Polyurethane Foam-Based Ultramicroporous Carbons for CO2

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Polyurethane foam-based ultra-microporous carbons for CO2 capture Chao Ge, Jian Song, Zhangfeng Qin, Jianguo Wang, and Weibin Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04771 • Publication Date (Web): 04 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016

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ACS Applied Materials & Interfaces

Polyurethane foam-based ultra-microporous carbons for CO2 capture Chao Gea,b, Jian Song a,b, Zhangfeng Qina, Jianguo Wanga, and Weibin Fana∗ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, P. R. China b

∗

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Corresponding authors. Tel.: +86-351-4199009; Fax: +86-351-4041153.

E-mail address: [email protected] Postal Address: Prof. Weibin Fan State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese Academy of Sciences 27 South Taiyuan Road Taiyuan, Shanxi 030001 PR China

ACS Paragon Plus Environment

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Abstract

A series of sustainable porous carbon materials was prepared from waste polyurethane foam and investigated for capture of CO2. The effects of preparation conditions, such as pre-carbonization, KOH to carbon precursor weight ratio and activation temperature, on the porous structure and CO2 adsorption properties were studied for the purpose of controlling pore sizes and nitrogen content and developing high-performance materials for capture of CO2. The sample prepared at optimum conditions shows CO2 adsorption capacities of 6.67 and 4.33 mmol⋅g-1 at 0 and 25 °C under 1 bar, respectively, which are comparable to those of the best reported porous carbons prepared from waste materials. The HCl treatment experiment reveals that about 80% of CO2 adsorption capacity arises from physical adsorption, while the other 20% is due to the chemical adsorption originated from the interaction of basic N groups and CO2 molecules. The relationship between CO2 uptake and pore size at different temperatures indicates that the micropores with pore size smaller than 0.86 and 0.70 nm play a dominant role in the CO2 adsorption at 0 and 25°C, respectively. It was found that the obtained carbon materials exhibited high recyclability and high selectivity to adsorption of CO2 from the CO2 and N2 mixture.

Keywords: CO2 adsorption, porous carbon, polyurethane foam, waste materials, ultramicropore


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1. Introduction CO2 is considered to be a primary greenhouse gas. About 44% of CO2 comes from flue gases discharged from fossil fuel-fired power plant.1 Flue gas typically consists of 12% CO2, 80% N2 and 8% H2O.2 Thus, selective capture and storage of CO2 from flue gases is effective for the abatement of climate change. Up to now, solution absorption, membrane separation, chemical and physical adsorption and cryogenic methods have been developed to separate and capture CO2.3-7 Among these routes, adsorption is the most promising technology because of its low cost, low energy consumption and easy handling.8 The key of this technology is to develop a highperformance CO2 adsorbent. Porous materials used for capture of CO2 mainly include zeolites,9 metal-organic frameworks,10 amine-functionalized

mesoporous

silica,11

porous

organic

polymers,12

graphene-based

adsorbents13 and porous carbons.14-15 The porous carbon material is regarded as one of the most potential candidate due to its high surface area, developed porosity, low preparation cost, stable physicochemical properties, facile chemical modification and fast regeneration.16 Therefore, considerable researches have devoted to preparing carbon-based porous materials for capture of CO2.17-18 Recently, various precursors, including polymer,19-20 polypyrrole21 and fungi-based chars,1 were used to prepare porous carbons. From the viewpoint of environmental friendliness and preparation cost, use of waste materials, such as carpet,22 fly ash,23 coconut24 and celtuce leaves,25 to prepare high-performance carbonaceous CO2 adsorbents would be more attractive. One efficient way to enhance CO2 capture capacity is to introduce nitrogen-containing species in the carbon material. Xing and his coworkers prepared one type of nitrogen-rich activated carbon from waste bean dreg, and found that it exhibited a CO2 uptake capacity as high as 4.24 mmol⋅g-1


