Computational Study on Ammonia Adsorption on the X12Y12 Nano-Clusters (x = B, Al and Y = N, P)

The adsorption of ammonia (NH₃) on four X₁₂Y₁₂ (X = B, Al and Y = N, P) nano-cages was investigated using density functional theory calculations. Changes in Gibbs free energy, adsorption energy, frequency modes, natural bond orbitals, molecular electrostatic potential surfaces and density of states were explored. It was found that the relative order of the acidity for the surfaces is: Al₁₂N₁₂ > Al₁₂P₁₂ > B₁₂N₁₂ > B₁₂P₁₂. In the case of Al₁₂N1₂, hybridization and lower electron density of the adsorbing atom overcome the disadvantage of high HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) energy gap (E_g); therefore, this cage becomes the most favourable for the adsorption of NH₃. We found that changes in the electronic properties of the clusters are negligible after the adsorption process. Also, the value of E_g increases with the increase in the Hartree-Fock exchange per cent of the density functional.