Experimental study of CO2 and N2 adsorption on activated carbon

The present study as designed to assess the CO2 and N2 adsorption isotherms on activated carbon (AC). The AC was characterized with SEM, XRD and N2 isotherm. The CO2 and the N2 adsorption experiments on the AC are performed at 298, 308 and 328 K and modelled by Langmuir model. The preferred selectivity (α CO2/N2) was obtained with the use of the Langmuir properties and the Henry coefficient (K H ) was found from the virial equation. The thermodynamic analysis of CO2 and N2 adsorption which include the entropy loss (ΔS), the Gibbs free energy change (ΔG) and the surface potential (Ω) was calculated. The results reveal that the order of CO2 uptake on AC is higher than that of N2. α(CO2/N2) decreased with the increase of the temperature. The KH values of CO2 on the AC are the most important, while the KH values for N2 are the least. The thermodynamic parameters which include Ω, Qst, ΔG and ΔS are impacted by the porous structure of the AC.The adsorption affinity and the adsorption spontaneity of CO2 is the highest, while N2 has the lowest adsorption spontaneity. Subsequently, CO2 exhibits a higher interaction with AC, in contrast to N2 which displays a slight interaction.the obtained results can be used to CO2 capture and to improve the efficiency of CO2 adsorption devices.


Introduction
Carbon dioxide (CO 2 ) emitted by power stations and cement kilns are responsible for global climate change [1]. One of the solutions to reduce these emissions is to capture CO 2 and store it in geological formations [2]. Several techniques have been used in the process of CO 2 capture and storage such as absorption, adsorption and membrane separation [3]. The use of an adsorbent with high capacity, good renderability and high selectivity is essential in the adsorption technique [4]. In this context different adsorbent are used such as clay, zeolite and activated carbon [5]. Activated carbon is a promising adsorbent since it has a high surface and a great adsorption capacity [6]. The adsorption technique using AC as a raw material is used extensively for the gas separation, purification and in the gas storage systems [7]. Recently, numerous contributions have been made to investigate the adsorption of fluids on AC on laboratory experiment. Chunzhi Shen et al, [8] have examined the kinetic parameters and the thermodynamic characteristics of CO 2 and N 2 on the activated carbon. Elham Khoramzadeh et al, [9] have analyzed the adsorption heats, the adsorption rate and the selectivity of CO 2 over N 2 on synthesized carbon. Honghong Yi et al, [10] have used a microwave (MAC) to analyze different gas isotherms such as CH 4 , N 2 and CO 2 on activated carbon and modeled it with Langmuir and Toth isotherm models. Ru Yang et al, [11] have explored the influenceof the pore size and the specific surface area on the adsorption of CO 2 and N 2 on activated carbons (ACs).
Another study on the adsorption process, Maciej et al, [12] have studied the performance of carbon fibers in the removal of Cu and Co from aqueous solutions. Similarly, Junwei Y et al, [13,14]  of activated carbon fibers during the adsorption of Cu (II). In fact, Mangun et al, [15] have investigated the adsorption of sulfur dioxide on ammonia-treated carbon fiber.
The aim of the current research is to investigate the adsorption mechanism of the N 2 and the CO 2 on activated carbon. The activated carbon has been characterized through various methods and the adsorption isotherms are performed at 298-320 K. The thermodynamic variables of Q , st ΔΩ, ΔG and ΔS of the gases were discussed. Based on the results found, the analyses parameters can be used as a basis for the conception of a gas storage system by adsorption process.

Materials and experiments
The activated carbon which is utilized in this research is obtained from Tunisian olive waste. [6]. The olive waste has been treated with 85% of phosphoric acid (H 3 PO 4 ) in a weight ratio of 1:1, and the mixture was then heated in an electric oven for 24 h at 100°C. Then the olive has been washed with distilled water and sodium bicarbonate (NaHCO 3 ) for the acid neutralization. Finally, the sample was washed with distilled water and dried at 600°C. The gases used in this research are CO 2 and N 2 at the purities of 99.999%. The adsorption test was done using a Nova 4200e static volumetric analyzer. Before the experiment, 0.1 g of the AC is placed under vacuum at room temperature for 12 h. The reversibility of the CO 2 adsorption procedure on the AC was tested by multiple adsorption cycles of CO 2 adsorption. Regeneration tests were performed at 393 K for 12 h under vacuum, and the associated adsorption isotherms were obtained. X-ray diffraction was determined on a 'Philips MPD1880-PW1710' diffractometer. Textural parameters were analyzed by the Micromeritics 3-Flex Micromeritics analyzer. Pore size and BET surface area were determined with the N 2 adsorption desorption at 77K. The morphology was performed by Scanning Electron microscopy FEI Q250 Thermo Fisher.

