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 (ro ot 3 x root 3)R30 degrees-Sn/Pt(lll) surface alloy has been investigat ed 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 polycrystall ine tin forming multilayers of SO3 (2SO(2,a) --> SOgas + SO3,a). At th ese 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 decomp oses, 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 we akly 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). A t 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)R3 0 degrees-Sn/Pt(111) alloy can be attributed to a combination of ensem ble and electronic effects. On the other hand, the low reactivity of t he 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 comp lex, involving a Sn(5s,5p)--> Pt(6s,6p) charge transfer and a Pt(5d)-- > Pt(6s,6p) rehybridization that localize electrons in the region betw een the metal centers. These phenomena reduce the electron donor abili ty of Pt and Sn, and both metals are not able to respond in an effecti ve way to the presence of SO2. The Sn/Pt system illustrates how a redi stribution of electrons that occurs in bimetallic bonding can be usefu l for the design of catalysts that have a remarkably low reactivity to ward SO2 and for controlling sulfur poisoning.