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at 1 atm and 25 °C. They thought that introduction of nitrogen species into carbon materials could effectively increase the interaction between CO2 molecules and carbon surface.26 Another carbon material worthy of being pointed out is carbon molecular sieve prepared from petroleum pitch. It shows a CO2 adsorption capacity of 8.6 mmol⋅g-1 at 0 °C, and high CO2 selectivity for CO2/CH4 (100%) and CO2/N2 (14%) separations from flue and landfill gases. This is due to the presence of more micropores with pore openings in the range of 0.35-0.7 nm in this carbon material.27 Therefore, another way to improve CO2 capture capacity is increase of micropores with pore size < 1.0 nm. Polyurethane foams (PUF) (Figure S1, Supporting information) is one of the most important thermoset polymers. The global demand for polyurethanes was estimated to be 13.65 million tons (Mts) in 2010 and expected to grow by 4.76% per year until 2016.28 This led to generation of huge amounts of related wastes, including useless articles and spent products, which are about 9.6 Mts by the 2015. However, regeneration of spent polyurethane is not only a highly energyconsuming but also a heavy time-consuming process.29-30 Consequently, most of them are directly burnt, causing severe environmental pollution as a result of giving off large amounts of nitrogen oxides and carbon oxides. Actually, these wastes are potential to prepare value-added nitrogen-doped carbon materials, which, additionally, can alleviate environmental pollution. In this context, a new type of microporous carbon materials with small pore size was prepared from waste PUF, and the effects of pre-carbonization and KOH activation conditions on their textural properties and CO2 adsorption capacities were studied. In addition, CO2 adsorption mechanism on the prepared carbons was investigated. 2. Experimental 2.1. Sample preparation


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The procedures for the preparation of porous carbon materials with waste PUF are as follows: a certain amount of waste PUF was pre-carbonized at 400 °C for 1 h in a nitrogen flow (80 mL·min-1). Then, it (designed as PUF-400) was stirred in KOH aqueous solution (KOH/PUF-400 =0.5, 0.7, 1, 2 or 4 (g/g)) for 1 h at room temperature. After that, the black slurry was dried at 110 °C overnight to evaporate water. The dried sample was activated at 500 – 800 °C for 2 h in a nitrogen flow (80 mL·min-1). Finally, the black solid was washed with 2 M HCl and deionized water until the filtrate became neutral. The obtained solid was dried at 110 °C for 2 – 4 h, and labeled as PUF-400-KOH-x-y with x and y representing the KOH/PUF-400 weight ratio and the activation temperature respectively. For comparison, porous carbon was also prepared by direct activation of waste PUF with KOH (KOH/PUF=2 (g/g)) at 700 °C, and it was denoted to PUFKOH-2-700. For direct estimation of the CO2 adsorption capacity induced by basic nitrogen species, PUF-400-KOH-1-700 as an example was treated with concentrated HCl acid (37%) at room temperature for 2 h, washed with distilled water and dried at 110 °C to protonate neutralize all of its nitrogen species. 2.2. Sample characterization The morphology of the samples was examined by scanning electron microscopy (SEM) conducted on a JEOL JSM-700 microscope at accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were measured on a JEOL JEM-2001F apparatus operating at 200 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex II diffractometer with CuKα radiation (40 kV, 15 mA). X-ray photoelectron spectra (XPS) were measured on a Krato AXIS Ultra DLD spectrometer with AlKα radiation from a double anode Xray source. Fourier transform infrared (FTIR) spectra in the region from 400 to 4000 cm-1 were collected on a Bruker VERTEX 70 spectrometer by using the conventional KBr pellet method.


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Elemental analysis was performed on an Elemntar Vario EL Cube microanalyzer. Nitrogen sorption isotherms were measured at -196 °C on an ASAP 2020 apparatus. Before the measurement, the sample was degassed at 200 °C for 8 h. The surface area was calculated by the BET method at p/p0 ≤ 0.05. The total pore volume was determined from the amount of nitrogen adsorbed at the p/p0 = 0.99. The pore size distribution (PSD) was attained by the non-local density functional theory (NLDFT) method by assuming a slit pore model. The cumulative volumes of the micropores with pore sizes < 0.7 nm and < 0.86 nm were determined by the NLDFT method based on the CO2 adsorption data at 0 °C, as shown in Figure S2 (Supporting information). The thermogravimetric-mass-infrared spectrometry (TG-MS-IR) analysis of the PUF was carried out with nitrogen as carrier gas on a joint apparatus assembled with a Setaram Setsys Evolution thermal analysis instrument, a Pfeiffer Vacuum Omnistar mass spectrometer and a Bruker TENSOR 27 FTIR spectrometer. About 10 mg of sample was put in a Pt crucible and heated from 25 to 900 °C at a heating rate of 5 °C·min-1. The detected m/z signals were in the ranges of 13 – 18, 25 – 32, 35 – 46 or at 60, 65, 68, 73, 78, 91, 96, 105 and 106. 2.3. CO2 adsorption measurements The CO2 adsorption isotherms were measured at 0 and 25 °C on a BELSORP-max apparatus. Before the measurement, the sample was degassed at 150 °C for 2 h. 3. Results and discussion 3.1. Morphologies of porous carbons and pyrolysis behavior of PUF Figure 1a shows the SEM and HRTEM images of pre-carbonized and KOH-activated samples. The smooth surface of the pre-carbonized precursor was completely destructed by the KOH etching, and changed to rugged and multihole morphology to a large extent (Figure 1b), thus forming large numbers of macropores on the surface (Figure 1c). HRTEM images (Figure 1e and


