Investigations of CO2, CH4 and N2 physisorption in single-walled silicon carbon nanotubes using GCMC simulation

In this paper, we report N2, CH4 and CO2 adsorption in single-walled silicon carbon nanotubes (SiCNTs) using grand canonical Monte-Carlo and calculate the isosteric heat of gas adsorption. The results demonstrate that at ambient temperature and high pressure, gas adsorption in these nanotubes is in the order of CO2 > CH4 > N2 and nanotubes’ order is (10,10) < (20,20) < (40,40), while this order of adsorptivity of nanotubes will be inverted for N2 when the pressure is very low. Then, we fit our simulation results to Langmuir and Langmuir–Freundlich equations to illustrate the mechanism of gas adsorptivity. The fitting exhibits that the simulation data obtained are very close to Langmuir–Freundlich behavior, which emphasizes that the dominant adsorptivity has occurred in multi-layer adsorption. Moreover, the comparison between our simulation results and other reports, which studied these gases’ adsorption on different nanoporous materials, experimentally and theoretically, is presented to illustrate that SiCNTs still have the best gas adsorptivity ability at ambient temperature and low/high pressure.


Introduction
The useful discovery of carbon nanotubes by Iijima [1] has opened up the possibility of synthesis of nanometer-scale, one-dimensional materials including nanowires, nanoparticles, and nanotubes [2,3]. Silicon carbon nanotube (SiCNT) was first synthesized in 2001 [2]; furthermore, it has been improved to grow for several kinds of onedimensional nanostructures as a tube furnace in 2002 [3] and it is the one of the most important nanotubes with many excellent properties, especially; as promising membrane for gas adsorption applications [4].
In the past decade, much of nanoscientific investigations has focused on finding new materials in nanoscale with particular properties such as high-ability natural gas adsorption capacity, owing to their supplies of energy that satisfy industrial, governmental and business agencies [5,6].
From the environmental, an economical benefit and the predominant cause of greenhouse effect and urgent issue suppressing global warming [7,8], a large of worldwide attentions have been reported on adsorptivity properties of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrogen (N 2 ) (as natural gas components) on various nanoporous materials like nanotubes to reducing CO 2 global emission and remove it from natural gas and pollutant air which as mention above, they have been considered as alternative resources of energy [5,6].
In recent years, many researchers have studied CO 2 , CH 4 and N 2 adsorption, theoretically and experimentally. That in follow we are going to examplify number of them and then compare their results with our data: Mishra et al. [9] have reported CO 2 , CH 4 and N 2 adsorption properties of a zinc-based metal organic framework, which is commonly known as ZnDABCO. Yi et al. [10] also studied these gas adsorption equilibrium isotherms on microwave-activated carbon, experimentally. Likewise, Zhang et al. [8] have presented experimental and computational studies to evaluate the adsorption-based separation of CO 2 from flue gas and natural gas using micro-porous metal organic framework Cu-TDPAT as a sorbent material. In another work, Karra et al. [11] performed atomistic grand canonical Monte-Carlo (GCMC) simulations to understand pore size, heat of adsorption, open metal sites, electrostatics, and ligand functionalization contributing to adsorption of CO 2 , CO, and N 2 in MOFs: IRMOF-1, IRMOF-3, Cu-BTC, Zn 2 [bdc] 2 [dabco].
Billemont et al. [12] have studied using experiments and molecular simulations, the adsorption of CO 2 and CH 4 in nanoporous carbons in the presence of water. They observed a small volume of water; the shape of the adsorption isotherms remained similar; both the molecular simulations and experiments showed a slight decrease in the CO 2 and CH 4 adsorption amounts [12].
In 2007, Huang et al. investigated adsorption of CO 2 and CH 4 using GCMC on single-walled carbon nanotubes (SWCNTs) [5], and they proposed Harris and Yung's EPM2 [13] and the spherical models for CO 2 as different potential models.
In this paper, applying GCMC simulation, we study adsorption of CO 2 , CH 4 and N 2 to calculate their adsorptivity properties in the single-walled silicon carbon nanotubes (SWSiCNTs) purly.
In this research, all gas molecules are treated as one-site the simplest 12-6 Lennard-Jones (LJ) potential to calculate their interactions with together and with Si and C atoms of SWSiCNT and the LJ parameters between different atoms are calculated by Lorentz-Berthelot rules. We chose classical 12-6 LJ representation according to UFF parameterization for Si and C atoms of SWSiCNTs [17], whereas for CO 2 , we used the LJ potential parameters from Ref. [5], and for CH 4 and N 2 from Ref. [18], which all LJ parameters here, are listed in Table 1.
The standard GCMC simulation is a stochastic method that can compute the adsorption isotherms of various gases in each solid framework such as nanotubes, with having a constant volume (V) in equilibrium and an infinite reservoir of gases imposing its chemical potential (l) at fixed temperature (T) [19]. In this work we used the Music code for all GCMC calculations [20]. The last half of 15 million trials was used for calculating the ensemble averages in gas adsorption simulations. The periodic boundary condition (PBC) is imposed in tube length direction. Further GCMC details are available in our previous publications [14,15].
It should be noted in the results and discussion section, at first we present the CO 2 , CH 4 and N 2 adsorption isotherms and isosteric heats which are calculated and then the gas adsorption fitting to Langmuir and Langmuir-Freundlich equations will be exhibited and in the last subsection of results, we have been compared the gas adsorptivity of gases with other published experimental and simulation data.

