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

The reaction of ${\mathrm{S}}_{\mathrm{2}}$ with Rh(111) and Cu/Rh(111) 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 ${\mathrm{S}}_{\mathrm{2}}$ on Rh(111) produces multilayers of ${\mathrm{S}}_n$ species $\text{(n=2–8)}$ that desorb between 300 and 400 K, leaving a film of ${\mathrm{RhS}}_x$ on the sample. ${\mathrm{S}}_{\mathrm{2}}$ dissociates upon adsorption on clean Rh(111) at 300 K. An adsorption complex in which ${\mathrm{S}}_{\mathrm{2}}$ 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 ${\mathrm{S}}_{\mathrm{2}}{\text{(2π}}_g)$ 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 $\text{2p}$ 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 ${\mathrm{RhS}}_x$ films. The bonding of S to Rh(111) 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(111). Cu adatoms significantly enhance the rate of sulfidation of Rh(111) through indirect ${\mathrm{Cu\leftrightarrow Rh\leftrightarrow S}}_{\mathrm{2}}$ and direct $\mathrm{Cu\leftrightarrow S–S\leftrightarrow Rh}$ interactions. In the presence of Cu there is an increase in the thermal stability of sulfur on Rh(111). The adsorption of ${\mathrm{S}}_{\mathrm{2}}$ on Cu/Rh(111) surfaces produces ${\mathrm{CuS}}_y$ and ${\mathrm{RhS}}_x$ species that exhibit a distinctive band structure and decompose at temperatures between 900 and 1100 K: ${\mathrm{CuS}}_y/{\mathrm{RhS}}_x/{\mathrm{Rh(111)\to S}}_{\mathrm{2}}\mathrm{(gas)}$ +Cu(gas)+S/Rh(111).