Surface Chemistry of SO_2 on Zn and ZnO: Photoemission and Molecular Orbital Studies

摘要

The reaction of SO_2 with polycrystalline Zn and ZnO surfaces has been investigated using synchrotron-based high-resolution photoemission spectroscopy and ab initio self-consistent-field calculations. The chemistry of SO_2 on Zn surfaces in quite complex and depends on both the temperature of adsorption and the SO_2 exposure. At 300 K, SO_2 dissociates on a clean Zn surface to form atomic sulfur and atomic oxygen (SO_(2,gas) → S_a + 20_a; SO_(2,gas) → SO_(gas) + O_a). The Zn <-> SO_2 bonding interactions induce a significant weakening of the S-O bonds. The theoretical calculations suggest the η~2-O,O and η~2-S,O binding conformations of SO_2 as the two possible precursors for the dissociation of the molecule. The dissociation reactions are more exothermic than the formation of SO_3 or SO_4: SO_(2,gas) + nO_a → SO_x, where x = 3 or 4. At high SO_2 exposures (300 K), when most surface sites are blocked and dissociation of SO_2 cannot occur, SO_3 and SO_4 are formed on the Zn surface. Adsorption at 100 K suppresses the SO_2 dissociation completely, and SO_3 is formed through the disproportionation reaction 2SO_(2,a) → SO_(3,a). Zn shows a much higher reactivity toward SO__2 than late transition metals. All the results for the reaction of SO_2 with ZnO surfaces indicate oxygen to be the active site. The Zn-O bonds in ZnO substantially reduce the electron density of zinc, the metal centers become poor electron donors for π-back-donation into the LUMO of SO_2, and the molecule mostly bonds to the oxygen sites of the oxide surface. Dosing SO_2 on ZnO at 300 K results in the formation of surface SO_4 species, which are stable to temperatures above 500 K. Results from the ab initio SCF calculations indicate that SO_2 adsorbs on an oxygen site to form SO_3, which then reacts with a lattice oxygen atom to form SO_4. The last step in this process has a substantial activation energy, and after dosing SO_2 to ZnO at 100 K a mixture of SO_3 and SO_4 is produced on the surface.