Cold Nitrogen Plasma: A Groundbreaking Eco-Friendly Technique for the Surface Modification of Activated Carbon Aimed at Elevating Its Carbon Dioxide Adsorption Capacity

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In this scientific article, we present an environmentally friendly method for increasing the nitrogen content on the surface of activated carbons, which leads to enhanced CO₂ adsorption of these carbons.

Abstract

The commercially available activated carbon was modified using barrier and spark discharge low-temperature nitrogen plasma treatment. The samples were investigated using nitrogen sorption at a temperature of −196 °C, XRD, SEM, and FTIR methods, and elemental analysis. The nitrogen content on the surface was increased, but other properties, such as specific surface area, total pore volume, pseudocrystallite height, and pseudocrystallite width, remained unchanged. The activated carbons after nitrogen plasma treatment indicated higher CO₂ adsorption than the pristine ones. Since the investigated materials only differed in their nitrogen content, it has been unequivocally demonstrated that the increased presence of nitrogen is responsible for the enhanced adsorption of CO₂. The low-temperature nitrogen plasma treatment of activated carbon is a promising method for enhancing CO₂ capture.

1. Introduction

One of the foremost environmental challenges in recent years is the management of greenhouse gas (GHG) emissions. Among all the GHGs, anthropogenic CO₂ is responsible for over 60% of global warming due to the immense quantities released into the atmosphere [1]. Since monitoring began in 1958, the rate of CO₂ concentration increase in the atmosphere has shifted from less than 1 ppm per year before 1970 to over 2 ppm per year in recent times [2]. In response to current demands, numerous researchers have suggested various methods for CO₂ removal, including adsorption, absorption, electroreduction, photocatalytic reduction, and catalytic reduction [3,4,5,6]. The average concentration of CO₂ in February 2024 stood at 423 ppm [7], and projections suggest it could rise to between 720 and 1000 ppm by the end of the century if current trends persist [8].

Porous materials like activated carbons, zeolites, and polymeric substances are being explored as CO₂ adsorbents [9,10]. Activated carbon offers numerous benefits, including cost efficiency, excellent chemical and thermal stability, and low regeneration energy requirements [11,12,13]. Due to these advantages, many researchers view activated carbon as a promising adsorbent for CO₂ capture. Consequently, continuous efforts have been made to enhance the CO₂ adsorption capacity of activated carbon [14,15,16].

Several researchers have confirmed that the surface morphology [17,18,19,20] and chemical composition [18,21,22,23] of carbons significantly influence their ability to CO₂ adsorption. Investigations, such as those conducted by some authors [24,25], have demonstrated that the CO₂ adsorption capacity at atmospheric pressure on activated carbons correlates directly with their specific surface area. Other authors emphasize the significant role of total pore volume [26], while others focus on the importance of micropores [27].

Theoretical studies aiming to identify the optimal pore size in laminar-type structures have indicated that pore diameters ranging from 0.57 to 0.72 nm are optimal [22]. Experimental studies have corroborated this range. Li et al. [18] indicated the importance of ultra-micropores with diameters of 0.4–0.6 nm. Sreńscek-Nazzal et. al. [20] showed that the CO₂ adsorption is correlated with a high volume of pores with diameters of 0.30–0.82 nm.

It has been suggested that integrating nitrogen-containing groups into carbon is an effective strategy to enhance CO₂ adsorption capacity.

Nitrogen-enriched porous carbons derived from petroleum coke as the carbon precursor that were subsequently modified with urea and activated using KOH under different conditions were described [28]. The urea modification resulted in the introduction of a substantial amount of nitrogen groups into the carbon matrix. Materials with varying porosity and nitrogen content were obtained. The highest adsorption at 25 °C was 4.40 mmol/g. This material did not contain the highest amount of nitrogen nor the highest volume of micropores or pores with diameters less than 1 nm. Nevertheless, the authors claim that the highest capacity for CO₂ adsorption is attributed to the presence of fine micropores (<1 nm) and significant nitrogen doping.