Results and discussion
This work investigates gas adsorption properties of SWSiCNTs using GCMC.
The gases are CH 4 , CO 2 and N 2 . Therefore, our results are presented in some following sections.   Fig. 1 shows that the gases have less adsorption, especially at higher temperature. According to Fig. 1, maximum adsorption in all gases takes place almost at 10 MPa (moderate pressure) and range of adsorptivity of gases together is low, about between 7.5 and 9.5 mol/kg. This means that (10,10) SWSiCNT has low gas adsorptivity and it seems that CH 4 and CO 2 have better adsorption, which relates to amount of adsorption as shown in Fig. 1.
However, the adsorption is decreased as a function of increasing the temperature. According to Fig. 3, maximum of adsorption in all gases occurred almost at 15 MPa pressure, and range of adsorptivity of gases together was about between 13 and 19 mol/kg. This means that the (20,20) SWSiCNT has more gas adsorptivity rather than the (10,10) SWSiCNT, and CH 4 and CO 2 have better adsorptivity. Moreover, CO 2 has better amount of adsorption than CH 4 especially at low and moderate pressure (see Fig. 3). Figure 5 also indicates that the gases have more adsorptivity in (40,40) SWSiCNT than in (10,10) and (20,20) SWSiCNTs. The gas adsorptions are decreased with increasing the temperature. Although, according to Figs. 1, 3 and 5, the maximum gas adsorptions occur in the (40,40) SWSiCNT, the range of adsorptivity of gases together in the latter nanotube is about between 10 and 45 mol/kg. This means that the (40,40) SWSiCNT has more gas adsorptivity rather than the (10,10) and (20,20) SWSiCNTs. Furthermore, it is clear that CO 2 has better adsorptivity for this reason: the most adsorptivity we can see in Fig. 5. Figure 5 shows that the (40,40) SWSiCNT can adsorb CO 2 at ambient temperature at low and moderate pressure (\10 MPa). In addition, CH 4 is adsorbed better than N 2 . Consequently, the large diameter SWSiCNT can adsorb CO 2 much more than CH 4 and N 2 , while small diameter SWSiCNT can adsorb CO 2 the same as CH 4 . The important point is that CO 2 has more e/k B rather than CH 4 and N 2 (Table 1), so, its interaction with atoms of SWSiCNT nanotubes is more considerable.
Therefore, more adsorptivity of CO 2 is expected energetically. It means that natural adsorptivity directly depends on the atomic interactions.

Adsorption fitting to Langmuir and Langmuir-Freundlich equations
In the next investigation, we have fitted adsorption data to where h is the amount adsorbed, P is the equilibrium pressure, and h m , k, b, n are isotherm parameters. Figures 4 and 5 present CH 4 , CO 2 and N 2 plots at 300 and 343 K related to (10,10) and (40,40) SWSiCNTs, respectively. It is obvious that both figures show that the data of adsorption correspond to the Langmuir-Freundlich equation. These observations emphasize that the Langmuir-Freundlich is the best fitting that illustrates the adsorption of gases imply with varying energetic inhomogeneity in both (10,10) as smaller diameter and (40,40) as larger diameter SWSiCNTs [14,15]. However, the gases adsorption adaptation to the Langmuir-Freundlich model can distinguish that multi-layer adsorptivity is very feasible  [14,15]. Because the pressure is increased and temperature is lowered, additional layers are formed. This has led to the modern concept of multilayer adsorption. In Fig. 6, we can see that even in the (10,10) SWSiCNT, the secondary layer of gas adsorption is taking place. So, Langmuir's model of uni-molecular adsorption is valid here.

The comparison of gases adsorption
In this subsection, we compare the CO 2 , CH 4 and N 2 adsorptions in SWSiCNTs and other nanoporous materials. Therefore, we first focus on CO 2 adsorption.

