Molecular adsorption on ultrafine precious metal particles studied by density functional calculation

The microscopic adsorption behavior of molecules including SO₂ and NO on ultrafine precious metal particles was investigated by the density functional quantum chemical calculations. The precious metals particles, which are the sites of activation in the catalyst, were modeled by Pt₅, Kh₅ and Pd₅ clusters. The adsorption energies (E_ads) of the molecules calculated on the metal clusters were calculated. Three geometries of SO₂ on the apex of Pt₅ cluster (X) were considered(a) two O atoms interact with X (C_2v symmetry; model-1), (b) one S atom interacts with X (C_2v symmetry; model-2), and (c) one O atom interacts with X (C_s symmetry; model-3). The order of E_ads values for the SO₂ adsorption states on Pt₅ clusters can be described as model-2 > model-3 > model-1. This result revealed the adsorption state of SO₂ in which one Pt-S bond formed is the most stable energetically. The order of E_ads for the SO₂ adsorption state on different metal clusters can be described as Pt > Pd > Rh. It was also determined that the adsorption state of SO₂ on metal clusters is less stable than that of NO and the order of adsorption stability on metal clusters is different for SO₂ and NO molecules.