Effects of compacting activated carbons on their volumetric CO2 adsorption performance

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Effects of compacting activated carbons on their volumetric CO2 adsorption performance Dawei Lia, , Yu Wanga, Xiaoxiao Zhanga, Jiaojiao Zhoua, Yonghong Yangb, Zongbo Zhangc, ⁎ Ling Weia, Yuanyu Tiana,d, , Xuebo Zhaoe ⁎

a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China Tangshan Polytechnic College, Tangshan 063299, China c College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao 266580, China d Research Center of Low Carbon Energy and Chemical Engineering, Shandong University of Science and Technology, Qingdao 266590, China e Institute of New Energy, China University of Petroleum (East China), Qingdao 266580, China b

ARTICLE INFO

ABSTRACT

Keywords: CO2 capture Activated carbon Biomass Volumetric performance Density

The practical application of activated carbons (ACs) in CO2 capture requires the materials to possess all-round high volumetric CO2 adsorption performance, but influences of compaction of ACs on the all-round volumetric adsorption performance are unclear. In this research, coconut shell-based and rice husk-based ACs were compacted to study the effects of compaction on ACs’ volumetric CO2 uptake, adsorption rate, and CO2/N2 selectivity. The compaction increased ACs' volume-based low-pressure CO2 uptake (by up to 53.8 wt%), atmospheric-pressure uptake (by up to 43.2%), CO2 adsorption rate, and CO2/N2 selectivity, simultaneously. The main reason was that compaction changed the volumetric volume of micropores in various ranges (< 0.7 nm, < 1nm, and < 2 nm). This change was because compaction increased tap density of ACs via reducing macropores in the range of 2 to 10 μm and increasing carbon skeletal density. Compaction slightly affected ACs' surface functional groups, gravimetric nanopore parameters, and gravimetric CO2 uptake. The research showed that compaction enhanced ACs' all-round volumetric CO2 adsorption performance; this result was highly appealing for ACs' practical application, but was not found in previous research.

1. Introduction The capture and sequestration of CO2 have received increased attention in recent years [1], due to the fact that the CO2 is regarded as the main culprit for global warming and ocean acidification [2]. The CO2 can be captured by absorption [3], membrane separation [4], and adsorption [1]. The adsorption of CO2 by using ACs is pretty promising, as the ACs feature tunable pore structure, low cost, high stability, and ease of regeneration. So far, many efforts have been made to prepare ACs for CO2 capture [1,2,5–11]. For example, Hu et al [12] synthesized ACs from phenolic resin by using NaNH2 as an activator; the obtained ACs exhibited a CO2 uptake of 4.64 mmol/g at 25 °C and 1 bar. For another, our group [13] produced ACs from chitosan char by using KOH as an activator, with the CO2 uptake of the AC reaching 1.86 mmol/g at 25 °C and 0.15 bar. To sum up, previous research show that ACs with encouraging gravimetric CO2 adsorption performance can be prepared via using various carbon precursors (biomass [1,7,14], polymers [6,15], etc) and

⁎

activators (KOH [5,16], NaNH2 [12,17,18], K2CO3 [19], NaOH [20], etc). However, only a little attention has been paid to improving the ACs’ volumetric CO2 adsorption performance. Actually, volumetric CO2 adsorption performance controls the efficiency, size, and capital outlay of adsorption bed [21,22], because ACs have to be placed in adsorption bed during application. Thus, it is imperative to enhance volumetric CO2 adsorption performance of ACs. So far, only a few research groups have made efforts to improve the volumetric adsorption performance [23–26]. They found that regulation of activation temperature [23], careful selection of feedstock [24], and compaction of feedstocks before activation [26] changed the volumetric CO2 adsorption capacity or CO2/N2 selectivity of ACs. Robert Mokaya [25] reported that compaction of potassium hydrogen phthalate-based ACs improved their CO2 uptake, but they did not explore the mechanism, the adsorption rate, and selectivity. So far, the effects of compaction on ACs’ volumetric CO2 uptake, CO2/N2 selectivity, and adsorption rate remain unknown. Given the above research context, this research aims to explore the

