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 θO=0.4 ML, but this can be increased to θO=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), SnOx. Shifts in Sn AES peaks were used to follow Sn oxidation.