Microporosity development in phenolic resin-based mesoporous carbons for enhancing CO2 adsorption at ambient conditions

Microporosity development in phenolic resin-based mesoporous carbons for enhancing CO2 adsorption at ambient conditions

Applied Surface Science 289 (2014) 592–600 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 289 (2014) 592–600

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Microporosity development in phenolic resin-based mesoporous carbons for enhancing CO2 adsorption at ambient conditions Jerzy Choma a , Katarzyna Jedynak b , Weronika Fahrenholz a , Jowita Ludwinowicz c , Mietek Jaroniec c,∗ a

Institute of Chemistry, Military Technical Academy, 00-908 Warsaw, Poland Institute of Chemistry, Jan Kochanowski University, 25-406 Kielce, Poland c Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242 USA b

a r t i c l e

i n f o

Article history: Received 17 July 2013 Received in revised form 25 October 2013 Accepted 13 November 2013 Available online 21 November 2013 Keywords: Nitrogen adsorption CO2 adsorption Micro-mesoporous carbons Soft templating TEOS assisted synthesis KOH activation

a b s t r a c t Soft-templating method was used to prepare mesoporous carbons. The synthesis in the presence of hydrochloric and citric acids involved resorcinol and formaldehyde as carbon precursors and triblock copolymer Pluronic F127 as a template. The as-synthesized samples underwent carbonization in flowing nitrogen at various temperatures; namely 600 ◦ C, 700 ◦ C and 800 ◦ C. Two routes were used to develop microporosity in the mesoporous carbons studied. The first one involved introduction of tetraethyl orthosilicate to the reaction system. After silica dissolution with NaOH, an increase in microporosity was observed. The second method, chemical activation with KOH at 700 ◦ C, was explored as an alternative approach to create microporosity. It is noteworthy that the TEOS addition not only led to the development of microporosity but also to some improvement of mesoporosity. The post-synthesis KOH activation resulted in more significant increase in the microporosity as compared to the samples obtained by TEOS-assisted synthesis. The mesopore volume was somewhat lower for activated carbons as compared to that in mesoporous carbons. Both methods resulted in micro-mesoporous carbons with good adsorption properties; for instance, in the case of carbons prepared in the presence of TEOS, the best sample exhibited BET surface area of 1463 m2 /g and the total pore volume of 1.31 cm3 /g. For the KOH activated carbons the best adsorption parameters were as follows: the specific surface area = 1906 m2 /g, and the total pore volume = 0.98 cm3 /g. Both procedures used for microporosity development afforded carbons with good adsorption properties that can be useful for applications such as CO2 adsorption, air and water purification. © 2013 Elsevier B.V. All rights reserved.

Mesoporous carbons with pores in the range between 2 nm and 50 nm are of great interest mainly because such materials can be used for adsorption of larger organic molecules. In order to expand the scope of potential applications of mesoporous carbons, for instance, to adsorption of volatile organic compounds, hydrogen storage and carbon dioxide capture, it is essential to develop microporosity, and consequently, to enlarge their specific surface area. Recently, numerous carbonaceous materials such as active carbons [1], carbon-metal composites, biomass-derived carbons [2,3], nitrogen-doped carbons [4,5] have been widely studied for carbon dioxide capture. A significant part of research activities in this field has been devoted to microporous carbons with high specific surface area; however, the recent experimental studies and computer simulations [6,7] show that the proper sizes of micropores are

∗ Corresponding author. Tel.: +1 330 672 3790; fax: +1 330 672 3816. E-mail address: [email protected] (M. Jaroniec). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.051

crucial to maximize carbon dioxide uptake at ambient conditions. Presser et al. [8] and Hu et al. [6] studied the effect of the micropore size on CO2 adsorption. They showed that the micropores with sizes below 1 nm determine the amount of CO2 adsorbed. Moreover, Wickramaratne and Jaroniec fabricated microporous carbons with small micropores (below 0.8 nm) and large micropore volume by activation of polymeric spheres with KOH. They showed that under atmospheric pressure the aforementioned carbons adsorbed 4.6 mmol/g and 8.9 mmol/g at 23 ◦ C and 0 ◦ C, respectively [9]. Mesoporous carbons can be considered as potential candidates for carbon dioxide capture; however, apart from their mesoporous structure, they need to possess highly developed microporosity. While mesoporosity in carbon materials is often created by hard- and/or soft-templating methods, their microporosity is usually introduced by physical or chemical activation. In the hard-templating method, one can employ a variety of templates such as ordered mesoporous silica, siliceous colloids and colloidal silica crystals [10–15], which are removed in the last step of the synthesis process, resulting in the desired carbon replica. In

