A DFT Study on the Adsorption of H2S and SO2 on Ni Doped MoS2 Monolayer

In this paper, a Ni-doped MoS2 monolayer (Ni-MoS2) has been proposed as a novel gas adsorbent to be used in SF6-insulated equipment. Based on the first-principles calculation, the adsorption properties of Ni-MoS2 to SO2 and H2S molecules, the main decomposition components of SF6 under a partial discharge (PD) condition have been studied. The adsorption energy, charge transfer, and structural parameters have been analyzed to find the most stable gas-adsorbed Ni-MoS2. Furthermore, the density of states (DOS), projected density of states (PDOS), and electron density difference were employed to explore the interaction mechanism between SO2, H2S, and the Ni-MoS2 surface. It is found that the H2S molecule and SO2 molecule interact with the Ni-MoS2 surface by strong adsorption energy. Therefore, we conclude that the interaction between these two kinds of gases and the Ni-MoS2 monolayer belongs to chemisorption, and the Ni-MoS2 monolayer might be a promising gas adsorbent for the fault recovery of SF6-insulated equipment. Additionally, we have to point out that all of the conclusions only considered the final adsorption energy, the barrier in the transition state has not been analyzed in this paper.


Introduction
Due to the excellent insulation and arc extinguishing properties of SF 6 , it has obtained a wide application in gas-insulated equipment, such as gas-insulated switchgear (GIS), gas-insulated breaker (GIB), and gas-insulated transformer (GIT). In addition, SF 6 -insulated equipment exhibits a great deal of advantages, such as a small occupied area requirement, little electromagnetic pollution, and high safety and reliability [1,2]. However, a certain amount of insulation defects inevitably occur in SF 6 -insulated equipment during the long-term running process, which may lead to partial discharge (PD) and the decomposition from SF 6 to SF x under the operating voltage [3,4]. Simultaneously, the SF 6 gas-filled chamber inevitably contains trace amounts of impurities, such as H 2 O and O 2 [5]. SF x will quickly react with the H 2 O and O 2 into various decomposition components (such as H 2 S, SO 2 , SOF 4 , SO 2 F 2 , SOF 2 , HF, CF 4 , and CO 2 , etc.) [1,[6][7][8]. These decomposition components can significantly accelerate the corrosion and aging process of the insulation medium, resulting in insulation failure. In order to ensure the running stability of SF 6 -insulated equipment, the primary task is to maintain the purity of the filling gas, namely removing the decomposition components of SF 6 [9,10]. Considering all of the SF 6 decomposition components, H 2 S, SO 2 gases, the main decomposition components under all types of PD conditions are the key removing target gases [11][12][13]. Thus, it is urgent to explore an effective adsorbent for H 2 S and SO 2 removal.

Computational Details
All calculations were performed based on the density functional theory (DFT) [28,29]. Th generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) was chosen to calculate the geometry optimization with the energy convergence accuracy, maximum stress, and max displacement set to 1 × 10 −5 Ha, 2 × 10 −3 Ha/Å, and 5 × 10 −3 Ha, respectively [30,31]. The effect of spin-polarization was ignored in this paper, and Grimme dispersion correction has been introduced to describe the weak interactions, like van der Waals force. The double numerical plus polarization (DNP) was chosen as the basis set, the density functional semi-core pseudopotential (DSPP) was applied in core treatment, and the Monkhorst-pack k point mesh of 5 × 5 × 1 was employed [32,33]. The selfconsistent (SCF) field tolerance was set to 1 × 10 −6 Ha, and the DIIS size was set to 6 to speed up the convergence of SCF [34].
A 4 × 4 × 1 MoS 2 monolayer supercell with a 20 Å vacuum slab, including 32 S atoms and 16 Mo atoms, was built in order to avoid the interaction between the adjacent cells. The optimized lattice constant of MoS 2 is calculated to be 3.180 Å, which is in good agreement with other theoretical calculation results [35]. One Ni atom was placed on the top site of the Mo atom at the center of the 4 × 4 × 1 MoS 2 monolayer supercell, bonding with three S atoms.
