| Abstract: | Complementary infrared and x-ray photoelectron spectra have been interpreted in a detailed study of surface equilibria involving SO₂, NO₂, N₂O₄, and H₂O adsorbed on amophous silica and silica alumina. Heterogeneous oxidation of SO₂ to sulfuric acid and sulfate ions with NO₂ and water occurs on both surfaces. Reactants adsorb physically initially, and NO₂ forms a reactive nitric acid-like surface complex with adsorbed water. Strongly bound nitrates and nitrites are formed by adsorbing NO₂ onto dehydrated SiO₂ Al₂O₃. The surface reaction with SO₂ at 25ᴼ results in an extensive coverage of sulfate ions. Gas phase NO is produced in the reaction, but adsorbed oxo-cations of nitrogen were also detected. These species were identified as NO₂⁺ and NO⁺ by correlating infrared absorption peaks with N(1s) binding energies. The former ion is stabilized by a shift in acid-base equilibria that accompanies dehydration of the surfaces following the reaction. The latter ion probably exists as ionic NOHSO₄, which undergoes surface hydrolysis to H₂SO₄ and NO. Shifts of the bending mode of adsorbed water to higher wavenumbers following the reaction also suggest a strongly acidic surface environment. |
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