Langmuir model
The modeling of adsorption isotherms is made with the Langmuir model which depicts that adsorption occurs on an energetically and structurally homogeneous surface. This model is defined by the following equation [1]: Where p, q m and b represent the pressure, the greatest quantity of monolayer adsorption and the Langmuir constant, respectively. Preferential selectivity (α CO2 / N2 ), which is a quantified subscript for evaluating the displacement capacity of CO 2 , is expressed as [16]: Where y CO N 2, 2 and x CO N 2, 2 correspond to the molar amounts in the batch phase and the adsorbed layer, respectively.
In the case of the CO 2 and N 2 adsorption corresponds to the Langmuir model, equation (2) can be expressed as follow [16]: Where q m represent the highest adsorption capacities and b the Langmuir constants for the both gazes.

Henry's law
The adsorption affinity between the molecule and the surface of the adsorbent is evaluated by the Henry coefficient noted K H [16]. By means of the virial formula, the relation of the amount of adsorption and the equilibrium pressure can be written as [17] : Where A 1 , A 2 and A 3 represent the virial coefficients.  Subsequently, equation (5) can be derived.
H 0 ( ) Taking into account the lower adsorption amount, the high term in equation (5) can be eliminated, and equation (6) can be obtained [17] : in terms of the adsorption quantity q, K H can be determined. 0.676

Thermodynamics settings
The most thermodynamic properties addressed are the entropy loss (ΔS), the Gibbs free energy change (ΔG) and the surface potential (Ω). Ω was found via the equation (7 Figure 1 gives the XRD diffraction of the AC which presents two peaks occurringat 2θ roughly 25°and 42°. These peaks are related to the (001) and (101) planes of the hexagonal structure of graphite [6].

Structural characterization of the AC
The morphology of the AC is show in the figure 2. The activated carbon presents smooth areas in the form of parallel lines. The surface of the AC presented clearly visible macropores and the diameter of the pores was between 5 and 15 μm.
The parameters of pore structure and the specific surface area of the activated carbon are lists in table 1. N2 adsorption-desorption isotherms and the pore size distributions of the activated carbon are presented in figures 3(a), (b). As show from table 1, the specific surface area of the AC was 1565 m 2 g −1 , and the pore volume was 0.676 cm 3 g −1 . Using the analysis of the pertinent parameters, it can be found that the microporous surface area of the activated carbon is more dominant than the pore volume of the micropores of the total pore volume. This means that the majority of the surface area and pore volume of the activated carbon is assigned to the micropore. The findings suggest that activated carbon exhibits dominant micropores, which provides an opportunity for adsorption of small molecules [5].The N 2 isotherm proves that the Activated carbon is classified as a type I which characterizes the micropore structure according to the IUPAC classification [6]. From the micropore distribution graph depicted in figure 3, it can be observed that the internal of activated carbon is composed mainly of micropores, which are distributed in the range of 0.7 to 1.8 nm. Figure 4 shows the adsorption isotherm of CO 2 and N 2 on activated carbon at various temperatures. We can see that the uptakes of both gases are improved when the equilibrium pressure increases, which suggests that the adsorption of both gases is favored at high pressure [6]. By comparing the adsorption isotherms of the both gases on the activated carbon, we can see that the CO 2 isotherm is of type I whereas it is linear for the N 2 .This isotherm differences are related to the interaction between these gases and the activated carbon. Figure 4 shows again the adsorption isotherm analysis with Langmuir's model.The model settings are detailed in table 2.We can note a high agreement between the experimental findings and the modelled values for a wide range of temperature and pressure which means a good suitability of the Langmuir model to simulate the CO 2 and N2 adsorption on the AC.

Separation coefficients
The calculated separation coefficients of both gazes are displayed in table 3.The values of (co 2/ N 2) are greater than 1.0, showing that the adsorbed N 2 can be displaced by the CO 2 on the surface AC. With rising temperatures, α (CO2/N2) on the AC exhibited a declining trend. The lower temperature favors the CO 2 injected to displace the adsorbed N 2 molecules efficiently [16]. As discussed in section 4.1, activated carbon has a micropore structure. Therefore, this structure can enhance the adsorption capacity of both gases and it promotes the displacement efficiency of CO2 on the activated carbon [16].  Table 4 reported the K H of CO 2 and N 2 on the AC. It can be noted that increasing the temperature during the experiments decreases the values K H of the both gazes. Therefore, higher temperature reduces the CO 2 and N 2 affinities adsorption on the surface of the AC as well as the adsorption amounts ( figure 4).

