Highly Enhanced Gas Sorption Capacities of N-Doped Porous Carbon

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Article

Highly Enhanced Gas-Sorption Capacities of N-Doped Porous Carbon Spheres by Hot NH and CO Treatments 3

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Hee Soo Kim, Min Seok Kang, and Won Cheol Yoo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10552 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 10, 2015

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The Journal of Physical Chemistry

Highly Enhanced Gas-Sorption Capacities of N-doped Porous Carbon Spheres by Hot NH3 and CO2 Treatments Hee Soo Kim1, Min Seok Kang1 and Won Cheol Yoo1* 1

Department of Applied Chemistry, Hanyang University, Ansan, 426-791, Republic of Korea

[email protected]

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ACS Paragon Plus Environment


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Abstract

Highly enhanced CO2 and H2 adsorption properties were achieved with a series of phenolic resin-based carbon spheres (resorcinol-formaldehyde carbon (RFC) and phenolformaldehyde carbon (PFC)) by carbonization of RF and PF polymer (RFP and PFP) spheres synthesized via a sol-gel reaction, and subsequent activation with hot CO2 or NH3 treatment. Monodisperse and size-tunable (100 – 600 nm) RFC and PFC spheres had intrinsic nitrogen contents (ca. 1.5 wt%), which is attributed to the synthetic conditions that utilized NH3 as a basic catalyst as well as nitrogen precursor. A series of CO2-activated and N-doped RFC and PFC spheres showed almost perfect correlation (R2 = 0.99) between CO2 adsorption capacities and accumulated pore volumes of fine micropores (ultramicropore < 1 nm) obtained using the non-local density functional theory (NLDFT) model. Interestingly, NH3 activation served not only as an effective method for heteroatom doping (i.e., nitrogen) into the carbon framework but also as an excellent activation process to fine-tune the surface area and pore size distribution (PSD). Increased nitrogen doping levels up to ca. 2.8 wt% for NH3activated RFC spheres showed superior CO2 adsorption capacities of 4.54 (1 bar) and 7.14 mmolg-1 (1 bar) at 298 K and 273 K, respectively. Compared to CO2-activated RFC spheres with similar ultramicropore volume presenting CO2 uptakes of 4.41 (1 bar) and 6.86 mmolg-1 (1 bar) at 298 K and 273 K, respectively, NH3-activated nitrogen-enriched RFC was found to have elevated chemisorption ability. Moreover, prolonged activation of RFC and PFC spheres provided ultra-high surface areas, one of which reached 4079 m2g-1 with an unprecedented superb H2 uptake capacity of 3.26 wt% at 77 K (1 bar), which represents one of the best H2 storage media among carbonaceous materials and metal-organic frameworks (MOFs).

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Introduction

Highly porous carbonaceous materials have greatly contributed to solutions for energy and environment problems because of their characteristic features that include high surface area and large pore volume, capabilities of surface functionalization and heteroatom doping, chemical and physical stability, relatively low cost, and light weight.1-3 In particular among potential applications of carbon materials, gas sorption properties, the capture and storage of CO2 and H2, have been intensively studied because of huge demands for mitigating global warming and climate change that are mainly caused by greenhouse gas (e.g., CO2) emissions from burning fossil fuels and the need for effective storage of H2 for fuel cell and power generation applications.3-7 In attempts to capture CO2, there have been numerous efforts to develop various dry sorbents due to serious drawbacks in current CO2 capture systems based on aqueous amine solutions, energy-intensive regeneration processes, and poor stability of facilities from the corrosive and toxic nature of amines.6 Among various types of dry sorbents, carbonaceous materials in particular have been highlighted as CO2 adsorbents due to the aforementioned features. According to recent publications, enhanced CO2 uptake from carbon materials has been reported by fine-tuning physical and chemical properties: development of PSD of ultramicropores (< 1 nm) and incorporation of basic moieties into carbon frameworks by heteroatom doping (i.e., nitrogen) for more efficient adsorption of the acidic nature of CO2 caused by polarization of quadrupole moments.3,5,8-12 For the former issue, many research groups have suggested that the strong dependency of CO2 capacity of carbon materials is related to micropore volume, in which the amount of even smaller pore size less than 1 nm for carbon materials plays a pivotal role in CO2 uptake5,13,14 In order to develop ultramicropores through various activation methods, hot CO2 treatment (C(s) + CO2 (g) 2CO (g)) has been of great interest for fine-tuning pore structure due to facile experimental controls (i.e., dose amount, reaction time, temperature control, etc.)5,15,16 Another important way to enhance CO2 uptake is to incorporate basicity into the carbon framework by heteroatom doping (i.e., nitrogen). As mentioned above, the Lewis acid-base interaction between acidic CO2 and N-doped carbon frameworks provides increased isosteric heats of adsorption (Qst) of CO2, indicating the chemisorption ability of carbon materials bearing 3



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nitrogen. Such a property also plays an important role in selective adsorption of CO2 over N2 or CH4.6,9,17,18 There are two main methods to incorporate nitrogen atoms into carbon frameworks: use of nitrogen containing precursors2,6,7,9-11,17-22 (e.g., melamine, pyrrole, Ncontaining MOF, ionic liquid, nitrogen containing biomass, etc.) and post-synthetic approaches such as hot NH3 treatment.23,24 The former synthetic process has advantages for achieving relatively high contents of nitrogen but morphology control for monodisperse carbon spheres is scarcely achieved; whereas, post-synthetic treatment using hot NH3 gas readily incorporates nitrogen into carbon frameworks without significantly changing the morphology of parent carbon spheres. On the other hand, among physisorption-based porous materials, highly porous carbon materials have been of extensive interest as H2 storage media due to the aforementioned features such as high surface area, light weight, and chemical stability. Therefore, much attention has been focused on increasing the surface area of carbonaceous materials via various approaches such as use of activation processes, zeolite derived carbon, MOF derived carbon materials, templated carbons, and carbide derived carbon materials.1,3,4,7,15,16,25,26 In addition, recent studies have shown a strong relationship between H2 capacity and the microporosity of carbon materials, suggesting the importance of developing microporosity of carbon materials for enhanced H2 storage.1,25 Specific properties of carbonaceous materials that play an important role in CO2 adsorption are not clearly associated with satisfying high uptake for H2; in general, a greater amount of CO2 is adsorbed with base-functionalized (i.e., N-doped) carbons and more developed ultramicropores, whereas carbon with a high surface area shows moderate uptake for CO2 but a high uptake of H2. Due to these issues, specific synthetic designs for carbon materials have generally been applied to enhance the uptake of either CO2 or H2. Therefore, detailed study that combines relationships between structural properties (i.e., surface area, polarity (doping), PSD, microporosity including ultramicroporosity) and gas sorption capacities for both CO2 and H2 with an experimental series of carbon materials has rarely been reported. Herein, we report highly enhanced uptakes of both CO2 and H2 from a series of activated N-doped carbon materials made from two different phenolic resin-based precursors with identification of crucial structure-property relationships, which are linked to gas sorption 4


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properties. Firstly, monodisperse and size-tunable RFP and PFP spheres were synthesized using a basic catalyst (NH3) with various combinations of solvent mixtures (water and ethanol) under hydrothermal reaction conditions. Intrinsically N-doped RFC and PFC spheres were produced from carbonization of RFP and PFP spheres, in which it was identified that NH3 acted as a basic catalyst as well as a nitrogen donating precursor.27 Such N-doped RFC and PFC spheres were further activated via hot NH3 or CO2 treatment to incorporate more nitrogen into the carbon frameworks and to fine-tune the surface area and PSD, respectively. Importantly, utilization of hot NH3 turned out to be not only an effective means for nitrogen doping into carbon frameworks but also an excellent activation method for fine-tuning surface area and PSD. Structure-property relationships of a series of activated RFC and PFC spheres for CO2 uptake revealed that regardless of the different polymer spheres used (RFP and PFP), a perfect linear relationship (R2 = 0.99) between CO2 uptake and ultramicropore volume, determined by the NLDFT (non-local density functional theory), was found. The capacity of CO2 uptake with CO2-activated RFC spheres reached as high as 4.41 and 6.86 mmolg-1 at 298 K and 273 K at atmospheric pressure (1 bar), respectively. In addition, hot NH3 activation of carbon spheres gave a similar ultramicropore volume with almost twice the nitrogen content (2.83%) compared to that of the CO2-activated RFC counterpart (1.55%). As a result, ultrahigh CO2 uptakes for NH3-activated RFC were recorded as 4.54 and 7.14 mmolg-1 at 298 K and 273 K at atmospheric pressure (1 bar), respectively, which are higher than those of CO2activated RFC, strongly suggesting that the elevated chemisorption by heteroatom doping via hot NH3 treatment affects the CO2 uptake capacity. Furthermore, a prolonged activation process resulted in ultra-high surface area of RFC spheres up to 4079 m2g-1 with an unprecedented H2 uptake capacity of 3.26 wt% at 77 K (1 bar), which is comparable to the best H2 storage media among carbonaceous materials and MOFs.

Experimental section Reagents and Chemicals. Resorcinol (Junsei), formaldehyde (Amresco), phenol (TCl), Pluronic F127 (Aldrich), and ammonia solution (28%, Wako) were used without purification. High purity gases were used for all adsorption experiments (N2: 99.999%, CO2: 99.999%, H2: 5



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99.999%). Synthesis of Resorcinol-Formaldehyde (RF) Polymer and Carbon (RFC) spheres. The detailed synthesis parameters are shown in Table S1. Generally, resorcinol and formaldehyde were mixed with a solution containing (200 ml) deionized water (H2O), and aqueous ammonia (28% NH3 (aq)). After stirring for 24 h at room temperature, the reaction mixture was moved to an oven and was subsequently heated for 24 h at 90 °C without stirring. The solid product was recovered by centrifugation and was washed several times with deionized water. For the carbonization process, RF polymer spheres were heated from ambient temperature to 800 °C under a N2 atmosphere over 3 h at a heating rate of 4.3 ˚C /min. Synthesis of Phenol-Formaldehyde (PF) Polymer and Carbon (PFC) spheres. The detailed synthesis parameters are shown in Table S2. Typically, phenol and formaldehyde were mixed in a solution containing 115 ml of deionized water and 45 ml of ethanol. Next, Pluronic F127 was added to the mixture. After stirring for more than 4 h at room temperature, the reaction mixture was moved to an oven and subsequently heated at 90 °C or 100 °C for 24 h. The solid product was recovered by centrifugation and was washed several times with deionized water. For the carbonization process, the PF polymer spheres were heated from ambient temperature to 800 °C under a N2 atmosphere over 3h at a heating rate of 4.3 °C /min. CO2 activation. Carbon dioxide activations were performed using 250 mg of RFC or PFC spheres. The spheres were placed in an alumina boat in a quartz tube in the isothermal zone of a tubular furnace that was purged with a flow of nitrogen. The samples were heated to an elevated temperature of 900 °C with a heating rate (30 °C /min) under inert condition (N2), then followed by changing the gas flux to carbon dioxide (1000 cc/min) for various times (e.g., 45, 120, 240, and 380 min). After the programmed activation times, the gas flux was changed to nitrogen during the cooling down process. Activated carbon samples were denoted as RFC_CX and PFC_CX, where X is the CO2 activation time. NH3 activation. 200 mg of RF carbon spheres were placed in an alumina boat in a quartz tube in the isothermal zone of a tubular furnace that was purged with a nitrogen flow. The sample was then heated to an elevated temperature of 900 °C with a heating rate of 30 °C 6


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/min under the inert condition (N2), and the gas flux was changed to a mixture of ammonia gas (1000 cc/min) and nitrogen gas (200 cc/min) for 10 min. After the programmed activation time, the gas flux was changed to nitrogen during the cooling down process. Activated carbon was denoted RFC_NX, where X is the NH3 activation time. Characterizations. Low temperature nitrogen and hydrogen adsorption-desorption isotherms were measured at –196 °C and at –186 °C on an adsorption volumetric analyzer BEL MAX and BEL MINI manufactured by BEL, Inc (Japan), respectively. CO2 adsorption isotherms were measured at 0 °C and 25 °C on the adsorption volumetric analyzer BEL MINI. All samples were degassed at 220 °C for 24 h under static vacuum before adsorption measurements. Specific surface area was determined by the Brunnauer-Emmet-Teller (BET) method from nitrogen adsorption isotherms in the relative pressure range of 0.05 - 0.20. Total pore volume (Vt) was estimated from the amount of gas adsorbed at relative pressure P/P0 = 0.99. Pore size distributions (PSD) and accumulated pore volume of RFC and PFC spheres were calculated from adsorption branches of nitrogen sorption isotherms using the NLDFT (Nonlocal density functional theory) method for slit-like pores available in the BEL Master software from BEL Inc. Micropore volumes were calculated from the corresponding isotherms using the t-plot method. The volume of pores below 2 nm, 1 nm, and 0.7 nm were calculated on the basis of accumulated pore volume. The isosteric heat of adsorption (Qst) was calculated using the Clausius–Clapeyron equation. Surface functional groups of the asprepared carbon spheres were determined by recording FTIR spectra on a Varian Corporation (USA) Varian Scimitar 1000 FT-IR. Raman scattering spectra were recorded with a Renishaw RM1000 (514 nm). Samples for Raman scattering were prepared on a clean glass substrate, and the samples were excited using a green laser (514.5 nm and 0.05 mW power). Powder Xray diffraction (XRD) data were recorded on a Rigaku, D/MAX-2500/PC equipped with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were obtained with a HITACHI S-4800 microscope with an accelerating voltage of 15.0 kV and an applied current of 10 mA and a HITACHI SU8010 microscope with an accelerating voltage of 5.0 kV and an applied current of 10 mA. X-ray photoelectron spectroscopy (XPS) spectra were measured using a PHI Versa Probe system with a 100-W ALK Alpha X-ray source. Elemental analyses (EA) were obtained using FLASH EA1112. TEM images were recorded on a 7



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transmission electron microscope (JEM-3010) with an operating voltage at 300 kV. 13C solid state NMR spectra were recorded on a Bruker Avance II 500 MHz solid/micro-imaging high resolution NMR spectrometer.

Results and Discussion

Synthesis of phenolic resin spheres. Since the first report by Liu et al. who used the concept of the Stöber method for preparation of phenolic resin spheres with controllable size and narrow particle size distribution, various modified approaches that utilize diverse additives and precursors have been carried out to prepare spherical morphologies as well as more complex structures such as core@shell materials.28-30 From this inspiration, a modified sol-gel method that used two different precursors, RF and PF, was employed because we were interested in observing the relationship between structure-properties of carbon spheres from different precursors and gas sorption properties. First, monodisperse and size controllable RFP spheres were synthesized using water as a solvent and aqueous NH3 solution as a catalyst at room temperature for 24 h and subsequent aging at 100 °C for 24 h. As seen in Figure 1, highly monodisperse RFP spheres were successfully synthesized by simply varying the amount of precursors while keeping the amount of solvent and catalyst constant. According to scanning electron microscopy (SEM) measurements, with an increase in the resorcinol amount (the mole ratio of resorcinol and formaldehyde was fixed at 1:4.3) from 0.76 to 0.85, 1.42, and 1.98 g, the size of the RFP spheres gradually increased from 180 ± 22 to 307 ± 23, 575 ± 24, and 802 ± 20 nm, while the monodispersity of the spheres was improved, probably because the surface energy of such small polymer spheres was increasing (Table S1 and Figure 1a-d). On the other hand, similar synthetic conditions except use of ethanol cosolvent (H2O (80 mL) : EtOH (80 mL) = 1:1 in volume ratio) was applied to prepare PFP spheres. With an increase in the precursor amount of phenol (the mole ratio of phenol and formaldehyde was fixed at 1:5.4) from 0.97 to 1.29, and 2.59 g, the size of the as-synthesized PFP spheres increased from 1.1 ± 0.21 to 1.9 ± 0.38, and 2.8 ± 0.61 µm (Table S2, Figure S1a-c). When a smaller amount of phenol, 0.63 g, was employed, a negligible amount of product was obtained. Notably, the relatively poor monodispersity and diminished productivity for small 8


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size PFP spheres compared to RFP was observed probably because the less reactive nature of phenol compared to resorcinol retards a sol-gel polymerization under the specific conditions. In order to increase the monodispersity through mitigating surface free energy, a non-ionic surfactant (F127) was chosen to use. When F127 (0.045 g) was applied to prepare PFP spheres of 1.1 ± 0.21 µm, a relatively increased monodispersity of 1 ± 0.14 µm was observed by SEM (Figure 1e). In addition, with an increase in the reaction temperature to 110 °C, the size of PFP increased to 2.2 ± 0.15 µm (Figure 1f); however, the amount of catalyst and the water to ethanol solvent ratio had little impact on size control under the specific conditions (not shown here). The RFP of 575 nm and PFP of 1000 nm were utilized for the following carbonization and activation processes, which were further investigated for gas sorption properties. Before moving on to the carbonization of phenolic resin-based polymer spheres, compositional information of RFP and PFP spheres, especially nitrogen content and different polymerization degree, were investigated using IR,

