Activated carbons prepared by the KOH activation of a hydrochar from garlic peel and their CO2 adsorption performance

NEW CARBON MATERIALS Volume 34, Issue 3, Jun 2019 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2019, 34(3): 247-257

RESEARCH PAPER

Activated carbons prepared by the KOH activation of a hydrochar from garlic peel and their CO2 adsorption performance Ge-ge Huang1,2, Yi-fei Liu1, Xing-xing Wu1, Jin-jun Cai1,2,* 1

Hunan Key Laboratory of Environment Friendly Chemical Process, School of Chemical Engineering, Xiangtan University, Xiangtan 411105,

China; 2

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Abstract:

Biomass is regarded as a promising low-cost precursor for the preparation of activated carbons. However, direct carbonization of

biomass usually produces a low-surface-area or even non-porous carbons that are useless for CO2 capture. In this work, garlic peel was first transformed to a hydrochar by hydrothermal carbonization and then chemically activated by KOH to obtain activated carbons with high-surface-areas and large pore volumes. The microstructure and morphology of the activated carbons were characterized by N2 adsorption, SEM and XRD. Results indicate that their surface area and pore volume are mainly determined by the activation temperature and KOH/hydrochar mass ratio. Activated carbon (AC-28) obtained by KOH activation with a KOH/hydrochar ratio of 2 at 800 °C has a well-developed porosity with a surface area and pore volume of 1 262 m2/g and 0.70 cm3/g, respectively, while a reduction of the activation temperature to 600 °C (AC-26) results in a material whose corresponding values are 947 m2/g and 0.51 cm3/g. Although AC-26 exhibits a much lower surface area and pore volume compared with AC-28, it has the larger CO2 uptake of up to 4.22 mmol/g at 25 °C and 1 bar due to its higher microporosity of up to 98% and abundant narrow micropores, implying that the microporosity is one of the main factors for CO2 capture besides the traditionally-believed surface area and pore volume. The isosteric heat of CO2 adsorption indicates that the affinity between the activated carbon and CO 2 molecules increases with the volume of narrow micropores less than 0.8 nm and the number of surface oxygen-containing functional groups. Key Words: Biomass; Hydrothermal carbonization; Porous carbons; Activation; CO2 capture

1 Introduction The rapid growing CO2 emission has drawn great attention for climate change, which is mostly from the combustion of conventional fossil fuels and industrial activities [1]. Efficiently CO2 capture has been proposed as an effective strategy to restrict an increase of the CO2 concentration in atmosphere. Among various technologies and processes for CO 2 capture, adsorption using solid adsorbents such as zeolites [2], porous silica[3], porous carbons[4-6], metal-organic frameworks[7] and organic hybrid matters[8] is a promising alternative method owing to the advantage in low energy requirement, simplicity in operation and clean process. Each of those adsorbents has its own advantages and disadvantages on the process for CO2 capture. For instance, a large-scale production of organic-inorganic framework and zeolites usually need complicated procedures with a high cost. On the other hand, different types of carbons like activated carbons, carbon fibers, nanotubes, and carbon molecular sieves have attracted great

attention for CO2 capture owing to their multi-merits in operation with non-pollution, high efficiency and low energy consumption[9]. Carbons are also regarded as promising adsorbents for CO2 capture owing to their well-developed pores and good chemical stability[10,11]. The widely reported activated carbons can be produced from a wide range of organic matters through carbonization and activation. However, the non-renewability and high cost of carbon source as well as the sophisticated procedure constrain their application. It is still a great challenge to find cost-effective resources for carbon production in the area of CO2 capture. With a sustainable principle in mind, many kinds of carbons from biomass have been reported including corncob [12], banana peel[13], rice husk[14], pine cone[15], lotus stem[16], bamboo[17], and nut shells[5,18], and the CO2 uptakes on biomass-derived carbons are usually in the range of 3-6.5 mmol/g at 1 bar and 0°C, which is mainly determined by porosity[16]. Therefore, a rational improvement of porosity in carbons especially for the small-sized micropores is very crucial for CO2 capture. In general, carbons from direct biomass

Received date: 02 Apr. 2019; Revised date: 02 Jun. 2019 *Corresponding author. E-mail: caijj @ xtu.edu.cn Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60014-4