The comparison of CO 2 adsorption
Mishra et al. [9] have reported CO 2 adsorption property of ZnDABCO at 294 and 350 K, experimentally. Figure 7a that is related to 294 K, shows that ZnDABCO has more CO 2 adsorptivity than the (10,10) SWSiCNT, but it has less than the (20,20) and (40,40) SWSiCNTs. However, CO 2 adsorption properties of ZnDABCO and (20,20) SWSiCNT are very close. Figure 7b also is related to 350 K, and shows that ZnDABCO has CO 2 adsorptivity close to the (10,10) SWSiCNT, but it is less than the (20,20) and (40,40) SWSiCNTs. As a result, it seems, in relatively high temperature, SWSiCNT has good CO 2 adsorption than inorganic nanoporous materials. In fact, many inorganic nanoporous materials such as MOFs are destroyed in high temperature, while nanotubes, especially SiCNTs, have the most thermal stability [21]. Figure 7c also presents the CO 2 adsorption equilibrium isotherms reported from Refs. [8,10] and our data at 298 K and range 1-100 kPa. This figure again indicates that the (10,10) and (20,20) SWSiCNTs have more CO 2 adsorptivity than activated carbon and Cu-TDPAT. It should be noted that our obtained simulation data values are above the simulation data values of Cu-TDPAT [8].
Another study is performed by Billemont et al. that they have been used both experimental and simulation approaches to indicate the adsorption of CO 2 on the Filtrasorb F400 activated carbon (experimental absolute adsorption) and on CS1000A and CS1000AF (simulated adsorption) as nanoporous carbons in the presence of water at 318.15 K Fig. 4 The data of adsorption fitted on Langmuir (dash lines) and Langmuir-Freundlich (color lines) equations in (10,10) SWSiCNT at 300 and 343 K. a CH 4 , b CO 2 , c N 2 [12]. They show that CS1000AF has more adsorption of CO 2 than CS1000A theoretically. Then, we compare our data with simulation data of CO 2 adsorptivity on CS1000A and also Filtrasorb F400. In the section that is related to adsorption of CH 4 comparison, we will remind about Billemont et al. (Ref. [12]) research again. Figure 7e presents this comparison.

A r c h i v e o f S I D
Huang et al. [5] using GCMC have investigated the adsorption of CO 2 (applying Harris and Yung's EPM2 and spherical models for CO 2 as different potential models) and CH 4 at 343 K. Their obtained data of simulation are shown in Fig. 7-e. Our results that we consider the spherical model for CO 2 at 300 and 343 K with comparing to Billemont et al. of simulation and experimental indication (Ref. [12]), illustrate that (20,20) SWSiCNT has more CO 2 adsorptivity than nanoporous carbon, but (10,10) SWSiCNT is less CO 2 adsorptivity than nanoporous carbon. In addition, the results of Huang et al. (Ref. [5]) have shown CO 2 adsorptivity close to the (10,10) SWSiCNT in the desired pressure range and at 343 K. Therefore, the (20,20) SWSiCNT can adsorb CO 2 more than nanoporous carbon and SWCNTs at the same thermodynamic conditions. As a result, Billemont et al. of simulation and experimental research (Ref. [12]) has been done at 318.15 K, and our investigation is at 300 and 343 K, and we know the adsorption decreases with increasing temperature, so, we can realize that (20,20) SWSiCNT even at 343 K has more CO 2 adsorptivity than nanoporous carbon at 318.15 K.
Consequently, as final investigation of the adsorption of CO 2 , we can find that SWSiCNT with diameter large enough (diameter larger than (10,10) tube) is proper to adsorb CO 2 as compared to some MOFs, nanoporous carbon and CNTs.

