Significantly Increased CO2 Adsorption Performance of

Significantly Increased CO2 Adsorption Performance of Nanostructured Templated Carbon by Tuning Surface Area and Nitrogen Doping. Lifeng Wang and ...
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Significantly Increased CO2 Adsorption Performance of Nanostructured Templated Carbon by Tuning Surface Area and Nitrogen Doping Lifeng Wang and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States

bS Supporting Information ABSTRACT: Carbon dioxide adsorption properties of a series of templated carbon adsorbents with high Brunauer Emmett Teller surface areas (1361 3840 m2/g) and with/without nitrogen doping (6 7 wt % N) were systematically studied. Two linear relationships between CO2 adsorption capacities and surface areas of nitrogen-doped/ undoped nanostructured templated carbons were first established. The doped nitrogen was present in the forms of pyridinic nitrogen, pyrrolic/pyridonic nitrogen, quaternary nitrogen, and an oxidized form of nitrogen. The interaction energies with CO2, as approximated by the isosteric heats of adsorption, were increased from 30 kJ/mol on the undoped carbon to 50 kJ/ mol on the N-doped carbon as a result of these nitrogen sites. The increased interactions led to an enhancement in CO2 adsorption capacity by a factor of 2, while N2 uptake was not enhanced. The optimized N-doped templated carbon, N-TC-EMC, possessed remarkable CO2 capacity (4 mmol/g at 1 atm and 298 K) and selectivity (CO2/N2 at 1 atm = 14). Postdoping ammonia treatment was found beneficial to CO2 adsorption. CO2 performance of N-doped carbon under wet condition and conditions relevant to flue gas, rates of adsorption, and regeneration requirement, which are important for practical applications, were also investigated. The results showed that N-doped templated carbon exhibited all prerequisite attributes for CO2 capture and storage applications: high CO2 capacity and CO2/N2 selectivity, fast and reversible adsorption, thermal and moisture stabilities, and ease in CO2 desorption.

1. INTRODUCTION Carbon dioxide emission from combustion of fossil fuels has caused increasing concerns for global climate change and urgent demand for reduction in its emission. This has promoted enormous research efforts in developing novel technologies and adsorbents for carbon capture and storage (CCS).1 As a promising alternative to liquid amine and ammonia absorption processes, CO2 adsorption on porous adsorbents, including metal organic frameworks (MOFs), 2 16 amine silica, 17 23 zeolites, 24 29 carbons, 30 35 and mesoporous alumina, 36 has been investigated extensively. Among these sorbents, carbon materials encompass desired attributes for potential CCS applications: high uptake rates, hydrophobicity, isotherm reversibility, stability, and low-energy requirement for desorption. However, the CO2 adsorption capacities and CO2/N2 selectivities of carbon sorbents are relatively low compared to some of the other types of sorbents. To increase the interactions between CO2 and carbon sorbent, modification of amine on carbon surface or incorporation of nitrogen groups into carbon framework was employed.37 48 Modification of amine on carbon can enhance CO2 adsorption but is plagued by the amine stability over regeneration cycles. Incorporation of nitrogen groups into carbon framework was reported earlier by Pevida et al., who obtained a high CO2 capacity (2.25 mmol/g at 298 K and 1 atm) on their melamine-based mesoporous carbon.44 More recently, Hao et al. achieved a high capacity of 3.1 mmol/g on the r 2011 American Chemical Society

nitrogen-doped carbon monolith by using amino acid (L-lysine) as the nitrogen precursor.45 Although progress has been made on these modified or doped carbons, to develop carbon sorbents with applicable capacities and selectivities still remains a challenge, and an understanding of how the sorbent structure and nitrogen doping affect CO2 performance is desirable for optimizing/maximizing CO2 adsorption on carbon sorbents and their potential application. In addition, most of reported studies were focused on measuring adsorption under pure or dry CO2. For practical applications, investigation of nitrogen-doped carbon performance under wet conditions and under conditions relevant to flue gas, rates of adsorption, and regeneration requirement are also important. Herein we synthesized a series of nanostructured templated carbons with various surface areas (1361 3840 m2/g) and with/ without N-doping (6 7 wt % N) by varying the synthesis conditions and investigated the effects of surface area and nitrogen doping on CO2 adsorption performance of these sorbents. The approach taken in this work was twofold: to tailor/maximize the Brunauer Emmett Teller (BET) surface area of the carbon and to increase the interactions with CO2 by N-doping or N-substitution. Templated carbons were employed Received: October 19, 2011 Revised: December 2, 2011 Published: December 06, 2011 1099

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The Journal of Physical Chemistry C because of their adjustable textual structures and high surface areas among carbons,49 57 and they yielded the highest hydrogen storage capacities.55 57 The sorbent attributes including CO2 adsorption under dry, wet, and flue gas conditions, CO2/N2 selectivity, rates of adsorption, adsorption reversibility, thermal and moisture effects, and regeneration were studied.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Templated Carbon (TC-Y1) from Y Zeolite. TC derived from Y zeolite was prepared according to a

procedure reported by Ma et al.52 Y zeolite was degassed in a flask for 12 h at 473 K, and then furfuryl alcohol (FA) was introduced into the flask. The mixture of zeolite and FA was stirred overnight under a helium flow, then filtered, and washed with mesitylene to remove FA on the external surface of Y zeolite. The polymerization of FA in Y zeolite was carried out by heating the composite under a flow of helium at 353 K for 24 h and then at 423 K for another 8 h. The Y/PFA composite was placed in a vertical quartz tube and heated to 973 K under a nitrogen flow. When the temperature reached 973 K, propylene gas (2% in nitrogen by volume; flow rate 150 cm3/min) was passed through the tube for 6 h. After the chemical vapor deposition (CVD) treatment, the composite was further heated at 1173 K for 2 h under a nitrogen flow. The obtained zeolite/carbon composite was treated in HF solution (40%) for 24 h and subsequently refluxed in a concentrated HCl solution for 4 h to dissolve the zeolite template. The resulting TC was washed with ammonia solution to neutralize the residue acid and finally washed with copious distilled water and collected by filtration. 2.2. Synthesis of Templated Carbon (TC-Y2) from Y. Y was degassed in a flask for 12 h at 473 K, then placed in a vertical quartz tube, and heated to 1073 K under a nitrogen flow. When the temperature reached 1073 K, propylene gas (2% in nitrogen by volume; flow rate 150 cm3/min) was passed through the tube for 6 h. After the CVD treatment, the composite was further heated at 1173 K for 2 h under a nitrogen flow. The obtained Y/ carbon composite was treated with the same procedure as described in 2.1. to remove Y zeolite. 2.3. Synthesis of Templated Carbon (TC-EMC) from EMC2. The synthesis procedure for TC-EMC was similar to that for TC-Y1, except that EMC-2 was used instead of Y, and 15 h of CVD of propylene was used instead of 6 h. 2.4. Superactivated Carbon Maxsorb. The superactivated carbon Maxsorb was obtained from Tokyo Zairyo Co. Ltd. It was produced by activation in molten KOH. 2.5. Synthesis of Nitrogen-Doped Templated Carbon (N-TC-Y1) from Y. Nitrogen-doped carbon derived from Y zeolite was prepared by using acetonitrile as the precursor, which was reported to be an effective precursor for nitrogen doping.58 60 Y was degassed in a flask for 12 h at 473 K, then placed in a vertical quartz tube, and heated to 1023 K under a nitrogen flow. When the temperature reached 1023 K, the nitrogen flow was switched to acetonitrile (saturated in nitrogen at a flow rate of 150 cm3/ min) to pass through the tube for 4 h. After the CVD treatment, the composite was further heated at 1173 K for 2 h under a nitrogen flow. The obtained Y/carbon composite was treated with the same procedure as described in section 2.1 to remove Y zeolite. 2.6. Synthesis of Nitrogen-Doped Templated Carbon (N-TC-Y2) from Y. The synthesis procedure for N-TC-Y2 was similar to that for N-TC-Y1, except that the CVD time was reduced from 4 to 2 h.

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2.7. Synthesis of Nitrogen-Doped Templated Carbon (NTC-EMC) from EMC-2. The synthesis procedure for N-TC-EMC

was similar to that for N-TC-Y1, except that zeolite EMC-2 was used instead of zeolite Y. 2.8. Synthesis of Amine-Grafted SBA-15. SBA-15 was synthesized according to the procedure reported by Zhao et al.61 The grafting of amine SBA-15 surfaces was achieved by refluxing 1.0 g of SBA-15 with 50 mL of toluene and 5.0 mL of 3-aminopropyltriethoxysilane. The mixture was stirred and refluxed at 353 K for 12 h. The resulting 3-aminopropyl-grafted SBA-15 was washed repeatedly with toluene and then dried under vacuum at 333 K overnight. 2.9. Characterization. Nitrogen adsorption isotherms and CO2 adsorption isotherms (0 1 atm) were measured with a Micromeritics ASAP 2020 analyzer. Nitrogen adsorption isotherms were measured at 77 and 298 K. To measure the 298 K N2 isotherm by Micromeritics ASAP 2020, the sample was in situ degassed at the measuring port at 423 K for 12 h and then cooled to room temperature under degassing. The in situ degassing at the measuring port of ASAP 2020 avoided the transfer of the sample from the degassing port to the measuring port, thus avoiding the exposure of sample to air that remained in the measuring port during transfer. The turbo pump connecting to the measuring port also provided higher vacuum than general degassing port. This helped to completely degas the sorbent surface for measuring a low amount of nitrogen adsorption at 298 K. The surface area is calculated using the Brunauer Emmett Teller (BET) model based on adsorption in the P/P0 range of 0.01 0.20. CO2 adsorption isotherms were measured at 298, 323, 348, and 373 K. Constant temperature circulating baths were used to maintain the sample at 298 and 323 K. Heating tape was used to maintain 348 K and higher temperatures. Liquid nitrogen was used for 77 K. The carbon sample was degassed at 423 K overnight before measurement. Amine-grafted silica sample was degassed at 373 K overnight. The rate of CO 2 adsorption (ROA) on adsorbent was measured by recording pressure change per dose vs time (or measuring the time it took a dose of gas to adsorb) with a built-in ROA function in Micromeritics ASAP 2020 analyzer. The heats of adsorption (HOA) were calculated with a built-in HOA function in Micromeritics ASAP 2020 analyzer, which is based on the Clausius Clapeyron equation. Moisture effects on CO2 adsorption on the N-TCEMC sample were studied by respectively performing CO2 adsorption under a dry CO2 stream (100% CO2), a humid CO2 stream (97% CO2; 3% H2O), and a humid CO2/N2/O2 stream (15% CO2; 78% N2; 4% O2; 3% H2O) at room temperature. The carbon sample was placed into a U-tube sample holder with switches for gas inlet and outlet. Before CO2 adsorption, the sample was heated at 423 K for 3 h under helium flow and cooled to room temperature under helium. After that, helium was switched to CO2 or a mixed-gas stream (total flow rate: 50 cm3/min). The flow time was 2 h to ensure adsorption equilibrium. The adsorbed CO2 on the N-TC-EMC sample was desorbed under flowing helium and collected by passing the desorption gas through three consecutive collectors containing barium hydroxide solution. The white precipitate (i.e., barium carbonate) was collected quantitatively by centrifugation at 4000 rpm for 10 min and dried at 373 K. The quantification of the adsorbed CO2 on N-TC-EMC was estimated by measuring the weight of the dry precipitate. The high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL 3011 analytical electron microscope equipped with EDX analysis operated at 300 kV. 1100

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Figure 1. CO2 adsorption isotherms on TC-EMC (]), TC-Y1 (O), TC-Y2 (0), and Maxsorb (4) at 298 K.

Figure 2. Relationship between the CO2 adsorption capacities (at 298 K and 1 atm) and BET surface area of carbon adsorbents.

X-ray photoelectron spectroscopy was recorded with a Kratos Axis ultra XPS spectrometer.

