3.1. Effect of oxygen-blowing and temperature on surface properties
At wave number 3800 − 3200 cm− 1, the CBAC sample has a broad and weak peak and is identified as a functional group with small single bonds of phenol, alcohol, and hydroxyl compounds (Fig. 3, left panels). Small functional groups of triple bond C ≡ C acetylenic and nitrogen compound identified at 2500 − 2000 cm− 1. The double bonds of oxygen-containing functional groups as a carboxylate and ketone carbonyl compound were identified at 1700 − 1500 cm− 1. However, the other functional groups of double bonds of alkenyl and aryl C = C stretch and aromatic ring C = C-C stretch were also detected in this region. At 1000 − 600 cm− 1, a fingerprint region consists of inorganic ions, hetero organohalogen compounds, aromatic rings, methylene, cyclohexane ring vibrations, and hetero-oxy compounds of CHN. Overall, CBAC raw material had a weaker peak than an oxidized sample.
As the CBAC sample was heated in the oxygen atmosphere, a new active site on the AC surface was formed. At the sample of O-100, the amount of hydroxyl groups of O-H single bond increased, which was generated from breaking O-H from water at a temperature of 100°C (Zhang et al. 2022). Similarly, peak spectrums of the triple bonds were also enhanced with a sharper peak. At wave number 2300 − 2200 cm− 1, oxygen functional groups are identified as a nitrogen compound of cyanate O-CN and C-OCN stretch and isocyanate -N = C = O stretch (Nandiyanto et al. 2019). In contrast, the triple bond of alkyne compound C ≡ C was detected at a wave number of 2260 − 2190 cm− 1 in a small amount. The oxygen-containing functional group of oxidized samples decreased at a temperature of 400°C and reappeared at 1300 − 1100 cm− 1 with a sharper peak than the other samples. Oxygen groups in this region are identified as C-O stretch ether compounds and a fraction of C-O alcohol and phenol compounds. The oxidized samples of O-200, O-300, and O-500 indicated the increasing hydroxyl groups with a weaker peak than an oxidized sample of O-100.
Nevertheless, the peak spectrum of oxygen carbonyl groups C = O increased at 1700 − 1500 cm− 1. Notably, the peak shape of carboxyl at O-500 appeared sharper. It is caused by a part of the hydroxyl compound released from the C-H bond. The formation of oxygen functional groups is usually through the breakdown of methyl groups (Li et al. 2019). The re-formation of higher oxygen functional groups can potentially occur at temperatures above 500°C. The study (Suresh Kumar et al. 2019) reported that the decomposition of functional (-COOH) and alcohol (C-OH) groups increased carbonyl groups at temperatures 500–800°C. This study indicated that the oxygen blowing into coal-based activated carbon and temperature influenced the existence of oxygen sites on the AC surface. The formation of oxygen functional groups results in a change of surface properties to become more acidic. The conversion of CBAC functional groups is represented in Table 2.
The calcination stage was carried out to verify the presence of oxygen groups. The calcination under the N2 atmosphere prevents the continuation of oxidation reactions. Moreover, a calcination temperature of more than 800°C was proposed to study AC's transformation pores structure of AC. After the calcination process at 900°C (Fig. 3, right panels), the sample of C-O100 experienced a transformation of chemical surface structure. The peak of hydroxyl groups was slightly weaker compared to before calcination. However, the peak of oxygen bonded in nitrogen compounds was slightly increased. Hydroxyl groups of C-O200 were sharper compared to the O-200 sample.
Meanwhile, carbonyl-nitrogen compounds appeared to have a weaker shape. The decrease in all of the oxygen-containing functional groups occurred in the C-O300 sample. The increased peak spectrum appeared in the samples of C-O400 and C-O500.
Oxygen-containing hydroxyl groups increased at calcination of oxidized samples of C-O400 and C-O500, followed by oxygen in nitrogen compounds at 2500 − 2200 cm− 1. Oxygen-containing ether, oxy compound, and a slight alcohol and phenol C-O stretch appeared with a sharper peak at 1300 − 1100 cm− 1. At the sample of C-O500, oxygen of carbonyl O = H compound at 1700 − 1500 cm− 1 saw a more significant amount with a sharper peak. Overall, at more than 900°C, the existing active site on the surface reacted with the nitrogen molecules.
Simultaneously, the chemical structure of the AC surface created a new active site. Calcination under the N2 atmosphere produced a nitrogen site and decreased oxygen molecules (Dąbrowska et al. 2023). Therefore, the increased peak spectrum represented increased alkaline properties from nitrogen functional groups. Its appearance with the sharper peak in Fig. 3.
