The adsorption of sulfur on Rh(111) and Cu/Rh(111) surfaces

reaction of S-2 with Rh(lll) and Cu/Rh(lll) surfaces has been investigated using synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy and ab initio self-consistent-field calculations. At 100 K, the adsorption of St on Rh(lll) produces multilayers of S-n species (n = 2 - 8) that desorb between 300 and 400 K, leaving a him of RhSx on the sample. S-2 dissociates upon adsorption on clean Rh(lll) at 300 K. An adsorption complex in which S-2 is bridge bonded to two adjacent Rh atoms (Rh-S-S-Rh) is probably the precursor state for the dissociation of the molecule. The larger the electron transfer from Rh(111) into the S-2(2 pi(g)) orbitals, the bigger the adsorption energy of the molecule and the easier the cleavage of the S-S bond. On Rh(111) at 300 K, chemisorbed S is bonded to two dissimilar adsorption sites (hollow and probably bridge) that show well separated S 2p binding energies and different bonding interactions. Adsorption on bridge sites is observed only at S coverages above 0.5 ML, and precedes the formation of RhSx films. The bonding of S to Rh(lll) induces a substantial decrease in the density of d states that the metal exhibits near the Fermi level, but the electronic perturbations are not as large as those found for S/Pt(111) and S/Pd(lll). Cu adatoms significantly enhance the rate of sulfidation of Rh(lll) through indirect Cu<->Rh<->S-2 and direct Cu<->S-S<->Rh interactions. In the presence of Cu there is an increase in the thermal stability of sulfur on Rh(111). The adsorption of S-2 on Cu/Rh(lll) surfaces produces CuSy and RhSx, species that exhibit a distinctive band structure and decompose at temperatures between 900 and 1100 K: CuSy/RhSx/Rh(111)-->S-2(gas) +Cu(gas)+S/Rh(111). (C) 1998 American Institute of Physics.