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Abstract

The effect of an alkali treatment (potassium hydroxide) on the properties of a commercial activated carbon has been studied. The aim of the treatment was to improve the adsorption properties of the material toward carbon dioxide. In the result of the treatment, silica contained in the raw carbon was removed and the density of the material increased. The changes in the surface chemistry were observed as well. The treatment of the activated carbon with KOH resulted in a complete removal of carboxy and lactone groups and a decrease of the general content of the acidic groups (more significant than that of basic groups). Simultaneously, the surface concentration of hydroxyl groups increased. The alkali treatment of activated carbon resulted in an increase of carbon dioxide uptake of 14% (measured using a volumetric method at 0℃). The adsorption of carbon dioxide on activated carbon has a mixed (physicochemical) character and that two types of adsorption sites are present at the surface. The adsorption energy varies roughly from 25 to 60 kJ/mol.

Keywords

Activated carbon carbon dioxide adsorption surface chemistry alkali treatment

Introduction

A decrease of carbon dioxide emission is one of the most important challenges today. It is believed that it would be a remedy to inhibit the global warming of our planet and dangerous climate changes. Fossil fuel power plants are the main source of anthropogenic CO₂ emissions, but industrial production plants (e.g. chemical industry, cement industry, iron and steel production) are also large contributors. There are a lot of solutions proposed for carbon dioxide storage, conversion, and utilization (Song, 2006), but in all the cases the first step has to be a CO₂ capture, its separation from exhaust gases. CO₂ can be separated directly from the flue gas (so-called PCCC—postcombustion carbon capture) by absorption, adsorption, membrane separation, cryogenic distillation, and solidification (Fu and Gundersen, 2012).

The existing processes used for PCCC and based mainly on absorption in liquids (as amines, potassium carbonate, or methanol) are costly, inefficient, and may have inherent environmental problems. There is a need to develop new PCCC technologies, more cost and energy efficient and more friendly for environment. Solid sorbents can be used in such processes, instead of traditional liquids. In a review paper on solid sorbents for PCCC (Choi et al., 2009), such materials as zeolites, activated carbons, calcium oxides, hydrotalcites, organic–inorganic hybrids, and metal–organic frameworks are mentioned. In another recent review paper on PCCC using solid sorbents, the criteria for choosing the best CO₂ sorbents are determined (Samanta et al., 2012) as: adsorption capacity and selectivity for CO₂, adsorption/desorption kinetics (fast enough under operating conditions), mechanical strength of sorbent particles, chemical stability/tolerance to impurities, low heat needed for a regeneration, and low cost of the adsorbent ($15/kg sorbent—would be not economical).

As carbonaceous materials can fulfill most of the above criteria, they seem to be a good candidate for the application at industrial scale.

Activated carbons are well-known sorbent materials for various applications, as removal of organic and inorganic impurities from water (Mohan and Pittman, 2006; Otowa et al., 1997) and gases, then it is not surprising that they can also be applied for CO₂ separation. A range of raw materials can be used to obtain activated carbons, for example coals, industrial by-products, or biomass products (as plum stones, coconut shells, or algae) (Aravindhan et al., 2009). Characteristic features of activated carbons are high specific surface area, well developed micropores structure, and interesting surface chemistry created by the presence of various heteroatoms and functional groups. The surface chemistry of the activated carbons can be modified to change their adsorption behavior. The modification can be thermal or chemical and in the latter case it can be carried out in gas or liquid phase. For the modification in liquid phase often nitric or sulfuric acid is applied (or their mixture) or hydrogen peroxide. Activated carbon can be also modified during the production process through chemical or physical activation. In the case of chemical activation, the raw material is impregnated with a chemical compound (as KOH, K₂CO₃, NaOH, Na₂CO₃, salts of alkali earth metals—AlCl₃ and ZnCl₂, and some acids as H₃PO₄ i H₂SO₄) and then heated under inert gas.

Maroto-Valer et al. (2005) investigated the CO₂ capture behavior of steam-activated anthracite and concluded that CO₂ uptake did not depend proportionally on the specific surface area of the material. The highest CO₂ adsorption capacity (65.7 mg CO₂/g of adsorbent) was reached on the sample with a specific surface area of 540 m²/g only, when the sample having almost doubled highest specific surface area of 1071 m²/g adsorbed much less of CO₂–40 mg CO₂/g. It means that despite the specific surface area there are other factors influencing the adsorption capacity toward CO mg CO₂/g as porous structure and surface chemistry.

Pevida et al. (2008) proved that nitrogen functionalities could improve the adsorption capacity of activated anthracites toward carbon dioxide.

Another way to introduce a basic surface functionality, which promotes CO₂ capture capacity, can be activation with potassium hydroxide. KOH has been reported as a carbon activator to improve the adsorption properties toward hydrogen sulfide (Sitthikhankaew et al., 2014) and nitrogen oxides (Lee et al., 2002) but also for carbon dioxide adsorption.

According to Lee et al. (2014), an activation of ordered nanoporous carbons with KOH caused an increase of specific surface area and total pore volume, which resulted in the enhancement of CO₂ adsorption capacity. A similar conclusion was drawn by de Andres et al. (2013); however, better results were reached using solid NaOH.

Wei et al. (2012) reported a high surface area and CO₂ uptake at high pressure (3.57 MPa) of activated carbon produced from Finger Citron residue and treated with KOH. A biological material (olive stones (Ubago-Perez et al., 2006)) was also used for fabrication of different granular and powder-activated carbons obtained by KOH activation.

To prepare efficient carbon dioxide sorbent with a basic surface, the doping with nitrogen is also recommended (Xing et al., 2012). The authors showed that introduction of nitrogen into a carbon surface facilitates the hydrogen-bonding interaction between the carbon surface and CO₂ molecules. Recently, an interesting paper of Zhou et al. (2015) was published, reporting a superior CO₂ sorption on N-doped microporous carbons derived from direct carbonization of K+ exchanged meta-aminophenol–formaldehyde resin. The authors concluded that a good performance of such a sorbent toward carbon dioxide is connected with an appropriate porous structure (uniform ultramicropores around 0.5 nm) and N-doping. The same research group of Zhou et al. (2016) followed the research on ultramicroporous sorbents for carbon dioxide enhanced adsorption and proposed a direct pyrolysis of alkali salts of carboxylic phenolic resins. The size of pores in the final product can be finely tuned depending on the applied alkali metal.

In our studies, a commercial activated carbon was modified with KOH. In the previous paper (Sreńscek-Nazzal et al., 2015), we showed the promising porous structure of such a commercial activated carbon, the total pore volume V_p was equal to 0.5 cm³/g with 80% of pores occupied by micropores (V_mic = 0.4 m³/g). The specific surface area (Brunauer–Emmett–Teller (BET)) of the carbon was high and equal to 1186 m²/g. The CO₂ adsorption isotherms measured at 40℃ up to 40 bar showed a significant increase of the carbon dioxide uptake caused by the addition of KOH, 1.4 times at 40 bar.

In the present paper, we are describing the properties of the same material (commercial WG12 carbon), but activated with KOH in a microwave-assisted hydrothermal reactor, because recently there are reports about a positive effect of microwave treatment (Kim et al., 2014; Schwenke et al., 2015; Zhang et al., 2013). The issue of the use of hydrothermal processes in carbon activation (HTC-based carbons) has been thoroughly treated in the book of Titirici (2013). Both the surface area and pore volume of hydrothermally produced carbons activated with KOH are formed mainly by micropores.

The aim of the present paper is to study the effect of KOH treatment on the surface properties of a commercial carbon activated with KOH.

Experimental

A sample of 3 g of commercial activated carbon WG12 (Gryfskand) was suffused with a solution of KOH (15 g in 50 ml of water) and put into a microwave-assisted hydrothermal reactor Magnum II (Ertec Poland). The process was carried out at 3 MPa for 15 min. The obtained material was washed with distilled water until a neutral pH was reached and next heated at 100℃ for 24 h.

