**3. Results and Discussion**

*3.1. Model Verification*

Molybdenum (Mo) has a body centered cubic (bcc) structure with an experimental lattice constant of 3.14 A˚ [34] and each conventional cell has 2 atoms. The theoretical lattice constant found in this study is 3.15 A which agrees well with the experimental value. ˚

In order to find out the thermodynamically most stable surface structure of bcc-Mo, the surface energy is calculated by the following equation:

$$\sigma = (E\_{slab} - \mathbf{n} \times \mathbf{E}\_{bulk}) / (2\mathbf{A}) \tag{1}$$

where A is the surface area and n is the number of atoms in the slab model, *Eslab* is the total energy of the surface, E*bulk* is the energy per Mo atom of the ground state structure.

The surface energies of low-index Mo surfaces are shown in Table 1. The surface energies of Mo (100), Mo (110) and Mo (111) are 0.22 eV/Å2, 0.17 eV/Å2 and 0.18 eV/Å2, respectively. Our results are the same with date reported in Materials Project Database [31]. All above results demonstrate that the input parameters and surface models used in the present study are reliable. In addition, considering that Mo (110) surface has the lowest surface energy, we focus on understanding Na and O adsorption properties on this energetically most stable surface.

**Table 1.** Surface energies of low-index surfaces.

