INTERACTION OF SULFUR WITH BIMETALLIC SURFACES - COADSORPTION OF SULFUR AND NOBLE-METALS ON RU(001)

The coadsorption of S with Cu or Ag on Ru(001) has been investigated using TDS, XPS, XAES, and CO chemisorption. At 300 K, copper and silver atoms in contact with Ru(001) react with S-2 to form noble-metal sulfides. The Cu-S and Ag-S bonds in these surface compounds break at high temperatures (>800 K) producing sulfur and noble-metal adatoms that compete for the ruthenium electrons. This competition leads to a weakening of 5-6 kcal/mol in the strength of the Ru-Cu and Ru-Ag bonds. A sulfur adatom produces long-range perturbations on the surface, diminishing the ability for bimetallic bonding of several (5-10) adjacent ruthenium atoms. At theta(s) = 0.2 ML (ML = monolayer), all the ruthenium sites show a strong weakening in their bonding interactions with copper or silver adatoms. Photoemission experiments examining the interaction of S-2 with copper and silver multilayers at 300 K show the formation of thick films of Cu2S and Ag2S at a fast rate. The decomposition pathways for these films are similar: evolution of S-2 into gas phase, with the noble metal remaining solid. For Ag2S films the decomposition process starts around 800 K, whereas Cu2S films are stable up to 950 K.