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C solid-state NMR (13C ssNMR), and

elemental analysis (EA). The IR spectra of both RFP and PFP spheres showed similar functional groups including C=C stretching in aromatic rings (ca. 1604 cm-1 and 1498 cm-1), C-H bend of methylene bridges (1473 cm-1), O-H bending (1352 cm-1), asymmetric stretching of the C-O of phenol (ca. 1230 cm-1), asymmetric stretching of the C-O-C of dimethylene ether bridges (ca. 1101 cm-1), and C-O stretching of hydroxymethyl groups (1043 cm-1) (Figure 2a).31-33 The difference between these polymer spheres was attributed to the degree of polycondensation of RF and PF precursors. C-H out of plane bending for the ortho- and parapositions at 748 cm-1 and 813 cm-1, respectively, and 2,4,6-trihydroxymethyl phenol at 948 cm-1 still remained for the less reactive PFP spheres; on the other hand, there were featureless patterns in the same region for the much more reactive RFP spheres that have fourcoordination geometry like a silica framework.28 Importantly, chemical information for C-N bonding of tertiary amines was observed for both polymer spheres at 1381 cm-1, strongly indicating incorporation of nitrogen into the materials. From

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C ssNMR, similar information was identified for both polymer spheres. The

prominent difference was the high intensity of hydroxymethyl groups at ca. 55 ppm for PFP spheres in which further formation of methylene bridges (ca. 27 ppm) and/or dimethylene ether bridges (ca. 80 ppm) were retarded by the less reactive nature of the PF precursor 9



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(Figure 2b).34-37 In addition, it was confirmed that the CH2 peak of the methylene bridge appeared significantly for RFP spheres. Again, C-N information for both polymer spheres was observed around 38-40 ppm,38 greatly suggesting the existence of nitrogen in the polymer matrix. The possibility of nitrogen incorporation was finally identified by EA measurement. Nitrogen contents of 3.25 wt% and 3.12 wt% were recorded for RFP and PFP spheres, respectively (Table 1). Thus, the IR,

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C ssNMR, and EA data suggest that nitrogen

incorporation during the polymerization is possible probably due to the chemical interaction of ammonia with formaldehyde to form trihydroxymethyl amine and subsequent chemical reactions with phenol or resorcinol to produce tertiary amine structures (Figure S2).27 Thus, size-controllable and monodisperse RFP and PFP spheres were successfully synthesized, and different degrees of polymerization caused by the different chemical reactivities of the precursors (resorcinol and phenol) with formaldehyde were investigated via spectroscopic techniques. Furthermore, a possible nitrogen incorporation pathway was proposed, and the existence of nitrogen in the polymer matrix was confirmed. Synthesis and activation of N-doped carbon spheres. RFP of 575 nm and PFP of 1000 nm were chosen to produce RFC and PFC spheres. First, carbonization at 800 °C was performed with both polymer spheres. Using RFP, as-made carbon (RFC) spheres with a size of 467 ± 19 nm were produced, which amounted to around 19% shrinkage probably due to radial contraction during the carbonization process (Figure 3a). Subsequent activation of RFC with CO2 treatment at 900 °C was performed with varied reaction times of 45, 120, 240, and 380 min, denoted as RFC_C45, RFC_C120, RFC_C240, and RFC_C380, respectively. According to SEM measurements, the sizes of the activated carbon spheres decreased to 372 ± 27 nm (RFC_C45), 319 ± 29 nm (RFC_C120), 278 ± 23 nm (RFC_C240), and 231 ± 20 nm (RFC_C380) compared to the parent RFC because hot CO2 treatment involves the following chemical reaction: C(s) + CO2 (g) 2CO (g) that acts to reduce the carbon mass (Figure 3 b-e).5,15,16 This phenomenon was further investigated in a TEM study. Not only reduced sizes but also much less dense features inside CO2-activated RFC spheres of RFC_C120 and RFC_C380 compared to the parent RFC sphere were observed, which demonstrates the effectiveness of the hot CO2 gas etching process through carbon spheres 10


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(Figure 3g-i). This effective CO2-activation process could develop and modify the porous nature of the RFC spheres associated with gas sorption capacity, which will be discussed in the next section. In addition, it is notable that even after the carbonization and CO2-activation processes, RFC and CO2-activated RFC spheres retained monodispersity with less than 9% variation, probably due to the inherent monodispersity (4%) of the parent RFC spheres. The same carbonization procedure was applied to PFP spheres of 1000 nm to produce PFC spheres and subsequent CO2-activation with reaction times of 45 and 120 min was also employed to generate PFC_C45 and PFC_C120, respectively. As expected, SEM measurements (Figure S3 a-c) indicated the size of the as-made PFC was reduced to 762 ± 79 nm, and the size of the CO2-activated PFC spheres further decreased to 710 ± 70 nm (PFC_C45). Notably, broken structures were observed for samples subjected to prolonged CO2-activation treatment (PFC_C45, 120), probably caused by use of less polymerized PFP spheres for preparation of PFC and CO2-activated PFC spheres (Figure S3b-c). In order to characterize the structural changes of carbon spheres during the carbonization and CO2-activation processes, XRD and Raman spectroscopy were utilized. Two broad peaks for RFC and PFC at ca. 25 and ca. 44 of 2-theta degrees corresponding to (002) and (101) peaks of micron-sized graphitic domain were observed, which has been generally reported for the amorphous nature of carbon spheres (Figure S4a-b).5,24 Upon CO2-activation of parent carbon spheres, gradually reduced peak intensities of (002) and (101) for CO2-activated RFC and PFC spheres were measured with longer treatment, and when further applied for 380 min for RFC sphere (RFC_C380), almost featureless patterns were observed. This result is due to less graphitization of carbon spheres through the CO2-activation process that corrupts graphitic structures via carbon loss. In addition, Raman analysis was applied, and the results were in good agreement with the XRD analysis. Characteristic vibration bands (i.e., D- and G-bands, 1350 and 1580 cm-1, respectively) for carbon materials appeared and the ratio (ID/IG) of these peaks, usually used to identify the degree of graphitization, gradually increased from 0.78 to 0.91 for the RFC series and from 0.79 to 0.86 for the PFC series with prolonged CO2activation times, indicating an increase of defect sites in the graphitic micron-domains by carbon loss via hot CO2 treatment (Figure S4 c-d).24 In an attempt to introduce heteroatoms (e.g., nitrogen) into the carbon matrix, which has 11