Corresponding authors at: State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China. E-mail addresses: [email protected] (D. Li), [email protected] (Y. Tian).

https://doi.org/10.1016/j.fuel.2019.116540 Received 22 August 2019; Received in revised form 2 October 2019; Accepted 29 October 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Dawei Li, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116540

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2. Experimental section

Table 1 Skeletal density and tap density of activated carbons. Sample

ρskeleton (g/cm3)

ρtap (g/cm3)

CSC-0 CSC-399 CSC-887 RHC-0 RHC-399 RHC-887

0.925 0.943 0.954 0.982 1.342 1.374

0.456 0.528 0.553 0.329 0.462 0.506

2.1. Raw materials Coconut shell char (CSC) and rice husk char (RHC) were separately made by carbonizing coconut shell and rice husk in a tubular furnace at 520 °C for 60 min in N2 flow of 400 cm3/min. The coconut shell or rice husk was directly transferred into the furnace heating zone already maintained at 520 °C, without a slow heating step. According to our previous research [24], CSC-based and RHC-based ACs displayed high volumetric and gravimetric CO2 uptake, respectively. Potassium hydroxide (AR) and hydrochloric acid (AR) were both bought from Sinopharm Chemical Reagent Limited Corporation. The used water during experiments was deionized water.

Table 2 Gravimetric pore parameters for activated carbons. Sample

SBET,m (m2/g)

Vmacro,m (cm3/g)

Vmic,m (cm3/g)

V < 1nm,m (cm3/cm3)

V < 0.7nm,m (cm3/g)

Vt,m (cm3/ g)

CSC-0 CSC-399 CSC-887 RHC-0 RHC-399 RHC-887

1258 1324 1370 1217 1172 1171

1.141 0.890 0.880 1.471 1.445 1.394

0.571 0.595 0.608 0.416 0.406 0.402

0.195 0.196 0.201 0.156 0.165 0.161

0.129 0.130 0.134 0.116 0.120 0.116

1.141 0.890 0.880 1.471 1.445 1.394

2.2. Preparation of ACs Two activated carbons, namely CSC-0 and RHC-0, were prepared from CSC and RHC without compaction, respectively. The number 0 in CSC-0 and RHC-0 meant no use of compaction. The detailed preparation method for the two ACs was the same as previously reported [24], except that the KOH activation products were treated with HCl solution before washing with water. In brief, CSC or RHC was physically mixed with powdered KOH (particle size < 0.2 mm) at char/KOH weight ratio of 1:1.5. The mixture was then heated at 650 °C for 90 min in N2 flow. Afterwards, the mixture was treated with 1 mol/L HCl solution, washed with distilled water until the elute was neutral, and dried at 110 °C for 2 h.

effects of compaction on all-round volumetric CO2 adsorption performance of ACs. The effects were discussed on basis of ACs’ density, fullscale pore structure, surface functional groups, and gravimetric CO2 adsorption performance. It was found that compaction of ACs increased their volumetric CO2 uptake, CO2/N2 selectivity, and adsorption rate, simultaneously, and could act as an effective method for increasing ACs’ all-round volumetric CO2 adsorption performance. The research results were considered appealing for practical application of ACs in CO2 separation or storage.

About 0.5 g CSC-0 or RHC-0 was placed in a die with an inner diameter of 1.3 cm. Then, the AC was compacted at predetermined 500 N 2 adsorption capacity(cm3/g STP)

(a)

300

200 RHC-0 RHC-399 RHC-887

100

0.0

0.2

0.4 0.6 Relative pressure(P/P0)

Differential pore volume (cm3/g)

1.6 (c)

0.8

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.5

1.0

1.5 2.0 2.5 Pore width(nm)

300 CSC-0 CSC-399 CSC-887

200 100

0.0

0.2

0.4 0.6 Relative pressure(P/P0)

(d)

RHC-0 RHC-399 RHC-887

1.4

(b)

400

0

1.0

Differential pore volume (cm3/g)

N2 adsorption capacity(cm3/g STP)

400

2.3. Posttreatment of ACs by compaction

46 48 50

0.8

1.0

CSC-0 CSC-399 CSC-887

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0 2.5 Pore width(nm)

