Repulsive Interactions between Au and S on Mo(110) and Rh(111):  An Experimental and Theoretical Study

Abstract

The coadsorption of Au and S on Mo(110) and Rh(111) has been examined using thermal desorption mass spectroscopy, photoemission, and ab initio self-consistent-field calculations. On both surfaces, the interactions between Au and S are repulsive. S reduces the adsorption energy of Au. In some cases, the weakening of the Mo−Au and Rh−Au bonds is so large (5−7 kcal/mol) that the Au adatoms “ball up” on the surface instead of “wetting” the Mo(110) and Rh(111) substrates. The ab initio SCF calculations show that a S adatom modifies the chemical behavior of adjacent Mo and Rh sites by reducing their contribution to the density of states around the Fermi level of the system, weakening in this way 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 transition-metal substrate. Gold produces dramatic changes in the reaction pathways for the removal of sulfur from the Mo and Rh surfaces. In the Au/S/Rh(111) system, S2 desorbs at much lower temperatures (100−250 K) than in S/Rh(111). For Au/S/Mo(110), the presence of Au favors desorption of S2 instead of migration of S into the bulk of Mo. These results are consistent with a model in which Au compresses the S overlayer into islands of high local coverage, reducing in this way its stability and favoring the “pairing” of sulfur atoms and desorption of S2.

Bimetallic Model Catalysts

“…The island density (and the average distance between islands) reaches a constant value around 0.1-0.15 monolayers (ML). The same is valid for the adsorption of sulfur on Au/Rh(111) [27], Au/Mo(110) [28], Au/Mo(100) [29], and Au/Pt(111) [30]. Figure 2b shows an STM image obtained after dosing gold to a Ru(001) surface that had been precovered with 0.05 ML of S [24].…”

Sulfur as a catalyst promoter or selectivity modifier in heterogeneous catalysis

“…In contrast the DOS for Pt and Pd decreases by approximately 50 and 55%, respectively indicating that both metals are more vulnerable to electronic effects from adsorbed sulfur. 11,12 However, the effect is perhaps not as simple as first perceived because calculations indicate that the electronic perturbations generated by adsorbed sulfur adatoms have a degree of long range character. 13,14 Indeed, the DOS on a rhodium surface is decreased for the nearest and next nearest surface atoms and this effect may extend for lengths of 3-3.5 Å 15 or up to as many as 13 surface atoms.…”

Adsorption of sulfur onTiC(001): Photoemission and first-principles studies

“…41,53 Thus, for 0.9 ML of sulfur on Mo͑110͒ only traces of S 2 are detected. 41 Typically, earlytransition metals bond S very strongly and a saturated chemisorbed monolayer is stable up to temperatures well above 1000 K. 53,54 In a TiC͑001͒ surface, interactions between C and Ti diminish the reactivity of the early-transition metal and similar amounts of adsorbed S and S 2 coexist at high coverages. In this respect, the behavior of the sulfur over- FIG.…”