Potassium Tethered Carbons with Unparalleled Adsorption Capacity

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3495−3505

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Potassium Tethered Carbons with Unparalleled Adsorption Capacity and Selectivity for Low-Cost Carbon Dioxide Capture from Flue Gas Hongyu Zhao,†,‡ Lei Shi,†,§ Zhongzheng Zhang,† Xiaona Luo,†,‡ Lina Zhang,† Qun Shen,† Shenggang Li,†,∥ Haijiao Zhang,‡ Nannan Sun,*,† Wei Wei,*,†,∥,⊥ and Yuhan Sun†,∥ †

CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China ‡ Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, and §Department of Chemistry, Shanghai University, Shanghai 200444, China ∥ School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China ⊥ Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China S Supporting Information *

ABSTRACT: Carbons are considered less favorable for postcombustion CO2 capture because of their low affinity toward CO2, and nitrogen doping was widely studied to enhance CO2 adsorption, but the results are still unsatisfactory. Herein, we report a simple, scalable, and controllable strategy of tethering potassium to a carbon matrix, which can enhance carbon−CO2 interaction effectively, and a remarkable working capacity of ca. 4.5 wt % under flue gas conditions was achieved, which is among the highest for carbon-based materials. More interestingly, a high CO2/N2 selectivity of 404 was obtained. Density functional theory calculations evidenced that the introduced potassium carboxylate moieties are responsible for such excellent performances. We also show the effectiveness of this strategy to be universal, and thus, cheaper precursors can be used, holding great promise for low-cost carbon capture from flue gas. KEYWORDS: carbon, adsorbents, adsorption, CO2 capture, CCS

1. INTRODUCTION

because of their unparalleled CO2 adsorption capacity from simulated flue gas.29−35 In general, operational conditions of practical PCC are very complicated, and aspects such as low pressure, low CO2 concentration, presence of contaminants (SOx, NOx, H2O, etc.), and others induced a trade-off between different adsorption−desorption behaviors. For example, one of the general principles for immobilized amines and MOFs to have high adsorption capacity is the presence of strong adsorption sites to facilitate selective CO2 adsorption from a low concentration stream such as PCC; however, these active sites are more difficult to regenerate and less stable in terms of cycling stability or even structural integrity of the materials.13,14 Thus, instead of considering adsorption capacities as an exclusive factor, balanced adsorption behaviors are of paramount importance to decrease the cost of PCC. One excellent example was recently reported by Nandi and co-workers, who prepared a nickel isonicotinate MOF with ultralow parasitic energy costs for PCC. Interestingly, this sample is not the highest performing material in any single aspect of adsorption

As fossil fuels will dominate the global energy consumption for at least more than a decade, effective means for CO2 reduction are urgently needed. According to the International Energy Agency, the “no more than 2 °C” target can hardly be achieved without CO2 capture and storage,1 but deployment of related technologies is still hindered particularly by the prohibitive high cost of CO2 capture.2 Because flue gas from power plant is the largest stationary emission point, targeted CO2 capture solutions are obviously the current priority (postcombustion capture, PCC). Amine scrubbing is a well-established process for CO2 separation.3 However, the technology is highly energyintensive and suffers from high environmental footprint (e.g., via amine evaporation). During the past few years, adsorptionbased CO2 capture using solid materials has been widely studied as an alternative to amine scrubbing that may lead to cost-effective CO2 capture from flue gas, and a wide spectrum of porous materials was investigated including zeolites,4,5 carbons, 6−8 metal oxides,9−11 immobilized amines, 12,13 metal−organic frameworks (MOFs),14−18 covalent organic frameworks,19,20 organic porous materials,21−28 and so forth. Among these materials, immobilized amines and MOFs with unsaturated metal centers (UMCs) are of particular interests © 2018 American Chemical Society

Received: September 22, 2017 Accepted: January 10, 2018 Published: January 10, 2018 3495

