Tuning porosity of coal-derived activated carbons for CO₂ adsorption

Abstract

A simple method was developed to tune the porosity of coal-derived activated carbons, which provided a model adsorbent system to investigate the volumetric CO₂ adsorption performance. Specifically, the method involved the variation of the activation temperature in a K₂CO₃ induced chemical activation process which could yield activated carbons with defined microporous (< 2 nm, including ultra-microporous < 1 nm) and meso-micro-porous structures. CO₂ adsorption isotherms revealed that the microporous activated carbon has the highest measured CO₂ adsorption capacity (6.0 mmol∙g^–1 at 0 °C and 4.1 mmol∙g^–1 at 25 °C), whilst ultra-microporous activated carbon with a high packing density exhibited the highest normalized capacity with respect to packing volume (1.8 mmol∙cm⁻³ at 0 °C and 1.3 mmol∙cm^–3 at 25 °C), which is significant. Both experimental correlation analysis and molecular dynamics simulation demonstrated that (i) volumetric CO₂ adsorption capacity is directly proportional to the ultra-micropore volume, and (ii) an increase in micropore sizes is beneficial to improve the volumetric capacity, but may lead a low CO₂ adsorption density and thus low pore space utilization efficiency. The adsorption experiments on the activated carbons established the criterion for designing CO₂ adsorbents with high volumetric adsorption capacity.

Key words： coal-derived activated carbons; porosity; CO₂ adsorption; molecular dynamics

1 Introduction

Serious environmental problems (such as acid rain, haze and greenhouse gases) have arisen due to the widespread use of fossil fuels. Especially, the sharp rise of concentration of CO₂ from ca. 325 ppm (1967) to 409.8 ppm (2019) has aroused wide public concern [1,2]. Nowadays, several chemical absorption and/or physical adsorption technologies (based on membranes [3,4], alkanol amine solvents [5], ionic liquids [6] and solid adsorbents [7,8]) have been used for CO₂ capture and storage. Among them, CO₂ adsorption by solid adsorbents is considered one of the most promising technologies due to low energy requirement, good stability and relatively wide operation ranges of temperatures and pressures. Solid adsorbents mainly include carbon-based materials [9,10] zeolites [11], metal-organic frameworks [12] and amine functionalized/supported adsorbents [13]. Among them, carbon-based materials have been widely explored and applied in CO₂ capture due to their favorable properties of high adsorption capacity, superior cycle stability, adjustable pore structures and, more importantly, reasonable costs. For example, coal/biomass-based activated carbons (ACs) are common adsorbents which are derived from low-cost raw materials and can be produced at scales. Among various approaches for preparing ACs, chemical activation using KOH [14], K₂CO₃ [15], H₃PO₄ [16] and NaOH [17] is commonly employed, as well as being practical for applications at scales. Especially, K₂CO₃ is more environmentally friendly compared to hazardous KOH and NaOH, but being comparable regarding its performance for activation. Yue et al. [18] investigated the activation of coconut shell using K₂CO₃, and the optimized microporous sample showed a high CO₂ capacity of 3.71 mmol∙g^‒1 at 25 °C and 5.12 mmol∙g^‒1 at 0 °C. Kim et al. [19] prepared a series of coffee ground-based microporous carbons using K₂CO₃ activation, which also showed good CO₂ uptakes, proving that K₂CO₃ activation was capable of producing high-performance CO₂ adsorbents.

Textural and surface chemical properties of ACs are key to determine their performance for CO₂ capture. In this regard, continuous efforts have been made to tune the pore structure and surface functionality of ACs to improve their CO₂ adsorption performance. It is generally agreed that CO₂ adsorption on ACs is based on a micropore filling mechanism, and the CO₂ adsorption capability of ACs is mostly related to narrow micropore structure [20]. Previous studies have demonstrated micropores with sizes smaller than 1 nm, i.e., ultra-micropores, are mainly responsible for CO₂ uptake at ambient pressure [21,22]. Additionally, chemical environment of ACs is also an important factor of affecting CO₂ adsorption, which can induce chemisorption, hydrogen bonding, and/or electrostatic interactions between the CO₂ molecule and surface functional groups [23,24]. The porosity of ACs can be effectively adjusted by carbon precursors [25] and activation methods [26], as well as the conditions of the method [27]. Based on pore adjustment and surface modification, CO₂ adsorption capabilities of the reported ACs have been significantly increased, and some of the reported ACs with the appropriate pores and active sites could achieve the CO₂ adsorption capacity of > 7 mmol∙g ^–1 at 0 °C and 1 bar [28]. However, it is worth mentioning that the majority of previous studies focused mainly on improving the CO₂ gravimetric capacitive performance of porous carbons with the low packing density, which could cause the insufficient use of pores in ACs. Conversely, in comparison with the gravimetric performance, CO₂ adsorption capability per volume of porous carbons may provide a relatively realistic indication of the performance. Besides, the volumetric CO₂ adsorption capability of porous carbons is also an important indicator to determine the scale and efficiency of adsorption systems [29–31], which, however, was commonly overlooked in most of the previous studies [20,32]. To achieve a high volumetric CO₂ adsorption capacity, porosity regulation of carbon materials is the prerequisite since pore configuration and size greatly correspond to the packing density of porous carbons [33]. Regardless gravimetric and volumetric adsorption, it is very necessary to explore the critical factors, which determine volumetric and/or gravimetric capability of porous carbons, and hence devise controllable methods for porosity regulation towards high-performance CO₂ adsorbents.

