6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
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Highly selective CO2 capture by S-doped microporous carbon materials Humaira Seema 1, K. Christian Kemp 1, Nhien H. Le, Sung-Woo Park, Vimlesh Chandra, Jung Woo Lee, Kwang S. Kim * Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
S-doped microporous carbon materials were synthesized by the chemical activation of a
Received 12 April 2013
reduced-graphene-oxide/poly-thiophene material. The material displayed a large CO2
Accepted 3 September 2013
adsorption capacity of 4.5 mmol g
Available online 10 September 2013
adsorption selectivity over N2, CH4 and H2. The material was shown to exhibit a stable recy-
at 298 K and 1 atm, as well as an impressive CO2
cling adsorption capacity of 4.0 mmol g 1. The synthesized material showed a maximum specific surface area of 1567 m2 g
and an optimal CO2 adsorption pore size of 0.6 nm.
The microporosity, surface area and oxidized S content of the material were found to be the determining factors for CO2 adsorption. These properties show that the as synthesized S-doped microporous carbon material can be more effective than similarly prepared Ndoped microporous carbons in CO2 capture. Ó 2013 Elsevier Ltd. All rights reserved.
The widely accepted scientific consensus is that global warming is due to an increase in anthropogenic greenhouse gases (i.e., CO2, CH4, NO2, etc.) [1–3]. The selective capture and storage of CO2 using a low cost and energy-efficient method is crucial for achieving a substantial reduction in atmospheric CO2 levels, thereby stalling global warming . Previous research studies have shown that adsorption using aqueous ammonia  or solid adsorbents are the methods most likely to be adapted in industrial applications for the efficient capture of CO2 from flue gases . When multiple CO2 adsorbents were compared (i.e., porous carbons, carbon–CaO composites, amine-modified mesoporous silicas, N-containing group-bridged metal phosphonates, covalent organic frameworks, metal–organic frameworks and microporous organic polymers), carbon based materials
displayed a comparatively high adsorption capacity for CO2 capture over a wide range of operating conditions [7–14]. Due to their impressive chemical stability and large theoretical surface area, graphene and reduced-graphene-oxide have attracted significant interest in basic scientific research [15–28]. These intrinsic properties should pave the way for graphenes application in environmental remediation, biosensors, etc. [29–32]. Conducting polymers have been extensively studied due to their chemical stability and ease of synthesis from abundant natural monomer resources. Polypyrrole, polythiophene (PTh), and their derivatives have attracted special interest for practical applications in oxidized or reduced states . It has been demonstrated by Chandra et al. that N-doped carbon, produced by the chemical activation of polypyrrole functionalized graphene sheets, is important in the selective CO2 adsorption of 4.3 mmol g 1 at 298 K and 1 atm . This work motivated us to synthesize a S-doped
* Corresponding author: Fax: +82 54 279 8137. E-mail address: [email protected]
(K.S. Ki. 1 Both authors are contributed equally to this work. 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.09.006
6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
microporous carbon material by chemical activation of a reduced-graphene-oxide/PTh material (SG).
Synthesis of SG
In a typical synthesis, reduced-graphene-oxide was synthesized by the hydrazine reduction of graphene oxide (Figs. S1 and S2) . To synthesize SG, the required amount of reduced-graphene-oxide (10 wt.%) and thiophene were dispersed in 50 mL chloroform (CHCl3) at room temperature using sonication. To this solution 8 g of FeCl3 dissolved in 100 mL CHCl3 was added, while maintaining the temperature at 0 °C. The resulting solution was stirred and allowed to polymerize for 10 h. After polymerization, 100 mL methanol was added and the resulting mixture was filtered. The obtained precipitate was washed with 300 mL methanol. The precipitate was then resuspended in 100 mL 1 M hydrochloric acid at room temperature, and the resulting suspension was stirred for 2 h. This solution was filtered and the precipitate was washed with deionised water until a neutral pH was observed. Finally, the SG material was dried at 60 °C for 24 h under vacuum. The synthesis of PTh was conducted using the same synthetic procedure in the absence of reduced-graphene-oxide. The activation of SG was carried out using a 7 M potassium hydroxide solution [35,36]. Typically, 150 mg of SG was dispersed in 10 mL of 7 M KOH and stirred at room temperature for 24 h. The resulting solution was filtered and the precipitate dried in an oven at 70 °C. Chemical activation was achieved by heating the precipitates for 1 h in a tube furnace under a N2 atmosphere (100 sccm) at temperatures ranging from 400–700 °C (heating rate = 3 °C min 1). After activation the product was washed with 8% HCl solution to neutralize excess KOH. The solution was then filtered and the precipitate washed with excess water until a neutral pH was observed. Finally, the precipitate was dried under vacuum at 70 °C for 24 h.
