Article pubs.acs.org/JPCC
Sulfur-Doped Millimeter-Sized Microporous Activated Carbon Spheres Derived from Sulfonated Poly(styrene−divinylbenzene) for CO2 Capture Yahui Sun,†,‡ Jianghong Zhao,*,§ Jianlong Wang,† Nan Tang,† Rijie Zhao,†,‡ Dongdong Zhang,†,‡ Taotao Guan,†,‡ and Kaixi Li*,† †
Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, Shanxi, P. R. China University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, P. R. China § Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, 92 Wucheng Road, Taiyuan 030006, Shanxi, P. R. China ‡
S Supporting Information *
ABSTRACT: Millimeter-sized activated carbon spheres are potential candidates for industrial-scale CO2 capture. Millimetersized sulfur-doped microporous activated carbon spheres were synthesized from poly(styrene−divinylbenzene), a very cheap and easily operated resin product, in the present work and studied for CO2 uptake. A series of sulfur-doped spherical carbon materials were yielded through the sulfonation, oxidation, carbonization, and KOH activation of the polymer precursors. In addition to promoting the cross-linking of the polymer molecules, the sulfonic substituents directly introduced sulfur functional groups into the carbon materials after pyrolysis. The SCS-700 sample showed the best CO2 adsorption performance, whose sulfur content reached 0.69 wt %, and exhibited a high surface area of 1526 m2 g−1 and a large pore volume of 0.726 cm3 g−1. The adsorbent showed high CO2 uptake at both 25 °C (4.21 mmol g−1) and 50 °C (2.54 mmol g−1) under ambient pressure due to its abundant ultramicropores and a high proportion of oxidized sulfur functional groups. Thanks to its high microporous volume of 0.617 cm3 g−1, the CO2 performance at 8 bar was 10.66 mmol g−1 at 25 °C. The thermodynamics indicated the exothermic and spontaneous nature of the adsorption process, which was dominated by a physisorption mechanism. Furthermore, the CO2 uptake curves on a TGA analyzer were fitted with different kinetic models, and the fractional order model showed the best agreement with the experimental data. The recycling curve of SCS-700 exhibited excellent cyclic adsorption performance with no significant capacity loss even after ten adsorption−desorption cycles. It is suggested that this excellent CO2 uptake was due to the synergistic effect of the well-developed microporous structure and the oxidized sulfur-containing functional groups.
1. INTRODUCTION
controllable distinct porosities, and easy regeneration capabilities.14−16 A wide variety of investigations have been made to design novel carbon materials with new structures and morphologies.17−19 The pore architecture and surface chemistry of activated carbons are the key factors for controlling CO2 adsorption. It is widely accepted that micropores smaller than 1 nm (especially smaller than 0.8 nm) are vital for CO2 adsorption at low pressures.20,21 Except for the modulation of the pore architecture, heteroatom doping is another important way to tune and control the surface physical−chemical properties of carbon materials, thus greatly affecting their CO2 adsorption capability.22−24 It has been reported that doping heteroatoms, such as nitrogen,25−27 sulfur,23,24 boron,28
Currently, the continuously increasing carbon dioxide (CO2) concentration in the atmosphere is considered to play a big role in global climate deterioration. Efficient methods for CO2 capture and storage (CCS) have attracted great interest in many fields.1 Thus far, various practical techniques have been developed for CO 2 capture and separation, including adsorption on porous solid adsorbents,2 absorption in liquid amine solutions,3 membrane separation,4 cryogenic distillation, and others.5 Porous solid adsorbents, including carbon materials,6 porous polymers,7 zeolites,8 silica,9 organic− inorganic hybrid sorbents,10 and metal−organic frameworks (MOFs),11,12 have been investigated worldwide for postcombustion CO2 capture in terms of high adsorption efficiency, good recycle performance, and eco-friendly advantages.1,13 Among these solids mentioned, porous carbons are one of the most promising groups of candidates for CO2 capture due to their remarkable thermal and chemical stabilities, low costs, © XXXX American Chemical Society
Received: March 8, 2017 Revised: April 25, 2017 Published: April 25, 2017 A