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the soft-templating method [16–21], surfactants and block copolymers (e.g., Pluronic F127) are used as soft templates. In contrast to the hard-templating method, the soft-templating approach eliminates the need for synthesizing siliceous or related hard templates, which reduces the number of synthesis steps, makes it cheaper and easier for scaling up. In the soft-templating process the thermosetting polymers (usually phenolic resins) and thermally degradable surfactants (usually triblock copolymers) form ordered polymer–polymer composite mesostructures, which during carbonization process are transformed into ordered mesoporous carbons. Some applications require both types of pores present in carbons: mesopores (2–50 nm) and micropores (below 2 nm). Mesopores facilitate transport and can accommodate larger adsorbate molecules, while micropores are essential for adsorption of small molecules. Therefore, activation, for instance with the help of KOH, is often employed to develop microporosity and enlarge surface area of mesoporous carbons. Górka et al. [20] studied the KOH activation of phenolic resin-based carbons obtained by the soft-templating method. The optimal activation temperature was experimentally determined as 700 ◦ C. Micro-mesoporous carbons having the specific surface area of about 750 m2 /g, the total pore volume exceeding 0.50 cm3 /g, and the volumes of micropores and mesopores of about 0.20 cm3 /g and 0.30 cm3 /g respectively, were obtained. Even better results were reported by the same group for the carbons synthesized using phloroglucinol and formaldehyde as carbon precursors, carbonized at different temperatures and activated with KOH at 700 ◦ C [22]. Carbonization at 600 ◦ C followed by KOH activation at 700 ◦ C resulted in a microporous carbon that possessed the specific surface area of ∼2200 m2 /g, the total pore volume of 1.20 cm3 /g, and the micropore volume of about 0.70 cm3 /g. Kubota et al. [23] used microwave-assisted synthesis to prepare porous carbons, subsequently subjected to KOH activation. The highest surface area of 2208 m2 /g was obtained for the carbon sample activated with KOH using the KOH/resin ratio = 4. The resulting carbon showed the total pore volume of 1.55 cm3 /g, mesopore volume of 0.62 cm3 /g and micropore volume of 0.94 cm3 /g. Micro-mesoporous carbons with good electrochemical properties were fabricated by Jin et al. [24]. Resorcinol and formaldehyde (carbon precursors) and Pluronic F127 (soft template) were used to fabricate ordered mesoporous carbons. After carbonization of mesoporous phenolic resin at 800 ◦ C, the resulting mesoporous carbon was impregnated with KOH solution and activated at 800 ◦ C. The product, micro-mesoporous carbon with ordered mesopores, exhibited the specific surface area of 1685 m2 /g, total pore volume of 1.25 cm3 /g, micropore volume of 0.47 cm3 /g, mesopore volume of 0.75 cm3 /g and macropore volume of 0.03 cm3 /g. Electrochemical study of this carbon revealed its superior capacitance. Also, a carbon electrode material for supercapacitors was obtained by activation of mesoporous carbons with CO2 [25]. CMK-3 and CMK-1 mesoporous carbons, synthesized by using SBA-15 and MCM-48 silicas as hard templates, respectively, were activated with CO2 at 950 ◦ C. The activated CMK-3 carbon exhibited the highest gravimetric capacitance of 223 F/g and volumetric capacitance of 54 F/cm3 , whereas the other parameters were as follows: specific surface area = 2749 m2 /g, total pore volume = 2.09 cm3 /g and mesopore width of ca. 3–4 nm. A similar carbon was also used for hydrogen storage [26]. The amount of hydrogen adsorbed at −196 ◦ C under 1 bar was determined as 2.24 wt%. Choi and Ryoo investigated KOH activation of mesoporous carbons [27]. Mesoporous carbon CMK-8 was synthesized by replicating mesoporous KIT-6 silica. Pores of this silica were filled with carbon precursor, sucrose, and after carbonization, silica was dissolved. The product, CMK-8 carbon, was mixed with KOH in the ratio of 1:5 and activated at 800 ◦ C for 1 h. The resulting activated carbon featured high values of adsorption parameters; namely, the

