The adsorption and reactions of NO 2 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 NO 2 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 NO 3(ads). At intermediate annealing temperatures, however, reactions are observed within the bulk N 2O 4 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 N 2O 4 and NO 2 changes drastically, producing a pure NO 2 monomer condensed phase. No XPS spectrum of NO 2 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 N 2O 4 at 25 K produces a different sequence of events on annealing to 300 K. Only traces of NO 2 monomer are observed and only a small amount, if any, of NO 3(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.