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The Journal of Physical Chemistry C and fluorine,29 into carbon network can increase the CO2 adsorption capacity and selectivity. Until now, there have been two strategies, namely, in situ doping and post-treatment, proposed to incorporate heteroatoms into carbon matrices.22,27 In contrast to the posttreatment of carbon with certain agents (i.e., H2S, sublimed sulfur, and NH3), the in situ carbonization/activation of a heteroatom-containing precursor can not only functionalize the surface but also obtain well-distributed bulk-doped carbon without additional steps.22 Numerous studies have introduced nitrogen into carbon networks using this in situ pathway, but much less attention has been paid to sulfur-doped carbon materials.23 Sulfur-doped carbon in previous literatures has mainly been prepared by post-treatment methods and used as lithium−sulfur battery electrodes. Very recently, a few researchers began to explore the potential application of sulfur-doped porous carbons in CO2 adsorption. Xia et al.23 successfully prepared sulfur-doped, ordered microporous carbon utilizing zeolite EMC-2 and poly(2-thiophenemethanol) as a template and carbon/sulfur source, respectively. The sulfurdoped carbon adsorbent exhibited higher CO2 uptake capacity than the sulfur-free counterpart and showed an initial isosteric heat (Qst) for CO2 adsorption up to 59 kJ mol−1, indicating that the sulfur-containing sorbent is beneficial to CO2 adsorption. Furthermore, Bandosz and co-workers30 demonstrated that CO2 adsorption capacity depends highly on the sulfur functional groups mainly due to three types of interactions: acid−base interactions between CO2 and sulfur incorporated into the aromatic rings; polar interactions between CO2 and sulfoxides, sulfones and sulfonic acids; and hydrogen bonding between CO2 and acidic groups on the surface. For industrial-scale CO2 capture, macro-sized carbon spheres have several specific advantages over granular or powdered activated carbon materials, such as high purity, good fluidity, and low material abrasion.31−33 Millimeter-sized poly(styrene− divinylbenzene) spheres are easy to prepare by suspension polymerization, and the resin spheres can be modified by different functional groups to introduce heteroatoms.34−36 Among the various functionalized resins, sulfonated poly(styrene−divinylbenzene) is the most commonly used carbon precursor. During the pyrolysis process, the sulfonic acid groups work not only as cross-linking agents leading to a high carbon yield (>40% at 800 °C) but also as sulfur resources to introduce various sulfur functionalities into the carbon surface and framework.30,37 Literatures previously reported sulfonated poly(styrene−divinylbenzene)-based carbon spheres which were mainly derived from commercial ion-exchange resins.38−41 Choma and co-workers41 obtained a series of microporous carbons from commercially available styrene−divinylbenzene resin spheres with sulfo functional groups through carbonization and KOH activation. Unfortunately, the spherical morphology was destroyed during KOH activation, and the activated carbon possessed irregular particles of different sizes. Wang et al.42 prepared porous carbon spheres by direct carbonization of a commercially available ion-exchange resin with a highly dispersed potassium species induced by ion exchange. However, the CO2 adsorption capacity was only 3.09 mmol g−1 at 25 °C and 1 bar owing to the lower proportion of micropores. For commercial ion-exchange resins, abundant macropores are required and might seriously reduce the mechanical strength of the derived activated carbon sphere, leading to destruction of the carbon spheres. In addition, the
relatively small amount of micropores in the adsorbents results in a low CO2 adsorption capacity. Here, we presented a simple and effective method to produce sulfur-doped millimeter-sized microporous carbon spheres (SCSs) for CO2 capture using poly(styrene−divinylbenzene) ion-exchange resins. The poly(styrene−divinylbenzene) spheres were prepared by a traditional suspension polymerization without employing porogen. The polymer spheres were treated through sulfonation, oxidation, carbonization, and KOH activation to obtain sulfur-containing activated carbon spheres. The spherical carbon activated at 700 °C preserved the spherical morphology and possessed superior CO2 adsorption performance. The spherical shape, high CO2 uptake, fast kinetic rate, and good regeneration stability showed that the carbon spheres synthesized by our technique are promising candidates for industrial application of CO2 capture.