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specific surface area = 2700 m2 /g, total pore volume = 1.70 cm3 /g, micropore volume = 1.00 cm3 /g, mesopore volume = 0.68 cm3 /g. The amount of hydrogen adsorbed reached 0.75 wt% under pressure of 100 atm. An interesting route to the development of microporosity in mesoporous carbons was proposed by Jaroniec et al. [28] and Górka and Jaroniec [29]. In the latter work, mesoporous carbons were synthesized via soft-templating under acidic conditions. Resorcinol and formaldehyde were used as carbon precursors and triblock copolymer F127 as a template. Tetraethyl orthosilicate (TEOS) was introduced to the system as a silica precursor. After carbonization followed by silica dissolution with HF, microporosity was created. The best carbon sample possessed the specific surface area = 1462 m2 /g, total pore volume = 1.42 cm3 /g, mesopore volume = 0.97 cm3 /g and micropore volume = 0.47 cm3 /g. Mesopores in this carbon were about 9.6 nm. This work represents a comparative study of two strategies for the development of microporosity in mesoporous carbons. In the first case, tetraethyl orthosilicate (TEOS) was added into the synthesis gel containing phenolic resin precursors and block copolymer template under acidic conditions. Hydrolysis of TEOS in the aforementioned gel produced orthosilicic acid, the condensation of which resulted in uniform distribution of amorphous silica in the composite mesostructure. Dissolution of silica with sodium hydroxide resulted in the formation of micropores. The second strategy used for the development of microporosity in mesoporous carbons involved post-synthesis chemical activation. Both aforementioned methods used for development of microporosity were shown to be effective for creating microporosity and enlarging the specific surface area of mesoporous carbons. A similar synthesis approach was reported in Polish [30]. In the current study new carbon samples were prepared by using smaller amount of polymer template (35 wt% less), which resulted in slightly better adsorption characteristics (up to 13% higher specific surface area). Also, additional carbon samples were prepared at higher carbonization temperatures. Importantly, the main aspect of this work, which is not reported in [30], is the study of CO2 adsorption at ambient conditions on mesoporous carbons with introduced microporosity by using TEOS as a pore-generating agent or by post-synthesis KOH activation. 1. Experimental 1.1. Chemicals Triblock copolymer Pluronic F127 (EO106 PO70 EO106 ) produced by BASF (Germany), tetraethyl orthosilicate (TEOS) produced by Sigma-Aldrich (Germany), citric acid (C6 H8 O7 ·7H2 O) produced by POCH (Poland), ethanol (96%), hydrochloric acid (HCl 35–38%), formaldehyde (37%), potassium hydroxide and sodium hydroxide produced by Chempur (Poland) were used as received. 1.2. Synthesis Mesoporous carbons were prepared using a slightly modified method proposed by Wang et al. [31] and analogous to that reported in [30]. The current synthesis differs from that in [30] in the amount of the block copolymer template used, which is about 35 wt% smaller but gives a slightly better adsorption characteristics of the carbons studied. A simplified scheme of the synthesis routes is shown in Fig. 1. The TEOS-assisted synthesis (route A) and post-synthesis KOH activation (route B) were used to create microporosity in mesoporous carbons. Route A synthesis: 2.5 g of resorcinol and 3.79 g of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock

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Fig. 1. Simplified scheme of the synthesis of micro-mesoporous carbons: (A) with TEOS addition, (B) post-synthesis KOH activation (analogous to scheme in [30]).

copolymer Pluronic F127 were dissolved in the mixture of 28.64 cm3 of ethanol and 15.9 cm3 of deionized water and kept under stirring for 15 min. Subsequently, 5.3 cm3 of concentrated HCl or 18.9 g of citric acid was introduced and the mixture was stirred for additional 30 min. For the carbons with TEOS addition, 8.34 cm3 of this reagent was added dropwise under rapid stirring. Then, 2.5 cm3 of formaldehyde was added dropwise. The resulting solution was kept under stirring until two phases separated. The bottom part (polymer-containing) was transferred to Petri dish and dried at 100 ◦ C for 24 h. Thermal treatment of this material was performed in a tube furnace under nitrogen flow (20 dm3 /h) using a heating rate of 1 ◦ C/min up to 600 ◦ C and kept at 600 ◦ C for 3 h. For the samples with TEOS addition, the treatment with 3% NaOH solution at 70 ◦ C for 16 h was employed to dissolve silica; this treatment was used to develop additional microporosity in polymer-templated mesoporous carbons. The final products, micro-mesoporous carbons, were washed with deionized water and dried at 80 ◦ C for 12 h. The resulting samples were denoted: C-HA-6, C-HA-T, C-HA-T* and C-CA-6, C-CA-T and CCA-T* (C—carbon; HA—hydrochloric acid; CA—citric acid; T—TEOS; T*—silica dissolved with NaOH; 6-carbonization at 600 ◦ C). Post-synthesis KOH activation was used to develop microporosity in the carbons synthesized via route B. Synthesis of ordered mesoporous carbons via route B was carried out similarly as in route A, except TEOS, which was not used. Since silica precursor was not used, no additional treatment with NaOH was needed after carbonization. The carbonization temperature for these samples varied and was performed at 600 ◦ C, 700 ◦ C and 800 ◦ C. Also, KOH activation was added to develop microporosity; this process was carried out as follows: 0.5 g of mesoporous carbon was mixed with 2 g of KOH. The resulting mixture was placed in a tube furnace under nitrogen flow (20 dm3 /h) and the temperature was increased (heating rate 10 ◦ C/min) up to 700 ◦ C and maintained at this temperature

