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 (squar e-root 4 x square-root 3)R30-degrees Sn/111) surface alloys has been s tudied using LEED, TPD and HREELS. NO adsorption produces a (2 x 2) LE ED pattern on Pt(111) and a (2 square-root 3 x 2 square-root 3)R30-deg rees LEED pattern on the (2 x 2)Sn/ Pt(111) surface. The initial stick ing 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 l ayer 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 all oys show two vibrational frequencies at 285 and 478 cm-1 in the low fr equency range and only one N-O stretching frequency at 1698 cm-1. We a ssign 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/P t(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 a t any NO coverage studied.