DOI: 10.1021/acsami.7b14418 ACS Appl. Mater. Interfaces 2018, 10, 3495−3505

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ACS Applied Materials & Interfaces behaviors.36 Similarly, Jo et al. used different diamines to modify an MOF material to circumvent the mutually antagonistic adsorption behaviors during PCC, and it was also found that the sample with the highest equilibrium adsorption capacity did not lead to the highest working capacity.37 In essence, the adsorption behavior of a material is thermodynamically determined by the interaction between adsorbent and adsorbate, which should be optimized to achieve the “balanced adsorption behavior” mentioned above. To this end, one can either deactivate the excessively strong adsorption sites or enhance CO2 affinity of weak adsorption sites. For example, Choi and co-workers selectively deactivated the most active amine groups (primary amine) in polyethyleneimine (PEI), and by immobilizing this modified amine onto a silica support, heat of adsorption was reduced substantially as compared with unmodified PEI, leading to enhanced cycling and chemical stability.13 Similarly, Jung et al. used the primary amine groups in the PEI molecule as knots to cross-link the polymer chain; in addition to improved thermal stability, adsorption stability was also increased because of the avoidance of generating unfavorable urea structures.38 For the other approach, nitrogen-doped carbons (NDCs) have been widely proved to have better CO2 adsorption than pure carbons, and higher adsorption enthalpies were obtained as well.39−43 Alternatively, Nugent used a reticular chemistry strategy to insert periodically arrayed hexafluorosilicate into an MOF without UMCs, which resulted in contracted pores and thus higher CO2 affinity because of better overlap of attractive potential fields from the opposite walls, leading to higher CO2 uptake.44 Nevertheless, complicated and tedious preparation steps are involved in most of the above approaches, among which NDCs are relatively cheap and easy to synthesize; therefore, these materials have been investigated extensively. Unfortunately, the best carbonization temperature to achieve an appropriate porosity differs from the one to obtain optimized type/content of N-functionalities, which is due to the porosity increase of NDCs with the increase of carbonization temperature until a peak value is achieved, while N-functionalities, particularly the chemically more active ones (stronger adsorption sites), are prone to decompose with the increase of temperature; this issue manifests NDCs with limited enhancement on CO2 affinity, and thus, they are considered less favorable for PCC.45−47 As a matter of fact, the role of nitrogen-bearing functionalities in enhancing the CO2 affinity of NDC lies on the disturbance of the evenly distributed surface electric field of carbons, which led to stronger interaction with the quadrupole of CO2; this is to say that involving any element with different electron negativities from pure carbon should work similarly. As a proof-of-concept, both Zhao48 and some of us49 reported the involvement of potassium as extra-framework cations to enhance CO2 adsorption on carbons. In these works, carbons were first activated with KOH at elevated temperatures, followed by washing of the obtained mixture with ethanol or H2O instead of HCl solution, allowing preservation of some potassium species in the carbon matrix. Although the synthesized carbons were effective as CO2 adsorbents, their preparation suffered from use of large amounts of corrosive KOH and less flexibility in controlling the potassium content in the final materials.

On the basis of these pioneering works, we report herein a simple, scalable, and controllable synthesis of potassium tethered carbons (PTCs) from low-cost precursors, namely, the controlled oxidation of carbons followed by potassium exchange. On the basis of comprehensive investigations on their CO2 adsorption performance, we demonstrate that the current strategy is more effective in enhancing CO2 affinity compared to NDCs, which resulted in a higher and more appropriate adsorption enthalpy of 30 kJ/mol at a medium-tohigh CO2 loading range of 0.6−2.0 mmol/g. Consequently, excellent CO2 adsorption capacities of 4.50−5.76 wt % under flue gas conditions (40 °C, 1 bar, 15 vol % CO2) were achieved, which are among the highest for carbon-based materials. Meanwhile, the samples were also characterized by fast adsorption kinetics, high cycling stability, and easy regeneration (115 °C). Besides, density functional theory (DFT) calculations demonstrated for the first time that compared with unmodified carbons or NDCs, potassium tethering introduced new adsorption sites that attract CO2 molecules with appropriate strength, and owing to the electrostatic nature of such an interaction, several CO2 molecules could be accommodated by only one potassium atom without obvious decrease in their binding energies. On the other hand, the moderate surface areas (∼500 m2/g) of PTCs limited N2 adsorption; therefore, a high CO2/N2 adsorption selectivity was obtained for PTCs, which demonstrated their great potential to be used in PCC. Finally, we also established different preparation protocols according to the nature of different carbon precursors to achieve highly dispersed tethering of potassium so that cheaper carbons can be used, making these materials highly competitive for low-cost PCC.

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedure. Three parent carbons (AC, CB, and MC) were used for modification. AC and CB were activated carbon and carbon black commercially available from Sinopharm and Black Diamond Co., Ltd., respectively. Mesoporous carbon (MC) was prepared following a solvent-free method;50 typically, 1.12 g of pphthalaldehyde, 0.88 g of resorcinol, and 3 g of Pluronic F127 were thoroughly mixed and grounded and then subjected to thermal treatment in an autoclave for 8 h at 250 °C. The obtained material was further carbonized in nitrogen at 800 °C for 5 h (2 °C/min heating rate) to obtain the final MC. Mixed acid oxidation and potassium tethering: typically, 3 g of the parent carbons and 120 mL mixed acid (concentrated HNO3 and H2SO4 with a volume ratio of 3) were added to a three-necked flask and heated to different temperatures (60, 80, and 100 °C) for 8 h. Subsequently, the mixture was cooled to room temperature, washed adequately with deionized water, and then dried at 70 °C for 24 h. The oxidized samples were further stirred in 2 mol/L KOH aqueous solution (mass/volume = 1.5 g/300 mL) for 24 h at room temperature, followed by filtration, washing with deionized water to neutral to remove physically deposited potassium ions, and then drying at 70 °C to recover the PTCs. 2.2. Characterization and Evaluation. N2 adsorption at −196 °C was measured on a Micromeritics ASAP2420 apparatus (pressure and temperature accuracy are ±0.15% of reading and ±0.1 °C, respectively), and samples were degassed at 300 °C for 10 h under vacuum before any tests. A TA Q50 thermal gravimetric analyzer (weight and temperature accuracy are ±0.1% and ±0.1 °C, respectively) was used to measure CO2 adsorption under ambient pressure, and the samples were first treated at 115 °C in Ar (100 mL/ min) for 30 min. CO2 and N2 adsorption isotherms were measured and cross-checked by a Quantachrome iSorb HP1 analyzer, a Belsorp max instrument, and a Micromeritics ASAP2020 instrument, respectively, and a degas temperature of 120 °C and a vacuuming 3496