for 2 h. Then, the samples were treated with 0.1 M HCl solution until the filtrate was neutral and subsequently transferred to oven for drying at 105 ◦ C for 5 h. The carbon samples prepared by using route B (Fig. 1) are named as follows: C-HA-6, C-HA-6-KOH, C-CA-6, CCA-6-KOH, C-CA-7, C-CA-7-KOH, C-CA-8, C-CA-8-KOH (C—carbon; HA—hydrochloric acid; CA—citric acid; KOH–KOH activation; 6,7,8carbonization at 600 ◦ C, 700 ◦ C and 800 ◦ C, respectively). 1.3. Measurements Nitrogen adsorption isotherms were measured at −196 ◦ C on ASAP 2020 volumetric adsorption analyzer manufactured by Micromeritics (Norcross, GA, USA). All the samples were outgassed at 200 ◦ C for 2 h prior to measurements. Thermogravimetric measurements (TG) were made using TGA Q500 thermogravimetric analyzer manufactured by TA Instruments (New Castle, DE, USA). Data were recorded from 30 to 700 ◦ C under air flow with a heating rate 5 ◦ C/min. Carbon dioxide adsorption isotherms were measured at 25 ◦ C on ASAP 2020 volumetric adsorption analyzer manufactured by Micromeritics (Norcross, GA, USA). All the samples were outgassed at 200 ◦ C for 2 h prior to measurements. 1.4. Calculations Adsorption parameters for the micro- and mesoporous carbons studied were calculated from nitrogen adsorption–desorption isotherms measured at −196 ◦ C [32–37]. The specific surface area SBET was calculated from adsorption isotherms in the relative pressure range of 0.05 to 0.2; 0.162 nm2 was used for cross-sectional area of nitrogen molecule [33]. The total pore volume Vt was estimated from the amount adsorbed at a relative pressure of ∼0.99 [34]. The micropore volume Vmi was calculated using the ˛s method

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Fig. 2. Nitrogen adsorption isotherms for mesoporous carbon C-HA-6, mesoporous silica–carbon composite C-HA-T obtained with addition of TEOS, and micromesoporous carbon C-HA-T* obtained from C-HA-T by silica dissolution; all these samples were prepared in the presence of hydrochloric acid (HA).

[34] in the range of ˛s from 0.8 to 1.2; ˛s is the standard reduced adsorption on a reference solid defined as the amount adsorbed at a given pressure divided by the amount adsorbed at a relative pressure of 0.4. Non-graphitized Cabot BP280 carbon black was used as a reference material [35]. The external surface Sext of the carbon samples was calculated using the ˛s -plot method in the range of ˛s from 2 to 8. The mesopore volume Vme was calculated as the difference between the total pore volume Vt and the micropore volume Vmi . The pore size distribution functions (PSDs) were calculated from the adsorption branch of isotherms using the KJS method [36] based on the Barrett–Joyner–Halenda (BJH) calculation procedure for cylindrical pores [37]. The KJS method offers a significant improvement in calculating PSD for carbons in the range of small mesopores because of using the statistical film thickness curve (t-curve) derived for a reference carbon surface [38] and the experimental Kelvin-type relation reported in [36]. The aforementioned t-curve was obtained by fitting the low-temperature nitrogen adsorption isotherm measured on Cabot BP280 carbon black [35] to the multilayer segment of the t-curve derived for ordered mesoporous silica materials, MCM-41 [38]. Maxima of the pore size distributions were used to estimate the micropore wmi and mesopore wme widths. Microporosity % was calculated as the ratio of the micropore volume Vmi to the total pore volume Vt . 2. Results and discussion 2.1. Phenolic resin-based micro-mesoporous carbons obtained in the presence of TEOS Micro-mesoporous carbons were obtained from polymertemplated mesoporous silica–carbon composites by dissolving silica, which resulted in creation of micropores. The aforementioned silica–carbon composites were prepared by one-pot synthesis using phenolic resin precursors and TEOS in the presence of block copolymer Pluronic F127 under acidic conditions. Shown in Fig. 2 is nitrogen adsorption isotherm for a phenolic resin-based mesoporous carbon prepared in the presence of Pluronic F127 and HCl by self-assembly and carbonization at 600 ◦ C (C-HA-6) together with isotherms for a mesoporous silica–carbon composite obtained by using a similar synthesis route