Characterization

The studies of the adsorption of carbon dioxide on the prepared carbon sample were carried out using a temperature-programmed desorption method (TPD-CO₂) with a micrometrics equipment. The studies were performed under atmospheric pressure at three different temperatures: −30, 0, and 20℃. Before the CO₂ adsorption studies the carbon samples were heated under helium at 300℃ for 2 h. Then, the samples were cooled to the adsorption temperature under helium. To reach a selected adsorption temperature (−30, 0, or 20℃) a liquid nitrogen cooling system was applied (Cryocooler). The CO₂ adsorption was performed with a stream of CO₂ passing through the adsorbent bed with a rate of 50 cm³/min for 30 min. After the adsorption CO₂ was replaced by helium, passing through the bed for 30 min at the adsorption temperature, to remove carbon dioxide not bounded to the surface.

The TPD-CO₂ process was conducted by applying an increase of temperature with a rate of 15, 20, and 25°/min under a flux of helium through the adsorbent bed. Three rates of temperature increase were applied to determine a desorption energy (E_d) of CO₂ from the carbon surface.

To determine the energy of CO₂ desorption the first raw kinetics was applied according to equation (1) $ln (β / T_{max}^{2}) = - E_{d} / {RT}_{max} + ln (E_{d} / (R^{.} C))$ (1)

in which:

β—rate of the temperature increase (°/min),

E_d—desorption energy (J/mol K),

R—gas constant,

T_p—a temperature, in which the desorption rate is the highest (K)

The energies of desorption were determined as tangents to the lines traced according to the dependence ln (β/T²_max) = f(1/T_max) for all cases of adsorption on carbon under the study.

The textural properties of the ACs were determined by physical adsorption of N₂ at 77 K and CO₂ at 273 K using a Quadrasorb apparatus (Quantachrome Instruments). Before the experiments, the samples were outgassed under vacuum at 260℃ overnight. The specific surface area was measured by the multipoint BET method.

The helium density of the prepared samples was measured under helium using a Micro-Ultrapyc 1200e equipment at the pressure of 17 psi. The samples were purged with helium for 20 min.

The presence of functional groups was assessed using a modified Boehm (1994, 2002) titration method. The modification of the method involved an adaptation of the analysis procedure to the small amount of the samples.

A determination of oxygen surface groups using a method of alkalimetric titration is based on the assumption that acid constants of the carboxy, lactone, and phenyl groups differ greatly from one another (the difference should be of several orders of magnitude). In such a case it is possible to titrate the different groups by their neutralization with appropriate bases.

To carry out the titration the samples of 0.2 g were taken and suffused with the solutions of NaHCO₃, Na₂CO₃, KOH, and HCl. In the next step of the procedure, the suspensions were mixed in the closed small flasks for 48 h. Next, the samples remained for 24 h for a decantation. Finally, the samples were filtered through a filter paper and the solutions were titrated using methyl orange and phenolphthalein.

The FTIR spectrum of the tested carbons was recorded with the Thermo Scientific Nicolet 380 spectrometer.

The surface composition was studied by X-ray photoelectron spectroscopy (XPS). The X-ray photoelectron spectra were obtained using Al Kα (hν = 1486.6 eV) radiation with a Prevac system equipped with Scienta SES 2002 electron energy analyzer operating at constant transmission energy. Survey spectra were collected with pass energy of 100 eV, and high-resolution spectra were acquired with pass energy of 50 eV. The spectrometer was calibrated by using the following photoemission lines (with reference to the Fermi level): EB Cu 2p_3/2 = 932.8 eV, EB Ag 3d_5/2 = 368.3 eV, and EB Au 4f_7/2 = 84.0 eV. The instrumental resolution, as evaluated by the full width at half maximum of the Ag 3d_5/2 peak, was 1.0 eV. The samples were loosely placed into a grooved molybdenum sample holder. The analysis chamber during experiments was evacuated to better than 5·10⁻¹⁰mbar. The surface composition of the samples was obtained on the basis of the peak area intensities using the sensitivity factor approach and assuming homogeneous composition of the surface layer. Obtained spectra were processed with use of CasaXPS software. Binding energy corrections, background subtraction, and deconvolution of the spectra have been applied. The phase composition of the samples was determined by X-ray Diffraction (XRD) method with the PANalytical Empyrean X-ray diffractometer using a Cu Kα radiation (α = 1.5418Å) at room temperature.