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

carbonization exhibit low-surface-areas or even non-porous, which have no use for CO2 capture[19]. After activation treatments with various activators such as KOH, K2CO3, ZnCl2, and NaOH, carbons show high-surface-areas and large pore volumes owing to the etching process, which has been discussed in recent reviews[20, 21]. KOH is the most often used for creating small micropores in carbon skeleton, and a possible mechanism for activation is that K ions can be released during reaction with carbon, and insert into carbon lattices accompanied with carbon oxidation and activation with in-situ formed CO2 or CO molecules during high temperature treatments[20]. Sevilla et al[4]. reported a high CO2 uptake of 4.8 mmol/g at 25 °C and 1 bar on the carbons from KOH activation of hydrothermally carbonized biomass sawdust. Li et al[14]. reported a porous carbon from KOH activation of rice husk char using a small KOH/char ratio of 1:1, and the carbon with a surface area of 1 041 m2/g had a high CO2 uptake of 2.11 mmol/g at 0.1 bar and 0 °C. The high CO2 uptakes at low pressures were ascribed to the presence of pores less than 0.7 nm with a narrow pore size distribution. Deng et al[18]. developed peanut shell carbons from KOH activation with CO2 uptakes even up to 5.0 mmol/g under ambient condition. However, the corrosion behavior of KOH should be kept in mind during the reaction for reactors especially at high temperatures. A particular attraction of activated carbons for CO2 capture is that the desirable property of green credentials would be highly enhanced if they are produced from natural biomass[22]. The utilization of biomass as sustainable resource has been largely accepted by the society because of potential reduction in greenhouse gas emission, which can be close to zero emission through balance between biomass production and utilization in the future. Garlic as an important vegetable is often used for cooking as flavor, and lots of garlic were cultivated in north China each year. The massive consumption will lead to a large amount of garlic peels (GP) discarded with environmental pollution and resource waste, and the transformation of GP into other value-added matters will have great significance to sustainable chemistry. Jahagirdar et al[23]. reported the direct use of raw GP as adsorbents to remove metal ions of Pb2+ and Hg2+ from water, constituting the first report for a sustainable use of GP. GP powders was also used as adsorbents to remove methylene blue from water owing to its spiral trachea structure[24]. The previous studies have confirmed cellulose polysaccharide in the fiber structure of GP[25], indicating the high potential of GP to be used as a carbon source. However, only limited studies were reported for carbons from GP biowaste even if they are low-cost renewable precursors[26,27]. Selvamani et al[26] reported N-doped carbons from GP via carbonization at 300 °C followed by mild KOH activation at 800 °C, exhibiting high-performance as anode materials for Na ions batteries with surface areas of 1 700 m2/g and limited N-species. We have also reported carbons from similar two-step carbonization and KOH activation with

surface areas of 1 638 m2/g, in which the pre-carbonization temperature is settled at 360 °C, and the largest CO2 uptake in series of GP-based carbons is 4.1 mmol/g at 25 °C and 1 bar[28]. A hierarchical carbon with a surface area of 2 800 m2/g was recently reported by varying pre-carbonization temperature and KOH/char mass ratio, which exhibit a high capacitance of 427 F/g at 0.5 A/g as electrodes for a supercapacitor in 6 mol/L KOH[26]. As similar cellulose component in the structure, it should be pointed out that onion skins were also ever reported for carbons from one-pot carbonization and K2CO3 activation, which shows a high capacitance of 188 F/g at 1 A/g as the electrode materials for supercapacitors in organic electrolytes[29]. All the above-mentioned results indicated that GP is a promising carbon source for high-performance carbons used for energy storage and environmental applications. Although raw GP has been reported as adsorbents for liquid adsorption, and some types of carbons were also reported for energy storage applications, it should be mentioned that there is little report on GP-derived carbons from hydrothermal carbonization up to date for CO2 capture, to the best of our knowledge. As an alternative to direct carbonization, hydrothermal carbonization is promising in transforming biomass into carbons which can be achieved under mild conditions using H2O, H2SO4 or C2H5OH as the reaction media[30]. Detailed discussion on hydrothermal reaction can be found in latest reviews reported by Titirici et al[31]. Here, porous carbons were obtained from GP via the combination of hydrothermal carbonization and KOH activation under different activation temperatures and KOH/char ratios. The relationship between surface area, pore volume, pore size, and CO2 uptakes was discussed in detail. The results indicate that as-prepared carbons exhibit the highest surface area and pore volume of 1 262 m2/g and 0.70 cm3/g, respectively, while the highest CO2 uptake of up to 4.22 mmol/g at 25 °C and 1 bar was observed for the carbon with a much lower surface area but higher microporosity, indicating the great importance of small micropores in the frameworks for CO2 capture instead of traditionally believed high surface area or pore volume required under ambient conditions.

2 Experimental 2.1 Synthesis of GP-based activated carbons The same batches of garlic were collected from a local supermarket. After collection, raw GP were peeled off, washed with distilled water, and dried in an oven at 80 °C overnight. The synthesis procedure for carbons is shown in Scheme 1. Typically, 5 g of dried GP was dispersed into 60 mL of a 1 wt.% sulfuric acid solution and put into a 100 mL Teflon-lined autoclave for hydrothermal carbonization at 200 °C for 24 h according to our previous work[16]. The black products were collected by filtration, washed with distilled water, dried at 80 °C for 24 h, and then carbonized at 400 °C with a heating rate of 4 °C/min under N2 flow for 2 h to obtain a biochar.