the surface area, shown in Figure 2. An approximately linear relationship is seen. This linear correlation is in agreement with previous observation on MOF sorbents by Zhou et al.10 It is noted that, in present work, the highest CO2 capacity of 3.2 mmol/g at 298 K and 1 atm was obtained on TC-EMC with an ultrahigh BET surface area of 3840 m2/g. To investigate the nitrogen doping effects on the CO2 capacity, an N-doped templated carbon was initially synthesized using acetonitrile as the source precursor and Y zeolite as the hard template (designated as N-TC-Y1, see Experimental Section). The success of N-doping in N-TC-Y1 was confirmed by the X-ray photoelectron spectrum (XPS), which showed ∼7 wt % nitrogen content in N-TC-Y1 (Figure S3). The chemical states of nitrogen in N-TC-Y1 were further evaluated by the N 1s core scanning. As shown in Figure S3, the N 1s core spectrum was deconvoluted by contributions from four peaks with binding energies centered at 398.4, 400.1, 401.4, and 403.9 eV, which were assigned to pyridinic nitrogen, pyrrolic/pyridonic nitrogen, quaternary nitrogen, and oxidized form of nitrogen, respectively.64,65 These XPS results indicated that acetonitrile was a suitable precursor for N-doping in carbon. Since acetonitrile is the only precursor (working as both carbon and nitrogen precursors) used for the synthesis of N-doped templated carbon, nitrogen could be homogeneously distributed both on the surface and inside the bulk. This has been reported in a previous study of N-doped carbon from acetonitrile.66 These basic nitrogen groups induced by nitrogen doping can interact with the acidic CO2 gas. Further discussion on the enhanced interactions will be given shortly. The BET surface area of N-TC-Y1 was only 1762 m2/g (Table S1). The N2 (77 K) isotherm of N-TC-Y1 exhibited a rapid increase at low relative pressure (P/P0 < 0.02), indicating the presence of microporosity (Figure S4). DFT results further confirmed that the pore size of N-TC-Y1 was mainly distributed between 7 and 40 Å (Figure S5). The relatively low surface area of N-TC-Y1 compared with undoped carbon was caused by the less infiltration step. Although the surface area of N-TC-Y1 was not very high, the CO2 isotherm on N-TC-Y1 showed a remarkable CO2 capacity of 3.2 mmol/g, as high as that of the undoped TC-EMC with a surface area of 3840 m2/g (Figure 3). Normalized by BET surface area, the enhancement factor on N-TC-Y1 via N-doping was 2.2. To further investigate the effect of N-doping on CO2 adsorption and improve the final storage capacity, two other nitrogen-doped templated carbons, i.e., N-TC-Y2 and N-TC-EMC, were synthesized by

3. RESULTS AND DISCUSSION To investigate the influence of surface area on CO2 capacity, a series of templated carbons with various surface areas were synthesized by adjusting the synthesis conditions (see Experimental Section). These templated carbons were designated as TC-Y1, TC-Y2, and TC-EMC (TC stands for templated carbon; Y and EMC denote the zeolite templates used for the synthesis). As shown in Figure S1 (Supporting Information), the N2 (77 K) isotherms on TC-Y1, TC-Y2, and TC-EMC carbons were all of type I, indicating the presence of microporosity. Pore size distributions of these carbons were calculated from the adsorption branch of the isotherm using the density functional theory (DFT) method. As shown in Figure S2, the pore sizes of these carbons were mainly distributed between 7 and 40 Å. The results are consistent with PSD for activated carbons with a significant quantities of micropores and some mesoporosity.62 The BET surface areas of TC-Y1, TC-Y2, and TC-EMC were 3519, 1815, and 3840 m2/g, respectively (Table S1). These are comparable to the reported high-surface zeolite-templated carbons.51 54,63 The main reason for the highest surface area of TC-EMC was that TC-EMC was synthesized using the template EMC-2, a zeolite with large pore openings and straight channels running along the c direction, which were favorable for the infiltration of carbon precursor and consequently better replication. TC-Y1 and TC-Y2 were both synthesized using type Y zeolite; however, TC-Y1 showed a higher surface area than TC-Y2. This was because TC-Y1 was synthesized through a two-step infiltration of furfural alcohol and propylene into Y, while TC-Y2 was synthesized through one-step filtration of propylene. The two-step filtration resulted in a higher loading of carbon precursor and better replication. For comparison, the nitrogen isotherm of the commercial, superactivated carbon Maxsorb was also measured (Figure S1), which revealed a BET surface area of 3311 m2/g for Maxsorb. The DFT pore size of Maxsorb was also distributed between 7 and 40 Å, but with a larger fraction of pores distributed between 20 and 40 Å than those in the templated carbon (Figure S2). The CO2 isotherms for TC-Y1, TC-Y2, TC-EMC, and Maxsorb at 298 K are shown in Figure 1. It is seen that the CO2 adsorption capacities followed the BET surface area. The CO2 adsorption capacities at 1 atm were further plotted against

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Figure 3. Comparison of CO2 adsorption isotherms on N-doped N-TC-Y1 (4) with a BET surface area of 1762 m2/g and undoped TC-EMC (O) with a BET surface area of 3840 m2/g at 298 K.