The adsorption energy (E ads ) was calculated by Equation (1) [36]: E ads = E Ni-MoS 2 /gas − E Ni-MoS 2 − E gas (1) where E Ni-MoS 2 /gas is the total energy of the gas adsorbed Ni-MoS 2 , while E Ni-MoS2 and E gas represent the total energy of the Ni-MoS 2 monolayer and the total energy of free gas molecule, respectively.
The more negative E ads obtained after geometry optimization, the easier for the free gas molecule to be adsorbed on the Ni-MoS 2 monolayer surface, indicating the adsorption system is more stable. In addition, the charge transfer (Q t ) between the gas molecule and Ni-MoS 2 monolayer was calculated by Equation (2): where Q a and Q b represent the amounts of carried charge of the gas molecules after and before gas adsorption, which were calculated by electron population analysis [37]. It is worth putting out that the value of Q b is always 0 e in this paper. According to the definition, if Q t is positive, the electrons transfer from gas molecule to the Ni-MoS 2 monolayer. Additionally, the density of states (DOS) was calculated to analyze the interaction mechanism between gas molecules and the Ni-MoS 2 monolayer [38].

Structures and Electronic Properties of H 2 S, SO 2 and the Ni-MoS 2 Monolayer
Firstly, the adsorption property of the Ni atom on the MoS 2 monolayer was discussed according to the adsorption energy analysis and population analysis. The adsorption energy (E ads ) of Ni atom on MoS 2 monolayer was defined in the Equation (3): The negative E ads in Equation (3) indicates that the binding process is exothermic. As the most stable doping position of Ni on MoS 2 monolayer is the top site of the Mo atom according to previous studies [24], therefore, only the structure of Ni-MoS 2 with Ni doping on the top site of the Mo is discussed in this paper.
As shown in Figure 1, The Ni atom above the Mo3 atom bonds with other three surrounding S atoms with a length of 2.121 Å, and there is no chemical bond between the doped Ni atom and the Mo3 atom because of the long distance between them (2.596 Å). The bond angle of the Mo1-S1-Mo2 near the Ni atom (81.3 • ) has slightly changed compared with that of the MoS 2 monolayer without doping (82.0 • ), indicating that the doped Ni atoms have a strong interaction with the initial MoS 2 monolayer structure, and the doping structure could be quite stable. Moreover, the large binding energy (3.495 eV) further confirmed the conclusion above. In addition, the charge transfer from the Ni atom to the MoS 2 monolayer is 0.021 e.
To further analyze the structural properties of Ni-MoS 2 monolayer, the total density of states (TDOS) and projected density of states (PDOS) have been calculated as shown in Figure 2. For TDOS distribution, the electron distribution of the Ni-MoS 2 monolayer around the Fermi level has slightly increased compared with that of the MoS 2 monolayer, implying that the doping of the Ni atom has enhanced the metallic property of the MoS 2 monolayer. As a result, the doped Ni atom acts as the active site for building interaction between the Ni-MoS 2 monolayer and the target gas molecules. For PDOS distribution, the peaks of Ni-3d orbital and S-3p orbital overlap at −5.5 eV, −4.5 eV, −3.5 eV, −2.5 eV, and 2.0 eV, indicating that the S-3p orbital strongly hybridize with the Ni-3d orbital. Therefore, the Ni atom adsorbs on the surface of MoS 2 monolayer by a stable structure.
The structures of the gas molecules are exhibited in Figure 3, and its specific structural parameters of the gas molecules are listed in Table 1. Additionally, the carried charge of S and O atom in the SO 2 are 0.453 e and −0.227 e, respectively. The H atom in the H 2 S has a positive charge of 0.174 e, and the S atom has a negative charge of 0.348 e. For the SF 6 molecule, the charge of the S atom is calculated to be 1.194 e, and the F atom is −0.199 e. These results are in agreement with the other theoretical calculation reports [39].
The negative Eads in Equation (3) indicates that the binding process is exothermic. As the most stable doping position of Ni on MoS2 monolayer is the top site of the Mo atom according to previous studies [24], therefore, only the structure of Ni-MoS2 with Ni doping on the top site of the Mo is discussed in this paper.  As shown in Figure 1, The Ni atom above the Mo3 atom bonds with other three surrounding S atoms with a length of 2.121 Å , and there is no chemical bond between the doped Ni atom and the Mo3 atom because of the long distance between them (2.596 Å ). The bond angle of the Mo1-S1-Mo2 near the Ni atom (81.3°) has slightly changed compared with that of the MoS2 monolayer without doping (82.0°), indicating that the doped Ni atoms have a strong interaction with the initial MoS2 monolayer structure, and the doping structure could be quite stable. Moreover, the large binding To further analyze the structural properties of Ni-MoS2 monolayer, the total density of states (TDOS) and projected density of states (PDOS) have been calculated as shown in Figure 2. For TDOS distribution, the electron distribution of the Ni-MoS2 monolayer around the Fermi level has slightly increased compared with that of the MoS2 monolayer, implying that the doping of the Ni atom has enhanced the metallic property of the MoS2 monolayer. As a result, the doped Ni atom acts as the active site for building interaction between the Ni-MoS2 monolayer and the target gas molecules. For PDOS distribution, the peaks of Ni-3d orbital and S-3p orbital overlap at −5.5 eV, −4.5 eV, −3.5 eV, −2.5 eV, and 2.0 eV, indicating that the S-3p orbital strongly hybridize with the Ni-3d orbital. Therefore, the Ni atom adsorbs on the surface of MoS2 monolayer by a stable structure.  The structures of the gas molecules are exhibited in Figure 3, and its specific structural parameters of the gas molecules are listed in Table 1. Additionally, the carried charge of S and O atom in the SO2 are 0.453 e and −0.227 e, respectively. The H atom in the H2S has a positive charge of 0.174 e, and the S atom has a negative charge of 0.348 e. For the SF6 molecule, the charge of the S atom is calculated to be 1.194 e, and the F atom is −0.199 e. These results are in agreement with the other theoretical calculation reports [39].   To further analyze the structural properties of Ni-MoS2 monolayer, the total density of states (TDOS) and projected density of states (PDOS) have been calculated as shown in Figure 2. For TDOS distribution, the electron distribution of the Ni-MoS2 monolayer around the Fermi level has slightly increased compared with that of the MoS2 monolayer, implying that the doping of the Ni atom has enhanced the metallic property of the MoS2 monolayer. As a result, the doped Ni atom acts as the active site for building interaction between the Ni-MoS2 monolayer and the target gas molecules. For PDOS distribution, the peaks of Ni-3d orbital and S-3p orbital overlap at −5.5 eV, −4.5 eV, −3.5 eV, −2.5 eV, and 2.0 eV, indicating that the S-3p orbital strongly hybridize with the Ni-3d orbital. Therefore, the Ni atom adsorbs on the surface of MoS2 monolayer by a stable structure. The structures of the gas molecules are exhibited in Figure 3, and its specific structural parameters of the gas molecules are listed in Table 1. Additionally, the carried charge of S and O atom in the SO2 are 0.453 e and −0.227 e, respectively. The H atom in the H2S has a positive charge of 0.174 e, and the S atom has a negative charge of 0.348 e. For the SF6 molecule, the charge of the S atom is calculated to be 1.194 e, and the F atom is −0.199 e. These results are in agreement with the other theoretical calculation reports [39].

Adsorption of H 2 S Gas on the Ni-MoS 2 Monolayer
To analyze the adsorption properties of Ni-MoS 2 monolayer to the target gas molecules, various initial approaching sites of H 2 S to the Ni-MoS 2 monolayer were calculated in order to obtain the most stable adsorption structure. After optimization, only one typical adsorption structure was received, as shown in Figure 4 in the top view and side view, and its E ads , Q t , and specific structure parameters are shown in Table 2.