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f) show that worm-like micropores were formed through the stacking of graphene layers. In addition, an increase in the KOH activation degree led to generation of many mesopores (Figure 1d and f), forming the micro-meso-macro hierarchical porous structure.

Figure 1. SEM images of PUF-400 (a), PUF-400-KOH-1-700 (b and c) and PUF -400-KOH-2700 (d), and HRTEM images of PUF-400-KOH-2-700 (e and f).


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The pyrolysis behavior of PUF was investigated in a nitrogen flow (30 mL·min-1) by TG-MSIR spectrometry (Figure S3, supporting information). PUF is thermally stable below 250 °C in N2 atmosphere. It decomposed and carbonized via three stages. The first one is between 250 and 350 °C with the formation of primary and secondary amines, CO2 and H2O,31 ethers, hydroxyl-, methyl-, methylene- and methane- species.32 The second one occurs in the range of 350 – 430 oC. It accounts for the complete decomposition and/or evaporation of isocyanate groups. In the third stage (430 – 510 °C), the material was degraded to aromatics, quaternary N, pyridine N-oxide species and CH4.33 3.2. Surface group analyses Figure 2 shows the FTIR spectra of PUF-based carbon materials. Six bands were primarily observed. The intense bands at 1583 and 1375 cm-1 are attributed to the N-H in-plane deformation (and/or C = C stretching vibration) and the C-N stretching vibration respectively.15,16,21 The broad band around 3440 cm-1 is assigned to the N-H and/or O-H symmetric stretching vibrations.34 As for the bands at 2927 and 2851 cm-1, they are due to the CH stretching vibrations of – CH2 and – CH3 groups.16 The presence of nitrogen species was further confirmed by x-ray photoelectron spectroscopy, which shows the N 1s signal at the binding energy of 400 eV although it decreased in intensity with the activation temperature. In addition, an intense O 1s signal was observed at 530 eV, and it seemed to be independent of activation temperature (Figure S4, Supporting information). The oxygen and nitrogen species uniformly distribute in the samples (Figure S5, Supporting information).


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d

h

c

g f

b

3440

T(%)

T(%)

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


a

d: PUF-400-KOH-2-800 c: PUF-400-KOH-2-700 b: PUF-400-KOH-2-600 a: PUF-400-KOH-2-500

3000 2000 1000 -1 Wavenumber/cm

2927

2851 1583

1375

e

1108

h:PUF-400-KOH-4-700 g:PUF-400-KOH-2-700 f:PUF-400-KOH-1-700 e:PUF-400-KOH-0.7-700

3000 2000 1000 -1 Wavenumber/cm

Figure 2. FTIR spectra of N-doped porous carbons obtained at different activation conditions Generally, CO2 adsorption capacity is closely related to the type and amount of nitrogen species on the surface of porous carbons. Figure 3 shows that the N1s signal can be deconvolved into 3 or 4 components depending on the activation temperature or KOH/PUF-400 ratio. The components at the binding energies of 398.6, 399.9, 401 and 402.8 eV are attributed to pyridine-, pyrrole-, quaternary N and pyridine N-oxide species, respectively.35-36 Pyridine N-oxide species were not detected in PUF-400-KOH-2-500, but it was present in PUF-400-KOH-2-600, 700, 800 and its relative amount increased with the activation temperature. This also held true for the quaternary N species (Figure 3a-d). In contrast, the relative contents of pyridinic and pyrrole-type nitrogen species gradually decreased from 29% and 51% to 16% and 38%, respectively, when the activation temperature was increased from 500 to 800 °C (Table S1, Supporting information). This indicates that some of pyridinic and pyrrole-type nitrogen species are likely transformed into quaternary N and pyridine N-oxide species at high activation temperature despite that the total amount of different types of nitrogen species decreased. Similar phenomenon was also observed


