THE INTERACTION OF SULFUR WITH AG/PT(111) SURFACES - SILVER-PROMOTED SULFIDATION OF PLATINUM

The coadsorption of sulfur and silver on Pt(111) has been examined using thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and X-ray excited Auger electron spectroscopy (XAES). The results of XPS and XAES indicate that at temperatures between 300 and 700 K, S-2 gas reacts with Pt(111) producing a chemisorbed layer of sulfur without forming bulk-like platinum sulfides. The sulfidation of platinum occurs after vapor depositing Ag on S/Pt(111) surfaces, or after exposing Ag/Pt(111) surfaces to S-2 gas. At temperatures from 400 to 700 K, the reaction of S-2 gas with Ag/Pt(111) surfaces produces silver sulfides that catalyze the sulfidation of platinum by providing surface sites for the dissociation of S-2, and by favoring the diffusion of S into the bulk of the Pt substrate. The silver and platinum sulfides in the S/Ag/Pt(111) systems decompose at temperatures between 750 and 850 K, with S-2 and S evolving into gas phase and metallic Ag remaining on top of a Pt(111) substrate partially covered by chemisorbed S. At submonolayer coverages of Ag and S on Pt(111), theta(Ag) + theta(s) < 1 ML, the presence of sulfur significantly weakens the strength of the PT <----> Ag interactions, and the desorption temperature of Ag is similar to 90 K smaller than on clean Pt(111). The TDS results indicate that a S adatom reduces the ability for bimetallic bonding of several (3-4) platinum surface atoms. (C) 1995 Academic Press, Inc.