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been shown to enhance CO2 uptake capacity hot NH3 treatment, 3,8-10,12,17,18,20,22,39 known to be a method to incorporate nitrogen into the carbon matrix, was applied to RFC spheres.23,24 Thus, hot NH3 treatment of RFC was carried out at 900 °C for 10 min, denoted RFC_N10, and the sphere size was reduced to 357 ± 22 nm (Figure 3f). On the other hand, during hot NH3 treatment at an elevated temperature, significant weight loss (ca. 27%) was monitored, in which the activation process was suspected of being accompanied by the following chemical reaction: 3C(s) + 4NH3(g) 2N2(g) + 3CH4(g),23 which will be discussed in detail later. The structural features of RFC_N10 spheres were characterized with XRD and Raman spectroscopy in which reduced XRD peak intensities of (002) and (101) and a moderately increased ID/IG ratio of 0.84 were identified, similar to those of RFC_C45 (Figure S4a and S4c). Compositional analysis for RFC, PFC, and RFC_N10 were carried out by IR spectroscopy. First, featureless patterns except several peaks of the C-N vibration of pyridine and/or pyrrole (1385 cm-1), C=N vibration of pyridine (ca. 1560 cm-1), N-H bonding of pyrrole (ca. 1655 cm-1), vibration peaks of N=O (ca. 1400 cm-1), and C=C stretching in aromatic rings broadly appeared on around 1602 cm-1 were recorded for RFC and PFC spheres,40-44 indicating that all of the dangling bonds on the polymer matrix disappeared and more importantly, the existence of nitrogen atoms in both carbon frameworks were identified even after the carbonization process (Figure S5). It is noted that for RFC_N10 spheres, more developed peaks involving vibration peaks of C-N (1385 cm-1) and N=O (ca. 1400 cm-1) appeared in the spectrum (Figure S5).44 Further characterization of nitrogen doping of these carbon spheres was carried out using XPS analysis. According to survey scans of RFC, PFC, and RFC_N10 samples, small but non-negligible signals from nitrogen atoms were identified for all of the carbon samples, in good agreement with the results from IR analysis (Figure S6). For more detailed chemical bonding information of nitrogen atoms in the carbon frameworks, highresolution XPS spectra for nitrogen 1s were analyzed (Figure 4a-c). The deconvoluted peaks assigned to pyridinic nitrogen (ca. 398 eV), pyrrolic nitrogen (ca. 400 eV), quaternary nitrogen (ca. 402 eV) and oxidized nitrogen (ca. 405 eV) were analyzed, and their integral area percentages are presented in Figure S6.24,45 According to the XPS analysis, the majority of RFC, PFC, and RFC_N10 sample peaks corresponds to pyridinic and pyrrolic nitrogen atoms with combined percentages of 76.5%, 80.6% and 70.9%, respectively, suggesting 12


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nitrogen doping occurred within the carbon lattice instead of dangling on the carbon surface (Figure 4a-c).11,23 In particular comparing RFC and RFC_N10 samples, a slightly increased amount of oxidized nitrogen of ca. 7% for RFC_N10 was observed, which was also reported for similar hot NH3 treatment of a 3D ordered mesoporous carbon sample.24 Weight percentages of nitrogen doping for RFC, PFC, and RFC_N10 samples corresponding to 1.61 wt%, 1.49 wt%, and 2.83 wt%, respectively, were analyzed by EA measurements. As anticipated, nitrogen doping of RFC and PFC spheres synthesized from nitrogen-containing polymer precursors (RFP and PFP) were confirmed, and it was elucidated that hot NH3 treatment generated more nitrogen doping into the carbon matrix compared to the parent RFC spheres (Table 1). Nitrogen adsorption studies. Textural features of RFC, CO2-activated RFCs, PFC, CO2activated PFCs, and RFC_N10 samples were analyzed using nitrogen sorption measurements, and adsorption parameters such as specific surface area determined by the BET model, micropore volumes with different pore sizes derived from the t-method and the NLDFT model, pore size distribution (PSD) obtained from the NLDFT model, and total pore volume measured at P/P0 = 0.99 are listed in Table 1. First, as-made RFC and CO2-activated RFC_C45, 120, 240, and 380 samples show significantly increased uptakes at low relative pressure representing microporous features along with CO2-activation times (Figure 5a). As mentioned above, hot CO2 treatment activates RFC spheres via the following chemical reaction, C(s) + CO2 (g) 2CO (g), which generates small pores that become larger pores with increased reaction times. The RFC and RFC_C45, 120 samples showed characteristic type I isotherms; on the other hand, RFC_C240 and RFC_C380 samples showed type IV isotherms due to the presence of mesopores arising from prolonged activation times (Figure 5a). As anticipated, surface areas of the samples determined by the BET model were significantly increased from 724 to 1611, 2842, 3540, and 4079 m2g-1 for the RFC and RFC_C45, 120, 240, and 380 samples, respectively (Table 1). It is notable that a surface area as high as 4000 m2g-1 for the RFC_C380 has rarely been reported for carbon materials.