Adsorption of H2S Gas on the Ni-MoS2 Monolayer
To analyze the adsorption properties of Ni-MoS2 monolayer to the target gas molecules, various initial approaching sites of H2S to the Ni-MoS2 monolayer were calculated in order to obtain the most stable adsorption structure. After optimization, only one typical adsorption structure was received, as shown in Figure 4 in the top view and side view, and its Eads, Qt, and specific structure parameters are shown in Table 2.
For adsorption system with structure shown in Figure 4a,b, a Ni-S bond with a length of 2.205 Å forms in the adsorption process, and the amount of the electrons transferred from the H2S molecule to Ni-MoS2 monolayer is up to 0.254 e, which means the S-Ni bond is not easy to break. However, the structure of the H2S has slightly changed after adsorption. The length of the H-S bond increases to 1.362 Å , the angle of the H1-S-H2 turned into 91.5°. The Eads of H2S on the Ni-MoS2 monolayer is −1.319 eV, which is large enough to completely adsorb H2S. Though barrier exists in the transition state, but the change of the structure of H2S is not obvious, therefore, we conclude that the large Eads can provide the energy to cross over the barrier. As a result, from the amount of electrons transfer and Eads, we conclude that the structure is the most stable structure for H2S adsorption.    Figure 5 presents the TDOS and PDOS of Ni-MoS2 monolayer before and after H2S molecule adsorption. The TDOS after H2S molecule adsorption shows a significant increase near −7 eV and −15 eV. Due to the main contribution of the outer orbitals of interacted atom in the adsorption process, only the PDOS of the S-3p and Ni-3d was discussed. The PDOS analysis shows that the S-3p orbitals overlaps with the Ni-3d orbitals in the range from −6 eV to 0 eV, and the overlapped peaks of these two orbitals appear at about −5 eV, −4 eV, −3.5 eV, −2.5 eV, and 1 5 eV. The wide range of overlap fully verifies the strong hybridization between these two orbitals. The analysis of TDOS and PDOS furtherly confirms the strong interaction between H2S and Ni-MoS2 monolayer, and its structure is quite stable.  For adsorption system with structure shown in Figure 4a,b, a Ni-S bond with a length of 2.205 Å forms in the adsorption process, and the amount of the electrons transferred from the H 2 S molecule to Ni-MoS 2 monolayer is up to 0.254 e, which means the S-Ni bond is not easy to break. However, the structure of the H 2 S has slightly changed after adsorption. The length of the H-S bond increases to 1.362 Å, the angle of the H1-S-H2 turned into 91.5 • . The E ads of H 2 S on the Ni-MoS 2 monolayer is −1.319 eV, which is large enough to completely adsorb H 2 S. Though barrier exists in the transition state, but the change of the structure of H 2 S is not obvious, therefore, we conclude that the large E ads can provide the energy to cross over the barrier. As a result, from the amount of electrons transfer and E ads , we conclude that the structure is the most stable structure for H 2 S adsorption. Figure 5 presents the TDOS and PDOS of Ni-MoS 2 monolayer before and after H 2 S molecule adsorption. The TDOS after H 2 S molecule adsorption shows a significant increase near −7 eV and −15 eV. Due to the main contribution of the outer orbitals of interacted atom in the adsorption process, only the PDOS of the S-3p and Ni-3d was discussed. The PDOS analysis shows that the S-3p orbitals overlaps with the Ni-3d orbitals in the range from −6 eV to 0 eV, and the overlapped peaks of these two orbitals appear at about −5 eV, −4 eV, −3.5 eV, −2.5 eV, and 1 5 eV. The wide range of overlap fully verifies the strong hybridization between these two orbitals. The analysis of TDOS and PDOS furtherly confirms the strong interaction between H 2 S and Ni-MoS 2 monolayer, and its structure is quite stable.  Figure 6 shows the electron density difference of the H2S adsorbed Ni-MoS2 monolayer from different side views, where the increase and decrease of the electron density are represented by the red and blue region, respectively. From the electron density difference, it is intuitive to analyze the charge distribution after the gas adsorption. It can be found that both of the H atoms receive electrons, while the electron density near the S atom and Ni atom suffer a reduction and increase, respectively, which is in agreement with the conclusion that the H2S molecule transfers quite a number of electrons to the monolayer. It is also interesting to notice that the electron density near the Mo atom below the Ni atom suffers an obvious reduction, and we assume that the electrons from the Mo atom made a contribution to the increase of the electron density surrounding H atoms. Therefore, the H2S molecule brings a dramatic change of electron density to the Ni-MoS2 monolayer. In conclusion, considering the structure parameters, charge transfer, adsorption energy, DOS, and electron density difference of H2S adsorbed Ni-MoS2 monolayer, it is obvious that the interaction between the H2S and the Ni-MoS2 monolayer belongs to chemisorption. In consequence, this configuration is the most stable adsorption structure for H2S adsorption on a Ni-MoS2 monolayer.

