Effect of Spin Multiplicity in O<sub>2</sub> Adsorption and Dissociation on Small Bimetallic AuAg Clusters

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