Theoretical study of adsorption of SO₂ on Ni(1 1 1) and Cu(1 1 1) surfaces

Surface structures and electronic properties of sulfur dioxide, SO₂, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces were investigated using B3LYP density functional theory. Geometry and orientation of SO₂ were fully optimized on the metal clusters and three and two stable structures were obtained for Ni(1 1 1) and Cu(1 1 1) surfaces, respectively. For the Ni(1 1 1) surface, the most stable structure was that SO₂ adsorbs with its molecular plane nearly parallel to the surface, with all the S and O atoms in bridge sites, and in good agreement with the observed structures on the Ni(1 1 1) surface. For the Cu(1 1 1) surface, the most stable structure in which SO₂ adsorbs with its plane perpendicular to the surface and with O atoms in on-top sites, was also in accord with the observed one on the Cu(1 1 1) surface. The adsorption energy was much larger for the Ni surface than for the Cu surface. The result of a Mulliken population analysis showed that the σ donation from the SO₂ σ orbital to the surface, and π back donation from the surface to the SO₂ π* orbital, play important roles in the adsorption, and that the amounts of both σ donation and π back donation were larger for the Ni surface than for the Cu surface. Upon adsorption the S-O bond length was elongated and the O-S-O angle became contracted.