Adsorption of sulfur on bimetallic surfaces: Formation of copper sulfides on Pt(111) and Ru(001)

Abstract

The interaction of sulfur and copper on Pt(111) and Ru(001) has been examined using x-ray photoelectron spectroscopy and thermal desorption mass spectroscopy. Cu/Pt(111) and Cu/Ru(001) surfaces were exposed to S2 gas at 300 K. It was found that for both substrates stable copper–sulfide films were formed, of which the Cu–S/Ru(001) system was the more stable. The decomposition of the Cu2S films on Pt(111) occurred at 600–850 K and on Ru(001) at 900–1000 K. Breakdown of the films led to evolution into the gas phase of sulfur (mainly as S2) without desorption of copper or the sulfur chemisorbed on the substrate. This chemisorbed sulfur desorbed in a broad feature from 1000 to 1500 K. For the Ru(001) substrate the Cu atoms remained on the surface until they desorbed at 1080 K, while on Pt(111) the Cu atoms migrated into the Pt(111) surface to form a subsurface Cu–Pt alloy; no Cu desorption features were seen at temperatures as high as 1300 K. On Pt(111), copper sulfide promoted the formation of a bulklike platinum sulfide. No sulfidation of ruthenium was detected in the presence of copper–sulfide films. For both substrates the sulfur atoms were found to highly perturb the copper sites, decreasing the ability of the noble-metal adatoms to adsorb CO.