Green Self-Activating Synthesis System for Porous Carbons; Celery Biomass Wastes as a Typical Case for CO2 Uptake with Kinetic, Equilibrium and Thermodynamic Studies

A green self-activating synthesis system (SASS) has been introduced for porous carbons. In the presented system, there is no external support for the activation process, and the activating agents are the circulating gases released during the pyrolysis treatment. As a typical case, this system was used for the synthesis of hierarchical porous carbons from celery wastes in hydroponic greenhouses. Based on the adsorption-desorption results, the optimal porous carbons were synthesized at 700 °C, providing a surface area as high as 1126 m 2 g -1 and micropore volume of approximately 0.7 cm 3 g -1 . X-ray photoelectron spectroscopy indicated the presence of graphitic nitrogen in the synthesized porous carbon structure. The synthesized porous carbons were applied as an adsorbent for CO 2 capture. CO 2 adsorption was performed at low and high pressures at various temperatures. Under low pressures (0-1 bar), the synthesized carbons adsorbed 5 mmolg -1 at 0 °C and 2.03 mmolg -1 at 25 °C. The adsorption capacity of the synthesized carbon at 25 °C and a relatively high pressure of 9.5 bar was 9.57 mmolg -1 . Based on the thermodynamic and kinetic models, it was clarified that the adsorption process can be regarded as physisorption with an adsorption enthalpy of 23.2 kJ.mol -1 . Additionally, the fractional-order kinetic model was found to be the best match in the kinetic curves. The synthesis system described herein represents a promising strategy for producing green porous carbon from various waste organic precursors.


