Reaction of S2 and H2S with Sn/Pt(111) surface alloys: Effects of metal–metal bonding on reactivity towards sulfur

Abstract

The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a (∛×∛)R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. At 100–300 K, S2 chemisorbs and reacts on polycrystalline tin to form metal sulfides. The reactivity of pure tin toward sulfur is large even at a temperature as low as 100 K. In contrast, tin atoms in contact with Pt(111) interact weakly with S2 or H2S. Tin does not prevent the bonding of S to Pt in a (∛×∛)R30°-Sn/Pt(111) surface alloy, but the alloy is less reactive toward H2S than polycrystalline Sn or pure Pt(111). At room temperature, S2 and H2S adsorb dissociatively on Pt sites of (∛×∛)R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to (∛×∛)R30°-Sn/Pt(111), one sees the formation of only a chemisorbed layer of sulfur (i.e., no sulfides of tin or platinum are formed). The Pt–Sn bond is complex, involving a Sn(5s,5p)→Pt(6s,6p) charge transfer and a Pt(5d)→Pt(6s,6p) rehybridization that localize electrons in the region between the metal centers. These phenomena reduce the electron donor ability of Pt and Sn, and the metals are not able to respond in an effective way to the presence of species that are strong electron acceptors like S2, HS, and S. The redistribution of charge produces surfaces that have a remarkable low reactivity toward sulfur. When compared to other admetals (Cu, Zn, Ag, Au), tin is the best choice as a site blocker that can enhance the tolerance of Pt reforming catalysts to sulfur poisoning. The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that are less sensitive to the presence of S-containing molecules.