Adsorption properties of SO 2 on ultrafine precious metal particles studied using density functional calculation

Abstract

The microscopic adsorption properties of molecules including SO 2 on ultrafine precious metal particles such as Pd and Pt were investigated using density functional quantum chemical calculations. The precious metal particles which are used as the activation sites in a three-way catalyst (TWC) were modeled by Pd 3 and Pt 3 clusters. The adsorption energies ( E ads) of the molecules on the metal clusters were calculated. Different possible adsorption sites of SO 2 on the Pd 3 and Pt 3 clusters were considered. It was found that the SO 2 adsorption states on the Pd 3 cluster are energetically more stable than those on the Pt 3 cluster when SO 2 molecule was initially located perpendicular to the cluster plane. There were only small differences in the values of E ads for the SO 2 adsorption on each adsorption site of the Pd 3 clusters. However, in the case of the Pt 3 cluster, the values of E ads for SO 2 adsorption depended on the adsorption sites. Moreover, when SO 2 molecule and the cluster were on the same plane, the SO 2 adsorption state on the Pd 3 cluster was energetically less stable than that on the Pt 3 cluster. These results indicate that the adsorption stabilities of SO 2 on the Pd 3 cluster strongly depend on the adsorption geometries. The difference in the adsorption stabilities of SO 2 on the Pd 3 cluster can be explained by the difference in the orbital interaction near the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels.