Activator for the Synthesis of Nitrogen

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Iron nanoclusters as template/activator for the synthesis of nitrogen doped porous carbon and its CO2 adsorption application Ning Fu, Huanming Wei, Hualin Lin, Le Li, Cuihong Ji, Ningbo Yu, Haijun Chen, Sheng Han, and Guyu Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15723 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

Iron nanoclusters as template/activator for the synthesis of nitrogen doped porous carbon and its CO2 adsorption application Ning Fu,1 Huan-Ming Wei,1 Hua-Lin Lin,1 Le Li,2 Cui-Hong Ji,3 Ning-Bo Yu,1 Hai-Jun Chen,1 Sheng Han*1 and Gu-Yu Xiao*2 1

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100

Haiquan Road, Shanghai 201418, P. R. China. 2

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix

Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China. 3

Department of Chemistry, School of Chemistry and Molecular Engineering, East China

Normal University, 500 Dongchuan Road, Shanghai 200241, P. R. China. * Corresponding authors. E-mail: [email protected]; Fax: +86-21-60873228; Tel: +86-21-60873228; E-mail: [email protected]; Fax: +86-21-54741297; Tel: +86-21-54742664.

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Abstract We propose a facile synthesis approach for nitrogen doped porous carbon and demonstrate a novel pore-forming method that iron nanoclusters act as a template or activator at the different carbonization temperature based on

Fe3+−poly (4-vinyipyridine) (P4VP)

coordination. P4VP will completely decompose even in an inert atmosphere, but under the coordination and catalysis of Fe3+, it can be converted to carbon at very low temperature (400 °C). The aggregation of iron nanoclusters in the carbonization process showed different pore-forming methods at different temperatures. The as-prepared materials possess high specific surface area (up to 1211 m2 g-1), large pore volume (up to 0.96 cm3 g-1), narrow microporosity and high N content (up to 9.9 wt%). Due to these unique features, the materials show high CO2 uptake capacity and excellent selectivity for CO2/N2 separation. The CO2 uptake capacity of NDPC-2-600 are up to 6.8 mmol g−1 and 4.3 mmol g−1 at 0 °C and 25 °C; the CO2/N2 (0.15/0.85) selectivity at 0 °C and 25 °C also reach 18.4 and 15.2, respectively.

Keywords: poly (4-vinyipyridine), metal-polymer coordination, nitrogen doped porous carbon, pore forming method, carbon dioxide adsorption

1. Introduction Global warming caused by carbon dioxide (CO2) emission due to fossil fuel combustion has become one of the main environmental problems facing humanity today.1-3 In the case of the main body status of fossil fuels cannot be shaken, the development of efficient CO2 capture and sequestration technology is of vital significance. Recently, various technologies have been used for the CO2 capture including chemisorption, physisorption and membrane separation.4-7 Among these technologies, chemisorption using alkali solutions (e.g., monoethanol amine and sterically hindered amines) has been widely used in industry, but it suffers from intrinsic limitations which includes intrinsic corrosive, operational safety and the 2 ACS Paragon Plus Environment

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adsorbent basic nonrenewable.8,9 CO2 physisorption by using porous materials (e.g., silica,10,11 zeolite,12,13 metal–organic frameworks14,15 and porous carbon4,16,17) has also become a focus of research due to its easy maintenance, easy renewable and less energy intensiveness. Especially, porous carbon not only has the advantages mentioned above but also has the specific features, including wide raw material source and easy synthesis, which is regarded as the most promising CO2 capture technology.

