Expanded graphite/phenolic resin-based carbon composite adsorbents for post-combustion CO₂ capture

Abstract

Porous carbon composite adsorbents were prepared from a commercial phenolic resin mixed with a small proportion of thermally expanded graphite (EG) followed by carbonization and physical activation with CO₂. The addition of EG dramatically hastens the CO₂ activation and results in remarkably enhanced microporosity development in the EG composite compared to the activated phenolic resin alone. The resultant EG composite adsorbents exhibit high CO₂ adsorption capacities at 298 K and excellent CO₂/N₂ adsorption selectivity. In particular, the EG composite shows superior CO₂ uptake at low CO₂ pressures (47 mg g⁻¹ at 298 K and 0.15 bar), which is more important to actual flue gas applications in post-combustion capture (PCC). Moreover, EG composite adsorbents are especially attractive as the EG component is inexpensive, available in very large amounts and easy to handle and is only required at a low addition level of around 2 wt%. The rapid CO₂ activation and the low burn-off for excellent CO₂ adsorption performance at low pressures greatly reduce the energy required to produce the adsorbent and the waste generated in activation. This further enhances the cost and environmental advantages of physically activated EG composites over those PCC adsorbents prepared by chemical activation and functionalization of porous carbons. Hence, due to its superior CO₂ adsorption properties and favourable fabrication process, the newly developed EG carbon composite adsorbent holds great promise for large-scale deployment and commercial applications to PCC.

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1. Introduction

Growing concern with human-induced climate change has attracted widespread efforts to develop cost-effective and robust technologies for post-combustion capture (PCC) of carbon dioxide (CO₂) from large point sources such as coal-fired power stations.¹ The PCC of CO₂ with solid adsorbents is considered a promising alternative to the conventional solvent process which uses chemical absorption with aqueous amine solvents.^2,3 Although this is currently the most commercially available PCC technology, the solvent process is energy-intensive and requires strict pre-cleaning treatments to the flue gas. Moreover, it raises environmental concerns due to large volumes of waste water and sludge produced from the solvent process and secondary emissions resulting from the solvent degradation. By contrast, the solid adsorbent process is a low-energy and environmentally-benign technology. It is based on simple physisorption of CO₂ with lower heat of CO₂ adsorption compared to chemical absorption, which implies potentially lower energy consumption needed for CO₂ desorption in adsorbent regeneration. As solid adsorbents are thermally stable and chemically non-reactive with CO₂ adsorbates, there are no environmental impacts due to degradation in the adsorbent process. If solid adsorbents are also tolerant of impurities (i.e. SO_x and NO_x) in the flue gas stream, significant savings in the overall capture cost could be made by removal of flue gas pre-treatments.

A large number of solid porous materials have been investigated for CO₂ capture including zeolites, porous carbons, functionalized porous silica, metal–organic frameworks and covalent organic frameworks.^4–13 Carbonaceous adsorbents have advantages such as low cost and high chemical, thermal and mechanical stability necessary to operate in actual flue gas streams. However, they generally have poor CO₂ adsorption capacity, which reduces their cost-effectiveness because more adsorbents are needed to cope with a given flue gas flow and more energy is required to achieve regeneration because more sensible heat is required to heat up a larger amount of adsorbents for thermal regeneration.

Approaches to improving the CO₂ adsorption capacity of carbonaceous materials have been mainly focused on (1) development of high-surface-area microporous carbons in a chemical activation method by blending carbon precursors with a very large proportion of harsh chemicals such as KOH followed by pyrolysis;^10,14,15 and (2) functionalization of porous carbons with basic groups, such as nitrogen doping and amine loading.^16,17 Although the CO₂ adsorption capacity is improved, there is a high complexity, cost, waste and inconvenience with chemical activation as large amounts of hazardous chemicals are used and chemical residues have to be thoroughly washed out. The functionalization method also has greater complexity or cost, and some functionalized carbons show difficult and unstable regeneration due to strong interactions with CO₂.