Adsorption of SO2 Gas on a Ni-MoS2 Monolayer.
For the adsorption of SO2 gas, the gas molecule is initially placed at various sites to approach the surface of the Ni-MoS2 monolayer. Three typical adsorption structures were obtained after geometric optimization, as shown in Figure 7. Table 3 shows the structural parameters of these configurations.  Figure 6 shows the electron density difference of the H 2 S adsorbed Ni-MoS 2 monolayer from different side views, where the increase and decrease of the electron density are represented by the red and blue region, respectively. From the electron density difference, it is intuitive to analyze the charge distribution after the gas adsorption. It can be found that both of the H atoms receive electrons, while the electron density near the S atom and Ni atom suffer a reduction and increase, respectively, which is in agreement with the conclusion that the H 2 S molecule transfers quite a number of electrons to the monolayer. It is also interesting to notice that the electron density near the Mo atom below the Ni atom suffers an obvious reduction, and we assume that the electrons from the Mo atom made a contribution to the increase of the electron density surrounding H atoms. Therefore, the H 2 S molecule brings a dramatic change of electron density to the Ni-MoS 2 monolayer.  Figure 6 shows the electron density difference of the H2S adsorbed Ni-MoS2 monolayer from different side views, where the increase and decrease of the electron density are represented by the red and blue region, respectively. From the electron density difference, it is intuitive to analyze the charge distribution after the gas adsorption. It can be found that both of the H atoms receive electrons, while the electron density near the S atom and Ni atom suffer a reduction and increase, respectively, which is in agreement with the conclusion that the H2S molecule transfers quite a number of electrons to the monolayer. It is also interesting to notice that the electron density near the Mo atom below the Ni atom suffers an obvious reduction, and we assume that the electrons from the Mo atom made a contribution to the increase of the electron density surrounding H atoms. Therefore, the H2S molecule brings a dramatic change of electron density to the Ni-MoS2 monolayer. In conclusion, considering the structure parameters, charge transfer, adsorption energy, DOS, and electron density difference of H2S adsorbed Ni-MoS2 monolayer, it is obvious that the interaction between the H2S and the Ni-MoS2 monolayer belongs to chemisorption. In consequence, this configuration is the most stable adsorption structure for H2S adsorption on a Ni-MoS2 monolayer.
The Ni-MoS2 monolayer shows an outstanding adsorption property to H2S molecules.

Adsorption of SO2 Gas on a Ni-MoS2 Monolayer.
For the adsorption of SO2 gas, the gas molecule is initially placed at various sites to approach the surface of the Ni-MoS2 monolayer. Three typical adsorption structures were obtained after geometric optimization, as shown in Figure 7. Table 3 shows the structural parameters of these configurations. In conclusion, considering the structure parameters, charge transfer, adsorption energy, DOS, and electron density difference of H 2 S adsorbed Ni-MoS 2 monolayer, it is obvious that the interaction between the H 2 S and the Ni-MoS 2 monolayer belongs to chemisorption. In consequence, this configuration is the most stable adsorption structure for H 2 S adsorption on a Ni-MoS 2 monolayer. The Ni-MoS 2 monolayer shows an outstanding adsorption property to H 2 S molecules.