Base on this basic point, significant efforts have been spent on the investigation how to improve CO2 capture capacity of porous carbon materials. Due to the CO2 is highly quadrupolar and weakly acidic, researchers wanted to through introducing nitrogen functional groups to improve the adsorption capacity of CO2, that is, nitrogen doping.16,18-20 According to the location of the elements, nitrogen doping can be divided into two classifications, namely, surface and structural nitrogen doping. Surface nitrogen doping is conducted by alkali impregnating or grafted amino functional groups methods; structure nitrogen doping is conducted by carbonization of nitrogen containing precursor into carbon skeleton. Compared the surface nitrogen doping, the structure nitrogen doping is more stable and has been widely studied.21 Meanwhile, researchers are also committed to use various method (e.g., template,2229

activation,16,29-34 catalysis35,36 or combination of these methods) to increase its specific

surface area and pore structure. Among these methods, the template methods are often used to prepare meso- or macropore with the controllable structure. However, the as-prepared mesoor macroporoe structure is only suitable for the transportation and diffusion of CO2 molecules but the contribution to the total amount of adsorption is relatively small. Some studies have revealed that CO2 adsorption capacity at atmospheric pressure is mainly dependent on the micropore volume of pore size at 3.3 ~ 8 Å.37,38 The preparation of this sub-nanometer level microporous structure generally adopts the method of activation or catalysis. Carbon materials with the maximum CO2 adsorption capacity are basically prepared by the two methods. 3 ACS Paragon Plus Environment


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However, due to its pore-forming mechanism, these two methods rely on the activator or catalyst reaction with the carbon around it to produce gaseous substance and to generate the pore structure, which will cause high energy consumption and a decrease in the carbon yield, these situations are really disadvantageous.

In this paper, we propose a novel approach to synthesize structure nitrogen doped porous carbon (NDPC) with unique pore structure via a simple process that carbonization metalpolymer coordination compound, here with the metal iron ( Ⅲ ) chloride hexahydrate (FeCl3·6H2O) and the polymer poly (4-vinyipyridine) (P4VP). P4VP, due to its structure, will completely decompose even in an inert atmosphere; it has not been reported as a carbon source for the preparation of porous carbon. Therefore, the original idea of our work is to attempt to use this polymer to prepare carbon by means of the metal organic coordination, with Fe3+ act as a complexant and catalyst. More interestingly, thinking about some phenomena in the experimental process that drives us find iron nanoclusters have different pore forming ways of template or activator at different carbonization temperature. One of these ways is carburizing phase formed by iron agglomeration, which can act as inner templates to form a high accessible surface area, large micropore volume with a very narrow pore size distribution (≈4 ~ 6 Å) under mildly carbonization conditions (T ≤ 600 °C) and overcomes the defect of low yield of activation or catalysis method. Considering these factors, this novel nitrogen doped porous carbon materials used for CO2 adsorption exhibits excellent adsorption capacity and CO2/N2 selectivity, indicating excellent practical application prospects.

2. Experimental Section

2.1 Materials


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Poly (4-vinyipyridine) (P4VP, Mv =60000, Sigma-Aldrich), iron chloride hexahydrate (FeCl3·6H2O, 99.0 wt%, Sinopharm Chemical Reagent Co. Ltd.) ethanol (CH3CH2OH, 99.7 wt%, Greagent). hydrochloric acid (HCl, 37 wt%, Greagent). All chemicals were purchased and direct used without any further purification.

2.2 Synthesis of nitrogen doped porous carbon (NDPC)

In a typical synthesis, 0.21 g P4VP and 1.08g FeCl3·6H2O were dissolved in 50ml ethanol, respectively. The FeCl3·6H2O solution was added to the solution of P4VP and stirred for 4 hours. It was then vaporized at 60 °C in air for several hours, and a brown carbonated precursor was obtained (mole ratio: Fe3+/4VP=2). The precursor was pyrolysis in a tubular furnace at suitable temperature for 2 h under N2 flow of 50 ml min−1 with a ramp rate of 5 °C min−1 and nature cool to room temperature. The carbonated samples were denoted as FeNDC-X-Y (iron-coordinating nitrogen-doped carbon), Fe-NDC-X-Y was washed in 1M HCl with magnetic stirring for 24 hours to remove the residual metal iron and the corresponding samples was denoted NDPC-X-Y (nitrogen doped porous carbon), where X is the mole ratio of Fe3+ to 4VP monomer and Y is the pyrolysis temperature.

