Adsorption and reaction of NO₂ on a (√3 × √3)R30° Sn/Pt(1 1 1) surface alloy

Adsorption of nitrogen dioxide (NO₂) on a (√3 × √3)R30° Sn/Pt(1 1 1) surface alloy has been investigated using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED). At 120 K, NO₂ is adsorbed molecularly as the N,N-bonded dimer, N₂O₄, interacting with the surface through a single oxygen atom in an upright but tilted geometry. However, no N₂O₄ or NO₂ desorbs molecularly from the monolayer state. The dimer completely dissociates at 300 K, leaving coadsorbed NO₂, NO, and O on the surface. Adsorbed NO₂ further dissociates to coadsorbed NO and O at 300-400 K. The maximum oxygen atom coverage obtained by heating the N₂O₄ monolayer was about θ_O=0.4 ML, but this can be increased to θ_O=1.1 ML by NO₂ dosing on the alloy surface at 600 K to remove inhibition by coadsorbed NO. Under these latter conditions, adsorbed oxygen desorbs as O₂ in three clear desorption states, the lowest of which is associated with O₂ desorption from Pt sites and the other two are from decomposition of reduced tin oxide phase(s), SnO_x. Shifts in Sn AES peaks were used to follow Sn oxidation.