# Theoretical Study of SOF\({}_{2}\) Adsorption on Pd/Pt-Ni(111) Bimetallic Surfaces
## 3 Results and Discussion  
### Adsorption of SOF\({}_{2}\) on Pd-Ni(111) and Ni-Pd-Ni(111) Surfaces  
The Pd layer was used to replace the outermost layer and secondary outer layer of the Ni(111) surface to obtain stable Pd-Ni(111) and Ni-Pd-Ni(111) surfaces. Then, the SOF\({}_{2}\) molecule was placed in different initial positions and directions to approach these two surfaces respectively. After geometric optimization, several stable structures were obtained as shown in Figure 2. The adsorption energy and relevant structural parameters are listed in Table 1.  
Figure 1: (**a–e**) Structures of the Ni(111) monometallic and Pd/Pt-Ni(111) bimetallic surfaces, (**f**) Structure of SOF\({}_{2}\). Figure 2: The top and side views of adsorption structures. (**a**,**b**) SOF\({}_{2}\) adsorption on Pd-Ni(111) surface for M1 and M2 structures, (**c**,**d**) SOF\({}_{2}\) adsorption on Ni-Pd-Ni(111) surface for M3-M4 structures. Figure 2 shows the different adsorption structures of SOF\({}_{2}\) on Pd-Ni(111) and Ni-Pd-Ni(111) surfaces. In the M1 structure, the SOF\({}_{2}\) molecule approaches the Pd-Ni(111) surface through S atom, forming a new chemical bond between the S atom and the Pd atom. The distance of the S-Pd bond is 2.183 A, while the bond lengths in the SOF\({}_{2}\) molecule have almost not changed. In the M2 structure, three atoms interact with the Pd-Ni(111) surface, covering the hcp and fcc positions. Compared with the M1 structure, the deformation of M2 seems to be more serious. In the M3 and M4 structures, SOF\({}_{2}\) adsorption on the surface had an adsorption distance of 3.793 A and 3.555 A, indicating that the adsorption on Ni-Pd-Ni(111) surface was much more moderate. As listed in Table 1, the adsorption energy of the four structures in Figure 2 was negative, indicating that the adsorption process is exothermic. The \(E_{\text{ads}}\) of M1 and M2 are \(-\)1.964 eV and \(-\)1.880 eV respectively, which are higher than the \(E_{\text{ads}}\) of SOF\({}_{2}\) adsorption on the Ni(111) surface (\(-\)1.55 eV). During the adsorption process, charge transferred from the SOF\({}_{2}\) molecule to the Pd-Ni(111) surface, forming a SOF\({}_{2}\)+ cation. Compared with the results on the Ni-Pd-Ni(111) surface, Pd-Ni(111) showed greater capability of adsorbing SOF\({}_{2}\) for its higher adsorption energy. In the M3 and M4 structures, charge transfer occurred from the Ni-Pd-Ni(111) surface to the SOF\({}_{2}\) molecule, which is the opposite of M1 and M2. To further study the electronic behavior of SOF\({}_{2}\) adsorption, the total density of states (TDOS) and partial density of states (PDOS) were calculated. The structures of M1 and M3 were selected for comparison of SOF\({}_{2}\) adsorption on the Pd-Ni(111) and Ni-Pd-Ni(111) surfaces. As shown in Figure 3a, there are some obvious changes in TDOS after SOF\({}_{2}\) adsorption on the Pd-Ni(111) surface. The TDOS increased from \(-\)9.5 eV to \(-\)7.5 eV and \(-\)4 eV to \(-\)1 eV, and a little decline occured from \(-\)5.5 eV to \(-\)4.5 eV. However, there are some differences with SOF\({}_{2}\) adsorption on the Ni-Pd-Ni(111) surface in Figure 3c, the TDOS has a visible rise from \(-\)10 eV to \(-\)3 eV in the M3 structure. In viewing the PDOS in Figure 3b, the change of TDOS in M1 structure mainly consists of a 2p orbital of F atoms, a 3p orbital of S atom, a 3d of Ni atoms, and a 4d of Pd atoms. The large overlapped area between S 3p, Ni 3d, and Pd 4d signifies that their orbitals are highly hybridized during the adsorption process. As for the M3 structure, the PDOS has a large overlapped area between F 2p and Pd 4d, indicating a strong interaction between them. Comparing the change of TDOS and PDOS, it is concluded that the interaction between the O atom, S atom and Pd atoms in M1 is stronger than that in M3. The result is consistent with the analysis of adsorption energy in Table 1.