9


with increasing KOH/PUF-400 ratio (Figure 3e-h). It was reported that basic pyridinic and pyrrolic N might be more beneficial for capture of CO2 than quaternary N and pyridine N-oxide species.16,21 Therefore, CO2 adsorption capacity on the prepared carbon materials would decrease with the activation temperature or the KOH/PUF-400 ratio if their porous structures are the same. However, a volcano relationship was observed between CO2 adsorption capacity and activation temperature or KOH/PUF-400 ratio, suggesting that their porous structures should have a vital effect on the capture of CO2. Indeed, it was found that CO2 capture process included both physical and chemical adsorptions (Figure S6, Supporting information), and the physical adsorption were much stronger than the chemical adsorption.

Intensity (a.u.)

(b)

Intensity (a.u.)

(a)

408

404

400

396

408

392

404

400

396

392

Binding Energy (eV)

Binding Energy (eV)

(c)

(d) Intensity (a.u.)

pyrrolic-N

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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408

404

400

396

Binding Energy (eV)

392

quaternary-N pyridinic-N pyridine N-oxide

408

404

400

396

392

Binding Energy (eV)


10

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(f)

Intensity (a.u.)

Intensity (a.u.)

(e)

408

404 400 396 392 Binding Energy (eV)

408

404

400

396

392

Binding Energy (eV)

(g)

(h) Intensity (a.u.)

Intensity (a.u.)

408

404

400

396

392

408

Binding Energy (eV)

404

400

396

392

Binding Energy (eV)

Figure 3. N 1s XPS of PUF-400-KOH-2-500 (a), PUF-400-KOH-2-600 (b), PUF-400-KOH-2700 (c), PUF-400-KOH-2-800 (d), PUF-400-KOH-0.5-700 (e), PUF-400-KOH-0.7-700 (f), PUF400-KOH-1-700 (g) and PUF-400-KOH-4-700 (h). 3.3. Effect of pre-carbonization (002) (100) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PUF-400-KOH-2-700

PUF-KOH-2-700

20

40

60

80

2θ (degree)

Figure 4. XRD patterns of PUF-400-KOH-2-700 and PUF-KOH-2-700.


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Figure 4 compares the XRD patterns of PUF-400-KOH-2-700 and PUF-KOH-2-700. Two broad diffraction peaks were observed at 2θ of 23.4 (002 plane) and 42.4° (100 plane) for both the samples, indicating that they both are amorphous. Nevertheless, the (100) diffraction peak of PUF-400-KOH-2-700 is more intense than that of PUF-KOH-2-700, indicative of a higher graphitization degree.37 In addition, the former sample contains mainly micropores (Figure 5a and b, Table 1), which contribute to about 88% of pore volume. In contrast, the PUF-KOH-2-700 exhibits a typical type IV sorption isotherm, and more than 50% of pore volume comes from mesopores. Furthermore, the surface area of PUF-400-KOH-2-700 is as large as about 1.33 times of that of PUF-KOH-2-700 (Table 1). This may result from the increase in the graphitization degree by pre-carbonization, and consequently, the enhancement in the resistance to the destruction of micropores by KOH. Another point worthy of being pointed out is that PUF-400KOH-2-700 contains 6.92 wt.% of nitrogen in contrast to 2.06 wt.% for the PUF-KOH-2-700, showing that pre-carbonization process greatly benefits for the reserve of nitrogen species in the porous carbons (Table 2). Figure 5a and b shows that PUF-KOH-2-700 exhibits much lower CO2 uptake than PUF-400-KOH-2-700. This may be due to its smaller amounts of nitrogen species and smaller ultramicropore volume.38 3.4. Effect of activation temperature on the textural properties of PUF-based porous carbons 3.4.1. Textural properties and chemical compositions Figure 5 shows the nitrogen sorption isotherms and PSDs of porous carbons activated at different temperatures with the KOH/PUF-400 ratio kept at 2. It is clear that PUF-400-KOH-2500 and PUF-400-KOH-2-600 exhibit typical type I isotherm, indicative of their microporosity. The pore diameter is about 1.07 nm for both the samples. In contrast, a number of mesopores are


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also present in the PUF-400-KOH-2-800, and simultaneously, the micropore size distribution is much wider than those of PUF-400-KOH-2-500 and PUF-400-KOH-2-600 (Figure 5b). This can be accounted for by the more severe activation at higher temperature.

