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

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