41

Additionally, the micropore volumes determined from the t-method were also significantly increased from 0.31 to 0.83, 1.26, 1.27, and 1.28 cm3g-1 for the RFC and RFC_C45, 120, 240, and 380 samples, respectively (Table 1). Interestingly, the micropore volume seems to be 13



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saturated at around 1.26 for the RFC_C120, 240, and 380 samples, resulting from the creation of pores larger than 2 nm. Along with the trend of surface area, total pore volumes of the RFC and RFC_C45, 120, 240, and 380 samples were substantially developed from 0.28 to 0.73, 1.33, 2.0, and 2.56 cm3g-1, which is consistent with CO2-activation times (Table 1). PSD was derived using the NLDFT model in order to closely characterize the micropores of the carbon materials. According to PSD, ultramicropores centered around 0.7 nm quickly developed with a 45 min CO2-activation time and gradually decreased with prolonged activation times due to continuous weight loss during the activation process (Figure 5b-c). The accumulated ultramicropore volume of the RFC_C45 sample had the highest value of 0.61 cm3g-1 compared to the other CO2-activated samples that had a broader distribution up to 4 nm for RFC_C380 (Figure 5d-e and Table 1). Therefore, a controlled activation process could generate selective development of ultramicropores under the specific conditions. On the other hand, when hot NH3-activation was applied to RFC spheres, increased uptake at low relative pressure compared to the parent RFC spheres strongly suggested the development of micropores (Figure 5a). A specific surface area of 1458 m2g-1, micropore volume of 0.76 cm3g-1 derived from the t-method, accumulated ultramicropore volume of 0.59 cm3g-1, and the PSD were identified (Figure 5b, d and Table 1). The obtained values and distribution of the RFC_N10 sample are very similar to those of the RFC_C45 sample, which suggests that they have similar textural features except for different nitrogen doping levels. Meanwhile, similar patterns were observed for the PFC series. Rapid development of ultramicropores for RFC_C45 was observed, and decreased ultramicroporosity and increased larger microporosity (1-2 nm) and mesoporosity were identified for the PFC_C120 sample (Figure S7a-c). CO2 adsorption studies. It has been widely accepted that enhanced CO2 uptake for carbonaceous materials arises from development of ultramicropores and incorporation of nitrogen atoms into the carbon framework.23,47 Thus, we first tried to utilize hot CO2 treatment for activation of RFC and PFC spheres. CO2 adsorption isotherms for the RFC and PFC series were measured at 298 and 273 K at atmospheric pressure (1 bar) (Figure 6a-b and Figure S8a-b). According to CO2 adsorption isotherms for the RFC series measured at 273 K 14


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and 1 bar, the CO2 adsorption capacities of RFC and RFC_C45, 120, and 240 samples were 3.82, 6.86, 5.59, and 4.86 mmolg-1, respectively (Figure 6a, Table 1); meanwhile their nitrogen contents were similar (Table 1). The ultramicropore volumes for RFC and RFC_C45, 120, and 240 samples of 0.24, 0.61, 0.46, and 0.36 cm3g-1, respectively, strongly suggest that development of ultramicroporosity by the CO2-activation process mainly governs the CO2 uptake capacities in this series, which is in good agreement with previously reported results (Table 1).2,5,13,14 In order to verify relationships between CO2 adsorption capacity and different pore size regions of the samples, the accumulated pore volumes with different pore sizes (e.g., 0.7, 1, and 2 nm) were plotted with respective to CO2 uptake capacities for RFC and RFC_C45 and 120 samples (Figure 6c). As anticipated, higher amounts of accumulated ultramicropore volumes less than 0.7 nm and 1 nm for the RFC and RFC_C45 and 120 samples correspond to the CO2 uptake capacities, whereas accumulated pore volumes up to 2 nm have no bearing on the trend of adsorption capacities (Figure 6c), clearly showing the importance of ultramicropore development for enhanced CO2 adsorption. A similar trend was observed for CO2 isotherms of the RFC series at 298 K and 1 bar (Table 1 and Figure S8a). In addition, the Qst for CO2 of the samples, calculated by the Clausius–Clapeyron equation (Table 1), showed that samples subjected to a prolonged CO2-activation process compared to the parent RFC spheres (30.6 kJmol-1) had reduced heat of adsorption for CO2 probably due to the generation of larger pores (Figure 5b-c).47 The CO2 adsorption capacities of PFC and PFC_C45 and 120 samples measured at 273 K and atmospheric pressure were 4.55, 6.77, 5.77 mmolg-1, respectively (Figure 6b, Table 1), and the same trend for CO2 uptake capacities was also obtained at 298 K and 1 bar (Figure S8b and Table 1), again strongly suggesting the importance of development of ultramicroporosity. When the accumulated pore volumes with different pore sizes (e.g., 0.7, 1, and 2 nm) were plotted with respective to CO2 adsorption capacities for the PFC and RFC_C45, and 120 samples (Figure 6d), a similar trend was observed that creation of ultramicropores is the dominant feature affecting CO2 capacities. Furthermore, structure-property relationships (in this case, PSD and ultramicroporosity) of different carbonaceous materials synthesized from different precursors toward CO2 15



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adsorption capacities were compared in parallel. Surprisingly, an almost perfect relationship (R2 = 0.99) between accumulated pore volume of ultramicropores (< 1 nm, determined by the NLDFT) and CO2 adsorption capacities of RFC, CO2-activated RFC samples, PFC, and CO2activated PFC series regardless of the precursor for carbon spheres was identified under these specific conditions (Figure 7a). When we attempted to find relationships between accumulated pore volumes acquired with pore sizes less than 0. 7 nm and 2 nm, which were determined by the NLDFT, and microporosity (< 2 nm), calculated from the t-method, and CO2 uptake capacities for the samples, poor correlations were elucidated (Figure S9a-c). Such an accurate correlation between ultramicropores and CO2 uptake capacities could provide a roadmap for the design of carbonaceous materials for CO2 adsorption applications. Another important factor that can enhance CO2 uptake is the incorporation of nitrogen into the carbon framework to facilitate greater adsorption of CO2 onto the N-doped carbons.6, 18