Table 2
Identified functional groups at each wave number of samples.
| Wave number cm− 1 | Functional groups |
| 3800 − 3600 | hydroxy compound |
| 3600 − 3500 | hydroxy compound, alcohol and phenol, OH stretch |
| 3500 − 3300 | Primary NH stretch and secondary > N-H stretch of aliphatic and aromatic amines |
| 2300 − 2200 | Nitrogen compounds of nitrile, cyanate C-OCN, isocyanate -OCN |
| 2260–2190 | Medial alkyne-disubstituted, C ≡ C |
| 1700 − 1500 | Alkenyl C = C stretch, aryl-substituted C = C |
| | C = C-C Aromatic ring stretch, carboxyl, amide, quinone |
| 1300 − 1100 | ether and oxy compound C-O stretch, alcohol and phenol C-O stretch |
| 1000 − 600 | inorganic ion, organohalogen compound, aromatic ring, methylene, cyclohexane ring vibrations, hetero-oxy compounds of CHN |
At 2500 − 2200 cm− 1 and 1750 − 1500 cm− 1. In the functional groups of AC, oxygen, and nitrogen have a role as main substituents. Respectively, the type of nitrogen functional groups was identified as aliphatic and aromatic cyanide, cyanate C-OCN and -OCN stretch and isocyanate N = C = O stretch, and amide functional groups of carboxylate compounds. The experiment shows that the calcination temperature at 900°C under a nitrogen atmosphere has been created and increased new sites of oxygen in the form of nitrogen compounds.
3.2. Pore structure
The isothermal adsorption on the AC surface is described as a maximum capacity adsorbent in the N2 layer. A horizontal plateau at a higher relative pressure exhibited a smaller pore size distribution. In Fig. 4 (left panels), the increased N2 adsorption shapes at P/Po less than 0.1 exhibited the presence of micropores. The adsorption shapes were slightly increased at P/Po between 0.1 to 0.8. Sharper adsorption occurred at P/Po more than 0.9, indicating the tendency of mesopores presence(Su et al. 2021). The results indicate that the volume enhancement of N2 adsorbed at low relative pressure tends to decrease with the increased temperature. It appeared from the adsorption line shape, which tended to form straight lines, indicating low porosity. This was also obtained by the micropore volume, which tended to decrease as temperature oxidation reached 300°C and re-increased at temperatures 400°C and 500°C (Fig. 4, right panels). The sample of O-300 had the lowest micropore volume with nearly a horizontal line at a low relative pressure of 0.1. Heat oxidation under an oxygen atmosphere reduced N2 adsorbed and mesopore-micropore volumes. However, the changes are not significant. Pores formation occurs through the gradual release of volatile matter.
In contrast, the VM content of CBAC was low (3.35%, Table 1). In addition, the high ash content of CBAC is also suggested as a resistor in the toleration of a pore (Monika et al. 2024). However, their study (Dong et al. 2021) stated that the transformation of the hydroxyl functional group can increase pore volume and specific surface area. Meanwhile, the carbonyl functional group (double bond) will decrease pore volume and surface area. This was consistent with the oxygen functional group in Fig. 3 (left panels); the spectrums of carbonyl groups C = O at wavenumber 1700 − 1500 cm-1 tended to increase with the increased temperature.
In Fig. 5 (left panels), the significant results occurred at a sample of C-O300, which experienced the enhancement of micropore volume at P/Po 0.1 with sharper line adsorption. The number of N2 adsorbed in the micropore also increased, representing the enhancement of micropore volume. Referring to Fig. 5b, the micropore volume of all samples is relatively constant except the sample of C-O400. The mesopore volume of C-O100 decreased as the temperature was increased to 900°C. Meanwhile, the mesopore volume of the other samples tended to increase. Referring to the oxygen functional groups at the calcinated oxidized sample in Fig. 3 (right panels), the increase of carboxyl-carbonyl led to an increase in the pore volume and specific surface area of the AC. It can be concluded that the formation of the pores is influenced by temperature and related to the formation of oxygen functional groups. The increased calcination temperature to 900°C evidenced the enhanced pores structure of AC. In contrast, the stability of micropore volume in sample C-O500 is suggested by the presence of high ash content, possibly increasing as heated at 900°C. High ash content can block most pores in the AC structure (Chen et al. 2021).