The nitrogen-rich carbon precursor and KOH, as an activating agent, were applied for the preparation of N-doped activated carbons [29]. The authors obtained materials that exhibit different levels of porosity and nitrogen content. The highest CO₂ adsorption was obtained for material displaying remarkably high nitrogen content (22.3 wt%) and a substantial surface area (1317 m²/g), along with a significant pore volume (0.27 cm³/g) consisting predominantly of ultra-micropores less than 0.7 nm in diameter. While the values of textural parameters for this particular material were the highest, the nitrogen content was not the highest compared to other materials. The authors claim that the exceptional CO₂ adsorption capacity is attributed to the presence of ultra-micropores, a significant volume of large micropores, and elevated nitrogen content. In our opinion, the significance of nitrogen content was not demonstrated here; only the values of textural parameters were shown.

Nitrogen-enriched carbon, which is characterized by well-developed microporosity and exceptional CO₂ adsorption capacity, was synthesized via KOH activation of a nitrogen-containing polymer derived from sucrose and urea [30]. The authors acquired materials displaying diverse textural characteristics and nitrogen concentrations. The materials exhibiting the highest adsorption had the largest micropore volume and the second- and third-largest volumes of ultra-micropores, but the nitrogen content was relatively low. Nevertheless, the authors claimed that the CO₂ adsorption values of all the carbon materials prepared showed a strong correlation with both nitrogen content and microporosity. In our opinion, the influence of nitrogen content was not demonstrated in this case; rather, only the impact of micropore and ultra-micropore volumes was shown.

The three above-mentioned works have shown that the presence of nitrogen and the ultra-micropore are critical factors for achieving high CO₂ absorption. The significance of micropores or ultra-micropores has been previously described, so there is no certainty whether only the volume of micropores matters and the presence of nitrogen is irrelevant.

Díez et al. [31] applied non-doped and N-doped activated carbon fibers with similar shapes and diameters for CO₂ adsorption. The textural values differed before and after the introduction of nitrogen. The highest CO₂ adsorption was identified for non-doped fibers. The authors acknowledged that the porous structure was the primary determinant of the fibers’ CO₂ adsorption capacity and that subjecting them to high-temperature ammonia treatment did not yield any positive effects.

Morales-Ospino [32] explored the influence of O- and N-doping of three commercial activated carbons on CO₂ uptake. The nitrogen content was increased through urea treatment and hydrogen peroxide oxidation, combined with urea treatment. Nitrogen and oxygen–nitrogen doping primarily affected the larger pores, diminishing the material’s textural properties and, consequently, reducing CO₂ adsorption. The authors demonstrated the significant influence of textural properties, which had a stronger effect on CO₂ adsorption by carbon materials compared to N-doping.

Many authors introduce nitrogen into activated carbons in various ways. However, such actions always lead to changes in other parameters, especially textural parameters [33]. Consequently, although these authors attempt to demonstrate the positive impact of increased nitrogen content on CO₂ adsorption, the conclusions drawn from these studies are not reliable because the introduction of nitrogen involves changes in other properties. It is well-known that the textural parameters are crucial for CO₂ adsorption. Simultaneous changes in nitrogen content and textural parameter values do not justify conclusions indicating that nitrogen content is the cause of changes in CO₂ adsorption.

According to our knowledge, there is a lack of studies presenting materials with the same textural properties but different nitrogen contents. In our study, we present commercially available activated carbon modified using barrier and spark discharge low-temperature nitrogen plasma treatment. As a result of this treatment, no properties of the activated carbon changed, except for the nitrogen concentration on the surface. This allowed us to demonstrate that the increase in surface nitrogen is the reason for the increased CO₂ adsorption. The preliminary studies presented here concern a slight increase in nitrogen concentration and a slight increase in CO₂ adsorption. However, it is important that we have demonstrated, for the first time, an increase in CO₂ adsorption specifically due to the increase in nitrogen content while keeping other properties unchanged. The research will continue toward increasing nitrogen content using this method.

The undeniable advantage of nitrogen plasma treatment is the possibility of enriching activated carbon with nitrogen without changing the values of textural parameters. In this case, the conclusion about the positive impact of increased nitrogen content is fully justified. The article presents a method of introducing nitrogen into activated carbons using nitrogen plasma, which allowed for a slight increase in nitrogen content and, consequently, a slight increase in CO₂ adsorption. These are preliminary but pioneering studies, and that is why we decided to publish them. We are currently working on modifying cold plasma treatment to increase the amount of nitrogen on the surface of activated carbon.