The comparison of CH 4 adsorption
In our work, also, we compare CH 4 adsorptivity in SWSiCNTs with other nanoporous materials such as Cu-TDPAT and ZnDABCO as MOF, activated carbon and CNTs. At first, we report our obtained simulation data at 294 K and pressure under 3,000 kPa, with Mishra et al. (Ref. [9]) as experimental study that it has been considered the adsorption of CH 4 on a zinc based metal organic framework: ZnDABCO. Figure 8a illustrates the adsorption isotherms of CH 4 in this case. In Fig. 8a, we can find that CH 4 adsorptivity even in the (10,10) SWSiCNT is more than in ZnDABCO; however, our data are from simulation, and Mishra et al.'s (Ref. [9]) work is an experimental study. Furthermore, Fig. 8a, shows that CH 4 adsorptivity behavior is the same, in both ZnDABCO and SWSiCNT.  Figure 8b shows that considering GCMC simulation on adsorptivity of CH 4 in Cu-TDPAT and SWSiCNT, we can realize that SWSiCNT, even with (10,10) chirality is more CH 4 adsorbable than Cu-TDPAT at same thermodynamic conditions. Also, this figure emphasizes that experimental data from Ref. [10] on activated carbon are very close to experimental and simulation data from Ref. [8] on Cu-TDPAT. This is a very gratifying challenge in gas adsorption investigations, because reports contain both simulation and experimental studies on two different nanoporous materials. Simulation data and experimental data from Ref. [8] provide similar estimates of the adsorption of gases on MOFs. Also, our obtained data exhibit that SWSiCNT is a more CH 4 adsorbable nanoporous material. Therefore, we understand that this type of analysis (simulation calculations) can be accurate, while the achievement of this method can be significant, and results obtained from the simulation calculations are credible in this field.

A r c h i v e o f S I D
Finally, with considering to Fig. 8a, b and c, we can result that SWSiCNT with large enough diameter is more CH 4 adsorbable at room temperature, and also, GCMC simulation can predict suitable the CH 4 adsorption behavior, this approach can also be reliable in terms of accuracy.

The comparison of N 2 adsorption
In last comparison, we represent our N 2 adsorption from GCMC simulations with experiments reports and with few existing theoretical works. Figure 9a exhibits our data and Mishra et al. (Ref. [9]) experimental report at 294 K under pressure of 3,000 kPa. Mishra et al. [9] present their temperature investigations Fig. 7 a CO 2 adsorption property of ZnDABCO reported from Ref. [9] and our results at 294 K. b CO 2 adsorption property of ZnDABCO reported from Ref. [9] and our results at 350 K. c CO 2 adsorption equilibrium isotherm from Refs. [8,10] and our data at 298 K and range 1-100 kPa. d Adsorption simulation of CO 2 from Ref. [11] that is about CO 2 adsorption in Cu-BTC and our data at 298 K. e Adsorption simulation of CO 2 from Ref. [12] experimental and simulation results, Ref. [5] simulation data and our data at range of 1,000-25,000 kPa and they have been resulted that ZnDABCO has a little more N 2 adsorptivity at 294 K than 350 K. Also, our GCMC simulation data of N 2 is more than Mishra et al. (Ref. [9]) reports. In Fig. 9a, we can realize that N 2 cannot more adsorbable in one type of materiel at different thermodynamics conditions. It means that with varying the thermodynamic conditions, for instance in ZnDABCO, according to Ref. [9] as experimental work, we can see that the increase of pressure cannot significantly affect nitrogen uptake, and even though with decreasing the temperature, not significant changing occur in N 2 uptake. Also, we can observe from our simulation calculations that the increase of pressure cannot have considerable effect on nitrogen uptake in (10,10) and (20,20) SWSiCNTs. Here, like CO 2 and CH 4 adsorptions compared in previous subsections, we find that N 2 adsorption behavior from simulation data is similar to experimental study.
Then, from Fig. 9a, b and c we can understand that N 2 adsorption at least under 100 kPa (about 1 atm) is differ than over than 1,000 kPa (10 atm) in SWSiCNTs so that, at low pressure (about 1 atm and ambient pressure) SWSiCNT with small diameter is more N 2 adsorbable than Fig. 8 a CH 4 adsorption property of ZnDABCO reported from Ref. [9] and our results at 294 K. b CH 4 adsorption equilibrium isotherm from Refs. [8,10] and our data at 298 K and range of 1-100 kPa.
c Adsorption simulation of CH 4 from Ref. [12] experimental and simulation results, Ref. [5] simulation data and our data at range of 1,000-25,000 kPa other SWSiCNT, contrariwise, SWSiCNT with large diameter is more N 2 adsorbable at pressure over 1,000 kPa (10 atm). Also, N 2 can adsorb lower than other gases because of their weakness interaction potentials with atoms of adsorbent (here SWSiCNT) and this lower adsorptivity is proved even with experimental observations.

Conclusion
This work deals with CO 2 , CH 4 and N 2 physisorption in SWSiCNTs by using GCMC approach. The adsorption isotherms of gases are prepared, and interpretation of these quantities illustrates that CO 2 has more adsorption in the pure state at ambient temperate and high pressure, and with increasing the diameter of nanotube, we observe, the increase of gas adsorption, while at very low pressure this result is inverted. In addition, we compare our simulation results with other experimental and theoretical investigations, which consider these gas adsorptions on different nanoporous materials. The observations show that SWSiCNTs are the best candidates for gas adsorption.