Reaction of S-2 with ZnO and Cu/ZnO surfaces: Photoemission and molecular orbital studies

The adsorption of S-2 on ZnO and Cu/ZnO has been investigated using synchrotron-based high-resolution photoemission spectroscopy. On dosing a clean ZnO surface with S-2 at 300 K, the molecule dissociates, The S is associated first with Zn and at medium coverages with Zn-O sites. When the sulfur coverage is increased to theta( )= 0.5 ML, evidence is found for sulfur bound purely to the O sites of ZnO. The sulfur species associated with O and the Zn-O sires are unstable at temperatures above 500 K. Possible reaction pathways for the dissociation of S-2 on ZnO(0001)-Zn and Zn(1010) surfaces were studied using ab initio SCF calculations. At low sulfur coverages, an adsorption complex in which S-2 is bridge bonded to two adjacent Zn atoms (Zn-S-S-Zn) is probably the precursor state for the dissociation for the molecule, It is possible to gel much higher coverages of sulfur on ZnO (0.7 ML) than on Al2O3 (0.1 ML) at similar S-2 exposures. This, in conjunction with results previously reported for H2S adsorption on Cr2O3 and Cr3O4, indicates that the reactivity of metal oxides toward sulfur is inversely proportional to the size of their band gap. Oxides with a large band gap (e.g., Al2O3, similar to 9.0 eV) are less susceptible to sulfur adsorption than oxides with a small band gap (e.g., ZnO, similar to 3.4 eV), The presence of Cu atoms on both metal oxides enhances their respective reactivities reward S-2. Upon dosing Cu/ZnO with S-2 at 300 K, sulfur prefers to attack supported Cu followed by reaction with the Zn sites of the oxide, and at large sulfur coverages the adsorbate bonds simultaneously to metal and oxygen sites on the surface. The sulfur bonded to both the metal and oxygen sites on the surface is relatively weakly bound and desorbs by 500 K. The Cu <-> S interactions are strong and lead to the formation of copper sulfides that exhibit a distinctive band structure and decompose at temperatures above 700 K.