Effect of Spin Multiplicity in O₂ Adsorption and Dissociation on Small Bimetallic AuAg Clusters

To dispose of atomic oxygen, it is necessary the O₂ activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O₂ adsorption and dissociation on small pure Au_n and Ag_m and bimetallic Au_nAg_m (n + m ≤ 6) clusters using the density functional theory (DFT) and the zeroth-order regular approximation (ZORA) to explicitly include scalar relativistic effects. The most stable Au_nAg_m clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O₂ adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O₂ is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O₂ occupies the σ*_O-O antibonding natural bond orbital, which weakens the oxygen bond. The Au₃ (²A) cluster presents the lowest activation energy to dissociate O₂, whereas the opposite applies to the AuAg (³A) system. In the O₂ activation, bimetallic clusters are not as active as pure Au_n clusters due to the charge donated by Ag atoms being shared between O₂ and Au atoms.