2.3 Characterization

Fourier transform infrared (FT-IR) spectra measurements were measured on a Paragon 1000 spectrometer (Perkin Elmer, USA). Thermogravimetric analysis (TGA) measurements were measured on a TGA 7 Thermogravimetric Analyzer (Perkin Elmer, USA) under N2 atmosphere at 20 °C min-1. Field emission scanning electron microscopy (FE-SEM) observations were examined using an S-4800 microscope (Hitachi, Japan). High-resolution transmission electron microscopy (HR-TEM) measurements were performed on a JEOL2100F (JEOL, Japan) microscope at an operating voltage of 220 kv. X-ray diffraction (XRD) 5 ACS Paragon Plus Environment


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measurements were measured on a D/max-2200/PC (Rigaku, Japan) with Cu Kα radiation λ = 1.5418 Å, voltage: 40 kV and current: 40 mA. Raman spectra were obtained using a Senterra R200-L apparatus (Bruker Optics, Germany) with 532 nm line of Ar-ion laser. X-ray photoelectron spectroscopy (XPS) data were measured on an AXIS UltraDLD apparatus (Shimadzu-Kratos, Japan). Inductively Coupled Plasma (ICP) data were measured on an iCAP6300 (Thermo, USA) emission spectrometer. Elemental analysis (EA) data were performed on a Vario EL Cube (Elementar, Germany) microanalyzer. The textural characteristics were quantified by measuring the N2 physisorption at −196 °C in a 3H2000PM2 specific surface & pore size analysis instrument (Beishide, China). All samples were outgassed at 150 °C under vacuum for 6 h prior to measurements. The specific surface areas (SBET) were calculated by Brunauer−Emmett−Teller equation based on adsorption data in the partial pressure (p/p0) range of 0.04 ~ 0.32. The total pore volume (Vtotal) was estimated from the amount of N2 adsorbed at p/p0 ~ 0.99. The micropore specific surface area (St-plot) and volume (Vmicro) of samples were calculated by the t-plot method. The pore size distributions and cumulative pore volume of samples were determined via a non-local density functional theory (NLDFT) method.

2.4 CO2 adsorption measurement

CO2 adsorption equilibrium and kinetics of samples were measured volumetrically in a 3H-2000PM2 specific surface & pore size analysis instrument. The adsorption isotherms were recorded at 0 °C and 25 °C and pressures of reaching 1 bar. The adsorption site temperature was controlled by using a Dewar bottle as a thermostatic bath utilizing water as the coolant. About 100 mg of sample was used for each test. All the samples were degassed at 150 °C under vacuum for 6 h prior to each adsorption experiment. N2 adsorption at 25 °C was measured using an identical procedure. 6 ACS Paragon Plus Environment

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2.5 CO2 selectivity calculation.

The CO2/N2 selectivity was calculated from single-component gas adsorption data based on ideal adsorption solution theory (IAST) according to the following equation:

S= (q1/q2)/ (p1/p2)

(1)

where q1 and q2 are the amounts of CO2 and N2 adsorbed at their respective equilibrium partial pressures p1 and p2. We assumed the CO2/N2 mixtures contained 15% CO2 and 85% N2, following the earlier work.39,40

3. Results and discussion

The nature of the coordination structure was investigated by Fourier transform infrared (FT-IR) techniques. The FT-IR spectra of P4VP and Fe-P4VP-2 (mole ratio: Fe3+/4VP=2) were shown in Figure 1a. Clearly, all apparent peaks were in accordance with the characteristic bands of P4VP.41 The broad band at 3430 cm-1 was attributed to O–H stretching vibrations, which might originate from hydroxyl groups of the adsorbed water. The bands at 2931 cm-1 and at 2855 cm-1 belonged to aromatic and aliphatic C-H stretching frequencies, respectively. Accordingly, the characteristic absorption of the pyridine rings at 1602 cm-1, 1557 cm-1, 1496 cm-1 and 1418 cm-1 corresponded to C=C or C=N, C=N, C=C and C-N stretching vibrations, respectively. The out of plane vibrations of C-H bond appeared at 1002825 cm-1, and the characteristic vibration of the C-C=C band was also observed at 564 cm-1. In contrast to the spectra of Fe-P4VP-2, the C=N stretching vibration characteristic peak at 1557 cm-1 of the pyridine ring was blue-shifted to 1559 cm-1 and the C-N stretching vibration characteristic peak at 1418 cm-1 of the pyridine ring was red-shifted to 1406 cm-1. The blueand red-shifted peaks indicated that coordination bonds had formed between N atom and



