Study of odd-even effects in physisorption and chemisorption of Ar, N2, O2, and NO on open shell Ag11-13+ clusters by means of self-consistent van der Waals density functional calculations
We have studied the adsorption and coadsorption properties of one or more X = Ar, N2, O2, and NO adsorbates on cationic silver clusters Ag11-13+, whose sizes are in the open shell region of metal clusters, aimed to understand the observed odd-even effects in the abundance spectra of Ag11-13+. mX complexes. All calculations were performed self-consistently using a non-local van der Waals correlation functional, covering the different nature of interactions between silver substrate and the several adsorbates, which range from dispersion (London) forces for Ar, non covalent π-π interaction for N2, charge-transfer interactions for O2 and NO, and the covalent Ag-Ag bond in the nude silver cluster. Despite the wide interval of adsorption energies, spanning two order of magnitude, we have been able to explain the following experimental facts. For X = Ar, N2, and O2 reactions with Ag11-13+, it was observed in the mass spectra an abundance peak at n=12 [M. Schmidt et al, ChemPhysChem, 16, 855 (2015)]. In addition, it was observed the competitive adsorption of two or more N2 molecules, and the cooperative effect of adsorbing N2 together with O2 molecules. For X=NO, an abundance peak at n=12 has been also observed [J. Ma et al, Phys. Chem. Chem. Phys. 18, 12819 (2016)]. We find that the main factors determining these properties are the different core motifs of the cluster geometry (pentagonal bipyramid for Ag11+ and Ag13+, but triangular prism for Ag12+), and, on the other side, the odd number of valence electrons for Ag12+, leading to a smaller HOMO-LUMO gap than those of its neighbours. Further details about the preferred adsorption sites, dipole moments, and dipole polarizabilities are also discussed.