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

Afterwards, the biochar was mixed with two-fold KOH in mass, and activated at 600, 700 and 800 °C for 1 h with heating rate of 2 °C/min under N2 flow. The carbons were treated with a 2 mol/L HCl solution at 60 °C for 3 h followed by distilled water until a neutral pH value reached, and dried at 100 °C for 24 h. The yields of carbons are about 16 wt.%-28 wt.%, depending on activation conditions, on the basis of per gram of the precursor. The carbons were denoted as AC-xy, in which ‘x’ stands for the mass ratio of biochar/KOH, and ‘y’ refers the temperature in hundredth. As comparison, two samples named as AC-06 and AC-46 were also obtained at 600 °C with different biochar/KOH mass ratios.

Scheme 1, An illustration of GP-derived carbons from the combination of hydrothermal carbonization and KOH activation.

is only of 5%, indicating that the hydrothermal reaction has dehydrated most of water molecules in structure to form hydrochar. The second stage shows the largest weight loss of 56% and 32% for raw GP and hydrochar with a precipitous decrease trend in weight, attributing to the decomposition of volatile in structure such as carbohydrate and cellulose components of the materials[25]. It should be noted that the degradation peaks of GP and hydrochar are observed at about 340 and 400 °C, respectively and the higher degradation temperature of 60 °C for the hydrochar is probably caused by the structure change of cellulose during hydrothermal reaction to form some macromolecules with stable aromatic structures whereas the cellulose structure was still retained in raw GP materials. As compared with cellulose, lignin in biomass is usually decomposed at a higher temperature, and therefore the remaining weight in substance after the second stage mainly includes the lignin decomposition, formation of carbon structures, and probable inorganic impurities[27]. There is still a small weight loss when the temperature is higher than 700 °C, which is probable due to the cracking reactions of C=C bonds to form stable carbon frameworks. As can be observed from Fig.1, the remaining weight for the hydrochar is still higher than 48% up to 800 °C under N2 flow, which is much higher than that of 10% for the raw GP materials, indicating the treatment of hydrothermal carbonization will largely increase the yields of carbons as compared with the direct carbonization.

2.2 Material characterization Thermal behavior of raw GP and hydrochar was performed on a thermal analyzer (TGA 1/1600HT) under N2 flow ramping at 10 °C/min. The structures of carbons were studied by X-ray diffraction (XRD, Bruker D8) and Raman spectroscopy (Renishaw-inVia). The morphology was observed on scanning electron microscopy (SEM, JSM-6610LV, JEOL). The N2 sorption isotherms were measured on a sorption analyzer at -196 °C (Micro- meritics, TriStarII 3020) after vacuum degas at 200 °C for 10 h, and CO2 adsorption isotherms at 1 bar were collected at both 0 and 25 °C. The surface areas and micropore volumes of carbons were calculated from the Brunauer-Emmett-Teller (BET) and t-plot method, respectively. The total pore volumes (Vt) were calculated from the adsorbed amount of N2 at a p/p0 of 0.98 and average pore size (Da) was calculated with the Dubinin-Astakhov method.

3 Results and discussion Thermogravimetric curves for the hydrochar and raw GP under N2 flow are shown in Fig. 1, which exhibit an obvious three-step weight loss for both the raw GP and hydrochar as the temperature increases up to 800 °C. As observed from Fig. 1, the initial stage weight loss for raw GP materials is about 12.6% below 100 °C, which is mainly attributed to the loss of moisture outside the surface and absorbed water in the cell structure of raw GP[8]. The weight loss for the hydrochar at the initial stage

Fig. 1

Thermogravimetric analysis for (a) raw GP materials and (b) the hydrochar under nitrogen atmosphere.

The SEM images for the hydrochar and carbons from different biochar/KOH mass ratios are shown in Fig. 2. As can be observed from Fig. 2a, the SEM image of the hydrochar shows a dense bulk morphology with irregular granules without any microspheres, which is usually observed for the hydrochar from other lignocelluloses biomass such as sawdust, starch, bamboo and microalgae [4, 32-34]. There are also no microspheres observed in the morphology for the hydrochar from lotus stem in our previous work[16], indicating that the morphology of carbon will be highly varied with the kind of precursors. After activation with different biochar/KOH ratios, the morphology of carbons in Fig. 2b for AC-06, Fig. 2c for AC-26, and Fig. 2d for AC-46 varied little and remained the smooth surface with

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

the bulk morphology. There are more big pieces with grooves for AC-46 as compared with that of AC-06 at the same resolutions, indicating the severer activation will wreck bulk particle with more etching at elevated biochar/KOH ratios to form the well-developed pores in the framework. Therefore, the

Fig. 2

AC-46 sample will exhibit relatively more eroded cavities with larger surface areas than AC-06. The SEM images for AC-27 and AC-28 (not shown here) are also very similar to the situation of AC-46 with a smooth surface morphology and large quantities of grooves.

SEM images for GP-derived carbons: (a) the hydrochar, (b) AC-06, (c) AC-26 and (d) AC-46.