Adsorption of SO 2 Gas on a Ni-MoS 2 Monolayer.
For the adsorption of SO 2 gas, the gas molecule is initially placed at various sites to approach the surface of the Ni-MoS 2 monolayer. Three typical adsorption structures were obtained after geometric optimization, as shown in Figure 7. Table 3 shows the structural parameters of these configurations.
bond is 1.903 Å . The O1-S bond of SO2 adsorbed on the monolayer is 1.543 Å , which slightly longer than that of a free SO2 molecule (1.480 Å ). The angle of the O1-S-O2 has decreased 5°. Thus, the structure of the SO2 molecule changes very little during the adsorption process. The Eads of the M1 system is calculated to be −0.823 eV, 0.094 e transfers from the Ni-MoS2 monolayer to the SO2 molecule. Due to the strong interaction between SO2 and the Ni-MoS2 monolayer, the adsorption of the M1 system belongs to chemisorption.     Figure 7a,b shows the top view and side view of the M1 system. It can be seen that the SO 2 molecule adsorbs on the Ni-MoS 2 monolayer through the Ni-O1 bond, and the length of the Ni-O1 bond is 1.903 Å. The O1-S bond of SO 2 adsorbed on the monolayer is 1.543 Å, which slightly longer than that of a free SO 2 molecule (1.480 Å). The angle of the O1-S-O2 has decreased 5 • . Thus, the structure of the SO 2 molecule changes very little during the adsorption process. The E ads of the M1 system is calculated to be −0.823 eV, 0.094 e transfers from the Ni-MoS 2 monolayer to the SO 2 molecule. Due to the strong interaction between SO 2 and the Ni-MoS 2 monolayer, the adsorption of the M1 system belongs to chemisorption.
The top view and side view of the M2 system are given in Figure 7c,d, the SO 2 molecule adsorbs on the monolayer with a Ni-S bond length of 2.059 Å. From the structural parameters in Table 3, it is found that the structure of the SO 2 molecule changes little after adsorption. The E ads of the M2 system has increased to −1.382 eV compared to that in the M1 system, which manifests the stability of the M2 system. In Addition, only 0.016 e transfers from the SO 2 molecule to the Ni-MoS 2 monolayer in the M2 system, and the charge transfers from the S atom, O1 atom, and O2 atom are 0.050 e, −0.033 e, and −0.033 e, respectively.
The top view and side view of the M3 system are given in Figure 7e,f, the O1 atom and S atom are trapped by the Ni-MoS 2 monolayer with bond lengths of 1.948 Å (Ni-O1) and 2.258 Å (Ni-S). Due to the strong interaction of the Ni-O1 bond and Ni-S bond, the Q t of the M3 system is calculated to be −0.206 e, which is distinctly larger than that in the M1 and M2 system. From the structural parameters in Table 3, the bond distance and angle in the SO 2 molecule insignificantly change after adsorption. The E ads of M3 exhibited in Table 3 is −1.327 eV, which is slightly smaller than that in the M2 system. Above all, according to the large amount of E ads and charge transfer between the SO 2 molecule and the M2 system, chemisorption of Ni-MoS 2 monolayer to SO 2 can be concluded. Although the E ads of M3 system is very close to that of the M2 system, these two new built bonds between the SO 2 molecule and Ni atom in M3 system means a higher barrier during the adsorption process. Therefore, the M2 system is the most stable configuration. To further verify the conclusion, the DOS and the electron density difference are intensively discussed below. Figure 8a shows the TDOS of the M2 system. It is obvious that a small change occurs in TDOS around the area of −20 eV, −10.5 eV, −7 eV, −3 eV, and −1 eV for the SO 2 -adsorbed Ni-MoS 2 monolayer. Similarly, as the adsorption process mainly contributed by of the outmost orbitals of atoms, only the PDOS of the S-3p and Ni-3d are discussed, as shown in Figure 8b. According to the PDOS results, the peaks of S-3p orbital and the Ni-3d orbital overlap at −5.5 eV, −4 eV, −2 eV, and 2 eV, suggesting that the interaction between SO 2 and Ni-MoS 2 monolayer is strong chemisorption, and its electronic structures are relatively active. Considering the large contribution of the S-3p orbital in the adsorption process, we confirm that the SO 2 adsorption structure in the M2 system is very stable. The top view and side view of the M2 system are given in Figure 7c,d, the SO2 molecule adsorbs on the monolayer with a Ni-S bond length of 2.059 Å . From the structural parameters in Table 3, it is found that the structure of the SO2 molecule changes little after adsorption. The Eads of the M2 system has increased to −1.382 eV compared to that in the M1 system, which manifests the stability of the M2 system. In Addition, only 0.016 e transfers from the SO2 molecule to the Ni-MoS2 monolayer in the M2 system, and the charge transfers from the S atom, O1 atom, and O2 atom are 0.050 e, −0.033 e, and −0.033 e, respectively.