Here, we report a simple chemical activation method using K₂CO₃ as the activating agent to prepare coal-derived ACs. By simply varying the activation temperature (in a range of 600–900 °C), the porosity of the resulting ACs can be tailored. Specifically, the low-temperature (at 600 °C) activation produced the AC with mainly ultra-micropores (< 1 nm), whilst an increase in the activation temperature can increase the pore sizes. The series ACs obtained can be used as a model adsorbent system for investigating the correlation between pore parameters and volumetric CO₂ adsorption characteristics. Based on the combined experimental study and molecular dynamics simulation, the correlation between the CO₂ adsorption capacity and pore characteristics of the ACs is proposed.

2 Experimental

2.1 Preparation of ACs

To prepare the ACs, a low-rank coal from Xinjiang province (China) was used as the carbon source, and the proximate, ultimate analyses, ash compositions of the coal are presented in Tables S1 and S2 (cf. Electronic Supplementary Material, ESM). The activation of the coal precursor towards ACs is illustrated in Fig.1. In detail, coal particles (< 1 mm) were first washed using acids (mixture of 5 mol·L⁻¹ hydrochloric acid and 20 wt% hydrofluoric acid) to remove the minerals. To prepare ACs, the acid-washed coal particles were added into 0.2 g∙mL^–1 K₂CO₃ solution with a mass ratio of 1:3 to form the coal-water slurry under magnetic stirring. The mixture was then evaporated at 100 °C to obtain the dry solid and further carbonized at the setting temperature for 1 h under nitrogen atmosphere (200 mL∙min^–1) at a heating rate of 5 °C∙min^–1 in a horizontal tube furnace, as shown in Fig. S1 (cf. ESM). After activation, the sample was washed using 2 mol·L⁻¹ hydrochloric acid and deionized water (three times) sequentially, and then dried at 80 °C for 12 h to obtain the ACs. The resulting ACs were denoted as AC-600, AC-700, AC-800 and AC-900 for the activation temperatures of 600, 700, 800, 900 °C, respectively.

Fig.1 Schematic K₂CO₃ activation of the coal precursor to prepare ACs, and the associated pore formation process in the ACs.

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2.2 Structural characterization and CO₂ adsorption

The morphology, microstructure and element composition of the obtained ACs were characterized by scanning electron microscopy (SEM, Helios Nanolab600i) and transmission electron microscopy (TEM, JEM-2100, at 200 kV). Porous properties of the ACs including specific surface area, pore volume and pore size distribution were measured by N₂ adsorption/desorption analysis (Micromeritics ASAP 2020 adsorption analyzer) at liquid nitrogen temperature (–196 °C). Before the test, ACs samples were degassed at 160 °C for 8 h. Specifically, the Brunauer–Emmett–Teller (BET) theory was used to determine the specific surface area (S_BET); the total pore volume (V_total) was determined using the amount adsorbed at relative pressure (p/p⁰) of 0.975; the ultra-micropore (≤ 1 nm) volume (V_{(≤1 nm)}) are calculated using the density functional theory (DFT ) model, i.e., the accumulated pore volume with pore size less than 1 nm [34]. X-ray diffraction (XRD) patterns of materials were collected using a Rigaku D/Max 2400 diffractometer with CuKa radiation (40 kV, 40 mA, λ = 1.5406 Å). The packing density of the ACs (d_packing) was defined as the mass per unit volume measured after packing the AC samples in a 10 mL measuring cylinder by vibration method, in which the frequency was 120 min⁻¹ and the amplitude was 3 mm. CO₂ adsorption on the ACs was conducted using the Micromeritics ASAP 2020 adsorption analyzer at 0 and 25 °C to obtain the isotherms in a pressure range of 0 to 1 bar, and the CO₂ adsorption capacity directly acquired from this instrument was denoted as q. Before CO₂ adsorption, the ACs samples were degassed at 180 °C under N₂ for 8 h.

2.3 Molecular dynamics simulation

In this study, the 3-layer graphitic slit-pore models with different pore sizes were used to simulate CO₂ adsorption on the ACs [35,36]. The effective pore sizes (D_eff) were selected as 0.2, 0.4, 0.6, 0.9, 2.0 and 3.5 nm, and the actual pore spaces (D_act) were calculated by D_act = D_eff + 0.335 nm correspondingly, where 0.335 nm is the layer distance of graphite. The structure file (.pdb) of CO₂ molecule was built by Gview (GaussView 5.0). The corresponding topology file (.itp) and relevant parameters of CO₂ molecule in the forcefield file (such as C–O bond length and atom charge) were obtained from the automated topology builder online molecular library [37]. The topology file of CO₂ molecule was optimized using DFT at B3LYP 6-31(G) level. Graphite laminates were created by visual molecular dynamics software. Here we used the command x2top in GROMOS 54A7 to generate the topology file of the graphene-like structure. Specifically, the atomic type was aromatic carbon CR1, and the standard C–C bond length was 0.142 nm [38]. All simulations in this work were performed in the NVT ensemble (i.e., the number of particles N, the volume V and the temperature T of the system were kept constant), and a V-RESCALE thermostat method was used to maintain the system temperature with a relaxation constant of 1.0 ps. In Gromos force field, non-bonded force dominates the interactions between gas molecules and the inner wall of carbon slit pore, which is based on Lennard–Jones potential. Van der Waals interactions were calculated using the cut-off method within a cut-off distance of 1.0 nm, and the smooth particle-mesh Ewald method was employed to account for the long-range electrostatic interactions. The total simulation time is 10 ns for each system with an integration time step of 1 fs, which is sufficient to achieve the balance of total energy of the simulation system.

