**4. Conclusions**

In this study, the adsorption and diffusion behaviors of Na and O atoms on BCC-Mo (110) surface are investigated by a first-principles approach. It is found that the Hollow site is the most energetically preferred adsorption site for a single Na or O atom. The existence of a Re alloy atom in the first atomic layer of the surface can strengthen the attractive interaction between the adsorbate and the substrate. The diffusion barrier of the Na atom on the Mo (110) surface is only 0.037 eV, and the pre-adsorbed O atom and Re atom can significantly impede Na diffusion. The surface vacancy formation energy is calculated for evaluating the stability of the Mo (110) surface. It is found that the Na or O atom can decrease the formation energy of the surface vacancy. It is worth noting that the Na/O co-adsorption can significantly reduce the vacancy formation energy to 0.47 eV for the Mo (110) surface, which indicates that the dissolution of surface atoms is a potential mechanism for the Mo in the Na liquid with O impurity. However, Re as alloy element can increase the resistance to the dissolution induced by Na/O co-adsorption.

**Author Contributions:** Q.Z. performs simulation and data analyzation, and writes the original draft; Z.L. concept this work; W.L., M.M. and H.D. refine the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study is financially supported by the President Foundation of China Academy of Engineering Physics (YZJJLX2018002).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