The CO₂ adsorption capacity of porous carbons can also be improved by physical activation, which entails etching carbons with mild oxidant gases such as CO₂ and steam at elevated temperatures, a process that is much more economically and environmentally attractive than chemical activation.¹⁸ An important issue with physical activation, particularly for carbon monoliths desirable for practical applications, is mass transport/diffusion of the etchant gases during oxidation.¹⁹ Facilitated mass transport of oxidizing molecules within the carbon structure is requisite to boost the reaction of physical activation, thereby enhancing the development of the narrow microporous structure in physically activated carbons for improved CO₂ adsorption at ambient conditions.

We recently reported the preparation of carbon composite monoliths with exceptional CO₂ adsorption capacity and kinetics, and excellent CO₂/N₂ selectivity.²⁰ It used a commercial phenolic resin mixed with just 1 wt% of carbon nanotubes (CNTs) and physical activation with CO₂. The outstanding performance is attributed to the monolith possessing a hierarchical macroporous–microporous structure with a very high proportion of narrow micropores. The proposed formation mechanism is that the CNTs act as a nanoscale scaffold to establish a macroporous structure in the resin-derived carbon after carbonization, which allows easy access of activating agent CO₂ molecules to the monolith's interior. The CNTs also offer a large primary surface area by distributing the resin-derived carbon into micro/nanometre scale domains, thus providing more locations for rapidly creating a large population of narrow micropores in the course of CO₂ activation. As a result, the CO₂ activation for the CNT-incorporated composite is remarkably quick, exhibiting much greater burn-off than the phenolic resin alone derived carbon at a given period of time and the rapid activation also minimizes pore widening. Therefore, it can be seen that fast mass transport of activating agent molecules is very important to the development of an optimum microporous structure in physically-activated carbons.

In the present study, we incorporated expanded graphite (EG) into phenolic resins to produce carbon composite adsorbents by physical activation with CO₂. Compared to CNTs, EG is of lower cost, and easier to make, handle and disperse in the preparation of composites. The large surface area and high aspect ratio (platelet diameter to thickness) of EG is anticipated to function in a somewhat similar manner to CNTs for achieving well-developed narrow microporous structures in the EG/phenolic resin-based carbon composite. The resultant EG monoliths were characterized for surface morphologies, pore structures and gas adsorption properties to understand the effects of EG addition on the development of microporous structures in phenolic resin-derived activated carbons as well as to evaluate CO₂ capture performance of EG composites for PCC.

2. Experimental

EG was prepared from commercial expandable graphite flakes (Asbury Carbons, Grade 3772, 80%, >300 μm and carbon content 99%). The expandable graphite flakes were thermally expanded and exfoliated by heating 3 g batches in loosely capped 500 ml single-use tin cans at 1050 °C in a muffle furnace for 3 min. This amount produced approximately 2.5 g of EG which is sufficient to fill the can, which was allowed to cool before opening. The obtained EG powders were well dispersed in an aqueous gel solution containing 5 wt% of methyl cellulose (MC, Aldrich, viscosity (2% in H₂O) = 4000 cp).

EG carbon composites were prepared by thoroughly mixing 5 g of commercial Novolac phenolic resin (Durez 7716) powder with a given amount of the EG gel paste. The resultant highly viscous mixture was transferred into a cylindrical polypropylene mold (20 mm inner diameter) with five 2 mm diameter channel pins, and then cured at 150 °C for 2 h to solidify the structure. After de-molding, the honeycomb monolith with five channels was carbonized in a tube furnace at 650 °C for 1 h under N₂ flow. The carbonized sample was weighted, returned to the tube furnace and heated to 950 °C under N₂ flow. At the setpoint temperature, the gas flow was switched to CO₂ for a given period to activate the sample with CO₂, and then back to N₂ as the sample was cooled. The activated sample was weighed to determine the burn-off (defined as (mass of carbonized sample − mass of activated sample)/mass of carbonized sample × 100%) during activation. The activated EG composites were labeled as EG-x-y, where x = the % EG to resin (i.e. mass percentage of EG/phenolic resin) with values of 2, 5 and 10%, and y = the activation time with values of 15 and 30 min. For comparison, the activated sample made of phenolic resin alone without EG was prepared and labeled as Res-60, denoting 60 min activation time.