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Fig. 3. Nitrogen adsorption isotherms for mesoporous carbon C-CA-6, mesoporous silica–carbon composite C-CA-T obtained with addition of TEOS, and micromesoporous carbon C-CA-T* obtained from C-CA-T by silica dissolution; all these samples were prepared in the presence of citric acid (CA).

but with TEOS addition (C-HA-T) and for a micro-mesoporous carbon obtained from the latter composite by silica dissolution (C-HA-T*). Fig. 3 shows nitrogen adsorption isotherms for analogous mesoporous carbons as those in Fig. 2 but synthesized in the presence of citric acid instead of HCl; namely, adsorption isotherm curves are shown for C-CA-6, C-CA-T and C-CA-T*. All nitrogen adsorption isotherms for the carbons studied are type IV with H2 hysteresis loop [39]. Similar isotherms for phenolic resin-based carbons were reported elsewhere [40]. Adsorption parameters for all the samples studied are listed in Table 1. Data summarized in Table 1 show that the specific surface area for the C-HA-6 carbon is 751 m2 /g and its total pore volume Vt equals to 0.78 cm3 /g, whereas the C-CA-6 sample possesses the surface area of 704 m2 /g and the pore volume of 0.54 cm3 /g. These data indicate that adsorption characteristics of the samples obtained in the presence of hydrochloric acid is somewhat better than that for the corresponding samples prepared in the presence of citric acid; this finding is in agreement with previous study [31]. Silica–carbon composites C-HA-T and C-CA-T were prepared by similar method as that used for the synthesis of C-HA-6 and C-CA-6 carbons but with addition of TEOS. Hydrolysis of TEOS in water-alcohol solution containing phenolic resin precursors and block copolymer resulted in the formation of amorphous silica, which mixed well with phenolic resin to form mesoporous silica–polymer composite; the latter after carbonization gave mesoporous silica–carbon composite. Adsorption isotherms (Figs. 2 and 3) as well as data in Table 1 indicate that the surface area and pore volume values for these composites are smaller than the values for the corresponding carbons obtained without TEOS. This outcome is not surprising if one considers incorporation of amorphous silica into micro-mesoporous carbon. Importantly, the same samples after silica dissolution, C-HA-T* and C-CA-T*, exhibited significantly better adsorption properties than the samples prepared without TEOS. The BET surface area of 1463 m2 /g and the pore volume of 1.31 cm3 /g were obtained for the C-HAT* carbon, whereas somewhat lower values of the specific surface area of 1153 m2 /g and pore volume of 0.95 cm3 /g were evaluated for the C-CA-T* carbon. Remarkably, a substantial increase in the micropore volume Vmi was achieved for the samples prepared with

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Table 1 Adsorption parameters for the carbons and silica–carbon composites studieda . Mesoporous carbon

SBET (m2 /g)

Vt (cm3 /g)

Vmi (cm3 /g)

Vme (cm3 /g)

Sext (m2 /g)

wmi (nm)

wme (nm)

Microporosity (%)