(b)

PUF-KOH-2-700 PUF-400-KOH-2-800 PUF-400-KOH-2-700 PUF-400-KOH-2-600 PUF-400-KOH-2-500

3 -1

dV/dD/(cm ·g ·nm )

600


(a)

3

-1

Adsorbed volume/(cm ·g )

-1

400

2

3

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


200

0 0.0

1

0

0.2

0.4

0.6

P/P0

0.8

1.0

1

Pore width/nm

10

Figure 5. Nitrogen sorption isotherms (a) and PSDs (b) of the obtained porous carbons prepared at different activation temperatures (KOH/PUF-400 = 2) Table 1. Textural properties of porous carbons prepared at different activation temperatures. Textural properties Yield Samples

SBET

Vt a

Vmicro b

Vultramicro c (cm3·g-1)

DNLDFT d

(m2·g-1)

(cm3·g-1)

(cm3·g-1)

≤0.7 nm

≤ 0.86 nm

(nm)

(%) PUF-KOH-2700

---

1076.9

0.70

0.34

0.06

0.09

1.31,12.9

PUF-400-KOH2-500

63

169.1

0.11

0.10

0.10

0.10

1.08

PUF-400-KOH2-600

59

825.9

0.35

0.31

0.13

0.15

1.07

PUF-400-KOH2-700

41

1430.0

0.59

0.52

0.13

0.17

1.27

PUF-400-KOH2-800

30

1469.9

0.67

0.54

0.07

0.12

1.31


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a

Total pore volume at p/p0 of 0.99. bMicropore volume determined by the t-plot method. cThe cumulative volume of pores smaller than 0.7 and 0.86 nm determined using the CO2 adsorption data at 0 °C. dDNLDFT is the maximum value of the PSD.

Table 1 displays the textural properties of the prepared porous carbons. The surface area and total pore volume significantly increased from 169.1 m2·g-1 and 0.11 cm3·g-1 to 1430.0 m2·g-1 and 0.59 cm3·g-1, respectively, when the activation temperature ramped from 500 to 700 °C. In particular, the volumes of the micropores with openings smaller than 0.7 and 0.86 nm increased from 0.1 to 0.13 and 0.17 cm3⋅g-1 respectively (Table 1). Regardless of this, a further increase in the activation temperature to 800 °C led to a considerable reduction in the micropores but an enhancement in the mesopores due to severe KOH etching.39 The yield of the porous carbons, as expected, gradually decreased from 63% to 30% with increasing activation temperature from 500 to 800 °C (Table 1). This arises from the reaction of less graphitic carbon and KOH (eqs. (1) and (2)).40 6KOH + 2C → 2K + 3H2 + 2K2CO3

(1)

K2CO3 → K2O + CO2, T ≥ 700 °C

(2)

Developed pores in the carbon matrix are formed through the redox reaction between potassium compound and carbon,41 the gasification with generation of H2O and CO2 and the metallic K intercalation causing expansion of carbon lattices. The chemical compositions of porous carbons prepared at different activation temperatures with the KOH/PUF-400 ratio kept at 2 are presented in Table 2. The nitrogen and hydrogen contents steadily decreased with increasing activation temperature owing to the formation of gaseous nitrogen/hydrogen-containing compounds.42 Table 2. Chemical compositions of porous carbons prepared at different activation temperatures with the KOH/PUF-400 ratio of 2.


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Samples

Chemical compositions (wt.%)

N/Ca

H/Ca

N

C

H

Other

PUF-KOH-2-700

2.06

41.68

2.28

53.98

0.042

0.652

PUF-400-KOH-2-500

9.03

77.79

3.25

9.93

0.100

0.498

PUF-400-KOH-2-600

8.71

76.31

2.91

12.07

0.098

0.454

PUF-400-KOH-2-700

6.92

81.74

2.22

10.69

0.073

0.324

PUF-400-KOH-2-800

4.22

81.76

1.06

12.96

0.044

0.154

a

molar ratios

3.4.2. CO2 adsorption properties

4


3 2 1 0 0.0

0.2

0.4

0.6

(b)

-1

-1

(a)

Adsorbed CO 2/(mmol·g )

