Interactions of nitric oxide molecules with pure and oxidized silver clusters Ag_n ^±/Ag_nO^± (n=11-13): A computational...

In this work, we have studied, within density functional theory, the interaction of NO with pure and oxidized silver clusters, both anionic and cationic, composed from 11 to 13 Ag atoms. In that size interval, shell closing effects are not expected, and structural and electronic odd-even effects will determine the strength of interaction. First, we obtained that species Ag_n ^± and Ag_nO^± with odd number of electrons (n = 12) adsorb NO with higher energy than their neighbors (n = 11 and 13). This result is in agreement with the facts observed in recent mass spectroscopy measurements, which were performed, however, at finite temperature. The adsorption energy is about twice for oxidized clusters compared to pure ones and higher for anions than for cations. Second, the adsorption of another NO molecule on Ag_nNO^± forms Ag_n(NO)₂ ^±, with the dimer (NO)₂ in cis configuration, and binding the two N atoms with two neighbor Ag atoms. The n = 12 species show the higher adsorption energy again. Third, in the absence of reaction barriers, all complexes Ag_n(NO)₂ ^± dissociate spontaneously into Ag_nO^± and N₂O, except the n = 12 anion. The maximum high barrier along the dissociation path of Ag₁₃(NO)₂ ^- is about 0.7 eV. Further analysis of projected density of states for Ag_11-13(NO)_x ^± (x = 0, 1, 2) molecules shows that bonding between NO and Ag clusters mainly occurs in the energy range between -3.0 and 3.0 eV. The overlap between 4d of Ag and 2p of N and O is larger for Ag₁₂(NO)₂ ^± than for neighbor sizes. For n = 12, the d bands are close to the (NO)₂ 2π orbital, leading to extra back-donation charge from the 4d of Ag to the closer 2π orbital of (NO)₂.