INTERACTION OF SULFUR WITH AU PT(111) AND AG/PT(111) SURFACES - PHOTOEMISSION-STUDIES/

The reaction of sulfur with Au/Pt(lll) and Ag/Pt(lll) surfaces has bee n investigated using high-resolution photoemission spectroscopy with s ynchrotron radiation. The valence spectra of S/Au/Pt(lll) and S/Ag/Pt( 111) surfaces display weak signals at binding energies between 1.8 and 0 eV. The adsorption of S induces a large decrease in the density of states displayed by the bimetallic surfaces around the Fermi level. Th is phenomenon may be responsible for the decrease in chemical and cata lytic activity that accompanies sulfur poisoning. In S/Au/Pt(lll) surf aces, sulfur does not form compounds with Au or Pt. Au forms three-dim ensional clusters, while S segregates into two-dimensional islands of high local coverage where it is bonded to hollow and bridge sites of t he Pt substrate. In S/Ag/Pt(111) surfaces, silver reacts with sulfur a nd promotes the synthesis of platinum sulfides. The formation of silve r and platinum sulfides makes possible the adsorption of large amounts of sulfur on Ag/Pt(111) surfaces. The S/Au/Pt(111) and S/Ag/Pt(lll) s ystems illustrate different extremes of the phenomena that can occur w hen sulfur is present on a bimetallic surface. The differences in the behavior of the S/Au/Pt(111) and S/Ag/Pt(111) surfaces arise from vari ations in the nature of the admetal <----> S and admetal <----> Pt int eractions.