3 Results and discussion

3.1 Coal-derived ACs and their structural characteristics

During the activation with K₂CO₃, carbonization of the coal precursor with the main reactions of K₂CO₃ + C ↔ K₂O + 2 CO and K₂O + C ↔ 2 K + CO are expected at elevated temperatures, to form ACs with various pore structures, as illustrated in Fig.1. An increase in the activation temperature caused the variation in pore sizes of the ACs, which was evidenced by the N₂ adsorption/desorption analysis. Fig.2 and Tab.1 show the corresponding pore size distributions and detailed pore parameters of prepared ACs, which exhibit significant differences.

Tab.1 Textural properties of the ACs under investigation

Sample	SBET/(m2·g–1)	Vtotal/(cm3·g−1)	V(≤1 nm)/(cm3·g–1)	V(≤1 nm)/Vtotal
AC-600	415	0.22	0.20	0.95
AC-700	636	0.34	0.30	0.88
AC-800	941	0.50	0.40	0.82
AC-900	1592	0.95	0.46	0.48

Fig.2 (a) N₂ adsorption/desorption isotherms and (b) DFT pore size distribution of the ACs.

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N₂ adsorption/desorption isotherms of the ACs are shown in Fig.2(a), in which the ACs obtained at low activation temperatures of < 900 °C show the typical Type I adsorption/desorption isotherms with predominant N ₂ uptake at p/p⁰ > 0.1, that is, microporous structures. Conversely, AC-900 exhibits the IV isotherm with a hysteresis loop at 0.45 < p/p⁰ < 1.0, which suggests the presence of mesopores [ 39]. The N₂ amount adsorbed on the ACs (Fig.2(a)), the BET surface area and pore volumes of the ACs (Tab.1) showed the strong dependence on the activation temperature. In detail, S_BET and V_total of AC-600 were the lowest at about 415 m²·g^–1 and 0.22 cm³·g^–1, respectively. By increasing the activation temperature to 700 and 800 °C, S_BET was enhanced to about 636 and 941 m²·g^–1 for AC-700 and AC-800, respectively. For AC-900, its S_BET and V_total were the highest among the ACs under investigation at 1592 and 0.95 m²·g^–1, respectively. DFT pore size distributions (PSDs) of the ACs are shown in Fig.2(b), which show that all the ACs possess ultra-micopores of 0.6–0.9 nm and micropores of about 1.3 nm. By increasing the activation temperature to 900 °C, in addition to the ultra-micopores and micropores, development of larger pores was observed to have the PSD at 1.5–5 nm, which extended into the mesopores. It is also worth noticing that, with an increase in the activation temperature, the PSD of the ACs shifted slightly to the right. For example, in the ultra-microporous range (i.e., < 1 nm), PSD of AC-600 centered at ca. 0.7 nm, whiles that of AC-800 shifted to 0.8 nm. Also, the intensity of differential PSDs increased as well as a function of the activation temperature, which suggests the expansion of pore volume, being in line with the specific pore volume data ( Tab.1). To demonstrate this further, a ratio of ultra-micropore volume to total pore volume (V_{(≤1 nm)} /V_total) was defined, which represent the proportion of ultra-micropores in the porous network of the ACs, as shown in Tab.1. Clearly, a continuous decrease of the V_{(≤1 nm)} /V_total ratio was found from 0.95 (for AC-600) to 0.48 (for AC-900) as a function of activation temperature from 600 to 900 °C. In summary, based on the discussion above, variation of the temperature of the K₂CO₃ activation of the coal precursor created a series of ACs with different porous properties, which can serve as a model adsorbent system for exploring the relationship between the pore structure and CO₂ adsorption performance.

As shown in Fig.3, more characterization methods were employed to investigate the properties of ACs. Fig.3(a)–3(h) show SEM and TEM micrographs of the ACs under investigation, and all the ACs show uneven surfaces with wrinkles inherited from the parent. Comparatively, the ACs obtained from relatively high activation temperatures, i.e., AC-800 and AC-900, show the presence of surface cracks (dotted frames in Fig.3(e) and 3(g)) compared with AC-600 (Fig.3(c)), which could be due to the severer carbonization reactions at higher temperatures. TEM characterisation of the AC-600/700/800 (Fig.3(b), 3(d) and 3(f)) show that these AC samples have similar amorphous microstructures dominated by micropores. In comparison, TEM image of AC-900 also suggests the presence of local long-range ordered graphene-like structures, which might be associated with the formation of potassium-carbon complex at high activation temperatures, as reported in our previous work [39,40].

