Reaction of SO 2 with ZnO(0001̄)–O and ZnO powders: photoemission and XANES studies on the formation of SO 3 and SO 4

Abstract

Synchrotron-based high-resolution photoemission and X-ray absortion near-edge spectroscopy (XANES) have been used to study the interaction of SO 2 with ZnO(0001̄)–O and polycrystalline films and bulk powders of ZnO. The adsorption of SO 2 on the (0001̄) oxygen-terminated face of ZnO at 110 K produced SO 3 species which were stable up to temperatures well above 400 K. On polycrystalline ZnO, the Zn↔SO 2 interactions were very weak, there was no decomposition of the admolecule, and evidence for bonding of zinc to SO 2 was found only at low temperatures (∼100 K) when the adsorbate had a limited mobility on the surface. At 300 K and moderate pressures, SO 2 only reacted with the oxygen centers of ZnO forming mainly SO 3 groups. Part of the formed SO 3 decomposed at temperatures between 400 and 600 K with evolution of SO 2 into the gas phase (ZnSO 3,solid→SO 2,gas+ZnO solid). At 600 K, a significant amount of SO 3 was still present on the ZnO systems. Reaction with O 2 led to an SO 3→SO 4 transformation on ZnO(0001̄)–O and powders of ZnO previously exposed to SO 2. SO 4 was directly formed during the adsorption of SO 2 on polycrystalline surfaces of zinc oxide that were rich in oxygen atoms which had a low coordination number (<3) and a relatively high mobility. On ZnO(0001̄)–O and polycrystalline ZnO, there was no decomposition of sulfur dioxide. Promotion with potassium and cesium facilitated the cleavage of SO bonds. After dosing SO 2 to ( 3 × 3 )- K/ZnO(000 1 ̄ )– O and ( 3 × 3 )- Cs/ZnO(000 1 ̄ )– O at 300 K, several sulfur species (S, SO 3, SO 4) coexisted on the alkali-metal-promoted surfaces whereas only SO 3 was formed on the pure ZnO(0001̄)–O system.