Introduction
The increase in world population and the industrialization of many countries leads to an increase in demand for energy, which requires the combustion of fossil fuels to supply this energy.
Despite many efforts to find a suitable alternative, these fuels are still cost-effective, convenient, and viable energy [1]. This has led to the production of excess carbon dioxide (CO2) in the atmosphere and enhanced its concentration up to 450 ppm. It is anticipated that atmospheric CO2 concentrations will increase to 950 ppm in the near future [2]. Rising carbon dioxide emissions as a greenhouse gas from widespread sources are one of the most important worries of recent decades. Therefore, to mitigate or separate this polar gas from other nonpolar gases, exclusive strategies must be used to reduce greenhouse effects, acid rain formation, and climate change. In addition, the presence of CO2 in gaseous streams such as flue gases reduces the heating value and destroys transmission pipelines [3,4]. The carbon capture and storage (CCS) approach consists of a set of technologies for the uptake of CO2 emissions from industries, followed by compression, transportation, and storage to execute a crucial function to decarbonize industry and deliver low carbon heat [5,6]. Amine scrubbing, ionic liquid absorption, membrane separation, and solid adsorbent adsorption are the most common CO2 capturing approaches [7]. Traditional CO2 uptake techniques employ amine solutions, which have drawbacks including corrosion, a propensity for severe foaming, costly regeneration, and solid suspension, which reduce CO2 solvent loading.
Owing to its low disposal cost, easy accessibility, thermal stability/conductivity, high specific surface area, insensitivity to moisture, ability to tune porosity, high adsorption, and easy regeneration due to weak interactions, porous carbons have attracted much attention as a promising CO2 adsorbent for large-scale applications [6,16]. The physisorption of CO2 is influenced by intrinsic properties such as acidity-basicity on the carbon surface, hydrophobicity/hydrophilicity of the porous carbons, specific surface area, distribution and order of the porosities, the isosteric heat of adsorption value (Qst), the presence of structural elements such as nitrogen, and the presence of micropores smaller than 1 nm [1,[17][18][19][20][21]. Furthermore, extrinsic parameters such as modifying the surface by heteroatom-doping, increasing pressure, and decreasing temperature play an important role in raising the quantity of adsorption [1,22]. Meanwhile, porous carbon is suitable to moderate adsorption selectivity and separation for CO2 over N2 and CH4 at low pressures (0.15 bar), owing to its higher quadrupole moment and acidic molecules unlike others [6,14,19]. Porous carbons are traditionally obtained by the pyrolysis of a variety of carbon-rich materials as precursors that can be artificial such as titanium carbide [23], sodium alginate [24], and melamine/polyaniline [25] or natural (biomass, cheap agricultural, and forestry wastes) such as rice husk [26], cornstalk [27], and tobacco rods [28] through different synthesis methods [16,29].
The selection of an appropriate precursor for pyrolysis of porous carbon is important for the final product's porous texture, which is determined by several factors, including scalability, cost, availability, nonhazardous to nature, a high percentage of carbon in the precursor with low ash, and the presence of enough volatiles to produce porosity [17,30,31]. Due to the synthesis of heterogeneous micro-meso porous carbons, biomass is advantageous because it incorporates heterogeneous micro/nanostructures and morphology, leading to an oversized quantity of porosity and tunable pore size, resulting in high CO2 sorption [32].
There are two widespread classical route methods to activate/pyrolyze a variety of carbonaceous biomass, generally known as chemical and physical activity that may consist of one (pyrolyzation of raw materials and activator) or two-step processes (carbonization and activation) [29,31,33]. Commercial porous carbons generated by physical gasification (air, N2, O2, NH3, CO2, or steam) have a limited CO2 adsorption capability at ambient pressure, which makes them unsuitable for adsorption applications. As a result, microporosities must be introduced in such materials, resulting in a considerable increase in specific surface area. Because chemical activation may yield activated carbons with extraordinarily large surface areas, numerous publications have claimed that KOH activation produces porous carbons with significant CO2 adsorption capability [16,34,35]. Even though physical activation is less expensive and has a lower environmental impact, chemical activation is preferred owing to the inclination to be mesoporous and tunable porosities of the final material. Nevertheless, chemical activation usually suffers from toxic gases generated during pyrolysis, process complexity, and corrosion of the devices and is not ecofriendly [16,[36][37][38]. In addition to the methods mentioned above, there are other methods for the synthesis of porous carbon such as molten salt etching, decomposable salt etching, template methods, and self-template methods that can be used for specific applications [39].
Due to the aforementioned drawbacks, a novel self-activation technique for porous carbon synthesis has been developed in recent years. Self-activation is a technique that uses the gases generated during the pyrolysis of biomass to activate the converted carbon, which saves money on activating chemicals and has a lower environmental effect than traditional activation methods [40].
In a previous study, a one-step self-activation technique under a N2 atmosphere was used to create N-doped mesopore-dominant carbon materials using intrinsic hydroxyapatites as natural templates. As discussed, this method is not considered self-activation due to the use of inert gases and templates during the process [41,42]. Moreover, few reports have focused on the preparation of porous carbons derived from biomass. Recently, Bommier et al. reported that cellulose-derived activated carbon under an argon atmosphere at 1100 °C for 2 h possessed specific surface areas, ranging from 98 m 2 g -1 to values as high as 2600 m 2 g -1 for use in an electrochemical capacitor [43].
Celery, as carbonaceous biomass that belongs to the Apiaceae family, is known for its wealthy cellulose. Owing to the high sturdiness inside the surroundings, celery can be cultivated broadly in most regions of the world with an excessive tonnage and ensure a lasting large-scale supply. It is frequently used in the food industry, resulting in a large number of byproducts, since nonedible and malformed sections of the celery must be removed to meet customer needs [36,44].
Zhang et al. reported in a previous study that celery biomass waste is easily available for generating biochars that could be effective at removing heavy metal pollution [45]. Furthermore, Mohebali et al. used celery residue modified with H2SO4 as a low-cost sorbent for the removal of methylene blue cationic dye [46].
Herein, a green, facile, low-cost, and efficient self-activating system has been introduced for the preparation of porous (activated) carbons. In this system, the circulating gases produced during the pyrolysis treatment of porous carbon precursors are the main activating agent of the precursors. Chopped celery wastes have been used as a typical precursor to accomplish the activation process. The morphology, structure, and mechanism of porosity formation at different temperatures as well as the chemical constitution of the resulting porous carbons, were characterized by field emission scanning electron and high-resolution transmission electron microscopy, Raman and Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (DTA-TG) and N2 adsorption-desorption analysis.The CO2 adsorption behavior of the synthesized porous carbons was investigated at low (0-1 bar) and high pressures (2-9.5 bar). The results showed that the synthesized porous carbon has a high maximum CO2 adsorption capacity of 5 mmolg -1 at 1 bar, 0 °C, and 9.57 mmolg -1 at 10 bar, and 25 °C. Eventually, based on the CO2 adsorption behavior of the synthesized porous carbon, the kinetics and thermodynamics of the prepared adsorbent were determined.

Materials
All chemicals used in this study were of analytical grade and employed as received without any further purification. Celery was collected from the waste of a hydroponic greenhouse (Tehran, Iran). To remove visible impurities from this biomass, it was washed several times before activation. Hydrochloric acid (37%) was purchased from Dr. Mojallali™ Company (Tehran, Iran), and CO2 (99.999%) was supplied by Arman Gas Company (Tehran, Iran).

