Efficient CO2 Capture by Porous Carbons Derived from Coconut Shell

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Efficient CO2 Capture by Porous Carbons Derived from Coconut Shell Jie Yang, Limin Yue, Xin Hu, Linlin Wang, Yongle Zhao, Youyou Lin, Yan Sun, Herbert DaCosta, and Liping Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00633 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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Efficient CO2 Capture by Porous Carbons Derived from Coconut Shell Jie Yang a, Limin Yuea, Xin Hu*,a, Linlin Wangb, Yongle Zhaoa, Youyou Lina, Yan Suna, Herbert DaCostac, Liping Guoa a

College of Chemistry and Life Sciences, Zhejiang Normal University, 688 Yingbin

Ave. Jinhua 321004, PR China b

College of Engineering, Zhejiang Normal University, 688 Yingbin Ave. Jinhua

321004, PR China c

Math, Science, and Engineering Division, Illinois Central College, 1 College Drive

East Peoria, IL 61635, USA *

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

86-579-8228-8269

1

ACS Paragon Plus Environment

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Abstract A series of porous carbons for CO2 capture were developed by simple carbonization and KOH activation of coconut shell under very mild conditions. Different techniques such as nitrogen sorption, X-ray diffraction, scanning emission microscope, transmission electron microscopy were used to characterize these sorbents. Owing to the high amount of narrow micropores within the carbon framework, the porous carbon prepared at KOH/precursor ratio of 3 and 600 °C exhibits an enhanced CO2 adsorption capacity of 4.23 and 6.04 mmol/g at 25 and 0 °C under 1 bar, respectively. In addition to the high CO2 uptake, these samples also show fast adsorption kinetics, moderate heat of adsorption, high CO2 over N2 selectivity, excellent recyclability and stability, and superior dynamic CO2 capture capacity. The application of coconut shell as precursors for porous carbons provides a cost-effective way for the development of better adsorbents for CO2 capture. Keywords: CO2 capture, porous carbon, coconut shell, KOH activation

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1. Introduction CO2 capture has attracted world-wide attention1. It has been reported that the CO2 concentration in the atmosphere by November 2016 has approached 404 ppm, which is 124 ppm higher than the pre-industrial time2. Emission of CO2 has strong impact on climate change, which has brought a series of environmental problems such as melting of glaciers, rising of sea level, and deterioration of the environment. Among the different approaches and technologies to mitigate the CO2 emission, CO2 capture using solid adsorbents is a promising option3-8. The advantages of the adsorption process include less energy requirements for sorbent regeneration, ease of operation, and low capital investment costs9. The key to this technology is to find sorbents with the following features: (1) high CO2 adsorption capacity, (2) high CO2/N2 selectivity, (3) moderate heat of adsorption, (4) fast adsorption kinetics, and (5) excellent chemical and mechanical stability. In recent years, much attention has been paid to the research of solid CO2 adsorbent, including carbons10-12, zeolite13, 14, metal organic materials15, 16 (MOF) and porous polymers17-19. Among these sorbents, porous carbons have attracted a great deal of research interest due to its simple preparation, low production cost, chemical and moisture inertness, high stability, easy to regenerate, and easy-to-design pore structure10, 20. Different precursors have been used to prepare porous carbons including coal21, petroleum coke10, 22-24, wood25 and polymers12, 26-28. Taking into consideration the possible scale and sustainable reasons in the process of synthesizing porous carbons for CO2 capture, the use of renewable sources as carbon precursor would be highly desirable29-31. One typical biomass,