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Figure 1. SEM images of (a) AC, (b) CB, and (c) MC.

Table 1. Properties of Parent Carbons, Oxidized Carbons, and PTCs bulk composition (mass ratio, wt %) from EA

textural parameters sample code AC CB MC AC-60ox CB-60ox MC-60ox AC-60ox-K CB-60ox-K MC-60ox-K

2

−1

SBET/m g 1300 1059 599 48 635 501 NA 512 473

3

−1

V/cm g 0.82 1.85 0.38 0.03 1.01 0.34 NA 0.82 0.38

3

Vmicro/cm g 0.21 0.02 0.16 0.03 0.05 0.13 NA 0.04 0.10

−1

surface composition (mass ratio, wt %) from XPS

graphitic degree

C

H

N

C

O

N

K

ID/IG

86.38 92.99 90.41 62.75 86.01 82.39 48.89 79.24 74.61

1.61 0.34 0.17 1.43 0.24 0.54 1.40 0.45 1.01

NA NA NA 2.45 NA 0.17 1.63 NA NA

81.42 94.36 90.24 74.25 90.43 84.83 47.55 82.97 72.09

18.58 5.64 9.76 23.67 9.10 14.37 25.14 10.45 14.44

NA NA NA 2.62 0.70 0.90 NA 1.13 NA

NA NA NA NA NA NA 27.30 5.87 12.19

2.52 1.49 1.97 NA NA NA NA NA NA

Figure 2. (a) N2 isotherms at −196 °C and (b) NLDFT pore size distribution of parent carbons. level of ca. 1 × 10−2 mbar were used. Purities of all gases (CO2, N2, and He) used in adsorption measurement are higher than 99.99%. Raman spectra were collected on an RM2000 microscopic confocal Raman spectrometer with an excitation laser of 532 nm. Fourier transform infrared (FT-IR) spectrum was collected using the KBr/ sample pellet method on a Thermo Fisher Nicolet 6700 FT-IR spectrometer at ambient temperature. X-ray photoelectron spectroscopy (XPS) of the samples was analyzed on a PHI-5000C ESCA system. The morphology of samples was investigated by a scanning electron microscopy (SEM) instrument on a SUPRRATM55 apparatus with an acceleration voltage of 2.0 kV, and transmission electron microscopy (TEM) images were obtained with a JEOL 2100F microscope operating at 200 kV. Elementary analysis (EA) was performed on a Thermo FLASH 2000 CHNS analyzer. A C22H12 molecule with six fused benzene rings and its derivatives with −COOH and −COOK groups were used as the models of pure graphite, oxidized graphite, and potassium tethered graphite during DFT calculations. The AUG-cc-pVDZ basis set51 was used for the nonmetal atoms, whereas the Stuttgart relativistic small core effective core potential-based basis set52 was used for potassium. The B97 exchange−correlation functional, that is, B97D,53 was used to improve the description of the dispersion interaction between CO2 and graphite at the DFT level because of the consideration of Grimme’s D2 dispersion correction and reduced computational cost by ignoring the Hartree−Fock exchange. Geometry optimizations were carried out

in redundant internal coordinates with the Berny algorithm54 for local minima, and analytic harmonic frequencies were calculated to verify the nature of the stationary states and to obtain the zero-point energy corrections. All calculations were performed with the Gaussian 09 program package,55 molecular visualization was accomplished using the AGUI graphical interface from the AMPAC program package,56 and atoms-in-molecule (AIM) analysis was carried out with the Avogadro program.57

3. RESULTS AND DISCUSSION 3.1. Parent Carbons. In this study, three carbons, namely, AC, CB, and MC, were selected for surface modification. Among these, AC is an activated carbon commercially available from Sinopharm and CB is a carbon black sample supplied from Black Diamond Co., Ltd. MC is prepared via a solventfree method reported previously with minor modifications.50,58 Figure 1a−c shows SEM images of the three parent carbons. At lower magnification, AC and MC showed granular morphologies, while CB was strawberry-like with rough surface (insets of Figure 1). In high-magnification SEM images, highly developed porous structures can be seen on the surfaces of AC and CB. The pores in AC were mainly located within the carbon matrix, indicating their formation by steam activation, 3497


