Interaction of sulfur with Pt(111) and Sn/Pt(111): Effects of coverage and metal-metal bonding on reactivity toward sulfur

In the chemical and petrochemical industries, Pt-based catalysts are very sensitive to sulfur poisoning. Synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy (TDS), and first-principles density-functional slab calculations were used to study the adsorption of sulfur on Pt(111) and a p(2x2)-Sn/Pt(111) surface alloy. Our results show important variations in the nature of the bonding of sulfur to Pt(111) depending on the coverage of the adsorbate. For small coverages, theta (S)<0.3 ML, atomic sulfur is the most stable species. The adsorbate is bonded to hollow sites, has a large adsorption energy (> 75 kcal/mol), and desorbs as S. The Pt-S bonds are mainly covalent but sulfur induces a significant decrease in the density of Pt 5d states near the Fermi level. When the sulfur coverage increases on the surface, theta (S)>0.4 ML, there is a substantial weakening in the Pt <---->S interactions with a change in the adsorption site and a tendency to form S-S bonds. Desorption of S-2 is now observed in TDS and the S2p core levels shift to higher binding energy. At coverages near a full monolayer, S-2 is the most stable species on the surface and its adsorption energy is similar to 45 kcal/mol. Similar trends are observed for the adsorption of sulfur on a p(2x2)-Sn/Pt(111) surface alloy, but the adsorbate <----> substrate interactions are weaker than on Pt(111). The formation of Pt-Sn bonds reduces the reactivity of Pt toward sulfur. Electronic effects associated with bimetallic bonding can be useful for controlling or preventing sulfur poisoning. (C) 2000 American Institute of Physics. [S0021-9606(00)70348-5].