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Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture
Accepted Manuscript Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture
Xiancheng Ma, Liqing Li, Zheng Zeng, Ruofei Chen, Chunhao Wang, Ke Zhou, Hailong Li PII: DOI: Reference:
S0169-4332(19)30784-6 https://doi.org/10.1016/j.apsusc.2019.03.162 APSUSC 42117
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
25 October 2018 4 March 2019 16 March 2019
Please cite this article as: X. Ma, L. Li, Z. Zeng, et al., Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.03.162
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ACCEPTED MANUSCRIPT Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture Xiancheng Maa, Liqing Lia*, Zheng Zenga, Ruofei Chena, Chunhao Wanga, Ke Zhoua, Hailong Lia a
School of Energy Science and Engineering, Central South University, Changsha 410083, Hunan, China
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* To whom correspondence should be addressed:
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TEL: +86-731-8887-9863
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Email: [email protected]
ACCEPTED MANUSCRIPT Abstract: Will the CO2 capture be affected by the N-doping? This question remains conflict due to the effect of oxygen content in N-doped porous carbons on CO2 uptake has not been systematically investigated. Herein, the effects of N-free and N-doped porous carbons on CO2 uptake were investigated by experiments and theoretical calculations. To elucidate the relative influences of nitrogen functional groups, we
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synthesized a series of carbons without or with N-doping (2.73-9.44% N) by varying the synthesis
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conditions. Experimental results show that the introduction of oxygen and nitrogen into carbon framework
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improves CO2 capture in porous carbons (PCs) and N-doped porous carbons (NPCs). Among these samples, the NPC600 exhibits an exceptionally high CO2 adsorption capacity (5.01 mmol g-1 at 1 bar and 25 ℃).
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Based on the theoretical calculations, the introduction of nitrogen into carbon framework with high oxygen
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content further enhances electrostatic interaction for CO2 adsorption. Moreover, the doping of nitrogen to carbon framework also has a greater effect on both the selectivity for CO2/N2 and the isosteric heat of CO2
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influence of N-doping on CO2 capture.
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adsorption. It is predicted that this investigation will eliminate any ambiguities and better explain the
1. Introduction
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Keywords: porous carbon, N-doping, O-doping, CO2 adsorption
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The main factor currently considered to lead to global warming is generated CO 2 from the burning of fossil fuels [1, 2]. Despite many efforts made worldwide, however, fossil fuels remain the most economical source of energy. Therefore, in order to reduce CO2 emission into the atmosphere, it is urgent and necessary to develop low-cost and sustainable methods to improve the capture and storages of CO2. Recently, various CO2 adsorption technologies have been applied, including membrane separation, physisorption, and chemisorption [3-5]. Among these methods, amine-based chemisorption has been extensively applied to CO2 capture in industry, but it suffers from intrinsic limitations such as the adsorbent basic non-renewability, the
ACCEPTED MANUSCRIPT operational safety, and intrinsic corrosiveness[6]. CO2 physisorption using porous materials has also caused extensive researches due to its easy maintenance, less energy intensiveness, and easy renewability. For example, porous materials such as zeolite[7, 8], metal-organic frameworks[9, 10], silica[11], and porous carbon[12, 13] have recently been used as adsorbents for CO2 uptake. In particular, porous carbon materials
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not only have the above-mentioned advantages but also have wide source of raw material, which are considered to the most promising CO2 adsorbents [14, 15].
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Fundamentally, the adjustment of porous structure and surface chemistry provides an effective way to
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improve the CO2 adsorption performance [13, 16-19]. Previous investigates have shown that the CO2
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adsorption performance depends strongly on the porous structure parameters of carbon, in particular to narrow micropore volume (<1 nm)[20, 21]. For example, Sevilla et al.[22] synthesized activated carbons
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exhibiting outstanding micropore surface areas by hydrothermal carbonization of biomass. The CO2 adsorption capacity was 4.8 mmol g-1 at 1 bar and 25 ℃. They reported that the significant CO2 adsorption
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capacity is mainly due to the presence of narrow micropore volume. Meanwhile, the introduction of heteroatom can enhance the electronegativity of the carbon framework, thereby improving CO2 adsorption
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capacity in comparison with non-doped carbon materials [23, 24]. Therefore, the carbon material with ideal porosity structure and suitable surface chemistry is considered as an optimal candidate for CO2 capture.
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However, most works about investigating CO2 adsorption performance of the porous carbon materials focused on a single factor, such as either narrow micropore volume or surface chemistry (N- or S- or O-containing functional groups)[25-27]. The synergistic effect of the porosity structure and surface chemistry on the CO2 adsorption has rarely been reported. In addition, several previous literatures report that N-doping does not promote the CO2 capture, and the CO2 capture on carbons is primarily determined by the narrow micropore volume [28, 29]. These investigates are inconsistent with the results of Xing et al, who studied N-doped porous carbons and found that CO2 adsorption capacity was not related to the micropore
ACCEPTED MANUSCRIPT volume and the specific surface area of the porous carbons, but closely associated with the N functionalities on carbons[30]. However, most porous carbons used for CO2 uptake have oxygen-containing functional groups, which may be derived from carbon precursors and activated reagent (i.e. KOH, H3PO4, H2O and CO2)[31-34]. The beneficial effect of oxygen functional groups on CO2 uptake has also been ignored.
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Herein, we synthesize a series of N-free and N-doped porous carbons from glucose by the KOH chemical activation. The resulting carbons exhibit the superior CO2 adsorption performance with the capacities of
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5.01 mmol g-1 and 7.60 mmol g-1 at 25 ℃ and 0 ℃, respectively. We carry out theoretical calculations and
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various material characterizations to demonstrate the relative effects of O-doping and N-doping in porous
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carbons for CO2 capture and to find whether oxygen groups play an important role in PCs and NPCs, which paves a new way for designing and preparing a new generation of CO2 adsorbents with the superior
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performance. 2. Experimental section
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2.1 Preparation of porous carbons (PCs) and N-doped porous carbons (NPCs) Preparation of PCs. The glucose was subjected to a hydrothermal treatment as follow; 4 g of the glucose
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was dispersed in 60 mL of distilled water, placed in a stainless steel autoclave and then heated to 180 ℃ for 10 h. The resulting product represented as hydrochar (HC) was recovered and dried. The product was
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chemically activated using KOH. Briefly, 1g of HC sample combined with 2 g of KOH was transferred in a furnace, after the composite was activated under N2 atmosphere at 500, 600, 700, or 800 ℃ with a heating rate of 3 ℃ min-1 for 1 h. The resulting samples were washed thoroughly with 5 wt% HCl solution and deionized water. Finally, the PCs was obtained and dried at 120 ℃ overnight. Preparation of NPCs. Hydrochars were prepared by a hydrothermal treatment of glucose and ethylenediamine; 8 g glucoses and 4 g ethylenediamine were added into 60 mL water and transferred to a stainless steel autoclave, and then heated to 180 ℃ for 10 h. The obtained HC was washed with deionized
ACCEPTED MANUSCRIPT water and then dried at 120 ℃. The KOH activation process is similar to that for preparing PCs. The resulting samples were washed thoroughly with 5 wt% HCl solution and deionized water. Finally, the NPCs was obtained and dried at 120 ℃ overnight. 2.2. Characterization of materials
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The X-ray diffraction (XRD) patterns were examined by X-ray Diffractometer (XRD; Bruker D8 Advance) using Cu/Ka radiation. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2
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20S-Twin electron microscope. The chemical structure was collected using a confocal Raman microscope
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with a 532 nm laser as the excitation source. N2 adsorption adsorption were measured by using a
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JW-BK132Z (Beijing JWGB Sci & Tech Co., Ltd) static volumetric analyzer at 77 K. The elemental composition on the carbon surface was measured using a X-ray photoelectron spectroscopy (XPS) analyzer
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with Al Kα (Thermo Fisher Scientific Inc.). CO2 adsorption isotherms were measured using a JW-BK132Z instrument (Beijing JWGB Sci & Tech Co., Ltd) at 0 ℃ and 25 ℃. CO2 adsorption isotherms were also used
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to calculate pore size distribution and pore volume for pores (0.3–1.5 nm) using the NLDFT. The selected NLDFT model is assumed to be a slit-shaped pores having a uniform density of carbon pore wall.