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Fe3+.42-44 Furthermore, an obvious new adsorption peak appeared at 1635 cm-1, which also proving the cross-linking between the Fe3+ with the polymer.45,46

Thermo-gravimetric analysis (TGA) of the P4VP, FeCl3·6H2O and Fe-P4VP-2 was shown in Figure 1b. For the P4VP, initial weight loss observed below 100 °C was attributed to the moisture removal. The subsequent weight loss corresponded to the decomposition of P4VP. The weight loss started at around 350 °C and continued up to 460 °C with a weight loss of 100 %, indicating the complete decomposition of P4VP. The TGA curve of FeCl3·6H2O exhibited three major stages of weight loss. In the first stage, the weight loss was observed below 190 °C due to dehydration. In the second stage (190 ~ 360 °C), the emergence of the first platform showed the formation of a stable hydrate Fe(OH)2Cl. In the last stage (above 400 °C), a second plateau corresponded to the production of mostly Fe2O3, which however retains some Cl- ions and -OH functional groups.47 The TGA curve of Fe-P4VP-2 exhibited by initial temperature to 1000°C showed a decreasing trend. Until 1000 °C, the weight still remained 26 % due to the reaction residual carbon and iron. Obviously, the carbon was formed by the decomposition of P4VP only after it formed the Fe-P4VP complex compounds.

The microstructure and morphology of the NDPC-2-600 sample were characterized by field-emission scanning electron microscopy (FE-SEM). As shown in Figures 2a-b, the asprepared NDPC-2-600 showed a packing structure, which was formed by interconnected nanoclusters. The SEM-mapping of Fe-NDC-2-600 and NDPC-2-600 were shown in Figure S1. First, a large number of nitrogen element sites can be observed on the surface of the materials. Second, the surface of the Fe-NDC-2-600 sample produced a high amount of iron oxide nanoclusters, which then disappeared after washing by hydrochloric acid. In addition, the porosity of NDPC-2-600 was composed of randomly distributed uniform micropores (d ≈


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4.6 Å) as demonstrated by the high-resolution transmission electron microscopy (HR-TEM) images (Figures 2c-d and Figure S2a).

The N2 adsorption isotherms and corresponding pore size distributions of carbon samples prepared at temperature between 500 °C and 800 °C and with Fe3+/4VP = 2 were shown in Figure 3. As shown in Figure 3a, the N2 adsorption isotherms of Fe-NDC materials below 650 °C were in the shape of type III, indicating a non-pore characteristic. When the temperature reached or higher than 650 °C, the adsorption isotherm was transformed into type IV. Moreover, the N2 adsorption-desorption curves showed a typical H2-type hysteresis loop, indicating an interconnected porous structure. Changes in the shape of the isotherms at carbonization temperature of 650°C indicated that the Fe-NDC samples from nonporous materials transformed to porous materials. As shown in Table S1, the specific surface area based on BET equation increased significantly with the carbonization temperature below 650 °C (< 20 m2 g-1) and above it (> 200 m2 g-1). This was the first interesting phenomenon we found: with the temperature to 600 °C as the dividing line, the surface area and pore volume suddenly increased of Fe-NDC samples. In contrast, the N2 adsorption isotherms of NDPC below 650 °C was in the shape of type I, however, it showed the type IV when above 650 °C (Figure 3c). Changes in the shape of the N2 adsorption curves at the carbonization temperature reached 650 °C indicated changes in the pore size of the NDPC. Obviously, the NDPC from the microporous materials transformed to the hierarchical porous materials. All Fe-NDC activated samples and NDPC samples had very narrow pore size distributions (≈ 0.4 ~ 0.6 nm) in the micropores region (Figures 3b-d and Figures S3a-b). We noticed that the specific surface area and the pore volume increased significantly by comparing the Fe-NDC and NDPC samples (Table S1 and Table 1). This was the second interesting phenomenon we found: the surface area and pore volume of the materials significantly increased only after a simple pickling treatment. In order to study the effects of different ratios on the textural 9 ACS Paragon Plus Environment


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properties, we prepared porous carbon with 4 different Fe/4VP ratios (Fe: 4VP= 4:1, 2:1, 1:1, 1:2) at 600 °C, and the specific surface area and pore volume of these samples were summarized in Table S2.

