Au Catalyst-Modified MoS2 Monolayer as a Highly Effective Adsorbent for SO2F2 Gas: A DFT Study.

To ensure the stable operation of gas-insulated equipment, removal of SF6 decomposition products of sulfur hexafluoride (SF6) is one of the best methods. SO2F2 is one of the typical decomposition products of SF6, while the Au-modified MoS2 (Au-MoS2) monolayer is a novel gas adsorbent. Therefore, based on the first-principles calculation, the adsorption properties of the SO2F2 molecule on the Au-MoS2 monolayer are calculated. Furthermore, the adsorption energy, charge transfer, and structure parameters were analyzed to obtain the most stable adsorption structure. These results indicate that all of the adsorption processes are exothermic. To better study the adsorption mechanism between the SO2F2 molecule and the Au-MoS2 monolayer, the density of states, the highest occupied molecular orbital, the lowest unoccupied molecular orbital, and electron density difference were obtained. At last, we conclude that the interaction between the SO2F2 molecule and the Au-MoS2 monolayer was chemisorption. This study provides a theoretical basis to prepare the Au-MoS2 monolayer for the removal of SF6 decomposition products.

Introduction

Over the past decades, sulfur hexafluoride (SF6) has been widely used in gas-insulated switchgear (GIS) and other gas-insulated equipment owing to its excellent insulating and arc extinguishing properties.[1] Even so, during the long-term running equipment, partial discharge (PD) frequently occurs due to the inevitable internal insulation defects.[2,3] Previous studies found that SF6 insulation gas will decompose to low-fluorine sulfides (SF, x = 1–5) under partial discharge.[4,5] These decomposed products would subsequently react with traces of gaseous O2 and H2O existing in SF6-insulated equipment and form various characteristic decomposition products, including SOF2, SO2F2, SO2, etc.[6,7] The running stability of gas-insulated equipment can be greatly affected by the decomposition products of SF6, which may ultimately lead to the breakdown of the power system.[8,9] To ensure the strong insulation of SF6-insulated equipment, one of the effective approaches is to maintain the purity of the filled gas, that is, removing the decomposition products of SF6 online.[10,11] Considering the large concentration and high chemical stability of SO2F2, it is urgent to find an effective adsorbent for its removal.

Currently, the MoS2 monolayer, with good chemical stability, good thermal stability, high specific surface area, and high surface activity, has become one of the research hotspots of gas adsorbent.[12,13] Metal-modified MoS2 materials have been investigated for application in gas molecules adsorption, which shows a broad application prospect.[14−16] Furthermore, based on the first-principles calculations, researchers verified that a noble metal-modified MoS2 monolayer shows great adsorption ability toward specific gas molecules.[15,17] For MoS2 modification, Wang et al. studied the adsorption and diffusion of noble metal atoms on MoS2 based on the density functional theory (DFT).[18−20] Studies proved that metal modification can effectively enhance the chemical activity and sensing sensitivity of modified MoS2 compared to that of the intrinsic monolayer.[21−23] Chen et al. studied the physical adsorption between Au-modified MoS2 and gas molecules (SOF2 and SO2F2) that did not break the structure of gas molecules.[24] However, SOF2 and SO2F2 may also chemically adsorb on metal-modified MoS2. For this reason, a Au catalyst-modified MoS2 monolayer was proposed as a highly effective adsorbent for SO2F2 removal in SF6-insulated equipment.[25]

In this study, based on the density functional theory (DFT), the adsorption of SO2F2 on the Au-modified MoS2 (Au–MoS2) monolayer with different initial conditions was studied to systemically analyze the adsorption mechanism.[26,27] The adsorption energy, charge transfer, electron density difference, and density of states (DOS) were calculated to explore the interaction mechanism between SO2F2 and Au–MoS2. This study results provide a theoretical basis for preparing a Au–MoS2 monolayer adsorbent for SO2F2 removal in the experiment.

