A LEED, TPD AND HREELS INVESTIGATION OF NO ADSORPTION ON SN/PT(111) SURFACE ALLOYS

The adsorption of NO on Pt(111), and the (2 x 2)Sn/ Pt(111) and (square-root 4 x square-root 3)R30-degrees Sn/111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 x 2) LEED pattern on Pt(111) and a (2 square-root 3 x 2 square-root 3)R30-degrees LEED pattern on the (2 x 2)Sn/ Pt(111) surface. The initial sticking coefficient of NO on the (2 x 2)Sn/ Pt(111) surface alloy at 100 K is the same as that on Pt(111), S0 = 0.9, while the initial sticking coefficient of NO on the (square-root 3 x square-root 3)R30-degrees Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of beta-state NO is reduced by 8-10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm-1 in the low frequency range and only one N-O stretching frequency at 1698 cm-1. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm-1 was also observed on the (2 x 2)Sn/Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (square-root 3 x square-root 3)R30-degrees Sn/Pt(111) surface alloy at any NO coverage studied.