46 48 50

Fig. 1. (a, b) N2 adsorption-desorption isotherms at −196 °C and (c, d) DFT-derived pore size distributions for activated carbons. 2

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Table 3 Volumetric pore parameters of activated carbons. SBET,

CSC-0 CSC-399 CSC-887 RHC-0 RHC-399 RHC-887

573 699 758 400 541 593

2.5

v

(m2/cm3)

Vt,

v

(cm3/cm3)

0.280 0.345 0.377 0.189 0.246 0.269

Vmic,

v

(cm3/cm3)

0.260 0.314 0.336 0.137 0.188 0.203

V < 0.7nm,

v

(cm3/cm3)

0.059 0.069 0.074 0.038 0.055 0.059

V < 1nm,

v

(cm3/cm3)

0.089 0.103 0.111 0.051 0.076 0.081

V1-2nm,

v

(cm3/cm3)

0.171 0.211 0.225 0.086 0.111 0.122

(a)

2.0 1.5 1.0

0.520 0.470 0.487 0.484 0.668 0.705

0.226 0.228 0.220 0.279 0.296 0.289

CSC-399

CSC-887 3430

0.1

1 Pore diameter(μm)

10

100

4000

3500

2919

3000

1625

2500 2000 1500 Wavenumbers (cm-1)

4 (b)

3

1024

1000

500

(b)

CSC-0 CSC-399 CSC-887

RHC-0

Transmittance

Differential pore volume(cm 3/g)

V < 0.7nm, v/Vmic,v

CSC-0

0.5

0.01

Vmacro,v (cm3/cm3)

(a) RHC-0 RHC-399 RHC-887

Transmittance

Differential pore volume(cm 3/g)

Sample

2

1

RHC-399

RHC-887

0 0.01

0.1

1 10 Pore diameter(μm)

3445

100

4000

3500

1629

2945

3000

2500

2000

1500

1102

1000

500

Wavenumbers (cm-1)

Fig. 2. Macropore distribution curves of (a) RHC-based and (b) CSC-based activated carbons.

Fig. 3. FTIR spectra of (a) CSC-based and (b) RHC-based activated carbon before and after compaction.

pressures (399 or 887 Mpa) for 10 min. This process was similar to that of previous researchers [27,28]. The resulting sample was recorded as CSC-x or RHC-x, where x (=399 or 887) represented the compaction pressure. For example, CSC-399 referred to the AC obtained by compacting CSC-0 at 399 Mpa for 10 min.

2460, Micromeritics Corp., USA) after the samples were degassed under vacuum at 300 °C for 4 h. The mass-based specific surface area (SBET,m), total pore volume (Vt,m), micropore volume (Vmicro,m), and pore size distribution curve above 1 nm were obtained using the N2 adsorption isotherms at −196 °C, with the detailed methods described previously [24]. The mass-based ultramicropore volume (V < 0.7nm,m) and fine micropore volume (V < 1nm,m) were obtained using the CO2 adsorption isotherms at 0 °C along with density function theory (DFT). The pore size distribution over the range of 0.5 to 50 nm was obtained by applying DFT to the N2 adsorption isotherms at −196 °C. The ACs' volume-based pore parameters, such as volumetric surface area (SBET,v), total pore volume (Vt,v), micropore volume (V < 2nm,v), fine micropore volume (V < 1nm,v), and ultramicropore volume (V < 0.7nm,v), were obtained by multiplying their corresponding gravimetric pore parameters with tap density. The skeletal density (ρskeleton), macropore volume (Vmacro,m), and macropore size distribution (from 3 nm to 171 μm) of ACs were

2.4. Characterization Tap density (ρtap) of ACs was determined as previously reported [23]. In brief, an AC with particle sizes of 50–74 μm was placed in a measuring cylinder which was shaken manually till the sample volume was constant for 2 min. The ratio of sample mass to this volume was recorded as tap density. The error for the reported tap density was ± 0.005 g/cm3. The use of particles with narrow particle distribution (50–74 μm) was important for decreasing the measurement error for the tap density. The adsorption or desorption isotherms for N2 or CO2 at −196, 0, 25, or 50 °C of the compacted samples and their uncompacted counterparts were collected on a volumetric adsorption analyzer (ASAP 3