2. EXPERIMENTAL SECTION Materials. Styrene, divinylbenzene (55%), 1,2-dichloroethane (DCE), gelatin, absolute ethanol, concentrated sulfuric acid (98 wt %), hydrochloric acid (37−38 wt %), and KOH were purchased from Tianjin Fuchen Chemical Corp. α,α′Azoisobutyronitrile (AIBN) was acquired from Shanghai no.4 Reagent & H. V. Chemical Corp. China. All chemicals were AR-grade and utilized without further purification, unless otherwise noted. Preparation of the Poly(styrene−divinylbenzene) Spheres. The millimeter-sized poly(styrene−divinylbenzene) spheres were synthesized via suspension polymerization. In a typical synthesis, the gelatin dispersing agent (0.75 g) was added into deionized water (125.0 mL) in a 500 mL threenecked round-bottom flask equipped with a mechanical stirrer, a reflux condenser, and a thermometer. Then, the mixture was heated to 50 °C to obtain a transparent solution, designated as the water phase. The AIBN initiator (0.142 g) was dissolved in a mixture of the styrene monomer (20.0 mL) and the divinylbenzene cross-linking agent (5.93 mL), denoted as the oil phase. Afterward, the oil phase was poured into the water phase under rigorous mechanical stirring and then kept at 50 °C for 20 min to make the oil phase disperse homogeneously in the water phase. Then, the suspension was heated to 70 °C to initiate the polymerization and was kept at this temperature for 4 h. Subsequently, the resin curing reaction was accomplished at 90 °C for 4 h. The products were washed by ethanol and water several times to remove the remaining unreacted monomer and dispersing agent. After drying at 110 °C for 10 h, the transparent and colorless millimeter-scaled polymer spheres were obtained and labeled PS. Sulfonation and Oxidation of the Poly(styrene− divinylbenzene) Spheres. After swelling in DCE (20.0 mL) overnight, the resin spheres (10.0 g) were sulfonated with concentrated sulfuric acid (98%, 21.74 mL) at 180 °C for 2 h. Then, the product was treated by Soxhlet extraction using deionized water until the pH was close to 7. Black sulfonated poly(styrene−divinylbenzene) spheres were obtained after drying at 110 °C for 10 h and are denoted as SPS. The oxidation of SPS was processed under air flow at 290 °C for 4 h. The temperature was first increased from room temperature to 100 °C with a ramping rate of 2 °C min−1 and then at 0.5 °C min−1 from 100 to 290 °C. The black product was marked as SPS-O. B
DOI: 10.1021/acs.jpcc.7b02195 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Carbonization and KOH Activation. The SPS-O was first carbonized at 600 °C under N2 flow for 1 h with a heating rate of 2 °C min−1. The carbonized spheres were denoted as SCS-C. The obtained carbonized sample was immersed into an aqueous KOH solution for 5 h with a weight ratio of KOH:carbon = 2:1 followed by drying at 120 °C for 12 h. Then, the dried mixture was activated by heating at 600, 700, or 800 °C for 1 h in a tube furnace under N2 with a ramp rate of 3 °C min−1. The activated sample was then repeatedly washed with 2 M HCl and deionized water until a neutral pH was obtained. Subsequently, they were dried in an oven at 110 °C for 12 h. The obtained activated carbon spheres were marked as SCS-600, SCS-700, and SCS-800 (to denote the different heating temperatures). For comparison, the SCS-600 sample was treated with concentrated HCl (37 wt %) for 24 h at ambient condition and then washed with excess deionized water until the pH of the filtrate reached approximately 7. The obtained dried sample was designated as SCS-600-HCl. Characterization. The pore structure was obtained from N2 adsorption isotherms measured at −196 °C by a Micromeritics ASAP2020 sorption analyzer. Before measurement, the sample was treated under vacuum at 200 °C for 6 h. The Brunauer−Emmett−Teller (BET) specific surface area (SBET) was calculated based on the adsorption data in the relative partial pressure (p/p0) range of 0.04−0.20, whereas the total pore volume (Vtotal) was calculated from the adsorbed amount at a relative pressure of 0.99. The micropore surface area (Smicro) and micropore volume (Vmicro) were calculated according to the t-plot analysis. The pore size distribution (PSD) was calculated by the density functional theory (DFT) model assuming split-shaped pores. Thermogravimetric analysis (TGA) was performed on a TA Q50 analyzer from ambient temperature to 800 °C with a ramping rate of 10 °C min−1 under a constant nitrogen flow of 50 mL min−1. The morphologies of the samples were investigated using a JEOL JSM-700 scanning electron microscope (SEM) at an accelerating voltage of 10.0 kV. Elemental analysis was obtained on an Elementar Vario Macro EL Cube microanalyzer. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Ultra DLD spectrometer with an exciting source of Mg Kα (1486.6 eV). Fourier transform infrared (FT-IR) spectroscopy was performed on a Bruker Vertex70 spectrometer over the wavenumber range of 4000−400 cm−1. CO2 Sorption Measurement. The CO2 isotherms were measured using an IGA-002 gravimetric adsorption instrument (Hiden Isochema, Warrington, UK) over a pressure range of 0−8 bar. Before the adsorption experiments, all the samples were degassed under high vacuum at 120 °C for 2 h. To determine the CO2 adsorption kinetics and the adsorption−desorption regeneration, the CO2 uptake as a function of time was obtained using a thermal gravimetric analyzer (Q50, TA Instruments, USA) at ambient pressure. A small amount of sample (approximately 10 mg) was treated at 120 °C for 2 h under a constant flow of N2 (50 mL min−1) to remove moisture and other molecules adsorbed in the pores. Then, the temperature was cooled to 25 °C under an N2 atmosphere and kept for 20 min. Subsequently, N2 was switched to CO2 flow with the same flow rate. After adsorption for 1 h, CO2 was changed back to N2 to perform the desorption process at 200 °C for 1 h. To investigate the recycling ability, the SCS-700 sample was tested for CO2 uptakes over multiple cycles of adsorption and desorption by the same temperature
swing method mentioned above. The temperatures for adsorbing and desorbing CO2 were 25 and 200 °C, respectively.
3. RESULTS AND DISCUSSION As illustrated in Figure 1, sulfur-doped millimeter-sized microporous activated carbon spheres were produced by
Figure 1. Schematic diagram of the preparation of the SCS samples.
suspension polymerization followed by sulfonation, oxidation, carbonization, and KOH activation processes, as suspension polymerization is often used for the preparation of millimeterscale resin spheres. In addition, this polymerization technology has the advantages of limit use of organic solvents and easy separation of the products. However, the poly(styrene− divinylbenzene) spheres without modification cannot function as the carbon precursor due to their decomposition at approximately 430 °C under an N2 atmosphere (the inset of Figure 2b). After decoration with sulfonic acid groups, the thermal stability of the functional resin is improved greatly, and the carbon residue of SPS is approximately 45.9% (Figure 2a).35,43 Evidently, decorated sulfonic acid groups can act as cross-linking agents during the carbonization. TG/DTG curves recording the thermal behaviors of the polymer samples under an N2 atmosphere (Figure 2) confirm the above viewpoint and exhibit that postoxidation can further improve the degree of cross-linking. As shown in Figure 2a, the weight loss of SPS-O is almost 16.7% less than that of SPS. The significant mass loss at approximately 290 °C of SPS can be assigned to the decomposition and the complex cross-linking of the sulfo groups,44 which might affect the porosity and chemical properties of the derived activated carbon.45 Moreover, the higher stability of SPS-O may be beneficial to keep the spherical shape of the derived activated carbons. For both SPS and SPSO, significant mass loss occurs at 350−450 °C, which can be attributed to the decomposition and carbonization of the poly(styrene−divinylbenzene) segments.37 The TG curve of SCS-C shows negligible weight loss in the range of 100−500 °C and a gradual weight loss of 11.3 wt % at 500−800 °C, indicating the superior thermal stability of the carbon char. The polymer precursor, intermediates, and final activated carbon were characterized by FT-IR spectroscopy to investigate the evolution of the bonding structures during the different treatment processes (Figure 3). In the spectrum of the PS copolymer, a sharp peak at 3024 cm−1 belongs to the stretching vibration of the aromatic C−H groups. The peaks at approximately 1028 cm−1 are associated with the bending vibration bands of the aromatic ring plane.46 The two peaks at 2922 and 2850 cm−1 are assigned to the symmetric/asymmetric stretching vibration of methylene, while the peaks between C
DOI: 10.1021/acs.jpcc.7b02195 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. TG and DTG curves of PS, SPS, SPS-O, and SCS-C.
