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

Abstract

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(2×2)-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, θS<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, θS>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 S2 is now observed in TDS and the S2p core levels shift to higher binding energy. At coverages near a full monolayer, S2 is the most stable species on the surface and its adsorption energy is ∼45 kcal/mol. Similar trends are observed for the adsorption of sulfur on a p(2×2)-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.