Prakash Ramakrishnan and
Sangaraju Shanmugam*
Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1 Sang-Ri, Hyeongpung-Myeon, Dalseong-gun, Daegu, 711-873, Republic of Korea. E-mail: sangarajus@dgist.ac.kr; Fax: +82-53-785-6409; Tel: +82-53-785-6413
First published on 31st October 2014
We report arch and hollow nanocarbons with high nitrogen content and appreciable surface area that are highly capable of adsorbing CO2 (4.23 mmol g−1), are selective (CO2/N2: ∼13%) and have high facile regeneration properties (98%) under ambient conditions, respectively.
In recent decades, porous carbon materials, a class of nanocarbons, have found potential applications in energy storage, gas adsorption, molecular separation, catalysis, and so on due to their tunable structural texture, adjustable surface functionality, chemical stability, low price and easy availability in versatile forms such as fibers, powders, sheets, foams, composites and tubes, etc.6–10 Porous carbons with surface functionalized groups that have high surface area are the most desirable for obtaining high electro-sorption and gas sorption properties.11 Nitrogen functionalized carbon materials have drawn much attention for CO2 adsorption, as N-doping generates basic groups that promote the adsorption of acidic CO2 gas.12 Mostly, N-doped porous carbons are prepared by two common approaches: post heat treatment synthesis using high surface area carbon materials under a N-source, and an in situ synthesis approach, i.e., a hard and soft template approach.13–16 However, the former approach results in low N-content, and in the latter approach complicated microphase separation, expensive templates for synthesis, toxic chemicals for etching and a time consuming tedious carbonization process are required.17 Alternatively, by using electrospinning, a high N-content can be obtained using N-source polymers.18 However, the N-doped carbon materials obtained using this approach often have limited surface area due to their cylindrical fibrous morphology. Thus, a post activation process using KOH treatment (physical or chemical activation) was adopted to increase the surface area, but at the cost of the high N-content.19 Thus, a facile and cost-effective method to fabricate N-doped nanocarbons with high surface area for CO2 adsorption is also necessary for use in CO2 capture applications.
Herein, we meticulously designed a new class of carbon material with a high level of N-doping and appreciable surface area. Porous carbon nanostructures with different morphologies were fabricated via a co-axial electrospinning approach, followed by leaching of sacrificial material using hot de-ionized water and a subsequent carbonization under inert atmosphere. Thus, two different N-doped carbon nanostructures, hollow (HCNR) and arch (ACNR) shaped morphologies, were obtained. The developed N-doped ACNR materials exhibit good textural properties along with a high N-content under ambient conditions, 298 K and 1 bar. In CO2 capture studies, a maximum CO2 adsorption of 4.23 mmol g−1, a good CO2 selectivity of ∼13%, and a regenerative capability of 98%, were obtained.
The fabricated N-doped porous HCNR and ACNR are illustrated in Scheme 1. The co-axially prepared electro-spun fibrous membranes of the HCNR and ACNR samples display average diameters of 270 and 330 nm, respectively. During the leaching process, the hydrophilic PVP core material was leached out completely from the electro-spun HCNR and ACNR membranes. Finally, carbonization of the HCNR material results in a hollow structure with a core diameter of 40 to 70 nm, while the carbonized ACNR material has an arch-shaped or semi hollow carbon nanostructure. Furthermore, during the carbonization, the diameters of the HCNR and ACNR decrease to 180 and 155 nm, respectively (Fig. 1(b) and (d)).
Fig. 1 HCNR sample FE-SEM (a and b) and FE-TEM (e) images. ACNR sample FE-SEM (c and d) and FE-TEM (f) images. |
The FE-TEM images in Fig. 1(e) and (f) confirm the morphologies of the HCNR and ACNR samples. For comparison, carbon nanorods (CNRs) were fabricated using a single spinneret electrospinning approach and carbonized using the same parameters as used for the ACNR and HCNR samples. The carbonized CNRs display a typical cylindrical rod shaped morphology as shown in Fig. S1(a)–(d).†
The XRD spectra reveal that all the carbonized samples were turbostatic carbon in nature (Fig. S2†). Through CHNS elemental analysis, the N-contents were found to be 8.51, 7.53 and 8.07 weight% for the CNR, HCNR and ACNR samples, respectively. The XPS spectra of the N-doped carbon nanostructures show strong signals for carbon, nitrogen and oxygen. The deconvoluted N 1s spectra of all the samples show the existence of pyridinic-N and quaternary-N (Fig. S3†). It is noted that the nitrogen functionalities act as Lewis base sites, which are active for CO2 sequestration.20 The peak deconvolution analysis shows that the amounts of pyridinic (N1) in the CNR, HCNR and ACNR samples were found to be 30, 37 and 35%, respectively.
Brunauer–Emmett–Teller (BET) measurements (Fig. S4 and Table S2†) reveal that the ACNR, HCNR and CNR samples exhibit BET surface areas (SBET) of 619, 557 and 484 m2 g−1, respectively. The t-plot micropore surface areas for ACNR, HCNR and CNR are 432, 417 and 334 m2 g−1, indicating 70–74% microporosity in all the samples (Table S2†). The total pore volumes from NL-DFT studies for the ACNR, HCNR and CNR samples are 0.6589, 0.5681 and 0.4803 cm3 g−1, respectively. The ACNR and HCNR samples exhibit high surface area, pore volume and micro-pore area, which means they could be highly suitable for gas sorption applications, such as H2, CH4, or CO2 capture.