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3. Results
3.1 Textural properties of PCs and NPCs
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The XRD patterns of PC600 and NPC600 are displayed in Figure 1a. Note that the number of 600 represents the activation temperature of 600 ℃ during the sample preparation. The two diffraction peaks at 25°and 44°can be attributed to the (002) and (101) plane of the graphitic carbon, indicating a low degree of graphitization[35]. Correspondingly, as shown in Figure 1b, the Raman spectra of PC600 and NPC600 show two prominent bands pointing to D-band (1330 cm-1) and G-band (1580 cm-1), which are related to the disordered carbon structures and crystalline graphitic carbon. Considering that the intensity ratio of the G and D bands (IG/ID) can be used to reflect the graphitic degree of carbon, and the IG/ID ratio of PC600 (0.89)
ACCEPTED MANUSCRIPT is larger than that of NPC600 (0.85), the introduction of nitrogen into carbon framework increases the levels of defects and disorder in carbon materials. In addition, compared with NPC600, PC600 shows a higher second-order band at about 2750 cm-1 (Figure S1), suggesting the presence of stronger feature of few-layered graphene[36, 37]. (b)
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Figure 1 X-ray diffraction (XRD) (a), and Raman spectra (b) of PC600 and NPC600.
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ACCEPTED MANUSCRIPT Figure 2 Nitrogen sorption isotherms (a, b) and micropore size distributions based on CO2 adsorption isotherms (c, d) of carbon materials. The N2 sorption-desorption isotherms and pore size distribution (PSD) based on CO2 adsorption isotherms of PCs were shown in Figure 2a,c, and the corresponding textural properties are listed in Table 1. As illustrated in Figure 2a, high microporosity of PCs presents since the isotherms of PCs exhibit type I and
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these isotherms exhibit narrow knees at a low relative pressure (P/P0＜0.03). As the activation temperature
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increases, the knee of the isotherms slightly widened, indicating that the micropore size increases. This is
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corroborated by our data analysis, the porosity of PCs is mainly composed of very narrow micropores, and their sizes are generally less than 1 nm, and the peak center of pore size are mainly 0.35, 0.55 and 0.85 nm.
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As illustrated in Table 1, the BET surface area of PCs rises from 972 m2 g-1 for PC500 to 2305 m2 g-1 for
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PC800. As the activated temperature increases, the total pore volume shows a similar trend, increasing from 0.493 mL·g-1 to 1.123 mL·g-1.
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The N2 sorption-desorption isotherms for the NPCs at 500 ℃, 600 ℃ and 700 ℃ (Figure 2b) are mainly
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type I and these samples are predominantly microporous. The NPC800 changed from type I to nearly type IV, indicating that the small mesopore range becomes larger. As shown in Figure 2b, the adsorption isotherm
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of the NPC800 has a very wide knee, and the adsorption linearity increases to a P/P0 of 0.4, suggesting that
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the small mesopores significant increase. Associated with the surface area rising from 1082 m2 g-1 for NPC500 to 2940 m2 g-1 for NPC800 (Table 1), the pore volume increases from 0.766 mL·g-1 to 2.126 mL·g-1 with the activation temperature increased. Table 1 Texture properties surface chemistry of PC and NPC samples SBET/ Smicroa/ Vpb/ Sample m2 g-1 m2 g-1 cm3·g-1 PC500 PC600 PC700 PC800 NPC500
972 1515 1815 2305 1082
912 1400 1665 2167 952
0.493 0.898 1.017 1.123 0.578
O N yield Vmicroa/ Voc/ 3 -1 3 -1 cm ·g cm ·g at. % at. % 0.393 0.619 0.740 0.934 0.441
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14.24 11.18 11.26 7.83 12.65
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Evaluated by t-plot method. b Total pore volume at p/p0 ≈ 0.99. c Pore volume of narrow micropores (<0.7
nm) based on CO2 adsorption isotherms at 273K. 3.2. Surface chemical properties of PCs and NPCs
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The surface composition of PCs and NPCs were investigated by XPS. The O 1s spectra for PC600 and NPC600 are shown in Figure S3 and S4. As expected, the PC600 and NPC600 can be resolved into four
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peaks centered at binding energies around O1 (530.2 eV), O2 (531.2 eV), O3 (531.9 eV), O4 (532.8 eV), O5
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(533.4 eV), and O6 (535.0 eV) components, which are assigned to quinones, -COOH, -C=O, -C-O, –OH and
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H2Oadsorption related groups, respectively[38-40]. With the increase of activation temperature, COOH and OH related species are decreasing remarkably. As shown in Figure 3, the percentage of nitrogen-containing
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functional groups in NPC samples is calculated after fitting the spectra identically to nitrogen. The peaks at about 398.6, 399.8, 401.0, and 402.0 eV are attributed to pyridinic-N, pyrrolic-N, graphitic-N and
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oxidized-N, respectively[18, 41]. To understand the relationship between the preparation conditions and nitrogen-containing functional groups of the carbons, the total oxygen or nitrogen content and the amount of
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various oxygen or nitrogen-containing functional groups were calculated (Figure S5 and Figure S6).
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Figure 3 XPS species fitted N 1s for NPCs samples. (a) 500 ℃, (b) 600 ℃, (c) 700 ℃ and (d) 800 ℃.
