The adsorption and activation properties of precious metal clusters toward NO: A density functional study

Ultrafine precious metal particles supported on the surface of a metal oxide are widely used as the important heterogeneous catalysts such as those for air pollution control. A considerable amount of studies regarding three way catalysts have been carried out. Rhodium is effective in reducing NO_x in the exhaust gas from automobiles. In numerous studies, the adsorption of NO on the surfaces of single metals, e.g., Rh, Pd, and Pt, has been examined from the viewpoint of surface science. However, different types of complex surface structures were found and the consensus has not been met. The adsorption and activation characteristics of precious metal clusters, e.g., Ir, Pt, and Au toward NO were studied by density functional computations, focusing on the geometrical features of model clusters that could determine the ability for the adsorption and NO activation. The order of the energetical stability of the adsorption states of NO could be described as Ir > Pt > Au. It depended on neither the shape of the pentaatomic clusters nor the number of atoms in the model clusters. The ability of the precious metal clusters for the activation of the N-O bond were discussed from vibrational and geometrical points of view. The substantial activation of the N-O bond was found on the NO/Ir₅ and the NO/Pt₄ systems. This indicated that the specific adsorption geometries enhanced the ability for the activation of the N-O bond. The Ir cluster had the best properties for NO adsorption and activation.