the carbon framework in the same time. The NH3 released in this process can further erode the carbon to develop some porosity. Additional, some Na2O and/or NaOH may also be formed during the reaction, which can create pores through a redox mechanism. More detailed and in-depth research should be conducted to clarify the mechanism of NaNH2 activation. The narrow microporosity of the as-synthesized adsorbents was calculated by the CO2 adsorption data measured at 0 °C using the Dubinin−Radushkevich (D−R) equation. As shown in Table 1, the Vn for these adsorbents is from 0.42-0.70 cm3/g. Higher Vn than Vt was found in the samples synthesized at NaNH2/LC ratio of 1, suggesting the preferential generation of narrow microporous structures at small NaNH2 amount. Although LC-550-2 possesses the most developed porous texture among all the adsorbents, it possesses the largest difference between Vn and Vt 7


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suggesting that high activation temperature is unfavorable for creation of narrow micropores. 3.2 Morphology, phase structure and chemical information of lotus leaf-derived carbonaceous adsorbents The morphology of LC and the selected N-doped porous carbon (LC-550-1) was observed by scanning electron microscopy (SEM). As illustrated in Figure 3a, LC exhibits a bulky morphology with some winkles but no holes found on the surface. After treatment by NaNH2, the LC was broken into smaller pieces, but still no obvious pores can be found on the surface of LC-550-1 (Figure 3b and Figure 3c). Transmission electron microscopy (TEM) was further explored to explore the detailed morphology of LC-550-1. As demonstrated in Figure 3d, abundant disordered slit-like microspores can be clearly observed for LC-550-1, which agrees well with the results of nitrogen sorption. The phase structural information of the lotus leaf-derived porous carbon was obtained by XRD measurement. As illustrated in Figure S1 (supporting information), two weak and broaden peaks at 2θ equal to ca. 23 and 43° were found, which can be indexed as (002) and (100) diffractions of amorphous carbon.38 The amorphous nature judged from the XRD results is in line with the finding through the TEM observation. From elemental analysis, LC was found to contain 64.37 wt% C, 2.57 wt% H and 1.55 wt% N. Upon NaNH2 activation, the N content of the activated adsorbents increased compared with LC, suggesting the successful integration of N into the carbon skeleton. N content in the activated carbons increases with the increase of NaNH2 amount but decreases with the increase of activation temperature. The more NaNH2 8


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was used, more N-containing functionalities are available to dope into the carbon skeleton. While higher activation temperature was explored, more unstable N species will be decomposed leading to the decreased N content in the resultant sample. X-ray photoelectron spectroscopy (XPS) was applied to identify the essence of N moieties of samples under different treatment conditions. All the selected samples exhibit two peaks centered at 398.4 and 400.0 eV (Figure 4), indexed to pyridinic-N (N-6), and pyrrolic-/pyridonic-N (N-5), respectively. The lack of high-temperature N species such as quaternary-N and oxidized nitrogen was possibly due to the low activation temperature in this study. However, it has been reported that N-5 and N-6 are favorable for the CO2 adsorption than quaternary-N or oxidized nitrogen39. Moreover, the XPS O 1s spectra of these carbons were deconvoluted to two peaks with binding energies of 531.2 and 533.0 eV (Figure S2), corresponding to C-O and C-OH groups40, respectively. For all the samples, the oxygen presents primarily in the form of C-OH group, which was reported to be the Lewis base sites that can bind CO2 molecules during the CO2 adsorption process.41 3.3. CO2 Adsorption performances The CO2 adsorption isotherms (Figure 5) for lotus leaf-derived porous carbons were measured up to 1 bar under 0 and 25ºC by the Beishide 3H-2000PS2 sorption analyzer, respectively. For the isotherms at both temperature, CO2 uptake increases with the increasing of the pressure without the sign of adsorption saturation. Higher CO2 adsorption capacity is expected if the pressure is further increased for these samples. Table 1 listed the CO2 uptake of these carbons at 0 and 25ºC and 1 bar. It is 9


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clearly seen that these lotus leaf-derived porous carbons possess CO2 uptake ranging from 2.21 to 3.50 mmol/g at 25°C and 3.58 to 5.18 mmol/g at 0°C. While in 0.15 bar and 25°C, the CO2 adsorption capacities for these adsorbents is in the ranges of 0.83 to 1.26 mmol/g. The maximum CO2 adsorption ability of 3.50 mmol/g at 25°C and 1 bar in this work is far below some carbonaceous adsorbents obtained by KOH activation,4245

but it is similar to or higher than various previously reported carbon adsorbents13, 46-

51

and other typical porous materials such as COFs,52 ZIFs 53 and MOFs. 18, 54 Table S1