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ACS Applied Materials & Interfaces whereas the pores in CB were mainly generated by the voids between large amounts of microspheres. In comparison, MC is more condense, which might be related to severe contraction of its precursor during high-temperature carbonization. Detailed textural properties of the samples were further characterized by N2 adsorption isotherms. As can be seen from Table 1, Brunauer−Emmett−Teller (BET) surface areas higher than 1000 m2/g were obtained for AC and CB, while the value for MC is lower (599 m2/g). As can be seen from Figure 2a, all the N2 isotherms showed rapid increase of N2 adsorption at low pressures, indicating the presence of micropores. According to a recent update from IUPAC,59 the hysteresis loop of AC and MC can be classified as H2 (b) type, and the sharper step-down of the desorption branch of MC suggests higher ordering of its mesoporous structure as reported previously.50,58 For sample CB, a H4 hysteresis loop was obtained, which agrees with the accumulation of voids between the microspheres (Figure 1b). Accordingly, pore size distribution calculated by nonlocal DFT (NLDFT) confirmed the presence of both micro- and mesopores in AC and MC, while larger pores were favorably found in CB (Figure 2b). Surface composition of the three parent carbons was quantified by XPS. From the full-scale survey (Figure S1), oxygen element was identified apart from carbon for all the three samples, which is due to the presence of oxygencontaining functionalities. Furthermore, the oxygen concentrations of the samples increased in the order of CB < MC < AC (Table 1), probably because of their preparation/ production processes (high temperature, steam activation, and others). Raman spectroscopy of the parent carbons was recorded (Figure S2), and the ID/IG ratios were calculated, as the graphitic degree is a determining factor influencing the chemical reactivity, leading to great influence on the modification of the carbons. As summarized in Table 1, CB is highly graphitic, while more structural defects can be found in AC, which might be due to the presence of a larger amount of oxygen functionalities in this sample. 3.2. Potassium Tethering. Modification of AC, CB, and MC was carried out in two steps, namely, oxidation and potassium tethering. First, the parent carbons were oxidized by a mixture of concentrated HNO3 and H2SO4 to introduce more surface functionalities, and the resulted samples were designated as precursor-xxox, where xx indicates the oxidation temperature in °C, for example, AC-60ox stands for AC oxidized at 60 °C. According to XPS measurement (Table 1), higher oxygen contents were obtained for all the oxidized carbons, suggesting the introduction of extra oxygen functionalities. Notably, FT-IR spectra of the oxidized samples show either intensified or newly emerged peaks at ca. 1570 and 1720 cm−1, corresponding to the aromatic rings coupled with highly conjugated −CO moieties and/or −COO bonds, respectively (Figure S3).60,61 On the basis of elementary analysis and XPS results (Table 1 and Figure S4), a very small amount of nitro group was also generated during oxidation; however, their content is too low to obviously influence CO2 adsorption (Table 2 and Figure S5). The introduced functionalities have a detrimental effect on the textural properties (Figure S6 and Table 1), for example, CB-60ox and MC-60ox have BET surface areas of 635 and 501 m2/g, respectively, which are moderately lower than their unmodified counterparts. In comparison, the porous structure of AC was severely damaged

Table 2. CO2 Adsorption Capacities of Parent Carbons, Oxidized Carbons, and PTCs Measured by the Gravimetric Method 15 vol % CO2