The Raman spectra for all GP-based carbons are shown in Fig. 3 to evaluate the graphitization degree. As can be seen from Fig. 3, all carbons have two prominent peaks at around 1 368 cm-1 (D-band) and 1 612 cm-1 (G-band), in which D-band represents the presence of disordered or defective carbon structures, and G band indicates the presence of some order-layered graphite with vibration of sp2-hybridized carbon atoms[1]. The broad peak for G-band with a strong intensity of D-band in Fig. 3 have indicated the amorphous characteristics in the structure of carbons. In general, the intensity ratio (ID/IG) can be used to evaluate the graphitization degree of carbons, where a smaller value indicates a higher graphitic degree. The calculated ID/IG ratio for AC-26, AC-27, and AC-28 with the same biochar/KOH ratio but different temperatures are about to be 0.91, 0.89, and 0.83 from integral area of peaks, respectively. The results indicate that all carbons show a very low graphitic degree but the severer activation will increase the limited degree. The calculated ID/IG values for AC-06, AC-26 and AC-46 increase first and then decrease with the biochar/KOH ratios, indicating that the graphitic degree varies with the biochar/KOH ratio probably due to the fact that the severer etching of carbon atoms with K+ ion will affect the interlayer distance of graphene layers in carbon frameworks[19].

Fig. 3

Raman spectra for GP-derived carbons.

The XRD patterns of AC-26, AC-27 and AC-28 are shown in Fig. 4 to further investigate the effect of activation extent on the graphitization degree of carbons. As can be observed from Fig. 4, all carbons show two weak broad diffraction peaks at ranges of 18°-26°and 41°-44°. It should be pointed out that the XRD pattern for AC-06 (not shown) is very similar to that of AC-26, while the XRD pattern for AC-46 is similar to that of AC-27. The two peaks are ascribed to (002) and (100) plane for carbon materials, respectively[1, 4-6] and the weak broad peaks

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

indicate that all the carbons have an amorphous structure with a very low graphitization degree, in well agreement with results of Raman spectra in Fig. 3. However, there are some shift to the small angles from 25° to 19° as the activation temperature increases, indicating that increasing the temperature will accelerate the intercalation of K+ ions into carbon skeleton for etching, and then affect the graphene interlayer distance in the frameworks.

Fig. 4 Representative XRD patterns of GP-derived carbons.

The microstructure of carbons were studied by N2 sorption at the liquid nitrogen temperature, and the results are described in Fig. 5 with the parameters summarized in Table 1. As shown in Fig. 5a, all the adsorption branches of N2 isotherms for carbons are almost overlapped with the desorption branches, and the adsorption also reaches horizontal plateau as the relative pressure (p/p0) increases up to 0.1, indicating a typical type I isotherm for carbons with large quantities of micropores in the carbon framework and mesopores are practically little or non-existing[2]. Some researches confirmed that small micropores less than 1.0 nm are important for CO2 capture at ambient conditions[1, 4, 5]. It is believed that the present carbons from KOH activation of GP hydrochar would exhibit an excellent CO2 capture performance owing to the high proportion of small micropores in the framework. By comparing the adsorption isotherms in series of carbons, it is found that AC-28 and AC-46 exhibit a relative wide isotherm ‘knee’ with the adsorbed N2 amounts largely increased as the

Fig. 5

p/p0 value increases up to 0.3, indicating the presence of some large-sized micropores or even small-sized mesopores in frameworks[18, 33]. Pore size distributions (PSDs) for carbons were calculated from non-local density functional theory using N2 adsorption branches with results illustrated in Fig. 5b. As observed from Fig. 5b, all carbons exhibit almost no pores larger than 10 nm with most pores in the range of micropores. In addition, AC-28 and AC-46 exhibit some pores in the range of 2-10 nm, indicating the severer activation conditions such as higher temperatures and higher contents of activators will widen these micropores into small-sized mesopores. The PSD curves for all the carbons in Fig. 5b indicate that micropores constitute the main structures with little small-sized mesopores except for the AC-28 and AC-46. As summarized in Table 1, the calculated BET surface areas for AC-06, AC-26, and AC-46 increase from 306 to 1 116 m2/g, and the total pore volumes and micropore volumes are 0.17 and 0.13 cm3/g for AC-06, 0.51 and 0.50 cm3/g for AC-26, and 0.63 and 0.57 cm3/g for AC-46. The results indicate that the direct carbonization of hydrochar at high temperatures is not enough to create pores in carbon framework, and the surface areas of AC-06 are limited to be 306 m2/g without the use of KOH. However, as the mass ratios of KOH/hydrochar and/or the activation temperatures increase, the BET surface areas of carbons increase with the values for AC-46 and AC-28 up to 1 116 and 1 262 m2/g, respectively. It should be noted that the increase of BET surface areas of carbons is at the expense of reducing microporosity, where the surface area for AC-26 is only 947 m2/g with the microporosity highly up to 98.04% while the surface area for AC-46 is 1 116 m2/g with the microporosity of 90.48 %. A possible reason for the variation of the surface area and microporosity is that as the activator contents or activation temperature increase within an appropriate range, the carbon skeleton can form high quantities of micropores and results in the increase of surface areas and total pore volumes. However, excessive KOH amount and higher temperature will seriously destroy the already existing micropores and small mesopores, leading to a decrease of microporosity[20]. Therefore, AC-28 shows the highest surface

(a) N2 adsorption-desorption isotherms and (b) corresponding pore size distributions for GP-derived carbons.