Self-activating system for preparation of porous carbon
The introduced self-activating system consists of a sealed chamber, a tubular electric furnace an air pump, and finally a condenser that is connected through pipes. Due to the sealing of the system, no gas is transmitted to the environment during the synthesis process. The gases created throughout the pyrolyzation of the feeding biomass circulate in a closed-loop channel with the assistance of an air pump positioned at the system and activation is combined into one step.
The activation process is carried out with the aid of these gases. Therefore, there is no need for another activating agent. A condenser is also installed along the route of the gases, collecting a portion of the exhaust gases, and transforming them into liquid.
For the preparation of porous carbons, in the first step, collected celery was dried overnight at 80 °C to remove moisture. Afterward, it was vigorously ground into fine powders to the optimum micron size in a ball mill. 10 g of the dried celery powder was directly pyrolyzed in the selfactivating system at 300, 600, 700 and 900 °C and maintained at final temperatures for 3 h. After cooling to room temperature, the obtained products were washed with 1 M HCl to remove the remaining impurities and then with deionized water until neutral pH. Finally, the green celeryderived porous carbons, hereafter named GC-PCs, were dried at 85 °C for 12 h.
The initial and final product yields were calculated by equations (1) and (2): where Y demonstrates the yield of porous carbon after pyrolysis and Yac-pick displays the yield of porous carbon after acid pickling. Winitial and Wfinal specify the weight of the initial biomass weight and final weight of porous carbon and Wac-pick represents the final weight of porous carbon after pickling. GC-PCs derived at different temperatures were named C-T where T is 300, 600, 700, and 900 (the pyrolyzation temperatures). A schematic of the green self-activating synthesis system and the process of preparing porous carbons for CO2 capture is shown in Fig. 1. Fig. 1. Schema representing the production of porous carbon from celery in a self-activating system and its application for CO2 capture

Characterization
The physicochemical parameters of the synthesized samples were investigated quantitatively and qualitatively using various characterization methods. Micromeritics ASAP2020 (US) adsorption analyzers were employed to measure the N2 adsorption-desorption isotherms at −196 °C. Before performing the adsorption-desorption analyses, samples were degassed under dynamic vacuum conditions to constant weight at a temperature of 150 °C for 6 h. The total surface area was calculated using the multipoint Brunauer-Emmett-Teller (BET) technique. The Barrett-Joyner-Halenda (BJH) technique is equipped to calculate the mesopore surface area, pore volume, and pore diameter. The t-method is used to calculate the micropore surface area and pore volume, whereas the Dubinin-Astakhov (DA) technique is used to determine the micropore diameter.

CO2 adsorption-desorption
The CO2 uptake behavior of the prepared porous carbons was investigated at low and high pressures. Low-pressure CO2 adsorption isotherms of the synthesized porous carbon were measured at 0 and 25 o C on an ASAP 2020 (US) Micromeritics between 0 and 1 bar. To evaluate the high-pressure CO2 capturing behavior of the synthesized porous carbon, a fixed bed adsorption reactor was constructed to evaluate the CO2 adsorption-desorption performance of GC-PC adsorbents. Pure CO2 and N2 were utilized as feeds in the studies to investigate the GC-PC surface adsorption ability. 1g of GC-AC was loaded in the cylindrical reactor of the instrument, and the reactor was then fully sealed. To assess the amount of gas adsorption, N2 was introduced into the device compartment as a purge gas. To eliminate moisture and all preadsorbed CO2, GC-PC was preheated under pure N2 gas at 110 °C for 30 minutes before being placed under vacuum for another 40 minutes. Experiments were conducted for 60 minutes at temperatures of (25, 35, and 45 °C) and pressures of (2-9.5 bar). The CO2 pressure and temperature were stabilized thanks to the mixing tank in the passage, and the stable gas was then delivered to the adsorbent reactor. The reactor's heat is provided by electric heat tracing, and the temperature and pressure fluctuations of CO2 are recorded in real-time by the computer.

Characterization
The differential thermal and thermogravimetric analysis method (DTA/TG) was accomplished in air and argon atmospheres to follow the mass change of celery from 25 °C to 1000 °C, (Fig. 2). This analysis provides crucial details about the decomposition behavior that occurs as the temperature rises, allowing the mechanism and temperatures of the reaction to be determined [47][48][49]. As shown in Fig compounds. Furthermore, an exothermic peak at ~450 °C corresponds to carbon matrix combustion as well as cellulose and lignin decomposition. A similar exothermic peak can be found in Fig.2