Fig.3 (a) SEM and (b) TEM images of AC-600; (c) SEM and (d) TEM images of AC-700; (e) SEM and (f) TEM images of AC-800; (g) SEM and (h) TEM images of AC-900; (i) Raman spectra of the ACs; (j) XRD patterns of the ACs.

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Raman spectroscopy and XRD analyses of the ACs, and the results are shown in Fig.3(i)–3(j). Raman spectra of the ACs (Fig.3(i)) exhibit the two peaks at 1350 and 1596 cm^–1, respectively, which represent the D band of sp² carbon atoms with defects and G band of planar sp²-configured carbon atoms, and are typical for the amorphous carbon structure of the ACs. XRD patterns of the ACs (Fig.3(j)) show some differences, that is, AC-600 and AC-700 exhibit two broad diffraction peaks at ca. 23° and 43°, which can be assigned to the (002) and (100) planes of amorphous carbon, being in good agreement with the amorphous structures shown in TEM images (i.e., Fig.3(b) and 3(d)). As compared with AC-600 and AC-700, the diffraction peaks of the (002) and (100) planes in AC-800 and AC-900 were less obvious. This should be ascribed to the well-developed porosity, which was also reported in the previous study on developing porous carbons [34].

3.2 CO₂ adsorption on the ACs

Fig.4 illustrates CO₂ adsorption performance of the ACs developed by this work. As shown in Fig.4(a) and Fig. S2 (cf. ESM), CO₂ adsorption isotherms were measured at 0 and 25 °C, respectively. All adsorption experiments were conducted with the cut-off pressure of 1 bar. As one can see in Fig.4(a), at 0 °C, isotherms of AC-600 and AC-700 leveled off with an increase in adsorption pressure, which suggest that CO₂ adsorption on them was approaching the equilibrium. Conversely, that of AC-800 and AC-900 increased continuously with an increase in pressure. The findings are in good agreement with the porous property of the ACs, that is, the enhanced pore volume of the ACs with an increase in the activation temperature. Interestingly, at the end of the adsorption analysis (at 1 bar), the measured CO₂ adsorption capacity (q) of AC-800 was the highest (i.e., 6.0 mmol∙g^–1 at 0 °C and 4.1 mmol∙g^–1 at 25 °C), rather than AC-900, which has the highest S_BET and V_total values, as well as mesopores. Such findings suggest that the micro-mesopore network in AC-900 might not be hierarchical, which could hinder the uptake of CO₂ molecules during adsorption assessment. This can be proved by examining the slopes of adsorption isotherms of AC-800 and AC-900. Specifically, from 0 to 1 bar, the increasing rate of CO₂ uptake of AC-800 is 6.1 mmol∙g^–1∙bar^–1, which is much higher than that of the AC-900 (4.7 mmol∙g^–1∙bar^–1). One possible explanation for the relatively poor CO₂ adsorption of AC-900 is the possible creation of slit-shaped ink-bottle pores or closed-end slit pores at 900 °C. Although AC-900 has the highest specific surface area and pore volume, its CO₂ adsorption capacity was still poor compared to AC-800, which is mainly microporous with the V_{(≤1 nm)}/V_total ratio of 0.82. Similar adsorption performance was also measured at 25 °C, as shown in Fig. S2. Regarding the microporous ACs (i.e., AC-600/700/800), the measured CO₂ adsorption performance corresponds to their pore characteristics well. Importantly, as the best-performed material under investigation, the capacity of AC-800 is much higher than that reported for other coal/biomass-based ACs, as shown in Table S3 (cf. ESM).

Fig.4 (a) CO₂ adsorption isotherms of the ACs measured at 0 °C; (b) photos of the packing patterns of the ACs at ca. 150 mg; (c) packing density (d_packing) corrected volumetric CO₂ adsorption capacities (q_cv,d) of the ACs at 0 °C and 1 bar; (d) q–V_total correlation of the ACs at 0 °C and 1 bar; (e) q_sv,Vt–(V_{(≤1 nm)}/V_total) correlation of the ACs at 0 °C; (f) comparison of q_sv,Vt and q_sv,BET values of different biomass/coal-derived ACs with AC-600 for CO₂ adsorption at 0 °C.

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Fig.4(b) shows the packing patterns of the ACs (with the mass of ca. 150 mg), which suggest the difference in the packing density of the ACs. The calculated packing densities of the ACs (d_packing) are 0.45 g∙cm⁻³ (AC-600), 0.23 g∙cm⁻³ (AC-700), 0.17 g∙cm⁻³ (AC-800) and 0.11 g∙cm⁻³ (AC-900). Accordingly, measured CO₂ adsorption capacity (q) was corrected using the packing density to obtain the adsorption capacity of packing bed (i.e., q_cv,d = q × d_packing) and normalized with respect to S_BET (i.e., q_sv,BET) and V_total (i.e., q_sv,Vt) the to further understand their CO₂ adsorption behaviors (all the calculated values of q_cv,d, q_sv,BET and q_sv,Vt are presented in Tab.2 and Fig. S3 (cf. ESM)). Fig.4(c) shows the calculated q_cv,d values of the ACs, in which AC-600 exhibits the highest value of 1.8 mmol∙cm^–3. Therefore, considering the scenario for practical applications, especially industrial fixed adsorption beds/towers for dealing with a large amount of gaseous pollutants, ACs with the high packing density have the potential to achieve a high CO₂ capacity with the reduced reactor volume.