2. Materials and Methods

2.1. Materials

Commercial activated carbon DT0 was kindly supplied by Gryfskand Ltd., Hajnówka, Poland.

2.2. Modification of Commercial Activated Carbons

Pristine activated carbon (DT0) underwent plasma treatment in the installation depicted in Figure 1 in the following two types of discharges: dielectric barrier or spark.

Both discharges were powered by a pulse current with positive polarization. The repetition rate of voltage pulses (f) was 340 Hz and 2300 Hz in the dielectric barrier discharge and spark discharge, respectively. The course of current (i) and voltage (u) over time (t) is shown in Figure 2 and Figure 3.

These waveforms were recorded with a two-channel Tektronix TDS 3032B oscilloscope with Tektronix TCP202 current and Tektronix P6015A voltage probes. The power (P) of the dielectric barrier discharge was 1 W, and the spark discharge was 5.9 W. The power was calculated using Equation (1), as follows:

$$\mathrm{P}=\mathrm{f}·\int \mathrm{i}(\mathrm{t})·\mathrm{u}(\mathrm{t})\mathrm{d}\mathrm{t}$$

(1)

In both discharges, activated carbon was modified in a flow system, and a mixture of nitrogen and helium was used. The gas flow rate was 10 L/h, and the volume ratio of nitrogen to helium was 1:1. The gas flow rate was regulated by Bronkhorst mass flow controllers. Before turning on the discharge, the reactors were flushed with a mixture of nitrogen and helium for 15 min. The time of the plasma treatment of activated carbon in both reactors was the same and amounted to 30 min.

The dielectric barrier discharge was generated in a reactor, in which the high-voltage electrode was placed centrally in a quartz tube. The inner diameter of the quartz tube was 12 mm, and the wall thickness was 1.2 mm. The quartz tube was covered from the outside with a layer of electrically conductive paint, which was a grounded electrode. The length of the grounded electrode was 190 mm. The high-voltage electrode was a stainless-steel rod with a diameter of 3.2 mm. The reactor was mounted vertically. In the lower part, there was a 5 mm layer of quartz wool, which served as a grate, in which activated carbon was placed. A diagram and photo of the dielectric barrier discharge reactor are shown in Figure 4.

The spark discharge was generated in a reactor, in which electrodes were placed in two arms of a quartz four-piece. The internal diameter of the quartz tubes from which the four-piece was made was 12 mm. The electrodes were made of graphite rods with a diameter of 5 mm. One electrode was connected to high voltage, and the other was grounded. The distance between the electrodes was 6 mm. Activated carbon was placed 15 mm below the electrodes. The activated carbon layer was 110 mm high and rested on a quartz wool grate. A diagram and photo of a barrier discharge reactor are shown in Figure 5.

2.3. Characterization of Materials

Nitrogen sorption isotherms and textural properties (specific surface area–BET, total pore volume-V_tot, micropore volume–V_micro, pore size distribution) were measured at −196 °C using volumetric techniques with ASAP 2460 Sorption Surface Area and Pore Size Analyzer (Micrometrics, Novcross, USA). X-ray diffraction (XRD) patterns were recorded using X’Pert–PRO, Panalytical, an X-ray diffractometer. To investigate the morphological structures of the samples, an ultra-high-resolution field emission scanning electron microscope (SU8020 Hitachi Ltd, Tokyo, Japan) was used. The FTIR spectra of the modified carbon samples were acquired using a Nicolet 380 ATR-FTIR spectrometer (Thermo Scientific, Waltham, MA, USA). Prior to recording the sample spectrum, the background line was automatically subtracted. The spectra were measured across the range of 4000–400 cm⁻¹. The elemental analysis was performed using a CN 628 elemental analyzer (LECO Corporation, St. Joseph, MI, USA).

2.4. CO₂ Uptake Measurements

CO₂ adsorption was investigated up to a pressure of 100 kPa at temperatures of 0, 10, 20, and 30 °C at room temperature using volumetric techniques with an ASAP 2460 Sorption Surface Area and Pore Size Analyzer (Micrometrics).