The top view and side view of the M3 system are given in Figure 7e,f, the O1 atom and S atom are trapped by the Ni-MoS2 monolayer with bond lengths of 1.948 Å (Ni-O1) and 2.258 Å (Ni-S). Due to the strong interaction of the Ni-O1 bond and Ni-S bond, the Qt of the M3 system is calculated to be −0.206 e, which is distinctly larger than that in the M1 and M2 system. From the structural parameters in Table 3, the bond distance and angle in the SO2 molecule insignificantly change after adsorption. The Eads of M3 exhibited in Table 3 is −1.327 eV, which is slightly smaller than that in the M2 system.
Above all, according to the large amount of Eads and charge transfer between the SO2 molecule and the M2 system, chemisorption of Ni-MoS2 monolayer to SO2 can be concluded. Although the Eads of M3 system is very close to that of the M2 system, these two new built bonds between the SO2 molecule and Ni atom in M3 system means a higher barrier during the adsorption process. Therefore, the M2 system is the most stable configuration. To further verify the conclusion, the DOS and the electron density difference are intensively discussed below. Figure 8a shows the TDOS of the M2 system. It is obvious that a small change occurs in TDOS around the area of −20 eV, −10.5 eV, −7 eV, −3 eV, and −1 eV for the SO2-adsorbed Ni-MoS2 monolayer. Similarly, as the adsorption process mainly contributed by of the outmost orbitals of atoms, only the PDOS of the S-3p and Ni-3d are discussed, as shown in Figure 8b. According to the PDOS results, the peaks of S-3p orbital and the Ni-3d orbital overlap at −5.5 eV, −4 eV, −2 eV, and 2 eV, suggesting that the interaction between SO2 and Ni-MoS2 monolayer is strong chemisorption, and its electronic structures are relatively active. Considering the large contribution of the S-3p orbital in the adsorption process, we confirm that the SO2 adsorption structure in the M2 system is very stable. With respect to the electron density difference in the M2 system, shown in Figure 9, the increase and decrease of the electron density are represented by the red and blue regions, respectively. It is found that two O atoms in SO2 receive electrons, and the electron density near the S atom decreases during the adsorption. In generally, the SO2 molecule acts as an electron acceptor according to the electron density distribution. With respect to the electron density difference in the M2 system, shown in Figure 9, the increase and decrease of the electron density are represented by the red and blue regions, respectively. It is found that two O atoms in SO 2 receive electrons, and the electron density near the S atom decreases during the adsorption. In generally, the SO 2 molecule acts as an electron acceptor according to the electron density distribution.

Adsorption of SF6 Gas on the Ni-MoS2 Monolayer
In order to ensure the practicability of the Ni-MoS2 adsorbent, the adsorption property of the Ni-MoS2 monolayer towards the SF6 molecule has also been studied, as SF6 will always be the largest part of components in SF6-insulated equipment. Various initial approaching sites of SF6 to the Ni-MoS2 monolayer were calculated in order to obtain the most stable adsorption structure. Two adsorption structures were received after geometric optimization, as shown in Figure 10 with different views. In addition, its adsorption energy, charge transfer, and other specific structural parameters are given in Table 4.