Tab.2 CO₂ adsorption capacities at 0 °C of the ACs under investigation

Sample	q a)/(mmol∙g–1)	qcv,d/(mmol∙cm–3)	qsv,BET/(mmol∙cm–2)	qsv,Vt/(mmol∙cm–3)
AC-600	4.1	1.8	96.4	18.2
AC-700	5.3	1.2	82.6	15.4
AC-800	6.0	1.0	64.1	12.1
AC-900	5.0	0.5	31.5	5.3

a) q values in this table are selected from Fig.4(a) at the condition of 0 °C and 1 bar.

The correlation between the measured CO₂ adsorption capacity (q) and the total pore volume of the ACs (V_total) is shown in Fig.4(d), which shows that there is positive linear relationship between the q values of the microporous ACs (i.e., AC-600/700/800) and their total pore volumes. Conversely, for the meso-micro-porous AC-900, although it has the highest V_total value, the distribution of its q value deviated significantly from the linear correlation of AC-600/700/800. This again proves that the pore volume of the meso-micro-porous AC-900 was not fully utilized during CO₂ adsorption at 0 °C. The specific CO₂ adsorption capacities (q_sv,Vt) of the ACs were further correlated with the V_{(≤1 nm)}/V_total ratio, as shown in Fig.4(e). A linear relationship was established for all the ACs, which shows the increase of q_sv,Vt values as an increase in the V_{(≤1 nm)}/V_total ratio for the ACs under investigation. The addition analysis clearly demonstrates that, during CO₂ adsorption at 0 °C, packing of CO₂ molecules was preferably in the ultra-micropores of the ACs. In addition, it also suggests that an increase in the activation temperature caused the formation of ink-bottle or closed-end slit mesopores, which was not desirable for CO₂ adsorption since the mesopore and/or micropore volumes in the ACs could become inaccessible after the occupation of ultra-micropores. Same practice was conducted for the adsorption data at 25 °C (Table S4, cf. ESM), and same conclusions were drawn as shown by the findings presented in Fig. S4 (cf. ESM).

Fig.4(f) presents the q_sv,Vt–q_sv,BET plots of various reported porous carbons [20,26,28,41–50] in comparison with AC-600 which has shown the best CO₂ adsorption performance (on the basis of q_sv,Vt) due to its well-developed ultra-micropore structure. One can see that the developed AC-600 outperformed the state-of-the-art materials regarding both the CO₂ adsorption per total pore volume and BET surface area, which suggests the very efficient use of its micropore space for packing CO₂ molecules and the great potential for being adopted by industrial adsorption systems for practical CO₂ capture. Further comparison based on q_sv,Vt and q_sv,BET values is presented in Table S3, again AC-600 demonstrated the best performance. To assess the selectivity of the developed ACs, AC-800 with the highest measured CO₂ capacity was selected to perform N₂ adsorption at 0 °C. Figure S5 (cf. ESM) compares the relevant N₂ and CO₂ isotherms. At 1 bar, the measured N₂ adsorption capacity of AC-800 was 1.17 mmol∙g^‒1, which was much lower than the measured CO₂ adsorption capacity of AC-800, suggesting the good selectivity to CO₂ adsorption against N₂ for the developed ACs.

3.3 Molecular dynamics simulation

To further investigate the correlations between pore parameters of ACs and CO₂ adsorption performance, molecular dynamics simulation was carried out based on the typical 3-layer slit pore models, and relevant results were summarized and shown in Fig.5. Fig.5(a) shows snapshots of the simulation system when the total energy reaches equilibrium, from which it can be found that: (i) pores with effective size (D_eff) smaller than the diameter of CO₂ molecule (i.e., 0.33 nm) have no CO₂ capacity; (ii) in the pore with D_eff = 0.4 nm, mono-layer CO₂ adsorption dominates to give the highest local density (205 kg·m⁻³) and overall number density (24.60 nm⁻³) as shown in the narrow peak of Fig.5(b); (iii) in the pore with D_eff = 0.6 nm, double-layer CO₂ adsorption exists with a local density of 128 kg·m⁻³ in each layer. Interestingly, the CO₂ layer density in ultra-micro pores is close to the magnitude of liquid (e.g., water density is 997 kg·m⁻³ under standard condition), which indicates the dense adsorption of CO₂; (iv) in the pores with D_eff > 2.0 nm, multi-layer adsorption appears, in which only the layers close to the pore wall show high density, whilst the CO ₂ molecules in the outer layers are loosely packed with insignificant density peaks, as shown in Fig.5(b). Thus, based on the simulation results, the ultra-micropores possess extremely high CO₂ adsorption density as compared to mesopores, which is the microscopic explanation of the positive correlation between volumetric capacity q_sv,Vt and the volume of ultra-micro pores (i.e., V_{(≤1 nm)}). In addition, as shown in Fig.5(c) and Table S5 (cf. ESM), the overall CO₂ density shows a dramatic fall as the pore size increases from 1 nm to 2 nm, which corresponds with the drop of q_sv,Vt between AC-800 and AC-900 as listed in Tab.2.