3. Theory

The Sips isotherm model is effective in predicting adsorption on heterogeneous surfaces. At low adsorbate concentrations, it simplifies to the Freundlich model, while at high adsorbate concentrations, it resembles the Langmuir model [34]. Equation (2) expresses the Sips isotherm model, as follows:

$$\mathrm{q}={\displaystyle \frac{q_{mS}·b_S{p}^{ns}}{1+b_S{·p}^{ns}}}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}]$$

(2)

q_mS—the maximum adsorption capacity [mmol/g]

b_S—the Sips constant [kPa⁻¹]

n_S—the heterogeneity factor

The Toth isotherm model is a different empirical equation designed to enhance the fitting of isotherms between experimental and predicted equilibrium data. This model is particularly effective for characterizing heterogeneous adsorption systems, accurately capturing both low- and high-end concentration boundaries [35]. Equation (3) outlines the Toth equation.

$$\mathrm{q}={\displaystyle \frac{q_{mT}{b}_T\mathrm{p}}{(1+(b_{T}p{)}^{nT}{)}^{\frac{1}{nT}}}}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}]$$

(3)

q_mT—the maximum adsorption capacity [mmol/g]

b_T—the Toth constant [kPa⁻¹]

n_T—the heterogeneity factor

The Radke–Prausnitz model [36] possesses several key attributes that render it the preferred option for most adsorption systems with low adsorbate concentrations. At low adsorbate concentrations, this model simplifies to a linear isotherm. Conversely, at higher adsorbate concentrations, it approaches the Freundlich isotherm, and when n_RP = 0, it resembles a Langmuir isotherm. Another important characteristic of this isotherm is its ability to provide a good fit across a broad range of adsorbate concentrations. The Radke–Prausnitz equation is formulated as follows in Equation (4):

$$\mathrm{q}={\displaystyle \frac{q_{mRP}·b_{RP}·\mathrm{p}}{1+b_{RP}·p^{n_{RP}}}}[\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}]$$

(4)

q_mRP—the maximum adsorption capacity [mmol/g]

b_RP—the Radke–Prausnitz constant [bar⁻¹]

n_RP—Radke–Prausnitz model exponent

The Langmuir isotherm was developed to explain gas–solid phase adsorption and remains a fundamental method for quantifying and comparing the maximum adsorption capacities of different sorbents. This theory assumes monolayer coverage of the adsorbate, with adsorption taking place at specific homogeneous sites, in which all sites are equal in their adsorption energies. Once an adsorbate molecule occupies a site, no additional adsorption can occur at that site. The sorbent has a finite capacity for the adsorbate [37]. The Langmuir isotherm equation is represented by Equation (5), as follows:

$$q={\displaystyle \frac{q_{mL}{·b}_L·p}{1+b_L·p}} [\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}]$$

(5)

q_mL—the maximum adsorption capacity [mmol/g]

b_L—the Langmuir constant [kPa⁻¹]

p—pressure [bar]

q—the adsorbed quantity under p pressure [mmol/g]

The Freundlich isotherm is an empirical equation used to describe adsorption on heterogeneous surfaces with varying adsorption energies [38]. The Freundlich equation is represented by Equation (6), as follows:

$$q=k_F ·p^{nF} [\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{g}]$$

(6)

k_F—the Freundlich constant [mmol/g]

n_F—the heterogeneity factor

To assess the optimal fittings of isotherm models to the experimental data, the hybrid error function (HYBRID) [39] has been employed. The sum of squared errors (SSE) is widely used to quantify the deviation of measured data from the true mean of the data. To improve the SSE at lower values of pressure, each of the sum of the squares of the error values was divided by the experimental value of CO₂ adsorption.

$$HYBRID={\displaystyle \frac{100}{n-p}}\sum _{i=1}^n{\displaystyle \frac{{\left(q_{i,exp}-q_{i,calc}\right)}^2}{q_{i,exp}}}$$

(7)

q_e,calc—theoretical value of adsorption on the activated carbon surface calculated based on the model

q_e,exp—experimental value of adsorption on the activated carbon surface

n—total number of measurements

p—number of model parameters

4. Results and Discussion

Nitrogen adsorption–desorption isotherms for pristine and modified carbon samples are shown in Figure 6. All recorded isotherms are very similar and conform to the IUPAC classification of Type IV. The observed hysteresis loop beyond the P/P₀ threshold of 0.43 signifies the occurrence of capillary condensation, providing evidence for the presence of mesopores. The pronounced incline at lower relative pressures signifies the presence of supplementary micropores. The mesopores enhance the mass transfer of adsorbate molecules, proving advantageous in sorption applications.

