2. Computational Methods
All the spin-polarized calculations are performed by Vienna Ab Initio Simulation Package (VASP) on the framework of density functional theory (DFT) study35, where the interaction between ions and electrons is described by Projector-augmented wave (PAW) method36, and cutoff energy of 500 eV is used to certain the basis sets. Geometric optimization is carried out by the Perdew-Burke-Ernzerhof (PBE) functional with the generalized gradient approximation (GGA)37. To avoid the interlayer interaction induced by the periodic boundary condition, a 20 Å vacuum layer is applied in the z-direction. The DFT-D3 method38 is implemented to describe the Van der Waals dispersion. The 3×3×1 Γ-center k-mesh is used for structural relaxation, while a 5×5×1 k-mesh is used for electronic properties calculation. The energy and force convergence criteria are set to 10-5eV and -0.02 eV/Å, respectively. The Bader analysis 39is employed to explore the charge transfer quantitively. The projected Crystal Orbital Hamilton Population (COHP)40 is used to analyze the N-N triple and TM-N bonds’ bonding nature for the adsorbed systems by implementing LOBSTER code 41. To test the thermostability of the selected eNRR candidates, ab Initio molecular dynamics simulations (AIMD)42 are performed for 9 ps with a time step of 1.5 fs. Since the eNRR usually occurs under the aqueous condition, the solvent effect is considered using the VASPsol model 43. The VASPKIT code44developed by Wang et al. is employed for pre- and post-data extraction.
For the eNRR process, the Gibbs free energy of each hydrogenation step is calculated using the computational hydrogen electrode model proposed by Nørskov et al. 45. The eNRR to ammonia catalytic reaction contains six proton-coupled electron transfer processes, for each process, the Gibbs free energy change can be obtained as:
ΔG = ΔE + ΔZPE -ΔTS + ΔGU + ΔGpH (1)
where ΔE, ΔZPE, T, and S are the total energy changes obtained from DFT calculation, zero-point energy, temperature, and entropy, respectively. GU is the electrode potential, and GpHis the effect of the pH.