Results and discussion

The comparison of the effect of the XRD spectra of the pristine and KOH-modified WG12 carbon is shown in Figure 1. There are no significant differences between the two spectra. No phases containing any potassium compounds can be detected using XRD method for the carbon treated with KOH.

Figure 1.

The effect of the KOH on the XRD spectra of the WG12 carbon. XRD: X-ray Diffraction.

The results of the Boehm’s titration are shown in Table 1. The treatment of the activated carbon with KOH resulted in a complete removal of carboxy and lactone groups. A decrease of the general content of the acidic groups was at the level of 9.35% roughly and a decrease of the general content of basic groups was 8.01%.

Table 1.

An average content of the functional groups (mmol/g).

Reactant	Group	WG-12	WG-12 15gKOH
Distilled H₂O	Active acidity	0.0000	0.0000
0.05 M NaHCO₃	The strongest carboxy groups, e.g. –COOH	0.0000	0.0000
0.05 M Na₂CO₃	Carboxy and lactone groups	0.5703	0.0000
0.05 M NaOH	General content of acidic groups	0.1155	0.1047
0.05 M HCl	General content of basic groups	0.3893	0.3581

The effect of the KOH treatment on the IR spectra is shown in Figure 2.

Figure 2.

An influence of the modification of carbon with KOH on the IR spectra.

Figure 3.

Decrease of the helium density with a KOH addition.

Comparing the IR spectrum of commercial coal WG12 and WG12-KOH15 after chemical modification we do not notice significant differences. In both spectra in the range of 3000–3750 cm⁻¹ wide band derived from the group –OH is visible. The intensity of this band is greater after modification with KOH.

The peaks in the range 2700–3000, 1250–1500, and 500–700 cm⁻¹ can be attributed to the C–H bending vibration and C–O stretching. The adsorption peak at 1630 cm⁻¹ corresponds to the band C=C.

The treatment of the carbon with potassium hydroxide caused a decrease of the helium density, resulting from some mineral impurities (ash) removal.

The chemical composition of the surface was studied by XPS. In Figure 4 survey XPS spectra acquired before and after KOH treatment are shown. On the surface of the sample before treatment, carbon, oxygen, and silicon atoms were identified, while after KOH treatment the XPS signal originating from silicon atoms disappeared. Atomic concentrations of carbon, oxygen, and silicon present on the surface of carbon WG12 are shown in Table 2.

Table 2.

Atomic concentration of carbon, oxygen, and silicon atoms on the surface of carbon WG12.

Sample	C	O	Si
Sample	at. %
WG12	90.1	6.9	2.9
WG12-KOH15	95.7	4.3	–

Figure 4.

Survey XPS spectra of the carbon WG12 before and after KOH treatment.

High-resolution XPS spectra give additional insight into the chemical state of the surface of the WG12 samples. The maximum of XPS Si 2p line is at 104.0 eV the position typical for silica (Chourasia, 2006). In Figure 5 the XPS spectrum of O 1s transition is shown. The deconvolution of the O 1s line acquired from the surface of carbon WG12 before KOH treatment is shown as shaded areas. The component of highest intensity is located at the binding energy of 533.6 eV. This position is characteristic for oxygen atoms in silicon oxide SiO₂ (Chourasia, 2006) and confirms the presence of silica on the surface of this sample.

Figure 5.

High-resolution XPS spectra of O 1s line before (thick solid line) and after (dashed line) KOH treatment. The components of O 1s line for the former case are depicted as shaded areas while for the latter case as thin solid lines.