Fig.5 (a) CO₂ adsorption on AC models of different effective pore sizes; (b) density profiles of CO₂ molecules along the pores with pore diameters ranging from 0.2 to 3.5 nm; (c) number density and percentage of CO₂ molecules adsorbed inside the slit pore; (d) self-diffusion coefficients of CO₂ in pores with different effective sizes.

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Molecular dynamics simulation also suggested that an increase in pore sizes provides sufficient space to allow the transportation of CO₂ molecules, and thus the self-diffusion coefficient of CO₂ molecules in the slit pore space increases (Tables S5–S6 (cf. ESM) and Fig.5(d)). This verifies the conclusion that the measured CO₂ capacities (q) increase linearly with the total pore volumes (i.e., Fig.4(d)). However, based on this model, it is difficult to explain the result that meso-micro porous AC-900 has relatively lower measured CO₂ capacity (q) than microporous AC-800, which may be due to the developed ink pore structure of AC-900 as discussed above.

4 Conclusions

In summary, a facile chemical activation method was developed (by simply varying the temperature of K₂CO₃ solution) to convert coal to ACs with the controlled porosity, i.e., microporous and meso-micro porous ACs with different proportion of ultra-micropores (< 1 nm). The obtained ACs were used as model adsorbents to study the relationship between CO₂ adsorption capacity and porosity of ACs, which revealed the importance of ultra-micropores of the ACs for CO₂ uptake. Sample AC-800 (i.e., activation at 800 °C) possesses the highest measured CO₂ adsorption capacity (6.0 mmol∙g^–1 at 0 °C and 4.0 mmol∙g^–1 at 25 °C), while AC-600 shows the highest volumetric capacity normalized with respect to packing volume (1.8 mmol∙cm⁻³ at 0 °C and 1.3 mmol∙cm^–3 at 25 °C) and total pore volume (18.2 mmol∙cm^–3 at 0 °C and 12.8 mmol∙cm^–3 at 25 °C), which are among the highest levels of reported ACs. Both experimental results and molecular dynamics simulation demonstrate that the measured CO₂ adsorption capacities increase linearly with the pore volume of microporous carbons while CO₂ volumetric capacities are directly proportional to the ultra-micropore volume. This work offers a design principle towards solid porous adsorbents with high CO₂ volumetric or specific capability of packing bed.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51806050) and the Fundamental Research Funds for the Central Universities of Harbin Institute of Technology. The authors also acknowledge the financial support from the Taif Researchers Supporting Project (Grant No. TURSP-2020/162) by the Taif University, Taif, Saudi Arabia.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-022-2155-1 and is accessible for authorized users.

References

1 Li J, Michalkiewicz B, Min J, Ma C, Chen X, Gong J, Mijowska E, Tang T. Selective preparation of biomass-derived porous carbon with controllable pore sizes toward highly efficient CO₂ capture. Chemical Engineering Journal, 2019, 360 : 250– 259

2 Kierzkowska A M, Pacciani R, Müller C R. CaO-based CO₂ sorbents: from fundamentals to the development of new, highly effective materials. ChemSusChem, 2013, 6( 7): 1130– 1148

3 Wang S, Li X, Wu H, Tian Z, Xin Q, He G, Peng D, Chen S, Yin Y, Jiang Z, Guiver M D. Advances in high permeability polymer-based membrane materials for CO₂ separations. Energy & Environmental Science, 2016, 9( 6): 1863– 1890

4 Du N, Park H B, Dal-Cin M M, Guiver M D. Advances in high permeability polymeric membrane materials for CO₂ separations. Energy & Environmental Science, 2012, 5( 6): 7306– 7322

5 Yu Q, Delgado J P, Veneman R, Brilman D W F. Stability of a benzyl amine based CO₂ capture adsorbent in view of regeneration strategies. Industrial & Engineering Chemistry Research, 2017, 56( 12): 3259– 3269

6 Li X, Hou M, Zhang Z, Han B, Yang G, Wang X, Zou L. Absorption of CO₂ by ionic liquid/polyethylene glycol mixture and the thermodynamic parameters. Green Chemistry, 2008, 10( 8): 879– 884

7 Lee Z H, Lee K T, Bhatia S, Mohamed A R. Post-combustion carbon dioxide capture: evolution towards utilization of nanomaterials. Renewable & Sustainable Energy Reviews, 2012, 16( 5): 2599– 2609

8 Yaumi A L, Bakar M Z A, Hameed B H. Recent advances in functionalized composite solid materials for carbon dioxide capture. Energy, 2017, 124 : 461– 480

9 Li D, Chen Y, Zheng M, Zhao H, Zhao Y, Sun Z. Hierarchically structured porous nitrogen-doped carbon for highly selective CO₂ capture. ACS Sustainable Chemistry & Engineering, 2016, 4( 1): 298– 304

10 Qie Z, Sun F, Gao J, Pi X, Wang L, Liu M, Qu Z, Zhao G. Enhanced SO₂ fluidized adsorption dynamic by hierarchically porous activated coke. Journal of the Energy Institute, 2020, 93( 2): 802– 810

