IRAS studies of NO₂, N₂O₃, and N₂O₄ adsorbed on Au(111) surfaces and reactions with coadsorbed H₂O

Adsorption or bonding geometries of pure adlayers of several N_xO_y species, i.e., nitrogen dioxide (NO₂), dinitrogen trioxide (N₂O₃), and dinitrogen tetroxide (N₂O₄), on Au(111) were determined by utilizing infrared reflection-absorption spectroscopy (IRAS). Dosing NO₂ on Au(111) at 85 K produced, in our experiments, mixtures of NO₂ and N₂O₃ (from contaminant NO) at submonolayer coverages and NO₂, N₂O₃, and N₂O₄ at monolayer coverages. However, a pure adlayer of chemisorbed NO₂ could be prepared by forming the monolayer on Au(111) at 85 K and then heating to 185 K or by NO₂ exposures on Au(111) at 185 K. Chemisorbed NO₂ is bonded to the surface in an O,O′-chelating geometry with C_2ν, symmetry. A monolayer of adsorbed N₂O₃ was produced by exposing the pure, chelating NO₂ adlayer to NO(g). The adsorbed complex with N₂O₃ has C_s symmetry, and we believe that N₂O₃ is bonded to the surface through one oxygen. Large NO₂ exposures can be used to produce crystalline N₂O₄ multilayers that have a preferential orientation of the N-N bond perpendicular to the Au(111) surface. To probe important aspects of the reactivity of these species with water and to investigate structure-reactivity relationships in this chemistry, we studied the reaction of each of these species with coadsorbed H₂O. Upon being heated, reactions proceed via two pathways. One route produces nitrous acid (HONO) and nitric acid (HNO₃) and occurs for all of the nitrogen oxide species listed above. These reactions do not depend on the degree of crystallinity of the condensed water clusters. A separate path occurs only for co-condensed amorphous ice clusters and multilayer N₂O₄ films, as signaled by the formation of oxygen adatoms on the Au(111) surface. These results reveal new information about fundamental interactions of nitrogen oxides and water in condensed phases.