Adsorption properties of SO2 on ultrafine precious metal particles studied using density functional calculation

The microscopic adsorption properties of molecules including SO2 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 Pd3 and Pt3 clusters. The adsorption energies (Eads) of the molecules on the metal clusters were calculated. Different possible adsorption sites of SO2 on the Pd3 and Pt3 clusters were considered. It was found that the SO2 adsorption states on the Pd3 cluster are energetically more stable than those on the Pt3 cluster when SO2 molecule was initially located perpendicular to the cluster plane. There were only small differences in the values of Eads for the SO2 adsorption on each adsorption site of the Pd3 clusters. However, in the case of the Pt3 cluster, the values of Eads for SO2 adsorption depended on the adsorption sites. Moreover, when SO2 molecule and the cluster were on the same plane, the SO2 adsorption state on the Pd3 cluster was energetically less stable than that on the Pt3 cluster. These results indicate that the adsorption stabilities of SO2 on the Pd3 cluster strongly depend on the adsorption geometries. The difference in the adsorption stabilities of SO2 on the Pd3 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.