The adsorption of CO and NO molecules on bimetallic Cu5Sc and Cu6Sc+ clusters is investigated using density functional theory calculations. The obtained adsorption energies indicate that the binding of CO and NO molecules on Cu5Sc and Cu6Sc+ clusters via carbon and nitrogen atoms, respectively, is stronger than that via oxygen atom. CO and NO adsorption energies on Cu5Sc vary from -0.89 to -74.68 and -2.67 to -106.92 kcal/mol, respectively, whereas those on Cu6Sc+ range from -4.28 to -28.13 and -4.27 to -88.87 kcal/mol. The results indicate that the doping of a Sc atom into pure Cu5 to form Cu5Sc cluster improves the adsorption strength of CO and NO molecules. The natural bond orbital (NBO) analysis demonstrates that charge-transfer from the cluster into the 2π* orbital of CO and NO molecules is essential for the stability of the derived complexes. There is an acceptable linear correlation between the transferred NBO charges and the adsorption energies for the CO or NO complexes with Cu5Sc, while a poor linear correlation is observed for the Cu6Sc+ complexes. According to a topological analysis of the electron density, the adsorption of CO and NO onto Cu5Sc and Cu6Sc+ in the resulting most stable complexes is primarily electrostatic with a partially covalent character.