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3.3. CO2 adsorption properties
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The CO2 adsorption isotherms of PCs and NPCs measured at 0 and 25 ℃ are displayed in Figure 4 and Figure 5, respectively. The adsorption capacity of CO2 at 1 bar is summarized in Table 2. At 1 bar, these
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PCs and NPCs have superior CO2 capture in the range of 4.33-7.60 mmol g-1 (190.52- 334.40 mg g-1) and 3.01- 5.01 mmol g-1 (132.40- 220.40 mg g-1) at 0 and 25 ℃, respectively. Among these samples, NPC600
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exhibits the highest CO2 adsorption capacity of 7.60 mmol g-1 at 0 ℃ and 5.01 mmol g-1 at 25 ℃, respectively. This adsorption capacity is higher than most of the reported adsorbents at 1 bar: porous carbon, graphene,
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carbon nanotubes, MOFs and ZIFs. For example, Zhu et al. reported that metal-and/or nitrogen-doped porous carbon derived from biomass obtained CO2 uptake of 7.30 mmol g-1 at 0 ℃ and 4.40 mmol g-1 at
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25 ℃[42]. Park and co-workers prepared borane-modified graphene-based materials with the highest CO2 adsorption capacity of 1.81 mmol g-1 at 25 ℃[43]. The nitrogen-doped carbon nanotubes were found to have the CO2 uptake of 1.53 to 1.92 mmol g-1[44]. The CO2 adsorption capacity of RMOF, MOF-210, MOF-177 were found to be about 1.00 mmol g-1[45]. At 0 ℃ and 1 bar, a series of ZIFs were found to have the CO2 adsorption capacity of less than 5.00 mmol g-1[46].
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Figure 5 CO2 adsorption isotherms at (a) 0 ℃, (b) 25℃ for NPCs. Table 2 CO2 capture capacities of the porous carbons at different adsorption temperatures
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Figure 4 CO2 adsorption isotherms at (a) 0 ℃ and (b) 25 ℃ for PCs.
CO2 capacity per mmol g-1
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0 ℃ 1 bar 4.33 (190.52) 6.38 (280.72) 6.25 (275.00) 6.99 (307.56) 5.36 (235.84) 7.60 (334.40) 7.18 (315.92) 6.24 (302.28)
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25 ℃ 0.15 bar 0.95 (41.80) 1.18 (51.92) 0.87 (38.28) 0.93 (40.92) 1.29 (56.76) 1.38 (60.72) 0.93 (40.92) 0.65 (28.60)
CO2 uptake density (μmol m-2) 25 ℃ 1 bar 6.11 4.67 3.84 3.53 6.54 5.79 3.88 2.08
ACCEPTED MANUSCRIPT In terms of the influence of O-containing functional groups and porosity texture of the PCs on their CO2 adsorption performance, we found that there is no direct relationship between the single factor (O-containing functional groups or porosity properties (i.e., SBET, VP, Vmicro or V0)) and the CO2 capture at 25 ℃ and 1 bar. For example, PC600 with intermediate level of oxygen content and SBET, VP, Vmicro, V0 showed the highest
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CO2 adsorption capacity in PCs samples. However, both PC500, which has the highest oxygen content but the lowest developed narrow micropore volume, and PC700, which has the highest developed narrow
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micropore volume but the lower oxygen content than that of PC600, show lower CO2 adsorption capacity
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than that of PC600. We can firstly assume that both oxygen content and porosity texture simultaneously
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determine the CO2 uptake.
Next move to the addition of nitrogen, the introduction of nitrogen has been proposed to mimic the amine
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scrubbing process for enhancing the CO2 adsorption capacity. It is noteworthy that NPC800 has higher specific surface area (2958 m2 g-1) and narrow micropore volume (0.207 mL g-1) than the NPC500, but it
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exhibits lower CO2 adsorption capacity of 3.36 mmol g-1 (25 ℃) than that of NPC500 (see Table 2). This implies that the doping of nitrogen and oxygen groups improve the affinity of carbon toward CO2 molecules.
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To further elucidate the greater important of nitrogen groups in determining CO2 uptake, the influence of porosity texture and functional groups on CO2 capture can be evaluated by the CO2 uptake density, given in
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μmol mL-1 in Table 2. NPC500, NPC600 and NPC700 have higher CO2 uptake density than that of N-free PCs. Therefore, the doping of N has beneficial effect on CO2 uptake density. Note that the NPC800 has lower CO2 uptake density in comparison with the PC800, since the decrease of oxygen contents in NPC800 makes the N-doping has no beneficial effect on the CO2 adsorption. It should be point out that although the NPC600, NPC700 and NPC800 have 8.02%, 5.05% and 2.73% nitrogen contents, they exhibit lower CO2 uptake density than that of PC500 (N-free). A comparison of the oxygen contents of samples reveals a significant difference (see Table 1). Indeed, whereas PC500 has the highest oxygen contents. This gives the
ACCEPTED MANUSCRIPT hints why the N-doping has no influence on CO2 adsorption in previous literatures[47]. It is important to investigate the trend of the CO2 uptake performances across the porosity structure and surface chemistry of the PCs and NPCs. No obvious trend was found between the CO2 adsorption capacity (25 ℃, 1 bar) and specific surface area or micropore volume for PCs (see Figure S7a,b). Furthermore, a
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linear trend can be found between the CO2 uptake and narrow micropore volume, as well as between the CO2 adsorption density and oxygen contents (see Figure S7c,d). A similar linear correlation can be found for
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CO2 adsorption capacity with surface area, micropore volume, narrow micropore volume and oxygen
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contents for NPCs (see Figure S8a,b,c,d). In addition, the CO2 adsorption density indicates a strict linear
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correlation with nitrogen contents. However, the presence of nitrogen makes the lower linear correlation between CO2 adsorption capacity and porosity structure for NPCs than that of PCs. This implies that the
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introduction of nitrogen groups into porous carbons decreases the effect of porosity structure (see Figure S8). These result shows that the doping of nitrogen can improve CO2 adsorption capacity.
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To clearly explain the interaction between the CO2 molecules and PC600 or NPC600, the isostheric heat of adsorption (Qst) was calculated based on the CO2 adsorption isotherms at 25 and 0 ℃ using the
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Clausius-Clapeyron equation. As shown in Figure 6a，the initial Qst (0.5 mmol g-1) varies cross a range of 31.4-35.1 kJ mol-1. Notely, the NPC600 has higher Qst value at low CO2 uptake than that of PC600. The
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possible reason is that the doping of nitrogen atoms to porous carbon enhances the interaction between CO2 molecule and carbon surface.
The NPC600 exhibits higher Qst than that of PC600 at low CO2 adsorption capacity is owing to its nitrogen content, which can enhance adsorption preference towards CO2 over N2, i. e. a higher CO2/N2 selectivity. The N2 adsorption isotherms at 25 ℃ of PC600 and NPC600 was shown in Figure 6b,c. Apparently, these two materials have a much smaller N2 adsorption capacity than that for CO2. To calculate the gas selectivity based on pure gas adsorption results, the ideal adsorption solution theory (IAST) was used
ACCEPTED MANUSCRIPT to determine the CO2/N2 selectivity of PC600 and NPC600. The CO2/N2 selectivity increases with the N2 molar fraction for both materials are shown in Figure 6d. At the typical CO2 concentration range in flue gas (15-3%), the IAST CO2/N2 selectivity reaches 9.8-21.4 and 17.7-28.2 for PC600 and NPC600, respectively. This further emphasizes the greater importance of nitrogen groups in affecting both the CO2 adsorption capacity and the selectivity. )
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Figure 6 (a) CO2 adsorption heat, (b) CO2 and N2 adsorption isotherms for PC600 at 25 ℃, (c) CO2 and N2 adsorption isotherms for NPC600 at 25 ℃ and (d) CO2/N2 selectivity (at 25 ℃) versus N2 molar fraction at 1 bar for PC600 and NPC600.