The hysteresis loop has been identified as H4. Hysteresis loops of type H4 are commonly encountered in complex materials that consist of both micropores and mesopores. It is characteristic of activated carbons.

The textural parameters of pristine and modified carbon samples are listed in

Table 1. Textural properties and nitrogen content of pristine (DT0) and modified by N₂ plasma samples (DT0_1, DT0_2).

AC	BET	Vtot	Vmicro	CN2
	m2/g	cm3/g	cm3/g	wt %
DT0	1177	0.631	0.383	0.9
DT0_1	1164	0.622	0.380	1.0
DT0_2	1181	0.632	0.385	1.0

.

The BET surface area of the starting materials was equal to 1177 m²/g, the total pore volume was 0.631 cm³/g, and the micropore volume was 0.383 cm³/g. The values of the textural parameters after spark discharge treatment increased slightly, no more than 1.4%. After barrier discharge, the textural parameters decreased very slightly, no more than 0.5%. After plasma treatment, the nitrogen content increased by 10% for both samples.

The DFT model was employed to derive the pore size distribution of the AC. The results are presented in Figure 7. Evidently, all examined samples displayed a similar pore size distribution, demonstrating comparable structural characteristics with abundant micropores and mesopores. The results shown in Figure 7 are consistent with the values presented in Table 1.

The X-ray diffraction profiles of pristine plasma-activated carbons and those modified by cold nitrogen are presented in Figure 8. The profiles are very similar. All the samples demonstrate extremely wide diffraction peaks, and the lack of a distinct peak indicates a primarily amorphous structure.

The XRD patterns of all AC typically show three broad bands due to reflections from the following planes (002)—2θ = 24.6°, (10)—2θ = 43.4°, and (11)—2θ = 73.7°. The first one is associated with the pseudocrystallite height (Lc), and the second one is associated with the pseudocrystallite width or average hexagonal carbon layer diameter (La). Both parameters can be obtained using the Scherrer equation, as follows:

$$L={\displaystyle \frac{K \cdot \lambda }{\beta \cdot cos\left(\theta \right)}}$$

(8)

where:

θ—angle of incidence

λ—wavelength of characteristic line utilized in X-ray spectrometry, equal to 0.154 nm for CuKα

β—width at half peak height

The stacking size Lc is calculated on the basis of data related to the first peak (2θ = 24.6°) and K equal to 0.9. The pseudocrystallite width (La) is calculated on the basis of data related to second peak (2θ = 43.4°) and K equal to 1.94 [40].

The interlayer spacing d₀₀₂ can be calculated from the position of the (002) band using Bragg’s Law, as follows:

n∙λ = 2d_hkl sin(θ)

where

n—order of diffraction, equal to 1 here

The average number of layer planes in the pseudocrystallites can be calculated by dividing Lc by d₀₀₂.

The interlayer spacing is the same for all the AC and was equal to 0.36 nm, demonstrating a significant deviation from the dimensions observed in graphene (0.334 nm).

The pseudocrystallite height (Lc) and width (La) were equal to 1.2 and 3.9 nm, respectively. The average number of layer planes is calculated as 3. The values of interlayer spacing, the pseudocrystallite height and width, and the average number of layer planes are typical for poorly graphitized and non-graphitizing carbons [41].

All activated carbons exhibited a similar porous structure. In the SEM images (Figure 9), clear entrances to macropores are visible, likely sequentially leading to mesopores and micropores. It was observed that cold plasma treatment did not alter the morphology of the DT0 carbon.

FTIR spectra of pristine plasma samples (DT0) and those modified by N₂ (DT0_1, DT0_2) are presented in Figure 10. The presence of numerous N-containing groups resulting from interactions between N₂ plasma and surface functional groups has been validated through FT-IR analysis. The pronounced and extensive spectral features observed at approximately 3400 cm⁻¹ are attributed to symmetric stretching vibrations of N–H bonds and/or the presence of hydrogen-bonded hydroxyl nests [42].