X-ray diffraction patterns (XRD) were recorded on a Riguka, Japan, RINT 2500 V X-ray diffraction-meter with Cu Ka irradi˚ ). Fourier transformed infrared (FTIR) specation (k = 1.5406 A tra were recorded in KBr pellets with a Bruker FTIR. Raman spectra were carried out using a Senterra Raman Scope system with a 532 nm wavelength incident laser light and power 20 mW. Scanning electron microscopy (SEM) images of the materials were taken using a field emission scanning electron microscope (FESEM, JEOL, FEG-XL 30S). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEM2100F (Cs corrected STEM) electron microscope with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALAB-220I-XL (THERMO-ELECTRON, VG Company) device. Photoemission was stimulated by a nonmonochromatized Mg Ka source (1253.6 eV) for all samples.
Gas adsorption measurements
Gas adsorption measurements were carried out using a Belsorp mini II (Japan) device. Before each measurement the samples were heat treated at 150 °C in vacuum for 24 h. BET (Brunauer–Emmett–Teller) surface area and pore size distributions were calculated from N2 adsorption–desorption isotherms measured at 77 K. H2, N2, CH4 and CO2 adsorption isotherms were measured at 298 K and up to 1 atm.
Results and discussion
Material synthesis and characterization
The SG material was synthesized by the polymerization of thiophene in the presence of reduced-graphene-oxide. The as synthesized SG material was chemically activated using KOH and heat treatment at temperatures from 400 to 700 °C (a-SG4–a-SG7). SEM images of the PTh, SG and a-SG materials are shown in Fig. 1 and Fig. S3. The SEM image of bulk PTh shows a typical granular morphology (Fig. 1b), whereas the SEM image of SG shows a graphene like surface similar to reduced-graphene-oxide (Fig. 1a and c). In the SG sample, the absence of PTh granules indicates growth of PTh on the reduced-graphene-oxide sheets (Fig. 1c). The SEM image of aSG6 shows the formation of a porous carbon structure (Fig. 1d). Additionally, the TEM image of a-SG6 shows a porous surface due to formation of holes on the SG surface after activation with KOH (Fig. 1e), while the high resolution TEM image of a-SG6 displays the amorphous nature of the material (Fig. 1f). From the electron energy loss spectroscopy (EELS) mapping study of the a-SG6 material, a uniform distribution of S on the surface can be seen (Fig. 2). This result shows that S has been successfully included in the activated hybrid material. XRD spectra were recorded for SG and a-SG samples to study the effect of activation temperature on graphitization in the sample (Fig. S4). The characteristic reduced-graphene-oxide broad (0 0 2) peak located at around 2h 23° in SG is found to be present in all the a-SG samples indicating the presence of reduced-graphene-oxide in the materials (Figs. S2b and S4) [37–39]. Upon activation the broad peak at 23° narrows, this indicates that graphitization occurs at higher temperatures. FTIR spectra of the samples were recorded using KBr pellets (Fig. S5). For the SG sample, the bands centered at 1719 and 1620 cm 1 correspond to the >[email protected]
and [email protected]
functional groups of reduced-graphene-oxide . The remaining peaks in the SG material are attributed to thiophene, with the peaks at 1480, 1450, 1033 and 786 cm 1 corresponding to [email protected]
asymmetric ring stretching, [email protected]
symmetric ring stretching, in-plane C–H aromatic bending and out of plane C–H aromatic bending, respectively. Additionally, the two peaks at 887 and 690 cm 1 indicate C–S stretching in the thiophene ring [37–39]. The FTIR spectra of a-SG show a decrease in the intensity of the graphene oxide absorption bands with an increase in activation temperature. This decrease in intensity indicates the graphitization of the hybrid material, which agrees well with the XRD data. Raman spec-
6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
Fig. 1 – SEM images of (a) reduced-graphene-oxide, (b) PTh, (c) SG and (d) a-SG6 along with (e) TEM and (f) HRTEM images of aSG6.