100 vol % CO2

sample code

40 °C/wt %

75 °C/wt %

40 °C/wt %

75 °C/wt %

MC MC-60ox MC-60ox-K CB CB-60ox CB-60ox-K AC AC-60ox AC-60ox-K

2.36 3.00 4.52 1.00 2.04 3.97 1.30 2.91 2.81

0.94 1.13 2.36 0.54 0.75 1.93 0.57 1.03 1.81

6.56 6.73 7.70 2.22 4.20 5.78 4.4 5.19 3.17

3.38 3.42 4.80 1.12 2.07 3.46 2.03 2.59 2.37

by oxidation, resulting in a much lower surface area of only 48 m2/g for AC-60ox. The above observation illustrates that the nature of the parent carbon has a substantial influence on the oxidation process. First, the reactivity toward oxidation is closely related to the graphitic degree because defects in the carbon matrix such as edges, steps, and kinks are more prone to be converted into oxygen functionalities.62,63 On the basis of the ID/IG ratio from Raman spectroscopy (Figure S2 and Table 1), the graphitic degree of the parent carbons increases as AC < MC < CB; following this trend, sample AC is more prone to be oxidized, and thus, a large amount of oxygen functionalities can be introduced. In fact, oxidation is too intense for AC that either most of the nanopores were blocked by the generated functionalities or the entire carbon matrix collapsed during oxidation, and both mechanisms could lead to drastic decrease of textural properties (Figure S6). Control experiments were conducted by increasing the oxidation temperature to 100 °C, and under such conditions, only a highly dispersed sol can be obtained for AC-100ox, which cannot be recovered by vacuum filtration, indicating the breakdown of graphite layers of the sample. On the other hand, CB-100ox remains intact with a substantial surface area of 62 m2/g. Interestingly, we noticed that although MC has a lower graphitic degree than CB, the decrease in textural properties for oxidized MC is smaller than that for oxidized CB, which may be associated with the preexisting oxygen groups and their capability in enhancing hydrophilicity, namely, for sample MC with more polar groups (oxygen-containing, Table 1), its hydrophilic surface facilitated the spreading of HNO3 and H2SO4, leading to easier access of the oxidant into the pores and thus an evenly distributed oxidation of the entire pore surface. On the contrary, oxidation occurs preferably on the pore mouth of CB as the diffusion of aqueous oxidant into the pores is inhibited by the hydrophobic surface, particularly in the initial stage of oxidation; therefore, the pores in CB have greater chance to be blocked by the newly formed functionalities, leading to a greater decrease of textural properties. In the second step of modification, the oxidized samples were further mixed with 2 mol/L KOH aqueous solution at room temperature, followed by filtration, washing, and drying to recover the PTCs, and a postfix of “-K” was added in the sample names to indicate PTCs hereafter. One important feature of the current preparation strategy as compared with our pioneering work49,58 is to achieve controllable introduction of highly dispersed and tethered potassium species on the 3498


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ACS Applied Materials & Interfaces carbon surface. This was first evidenced by XPS as the presence of potassium can be easily detected, and its concentration shows a close relationship with the oxygen concentrations in the oxidized carbons (Table 1). TEM images of the PTCs from different carbon precursors (Figure 3) illustrated their significantly different morphologies.

detected by XPS, and negligible changes on adsorption capacity were observed (Table S2). N2 isotherms (−196 °C) of the PTCs are depicted in Figure S6, and the obtained porous parameters are listed in Table 1. We noted that the measurement of AC-60ox-K was unsuccessful because N2 uptakes very close to zero were obtained in repeated tests, probably indicating its literally nonporous nature. In contrast, thanks to the high dispersion of potassium, only slight influence in terms of porous structure was observed on MC-60ox-K, and the sample still has a high surface area of 512 m2/g, which endowed the sample with great potential to be used as an adsorbent. As demonstrated above, the oxidation degree of carbons can be simply adjusted by selecting a carbon precursor and/or varying the oxidation temperature; therefore, the current work provides a straightforward way to quantitatively adjust the tethered potassium and thus CO2 adsorption.48 Last but not the least, by using 50 wt % PEI supported on fumed silica (FS) as a benchmark for immobilized amine (PEI/FS),64,65 we show in Table S3 that the preparation cost of PEI/FS is strikingly higher than that of PTCs, and thus, there is great potential for large-scale manufacture of PTCs with 90% lower price than that of immobilized amines. 3.3. CO2 Adsorption. CO2 adsorption capacities of the parent, oxidized, and potassium tethered samples were first evaluated by a thermal gravimetric method in both 15 vol % CO2/N2 and pure CO2; before any tests, the samples were purged with Ar at 115 °C to remove any preadsorbed contaminants. From Table 2, it could be observed that CO2 adsorption was effectively improved after potassium tethering, particularly at lower pressures (see also CO2 isotherms in Figure S5). Meanwhile, CO2 adsorption capacities diminished with the increase of temperature and decrease of CO2 concentration for all the samples, which suggests that physical adsorption still represents the major adsorption mechanism. In general, PTCs do not show any competitive results in pure CO2 atmosphere as compared with recent publications;66−71 however, decent CO2 uptake of 4.52 wt % was obtained over MC-60ox-K under simulated flue gas conditions (15 vol % CO2/N2, 40 °C). With further optimizations, a remarkable adsorption capacity of 5.76 wt % was achieved on MC-80ox-K, and it should be mentioned here that the measurement of CO2 adsorption on carbons has not been widely carried out under flue gas conditions, and our result is among the highest values in the open literature (Table S4). Figure 4a depicts the CO2 isotherms of MC-60ox-K at different temperatures, and the obtained adsorption capacities are identical to those obtained from thermogravimetric analysis

Figure 3. TEM images of (a) AC-60ox-K, (b) CB-60ox-K, and (c) MC-60ox-K and (d) EDS mapping of MC-60ox-K.

