Density functional study of molecular nitrogen adsorption on gold-copper and gold-silver binary clusters.

Density functional theory calculations were performed to investigate the adsorption behaviors of nitrogen molecule on small bimetallic AunCum and AunAgm clusters, with n + m ≤ 5. In all cases the N2 forms a linear or quasi-linear M-N-N structure (M = Au, Cu or Ag). The adsorption energies of N2 on pure metal clusters follow the order CunN2 > AunN2 > AgnN2, which is due to the weaker orbital interaction between silver and N2. N2 prefers to bind to a copper atom in AunCumN2 complexes and prefers to bind to a silver atom in AunAgmN2 complexes. The combination of Cu atoms into Aun clusters makes the cluster more reactive toward N2 while the combination of Ag atoms into Aun clusters makes the cluster less reactive toward N2. The electrostatic interaction is strengthened while the back-donation from metal to N2 is reduced in bimetallic cluster nitrides, as compared to the mono cluster nitrides. The N-N stretching frequencies are all red-shifted upon adsorption and the M-N stretching frequencies are highly correlated to the atoms to which the N is attached.