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coconut shell, is a renewable and competitive carbon precursor, which has merits of low cost and abundant output. In addition, the carbon prepared by coconut shell has the benefits of high purity and dust-free nature32, as well as an optimum porosity 33. Several research groups have reported the CO2 adsorption capacity of coconut shell-based porous carbon sorbents under different conditions. For example, Giraldo et al. synthesized activated carbon monoliths by H3PO4 activation of coconut shell, which shows less than 4 mmol/g CO2 sorption capacity at 0 °C and 1 bar34. Jaroniec et al. synthesized microporous activated carbons by CO2 activation of coconut shell35. The maximum CO2 adsorption capacities on these carbons were 3.9 and 5.6 mmol/g at 25 and 0 °C under 1 bar, respectively. Another work by Himento et al. indicated that coconut shell derived activated carbon only showed CO2 uptake of 2 mmol/g at 25 °C and 1 bar36. To further reduce the CO2 capture cost, higher CO2 capacity on coconut shell based carbons is therefore needed. In this work, the carbonized coconut shell was activated by KOH under very mild conditions, compared with activation temperatures ranging from 700-900°C reported for KOH activation of different precursors such as biomass materials37-39, petroleum coke10,

23

or carbon fiber27,

40

in previous reports. Under the moderate

KOH/carbon ratio and activation temperature, the narrow micropores (< 1 nm) of the resulting products are well developed, which is proposed to be responsible for the CO2 adsorption under ambient condition10,

11, 37, 39, 41-43

. A further increase in the

activation degree could lead to generation of many mesopores, which is useless in CO2 adsorption under low pressure. In addition, low amount of activation agents and

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low temperature used in the activation process can further reduce the preparation cost of the carbon sorbents. It is shown that the simple carbonization and KOH activation of coconut shells under mild condition result in microporous carbons with high CO2 adsorption capacity, moderate heat of adsorption, fast adsorption kinetics, high CO2/N2 selectivity, and stable cyclic ability at ambient conditions. 2. Experimental 2.1. Carbonization Before carbonization, coconut shells were heated in an oven at 120 °C for 24 h to remove moisture, followed by grinding and sieving into the size between 100-200 mesh (74-150 µm). Carbonization was carried out in a horizontal tubular reactor under nitrogen flow. The terminal carbonization temperature was set to 500 °C and the carbonization time was 2 h. The heating rate and nitrogen flow rate were 5 °C/min and 200 ml/min, respectively. The sample was cooled down to room temperature under nitrogen atmosphere and was labeled as C. Elemental analysis shows that C contains 79.80 wt% C, 0.26 wt% N, 2.87 wt% H and 17.07% O. 2.2. KOH activation In a typical synthesis, 2 g C were stirred in a solution containing 6g KOH for 6 h at room temperature. Afterwards, the mixture was dried overnight at 120 °C in an oven. The activation was performed at 600°C for 1 hour in a horizontal quartz tubular reactor under nitrogen flow. During the activation process, the heating rate was 5 °C/min and the nitrogen flow rate was 400 mL/min. After activation, the sorbent was washed using distilled water until the pH value of the filtrate was approximately

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7. Then, the sample was further dried at 150 °C under vacuum for 24 h. The yield of this process varies from 78% to 90% depending on the activation conditions. The as-prepared samples were labeled as C-X-Y, where X is the activation temperature and Y stands for the ratio of KOH to C. For example, the above described sample can be designated as C-600-3. 2.3. Characterization X-ray diffraction (XRD) was performed using CuKα radiation (λ =0.15406nm) by a PHILIPS PW3040/60 diffractometer. The morphology of the carbon sample was observed by scanning electron microscopy (SEM, Hitachi S-4800). The detailed pore structure of the carbon sample was evaluated by transmission electron microscopy (TEM, JEOL-2100F) operated at 200 KV. Nitrogen adsorption/desorption isotherms were measured at -196 °C on a 3H-2000PS2 volumetric analyzer (Beishide, Inc., Beijing). Before adsorption measurements, the samples were outgassed at 200 °C for at least 12 h. The specific surface area (SBET) was measured from the isotherm in the relative pressure range of 0.005-0.05, according to the Brunnauer-Emmett-Teller (BET)

method.