Sorption measurements for N₂ and CO₂ were carried out on Micromeritics ASAP 2020 volumetric analyzer at various temperatures after degassing overnight under vacuum at 473 K. The Brunauer–Emmett–Teller (BET) specific surface area (S_BET) and the total pore volume (V_t) were obtained from N₂ adsorption isotherms at 77 K. S_BET calculation was performed below 0.1 relative pressure and V_t was calculated by the N₂ amount adsorbed at the relative pressure of about 0.99. Adsorption isotherms for CO₂ at 273 K were used to obtain the narrow micropore (<1 nm) size distributions, which were calculated by the density function theory (DFT) method. The surface area (S_nm) and the volume (V_nm) of narrow micropores were derived from the above calculations. For comparison, the DFT calculation was also conducted on the N₂ adsorption isotherm at 77 K to determine the NMPSD. The heat of CO₂ adsorption was calculated using CO₂ isotherms at 273, 298 and 323 K based on the Clausius–Clapeyron equation. Morphologies of samples were examined using scanning electron microscopy (SEM, Nava Nano SEM 430) at an operating voltage of 5 kV. The SEM observation was conducted on the sample without coating. Transmission electron microscopy (TEM) was performed on a Tecnai T12 at an operating voltage of 120 kV.

3. Results and discussion

Prior to thermal treatment, the expandable graphite shows a bulky micro-morphology composed of dense graphite flakes (Fig. 1a). Its specified expansion ratio is 270 ml g⁻¹ minimum; however if allowed to expand to this extent, the material is intolerably fluffy and difficult to work with. Thus samples were constrained to a density of 2.5 g to 500 ml or 200 ml g⁻¹. The obtained EG is nevertheless very fluffy and appears in a worm-like particulate shape. As shown in Fig. 1b, EG exhibits an irregular porous network comprising a mass of graphite nanosheets separated by voids and crevices of micron to nanometer. Such morphology is resulted from exfoliation and distortion of graphite nanosheets during rapid thermal expansion and consistent with the observation reported previously.²¹ The BET surface area (S_BET), obtained from N₂ adsorption isotherms at 77 K (ESI Fig. S1†), increases from 0.9 m² g⁻¹ for the expandable graphite to 34.3 m² g⁻¹ for the EG. In addition, the N₂ isotherm of the EG shows a hysteresis loop at relative pressures above 0.5, indicating the presence of mesopores.

Fig. 1 SEM images of (a) the expandable graphite and (b) the expanded graphite (EG).

The surface morphologies of the activated phenolic resin and the EG composite were analyzed by SEM which reveals a dramatic difference after incorporating EG (Fig. 2a–c). The resin alone derived Res-60 exhibits a bulky and dense structure with large vapor holes and little secondary structure such as pores and channels (Fig. 2a). By contrast, the EG composite (EG-5-30) presents a porous microstructure composed of interconnected irregular large particles and shows porosity and channeling at a range of scales from sub-millimeter down (Fig. 2b and c). The original porous network of the EG is covered with resin derived carbons in the EG composite (Fig. 2c), which splits the phenolic resin and subsequently the derived carbon (after carbonization) into small domains that would contribute to the rapid and uniform development of micropores in CO₂ activation. The TEM image of the EG composite, as shown in Fig. 2d displays the disordered micropores distributed all over the carbon material, which are responsible for CO₂ adsorption under ambient conditions.

Fig. 2 SEM images of (a) the activated phenolic resin Res-60 and (b and c) the EG composite EG-5-30 at different magnifications, and (d) TEM image of EG-5-30.