Fig. 2 – (a) TEM image of a-SG6 and EELS mapping for (b) C and (c) S. (A colour version of this figure can be viewed online.)
tra of the SG and a-SG4 samples show a distinct absorption band at 1450 cm 1, which is attributed to the presence of PTh in the samples (Fig. S6) . However, in the a-SG5–aSG8 samples the disappearance of the PTh bands and the emergence of the D and G bands of graphene indicate graphitization occurring in the sample. The emergence of these characteristic bands and the downward shift of the G peak compared to reduced-graphene-oxide indicate that doping of S into the graphene framework has occurred (Fig. S6) . The surfaces of the adsorbent materials were analyzed using XPS. The full survey spectra of SG and a-SG show the presence of C, O and S only (Fig. 3). The C, O, and S compositions of the materials were calculated from the corresponding peak areas of the XPS spectra (Table S1). Deconvolution of the C1s core level peak of the SG and a-SG samples show the presence of peaks at 284.5, 286.0 and 288.9 eV corresponding to the C–C, C–S and [email protected]
bonds, respectively (Fig. S7). Deconvolution of the S2p core level peak of the SG and a-SG samples show peaks located at 163.6 and 164.7 eV corresponding to neutral S (Fig. S8) [42,43]. In the a-SG samples an additional peak appears at 168 eV due to oxidized S.
Gas adsorption measurements
Fig. 4 shows the N2 adsorption–desorption isotherms of the SG material before and after chemical activation. The N2
Fig. 3 – XPS full survey spectra of SG and a-SG samples. (A colour version of this figure can be viewed online.)
isotherm of the a-SG samples display a type I shape according to the IUPAC classification . The micropore size distribution and micropore volume of the samples were calculated using the NLDFT method as shown in Table S1 and Fig. S9. The molecular size of CO2 is 0.209 nm, therefore only pores less than 1 nm are effective for efficient CO2 adsorption at atmospheric pressure [45–47]. In contrast at higher pressures
6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
Fig. 4 – The N2 adsorption (filled symbol)–desorption (open symbol) isotherms of SG and a-SG samples. The observed N2 adsorption in the low pressure regime indicates the existence of microporosity, while the small hysteresis loop at partial pressures of 0.5–0.8 shows the existence of mesopores in the material. (A colour version of this figure can be viewed online.)
CO2 is adsorbed in the supermicroporosity range (pore sizes between 0.7 and 2 nm). Current research efforts are focused on increasing the interaction between the adsorbent and CO2 which can be achieved by increasing the surface area, as well as modifying the surface and pore structure [48–52]. Fig. S9a shows that the a-SG6 sample exhibits the largest micropore volume at 0.6 nm pore size, when compared to other samples. Furthermore, the sample exhibits an optimum oxidized S content (Table S1). In contrast, the a-SG7 sample exhibits a large supermicroporosity but a small oxidized S content and 0.6 nm micropore volume as compared to aSG6. From this, we can conclude that these two factors (micropore volume and oxidized S content) play the most important role for large CO2 adsorption in the synthesized material. The specific surface area of the samples was calculated from the N2 adsorption isotherms using the BET theory (Table S1). It was found that the surface area of the SG material increases from 579–1567 m2 g 1 with an increase in activation temperature from 400 to 700 °C. It was found that the CO2 adsorption capacity of the material increases with an increase in activation temperature up to 600 °C, however a further increase in activation temperature leads to a decrease in adsorption capacity (Fig. 5). This decrease in adsorption capacity with an increase in activation temperature above 600 °C can be explained by a decrease in oxidized S content (Table S1). It is also evident that CO2 adsorption in the activated samples does not show a correlation with the S content (Fig. 6). We can as such conclude that the CO2 adsorption capacity is largest for the a-SG6 sample due to an enhanced interaction between adsorbate and adsorbent, where the adsorbent displays an optimal oxidized S content and microporosity. The a-SG6 sample displays a maximum adsorption capacity of 4.5 mmol g 1 at 298 K and 1 atm, which is far greater than recently synthesized S containing porous carbon materials  and better than that of recently reported N containing
Fig. 5 – CO2 adsorption isotherms for SG and a-SG samples at 298 K (filled symbols: adsorption, open symbols: desorption). (A colour version of this figure can be viewed online.)