The

pore

size

distribution

(PSD)

was

obtained

from

Harvath–Kawazoe (H–K) method. The micropore volume (Vt) was calculated according to the t-plot method. The total pore volume (V0) was determined from the amount of N2 adsorbed at a relative pressure of 0.99. The CO2 adsorption capacity of carbon samples was evaluated at 0 and 25°C by volumetric gas adsorption studies using a Beishide 3H-2000PS2 sorption analyzer. The narrow micropore volume (micropores < 1 nm), Vn, was calculated from CO2

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adsorption at 0 °C using the Dubinin–Radushkevich (D-R) equation. Before the adsorption measurement, the sample was outgassed at 200oC for at least 6 h. The dynamic CO2 adsorption capacity of the sorbents was studied on a fixed-bed reactor schematically illustrated in Figure S1 (Supporting information) at 1 bar and 25 °C. First, the bed was heated at 100 °C for 1 h under N2 at a ﬂow rate of 20 mL/min. The gas flow was shifted from nitrogen to a 10% mixture of CO2 in N2 and at a flow rate of 10 mL/min, when the sample temperature reached 25 °C. The effluent gases were monitored online using an Agilent 7820A gas chromatograph with a thermal conductivity detector (TCD). From the breakthrough curves, the dynamic CO2 capture capacity on an adsorbent was calculated. The adsorption kinetics of CO2 was determined by a gravimetric method using a thermogravimetric analysis instrument (NETZSCH STA 449C) with the following operation procedure. Approximately 15 mg carbon sorbent was placed on the test pan of the thermogravimetric analyzer and heated at 200 °C for 1 hour under a nitrogen flow rate of 30 mL/min for desorption of volatile components. The gas flow was shifted from nitrogen to CO2 gas when the sample temperature was lowered to 25 °C. The weight gain versus time was recorded. 3. Results and Discussion 3.1. Porous Properties The porous properties of the carbons prepared were characterized by N2 adsorption/desorption at -196°C and CO2 sorption at 0 °C. Figs. 1 show the nitrogen adsorption/desorption isotherms at -196 °C for porous carbons prepared from coconut

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shell under different conditions. The textural properties of carbons are summarized in Table 1. As can be observed from this table, an increase in KOH/precursor ratio or activation temperature is found to enhance the BET surface area (SBET), total pore volume (V0) and micropore volume (Vt) of the carbons with only a few exceptions. The BET surface area of this series of samples ranges from 878 to 1817 m2/g with total pore volume (V0) and micropore volume (Vt) in the range of 0.28-0.69 cm3/g and 0.27-0.67 cm3/g, respectively. All samples exhibit type I isotherms according to the IUPAC (International Union of Pure and Applied Chemistry) classification, with major adsorption at relative pressure P/P0 < 0.01. At P/P0 > 0.01, the isotherms show an almost flat sorption characteristic. The major nitrogen uptake at low relative pressures (P/P0< 0.01) and no hysteresis loops in the entire P/P0 range indicate that the porous carbons have a narrow micropore size distribution and almost no mesoporosity. This can be confirmed by the pore size distribution of representative carbons illustrated in Fig. 1d, which demonstrates the micropores with average diameter of 6-7 Å. Narrow micropores ( < 1 nm) analysis of the prepared carbon samples was performed by CO2 adsorption at 0 °C in order to avoid diffusional problems of nitrogen molecules at -196 °C inside the narrower pores. The corresponding values are in the range of 0.39 -0.58 cm3/g, as shown in Table 1. It should be noted that for any sample, except C-650-3 and C-700-3, which are prepared at the highest KOH/precursor ratio of this work, the volume of the narrow micropores (Vn) is always larger than Vt. This indicates that relatively mild synthesis condition is beneficial to the formation of narrow micropores, which play a key role in the enhanced CO2