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

areas and pore volumes with a lower microporosity as compared with AC-26 and AC-27. The average pore sizes of carbons are also varied with activation conditions, and most of

carbons have average sizes of about 2.0 nm (Table 1), indicating the presence of high quantities of micropores with very limited mesopores in the framework.

Table 1 Pore structure parameters calculated from N2 adsorption isotherms and CO2 uptakes at 1 bar for GP-based carbons. Samples

SBET (m2/g)

Smicro (m2/g)

Vt (cm3/g)

Vmicro (cm3/g)

CO2 uptakes (mmol/g) Vmicro/Vt (%)

Da (nm) 0 °C

25 °C

AC-06

306

250

0.17

0.13

76.47

2.24

2.49

1.65

AC-26

947

928

0.51

0.50

98.04

2.16

6.25

4.22

AC-27

1179

1166

0.63

0.61

96.82

2.15

5.96

3.99

AC-28

1262

1206

0.70

0.65

92.86

2.21

4.33

2.82

AC-46

1116

1055

0.63

0.57

90.48

2.24

4.60

2.84

Fig. 6

CO2 adsorption isotherms for GP-derived carbons measured at (a) 25 °C and (b) 0 °C.

The CO2 capture performance for all carbons were evaluated at the ambient pressure and 0 and 25 °C, and the detailed uptakes are summarized in Table 1. As can be observed from Fig. 6, the CO2 adsorption curves both at 0 and 25 °C are quite similar only with some difference in the adsorbed amounts. The uptakes sharply decrease with an increase of the adsorption temperature, indicating that the CO2 capture on carbons is a physisorption type[15,33]. The CO2 uptakes both at 0 and 25 °C on AC-06 are the smallest ones in series of carbons, due to its smallest surface area of 306 m2/g. The surface area and pore volume for AC-26 and AC-46 increase with the KOH/char ratios. However, the AC-46 has a lower CO2 uptake than the AC-26. It should be pointed out that the AC-26 shows the largest CO2 uptake of highly up to 6.2 mmol/g at 0 °C and 4.2 mmol/g at 25 °C at 1 bar even though its surface area is much lower than that of AC-46, AC-27 and AC-28. The AC-26 also exhibits an higher CO2 uptake as compared with other counterparts even at a low pressure range (Fig. 6), which is probably due to its narrow pore size distributions as shown in Fig. 5b and the ultrahigh microporosity (Table 1), which largely favors the CO2 capture[1, 35]. The AC-28 has the largest surface area and total pore volume whereas the CO2 uptake is only of 4.33 and 2.82 mmol/g at 0 and 25 °C, respectively. Moreover,

the comparison results on the AC-06, AC-26, and AC-46 also confirm the positive effects of the narrow micropore size on CO2 capture. In general, the large surface area and high micropore volume are basic requirements for a high CO2 uptake in carbons. As shown in Table 1, AC-28 has the largest surface area and pore volume, while the uptake for AC-28 is much lower than that of AC-27 and AC-26, indicating that carbons with a higher microporosity could possess a higher CO2 uptake[1]. The surface area for AC-26 is 947 m2/g, less than that of 1 116 m2/g for AC-46 while the microporosity for AC-26 is up to 98.04%, higher than that of 90.48% for AC-46. The lower CO2 uptakes for AC-46 indicate that the CO2 capture uptake is not determined by the surface area of carbons only. After the comparison between AC-27 and AC-46 with a similar surface area, it is found that the higher CO2 uptake for the AC-27 is probable due to its narrower pore size distribution and higher microporosity. The relationship for CO2 uptake with the physical pore characteristics of carbons are shown in Fig. 7. As can be observed from Fig. 7a that the CO2 uptake first increases linearly with an increase of the surface area, then decreases with a further increase in the surface area, indicating an appropriate surface area are favorable to create a desired pore

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

structure for CO2 capture under a specific condition. The CO2 uptakes on carbons in relation to the total pore volume and micropore volume show a very similar trend as compared with the surface area, and there is no evident linear relationship between the uptake and pore volume. These results imply that the CO2 uptake seem to have little linear relationship with surface area, micropore volume, and total pore volume for GP-based carbons from KOH activation of hydrochar (Fig. 7). However, it should be pointed out that there is a nearly linear relationship between the micropore volume and CO2 uptake, which is ever observed for GP-based carbons from direct carbonization and KOH activation in our previous work[28], where carbons with a larger micropore volume almost exhibit a larger uptake even though with some exceptions. The CO2 uptake for carbons is obvious proportional to the microporosity as shown in Fig. 7d, which is also observed for the GP-based carbons from direct carbonization and KOH activation [28]. All the information reveals that the enhancement of CO2 uptake on carbons could be affected by the characteristic of microporosity but not the whole micropore volume. For instance, Boyjoo et al[35]. reported a porous carbon from KOH activation of hydrothermally carbonized waste Coca Cola, and the carbon