(supporting information) lists the comparison of CO2 uptake for various sorbents at 25°C and atmospheric pressure. To understand the main elements that govern CO2 adsorption abilities for these adsorbents, the correlation between CO2 uptake and each porous textural characteristics (i.e. SBET, V0, Vt , Vn) along with N content was displayed in Figure S3. No proper correlation between these factors and CO2 uptake can be found for these adsorbents, suggesting that the CO2 uptake must not be decided by any of the single parameter. It has been commonly suggested in earlier studies that the synthetic effect of narrow micropores and content of N governs the CO2 adsorption capacity of porous carbons under the ambient conditions.25, 55 Some findings of this study are consistent with this conclusion. For instance, although LC-550-1 and LC-500-1 only have medium Vn and N content, respectively, these two samples possess the first and second highest CO2 uptake of all the samples. However, more results do not follow the above rule. Many samples such as LC-450-2, LC-500-2, LC-500-3 and LC-550-2 have both higher Vn and N content but much lower CO2 uptake than LC-550-1. These results illustrate that 10


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besides Vn and nitrogen content, there presents other important features that play important roles in determining the CO2 uptake for these carbons. Considering the relative high CO2 uptake for the adsorbents prepared at NaNH2/LC ratio of 1 among all the sorbents, the pore size and pore size distribution of the samples may be the crucial keys in determining CO2 adsorption capacities of these samples. As shown in Figure 2, the samples obtained at NaNH2/LC ratio of 2 and 3 at each temperature demonstrate both a larger pore size and broader PSD than the samples obtained at NaNH2/LC ratio of 1. This could be the reason why the samples with higher Vn and N content exhibit lower CO2 uptake than LC-550-1. Thus, it is believed that the synergistic effects of Vn, nitrogen content, pore size and PSD determine the CO2 adsorption abilities of these sorbents under the ambient conditions. For the practical applications, the good recyclability of a sorbent is needed. Herein, a five-time successive cyclic CO2 adsorption on LC-550-1 was carried out at 25°C. Before each adsorption measurement, LC-550-1 was heated at 200oC for 6 h under vacuum. As presented in Figure 6, negligible loss of CO2 adsorption capacity takes place for this adsorbent in the 5th cycle, indicating its great recyclability and stability. Along with the high CO2 adsorption capacity and good recyclability, a potential CO2 adsorbent must also have good CO2 selectivity over N2. To obtain the CO2/N2 selectivity, the N2 adsorption isotherm of LC-550-1 was also measured under 1 bar and 25°C, as shown in Figure 7a. Compared with the high CO2 uptake, only 0.44 mmol/g of N2 uptake is found for LC-550-1, suggesting a high selectivity towards CO2. Basing on the ideal adsorbed solution theory (IAST),56 the CO2/N2 selectivity of LC-550-1 is 11


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predicated to be 21 on a CO2-N2 mixture (10:90 v/v) at 25°C and 1 bar, which is similar to or higher than those of many carbonaceous adsorbents reported earlier.25, 47, 57-61 The isosteric heat of adsorption (Qst), which decides the energy penalty for regeneration, is also an important factor to judge the merits of the adsorbents. Based on the CO2 adsorption data at 25 °C and 0 °C, Qst was calculated by the Clausius– Clapeyron equation.62,

63

Qst at different CO2 loading amounts for some selected

samples are displayed in Figure 7b. At nearly zero loading, the value of Qst is between 26 to 43 kJ mol-1 for these samples, which is higher than some typical carbonaceous sorbents reported in previous reports.64-67 The higher Qst means stronger interaction between adsorbents and CO2, which is advantageous to capture of CO2 from gas mixture with dilute CO2 concentration. But a too high Qst implies a huge amount of energy needed to regenerate the sorbent. The overall Qst of these sorbents lies in the range of 18-43 kJ mol-1, which is still within the class of physisorption demonstrating the easiness of sorbent regeneration. For the potential application, the adsorbents should also have fast adsorption kinetics. The CO2 adsorption kinetics of LC-550-1 was tested at 25°C with results shown in Figure 7c. LC-550-1 exhibits a fast CO2 adsorption rate, which achieved adsorption saturation at about 6 mins. This finding points that LC-550-1 can efficiently adsorb CO2 with short adsorption cycle times, which is particularly favorable for potential applications. In the practical CO2 separation from the flue gas, the dynamic CO2 capture capacity of an adsorbent is important, which can truly reflect the actual application 12