It has been reported that N-doped carbon has an increased Qst value compared to its non-

doped counterpart, which identifies the chemisorption ability of N-doped carbon. Therefore, it was of interest to observe the nitrogen effect using two different carbon samples with almost the same textural feature (e.g., accumulated pore volume of ultramicropores) but different nitrogen contents. As mentioned above, textural features of the RFC_C45 and RFC_N10 samples were similar to each other (see Table 1 and Figure 5), except RFC_N10 had almost twice the nitrogen content (2.83 wt%) of RFC_C45 (1.55 wt%). Thus, the CO2 adsorption isotherm for RFC_N10 was measured at 298 and 273 K at 1 bar (Figure S10 and Figure 7b). According to the isotherm, the CO2 adsorption capacity of RFC_N10 at 273 K and atmospheric pressure was 7.14 mmolg-1, which is superior to the CO2 uptake of 6.86 mmolg-1 for RFC_C45 (Table 1), and that is located on the upper region of the plotted line showing almost perfect correlation for CO2 uptake capacities of the RFC and PFC series (Figure 7a). The reason for the ultra-high CO2 uptake capacity for RFC_N10 is attributed to the higher nitrogen content that is accompanied by a greater Lewis acid-base interaction between the acidic CO2 and N-doped carbon framework. To assess the strength of the interactions between CO2 and the carbon samples, Qst values for RFC_C45 and RFC_N10 were calculated using CO2 sorption isotherms measured at 273 and 293 K by the Clausius– Clapeyron equation.47 As seen in Figure 7c, through all the coverage regions, RFC_N10 has a higher Qst value than that of RFC_C45, arising from higher incorporation of electron-rich 16


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nitrogen sites in the RFC_N10 sample, which could facilitate more interactions between acidic CO2 and basic N-doped carbon. H2 adsorption studies. Light weight, high specific surface area, and development of microporosity of carbonaceous materials are particularly important features for enhanced storage of H2.4,16,48 In this article, as a means to augment the specific surface area and microporosity, activation processes using hot CO2 and NH3 treatments provided significantly increased textural features regardless of carbons produced with different precursors. According to the H2 isotherms for carbon samples measured at 77 K, 87 K and 1 bar (Figure S11), H2 uptake sharply increased along with activation times for the carbon samples with PFC_120 and RFC_240 and 380, exhibiting more than twice the H2 capacity compared to the parent carbon spheres (RFC and PFC) (Table 1 and Figure S11). In addition, the Qst for H2 of the samples was calculated by the Clausius–Clapeyron equation (Table 1). Similarly, the trend of the Qst of H2 adsorption resembled that of CO2 adsorption, probably resulted from the generation of larger pores for prolonged CO2-activated carbon samples (Table 1). In particular, the RFC_380 sample had an ultra-high H2 uptake capacity, as high as 3.26 wt%, which makes it one of the most efficient H2 storage adsorbents among carbonaceous materials and MOFs (Table S3).1,15,46,49-54 To elucidate relationships between H2 adsorption capacity and textural features of the carbon materials, H2 capacities with respect to surface area and pore volumes less than 2 nm determined by the NLDFT were plotted (Figure 8a and b). Generally, H2 uptakes for carbon samples regardless of precursor and nitrogen doping level were enhanced with respect to increased surface area (R2 = 0.86, Figure 8a) and pore volumes less than 2 nm (R2 = 0.90, Figure 8b). In particular, with up to ca. 2.5 wt% of H2 uptake for both plots, almost perfect correlations between H2 uptake and surface area and pore volumes less than 2 nm of carbon samples were observed (R2 = 0.99 for plots, Figure S12a and c). There are discrepancies, however, that for carbons with high H2 uptakes larger than 2.5 wt%, the H2 uptake with respect to surface area showed relatively high correlation (R2 = 0.86, Figure S12b), whereas H2 uptake plotted against pore volumes less than 2 nm showed a poor relationship (Figure S12d) probably due to generation of PSD up to 4 nm from materials (e.g., RFC_C120, 240, 380 and PFC_C120) subjected to prolonged activation processes (Figure 5b and S7b). 17



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Conclusion We have demonstrated that highly enhanced CO2 and H2 adsorption properties were acquired from the series of N-doped carbon materials via carbonization of different polymer spheres (RF and PF) and subsequent activation processes with hot CO2 or NH3 treatment in order to control textural properties such as PSD, specific surface area, and microporosity, and the nitrogen doping level. Monodisperse RFP with size ranges of 100 - 800 nm and PFP of 1000 - 2000 nm were successfully synthesized via a sol-gel process using a solvent mixture of water and ethanol and a base catalyst (NH3). During polymerization, possible nitrogen moiety incorporation within the polymer matrix was proposed, and it was found that NH3 served both as a basic catalyst as well as a nitrogen precursor, resulting in intrinsically Ndoped carbons after carbonization (ca. 1.5 wt% for RFC and PFC). Such monodisperse and size-tunable N-doped carbons were further activated via hot CO2 treatment to increase textural features, for example surface area up to 4079 m2g-1 (RFC_C380) and ultramicropore of 0.61 cm3g-1 (RFC_C45), or via hot NH3 activation for incorporation of nitrogen atoms into the carbon framework to give enhanced N-doped carbon with ca. 2.83 wt% nitrogen (RFC_N10). Structure-property relationships of a series of CO2-activated carbon spheres regardless of the precursor polymer spheres demonstrated that there was a perfect linear relationship (R2 = 0.99) between CO2 uptake and ultramicropore volume. Furthermore, comparing the CO2 uptake capacities for different N-doped carbons (e.g., RFC_C45 and RFC_N10) with similar textural features, the higher incorporated nitrogen content of RFC_N10 showed an enhanced CO2 uptake of 7.14 mmolg-1 at 273 K and 1 bar, and the higher Qst value over all CO2 coverage regions compared to its counterpart (RFC_C45), strongly suggesting that a higher nitrogen content facilitated interactions between the polar acidic CO2 and electron-rich nitrogen in the carbon framework. Moreover, ultra-high specific surface area could be achieved through prolonged activation processes, one of which reached 4079 m2g-1 with an unprecedented superb H2 uptake capacity of 3.26 wt% at 77 K (1 bar), which makes it one of the best H2 storage media among carbon materials and MOFs. Consequently, various relationships between structure-properties and gas sorption capacities using a well-designed series of carbonaceous materials gave rise to a useful principle for the design of carbonaceous materials for gas sorption applications. 18


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Acknowledgements This work was supported by the research fund of Hanyang University (HY-2013-N). The authors thank Prof. H. Chun (Hanyang University) regarding H2 adsorption experiments.