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Isostetric heat of adsorption (kJ/mol)

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the partial pressure of CO2 was 0.15 bar. In other words, the selectivity was calculated from (QCO2,0.15,v/QN2,0.85,v) × 0.85/0.15, where QCO2,0.15,v and QN2,0.85,v referred to the amount of CO2 and N2 adsorbed at 0.15 bar and 0.85 bar by per volume of AC, respectively. The isosteric CO2 adsorption heats were calculated from the CO2 adsorption isotherms at 0, 25, and 50 °C (Fig. S1 in Supporting Information) by applying the Clausius-Clapeyron equation, as described previously [24]. The dynamic CO2 adsorption performance of ACs was investigated in a thermogravimetric analyzer (TGA Q500, TA Corp, USA). During investigation, about 20 mg of AC was placed in a crucible, degassed in N2 flow at 220 °C for 1 h, and then cooled to 50 °C. After elapse of 2 min, the gas was changed to CO2 for adsorption. The CO2 uptake was calculated by using the sample mass at the end of the degassing step as the reference mass during the whole experiment. The CO2 dynamic adsorption curves were fitted to the Lagergren’s first order kinetic model (Eq. (1)), pseudo second order model (Eq. (2)), and intra-particle diffusion model (Eq. (3)). The models are described as:

(a)

30 24 RHC-0 RHC-399 RHC-887

18 12 6 0 0.0

0.5

1.0 1.5 CO2 loading (mmol/g)

2.0

ln(q e-qt ) =lnq e-k1t

(1)

t 1 t = + qt k2q 2e q e

(2)

qt = k p ×

1 t2

+C

(3)

where k1 and k2 are the rate constants of pseudo-first-order model (min−1) and pseudo-second-order model (g·mg−1·min−1), respectively, kp (mg·g−1·h−0.5) is intra-particle diffusion coefficient, and qe and qt are the CO2 uptakes at adsorption equilibrium and time t., respectively. 3. Results and discussion 3.1. Effects of compaction on densities

Fig. 4. Isosteric adsorption heats of (a) RHC-based and (b) CSC-based activated carbons.

The tap density (ρtap) of ACs was presented in Table 1. For the CSCderived ACs, the compacted samples (CSC-399 and CSC-887) displayed a higher ρtap than the uncompacted sample (CSC-0), and the AC compacted at a high pressure (CSC-887) showed a larger ρtap than the one compacted at a low pressure (CSC-399). Similar observation was also found for the RHC-derived ACs. The observations indicated that compaction increased the tap density of biomass-derived ACs. The result was explained from two aspects. Firstly, as shown in Table 1, the skeletal density (ρskeleton) of the CSC-based ACs increased in the order of CSC-0 < CSC-399 < CSC-887. This order suggested that the skeletal density of ACs tended to increase after compaction. Similar increment trend was shown by the RHC-based ACs. Secondly, as shown in Table 2, the macropore volume (Vmacro,m) of the RHC-based ACs decreased in the order of RHC-0 > RHC-399 > RHC-887. This order showed that compaction of the ACs tended to reduce the macropore volume of ACs. The reduction was attributable to macropore collapse as a result of compaction, which would be further discussed in Section 3.3. In summary, the tap density of ACs increased after compaction, as the ACs' skeletal density increased and their macropore volume decreased.

Table 4 Gravimetric CO2 uptake, volumetric CO2 uptake, and volumetric CO2/N2 selectivity of ACs at 0 °C. Sample

CSC-0 CSC-399 CSC-887 RHC-0 RHC-399 RHC-887

Gravimetric CO2 uptake (mmol/g)

Volumetric CO2 uptake (mmol/cm3)

