IRAS Studies of NO₂, N₂O₃, and N₂O₄Adsorbed on Au(111) Surfaces and Reactions with Coadsorbed H₂O

Adsorption or bonding geometries of pure adlayers of several N x O y species, i.e., nitrogen dioxide (NO 2 ), dinitrogen trioxide (N 2 O 3 ), and dinitrogen tetroxide (N 2 O 4 ), on Au(111) were determined by utilizing infrared reflection−absorption spectroscopy (IRAS). Dosing NO 2 on Au(111) at 85 K produced, in our experiments, mixtures of NO 2 and N 2 O 3 (from contaminant NO) at submonolayer coverages and NO 2, N 2 O 3, and N 2 O 4 at monolayer coverages. However, a pure adlayer of chemisorbed NO 2 could be prepared by forming the monolayer on Au(111) at 85 K and then heating to 185 K or by NO 2 exposures on Au(111) at 185 K. Chemisorbed NO 2 is bonded to the surface in an O,O‘-chelating geometry with C 2 v symmetry. A monolayer of adsorbed N 2 O 3 was produced by exposing the pure, chelating NO 2 adlayer to NO(g). The adsorbed complex with N 2 O 3 has C s symmetry, and we believe that N 2 O 3 is bonded to the surface through one oxygen. Large NO 2 exposures can be used to produce crystalline N 2 O 4 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 H 2 O. Upon being heated, reactions proceed via two pathways. One route produces nitrous acid (HONO) and nitric acid (HNO 3 ) 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 N 2 O 4 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.