4. Discussion To better understand the interactions between oxygen or nitrogen-containing functional groups and CO2 molecules, we performed GCMC simulations and DFT calculations. The calculation details are described in Supplementary Information. Based on XRD and Raman characterization, multiple graphene sheets present on the PC600 and NPC600.As shown in Figure S9 and Figure 7, we constructed a slit-pore graphene model
ACCEPTED MANUSCRIPT (SGM) with different pore size to understand the interactions between the doping N in carbons with oxygen
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atoms and CO2 molecules.
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Figure 7 Carbon model: graphite wall with functional groups Considering that the PC600 can be assumed as an aggregated of small graphene planes with oxygen atoms,
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we defined the model of oxygen atoms inside a slit-pore graphene model as OSGM. Figure 8a shows the
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CO2 adsorption of OSGMs with different pore sizes (0.5 nm-1.1 nm) at 25 ℃ and at 1 bar. SGM achieves maximal CO2 capture at a pore size of approximately 0.6 nm. However, its CO2 capture gradually decreases
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with increasing pore size. Moreover, it also demonstrates that the introduction of oxygen into SGM
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improves CO2 uptake. For example, the maximum CO2 adsorption capacity of the OH group can reach 26 mmol mL-1. Compared with the SGM, the 0.6 nm of the OH group shows a 67.3% growth. However, the C-O and C=O groups have maximal CO2 adsorption capacity of 19.2 and 14.6 mmol mL-1 in the pore size of 0.7, which exhibit 266% and 178% compared with that of SGM. Based on the results, it can be concluded that the doping of oxygen can boost CO2 capture and the optimum pore size for CO2 adsorption is about 0.6-0.7 nm.
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Figure 9 Isosteric heat of adsorption as a function of pore size of OGSMs and N-doped SGM-OH At 25 ℃ and at 1 bar, we investigate CO2 uptakes of N-doped SGM-OHs and SGM-OH with different
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pores (0.5-1.1 nm). As shown in Figure 8b, it can be found that the maximal CO2 uptakes of N-doped SGM-OHs and SGM-OH occur among 0.6 nm. Besides, the CO2 uptake with different pore sizes follows the sequence of Graphitic-N> Pyridinic nitrogen-N> Pyrrolic-N>SGM-OH at 1 bar, supporting that the nitrogen-containing functional groups improve the interaction between the CO2 molecules and the carbon surface. Figure 9a shows the relationship between GSMs and Qst value at 25 ℃ and 1 bar. It is noteworthy that Qst value for CO2 capture on OGSMs is higher than that on GSM under the same conditions. For example, Qst
ACCEPTED MANUSCRIPT value of CO2 adsorption on OH group is 34.7 KJ mol-1, which is a 14.7 % rise than that on GSM. The above discussions indicate that the strong interaction between CO2 molecules and OGSMs contributes to its superior CO2 capture. Similarly, Figure 9b shows that the introduction of nitrogen into carbon surface enhances Qst value of CO2 adsorption.
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To investigate the influence of electrostatic interactions on CO2 uptake, we conducted an electrostatic contribution analysis. The electrostatic contribution of carbon framework changes can be estimated by using
Swith -Swithout 100% Swith
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Electrostatic contribution =
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where Swith and Swithout are the adsorption capacity with and without the gas-framework electrostatic
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interaction. The electrostatic contribution of the O-doped SGMs and N-doped SGM-OH with the pore size of 0.6 nm on CO2 uptake at 25 ℃ is shown in Figure 10. The doping oxygen into carbon framework
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interaction plays an important role for CO2 uptake. Then, similarly, the introduction of nitrogen into carbon framework further enhances electrostatic interaction for CO2 uptake. In this process, the doping of
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oxygen/nitrogen into carbon surface show stronger electrostatic interaction owing to their higher
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accepting/donating electron densities, resulting in enhancing CO2 uptake[48]. In addition, Xing et. al[30] has pointed out that the doping of N to porous carbon materials improve the hydrogen-bonding interaction between the CO2 molecules and carbon framework, which explains that the N-doping porous carbons has superior CO2 adsorption. As shown in Figure 11, computational results indicate that the oxygen and nitrogen functional groups have higher adsorption energy than that of pure graphene. Also, the distance of CO2-functional groups achieves shorter than that of pure graphene. These results suggest that the hydrogen-bonding interaction between functional groups and CO2 is stronger than that of pure graphene.
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Figure 10 Electrostatic contribution of O-doped GSMs and N-doped OSGM-OH with the pore size of 0.6
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Figure 11 Hydrogen-bonding interaction of CO2 with functionalized carbon surface. 5. Conclusions The effect of N-doping of porous carbons on CO2 uptake has been studied by analyzing the CO2 adsorption properties of two porous carbons with similar textural properties: (a) N-undoped porous carbon materials and (b) N-doped porous carbon materials. The N-doped porous carbon contains 2.73-9.44% of
ACCEPTED MANUSCRIPT nitrogen by varying the synthesis conditions. The resulting carbons exhibit superior CO2 adsorption capacity. In particular, the NPC600 exhibit an exceptionally high CO2 adsorption capacity (5.01 mmol g-1). However, previous reports indicate that the doping of nitrogen does not improve CO2 capture. The reason for this inconsistent analysis result is that there is no systematic analysis of the presence of oxygen in the porous
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carbons, since the introduction of nitrogen into carbon framework with high oxygen content further enhances electrostatic interaction for CO2 uptake. In addition, we also show that the N-doped exhibits a
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greater impact on both the selectivity for CO2/N2 and the isosteric heat of CO2 adsorption.
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Acknowledgements
Nature Science Foundation China (No. 21878338).
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References
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This work was supported by the Key R & D Projects in Hunan, China (No.2018SK2038) and the National
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ACCEPTED MANUSCRIPT Highlights We synthesized a series of carbons without N-doping and with N-doping.
The porous carbons exhibited the superior CO2 adsorption capacity.
O-doping and N-doping improve CO2 adsorption.
The interaction between functional groups and CO2 was elucidated by GCMC calculations.
