Surface Chemistry of SO₂on Sn and Sn/Pt(111) Alloys:  Effects of Metal−Metal Bonding on Reactivity toward Sulfur

The surface chemistry of SO 2 on polycrystalline Sn, Pt(111), and a ( x )R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent field calculations. Metallic tin has a large chemical affinity for SO 2 . At 100−150 K, SO 2 disproportionates on polycrystalline tin forming multilayers of SO 3 (2SO 2,a → SO gas + SO 3,a ). At these low temperatures, the full dissociation of SO 2 (SO 2,a → S a + 2O a ) is minimal. As the temperature is raised to 300 K, the SO 3 decomposes, yielding SO 4, S, and O on the surface. Pure tin exhibits a much higher reactivity toward SO 2 than late transition metals (Ni, Pd, Pt, Cu, Ag, Au). In contrast, tin atoms in contact with Pt(111) interact weakly with SO 2 . A ( × )R30°-Sn/Pt(111) alloy is much less reactive toward SO 2 than polycrystalline tin or clean Pt(111). At 100 K, SO 2 adsorbs molecularly on ( × )R30°-Sn/Pt(111). Most of the adsorbed SO 2 desorbs intact from the surface (250−300 K), whereas a small fraction dissociates into S and O. The drastic drop in reactivity when going from pure tin to the ( × )R30°-Sn/Pt(111) alloy can be attributed to a combination of ensemble and electronic effects. On the other hand, the low reactivity of the Pt sites in ( × )R30°-Sn/Pt(111) with respect to Pt(111) is a consequence of electronic effects. 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 both metals are not able to respond in an effective way to the presence of SO 2 . The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that have a remarkably low reactivity toward SO 2 and for controlling sulfur poisoning.