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Abstract

Separation of CO₂ based on adsorption, absorption, and membrane techniques is a crucial technology necessary to address current global warming issues. Porous media are essential for all these approaches and understanding the nature of the porous structure is important for achieving highly efficient CO₂ adsorption. Porous carbon is considered to be a suitable porous media for investigating the fundamental mechanisms of CO₂ adsorption, because of its simple morphology and its availability in a wide range of well-defined pore sizes. In this study, we investigated the dependence of CO₂ adsorption on pore structures such as pore size, volume, and specific surface area. We also studied slit-shaped and cylindrical pore morphologies based on activated carbon fibers of 0.6–1.7 nm and carbon nanotubes of 1–5 nm, respectively, with relatively uniform structures. Porous media with larger specific surface areas gave higher CO₂ adsorption densities than those of media having larger pore volumes. Narrower pores gave higher adsorption densities because of deep adsorption potential wells. However, at a higher pressure CO₂ adsorption densities increased again in nanopores including micropores and small mesopores. The optimal pore size ranges of CO₂ adsorption in the slit-shaped and cylindrical carbon pores were 0.4–1.2 and 1.0–2.0 nm, respectively, although a high adsorption density was only expected for the narrow carbon nanopores from adsorption potentials. The wider nanopore ranges than expected nanopore ranges are reasonable when considering intermolecular interactions in addition to CO₂–carbon pore interactions. Therefore, cooperative adsorption among CO₂ in relatively narrow nanopores can allow for high density and high capacity adsorption.

Keywords

cooperative adsorption optimal pore size carbon nanotube activated carbon

Introduction

Emissions of CO₂ are a serious global issue because of the contributions of CO₂ to global warming as a major greenhouse gas. To reduce CO₂ emissions, CO₂ separation and decomposition technologies have been considered as well as the use of more efficient energy systems. Separation of CO₂ is a necessary technology for continued use of conventional fuel systems. Three main techniques have been proposed for CO₂ separation, namely refrigeration, adsorption/absorption, and membrane separation. Refrigeration is an expensive technique and not suitable from an industrial point of view. However, adsorption, absorption, and membrane separation can be used under ambient conditions. These techniques depend on porous media such as activated carbon, porous silica, zeolites, and metal organic frameworks (Bhatia and Nguyen, 2011; Choi et al., 2009; D’Alessandro et al., 2010; Presser et al., 2011; Sevilla et al., 2011; Silvestre-Albero and Rodríguez-Reinoso, 2012; Silvestre-Albero et al., 2011; Wahby et al., 2010; Xia et al., 2011; Yang et al., 2008). Amine-functionalized solutions have shown high potential for CO₂ absorption by strong amide bonding. However, it is difficult to recover the absorbed CO₂ by this approach. Adsorption of CO₂ by porous media may allow easier recovery and recyclability for both adsorption and membrane separation approaches. To optimize porous media for CO₂ adsorption, the CO₂ adsorption mechanisms and CO₂ adsorption dependence on pore structure should be clarified. Porous carbon media with a variety of well-defined morphologies are available for evaluating CO₂ adsorption mechanisms. Activated carbons are composed of stacked graphene layers, which give slit-shaped nanopores. Mays proposed that range of nanopores was 0.1–100 nm (Mays, 2007). Pores smaller than 5 or 10 nm, however, have apparently strong interaction potential with adsorbed molecules and we thus define nanopores as smaller pores than 5 nm (Ohba et al., 2015). These materials have been widely used as adsorbents for various gasses and liquids (Chandra et al., 2012; D’Alessandro et al., 2010; Hornbostel et al., 2013; Marco-Lozar et al., 2012; Martín et al., 2011, 2010; Plaza et al., 2010; Samanta et al., 2012; Sevilla et al., 2012; Sevilla and Fuertes, 2012; Suzuki, 1994; Wang et al., 2011; Xia et al., 2011; Yoon et al., 2004; Zhao et al., 2012). Carbon nanotubes (CNTs), a relatively new class of porous material, have cylindrical nanopores. The specific surface area and pore volume dominate the adsorption properties of these materials (Ello et al., 2013; Jiménez et al., 2012; Lee et al., 2014). Furthermore, narrow pore sizes have been shown to be important for CO₂ adsorption (Casco et al., 2014; Lee and Park, 2013; Marco-Lozar et al., 2014; Presser et al., 2011; Wickramaratne and Jaroniec, 2013). However, further investigations on the pore size and morphology dependences of CO₂ adsorption are necessary to enable improvements to CO₂ adsorbents. Here we conducted experiments and computations to study how CO₂ adsorption properties depend on pore size and morphology (slit or cylinder) using activated carbon fibers (ACFs) and CNTs.

