Surface chemistry of SO2 on Sn and Sn/Pt(111) alloys:: Effects of metal-metal bonding on reactivity toward sulfur

The surface chemistry of SO2 on polycrystalline Sn, Pt(lll), and a (root 3 x root 3)R30 degrees-Sn/Pt(lll) 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 SO2. At 100-150 K, SO2 disproportionates on polycrystalline tin forming multilayers of SO3 (2SO(2,a) --> SOgas + SO3,a). At these low temperatures, the full dissociation of SO2 (SO2,a --> S-a + 2O(a)) is minimal. As the temperature is raised to 300 K, the SO3 decomposes, yielding SO4, S, and O on the surface. Pure tin exhibits a much higher reactivity toward SO2 than late transition metals (Ni, Pd, Pt, Cu, Ag, Au) In contrast, tin atoms in contact with Pt(111) interact weakly with SO2. A (root 3 x root 3)R30 degrees-Sn/Pt(111) alloy is much less reactive toward SO2 than polycrystalline tin or clean Pt(lll). At 100 K, SO2 adsorbs molecularly on (root 3 x root 3)R30 degrees-Sn/Pt( ill). Most of the adsorbed SO2 desorbs intact from the surface (250-300K), whereas a small fraction dissociates into S and O. The drastic drop in reactivity when going from pure tin to the (root 3 x root 3)R30 degrees-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 (root 3 x root 3)R30 degrees-Sn/Pt(lll) with respect to P(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 SO2. 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 SO2 and for controlling sulfur poisoning.