adjusting the synthesis conditions and using Y and EMC-2 zeolites as hard templates. The nitrogen isotherms of N-TC-Y2 and N-TC-EMC were then compared with N-TC-Y1 in Figure S4. The surface areas of these nitrogen-doped carbons followed the order N-TC-EMC (2559 m2/g) > N-TC-Y1 (1762 m2/g) > N-TC-Y2 (1361 m2/g). Again, the highest surface area of N-TCEMC among the doped carbons was due to better infiltration of carbon precursor into EMC-2 because of its large pore openings and straight channels. The surface area difference between N-TC-Y1 and N-TC-Y2 was caused by the different precursor loading amounts. This is also confirmed by DFT results. Although DFT pore sizes of these N-doped carbons were distributed in the same range (7 40 Å), the corresponding pore volume of N-TCEMC is larger than those of N-TC-Y1 and N-TC-Y2, indicating a better replication in N-TC-EMC (Figure S5 and Table S1). The success of N-doping in the N-TC-EMC framework was also confirmed by the XPS spectra (Figure S6), which are identical to those of N-TC-Y1 due to the same doping method. The successful structural replica from EMC-2 to N-TC-EMC was confirmed by the high-resolution transmission electron microscopy (HRTEM) image of N-TC-EMC, on which ordered microporous channels can be clearly observed (Figure S7). CO2 adsorption results showed N-TC-Y2 adsorbed 2.6 mmol/g CO2, while N-TC-EMC adsorbed 4.0 mmol/g CO2 at 298 K and 1 atm, which is the highest among all the tested undoped carbons and nitrogen-doped carbons (Figure 4). This high capacity was 1.8 times the previously reported capacity of mesoporous carbon (2.25 mmol/g)44 and 1.3 times the recently reported capacity of nitrogen carbon monolith (3.1 mmol/g).45 It is worth noting that the desorption isotherm of CO2 on N-doped carbons followed completely the adsorption isotherms, showing complete reversibility. The CO2 adsorption capacities (at 298 K and 1 atm) of N-TCEMC, N-TC-Y1, and N-TC-Y2 were further plotted against their surface areas. As shown in Figure 5, an approximately linear relationship was again seen. From the linear relationship, a CO2 capacity of 6.4 mmol/g can be expected on a N-doped carbon with a surface area of 4000 m2/g. Comparison of the linear relationships of N-doped carbon and undoped carbon showed the slope of the N-doped carbon doubled that of the undoped carbon; i.e., the CO2 capacity was doubled by N-doping. To further understand the N-doping effect, the isosteric heats of adsorption on N-doped carbon (N-TC-EMC) and undoped

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Figure 4. CO2 adsorption isotherms on N-TC-EMC (]), N-TC-Y1 (4), and N-TC-Y2 (O) at 298 K; CO2 desorption isotherm on N-TCEMC (/).

Figure 5. Relationships between the CO2 adsorption capacities (at 298 K and 1 atm) and BET surface area of nitrogen-doped carbons (]) and undoped carbons (0). The N-doped carbons contain 6 7 wt % N.

cabon (TC-EMC) were calculated from the CO2 isotherms at 298 and 323 K by using the Clausius Clapeyron equation and compared in Figure 6. The isosteric heats of adsorption were determined by evaluating the slope of the plot of ln(P) versus (1/T) at the same adsorption amount. As shown in Figure 6 insets, the heats of adsorption on the N-doped carbon were 50 kJ/mol at near zero loading, declining to 33 kJ/mol at 2 mmol CO2/g, and those on the undoped carbon were 30 26 kJ/mol. At high loadings, the heats of adsorption on N-TC-EMC (33 kJ/mol) were also close to the recently reported value (30 32 kJ/mol) on deep eutectic solvents derived N-doped carbons.46 The declines in the isosteric heats with surface coverage were caused by energy heterogeneity; i.e., sites with stronger interactions with CO2 were filled first. The amount of N-doping was ∼7 wt %, corresponding to 5 mmol N/g. Knowing the BET surface area and assuming the uniform dispersion of N and C in the doped sample, the surface N atoms on N-TC-EMC were ∼3 mmol N/g. Thus, below 2 mmol/g loading, it is safe to assume that all CO2 molecules were adsorbed on N-sites. The surface N-sites were mostly pyridinic and pyrrolic/pyridonic types (as shown by XPS), which contain lone pair electrons of nitrogen, capable of Lewis acid base interactions with CO2. The stronger interactions of CO2 with these Lewis base N-sites were reflected 1102

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Figure 7. CO2/N2 selectivities on carbon adsorbents at 298 K. CO2 adsorption isotherms on N-TC-EMC ([), TC-EMC (2), and Maxsorb (b); N2 adsorption isotherms on N-TC-EMC (]), TC-EMC (4), and Maxsorb (O).

