Sulphur-induced structural chemistry of oxide surfaces

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 SOx species: chemisorbed SO2 and SO2–4. 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.