*3.4. Impact of O/Na atom Adsorption on the Vacancy Formation of Mo (110) and Mo-Re (110) Surfaces*

We propose that Mo dissolution into liquid Na is a potential scenario of Mo corrosion. In this study, the Mo vacancy formation energy is also calculated to evaluate the effect of O and Re on the corrosion resistance of Mo surface. The vacancy formation energy is defined using the following Equation (5):

$$E\_{\text{vac}}^f = E\_{\text{vac}-surface} + E\_{\text{Mo}} - E\_{\text{surfacc}} \tag{5}$$

where *Esur f ace* stands for the energy of surfaces without vacancies, *EMo* is the energy of a Mo atom in its BCC structure and *Evac*−*sur f ace* is the surface models with a Mo vacancy in the upmost layer.

Table 5 shows that the vacancy formation energies are 1.45 eV and 1.32 eV for the clean Mo (110) surface and the clean Mo-Re (110) surface, respectively. Therefore, introducing Re atom into the Mo (110) surface can enhance forming surface vacancies. The surface vacation formation energies of Mo (110) or Mo-Re (110) surface with an Na or O atom is also calculated. A single Na atom or O atom can reduce the surface vacancy formation energy by 0.4~0.5 eV. It can be inferred that a low coverage of Θ = <sup>1</sup> <sup>8</sup> ML of Na or O can destabilize the Mo (110) and Mo-Re (110) surface. The effect of Na/O synergetic effect on vacancy formation is also studied. The present theoretical results show that the vacancy formation energy is significantly decreased when a Na atom and a O atom are co-adsorbed on the Mo (110) surface. Therefore, it can be inferred that impurity can facilitate the dissolution of Mo atoms and lead to corrosion. For the Na/O co-adsorption condition, the existence of Re atom can increase the surface vacancy formation energy to 1.06 eV. Hence, Re can prevent the Mo alloy from corrosion in liquid Na with O impurities.


**Table 5.** Mo vacancy formation energy on different surface models.