Result and Discussion

Structural Property of Gas Molecule Models and Au–MoS2 Monolayer

Before gas adsorption, to gain the most stable structure, the structures of a SF6 gas molecule and its decomposed product SO2F2 and the Au–MoS2 monolayer model were built and optimized. Figure shows the optimized structure of SO2F2 and SF6 molecules. The bond lengths and the bond angles are marked in the molecular structures. The central S atom in the SO2F2 molecule has an sp3 hybridization with two O atoms and two F atoms. The same situation exists in the SF6 molecule, where the S atom and the four F atoms have an sp3 hybridization. The bond length of S–F in SO2F2 is 1.611 Å, which is longer than the S–O bond (1.442 Å) because of the smaller radius of the O atom. The bond angle of O–S–F, F–S–F, and O–S–O is 107.738, 94.355, and 126.741°, respectively. The S–F bonds in the SF6 molecule are perpendicular to each other, and the bond length is similar to the S–F bond length in the SO2F2 molecule, which is 1.616 Å. Analysis of Mulliken atomic charges shows that the S atom in the SO2F2 molecule has a positive charge of 0.868e, while the O and F atoms have negative charges of 0.214e and 0.220e, respectively. However, the S atom in the SF6 molecule obtains a positive charge of 1.194e, which is transferred from the F atoms. A large number of electron transfers indicate that there is a strong interaction between F atoms and S atom in the SF6 molecule, which ensures that the SF6 molecule has a very stable structure.

Figure 1

Optimized structures of (a) SO2F2 molecule and (b) SF6 molecule.

As shown in Figure , the Au atom approaches the Mo atom from the top site and forms bonds with three surrounding S atoms. The bond lengths of the Au–S bond are 2.805 Å, and the Au atom shows a weak interaction with the Mo atom because of the long distance between them (3.767 Å). The bond angles of the Mo–S–Mo and S–Mo–S in the MoS2 monolayer without Au atom modification are both 81.55°. However, the bond angles slightly decreased to 81.34 and 80.99°, respectively, after Au modification. The obvious changes of angle and the binding energy (−0.448 eV) indicate that the modified Au atom has a strong interaction with the MoS2 monolayer. The carried charge of the Au atom after modification was also calculated, which is −0.038e. This indicates that the electron transfer from the Au atom to MoS2 and further verifies the above conclusion.

Figure 2

Top view (a) and side view (b) of the Au–MoS2 monolayer.

To further investigate the influence of Au atom modification on the electronic properties of MoS2, the total density of states (TDOS) and projected density of states (PDOS) of intrinsic MoS2 and the Au–MoS2 monolayer were calculated, as shown in Figure . It can be found that the waveform of TDOS nearly not changes except a distinct left shift after Au atom modification. This means that after the MoS2 monolayer is doped with Au atoms, the number of electrons in the system increases and the metallicity is enhanced. Near the Fermi level, the value of TDOS decreases significantly after Au atom doping, which means that doping of Au atom evidently enhances the metallic properties of MoS2. According to the PDOS, the peaks of Au 5d orbitals and S 3p orbitals overlap at −5, −3, and −2 eV, indicating that the d orbital strongly hybridizes with the p orbital. Strong orbital hybridization means that the Au atom and MoS2 monolayer can be well combined.

Figure 3

(a) TDOS and (b) PDOS of Au–MoS2.

Furthermore, the band gap of MoS2 and Au–MoS2 was also calculated. It is found that the band gap changes from 2.057 to 0.266 eV after modification. The reduction of the band gap is conducive to electron transfer from the valence band and conduction band. As a result, the modified Au atom acts as an active site to form strong interaction between the Au–MoS2 monolayer and the target gas molecules.

Adsorption Structures of the SF6 Molecule

To ensure that the Au–MoS2 monolayer can act as an adsorbent for decomposition products of the SF6 molecule, we first studied the interaction mechanism between the Au–MoS2 monolayer and the SF6 molecule so as to ensure that the adsorbent would not affect SF6 molecules. Figure a–c shows one F atom, two F atoms, and three F atoms close to the Au–MoS2 monolayer, respectively. According to our analysis, the adsorption energies of the SF6 molecule are −0.004, −0.001, and −0.003 eV and the transfer charges are −0.116e, −0.021e, and −0.018e, respectively, in three cases close to those of the Au–MoS2 monolayer, indicating that there is only a weak force between the SF6 molecule and the Au–MoS2 monolayer. In the three cases shown in Figure , the distance between the SF6 molecule and the Au–MoS2 monolayer is so far that it is difficult to form a bond and the closest distance is 2.749 Å, which further verifies the above conclusion. Therefore, we believe that the Au–MoS2 monolayer can be used to study as an adsorbent for the decomposition products of the SF6 molecule.

Figure 4

Adsorption structures of SF6 on the Au–MoS2 monolayer: (a) P1 structure; (b) P2 structure; and (c) P3 structure.

Adsorption Structures of Single and Double SO2F2 Molecules

To obtain the most stable adsorption structure, several adsorption structures with different initial approaching sites were built for calculation. In this study, adsorption of single and double SO2F2 molecules was taken into account at the same time to systematically analyze the adsorption ability of the Au–MoS2 monolayer to the SO2F2 molecule. Figure shows a typical adsorption of single SO2F2 molecule on the Au–MoS2 monolayer, which shows the largest adsorption energy in all monomolecular adsorption structures. The corresponding structural parameter, adsorption energy, and total charge transfer of the SO2F2 molecule adsorbed on the Au–MoS2 monolayer are given in Table . For double SO2F2 molecule adsorption, two typical structures were obtained, as shown in Figure , with its parameters exhibited in Table . As the negative adsorption energy shown in Tables and 2, the adsorption process of the SO2F2 molecule on the Au–MoS2 monolayer is exothermic, indicating that the adsorption process is spontaneous.

Figure 5

Adsorption structures of single SO2F2 on the Au–MoS2 monolayer: (a) top view and (b) side view.

Table 1

Parameters of Single SO2F2 Molecule Adsorption System: Adsorption Energy (Eads in eV), Total Charge Transfer (Qt in e), and Bond Length between the Major Atoms (d in Å)

system	Eads (eV)	Qt-Mulliken (e)	Qt-Hirshefeld (e)	bond length (Å)
SO2F2/Au–MoS2	–0.559	–0.614	–0.461	1.483(S–O1), 1.485(S–O2), 1.689(S–F1), 2.012(Au–F2)

Figure 6

Adsorption structures of double SO2F2 on the Au–MoS2 monolayer: (a) top view and (b) side view.

Table 2

Parameters of Double SO2F2 Molecule Adsorbed on the Au–MoS2 Monolayer: Adsorption Energy (Eads in eV), Total Charge Transfer (Qt in e), and Bond Length (d in Å)

system	Eads (eV)	Qt-Mulliken (e)	Qt-Hirshefeld (e)	bond length (Å)
2SO2F2/Au–MoS2	–0.681	–0.664	–0.572	1.471(S–O1), 1.473(S–O2), 1.701(S–F1), 2.001(Au–F2)

As shown in Figure and Table , the Au–MoS2 monolayer shows strong adsorption to the SO2F2 molecule with an adsorption energy of −0.559 eV. When the SO2F2 molecule approaches the surface of the Au–MoS2 monolayer, the F1 atom separates from the SO2F2 molecule and approaches the modified Au atom with an Au–F1 bond length of 2.012 Å, indicating a chemical interaction between the SO2F2 molecule and the Au–MoS2 monolayer. The corresponding charge transfer values, Qt, calculated by Mulliken and Hirshfeld methods are −0.614e and −0.461e, respectively. Therefore, the SO2F2 molecule acts as an electron donor and the Au–MoS2 monolayer acts as an electron acceptor. The bond lengths of S–O1 and S–O2 increase to 1.483 and 1.485 Å, respectively. The distance between the Au atom and the adjacent Mo atom changes from 3.740 to 4.223 Å, and the bond length of S–F2 also increases. Therefore, a preliminary conclusion can be drawn from the above analysis that the Au–MoS2 monolayer can effectively adsorb the SO2F2 molecule.

To fully analyze the adsorption capacity of the Au–MoS2 monolayer to the SO2F2 molecules, it is necessary to explore the adsorption of double SO2F2 molecules. Therefore, we constructed all kinds of configurations with double SO2F2 molecules adsorbing on the Au–MoS2 monolayer. Figure shows the most stable adsorption structure of double SO2F2 molecules adsorbed on the Au–MoS2 monolayer; the S–F1 bond of the SO2F2 molecule is broken in the adsorption process, and the S atom and F1 atom rebond to the Au atom. However, the other SO2F2 molecule keeps away from the Au modification site with a really long distance (3.778 Å) due to the repulsive force between gas molecules. The Qt by Mulliken for double molecules is −0.664e and −0.007e, manifesting the weak interaction of the Au–MoS2 monolayer toward the second molecule. In addition, comparing with the adsorption structures for the corresponding parameters of single and double SO2F2 molecule adsorption, we conclude that double gas molecule adsorption is basically similar to that of single gas molecule adsorption.

Electronic Property

To explore the interaction mechanism between the SO2F2 molecule and the Au–MoS2 monolayer surface, the density of state (DOS), the molecular orbital theory (highest occupied molecule orbital, HOMO and lowest unoccupied molecule orbital, LUMO), and the electron density difference are discussed.

Figure a1,b1 shows the TDOS before and after SO2F2 molecule adsorption, and Figure a2,b2 presents the PDOS of the characteristic atoms after SO2F2 molecule adsorption. For bimolecular SO2F2 adsorption, only one molecule is closer to the surface of the Au–MoS2 monolayer. Therefore, only the SO2F2 molecule that is close to the Au–MoS2 monolayer is analyzed in PDOS analysis.

Figure 7

TDOS and PDOS: (a1)–(a2) single SO2F2 adsorption and (b1)–(b2) double SO2F2 adsorption.

It is obvious that SO2F2 molecule adsorption significantly changes the distribution of TDOS, which is mainly because of the interaction among Au 5d orbital, F 2p orbital, O 2p orbital, and S 3p orbital. For single SO2F2 molecule adsorption, the 6s and 5d orbitals of the Au atom, the 2p orbital of the F atom, and the 2p orbital of the O atom overlap in the range of −0.3–0 eV, as the PDOS shows in Figure a2. The large overlapped area among Au 5d, F 2p, and O 2p indicates that these orbitals are highly hybridized during the adsorption process. The overlap between the F 2p orbital and the Au 5d orbital is larger than the overlap between the O 2p orbital and the Au 5d orbital, resulting in the formation of a new Au–F bond during the adsorption process. For the TDOS of double SO2F2 adsorption, as shown in Figure b2,c2, a large overlapping area exists among Au 5d, F1 2p, and S1 3p from −0.3 to 0 eV, indicating a strong hybridization between the SO2F2 molecule and the Au–MoS2 monolayer. Comparing the changes in TDOS and PDOS, the hybridization degree of double SO2F2 molecule adsorption is similar to that of single SO2F2 molecule adsorption.

To further analyze the electronic property of the SO2F2 molecule adsorbed on the Au–MoS2 monolayer, molecule orbital theory is taken into consideration. The distributions for highest occupied molecule orbital (HOMO) and lowest unoccupied molecule orbital (LUMO) are exhibited in Figure , and the HOMO energy (EHOMO), LUMO energy (ELUMO), and the energy gap (Eg) between HOMO and LUMO are also given in the figure.

Figure 8

HOMO and LUMO distributions and related energies for Au–MoS2 systems: (a) Au–MoS2; (b) Au–MoS2/SO2F2; and (c) Au–MoS2/2SO2F2.

HOMO and LUMO energies for intrinsic Au–MoS2 are −5.402 and −5.112 eV, respectively, with an Eg of 0.290 eV. According to the orbital distributions of HOMO and LUMO for single SO2F2 molecule adsorption, the energy gap of Au–MoS2/SO2F2 increases to 0.981 eV, indicating that the SO2F2 molecule adsorption greatly decreases the conductivity. For the orbital distributions of double SO2F2 molecule adsorption systems, the HOMO and LUMO only distribute on one gas molecule that is close to the Au modification site. The adsorption results of double gas molecule adsorption are similar to that of single gas molecule adsorption, and the HOMO and LUMO energies decline after adsorption. When the double SO2F2 molecules adsorb on the Au–MoS2 monolayer, the energy gap has a little increase to 0.507 eV because of the repulsive force between gas molecules. Since the electrons in HOMO and LUMO are mainly located in the surrounding gas adsorbates, the energy reduction of EHOMO, ELUMO, and Eg confirms the strong interaction between the SO2F2 molecule and the Au–MoS2 monolayer. As a result, SO2F2 molecule adsorption decreases the conductivity of the Au–MoS2 system. Therefore, we can draw a conclusion that there is chemical adsorption between the single SO2F2 molecule and the Au–MoS2 monolayer.

The electron density difference of the SO2F2 molecule adsorbed on the Au–MoS2 monolayer with different systems is shown in Figure , where the increase and decrease of the electron density are represented by red and blue regions, respectively.

Figure 9

Electron density difference: (a) single SO2F2 adsorption and (b) double SO2F2 adsorption.