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uptake at ambient temperature and pressure10-12, 37, 39, 41. 3.2. Surface Morphology and Phase Structure The pore structures and surface morphology of carbon materials were further studied with XRD, SEM, and TEM techniques. XRD characterization of representative sample C-600-3 indicates the lack of a long-range structural order for these carbons. A broad diffraction peak at around 2θ = 43° is observed, corresponding to reflections from (100) plane of amorphous carbon (Figure 2a)39. According to the SEM images, the carbonized coconut shell (C) has a bulky morphology, with no pores on its smooth surface (Figure 1b). After activation, some large cavities with pore sizes of 1 to 5 µm were observed on the surface of porous carbon (C-600-3 as a representative example) (Figure 2c). The TEM images shown in Figure 2d evidently demonstrate the presence of worm-like micropores in the carbon material, which are formed by the stacking of curved graphene layers44. The sizes of most micropores are smaller than 1 nm. The disorder structure observed from the TEM is consistent with the XRD result. 3.3. CO2 Adsorption Properties The CO2 adsorption isotherms of the prepared carbons at 25 °C and 0 °C are shown in Figs. 3. The CO2 adsorption capacities were collected in Table 1 for comparison. These coconut shell –based carbons show high CO2 capture capacities in a range of 3.56-4.23 mmol/g at 25 °C under 1 bar. While at 0.15 bar, which is the typical pressure of CO2 in flue gas, the CO2 uptakes are in the range of 1.26-1.45 mmol/g at 25 °C for these samples. When the temperature was lowered to 0 °C, the

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CO2 adsorption capacities increased to 4.63 - 6.04 mmol/g under 1 bar. C-600-3 has the highest CO2 capture capacity of 4.23 mmol/g at 25 °C, which is higher than many porous materials reported in the literatures. For example, at 25 °C and 1 bar, CO2 uptakes of MIL-5345, IRMOF-115, and MOF-17715 were less than 2.5 mmol/g. In addition, this CO2 adsorption capacity is higher than many other biomass-derived carbons11,

37, 46-53

(Table S1, Supporting information). For example, Plaza et al.

developed porous carbons at carbonization temperature of 600°C and ammonia activation temperature of 900°C using biomass material almond shell46 and Oliver stone49 as the precursors, respectively. The resulting sorbents only showed 2.2 and 2.4 mmol/g CO2 adsorption capacities at 25°C and 1 bar. Wang et al. synthesized a Fungi-derived porous carbon sorbent at carbonization temperature of 500°C and KOH activation temperature of 700°C37, which showed 3.5 mmol/g of CO2 uptake at 25°C and 1 bar. In another work, Parshetti et al. synthesized palm empty fruit bunch-derived porous carbon by hydrothermal carbonization at 250°C and KOH activation at 800°C, which shows CO2 uptake of 3.7 mmol/g at 25°C and 1 bar53. However, some previous studies reported very high CO2 uptake for porous carbons derived from biomass or polymer precursors. For example, Jaroniec et al. synthesized highly microporous carbon spheres using phenolic resin as the precursor by a modified Stöber method, which showed the maximum 4.7 mmol/g of CO2 uptake at 25°C and 1 bar54, 55. Mokaya et al. have developed porous carbons with very high CO2 uptake at 25°C and 1 bar using sawdust and lignin (5.8 mmol/g)56, Jujun grass and Camellia japonica (5.0 mmol/g)57 and polypyrrole (5.5 mmol/g)58 as the precursors,

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respectively. It should be also noted that our results are still lower than those of high N-containing

porous

carbon

materials22,

26,

59-63

.