Thinking about the first phenomenon was our motivation to perform the X-ray diffraction (XRD) test for the Fe-NDC samples, as shown in Figure S4. Figure S4a indicated the XRD pattern of Fe-NDC-2-600. The diffraction data which can match to the rhombohedral phase of hematite (α-Fe2O3, ICCD No. 33–0664), indicating that the iron salt in Fe-NDC-2-600 existed in the form of α-Fe2O3 crystals. In contrast, Figure S4b confirmed that graphitic carbon, metallic Fe0 and Fe3C coexisted in Fe-NDC-2-700, which was in accordance with the diffraction data for α-Fe0 (ICCD No. 06-0696) and Fe3C (ICCD No. 76-1877). Clearly, with the temperature increasing from 600 °C to 700 °C, the phase of iron salt from α-Fe2O3 transformed to Fe0 and Fe3C. It may be one reason underlying the type changes in the N2 adsorption isotherm of Fe-NDC-2-600 and Fe-NDC-2-700 (Figure 3a). The reaction occurring during carbonization might be described as follows, showing that when the temperature was above 600°C, the Fe2O3 nanoclusters acted as an activating porogen to induce Fe-NDC to form porous structure. 400 °C ≤ T ≤ 600 °C P4VP → CO2 + H2O + N2O + C

(2)

FeCl3·6H2O → Fe2O3 + HCl + H2O + Cl2

(3)

600 °C < T ≤ 800 °C Fe2O3 + C → Fe + CO2 + CO

(4)

Fe + C→ Fe3C

(5)

Fe + CO → Fe3C+ CO2

(6)

The abovementioned second phenomenon motivated us to perform the inductively 10 ACS Paragon Plus Environment

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coupled plasma (ICP) test for the Fe-NDC and NDPC samples, as shown in Table S3. The FeNDC samples contained considerable amounts of iron (21.88 ~ 39.98 wt%). In contrast, the NDPC samples contained only very little iron (< 2.2 wt%). The specific surface area and pore volume of samples significantly increased only after simple pickling. Thus, we assumed that the iron particles might act like an inner hard-template and a living group to generate the pore structure in the whole carbonization temperature range.

In summary, we presented the method of iron nanoclusters as a porogen, including the dual effect of the template method and activation method. In the whole stage, iron nanoclusters in different phases (Fe2O3, Fe0, Fe3C) at the different carbonization temperature had the similar pore-forming effect of the template. During the heating process, the iron nanoclusters were gradually formed with a uniform size covered in carbon materials (Figures S5b-c). After hydrochloric acid etching, iron nanoclusters were washed away to form pore structures (Figures S5d-e). Furthermore, when the temperature was high enough (650 °C), the generated Fe2O3 nanoclusters started playing the role of activating agents, similar to the ZnO produced by ZnCl2 activation (Figure S5c).48,49 However, the difference between them was that the reaction of ZnO and carbon to generate gaseous Zn0 and leave in the process of activation, but the reaction of Fe2O3 and carbon to produce Fe and Fe3C and coated in the material. Interestingly, the aggregated Fe2O3 nanoclusters only acted as hard-templates to form the micropore structure with a narrow pore size distribution under mildly carbonization conditions (T≤ 600 °C). Only when temperature continued to rise, the Fe2O3 nanoclusters started to activate of the carbon around them to form the pore structure. Simultaneously, the transformed iron phases of Fe0 and Fe3C also acted as templates to form the pore structure, thereby proving that iron nanoclusters acted as both a template and activator at the high temperature conditions (T > 600 °C).