Fig. 7

shows a surface area of 1 405 m2/g with a CO2 uptake of highly up to 5.22 mmol/g at 25 °C and 1 bar, constituting probable the largest uptake reported for carbons up to date, which is due to a high proportion of micropores with 55.8% pores are less than 0.8 nm. Significantly, the present highest CO2 uptake is observed for AC-26 with the uptake of 6.2 mmol/g at 0 °C and 4.2 mmol/g at 25 °C at 1 bar, which is comparable to or slightly higher than those reported from other biomass like algae[34], olive stone[36], horse manure[8], birch wood bark[37], and chitosan[38]. Moreover, the uptake at 25 °C is higher than that of 3.8 mmol/g in our previous work for the carbon from KOH activation of lotus stem hydrochar with a surface area of 2 091 m2/g[16], indicating the importance of the effective surface area and micropore size for CO2 uptake. It should be admitted that the CO2 uptake is still lower than N-doped carbons from other biomass such as longan shells[11] and water chestnut[39] owing to the introduction of nitrogen atoms, while the AC-26 is obtained from the low-cost hydrothermal carbonization and KOH activation without nitrogen. These comparison implies that an excellent CO2 uptake for the carbon from hydrothermally carbonized GP waste, which is a promising adsorbent for CO2 capture under ambient condition.

elationship between porous characteristics of carbons and CO2 uptakes at 0 °C and 1 bar: (a) surface area, (b) micropore volume, (c) total pore volume and (d) Vmicro/Vtotal.

Isosteric heat (Qst) of CO2 adsorption on carbons is also an important indicator for the recycling use of carbons as adsorbents. The calculated Qst values using the Clausius-Clapeyron equation with CO2 isotherms at 25 and 0 °C,

and the variation of Qst values with the adsorbed CO2 amount are shown in Fig. 8. As observed from Fig. 8, Qst values on all the carbons decrease sharply as the adsorbed CO2 amount increases at the first stage, and then are kept to be steady as the

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

surface coverage increase further, due to an increase of interaction between the adsorbed CO2 molecules[7,16]. The largest Qst is observed for AC-06 with the value close to 40 kJ/mol at the zero loading, and then decreases to around 35 kJ/mol and kept steady as the surface coverage increase further. Considering the fact that the AC-06 shows the smallest surface area, the largest Qst is probably due to surface functional groups such as oxygen-based groups that can enhance the formation of hydrogen-bond with adsorbed CO2 molecules during adsorption[40]. It should be pointed out that the presence of oxygen species can not be avoided, and the carbons from hydrothermal carbonization using water as the reaction medium will do have much of oxygen groups as compared with the one from the other method. The effect of the surface functional groups on the CO2 uptakes should be further confirmed through more experiments in future. However, all the Qst values for the other activated carbons vary in the same trend, where the Qst varies in the range of 15-25 kJ/mol as an increase of an adsorbed amount. The variation trend for Qst indicates that small-sized micropores in carbons are occupied first by adsorbate molecules, indicating energetic heterogeneity properties of CO2 adsorption on carbons. All Qst values are lower than the typical energy required for chemisorption (>60 kJ/mol), suggesting a dominant physisorption character between pores and adsorbed CO2 molecules. Qst values on carbons from KOH activation of GP hydrochar are similar to that of carbons from other biomass like lotus stem[16] and coconut shell[41] using the same route, which is much larger than that of carbons from direct carbonization and KOH activation [28] . Although AC-28 and AC-46 show the larger

regeneration indicate the GP-based carbons are promising candidates for CO2 capture.

4

Conclusions

GP was used as the precursor for carbons via hydrothermal carbonization and KOH activation for CO2 capture. The activation condition largely affected the pore structure of carbons and CO2 uptake. The AC-26 shows the largest CO2 uptake of up to 6.2 mmol/g at 0 °C and 4.2 mmol/g at 25 °C and 1 bar, resulting from an activation temperature of 600 °C with a KOH/hydrochar ratio of 2:1 due to a ultrahigh microporosity of 98.04% and a moderate surface area of 947 m2/g. Both AC-46 and AC-28 also show well-developed micropores with surface areas of 1 116 and 1 262 m2/g, respectively, while the presence of small quantity of small-sized mesopores could lower the microporosity and then make CO2 uptake lower than AC-26. This is the first report on high-surface-area carbons from GP for CO2 capture from hydrothermal carbonization and KOH activation, which is an effective way to explore low-cost carbons for a high CO2 uptake.