Experimental and simulation procedures

Experimental

Four types of ACFs (A5, A15, A20, and W15; Ad’All Co. Ltd, Kyoto, Japan, which were named as ACF06, ACF11, ACF13, and ACF17, respectively, as mentioned later) and four kinds of CNTs (1 nm CNT, Unidym Inc., California, USA; 2 nm CNT, Hata Group, AIST, Japan; 3 nm CNT and 5 nm CNT, NanoLab Inc., Massachusetts, USA, which were named as CNT14, CNT24, CNT29, and CNT46, respectively, as mentioned later) were used in this study (Hata et al., 2004). The CNT samples are named based on the average CNT diameters, as reported elsewhere (Ohba, 2014). The CNTs were partially oxidized in O₂ atmosphere at 673 K for 1 h to remove CNT caps. We measured the N₂ adsorption isotherms at 77 K and CO₂ adsorption isotherms at 273 K using volumetric apparatuses (Autosorb-1, Quantachrome Instrument, Florida, USA) after vacuum evacuation at 423 K for 2 h. The specific surface area, micropore volume, and average micropore size were determined from the α_s analysis of the N₂ adsorption isotherms. The total pore volumes were calculated from the adsorption amounts around a relative pressure of 0.99. The mesopore volumes were calculated by subtraction of the micropore volumes from the total pore volumes.

Simulation procedures

Grand canonical Monte Carlo (GCMC) simulations of CO₂ in carbon slit pores and CNTs at 273 K were performed to predict the CO₂ adsorption properties. The potential model of CO₂ we used was a three-centered model with Lennard–Jones and Coulomb interactions; σ_C = 0.2753 nm, ɛ_C/k_B = 29.07 K, σ_O = 0.3029 nm, ɛ_O/k_B = 83.2 K, q_C = +0.6466 e, and q_O = −0.3233 e (Makrodimitris et al., 2001). The distance between C and O atoms in a single CO₂ molecule was 0.1143 nm. Carbon slit pores and nanotubes were represented by structureless potential models proposed by Steele (1973) and Steele and Bojan (1998). The collision diameter and potential well depth of C atoms were 0.3416 nm and 30.14 K, respectively. Lorentz–Berthelot mixing rules were adopted for intermolecular interaction between heteroatoms. The unit cell sizes for carbon slit pores and CNTs were 6.0 × 6.0 × 10.0 and 10.0 × 10.0 × 6.0 nm³, respectively. A three-dimensional periodic boundary condition was applied to the unit cells and the cutoff length was 3.0 nm. The calculation cycle for each equilibrium point was 3 × 10⁶.

Results and discussion

Pore structure characterizations

The N₂ adsorption isotherms of ACFs and CNTs measured at 77 K are shown in Figure 1. All adsorption isotherms were type I indicating the main contributions to adsorption arose from the carbon micropores or nanopores. The amount of N₂ adsorbed on ACF06 was smaller than that of other ACFs suggesting that ACF06 featured smaller nanopores. The CNT bundles showed an adsorption increase at a high relative pressure around 0.9, except for 3 nm CNTs, indicating that the 3 nm CNTs rarely formed bundles. Adsorption properties generally depend on specific surface area, pore volume, and pore size. Thus, the wide variety of porosities available in nanoporous carbon samples is useful for studying the mechanism of CO₂ adsorption to optimize adsorption performance. Table 1 shows the pore structures, which were obtained from α_S-analysis of the N₂ adsorption isotherms. CNT diameters were determined from transmission electron microscope images as reported in our previous study (Ohba, 2014). The ACFs and CNTs we used here had different porosities: The specific surface areas were in the range 900–1990 m²/g for ACFs and 340–1540 m²/g for the CNTs; the micropore volumes were 0.30–0.99 cm³/g for ACFs and 0.17–0.81 cm³/g for the CNTs; the average nanopore sizes were 0.6–1.7 nm for the ACFs and 1–5 nm for CNTs. The slit pore width and CNT diameter are expected to show the greatest effect on the CO₂ adsorption properties; thus, we named the ACF and CNT samples according to their slit pore width or CNT diameter as ACF06, ACF11, ACF13, ACF17, CNT14, CNT24, CNT29, and CNT46.

