INFLUENCE OF ALLOYED SN ATOMS ON THE CHEMISORPTION PROPERTIES OF NI(111) AS PROBED BY RAIRS AND TPD STUDIES OF CO ADSORPTION

The chemisorption of CO on Ni-Sn alloys formed on a Ni(111) surface has been studied with reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Formation of the (root 3 x root 3)R30 degrees-Sn/Ni(111) surface alloy strongly suppresses CO adsorption. Only 0.04 ML CO can be chemisorbed on this surface at 110 K, exclusively at atop sites. The binding energy of this adsorbed CO is reduced to only about 15 kcal/mol. Additionally, no significant effect of subsurface Sn on CO chemisorption on this alloy was observed. These results are in sharp contrast to our previous results for CO chemisorption on the (root 3 x root 3)R30 degrees-Sn/Pt(111) surface alloy, where the chemisorbed CO saturation coverage, adsorption site distribution and desorption temperature were quite similar to those properties of the Pt(111) surface. We ascribe the influence of alloyed Sn atoms on the CO chemisorption properties of this Sn/Ni(111) surface alloy to be due to repulsive Sn-CO interactions at the NI-CO distance required for chemisorption. The differences in the chemisorption properties of these Pt-Sn and Ni-Sn alloys are rationalized by considering the different sizes of the surface unit cells and the location of Sn with respect to the surface plane (the Sn buckling distance). These results have important implications for the discussion and determination of ensemble sizes at bimetallic surfaces since the surface chemistry depends strongly on the vertical position of the modifier.