C-HA-6 C-HA-T C-HA-T* C-CA-6 C-CA-T C-CA-T* C-HA-6 C-HA-6-KOH C-CA-6 C-CA-6-KOH C-CA-7 C-CA-7-KOH C-CA-8 C-CA-8-KOH

751 476 1463 704 560 1153 751 1906 704 1708 705 1466 695 928

0.78 0.60 1.31 0.54 0.55 0.95 0.78 0.98 0.54 0.81 0.50 0.79 0.49 0.57

0.14 0.06 0.32 0.12 0.07 0.22 0.14 0.75 0.12 0.75 0.04 0.45 0.04 0.15

0.64 0.54 0.99 0.42 0.48 0.73 0.64 0.23 0.42 0.06 0.46 0.34 0.45 0.42

2.0 2.1 7.0 1.1 2.1 4.0 2.0 3.0 1.1 0.2 0.3 0.5 0.1 0.3

1.91 1.90 1.90 1.92 1.92 1.92 1.91 1.91 1.92 1.91 1.94 1.91 1.93 1.92

7.92 9.08 9.10 6.19 8.58 8.71 7.92 – 6.19 – 5.60 4.43 5.59 5.14

18 10 24 22 13 23 18 77 22 93 8 57 8 26

a Notation: C—carbon; HA—hydrochloric acid; CA—citric acid; T—TEOS; T*—dissolution of SiO2 ; KOH–KOH activation; SBET , BET specific surface area; Vt , total pore volume; Vmi , volume of micropores obtained by the ˛s -plot method; Vme , volume of mesopores obtained by subtraction of the micropore volume Vmi from the total pore volume Vt ; wmi , micropore diameter at the maximum of the PSD curve obtained by the KJS method; wme , mesopore diameter at the maximum of the PSD curve obtained by the KJS method; Microporosity—the percentage of the volume of micropores to the total pore volume; 6, 7, 8, carbonization temperature corresponding to 600 ◦ C, 700 ◦ C, 800 ◦ C, respectively.

Fig. 4. Pore size distribution functions for mesoporous carbon C-HA-6, mesoporous silica–carbon composite C-HA-T obtained with addition of TEOS, and micromesoporous carbon C-HA-T* obtained from C-HA-T by silica dissolution; all these samples were prepared in the presence of HCl acid (HA).

Fig. 5. Pore size distribution functions for mesoporous carbon C-CA-6, mesoporous silica–carbon composite C-CA-T obtained with addition of TEOS, and micromesoporous carbon C-CA-T* obtained from C-CA-T by silica dissolution; all these samples were prepared in the presence of citric acid (CA).

TEOS and subjected to NaOH treatment as compared to the carbons synthesized without TEOS or to the corresponding silica–carbon composites. As can be seen from Table 1, there is also a significant increase in the mesopore volume Vme for the carbons that underwent silica etching in contrast to the corresponding silica–carbon composites and to the mesoporous carbons prepared without TEOS addition. These results clearly show a substantial improvement in the adsorption characteristics of the phenolic resin-based carbons obtained by TEOS-assisted synthesis. This synthesis route combined with silica etching resulted in the development of additional microporosity in mesoporous carbons, which is reflected in the PSD curves shown in Figs. 4 and 5. It is noteworthy that the mesopore widths of silica–carbon composites and the corresponding carbons after silica etching are larger than those of the corresponding carbons prepared without TEOS addition. This mesopore enlargement was discussed in [28] and it is caused by favorable interactions between poly(ethylene oxide) (PEO) blocks of Pluronic F127 template and silica species (pre-hydrolyzed TEOS). Some fraction of the latter species is probably accumulated near PEO blocks, which leads

to the enlargement of mesopores formed during soft templating process carried out in the presence of TEOS. Each of the PSD curves for the carbons studied: C-HA-6, C-HA-T, C-HA-T* and C-CA-6, C-CA-T, C-CA-T*, has two maxima corresponding to micropores and mesopores; areas of both peaks reflect contributions of micropores and mesopores to the total pore volume. For the C-HA-6 and C-CA-6 carbons a significant peak can be observed in the range of mesopores along with a small one associated with micropores. For the silica–carbon composites (C-HA-T and C-CA-T) a decrease in the peak height in the region of mesopores as well as the shift of its maximum to the larger mesopore widths can be observed. The change in the peak height corresponding to micropores is minimal, whereas the position of the peak maximum does not change. The micropore widths wmi and mesopore widths wme for all the samples studied are listed in Table 1. The course of the pore size distribution functions varies significantly for the samples after silica dissolution treatment. For both C-HA-T* and C-CA-T* carbons there is a significant increase in microporosity as evidenced by a

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Fig. 6. TG and DTG curves recorded under air flow for the silica–carbon composite prepared using HCl acid (C-HA-T) and the respective carbon obtained after silica etching (C-HA-T*).