It can be seen from Table 1 that the composites with EG incorporated are much more reactive with CO₂ during activation, exhibiting significant burn-off within much shorter activation duration. Although 7.6 wt% of burn-off was achieved in resin alone derived Res-60 when activated at 950 °C for 60 min, the EG composites took just 30 min to reach the levels of burn-off 2 to 4 times higher. The more EG was incorporated, the higher burn-off was achieved. When the EG composites were activated at 950 °C for 30 min, the burn-off reached 14.3 wt% for EG-2-30, went up to 19.9 wt% for EG-5-30 and doubled to 27.4 wt% for EG-10-30. The dramatic increase of burn-off with the inclusion of EG is likely attributed to the EG's modifications to the microstructure of phenolic resin derived carbons. As discussed above, in contrast to resin alone Res-60, the EG composite has a porous microstructure which would favor mass transport of activating agent CO₂ within the monolith, thus leading to rapid burn-off.

Table 1 Burn-off, textural properties and CO₂ adsorption capacities of activated adsorbents

Samples	Burn-off (wt%)	N2 adsorption at 77 K		CO2 adsorption at 273 K		CO2 uptake at 298 K (mg g^−1)
		SBET (m^2 g^−1)	Vt (cm^3 g^−1)	Snm (m^2 g^−1)	Vnm (cm^3 g^−1)	1 bar, C100	0.15 bar, C15
Res-60	7.6	237	0.108	317	0.092	82	24
EG-2-30	14.3	723	0.290	527	0.157	135	45
EG-5-30	19.9	908	0.367	477	0.143	128	40
EG-10-30	27.4	1028	0.439	413	0.125	113	33
EG-2-15	7.6	523	0.205	542	0.159	127	47

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To study the effects of EG addition on the textural properties, N₂ and CO₂ sorption measurements were carried out at 77 and 273 K, respectively. Fig. 3a shows N₂ adsorption–desorption isotherms for the activated phenolic resin (Res-60) and three EG composites activated for 30 min. All the isotherms display significant steep N₂ uptake at low relative pressures followed by an adsorption plateau, typical for microporous materials.²² A slight hysteresis for EG-10-30 at relative pressures above 0.5 is similar to the N₂ sorption behavior of pure EG (ESI Fig. S1†), suggesting that the EG content in this sample may be excessive and some has failed to disperse effectively. As seen from Table 1, the EG composites exhibit a significantly enhanced porous structure compared to resin alone derived Res-60. The surface area S_BET is 237 m² g⁻¹ for Res-60 and dramatically increased to 723 m² g⁻¹ for EG-2-30. With more EG incorporated, S_BET is increased to 908 m² g⁻¹ for EG-5-30 and further to 1028 m² g⁻¹ for EG-10-30. Similarly, pore volumes (V_t) of the composites are remarkably higher than Res-60, and V_t is increased as the addition amount of EG is increased. Therefore, the results clearly indicate that the incorporation of EG greatly enhances the development of microporous structures in the phenolic resin derived carbons.

Fig. 3 (a) N₂ sorption isotherms at 77 K for the activated phenolic resin (Res-60) and the EG composites activated for 30 min and (b) their narrow micropore (<1 nm) size distributions obtained from CO₂ adsorption at 273 K.

It is well established that adsorption of CO₂ on porous carbons under ambient temperature and pressure conditions is attributed to narrow micropores less than 1 nm.²³ Smaller micropore sizes below 0.8 and 0.7 nm have been reported to be responsible for CO₂ uptake at atmospheric pressure of 1 bar.^22,24,25 The flue gas is typically only 10–15% CO₂, i.e. CO₂ partial pressure of 0.1–0.15 bar, which suggests that narrow micropores with a even smaller pore size contribute to CO₂ adsorption in PCC. Such ultra-narrow micropores are difficult to be detected due to slow diffusion of N₂ molecules at 77 K. In this regard, CO₂ physisorption at 273 K is more effective to obtain rapid high-resolution characterization of narrow micropores in carbons. For comparison, the narrow micropore (<1 nm) size distribution (NMPSD) of ES-2-30 was obtained from both N₂ adsorption isotherm at 77 K (Fig. 3a) and CO₂ adsorption isotherm at 273 K (ESI Fig. S2†), as presented in ESI Fig. S3.† Compared to N₂ adsorption, features of the NMPSD are captured in much greater detail with the DFT calculation from CO₂ adsorption, and pore filling for ultra-small narrow micropores less than 0.5 nm can be observed from CO₂ adsorption.