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Xiancheng Ma, Liqing Li, Zheng Zeng, Ruofei Chen, Chunhao Wang, Ke Zhou, Hailong Li PII: DOI: Reference:
S0169-4332(19)30784-6 https://doi.org/10.1016/j.apsusc.2019.03.162 APSUSC 42117
To appear in:
Applied Surface Science
Received date: Revised date: Accepted date:
25 October 2018 4 March 2019 16 March 2019
Please cite this article as: X. Ma, L. Li, Z. Zeng, et al., Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.03.162
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ACCEPTED MANUSCRIPT Experimental and theoretical demonstration of the relative effects of O-doping and N-doping in porous carbons for CO2 capture Xiancheng Maa, Liqing Lia*, Zheng Zenga, Ruofei Chena, Chunhao Wanga, Ke Zhoua, Hailong Lia a
School of Energy Science and Engineering, Central South University, Changsha 410083, Hunan, China
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* To whom correspondence should be addressed:
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TEL: +86-731-8887-9863
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Email: [email protected]
ACCEPTED MANUSCRIPT Abstract: Will the CO2 capture be affected by the N-doping? This question remains conflict due to the effect of oxygen content in N-doped porous carbons on CO2 uptake has not been systematically investigated. Herein, the effects of N-free and N-doped porous carbons on CO2 uptake were investigated by experiments and theoretical calculations. To elucidate the relative influences of nitrogen functional groups, we
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synthesized a series of carbons without or with N-doping (2.73-9.44% N) by varying the synthesis
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conditions. Experimental results show that the introduction of oxygen and nitrogen into carbon framework
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improves CO2 capture in porous carbons (PCs) and N-doped porous carbons (NPCs). Among these samples, the NPC600 exhibits an exceptionally high CO2 adsorption capacity (5.01 mmol g-1 at 1 bar and 25 ℃).
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Based on the theoretical calculations, the introduction of nitrogen into carbon framework with high oxygen
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content further enhances electrostatic interaction for CO2 adsorption. Moreover, the doping of nitrogen to carbon framework also has a greater effect on both the selectivity for CO2/N2 and the isosteric heat of CO2
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influence of N-doping on CO2 capture.
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adsorption. It is predicted that this investigation will eliminate any ambiguities and better explain the
1. Introduction
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Keywords: porous carbon, N-doping, O-doping, CO2 adsorption
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The main factor currently considered to lead to global warming is generated CO 2 from the burning of fossil fuels [1, 2]. Despite many efforts made worldwide, however, fossil fuels remain the most economical source of energy. Therefore, in order to reduce CO2 emission into the atmosphere, it is urgent and necessary to develop low-cost and sustainable methods to improve the capture and storages of CO2. Recently, various CO2 adsorption technologies have been applied, including membrane separation, physisorption, and chemisorption [3-5]. Among these methods, amine-based chemisorption has been extensively applied to CO2 capture in industry, but it suffers from intrinsic limitations such as the adsorbent basic non-renewability, the
ACCEPTED MANUSCRIPT operational safety, and intrinsic corrosiveness[6]. CO2 physisorption using porous materials has also caused extensive researches due to its easy maintenance, less energy intensiveness, and easy renewability. For example, porous materials such as zeolite[7, 8], metal-organic frameworks[9, 10], silica[11], and porous carbon[12, 13] have recently been used as adsorbents for CO2 uptake. In particular, porous carbon materials
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not only have the above-mentioned advantages but also have wide source of raw material, which are considered to the most promising CO2 adsorbents [14, 15].
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Fundamentally, the adjustment of porous structure and surface chemistry provides an effective way to
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improve the CO2 adsorption performance [13, 16-19]. Previous investigates have shown that the CO2
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adsorption performance depends strongly on the porous structure parameters of carbon, in particular to narrow micropore volume (<1 nm)[20, 21]. For example, Sevilla et al.[22] synthesized activated carbons
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exhibiting outstanding micropore surface areas by hydrothermal carbonization of biomass. The CO2 adsorption capacity was 4.8 mmol g-1 at 1 bar and 25 ℃. They reported that the significant CO2 adsorption
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capacity is mainly due to the presence of narrow micropore volume. Meanwhile, the introduction of heteroatom can enhance the electronegativity of the carbon framework, thereby improving CO2 adsorption
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capacity in comparison with non-doped carbon materials [23, 24]. Therefore, the carbon material with ideal porosity structure and suitable surface chemistry is considered as an optimal candidate for CO2 capture.
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However, most works about investigating CO2 adsorption performance of the porous carbon materials focused on a single factor, such as either narrow micropore volume or surface chemistry (N- or S- or O-containing functional groups)[25-27]. The synergistic effect of the porosity structure and surface chemistry on the CO2 adsorption has rarely been reported. In addition, several previous literatures report that N-doping does not promote the CO2 capture, and the CO2 capture on carbons is primarily determined by the narrow micropore volume [28, 29]. These investigates are inconsistent with the results of Xing et al, who studied N-doped porous carbons and found that CO2 adsorption capacity was not related to the micropore
ACCEPTED MANUSCRIPT volume and the specific surface area of the porous carbons, but closely associated with the N functionalities on carbons[30]. However, most porous carbons used for CO2 uptake have oxygen-containing functional groups, which may be derived from carbon precursors and activated reagent (i.e. KOH, H3PO4, H2O and CO2)[31-34]. The beneficial effect of oxygen functional groups on CO2 uptake has also been ignored.