significant enlargement of the PSD peak in the micropore range. In the case of C-HA-T* there is also some enlargement of the peak related to mesopores. In order to determine the amount of silica in the silica–carbon composites (C-HA-T and C-CA-T) and the amount of silica present in the composites after treatment with NaOH (C-HA-T* and C-CAT*), thermogravimetric analysis under air flow was performed in the temperature range from 30 ◦ C to 700 ◦ C. The results for both silica–carbon composite C-HA-T and NaOH etched carbon prepared in the presence of hydrochloric acid, C-HA-T*, are shown in Fig. 6; the results for the composite C-CA-T and the etched carbon C-CA-T* prepared in the presence of citric acid are presented in Fig. 7. The TG curves – a weight change as a function of temperature – are presented in the upper panels, whereas the lower panels show the DTG curves – derivative weight change as a function of temperature. The results for the samples studied show that the amount of silica introduced into silica–carbon composite C-HA-T reached 50 wt% and in the case of C-CA-T composite—52 wt%. After NaOH etching, the silica residue amount was identical for both C-HA-T* and C-CAT* carbons, namely 3.2 wt%. The amount of silica left in the carbon samples could be inaccessible to NaOH, which was used to etch SiO2 . The DTG curves presented in the lower panels of Figs. 6 and 7 allowed estimating the stability of silica–carbon composites as well as the carbons after silica dissolution. The analysis under air flow revealed that the silica–carbon composites C-HA-T and C-CA-T are more stable than the carbons C-HA-T* and C-CA-T* obtained after silica dissolution. For the composites the temperature at the maximum rate of oxidation was determined to be in the range of 477–479 ◦ C, whereas for the carbons in the range of 355–366 ◦ C. Therefore, the silica presence hinders the carbon oxidation process in silica–carbon composites. 2.2. Microporous carbons obtained by KOH activation Carbons synthesized via route B shown in Fig. 1, namely CHA-6-KOH, C-CA-6-KOH, C-CA-7-KOH and C-CA-8-KOH possessed different structural properties than the carbons discussed above. Nitrogen adsorption isotherms measured on the carbons prepared

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Fig. 7. TG and DTG curves recorded under air flow for the silica–carbon composite prepared using citric acid (C-CA-T) and the respective carbon obtained after silica etching (C-CA-T*).

Fig. 8. Nitrogen adsorption isotherms for the carbon samples prepared in the presence of HCl and carbonized at 600 ◦ C: mesoporous C-HA-6 and microporous C-HA-6-KOH obtained by KOH activation of C-HA-6.

in the presence of hydrochloric and citric acids (Figs. 8 and 9) show that the resulting carbons are highly microporous, although some mesoporosity has been retained, especially for the samples carbonized at higher temperatures. Note that an increase in the carbonization temperature resulted in reduction of the mesopore widths due to the structure shrinkage; this process can cause disappearance of some small micropores, leading to the diminishment of the overall microporosity. Mesoporosity of the carbons synthesized in the presence of hydrochloric and citric acid, carbonized at 600 ◦ C, was significantly destroyed during KOH activation; however this treatment afforded highly microporous carbons with the following structural parameters: the specific surface area SBET of 1906 m2 /g and the total pore volume of 0.98 cm3 /g were obtained for the C-HA-6-KOH carbon, whereas SBET = 1708 m2 /g and Vt = 0.81 cm3 /g for C-CA-6-KOH. The

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Fig. 9. Nitrogen adsorption isotherms for the carbon samples prepared in the presence of citric acid: mesoporous samples (C-CA-6, C-CA-7, C-CA-8) carbonized respectively at 600 ◦ C, 700 ◦ C, 800 ◦ C and micro-mesoporous obtained by KOH activation (C-CA-6-KOH, C-CA-7-KOH, C-CA-8-KOH).

samples carbonized at 700 ◦ C and 800 ◦ C were less susceptible to KOH activation, which resulted in partial preservation of mesoporosity. Activation is more efficient for the samples carbonized at 600 ◦ C as compared to those carbonized at 800 ◦ C because the matrix of the former is less strong and consequently more vulnerable to the activation process [20]. Pore size distribution functions shown in Figs. 10 and 11 support this observation. A significant increase in the height of the peak located in the micropore range with the simultaneous peak decrease in the mesopore range is observed for the KOH activated carbon samples: C-HA-6-KOH, C-CA-6-KOH, C-CA-7-KOH and C-CA-8-KOH. In particular, one can see this effect for the sample carbonized at 600 ◦ C and later subjected to KOH activation; the mesoporous structure of this carbon was not sufficiently resistant to KOH activation at 700 ◦ C. As expected, the micropore volume of C-HA-6-KOH as compared to that of C-HA-6 increased from 0.14 cm3 /g to 0.75 cm3 /g; a similar increase was observed for the C-CA-6-KOH and C-CA-6

Fig. 10. Pore size distribution functions for the carbon samples prepared in the presence of HCl and carbonized at 600 ◦ C: mesoporous C-HA-6 and microporous C-HA-6-KOH obtained by KOH activation of C-HA-6.