sulfur species during the harsh alkali activation. From the analysis above, the −SO3H functional groups can be easily grafted onto the benzene rings via concentrated sulfuric acid treatment and act as cross-linking agents and sulfur resources in the follow-up heat treatment process. The elemental compositions of the sulfur-doped microporous carbon spheres were evaluated by elemental analysis (Table S1). It shows that the sulfur content of the carbonized sample is 3.01 wt %. However, the mass percentage of sulfur decreases dramatically after KOH activation. During the KOH activation, the sulfur functional groups in the carbon network and surface may react with KOH or other potassium-containing intermediates to produce K2S.37,52,54 When the sample was washed with H2O, the S2− species, and even some S0/S2+/S4+ species in the active sulfur functional groups, could bond with H+ to release H2S with a typical “rotten egg” odor.55 For comparison, the SCS-600 sample was further treated with concentrated HCl that could provide a large quantity of protons (H+). When adding the acidic solution, bubbles formed on the surface, and a “rotten egg” odor escaped simultaneously. After thoroughly washing with deionized water to a neutral pH and drying, the sulfur content of SCS-600-HCl decreases sharply from 0.66 to 0.13 wt % (Table S1). This result also attests that part of the active sulfur functionalities were lost by the production of H2S gas during the washing process. To further confirm the compositional information on the sulfur species, XPS analysis was performed (Figure 4 and Table S2). Significantly, the types of sulfur in the activated carbon samples are quite different from the carbonized one. The sulfur species of the SCS-C carbonized sample are mainly aromatic sulfur (164.7 eV) and mono-oxidized sulfur (166.8 eV).30,56,57 After KOH activation, the S 2p peak splits into two distinct groups. The broad peaks with high bonding energies
Figure 3. FT-IR spectra of the samples.
1602 and 1452 cm−1 are typical vibrations of the benzene ring.47 Moreover, the peaks in the fingerprint range from 900 to 650 cm−1 are ascribed to the out-of-plane deformation vibrations of the aromatic C−H groups, and the two sharp and strong adsorption peaks at 756 and 698 cm−1 indicate that only one substituent exists on most of the benzene rings.48,49 It is clear that PS is mainly composed of polystyrene segments. After sulfonation with concentrated sulfuric acid at a high temperature, there are some new peaks in the spectrum. The newly presented peaks at 1180, 1127, 1038, and 1008 cm−1 are assigned to the −SO3H groups.47,50,51 The broad bands at 1180 and 1127 cm−1 are the asymmetric stretching vibrations of S−O and SO groups, respectively.46,51 Furthermore, the adsorptions at 1039 and 1009 cm−1 resulted from the symmetric stretching vibration of the SO groups affiliated with the SO3− groups.46 These peaks indicate that −SO3H functional groups were successfully grafted onto the benzene rings. Furthermore, the absence of two sharp peaks at 756 and 698 cm−1 indicates that multiple substituents exist on the benzene ring. After oxidation, the two peaks at 756 and 698 cm−1 reappear. In addition, another new remarkable peak at 1384 cm−1 is attributed to the combination of the O−H deformation vibration and the C−O stretching vibration.45 Clearly, part of the sulfo groups decompose and recombine to form a threedimensional network during oxidation, leaving some monosubstituted benzene rings. For the SCS-C carbonaceous sample, the new band at 1270 cm−1 and the small peaks at 874 and 745 cm−1 belong to the S−CH2−R wagging vibration and the C−S stretching vibrations of the sulfur species.52,53 The spectrum of SCS-700 is very similar to that of SCS-C except that the C−S vibration peak of SCS-700 disappeared, implying the loss of
Figure 4. XPS S 2p spectra of the SCS samples. D
DOI: 10.1021/acs.jpcc.7b02195 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. SEM images of (a) SCS-600, (b) SCS-700, and (c) SCS-800. (d) EDS mapping of C and S for SCS-700.
Figure 6. (a) N2 adsorption−desorption isotherms and (b) DFT pore-size distributions for the SCS samples.
