Sulphur-induced structural chemistry of oxide surfaces

Abstract

Surface EXAFS, NEXAFS and photoemission have been used to study the structure of surface phases resulting from reaction of H₂S with NiO(100) and SO₂ with TiO₂(110). S K-edge SEXAFS data, in conjunction with the LEED pattern symmetry, suggest that the NiO(100)/H₂S reaction at 570 K results in reduction of the substrate to form an azimuthally aligned Ni(100)c(2 × 2)S raft, with S occupying the fourfold Ni(100) hollow site. S K-edge NEXAFS results for SO₂ adsorption on low-temperature TiO₂(110) identify two surface SO_x species: chemisorbed SO₂ and SO^2–₄. The polarisation dependence of the NEXAFS suggests that the SO₂ molecular plane lies close to parallel to the surface. Photoemission data also evidence chemisorption at 110 K and show that this precursor phase reacts further in a thermally activated process to form a stable surface sulphate-like species. A mechanism is suggested which involves SO₂ bonding to a terrace Ti site, with an activated hop to an adjacent site in which sulphur bonds to two oxygen atoms on the raised row.