Fig. 11. Pore size distribution functions for carbon samples prepared in the presence of citric acid: mesoporous samples (C-CA-6, C-CA-7, C-CA-8) carbonized respectively at 600 ◦ C, 700 ◦ C, 800 ◦ C and micro-mesoporous ones obtained by KOH activation (C-CA-6-KOH, C-CA-7-KOH, C-CA-8-KOH).

pair. However, the mesopore volume decreased from 0.64 cm3 /g to 0.23 cm3 /g and from 0.42 cm3 /g to 0.06 cm3 /g for respective carbons. The samples carbonized at 700 ◦ C and 800 ◦ C were less susceptible to KOH activation—only a slight change in the mesopore volume and a noticeable increase in microporosity were achieved. The mesopore volume decreased from 0.46 cm3 /g to 0.34 cm3 /g for C-CA-7 and C-CA-7-KOH, respectively; whereas the micropore volume increased from 0.04 cm3 /g to 0.45 cm3 /g. Similarly, for the C-CA-8 and C-CA-8-KOH carbons the mesopore volume decreased from 0.45 cm3 /g to 0.42 cm3 /g, and the micropore volume increased from 0.04 cm3 /g to 0.15 cm3 /g 2.3. Carbon dioxide adsorption Adsorption of CO2 at 25 ◦ C on the carbons studied is shown in Fig. 12. As can be seen from this figure the CO2 capture is much more effective for the KOH activated carbons; CO2 adsorption on

Fig. 12. CO2 adsorption isotherms for the samples obtained after silica dissolution (C-HA-T* and C-CA-T*) and KOH activated (C-HA-6-KOH and C-CA-6-KOH).

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capture and at the same time attractive adsorbents for air and water purification.

Acknowledgment JC and WF acknowledge the National Science Centre (Poland) for support of this research under Grant UMO 2011/03/N/ST5/04444. KJ acknowledges the National Science Centre (Poland) for support of this research under Grant DEC-2012/05/N/ST5/00246.

References

Fig. 13. CO2 adsorption isotherms for the samples carbonized at 600 ◦ C, 700 ◦ C, 800 ◦ C (C-CA-6, C-CA-7 and C-CA-8) and KOH activated (C-CA-6-KOH, C-CA-7-KOH and C-CA-8-KOH).

these carbons is about twice higher as compared to that on the carbons obtained in the presence of TEOS under acidic conditions (hydrochloric and citric acids). Since KOH activated carbons synthesized in the presence of citric acid (CA) showed somewhat better CO2 uptake, these samples were selected to study the effect of carbonization temperature on the effectiveness of KOH activation. A comparison of adsorption isotherms for mesoporous carbons synthesized in the presence of citric acid, carbonized at 600 ◦ C, 700 ◦ C and 800 ◦ C respectively and for KOH activated carbons is presented in Fig. 13. As expected, the KOH activation of mesoporous carbons produced micro-mesoporous carbons that exhibited higher CO2 adsorption than the corresponding mesoporous carbons not subjected to the activation process. It is also noteworthy that 600 ◦ C and 700 ◦ C are the optimal carbonization temperatures to achieve the high CO2 adsorption capacity. The C-HA-6-KOH, C-CA-6-KOH and C-CA-7-KOH carbon materials possess the best structural parameters as well as show good adsorption properties towards CO2 . 3. Conclusions In summary, mesoporous carbons were obtained by softtemplating method in the presence of hydrochloric and citric acids using resorcinol and formaldehyde as carbon precursors and triblock copolymer Pluronic F127 as a template. The main focus of this study was the development of microporosity in the mesoporous carbon materials obtained by soft-templating in the presence of hydrochloric and citric acids, which is beneficial for CO2 adsorption. Two approaches were employed to create additional microporosity: TEOS-assisted synthesis and post-synthesis KOH activation. The latter method provided better results; hence micro-mesoporous carbons after KOH treatment were tested for CO2 adsorption. KOH activated carbon C-HA-6-KOH exhibited BET surface area of 1906 m2 /g and total pore volume of 0.98 cm3 /g. Although the chemical activation was more effective for the development of microporosity, it led to a significant deterioration of mesoporous structure of some carbons. Silica etching was shown to be useful because did not cause the deterioration of the mesoporous structure. Interestingly, in the case of the C-HA-T* and C-CA-T* carbons not only microporosity was developed but also the mesopore volume was slightly increased. The carbons with highly developed microporous structures were shown to be good adsorbents for CO2

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