However,

some

of

the

nitrogen-enriched samples require complicated and tedious multi-step synthesis process22, 24, 59, 60, while only simple carbonization and KOH activation of coconut shells is needed to achieve high CO2 adsorption capacity under ambient condition, as shown in this study. The effect of factors such as BET surface area (SBET,), total pore volume (V0), micropore volume (Vt), and narrow micropore volume (Vn) on the CO2 uptake of synthesized carbons is also examined. Figure S2 shows the correlation between CO2 uptake of the prepared carbons and each of the textural characteristics. No direct correlation between CO2 uptake and any of the porous properties characteristics (i.e. SBET, V0, Vt or Vn) was found, since the regression coefficients (R2) of the linear fitting for above parameters are only 0.1, 0.095, 0.089 and 0.562, respectively. C-600-3 shows the highest CO2 uptake, which can be due to its highest narrow microporosity in this study, while C-600-1 with the lowest narrow micropore volume shows the lowest CO2 adsorption capacity. One interesting finding is that C-650-3 has a much higher Vn than C-700-1, but a lower CO2 uptake. This can be possibly attributed to the bigger pore size of C-650-3 than C-700-1, as shown from the PSD figure (Figure 1d). The isosteric heat of adsorption (Qst), which reflects the strength of the interaction between CO2 molecules and adsorbent, was calculated from the CO2 adsorption isotherms at 25 °C and 0 °C using the Clausius–Clapeyron equation64, 65. The Qst for C-600-3 is in the ranges of 23-37 kJ mol-1, typical for a physisorption

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process and similar to other porous carbons11, 66-68 (Figure 3d). The Qst decreased gradually with increasing CO2 loading indicating the energetic heterogeneity of adsorption sites for C-600-3. From the standpoint of potential application, this moderate Qst is favorable, which warrants both efficient CO2 capture and succeeding easy desorption to regenerate the adsorbents. Besides CO2 adsorption capacity, cyclic stability towards CO2 adsorption, CO2 uptake kinetics, and CO2 over N2 selectivity are also key parameters when assessing a sorbent. These aspects will be cautiously examined and discussed. In order to test the recyclability and stability, the sample C-600-3 was submitted to a ten-cycle adsorption/desorption test at 25 °C (Figure 4). During the cyclic adsorption process, the sample was heated at 200oC for 6 h in vacuum before acquiring the corresponding adsorption isotherms. The results show only 2.4% loss after ten successive adsorption/desorption runs, indicating the high potential of multiple-time use in CO2 capture processes. The CO2 over N2 selectivity of C-600-3 was determined according to its N2 and CO2 adsorption isotherms (Figure S3). Langmuir-Freundlich model was used to fit the isotherm data for CO2 and N2 adsorption isotherm was fitted by a linear equation. According to the ideal adsorption solution theory (IAST)69, CO2/N2 selectivity was predicted for CO2-N2 binary mixtures with N2 molar fraction ranging from 70% to 99%, which is typical composition range of flue gases66. As shown in Figure S4a, the calculated CO2/N2 selectivities increase with the N2 molar fraction. At the total pressure of 1 bar and CO2 concentration of 10%, a selectivity of 22 is predicted for

12


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C-600-3, which is compared to or better than those of carbonaceous sorbents reported previously24, 43, 70-73. In addition, IAST-predicted adsorption selectivity of CO2/N2 for C-600-3 using a binary mixture of 10 : 90 CO2/N2 at 25 °C and up to 1 bar was illustrated as Figure S4b. From this figure, it can be seen that CO2/N2 selectivity can be up to 98 for C-600-3 indicating its potential in post-combustion CO2 capture. Figure S5 shows the CO2 adsorption kinetic for C-600-3 at 25 °C conducted on a thermogravimetric (TG) analyzer. A steep linear weight gain, which holds about 90% of the total uptake, completed in less than 5 mins. A classical Fick’s diffusion model shown as the Equation 1 was applied to correlate the adsorption kinetics data and to calculate the diffusion time constants for CO2 of C-600-374, 75.

1−

 − π 2 Dct  mt 6  = 2 exp 2 mmax π r c  

1

where mt/mmax is the fractional adsorption uptake and the ratio Dc/rc

2

(min-1) is

known as the diffusion time constant. It can be seen from Figure S6a that this Fick’s diffusion model is unable to illustrate the whole experimental curve with the squared correlation coefficient (R2) for regressions of only 0.74. The value of diffusion time constant is found to be 0.022 min-1. Instead, Avrami’s fractional-order kinetic model shown as the equation 2 was chosen to fit the experimental data76, 77. qt = qe 1 − e − 

( k A t )n A

  