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The NLDFT pore size distributions and cumulative pore volume of Fe-NDC and NDPC samples were shown in Figure S6. Fe-NDC-2-X (X= 650, 700, 800) data indicated that activation produce many micropores with a narrow pore size distribution (0.4~1 nm) and a part of meso- or macropore structure (Figures S6d-f). In addition, the proportion of micropore volume decreased when temperature increased, indicating that the pore structure of Fe-NDC samples is controlled by the activation degree. Meanwhile, the Fe-NDC-2-700 and Fe-NDC2-800 had similar pore structure and pore volume which might be attributed to the activation process relying on the reaction between iron oxide and carbon, and it would terminate when the Fe2O3 was completely transformed into Fe0 and Fe3C (Figures S6e-f).

The XRD patterns of NDPC samples shown in Figure S7a revealed their architectures prepared at different temperatures. The pattern of the materials prepared at lower than 650 °C demonstrated a broad peak from 20° to 35°, corresponding to amorphous carbon. The pattern of the materials prepared at higher than 600 °C demonstrated the peak around 26°, which became stronger and narrower with increasing temperature, showing that a higher pyrolysis temperature might cause more ordered graphitic carbon.50-52 Figure S7b indicated the Raman spectra of NDPC synthesized at different pyrolysis temperatures. The D band present at 1332 cm-1 corresponded to the defects, such as edges, boundaries or disorders of the carbon, and the G-band presented at 1588 cm-1 gave evidence for the existence of sp2-hybridized carbon atoms and ordered graphitic structure.53 The ratios of the D band to G band integrated intensities (ID/IG) were 1.58 (500 °C), 1.63 (550 °C), 1.73 (600 °C), 1.94 (650 °C), 1.60(700 °C) and 1.53 (800 °C). At first, the values of ID/IG increased, then it decreased with the increasing temperature. This result might be because when the pyrolysis temperature was lower than 650 °C, the specific surface area and pore volume of NDPC samples changed with increasing temperature lead to more defects in materials, making the D band became stronger, which caused the ID/IG becoming higher. However, although the specific surface area at 650 12 ACS Paragon Plus Environment

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°C decreased, this stage which as beginning of the activation reaction made the pore size distribution more disordered and the pore structure more complex, thereby further strengthening the D band and increasing the ID/IG. When the pyrolysis temperature was higher than 650 °C, the value of ID/IG decreased with increasing temperature owing to the formation of similar pore structure but graphitization degree increased gradually.

The chemical compositions of the NDPC samples measured by EA and ICP were exhibited in Table 1. A common trend of the decrease in the nitrogen content was observed with the increase in carbonization temperature from 9.9 wt% for the sample prepared at 400 °C (NDPC-2-400) to 1.36 wt% for the sample prepared at 800 °C (NDPC-2-800). In addition, the surface chemical compositions were measured by X-ray photoelectron spectroscopy (XPS). The N1s XPS spectra displayed the changes occurring in nitrogen species on the surfaces of the carbon samples after pyrolysis (Figure 4 and Table S4). The asymmetric N1s peaks can be fitted to three peaks of pyridinic N (398.6 ± 0.2 eV), pyrrolic N (399.5 ± 0.2 eV) and quaternary N (400.7 ± 0.5 eV).16,53-56 The surface nitrogen contents of samples NDPC-2-X (X= 400, 500. . .) were 8.15,9.27, 9.80, 7.42, 5.34, 4.57 and 3.11 at%, which were a little bigger than the corresponding values from the EA. This finding revealed that more nitrogen elements distributed on the surface of the materia was beneficial for CO2 adsorption because these surface nitrogen active sites might act as anchors for CO2 capture .56

The CO2 adsorption isotherms of the NDPC-2-X samples at 0 °C and 25 °C were shown in Figures 5a-b, respectively. The CO2 adsorption capacity increased steadily along with the increasing pressure and up to maximum at 1 bar. No distinct plateau was observed in these isotherms for the whole pressure range, indicating these as-prepared porous carbon samples can possibly adsorb more CO2 at higher pressure.57 The CO2 capture capacities of the NDPC samples under atmospheric pressure at 0 °C and 25 °C were summarized in Table 2. Amount 13 ACS Paragon Plus Environment