0.15 bar

1 bar

0.15 bar

1 bar

2.21 2.21 2.28 1.96 2.02 1.97

6.95 6.89 7.05 5.63 5.33 5.23

1.01 1.16 1.26 0.65 0.93 1.00

3.17 3.64 3.90 1.85 2.46 2.65

Volumetric CO2/ N2 selectivity

20.17 23.65 22.90 14.22 26.95 23.21

determined by mercury intrusion porosimetry (PoreMaster60, Quantachrome Instruments, USA). The volume of intruded mercury was considered as a function of pressure. The mercury contact angle on the microparticles and the mercury surface tension were 130° and 4850 N/ m, respectively. To investigate the surface functional groups of AC, the AC was mixed with KBr at a weight ratio of about 1/100, ground, and pressed to form a pellet for collecting spectra using a Fourier Transform Infrared Spectroscopy (FTIR, TENSOR II, Bruker Corp.) in a wavenumber range of 400–4000 cm−1 with a resolution of 4 cm−1.

3.2. Effects of compaction on nanometer-sized pores The N2 adsorption/desorption isotherms of ACs were presented in Fig. 1a and 1b. The isotherms of the ACs before and after compaction all belonged to typeⅠisotherms, which featured a horizontal curve over a wide relative pressure range. Thus, all the samples were microporous, which was also shown by the pore size distribution curves in Fig. 1c and 1d. The micropore parameters on mass basis were listed in Table 2. The V < 0.7nm,m, V < 1nm,m, and Vmicro,m of CSC-887 were higher than the corresponding value of CSC-0 by only 3.9%, 3.1%, and 6.5%, respectively, which indicated that the effects of compaction on ACs’ massbased micropore volume were small. Similar results were shown by the

2.5. Adsorption performance analysis The CO2/N2 adsorption selectivity was obtained via applying ideal adsorption solution theory to the CO2 and N2 adsorption isotherms at 0 °C, assuming that the total pressure of CO2-N2 mixture was 1 bar and 4

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(a)

5 4

RHC-0 RHC-399 RHC-887

3 2 1 0

Volumetric CO2 uptake at 0 o C ( mmol/cm3 )

Gravimetric CO 2 uptake at 0o C ( mmol/g )

6

0.2

0.4

0.6

Pressure (bar)

0.8

1.0

Volumetric CO2 uptake at 0 o C ( mmol/cm3 )

Gravimetric CO 2 uptake at 0o C ( mmol/g )

D. Li, et al.

3 (c)

2

RHC-0 RHC-399 RHC-887

1

0

0.2

0.4

0.6

Pressure (bar)

0.8

1.0

8 7

(b)

CSC-0 CSC-399 CSC-887

6 5 4 3 2 1 0

4

0.2

(d)

0.4

0.6

Pressure (bar)

0.8

1.0

CSC-0 CSC-399 CSC-887

3

2

1

0

0.2

0.4

0.6

Pressure (bar)

0.8

1.0

Fig. 5. (a,b) Gravimetric and (c,d) volumetric CO2 adsorption isotherms of rice husk-based and coconut shell-based activated carbons.

RHC-based ACs. Table 3 shows the micropore parameters of ACs on volume basis. The compacted CSC-based ACs showed higher V < 0.7nm,v, V < 1nm,v, and Vmicro,v than the uncompacted sample (CSC-0), and the higher the compaction pressure was, the larger the pore parameters were. This result demonstrated that compaction increased volumetric volume of micropores in the range of < 0.7 nm, < 1 nm, or < 2 nm. Similar increment role was shown by the RHC-based ACs (Table 3). The reason was understandable. The volumetric micropore parameters were the product of tap density and the corresponding gravimetric micropore parameters. The gravimetric micropore parameters were slightly affected by compaction, as already discussed in previous paragraph, but the tap density was increased obviously, as shown in Table 1. Consequently, the compaction increased the volumetric micropore parameters of ACs.

result was shown by the RHC-based ACs.