Fig. 3b displays the NMPSDs of the activated phenolic resin and three EG composites obtained from CO₂ adsorption isotherms at 273 K (ESI Fig. S2†). The corresponding surface areas (S_nm) and pore volumes (V_nm) of narrow micropores are listed in Table 1. It is evident that the EG composites have considerably enhanced development of narrow micropores compared to resin alone derived Res-60, exhibiting higher values of S_nm and V_nm. Among the three EG composites activated for 30 min, EG-2-30 contains the least amount of EG but exhibits the highest narrow microporosity in the pore size range below 0.7 nm. It is worth noting that only a small amount of EG addition (2%) to the phenolic resin can dramatically improve the narrow microporosity of the resultant carbon material.

The CO₂ equilibrium adsorption capacities at different CO₂ partial pressures were obtained from CO₂ adsorption isotherms at 298 K (Fig. 4). CO₂ molar capacity (mmol g⁻¹) was plotted in Fig. 4 whilst CO₂ mass (mg g⁻¹) capacity was listed in Table 1. The values of molar and mass capacity represent mmol and mg of CO₂ adsorbed per g of adsorbent, respectively. The CO₂ uptake at ambient saturated CO₂ pressure of 1 bar was denoted as C₁₀₀. As noted above, the content of CO₂ is low in the flue gas, so in practice CO₂ adsorption at a low CO₂ partial pressure is a more realistic measure of adsorbent performance for PCC. The CO₂ uptake at 0.15 bar corresponding to 15% CO₂ partial pressure, denoted as C₁₅, was defined as an indicator of the low-pressure CO₂ adsorption capacity. Fig. 4 clearly shows that the EG composite possesses significantly enhanced CO₂ adsorption capacities compared to resin alone Res-60 at 298 K and over the measured pressure range up to 760 mmHg. The CO₂ uptake of the EG composite declines with an increased incorporation amount of EG. EG-2-30 exhibits the highest CO₂ uptake amongst the three EG composites activated for 30 min. The C₁₀₀ of EG-2-30 (135 mg g⁻¹) is 65% higher than that of resin alone Res-60 (82 mg g⁻¹), while the improvement in the CO₂ uptake at the low pressure (C₁₅) is even more striking, increasing by 88% to 45 mg g⁻¹ for EG-2-30 compared to 24 mg g⁻¹ for Res-60 (Table 1). In addition to greatly enhanced CO₂ adsorption capacity, sorption of CO₂ in the EG composite is reversible and no significant hysteresis in adsorption–desorption curves was observed (Fig. 4). This is desirable for recycle and regeneration of solid adsorbents after CO₂ capture.

Fig. 4 CO₂ adsorption–desorption isotherms at 298 K for the EG composites activated at 950 °C for 30 min. The CO₂ adsorption isotherm of the activated phenolic resin (Res-60) at 298 K is plotted for comparison.

The above performance of CO₂ adsorption is in good agreement with the variation in NMPSDs of the adsorbents as revealed in Fig. 3b. The CO₂ uptake in the EG composite is remarkably improved compared to resin alone, which is attributed to a substantial improvement in the narrow microporosity with an incorporation of EG. For a given activation time of 30 min, the narrow microporosity (<0.7 nm) of the EG composite responsible for CO₂ uptake at ambient conditions decreases in proportion to the EG content, thereby with EG-2-30 having the largest CO₂ uptake among the three EG composites. In addition, there are no correlations between the CO₂ uptake of the adsorbents and their textural properties other than characteristics of narrow micropores, such as surface areas S_BET and total pore volumes V_t. As listed in Table 1, the more EG the composite contains, the higher values are obtained for both S_BET and V_t. Nevertheless, the best CO₂ capacity was observed in EG-2-30 containing the least amount of EG.