AC-60ox-K is composed by a few layers of graphite, which is expected because of its poor resistance toward oxidation (Figure 3a). In contrast, a curly morphology with clear graphitic lattice can be seen in CB-60ox-K (Figure 3b), in line with its high graphitic degree and spherical particles in the SEM image of CB (Figure 1b). For MC-60ox-K, a highly developed spongelike porous structure without observable graphitic lattice was obtained (Figure 3c). Nevertheless, one fact in common is that no potassium species is visible irrespective of its concentration or the used carbon, indicating the high dispersion of the potassium species, which can also be evidenced by the elementary mapping of the samples (Figure 3d). Together with the above-discussed close relationship between potassium and oxygen concentrations, it is highly possible that carboxyl and a minor amount of hydroxyl groups induced during oxidation served as “anchoring sites” for potassium ions, leading to potassium tethering on the surface in a highly dispersed way. Control experiments were carried out by treating the parent carbon directly with KOH solution, followed by similar washing and drying steps, no potassium was

Figure 4. (a) CO2 adsorption isotherms of MC-60ox-K and (b) isosteric adsorption heat of MC-60ox-K. 3499


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publications, ultrahigh surface areas were pursued for carbonbased materials, which increased the CO2 uptake indeed (particularly at elevated pressure), but N2 adsorption capacity would increase at the same time, leading to inferior CO2/N2 selectivity, and similar considerations were also emphasized recently by Wang et al.79 Moreover, PTCs also showed excellent adsorption kinetics, and by monitoring the pressure drop of the analyzer sample cell during the isotherm analysis, we observed that even at an equilibrium pressure as low as 0.18 bar, less than 1 min was needed to achieve over 90% of adsorption (Figure 6a). Excellent cycling stability can be achieved over PTC as well, for example, MC-60ox-K showed stable CO2 adsorption capacity at ca. 4.50 wt % during 25 cycles when desorption was performed at 115 °C, and a slight increase of CO2 uptake can be achieved when the sample was regenerated at 150 °C because of the recovery of the stronger adsorption sites (Figure 6b). Similarly, sample MC-80ox-K also maintains its adsorption capacity of ca. 5.00 wt % with 115 °C regeneration in 50 cycles (Figure 6c). Finally, we took the desorption process into consideration, which was overlooked in most previous publications. Normally, an inert gas was used during desorption, which helps to evaluate and compare equilibrium capacity between different samples, however this also dilutes the adsorbed CO2. For practical PCC, pure CO2 is a perfect candidate as a purging gas because no phase transition is involved (as compared with steam purging), and thus, lower regeneration energy will be needed. We measured CO2 uptake of MC-60ox-K in a pure CO2 stream at elevated temperatures, and it was found that full desorption could be achieved at 150 °C in pure CO2 (Figure S11), which resulted in a high working capacity of larger than 4.5 wt % of the developed PTCs together with their potential to produce concentrated CO2 from flue gas. 3.4. Adsorption Mechanism. Owing to the unique adsorption behavior of PTCs as compared to normal carbons discussed above, we further carried out DFT calculations to reveal the detailed adsorption mechanism at the molecular level. To this end, a graphite layer composed of six benzene rings was used to model the pure carbon surface, and based on this model, oxidized and potassium tethered carbon surfaces were derived, as shown in Figure S12. Cartesian coordinates for the optimized structures of our models including those with CO2 molecules adsorbed are given in Table S5. In our calculations, we gradually increased the number of CO2 molecules interacting with the model surface from one to four, and the optimized CO2 adsorption location and energy were determined. For the pure carbon model, the first CO2 molecule resides ca. ∼3.3 Å above the graphite layer with an adsorption energy of 13.1 kJ/mol (Figure S13a), and longer distances were predicted for the second and the third CO2 molecules, but their adsorption energies did not alter significantly, probably because of the low intermolecular repulsion between CO2 molecules. In contrast, the fourth CO2 molecule is no longer in direct contact with the graphite layer, and its adsorption energy decreased substantially to 10.9 kJ/mol (Figure 7a, see Figure S14a for the distance between adsorbed CO2 molecules). For the oxidized carbon, weak hydrogen bonding interaction between CO2 and the −COOH group at a distance of ∼2.3 Å between the H atom of −COOH and the O atom of CO2 was predicted, and in addition to this hydrogen bonding interaction, the graphite layer is capable of accommodating another three CO2 molecules, with an average

(note the adsorption of MC-60ox-K and MC-80ox-K is far from saturation at 1 bar, see Figures S7 and S8, respectively). Slight hysteresis was observed, probably because of the presence of a minor amount of strong adsorption sites, and temperature increase is needed for their regeneration. We also performed repeated adsorption−desorption cycles (Figure S9); a very slight decrease of capacity (avg. 1.3% in each cycle) was noticeable from the 1st to 3rd cycle. By applying the Clausius−Clapeyron equation to these isotherms, isosteric heat of adsorption (Qst) was calculated (Figure 4b); remarkably, the initial Qst value was found as high as 70 kJ/mol, indicating again the presence of strong adsorption sites. Because of the preferential occupation of more energetic sites upon CO2 loading, adsorption heat decreased gradually afterward and reached a plateau of ca. 30 kJ/mol. It has been widely demonstrated that for unmodified carbons, Qst for CO2 normally ranges from 17 to 25 kJ/mol,72,73 which indicated relatively weak interaction between the carbon surface and CO2 and thus dissatisfied CO2 adsorption at low pressures and/or concentrations. Modification of carbons by heteroatoms, nitrogen in particular, was proved to be effective to enhance CO2 affinity, and initial isosteric adsorption heat of 25−30 kJ/ mol was frequently reported.43,74 All these values are significantly lower than those of the PTCs presented here. For immobilized amines, Qst values of ca. 80 kJ/mol were reported,65 and by selective functionalization of primary amines, lower Qst of ca. 56 kJ/mol could be obtained, but these values are still approximately twice higher than those of PTCs, indicating potentially higher regeneration energy costs. Because the effectively improved CO2 affinity has been evidenced by the adsorption heat values, we expect the PTCs to possess an enhanced CO2 adsorption selectivity over N2, which is a great concern when considering physical adsorbents (e.g., zeolites, carbons, and so forth) for PCC. Figure 5 shows the

Figure 5. Comparison of CO2 and N2 isotherms of MC-60ox-K at 25 °C.