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Herein, we synthesize a series of N-free and N-doped porous carbons from glucose by the KOH chemical activation. The resulting carbons exhibit the superior CO2 adsorption performance with the capacities of
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5.01 mmol g-1 and 7.60 mmol g-1 at 25 ℃ and 0 ℃, respectively. We carry out theoretical calculations and
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various material characterizations to demonstrate the relative effects of O-doping and N-doping in porous
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carbons for CO2 capture and to find whether oxygen groups play an important role in PCs and NPCs, which paves a new way for designing and preparing a new generation of CO2 adsorbents with the superior
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performance. 2. Experimental section
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2.1 Preparation of porous carbons (PCs) and N-doped porous carbons (NPCs) Preparation of PCs. The glucose was subjected to a hydrothermal treatment as follow; 4 g of the glucose
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was dispersed in 60 mL of distilled water, placed in a stainless steel autoclave and then heated to 180 ℃ for 10 h. The resulting product represented as hydrochar (HC) was recovered and dried. The product was
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chemically activated using KOH. Briefly, 1g of HC sample combined with 2 g of KOH was transferred in a furnace, after the composite was activated under N2 atmosphere at 500, 600, 700, or 800 ℃ with a heating rate of 3 ℃ min-1 for 1 h. The resulting samples were washed thoroughly with 5 wt% HCl solution and deionized water. Finally, the PCs was obtained and dried at 120 ℃ overnight. Preparation of NPCs. Hydrochars were prepared by a hydrothermal treatment of glucose and ethylenediamine; 8 g glucoses and 4 g ethylenediamine were added into 60 mL water and transferred to a stainless steel autoclave, and then heated to 180 ℃ for 10 h. The obtained HC was washed with deionized
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The X-ray diffraction (XRD) patterns were examined by X-ray Diffractometer (XRD; Bruker D8 Advance) using Cu/Ka radiation. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2
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20S-Twin electron microscope. The chemical structure was collected using a confocal Raman microscope
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with a 532 nm laser as the excitation source. N2 adsorption adsorption were measured by using a
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JW-BK132Z (Beijing JWGB Sci & Tech Co., Ltd) static volumetric analyzer at 77 K. The elemental composition on the carbon surface was measured using a X-ray photoelectron spectroscopy (XPS) analyzer
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with Al Kα (Thermo Fisher Scientific Inc.). CO2 adsorption isotherms were measured using a JW-BK132Z instrument (Beijing JWGB Sci & Tech Co., Ltd) at 0 ℃ and 25 ℃. CO2 adsorption isotherms were also used
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3. Results
3.1 Textural properties of PCs and NPCs
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The XRD patterns of PC600 and NPC600 are displayed in Figure 1a. Note that the number of 600 represents the activation temperature of 600 ℃ during the sample preparation. The two diffraction peaks at 25°and 44°can be attributed to the (002) and (101) plane of the graphitic carbon, indicating a low degree of graphitization[35]. Correspondingly, as shown in Figure 1b, the Raman spectra of PC600 and NPC600 show two prominent bands pointing to D-band (1330 cm-1) and G-band (1580 cm-1), which are related to the disordered carbon structures and crystalline graphitic carbon. Considering that the intensity ratio of the G and D bands (IG/ID) can be used to reflect the graphitic degree of carbon, and the IG/ID ratio of PC600 (0.89)
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ACCEPTED MANUSCRIPT Figure 2 Nitrogen sorption isotherms (a, b) and micropore size distributions based on CO2 adsorption isotherms (c, d) of carbon materials. The N2 sorption-desorption isotherms and pore size distribution (PSD) based on CO2 adsorption isotherms of PCs were shown in Figure 2a,c, and the corresponding textural properties are listed in Table 1. As illustrated in Figure 2a, high microporosity of PCs presents since the isotherms of PCs exhibit type I and
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these isotherms exhibit narrow knees at a low relative pressure (P/P0＜0.03). As the activation temperature
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increases, the knee of the isotherms slightly widened, indicating that the micropore size increases. This is
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corroborated by our data analysis, the porosity of PCs is mainly composed of very narrow micropores, and their sizes are generally less than 1 nm, and the peak center of pore size are mainly 0.35, 0.55 and 0.85 nm.
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As illustrated in Table 1, the BET surface area of PCs rises from 972 m2 g-1 for PC500 to 2305 m2 g-1 for
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PC800. As the activated temperature increases, the total pore volume shows a similar trend, increasing from 0.493 mL·g-1 to 1.123 mL·g-1.
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The N2 sorption-desorption isotherms for the NPCs at 500 ℃, 600 ℃ and 700 ℃ (Figure 2b) are mainly
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type I and these samples are predominantly microporous. The NPC800 changed from type I to nearly type IV, indicating that the small mesopore range becomes larger. As shown in Figure 2b, the adsorption isotherm
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of the NPC800 has a very wide knee, and the adsorption linearity increases to a P/P0 of 0.4, suggesting that
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the small mesopores significant increase. Associated with the surface area rising from 1082 m2 g-1 for NPC500 to 2940 m2 g-1 for NPC800 (Table 1), the pore volume increases from 0.766 mL·g-1 to 2.126 mL·g-1 with the activation temperature increased. Table 1 Texture properties surface chemistry of PC and NPC samples SBET/ Smicroa/ Vpb/ Sample m2 g-1 m2 g-1 cm3·g-1 PC500 PC600 PC700 PC800 NPC500
972 1515 1815 2305 1082
912 1400 1665 2167 952
0.493 0.898 1.017 1.123 0.578
O N yield Vmicroa/ Voc/ 3 -1 3 -1 cm ·g cm ·g at. % at. % 0.393 0.619 0.740 0.934 0.441
0.190 0.311 0.318 0.303 0.197
14.24 11.18 11.26 7.83 12.65
9.44
11.2 8.7 6.2 3.8 10.1
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1793 2328 2958
1683 2187 2084
0.865 1.113 1.613
0.731 0.937 1.163
0.338 0.274 0.207
10.86 10.04 4.47
8.02 5.05 2.73
7.9 4.8 3.2
Evaluated by t-plot method. b Total pore volume at p/p0 ≈ 0.99. c Pore volume of narrow micropores (<0.7
nm) based on CO2 adsorption isotherms at 273K. 3.2. Surface chemical properties of PCs and NPCs
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The surface composition of PCs and NPCs were investigated by XPS. The O 1s spectra for PC600 and NPC600 are shown in Figure S3 and S4. As expected, the PC600 and NPC600 can be resolved into four
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peaks centered at binding energies around O1 (530.2 eV), O2 (531.2 eV), O3 (531.9 eV), O4 (532.8 eV), O5
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(533.4 eV), and O6 (535.0 eV) components, which are assigned to quinones, -COOH, -C=O, -C-O, –OH and
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H2Oadsorption related groups, respectively[38-40]. With the increase of activation temperature, COOH and OH related species are decreasing remarkably. As shown in Figure 3, the percentage of nitrogen-containing
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functional groups in NPC samples is calculated after fitting the spectra identically to nitrogen. The peaks at about 398.6, 399.8, 401.0, and 402.0 eV are attributed to pyridinic-N, pyrrolic-N, graphitic-N and
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oxidized-N, respectively[18, 41]. To understand the relationship between the preparation conditions and nitrogen-containing functional groups of the carbons, the total oxygen or nitrogen content and the amount of
Intensity a./u.
(b)
Intensity a./u.
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various oxygen or nitrogen-containing functional groups were calculated (Figure S5 and Figure S6).
406
404
402
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Binding energy (eV)
398
396
406
404
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398
Binding energy (eV)
396
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Intensity a./u.
Intensity a./u.
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404
402
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398
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Binding energy (eV)
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398
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Binding energy (eV)
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Figure 3 XPS species fitted N 1s for NPCs samples. (a) 500 ℃, (b) 600 ℃, (c) 700 ℃ and (d) 800 ℃.
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3.3. CO2 adsorption properties
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The CO2 adsorption isotherms of PCs and NPCs measured at 0 and 25 ℃ are displayed in Figure 4 and Figure 5, respectively. The adsorption capacity of CO2 at 1 bar is summarized in Table 2. At 1 bar, these
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PCs and NPCs have superior CO2 capture in the range of 4.33-7.60 mmol g-1 (190.52- 334.40 mg g-1) and 3.01- 5.01 mmol g-1 (132.40- 220.40 mg g-1) at 0 and 25 ℃, respectively. Among these samples, NPC600
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exhibits the highest CO2 adsorption capacity of 7.60 mmol g-1 at 0 ℃ and 5.01 mmol g-1 at 25 ℃, respectively. This adsorption capacity is higher than most of the reported adsorbents at 1 bar: porous carbon, graphene,
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carbon nanotubes, MOFs and ZIFs. For example, Zhu et al. reported that metal-and/or nitrogen-doped porous carbon derived from biomass obtained CO2 uptake of 7.30 mmol g-1 at 0 ℃ and 4.40 mmol g-1 at
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25 ℃[42]. Park and co-workers prepared borane-modified graphene-based materials with the highest CO2 adsorption capacity of 1.81 mmol g-1 at 25 ℃[43]. The nitrogen-doped carbon nanotubes were found to have the CO2 uptake of 1.53 to 1.92 mmol g-1[44]. The CO2 adsorption capacity of RMOF, MOF-210, MOF-177 were found to be about 1.00 mmol g-1[45]. At 0 ℃ and 1 bar, a series of ZIFs were found to have the CO2 adsorption capacity of less than 5.00 mmol g-1[46].