3.3. Effects of compaction on macropores

The CO2 adsorption heats for ACs before or after compaction were in the range of 24 to 36 kJ/mol (Fig. 4). These adsorption heats were much lower than those for chemisorption (60–90 kJ/mol [31]), and lay in the typical range for physisorption process [32]. Thus, the CO2 molecules were physically adsorbed onto the surface, which agreed with the above statement that the ACs’ different volumetric CO2 adsorption performance was not due to their surface chemistry. In other words, the different volumetric CO2 adsorption performance was probably governed by different pore structure, which was discussed below. Table 4 shows the volumetric CO2 uptake of ACs at 0.15 bar. As the compaction pressure increased from 0 to 887 Mpa, the volumetric lowpressure CO2 uptake of CSC-based and RHC-based ACs both increased; this trend can also be found from the volumetric CO2 adsorption

3.4. Effects compaction on surface functional groups The FTIR spectra of the obtained ACs were presented in Fig. 3. The adsorption bands around 3440, 2900, 1620, and 1100 cm−1 were ascribable to the stretching of O–H, C–H, C]O, and C-O, respectively [29,30]. The FTIR spectra of the ACs in Fig. 3a were similar, and so were the spectra shown in Fig. 3b. These results indicated that the compaction of ACs did not affect their surface functional groups. Thus, the different volumetric CO2 adsorption performance of the compacted and uncompacted samples, which would be shown below, should not be owing to their difference in surface chemistry. 3.5. Effects of compaction on volumetric and gravimetric CO2 uptake

The macropore volume (Vmacro,m) of ACs was listed in Table 2. Whether for the CSC-based or RHC-based ACs, the compacted samples exhibited smaller Vmacro,m than the corresponding uncompacted sample. This feature was particularly obvious for the samples compacted at high pressures (CSC-887 or RHC-887). The result indicated that compaction of ACs decreased their macropore volume. To gain more information, we obtained the macropore size distribution curves of ACs, as presented in Fig. 2. The macropore distribution curves of CSC-887 and CSC-399 were lower than the curve of CSC-0 in the range of 2–10 μm, but were higher than the latter in the range of 0.1–2 μm. This result meant that the large macropores with sizes of 2–10 μm collapsed into small ones (0.1–2 μm) as a result of compaction. Similar 5

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Volumetric CO2 uptake at 1 bar (mmol/cm 3)

(a) 1.2

R2=0.999

1.0

0.8

0.6

0.04

0.05

0.06

0.07

V<0.7nm,v(cm3/cm3)

(b) 1.2

R2=0.981 1.0

0.8

0.6 0.05

0.06

0.07

0.08

V <1nm,v

0.09

(cm3/cm3)

3.78

(a)

2

R =0.900

3.15

2.52

1.89

0.04

Volumetric CO2 uptake at 1 bar (mmol/cm 3)

Volumetric CO2 uptake at 0.15 bar (mmol/cm3)

Volumetric CO2 uptake at 0.15 bar (mmol/cm3)

D. Li, et al.

0.10

0.11

Fig. 6. Correlation of volumetric CO2 uptake at 0.15 bar (0 °C) with (a) volumetric volume of ultramicropores (< 0.7 nm) and (b) that of fine micropores (< 1 nm).

3.78

0.05

3

0.06

0.07

3

V<0.7nm,v(cm /cm )

(b) 2

R =0.969 3.15

2.52

1.89

0.05

0.06

0.07

0.08 0.09 3 3 V<1nm,v(cm /cm )

0.10

0.11

Fig. 7. Correlation of volumetric CO2 uptake at 1 bar (0 °C) with volumetric volume of pores below (a) 0.7 nm and (b) 1 nm.

controlled the volumetric CO2 uptake at 1 bar. The compacted samples had high V < 1nm,v (Table 3), thus showing high volumetric CO2 uptake at 1 bar. The gravimetric CO2 uptake was also listed in Table 4. The gravimetric CO2 uptake at 0.15 bar (or 1 bar) of the compacted ACs was similar to that of their uncompacted counterpart. For example, CSC-887 resembled CSC-0 in gravimetric CO2 uptake at 0.15 bar (or 1 bar). The observations indicated that compaction slightly affected gravimetric CO2 uptake. The reason was understandable. The gravimetric CO2 uptakes at 0.15 bar and 1 bar were determined by micropores with sizes of < 0.7 nm [33,36] and < 1 nm [37,38], respectively; these micropores were only slightly affected by compaction, as already discussed in Section 3.2.

