Adsorption of small molecules on silver clusters

We report investigations of adsorption of N ₂ and O ₂ molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n 6 to 7 and for cations, in agreement with experiments, from n 4 to 5. With the refined structures, adsorption energies of N ₂ and O ₂ molecules have been calculated. We have identified characteristic drops in the adsorption energies of N ₂ that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag ₇ structure slightly more than the 2D counterpart. Results for pure O ₂ adsorption indicate that charge transfer from Ag _n to O ₂ occurs when n 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N ₂-induced increase in charge transfer from Ag _n cations to O ₂.