11 Kim J, Lin L C, Swisher J A, Haranczyk M, Smit B. Predicting large CO₂ adsorption in aluminosilicate zeolites for postcombustion carbon dioxide capture. Journal of the American Chemical Society, 2012, 134( 46): 18940– 18943

12 Al-Maythalony B A, Shekhah O, Swaidan R, Belmabkhout Y, Pinnau I, Eddaoudi M. Quest for anionic MOF membranes: continuous sod-ZMOF membrane with CO₂ adsorption-driven selectivity. Journal of the American Chemical Society, 2015, 137( 5): 1754– 1757

13 Qi G, Wang Y, Estevez L, Duan X, Anako N, Park A H A, Li W, Jones C W, Giannelis E P. High efficiency nanocomposite sorbents for CO₂ capture based on amine-functionalized mesoporous capsules. Energy & Environmental Science, 2011, 4( 2): 444– 452

14 Oginni O, Singh K, Oporto G, Dawson-Andoh B, McDonald L, Sabolsky E. Influence of one-step and two-step KOH activation on activated carbon characteristics. Bioresource Technology Reports, 2019, 7 : 100266

15 Deng H, Li G, Yang H, Tang J, Tang J. Preparation of activated carbons from cotton stalk by microwave assisted KOH and K₂CO₃ activation. Chemical Engineering Journal, 2010, 163( 3): 373– 381

16 Oginni O, Singh K, Oporto G, Dawson-Andoh B, McDonald L, Sabolsky E. Effect of one-step and two-step H₃PO₄ activation on activated carbon characteristics. Bioresource Technology Reports, 2019, 8 : 100307

17 Tseng R L. Physical and chemical properties and adsorption type of activated carbon prepared from plum kernels by NaOH activation. Journal of Hazardous Materials, 2007, 147( 3): 1020– 1027

18 Yue L, Xia Q, Wang L, Wang L, DaCosta H, Yang J, Hu X. CO₂ adsorption at nitrogen-doped carbons prepared by K₂CO₃ activation of urea-modified coconut shell. Journal of Colloid and Interface Science, 2018, 511 : 259– 267

19 Kim M J, Choi S W, Kim H, Mun S, Lee K B. Simple synthesis of spent coffee ground-based microporous carbons using K₂CO₃ as an activation agent and their application to CO₂ capture. Chemical Engineering Journal, 2020, 397 : 125404

20 Deng S, Wei H, Chen T, Wang B, Huang J, Yu G. Superior CO₂ adsorption on pine nut shell-derived activated carbons and the effective micropores at different temperatures. Chemical Engineering Journal, 2014, 253 : 46– 54

21 Rashidi N A, Yusup S. An overview of activated carbons utilization for the post-combustion carbon dioxide capture. Journal of CO₂ Utilization , 2016, 13 : 1– 16

22 Jagiello J, Kenvin J, Celzard A, Fierro V. Enhanced resolution of ultra micropore size determination of biochars and activated carbons by dual gas analysis using N₂ and CO₂ with 2D-NLDFT adsorption models. Carbon, 2019, 144 : 206– 215

23 Feng S, Li W, Shi Q, Li Y, Chen J, Ling Y, Asiri A M, Zhao D. Synthesis of nitrogen-doped hollow carbon nanospheres for CO₂ capture. Chemical Communications, 2014, 50( 3): 329– 331

24 Kim J, Han J, Ha D, Kang S. Synthesis of nitrogen and boron co-doped carbon (CNB) and their CO₂ capture properties: from porous to hollow granule structure. Journal of Materials Chemistry A, 2014, 2( 39): 16645– 16651

25 He X, Zhang H, Zhang H, Li X, Xiao N, Qiu J. Direct synthesis of 3D hollow porous graphene balls from coal tar pitch for high performance supercapacitors. Journal of Materials Chemistry A, 2014, 2( 46): 19633– 19640

26 Alabadi A, Razzaque S, Yang Y, Chen S, Tan B. Highly porous activated carbon materials from carbonized biomass with high CO₂ capturing capacity. Chemical Engineering Journal, 2015, 281 : 606– 612

27 Qie Z, Zhang Z, Sun F, Wang L, Pi X, Gao J, Zhao G. Effect of pore hierarchy and pore size on the combined adsorption of SO₂ and toluene in activated coke. Fuel, 2019, 257 : 116090

28 Deng S, Hu B, Chen T, Wang B, Huang J, Wang Y, Yu G. Activated carbons prepared from peanut shell and sunflower seed shell for high CO₂ adsorption. Adsorption, 2015, 21( 1-2): 125– 133

29 Li D, Zhou J, Wang Y, Tian Y, Wei L, Zhang Z, Qiao Y, Li J. Effects of activation temperature on densities and volumetric CO₂ adsorption performance of alkali-activated carbons. Fuel, 2019, 238 : 232– 239

30 Liu J, Liu X, Sun Y, Sun C, Liu H, Stevens L A, Li K, Snape C E. High density and super ultra-microporous-activated carbon macrospheres with high volumetric capacity for CO₂ capture. Advanced Sustainable Systems, 2018, 2( 2): 1700115