isotherms in Fig. 5c and Fig. 5d. More specifically, the volumetric CO2 uptake at 0.15 bar for CSC-887 and RHC-887 increased by 24.8% and 53.8%, respectively, compared with the value for their uncompacted counterparts (CSC-0 and RHC-0). The result indicated that compaction effectively enhanced volumetric low-pressure CO2 uptake of biomassbased ACs. To explore the reason, we correlated the uptake of the six ACs with their V < 0.7nm,v and V < 1nm,v, with the correlation results presented in Fig. 6. The adjusted determination coefficient (R2) for the correlation of the volumetric CO2 uptake at 0.15 bar with V < 0.7nm,v reached 0.999. The result meant that volumetric ultramicropore volume of ACs determined the volumetric CO2 uptake at 0.15 bar, which well agreed with previous observation [33]. In such narrow pores, the potential fields from neighboring walls can overlap, thus endowing the narrow pores with enhanced interaction energy [34]. In general, the adsorption potential of the narrow pores increased with the decrease in their pore size [35]. Table 4 also shows the volumetric CO2 uptake of ACs at 1 bar. Whether for the CSC-based or the RHC-based ACs, increasing the compaction pressure from 0 to 887 Mpa enhanced the CO2 volumetric uptake of ACs at 1 bar, which can also be seen from the volumetric CO2 adsorption isotherms in Fig. 5c and 5d. More specifically, the volumetric CO2 uptake at 1 bar for CSC-887 and RHC-887 increased by 23% and 43.2%, respectively, compared with that for their corresponding counterpart. To explain the trend, the uptake at 1 bar was correlated with volumetric ultramicropore volume and fine micropore volume (Fig. 7). The R2 for the correlation of volumetric CO2 uptake at 1 bar with the volumetric fine micropore volume (V < 1nm,v) was as high as 0.969, which indicated that the volumetric volume of pores below 1 nm

3.6. Effects of compaction on volumetric CO2 adsorption rate The CO2 dynamic adsorption curves of ACs at 50 °C were collected by TG instruments. The CO2 uptake measured by TG was close to that measured by volumetric analyzer (Table S1 in Supporting Information), which suggested the reliability of the used TG method. Fig. 8 shows the CO2 dynamic adsorption curves of ACs at 50 °C. The CO2 adsorption rate of ACs can be evaluated by the period required for achieving the same specified capacity, as shown by the dashed line in Fig. 8a and 8b. The shorter the period is, the higher the CO2 adsorption rate is. Accordingly, it was found that the volumetric CO2 adsorption rate of CSCbased ACs increased in the order of CSC-0 < CSC-399 < CSC-887, and that of RHC-based ACs rose in the sequence of RHC-0 < RHC399 < RHC-887. These facts indicated that compaction increased the 6

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Volumetric CO2 uptake (mmol/cm3 )

D. Li, et al.

1.0

compacted ACs had more large pores than their uncompacted counterparts in unit volume, as shown by their higher V1-2nm,v and Vmacro,v (Table 3). Thus, the diffusion of CO2 in the compacted samples was fast. As a result, the unoccupied pores effective for CO2 adsorption determined the CO2 adsorption process.

(a)

0.8 0.6

3.7. Effects of compaction on volumetric CO2/N2 selectivity

QRHC-0,v×90% 0.4

Table 4 shows the CO2/N2 selectivity of ACs obtained by ideal adsorption solution theory. Two interesting features were noticed. Firstly, the volumetric CO2/N2 selectivity for the compacted ACs was higher than that of uncompacted ACs. This feature was particularly obvious for the RHC-based ACs. The feature indicated that compaction increased ACs' volumetric CO2/N2 selectivity. Secondly, the ACs compacted at low pressure (RHC-399 or CSC-399) showed higher volumetric CO2/N2 selectivity than those compacted at high pressure (RHC-887 or CSC887). The two features were understandable. The volumetric CO2/N2 selectivity was governed by the volumetric ultramicropore/micropore ratio (V < 0.7nm,v/Vmicro,v) and volumetric ultramicropore volume (V < 0.7nm,v) [23,24], as a narrow micropore size distribution and a large number of ultramicropores were favorable for overlapping of adsorption potentials and promotion of CO2 adsorption [23]. Thus, the sample with a high V < 0.7nm,v/Vmicro,v and V < 0.7nm,v showed high volumetric CO2/N2 selectivity.

