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.