2

where KA is the Avrami kinetic constant and nA is the Avrami exponent. qt and qe are the CO2 adsorbed amounts at time t and at equilibrium, respectively. This Avrami’s fractional-order kinetic model was found to fit the whole experimental data well with

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the squared correlation coefficient (R2) for regressions of over 0.95 as illustrated by Figure S6b. The Avrami kinetic constant is calculated to be 0.391 min-1 for C-600-3. This relatively high Ka confirms the high CO2 diffusion rate in this sorbent. It is worth pointing out that for practical applications in CO2 capture, the sorbents should be evaluated under dynamic conditions to find out the extent of equilibrium adsorption capacity converted into breakthrough capacity. The dynamic CO2 capture capacity of C-600-3 is characterized using a dynamic setup. From the breakthrough curves shown in Figure S7, its dynamic CO2 capture capacity is calculated to be 1.0 mmol/g from a CO2/N2 (10:90 v/v) gas mixture at 25 °C, proving its capability in capturing CO2 from flue gas. From practical aspects, the low-pressure CO2 capture capacity (~0.1 bar) is more important than that at 1 bar. More research efforts should be put on this topic.

4. Conclusion To summarize, the porous carbons developed in this work by simple carbonization and KOH activation of coconut shell exhibit high CO2 adsorption capacity, fast adsorption kinetics, moderate heat of adsorption, high CO2 over N2 selectivity, excellent recyclability and stability, and superior dynamic CO2 capture capacity. The low cost and wide availability of coconut-shell precursor as well as the mild synthesis condition for these sorbents imply that they are promising for industrial applications

Acknowledgement The authors acknowledge the support of Zhejiang Provincial Natural Science

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Foundation (LY15B060004), the NSF of China (21106136), and National Undergraduate Training Program for Innovation and Entrepreneurship of China (201510345015).

Supporting Information: Schematic diagram of the CO2 capture system, N2 adsorption isotherm, CO2 adsorption kinetic and kinetic model simulation, IAST CO2/N2 selectivity, and CO2 breakthrough curve under simulated flue gas condition of C-600-3, the plots of each porous properties characteristics versus CO2 uptake. This material is available free of charge via the internet at http://pubs.acs.org.

Nomenclature: XRD = X-ray diffraction SEM = scanning emission microscope TEM = transmission electron microscopy BET = Brunnauer-Emmett-Teller PSD = pore size distribution H-K = Harvath–Kawazoe D-R = Dubinin–Radushkevich TCD = thermal conductivity detector Qst = isosteric heat of adsorption (Qst) IAST = ideal adsorption solution theory

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Nitrogen-Doped

Porous

Coconut

Shell-Based

CO2

Sorbent

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Table 1. Porous properties and CO2 uptakes of sorbents derived from coconut shell under different conditions Sample

SBETa 2

V0b 3

Vtc 3

Vnd

Yield

3

(%)

25°C

0°C

(m /g) (cm /g) (cm /g) (cm /g)

CO2 uptake (mmol/g)

C

21

0.02

0.01

0.17

-

1.51

2.02

C-600-1

718

0.28

0.27

0.39

90.0

3.56

4.63

C-600-2

1082

0.42

0.41

0.57

88.6

3.99

5.82

C-600-3

1172

0.44

0.43

0.58

87.1

4.23

6.04

C-650-1

885

0.35

0.33

0.39

87.2

3.83

5.01

C-650-2

1211

0.46

0.44

0.58

85.9

4.13

6.20

C-650-3

1513

0.58

0.57

0.55

82.3

3.69

4.95

C-700-1

878

0.33

0.32

0.44

82.4

3.83

4.98

C-700-2

1233

0.46

0.45

0.47

81.2

3.71

5.14

C-700-3

1817

0.69

0.67

0.55

78.4

3.97

5.89

a

Surface area was calculated using the BET method at P/P0=0.005-0.05. Total pore volume at P/P0 = 0.99. c Evaluated by the t-plot method. d Pore volume of narrow micropores (

Efficient CO2 Capture by Porous Carbons Derived from Coconut Shell

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