Associated Content Supporting Information Available. Detailed material characterizations including Tables and SEM for polymer spheres, gas sorption isotherms (N2, CO2, and H2), IR spectra, XPS spectra, and plots for CO2 and H2 capacities with respective to various textural features. This information is available free of charge via the Internet at http://pubs.acs.org

Author Information Corresponding Author *

Prof. Won Cheol Yoo, Tel: +82-31-400-5504, E-mail: [email protected]

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the Metal Oxide and Organic Linking Units on the Low-Pressure Hydrogen Adsorption Properties of Metal-Organic Frameworks. J. Am. Chem. Soc. 2006, 128, 1304-1315. (52) Wong-Foy, A. G.; Lebel, O.; Matzger, A. J. Porous Crystal Derived from a Tricarboxylate Linker with Two Distinct Binding Motifs. J. Am. Chem. Soc. 2007, 129, 15740-15741. (53) Wang, X. S.; Ma, S.; Forster, P. M.; Yuan, D.; Eckert, J.; López, J. J.; Murphy, B. J.; Parise, J. B.; Zhou, H. C. Enhancing H2 Uptake by “Close‐Packing” Alignment of Open Copper Sites in Metal–Organic Frameworks. Angew. Chem., Int. Ed. 2008, 47, 7263-7266. (54) Sevilla, M.; Foulston, R.; Mokaya, R. Superactivated Carbide-Derived Carbons with High Hydrogen Storage Capacity. Energy Environ. Sci. 2010, 3, 223-227.

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Figure 1. SEM images of monodisperse RFP spheres (a) ~ (d) using different precursor amounts, and PFP spheres using different reaction temperatures of 100 °C (e) and 110 °C (f).

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Figure 2. FT-IR (a) and 13C ssNMR (b) spectra of RFP (575 nm) and PFP (1000 nm) spheres.

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Figure 3. SEM images of RFC (a), RFC_C45 (b), RFC_C120 (c), RFC_C240 (d), RFC_C380 (e), RFC_N10 (f), and TEM images of RFC (g), RFC_C120 (h), RFC_C380 (i).

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Figure 4. High resolution N1S XPS spectra of RFC (a), RFC_C45 (b), and RFC_N10 (c).

Figure 5. Nitrogen sorption isotherms (a), pore size distribution curves for RFC, RFC_C45 and RFC_N10 samples (b), pore size distribution curves for RFC_C120, RFC_C240 and RFC_C380 samples (c), accumulated pore volume curves for RFC, RFC_C45 and RFC_N10 (d), and accumulated pore volume curves for RFC_C120, RFC_C240 and RFC_C380 samples (e) of RFC series. 27



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Figure 6. CO2 sorption isotherms at 273 K for RFC (a) and PFC (b) series. Distribution of pore volumes between pores of less than 0.7 nm (blue), less than 1 nm (red), and less than 2 nm (black) of RFC series (c) and PFC series (d) along with CO2 adsorption capacities .

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Figure 7. The relationship plot of CO2 adsorption capacities with respect to the accumulative pore volume of ultramicropores (< 1 nm, determined by the NLDFT) of RFC and PFC series (a), CO2 sorption isotherms at 273 K for RFC_C45 and RFC_N10 samples (b), and isosteric heats of adsorption of the RFC_C45 and RFC_N10 samples (c).

Figure 8. The relationship plot of H2 adsorption capacities with respect to the specific surface areas (a) and accumulative micropore volumes (< 2 nm, determined by the NLDFT) (b) of the RFC and PFC series 29



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Table 1. Textural Properties including the specific surface areas, pore volumes with different pore size regions, and CO2 and H2 uptake capacities, isosteric heats of adsorption (Qst), and EA results for the carbon samples.

Pore volume 3

-1

[cm g ]

Surface Area

CO 2 uptake -1

Qst CO2 [kJ molg-1]

H uptake 2

-1

EA (N) [wt%]

Micropore

1nm Accumulated poreb

2nm Accumulated poreb

Total porec

[m g ]d

[mmol g ] (273K / 298K at 1bar)

RFC

0.31

0.24

0.32

0.281

724

3.82 / 2.82

30.6

1.4/1.2

9.1

1.61

RFC_C45

0.83

0.61

0.83

0.732

1611

6.86 / 4.41

27.2

2.51/2.11

9.2

1.55

RFC_C120

1.26

0.46

1.26

1.334

2842

5.59 / 3.32

25.1

2.73/2.3

8.7

1.34

RFC_C240

1.27

0.36

1.27

1.99

3540

4.86 / 2.5

23.6

3.16/2.63

8.4

-

RFC_C380

1.28

0.27

1.28

2.56

4079

-

-

3.26/2.75

8.3

-

RFC_N10

0.76

0.59

0.77

0.647

1458

7.14 / 4.54

27.8

2.28/1.89

9.0

2.83

PFC

0.38

0.35

0.37

0.301

743

4.55 / 3.18

29.8

1.52/1.3

9.3

1.46

PFC_C45

0.81

0.6

0.81

0.637

1588

6.77 / 4.27

26.7

2.48/1.98

9.5

1.29

PFC_C120

1.45

0.49

1.46

1.48

3283

5.77 / 3.32

24.2

3.05/2.54

8.6

0.81

Sample a

2

-1

a

calculated by the t-plot determined by the NLDFT model c measured at P/P0 = 0.99 d obtained by the BET method b

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[mmol g ] (77K/87K at 1 bar)

Qst H2 [kJ molg-1]

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Table of contents

31


Highly Enhanced Gas Sorption Capacities of N-Doped Porous Carbon

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