Figure 6. CO2 adsorption isotherms on (a) N-TC-EMC and (b) TC-EMC at 298 K (0) and 323 K (]). Inset: heats of adsorption (Hs.A.).

in the increased isosteric heats of adsorption, from 30 kJ/mol on undoped carbon to 50 kJ/mol on N-doped carbon, at near zero loading, and consequently resulted in the significantly enhanced CO2 uptake. As shown above, we observed two separate linear relationships between CO2 uptakes and surface areas of the two types of sorbents (N-doped carbons and undoped carbons). It is known that surface area is a dominant factor for CO2 adsorption in the high pressure range. While for CO2 adsorption in the low pressure range (e.g., below 1 atm in our study), the CO2 adsorption capacity was determined by both interactions between CO2 and sorbent surfaces and the sorbent surface area. For synthesis of N-doped templated carbons, the same N-doping method and N precursor were used, so the obtained carbons have the similar N content and N type; i.e., their interactions with CO2 were similar. Thus, the CO2 uptakes on these N-doped carbons were affected by their surface areas. But if we compare the capacity between an N-doped carbon and an undoped carbon (N-free), we can observe an N-doped carbon with lower surface area showed a higher CO2 capacity than an undoped carbon, simply because of the enhanced CO2 interactions with the N-doped carbon. As a reference, surface areas of these carbons were also calculated based on DFT model.67 Although DFT surface areas of these carbons were lower than their BET surface areas (Table S1), the DFT surface areas of these carbon sorbents still followed the same order as their BET surface areas. For the undoped carbons, the DFT surface area followed the order of TC-EMC > TC-Y1 > Maxsorb > TC-Y2. For the N-doped carbons, the DFT surface area followed the order of N-TC-EMC > N-TC-Y1 > N-TC-Y2. These results further confirmed that CO2 uptake on

the same type of carbon sorbents increased with surface area (both BET and DFT surface areas). In addition, DFT results showed the pore sizes of these carbon sorbents were mainly distributed between 7 and 40 Å. Considering the small size of CO2 (kinetic diameter 3.3 Å) and the similar pore size range for these carbon sorbents, the effects of pore size of these carbons on CO2 uptake would be similar. It is worth noting that, in our study, the synthesized carbon sample was treated with ammonia solution before CO2 adsorption. The effects of ammonia treatment on CO2 adsorption were studied by comparing CO2 adsorption isotherm on N-doped carbon with or without ammonia treatment. As shown in Figure S8 in the Supporting Information, the N-TC-EMC without ammonia treatment had lower CO2 adsorption capacity than N-TC-EMC with ammonia treatment. We attribute the lower CO2 adsorption by the untreated N carbon to occupation of the basic nitrogen sites by protons and F or Cl (as the counterion) which were caused by the HF and HCl treatment (for template removal), and this was detrimental to CO2 adsorption. Ammonia could neutralize the preadsorbed protons during the HF and HCl treatment and free the basic sites, resulting in a higher CO2 capacity. The detrimental effects of adsorbed protons on CO2 adsorption on carbon were also reported by Hao et al.,45 who observed reduced CO2 capacity after treating their N-doped carbon monolith with HCl. They proposed that the basic nitrogen groups were the anchor sites for CO2. Our results were in agreement with their observation. It is also reported that tetramethylammonium hydroxide (TMAOH) or NaOH can be used to remove the residual Cl ions and to neutralize the protonated amine groups in the aminopropyl functionalized materials.68,69 Considering the use of HF and HCl during the synthesis of the carbon sample, we used ammonia solution to treat these carbons. To study the selectivity of CO2 over N2, N2 isotherms at 298 K on N-TC-EMC, TC-EMC, and Maxsorb were measured and compared with CO2 isotherms. As shown in Figure 7, the N2 isotherms were not influenced by N-doping and followed directly the BET surface area. Thus, N-doping also significantly increased the CO2/N2 selectivity. For N-TC-EMC, CO2/N2 (at 1 atm) = 14. A ratio >10 is necessary because the CO2 concentration in the desorbed product needs to be higher than 90% for underground storage. By narrowing the pores would increase both CO2 and N2 1103

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Figure 8. Rates of CO2 adsorption on N-TC-EMC (4), TC-EMC (O), and amine-grafted SBA-15 (], inset figure) at 298 K and pressure