Coadsorption of Nitrogen Dioxide and Oxygen on Pt(111)

Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) have been used to determine the NO₂ adsorption energy and geometry in the presence of coadsorbed oxygen atoms on Pt(III) at 100 K. NO₂ is adsorbed molecularly, regardless of the oxygen atom precoverage. Coadsorption with oxygen atoms causes a decrease in the amount of irreversible NO₂ adsorption and also a decrease in the amount of the Pt(III) μ-N,O-nitrito surface complex (bridge-bonded NO₂). Coadsorption also causes the formation of a new, low-temperature desorption state for NO₂ which is associated with an NO₂ bonding geometry possessing C2υ symmetry. It is proposed that this NO₂ species is bonded to a platinum atom through the nitrogen atom in a nitro configuration. An NO₃ species is not observed spectroscopically at any oxygen atom coverage in this work. However, an NO₃ intermediate may be responsible for some of the features in the adsorption and desorption kinetics. At θ_o = 0.75 monolayer (ML), a chemisorbed NO₂ saturation coverage of 0.15 ML can be achieved and all NO₂ adsorption is reversible, having a chemisorption bond energy of 11 kcal/mol. When the chemisorbed state is saturated, an N₂O₄ multilayer can be formed at 100 K with greater NO₂ exposures.