(b) from which it can be inferred that celery combustion takes place in an argon (inert)
atmosphere. This means that there is enough oxygen in the celery constituents from which the combustion reaction can take place [31,50]. No. . This is an indicator of amorphous graphitic carbon which is possibly formed during carbonization. The lower proportion of crystallinity in these two cases may be attributed to the broader intensities for the peak occurring at 22-23 o . Broadening of these peaks implies a very small crystallite size and hence the development of a nanostructured skeleton. These results indicate that when the temperature rises, the porosity of graphitic carbons gradually increases until the structure collapses [53][54][55][56]. The (002) peak moves towards higher diffraction angles as the activation temperature rises from 300 to 900 °C. The change in peak location indicates a decrease in the interlayer distance, and the increased intensity correlates to greater translational ordering in porous carbons pyrolyzed at more elevated temperatures [51]. The lack of any sharp peak in the XRD plots for acid-washed samples indicates that the synthesized compounds are to some extent amorphous in nature, and it also confirms that there are no left-over inorganic residues after acid pickling [57,58].
Raman analysis was employed to investigate the crystallinity and graphitization level of the synthesized porous carbons. The Raman spectroscopy results of C-700 and C-900 (See Fig. 3 (b)) show D-band peaks at approximately 1345 cm -1 and G-band peaks at around 1603 cm -1 , indicating the presence of a carbon lattice defect with a distorted structure (sp3) and some orderlayered graphite with vibration sp2 hybridized carbon atoms, respectively. Moreover, the presence of a 2D-band around 2873 (cm -1 ) is a sign of a local graphene-like structure [16,[59][60][61][62][63]. The integrated intensity ratio of the D and G bands (Id to Ig) can be used to evaluate the surface graphitization, higher of which shows lower graphitization level. For C-700 and C-900, these ratios are 0.95 and 0.98, respectively, which is sign of an intermediate amorphous-crystalline structure.
According to the results, the quantity of graphitization in these two samples is very little different from each other. C-900 is carbonized at a higher temperature and has a higher Id to Ig ratio than C-700 which is compatible with the XRD results. While a higher temperature was appropriate for the graphitization stage, high temperatures in combination with a reactable (corroding) atmosphere which is the case in the introduced system can destroy a portion of the synthesized carbon structure and reduce the graphitization level [14,[64][65][66].
Nitrogen adsorption-desorption analysis results of the pyrolyzed celery wastes in Figs. 3(ce) and Table 1, demonstrate the activation strength of the introduced self-activating system. The specific surface area, mean pore diameter, and total pore volume of the celery wastes pyrolyzed at 300 and 600 o C are 1.7 m 2 .g -1 , 53.5 nm, 0.02 cm 3 .g -1 , and 9.75 m 2 .g -1 , 11.7 nm, 0.03 cm 3 .g -1 , respectively, which confirms that these two pyrolyzed celery wastes are not porous in nature.
Conversely, as the activation temperatures increase, the mesopore volumes of porous carbons increase as seen by the much higher N2 adsorption. From the DTA-TG results of celery wastes in an argon atmosphere, Fig. 2 (b), it can be deduced that the combustion of the carbonaceous components of celery results in mass losses of approximately 30% and 70% at 300 and 600 o C, respectively. This accompanies the production of gases such as CO2, CO, H2, and H2O. Based on the following reactions, all of these gases have activating potential on the carbon precursors [61,67].
The specific surface area, mean pore diameter, and pore volume of the synthesized carbons at 300 and 600 o C in Table 1 confirm that at these pyrolyzation temperatures, the gaseous activating agents have no significant effect on the activation of celery biomass. On the other hand, celery wastes pyrolyzed at 700 and 900 o C possess a specific surface area, mean pore diameter, and total pore volume of 1126 m 2 .g -1 , 2.5 nm, 0.69 cm 3 .g -1 and 1169 m 2 .g -1 , 2.7 nm, 0.84 cm 3 .g -1 , respectively. Therefore, these temperatures are high enough for the activation of celery. Based on the TG results, at 700 o C about 75%, 5% greater than that at 600 o C, and at 900 o C about 90% of the initial weight of the biomass was lost. Therefore, combustion of the carbon precursor which takes place up to 600 o C cannot have a direct effect on the activation of the biomass. When the activation temperature increases (≥700 o C) the standard Gibbs free energy change of reactions (4) and (7) becomes negative. Therefore, CO2 and H2O, which are the combustion reaction products of celery biomass can react with the carbon atoms and, in fact, etch the surface of carbon layers.
This etching process results in porosity formation and increases in the specific surface area. During self-activation, two situations may occur: (1) pore expansion and (2) pore combination. The narrow boundaries between the pores are removed, allowing the pores to merge. The specific surface area is enhanced by pore expansion during the self-activation process, but at high temperatures (≥700 o C) it is unchanged due to pore combination. At higher temperatures, the pore combination mechanism seems to dominate the whole process. As a result, as shown in the XRD plots, the order of the structure does not increase with increasing temperature, contrary to expectations [68][69][70]. It can thermodynamically be concluded that temperatures lower than about 705 o C are not high enough to effectively initiate the aforementioned reactions of carbons with the combustion product gases. Pyrolyzation at ≥850 o C is high enough to effectively initiate reaction (6) [70]. In this reaction, CO2 reacts with H2, which is the product of carbonization treatment and reaction (4). On the other hand, H2O which is one of the products of this reaction can be helpful for the activation process in reaction (4). The dual effect (both positive and negative) can be regarded as the reason for the approximately same activation response of C-700 and C-900. This can also be inferred from the TG result of celery in the argon atmosphere (See Fig. 2 (b)). Circulation and transfer of the gases to atmospheric temperature (approximately 25 o C) hinders the increase in the pressure of the hot reaction chamber. This, in turn, prevents the stopping of the activation reactions, especially reactions (4), (5), and (7) [68,70]. Another important beneficial role of the circulation process is the possibility of saving the large amounts of the activation gases produced during the carbonization process at low temperatures (T≤700 o C). The amounts of the activating gases can be inferred from the TG result of celery in the argon atmosphere, Fig.2 (b). These gases can participate in the activation process at T≥700 o C. In conventional self-activating processes, the active gases produced at low temperatures are exhaust gases [36,43,59]. Therefore, they do not have a considerable role in the activation process but are also harmful to the environment. Based on the BJH and MP plot results in Fig.3 (d, e), C-700 has mesopore and micropore volumes of 0.27 cm 3 g -1 and 0.42 cm 3 g -1 , and C-900 has mesopore and micropore volumes of 0.41 cm 3 g -1 and 0.43 cm 3 g -1 , respectively. Moreover, based on BJH pore size distributions, porous carbons are composed of microspores (1-2 nm), small mesopores (2-5 nm), and large mesopores-macropores (10-60 nm).
Therefore, both of these porous carbons can be regarded as hierarchical [71,72].  In addition to the carbon atoms, intrinsic heteroatoms, such as nitrogen and oxygen which are normally the constituents of the initial structure of the precursors can affect and generally and alcohol are responsible for the peaks at 1467 and 1095 (cm -1 ), respectively [49,52,78]. As the pyrolyzation temperatures increased, the total intensity of the peaks decreased and the hydrophobic properties of the synthesized carbons increased. Moreover, the number of acidic functional groups in the carbon decreased while the number of alkaline surface groups increased. Due to the relatively high pyrolyzation temperatures, many functional groups decompose to form volatile CO and CO2 and move away from the structure. This is caused by the heterogeneity of acidic groups at high temperatures. Basic types, on the other hand, increase as the temperature rises. These groups can form during the cooling process. As a result, CO2 and CO formation in porous carbons (C-700, C-900) was confirmed by the detection of minor peaks at around 1900-2300 (cm -1 ) in the FTIR spectra [49,52,79]. The pyrolysis temperature of 900 o C used for C-900 degraded the functional groups that were originally comprised within the raw celery biomass material.
Therefore, the bonds of carboxyl, alcohol, and ether groups that formed the lignocellulosic structure were fractured. This is due to the elimination of oxygen-containing functional groups and the high degree of graphitization at high temperatures [77]. On the other hand, the surface of C-700 synthesized carbons is highly aromatic and contains many oxygenated functional groups such as phenolic and carboxylic acid groups, which add a negative charge to the surface. All of these functional groups can increase the CO2 adsorption capacities of these porous synthesized carbons [55]. X-ray photoelectron spectroscopy (XPS) analysis was used to determine the chemical bonding states, elemental compositions, and atomic percentage of C-700. The carbon, nitrogen, and oxygen peaks and quantities can be seen in Fig.4 (b-e) and Table 2, respectively. From the survey analysis in Fig.4 (b) and Table 2 [55,81]. Both of these forms of oxygenated groups give a negative charge or basic character to the surface of porous carbons. This helps to anchor mildly acidic CO2 molecules by acid-base interactions [55]. For N1 s, [Fig.4 (e)], there is one detectable graphitic nitrogen (N-Q) bond centered at 402 eV [19,82].  two adjacent planes can be detected in Fig. 5 (e) and the yellow square in Fig. 5 (f), which is to some extent larger than that of the bulk graphite planes. This can be attributed to the presence of functional groups and nitrogen and oxygen-doped atoms in the porous carbon structure [83]. The edges of these graphitic layers are distinctly crystalline, and the crystallinity in-plane was fine, which confirmed the Raman spectroscopy results in Fig. 3 (b). Another characteristic of the graphitic layers is that they appear to adhere to the surface of porous carbon, creating an ultrathin conductive layer on the surface of the disordered carbon matrix. Besides, the amorphouscrystalline intermediate structure in Fig. 5 (f) verifies the formation of a turbostratic carbon structure as claimed in the X-ray diffraction pattern analysis of this synthesized carbon, Fig.3 (a).