Table 1. Pore Structural Properties and CO2 Adsorption Performance of the SCS Samples CO2 uptake (mmol g−1) at 25 °C
at 50 °C
samples
SBET (cm2 g−1)
Smicro (cm2 g−1)
Vtotal (cm3 g−1)
Vmicro (cm3 g−1)
Vmicro/Vtotal (%)
0.15 bar
1 bar
8 bar
0.15 bar
1 bar
8 bar
SCS-600 SCS-700 SCS-800 SCS-600-HCl
1324 1526 1631 1582
1157 1354 1296 1342
0.635 0.726 0.771 0.761
0.536 0.617 0.598 0.620
84.4 85.0 77.6 81.5
1.08 1.03 0.83 0.68
4.00 4.21 3.40 2.65
9.30 10.66 9.69 6.61
0.53 0.53 0.50 0.34
2.44 2.54 2.16 1.60
7.16 8.21 7.49 5.22
correspond to oxidized sulfur species, such as sulfoxides (168.0 eV) and sulfonic acids (169.2 eV).58 The peaks at ca. 164.7 and 163.5 eV belong to neutral sulfur.57−59 It shows clearly that KOH activation has a significant effect on both the contents and configurations of the sulfur species.57 As presented in Figure 4 and Table S2, the sulfur functional groups of the carbonized sample (SCS-C) oxidized into di/tri-oxidized sulfur species at a lower activation temperature (600 or 700 °C), with fewer mono-oxidized sulfur species and a relatively low content of neutral sulfur. When the activation temperature increased to 800 °C, the proportion of neutral sulfur rises to nearly half of the total sulfur content, implying that a portion of sulfur in this oxidation state on the surface are incorporated into the carbon backbone at elevated temperature.30 Comparing the XPS S 2p spectra of the samples before and after acidic washing (Figure S1), the neutral sulfur proportion becomes higher after the acidic treatment. Combined with the elemental analysis (Table
S1), it can be inferred that the thermal and chemical stability of neutral sulfur is much higher than that of the oxidized one. In addition, this is the reason why the sulfur content of the sample activated at an elevated temperature is higher. Figure S2 and Figure 5 show the SEM images of the carbonized and KOH-activated samples. The carbonized sample (SCS-C) shows nearly perfect spheres with very smooth external surface (Figure S2a−c). The activated samples maintain the spherical shapes with diameters of approximately 0.5−0.8 mm (Figure 5a−c). With an increase of the activation temperature from 600 to 800 °C, the surface of the activated carbon spheres is etched severely. At 800 °C, the sample is badly damaged and even loses sphericity. The SCS samples have layered structure, as exhibited in Figure 5b,c. The highresolution SEM image of SCS-700 (Figure 5d) displays vesicles on the surface rather than the smooth surface of SCS-C, which might be due to the release of gaseous species or potassium E
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The Journal of Physical Chemistry C Table 2. Comparison of the CO2 Uptake at 1 bar for Different Sulfur-Doped Carbon Adsorbents carbon adsorbents
precursor
SBET (m2 g−1)
S content
CO2 uptake (mmol g−1)
ref
SCS-700 SCEMC SPC a-SG6 C-AO CS10-O
sulfonated poly(styrene−divinylbenzene) poly[(2-hydroxymethyl)thiophene] poly[(2-hydroxymethyl)thiophene] reduced-graphene-oxide/polythiophene material poly(ammonium-4-styrenesulfonate) poly(sodium-4-styrenesulfonate) and graphite oxide composite
1526 729 2500 1396 727 831
0.69 wt % 6.56 wt %
4.21 (25 °C) 2.46 (25 °C) 4.15 (23 °C) 4.5 (25 °C) 2.30 (30 °C) 2.90 (30 °C)
this work 23 24 57 66 30
the CO2 uptakes decrease under the same pressures. The sample activated at 700 °C exhibits the highest CO2 capacity at pressures of 1 and 8 bar. Since the chemical surface properties of SCS-600 and SCS-700 are similar based on the above XPS analyses, this superior CO2 uptake of SCS-700 is attributed to its more abundant ultramicropores (especially micropores ≤0.8 nm) than those in SCS-600.21 After raising the temperature to 800 °C, the CO2 adsorption capacity drops from 4.21 to 3.40 mmol g−1. According to the pore structure analyses, the Smicro and Vmicro of SCS-800 are smaller than those of SCS-700 due to the overetching reaction at a high temperature, as evidenced in Figure 6b, in which SCS-800 possesses more supermicropores (>1.0 nm) and fewer ultramicropores (