After KOH treatment XPS signal of silicon disappears. The XPS O 1s spectrum collected after KOH treatment is apparently shifted to the low energy side (dashed line in Figure 5). The deconvolution of this spectrum is shown as thin lines. It is noteworthy that the component previously ascribed to the Si–O bonds, located at 533.6 eV, disappeared completely. The other components corresponding to the C–O bonds (at 532.5 eV) and C=O bonds (at 531.4 eV) are slightly shifted within model constraints. However, the ratio of these two components area is almost identical before and after KOH treatment. It is concluded that KOH treatment removes SiO₂ from the surface of carbon WG12. Simultaneously, the exposure of the surface of carbon WG12 to the KOH solution does not result in the formation of any new form of carbon–oxygen bonds. Before and after KOH treatment only hydroxyl (CO) and carbonyl (C=O) groups are detected on the surface.

The latter statement is supported by the analysis of high-resolution XPS C 1s spectra. A spectrum originating from carbon WG12 before KOH treatment is shown in Figure 6. The spectrum obtained from carbon WG12 after KOH treatment (not shown) is virtually identical. The envelope of the C 1s line is typical for the carbon black (Gardner et al., 1995) or other graphite-like specimens (Pełech et al., 2012). At the low-energy side there is a slim and intense peak with maximum at 284.4 eV which is accompanied by a long tail extending to the binding energy of about 293 eV. It was numerically deconvoluted into six components shown as thin lines below the envelope of XPS data. Considering components from the lowest binding energy one can attribute the given peak to the following chemical bindings of carbon atoms: position 284.4 eV (black line) corresponds essentially to carbon atoms located in graphitic rings; position 285.5 eV (brown line) is attributed to sp³ carbon atoms, bonded either with second carbon or with hydrogen atoms; position 286.4 eV (green line) is ascribed to a group of differently bonded carbon atoms linked to one atom of oxygen, i.e. the functional groups as C–O–C or C–OH; position 288.0 (pink line) corresponds to functional groups as C=O or O–C–O; position 289.8 eV (gray line) attributed to carbon atoms indicated by asterisk in the functional groups like C–O–C*=O or HO–C*=O; position 291.4 eV (blue line) attributed to shake-up structure caused by the π→ π* transition in the graphite rings (Dieckhoff et al., 1995). The binding energy assignments described above are based on the energy shifts given in Appendix E of the book of Briggs and Grant (2003).

Figure 6.

High-resolution spectrum of XPS C 1s line from KOH-treated carbon WG12.

The treatment of the commercial activated carbon with potassium hydroxide did not result in significant changes in porosity and specific surface area of the material. In the case of the untreated commercial carbon, the total pore volume V_p was equal to 0.50 cm³/g with 80% of pores occupied by micropores (V_mic = 0.40 m³/g) and the specific surface area was equal to 1128 m²/g. After the alkali treatment the specific surface area increased slightly to 1181 m²/g and the total pore volume and micropore volume to 0.56 and 0.42 m³/g, respectively.

The effect of KOH treatment on the porous structure of material is shown in Figure 7. The changes in porosity after KOH treatment are not very significant. The pores between 3 and 4.5 nm disappeared but also those between 1.3 and 2 nm. On the contrary, there are more pores between 2 and 3 nm.

Figure 7.

Effect of KOH treatment on the porous structure.

The addition of KOH resulted in an increase of carbon dioxide uptake measured using a volumetric method at 0℃ of 14% (from 3.16 to 3.67 mmol/g). With an increasing temperature the CO₂ uptake of the modified carbon dropped to 2.43 mmol/g at 25℃.

The results of TPD-CO₂ measurements are shown in Figure 8. The lines of desorption of CO₂ from the surface of WG12 and from the surface of the carbon modified with a solution of potassium hydroxide WG12-KOH15 correspond to the various rates of temperature increase.

Figure 8.

TPD-CO₂ after the adsorption of carbon dioxide at −30℃ on (a) carbon WG12 and (b) carbon modified with KOH WG12-KOH15.

For the sample of unmodified carbon two desorption peaks are visible—one with a maximum at 18.6℃ (temperature increase rate 25°/min), and a second one with a maximum at 119.9℃. The modification of carbon with potassium hydroxide caused a shift of the first maximum to higher temperatures (peak at 27.2℃, increase rate 25°/min), whereas the second peak disappeared.

An increase of the CO₂ adsorption temperature to 0℃ (Figure 9) caused a shift of the first maximum of the desorption peak to temperature of 60℃. The second maximum in the case of unmodified carbon appears at about 115℃, whereas in the case of KOH-modified carbon—at about 195℃.

