NO2 ADSORPTION ON NI(100) - A COMPARISON OF NO2 WITH CO2 ADSORPTION

NO2 adsorption has been studied on Ni(100) at temperatures between 90 and 400 K via HREELS, ARUPS, XPS and NEXAFS. It is shown that NO2 dissociates at low temperatures and small exposures forming atomic oxygen and molecularly adsorbed NO. HREELS data of NO Ni(100) in comparison with those of NO + O Ni(100) indicate that the molecular axis of NO in the coadsorbed layer is tilted away from the surface normal. After saturation of the dissociative adsorption NO2 will chemisorb on the surface. This has been followed by HREELS and XPS. NEXAFS data indicate that the chemisorbed NO2 moiety is adsorbed with the molecular plane perpendicular to the surface plane and the nitrogen end down. At high NO2 exposures and at low temperatures physisorbed N2O4 is formed on top of this relatively complex chemisorbed layer. It is likely that the molecular plane of N2O4 is oriented parallel to the metal surface. The adsorption of NO2 on Ni(100) is compared with other NO2 adsorption systems, and it is shown in comparison with the HREELS data of another triatomic, i.e. CO2, that the vibrational spectra represent finger prints of the adsorption geometry of these triatomic molecules. © 1990.