Comparing B3LYP and B97 Dispersion-corrected Functionals for Studying Adsorption and Vibrational Spectra in Nitrogen Reduction

Electrochemical ammonia synthesis is being actively studied as a low temperature, low pressure alternative to the Haber-Bosch process. This work studied pure iridium as the catalyst for ammonia synthesis, following promising experimental results of Pt-Ir alloys. The characteristics studied include bond energies, bond lengths, spin densities, and free and adsorbed vibrational frequencies for the molecules N₂, N, NH, NH₂, and NH₃. Overall, these descriptive characteristics explore the use of dispersion-corrected density functional theory methods that can model N₂ adsorption – the key reactant for electrochemical ammonia synthesis via transition metal catalysis. Specifically, three methods were tested: hybrid B3LYP, a dispersion-corrected form B3LYP-D3, and semi-empirical B97-D3. The latter semi-empirical method was explored to increase the accuracy obtained in vibrational analysis as well as reduce computational time. Two lattice surfaces, (111) and (100), were compared. The adsorption energies are stronger on (100) and follow the trend E_B3LYP>E_B3LYP-D3>E_B97-D3 on both surfaces.