CO2 and N2 isotherms measured at 25 °C, and compared with the CO2 uptake, the N2 uptake is exceptionally low as compared with other carbon materials, probably because of the low surface area of the sample. As the linear shape of the N2 isotherm posed large uncertainty when using the data for isotherm fitting by empirical equations such as Langmuir, we are unable to calculate adsorption selectivity by the ideal adsorption solution theory. Alternatively, by using the virial approach,75 Henry constants of 1.21834 × 10−6 and 3.01858 × 10−9 were obtained for CO2 and N2, respectively (Figure S10); this gave an adsorption selectivity of 404, which is significantly higher than reported values,43,76−78 clearly demonstrating the high preference of CO2 capture in flue gas. Note that in most 3500


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Figure 6. (a) Adsorption kinetics of MC-60ox-K at 25 °C; (b) cycling stability of MC-60ox-K with desorption temperatures of 115 and 150 °C; and (c) cycling stability of MC-80ox-K with a desorption temperature of 115 °C (the adsorption conditions used in (b,c) are all 15 vol % CO2/N2 at 40 °C).

shown here suggest that by potassium tethering, new adsorption sites with stronger adsorption energies can be generated, which is responsible for the excellent PCC performance of PTCs. To further understand the interaction between the adsorbed CO2 molecules and the carbon surface, we performed AIM analysis on the models. As shown in Figure S15, for the adsorption of only one CO2 molecule on the graphene model, the OCO bond angle of the adsorbed CO2 is ∼179°, and the interaction of the CO2 molecule and the adsorbent is mostly between the O atom of the CO2 molecule and the C atom on graphene with a distance of ∼3.4 Å. When a second CO2 molecule was adsorbed, we noticed interaction between the two CO2 molecules via their O atoms. For oxidized carbons (Figure S16), CO2 interacts mainly with the H and O atoms from the −COOH groups with distances of ∼2.3 and ∼3.2 Å, respectively (distance of O(carboxyl)−C(CO2) is 2.98 Å, Figure S17). The OCO bond angle of the adsorbed CO2 molecule is ∼178°. On the surface of the PTC model (Figure S18), a slightly decreased OCO bond angle of ∼176° was calculated for the first adsorbed CO2 molecule, and the interaction is between the K atom of PTC and the O atom of CO2 (∼2.9 Å); additional interaction can also be determined between the C atom of CO2 and the O atom of −COOK (∼2.9 Å). For PTC, interaction between adsorbed CO2 molecules occurred only when the fourth CO2 molecule is adsorbed at the −COOK site through the O atoms (∼3.6 Å). Thus, our AIM analysis also suggests the K site as a high-affinity site for CO2. 3.5. Cheaper Precursors. On the basis of the above discussion, tethering of potassium is of great importance to improve the PCC performance of carbons, which inspired us to selectively introduce carboxyl groups onto the carbon matrix as these groups played a decisive role in anchoring potassium, and because liquid oxidation did not show any considerable selectivity,80 we used a mechanochemical method for the oxidation of carbon with two possible benefits: first, the carbon matrix can be torn into smaller pieces by the strong sheering force of ball-milling, allowing more edges for carboxylation. Second, ammonium peroxydisulfate (APS) was also added during ball-milling to in situ functionalize the newly formed edges. We assume this oxidation method to be “milder”, and therefore, the least graphitic carbon, namely, AC, was used in this case. Potassium tethering was then carried out similar to the procedure employed previously. By adjusting the ballmilling conditions including duration, rotation rate, APS/AC mass ratio, and so forth, we finally achieved an optimized sample, AC-APS-K, which showed a potassium loading of 12.91

Figure 7. Adsorption configurations. (a) Pure carbon, (b) oxidized carbon, and (c) PTCs.