porous carbon materials [34,54–56]. Additionally, it should be noted that the synthesized material shows a larger adsorption capacity at low partial pressures (1.82 mmol g 1 at P/ P0 = 0.2) than other recently synthesized materials [34,54– 56]. For this reason we believe that this material has potential to be applied as an industrial CO2 adsorbent for post combustion capture applications. The graphs of adsorption capacity vs. BET surface area, micropore volume and oxidized S content show a clear correlation, while the plot of S content vs. adsorption capacity shows an inverse relation (Fig. 6 and Fig. S10). These correlations indicate that pore volume, surface area and oxidized S are the determining factors in the large adsorption capacity of the a-SG6 material. In the oxidized S vs. adsorption capacity correlation graph, the SG material data point is omitted as it displays no oxidized S. The correlation graphs show that to further increase CO2 adsorption capacity in the material it is important to increase the oxidized S content and the microporosity of the samples. From calculations using Density Functional Theory (DFT), it is evident why the oxidized S containing porous compounds outperforms previously reported N containing porous compounds [34,54–56]. As can be seen in Fig. 7b, when the CO2 molecule interacts with the N moiety in pyrrole  the binding energy (3.4 kcal mol 1) is less than when it interacts with the di-/mono-oxidized S moiety in thiophene (4.3/ 6.0 kcal mol 1). Since the mono-oxidized S moiety is known to be less stable, the di- (or tri/tetra-oxidized) S moiety is likely to strongly interact with CO2. The existence of the dioxidized S moiety was verified using XPS (Fig. S8). This larger binding energy indicates a more stable and favorable bond formation between adsorbent and adsorbate, which translates to an increased adsorption capacity. According to natural bonding orbital (NBO) calculations, both O in thiophene ( 0.94) have a significantly larger negative charge than the N in pyrrole ( 0.59). This negative charge on the O is due to the highly positive charge of S (Figs. S11 and S12). The attraction energy of SO2 with CO2 [due mainly to the attraction energy between Othiophene ( 0.94) and CCO2 (+1.07) after removing
6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
Fig. 6 – Plot showing the relationship between CO2 adsorption versus (a) S content and (b) oxidized S content for SG and a-SG samples. (A colour version of this figure can be viewed online.)
Fig. 7 – Molecular interactions of CO2 with (a) di-oxidized S of thiophene, (b) mono-oxidized S of thiophene, (c) pyrrolic N and (d) pyrrolic hydrogen. The binding energies are 4.3, 6.0, 3.4 and 3.1 kcal mol 1, respectively. The dotted distances are denoted in Angstrom. All calculations were performed using the DFT (M062X/6-31+G*) method. (A colour version of this figure can be viewed online.)
the two Othiophene OCO2 repulsions] is much larger than that of pyrrole NH with CO2 [due mainly to the attraction energy between Npyrrole ( 0.59) and CCO2 (+1.06) after removing the two Npyrrole OCO2 repulsions]. These results reflect that the oxidized S content plays a significant role in increased CO2 adsorption. For application in industry, adsorbents should exhibit high adsorption capacity and stability during recycling experiments. On the basis of its inherently large adsorption capacity, the a-SG6 material was chosen for recycling and selectivity studies. Fig. S13 shows the adsorption capacity of the a-SG6 material during CO2 recycling at 298 K for 10 cycles. During recycling, desorption was achieved using a combination of thermal treatment (150 °C) and evacuation. The a-SG6 material shows an initial decrease of 10% in its adsorption capacity (4.5–4.0 mmol g 1) after 9 cycles, where after the material retains its adsorption capacity. This initial decrease in adsorption capacity could be attributed to the instability of the various oxidation states that S can adapt during the harsh activation process. Selectivity studies were conducted, showing that the material adsorbs CO2 selectively over H2, N2 and CH4 (Fig. 8). Using the initial slopes method, the selectivity was calculated to be 51, 12 and 214 for N2, CH4, and H2,
Fig. 8 – Adsorption–desorption curves of a-SG6 at 298 K in H2, N2, CH4 and CO2 atmospheres (filled symbols: adsorption, open symbols: desorption). (A colour version of this figure can be viewed online.)
respectively (Fig. S14). These values are very impressive from an industrial applications point of view, due to the very large
6 6 ( 2 0 1 4 ) 3 2 0 –3 2 6
selectivity that exists between CO2 and N2 which is the other major component of flue gas.
S-doped microporous carbon materials were synthesized by the chemical activation of a SG material. CO2 adsorption experiments show a large adsorption capacity of 4.5 mmol g 1 for a-SG6 at 298 K and 1 atm and a stable recycling adsorption capacity. Additionally, this material shows a high adsorption capacity at low partial pressures, 1.82 mmol g 1 at P/P0 = 0.2. The material shows impressive selectivity for CO2 adsorption over N2, CH4 and H2. The factors determining the CO2 adsorption capacity for this material were found to be surface area, oxidized S content and microporosity.
Acknowledgements This work was supported by NRF (National Honor Scientist Program: 2010-0020414, WCU: R32-2008-000-10180-0) and KISTI (KSC-2011-G3-02).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon.2013. 09.006.
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