For single SO2F2 molecule adsorption, as shown in Figure a, both of the F atoms receive tiny electrons, while the electron density near the S atom decreases, which is in agreement with the conclusion that the SO2F2 molecule transfers quite a part of electrons to the adsorbent. For the double SO2F2 molecule adsorption, as shown in Figure b, the electron density near the Au atom decreases, which is similar to that of single SO2F2 molecule adsorption. For the SO2F2 molecule close to the Au atom, the electron density of the S atom suffers a decrease. For the other SO2F2 molecule away from the Au–MoS2 monolayer surface, the electron density changes slightly. Therefore, we conclude that the SO2F2 molecule acts as the electron acceptor and the Au–MoS2 monolayer acts as the electron donor in the SO2F2 molecule adsorption process. All of these indicate that there is a certain amount of electron transfer between the SO2F2 molecule and the Au–MoS2 monolayer, which provides a factual basis for the stable adsorption of the SO2F2 molecule on the Au–MoS2 monolayer.

Conclusions

In this study, the interaction mechanism between the SO2F2 molecule and the Au–MoS2 monolayer surface was analyzed based on DFT calculations. First, the most stable structures of SO2F2 and the Au–MoS2 monolayer were discussed. Then, the most stable structures of single SO2F2 molecule adsorption and double SO2F2 molecule adsorption structures were obtained based on lots of initial adsorption structures. The density of states, HOMO and LUMO distributions, and electron density difference were analyzed to further explore its adsorption mechanism. It is found that the SO2F2 molecule tends to chemically adsorb on the surface of the Au–MoS2 monolayer and the adsorption energies of the single SO2F2 molecule and the double SO2F2 molecules are up to −0.559 and −0.681 eV, respectively, indicating a strong interaction between the SO2F2 molecule and the Au–MoS2 monolayer. In addition, the adsorption of the SO2F2 molecule can significantly change the conductivity and charge distribution of the Au–MoS2 monolayer. Due the remarkable interaction between the SO2F2 molecule and the Au–MoS2 monolayer, the Au-modified MoS2 monolayer can be used as a promising adsorbent on removing the typical decomposition products of SF6.

Computational Details

All of the calculations in this paper were carried out based on DFT.[28−30] The generalized gradient approximation with the Perdew–Burke–Ernzerhof functional was chosen to calculate the electron exchange and correlation energy.[31,32] The double numerical plus polarization was used as the basis set, and the density functional semicore pseudopotential was applied for core treatment.[33,34] All of the geometry optimizations were performed with the energy tolerance accuracy, maximum force, and maximum displacement of 1.0 × 10–5 Ha, 2.0 × 10–3 Ha/Å, and 5.0 × 10–3 Å, respectively.[35,36] The self-consistent field was set to 1.0 × 10–6 Ha.[37,38] The Brillouin zone was determined by the 5 × 5 × 1 Monkhorst–Pack k-point sampling method.[39,40]

To avoid the interaction between adjacent structures, a 4 × 4 × 1 MoS2 monolayer supercell with a 15 Å vacuum layer was constructed. The optimized lattice constant of MoS2 is 3.180 Å in this study, which is consistent with other experimental and theoretical results.[41] In the center of the 4 × 4 × 1 MoS2 monolayer supercell, one Au atom was added at the top of the Mo atom, which bonds with three S atoms.

The adsorption energy (Eads) of the gas molecule on the Au–MoS2 monolayer is defined by eq and the unit is eV, where EAu–MoS is the total energy of the system after adsorption and EAu–MoS and Egas molecule represent the total energies of the Au–MoS2 monolayer and the free gas molecule, respectively. The negative value of Eads indicates that the adsorption process is exothermic and happens spontaneously.The charge transfer (Qt) between the gas molecule and the Au–MoS2 monolayer can be obtained by eq and the unit is e, where Qads represents the carried charge of the gas molecule after adsorption and Qiso represents the carried charge of the gas molecule before adsorption. A negative Qt means charges transfer from the Au–MoS2 monolayer to the gas molecule.The binding energy of the Au–MoS2 monolayer is defined in eq and the unit is eV, where EAu–MoS is the total energy of the Au atom-modified MoS2 monolayer and EAu and EMoS represent the total energy of the isolated Au atom and the intrinsic MoS2 monolayer, respectively. The negative Ebind indicates that the binding process is exothermic.