CO2 adsorption performance
The CO2 adsorption efficiency of C-700 was obtained under 1 bar at 0 °C and 25 °C, respectively, as shown in Fig.6. The CO2 adsorption capacity at 1 bar was 5 mmolg -1 at 0 °C and d e 24 2.03 mmolg -1 at 25 °C, which is comparable to that of other biomass-derived porous carbons. This sample was chosen for adsorption testing based on the adsorption-desorption findings, which showed a higher microporous surface area (1012.61 m 2 . g -1 ) than C-900. The non-activated samples (C-300, 600) were extremely nonporous and had a very low specific surface area, so CO2 adsorption was not performed on them. CO2 uptake increases with increasing pressure without indicating adsorption saturation at both temperatures, indicating an adsorbents can sustain increased CO2 uptake at higher pressures. The overall output pattern revealed that higher loadings occur at lower temperatures, indicating that the exothermic nature physisorption process.
Moreover, isotherms display almost no adsorption-desorption hysteresis, indicating that this adsorption mechanism is physically invertible. The appearance of oxygen-nitrogen-containing groups on carbon surfaces (without any external source) and micropores, which may increase the carbon materials affinity for CO2 due to acid-base interactions and pole-pole interactions, are the key factor determinants of CO2 adsorption by porous materials [57,80,[84][85][86]. Physisorption mechanism created by van der Waals and quadrupole nature of the CO2 molecule interactions between the gas and the surface molecules at the walls of the carbon pores. Owing to this mechanism, interaction is so weak and regeneration does not necessitate additional energy [67,87].