Acknowledgements National Natural Science Foundation of China (21506184); Natural Science Foundation of Hunan Province (2019JJ50597); State Key Laboratory of Powder Metallurgy of Central South University (621011821); Hunan 2011 Collaborative Innovation Center of Chemical Engineering with Environmental Benignity and Effective Resource Utilization.

References [1] Cai J, Qi J, Yang C, et al. Poly(vinylidene chloride)- based carbon with ultrahigh microporosity and outstanding performance for CH4 and H2 storage and CO2 capture [J]. ACS Applied Materials & Interfaces, 2014, 6 (5): 3703-3711. [2] Regufe M J, Ferreira A F P, Loureiro J M, et al. New hybrid composite honeycomb monolith with 13X zeolite and activated carbon for CO2 capture [J]. Adsorption, 2018, 24 (3): 249-265. [3] Yang S, Zhan L, Xu X, et al. Graphene-based porous silica sheets impregnated with polyethyleneimine for superior CO2 capture [J]. Advanced Materials, 2013, 25 (15): 2 130-2134.

Fig. 8

The isosteric heat of CO2 adsorption on carbons as a function of adsorbed amounts.

surface areas as listed in Table 1, it should be noted that the Qst of CO2 adsorption on the two samples are almost the lowest at the zero loading due to the presence of large quantities small-sized mesopores from severe KOH activation (Fig. 5b). The present Qst value at the whole surface coverage of CO2 on the carbons suggests the easy desorption, that is, the easily recycling use of carbon adsorbents. Therefore, the combination of a good stability of carbons with a high CO2 uptake and facile

[4] Sevilla M, Fuertes A B. Sustainable porous carbons with a superior performance for CO2 capture [J]. Energy & Environmental Science, 2011, 4 (3): 1765-1771. [5] Yue L, Xia Q, Wang L, et al. CO2 adsorption at nitrogen-doped carbons prepared by K2CO3 activation of urea-modified coconut shell [J]. Journal of Colloid and Interface Science, 2018, 511: 259-267. [6] Wang Z, Zhan L, Ge M, et al. Pith based spherical activated carbon for CO2 removal from flue gases [J]. Chemical Engineering Science, 2011, 66 (22): 5504-5511.

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

[7] Millward A R, Yaghi O M. Metal-organic frameworks with

[21] Spigarelli B P, Kawatra S K. Opportunities and challenges in

exceptionally high capacity for storage of carbon dioxide at room

carbon dioxide capture [J] Journal of CO2 Utilization, 2013, 1:

temperature [J]. Journal of the Americal Chemical Society, 2005, 127 (51): 17998-17999.

69-87. [22] Balahmar N, Al-Jumialy A S, Mokaya R. Biomass to porous

[8] Heydari-Gorji A, Sayari A. Thermal, oxidative, and CO2-induced

carbon in one step: directly activated biomass for high

degradation of supported polyethylenimine adsorbents [J].

performance CO2 storage [J]. Journal of Materials Chemistry A,

Industrial & Engineering Chemistry Research, 2012, 51 (19):

2017, 5 (24): 12330-12339.

6887-6894.

[23] Jahagirdar D V, Nigal J N. Adsorption of Cd2+ and Pb2+ on

[9] Shen C, Yu J, Li P, et al. Capture of CO2 from flue gas by vacuum pressure swing adsorption using activated carbon beads [J]. Adsorption, 2011, 17 (1): 179-188.

agricultural byproducts [J]. Asian Journal of Chemistry, 1997, 9 (1): 122-125. [24] Hameed B H, Ahmad A A. Batch adsorption of methylene blue

[10] Singh G, Kim I Y, Lakhi K S, et al. Heteroatom functionalized

from aqueous solution by garlic peel, an agricultural waste

activated porous biocarbons and their excellent performance for

biomass [J]. Journal of Hazardous Materials, 2009, 164 (2-3):

CO2 capture at high pressure [J]. Journal of Materials Chemistry

870-875.

A, 2017, 5 (40): 21196-21204.

[25] Reddy J P, Rhim J W. Isolation and characterization of cellulose

[11] Wei H, Chen H, Fu N, et al. Excellent electro- chemical properties and large CO2 capture of nitrogen-doped activated porous carbon synthesised from waste longan shells [J]. Electrochimica Acta, 2017, 231: 403-411.

nanocrystals from garlic skin [J]. Materials Letters, 2014, 129: 20-23. [26] Selvamani V, Ravikumar R, Suryanarayanan V, et al. Garlic peel derived high capacity hierarchical N-doped porous carbon anode

[12] Sun Y, Webley P A. Preparation of activated carbons from corncob with large specific surface area by a variety of chemical activators and their application in gas storage [J]. Chemical Engineering Journal, 2010, 162 (3): 883-892. [13] Liu R, Ji W, He T, et al. Fabrication of nitrogen-doped hierarchically porous carbons through a hybrid dual-template

for sodium/lithium ion cell [J]. Electrochimica Acta, 2016, 190: 337-345. [27] Zhang Q, Han K, Li S, et al. Synthesis of garlic skin-derived 3D hierarchical porous carbon for high-performance supercapacitors [J]. Nanoscale, 2018, 10 (5): 2427-2437. [28] Huang G, Wu X, Hou Y, et al. Sustainable porous carbons from

route for CO2 capture and haemoperfusion [J]. Carbon, 2014, 76:

garlic peel biowaste and KOH activation with an excellent CO2

84-95.

adsorption performance [J]. Biomass Conversion Biorefinery,

[14] Li D, Ma T, Zhang R, et al. Preparation of porous carbons with high low-pressure CO2 uptake by KOH activation of rice husk char [J]. Fuel, 2015, 139: 68-70.