Based on our previous study,²⁰ the extent of burn-off greatly influences the CO₂ capacity of CNT composite adsorbents prepared by CO₂ activation. To investigate this effect in the EG composite, we chose the best-performed EG composite containing 2% EG to be activated at 950 °C for 15 min, labeled as EG-2-15. The CO₂ adsorption isotherm of EG-2-15 at 298 K is shown in Fig. 5. Compared to EG-2-30, EG-2-15 has much lower burn-off of 7.6 wt%, but improved C₁₅ at 4.7 mg g⁻¹ and reduced C₁₀₀ at 127 mg g⁻¹ (Table 1). As shown in their NMPSDs (ESI Fig. S4†), EG-2-15 has a larger proportion of micropores below 0.55 nm but less microporosity in the pore size range above 0.55 nm compared to EG-2-30. The variation of the NMPSD well elucidates the increase of C₁₅ and decrease of C₁₀₀ in EG-20-15. The extended activation widens narrow micropores, which may benefit CO₂ uptake at higher CO₂ pressures but deteriorate CO₂ adsorption at lower pressures. While the narrow micropore size range responsible for CO₂ adsorption at 298 K and various pressures cannot yet be defined exactly, it is clear from our results that the C₁₅ value – the specific performance indicator of adsorbents in use for PCC – is most influenced by narrow micropores smaller than about 0.55 nm. This accords with the observation that the CO₂ uptake at lower pressures purely depends on the fraction of fine micropores below 0.5 nm for phenolic resin derived activated carbon spheres prepared by KOH chemical activation.²² In addition, with a same level of burn-off as resin alone Res-60, EG-2-15 has dramatically enhanced CO₂ uptake, particularly for C₁₅ at 47 mg g⁻¹ almost twice (96% increase) that of Res-60.

Fig. 5 CO₂ and N₂ adsorption isotherms of EG-2-15 at 298 K.

Values of up to 135 mg g⁻¹ (3.07 mmol g⁻¹) and 47 mg g⁻¹ (1.06 mmol g⁻¹) were achieved in the EG composites for C₁₀₀ and C₁₅, respectively. They are lower than those of our previous CNT-incorporated phenolic resin-based composite carbons (maximum C₁₀₀ at 159 mg g⁻¹ and C₁₅ at 52 mg g⁻¹).²⁰ However, the EG composites outperform other phenolic resin-based activated carbons prepared with organic additives by CO₂ activation, in which the C₁₀₀ up to 108 mg g⁻¹ was obtained.²⁶ Moreover, indicative of CO₂ adsorption capacity under the flue gas condition, the C₁₅ exhibited by the EG composite adsorbent is among the highest reported for porous carbons based on recently published comparisons (17–52 mg g⁻¹ at 298 K and 0.15 bar).^20,27

As the flue gas is comprised of around 80% nitrogen, a key PCC performance factor is the selectivity of the adsorbent for capturing CO₂ over N₂. A high CO₂/N₂ selectivity both increases the overall efficiency by boosting the CO₂ uptake, and results in a purer CO₂ capture product. CO₂ and N₂ adsorption capacities of EG-2-15 were compared at 298 K. As shown in Fig. 5, the amount of CO₂ adsorbed in EG-2-15 is much higher than that for N₂. Its equilibrium CO₂/N₂ adsorption ratio at 1 bar is 6.0 (2.89 mmol g⁻¹ CO₂ vs. 0.48 mmol g⁻¹ N₂), close to the ratio 6.3 of our composite with CNTs.²⁰ This value is even superior to those of porous carbons prepared by KOH activation from petroleum pitch (2.8),²⁸ petroleum coke (5.1)²⁹ and sawdust-based chars (5.4).¹⁰ In addition, CO₂ and N₂ adsorption isotherms at 298 K for other three EG composites activated for 30 min were also compared (ESI Fig. S5†). With an increase of EG content, the CO₂ uptake is decreased and the amount of N₂ adsorbed is also slightly reduced with the N₂ uptake at 1 bar being 0.49, 0.47 and 0.41 mmol g⁻¹, respectively for EG-2-30, EG-5-30 and EG-10-30. Similar to that of EG-2-15, the equilibrium CO₂/N₂ adsorption ratios at 1 bar are around 6.2–6.3 for the above three EG composites.