Figure 1.

N₂ adsorption isotherms on ACFs (a) and CNTs (b) at 77 K. (a) •: ACF06, ▪: ACF11, ▴: ACF 13, and ♦: ACF17 and (b) •: CNT14, ▪: CNT24, ▴: CNT29, and ♦: CNT46. ACF: activated carbon fiber; CNT: carbon nanotube.

Table 1.

Porosities of ACFs and CNTs obtained from N₂ adsorption isotherms at 77 K.

	Specific surface area (m²/g)	Total pore volume (cm³/g)	Micropore volume (cm³/g)	Mesopore volume (cm³/g)	Average pore size (nm)
ACF06	900	0.31	0.30	0.00	0.6
ACF11	1820	1.18	0.82	0.35	1.1
ACF13	1990	1.40	0.99	0.41	1.3
ACF17	1320	1.29	0.51	0.78	1.7
CNT14	970	1.12	0.17	0.96	1.4
CNT24	1540	4.12	0.81	3.31	2.4
CNT29	1010	0.97	0.71	0.26	2.9
CNT46	340	1.69	0.29	1.40	4.6

ACF: activated carbon fiber; CNT: carbon nanotube.

CO₂ adsorption

CO₂ adsorption isotherms on ACFs and CNTs at 273 K are shown in Figure 2. We observed a considerable CO₂ adsorption for ACF06 and slight adsorption hysteresis owing to the strong adsorption potential of the narrow micropores, suggesting strong physical adsorption and/or weak chemisorption on carbons. The adsorption isotherms of CO₂ on other ACFs were Freundlich type, suggesting relatively strong adsorption. Conversely, the CO₂ adsorption isotherms on CNTs were of a Henry type except for that of CNT14. Regardless of the pore shape the micropores showed considerable adsorption of CO₂. The adsorption density of CO₂ in micropores and that in all pore types combined was evaluated from the micropore and total pore volumes in Table 1, as shown in Figure 3. The adsorption densities in the total pore volumes, shown in Figure 3(c) and (d), indicated a particularly high adsorption density for ACF06. For this reason, ACF06 has a higher amount of narrow micropores than that of other samples. We observed greater adsorption densities for all pore types combined for ACFs with narrower pores, as shown in Figure 3(c). However, ACF17, which had the widest pores in the ACF series (both micropores and narrow mesopores) showed the second highest adsorption densities in micropores, as shown in Figure 3(a). In this case the difference between CO₂ adsorption in micropores and that in mesopores could not be distinguished experimentally; thus, adsorption of CO₂ in mesopores of ACF17 was also considered to be adsorption in micropores. For this reason, ACF17 showed the second highest adsorption density. The CO₂ adsorption density for all pore types combined did not show a clear trend for the CNTs; however, adsorption density was related to the size of micropores in the CNTs. These results suggest that only micropores and narrow mesopores strongly contribute to CO₂ adsorption.

Figure 2.

CO₂ adsorption isotherms on ACFs (a) and CNTs, (b) at 273 K. (a) •: ACF06, ▪: ACF11, ▴: ACF 13, and ♦: ACF17 and (b) •: CNT14, ▪: CNT24, ▴: CNT29, and ♦: CNT46. ACF: activated carbon fiber; CNT: carbon nanotube.

Figure 3.

Adsorption densities of CO₂ in micropores of ACFs (a) and CNTs (b), and total pores of ACFs (c) and CNTs (d) at 273 K. (a and c) •: ACF06, ▪: ACF11, ▴: ACF 13, and ♦: ACF17; (b and d) •: CNT14, ▪: CNT24, ▴: CNT29, and ♦: CNT46. ACF: activated carbon fiber; CNT: carbon nanotube.