The spectral analysis reveals discernible bands in the range of about 1441  cm⁻¹, which are ascribed to amides, pyridine, and C=N functionalities [43]. Moreover, discernible peaks manifest in the spectral region spanning about 1100 cm⁻¹, corresponding to the stretching vibrations of C–N single bond bonds [44].

The spectral band observed within about 880 cm⁻¹ is indicative of out-of-plane N-H deformation vibrations [45]. Consequently, the Fourier-transform infrared (FT-IR) analysis corroborates the presence of numerous nitrogen-containing groups within the carbon samples. Therefore, the FT-IR analysis substantiates the presence of N–H, amide, pyridine, C = N, and C –N species in all the samples. All of the bands listed above are more intense for samples modified by N₂ plasma. This observation indicates the successful introduction of nitrogen functional groups in comparison to the pristine sample

In summary, regarding the properties of the investigated carbons, it should be emphasized that their textural parameters and pseudocrystal sizes remained essentially unchanged due to plasma treatment. Only the nitrogen content increased, as confirmed using elemental analysis and FTIR spectroscopy. This enables the investigation of the sole influence of nitrogen presence on CO₂ adsorption.

This study involved the analysis of pristine and modified activated carbons with regard to their adsorption of CO₂. Tests were conducted at temperatures of 0 and 30 °C under pressures up to 100 kPa. The findings are illustrated in Figure 11.

For all activated carbon, CO₂ adsorption isotherms at temperatures of 0 and 30 °C, demonstrate a comparable trend. These isotherms showed a rapid rise at low pressure, followed by a relatively swift increase at higher pressure. The modification of the activated carbon DT0 by cold nitrogen plasma results in a slight increase in CO₂ adsorption. The higher the adsorption temperature, the greater the difference between the initial carbon and the modified ones. This suggests the potential occurrence of chemical adsorption.

The CO₂ adsorption capacities acquired at 100 kPa pressure and temperatures ranging from 0 to 30 °C are detailed in

Table 2. CO₂ adsorption capacities on pristine plasma samples (DT0) and those modified by N₂ (DT0_1, DT0_2) at 100 kPa pressure and temperatures ranging from 0 to 30 °C.

	The CO2 Adsorption at 100 kPa [mmol/g]				Percentage Increase in CO2 Adsorption
	0 °C	10 °C	20 °C	30 °C	0 °C	10 °C	20 °C	30 °C
DT0	3.2	2.6	2.08	1.67
DT0_1	3.24	2.65	2.13	1.72	1.25%	1.92%	2.40%	2.99%
DT0_2	3.28	2.69	2.15	1.73	2.50%	3.46%	3.37%	3.59%

. The values confirm the conclusions drawn from Figure 11.

As a result of cold nitrogen plasma modification, there was a slight increase in CO₂ adsorption compared to the original carbon. However, we present preliminary studies here, in which the goal was to demonstrate that cold nitrogen plasma modification can be conducted in such a way that without reducing the values of textural parameters crucial for CO₂ adsorption, only the nitrogen content on the surface increases, leading to an enhancement in CO₂ adsorption. To the best of our knowledge, this is the first such report. Although Phan et al. [46] conducted a study on the impact of nitrogen plasma modification on CO₂ adsorption, they did not achieve an increase in CO₂ adsorption, only a slight decrease.

Experimental data of CO₂ adsorption at temperatures of 0, 10, 20, and 30 °C on pristine plasma samples (DT0) and those modified by N2 (DT0_1, DT0_2) were fitted to Sips, Toth, Radke–Prausniz, Langmuir, and Freundlich equations.

Mathcad Prime 7.0.0.0 was used to determine which of the abovementioned models best described the experimental data. Five mcdx files were created to perform calculations for each model (Supplementary Material data: Model_1_Sips_ang.mcdx, Model_2_Toth_ang.mcdx, Model_3_R_P_ang.mcdx, Model_4_ Langmuir_ang.mcdx, and Model_5_Freundlich_ang.mcdx). The parameter values of the models and HYBRID values for the temperatures of 0 °C, 10 °C, 20 °C, and 30 °C were calculated based on the experimental data (Supplementary Material data: Data.xlsx). The obtained results were collected in an Excel file (Supplementary Material data: Models results.xlsx). The lowest HYBRID values were obtained for the Toth model. The parameters of Equation (3) at different temperatures and HYBRID values are presented in

Table 3. The parameters of the Toth equation at different temperatures and HYBRID values for CO₂ adsorption over activated carbons before and after cold nitrogen plasma modification.