5 Adsorbed CO 2/(mmol· g )

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


0.8

1.0

6


4

2

0 0.0

0.2

Pressure/bar

0.4

0.6

0.8

1.0

Pressure/bar

Figure 6. CO2 adsorption isotherms at 25 (a) and 0 °C (b) of samples activated at different temperatures with the KOH/PUF ratio kept at 2. Figure 6 shows the CO2 adsorption isotherms at 0 and 25 °C of samples activated at different temperatures with the KOH/PUF ratio kept at 2. All the samples exhibit much higher CO2 uptake at 0 °C than at 25 °C (Table S2, Supporting information). This is due to the exothermic feature of CO2 adsorption. The CO2 adsorption capacities increased from 2.92 and 2.21 to 5.85 and 3.82 mmol·g-1 at 0 and 25 °C, respectively, when the activation temperature was increased from 500 to 700 oC. One may think that it is caused by the significant increase in the surface area and pore volume (Table 1). However, this cannot account for the CO2 adsorption capacity of PUF-400-


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KOH-2-800, which possesses larger surface area, pore volume and micropore volume than PUF400-KOH-2-700 but a lower CO2 uptake, being 4.36 and 2.44 mmol·g-1 at 0 and 25 °C respectively. Recently, it was suggested that CO2 adsorption capacity of porous carbons might depend on ultramicropore volume as the ultramicropores are beneficial to the CO2 molecule filling.38 Figure 7 shows the correlations of CO2 uptake with the volumes of micropores with opening smaller than 0.7 and 0.86 nm. Clearly, the CO2 adsorption capacity at 0 °C linearly increased with increasing volumes of both types of ultramicropores, but a more linear relationship was observed for the pore opening < 0.86 nm. This indicates that the micropores smaller than 0.86 nm all contribute to the high CO2 uptake. In contrast, the CO2 capture amount at 25 °C seems to be more correlated to the volume of pores < 0.7 nm (Figure 7c). The linear relationship between the CO2 uptake and the volume of micropores less than 0.86 or 0.7 nm can well account for the higher CO2 adsorption amount on the PUF-400-KOH-2-700 than on the PUF-400-KOH-2-800. The CO2 adsorption behavior on the porous carbons not only depends on the ultramicropore, but also related to the adsorption conditions. Figure 6 shows that PUF-400-KOH-2-500 exhibits higher CO2 uptake than PUF-400-KOH-2-800 below 0.8 bar at 25 °C but 0.3 bar at 0 °C. At high temperature and low pressure, small micropores contribute more to CO2 adsorption. With decreasing adsorption temperature and increasing adsorption pressure, large micropores and even mesopores also play a great role, as supported by the linear increase of the CO2 adsorption capacity at 50 bar on the porous carbon materials prepared from polar anthers with the total pore volume.43 This behavior is closely related to the pore filling mechanism of CO2 adsorption.38 The cumulative pore volume of the micropores < 0.7 nm is 0.10 cm3·g-1 for the PUF-400-KOH-2-500 in contrast to 0.07 cm3·g-1 for the PUF-400-KOH-2-800. Consequently, PUF-400-KOH-2-500 shows higher CO2 uptake at 25 °C below 0.8 bar, while PUF-400-KOH-2-800 exhibits higher


16

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CO2 adsorption capacity at 0 °C and > 0.3 bar as its cumulative pore volume of micropores smaller than 0.86 nm reaches 0.12 cm3·g-1, being larger than that (0.10 cm3·g-1) of PUF-400KOH-2-500 (Table 1). Generally, the N content in the porous carbons is considered to be another factor determining its CO2 uptake. However, it was found that the correlation coefficients between the CO2 uptakes at 0 and 25 oC and the N content (4 – 10 wt.%) were only 0.19 and 0.02 respectively (Figure 7e and 7f). Such low correlation coefficients suggest that the N content in the sample may not be the predominant factor in the experimental range. Indeed, the CO2 adsorption capacity of concentrated HCl-treated PUF-400-KOH-1-700 (designed as PUF-400-KOH-1-700-HCl) (Figure S7, Supporting information) shows a CO2 uptake of 3.43 mmol g-1 at 25 °C, which is 79.2% of that of PUF-400-KOH-1-700 (4.33 mmol g-1), revealing that only about 20% of its CO2 uptake is

P=1 bar T= 0 °C 2 R = 0.96

6.0

4.5

3.0

(b)

-1

Adsorbed CO2/(mmol·g )

(a)

-1

Adsorbed CO2/(mmol·g )

caused by the nitrogen species.

0.12

0.15

4.5

3.0 0.06

0.18 3

0.06

0.09

0.12

0.15 3

-1

Micropore Volume(

Polyurethane Foam-Based Ultramicroporous Carbons for CO2

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