The adsorption and reactions of NO₂ on the Ag(111) surface II. Adsorption at 25 K and annealing to 300 K

Abstract

XPS has been used to follow the adsorption of NO2 on Ag(111) at 25 K followed by annealing to 300 K. For initial conditions of thick multilayers at 25 K reactions with the substrate are identical to those at 90 K; initial adsorption is dissociative to NO(ads) + O(ads) and the final state on warming from 25 to 300 K. is NO3(ads). At intermediate annealing temperatures, however, reactions are observed within the bulk N2O4 condensed phase which were not observed for the studies at 90 K where these initial thicknesses of condensate were not achieved. Between 112 and 145 K the equilibrium between N2O4 and NO2 changes drastically, producing a pure NO2 monomer condensed phase. No XPS spectrum of NO2 monomer, gaseous or condensed, seems to have been reported before. Desorption occurs rapidly at temperatures at and above this conversion. Starting with a low coverage of N2O4 at 25 K produces a different sequence of events on annealing to 300 K. Only traces of NO2 monomer are observed and only a small amount, if any, of NO3(ads) is produced. By 300 K these species and most of the initially dissociated product, NO(ads), have all desorbed leaving dominant a oxygen species which we believe must be OH(ads) on the surface, based on its O(1s) BE. © 1990.