Adsorption of nitrogen dioxide on polycrystalline gold

Abstract

The adsorption of nitrogen dioxide (NO2) on a polycrystalline Au surface was studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Three desorption states due to chemisorbed NO2 were observed using TPD, with desorption activation energies,Ed, of 11,13, and 17 kcal/mol. The desorption energies reflect the heats of adsorption of NO2 on the polycrystalline gold surface, since NO2 adsorption is not an activated process. Desorption of physisorbed NO2 from N2O4 multilayers was also seen at 130–140 K. The sticking probability of NO2 at 120 K is independent of coverage indicating a strong influence of a precursor state in the adsorption kinetics. Vibrational spectra using HREELS show that chemisorbed NO2 is molecularly adsorbed on the surface, probably as a Au O,O'-nitrito surface chelate. No evidence for the dissociation of NO2 on Au was found using AES, TPD, or HREELS, even for large exposures of NO2 at surface temperatures up to 500 K. Comparison of these results with those for NO2 adsorption on a Au(111) surface is made. High energy sites, such as steps and kinks, and other crystal faces of Au can chemically bond NO2 more tightly than occurs on Au(111), but the activation energy for dissociation of NO2 at all of these sites exceeds 17 kcal/mol, and thus NO2 adsorption is reversible on Au under low pressure conditions.