REPULSIVE INTERACTIONS BETWEEN AU AND S ON MO(110) AND RH(111) - AN EXPERIMENTAL AND THEORETICAL-STUDY

The coadsorption of Au and S on Mo(110) and Rh(111) has been examined using thermal desorption mass spectroscopy, photoemission, and ab init io self-consistent-field calculations. On both surfaces, the interacti ons between Au and S are repulsive. S reduces the adsorption energy of Au. In some cases, the weakening of the Mo-An and Rh-Au bonds is so l arge (5-7 kcal/mol) that the Au adatoms ''ball up'' on the surface ins tead of ''wetting'' the Mo(110) and Rh(111) substrates. The ab initio SCF calculations show that a S adatom modifies the chemical behavior o f adjacent Mo and Rh sites by reducing their contribution to the densi ty of states around the Fermi level of the system, weakening in this w ay their ability to form strong bonds with Au. In the Au/S/Mo(110) and Au/S/Rh(111) surfaces, Au and S compete for the electrons of the tran sition-metal substrate. Gold produces dramatic changes in the reaction pathways for the removal of sulfur from the Mo and Rh surfaces. In th e Au/S/Rh(111) system, S-2 desorbs at much lower temperatures (100-250 K) than in S/Rh(111). For Au/S/Mo(110), the presence of Au favors des orption of St instead of migration of S into the bulk of Mo. These res ults are consistent with a model in which An compresses the S overlaye r into islands of high local coverage, reducing in this way its stabil ity and favoring the ''pairing'' of sulfur atoms and desorption of S-2 .