To counter a major greenhouse gas component, CO2, our N-doped porous carbon materials were employed for CO2 sequestration and their performance was measured. Fig. 2(a) shows the CO2 and N2 adsorption isotherms of ACNR, HCNR and CNR at 298 K, which show that a maximum CO2 adsorption of 4.23 mmol g−1 at 1 bar was observed for ACNR, higher than that of the other materials. Also, at 273 K, ACNR exhibits a high adsorption of 6.53 mmol g−1 at 1 bar (Fig. S5(a)† and Table 1). The high CO2 adsorption of ACNR is due to its high surface area (619 m2 g−1), high pore volume (0.6589 cm3 g−1) and high N-content (8.70 wt%). Also, the N-species from pyridinic groups, which as explained in the XPS studies exist in all N-doped carbon nanostructures and act as Lewis base sites for CO2 adsorption, play a vital role, particularly in low pressure regions.20 In addition, the ACNR and HCNR samples display no saturation limit for CO2 at 1 bar in comparison to the CNRs, which indicates that high CO2 adsorption could be possible at elevated pressures. The same phenomenon of CO2 capture was even observed at various temperatures, including 273 K, 293 K and 303 K (Fig. S5(a)–(c)†), and the obtained results are listed in Table 1. In comparison, the cylindrical CNR sample shows poor CO2 adsorption because of its low surface area, despite it having a high N-content.
Fig. 2 (a) CO2 adsorption and CO2/N2 selectivity tests conducted at 298 K for all samples. (b) CO2 regenerative test for ACNR at 298 K using high purity CO2 gas. |
Comparison with the CO2 capture of various N-doped and N-free state-of-the-art carbon sorbents suggests that higher and comparable CO2 adsorption capacities for the ACNR and HCNR samples were achieved (Table S3†). It is clear from this comparison that N-doped ACNR could be considered amongst the potential N-doped carbon materials for CO2 adsorption applications.
In addition to CO2 adsorption evaluation, the selectivity for CO2 over other flue gas components, predominately N2 gas (70–80%), is crucial for power plants during the post combustion process. It is also to be expected that the presence of N-content could result in the preferential adsorption of CO2 over N2.21 Thus the ideal adsorption solution theory (IAST) was used to evaluate the selectivity for CO2/N2 of all the samples. As shown in Fig. 2(a), the selectivities for CO2 over N2 gas at 298 K for ACNR, HCNR and CNR were found to be 13.53, 9.63, and 8.87%, respectively, which reveals that the ACNR sorbent has good selectivity for CO2 gas. Also, ACNR selectivity is higher than that of other porous materials, such as MOF-253, which possesses a selectivity of 12%.22
For industrial CO2 capture applications, the adsorbents should possesses the critical criteria of high cyclic stability towards CO2 adsorption. To ascertain this, a CO2 regenerative/cycle test for the best CO2 adsorption sample, ACNR, was tested by performing ten subsequent adsorption–desorption processes at 298 K under vacuum evacuation, and monitoring each cycle (Fig. 2(b)). This revealed that ACNR has 98% CO2 retention after 10 cycles. Thus, the ACNR sample with high CO2 adsorption, good selectivity and regenerative capability characteristics is impressive for industrial applications.
Also, to determine the strength of interaction between CO2 and our N-doped carbon materials the isosteric heat of adsorption (Qst) was calculated using the Clausius–Clapeyron equation for CO2 isotherms performed at 273, 293, 298 and 303 K (Fig. S5(d)†). The Qst evaluated for all samples indicate the physisorption nature of an absorbent, i.e., weak van der Waals forces. Also, it was found that N-content in all the samples could substantially improve the isosteric heat of adsorption at low coverage, but the micropores play a vital role in high coverage, which overshadows the N-content role. From the results, ACNR had the highest Qst value, 36.27 kJ mol−1; HCNR and CNR had lower Qst values of 20.80 kJ mol−1 and 7.8 kJ mol−1, respectively. In general, the high Qst is a necessary criteria for flue gas separation, owing to the high CO2 uptake at very low pressure. At the same time, too high a Qst, which would result in chemisorption, would cause difficulty in the regeneration of the sorbents.23 Notably, ACNR with a Qst of 36.27 kJ mol−1 is highly suitable for flue gas separations.
In summary, we have successfully developed high N-content and appreciable surface area of arch and hollow nanocarbons, by using a simple co-axial electrospinning technique. Their CO2 uptake properties were demonstrated to be superior in all aspects, including higher CO2 uptake amounts, selectivity of CO2 at low pressure, regenerative capability and much higher Qst. Nanocarbons enabling such cost effective and appreciable CO2 sequestration could possibly meet the criteria of commercial CO2 sorbents. Further, we hope that these unique N-doped nanocarbon structures will open new opportunities in applications related to H2 adsorption, electrocatalysts for CO2 reduction, energy storage, separation, and medicine.
Footnote |
† Electronic supplementary information (ESI) available: Figures of FE-SEM, FE-TEM, and N2 adsorption–desorption, and Tables of deconvoluted XPS analysis for all samples. See DOI: 10.1039/c4ra09200g |
This journal is © The Royal Society of Chemistry 2014 |