The effect of temperature on CO2 adsorption
Another variable that has a significant impact on CO2 adsorption capacity is temperature.
The C-700 behavior and the quantity of CO2 adsorption capability are revealed in Fig. 7. Figure 7 shows the CO2 adsorption on GC-PC as determined using an experimental setup over 75 minutes at various isotherm temperatures. The observed CO2 adsorption capacity findings for GC-PC in the temperature range of 25 °C -55 °C show that the decrease in adsorption temperature can enhance CO2 adsorption capacity at lower temperatures. Due to the Boltzmann equation, the increase in interaction is due to a rise in the kinetic energy of the solid/gas molecules engaged in the adsorption process, which generates increased molecular interaction and decreases the efficient adsorption surface. At high temperatures, the interactions between the sorbent and the sorbate weaken, and repulsion becomes more desirable, resulting in a shift in equilibrium in the opposite direction of the adsorption process. As the adsorption temperature rises, molecular activity rises as well, intensifying the struggle for limited adsorption sites. As a result, the repulsions between molecules increase, resulting in a decrease in the quantity of adsorption. Therefore, the lowest

Kinetic modeling
To assess their adsorption performance and comprehend the total mass transfer in CO2 capture, kinetic studies of adsorption on the produced porous carbon are necessary. Furthermore, adsorption kinetics are required for the logical design and modeling of gas-treatment facilities.
Moreover, to estimate the capacity and rate of CO2 adsorption in adsorbents, kinetic models can be used. As a result, research into CO2 adsorption kinetics is critical. Micropores, mesopores, the surface area of the adsorbent, and the form of different kinetics all play a key role in gas adsorption by porous materials. The physical and chemical structure of the adsorbent also affects the mechanism, according to a review and study of the uptake kinetics [91]. We explored different theoretical kinetic models for the kinetic study, including pseudo-first-order, pseudo-second-order, Elovich, and fractional order, models (Eqs. 8-11).
where qe and qt (mmol.g -1 ) are the adsorptive capabilities at equilibrium and at time t (minutes), respectively, and kf and ks are the rate constants (1.min -1 ). The rate constant for the fractional-order is kn, whereas the model constants are m and n. Reversible adsorption with equilibrium between the gas and solid surface is depicted in the pseudo-first-order kinetic model based on the chemisorption process as the adsorption controlling factor, whereas the pseudo-second-order kinetic model was based on the hypothesis that chemical adsorption was the rate-controlling step.
The fractional-order kinetic model represents the complexity of the reaction mechanism that could involve more than one reaction pathway. This means that this model is capable of calculating the CO2 capture occurring by both physisorption and chemisorption mechanism [92][93][94]. Since predicting kinetic parameters is complex, a typical method is to adjust experimental data to a series of models and then choose the best model [95]. The kinetic parameters are mentioned in Table 3, bar. This model is based on the assumption that the rate of sorption is influenced by the n th power of the driving force, and the m th power of the sorption time. The best CO2 adsorption kinetic model was also fitted to the experimental data, and the GC-PC kinetic curves are presented in Fig. 8.