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CO2 uptake (mmol g-1)
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Figure 5 CO2 adsorption isotherms at (a) 0 ℃, (b) 25℃ for NPCs. Table 2 CO2 capture capacities of the porous carbons at different adsorption temperatures
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CO2 uptake (mmol g-1)
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Figure 4 CO2 adsorption isotherms at (a) 0 ℃ and (b) 25 ℃ for PCs.
CO2 capacity per mmol g-1
Samples PC500 PC600 PC700 PC800 NPC500 NPC600 NPC700 NPC800
0 ℃ 1 bar 4.33 (190.52) 6.38 (280.72) 6.25 (275.00) 6.99 (307.56) 5.36 (235.84) 7.60 (334.40) 7.18 (315.92) 6.24 (302.28)
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(mg g ) 25 ℃ 1 bar 3.01 (132.44) 4.19 (184.36) 3.91 (172.04) 3.96 (174.24) 3.78 (166.32) 5.01 (220.44) 4.32 (190.08) 3.36 (162.80)
25 ℃ 0.15 bar 0.95 (41.80) 1.18 (51.92) 0.87 (38.28) 0.93 (40.92) 1.29 (56.76) 1.38 (60.72) 0.93 (40.92) 0.65 (28.60)
CO2 uptake density (μmol m-2) 25 ℃ 1 bar 6.11 4.67 3.84 3.53 6.54 5.79 3.88 2.08
ACCEPTED MANUSCRIPT In terms of the influence of O-containing functional groups and porosity texture of the PCs on their CO2 adsorption performance, we found that there is no direct relationship between the single factor (O-containing functional groups or porosity properties (i.e., SBET, VP, Vmicro or V0)) and the CO2 capture at 25 ℃ and 1 bar. For example, PC600 with intermediate level of oxygen content and SBET, VP, Vmicro, V0 showed the highest
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CO2 adsorption capacity in PCs samples. However, both PC500, which has the highest oxygen content but the lowest developed narrow micropore volume, and PC700, which has the highest developed narrow
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micropore volume but the lower oxygen content than that of PC600, show lower CO2 adsorption capacity
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than that of PC600. We can firstly assume that both oxygen content and porosity texture simultaneously
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determine the CO2 uptake.
Next move to the addition of nitrogen, the introduction of nitrogen has been proposed to mimic the amine
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scrubbing process for enhancing the CO2 adsorption capacity. It is noteworthy that NPC800 has higher specific surface area (2958 m2 g-1) and narrow micropore volume (0.207 mL g-1) than the NPC500, but it
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exhibits lower CO2 adsorption capacity of 3.36 mmol g-1 (25 ℃) than that of NPC500 (see Table 2). This implies that the doping of nitrogen and oxygen groups improve the affinity of carbon toward CO2 molecules.
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To further elucidate the greater important of nitrogen groups in determining CO2 uptake, the influence of porosity texture and functional groups on CO2 capture can be evaluated by the CO2 uptake density, given in
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μmol mL-1 in Table 2. NPC500, NPC600 and NPC700 have higher CO2 uptake density than that of N-free PCs. Therefore, the doping of N has beneficial effect on CO2 uptake density. Note that the NPC800 has lower CO2 uptake density in comparison with the PC800, since the decrease of oxygen contents in NPC800 makes the N-doping has no beneficial effect on the CO2 adsorption. It should be point out that although the NPC600, NPC700 and NPC800 have 8.02%, 5.05% and 2.73% nitrogen contents, they exhibit lower CO2 uptake density than that of PC500 (N-free). A comparison of the oxygen contents of samples reveals a significant difference (see Table 1). Indeed, whereas PC500 has the highest oxygen contents. This gives the
ACCEPTED MANUSCRIPT hints why the N-doping has no influence on CO2 adsorption in previous literatures[47]. It is important to investigate the trend of the CO2 uptake performances across the porosity structure and surface chemistry of the PCs and NPCs. No obvious trend was found between the CO2 adsorption capacity (25 ℃, 1 bar) and specific surface area or micropore volume for PCs (see Figure S7a,b). Furthermore, a
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linear trend can be found between the CO2 uptake and narrow micropore volume, as well as between the CO2 adsorption density and oxygen contents (see Figure S7c,d). A similar linear correlation can be found for
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CO2 adsorption capacity with surface area, micropore volume, narrow micropore volume and oxygen
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contents for NPCs (see Figure S8a,b,c,d). In addition, the CO2 adsorption density indicates a strict linear
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correlation with nitrogen contents. However, the presence of nitrogen makes the lower linear correlation between CO2 adsorption capacity and porosity structure for NPCs than that of PCs. This implies that the
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introduction of nitrogen groups into porous carbons decreases the effect of porosity structure (see Figure S8). These result shows that the doping of nitrogen can improve CO2 adsorption capacity.
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To clearly explain the interaction between the CO2 molecules and PC600 or NPC600, the isostheric heat of adsorption (Qst) was calculated based on the CO2 adsorption isotherms at 25 and 0 ℃ using the
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Clausius-Clapeyron equation. As shown in Figure 6a，the initial Qst (0.5 mmol g-1) varies cross a range of 31.4-35.1 kJ mol-1. Notely, the NPC600 has higher Qst value at low CO2 uptake than that of PC600. The
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possible reason is that the doping of nitrogen atoms to porous carbon enhances the interaction between CO2 molecule and carbon surface.
The NPC600 exhibits higher Qst than that of PC600 at low CO2 adsorption capacity is owing to its nitrogen content, which can enhance adsorption preference towards CO2 over N2, i. e. a higher CO2/N2 selectivity. The N2 adsorption isotherms at 25 ℃ of PC600 and NPC600 was shown in Figure 6b,c. Apparently, these two materials have a much smaller N2 adsorption capacity than that for CO2. To calculate the gas selectivity based on pure gas adsorption results, the ideal adsorption solution theory (IAST) was used
ACCEPTED MANUSCRIPT to determine the CO2/N2 selectivity of PC600 and NPC600. The CO2/N2 selectivity increases with the N2 molar fraction for both materials are shown in Figure 6d. At the typical CO2 concentration range in flue gas (15-3%), the IAST CO2/N2 selectivity reaches 9.8-21.4 and 17.7-28.2 for PC600 and NPC600, respectively. This further emphasizes the greater importance of nitrogen groups in affecting both the CO2 adsorption capacity and the selectivity. )
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N2 Molar Fractiion (%)
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Figure 6 (a) CO2 adsorption heat, (b) CO2 and N2 adsorption isotherms for PC600 at 25 ℃, (c) CO2 and N2 adsorption isotherms for NPC600 at 25 ℃ and (d) CO2/N2 selectivity (at 25 ℃) versus N2 molar fraction at 1 bar for PC600 and NPC600.