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of them, the NDPC-2-600 had a highest surface area (1211 m2 g-1), largest pore volume (0.96 cm3 g-1) and superior surface N content (7.42 at%). It showed the highest CO2 uptakes of 6.8 mmol g−1 and 4.3 mmol g−1 at 0 °C and 25 °C, respectively, which are comparable to the best reported CO2 uptakes by nitrogen doped porous carbons (Table S5). NDPC-2-550, which had the highest surface N content (9.80 at%) but lower surface area (836 m2 g-1) and smaller pore volume (0.74 cm3 g-1), displayed CO2 uptakes of 4.5 and 3.1 mmol g-1 at 0 °C and 25 °C, respectively. Meanwhile, the very high CO2 uptake by NDPC-2-600 (1.74 mmol g-1 at 0.1 bar and 0 °C) confirmed the stronger interaction between the CO2 molecules and the active sites associated to N groups at low pressure (Figure 5c).58,59 Taken together, both the textural properties and surface N content were important factors in determining the CO2 uptake capacity.

To clarify the interactions strength between CO2 and the NDPC samples, the isosteric heats of adsorption (Qst) for NDPC-2-X samples were further evaluated from the CO2 adsorption isotherms determined at 0 °C and 25 °C using the Clausius − Clapeyron equation (Figure S8). The high initial Qst values might suggest that the CO2 molecules were selectively adsorbed on the surface N active sites at low CO2 loading moment for NDPC samples.16-18 At high CO2 loading moment, the comparatively low Qst values could indicate that the CO2 physisorption proceeds occupy the dominant position.59,60 NDPC-2-600 exhibited a relatively high heat of adsorption compared with the other NDPC samples which was in good agreement with CO2 adsorption studies. To develop practical CO2 capture applications, in addition to high CO2 uptake capacity, we must also consider the adsorption selectivity of the carbon sorbents. Significantly, the CO2/N2 (0.15/0.85) selectivity for NDPC-2-600 sample at 0 °C and 25 °C up to 18.4 and 15.2, which indicated excellent industrial application prospects (Figures 5c-d).


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4. Conclusion

In summary, we propose a facile synthetic method for the preparation of nitrogen doped porous carbon with high specific surface area (up to 1211 m2 g-1) and high N content (up to 9.9 wt%) by using P4VP as a carbon and nitrogen source, Fe3+ acts as a complexant and catalyst and iron nanoclusters act as a porogen based on metal−polymer coordination. Owing to the nitrogen atom in the aromatic ring having lone pair electrons, Fe3+ can serve as a complexant to connect them via electrostatic interactions to form the coordination structures (P4VP-Fe). Under the catalysis of Fe3+ during in pyrolysis process the P4VP did not decompose completely and transform to carbon. Then, via washed with 1M HCl to remove residual metal to obtain the NDPC samples, possessing a narrow microporosity and high surface nitrogen contents (up to 9.82 at%) which can be obtained by using only mild pyrolysis temperature (T < 650 °C). These carbons exhibite a high CO2 capture capacity, especially, the NDPC-2-600 has a highest surface area (1211 m2 g-1), biggest pore volume (0.96 cm3 g-1) and superior surface N content (7.42 at%). It shows the highest CO2 uptakes of 6.8 mmol g−1 and 4.3 mmol g−1 at 0 °C and 25 °C, the CO2/N2 (0.15/0.85) selectivity at 0 °C and 25 °C can reach 18.4 and 15.2, indicating excellent industrial application prospects. We demonstrated a novel pore-forming method in which metal act as a template or activator at the different stages of formation process of the carbons based on metal−polymer coordination. We believe that this synthetic approach and the proposed method may open a new avenue for us to synthesis the carbon-based porous materials.