	Temperature [°C]
	0	10	20	30
DT0
qmT	326	167	115	44
bT	0.0016	0.0015	0.0012	0.0016
nT	0.1915	0.2228	0.2474	0.3104
HYBRID	3.94 × 10−05	3.76 × 10−05	1.45 × 10−05	5.34 × 10−06
DT0_1
qmT	364	244	125	56
bT	0.0015	0.0011	0.0011	0.0013
nT	0.1884	0.2112	0.2468	0.2996
HYBRID	3.97 × 10−05	2.55 × 10−05	1.70 × 10−05	1.64 × 10−05
DT0_2
qmT	306	201	129	49
bT	0.0017	0.0013	0.0011	0.0015
nT	0.1932	0.2179	0.2429	0.3059
HYBRID	1.14 × 10−04	5.40 × 10−05	2.40 × 10−05	1.39 × 10−05

. The values of the coefficient n_T are far from unity, indicating the heterogeneity of the surface of all carbons.

The isosteric heat of adsorption (Q_iso) is very important to gain deeper insights into the interaction between the adsorbed molecules and the internal surface of the adsorbent. Q_iso gives a direct assessment of the intensity of the binding forces between the adsorbate molecules and the surface of the adsorbent.

The isosteric heat of adsorption is ascertainable through the application of the Clausius–Clapeyron equation, as follows:

$$Q_{iso}=-R{\left({\displaystyle \frac{\partial ln\left(p\right)}{\partial \left(\frac{1}{T}\right)}}\right)}_q$$

(9)

To utilize Equation (9), the Toth equation and parameters from Table 3 were used to calculate the pressure values corresponding to the seven levels of adsorption. The determination of pressure from Equation (3) is not algebraically feasible. Therefore, numerical methods were employed. Mathcad Prime 7.0.0.0 (Supplementary Material data: Adsorption Model_2_Toth ang.mcdx) was utilized to compute the pressure values for corresponding surface loadings levels (q). The obtained values have been compiled in the file (Supplementary Material data: Models results.xlsx, Toth model sheet).

Plotting ln(p) against the inverse of absolute temperature (1/T) for each loading, straight lines were obtained, with a slope of −Q_iso/R (Figure 12).

This approach allows for the determination of the heat of adsorption across all relevant loading (see Figure 13). This graph is crucial, as it provides insight into the energetic uniformity of the surface under investigation.

Figure 13 illustrates that the isosteric heat of adsorption, calculated using the Clausius–Clapeyron and Tooth equations, exhibited a consistent trend, as follows: decreasing as CO₂ loading increased. Initially, CO₂ molecules preferentially entered smaller pores during adsorption, resulting in a stronger gas–adsorbent interaction and, consequently, higher isosteric heat at lower loading. As loading increased, the interaction weakened due to pore saturation, causing a decline in the isosteric heat of adsorption. The values of the isosteric heat of adsorption are lower than 50 kJ/mol, indicating the physical nature of CO₂ adsorption over DT0 samples.

5. Conclusions

Nitrogen plasma treatment does not affect the textural parameters of activated carbon. Similarly, spark discharge and dielectric barrier discharge do not change the values of the pseudocrystallite height and average hexagonal carbon layer diameter. Only an increase in nitrogen content was observed based on elemental analysis. The introduction of nitrogen onto the surface was also confirmed by FTIR investigations. The increased nitrogen content led to an increase in CO₂ adsorption. The higher the adsorption temperature, the more noticeable this effect is. Preliminary studies have shown an increase in CO₂ adsorption solely due to the increase in nitrogen content while maintaining unchanged textural parameters, which are considered key for CO₂ adsorption. Further research on the influence of the spark and dielectric barrier discharge of cold nitrogen plasma is necessary to develop this technique, which could be applied to enhance CO₂ adsorption on any activated carbons.

6. Patents

Joanna Siemak, Bogdan Ulejczyk, Joanna Sreńscek-Nazzal1, Michał Młotek, Krzysztof Krawczyk, and Beata Michalkiewicz are authors of patent P.445472 pending to West Pomeranian University of Technology in Szczecin and Warsaw University of Technology resulting from the work reported.