31 Haffner-Staton E, Balahmar N, Mokaya R. High yield and high packing density porous carbon for unprecedented CO₂ capture from the first attempt at activation of air-carbonized biomass. Journal of Materials Chemistry A, 2016, 4( 34): 13324– 13335

32 Guo L, Yang J, Hu G, Hu X, Wang L, Dong Y, DaCosta H, Fan M. Role of hydrogen peroxide preoxidizing on CO₂ adsorption of nitrogen-doped carbons produced from coconut shell. ACS Sustainable Chemistry & Engineering, 2016, 4( 5): 2806– 2813

33 Fan Z, Cheng Z, Feng J, Xie Z, Liu Y, Wang Y. Ultrahigh volumetric performance of a free-standing compact N-doped holey graphene/PANI slice for supercapacitors. Journal of Materials Chemistry A, 2017, 5( 32): 16689– 16701

34 Wang L, Sun F, Gao J, Pi X, Pei T, Qie Z, Zhao G, Qin Y. A novel melt infiltration method promoting porosity development of low-rank coal derived activated carbon as supercapacitor electrode materials. Journal of the Taiwan Institute of Chemical Engineers, 2018, 91 : 588– 596

35 Kommu A, Singh J K. Separation of ethanol and water using graphene and hexagonal boron nitride slit pores: a molecular dynamics study. Journal of Physical Chemistry C, 2017, 121( 14): 7867– 7880

36 Zhou H, Xie J, Liu B, Ban S. Molecular simulation of methane adsorption in activated carbon: the impact of pore structure and surface chemistry. Molecular Simulation, 2016, 42( 9): 776– 782

37 Malde A K, Zuo L, Breeze M, Stroet M, Poger D, Nair P C, Oostenbrink C, Mark A E. An automated force field topology builder (ATB) and repository: version 1.0. Journal of Chemical Theory and Computation, 2011, 7( 12): 4026– 4037

38 Miller D L, Kubista K D, Rutter G M, Ruan M, De Heer W A, Kindermann M, First P N, Stroscio J A. Real-space mapping of magnetically quantized graphene states. Nature Physics, 2010, 6( 10): 811– 817

39 Qie Z, Sun F, Zhang Z, Pi X, Qu Z, Gao J, Zhao G. A facile trace potassium assisted catalytic activation strategy regulating pore topology of activated coke for combined removal of toluene/SO₂/NO. Chemical Engineering Journal, 2020, 389 : 124262

40 Wang L, Sun F, Hao F, Qu Z, Gao J, Liu M, Wang K, Zhao G, Qin Y. A green trace K₂CO₃ induced catalytic activation strategy for developing coal-converted activated carbon as advanced candidate for CO₂ adsorption and supercapacitors. Chemical Engineering Journal, 2020, 383 : 123205

41 Sun F, Gao J, Yang Y, Zhu Y, Wang L, Pi X, Liu X, Qu Z, Wu S, Qin Y. One-step ammonia activation of Zhundong coal generating nitrogen-doped microporous carbon for gas adsorption and energy storage. Carbon, 2016, 109 : 747– 754

42 Plaza M G, González A S, Pis J J, Rubiera F, Pevida C. Production of microporous biochars by single-step oxidation: effect of activation conditions on CO₂ capture. Applied Energy, 2014, 114 : 551– 562

43 Vargas D P, Giraldo L, Erto A, Moreno-Piraján J C. Chemical modification of activated carbon monoliths for CO₂ adsorption. Journal of Thermal Analysis and Calorimetry, 2013, 114( 3): 1039– 1047

44 Zhu X L, Wang P Y, Peng C, Yang J, Yan X B. Activated carbon produced from paulownia sawdust for high-performance CO₂ sorbents. Chinese Chemical Letters, 2014, 25( 6): 929– 932

45 Plaza M G, González A S, Pevida C, Pis J J, Rubiera F. Valorisation of spent coffee grounds as CO₂ adsorbents for postcombustion capture applications. Applied Energy, 2012, 99 : 272– 279

46 Parshetti G K, Chowdhury S, Balasubramanian R. Biomass derived low-cost microporous adsorbents for efficient CO₂ capture. Fuel, 2015, 148 : 246– 254

47 Wang J, Heerwig A, Lohe M R, Oschatz M, Borchardt L, Kaskel S. Fungi-based porous carbons for CO₂ adsorption and separation. Journal of Materials Chemistry, 2012, 22( 28): 13911– 13913

48 Sevilla M, Fuertes A B. Sustainable porous carbons with a superior performance for CO₂ capture. Energy & Environmental Science, 2011, 4( 5): 1765– 1771

49 Silvestre-Albero A, Silvestre-Albero J, Martínez-Escandell M, Rodríguez-Reinoso F. Micro/mesoporous activated carbons derived from polyaniline: promising candidates for CO₂ adsorption. Industrial & Engineering Chemistry Research, 2014, 53( 40): 15398– 15405

50 Wang L, Sun F, Gao J, Zhu Y, Pei T, Li L, Zhao G, Qin Y. Pore reorganization of porous carbon during trace calcium-catalyzed coal activation for adsorption applications. Energy & Fuels, 2018, 32( 9): 9191– 9201