The CO₂ adsorption densities are also related to the pore structure of the material. Figure 4 shows the dependence of the adsorption density of CO₂ on specific surface area and pore volumes. The CO₂ adsorption densities for all pore types combined increased linearly with increasing specific surface area except for ACF06, as shown in Figure 4(a). The trend of CO₂ adsorption in micropores only, shown in Figure 4(b), was similar to that for all pore types combined except for ACF06 and CNT14. Thus, CO₂ can be adsorbed more effectively by nanoporous carbons with larger specific surface areas. Those trends of CO₂ adsorption amounts with specific surface area were similar to the preceding report by Casco et al. (2014). The difference in the extremely narrow micropores of the ACFs and CNTs was caused by the considerable enhancement of the CO₂ adsorption potentials by surrounding CO₂ molecules adsorbed to the carbon walls. The pore volume dependence of the adsorption densities was observed as shown in Figure 4(c) and (d). The adsorption densities decreased with increasing total pore volume and micropore volume, which were inverse relationship with the results of high pressure CO₂ adsorptions by Casco et al. (2014). The adsorption pressure in this study was below 1 atm and CO₂ could not be adsorbed in wide carbon nanopores below 1 atm, whereas plenty amounts of CO₂ were adsorbed in high pressure range. The CO₂ adsorption density showed an undesirable decrease with increasing pore volume below ambient pressure. The dependence of CO₂ adsorption amounts with pore volumes on adsorption pressure was clearly shown in Marco-Lozar et al. (2014) and Presser et al. (2011). The trend of CO₂ adsorption with micropore volume was negative below 1 atm, whereas the trend was changed to positive with increasing pressure.

Figure 4.

Adsorption densities of CO₂ on ACFs (•) and CNTs (▪), determined from CO₂ adsorption, and total pore volumes or micropore volumes, as functions of specific surface area (a and b), total pore volumes (c), and micropore volumes (d). ACF: activated carbon fiber; CNT: carbon nanotube.

The average pore size might be one factor, which controls CO₂ adsorption density (Wickramaratne, and Jaroniec, 2013; Zhang et al., 2013). Figure 5 shows the adsorbed densities as a function of average pore sizes of ACFs and CNTs. Higher pressures induced greater adsorption densities in all cases. Smaller pores typically have a greater adsorption potential for CO₂ and we found that CO₂ adsorption stability decreased exponentially with increasing pore size, in agreement with previous reports (Keffer et al., 1996; Samios et al., 2000). The ACFs had micropores with average pore sizes of 0.6–1.7 nm except for those of ACF17. The adsorption densities determined from the total pore volume, shown in Figure 5(a), indicated that the CO₂ adsorption densities decreased exponentially with increasing pore size, as expected from the adsorption potential of CO₂. For the CNT samples, adsorption densities decreased with increasing pore size, except for CNT29. This CNT sample featured mainly a microporous structure, whereas other CNTs had both micro- and mesopores. Mesopores weakly adsorb CO₂ and the CO₂ adsorption densities in these CNTs were less than the values expected from the densities measured in the ACFs. Figure 5(b) shows the adsorption densities determined from the micropore volumes. The adsorbed densities for ACF17 were larger than those for ACF13, because the CO₂ adsorption in these mesopores was counted as micropore adsorption. CNTs have both micro- and mesopores and the adsorption densities obtained based on the micropore volume showed a monotonic decrease with increasing pore size. Adsorption of CO₂ occurs mainly in micropores and is somewhat low in mesopores. Adsorbed densities are determined by assessment of the adsorption densities of all pore types combined and micropores as well as the pore shape (slit or cylinder). Thus, both micropores and mesopores should be considered for obtaining porous carbon media with greater CO₂ adsorption density.

Figure 5.

Adsorption densities of CO₂ in total pores (a) and micropores (b) at P/P₀ = 0.1 (•), 0.2 (▪), 0.5 (▴), and 1.0 (♦).

Adsorption mechanism of CO₂

To clarify the relationship between the adsorption densities of CO₂ and pore sizes in the ACFs and CNTs, we performed a GCMC simulation of CO₂ in slit-shaped and cylindrical carbon nanopores at 273 K. Figure 6 shows the adsorption isotherms of CO₂ in carbon nanopores at 273 K. The illustrations of CO₂ adsorbed in slit and cylindrical carbon nanopores were obtained from simulations, as shown in Figure 7. The simulations indicated that CO₂ rarely adsorbed in the 0.3 nm slit pores, as shown in Figure 6(a), owing to these pores having a smaller size than that of the CO₂ molecule. However, the highest adsorption density was observed for the 0.4 nm slit pores. The shapes of adsorption isotherms in slit pores changed from Langmuir to Henry type with increasing pore size. The borders between Langmuir and Henry type behaviors were 1.2–1.4 nm for the slit pores and ∼2 nm for cylindrical pores. The enhanced adsorption potentials were weakened for larger pore sizes. In the case of CNTs, the smallest pore size that allowed adsorption of CO₂ was 1.0 nm, where the pore size is defined as the distance from the centers of carbon atoms. The pore size of the slit pores was defined as the carbon surface–surface distance. Thus, 0.3 nm should be subtracted from the cylindrical pore size to allow for a comparison with the slit pore size. Even with this correction the effective cylindrical pores size for CO₂ adsorption was larger than that of slit pores.

