Adsorption and reaction of NO 2 on a (√3×√3) R30° Sn/Pt(1 1 1) surface alloy

Abstract

Adsorption of nitrogen dioxide (NO 2) 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, NO 2 is adsorbed molecularly as the N, N-bonded dimer, N 2O 4, interacting with the surface through a single oxygen atom in an upright but tilted geometry. However, no N 2O 4 or NO 2 desorbs molecularly from the monolayer state. The dimer completely dissociates at 300 K, leaving coadsorbed NO 2, NO, and O on the surface. Adsorbed NO 2 further dissociates to coadsorbed NO and O at 300–400 K. The maximum oxygen atom coverage obtained by heating the N 2O 4 monolayer was about θ O=0.4 ML, but this can be increased to θ O=1.1 ML by NO 2 dosing on the alloy surface at 600 K to remove inhibition by coadsorbed NO. Under these latter conditions, adsorbed oxygen desorbs as O 2 in three clear desorption states, the lowest of which is associated with O 2 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.