4. Discussion To better understand the interactions between oxygen or nitrogen-containing functional groups and CO2 molecules, we performed GCMC simulations and DFT calculations. The calculation details are described in Supplementary Information. Based on XRD and Raman characterization, multiple graphene sheets present on the PC600 and NPC600.As shown in Figure S9 and Figure 7, we constructed a slit-pore graphene model
ACCEPTED MANUSCRIPT (SGM) with different pore size to understand the interactions between the doping N in carbons with oxygen
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atoms and CO2 molecules.
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Figure 7 Carbon model: graphite wall with functional groups Considering that the PC600 can be assumed as an aggregated of small graphene planes with oxygen atoms,
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we defined the model of oxygen atoms inside a slit-pore graphene model as OSGM. Figure 8a shows the
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CO2 adsorption of OSGMs with different pore sizes (0.5 nm-1.1 nm) at 25 ℃ and at 1 bar. SGM achieves maximal CO2 capture at a pore size of approximately 0.6 nm. However, its CO2 capture gradually decreases
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with increasing pore size. Moreover, it also demonstrates that the introduction of oxygen into SGM
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improves CO2 uptake. For example, the maximum CO2 adsorption capacity of the OH group can reach 26 mmol mL-1. Compared with the SGM, the 0.6 nm of the OH group shows a 67.3% growth. However, the C-O and C=O groups have maximal CO2 adsorption capacity of 19.2 and 14.6 mmol mL-1 in the pore size of 0.7, which exhibit 266% and 178% compared with that of SGM. Based on the results, it can be concluded that the doping of oxygen can boost CO2 capture and the optimum pore size for CO2 adsorption is about 0.6-0.7 nm.
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SGM COOH OH C-O C=O
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16 12
CO2 uptake (mmol cm-3)
8 4 0
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OH Pyridinic-N Graphitic-N Pyrrolic-N
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20 15 10 5 0
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Isosteric heat (kJ mol-1)
(a)
Isosteric heat (kJ mol-1)
40
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Figure 8 CO2 adsorption of OSGM (a) and N-doped OSGM-OH (b) with different pore sizes
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Figure 9 Isosteric heat of adsorption as a function of pore size of OGSMs and N-doped SGM-OH At 25 ℃ and at 1 bar, we investigate CO2 uptakes of N-doped SGM-OHs and SGM-OH with different
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pores (0.5-1.1 nm). As shown in Figure 8b, it can be found that the maximal CO2 uptakes of N-doped SGM-OHs and SGM-OH occur among 0.6 nm. Besides, the CO2 uptake with different pore sizes follows the sequence of Graphitic-N> Pyridinic nitrogen-N> Pyrrolic-N>SGM-OH at 1 bar, supporting that the nitrogen-containing functional groups improve the interaction between the CO2 molecules and the carbon surface. Figure 9a shows the relationship between GSMs and Qst value at 25 ℃ and 1 bar. It is noteworthy that Qst value for CO2 capture on OGSMs is higher than that on GSM under the same conditions. For example, Qst
ACCEPTED MANUSCRIPT value of CO2 adsorption on OH group is 34.7 KJ mol-1, which is a 14.7 % rise than that on GSM. The above discussions indicate that the strong interaction between CO2 molecules and OGSMs contributes to its superior CO2 capture. Similarly, Figure 9b shows that the introduction of nitrogen into carbon surface enhances Qst value of CO2 adsorption.
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To investigate the influence of electrostatic interactions on CO2 uptake, we conducted an electrostatic contribution analysis. The electrostatic contribution of carbon framework changes can be estimated by using
Swith -Swithout 100% Swith
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Electrostatic contribution =
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eq. (1)
(1)
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where Swith and Swithout are the adsorption capacity with and without the gas-framework electrostatic
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interaction. The electrostatic contribution of the O-doped SGMs and N-doped SGM-OH with the pore size of 0.6 nm on CO2 uptake at 25 ℃ is shown in Figure 10. The doping oxygen into carbon framework
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enhances 31.9 % for CO2 uptake at 1 bar by the electrostatic interaction, which shows that the electrostatic
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interaction plays an important role for CO2 uptake. Then, similarly, the introduction of nitrogen into carbon framework further enhances electrostatic interaction for CO2 uptake. In this process, the doping of
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oxygen/nitrogen into carbon surface show stronger electrostatic interaction owing to their higher
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accepting/donating electron densities, resulting in enhancing CO2 uptake[48]. In addition, Xing et. al[30] has pointed out that the doping of N to porous carbon materials improve the hydrogen-bonding interaction between the CO2 molecules and carbon framework, which explains that the N-doping porous carbons has superior CO2 adsorption. As shown in Figure 11, computational results indicate that the oxygen and nitrogen functional groups have higher adsorption energy than that of pure graphene. Also, the distance of CO2-functional groups achieves shorter than that of pure graphene. These results suggest that the hydrogen-bonding interaction between functional groups and CO2 is stronger than that of pure graphene.
40
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cN ra ph iti cN Py rro lic -N G
Py rid
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C= O
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O CO
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Figure 10 Electrostatic contribution of O-doped GSMs and N-doped OSGM-OH with the pore size of 0.6
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Figure 11 Hydrogen-bonding interaction of CO2 with functionalized carbon surface. 5. Conclusions The effect of N-doping of porous carbons on CO2 uptake has been studied by analyzing the CO2 adsorption properties of two porous carbons with similar textural properties: (a) N-undoped porous carbon materials and (b) N-doped porous carbon materials. The N-doped porous carbon contains 2.73-9.44% of
ACCEPTED MANUSCRIPT nitrogen by varying the synthesis conditions. The resulting carbons exhibit superior CO2 adsorption capacity. In particular, the NPC600 exhibit an exceptionally high CO2 adsorption capacity (5.01 mmol g-1). However, previous reports indicate that the doping of nitrogen does not improve CO2 capture. The reason for this inconsistent analysis result is that there is no systematic analysis of the presence of oxygen in the porous
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carbons, since the introduction of nitrogen into carbon framework with high oxygen content further enhances electrostatic interaction for CO2 uptake. In addition, we also show that the N-doped exhibits a
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greater impact on both the selectivity for CO2/N2 and the isosteric heat of CO2 adsorption.
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Acknowledgements
Nature Science Foundation China (No. 21878338).
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References
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This work was supported by the Key R & D Projects in Hunan, China (No.2018SK2038) and the National
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ACCEPTED MANUSCRIPT Highlights We synthesized a series of carbons without N-doping and with N-doping.
The porous carbons exhibited the superior CO2 adsorption capacity.
O-doping and N-doping improve CO2 adsorption.
The interaction between functional groups and CO2 was elucidated by GCMC calculations.
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