We also calculated the CO₂/N₂ adsorption selectivity at 298 K using the ratio of the initial slopes of CO₂ and N₂ adsorption isotherms at very low pressures (ESI Fig. S6†). EG-2-15 has a high initial selectivity of 18.1. This is comparable to that of the CNT composite (19.8),²⁰ substantially better than reported for pristine microporous carbons (∼7)^30,31 and N-doped microporous carbons based on polyimine (12.5)³² and even better than graphene–polyaniline composites (17.9).³³ However, the carbon-based adsorbents generally show much lower CO₂/N₂ adsorption selectivities than recently reported microporous polymer adsorbents such as microporous polyaminals (104)³⁴ and microporous polyimides (56).³⁵ Such remarkable CO₂/N₂ selectivities are attributed to the abundant CO₂-philic functional groups in the microporous polymer in addition to the contribution from the microporous structure.

To understand the affinity of the EG composite to CO₂ molecules, the isosteric heat of CO₂ adsorption for EG-2-15 was calculated from isotherms at 273, 298 and 323 K (ESI Fig. S7†). It is obvious that the CO₂ uptake is decreased with an increase of adsorption temperature, which is consistent with the physisorption nature for CO₂ adsorption on pristine microporous carbons. At 323 K, the values of C₁₅ and C₁₀₀ for EG-2-15 are reduced to 32 and 107 mg g⁻¹, respectively. At near zero CO₂ loading the isosteric heat is 31.0 kJ mol⁻¹ and declines to 22.4 kJ mol⁻¹ at 2 mmol g⁻¹ of CO₂ adsorbed (Fig. 6). The value for EG-2-15 (31.0 kJ mol⁻¹) is slightly lower than that of the CNT composite (32.6 kJ mol⁻¹),²⁰ but higher than for N-free pristine porous carbons (20–30 kJ mol⁻¹).^10,36 The heat of adsorption at low surface coverage indicates the interaction between the adsorbent and adsorptive molecules, relying on the pore size distribution in the case of CO₂ physisorption on pristine porous carbons by considering the fact that smaller micropores engender stronger interactions between CO₂ molecules and porous carbons than larger micropores. Hence, the high heat of CO₂ adsorption for EG-2-15 should be attributed to its well-developed narrow microporosity.

Fig. 6 Isosteric heat of CO₂ adsorption for EG-2-15.

4. Conclusions

The addition of a small proportion of EG to the phenolic resin dramatically hastens the CO₂ activation and improves the development of microporous structures in resin-derived activated carbons. The resultant EG composite carbons exhibit large pore surface areas and volumes up to more than three times higher than for resin alone. The narrow microporosity responsible for the CO₂ uptake at ambient conditions is also remarkably enhanced with the inclusion of EG. The improvement of CO₂ uptake in the EG composites is striking with an increase of up to 65% and 96% for C₁₀₀ and C₁₅, respectively. The EG composites have lower CO₂ adsorption capacities than our previous CNT composite carbons but outperform other reported phenolic resin based activated carbons. In particular, they show superior CO₂ uptake at low pressures, which is more important to PCC applications. The EG composite adsorbent exhibits excellent CO₂/N₂ selectivity of 18.1 at 298 K, and a strong affinity to CO₂ molecules indicated by its high heat of CO₂ adsorption (31.0 kJ mol⁻¹).

In addition to their superior CO₂ adsorption properties, the physically activated EG composite adsorbents are especially attractive as the EG component is inexpensive, available in very large amounts and easy to handle and is only required at a low addition level of around 2 wt%. The EG component dramatically reduces the activation time and the extent of burn-off necessary to give an excellent PCC adsorbent. This greatly reduces the energy required to produce the adsorbent and the waste generated in activation and further enhances its cost and environmental advantages over chemical activation and functionalization of porous carbons. Therefore, the newly developed EG composite carbon adsorbents are promising for large-scale deployment and commercial applications to PCC.

Acknowledgements

This research was funded by CSIRO. The expandable Graphite (Grade 3772) was donated by Asbury Carbons.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09853j

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