Effect of Spin Multiplicity in O 2 Adsorption and Dissociation on Small Bimetallic AuAg Clusters.

To dispose of atomic oxygen, it is necessary the O ₂ activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O ₂ adsorption and dissociation on small pure Au _n and Ag _m and bimetallic Au _n Ag _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 _n Ag _m clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O ₂ adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O ₂ is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O ₂ occupies the σ* _O-O antibonding natural bond orbital, which weakens the oxygen bond. The Au ₃ ( ² A) cluster presents the lowest activation energy to dissociate O ₂ , whereas the opposite applies to the AuAg ( ³ A) system. In the O ₂ activation, bimetallic clusters are not as active as pure Au _n clusters due to the charge donated by Ag atoms being shared between O ₂ and Au atoms.