Theoretical study of oxygen adsorption on pure Aun+1+ and doped MAun+ cationic gold clusters for M = Ti, Fe and n=3-7

A comparative study of the adsorption of an O-2 molecule on pure Au-n+1(+) and doped MAun+ cationic gold clusters for n = 3-7 and M = Ti, Fe is presented. The simultaneous adsorption of two oxygen atoms also was studied. This work was performed by means of first principles calculations based on norm-conserving pseudo-potentials and numerical basis sets. For pure Au-4(+), Au-6(+), and Au-7(+) clusters, the O-2 molecule is adsorbed preferably on top of low coordinated Au atoms, with an adsorption energy smaller than 0.5 eV. Instead, for Au-5(+) and Au-8(+), bridge adsorption sites are preferred with adsorption energies of 0.56 and 0.69 eV, respectively. The ground-state geometry of Au-n(+) is almost unperturbed after O-2 adsorption. The electronic charge flows towards O-2 when the molecule is adsorbed in bridge positions and towards the gold cluster when O-2 is adsorbed on top of An atoms, and both the adsorption energy and the O-O bond length of adsorbed oxygen increase when the amount of electronic charge on O-2 increases. On the other hand, we studied the adsorption of an O-2 molecule on doped MAun+ clusters, leading to the formation of (MAunO2+)(ad) complexes with different equilibrium configurations. The highest adsorption energy was obtained when both atoms of O-2 bind on top of the M impurity, and it is larger for Ti doped clusters than for Fe doped clusters, showing an odd-even effect trend with size n, which is opposite for Ti as compared to Fe complexes. For those adsorption configurations of (MAunO2+)ad involving only Au sites, the adsorption energy is similar to or smaller than that for similar configurations of Aun+1O2+ complexes. However, the highest adsorption energy of (MAunO2+)(ad) is higher than that for (Aun+1O2+)ad by a factor of similar to 4.0 (1.2) for M = Ti (M = Fe). The trends with size n are rationalized in terms of O-O and O-M bond distances, as well as charge transfer between oxygen and cluster substrates. The spin multiplicity of those (MAunO2+)(ad) complexes with the highest O-2 adsorption energy is a maximum (minimum) for M = Fe (Ti), corresponding to parallel (anti-parallel) spin coupling of MAun+ clusters and O-2 molecules. Finally, we obtained the minimum energy equilibrium structure of complexes (AunO2+)(dis) and (MAunO2+)(dis) containing two separated O atoms bonded at different sites of Au-n(+) and MAun+ clusters, respectively. For (MAunO2+)(dis), the equilibrium configuration with the highest adsorption energy is stable against separation in MAun+ and O-2 fragments, respectively. Instead, for (AunO2+)(dis), only the complex n = 6 is stable against separation in Au-n(+) and O-2 fragments. The maximum separation energy of (MAunO2+)(dis) is higher than the O-2 adsorption energy f (MAunO2+)(ad) complexes by factors of similar to 1.6 (2.5), 1.6 (1.7), 1.5 (2.4), 1.5 (1.3), and 1.6 (1.8) for M = Ti (Fe) complexes in the range n = 3-7, respectively.