Reaction of SO2 with ZnO(000(1)over-bar)-O and ZnO powders:: photoemission and XANES studies on the formation of SO3 and SO4

Synchrotron-based high-resolution photoemission and X-ray absortion near-edge spectroscopy (XANES) have been used to study the interaction of SO2 with ZnO(000 (1) over bar)-O and polycrystalline films and bulk powders of ZnO. The adsorption of SO2 on the (000 (1) over bar) oxygen-terminated face of ZnO at 110 K produced SO3 species which were stable up to temperatures well above 400 K. On polycrystalline ZnO, the Zn<->SO2 interactions were very weak, there was no decomposition of the admolecule, and evidence for bonding of zinc to SO2 was found only at low temperatures (similar to 100 K) when the adsorbate had a limited mobility on the surface. At 300 K and moderate pressures, SO2 only reacted with the oxygen centers of ZnO forming mainly SO3 groups. Part of the formed SO3 decomposed at temperatures between 400 and 600 K with evolution of SO2 into the gas phase (ZnSO3,solid-->SO2,gas + ZnOsolid). At 600 K, a significant amount of SO2 was still present on the ZnO systems. Reaction with O-2 led to an SO3-->SO4 transformation on ZnO(000 (1) over bar)-O and powders of ZnO previously exposed to SO2. SO4 was directly formed during the adsorption of SO2 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(000 (1) over bar)-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 SO2 to (root 3 x root 3)-K/ZnO(000 (1) over bar)-O and (root 3 x root 3)-Cs/ZnO(000 (1) over bar)-O at 300 K, several sulfur species (S, SO3, SO4) coexisted on the alkali-metal-promoted surfaces whereas only SO3 was formed on the pure ZnO(000 (1) over bar)-O system. (C) 1999 Elsevier Science B.V. All rights reserved.