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 NxOy spec ies, i.e., nitrogen dioxide (NO2), dinitrogen trioxide (N2O3), and din itrogen tetroxide (N2O4), on Au(lll) were determined by utilizing infr ared reflection-absorption spectroscopy (IRAS). Dosing NO2 on Au(lll) at 85 K produced, in our experiments, mixtures of NO2 and N2O3 (from c ontaminant NO) at submonolayer coverages and NO2, N2O3, and N2O4 at mo nolayer coverages. However, a pure adlayer of chemisorbed NO2 could be prepared by forming the monolayer on Au(lll) at 85 K and then heating to 185 K or by NO2 exposures on Au(lll) at 185 K. Chemisorbed NO2 is bonded to the surface in an O,O'-chelating geometry with C-2v symmetry . A monolayer of adsorbed N2O3 was produced by exposing the pure, chel ating NO2 adlayer to NO(g). The adsorbed complex with N2O3 has C-s sym metry, and we believe that N2O3 is bonded to the surface through one o xygen. Large NO2 exposures can be used to produce crystalline N2O4 mul tilayers that have a preferential orientation of the N-N bond perpendi cular to the Au(111) surface. To probe important aspects of the reacti vity of these species with water and to investigate structure-reactivi ty relationships in this chemistry, we studied the reaction of each of these species with coadsorbed H2O. Upon being heated, reactions proce ed via two pathways. One route produces nitrous acid (HONO);md nitric acid (HNO3) and occurs for all of the nitrogen oxide species listed ab ove. These reactions do not depend on the degree of crystallinity of t he condensed water clusters. A separate path occurs only for co-conden sed amorphous ice clusters and multilayer N2O4 films, as signaled by t he formation of oxygen adatoms on the Au(lll) surface. These results r eveal new information about fundamental interactions of nitrogen oxide s and water in condensed phases.