Equilibrium adsorption isotherms
The adsorption mechanism and expression of surface attributes throughout the adsorption process may be predicted using an adsorption isotherm analysis, which is highly essential.
Adsorption isotherms are essential in improving the use of carbon as an adsorber because they represent how adsorbates interact with porous carbon. In Fig. 9, the CO2 adsorption isotherms were isotherm also describes adsorption on heterogeneous surfaces, albeit with the assumption that the adsorption heat of the molecules in the layer drops linearly rather than logarithmically with coverage [96]. Table 4 shows the experimental results and related R 2 correlation coefficients for all coefficients of isotherm parameter models. The adsorption isotherm formula calculated from Eq.
= ( ) + ( ) (15) where qe is the value of CO2 adsorption capacity (mmolg -1 ), qm is the max adsorption value of CO2 (mmolg -1 ), P is the equilibrium pressure (bar), and KF is the that the sorbent surface is homogeneous; 5) there is no interaction among molecules adsorbed on neighboring sites; and 6) the adsorption system is in dynamic equilibrium, with a sorption rate equal to the desorption rate [97]. Furthermore, two isotherm models developed by D-R and Temkin have useful data allocated to the energy parameters, where Ꞷ is the mean adsorption free energy and bT is the heat of adsorption. The normal physisorption of CO2 adsorption is shown by average λ values in the 1-2 (kJmol -1 ) range [93]. The Freundlich constant, n, in the range of 1 to 2, illustrates the desirability of physisorption based on the findings of Table 4. According to the results, the adsorption process is multi-layer, with CO2 adsorbed and penetrated in the surface and inner layers of C-700. According to the nonlinear R 2 values derived in Table 4 and by the nonlinear regression method, Langmuir > Freundlich > D-R > Temkin was the order of effectiveness of the stated isotherms in the explanation and prediction of the adsorption behavior [89,98]. Fast adsorption kinetics are one of the most significant features anticipated in a good sorbent, because the effectiveness of a sorbent in dynamic processes and its capacity to endure huge sorbate fluxes are linked to its rate of sorption. Indeed, even if the equilibrium adsorption capacity is quite high, the adsorbent cannot be employed in industrial applications if the adsorption rate is too low.

