Trends in the bonding of CO to the surfaces of Pt3M alloys (M=Ti, Co, and Sn)

Abstract

Studies of the surface composition and CO chemisorption on the [111] and [100] oriented single crystals of the isostructural [face-centered-cubic (fcc)] alloys of bulk stoichiometry Pt3M (M=Ti, Co, and Sn) are reviewed. In principle, the study of these alloys is ideal for separating ensemble effects from electronic effects in adsorbate bonding, since the surfaces of these ordered alloys formed by bulk truncation have the M atoms in fixed positions without any M–M pair sites. Also by varying M from Ti to Co to Sn one can examine the effect of the d and s–p orbital contributions to the intermetallic bond on the binding energy of CO. In practice, one must account for surface segregation, and the formation of surfaces that do not have the ideal structure and composition. The strength of the intermetallic bond is found to play a major role in determining the composition of the surface. In the very exothermic alloys Pt3Ti and Pt3Sn, the thermodynamic tendency for the atom with the lower surface energy to be at the surface determines which of the two bulk planes forms the surface plane in the [100] orientation, either pure Pt(Pt3Ti) or 50% M(Pt3Sn). In CoPt3 where the intermetallic bonding is relatively weak, the lower surface energy of Pt produces pure Pt planes on both the [111] and [100] orientations. Photoemission and thermal desorption studies show that the intermetallic bonding in all three alloys has only a small effect on the binding energy of CO, a decrease in adsorption energy of 20 kJ/mol. The surprising invariance of adsorption energy on Pt3M surfaces having such a large variation in the d-orbital configuration of the M atom is attributed to the predominance of s–p orbital contributions to the intermetallic bond in these alloys.