Chemistry of NO₂ and SO₂ on Ice Layers and H₂O/Zn Interfaces:  Photoemission Studies on the Formation of Acid Water and Metal Corrosion

Synchrotron-based photoelectron spectroscopy was used to study adsorption of sulfur dioxide and nitrogen dioxide on ice multilayers deposited on zinc. The formation of acid water (H 2 O/H x SO 4 or H 2 O/H x NO 3 ) and the corrosion of the underlying Zn metal were investigated. Small amounts of dosed SO 2 react with an ice multilayer at 100 K producing sulfuric acid-like species (H x SO 4 ) on the surface. Extensive dosing leads to a condensed SO 2 multilayer which is thermally unstable and desorbs upon mild heating to 130−150 K. Above 170 K, when also the ice multilayer desorbs, the only sulfur species on the Zn surface is H x SO 4 . The {H 2 O/H x SO 4 } mixture eventually oxidizes the metal, and the adsorbed SO 4 groups are stable to at least 350 K. At 100 K, the initial stage of a NO 2 adsorption on ice is nonreactive. NO 2 reacts with ice only at higher exposures, producing a small amount of nitric acid-like species (H x NO 3 ), followed by condensation of an O 2 N−NO 2 multilayer. The N 2 O 4 multilayer partially desorbs at temperatures around 150 K and partially reacts with ice, producing more H x NO 3 . This species transforms to H x NO 2 (perhaps HONO) or chemisorbed NO 2 upon removal of the ice multilayer (above 170 K). Further decomposition of H x NO 2 occurs between 250 and 350 K, and then the Zn surface is essentially free of any nitrogen species but contains lots of oxygen.