Thermodynamic modeling
Aside from the isotherm parameters, thermodynamic parameters such as the standard Gibbs free energy change (ΔG 0 ), the standard enthalpy change (ΔH 0 ), and the standard entropy change (ΔS 0 ) can be used to determine the nature of adsorption (i.e. the nature of the sorbentsorbate interactions), whether physisorption and chemisorption. Thermodynamic data only describe a system's final state (i.e., its equilibrium adsorption capacity), whereas kinetics are concerned with how the system changes over time, with a focus on the rates of change. The laws of thermodynamics are used to calculate the mentioned parameters using the following equations.
The adsorption enthalpy was evaluated using the Van't Hoff equation, Eq. 14, and from the q [ mg/g ] unification of two equations, Eq. 16 and Eq. 19 [99]: where R is the gas constant [8.314 (J.mol -1 K -1 )], T is the absolute temperature (°K), enthalpy (ΔH 0 ) is the slope, and entropy (ΔS 0 ) is the intercept that they are obtained from the plotting of lnkd against 1/T. Equations 12 and 13 were used to calculate the standard Gibbs free energy change of adsorption [93]. Table 5 shows the results of the estimated CO2 thermodynamic parameters. In terms of ΔH 0 , a negative or positive value indicates whether a process is exothermic or endothermic. Since the total energy adsorbed in bond breaking is less than the total energy released in bond formation between gas molecules and the surface of the sorbent in the negative form, the process is characterized by a release of energy in the form of heat to its surroundings. In the positive situation, however, the process is distinguished by the adsorption of energy in the form of heat from its surroundings. The physisorption process was represented by a value of less than 20 (kJ.mol -1 ), whereas the chemisorption process was represented by a value of more than 40 (kJ.mol -1 ) [99]. ΔS 0 is a representation of the randomized and organized gas-solid interfaces, with ΔS 0 > 0 representing more randomness and ΔS 0 < 0 representing less randomness [90]. The Van't Hoff plot of the equilibrium constant of GC-PC adsorbent used for the estimation of ΔS 0 and ΔH 0 of the CO2 adsorption reaction in the temperature range from 298 °K to 328 °K is plotted in Fig. 10. The negative values of Gibbs free energy change (ΔG 0 ) and (ΔS 0 ), indicates that the adsorption process is exothermic and spontaneous [98]. Figure 11 depicts the percentage changes in CO2 adsorption as a function of temperature on the GC-PC adsorbent. This physisorption process is confirmed by the fact that the percentage of adsorption capability for CO2 rises as the temperature drops.  To determine the affinity of the guest adsorbate for the adsorbent, the Clausius-Clapeyron equation was used to compute the isosteric heat of adsorption (Qst) based on the pure component adsorption isotherms at three temperatures (298, 308, and 318 °K). Figure 12 (a) depicts the isosteric heat of CO2 adsorption in the range of 33-44 kJ.mol −1 . The slopes of the straight lines may be utilized to determine the Qst value after graphing lnPCO2 vs. 1/T at a set specified adsorbed quantity of CO2 [100].
The computed value of Qst (45 kJ.mol -1 ) for the C-700 sorbent is less than 80 (kJ.mol -1 ), implying that CO2 adsorption is physical and similar to other adsorbents utilizing porous carbon.
The Qst value declined quickly and became steady as the surface coverage and CO2 loading rose.
This variability in Qst can be ascribed to an energetically heterogeneous surface for CO2 capture [101]. The higher values of Qst at the beginning of the sorption process can be correlated to CO2 adsorption on strong binding sites and the filling of ultrafine micropores. The Qst, conversely, drops as the surface coverage rises. This is due to weaker interactions between restricted CO2 in larger pores and the surface [102].
The reusability of C-700 was investigated by repeating ten CO2 adsorption cycles with regeneration. As seen in Fig. 12(b), for the CO2 adsorption isotherms, there is no discernible decrease in the measured CO2 uptake over ten cycles. Aside from high CO2 capture, the adsorbent's ability to be recycled is a critical requirement for practical applications implying that celery can be used as a high-performance reusable sorbent for CO2 capture applications [1,103]. One of the most important factors to consider is adsorbent reuse for economic reasons. The adsorbent potential does not change significantly after each cycle, as seen in Fig. 12(b). GC-PC can be used in manufacturing applications as a low-cost and cost-effective adsorbent due to the benefits of the regeneration process.

Comparison of various adsorbents
The adsorption capacities of GC-PC used in this study are compared to those of a variety biomass precursor-derived porous adsorbents previously used for CO2 capture, as shown in Fig.   13. As shown, GC-PC has significantly higher adsorption capacities than many other adsorbents previously mentioned. The findings of this research can be used to develop a novel CO2 capture GC-PC synthesis adsorbent that is both effective and high-performing [60,[103][104][105][106][107][108][109][110][111].

Conclusions
In this work, a green self-activating synthesis system was introduced for the preparation of porous carbon from waste carbon precursors. As a typical case, hierarchically porous carbon was synthesized from celery waste. This system is environmentally friendly, simple, cost-effective, and a one-step self-activating method. During pyrolysis, the activating reagents are the gaseous pyrolyzation products of the precursors, resulting in the synthesis of porous carbon without the need for an additional activator. Achievement specific surface areas as high as about 1170 m 2 g -1 confirm the proper performance of the designed activating system. Based on the observed characteristics, the CO2 capture capacities of the best sorbent were evaluated. The celery precursor activated at 700 o C validates an important perspective for CO2 capture. At low temperature and pressure, the synthesized porous carbons adsorbed 5 (mmolg -1 ) at 0 °C and 2.03 (mmolg -1 ) at 25 °C under 1 bar. Adsorption experiments in the temperature range of 25 °C to 55 °C under pressures up to 10 (bar) were conducted with an adsorption duration of 75 minutes. The CO2 adsorption kinetics on the porous carbon closely followed the fractional-order kinetic model. After comparing experimental adsorption effects by models, the Langmuir, fractional-order, and pseudo-secondorder isotherm models were well equipped with adsorption isotherm and kinetic adsorption models, respectively. The Langmuir isotherm equation matched the equilibrium results very well, and the maximal adsorption potential for CO2 was found to be 9.57 (mmolg -1 ) at 298 °K and 10 bar. Adsorption was a possible, spontaneous, and physisorption process, as shown by the negative values of thermodynamic parameters. The physisorption process was reported by enthalpy 23.2 (KJ.mol -1 ). The effectiveness of porous carbon was very successful after 10 cycles of regeneration.
The produced porous carbon could be promising for industrial CO2 adsorption technologies based on improved adsorbents, compared to the other biomass adsorbents investigated, due to its highperformance efficiency and high-efficiency regeneration.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could influence the work reported in this research.