adsorption energy of 13.2 ± 0.5 kJ/mol, indicating a slight increase of CO2 affinity compared to pure carbon (Figures 7b and S13b). For the PTC model, much stronger interaction of 27.1 kJ/ mol between the potassium atom and the CO2 molecule was predicted (Figure S13c); note that this value cannot be directly compared with the experimental ones (Figure 4b) because the used model did not fully resemble the three-dimensional matrices of PTCs. Nevertheless, the trend of increasing CO2 affinity by potassium tethering is identical to the experimental observations. Remarkably, because the carboxyl group (the anchor site of potassium) exists exclusively at the edge of the graphite layer, the interaction between the potassium atom and the CO2 molecule is less sterically hindered, and our calculations demonstrated that more than one CO2 molecule could be strongly adsorbed around the K atom without significant decrease in the adsorption energy (25.9 kJ/mol for the fourth CO2 molecule, Figure 7c, the distance between adsorbed CO2 molecules is presented in Figure S14b). It should be mentioned here that the purpose of the current DFT calculations is to verify the enhancement of CO2−adsorbent interaction due to potassium tethering, and at higher CO2 loadings and/or higher pressures, interaction between adsorbed CO2 molecules may also play a role. Nevertheless, results 3501


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ACS Applied Materials & Interfaces wt % and a promising CO2 uptake of 4.56 wt % under flue gas conditions (Figure S19).

4. CONCLUSIONS A simple, scalable, and controllable strategy was developed for the preparation of PTCs, which showed CO2 adsorption capacities of ca. 5 wt % under flue gas conditions, which are among the highest to date for carbon-based materials. Meanwhile, a high CO2/N2 selectivity was obtained together with fast adsorption kinetics and stable cycling stability. Adsorbed CO2 can be fully desorbed at 150 °C in pure CO2 allowing the production of concentrated CO2 from flue gas, which was normally overlooked. The excellent performance is related to the appropriate tuning of adsorbent−adsorbate interactions; PTCs have an adsorption enthalpy value of ca. 30 kJ/mol at substantial CO2 loading, which is obviously higher than NDCs but far lower than that of immobilized amines, which not only guaranteed the balanced adsorption behavior but also avoided an energy-intensive regeneration process. The DFT study demonstrated that the highly dispersed potassium carboxylate moieties are the major adsorption sites in PTCs, and owing to their electrostatic interaction with the CO2 molecules, more than one CO2 molecule can be attracted by only one potassium atom without any obvious decrease in binding energies, which explained the extraordinary high CO2/ N2 adsorption selectivity of PTCs. We also developed a mechanochemical method to tether potassium onto commercially available activated carbons, which further decreased the preparation cost of PTCs without any compromise on the adsorption capacity. On the basis of the above discussion, we envision the current work paved a potential way for low-cost PCC by using modified carbons; further investigations will focus on the influence of using alternative alkali metal cations and evaluating the materials in the presence of moisture, SOx, NOx, and so forth.

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PTCs; AIM analysis for the interaction between (a) one and (b) two CO2 molecules with our graphene model; AIM analysis for the interaction between one CO2 molecule and our graphene oxide model; adsorption configuration of oxidized carbon adsorbing four carbon dioxide molecules on its surface; AIM analysis for the interaction between one and four CO2 molecules with our PTC model; thermogravimetric curve of the ACAPS-K under 15 vol % CO2 as the adsorbate; CO2 adsorption energies calculated with B97D/B3LYP and the DZVP2/DZVP basis set; adsorption capacity and surface composition of controlling experiments; material cost of the adsorbent per ton; CO2 adsorption capacity of representative carbonaceous adsorbents under 15 vol % CO2 as the adsorbate; and Cartesian coordinates for our optimized structures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.S.). *E-mail: [email protected] (W.W.). ORCID

Hongyu Zhao: 0000-0002-5001-4692 Zhongzheng Zhang: 0000-0001-6926-7887 Shenggang Li: 0000-0002-5173-0025 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB10040200) and Shanghai Natural Science Foundation (nos 17ZR1433600 and 17ZR1433700).

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14418. XPS survey of the parent carbons (AC, CB, and MC); Raman spectroscopy of parent carbons (AC, CB, and MC); FT-IR spectroscopy of the parent and oxidized carbons; N 1s spectra of (a) AC-60ox, (b) MC-60ox, (c) CB-60ox, and (d) CB-60ox-K; CO2 isotherms of MC, MC-60ox, and MC-60ox-K at 40 °C; N2 isotherms (−196 °C) of the parent and oxidized carbons and PTCs: (a) MC series, (b) CB series, and (c) AC series; high-pressure CO2 isotherms of MC-60ox-K at 40 °C measured by different apparatuses; high-pressure CO2 isotherms of MC-80ox-K at different temperatures measured by a Quantachrome iSorb HP1 analyzer; CO2 adsorption isotherms at 25 °C of MC-60ox-K; virial characteristic curve of (a) CO2 and (b) N2 for sample MC-60ox-K at 25 °C; thermogravimetric curve of MC60ox-K under pure CO2 as the adsorbate; model of (a) pure carbon surface, (b) oxidized carbon surface, and (c) PTC surfaces; adsorption configurations of the carbon material adsorbing a carbon dioxide molecule on its surface: (a) Pure carbon surface, (b) oxidized carbon surface, and (c) PTC surfaces; distance between adsorbed CO2 molecules on (a) pure carbon and (b) 3502


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