2019, doi:10.1007/s13399-019-00412-6. [29] Wang D, Liu S, Fang G, et al. From trsh to treasure: Direct transformation

of

onion

husks

into

three-dimensional

[15] Li K, Tian S, Jiang J, et al. Pine cone shell-based activated carbon

interconnected porous carbon frameworks for high-performance

used for CO2 adsorption [J]. Journal of Materials Chemistry A,

supercapacitors in organic electrolyte [J]. Electrochimica Acta

2016, 4 (14): 5223-5234.

2016, 216: 405-411.

[16] Wu X, Zhang C, Tian Z, et al. Large-surface-area carbons derived from lotus stem waste for efficient CO2 capture [J]. New Carbon Materials, 2018, 33 (3): 252-261. CO2 adsorption capacity synthesized from clay-reinforced biobased chitosan-polybenzo- xazine nano-composites [J]. ACS Sustainable Chemistry and Engineering, 2016, 4 (3): 1286-1295. [18] Deng S, Hu B, Chen T, et al. Activated carbons prepared from peanut shell and sunflower seed shell for high CO2 adsorption [J]. Adsorption, 2015, 21 (1-2): 125-133. [19] Tian Z, Xiang M, Zhou J, et al. Nitrogen and oxygen-doped porous carbons

from algae biomass: Direct

carbonization and excellent electrochemical properties [J]. Electrochimica Acta, 2016, 211: 225-233. [20] Wang J, Kaskel S. KOH activation of carbon- based materials for energy storage [J]. Journal of Materials Chemistry, 2012, 22 (45): 23710-23725.

carbons for supercapacitors [J]. Carbon, 2016, 103: 181-192. [31] Titirici M M, White R J, Brun N, et al. Sustainable carbon

[17] Almahdi A A, Hatsuo I, Syed Q. Carbon aerogels with excellent

hierarchical

[30] Sun W, Lipka S M, Swartz C, et al. Hemp-derived activated

materials [J]. Chemical Society of Reviews, 2015, 44 (1): 250-290. [32] Yue L, Rao L, Wang L, et al. Efficient CO2 capture by nitrogen-doped biocarbons derived from rotten strawberries [J]. Industrial & Engineering Chemistry Research, 2017, 56 (47): 14115-14122. [33] Wei H, Deng S, Hu B, et al. Granular bamboo- derived activated carbon for high CO2 adsorption: the dominant role of narrow micropores [J]. ChemSusChem, 2012, 5 (12): 2354-2360. [34] Zhang Z, Wang K, Atkinson J D, et al. Sustainable and hierarchical porous enteromorpha prolifera based carbon for CO2 capture [J]. Journal of Hazardous Materials, 2012, 229-230: 183-191.

Ge-ge Huang et al. / New Carbon Materials, 2019, 34(3): 247-257

[35] Boyjoo Y, Cheng Y, Zhong H, et al. From waste Coca Cola® to activated carbons with impressive capabilities for CO2 adsorption and supercapacitors [J]. Carbon, 2017, 116: 490-499. [36] Plaza M G, Pevida C, Arias B, et al. Development of low-cost biomass-based adsorbents for post- combustion CO2 capture [J]. Fuel, 2009, 88 (12): 2442-2447. [37] Dobele G, Dizhbite T, Gil M V, et al. Production of nanoporous carbons from wood processing wastes and their use in supercapacitors and CO2 capture [J]. Biomass Bioenergy, 2012, 46: 145-154. [38] Primo A, Forneli A, Corma A, et al. From biomass wastes to highly efficient CO2 adsorbents: Graphitisation of chitosan and

alginate biopolymers [J]. Chem Sus Chem, 2012, 5 (11): 2207-2214. [39] Wei H, Chen J, Fu N, et al. Biomass-derived nitrogen-doped porous carbon with superior capacitive performance and high CO2 capture capacity [J]. Electrochim Acta, 2018, 266: 161-169. [40] Xing W, Liu C, Zhou Z, et al. Oxygen-containing functional group-facilitated CO2 capture by carbide- derived carbons [J]. Nanoscale Research Letters, 2014, 9: 189. [41] Ello A S, de Souza L K C, Trokourey A, et al. Coconut shell-based microporous carbons for CO2 capture [J]. Microporous and Mesoporous Materials, 2013, 180: 280-283.