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potential of an adsorbent. Herein, dynamic breakthrough curve of LC-550-1 was measured to assess its dynamic CO2 capture performance. A binary CO2-N2 mixture (10:90 v/v) was used for the dynamic CO2 capture experiment. As illustrated in Figure 7d, the CO2 breakthrough happens at around 8 mins for LC-550-1. The dynamic CO2 capture capacity is calculated to be 0.82 mmol/g, which suggests its great possibility in the CO2 capture from the flue gas. It is worth mentioning that there exists some amount of water in real flue gas environment, which could deteriorate the CO2 uptake of these lotus leaf-derived carbons due to the competitive adsorption between CO2 and H2O. 3.4. Supercapacitance performance of LC-550-1 In order to verify the effectiveness of these lotus leaf-derived carbons as the electrode materials for supercapacitors, cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) were measured for LC-550-1 in 6M KOH electrolyte with a three electrode system. Figure 8a shows CV curves of LC-550-1 at different scan rates (5,10, 20, 40, 50, 60, 80, 100 and 200 mV/s). They all exhibited roughly rectangular shapes with minor distortion, indicating the well typical electrical double-layer capacitors (EDLC) feature for this sample. Figure 8b gives the GCD curves of LC-5501 at different current densities. Small deviation from linear behavior was found for the discharge curve of LC-550-1, especially at current density of 10 A/g. This phenomenon indicates both the double-layer capacitance resulting from the advanced porous structure and the pseudocapacitance due to the nitrogen incorporation matter in the capacitive performance of this sample. At the current density of 0.5 A/g, the LC-550-1 exhibited a specific capacitance of 266 F/g, comparable to or higher than some 13


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carbonaceous electrodes reported in previous literatures.68-71 The changing of capacitive performance with respect to current density is shown in Figure 8c. It is obviously seen that with the increasing of current density, the specific capacitance decreases, which is due to the limitation of ions diffusion and thus restriction of double layer capacitance at higher current density. The cyclic stability of LC-550-1 in capacitance was tested and the results are shown in Figure 8d. LC-550-1 exhibits an excellent cycling performance for consecutive 5000 cycles with a cycle retention of 97% indicating its excellent long-term stability. 4. Conclusion In this study, using lotus leaf as the raw materials, N-doped porous carbons were prepared by an one-step sodium amide activation process under relatively lowtemperature ranges. The as-synthesized carbonaceous materials possess well developed porous structure and high nitrogen content. These carbons demonstrate good CO2 uptake with maximum capacity of 3.50 and 5.18 mmol/g at 25 and 0°C under atmospheric pressure, respectively. Systematic studies found that the synergistic effects of narrow microporosity, N content, pore size and PSD of the sorbents decide their CO2 adsorption abilities under the ambient conditions. As a result, the sample prepared under NaNH2/LC ratio of 1 and 550°C shows the maximum CO2 uptake at 25°C and 1 bar. This sample also demonstrate other great CO2 adsorption properties including high selectivity of CO2 over N2, fast adsorption kinetics, moderate heat of adsorption, excellent recyclability and good dynamic adsorption capacity. In addition, preliminary electrochemical studies indicate that this lotus leaf-derived carbon has high capacitance 14


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(266 F/g) and excellent stability in cycling tests (retention of 97%). These results indicate that the lotus leaf-derived carbons have potential as the CO2 adsorbents and supercapacitor electrode materials. Acknowledgments This work was supported by Zhejiang Provincial Natural Science Foundation (LQ17B060001), the NSF of China (21706239), and National Undergraduate Training Program for Innovation and Entrepreneurship of China (201810345024). Supporting Information: Detailed material characterization, adsorption and electrochemical measurement of sorbents, schematic diagram of the CO2 capture system (Scheme S1), XRD pattern of LC-550-1 (Figure S1), O 1s XPS spectra of selected samples (Figure S2), plots of each porous property characteristic and nitrogen content versus CO2 uptake (Figure S3), a table summarizes the CO2 uptake for various sorbents. References 1.

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

V0b (cm3/g)

Vtc (cm3/g)

Vnd (cm3/g)

N (wt%)

C (wt%)

H (wt%)

LC

4

0.02

0

0.13

1.55

64.37

LC-450-1

792

0.42

0.29

0.42

3.53

LC-450-2

1651

0.83

0.67

0.64

LC-450-3

1557

0.78

0.54

LC-500-1

833

0.43

LC-500-2

1924

LC-500-3

Sample

CO2 uptake (mmol/g) 25°C

0°C

2.57

1.00

1.16

52.46

3.62

3.24

4.60

3.85

51.21

3.64

3.33

5.18

0.47

4.27

51.79

3.68

2.67

3.81

0.31

0.44

3.24

52.98

5.06

3.38

4.87

1.00

0.79

0.70

3.64

53.24

5.75

3.22

4.92

1667

0.92

0.72

0.59

4.04

53.35

5.81

3.04

4.44

LC-550-1

1087

0.54

0.45

0.54

2.55

53.30

5.19

3.50

5.04

LC-550-2

1883

1.24

0.90

0.58

3.04

58.75

5.04

2.21

3.58

LC-550-3

1311

0.85

0.56

0.47

3.65.

59.62

5.52

2.44

3.68

a

Surface area was calculated using the BET method at P/P0=0.01-0.1. pore volume at P/P0= 0.99. c Evaluated by the t-plot method. d Pore volume of narrow micropores (

Nitrogen doped porous carbons from lotus leaf for CO2 capture and

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