Supporting Information

SEM-mapping, HR-TEM image, XRD patterns, Raman spectra, scheme for pore formation, isosteric heat of adsorption (Qst), textural properties (BET, pore volume, pore size distributions), ICP data of Fe content, surface chemical composition (XPS), and CO2 uptake 15 ACS Paragon Plus Environment


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comparison of various nitrogen doped porous carbon materials. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (No. 20976105, 21320102006, 21274089 and 91127047), the National Basic Research Program (No. 2013CB834506), Shanghai Leading Academic Discipline Project (No. J51503), Shanghai Association for Science and Technology Achievements Transformation Alliance Program (No. LM201559), Shanghai Municipal Education Commission boosting project (No. 15cxy39), Science and Technology Commission of Shanghai Municipality Project (No. 14520503200), Shanghai Talent Development Funding (No. 201335).

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Table and Figures Table 1. Textural properties and chemical composition of NDPC. Textural properties St-plot Vmicro (m2 g-1)b (cm3 g-1)c

Chemical composition (wt%) Vtot (cm3 g-1)d

N

-

-

9.90

22.75

64.25

0.3547

505

0.26(0.37)

0.70

7.87

19.65

68.49

0.3765

836

702

0.36(0.49)

0.74

6.85

21.26

68.32

0.3258

NDPC-2-600

1211

1079

0.55(0.57)

0.96

7.43

17.67

70.30

1.4784

NDPC-2-650

737

468

0.24(0.25)

0.96

4.43

17.17

73.05

2.9293

NDPC-2-700

322

114

0.06(0.07)

0.88

2.56

11.23

84.26

0.4271

NDPC-2-800

309

120

0.06(0.08)

0.71

1.36

6.90

88.67

2.1972

NDPC-1-600

1079

1006

0.51(0.80)

0.64

64.04

0.9734

Sample

SBET (m2 g-1)a

NDPC-2-400

3

0

NDPC-2-500

659

NDPC-2-550

6.25

O

25.43

C

Fee

(a) specific surface areas were calculated by BET equation based on N2 adsorption datas in the partial pressure (p/p0) range of 0.04 ~ 0.32. (b) micropore specific surface area and (c) pore volume of samples were determined by the t-plot method. The values in parenthesis are the percentage of Vmicro/ Vtot. (d) total pore volume at p/p0 ~ 0.99. (e) the Fe contents were determined by ICP. Table 2. CO2 uptake at (a) 0 °C and (b) 25 °C of NDPC samples. CO2 uptake, mmol g−1 (mg g−1) samples

0 °C

25 °C

NDPC-2-500

3.1 (136.4)

2.2 (96.8)

NDPC-2-550

4.5 (198.0)

3.1 (136.4)

NDPC-2-600

6.8 (299.2)

4.3 (189.2)

NDPC-2-650

4.2 (184.8)

3.0 (132.0)

NDPC-2-700

1.3 (57.2)

0.8 (35.4)

NDPC-2-800

1.2 (52.8)

0.8 (35.4)

NDPC-1-600

5.2 (228.8)

3.4 (149.6)


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Figure 1. (a) FT-IR spectra of P4VP and Fe-P4VP-2. (b) TGA curves of P4VP, FeCl3·6H2O and Fe-P4VP-2 measured at heating rate of 10 °C min-1 under nitrogen flux of 50 ml min-1.

Figure 2. (a, b) FE-SEM and (c, d) HR-TEM images of NDPC-2-600.



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Figure 3. (a, c) N2 adsorption/desorption isotherms of Fe-NDC-2-X and NDPC-2-X, respectively. (b, d) pore size distributions of Fe-NDC-2-X and NDPC-2-X, respectively.

Figure 4. N1s XPS spectra of (a) NDPC-2-400 (b) NDPC-2-500 (c) NDPC-2-550 (d) NDPC2-600 (e) NDPC-2-650 (f) NDPC-2-800. 26 ACS Paragon Plus Environment

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Figure 5. CO2 adsorption isotherms at (a) 0 °C and (b) 25 °C of NDPC samples. CO2 and N2 adsorption isotherms of NDPC-2-600 as measured at (c) 0 °C and (d) 25 °C. Insets are the calculated CO2/N2 selectivities.



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TOC Art


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Activator for the Synthesis of Nitrogen

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