RHC-0 RHC-399 RHC-887

0.2

0

1

2 3 Time (min)

4

5

Volumetric CO2 uptake (mmol/cm3)

(b)

1.5

1.0 QCSC-0,v×90%

0.5

4. Conclusions

CSC-0 CSC-399 CSC-887

0

1

2

3 Time (min)

4

(1) Compaction significantly increased ACs' volumetric volume of micropores in the range of < 0.7 nm, < 1 nm, and < 2 nm, mainly because their tap density was increased. The rise in tap density was because compaction reduced the macropores in the range of 2 to 10 μm and increased skeletal density. (2) Compaction of ACs significantly increased volumetric CO2 uptake at low and atmospheric pressure, which was ascribable to the enhanced volumetric volume of pores below 0.7 nm and 1 nm, respectively. The compaction-induced increase in volumetric CO2 uptake at low and atmospheric pressure reached 53.8% and 43.2%, respectively. Besides, the compaction increased the volumetric CO2 adsorption rate and CO2/N2 selectivity, which was ascribable to the increased volumetric volume of micropores and/or ultramicropores of ACs. (3) Compaction of ACs slightly affected surface functional groups, gravimetric nanopore parameters, and gravimetric CO2 uptake of ACs. (4) Compaction of ACs was an effective method for enhancing ACs' volumetric CO2 uptake, adsorption rate, and CO2/N2 selectivity, simultaneously.

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Fig. 8. Volumetric CO2 kinetic adsorption curves at 50 °C for (a) RHC-based and (b) CSC-based activated carbons. QRHC-0,v and QCSC-0,v referred to the volumetric equilibrium CO2 uptake for RHC-0 and CSC-0 at 50 °C, respectively. The volumetric equilibrium CO2 uptake was the product of gravimetric equilibrium CO2 uptake and tap density.

volumetric CO2 adsorption rate of ACs. More specifically, the adsorption time required for CO2 uptake to reach 0.9*QRHC-0,v for RHC-0 was 0.6 min, whereas that for RHC-887 was only 0.26 min (Fig. 8a). Namely, the adsorption time was reduced by more than 50% after compaction. Similar result was shown by CSC-0 and CSC-887. The increased adsorption rate was understandable. The adsorption rate was governed by volumetric volume of pores in the range of < 1 nm and those in the 1 to 2 nm range [24]. The former pores can offer high adsorption potentials whereas the latter can facilitate the diffusion of CO2 molecules into narrow micropores and even provide adsorption potential[24]. The compacted ACs had higher V < 1nm,v and V < 1-2nm,v than their uncompacted counterparts; namely, the compacted ACs possessed higher Vmicro,v, thus showing superior volumetric CO2 adsorption rate. The high Vmicro,v of the compacted ACs was due to the fact the compaction increased ACs’ tap density without decreasing their mass-based micropore parameters, as already discussed in Section 3.1 and 3.2 To gain further insight into the adsorption process, the CO2 dynamic adsorption curves were fitted to the Lagergren first-order kinetic model, pseudo-second-order model, and intraparticle diffusion model. The fitting results (Fig. S2 and Table S2 in Supporting Information) showed that the Lagergren first-order kinetic model gave the best fit of the kinetic adsorption curves (R2 = 0.999). Therefore, the unoccupied pores effective for CO2 adsorption governed the CO2 adsorption process. This was understandable, as the CO2 molecules have to diffuse from the external surface of ACs into the internal pores before adsorption. The

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the Fundamental Research Funds for the Central Universities (18CX02121A, 19CX02020A, and 16CX05012A), Shandong Natural Science Foundation (ZR2017QEE006), Key Technology Research and Development Program of Shandong (2018GGX103034), Shandong Major R&D plan (2018GGX103034), National Natural Science Foundation of China (21576294, 21576293, and 21604094), and Special Project Fund of Taishan-Scholars of Shandong Province (ts201511017). 7

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Appendix A. Supplementary data

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