3.3. Performance of CO2 adsorption
The micropore size and the type of functional groups that bond with CO2 gas affect the performance of CO2 adsorption capacity. This is related to the co-existence of N2 gas molecules, which allowed overlapping bonds with CO2 gas because both of these gases have a similar kinetic diameter, with molecule sizes of 3.3 and 3.64 Å, respectively (Amagai et al. 2020). The CO2 adsorption capacity in the column was continuously measured as a time function or a breakthrough until the CO2 concentration neared the inlet gas concentration value. It was also presented that the gas has reached saturation level (García et al. 2011).
Figure 6 (left panels) shows a breakthrough of CBAC and oxidized samples. The Y-axis is a ratio of CO2 concentration at the output adsorption column (Co) with CO2 concentration at the inlet of the adsorption column (Ci), where the Ci value is constant of 15 vol.%. At the initial adsorption process, the C0/Ci value is zero; namely, all of the CO2 gas was adsorbed by activated carbon until, at a specific period, the C0/Ci value increased and finally reached the saturation point, when the Co value similar with the Ci value (C0/Ci = 1). The breakthrough was determined since CO2 gas flows into the adsorption column to the time CO2 gas was detected at the outlet. Generally, the adsorption curve is S-shaped. The Boltzmann equation obtains the Breakthrough curve. Boltzmann equation generally investigates the gas flow in porous media (Amagai et al. 2020). The cumulative percentage of CO2 adsorbed was calculated as the area above the curve, which is divided by the calculated area in total (Eq. 1).
Adsorption %, t =\({\int }_{0}^{t}1 dt-\underset{0}{\overset{t}{\int }}\frac{1}{1+K x {exp}\left(A x t\right))}dt\) (1)
Where t; residence time (min) for the adsorption process, K and A are constants of the Boltzmann equation. The values of P, V, R, and T are known. Subsequently, a mole of CO2 gas is calculated using the ideal Eq. 2.
N, mole = \(\frac{\text{P}\text{ x V}}{\text{R x T}}\) (2)
Where P is the pressure in the adsorption process (1 atm), V is the volume of CO2 gas (0.015 L), R is the value of gas constant (0,082 atm L/K), and T is room temperature (298°K). The mass of CO2 gas during adsorption is calculated with Eq. 3.
CO2 mass = \(\text{n x }\text{Mr}\text{ x t}\) (3)
Where, Mr; molecular weight of CO2 (44 g/mole). Determination of a value of adsorption capacity using Eq. 4
CO2 adsorption (mg/g) = Adsorption% x CO2 mass (4)
Coal-activated carbon is an adsorbent with isotherm type 2 (Weidenthaler 2011); the adsorbate is adsorbed in one layer (monolayer). The adsorption capacity of CO2 on activated carbon is limited by the availability of the adsorption layer, which consists of one layer until it reaches saturation point when the adsorbent encounters the maximum capacities (Hossain et al. 2019). Results in Fig. 6 (left panels) show that the required time to reach a breakthrough of CBAC is 25 min, while for O-100, O-200, O-300, O-400, and O-500 for 20, 20, 15, 15, and 15 min, respectively. The decreased breakthrough time showed that the adsorbent in the adsorption column had started with the gas filling. Thus, the adsorbent cannot accommodate the CO2 gas passed through in the column. Meanwhile, the calcination of oxidized samples has a breakthrough with a longer time of 10 min compared to the oxidized sample (Fig. 6, right panels). Breakthrough times of C-O100, C-O200, C-O300, C-O400, and C-O500 are 30, 30, 25, 25, and 25 min, respectively.
The enhancement of breakthrough time is in line with the adsorption capacity of CO2 gas in Fig. 7. Compared with the oxidized sample, CO2 gas adsorption of the calcination samples shows increased capacities. The highest adsorption capacity was obtained at C-O100. The increased oxidation and calcination temperatures produced the decreased adsorption capacity. The data deviation occurred at sample O-300, where the micropore volume is the lowest value (Fig. 4, right panels). However, the adsorption capacity is relatively high. Referring to the FTIR analysis in Fig. 3, part of the hydroxyl changed to the carbonyl functional group. Likewise, in the C-O300 sample, the presence of nitrogen gas produced a new site of the nitrogen functional group. Thereby, the surface had more fundamental properties. It was proved that oxygen-blowing and temperature affect the formation of the chemical surface and pore structure of CBAC products. However, the formation was not significant because the development of pore structure and surface chemical properties are related to the VM released. In contrast, the VM of CBAC products is very low.