Figure 6.

Simulated CO₂ adsorption isotherms for different slit widths (a) and cylindrical diameters (b) at 273 K.

Figure 7.

Snapshots of CO₂ in slit pores (a) and cylindrical pores (b) at 1 atm.

The simulated adsorbed densities of CO₂ in slit and cylindrical pores were calculated, as shown in Figure 8. The 0.4 nm slit pores showed the highest density at low pressure and the distributions shifted to wider pores 0.6–0.8 nm with increasing pressure. In the same manner, the highest density found for the 1.2 nm cylindrical pores at low pressure shifted to 1.5 nm pore size at high pressure. The distributions decreased exponentially with increasing pore size, corresponding to the experimental density distribution shown in Figure 5. Therefore, the preferable slit and cylindrical pore sizes for CO₂ adsorption were 0.4–1.2 and 1.0–2.0 nm, respectively, assessed from the experimental and simulated adsorption densities in Figures 5 and 8.

Figure 8.

Adsorption densities of CO₂ in carbon slit pores (a) and cylindrical pores (b) at P = 0.1 (•), 0.3 (▪), 0.5 (▴), and 1.0 atom (♦).

The adsorption potentials of CO₂ in slit and cylindrical pores were calculated from the intermolecular potentials, as shown in Figure 9(a) and (c). The deepest potential wells were 35 and 28 kJ/mol in the 0.3 nm slit pores and the 1.0 nm cylindrical pores, respectively. The stabilization energy of CO₂ adsorbed in nanopores was evaluated from the potential minima of the adsorption potentials in Figure 9(a) and (c), as shown in Figure 9(b) and (d). The stabilization energies of CO₂ in slit pores decreased suddenly for pore widths larger than 0.4 nm, while those in cylindrical pores decreased gradually. The profiles of the stabilization energies were similar to those of the adsorption densities at low pressure. This is because interactions between CO₂ molecules and carbon pores dominate CO₂ adsorption. However, in the high pressure region, the distributions in Figure 8 shifted to larger pores sizes. Another peak of adsorption density for the slit pores was observed at P = 1.0 atm. The intermolecular interaction of CO₂ induced high density adsorption of CO₂ in the larger nanopores than that expected from the CO₂–carbon pore interaction.

Figure 9.

Adsorption potentials of CO₂ in slit pores (a) of 0.4 (○), 0.5 (□), 0.8 (⋄), 1.0 (Δ), 1.2 nm (•) and cylindrical pores (c) of 1.0 (○), 1.2 (□), 1.5 (⋄), 2.0 (Δ), and 2.5 nm (•). Stabilization energy of CO₂ adsorbed in slit pores (b) and cylindrical pores (d).

Conclusion

CO₂ adsorption properties in carbon nanopores with various specific surface areas, nanopore volumes, and nanopore sizes were investigated in this study. Larger specific surface areas induced higher adsorption densities of CO₂, while larger nanopore volumes decreased the adsorption densities. Nanopore sizes related to adsorption potentials were particularly important for higher adsorption density in the low pressure region. Extremely narrow nanopores have strong adsorption potentials and high adsorption density was thus observed in those nanopores, while the adsorption capacity was generally small. However, the nanopores typically had relatively weak adsorption potentials but could have an inherently high capacity. Relatively narrow nanopores between 1 and 2 nm in size induced considerable intermolecular interactions between CO₂ molecules, which could compensate the weak adsorption potentials of those nanopores at a high pressure.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the Japan Society for the Promotion of Science KAKENHI Grant Numbers 26706001 and 15K12261, the Futaba Electronics Memorial Foundation, and the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

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Cooperative CO 2 adsorption promotes high CO 2 adsorption density over wide optimal nanopore range

Abstract

Keywords

Introduction

Experimental and simulation procedures

Experimental

Simulation procedures

Results and discussion

Pore structure characterizations

CO2 adsorption

Adsorption mechanism of CO2

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References

CO₂ adsorption

Adsorption mechanism of CO₂