REACTION OF S-2 AND H2S WITH SN PT(111) SURFACE ALLOYS - EFFECTS OF METAL-METAL BONDING ON REACTIVITY TOWARDS SULFUR/

The surface chemistry of S-2 and H2S on polycrystalline Sn, Pt(111), a nd a (root 3 X root 3)R30 degrees-Sn/Pt(111) surface alloy has been in vestigated using synchrotron-based high-resolution photoemission and a b initio self-consistent-field calculations. At 100-300 K, S-2 chemiso rbs and reacts on polycrystalline tin to form metal sulfides. The reac tivity 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 wea kly with S-2 or H2S. Tin does not prevent the bonding of S to Pt in a (root 3 X root 3)R30 degrees-Sn/Pt(111) surface alloy, but the alloy i s less reactive toward H2S than polycrystalline Sn or pure Pt(111). At room temperature, S-2 and H2S adsorb dissociatively on Pt sites of (r oot 3 X root 3)R30 degrees-Sn/Pt(111). Upon the dosing of S-2 and H2S to (root 3 X root 3)R30 degrees-Sn/Pt(111), one sees the formation of only a chemisorbed layer of sulfur (i.e., no sulfides of tin or platin um are formed). The Pt-Sn bond is complex, involving a Sn(5s, 5p) --> Pt(6s, 6p) charge transfer and a Pt(Sd) --> Pt(6s, 6p) rehybridization that localize electrons in the region between the metal centers. Thes e phenomena reduce the electron donor ability of Pt and Sn, and the me tals are not able to respond in an effective way to the presence of sp ecies that are strong electron accepters like S-2, HS, and S. The redi stribution of charge produces surfaces that have a remarkable low reac tivity toward sulfur. When compared to other admetals (Cu, Zn, Ag, Au) , tin is the best choice as a site blocker that can enhance the tolera nce of Pt reforming catalysts to sulfur poisoning. The Sn/Pt system il lustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that are less sensit ive to the presence of S-containing molecules. (C) 1998 American Insti tute of Physics. [S0021-9606(98)70234-X].