Adsorption of SF 6 Gas on the Ni-MoS 2 Monolayer
In order to ensure the practicability of the Ni-MoS 2 adsorbent, the adsorption property of the Ni-MoS 2 monolayer towards the SF 6 molecule has also been studied, as SF 6 will always be the largest part of components in SF 6 -insulated equipment. Various initial approaching sites of SF 6 to the Ni-MoS 2 monolayer were calculated in order to obtain the most stable adsorption structure. Two adsorption structures were received after geometric optimization, as shown in Figure 10 with different views. In addition, its adsorption energy, charge transfer, and other specific structural parameters are given in Table 4.

Adsorption of SF6 Gas on the Ni-MoS2 Monolayer
In order to ensure the practicability of the Ni-MoS2 adsorbent, the adsorption property of the Ni-MoS2 monolayer towards the SF6 molecule has also been studied, as SF6 will always be the largest part of components in SF6-insulated equipment. Various initial approaching sites of SF6 to the Ni-MoS2 monolayer were calculated in order to obtain the most stable adsorption structure. Two adsorption structures were received after geometric optimization, as shown in Figure 10 with different views. In addition, its adsorption energy, charge transfer, and other specific structural parameters are given in Table 4.   As the parameters show in the Table 4, the E ads is only −0.174 eV for M1, and 0.181 eV for M2. Q t is −0.445 e and −0.454 e for the M1 and M2 structures, respectively. Though the d F1-S of SF 6 suffers a very small increase compared with that of free SF 6 molecule, it is still difficult to break its chemical bonds by the weak adsorption energy. Therefore, the SF 6 molecule interacts with Ni-MoS 2 monolayer by physisorption. Once H 2 S and SO 2 decomposition components occur in SF 6 -insulated equipment, H 2 S and SO 2 quickly fill the role of the adsorption of SF 6 because of its strong adsorption energy of H 2 S and SO 2 . Additionally, the repulsion between gas molecules will block the interaction between the SF 6 molecule and Ni-MoS 2 monolayer. As a result, the Ni-MoS 2 monolayer can be a good adsorbent to H 2 S and SO 2 in a SF 6 atmosphere.

Conclusions
In this study, a Ni-MoS 2 monolayer material has been proposed as a potential adsorbent to remove the typical decomposition components of SF 6 under partial electric discharge: H 2 S and SO 2 . All of the calculations performed with respect to density functional theory analysis and all of the conclusions only considered the final adsorption energy; the barrier in transition state has not been analyzed in this paper. Various adsorption models of H 2 S and SO 2 molecules on the Ni-MoS 2 monolayer were built to find the most stable adsorption structure by analyzing the adsorption energy, charge transfer, and other structural parameters. To further analyze the interaction mechanism, the DOS, PDOS and electron density difference were presented and analyzed. We concluded that H 2 S and SO 2 tend to adsorb on the surface of Ni-MoS 2 monolayer by chemisorption, and the adsorption energy of the H 2 S and SO 2 is up to −1.319 eV and −1.382 eV, respectively, indicating that the interaction between these two kinds of gases and the Ni-MoS 2 monolayer is pretty strong. Additionally, the weak physisorption between SF 6 and the Ni-MoS 2 monolayer provides the basis for selectively adsorbing H 2 S and SO 2 from the SF 6 atmosphere. Therefore, the Ni-MoS 2 monolayer might be a promising gas adsorbent to remove these two typical decomposition components of SF 6 , which plays a key role in enhancing the running stability of SF 6 -insulated equipment.
Author Contributions: Y.G. proposed the project and analyzed the simulation results. H.W. and Y.G. contributed to the DFT simulations. J.K., W.W. and C.T. performed the analysis of the data and provided some revision of the manuscript. All authors read and approved the final manuscript.