First-principle calculations of sulfur dioxide adsorption on the palygorskite

This study uses the density functional theory (DFT) method of first-principle to provide theoretical guidance for a deeper understanding of the adsorption performance of palygorskite (PAL). An analysis was conducted to determine the optimal surface configurations and binding sites for SO2 on the PAL (100) surface. The results show that for the SO2/PAL adsorption system, the adsorption energies (-0.13 and -0.10 eV) and charge transfer amounts (0.017 and 0.029 e) at the top and bridge sites are greater than those at the hollow site (0.55 eV and 0.015 e), which determine the steadiest adsorption sites. Moreover, the fermi level is altered by orbitals in SO2, revealing interactions between SO2 and the (100) surface of PAL.


Introduction
SO 2 emissions can result in acid rain, which can have adverse effects on the ecosystem [1] .To limit their detrimental effects, poisonous gases must be detected, captured, and eliminated, which requires innovative technical materials and procedures.With benefits like structural stability, diversified morphology, rich element composition, good adsorption and high thermal stability, clay minerals may hold promise for adsorption.Palygorskite (PAL) is a highly promising clay mineral that exhibits a very large specific surface area and powerful adsorption capabilities, as well as outstanding colloidal properties and high-temperature stability [2] .It has the ideal chemical formula Mg 5 Si 8 O 20 (HO) 2 (OH 2 ) 44 H 2 O [3] . Figure 1 shows that there are several pore channels between the chains.Moreover, PAL has the capacity to capture a variety of contaminants [4][5][6] .Although PAL has been the subject of extensive investigation, little is known about the material's capacity to absorb gas molecules by itself.To more fully comprehend the adsorption capabilities of the clay material using first-principles calculations, this study estimated and analyzed the SO 2 adsorption by PAL from a microscopic perspective.

Computational Method and Theoretical Model
The Dmol 3 programme package within Materials Studio 8.0 is used in this study, which is based on DFT, to analyze the interaction between the PAL (100) surface and SO 2 [7-9]   .The interaction between the PAL (100) surface and SO 2 is analyzed using the adsorption energy, and the calculation formula is provided by [10] : ) where  denotes the energy of adsorption, and  ,  , and  denote the energy of the system, PAL (100) surface, and free SO 2 .From Equation (1), it follows that the greater the contact between the surface and SO 2 , the lower the adsorption energy value.

Results and discussion
3.1 Geometry model SO 2 's adsorption behavior was studied on the PAL (100) surface.Figure 2 (a) depicts the computed adsorption sites using the Monte Carlo (MC) approach, with colorful dots designating the active adsorption sites, which are primarily located around the surface Si-O bonds.As a result, three highdensity adsorption sites including the top, hollow, and bridge site, were taken into account for the calculations and analyses that followed, as shown in Figure 2 (b).It should be noted that the SO 2 is placed on the three sites throughout the (100) surface calculation procedure, and the positions of all the atoms in the calculation are adjustable.
Figure 3 (a) exhibits PAL (100) surface geometry.Figures (b-d) exhibit adsorption configurations' top and lateral perspectives at the top, hollow, and bridge sites on PAL (100) surface.The top site has the shortest adsorption distance when compared to the other two adsorption geometries, with an S-O distance of 3.344 Å.The adsorption energies ( ), charge transfers (Q), and adsorption distances (d) for SO 2 at three sites are all listed in Table 1.It is discovered that the energies at which SO 2 adsorbs at the top, hollow, and bridge sites are -0.13,0.55, and -0.10 eV.
PAL is a kind of thermally stable adsorption, according to the results of adsorption energies.The charge (Q) transfers via the PAL (100) surface to SO 2 and is a crucial aspect of adsorption geometry.It is observed that, among the adsorption geometries, the bridge site of the PAL (100) surface has the highest charge transfer to SO 2 (0.029 e), while the hollow site has the lowest charge transfer to SO 2 (0.015 e).As a result, the PAL (100) surface exhibits stable adsorption of SO 2 at the top and bridge sites.  of SO 2 decreased to 1.479 Å.The bond angle of SO 2 experienced only slight changes, with values of 119.548° and 119.485° at hollow and bridge sites, respectively, as shown in Figure 4 (b).The fact that SO 2 adsorbed at the top and bridge sites showed a significant shift in bond length and bond angle suggested that there was more interaction between the SO 2 and these two sites on the PAL (100) surface during the adsorption process.

Difference Charge Density
The difference charge density shown in Figure 5 provides information on the redistribution of electrons around the active sites on the PAL (100) surface upon SO 2 adsorption.From Figure 5 (a-c), regardless of the adsorption configuration at the top, hollow, or bridge sites, a positive charge is formed around the adsorption site, indicating a decrease in electron density in this area and electron transfer to the SO 2 .
According to the charge transfer trends in Table 1, the electron values at the top and bridge sites are higher than those at the hollow site, indicating that the PAL (100) surface has a greater capacity to adsorb SO 2 at these locations.

Density of states analysis (DOS)
Analysis of DOS, which depicts the distribution of electrons in different orbitals, can help to clarify the connection.Figure 6 illustrates the results of this study's calculations of total and partial DOS (TDOS, PDOS) of SO 2 adsorbed on top, bridge, and hollow sites both before and post adsorption.The TDOS curve changes as a result of interactions between SO 2 gas and various active sites on the PAL (100) surface, particularly in the Fermi level area.TDOS curves for three distinct locations are extremely similar, as shown in Figure 6 (a).A minor increase in multiple peaks nearby the fermi level is brought on by the activated state of S, O 1 , and O 2 atoms, which is compatible with the behavior of electron transfer.Moreover, a new peak at -14 and -5 eV can be seen, which is typically a marker of the new material production.Analyzing the PDOS in Figure 6 (b-d), the major source of the observed features is attributed to the p orbitals of S, O 1 , and O 2 atoms of SO 2 , leading to changes in the total electron density curve at -1~4 eV and -5 eV.Based on the PDOS analysis in Figure 6 while the S atom's 3s and 3p orbitals and the O 1 atom's 2s and 2p orbitals at the top site experience an upward shift in energy within the -10 to 5 eV range.In contrast, the three atoms' orbitals all shift to lower energy areas close to the fermi level as shown by the PDOS in Figure 6 (c) and (d).This occurrence is consistent with the top site's shorter adsorption distance in Table 1 and suggests a greater connection between the SO 2 adsorbed there and the PAL (100) surface.

Conclusion
This study offers important new understandings of SO 2 adsorbed behavior on the PAL (100) surface.The calculated adsorption energies and charge transfer amounts reveal that the top and bridge sites on the surface are stable adsorption sites for SO 2 .The analysis of bond lengths, bond angles, and changes in TDOS and PDOS also provides a comprehensive understanding of the adsorption process.The present research lays the foundation for further exploration into the development and improvement of adsorbents for environmental purposes and highlights the possibility of using the PAL (100) surface as a promising material for the removal of SO 2 .Future studies might examine how surface alterations like doping or functionalization affect the way that dangerous gases like SO 2 adsorb on surfaces.

Figure 6 .
Figure 6.(a) TDOS of the PAL system for SO 2 pre-and post-adsorption.Fractional wave density of states at sites: (b) Top; (c) Hollow; (d) Bridge.
(b), the O 2 atom's 2s and 2p orbitals move towards lower energy regions,

Table 1 .
The most stable conformation at each adsorption site's geometric properties.