Figure 9.

TPD-CO₂ after the adsorption of carbon dioxide at 0℃ on (a) carbon WG12 and (b) carbon modified with KOH WG12-KOH15.

The shift of maxima (at the same rate of temperature increase) into the direction of higher temperatures is connected with an increase of binding energy of carbon dioxide molecules to the surface.

The desorption characteristics after an adsorption of carbon dioxide at 20℃ (Figure 10) is similar to those at lower adsorption temperatures. The maxima of desorption peaks were shifted to higher temperatures, as it was in the previous case.

Figure 10.

TPD-CO₂ after the adsorption of carbon dioxide at 20℃ on (a) carbon WG12 and (b) carbon modified with KOH WG12-KOH15.

On the basis of the performed experiments the desorption energy of carbon dioxide from the surface was determined as well as the volume of CO₂ (Table 3). The energy E1 was determined on the basis of the desorption peak at lower temperature, E2 corresponds to a higher temperature. At the temperature of −30℃ the adsorption energy is equal to 25.8 and 26.9 kJ/mol on the unmodified WG12 i and modified WG12+KOH15, respectively. Taking into account the value of the adsorption energy, it can be stated that the adsorption of carbon dioxide on activated carbon has a physical character (physisorption). However, the value of adsorption energy 73.8 kJ/mol can be considered as a chemisorption. Then, it can be concluded that the adsorption of carbon dioxide on activated carbon has a mixed (physical/chemical) character and that two types of adsorption sites are present at the surface. It can also be concluded that on the unmodified carbon there are adsorption sites with higher bond strength. Adsorption at 0 and 20℃ occurs at adsorption sites with a bond strength of 46–58 and 54–75 kJ/mol, respectively. Modification of carbon with KOH resulted in higher CO₂ uptake at −30 and 0℃. At 20℃ the adsorbed volume is similar for both modified and unmodified carbon.

Table 3.

The effect of KOH treatment on energy of carbon dioxide adsorption.

Temperature adsorption (℃)	WG12			WG12-KOH15
Temperature adsorption (℃)	E1 (kJ/mol)	E2 (kJ/mol)	V_CO2 (cm³/g)	E1 (kJ/mol)	E2 (kJ/mol)	VCO₂ (cm³/g)
−30	25.8	73.8	0.878	26.9	–	3.030
0	46.5	53.9	0.272	45.4	57.0	0.405
20	58.6	75.5	0.240	58.4	64.2	0.216

Conclusions

The treatment of commercial carbon WG12 with KOH in the microwave-assisted hydrothermal reactor results in increased carbon dioxide uptake, even if the modified material was completely free of KOH (proved using XPS and XRD). According to the XPS studies, the elimination of silica from the surface of carbon WG12 occurred as well due to the KOH treatment.

Only hydroxyl (C–O) and carbonyl (C=O) groups are detected on the surface before and after KOH treatment. Only hydroxyl (C–O) and carbonyl (C=O) groups are detected on the surface before and after KOH treatment; however, the treatment does not affect the chemical state of oxygen atoms bound to the carbon.

The treatment with KOH is connected with an increased concentration of hydroxyl groups on the surface, demonstrated using IR spectroscopy.

According to Boehm’s analysis, the treatment of the activated carbon with KOH resulted in a complete removal of carboxy and lactone groups. A decrease of the general content of the acidic groups was at the level of 9.35% roughly, and a decrease of the general content of basic groups was 8.01%.

The adsorption of carbon dioxide on the commercial activated carbon can have both chemical and physical character and both types of adsorption sites are present at the carbon surface.

The general conclusion is that a treatment of activated commercial carbon with potassium hydroxide causes an increase of hydroxyl groups on the surface, resulting in an increased carbon dioxide uptake.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: The studies presented in the paper were performed within the framework of Project Contract No. Pol-Nor/237761/98/2014 funded from the Polish-Norwegian Research Programme operated by the National Centre for Research and Development under the Norwegian Financial Mechanism 2009–2014.

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