Henry's coefficients
The K H values of N 2 are smaller than those of CO 2 , showing that the adsorption force of CO 2 on the AC surface is higher. The K H is more important in the case of CO 2 which is attributed to the presence of a quadrupole moment, which enables the strong binding between the AC surface and the CO 2 through an electrostatic interaction [17].     Figure 5 shows the acquired Ω of both gases on the AC, which have negative values. It can notice that with increasing pressure, the absolute Ω values of the gases continue to grow. Moreover, the rates of increase in ΔΩ for CO 2 are highest at the beginning, then gradually decrease with pressure. In contrast, the rate of increase in ΔΩ for N 2 reduces over the entire pressure range.We can also find that Ω of CO 2 is higher than thatof N 2 , showing that the CO 2 adsorption on the AC is more favorable. As discussed in section 4.1, the micropore structure in AC is the more numerous, and the fluctuation of adsorbate molecules from one cavity to the another in the microporous structure of AC needs more isothermal energy [18].Therefore, CO 2 has thehighest Ω compared to the N 2 .

Gibbs free energy
According to the pore structure of the AC, there would be a force between the surface and the carbon atoms, which allows the movement of the atoms into the interior of the AC. This will result in the carbon atoms generating supplementary energy, i.e. surface free energy [17]. Figure 6 describes the ΔG of adsorbed gases in the AC, that are negative, suggesting that adsorption is a spontaneous phenomenon. The absolute value of ΔG rises with increasing pressure, indicating that increasing pressure enhances the spontaneity of the adsorption [16]. It can be observed that the CO 2 value of ΔG is higher than N 2 , which indicates that the CO 2 adsorption on the AC has an important spontaneous degree than that of N 2 .We note that the porous surface material allows the molecules to be adsorbed in order to reduce the surface free energy. Therefore, the CO 2 which has a large ΔG has the largest adsorption on the AC than N 2 . Figure 7 plot the acquired Q st of the gases as a function of loading in the AC. The ΔH of both gases being negative that means an exothermic adsorption. Besides, all Q st were less of 40 kJ mol −1 , which means that the CO 2 adsorption is physisorption. The decline of Q st with loading indicates that the surface of the AC is heterogeneous [6]. In the adsorption step, many active sites are available and the adsorbates are directly adsorbed onto these sites, leading to a strong force between the adsorbate and the adsorbent [16]. Throughout the adsorption process, the value of Q st is influenced by the interaction energy between the adsorbate and the adsorbate. We notice that CO 2 has the most Q st compared to N 2 which is the lowest. The greater Q st shows that the CO 2 -AC interaction is the strongest when comparing with N 2 . Figure 8 depicts the ΔS of the studied gases to be negative, which suggests that the adsorption of the gas in the AC changes from unbound to a bound phase. The adsorption involves the creation of a more ordered reorganization on the AC surface and the reduction of the freedom degree molecules. We note that when the adsorption loading is important implies that the ΔS is lower, which confirms that the ΔS values for the two gases decrease inversely with the loading [17]. Moreover, when the available pore is taken by adsorbate molecules, the degree of disorder of the adsorption system grows. Therefore, initially the molecules are adsorbed in the larger pores and the adsorbed molecules are more contained [17]. With growing charge, the major part of the molecules is trapped in the larger pores. In these bigger pores, the mobility of the adsorbed molecule is not efficient restrained, resulting in a lower ΔS. We can see that the ΔS of CO 2 is greatest than that of N 2 . In fact, the interaction between the AC and the CO 2 is affected by the quadrupole moment enhancement as well as the polarizability. Therefore, the ΔS value is less for N 2 than for CO 2 . This is due to the low polarity of N 2 which causes a stronger interaction between the electrostatic field and the N 2 molecules.

Conclusion
In this work, the CO 2 and N 2 adsorption at 298, 318 and 322 K on activated carbon were performed. The AC were described via SEM, N 2 adsorption-desorption and XRD. The adsorption isotherms of CO 2 and N 2 are modelled by Langmuir model. To understand the adsorption mechanism and to explain the adsorption process, the thermodynamic analysis of CO 2 and N 2 adsorption has been realized. The (CO 2/ N 2) values all show a decreasing trend with temperature.The adsorption affinity and the degree of adsorption spontaneity of the CO 2 on the AC is higher than the N 2 . Subsequently, the CO 2 shows a more significant interaction with the AC when compared to that of N 2 .